MINISTERUL MINELOR, PETROLULUI Si GEOLOGIE! INSTITUTUL DE GEOLOGIE SI GEOFIZICĂ ANUARUL INSTITUTULUI d© GEOLOGIE Geofizica «Sili® H.P. HA NN: Petrographic Investigation of Pegmatites Located between Teregova and Marga (Eastern Banat, South Carpathians) Ș. VELICIU •• Geothermics of the Carpathian Ârea VOL. 67 BUCUREȘTI - 1387 «Jir Institutul Geologic al României XIGRZ Couverture: C. Vasile Les auteurs assument la responsabilite des donn^es publiees MINISTERUL MINELOR, PETROLULUI Șl GEOLOGIEI INSTITUTUL DE GEOLOGIE Șl GEOFIZICĂ ANUARUL INSTITUTULUI DE GEOLOGIE ȘI GEOFIZICĂ VOL. 67 BUCUREȘTI 198 7 Institutul Geological României CONTENTS Page Hann H. P. Petrographic Investigation of Pegniatites Located Between Teregova and Marga (Eastern Banat, South Carpathians) ...................................... 5 Studiul petrografic al pegmatitelor dintre Teregova și Marga (Banatul de Est, Carpații Meridionali) ....................................................... 75 Veliciu Ș. Geothermics of the Carpathian Area .................................... 84 Regimul geotermic al ariei carpatice............................................. 111 Redactor : FELICIA ISTOC'ESCU Ilustrația : V. NIȚU Traducător : RUXANDRA NEGREA Dat la cules : iunie 19S7. Bun de tipar : sept. 19S7. Tiraj : 750 ex. Hirtie scris IA 70x100/56 g. Coli de tipar 7 Ifî. Co- manda 231. Pentru biblioteci indicele de clasificare 55 (058) Poligrafică „Informația" str. Brezoianu 23—25, București — România Geological României PETROGRAPHIC INVESTIGAT1ON OF PEGMATITES LOCATED BETWEEN TEREGOVA AND MARGA (EASTERN BANAT, SOUTH CARPATHIANS)1 HORST PETER HANN 2 Abstract The occurrenee of the peginatitc bodies is related to highly migmalized metamorphic terrains. Several genetic types are defined by the study of the shape of pegmatite bodies aud of the relations to the adjoining rocks, the classification of the contact types and the inter- pretation of peripheral mafie accumulations. The study of zoning points to the succession of different stages of pegmatitic minerals genesis. The graphic textures are of metasomatic origin. The geochemistry of pegmatites shows the simultaneous developmcnt of the former and of adjoining paragneisses. The rare mineral content is low. The pegmatites originale in different petrologie processes, mostly of analectic-metasomatic nature, within the metamorphic field. Restune L’etude pitrologigue des pegmatites situies entre Teregova et Morga (le Banat Oriental). Ea prfaence des corps pegmatitiques est reliee aux terrains mHamorphiques intens6ment mig- matisâs. L’itude de la configuration des corps pegmatitiques et des relations avec les roches environnantes, la classification des types de contact et 1’interprMation des concentrations mafiques piriphâriques indiquent la coexistence de plusieurs types g6n£tiques. La zonation informe sur Ia succession des fetapes de la gen^se des miniraux pegmatitiques. Les structures graphiques sont d’origine mătasomatique. La giochimie des pegmatites caracWrise l’6volution simultanee avec celle des paragneiss environnantes. La teneur en min6raux rares est râduite. Les pegmatites se sont formGes par des processus p^trologiques distincts, pour la phipart d’ori- gine anatectique-mitasomatique, dans le domaine m6tamorphique. 1 Thesis of doctor’s degree held on November 7, 1983 at the University of Bucharest; paper received on December 3, 1983 and accepted for publication on January, 1984. 2 Institutul de Geologie și Geofizică, str. Caransebeș 1, 79678 București — 32. (GR. i Institutul Geologic al României 6 H. P. HANN 2 1. INTRODUCTION The pegmatite investigation on Romanian territory is restricted to minor occurrences in the East Carpathians (Rodna Mts) and in the Apuseni Mts (Muntele Mare, Preluca Mts), and to a very large area in the South Carpathians wherein mesometamorphic rocks assigned to the Sebeș-Lotru Group contain numerous pegmatite mobilisates. Thus, the Teregova-Marga region containing many pegmatite bodies is of major importance. The present study required, during the early stage, a revision of all geological investigations carried out by previous geologists in this area. Then, the different types of crystalline schists hosting pegmatite bodies and the problems regarding their structure and lithostratigraphy were taken into account. The relationships between the metamorphism grade, the migmatite textures and the numerous pegmatite bodies are also worth studying and will be treated further on. The shapes of pegmatite bodies, their systematics, the classification of different types of contact zones between pegmatites and surrounding rocks, the description and discussion of the significance of mafie concen- trations at the periphery of the pegmatite bodies are relevant for the interpretation of pegmatite genesis. The investigation of the inner structure of pegmatite bodies ac- counts for the presence or absence of zoning and for its formation. The description of different types of zoning should be based on data concer- ning the grain size of minerals, the relative amounts of major and accessory pegmatite-bearing minerals and the occurrence of graphic texture. The study of the latter is a complex one — petrographic and geochemical — and is important with respect to their origin. Structural infonnation reconstituie the sequence of pegmatite mineral formation and the thermo- baric conditions which controlled their crystallization. Concerning the pegmatite mineralogy the present author is interested in investigating both their major (feldspars, micas, quartz) and accessory minerals (tourmaline, beryl, gârneț). The relationships between minerals, the study of the physico-chemical conditions are of peculiar interest. The geochemical characterization of minerals is also worth considering in order to complete the study; the mineralogical-geochemical data are also used as discriminating criteria of pegmatite genesis. The geochemical study of pegmatites implies the interpretation of bulk analyses for defining the geochemical evolution and the relations between pegmatites and surrounding metamorphic rocks, which provide Information on pegmatite genesis. The pegmatite genesis is based on data concerning the morpho- structural features of pegmatites and the interpretation of the mineralo- gic-geochemical investigation. Such analysis is an attempt to establish the origin of pegmatites from the investigated area, which represents in fact the purpose of the present study. i Institutul Geological României 16 R/ 3 PEGMATITES BETWEEN TEREGOVA AND MARGA 7 2. PREVIOUS STUDIES AND EVOLUTION OF IDEAS CONCERNING THE PEGMATITE BODIES The oldest data on the pegmatite occurrences in the Armeniș-Sadova Veche region were provided at the end of the 19th century (Schafarzik, 1898). The author described pegmatite lenses and tabular bodies at Ar- meniș containing: potash feldspar, muscovite, quartz, gârneț and light- green apatite prisms, 3 cm long. At Sadova he described three pegmatite veins which cut the crystalline limestones; they contain perthite, oligo- clase, quartz, black tourmaline nests, biotite; feldspar and quartz form graphic intergrowths. The pegmatites exhibit zoned, symmetrical texture : tourmaline crystals surrounded by quartz in the inner zone, large size microcline, quartz, biotite crystals outwards, whereas both contact zones are marked by green tremolite stripes. These pegmatites deposited from former hot springs. This view on the pegmatite genesis was commonly accepted at that time. Schafarzik does not accept these pegmatites to be formed as a result of magmatism or lateral secretion. Later on, in 1931, Dittler and Kirnbauer studied the pegmatite veins at Teregova and described some new mineral occurrences, that is beryl and columbite; the pegmatites are considered of igneous origin. The geological literature on pegmatites is rather sparse until 1951, when Gherasi presented a report on mica-bearing pegmatites from the Voislova-Măru region. The pegmatites are considered to represent lens- like bodies concordant with the surrounding crystalline schists. They contain orthose, microcline, oligoclase, quartz, muscovite, biotite, tour- maline, gamet, beryl. Following this, several prospections Avere carried out in order to discover pegmatite bodies of economic interest. Thus, Avramescu (1954) described the pegmatite occurrences from the Luncavița-Teregova-Ar- meniș-Dalci zone. He described zoned textures and provided seven Che- mical analyses. The pegmatites bearing abundant plagioclase feldspar are considered of metamorphic origin, while those with prevailing potash feldspar and rare minerals are considered granitic pegmatites. Prospec- tions were carried out by Roșea (1954), Micșa and Gali (1956), Marinescu and Ardeleanu (1956), Codarcea and Stoenescu (1957), Minzatu and Mîn- zatu (1957, 1958), who described in detail the shape of pegmatite bodies and the relations between pegmatites and surrounding rocks. Zoned textures, mineralogical and geochemical features are discussed based on 13 Chemical analyses. Superceanu (1957) published a paper on the rare minerals contained by the pegmatites of the Banat region. Besides beryl and tourmaline, columbite, tantalite and montebrasite are described. Pegmatites are considered of granitic origin. All these data Avere later taken over by Schneiderhohn (1961). Grosu and Angelescu (1960) described the zoning of some pegmatites in this area. Savu and Micu (1964) described pegmatite veins and lenses in the Armeniș-Slatina-Timiș-Petroșnița area and divided them into muscovite, biotite, muscovite and biotite pegmatites, feldspar pegmatites and tour- maline pegmatites. Suru (1966) presented a Avell documented report on pegmatites, mostly already exploited, including detailed mineralogic Ja Institutul Geological României 16 R 7 8 H. P. HANN 4 descriptions and problems conceming their zoning. These pegmatites are considered of magmatic origin. He stated that pegmatites with major potash feldspar contents and those characterized by graphic textures — potash feldspar and quartz — exhibit a low muscovite content. Gherasi and Zimmermann (1968) mentioned concordant and discor- dant pegmatites occurring in the Getic Crystalline north of Muntele Mic, considering them of different ages (that is the discordant bodies are more recent). Pomârleanu and Movileanu (1968) determined the temperature of 338—408°C for the muscovite formation by using the muscovite-para- gonite diagram. Deaș et al. (1968) presented a report of economic interest concerning the geological study of mica-bearing pegmatites, carried out during the 1961—1967 period of time. According to Savu (1970) the sillimanite z.one and the disthene zone contain the major bulk of the pegmatite bodies. Concordant pegma- tites are mica-abundant and were formed in situ, while discordant bodies are feldspar-abundant, in association with rare minerals, and resulted from pegmatoids migrated from deep-seated sources. Marinescu et al. (1973) described pegmatite occurrences from this region with emphasis on the chemistry of pegmatites. The pegmatite occurrences from Teregova-Luncavița area are the object of a prospecting report (Hann, 1973) including the description of pegmatite shape, different types of relation between pegmatites and the host rock, their inner structure. The pegmatite genesis is interpreted in terms of secretion processes during metamorphic differentiation of rocks during almandine amphibolite facies. Gherasi et al. (1974) divided the pegmatite occurrences north of Muntele Mic into muscovite-bearing calc-alkaline pegmatites and quartz -feldspar pegmatites. The largely crystallized tourmaline of pegmatites assigned to the muscovite-bearing group is considered as a result of pneu- matolytic activity. The quartz-feldspar pegmatites are considered to result from lateral secretion smce the plagioclase (An14) of their marginal micropegmatite zone and the plagioclase of the host rock are similar. The zoned textures, asymmetrical in places, of discordant pegma- tites found in the metamorphic rocks of the Getic Nappe from the area situated north of Muntele Mic, and the different mineralogic features of pegmatites were described by Hann (1976). Savu (1977) described several textural and petrographic characte- ristics of pegmatite occurrences from the Banat region. Taking into account the Chemical analyses listed by Avramescu (1954) and Mânzatu, Mânzatu (1957, 1958) the author made some petrochemical and genetic comments. Thus, pegmatites are supposed to be formed by anatexis, on the level of sillimanite zone and disthene zone, at a temperature of 600 —700°C. Considerations on the evolution of pegmatitisation, the succession of different mineralogic types and the geochemical evolution were made by Hann (1977). Pomârleanu and Movileanu (in Hann et al., 1977) gave the following geothermometrical data: 554—600°C for microcline, 560—585°C for bio- Institutul Geological României IGR/ 5 PEGMATITES BETWEEN TEREGOVA AND MARGA 9 tite, 295—560°C for muscovite, 210—470°C for quartz and 237—470°C for beryl. Gridan (1981) described the pegmatite occurrences from the north -eastern area of the Semenic Mts and their unhomogeneous mineralogic and structural features. According to this author the pegmatites are of quartz-feldspar and micaceous types; he found no evidence of their mag- matic origin. Diaconu (1979) reported exhaustive prospecting and exploration data in order to promote the discovery of other muscovite or feldspar- bearing pegmatite bodies. 3. LOCATION AND DESCRIPTION OF MAJOR PEGMATITE BODIES The major pegmatite occurrences are found north of Muntele Mic, in the Tîlva, Măgura and Dalei-Var areas and in the Semenic Mts, that is Slatina-Timiș, Armeniș and Teregova areas (Fig. 1). I. Tîlra area. The pegmatite bodies occur north of Măru and south of Voislova in the Curcan and Fața Lungă hills, in the basins of Slatina, Valea Mare, Plopi, Pîrîul Pascului valleys. The various pegmatite bodies have been entirely or partly mined out for muscovite and beryl, and partly for feldspar (e.g. the so-called “main body” — vein 1 Tîlva, situated north-west of Curcan hill). Other pegmatite bodies were exploited north of the Curcan hill or in the Fața Lungă hill — „Rîpi” pegmatite body. The pegmatites from this area are tourmaline-rich, too. Some pegmatite dykes are discordant (e.g. the body 1 Tîlva and another body situated at ca 200 m southwards) and most of the bodies are concordant and lens- like shaped. II. Măgura area. West-southwest of Măgura pegmatite occurrences are frequent in the Pietroasa Valley basin, a left tributary of the Bistra Mărului Valley. An important body — „Cîmiș” (180 m long, 20 m wide) — cuts the main valley and is still mined. Discordant and beautifully zoned pegmatite veins occur in the right tributary of Pietroasa Valley- Pîrîul cu Mărul. Other bodies are located along the Ogașul Strîmb brook and Iedera Valley, right-side tributaries of the Pietroasa Valley. Several pegmatite bodies, usually concordant, occur in the Socetul Mare summit, south of Pietroasa Valley and north of Șasa Valley, left-side tributary of Bistra Mărului Valley. In this area, the pegmatite lenses may form elongated elevations, prominent due to their high erosion resistance. Pegmatite bodies lie in the Cermaz Valley, left-side tributary of the Șasa Valley, near its confluence with Bistra Mărului Valley. Concordant and discordant pegmatites, completely or incompletely zoned, or unzoned in places are also present. Pegmatite occurrences are reported at Dobrotin, Runcurelu Mare and Runcurelu Mic valleys, left-side tributaries of Bistra Mărului Valley, south of Pietroasa-Bistrâ Mărului confluence. The west- ern margin oi' the Măgura area contains several lens-like, concordant pegmatite bodies along the upper course of the Scoarța Valley. Institutul Geological României 10 H. P. HANN 6 Fig. 1. Localisation of pegmatites in Teregova—Marga area, I, Tîlva arca ; II, Măgura area; III, Dalci-Var area; IV, Slatina Timiș area; V, Armeniș arca; VI, Teregova area. Institutul Geologic al României 7 PEGMATITES BETWEEN TEREGOVA AND MARGA 11 The muscovite contents of the pegmatite occurrences from the Măgura area are of economic interest. The bodies located along Runcurelu Mare and Runcurelu Mic valleys and the upper course of Scoarța Valley arc rich in potash feldspar. III. Dald-Var area. The bodies lying east and north-east of Var aud Dalei localities occur in the basin of the Văruț Valley, in Valea Satului and in its left-side tributary, Străului brook, in Dalei Valley and in Găina Mică brook, between Dalei and Var. The pegmatite bodies re- present the south-western extension of Pietroasa Valley-ScoarțaValley portion from the Măgura area. These pegmatites are commonly concor- dant and have rather high muscovite and feldspar contents. Muscovite was exploited in Valea Satului and Străului brook, whereas feldspar in Dalei Valley. IV. Slatina-Timiș area. It is located in the eastern part of the Semenic Mts. The pegmatite bodies occur south of Slatina-Timiș village in the Seeaș brook, and then northwards, in the Slatina Valley, Goleț Valley and Bucoșnița brook. All these valleys are left-side tributaries of the Timiș Valley. An important body crops out in the Slatina Valley, ca 1.5 km off its confluence with the Timiș Valley. Other pegmatite occurrences are mentioned 600in upstream, in the left slope of the Slatina Valley, the main pegmatite bodies occur at 4.5—5 km off its confluence ■with the Timiș Valley. These pegmatites are characterized by their high feldspar or quartz contents. V. Armeniș area. The pegmatite bodies occur both north and south of Armeniș along the Timiș Valley. To the north, important occurrences are mentioned up to Sat Bătrîn, in the right slope of the. Timiș Valley, and to the south up to Valea Mare, near the tunnel, in the left slope of the Timiș Valley. Pegmatite occurrences are also reported in the Biban brook and Frîncu brook, left-side tributaries of the Timiș Valley. The most important body is situated near the Armeniș railway-station and crosses the Timiș Valley. It is the most important body of the region and one of the largest aii over the country (550 m long, 40 m wide). It is mined for feldspar only in the right slope of the Timiș river where it crops out on 420 m; the open-pit working has developed along 340 m since 1954. The concordant body trends mainly NW—SE. It seems that, at least partly, it is delimited by disconformity from the adjoining gneisses. The pegmatites do not exhibit large muscovite crystals, are mostly concordant and are mined or might be mined for feldspars. VI. Teregova area. Pegmatite bodies are found north, south and west of Teregova locality. North-west of Teregova village, in the left slope of Teregova Valley, there are two large discordant and well zoned dykes mined intermittently for „glaze” potash feldspar since the begin- ning of the century till 1957. Big size beryl crystals have also been 12 H. P. HANN 8 reported. The pegmatite veins are 220 m long and 10 m wide. They were mined at four horizons; no investigations were carried out beneath the lower horizon (IV). Commonly concordant pegmatite bodies occur bet- ween Teregova and Luncavița villages, in the Lazul hill situated South- west of Teregova, in the Cerbul ravine (right-side tributary of Teregovița Valley), in the Teregova and Timiș valleys, in the north of this area. All these pegmatite occurrences are important owing to their feldspar content and, subordinately, to the largely crystallized muscovite. 4. PETROGRAPHIC AND STRUCTURAL SETTING OF PEGMATITES - THE SEBEȘ-LOTRU GROUP The mesometamorphic crystalline schists of the Sebeș-Lotru Group, which host the pegmatite bodies, constitute the Getic Nappe and occur both in the north of the Muntele Mic massif and east of the Semenic Mts (PI. I; PI. II). North of Muntele Mic, the Sebeș-Lotru crystalline schists thrust over the Măru amphibolites of the Danubian units. West of the thrust line, inside the Getic Nappe, a digitation builds up the Turnu Ruieni scale (PI. I). According to geological evidence, palynologic data (Visarion, in Savu et al., 1975) and the Rb/Sr dating of 837 m. y. (Bagdasarian, 1972), the Sebeș-Lotru Group is assigned to the. Upper Precambrian A. Since the second half of the last century, some studies have been designated to the general geologic characterization and description of the crystalline schists. It is to mention here Stur’s data (1862—1863) cited by Hauer and Stache (1869), the studies of Bockh (1879, 1883), Inkey (1889) and Schafarzik (1898, 1899). Mrazec (1897) assigned the South Carpathian crystalline schists to two groups with specific petrographic and metamorphic characieris- tics; following this, Murgoci (1905) put forward the alpine thrust of the Getic Nappe, that is the crystalline schists of the group I over the group II, which represents the Autochthon. In 1908, Murgoci figured on a map the Getic thrust line, which also crosses the present area of study. Twenty-five years later, Streckeisen and Gherasi (1932) confirmed the tectonic relations between the Getic Nappe and the Danubian units in this region, and they drew up a tectonic sketch including some changes as compared to the former map by Murgoci (who had assigned the Măru amphibolites to the Getic Nappe based on Schafarzik’s data). This tecto- nic sketch also includes the Socetul Mare area. New data concerning the crystalline schists are given by Gherasi (1952), Roșea (1954), Avramescu (1954), îlicșa and Gali (1956), Mânzatu (1957, 1958), Codarcea and Stoenescu (1957), Rădulescu and Rădulescu (1957, 1958), Hurduzeu (1962), Savu and Micu (1964), Popescu and Ștefan (1964), Savu (1965), Gherasi and Zimmermann (1968), Gherasi et al. (1969, 1970), Savu (1970), Hann (1973), Gherasi et al. (1974), Hann (1976), Hann et al. (1977), Gridan (1981), Hann (in Savu et al., 1981) Savu and Hann (1982). Institutul Geological României 9 PEGMATITES BETWEEN TEREGOVA AND MARGA 13 4.1. Petrographic Characteristics 4.1.1. Paragneisses. They represent the main rock type. Either they are only bi'otitic or they contain both mica minerals; the increase of mica content is simultaneous with the decrease of feldspar content, and thus the initially granolepidoblastic rock beeomes lepidoblastic and the rocks are to be named micaceous paragneisses. The common mineral association also includes plagioclase (15—20% An) and quartz which provide the ribbon structure of the rock. Microcline, almandine, disthene and sillimanite occur sparsely. Sillimanite (or fibrolite) is usually closely related to biotite and forms needles or sheaves. Epidote, zircon, apatite, rutile and opaque minerals occur in restricted amounts. 4.1.2. Quartz-îeldspar-bearing-leueogneisses. They form an impor- tant level both north of Muntele Mic and to the east of the Semenic Mts (Savu, 1970). They are associated with amphibolites, rarely with mica- chists, and might be considered a leptino-amphibolite formation. The quartz-feldspar gneisses are interbedded in the paragneisses. Their texture is granoblastic to granolepidoblastic (determined by subparallel mica sheets). The main mineralogic componenta are microcline, plagioclase (15% An) and quartz. Mirmekite intergrowths are also present. Muscovite, biotite and isolated hornblende crystals occur subordinately. 4.1.3. Mieasehists. Micaceous paragneisses often grade into mica- schists which may also become phaneroblastic. Only thick micaschist interlayerings have been figured on the map. North of Muntele Mic, the mieasehists and the micaceous paragneisses represent a main horizon and are usually migmatized. Most of the pegmatites from this region occur either in this horizon or in its vicinity. Besides muscovite and biotite, which are largely crystallized, they contain also quartz, subordinately plagioclase (16—20% An) and almandine porphyroblasts. Sillimanite and disthene do also occur, the former in association with biotite. Some thin sections show sillimanite needles and sheaves associated with biotite, but unrelated to short prismatic disthene crystals. 4.1.4. Amphibolites, amphibole gneisses. They occur as interlaye- rings of varying thickness and length and exhibit a mainly nematoblastic texture. They are either banded or massive. They consist of green horn- blende, plagioclase (34—40% An, which may exceed 50% of the ground- mass of the amphibole gneisses) and subordinate quartz. Some amphi- bolites also contain gârneț porphyroblasts which lend to the rock the aspect of eclogitic amphibolite. The thin sections do not exhibit diablastic textures nor relict pyroxene which could account for the generation of amphibolite by eclogite retromorphism. However, the minor element content strongîy suggests an obvious difference between this amphibolite. (Cr — 5 ppm, V — 60 ppm, Ni — 19 ppm, Zr — 630 ppm, analyst C. Udrescu) and the eclogitic amphibolites reported from the Sebeș-Lotru Group of the Căpățîna Mts, and described by Hann (1983) as yielding a mean content of 335 ppm Cr, 315 ppm V, 113 ppm Ni, 113 ppm Zr. JA Institutul Geological României igr7 14 H. P. HANN 10 Some amphibolites also contain biotite, epidote, chlorite. Umenite, tita- nite and uralite amounts point to the orthorock character. According to Savu, Vasiliu (.1970) the amphibolites associated with the crystalline limestones and crystalline dolomites bearing magnetite lenses, in the Armeniș area, contain hastingsite resulting from intense migmatization. 4.1.5. Crystalline limestones, dolomites. They occur rarely and form lenses or bands and are white or grey in colour. Gherasi et al. (1974} mentioned from north of Pietroasa Valley some diverging grammatite prisms within granoblastic dolomite. Other crystalline limestone lenses, also occurring north of Muntele Mic, contain diopside, quartz, zoisite. In the Semenic Mts, the crystalline dolomites were described by Schafarzik (1898), Hurduzeu (1962), Savu and Micu (1964). At Armeniș, besides tremolite, quartz, diopside, plagioclase (45—50% An), phlogopite, tita- nite, zoisite and apatite, dispersed pyrrhotite and magnetite bands are also present. 4.1.6. Quartz paragneisses. They occur as thin, usually biotitic layers in the paragneiss pile. They are fine-grained rocks and exhibit typical granolepidoblastic texture. Quartz, biotite and minor plagioclase (i5-25% An) are accompanied by subordinate muscovite, almandine, zircon and opaque minerals. 4.1.7. Migmatites. The migmatization is characteristic of the above mentioned crystalline schists. There is a correlation between the intensity of this process, namely the feldspathization of schists, and the frequency of pegmatite bodies. Migmatization has often been conformable to the foliation resulting in the stromatitic structure. The ophthalmitic structure is often present too. All migmatic structures in terms of Mehnert’s classification (1962) are encountered. The micaceous paragneisses and the micaschists were much more migmatized than the amphibolites and quartz paragneisses. The leucosome is from 1 mm to several cm thick. An increased thickness promoted an increased grain size of neosomatic microcline, plagioclase or quartz, resulting in pegmatoid structures. 4.1.8. Serpentinized ultrabasites. Most of such lenses are interlayered in the paragneisses in the Pietroasa Valley (left-side tributary of Bistra Mărului Valley), as well as in other areas. These rocks were described by Schafarzik (1899), Gherasi (1952), Mînzatu, Mînzatu (1957), Gherasi, Zimmermann (1968), Gridan (1979). If serpentinization is incomplete, the thin sections show cellular textures with olivine relics. Bastitized orthopyroxene relics are also present. The amphiboles are represented by anthophyllite and grammatite, both with needle-like habit and different extinctions. According to Mărunțiu (1978) the alteration of primary minerals of the ultramafic rocks is due to some retromorphic processes, and the orthopyroxene-rich rocks grade to anthophyllite and talc-rich rocks. Thus, the most common primary rocks correspond presumably to orthopyroxene peridotites (harzburgite). M- Institutul Geological României 16 R/ 11 PEGMATITES BETWEEN TEREGOVA AND MARGA 15 4.1.9. Granites. South of Glimboca there is an elongated, confor- mable granitic body. The rocks are characterized by the coarse grains of white-pink feldspar, accompanied by biotite, usually chloritized, and quartz. Granițe is highly tectonized due to the faults which either cut oi' delineate it; the most important fault delineates the Upper Cretaceous rocks of the Rusca Montană basin from the Sebeș-Lotru Group. In weath- ering areas granițe is converted into typical gruss. The study of thin sections shows a hypidiomorphic structure; microcline, plagioclase and quartz are the main components. Chloritized biotite, epidote, calcite and sericite are also present. These rocks also occur north-east of Glimboca in the Poiana Ruscă Mts and have been described by Maier et al. (1975) as the „Criva grani- toid” of anatectic origin (Pl. II). 4.1.10. Banatites. Small discordant igneous bodies represented by hornblende and biotite granodiorites, biotite granodiorites and quartz- diorite porphyries occur in the Semenic Mts near Luncavița, Teregova and Armeniș. These rocks, -which penetrated the crystalline schists along some fractures, are the result of Laramian (banatitic) calc-alkaline sub- duction magmatic activity in the South Carpathians (Rădulescu, Săndu- lescu, 1973) and occur along the eastern banatitic Teregova-Lăpușnicel alignment trending, according to Savu and Hann (1982), north-eastward to Turnu Ruieni. The banatitic rocks have been described by Savu (1962), Para- schivescu, Serghie (1963), Hann (1973) and Gunnesch et al. (1978), according to whom the banatitic intrusions are related to a main north- south trending fault, and by Vlad et al. (1980) and Savu (1982) who consider that the banatite occurrences line a NNE—SSW trending fault System. 4.2. Metamorphism, Retromorphism Two index minerals — kyanite and sillimanite — have been reported north of Muntele Mic. In some thin sections both minerals have been identified without mutual relations. In fact the direct alteration of kya- nite into sillimanite is very rare. The transition of one index mineral to the other results from intermediate reactions (Hârtopanu, 1976). There- fore, kyanite is replaced by muscovite, while sillimanite is characterized by subsequent nucleation on muscovite or other mineral, usually biotite. A peculiar feature is the occurrence of both minerals on the isograde plane which thus preserves frozen reactions. This might be due to the fact that one mineral co-exists metastably with the other, namely kyanite coexists with sillimanite as far as it has not surpassed the kinetic limit of its decomposition. On the other hand, the spatia! position of the isograde plane should be considered. Therefore, the different thermo-baric gradient potentials in different places of the area affected by metamorphism may result in an unconformity between the inițial structure and the isograde plane. However it is possible that the two planes be conformable. In this case, the simultaneous occurrence of the two minerals accounts for the Institutul Geologic al României igrZ 16 H. P. HANN 12 presence of the isograde plane approximately parallel to the present-day erosion level. Pervasive retromorphism of schists is widespread between Șasa Valley and Slatina Valley. The retromorphism results in new minerals mostly of mimetic growth on pre-existing ones. Chlorite is the most cha- racteristic mineral which replaces, more or less completely, gamet and biotite. Some zones also show textural redistribution following a young „s” plane, accoinpanied by the simultaneous formation of chlorite and albite. This is an incipient process which has not become penetrating nor almost homogeneous as reported in retromorphic areas elsewhere (e.g. the Chirii Series in the Rarău Mts — Krăutner et al., 1981 — and the Uria Formation in the Lotru Mts — Hann, Szâsz, 1983). This retromor- phic zone is also characterized by some features which are due to mechanic deformations and laminations that affected the rocks. These deformations are partly related to the thrust line of the Getic Nappe or are present inside the nappe along some fractures or reverse faults. The discont- inuous nature of the mylonitization area along the thrust line could be due to the fact that the mechanic effects of thrusting upon the rocks assigned to the two units depend onthe position of the sliding plane with respect to the rock foliation. If the thrust plane parallels the metamor- phic foliation plane, the frictional force is consumed along the latter and the effect is less spectacular. If the position of the two planes differs, the effet of mylonitisation is stronger. According to Higgins (1971) there are several types of cataclastic rocks, from breccias, microbreccias to ultraniylonites. The study of retromorphic processes and their different effects resulted in the actual image of the contact line between the Getic Nappe and the Danubian units; this line was estimated westward by early authors. Savu (1970) recognized three metamorphic areas assigned, like the entire Semenic crystalline massif, to a metamorphic province of barro- wian type. From east to west the sillimanite zone superposed on the sillimanite gneiss complex, the kyanite zone overlay the base of paragneiss and quartz-feldspar gneiss complex and the kyanite and staurolite zone superposed on several complexes, partly on the quartz-feldspar gneiss complex and on the micaschist complex from the investigated region. 4.3. Lithostratigraphy According to Gherasi et al. (1974), the crystalline schists occurring north of Muntele Mic and belonging to the Sebeș-Lotru Group have been assigned to two horizons : the lower horizon including biotite parag- neisses and amphibolites, metamorphosed at the sillimanite isograde level, and the upper micaschist horizon metamorphosed at the kyanite isograde level. In the Dalci-Măru area the lower horizon overlies the upper one. The crystalline schists are considered to form the reverse limb of a recumbant fold lying tectonically on the Danubian units. The geologic map, scale 1 : 50,000, sheet Muntele Mic (Hann, in Savu et al., 1981) promoted a different image of the lithostratigraphic 13 PEGMATITES BETWEEN TEREGOVA AND MARGA 17 sequences characteristic of the Sebeș-Lotru Group from this region. The alterations are based on the following : 1) sillimanite occurs all over the area covercd by the Sebeș-Lotru metamorphics, accompanies the kyanite amounts and is therefore inadequate for defining this horizon; 2) the Turnu Ruieni scale was delimited within the Getic Nappe being charac- terized by a typical lithostratigraphic sequence showing a quartz-feldspar leucogneiss horizon; 3) the lithostratigraphic sequence of the crystalline schists assigned to the Getic Nappe also includes a mainly micaschist and micaceous paragneiss horizon, usually highly migmatized. Most of the pegmatites are present in this horizon or in the vicinity. As a result, the following lithostratigraphic sequence is listed below : the base of the Turnu Ruieni scale shows alternating amphibolites, quartz paragneisses, quartz-feldspar gneisses and mieasehists developed on two micas para- gneisses. Then follows the horizon of quartz-feldspar leucogneisses with interbedded amphibolites and mieasehists, and the top built up of alter- nating mieasehists and amphibolites on biotite and muscovite paragnei- sses. Krăutner (1980) named the quartz-feldspar leucogneisses the lepti- noamphibolite formation and related it to similar sequences of the Sebeș- Lotru Group in the Semenic Mts (according to Savu, 1970), the Godeanu Mts (Bercia, 1975), the Poiana Ruscă Mts (according to Maier et al., 1975) and the Căpățîna Mts (according to Hann, Gheuca, in Lupu et al., 1978). The lithostratigraphic sequence of the Sebeș-Lotru Group assigned to the Getic Nappe includes at its base several crystalline limestone levels and alternating mieasehists and amphibolites on biotite and muscovite paragneisses. Between the base and the top lies the level of mieasehists and micaceous paragneisses, while the top consists of alternating mica- schists, amphibolites, crystalline limestones and quartz paragneisses on two mica paragneisses. In the Semenic Mts, the stratigraphic sequence of the Sebeș-Lotru Group was described by Savu (1970) who distinguished five rock complexes ; three of them are found in the area of the present investigation. The sequence starts with the complex of sillimanite paragneisses and carbo- nate rocks (CJ including biotite and sillimanite paragneisses and biotite, muscovite and almandine paragneisses with associated migmatites. The paragneisses contain interbedded amphibolites and crystalline limestones bearing lenses of silicates and crystalline dolomites. The next complex (C2) consists of paragneisses and quartz-feldspar gneisses, muscovite and biotite-bearing paragneisses interbedded with kyanite mieasehists or kyanite, staurolite and almandine mieasehists, minor crystalline lime- stone lenses containing silicates or crystalline dolomites related in places to pyroxene gneisses. According to Gridan (1979) these two complexes constitute a unique unit named the lower complex of quartz-feldspar micaceous and carbonate rocks. Then the quartzite complex C follows, but it is not found in the area of the present investigation; however the micaschist complex (C4) is present and consists of kyanite, staurolite and almandine mieasehists, muscovite and biotite mieasehists, musco- vite and almandine quartz schists interlayered with paragneisses, quartz- feldspar gneisses, amphibolites and quartzites. 2 - c. 231 Institutul Geological României 18 H. P. HANN 14 4.4. Mineralizations of the Turnu-Ruieni Metallogenetic Field Minor pyrite, pyrrhotite with related chalcopyrite and rare molyb- denite occur as impregnations and thin veins (0.15—0.25 m thick) in the crystalline schists. Pyrite and pyrrhotite impregnations are reported by Rădulescu, Rădulescu (1957, 1958) in the gneiss in the Slatina basin, assig- ned at that time to the Danubian units. These ores have also been studied by Popescu, Ștefan (1964). Later on, Gherasi et al. (1970, 1974) assigned the disseminated pyrite ores to the alpine cycle and mentioned that the minor molybdenum ore in the Slatina Valley might be related to a deep- seated banatite body. The ores have been recently studied by Savu and Hann (1982) • they are located along fractures and mylonitization areas of the crystal- line schists of the Sebeș-Lotru Group. The lineation controlling the ores trends NNE —SSW parallel to the thrust plane. The emplacement was controlled by the Șasa Valley-Borlova reverse fault and a related fault. The ores are found on highly mylonitized rocks also affected by high temperature hydrothermal metamorphism. The Chemical analyses show high Pe (22—24%) and S (12—14%) contents corresponding to pyrite and pyrrhotite rich ore. Minor Cu (0.044—0.49%), Ni (0.04—0.05%) and Mo (ca 0.05%) amounts have also been identified. Gold and silver contents are very restricted. The ores may be assigned to the catathermal type passing to the pneumatolitic one, as far as both the characteristic pyrite-pyrrhotite (± chalcopyrite; ± molybdenite) association and the results of hydrothermal metamorphism (gârneț occurrenee in hydro- thermalized amphibolites) show a high temperature of formation (350 — 570°C). The ores located in a tectonic reactivation area associated with the constitution of the Getic Nappe. The pyrite crystals and the quartz gangue are not deformed tectonically and cement the mylonites and the breecias, thus proving their younger age. Therefore, one may consider that these ores are related to the Laramian (banatitic) magmatic activity. The ores from this metallogenetic field are located in the northward prolongation of the eastern banatitic alignment reported in the Semenic Mts between Mehadia and Teregova. The ores from the Turnu Ruieni- Borlova metallogenetic field are thus ascribed to the metallogenetic district related to the eastern Laramian lineament, which is to be named the Turnu Ruieni—Teregova—Lăpușnicel metallogenetic district.lt be- longs to the copper-bearing metallogenetic zone of the banatitic province (sensu Vlad, 1979). 4.5. Tectonic Data The tectonic character of this region is given by alpine deformations, that is thrust of the Getic Nappe over the Danubian units following a WNW—ESE strike. In the Muntele Mic area, west of the thrust line, there are tectonic lines represented by faults, reverse faults and a digita- tion which borders the Turnu Ruieni scale. All these ruptures are appro- ximately parallel to the thrust plane and they formed simultaneously with it. There are also transverse faults, some of them shifting the thrust 'A Institutul Geological României igrA 15 PEGMATITES BETWEEN TEREGOVA AND MARGA 19 plane. An important fault cuts the basins of Mâloasa, Conmlețu and Slatina valleys shifting both the thrust plane and the other tectonic lines inside the nappe. The fault which delineates the Upper Cretaceous rocks of the Rusca Montană basin from the rocks of the Sebeș-Lotru Group, Crossing the valleys of Scoarța, Măceș and Văruț is worth mentioning. In the Semenic Mts a generally N—8 trending fault System is prevalent. 5. MORPHOSTRUCTURAL CHARACTERISTICS OF THE PEGMATITE BODIES The characteristics regarding the shape of pegmatite bodies, their relations with the surrounding rocks and their inner structure are to be taken into account in relation with certain genetic features. It is well known that the early meaning of pegmatite was confined to a specific texture, that is the graphic texture, This was used by Hauy before 1820. The notion of pegmatite synonymous with graphic granites was first used by Brogniart in 1813, who defined it as a rock built up of lamellar feldspar and quartz. Later, during the second half of the 19th century, according to new and more accurate petrographic data, peg- matites were considered coarse crystallized rocks, usually of granitic com- position, often zoned due to the different grain size, the presence of gra- phic structure or the varying amounts of main or accessory component minerals. 5.1. The Pegmatite Shape and Their Relations with the Surrounding Rocks The pegmatite bodies show rather various shapes, that is from lenses or veins to bodies with irregular and elaborate, even strânge mar- gins. According to Schneiderhohn (1961) the pegmatite shapes within Precambrian crystalline schists, considered on the whole, are similar all over the world. However, there are several peculiar aspects rather mis- leading as regards their variety. This is also the case of the pegmatites occurring in the Teregova-Marga region. The common shapes are the following: lenses, concordant veins, dykes, nests and large irregular bodies. 5.1.1. Lenses. They are concordant pegmatite bodies which occur very frequently. The lenses exhibit slightly curved or very undulated margins following the folds of the adjacent rock. The high undulation of the contact line may show that its shape depends on the type of rock : a pegmatite occurring between quartz paragneisses and micaschists (Fig. 2) shows a curved shape as compared to the former and a sinuous one as compared to the latter. In other cases, the pegmatite lenses from the anticlinal zone of a fold show their maximum thickness in the area corres- ponding to the fold axis (Fig. 3) showing that the lens margin is directly influenced by the folding of schists and by the shape of that fold. -^.JX Institutul Geologic al României \JGR> 20 H. P. HANN 16 It may be thus inferred that pegmatites and folds formed simul- taneously. Fig. 2. Contour of a lens- like pegmatite body situa- ted at the contact between two different rock types. 1, pegmatite ; 2, micaschist; 3, quartz paragneiss. Fig. 3. Pegmatite lenses showing maximum thick- ness in the fold axis : the lens contour is influenced by the fold shape. 1, pegmatite ; 2, paragneiss. The pegmatite lenses are sometimes isolated in the paragneisses and commonly they form assemblages of distinct types: a) clusters of lenses forming a swarm as a whole (Fig. 4); b ) rather large pegmatite lenses and smaller, elongated ones situated at a distance of a few cm, parallel to the contour of the big lens (Fig. 5); Fig. 4. Swarm of lens-like pegmatite bodies. 1, pegmatite; 2, paragneiss. 17 PEGMATITES BETWEEN TEREGOVA AND MARGA 21 c) rows of lenses situated on the same level or on different levels within the paragneisses sequence (Fig. 6a, 6b, PI. IV, Fig. 1). Iț is to note that for type “a” the contact between pegmatites and the adjacent rock is usually diffuse3. For type “i” the pegmatite grades into the adjacent rock by several pegmatite stripes or bands parallel to the main body4. As for type “c” the contact is clear-cut and in certain Fig. 5. Large pegmatite lens closely associated with thin, elongated, parallel lenses. 1, pegmatite; 2 paragneiss. Fig. 6. a, Row of lenses resulting from the bou- dinage of some concordant vein-like pegmatite bodies. 1, pegmatite; 2, para- gneiss. Fig. 6. b, Pegmatite lenses situated at different levels and resulting from dyke fragmentation. 1, pegma- tite ; 2, paragneiss. parts it may be delineated by slickensides. Thus these lenses have resulted from the boudinage of some concordant veins (PI. IV, Fig. 1) or, according to Șeclăman (1972) from the breaking-down of a dyke due to unaffine lamination movements parallel to the gneiss foliation. 5.1.2. Concordant veins. The pegmatite bodies occur frequently as interlayerings within the migmatized paragneisses (Fig. 7) and show branchings out or thickening, then become thinner and thinner to their complete disappearance. In other cases they follow the schist undulated contours or, if secondary veins start from the main body, local unconfor- mities are generated. The contact with the adjacent rock is often sinuous, and it may be both diffuse and Sharp. In other cases the contact is Sharp and inside Institutul Geologic rfi an iVi 22 H. P. HANN 18 the pegmatite, near the contact with the schists, there is a narrow pa- ragneiss stripe parallel to the contact line (Fig. 8). As regards the first case, the pegmatite genesis rnay be related to the pegmatite evolution of quartz-feldspar mobilizates. The second case is characteristic of dilation pegmatites. Fig. 7. Concordant pegma- tite vein showing thin- nings and thickenings, with secondary veins ge- nerating local unconformi- ties. 1, pegmatite; 2, pa- ragneiss. Fig. 8. Concordant pegma- tite vein with parallel pa- ragneiss stripes next to thc contact line. 1, pegma- tite ; 2, paragnciss. 5.1.3. Nests. They occur less frequently, and form small bodies with irregular margins (Fig. 9a, b). The contact with the adjacent rock may be Sharp or diffuse. The pegmatite illustrated in Fig. 9 a shows both contact types within the same nest. Fig. 9. Pegmatite bodies forming nests. a, generated by migmati- zation processes or assigned to concretionary pegmatites ; b, ge- nerated by filling of rock fissures or cavitics through secretion pro- cesses. 1, pegmatites; 2, para- gneisses. Institutul Geological României 19 PEGMATITES BETWEEN TEREGOVA AND MARGA 23 These pegmatite bodies may also form during the migmatization process, or may be ascribed to the concretionary pegmatites described by Bamberg (1952). The pegmatite nests of type “fe” result from filling rock fissures or cavities with quartz-feldspar mobilizates by secretion (Mehnert, 1971, Bamberg, 1952). 5.1.4. Dykes. They occur less frequently than the concordant lenses or veins, but may form several important bodies in places. The sizc of these bodies varies broadly. The unconformity with respect to thehost crystalline schists varies and may be rapidly modified on short distances. As regards the shape of the contact line, there are dykes which cross the adjacent rocks along a predominantly straight line, resulting in a sharp contact (Fig. 10) and dykes characterized by a highly sinuous contact line, the pegmatitic body including parts of the adjacent rock (Fig. 11), the contact being both sharp and diffuse. Some dykes belonging to the former group show a contact line with fine, millimetric inlets a few cm long, which favour the advancement of pegmatites inside the paragneisses (Fig. 10 a, PI. III, Fig. 1). The contact is still sharp, but in this case the later intrusion of pegmatites along low resistance lines during some anatectic processes is obvious. Fig. 10. Pegmatite dykes unconformably pier- cing the alternating paragneisses and mica- schists. a, contact line showing fine pegmatite intrusions in the adjacent rocks. 1. pegma- tites; 2. micaschists ; 3, paragneisses. Fig. 11. Pegmatite dyke with a highly sinuous contact line and visible conformity features. 1, pegmatite; 2, amphibolite; 3, paragneiss. The unconformable feature of dykes points to the late generation of the main metamorphism and migmatization stages of crystalline schists independent of their genesis (lateral secretion, intrusion along fractures of some anatectic mobilizates within open or closed Systems, or metaso- matic processes characteristic of “replacement” pegmatites, Bamberg, 1949). 24 H. P. HANN 20 5.1.5. Large irregular bodies. In the area of study there are also some pegmatite bodies, usually of large size and irregular shape (e.g. Armeniș, Dalei, Cîmiș—Valea Pietroasa, Curcan hill). Following the contact line, some parts of it are highly curved, then it becomes straight or slightly winding. The nature of the contact differs along the same body : it may be Sharp (Armeniș body, in certain zones the contact is of tectonic nature and is due to slickenside) or diffuse. Their relation with the adjacent schists varies as well. Thus, at Armeniș the pegmatite body is confor- mable on the whole. However, following the contact line, on tens of me- ters, the pegmatite body is obviously unconformable. There is also the case (Pietroasa Valley, Curcan peak) in which the pegmatite body is mainly unconformable and, on limited areas, conformable. Thus, these pegmatite bodies show features which, when first examined, are not concordant. These apparent contradictions may be understood by taking into account that the different features of normal size dykes or lenses occur in the former case on a different scale. For example, if in the case of a small size body the unconformity is decimetric, in the other case it is of metric size and the unconformity between the body and the schists is obvious. The complexity of shape and of the contact with the adjacent rocks might also be due to the inclusion of several pre-existing pegmatite bodies with peculiar genetic features which influenced differently their charac- teristics. Several genetic processes are supposed to have influenced suc- cessively or altematively the constitution of these large size bodies and have thus generated obvious features. 5.2. Types of Contact Between the Pegmatite Bodies and the Surrounding Rocks The above listed descriptions lead to the following classification of the different contact types : AJ Sharp contact: «, tectonic; b, normal B) Graded contact : a, diffuse; b, altemating C) Mixed contact A) The Sharp tectonic contact (a) (Fig. 12a) occurs along slicken- side usually typical of young, alpine tectonic planes. This type of contact may not be associated with other mentioned types, as it is not involved in the genesis and evolution of pegmatite with respect to the adjacent rock. Therefore its assignment to this classification is formal. The budined or fragmented veins have previously been considered to show a tectonic, contact, but at present this is no longer valid as the slickenside is commonly absent. The deformations seem to belong to a late metamorphic stage marked by similar metamorphic conditions. The recrystallization of mi- nerals along the lamination planes between the pegmatite bodies and the adjacent rock took place to their complete closing. This results in a Sharp normal contact (b ) with an abrupt transition from the pegmatite to the Institutul Geological României 21 PEGMATITES BETWEEN TEREGOVA AND MARGA 25 adjacent rock (Fig. 12 b). On the other hand, this type of contact may be due to the petrographic features of the adjacent rock as inferred from the contact area between pegmatites and amphibolites (Fig. 12c). Fig. 12. Types ol Sharp contact: a, tectonic; b, normal: c, Sharp contact rcsulting from the pclro- graphic features of surrounding rock. 1, pegmatite ; 2, paragneiss ; 3, amphibolite; 4, mylonites oa the friclion mirror. B) The graded diffuse contact (a) is of two types: 1, the tran- sition results from the gradual decrease of the grain size of pegmatitic minerals and the simultaneous occurrence of minerals hosted by the adjacent rock (Fig. 13a); 2, the pegmatite groundmass exhibits parts built up of paragneisses and the paragneisses include pegmatitic zones, the transition zone being represented by the area between the pure peg- matite and the paragneiss unaffected bv pegmatitization (Fig. 13b. Pl. III, Fig. 2). Fig. 13. Types of graded con- tact : a and b, diffuse. a, by gradual decrease of grain size of pegmatitic minerals; b, parag- neiss stripes in pegmatite ground- mass and pegmatite zones in paragneiss; c, alternating graded contact. 1, pegmatite; 2, para- gneiss. c The alternating graded contact (b) is represented by some rock stripes within the pegmatite, alternating with pegmatite stripes to final settlement of one or other feature (Fig. 13a), depending on the direction in which it is looked at. Institutul Geological României 26 H. P. HANN 22 C) Mixed contact; in some cases different contact types occur along the boundary of a pegmatite body. The most common example is represented by a tectonic contact partly accompanied by another contact type. There are also some other associations of different contact types within the same pegmatite body, Such as the Sharp normal contact and the diffuse contact. Considering the data presented above, one may infer that the con- tact type, associated with the shapeof the body, may provide genetical information ; however, it is not advisable to draw hastly general conclu- sions based on these relations. 5.2.1. Mafie aceumulations at the periphery of some pegmatite bodies. The last characteristic implied by the study of the boundary between the pegmatite bodies and the adjacent rocks concems the ac- cumulation of mafie minerals (biotite, tourmaline or homblende) in the host paragneiss, in the contact area with some pegmatite bodies. These aceumulations are up to 1 cm thick and the minerals are always parallel to the contact plane. This feature is noted in connection with both the mafie cover of conformable pegmatite bodies and the one of unconfor- mable bodies, pointing to a Sharp contact. The occurrence of a mafie border is irrelevant at first sight as its presence is related to a certain shape or a certain genetic type of pegmatite bodies. As it is known, these aceumulations of mafie minerals may be con- sidered restites resulting from anatectic differentiation processes (Scheu- mann, 1937 ; Mehnert, 1951, 1962). However, they may also result from metasomatic processes due to the reaction between the host rock and the mobilised pegmatite, constituting according to Reynolds (1946, 1949) the “basic front”. These stripes of mafie mineral aceumulations are ge- nerated by lateral secretion, as well, in which case the mineral substance is carried away by diffusion, marked by the migration of active minerals — quartz, feldspar — and the simultaneous accumulation of inert mi- nerals — biotite, tourmaline, hornblende (Ramberg, Șeclăman, 1972). Taking into account the fact that the stripe of mafie minerals resul- ting from anatectic differentiation may be then destroyed by recrystal- lizations or local revival of anatexis, one may account for its absence from some pegmatite bodies of this genetic type, therefore for its appa- rently unstabie feature. The mafie mineral aceumulations resulting from metasomatosis processes may be similarly approached : the absence of basic phenomena might be due to incomplete reactions or to their revival, or even the later annihilation of their effects because of, for example, some mechanic deformations. As regards the mafie cover of the pegmatite bodies generated by lateral secretion, it may be destroyed by subsequent blastesis of leucocrate minerals. It is to infer that the mafie accumulation as stripes at the peri- phery of pegmatite bodies takes place during some distinct petrological processes. In other words, different phenomena determine the same features. 'A Institutul Geological României IGR/ 23 PEGMATITES BETWEEN TEREGOVA AND MARGA 27 5.2.2. Conchisions. The study of the different characteristics of pegmatites from the Teregova-Marga region shows primarily the co- existence of several genetic types. Pegmatite. bodies with different shapes formed during the same genetic proeess and the other way round, similar shapes were generated by different genetic processes. The assignment of these pegmatites to the different genetic types is difficult because of the concentration of phenomena. It is possible to determine the genetic type of the pegmatite bodies by using alî data available and by studying the different aspects in detail. 5.3. Inner Structure The study of the inner structure of the pegmatite bodies allows to reconstruct the successive stages of pegmatitic minerals formation and the thermo-baric conditions of their crystallization. All data relevant of the inner structure regard the grain size, the relative amounts of major and accessory minerals and the presence of the graphie structure. Considering these data, the zoned structure may be recognized, as well as the occurrenee inside pegmatites of some tabular bodies which represent in fact fractures or fissures filled with younger pegmatitic minerals, or the presence of substitution bodies sensu Cameron et al. (1949). 5.3.1. Zoning. Most of the pegmatite bodies exhibit this structural feature, even if restricted cases consist of an aplite contact zone contras- ting to a largely crystallized, homogeneous pegmatitic mass. When several zones are delineated, they are proved to be generally parallel to the outer contour of the pegmatite body. Both zoning and other structural features show a wide range of aspects often difficult to recognize. These zonings may be complete, incomplete, symmetrical, asymmetrical, simple or composed. There are also numerous bodies marked either by incipient zoning or by destroyed zoning. The most representative example is pro- vided by the large size Armeniș body. It also exhibits wonderful graphie structures, big clusters of largely crystallized microcline-perthite or large areas consisting of plagioclase (oligoclase) associated with quartz, subor- dinate microcline, muscovite, gârneț, biotite. Each area. apari, shows features characteristic of a zone, but the lack of their continuity makes us speak of “pieces” of zones, whereas the pegmatite body is unzoned on the whole. A similar instance is represented by the big Cîrniș body in the Pietroasa Valley, Măgura sector. These situations may result from the occurrenee of substitution bodies which pierce and obliterate the inițial zonings, as well as from intense cataclasis which changed the pri- mary inner structure of pegmatite bodies. Zonings have been mentioned by different authors, concerning both the pegmatite bodies in the Semenic Mts and those occurring north of Muntele Mic. Thus, Superceanu (1957) recognizes, within the two large pegmatite dykes at Teregova, an outer aplite zone also containing tourmaline, biotite and hornblende, an intermediate zone with large microcline blocks, quartz and abunding rare minerals such as beryl, nio- bate, tantalite, monazite and zircon, and an inner zone mainly built of Institutul Geological României 28 H. P. HANN 2+ quartz. The contact area between the inner and the intermediate zone includes column-like tantalite crystals weighing 0.5 kg, zircon, niobate, lepidolite crystals and large beryl crystals weighing up to 11 kg. This author also describes the complex zonings of the pegmatite bodies in the Tîlva hill. Mînzatu and Mînzatu (1957, 1958) also mention zoned structures of pegmatites from Slatina-Timiș area and from Bocetul Mare peak lying north of Muntele Mic. They contain an outer apiite zone, then a „pegma- tite” zone of varying grain size, which also contains quartz and both micas besides feldspars, then a central (inner) zone of coarse grain size including quartz cores with miarolitic cavities. Sura (1966) treats the zoning of some relevant pegmatite bodies in the Tîlva hill (body 1 Tîlva) and in the Pietroasa Valley (Pîrîul cu Mărul body). Thus he describes a micropegmatitic outer zone, of centi- metric size, a medium grained marginal zone with important muscovite accumulations, an intermediate zone with „block”-like structure in which the crystals show the largest dimensions, and an inner zone represented by the quartz core. As regards the body 1 Tîlva, the marginal zone consists of plagioclase, quartz, muscovite, as well as gârneț and tourmaline, the intermediate zone with block-like structure contains microcline, plagio- clase, scarce quartz and muscovite, and the central zone consists of quartz with large beryl crystals. The Pîrîul cu Mărul body shows a quartz core in marginal position next to the dyke hanging-wall with a poorly developed and discontinuous marginal zone. Nevertheless, the muscovite accumu- lations of economic interest are present in this zone. Zoned structures are also mentioned by Savu (1977) and have been described by Hann (1973, 1977). The review of the different shapes of the pegmatite bodies points to the following remarks on their zoning. Lenses.Most of the small size lenses (2—3 m long, as much as 1 m wide) are unzoned. The grain size increases almost always from the margin to the core of the lens wherein feldspars abund. Concomitantly the mica content increases slightly next to the contact with the surrounding rocks. After detailed investigation,most of these lenses exhibit randomly spread, small size cores abunding in one or two minerals, such as quartz and micas, or isolated zones with coarse-grained minerals. Thus, the inner structure of these lenses is however heterogeneous, unzoned and unho- mogeneous. These data are relevant as far as these individualized spots account for an incipient zoning and allow to assign the pegmatite body to the sequence of processes resulting in pegmatites, in our case an early evolution stage. Graphic textures are scarce, mainly consisting of plagioclase and quartz. Plagioclase is however the main component of these lenses. Quartz and mica amounts are relatively high. Microcline ones are reduced. Under the microscope potash feldspar grows by corroding plagioclase. The de- crease of the grain size from inside to the periphery might be due to defor- mations resulting in the boudin structure, namely the breaking up of some pegmatite veins or dykes. Șeclăman (1972) has shown that decrease of grain size points in fact to the increased intensity of mineral cataclasis. Institutul Geological României IGRZ 25 PEGMATITES BETWEEN TEREGOVA AND MARGA 29 The same explanation may be given for the limited occurrence or the absence of graphic textures, which, as shown by Drescher-Kaden (1969) and Șeclăman (1971, 1972) are easily destroyed during deformation. The larger lenses frequently show the central zone which consists almost exclusively of quartz and may be discontinuous. There is also an intermediate zone of plagioclase, more and more substituted by mi- crocline, as well as of quartz and micas. It gradcs into the outer zone by decrease of grain size, quartz enrichment and concomitant increase of mica amount characterized by biotite prevalence and gradual decrease of microcline content. Concordant veins. The small size ones are never zoned. The size increase is accompanied by grain size increase and the appearance of zoning, which in certain cases is complete. The small size veins consist of plagioclase, microcline, quartz, biotite, muscovite. Under the micro- scope, the potash feldspar is noticed to replace the plagioclase. Biotite and muscovite amounts are equal or muscovite ones are prevailing. Mus- covite formed at the expense of biotite is scarce. Most of the specialists called the large conformable veins lenses, although there is an obvious difference between the typical lens-shape and the shape of these bodies. The confusion is probably due to both their layered aspects and their gradual thinning out to pointed ends. Most of these veins are clearly zoned. Each body stands out owing to its inner structure. Some veins exhibit between the quartz core and the contact (outer) zone an intermediate zone built of largely crystallized microcline, frequently graphically intergrown with quartz, and a marginal zone mainly consisting of plagioclase, muscovite and quartz; other veins show only the contact zone, an intermediate zone and the central zone. In most cases, the intermediate zone may not be clearly delimited from the marginal one. All deviations from an ideal zoning pattern are due to the succession of the different stages of pegmatite formation and to the conditions undei’ which they took place. For example, in the vein located below Fața Lungă hill, to the left of the road to Măru village, the quartz core is placed laterally and almost the entire pegmatite ground- mass consists of largely crystallized microclineperthite, frequently gra- phically intergrown with quartz. This microcline mass contains only pla- gioclase, quartz and muscovite nests representing the marginal zone relics. Thus, it is to infer that the potassic metasomatosis phase was prevailing and took place in its anhydrous form, within an open system, accompanied by the decrease of the muscovite amount. The body located to the east, below the Curcan peak (Tilva area), shows an intermediate zone consisting of largely crystallized perthites, which grades outwards into a marginal zone containing plagioclase, abundant quartz and large muscovite plates. The muscovite amounts result from biotite substitution, the latter being however present as relict mineral. This is proved by plates in which the two minerals are entwinned, the muscovite area sho- wing “brown” patches of biotite or small iron oxide inclusions resulting from biotite deferrization. In other instances biotite disappears comple- tely and only the opaque inclusions are present. In these pegmatites the potassic metasomatosis took place initially in the conditions of water preservation which favoured the crystallization of muscovite, and then Institutul Geological României 30 H. P. HANN 25 anhydrous metasomatosis and large erystallization of potash feldspar occurred as a result of system opening. On the ridge to the left of the Pietroasa Valley there is a vein sho- wing a zone built of oligoclase and quartz graphic intergrowths (Pl. VI, Fig. 2). Similar intergrowths have been reported from other veins too, preserved as relict structures of limited extent within perthites. The potash feldspar occurs subordinately inside the body, replacing the pla- gioclase, while the central quartz zone is discontinuous. Therefore, the largely developed plagioclase, characterized by a high temperature of formation, represents a first stage of pegmatite generation. At the end of the plagioclase phase, due to quartz aggressiveness and relatively low water pressure, graphic intergrowths of the two minerals do form. A zone of plagioclase graphic textures stands thus out, and it has been preserved in the absence of any other stage of pegmatite evolution. Nests. These bodies are irrelevant with respect to zoning. Generally, the grain size is directiv proporțional to the size of the body. As far as the nests are always of reduced dimension, the grain sizeis never excepțional. No proper zoning is noted; micas occur in somewhat higher amounts at the periphery of the body. Microcline amounts are at least equal to pla- gioclase ones, or in other instances are subordinate, but the fonner always corrodes the latter. Dylces. These pegmatite bodies exhibit in places complete zonings often of complex type owing to the diversity and importance of their mineralogic composition. However, there are also unzoned dykes or simple zoned dykes, which include those resulting from metasomatic processes of “replacement” type which frequently preserve inclusions of the surrounding rock, as shown in Figure 10. As a result of more intense recrystallization innerwards there is a difference between the periphery and the inner zone. No mineralogic zoning is obvious, the whole body consisting of plagioclase, microcline, fine grained muscovite and scaree biotite. Another similar dyke (lying below the Ogaș hill, right slope of Bistra Mărului Valley, Tîlva area) exhibits a lateral, asymmetrical quartz zone and the rest of the body consists of plagioclase, microcline partly graphically intergrown with quartz, muscovite and biotite, no zoning being noticed so far. However, the quartz zone and the microcline. sub- stitution by graphic quartz show that at the end of a mainly alkaline metasomatic stage the quartz was redistributed and became active again. The evolution of these processes may result in the obvious zoning of these smaller dykes. The large dykes, unfortunately already completely mined, show quito excepțional size in this region (at Teregova they are 10 m thick and ovei’ 200 m long). It is worth mentioning that- these bodies underwent all or almost all the evolution stages characteristic of pegmatites and they exhibit different zoning patterns, while the typical rare minerals (beryl, monazite, tantalite) contribute to their complex features. Thus, these bodies show features similar to pegmatites of magmatic-granitic origin, with the difference that they had been generated by anatectic metamor- phic processes, as shown further on. These similarities, which are in fact a consequence of eonverging phenomena, allowed the assignment of these pegmatite bodies to a mag- 'A Institutul Geologic al României iGRy 27 PEGMATITES BETWEEN TEREGOVA AND MARGA 31 matic granitic origin (Dittler, Kimbauer, 1931 ; Avramescu, 1954; Su- perceanu, 1957 ; Suni, 1966). The first, eastward located, dyke at Teregova has a still unmined area (only 4.5 m wide) which shows asymmetrical zoning (Fig-14). It has an outer contact zone, finely crystallized, of aplite nature, 3—4 cm wide. Under the microscope, the rock shows equigranular texture and leucotonalite composition, consisting of albite and quartz (PI. VI, Fig. 1). Fig. 14. Pegmatite dyke with asym- mctric zoning (Teregova, eastern body). 1, central zone built of quartz ; 2, inter- mediate micropegmalile (aplite) zone; 3, largely crystallized intermediate zone built of microcline, subordinate mus- covite, quartz, plagioclase; 4, finer grained outer zone built of abundant muscovite, quartz and subordinate mi- crocline; 5, contact aplite zone with tourmaline ; 6, migmatized paragneisses. Fig. 15. Contact area (A) and part of the externai area (B). Sketch drawn according to polishcd hand specimen — Teregova eastern body. It is to note the Sharp boundary between the two zones and the constant width of the contact area within the aplitic, quarlz- feldspathic groundmass also including tourmaline crystals (black). The outer area consists of muscovite (hatched), quartz (dots) and feldspars (while). The symmetric extinctions of albite crystals are of at least 14° pointing to pure sodic feldspar composition. Considering that the surromrding rock never contains albite and only oligoclase (AnM), it is excluded that the contact zone had formed by secretion process. On the other hand, Schneiderhohn (1961) shows that the contact areas built of albite and quartz are characteristic of pegmatites yielding lithium minerals, as con- firmed by Superceanu (1957) who notes the scarce occurrence of lepido- lite and spodumene in the Teregova pegmatites. The contact area is not even. A few meters away the aplitic features are still present, but they are uneven as the finely grained groundmass also includes larger quartz, oligoclase and tourmaline crystals (Fig. 15). The contact area is continuous, of constant width, different from the other areas of varying width characteristic of numerous pegmatite Institutul Geological României 32 H. P. HANN 28 occurrences described by different authors, such as Cameron et al. (1941) and Jalms (1955). Taking into account the texture and mineralogic fea- tures of the contact area presented above. it is considered to result from the undercooling of anatectic-palingenetic fluids, which, according to Tuttle (1952), occurred at the same time with volatile loss. An outstanding characteristic is the presence of a micropegmatite zone next to one side of the quartz core; although it is not as fine grained as the contact area, it contrasts obviously with the surrounding interme- diate area, lending an asymmetric character to the zoning of these peg- matites. The microscopic mineralogic study points to oligoclase, quartz, scarce microcline and to a slightly inequigranular texture. These rocks might occur inside the pegmatites duc to the local loss of volatiles and the concomitant increase of crystallization nudei (Jahns, 1955). The proper intermediate area consists almost exclusively of largely crystallized microclineperthite with subordinate quartz, plagioclase and muscovite. The outer area contains acid plagioclase, quartz, muscovite, Subordinate microcline and less crystallized minerals as compăreți to the intermediate area. Considering the inner texture of one may get the graphic representa zoning, grain size and mineralogic the pegmatites under discussion (Fig. 16) of the relations among tures. 16. Relationship among zoning, grain size and mineralogic composition characteristic of pegmatites located between Teregova and Marga localitics. The appearance and growth of different minerals may be studied by taking into account their texture, thus defining the successive stages of pegmatite evolution. 5.3.2. Graphic texture. The graphic textures are of special impor- tance to the investigation of pegmatites. Micrographic intergrowths of quartz aud feldspar are also present in granites, but only pegmatites show exceptionally large ones, due to the specific growth conditions of pegmatite minerals with peculiar crystal size. Therefore, the pegmatite fluids should contain an increased amount of volatiles and a reduced number of crystallization nudei. Regardless of their origin, the graphic intergrowths of pegmatites are largely developed depending on these Institutul Geological României 29 PEGMATITES BETWEEN TEREGOVA AND MARGA 33 conditions. The graphie textures of pegmatites are also the result of quartz aggressiveness. Quartz may be often redistributed within the different stages of pegmatite evolution. The study of the graphie fabrics, of their genesis and changes also implies the investigation of pegmatite genesis. Therefore, many rescar- chers speak of “graphie granites” or “graphie pegmatites” or “runites”. The origin of graphie intergrowths is rather controversial. Accor- ding to different interpretations, there are three different means of un- derstanding the constitution of graphie textures. 1) Graphie textures are the result of simultaneous crystallization of feldspar and quartz. This is the classical view supported, among others, by Brbgger, 1881 (in Schneiderhbhn, 1961) and mainly by Fersman (1915, 1929, 1952) whose contribution is outstanding. According to this author, there is a definite reciprocal orientation within quartz and feldspar inter- growths owing to simultaneous crystallization (trapezohedron rule). Vogt (1929) mentions the crystallization of quartz and feldspar in eutectic conditions, based on a 26% quartz within graphie intergrowths. Later studies pointed to frequent important variations of quartz/ feldspar ratio, in the absence of eutectic conditions (Barth, 1962), as also proved by some other relatively recent studies (Winkler, 1967). 2) The quartz of graphie textures has formed by parțial corrosion and replacement of feldspar, as considered by several researchers such as Sederholm, 1916, (in Schneiderhbhn, 1961), Landes (1925), Schaller (1927), Schădel (1961), Augustithis (1962,1974), Dreschcr-Kaden (1969), Șeclăman (1971), Șeclăman, Constantinescu (1972). 3) The quartz and feldspar intergrowths are the result of both simultaneous crystallization and replacement of one of the intergrown minerals by the other. This is Wahlstrom’s (1939 a, b) opinion, who, on the one hand denies Fersman’s law and on the other supports the idea of the constitution of graphie textures as a result of simultaneous crystal- lization as well as parțial replacement of feldspar by quartz. Erdmans- dbrfer (1941) accepts both possibilities and according to Eskola the si- multaneous crystallization of quartz and feldspar should not respond to the eutectic parameters, while graphie textures are the result of late substitution of feldspar by quartz. Jahns (1955) and Schneiderhbhn <1961) have shown that the graphie textures formed in different ways. In the area under discussion there are graphie intergrowths of quartz and plagioclase (PI. VI, Fig. 2 a, b), perthite and microcline (PI. VI, Fig. 3), characteristic of intermediate or outer zones of pegmatite bodies. The study of three-dimensional graphie quartz shows that it is mostly skeletal and does not occur as isolated inclusions. Some faces of the graphie quartz exhibit a lamellar character (Fig. 17 a). The cut per- pendicular to this face points to an entirely different image (Fig. 17 b) of proper graphie quartz (PI. V, Fig. 1 a, b, c). The quartz lamellae intersect at depth other lamellae almost perpendicular to the former, unseen on the face bearing lamellar quartz as they are cut almost parallel to the plane on which they occur, or, owing to their thinness they have been detached or represent ununiform patch.es in case they had been o Ol 1O CQ T-< Be 1 7 1 1 1 1 1 .2 1 5.8 2.1 1.4 1 1 N 12 10 10 10 10 10 10 10 10 10 10 10 17 > Ol Ol CO Ol Ol Ol Ol Ol Ol Ol Ol Ol Ol o 1 2 2 1 1 1.5 1 1 2.5 4 2.5 2.5 1 Co Ol Ol Ol Ol Ol Ol Ol OI Ol Ol OI Ol Ol Z m Ol Ol Ol -COOIOICOCO^COOIOI Ol Mo Ol Ol Ol Ol o Ol Ol Ol Ol Ol Ol Ol Ol 1O Ga LO UO LO Ol [•' O O X • ■ C m O CC • T- o. TT ri M co Sn IO io Ol - CO Ol OI Ol Ol Ol • Ol Ol CO OI Ol co Cu kO tO kO kO CO CO xr • -TFT* • • co x* CO io kO TT CO CO Pb in o o o c o lo co o o c i.o CC O, O o > O « .O O C -< t—i r-4 r-< rH Location CC o « u ? - 5 u £ U C3 CC « s 2 « ® « te - u - > 2-S Ș V 2 ăb e •? es r* cs o u1 crk > - > O g , C3 - rt ° > o o o te E5S§5E2K£ES£S o o .2 >-• t- f_< u, o OP40QQS<0 1O un o oi xr CD LO m 10 0 0 CD OI 0 O co •»—< v—< V—< V—< T—< Q irca a Q CC Zi Gî Location Fața Lungă hill, Tîlva Rîpi hill, Tîlva arca Dobrotin valley, Măgun Curcan hill, Tîlva area Scoarța valley. Măgura Scoarța valley, Măgura Scoarța valley, Măgura Ogaș hill, Măgura arca CD s M 1/5 /5a 00 r~ 0 t ; »—1 •v—< cn V—' ■ ■■ ■ < > uO ( 61 T-^ v-* co r-t T”• No. v—t Ol CO un CD O- GO Analysts; 1 — 4, 7—8 Constanța Udrescu, 5 — 6 Ana Șerbănescu Institutul Geological României 41 PEGMATITES BETWEEN TEREGOVA AND MARGA 45 tution perthites or relict plagioclase inclusions in microcline are often present. It is thus inferred that potash feldspar represents a second evo- lution stage (II) of pegmatite genesis. The absence of water from the potash feldspar network accounts forits genesis by “dry” potassic meta- somatosis. Microelineperthite is the prevailing alkali feldspar. The perthite textures defined may be of mixed-metasomatic and exsolution nature, as far as the microelineperthite composition consists of a potassic stage, an exsolved sodic stage and a relict plagioclase stage. Irregular shapes (PI. VII, Fig. 2 a, b) of low temperature perthites resulted from potash feldspar substitution by plagioclase are prevailing (Wahlstrom, 1939; Drescher-Kaden, 1969). Perthites with fine albite bands uniformly ar- ranged and originating in high temperature exsolutions are sparse (Vogt, 1908; Andersen, 1938; Mehnert, 1971). A microelineperthite sample of irregular shape (collected from the Armeniș quarry) was studied by JEOL 100 microsonde at ICEM labo- ratory for physical metallurgy (PI. VIII). The topographic surface shows a relatively homogeneous, fine grained groundmass abunding in light elements (Al, K, Si). Considering the element distribution, Al abundances are uniformly spread, followed by Si. K, Na and Ca show complementary distribution, with irregular and uneven boundaries. K contains Na in places, while the reduced Ca amount is related to Na one and determines the unhomogeneous character of the mineral groundmass, visible to the right of the image. The analytical data from Table 5 point to the Chemical composition of alkali feldspars. The silica excess is due to quartz inclusions. A single sample yields an anomalous Fe content (3.5% Fe2O3) probably due to a hematite or magnetite inclusion. The potassium content varies from 12.50 to 7.60% K,O, with a mean content of 11.18%. The sodium content varies from 2.15 to 3.05% Na2O, with a mean content of 2.79%. The calcium content (1.05—0.20% CaO) shows the normal values characte- ristic of these minerals. The crystallochemical formulas and the normative composition of alkali fedspars were also calculated : S ample : Fk 1136 .21^*3.92 .35^^24. ( ’ 0.2 4$ * 1 i. 76^32 ) 1101/5 3.00^ a0.8'A a 0.12-^h.O3 Gr75.37Ab2i.60-^13.01 (^-^0.20®hl.80^32) 367 ■^2 .S0^ai .07*-a 0.08^3.98 bl 7O.8st^b27.QgAn2.02 05^’11.9,^32) 1245 M Or73.66Ab23.53An2.79 1101 Or6i.50Ab3i.36An7.12 1126 Or67.9oAb23 6oAn3 49 64, Or71.78Ab26.76An J 45 Taking into account the minor element distribution within micro- clineperthites (Table 6) the most important substitution types result from K replacement by Ba and Pb. Ba amount varies within wide limits Institutul Geological României 46 H. P. HANN 42 (180—5200 ppm) and is higher than Sr one. Ba : K ratio (Fig. 26) shows that K increase entails Ba increase. Side values account for albite occur- rence in the perthite texture. Fig. 26. Ba : K diagram of mi- croclineperlhites from Teregova- Marga area The mean Ba content is of 2.218 ppm, close to the mean value of 2.486 ppm reported for pegmatites from Rodna Mts (Murariu, 1979). According to the same author, a compârison with the alkali feldspars of granites (pean Ba content of 5.093 ppm, Liahovici, in Murariu, 1979). shows that the Ba content of pegmatites is highly decreasing. Taking into account the relation between Ba content of feldspars and crystalli- zation temperature, the alkali feldspars of the investigated pegmatites were generated at lower temperatures than those of granites or than the alkali feldspars of some igneous, granitic pegmatites. Sr content varies from 32 to 500 ppm and is lower than Ba content. The similar size of ionic radiuses of K (1.33 kX) and Pb (1.32 kX) results in identica! position of Pb and K ions within the potash feldspar. The investigated pegmatites show Pb accumulations of alkali feldspars varying from 73 to 170 ppm. 6.2. Micas Considering their textura! relations with the other minerals micas characterize different stages of pegmatite genesis. According to the alte- rations affecting them (biotite especialiy) and to the response to mechanic deformations (noticeable in the case of muscovite) micas point to both the alteration of chemistry typical of different evolution stages and the tectonic conditions of pegmatite genesis. 6.2.1. Biotite. Although biotite occurs subordinately, its presence is of petrogenetic importance. It is frequently fresh in small, mainly plagio- clase pegmatite bodies. The large zoned bodies contain biotite, largely crystallized in places, associated with plagioclase or microcline, which is to be wholly altered to muscovite, seldom chlorite, and represents a relict mineral. Biotite is characteristic of stage I of pegmatite genesis and crystallizes together with plagioclase, quartz, subordinate microcline. Then, it becomes unstable, is deferrized and the increased pressure of water vapours results in its alteration to muscovite. This phenomenon Institutul Geological României 43 PEGMATITES BETWEEN TEREGOVA AND MARGA 47 is also represented by brown biotite patches on some muscovite grains and small hematite or magnetite grain accumulations. The microscopic study of muscovites shows hematite grains along the cleavage planes. 6.2.2. Muscovite. Large muscovite plates are often present in the so-called "muscovite pegmatites”. Muscovite may show greenish shades due to different hematite (especially resulted from biotite alteration), tourmaline, apatite or gârneț inclusions. Muscovite is subordinate in “microcline pegmatites”. There are also pegmatites containing both minerals assigned to different zones. The microscopic study shows plagioclase replaced by muscovite, accom- panied by quartz, the two minerals being closely related. Muscovite was subsequent to plagioclase and was assigned to another stage (II A) of pegmatite genesis, characterized by potassic metasomatosis abunding in water vapours. During this stage, the crystallization of muscovite plates associated with quartz becomes prevaiîing and plagioclase occcurs as relict mineral. In the absence of water vapours, muscovite crystallization may cease and microcline may form, both minerals occurring in high amounts within the same body. It is also possible that potash feldspar hydrolysis may generate postmicrocline muscovit e according to the reaction 3 KAlSi3O8 4- 2H2O -» KAl2(OH)2Si3AlOi0 + 2K0H + 6 SiO2 Muscovite, commonly not too largely crystallized, occurs along some fissures or fracture areas, filling them together with quartz. This musco- vite is subsequent to the main evolution stages of pegmatite genesis, due to their parțial revival as a result of the gaps inside the pegmatite bodies generated by tectonisation. Intense and prevaiîing mechanic deforma- tions and the conditions characteristic of muscovite genesis favoured the appearance of “acicular muscovite” with bent lamellae, associated with fine-grained quartz, which accounts for the pressure conditions that had influenced the crystal growth. The analytical data of Table 7 point to constant silica content and high A12O3 amount varying from 29.95 to 41.06%, also in agreement with other authors’ data (lanovici, 1939 ; Murariu, 1979). These analyses also point to Fe3+, Fe2+, Mg2+, Ti4 ions contained by muscovite which may substitute octahedral aluminium. Ferrous oxide contents are almost always higher than ferric oxide ones, agreeing with literature data men- tioned above. The diagram (FeO + Fe2O3)/TiO2 (Fig. 27) shows two fields; the right side one with the plots of high TiO2 contents characteristic of mus- covites originating in biotite alteration and the left side field with plots of muscovite resulted from plagioclase or microcline hydrolysis. The K content of muscovites varies from 8.10 to 11.12% K2O with a mean value of 9.92% K2O. The Na content is of 0.50—1.00% Na2O. The K2O : Na2O ratio (Fig. 28) shows plots grouped within a hori- zontally elongated field, which accounts for independent K2O and Na2O contents. Institutul Geologic al României 44 H. P. HANN 48 TABLE 7 Chemical composition of some muscovites from pegmatites in Teregova — Marga area TÎIva hill 29.95 1.07 0.67 0.03 0.75 810 O» z TÎIva hill Cl o 0.58 0.97 0.02 0.92 CM Cl 0.06 z Tere- gova 36.49 0.58 0.97 0.02 0.75 10.17 0.05 a Bău- țari TÎIva area 6>n 12 S — 0 0 0.75 9.75 80'0 1009s Arme- niș 1.54 0.60 0 0.50 CM 0.05 2060j Scoar- ța Va- lley Măgu- |ra arca 37.80 0.70 0.82 0 1.00 9.62 0.22 2206 Cer- ni ez Zalley Jăgu- a arca 30.36 1 .00 0.75 0 0.50 10.17 0.52 r i—• r-* 1020 Cioaca cu Mă- rul Măgu-, ra arca 33.47 0.81 0.90 0 0.80 10.37 0.22 O Scoar- ța Va- lley Măgu- ra area| 32.55 0.97 0.82 0 0.75 9.75 0.46 o TÎIva hill TÎIva area 36.02 1.17 0.82 © 0.67 9.75 0.34 s Arme- niș 33.32 O Ol co Y"* O 0 0.62 10.00 0.05 Ci Cioaca Pie- troasă Măgu- o o 5 31.64 0.82 0.67 0 0.50 10.25 0.12 Mărul brook Măgu- ra arca co CM 1.30 0.82 0 0.87 10.12 0.26 Cioaca cu Tei Măgu- ra arca 41 .06 1.20 0.97 0 0.87 11.12 0.21 Mu 1106 ; Curcan hill 1 Tilva । CC CM O 1> in ~ 00 & 0.018 1.30 1.04 0.90 8.70 0.20 0 Y— © O T—< Mu 1136 Rîpi hill Tilva arca 44.42 36.42 Ci w O O0 r-’ O 0.015 1.08 0.98 0.90 0 Cl Cl ^0 0 3.73 99.54 369 Dobro- tin brook Măgu- ■s oo oo oi m m io oo oi o o o co 0 Ci co 0 0 CM 10 0 Ci 0 O r-< Ci 0.44 0.07 CM Cl 100.20 377 Ogaș Vallcy Măgu- ra area o o o r- O Ol H o O IO r< O »n 00 oi 0 0 co t-. co cc i> m 0 0 • r-< 0 O Ci O O 4.45 100.81 Sample Location SiO2% A12O3 % Fe2O3 FeO MnO MgO _ O Ol IO — a « «O “ O && H &OK TOTAL Z a oo W s A Institutul Geologic al României 45 PEGMATITES BETWEEN TEREGOVA AND MARGA 49 Taking into account the minor element content (Table 8), it is to mention the low Sn content (15—65 ppm) and the varying Cu one (2 — 75 ppm). Co and Ni amounts are sparse, 2—12 ppm and 5—13 ppm res- pectively. 2 Fig. 27. (FeO + Fe2 O3 )/TiO2 diagram of muscovites from Teregova-Marga arca. 0 0,2 0,4 0.6 0.8 1,0 1? NojO* Fig. 28. Na» O/I<2 O diagram of muscovites from Teregova-Marga area. The data mentioned above show that the muscovites yielded by these pegmatites are characterized by low minor element concentration coefficients, which, according to Stern (1966), is typical of metamorphic pegmatites. 6.3. Quartz Quartz occurs in all pegmatite types, is associated with all pegma- tite minerals and characterizes the different stages of their evolution. Quartz appears at the end of each crystallizing stage and contributes to the development of metasomatic textures. It is partly redistributed in each evolution stage, which makes difficult the recognition of different rock generations. The quartz from the central zone (III) usually marks the end of pegmatite evolution. It is translucent oi* of dark-grey colour due (acc. to Holden, 1920) to some free Si atoms resulted from irradiation. Far from the core, the quartz becomes of liglit grey or whitish colour and is of high temperature type. During stage I quartz occurs together with plagioclase and biotite or forms graphie textures with plagioclase. During stage II it occurs together with microcline and microclineperthite or represents graphie textures within the latter. The transition from one stage to another is marked by myrmekite quartz prior to microcline generation. Stage II A is characterized by peculiar affinity between quartz and muscovite. The minor element contents of a quartz sample collected from the central area of a pegmatite exposed on Mărul brook (Măgura area, analyst 4 — c. 231 50 H. P. HANN 46- Ana Șerbănescu), and of two samples collected from the quartz core of some pegmatites lying in the Ogaș hill (Măgura area, analyst Constanța Udrescu) are the following : Cu 3 — 9 ppm, Ni 2—3.5 ppm, Zr 3—9 ppm, Ba 17 — 38 ppm, and only the first sample yielded B 30 ppm as a result of abundant tourmaline crystals near the central area. 6.4. Accessory Minerals They show varying, usually low, contents in all pegmatite zones and associate with the main minerals. On the whole, most pegmatites contain very reduced amounts of accessory minerals or lack in them, while very few pegmatites yield varied and abundant amounts. The main accessory minerals encountered in the area under discus- sion will be presented below. 6.4.1. Tourmaline. It is the most frequently encountered accessory mineral. The dark variety (Schbrl) is present in the intermediate (to central) zone of some pegmatites (Teregova, Pîrîul cu Mărul — Măgura area, Curcan hill — Tîlva area), some grains being 20—30 cmlong and associated with both microcline and quartz. Tourmaline also forms small prisms in the marginal aplite zone of several pegmatites, or radial prisms (“Sonnenturmalin”), parallel to the contact line with the surrounding rock, pointing to a volatile-abunding environment. Tourmaline may form at the expense of biotite (espeeially outside the pegmatite bodies) under the influence of B-rich Solutions (Kunitz, 1929). In this case biotite is not altered to muscovite, but to schbrlite. Owing to its low alkali content, tourmaline occurs in all stages of pegmatite genesis and associates with final quartz. The large tourmaline grains were frequently corroded by quartz, resulting in „graphic tourmaline” (PI. IX, Fig. 2 a, 2 b, Fig. 3). Besides quartz, fine-grained muscovite occurs on fissures inside the grains. TA Minor element content of some tourmaline samples Sample Location Pb Cu Zn Sn Ga Mo Ni Co Cr 6h Curcan hill 5 6.5 850 43 48 <2 70 28 95 1106/80 Curcan hill 6.5 9 1150 16 41 3.5 15 <2 18 11263/80 Văruț Valley 13 3.5 60 6.5 17 2 3 <2 <2 The minor element content of some largely crystallized tourmaline grains from the intermediate zone (Table 9) points to Zn 60—1150 ppm. The isomorphism between zinc and ferrous iron and magnesium accounts for the zinc substitution of these elements eontained by tourmaline. 47 PEGMATITES BETWEEN TEREGOVA AND MARGA 51 Relatively high Sn (16—43 ppm), Ga (17—48 ppm), Zr (10—130 ppm), Mn (10—1000 ppm) and Li (3—56 ppm) contents are worth mentioning; these values are close to those reported by Pomârleanu and Murariu (1970) from Teregova and Voislova tourmaline occurrences. 6.4.2. Beryl. Some large, discordant pegmatite bodies (Teregova, Pîrîul cu Mărul, Tîlva hill) contain important beryl amounts as greenish, hexagonal prisma, of decimetric size in places. It occurs in the interme- diate zone within quartz or microcline. The pegmatites abunding in tourmaline also contain beryl accounting for similar genesis of the two minerals. Dittler and Kimbauer (1931) are the first to report beryl oc- currences at Teregova and to analyse them chemically; it is also cited by Avramescu (1954) and Savu (1977), who also calculated its crystal- lochemical formula : (Be2j7i •^^'0,25 F-o,o6 Ca0i04) 3.06 (A11j74 Fe;^Mg0>14) 1.94 (Si6,78 A1o,22) 6 (Oj7j27 OH0 73) 18 6.4.3. Gârneț. The investigated pegmatites contain brown-dark red gârneț, of probably almandine character, in intermediate zones (e.g. Văruț Valley, Dalci-Var area), represented by large idiomorphic grains (ca 1 cm in diamater) within plagioclase, quartz and few muscovite ground- mass (PI. TV, Fig. 2). Cherry red coloured gârneț associates with abundant translucid final quartz which may be slightly corroded, as well as with microcline or microclineperthite, and a few muscovite (e.g. Pîrîul cu Mărul body). Its colour is probably due to a high Mn content which resulted in spessar- tite enrichment, these garnets being characteristic of the last stages of pegmatite genesis. According to Vlasov (1961) garnets alter their compo- sition during pegmatite processes, the increase of Mn content taking place from outside innwards. BLE 9 rom pegmatites in Teregova — Marga area V Sc Y Yb Zr Be B Nb Ba Sr Mn Ti Li 120 13 14 <1 130 8.5 3000 <10 80 90 40 8 <2 16 2.5 60 5.5 .3000 14 34 <10 1000 370 56 2.5 <2 <10 <1 10 13 1000 18 10 12 80 72 <3 These large gârneț grains point to the high pressure which influenced the pegmatite genesis. Fine grained, rosy gamet also occurs on the fissures which cross the less chloritized pegmatite body (e.g. Armeniș). Institutul Geological României 52 H. P. HANN 48 TA Chemical composition of some pegmatite samples No Sample Location OXI SiO2 A12O3 TiO2 Fc2O3 FeO 1 40 Mărul brook — Măgura area 73.74 15.00 sld 0.46 0.13 2 41/4 Mărul brook — Măgura area 71.26 17.00 ii 0.21 0.13 3 50 Cioaca cu Mărul-Măgura area 77.67 12.65 i i 0.31 0.13 4 51 Mărul brook — Măgura area 74.19 15.45 ti 0.03 0.11 5 50/1 Cioaca cu Mărul-Măgura arca 65.68 18.75 fi 0.08 0.16 6 52 Cîrniș-(Mărul brook)-Măgura arca 73.23 16.70 ii 0.19 0.10 7 53 Armeniș 73.23 15.80 »> 0.09 0.10 8 54 Armeni ș 70.97 16.65 a 0.09 0.10 9 56 Armeniș 74.36 16.65 it 0.86 0.13 10 57 Armeniș 72.33 16.40 a 0.23 0.06 11 58 Teregova 69.40 16.25 ii 0.24 0.10 12 58/1 Teregova 70.77 15.65 ii 0.34 0.10 13 59 Teregova / 1 . 1 / 15.90 ii 0.14 0.10 14 60 Armeniș 73.48 17.50 ti 0.36 0.22 15 62 Sat Bălrîn-Armeniș area 73.42 16.75 it 0.26 0.13 16 63 Slatina Timiș 72.97 16.40 a 0.43 0.16 17 61 Tîlva 87.58 8.40 ii 0.33 0.16 18 67 Var 76.46 15.25 0.18 0.16 19 70 Scoarța Valley — Măgura area 69.08 19.40 0.05 0.46 0.08 20 74 Scoarța Valley — Măgura area 70.57 17.66 0.04 0.03 0.16 21 1754 Găina Mare Valley — Măgura area 73.67 15.23 0.075 0.39 0.55 22 1767 Strau Valley — Dalei-Var area 71.80 16.15 sld 0.36 0.31 23 2273 Armeniș 73.69 16.40 sîd 0.57 0.39 24 2492 Curcan hill — Tîlva arca 71.96 15.37 sld 0.55 0.23 25 2494 A Curcan peak — Tîlva area 72.08 14.10 sld 0.16 0.31 26 2491 13 Curcan peak — Tîlva area 73.33 12.73 sld 0.36 0.31 27 2495 Curcan peak — Tîlva area 71.95 17.02 0.075 0.61 0.32 28 2518 Rea Valley (Valea Satului)- Dalci-Var arca 71.30 14.70 sld 0.15 0.23 29 2275/2 Armeniș 75.20 15.03 traccs 0.10 0.11 30 2273/2 Armeniș 76.00 14.90 0.07 0.05 0.14 31 Rb 413 Ogaș hill — Tîlva area 79.40 13.73 0.08 0.10 0.11 32 1108/80 Armeniș 73.00 15.33 0.03 — — 33 1106/80 Curcan hill — Tîlva area 73.30 16.30 0.10 0.86 0.56 34 1108/60/1 Armeniș 74.30 14.63 0.06 0 0.07 35 Rn A 1 Ogaș hill — Tîlva arca 77.20 14.35 0.03 0.06 0.11 36 1107 Slatina-Timiș 72.50 15.60 — 0.22 0.36 37 1126 Var 74.93 14.50 — 0.20 0.28 38 1136 Rîpi hill — Tîlva area 76.22 13.00 0.025 0.21 0.72 39 1238 Găina Valley-Dalci-Var area 75.77 15.25 — 0.10 0.14 40 2103/2 Sasa Valley — Măgura area 72.05 17.62 — 0.92 0.38 41 352 Dobrotin brook — Măgura area 73.69 15.22 — 0.11 0.12 42 367 Dobrotin brook — Măgura area 71.78 17.62 — 0.17 0.03 Analyst: samples 1—28 Movileanu Aurelia, 29—35 ;jX Institutul Geologic al României XIGRZ 49 PEGMATITES BETWEEN TEREGOVA AND MARGA 53 BLE 10 in Teregova-Marga area (Semenic-Țarcu mountains') DES % MnO MgO 1 CaO | Na2O | 1 k2o P2O5 H2O~ II2O+ S Total P c2O3 Total 0.013 0.05 0.87 4.30 4.40 0.085 0.18 0.79 100.01 sld 0.10 0.56 3.00 8.00 0.095 0.16 0.47 100.98 0.005 0.40 0.59 5.00 2.70 0.050 0.26 0.24 100.00 0.005 sld 0.77 3.40 6.10 0.105 0.08 0.50 100.73 sld 0.27 1.68 9.80 3.60 0.105 0.28 0.22 100.62 0.010 sld 0.80 3.70 5.60 0.105 0.14 0.02 100.59 sld sld 1.12 8.00 1.30 0.105 0.28 0.07 100.09 sld sld 0.56 3.50 8.20 0.075 0.18 0.12 100.44 0.013 sld 0.40 4.30 3.00 0.025 0.16 0.97 100.86 sld 0.10 0.35 3.50 7.60 0.70 0.14 0.07 100.85 0.020 sld 0.45 10.60 2.50 0.41 0.14 0.20 100.31 0.029 sld 0.45 9.80 2.60 0.54 0.14 0.19 100.60 0.005 0.05 0.35 8.00 4.20 0.35 0.12 0.33 100.71 sld 0.15 0.87 6.80 1.00 0.040 0.08 0.53 101.03 0.010 0.15 0.87 8.00 0.50 0.025 0.16 0.37 100.64 0.005 0.07 0.50 4.30 5.00 0.050 0.06 0.57 100.51 sld 0.27 0.20 1.20 1.60 0.05 0.06 0.34 100.19 0.005 0.07 0.50 5.10 2.30 0.095 0.20 0.53 100.84 sld 0.40 0.50 4.20 6.10 0.10 0.14 0.18 100.69 0.16 0.02 0.52 3.50 6.40 0.025 0.18 0.28 99.54 0.030 0.55 0.70 4.40 4.90 0.110 sld sld 100.60 0.012 sld 3.22 6.50 1.70 0.550 100.61 0.020 0.30 1.61 6.70 0.80 0.090 ,, 100.57 sld 0.45 0.98 3.00 7.80 0.320 ,, 100.66 0.020 0.28 0.70 4.00 8.50 0.100 0.10 0.57 100.42 0.024 0.57 0.70 3.00 8.70 0.580 0.04 0.24 100.58 0.020 sld 2.10 4.40 2.40 0.250 0.12 0.04 100.52 sld 0.79 1.05 1.70 9.50 0.850 1.23 0.20 100.51 0.03 0.09 0.71 2.37 5.55 0.12 0.42 0.03 99.79 traces 0.10 1.08 0.23 0.94 0.03 0.64 0.03 100.24 0.26 0.02 0.19 0.83 4.70 0.49 0.05 0.45 0.03 100.21 0.25 — 0.09 0.32 3.02 8.06 0.15 0.22 0.03 100.28 0.26 0.28 0.25 1.25 3.50 0.94 0.12 0.54 0.03 100.02 — — 0.09 0.63 4.33 4.83 0.17 0.44 0.03 99.61 1.48 0.02 0.13 1.44 6.09 0.52 0.16 0.42 0.03 100.59 0.11 0.040 0.40 1.26 3.90 5.50 0.090 0.16 0.42 100.45 0.22 0.010 0.50 0.70 4.50 4.50 0.135 0.34 0.27 100.86 0.012 0.50 1.05 2.60 5.20 0.275 0.20 0.58 100.58 — 0.50 1.05 5.00 1.80 0.125 0.16 0.56 100.46 0.02 0.87 0.70 2.46 3.40 0.13 0.13 1.11 0.05 99.77 0.018 0.64 1.84 5.30 1.90 0.07 — 0.95 99.85 — 0.85 1.70 2.80 6.80 0.06 — 0.88 99.68 Vlad Catrinel, 36—42 Neacșu Vasilica. Institutul Geological României H. P. HANN 50 54 Analysts : samples 1-6 Mînzatu el al. (1958), 12-18 Mînzatu, Mînzatu (1957) 7-11 and 19-20 Avramescu (1954). Institutul Geological României 51 PEGMATITES BETWEEN TEREGOVA AND MARGA 55 Different pegmatite types correspond to the different structural- mineralogic stages of pegmatites genesis. Pegmatites with intermediate features are also added. The relations among pegmatite minerals point to the succession of evolution stages, which, by parțial superposition, generate zoned pegmatites. Each zone of a pegmatite body represents a petrogenetic type which may constitute an independent pegmatite. Plagioclase and biotite are typical of stage I, followed by quartz which under certain conditions generates graphic textures. Stage II — microcline or stage II A — muscovite depend on the pressure of water vapours. In an open system and low H2O pressure microcline crystallizes, while in closed System and high H2O pressure muscovite is generated, the entire process being influenced by the prevailing tectonic conditions. During this stage the “microcline block” zone defined by Vlasov (1952), namely a zone with large muscovite plates, is formed. Biotite is deferrized, but it may occur as relict mineral. Quartz is present again, substituting microcline and resulting in graphic textures or (during stage II A) acting as catalyser which facilitates the development of muscovite grains. This quartz type (stage III) marks the end of pegmatite evolution by concen- trating in a quartz core. In certain cases, this was followed by siight albi- tisation which generated the substitution perthites, but not a proper albite zone illustrative of an independent stage as mentioned by G-hinsburg (1960), Vlasov (1952). However, the pegmatite evolution is concluded by quartz, graphically developed in replacement microclineperthites and generating the quartz core. The different features implied by the minor element content of pegmatite minerals account for the relatively reduced intensity of sub- stitution, which together with the reduced contents of accessory minerals account for the metamorphic origin of pegmatites under discussion. 7. GEOCHEMISTRY OF PEGMATITES The Chemical sampling of pegmatites is rather difficult, as an accu- rate method based on all their characteristics is needed; the large grain size of minerals, zoning, fissures or fractures filled with later formed minerals, accidental distribution of different accessory minerals, etc. are to be considered. Even if all these features are taken into account, the result will be subjective, as the selection imposed by practicai conditions is objective. Taking into account the Chemical analyses (Tables 10,10 A) several features related to the evolution and characteristics of major and minor elements of pegmatites may be defined. The sequence of different evolution stages of pegmatite genesis reflected by the mineralogic characteristics may also be transferred to the Chemical properties : NaCa (I), K (II) oi- K + H2O (II A) and SiO2 (III), each stage being marked by the presence of quartz, and the appea- rance of final quartz is preceded by albitisation (Na) (replacement mi- croclineperthites) which does not represent a proper stage within these pegmatites, as no independent crystals are present. H. P. HANN 52 56 The characteristics of pegmatites geochemical evolution are shown bv different diagrams. Thus, on SiO2/Al2O3 diagram (Fig. 29) wereplotted the results of the Chemical analysis of some paragneisses collected from the rock pile which hosts the pegmatites (Table 11). It is to note a main Fig. 29. SiO2 /Ah O3 diagram of pegmatites from Teregova-Marga area. 1, pegma- tites ; 2, paragneisses; 3, Troger mean ; 4, mean value of Teregova-Marga area; 5, mean of graphic textures. TABLE 11 Chemical composilion of some paragneisses from Teregova-Marga arca Sample 359 360 366 367 368 369 370 371 372 373 374 375 SiO2 67.64 67.25 61 .03 62.98 62.32 70.85 68.74 57.48 58.81 59.98 65.46 69.01 TiO2 0.90 0.88 0.87 0.88 0.93 0.57 0.73 1.11 1 .23 0.93 0.77 0.61 15.21 15.24 18.41 17.61 18.77 12.51 13.59 19.44 17.66 18.31 16.46 13.37 Fe2O3 0.57 0.72 0.73 1 .05 0.65 0.73 0.80 1.51 1.41 1 .23 0.68 1.01 FeO 4.25 4.35 4.99 4.89 4.88 4.71 4.76 5.76 6.36 5.42 4.36 4.69 MnO 0.08 0.07 0.06 0.06 0.05 0.04 0.04 0.10 0.12 0.12 0.06 0.03 MgO 2.03 1.97 3.25 2.55 2.52 2.23 1.82 3.35 3.85 2.79 2.44 1 .97 CaO 1.53 1 .52 2.01 1.70 1.89 2.09 2.40 1.92 1.60 1.77 1.86 2.75 K2O 2.70 2.85 2.64 2.54 2.50 1.60 1.52 3.67 3.83 3.46 2.22 1.50 Na2O 2.31 2.19 5.20 2.90 3.06 2.60 3.15 3.20 2.54 3.23 3.19 3.05 p»65 0.24 0.24 0.22 0.19 0.17 0.16 0.27 0.34 0.22 0.17 0.31 0.18 H„O+ 1.66 1.66 2.11 2.14 1.82 1.36 1.58 1.87 1.92 2.01 1.85 1.30 CO. 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 S 0.26 0.34 0.09 0.07 0.09 0.14 0.13 0.03 0.08 0-08 0.07 0.18 Fe(S) 0.22 0.29 0.08 0.06 0.08 0.12 0.11 0.02 0.06 0.06 0.05 0.15 Total 99.60 99.57 99.69 99.62 99.73 99.71 99.64 99.80 99.69 99.56 99.78 99.80 Fe2O3tot. 5.70 5.96 6.38 6.56 6.18 6.13 6.24 7.94 8.55 7.33 5.59 6.47 Analyst: Elena Colios Institutul Geological României 53 PEGMATITES BETWEEN TEREGOVA AND MARGA 57 field of accumulation of plots between 75% SiO2 and 18% A12O3. The Troger mean (acc. to Schneiderhbhn, 1961) occurs at the top of this field, unlike the mean of investigated pegmatites. There is a left field and a right field with fewer plots. The left field represents the mainly muscovite pegmatites (quartz rich and attached to the preceding plagioclase member), and the right field contains the plots of mainly microcline pegmatites. At the base of this diagram the plots of paragneisses form an elongated field parallel to the three pegmatite fields pointing to the si- multaneous evolution and interdependence of the two rock types during the metamorphic processes. The diagram Na-O/K2O (Fig. 30) points to an almost uniform and widespread occurrence of pegmatite plots, and the concentration of pa- ragneiss plots at the base of the diagram. The Troger mean appears to the right of the diagram and does not correspond to the pegmatite mean. Fig. 30. Na2 O/K2 O diagram of pegmatites from Teregova-Marga area. 1, pegmatites; 2, paragneisses; 3, Troger mean; 4, mean value of Teregova-Marga area; 5,mean of graplijc textures. The plots of the mean of graphic textures appear to the right of the dia- gram too, as most of them are of microcline character. The diagonal line delimits the plots in two fields : above and below it. According to Mehnert (1971) the plots above the diagonal line corres- pond to the plots of graywaeke gneisses. It may be inferred that some Institutul Geological României 58 H. P. HANN 54 pegmatites were generated in situ from anatectic Solutions of pegmatoid character mobilized from surrounding micaschists and paragneisses, while other pegmatites originate in anatectic pegmatite fluids generated by gneisses, which may migrate from greater depth and may form complex pegmatites in certain conditions. Diagrams SiO2 : A12O3/ K2O : Na2O and Na2O + K2O : K2O + + Na2O (Figs. 31, 32) have been drawn in view of a varied characteri- sation of pegmatites based on their major element contents. According to the diagram of Figure 31, the different pegmatite petrographic types may be divided horizontally as follows : a field with maximum concentration of plots within a field with fewer plots, both 55 PEGMATITES BETWEEN TEREGOVA AND MARGA 59 Minor clement content (um) of seine pegmatites samples in Teregova — Marga arca >—< 7 V 3 7 3 3 3 3 3 3 3.5 4.5 3 3 3 3 4.5 7 3 3 10 o co 00 100 10 90 45 50 65 30 95 10 10 200 170 67 67 12 230 30 35 100 130 380 55 300 — 16 72 165 950 65 310 315 60 570 100 950 20 19 59(10 300 90 790 38 85 43 42 380 110 130 80 1100 50 200 30 30 30 30 30 30 36 30 30 30 30 30 30 30 30 35 30 30 30 30 30 30 30 30 O o I> co — 2 1.7 7.6 5.8 1 1 .4 5 1.3 130 140 4.8 1 1.9 2.1 1.4 3.1 5 11 11 1.9 O CO o CD in oi 280 2.6 Zr 60 10 10 10 12 10 24 10 22 26 10 10 21 10 10 16 10 17 17 10 o oo oi cr r-< CO v-< TF o o Yb 1 1 1 1 1.4 1 1 1 1 1.1 1 1 ■< T—1 >* io o o o o o o o r“. T~> V—< —1 V—« T-H T-< 10 10 10 14 11 10 10 10 o o o T-< r-< Ol r-4 O I- s» Ol Ol Ol Ol Ol Ol Ol Ol Ol Ol ol oi • oi oi oi oi oi oi co Ol Ol CO Ol Ol Ol > Ol Ol Ol Ol Ol Ol Ol Ol Ol Ol Ol ol CO Ol Ol Ol Ol Ol Ol [■- UO O • Ol o Ol ~ 3.5 3 o X 1 1 1 1 1 1 1 2 1 1 4 2.5 2 1 1.5 1.5 8 5 4.5 1.5 5 1.5 2 Co Ol Ol Ol Ol Ol Ol Ol Ol Ol Ol Ol ol Ol Ol Ol Ol Ol Ol Ol Ol Ol Ol Ol co o Ol z m m io io • Ol CC CO Ol • • Ol Ol Ol Ol Ol Ol Ol Ol i-O Ol T co Ol Ol Ci » CO Ol co Ol Ol Ga 15 15 9 11 13 10 16 7.5 15 15 3.5 12 10 15 9 15 10 15 16 12 23 18 13 13 CO co CO Y-1 Sn m oi oi oi oi io oi co oi oi oi iC 1.0 • Ol Ol • Ol Ol Ol o Ol Ol Ol Ol 5.5 3 Cu 4.5 4.5 5 6.5 4.5 8 4.5 6 8.5 7.5 3 5 3 4 4 3 3.5 3 10 9 8 4 4.5 16 5.5 Pb CC O I> Ol O O O, O O v ce o —• CO T-t TF CO O io Ci Ol Ol y-4 co O Ci O O CQ O Ol Ol > n o r, TF oi oi Ol IO cc CO »D O Ol LO Ol l> Location Mărul brook-Măgura area Mărul brook-Măgura area Cioaca cu Mărul-Măgura area Măgura arca Pietroasa (Cirniș) Valley Măgura arca Armeniș Armeniș Armeniș Teregova Armeniș Tîlva l Dobrolin brook-Măgura arca Dobrotin brook Armeniș Armeniș Var Dalei-Dalei Var Curcan-Tîlva hill-Tilva arca Var, Dalei-Var area Var, Dalei-Var area Ogaș hill-Tilva area Ogaș hill-Tilva area Ogaș hill-Tilva area Armeniș Strimba Valley-Dalci Var area ieregova Armnnis Sample O O qq £ < d « , CO CO O C O'’ CC <. ^M«r-OOOlOOlOlCCr; 0 t* T? 1O LO 1O LQi'^iOlOLDCDCOCOr-i'r-i'r-i'’—<-r- OOQ O T-< o 13 14 15 16 17 18 19 on Ol CO Ol Ol Ol Ol o co N Ol Institutul Geological României 60 H. P. HANN 56 Institutul Geologic al României 57 PEGMATITES BETWEEN TEREGOVA AND MARGA 61 pointing to the pegmatite evolution owing to the alkali content and thus accounting foi' its importance to the geochemieal characterisation of pegmatites. The diagram of Figure 32 shows distinct fields witli the plots of paragneisses and pegmatites respectively. However, it is to note the similarity of the two fields all oier the area of paragneiss occurrence. At the top of the paragneiss field, their plots are similar to pegmatite plots, partly parallel to them. Then, the pegmatite field covers the top of the diagram. This accounts for the pegmatite origin in the gradual division and migration of alkaline mobilisates (together with quartz) from the rock piles during anatectic metamorphic processes. In view of a thorough geochemieal characterisation the pegmatite samples were also analysed spectrally (Table 12). The minor elements (B, Li, Be, Zr, Y, Yb, Sn, etc.) characteristic of granitic pegmatites do not show high contents. The results of these analyses have been compared to the minor element content of some paragneisses (Table 13) : the vana- TABLE 13 Minor element content of some paragneisses from Teregova-Marga area No. Sample Pb Cu Ga Sn Ni Co Cr V Sc Zr Nb Y Yb La Ba Sr 1 359 3 - / 16 2 60 16 95 100 19 320 17 45 4.8 50 600 170 2 360 3 7 16.5 2 50 13 85 85 16 240 14 32 3.2 56 600 160 3 366 10 16 23 3 48 10 80 95 17 180 10 19 2.0 30 400 320 4 367 8 19 23 3.5 48 12 90 105 17 210 12 24 2.3 38 420 300 5 368 8 8.5 22 2.5 48 11 67 80 15 160 14 20 1.8 32 420 500 6 369 3 5 16.5 3.5 50 13.5 90 90 16 260 12 30 2.8 30 270 90 7 370 3 8 18 4 48 15 85 95 19 260 14 27 3.2 30 300 110 8 371 11 6.5 20 2.5 66 22 125 130 21 170 12 33 3.4 44 500 240 9 372 8 9 19 3 66 25 110 130 21 190 13 27 2.2 46 500 170 10 373 13 38 23 3 65 23 120 140 22 210 10 30 2.0 42 600 270 11 374 13 10 19 3 62 14 95 110 20 270 12 29 3.0 44 380 400 12 375 3.5 24 4 55 19 95 120 20 280 13 29 3.6 30 280 100 Analyst : Constanța Udrescu diurn of paragneisses did not migrate to pegmatites as the lat ier yielded very reduced contents (26 ppm at the most, compared to 130 ppm reported from paragneisses); zinc contents of pegmatites are of 10—60 ppm, and of rocks are of 160—320 ppm, while chromium and nickel contents of paragneisses are of 67—125 ppm and 48—66 ppm respectively, higher than those of pegmatites (1—4.5 ppm 2—8 ppm respectively). Beryl con- tent of pegmatites is reiatively high (1.3—280 ppm). Lithium amounts are usually reduced. The biotite-rich pegmatites also have high Li con- tents, as it may substitute Mg and Al of the mineral network. Proper minerals are exceptionally formed (Superceanu (1957) mentions spodu- mene and lepidolite at Terogova). It may also lack, mainly in microcline pegmatites, as for example those from Southern Norway (Bjorlikke, 1937). Institutul Geological României 62 H. P. HANN 58 The rather monotonous and reduced minor element content of peg- matites does not favour the drawing up of diagrams relevant of the petro- genetic characteristics. However, the diagram of Figure 33 is an attempt at rendering the variation of some minor elements compared to the sum total of alkalis. Besides Pb and partly Ba, marked by positive cor- relation, the other elements (Sr, Cu, Ga) occur in reduced amounts, which correspond to Hornung’s data (1962). Stan(1977)studiedlead and barium substitution during feldspathisation. The differentiation of alka- line fluids of the paragneiss pile results in pegmatite quartz-feldspar mo- bilisates. Fig. 33. Variation of some elements compared to alkali sum total of Tere- gova-Marga pegmatites. 1, pegmatites; 2, paragneisses. 8. PEGMATITE GENESIS All structural, mineralogic and geochemical data account for the metamorphic origin of the investigated pegmatites, their genesis by magmatic processes being excluded. There is no evidence of any connec- 59 PEGMATITES BETWEEN TEREGOVA AND MARGA 63 tion with an igneous mass, no feeding channels are present and there is no trace of shifting or disappearance during the deformations which affec- ted the rock pile. No systematic zonal variations of pegmatite characte- ristics are noted and no areal distribution against surrounding granitoid massifs relevant of the more or less rhythmical pulsatory character of pegmatite emplacement is present. In fact, Savu (1977) demons- trated the absence of petrogenetic relations between the pegmatites in the Semenic Mts and the Poniasca granitoid pluton. The metamorphic genesis of pegmatites implies the concomitant or successive development of independent genetic processes within the same rock pile affected by metamorphism. This may be proved by the different morphostructural features, also coinciding by convergence of phenomena. An example is given by the lens shape. In folded areas, the maximum thickness of lenses is shown by the foid axis, where local pressures are lower and the pegmatites formed concomitantly with the folds. Some lenses show features accounting for migmatisation processes (diffuse graded contact, Fig. 13 a, b; “swarm”-like arrangement, Fig. 4), others show the characteristics of metamorphic pegmatites of „dilation” type (alternating graded contact), while a third category of lenses originates in the boudinage of concordant veins or the fragmentation of some peg- matite dykes due to unaffine lamination, parallel to paragneiss foliation. The concordant pegmatite veins may also be the result of some “dilation” metamorphic processes, as described by Goodspeed (1940) and recognized by Gherasi et al. (1974) in the Măru-Voislova region, or of the pegmatite evolution of quartz-feldspar mobilisates. The morpho-structural features also account for other pegmatite types of metamorphic origin. Thus, there are discordant pegmatites resulted from metasomatosis and marked by “nondilation” pegmatiti- zation (Ramberg, 1952), while the structural features of surrounding rock have been partly preserved as relics within the pegmatite ground- mass (Fig. 11). The boundary of these pegmatites is very sinuous and the contact with the surrounding rock is of mixed nature — either clear or diffuse. Pegmatites of this type have been reported by Maier et al. (1975) from Băuțari, in the Poiana Rusca Mts. Pegmatite dykes are also gene- rated by anatectic metamorphic processes. These pegmatite bodies show clear contact (usually of large size), and along the contact line there are fine-grained pegmatite intrusions in the surrounding rock (Fig. 10 a) ac- counting for the subsequent intrusion of anatectic mobilisates, probably along some fractures. „Concretion” pegmatites (Ramberg, 1952, Barth, 1962) resulted from concretion growth, of porphyroblastic character outside, starting from some cores at the expense of a solid host rock (Fig. 9 a). The study of mafie concentrations at the periphery of some peg- matite bodies also points to the different petrological processes which influ- enced the constitution of metamorphic pegmatites. The mafie border may result from anatectic differentiation (restites — Scheumann, 1937 ; Mehnert, 1951, 1962), or from metasomatic processes between the host rock and the pegmatitic mobilisate (“mafie front” — Reynolds, 1946,. 1949), or from lateral secretion due to diffusion (Ramberg, 1952). —-x Institutul Geologic al României jgr/ 64 H. P. HANN 60 Some of the pegmatite bodies result from lateral secretion (Fig. 9 b). The metasomatism is proved by the spațial distribution of stages during these processes, marked by matter supply and release. The generation of pegmatites by lateral secretion was described by many authors, as for example Ramberg (1952), Barth (1962), Gresens (1967), Șeclăman (1972). It is to mention that the rock cavities or fissures due to high pres- sure and temperature are incompatible with the equilibrium state. There- fore, the basic gradients (free energy gradients implicitly) appear and cause the migration of rock fluids and mineral components from high pressure areas to lowei' pressure cavities or fissures. As the mobilisation speeds of different minerals are not equal, the cavities are filled with the easiest mobilized minerals (feldspar and quartz), namely the minerals consisting of alkalis and silica easily soluble in aqueous fluids. The ini- tiation and development of lateral secretion processes are due to the cons- titution of rock cavities during the folding accompanied by the generation of shearing fissures and străin fissures. In ductile metamorphic environ- ments deprived of any proof of vein shearing, the fissures are the result of hydraulic force (Yardley, 1975): as far as the crystalline schists repre- sent anisotropic environments, the hydraulic fissuring with a certain trending may also occur in the absence of any stress deviation, along some low resistance planes. According to Șeclăman (1972) the fissures result from unaffine laminations of rock piles. The mineral substance is transported by diffusion, represented in the solid stage (intracrystalline) by the crystal network of minerals or by their discontinuities, imperfec- tions respectively, and by the intergranular fluid Solutions or intergra- nular veins through stationary pore fluid contributing to substance trans- fer (Yardley, 1975). The aqueous fluid of rocks contains volatile elements which influence the parțial water pressure affecting the formation of pegmatitic minerals. For example, potash feldspar is subsequent to pla- gioclase and it appears by decrease of water pressure below the lower limit of mica stability. The diffusion of a certain substance is caused by its Chemical potențial. Diffusion takes place from the high Chemical potențial area to the low Chemical potențial and low pressure area, re- sulting in the thermo-dynamic condition of diffusion (Șeclăman, 1972). Most of the pegmatites were generated by metamorphic processes called “anatectic-metasomatic” by Barth (1962). They may be generally characterized as “lateral secretion” processes according to Holmquist (1924) who mentioned the “lateral secretion venites” originating in the surrounding rocks. The mobilisation of initially solid rock fragments and the presence of liquid components as molecular dispersion within the solid/liquid boundary area result in the accumulation of mobile compo- nents in veins and irregular masses — metatects, obviously different from the solid components — restites (Scheumann, 1937, Mehnert, 1951, 1968). The characteristic features are effaced in the palyngenetic field and lateral secretion processes are no longer noted. The crystalline schists of the Sebeș-Lotru Group were generated by high-grade metamorphism which also implied some anatectic processes, as proved by the different types of migmatitic textures. Winkler (1966, 1967) and von Platen (1965) present the thermo-baric conditions (pres- sures of 10 kb and temperatures of 600 — 700°C) which influenced these A Institutul Geologic al României ICR/ 61 PEGMATITES BETWEEN TEREGOVA AND MARGA 65 processes and caused the segregation of leucocrate mobilisates from mieasehists and gneisses. In appropriate conditions (the prevalence of a volatile stage, favourable tectonic conditions) a pegmatite evolution takes place. According to the evolution stages crossed simple or compound pegmatite bodies appear. The pegmatites commonly occur in areas with highly migmatized rocks and lack from those areas deprived of these occurrences, thus ae- counting for their anatectic genesis. The sillimanite (Armeniș) genetically related to muscovite (muscovite ± quartz = sillimanite + potash feld- spar) also demonstrates the anatectic genesis of pegmatites. The subse- quent pegmatite evolution is marked by gradual cooling of pegmatites, as demonstrated by the temperature at which some pegmatite minerals were formed (Pomârleanu, in Hann et al., 1977) : microcline between 554° and 600°C, biotite between 560°C and 585°C, muscovite between 285°C and 470°C and beryl between 237°C and 460°C. The relation bet- ween the Ba content of feldspars and the crystallisation temperature aecounts for decreased temperature of alkali feldspar generation in the Semenic-Țarcu pegmatites. With respect to the structural-mineralogic features of the pegmatite evolution, it is to note their generation by a main mineral stage (the hardly mobile anatectic fluids) consisting of plagioclase, biotite-micro- cline, microclineperthite-muscovite-quartz, which replace each other and account for the pegmatite evolution stage. Plagioclase and biotite are characteristic of stage I, followed by quartz, which also generates graphic textures. Microcline and muscovite mark the second stage. Microcline is mctasomatically developed on plagioclase, and muscovite results from biotite deferrization or plagioclase replacement, at the expense of a pre- existing sericite. The mineralogic features depend on the pressure of water vapours. An open system and decreased H2O pressure result in microcline (II) crystallization, while an equally open and increased H2O pressure generate muscovite (II A). Thus, the tectonic framework influ- ences the entire process. Quartz substitutes microcline and generates the graphic textures, or (stage II A) favours the growth of muscovite grains. The metasomatic character of this mineral is also evinced by the study of graphic textures. The data mentioned above account for the se- lective substitution of feldspar by quartz. Quartz (III) marks the end of pegmatite evolution and concentrates in the quartz core. Albitisation is also present, resulting in perthitic textures. The microscopic study points to its metasomatic character. A proper albite zone representing an independent stage is absent. However, the pegmatite evolution ends with quartz graphically developed in the substitution microclineperthites, subsequently forming the quartz core. Therefore, one may state that different pegmatite types correspond to the different structural-mineralogic stages. The relations between pegmatitic minerals point to the sequence of evolution stages, which commonly replace each other and by parțial superposition result in zoned pegmatites. Each zone of a pegmatite body represents a petrogenetic type which, isolated, constitutes an independent pegmatite. The geochemical study evinces that the sequence of pegmatite evo- lution stages is also revealed by the Chemical characteristics : NaCa $ — c. 231 Institutul Geologic al României yiGR/ 66 H. P. HANN 62 (I) — K(II) or K + H20 (II A) and SiO2 (III), that is a sequence of alkaline stages (with quartz intermediate stages) ending with final quartz. The geochemical data usually account for the interdependence between pegmatites and surrounding rocks during the metamorphic processes, as well as for the segregation and gradual migration of alkaline mobilisates and quartz contained by the rock pile during the anatectic processes. The reduced minor element content of pegmatites informs on their me- tamorphic origin, while the diagrams show that the differentiation of alkaline fluids results in quartz-feldspar pegmatite mobilisates. The accessory minerals occur in reduced amounts and accompany the different major mineralogic stages. Metamorphic pegmatites are also characterized by reduced minor element contents of different peg- matite minerals, as proved by the Sebeș-Lotru pegmatites too. The large pegmatite dykes (e.g. Teregova, Pîrîul cu Mărul, Tilva hill), characterized by complete evolution and varied mineralogic features, resulted from ascending migration of pegmatite fluids along several important fractures and imply the occurrence of some anatectic-palyn- genetic cores, thus engendering the characteristics of typical magmatic pegmatites. The convergence of phenomena is significant again, as far as different petrogenetic processes, even opposite ones, determine similar mineralogic and structural features. These pegmatites are the first pro- ducts of the palyngenetic processes and show the same features as the last products of magmatic differentiation. 9. CONCLUSIONS’ There are different opinions on the pegmatite genesis. Most of the authors, however, assign the large dykes to the magmatic origin ; as regards the other pegmatite bodies, some are considered magmatic, others metamorphic. The main petrographic type from the area under discussion is re- presented by paragneisses, but pegmatite bodies are frequently reported from micaschists or mica paragneisses and pierce the amphibolites in places. Due to the high grade of metamorphism (sillimanite isograde) frequent migmatisations are noted, and a positive correlation is mentioned between the intesity of this process and the pegmatite occurrence. The pegmatite bodies are lithostratigraphically related, at least north of Muntele Mic, to highly migmatized micaschists and mica paragneisses. Among the different pegmatite shapes, the most frequent are: lenses, concordant veins, dykes, nests and large irregular bodies. The contact between the pegmatite bodies and surrounding rocks is also described : sharp contact (tectonic or normal), graded contact (diffuse or altemating) and mixed contact. The contact type and the shape of the pegmatite body may inform on their genesis by different metamorphic processes. The mafie aecumulations at the periphery of some pegmatite bodies may result from distinct petrologie processes within a metamor- phic field; it is wortli mentioning that different phenomena generate identica! features. WRV I nstitutul G eolog ic a I Român iei VlCR/ 63 PEGMATITES BETWEEN TEREGOVA AND MARGA 67 The investigatei! pegmatite bodies are zoned (complete, incomplete, symmetrical, asymmetrical, simple or complex zoning), unzoned or show incipient zoning; other bodies show destroyed zoning. Most of the small lenses are unzoned and their inner structure is heterogeneous, unzoned and unhomogeneous. The prevailing minerals of these lenses are plagio- clase and quartz. The larger lenses also exhibit zoning, and contain besides plagioclase, quartz and micas, microcline which substitutes the plagio- clase. The large concordant veins are completely zoned : a contact aplite zone, a marginal zone consisting of plagioclase, quartz and muscovite ± biotite, an intermediate zone, mainly containing microcline and a central zone built up of quartz, which is the last to crystallize. Graphic textures characterize both the marginal and the intermediate zones. In other cases, it is to note only a contact zone, an intermediate zone and the quartz core. If microcline prevails in the intermediate zone, plagioclase and muscovite are relics of a former marginal zone. It is to infer that during this stage of pegmatite genesis, the K-metasomatosis phase was prevailing in anhydrous state within a closed System. If muscovite pre- vails in the intermediate zone and associates with quartz and plagioclase, K-metasomatosis took place in the conditions of water preservation which favoured the crystallization of muscovite. The small dykes are unzoned or show simple zoning and contain enclaves of the surrounding rock accounting for their generation by metasomatic processes. The large dykes, less numerous, often show complete zoning, which means that all evolution stages characteristic of pegmatite genesis had been crossed, while some rare minerals (beryl, monazite, tantalite) determine their complex nature. The graphic textures result from quartz intergrowths with both plagioclase and microcline or microclineperthite. They formed by meta- somatic corrosion, that is due to selective substitution of feldspar by quartz along deformation stripes, which constitute the reticular labile zones associated in one or several systems intersecting each other. Their metasomatic origin is demonstrated by the fact that the different positions of deformation stripes correspond to the positions of the quartz lamellae, by the radia! symmetrical graphic textures pointing to ever increased feldspar substitution by quartz next to the source. The geochemical study of graphic textures points to the graphic texture areas within the geochemical evolution of pegmatite bodies. The varying amounts of quartz and other minerals, as well as the correlation fields plotted on Ba : K, Ba: Sr, Sr : Ca and (Na + K)% diagrams account for graphic textures generated by metasomatic processes, not by simultaneous crys- tallization. The main mineral phase of pegmatites consists of biotite, acid plagioclase, microcline, muscovite, quartz. These minerals determine the characteristic zoning, the geochemical evolution of pegmatites and, by replacing each other, point. to the evolution stage’ of pegmatites. Acces- sory minerals — gârneț, tourmaline, beryl, apatite, columbite, orthite, kyanite, sillimanite — accompany the different stages of major minera- logic composition. Most of the pegmatites under discussion show simple mineralogic features, characteristic of metamorphic pegmatites. | ’A Institutul Geologic al României MGR/ 68 H. P. HANN 64 Feldspars are the most important pegmatite minerals as regards both their amount and the fact that the feldspar type points to the evolu- tion stage attained by a pegmatite. Feldspars also account for the defor- mations which had affected the pegmatites, implicitly the changes of tectonic conditions. The acid plagioclase (albite-oligoclase, oligoclase) represents the first evolution stage (I) generating inițial pegmatites. Alkali feldspars represented by microcline or microelineperthite occur in the triclinic modifieation, being metasomatically developed at the expense of plagioclase and constituting the second evolution stage (II). Most of the perthitic textures are metasomatic due to abundant irregular shapes and uneven disappearance as shown by the microprobe analysis. Mica occurrences also characterize different stages of pegmatite genesis. Owing to the changes underwent (especially by biotite) and to their sensitiveness to mechanie deformations (noticed in the case of mus- covite), micas evince both changes of chemistry typical of different evolution stages and the control of tectonic conditions during pegmatite evolution. Biotite is characteristic of stage I, crystallizing together with plagioclase. Later on, it becoines unstable, is defenized and altered to muscovite by increase of pressure of water vapours. Muscovite amounts are considerable and constitute large plates associated with quartz and plagioclase; however, muscovite is subordinate in microcline pegmatites. Muscovite replaces plagioclase, at the expense of pre-existing seri- cite, being accompanied by quartz. It may also result from biotite altera- tion. It represents stage IIA of pegmatite evolution marked by K-me- tasomatosis abunding in water vapours. Quartz occurs in all pegmatite types, associates with all pegma- tite minerals and characterizes the different stages of their evolu- tion. It is redistributed in the successive evolution stages, while central quartz (III) ends the pegmatite evolution. Aecessory minerals are few or evon absent from most of the pegma- tites and only in a few instances they are varied and abundant. The most frequent aecessory mineral is tourmaline, preserved in all stages owing to its low alkaline character. The pegmatites abunding in tourmaline also contain beryl amounts in the intermediate zone. Gârneț, characteristic of the high pressure which influenced the pegmatite genesis, occurs in the intermediate zone and next to the quartz eore, altering its composition from outside innerwards by increasing its Mn content. Finally, the different structural-mineralogic stages of pegmatite genesis are represented by different pegmatite types. The relations among pegmatite minerals point to the sequence of evolution stages, commonly replacing each other, and generating zoned pegmatites by parțial super- position. Each zone of a pegmatite body represents a petrogenetic type, which isolated, may form an independent pegmatite. The different features implied by the minor elements yielded by pegmatitic minerals account for the relatively low intensity of substitu- tions, which, together with the. reduced amount of aecessory minerals, accounts for the metamorphic origin of these pegmatites. The sequence of evolution stages, shown by the Chemical features, is the following : NaCa (I), K (II) or K + H2O (II A) and SiO2 (III); quartz precedes each stage and final quartz is preceded by albitisation Institutul Geological României 65 PEGMATITES BETWEEN TEREGOVA AND MARGA 69 (Na) — substitution microclineperthites which do not represent an inde- pendent stage as no independent crystals are formed. The different diagrams, based on bulk analyses of pegmatitic rocks, point to the simultaneous evolution of pegmatites and paragneisses as well as to the interdependence of the two rock types during metamorphic processes. Thus, the geochemieal data prove the generation of pegmatites by segregation and gradual migration of alkaline mobilisates and of quartz from the rock pile during the anatectic metamorphic processes. The minor element content of the investigated pegmatites is reduced and rather monotonous. The diagrams regarding the differentiation of alkaline fluids from the paragneiss pile point to the generation of quartz- feldspar pegmatitic mobilisates. The origin of studied pegmatites should be assigned to the meta- morphic field, as proved by all mineralogic and geochemieal structural data. No relations with a granitic magmatic mass are defined. The meta- morphic genesis of these pegmatites implies the simultaneous or succes- sive development of some independent petrologie processes within the same rock pile affected by metamorphism. Thus, some pegmatites have resulted from metasomatosis, marked by „nondilation” pegmatitisation, while others are of „dilation” or „concretion” type. Some pegmatite bodies are the product of lateral secretion : the fluids and mineral com- ponents of rocks migrate toward the rock cavities or fissures and because of varying mobilisation speeds, the latter are filled with the easiest mo- bilized minerals — feldspars and quartz. The mineral substance is trans- ported by diffusion. The aqueous fluid of rocks consists of volatile com- ponents which influence the genesis of pegmatitic minerals. However, most of the pegmatites under discussion were generated by „anatectic-metasomatic” metamorphic processes which are in fact (prior to palyngenesis) „lateral secretion” as they consist of mobilisates supplied by surrounding rocks. Under favourable conditions (a volatile phase, adequate tectonic conditions) some of these mobilisates undergo pegmatitic evolution. According to its duration, the evolution results in simple or complex pegmatite bodies. Commonly, these pegmatites ac- company the highly migmatized rocks, while the sillimanite amounts of pegmatites account for anatectic genesis. The large pegmatite dykes, characterized by complete evolution and varied mineralogic features, due to convergent phenomena, resem- bling to magmatic-granitic pegmatites. They resulted from ascending migration of pegmatitic fluids along some fractures and account for the occurrence of some anatectic-palyngenetic cores. The investigation of pegmatites from Teregova-Marga region also informs on the main trends of prospect studies of pegmatites. Further investigation of pegmatites is based on both their special petrogenetic features and the considerable amounts of muscovite, rare minerals and feldspars of real economic interest. New data may be obtained from field investigation of different pegmatite types and of the contact relations with surrounding rocks, as far as new features and new interpretations are to be expected. JM. Institutul Geological României iGsy 70 H. P. HANN 66 The structural-mineralogic study of pegmatites, mainly the relations among pegmatite minerals, also informs on the pegmatite genesis. The geochemical study, based on bulk analyses of pegmatites, brings no new data because of the practicai difficulty of obtaining ac- curate mean values. However, the most important means of future in- vestigations of pegmatites are offered by the study of pegmatitic minerals, of the deformations which had affected them aud of their geochemical characteristics. New progress is expected by using up-to-date methods of investigation of crystallo chemi cal characteristics of pegmatitic mine- rals. All detailed data informing on pegmatite genesis improve the con- clusions inferred. Acknowledgements I would like to offer my grateful thanks to professor dr. doc. Dan Rădulescu for having guided my geologic thinking and scientific activity since student years. I am grateful to dr. Nieolae Gherasi for having suggested this topic of study to me and for his first useful advice in the field work. I thank my colleague Radu Constantinescu for numerous useful discussions on this subject and for helping me with the processing of Chemical data. I am grateful to dr. Mircea Săndulescu for having facilitated my investigations in the area under diseussion. I also thank my colleagues Ana Șerbănescu, Antoneta Seghedi, Ana Baralia, Ovidiu Șerbănescu and Dumitru Danei for helpful contri- bution to elaborating the graphie material. 3 These characteristics account for the relation with migtnatisation processes being characteristic of a certain stage of leucosoma pegmatite evolution. 4 Features considered typical of dilational type metamorphic pegmatites (Goodspeed, 1940, Gherasi et al„ 1974). REFERENCES Andersen O. (1938) The genesis of some types of feldspar from granițe pegmatites. Norsk. Geol Tidskrift., 10, p. 114-205, Oslo. Augustithis S. S. (1962) Researches of blastic proeess in granitic rocks and later graphie quartz in pegmatites (pegmatoids) from Ethiopia. Nova Acta Leopoldina, N. 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Geol. Mag., 112(2), p. 184-189. STUDIUL PETROGRAFIC AL PEGMATITELOR DINTRE TEREGOVA ȘI MARGA (BANATUL DE EST, CARPAȚII MERIDIONALI) (Rezumat) în cadrul teritoriului ocupat de rocile mezometamorfice ale grupului Sebeș-Lotru din Carpații Meridionali, regiunea cercetată, situată atît în estul munților Semenic cît și în nordul Masivului Muntele Mic, prezintă o importanță deosebită pentru studiul pegmatitelor datorită apariției în această zonă a unui număr însemnat de diferite corpuri de pegmatite. Localizarea și prezentarea principalelor corpuri de pegmatite se face ținînd cont de faptul că acestea au în anumite sectoare o răspîndire mai mare. Astfel deosebim în regiunea situată la nord de Muntele Mic, sec- toarele Tîlva, Măgura, și Dalei-Var, iar în cadrul munților Semenic, sec- toarele Slatina-Timiș, Aimeniș și Teregova. Șisturile grupului Sebeș- Lotru reprezintă unitatea Pînzei Getice, care, la nord de Muntele Mic încalecă, în lungul planului de șariaj getic peste amfibolitele seriei de Măru care aparțin domeniului Danubian. La vest de linia de șariaj apare în interiorul pînzei getice o digitație care delimitează solzul Turnu Ruieni. Fondul petrografic al regiunii este constituit din paragnaise i sillimanit ± disten în care se întîlnesc intercalații și nivele constituite din gnaise albe cuarțo-feldspatice, micașisturi, amfibolite, calcare crista- line etc. Stiva de roci prezintă frecvent intense migmatizări. Succesiunea litostratigrafică a grupului Sebeș-Lotru din pînza getică de la nord de Institutul Geological României 7fi H. P. HANN 72 Muntele Mic este caracterizată de prezența orizontului micașisturilor și al paragnaiselor micacee intens migmatizate, în interiorul sau în veci- nătatea căruia apar majoritatea corpurilor de pegmatite. Caracterele morfo-strueturale ale corpurilor de pegmatite Forma și relațiile eu roca înconjurătoare. Cel mai frecvent se întâlnesc lentilele care reprezintă corpuri concordante și apar izolate sau formează asociații de tipuri distincte : a) lentile de dimensiuni ase- mănătoare dispuse în roi; b) lentile mai mari alături de care apar lentile mici paralele; c) șiruri de lentile situate la același nivel sau la nivele diferite. La tipul “a” contactul cu roca înconjurătoare este de obicei difuz ceea ce indică legătura cu fenomenele de migmatizare, la tipul contactul este gradat, pegmatitele fiind de tip dilational (Goodspeed 1940), iar la tipul “c” contactul este net, lentilele formîndu-se prin budinarea unor filoane de pegmatite concordante sau prin fragmentarea unor* dyke- uri (Șeclăman 1972). Filoanele concordante reprezintă corpuri mari cu gîtuiri și îngroșări, limita față de roca înconjurătoare poate avea atît un caracter difuz cit și net. Cuiburile de pegmatite se întâlnesc mai rar, au mărimi reduse și contururi neregulate, contactul fiind net sau difuz. Ele s-au format în timpul proceselor de migmatizare, pot fi pegmatite de concrețiune (Ramberg 1952) sau formate în urma unor procese de secreție laterală. Dyke-urile (filoane discordante) se întîlnesc mai rar, dar formează cîteva corpuri importante (Teregova) și s-au format în cadrul unor procese de natură anatectică prin pătrundere în lungul unor zone de minimă rezistență, sau sînt de tip „replacemcnt” (Ramberg 1949) și s-au format prin intermediul unor procese metasoma- tice. Corpurile mari cu forme neregulate prezintă în toate privințele caractere heterogene și au luat naștere prin înglobarea, în timpul formării lor, a mai multor corpuri de pegmatite preexistente cu caractere deosebite sau, la formarea lor au participat mai multe procese genetice a căror influență a predominat succesiv sau alternant, generînd aspecte distincte. Clasificarea tipurilor de contact (met și gradat) și caracterul formei corpurilor ne ajută la stabilirea unor indici genetici. Concentrațiile mafice periferice unor corpuri sînt constituite din biotit, turmalină sau hornblendă și pot fi interpretate ea „restite” ((liferențiere anatectică — Scheumann 1937, Mehnert 1951), se pot forma prin procese metasomatice ('Reynolds 1946), sau datorită pro- ceselor de secreție laterală, transportul substanței minerale produeîndu-se prin difuzie (Ramberg 1952, Șeclăman 1972). Remarcăm deci, că procese petrologice distincte generează aspecte asemănătoare. în concluzie, rezultă că în regiune coexistă mai multe tipuri genetice de pegmatite, iar la stabilirea acestora trebuie să ținem cont de dificultățile create de con- vergența fenomenelor. 2. Structura internă. Zonarea se rezumă la unele corpuri la prezența unei zone de contact aplitice în contrast cu o masă pegmatitică larg cristalizată și omogenă. Întîlnim însă și corpuri cu zonări complete, Institutul Geological României 73 PEGMATITES BETWEEN TEREGOVA AND MARGA 77 incomplete, simetrice, asimetrice, simple, complexe, corpuri cu o zonare incipientă sau a căror zonare a fost distrusă. Lentilele mici nu sînt zonate sau prezintă o zonare incipientă, ceea ce denotă că aparțin unui stadiu evolutiv timpuriu. Predomină plagioclazul, microclinul apare corodîn- du-1 pe acesta. Zonări complete întâlnim în cazul filoanelor concordante mari. Cînd zona intermediară este constituită din microclin deducem prezența unei faze de metasomatoză potasică intr-un sistem deschis, în forma ei anhidră. Cînd apare muscovitul larg cristalizat în locul micro- clinului, metasomatoza s-a produs în condițiile păstrării apei în sistem. Dyke-urile mari (Teregova) prezintă zonări complete și complexe, datorită varietății și semnificației conținutului lor mineralogic (berii, columbit, monazit). S t r u c t u r a g rafie ă este dată de con creșteri ale cuarț.ului cu plagioclaz, cu pertit și cu microclin și este caracteristică zonelor inter- mediare. Apar toate aspectele caracteristice formării cuarțului grafic prin înlocuirea selectivă a feldspatului, deci prin coroziune metasomatică. Compoziția normativă a granițelor grafice indică caracterul variabil al proporțiilor de cuarț, plagioclaz și feldspat potasic, fapt caracteristic formării metasomatice și nu unui eutectic granitic. Mineralogia pegmatitelor. .Majoritatea pegmatitelor din regiunea cercetată au o' mineralogie simplă, fapt care constituie o trăsătură carac- teristică pegmatitelor de origine metamorfică. Principala fază de minerale este constituită din biotit — plagioclaz acid — microclin, microclin pertit — este constituită din biotit — plagioclaz acid — microclin, microclin pertit — muscovit — cuarț. Minerale rare sau accessorii, care ar putea repre- zenta produsul pneumatolizei, apar doar izolat. Asociate diverselor stadii ale mineralogiei majore au fost întîlnite următoarele minerale: granat, turmalină, berii, apatit, columbit, monazit, ortit, disten, sillimanit, horn- blendă, magnetit, hematit, pirită, calcit și dorit. Feldspat ii. Tipul de feldspat indică treapta evolutivă la care a ajuns un pegmatit. Reflectă și deformările suferite de pegmatite, deci modificările regimului tectonic. Feldspații plagioclazi (albit, albit-oligo- claz, oligoclaz) reprezintă componentul mineralogic principal al corpurilor mici, uneori este substituit de către microclin și reprezintă un prim stadiu (I) de formare al pegmatitelor. Feldspații alcalini, reprezentați prin mi- croclin sau microclinpertit, corodează plagioclazul și reprezintă un al doilea (II) stadiu evolutiv. Structurile pertitice întîlnite sînt de origine metasomatică. M i c e 1 e caracterizează prin prezența lor anumite etape de formare a pegmatitelor, iar prin transformările suferite (biotitul) reflectă modificări ale chimismului caracteristic diferitelor stadii evolu- tive. Biotitul cristalizează alături de plagioclaz și aparține stadiului (I) de formare. Ulterior este deferizat sau se păstrează ca mineral relict. Mus- covitul se formează prin deferizarea biotitului sau înlocuirea plagioclazului și este întotdeauna însoțit de cuarț, reprezentând stadiul de formare II A, caracterizat de o metasomatoză potasică, bogată in vapori de apă. Prin hidroliza feldspaților potasici este însă posibilă și formarea unui muscovit postmicroclinic. Cuar ț u 1 apare în toate tipurile de pegmatite, intervine la sfîrșitul fiecărei faze de cristalizare, contribuind la dezvoltarea struc- Institutul Geologic al României \ (CR7 78 H. P. HANN 74 turilor metasomatice. Cuarțul zonei centrale (III) încheie evoluția procese- lor de formare a pegmatitelor. Geochimia pegmatitelor. Pe baza a 62 analize chimice complete de silicați și 42 analize spectrale s-au evidențiat unele caracteristici geochi- mice ale pegmatitelor. Evoluția elementelor majore prezintă următoarea succesiune : NaCa (I), apoi K (II) sau K -p H2O (II) și SiO2 (III) cu precizarea că între fiecare stadiu se interpune cuarțul, iar înainte de cuarțul final apare uneori o albitizare (Na — microclinpertitul de sub- stituție), dar care nu formează în acest caz un stadiu independent. Diferitele diagrame construite indică evoluția cvasiconcomitentă și interdependența dintre paragnaise și pegmatite în timpul proceselor metamorfice care le-au generat. Conținutul în elemente minore ale pegma- titelor este redus și are un caracter monoton. Din diagrame rezultă sensul proceselor de diferențiere a fluidelor alcaline din masa paragnaiselor înspre mobilizatele pegmatitiee. Geneza pegmatitelor. Datele structurale, mineralogice și geochimice converg spre ideea originii metamorfice a pegmatitelor. Dealtfel Savu (1977), demonstrează absența oricărei legături petrogenetice a pegmati- telor din munții Semenic cu plutonul granitoid de Poniasca. Prin geneza metamorfică a pegmatitelor înțelegem desfășurarea în paralel sau succesiv a unor procese petrologice distincte în cadrul aceleiași stive de roci supuse metamorfismului. Dovezi se găsesc începînd cu studiul aspectelor morfo- structurale, care atrag totodată atenția și asupra coincidențelor posibile ca urmare a convergenței fenomenelor. Astfel, pegmatitele s-au format în urma proceselor de migmatizare și pot fi de tip „dilational”, „concre- țional”, și au luat naștere în urma unor procese metasomatice, pegmatiti- zarea producîndu-se nedilational, sau reprezintă produsul unor procese de secreție laterală. Majoritatea pegmatitelor acestei regiuni au fost însă generate de procese „anatectie metasomatice” (Barth, 1962). Prezența proceselor anatectice este demonstrată de diferitele tipuri de structuri migmatice întîlnite. Unele dintre mobilizatele leucocrate urmează o evoluție pegmatitică și care, în funcție de desfășurarea ei în timp, gene- rează pegmatite simple sau complexe, după cum au fost parcurse stadiile evolutive. Plagioclazul și biotitul reprezintă stadiul I de formare, după care intervine cuarțul, ce poate genera în anumite condiții structuri grafice. Caracterul mineralogic al stadiului II este în funcție de presiunea vaporilor de apă : cînd sistemul se deschide și presiunea H2O scade, cris- talizează microclinul (II), cînd rămîne închis și presiunea H2O crește, cristalizează moscovitul (II. A). Deci, desfășurarea întregului proces este controlată de regimul tectonic dominant în perioada respectivă. Cuarțul intervine din nou, formînd concreșteri grafice cu microclinul sau însoțește muscovitul. Evoluția se încheie prin cuarțul (III) din nucleu, în concluzie se poate afirma, că diferitelor stadii structural mineralogice ale formării pegmatitelor le corespund diverse tipuri petrografice depegma- tite. Relațiile dintre mineralele pegmatitelor arată ordinea succesiunii stadiilor de evoluție, care în general, se înlocuiesc unele pe altele, rezultînd pegmatite zonate. Fiecare zonă a unui corp pegmatitic reprezintă un tip Institutul Geologic al României igr/ 75 PEGMATITES BETWEEN TEREGOVA AND MARGA 79 petrogenetic, care, atunci cînd apare singur, constituie un pegmatit in- dependent. Dyke-urile de dimensiuni mari, cu o evoluție completă și o minera- logie variată, prezintă datorită convergenței fenomenelor aspecte asemă- nătoare cu pegmatitele de origine magmatică — granitică. Ele s-au format însă în urma migrării ascensionale a fluidelor pegmatitice în lungul unor fracturi și presupun existența unor sîmburi anatectici palingenetici. Semnificația petrogenetică deosebită și importanța economică a pegmatitelor impun cercetarea lor și în viitor, progrese fiind posibile atît prin studiul relațiilor dintre minerale cit și prin acela al caracteris- ticilor geochimice și cristalocliimice ale acestora. EXPLANATION OF PLATES Plate III Fig. 1. Some dykes belonging to the former group show a contact line with fine, millimetric inlets a few cm long, which favour the advancement of pegmatites inside the paragneisses. Fig. 2. The pegmatite groundmass exhibits parts built up of paragneisses and the paragneisses include pegmatite zones, the transition zone being represented by the arca between the pure pegmatite and the paragneiss unaffected by pegmatitization. Plate IV Fig. 1. Pegmatite lenses resulted from the boudiuage of some concordant veins. Fig. 2. Gârneț (almandine) represented by large idiomorphic grains (ca 1 cm in diameler) within plagioclase (plg), quartz (qz) and scarcc muscovite (mu) groundmass. Plate V Fig. la, 1b, Ic. Graphic quartz. Some faces exhibit a lamellar character. The cut perpendicular to this face points to an entirely different image of proper graphic quartz. Plate VI Fig. 1. Contact zone with equigranular aplite texture and leucotonalitc composition, consis' ting of albite (ab) and quartz (qz). Fig. 2a, 2b. Graphic intergrowth of quartz (qz) and plagioclase (plg). Fig. 3. Graphic intergrowth of quartz (qz) and microcline (mi). Plate VII Fig. 1. Plagioclase (plg) with fine polysynthetic twins is substituted by microcline (mi) wherein it forms relict inclusions. Fig. 2a, 2b. Irregular shapcs of low temperature perthites resulted from potash feldspar subs- titution by plagioclase are prevailing. Fig. 3. Irregular shapes of perthites (plg) cut by a quartz (qz) vein. Institutul Geological României 80 H. P. HANN 76 Plate VIII A microclinepcrthite sample of irregular shape studied by microprobe analysis (x 1200). Fig- 1. K-distribution. Fig. 2. Na-distribution. Fig. 3. Ca-distribution. Fig. 4. Al-distribution. Fig- 5. Si-distribution. Fig- 6. The lopographic surfaee (composition pattern) Plate IX Fig. 1. Albite (ab) forming in places the covor of altered plagioclase (plg). Fig. 2a, 2b. Large tourmaline (tu) grains corroded by quartz (qz) resulting in “graphic tour- maline”. Fig. 3. Tourmaline (tu) is substituted by quartz (qz) wherein it forms relict inclusions. Institutul Geological României GROUP NORTH OF MUNTELE MIC (BANAT) p Măgură ^Oogng । Oagna i -1 - Oagno Oagna GETIC UNITS oCicleni o Dalei Turnu Ruieni îOagnax' VRACONIAN TURONIAN LOWER CRETACEOUS UPPER JURASSIC LOWER MIDDLE JURASSIC UPPER PRECAMBRIAN Dalslandian metamorphism of Barrowian type, in almandine am BĂUȚAR FORMATION ine limestones agna £23 £23 pgn Pgn.qț Micaschists t sillimanite Biotite * muscovite Micaschist horizon mi Crystalline limestones Biotite + muscovite t sillimanite Turnu Ruieni Scale BUCOVA FORMATION CONVENȚIONAL SIGNS MIGMATITIC ROCKS PI. I oBorlova pypac PRECAMBRIAN MAGMATITES Ol ist ol ith s ANUARUL INSTITUTULUI DE- GEOLOGIE SI GEOFIZICA. VOL. 67 pypocp UPPER PRECAMBRIAN DANUBIAN UNITS Măru Series Barrowian metamorphism in almandine amphibolite facies (metamorphosed volcano - sedimen tary formation) Institutul Geological României H l'.HANN Pegmatites between Teregovo and Ma«ga imprim Atei.Inst Geol.Geof. ANUARUL INSTITUÎULU- Dl IGR OGIE $1 GEOFIZICA. VOL 67 L Institutul Geologic al României Institutul Geological Româniej H. P. HANN. Pegmatites between Teregova and Marga. PI. IV. 1 2 Anuarul Institutului de Geologie și Geofizică, voi. 67. Institutul Geological României FI. P. HANN. Pegmatites between Teregova and Marga. PI. V. 1 a 1 b Anuarul Institutului 1 c de Geologie și Geofizică, voi. 67. Institutul Geological României H. P. HANN. Pegmatites between Teregova and Marga. PI. VI. 2a 2b Anuarul Institutului de Geologie și Geofizică, voi. 67. Institutul Geological României H. P. HANN. Pegmatites between Teregova and Marga. PI. VII. 1 3 2a 2b Anuarul Institutului de Geologie și Geofizică, voi. 67. 'A Institutul Geologic al României. igr/ H. P. HANN. Pegmatites between Teregova and Marga. PI. VIII. Anuarul Institutului de Geologie și Geofizică, voi. 67. Institutul Geological României H. P. HANN. Pegmatites between Teregova and Marga. PI. IX. 3 Anuarul Institutului de Geologie și Geofizică, voi. 67. PA Institutul Geological României-oT^^ -’6r7 GEOTHERMICS OF THE CARPATHIAN AREA1 BY ȘERBAN VELICIU2 Abstract The study presents the geothermal regime of the Romania» Carpathians arca on the basis of the measured values of heat flow, heat conductivitv and radiogenic heat generation of the rocks. Meat flow values in 54 deep wclls in various geological units were obtained. Cor- relations among heat flow within the Carpathian foldcd chain (40—126 mW*,' incliiding posl-tectonic depressiohs), the Moldavian Platform (39 — 55 m\Vm-2) as well as the Moesian Platform (39 — 78 mWm-1) and structural features of these major units have been cstablished. The distribution of temperature os. depth and its intcrrclation with geological and geophysical observations was examinfcd with a particular regard to the East Carpathians, Pannonian Basin and Transylvanian Basin. The territory under diSciission represehts a relalively youhg area, of less than 25 m. y. since the lasl thermal-tectbhic event thils some dynamic geothermal models have been adopted. In order to explain the high heat flow (73 — 126 ihWin-2) observed at the inner part of the East Carpathians, a model which takes into account the radiogchic heat generation and the descenl of a lithosphcric plate has been claboratfcd. Calculated gebtherms indicate the presenct of a high temperature ăhomaly located in the upper mantie. The geo- thermal data suggest that both the high heat flow and tlie building up of the Nbogenc volcanic chain arc consequcnccs of the lithosplieric Alpine subduction process in the Carpathian arca. The Transylvanian Basin exhibits abnonnally low heat flow which makes it distinct as com- pared with other typical cnsialic intra-arc basins. I.ithospheric sfretching as described for ex- tensional basins (i.e. Pimnoniân Basin) is not very likțly to be present here. Resuine Le regime gioliicrmigiic de l’aire carpalhique. L’âtude presente le regime giothermique des Carpathes roumaines en considerent les valeurs mesurtes du flux geothcrmique, la conduc- tibilitd thcrmique el la generation thcrmique radiog^nique des rochcs. On a obtenu des valeurs du flux gdothermique pour 54 forages profonds dans diff6rentes uniUs geologiques. On a etabli aussi des correlations entre le flux geothcrmique ă l’intirieur de la chaine plissee des Carpathes (40—120 m\Vm-2, depressions post-tectoniqucs y incluses) et de la Plate-forme Moesienne (39 — 78 mWm"4) et les traits structuraux de ces unitis majeures. La distribution de la tempe- rature par rapport â la profondeur et les corrâlations avec les donnees gâologiqucs et gâophy- 1 Paper presented on 5. IV. 1985, accepted on 16. IV. 1985 2 Institute of Geology and Geophysics, Caransebeș 1, 79678 Bucharest, Romania. 6 - c. 231 Institutul Geologic al României \JGR/ 82 Ș. VELICIU 2 siques soni exaniin6es surlout en tenant comptc des Carpathes Orientales, le Bassin Pannonien et le Bassin Transylvain. L'aire investigare est relativement jeunc, ă moins de 25 m.a- d6s le dernier 6v6ncment thermo-tcctoniqiie et par consequcnl on a adopta des inodelcs geolhcrmiques en regime dynamiqnc. Le flux geoUicriniquc e!ev6 (73 — 126 mWnT8) a l’interieur des Carpathes Orientales est expliqne par un modele bas6 sur la generation thermique radiogeniquc ct la plaque lithospherique baissie. Les geothermes calculees indiquent une anomalie de temperature elevee dans le manteau supârieur. Les donnâes geothcrmiques suggercnt que tant le flux g£othermique aulant que l edification de la chaîne volcanique neogene sont le resultat du processus de sub- duction lithospherique alpine dans la râgion des Carpathes. Le Bassin Transylvain se caraclerise par un flux geothermique particulierement bas qui le diffdrencie d’autres bassins interarc en- sialique typiques- L'extension lithospherique dderite pour les bassins post-tectoniqucs (ex. le Bassin Pannonien) n’est pas en concordance avec les donnees ci-presentecs. Introduction The geological structure of the earth’s crust is now viewed. as a col- lection of effects of the plate tectonies (Mc Kenzie, Parker 1968 ; Morgan 1968; Le Pichon 1968; Dewey, Bird 1970). Ou the other hand, it is pos- sible to construct various geothermal models that provide some insights into the tectonic history due to the long relaxation time of the heat con- duction in rocks. Von Herzen (1967) demonstrated that the time interval required by the surface heat flow is below 150 m.y., in order to be in equilibrium with the inner heat sources distributed within a lithosphere 100 km thick. If it is taken into account that the most important contri- bution of the radiogenic heat sources is concentrated within the first 30—40 km of the earth’s crust. less than 50 m.y. is necessary to reach thermal equilibrium. Such geothermal models should be reconciled with the knowledge of the structure of the crust, the petrophysical properties of the rocks and the thermal regime of the carth as inferred from the observations at its surface. The geothermal models for oceanic basins are mainly conditioned by thc time clapsed since the formation of the oceanic crust in the sprea- ding zones. Simple conductive geothermal models have been derived from the paleomagnetic data, cooling and contraction of the inițial magma as well as the distance from the spreading centres. A review of the geo- thermal models has been performed by Palmason (1973) for the Mid- Atlantic Ridge and by Lubimova, Nikitina (1978) for the Western Pacific. Near old trenches and extensional basins the situation is quite dif- ferent as compared to the ridges and oceanic basins ; a broader region is subjected to thermal disturbanccs by a variety of processes, the details of which are not as well understood yet. Models that take into considera- tion both the heat sources associated with descent of the lithosphere or with extension of the ensialic post-tectonic basins, provided reasonable explanations for the gross features of the observed heat flow density at the earth’s surface (Oxburg, Turcotte 1971; Lachenbruch, Sass 1978; Mc Kenzie 1978). It is the aim of this study to present the geothermal regime of the Romanian Carpathians area based on the measured values of heat flow 3 GEOTHERMICS OF THE CARPATHIAN AREA 83 density, heat conductivity and radiogenic heat generation of the rocks. So, the distribution of geotherms (temperature vs. depth) and its inter- relation with geologica! and geophysical observations is examined with a particular regard to the East Carpathians, Pannonian Basin and Transyl- vanian Basin. The territory under discussion represents a relatively young area, of less than 25 m.y. since the last thermal-tectonic event, thus some dynamic geothermal models should be adopted. A general Outlook on the geology shows that the Carpathian area, belonging to the northern branch of the peri-Mediterranean Alpine folded ehains has been divided into structural units (Dumitrescu, Săndulescu 1968 ; Săndulescu 1975) which generally group together nappes of similar type and of synchronous age of tectogenesis. From the last point of view the Carpathians show two main periods of compressions : Cretaceous and Miocene. The Cretaceous Carpathians group the Dacides, the Transyl- vanides and the Danubian (Fig. 1). The Miocene Carpathians (Moldavides) covei' the outer zone of the chain. Two big Neogene molasse basins (Tran- sylvanian and Pannonian) are superposed on the folded units and a Sar- mato-Pliocene molasse foredeep borders the folded chain outwards. Pre-Alpine crystalline formations crop out inside the Dacidian areas. Tire Transylvanides are the main suture of the Carpathians con- Fig. 1. Schematic tectonic map of the Carpathian area (after Săn- dulescu, 1975). l-Western Dacides, a-zone of the Metaliferi Mts. (Southern Apuseni); 2-Eastern Dacides; 3-Southern Dacides; 4-Pienides; 5-post-tectonic cover; 6-Moldavides; 7-Foredeep, a- inner, b-outer; 8-Neogene depressions : T. B. -Transylvanian basin ; P. B.-Pannonian basin; 9-Neogene volcanics; 10-Foreland. Institutul Geological României 84 Ș. VELICIU 4 taining units with ophiolitic complexes. The outer Dacides are the second suture, showing flysch deposits and ophiolitic rocks. Large development of flysch is known inside the Moldavides. Except the ophiolitic assemblages, the Alpine magmatic activity exhibits three igneous periods : an ensialic predominantly alkaline momen (Jurassic) known in the South Carpathians and the East Carpathians, andC two subduction calc-alkaline moments, the first in the Upper Cretaceous and Paleocene time, the second during the Neogene. The Carpathians foreland groups platform areas of different ages. The oldest one is the Moldavian Platform (belonging to the Epi-Algomian East-European ciaton) situated in front of the East Carpathians. An Epi-Hercynian platform (Moesian Platform) runs south and westward to the formei'. The outer part of the Danubian realm has been considered a direct prolongation of the Moesian Platform. Radiogenie heat generation in the Carpathians It is generally accepted that the distribution of radioactive heat sources is the major motive for the temperature field in the continental crust. All natural radioactive isotopes generate heat to a certain extent but only the contribution of the decay series U238, U235, Th232 and of the isotope K40 are significant for the heat flow from the earths’ interior. Two independent methods were developed for the determination of radiogenie heat generation constants (Birch, 1954). The first summed the kinetic energy of the particles emitted and of the recoil nucleus and the electromagnetic radiation involved in the radioactive decay. The second method used the mass difference between the parent and daughter nuclides. The heat generation constants based upon the work of Birch (1954) which gave the heat generated per unit time and gram U, Th, K, were extensively used in the literature. However, decay schemes, Jialf lives and mass differences have been subscquently revised. This fact called for the redetermination of the heat generation constants (Hamza, Beck 1972 ; Rybach 1976). Table 1 presents a compilation of the heat genera- TABLE 1 Heat generation constants after Birch, Hamza, Beck and Rybach Element Heat generation, in izW kg-1 Birch (1954) Hamza, Beck (1972) Rybach (1976) with neutrino energy without neutrino energy U Th K 97 (0.73) 27 (0.20) 0.0036 (27x10-°) 98 (0.74) 27 (0.20) 0.0034 (26xl0-«) 106 (0.80) 28 (0.21) 0.0068 (51X 10"°) 92 (0.718) 26 (0.193) 0.0035 (26.2x10-°) The more familiar figures in cal yr”1g“3 are given in parantheses 5 GEOTHERMICS OF THE CARPATHIAN AREA 85 tion constants determined for the main natural radioactive elements. The differences found between Birch’s values and those of other authors are minor. In the present study on the Carpathian area the radiogenic heat generation (A) for a certain rock has been calculated using the constants revised by Eybach (1976) : A/gWm-3/ = 0.133p(0.718 c^ + 0.193 cTh + 0.262 cx) The computation involved knowledge on the U, Th and K contents taking also the density p (gem-3) into account. Practicai concentrations units are weight ppm for U and Th, and weight per cent for K. A gamma-ray spectrometric technique using a Nai (TI) crystal was utilised because it enabled simultaneous determinations of the significant heat-produeing radio-elements. The basic principles and applications of gamma-ray spectrometric technique have. been thoroughly discussed by Lemne (1968, 1970). The selection of the rock samples for the heat generation determi- nations has been performed based on two criteria : (1) the samples should be representative for the uppermost part of the crust in the Carpathian area and (2) they should offer the possibility to investigate a variety of petrographic types ranging from the acid rocks to the basic ones. From the first point of view, deserved consideration the crystalline and magmatic formations of Archean, Proterozoic and Paleozoic age belonging to the Danubian Autochthon (South Carpathians) and the Gilău Autochthon (Apuseni Mts). Some magmatic rocks (Paleozoic granites, Paleocene granodiorites, Neogene andesites and Jurassic ophiolites) from the Apuseni Mts have also been analysed (Table 2). The number of analysed samples corresponds roughly to the relative surface abundance (in %) of the respective petrographic type in the Romanian Carpathians. Table 3 lists the average heat generation figures for the Romanian Carpathians grouped according to the petrographic facies (magmatic and metamorphic, respectively). For comparison the values determined for characteristic surface-rock types from the Carpathians are presented together with values from Swiss Alps (Rybach 1976). In terms of the surface radiogenic heat generation of the rocks, the differences found between these two Alpine orogenic regions are minor. Data from Table 3 clearly imply a marked upward concentration of radiogenic heat-producing isotopes in the crust. So, from a theoretical point of view, the crust in the Carpathians area constituted only of grano- diorites (A = 1.87 gWm-3) could generate enough heat in order to sustain the average surface heat flow (69 ± 26 mWm-2) measured in the Carpat- hians (including the post-tectonic Neogene depressions), without any caloric contribution from the mantie. However, on petrological and geophysical arguments it has been assumed (Smithson, Deker 1974) that the lower crust has to exhibit a granulitic facies which is less endowed with radiogenic heat-produeing elements. This assumption seerns to be valid also for the Carpathian area. igrZ Institutul Geologic al României 86 Ș. VELICIU 6 TABLE 2 Radiogenic heat generation of characteristic rocks from the Romanian Carpathians Tectonic unit Rock type Age (m.y.) Num- ber of sam- plcs Heat generation (p Wm-3) Width of variation Mean value Southern Carpathians (Southern Dacides): Danubian Autochthon Getic Nappe Epizone series (scricitic-chioritic schists) Late Proterozoic (850) 24 0.42-1.06 0.74 Granitoids Early Paleozoic (550) 300 1 .83-4.13 2.98 Mcsozone series (microschists; paragneisscs) Late Archean (1600) 391 1.81-2.22 2.02 Granites Late Proterozoic (800) 50 1.94-3.10 2.52 Apuseni Mts (Western Dacides) Gilău Autochthon Granites Early Paleozoic (530) 49 1.86-2.48 2.17 Neogene andesites Miocene (15-20) 61 0.52-1.18 0.85 Banatites (granodiorites) Palcocene (60) 41 1.71-1.99 1.85 Ophiolites Lower Jurassic (200) 53 0.14-0.56 0.35 Epizone series (Arada Series) Early Paleozoic (500) 22 0.70-1.43 1.09 Mesozone series (Someș Series) Archean (900-1600) 56 1.74-3.11 2.43 Cretaceous Flysch Maastrichtian (70) 91 0.86-1.31 1.09 Institutul Geological României 7 GEOTHERMICS OF THE CARPATHIAN AREA 87 The data from the Carpathians indicate a variation of radiogenic heat generation over two order of magnitude reflecting the geochemical conditions during the formation of the respective rocks (magmatic dif- ferentiation, metamorphism or sedimentation). The mechanism by which TABLE 3 A comparison of heat generation values for the Carpathians and Stviss Alps Rock type ■ Romanian Carpathians (this work) Swiss Alps (Rybach 1976) Number of samples Width of variation (pWm-3) Mean value Number of samples Width of variation (Wm-3) Mean value Magmatic rocks : Granițe 50 1.94-3.10 2.52 8 1.88-6.06 2.5 Granodiorite 41 1.71-1.99 1.87 1.5 Andesite 61 0.52-1.18 0.85 1.1 Basalt 53 0.14-0.57 0.35 8 0.08-1.05 0.3 Metamorphic rocks : ,,Green schists” facies (epizone) Amphibolites facies (mesozone) 22 0.70-1.49 1.09 18 0.25-2.42 1.5 391 1.74-3.11 2.43 55 0.86-5.02 2.42 Sedimentary rocks Cretaceous flysch (sandstones) 91 0.8G-1.31 1.09 Carbonate rocks 12 0.03-0.92 0.33 Continental crust 0.72 0.80 the segregation of the heat-producing isotopes occuired is not well un- derstood yet (Rybach 1976) but clearly these are closely associated with the above mentioned geological processes. Roy et al. (1978) first recognized that the regional variation in surfaee heat flow within a “heat flow province” is linearly related to the heat generation of the surfaee rocks. The relationship is expressed as : q0 = qr + b Ao (where ^is the surfaee heat flow, A0 is theheat generation of the surfaee rocks, qr is the “reduced heat flow” and b characterizes the vertical distribution of the heat sources within the crust). This obser- vation has been confirmed in 17 different “heat flow provinces” and the best fitting line through the data points after Vitorello, Pollack (1980) Institutul Geological României 88 Ș. VELICIU 8 is shown in Fig. 2. On the same graph the data from the Carpathian area have been plotted (Fig. 3). The graph suggests that the proportion qrlqa = 0.4/0.6 empirically established by Pollack, Chapman (1977) seems to be valid also for data from the Carpathians. Fig. 2. Reduced heat flow vs. mean surface heat flow for different 17 "heat flow provinces” of the Globe, after Vitorello, Pollack (1980). In the Carpathian area, affected by young tectogenetic phases (Cretaceous and Miocene) and returning to thermal equilibrium by passive eooling, it should be a decrease in the crusta! radiogenic contribution to Fig. 3. Reduced heat flow vs. mean surface heat flow for six tectonic units from the Carpathians (East Carpathians, ECț-Crystalline-Meso- zoic zone and flysch zone, ECg- Neogene volcanic zone ; F-Foredeep ; Ap. M. -Apuseni Mts.; T. B. -Tran- sylvanian Basin ; P. B. -Pannonian Basin). The bars represent the stan- dard deviation of the mean heat flow. Correlation factor of the regres- sion line is 0.87. The dotted line represents the best fitting line through the data points from Fig. 2. maintain the ratio of 0.4/0.6. This is very likely accomplished by the removal of radiogenic heat-producing elements through erosional pro* cesses. C M Institutul Geological României IGR 9 GEOTHERMICS OF THE CARPATHIAN AREA 89 Heat flow density distributiou The first references in the scientific literature to measurements of temperatures in boreholes on the Romanian territory (Bungețeanu 1910, Tănăsescu 1912) appear at the beginning of the 20-th eentury (fide Airinei 1981). However, it was not until the 4-th decade that the research in geothermics was intensified directly connected with the programmes of oii industry boreholes logging. Accumulated in a huge amount (over 4000 investigated boreholes) the temperature data have been collected, analysed and interpreted in order to disclose the thermal regime of the oii and gas fields (Cristian et al. 1969, 1971, Negoiță 1970, Neguț 1972, Paraschiv, Cristian 1973, 1976). Sonic attempts were made to use these data for heat flow calculation. However, the reported values (Negoiță 1970, Paraschiv, Cristian 1973, 1976) were mere estimations based on thermal conductivities quoted from literature, for the rock types from different regions with distinct geological characteristics. In the last years several heat flow researchers have carried out measurements, appropriate to the internațional standards for this kind of studies, on tectonic units of the Carpathians and their foreland. Veliciu et al. (1977) reported 21 values of heat flow covering the whole territory of Romania; 19 values for the Transylvanian Basin and the Neogene volcanic chain of the East Carpathians were determined by Demetrescu (1979, 1981); 11 values for the eastern part of the Moesian Platform were determined by Neguț (1982); Veliciu, Visarion (1984) published 4 values of heat flow from Oaș-Gutîi and Călimani Mts. Consequently the set of heat flow reliable data for the Romanian territory contains now 54 values (see Appendix I). For this study the mentioned set of data has been supplemented by the heat flow values determined in U.S.S.R. (Kutas, Gordienko 1970 ; Kutas 1978) and Hungary (Horvâth, et al. 1981). This enabled the com- pilation of the heat flow map from Plate I. A non-uniform geographic distribution of the heat flow observation points is a first remark about the heat flow map of Romania. Moreover, despite a higher density of determinations as compared with neighbou- ring territories, this density is still relatively low as referred to the geo- logic-structural complexity of the Carpathian area. Most observations were concentrated in the central and eastern part of the Transylvanian Basin, on the Neogene volcanics, the Outer Flysch and Foredeep zones of the East Carpathians as well as in the central and eastern part of the Moesian Platform. Heat flow values indicated in Plate I suggest some possible correla- tions between the thermal regime and the characteristics of the major structural elements of the Carpathians. Such correlations have been previously attempted for different regions of the Globe, taking into consideration an empirical relationship regarding the heat flow and the age of the earth’s crust. In a detailed and systematic analysis of the data from Europe, Polyak, Smirnov (1968) first demonstrated the decrease of heat flow versus tectonic age. Recently, ’.T- Institutul Geologic al României JGRZ 90 Ș. VELICIU 10 similar analyses performed on more than 1500 observations from the continents (Chapman, Furlong 1977 ; Sclater at al. 1980) confirmed this decrease (Ttble 4). TABLE 4 Variation of heat flow with age of the most recent tectonic-thermal event (comp lied from Chapman, Furlong 1977; Sclater et al. 19S0) Age group Mean heat flow (mWnr2) Standard deviation N Age group (m-y.) Mean heat flow Standard deviation N Cenozoic 71 37 587 Mesozoic 73 29 85 0-250 76 53 398 Late Paleozoic 61 18 514 Early Paleozoic 52 17 88 250-800 63 21 500 Late Proterozoic 54 20 265 Early Proterozoic 51 21 78 800-1700 50 10 138 Archean 41 11 136 >1700 46 16 375 Total 1753 Total 1411 ,,N” represents numbcr of analysed observations. For all geothermic studies the definition of the “tectonic age” cons- tituted a key point. Within the continents this was nsually considered as the age of the last tectonic-thermal event which mobilized the region in which the heat flow values were determined. Measurements performed in the undeformed platform sediments carry the age of stabilization of the platform basement; measurements from the magmatic areas were referred to the radiometric age of the magmatic extrusion or intrusion; a fold belt carries the youngest age of deformation and a metamorphic terrain carries the age of the latest thermal metamorphic episode. Obvious- ly, the assignment of age is somewhat subjective, particularly in partially or weakly remobilized terranes and this subjectivity contributes to some of the scatter about the age group-heat floiv means. Moreover, there are certain geologic and tectonic settings where the regional heat flow may depart significantly from the appropriate age-group mean from Table 4. One of the uses for the heat flow versus age relationship is the es- timation of heat flow in unsurveyed areas on the basis of the tectonic- thermal age of the terrain, supplementing in this way the existing heat flow observations. The decay of continental heat flow with age extends over a longer period of time and it is a complex phenomenon comprising the following three componenta (Vitorello, Pollack 1980) as is shown in Fig. 4 : compo- nent I, crusta! radiogenic heat with a time-dependent decrease introduced IGR/ Institutul Geologic al României 11 GEOTHERMICS OF THE CARPATHIAN AREA 91 through erosion; component II, heat from a transient thermal perturba- tion associated with tectogenesis and component III, background heat flow from deeper sources within the earth. On a similar graph (Fig. 5) data for the Romanian territory have been plotted (Table 5), using the tectonic age from Rădulescu, Dimitrescu Fig. 4. Decrease of continental heat flow with age and its three principal components after Vitorello, Pollack (1980): component I — radiogenic heat from the crust; component II — heat from a transient thermal perturbation associ- ated with tectogeneses ; com- ponet III — background heat flow from deeper sources. EPr-Early Proterozoic; LPr- Late Proterozoic; EPz-Early Paleozoic ; LPz-Late Paleozoic; M-Mesozoic; C-Cenozoic. Tectonic age ( KPyears) Fig. 5. Decrease of heat flow with age in the Carpathian area. N. V. -neo-volcanic zone of the East Carpathians; T. B. -Transyl- vanian Basin ; P. B. -Pannonian Basin ; N. D. -North Dobrogea ; Mo. P. (w)-Moesian Platform (western part); [Mo. P. (e)-Moesian Platform (eastern part); Md. P. -Moldavian Platform. Heat flow data are from Veliciu, Demctrescu (1979). The three principal components of heat flow from Fig. o. Age data from Rădulescu, Dimitrescu (1982). <92 Ș. VELICIU 12 TABLE 5 1 aricit ion of heat floiv with age of the most recent tectonic-thermal event in the Carpathian area. Age data from Hâdul eseu, Dimitrescu (1982) Tectonic unii Last tectonic-thermal event and its age (m.y.) Number of observations Mean heat flow (mWm-2) Standard Refe- deviation 'rences Neogene volcanic zone of East Carpathians Mio-Pliocene- Quaternary calcalkaline volcanics (3-12) 18 96 16 1,2, 3,4 Pannonian Basin (eastern limit) post-Miocene extension (10-25) 4 96 8 1,5 Transylvanian Basin pre-Senonian folded basement (80-100) 14 44 15 16 North Dobrogea Pcrmian and Triassic volcanics (250-390) 5 60 8 2,7 Moesian Platform : western part eastern part magmatic intrusions during Breton tectogenetic phasc (400) 4 70 7 1 metamorphoscd and folded epi-Hercynian basement (700) 6 47 9 1,8 Moldavian Platform metamorphoscd and folded Proterozoic basement (1000-1600) 7 44 6 1,7 References : (1) Veliciu et al. (1977); (2) Veliciu, Visarion (1984); (3) Demetrescu (1979); (1) Kutas, Gordienko (1970); (5) Horvăth et al. (1981); (6) Demetrescu et al. (1981); (7) Kutas (1978); (8) Neguț (1982). (1982) . The observational data from the Carpathian area fit the mean values calculated by Chapman, Furlong (1977) and Sdater et al. (1980), except for the Transylvanian Basin which is eharacterized by surprising low heat flow (mean 44 mWm“2). In areas affected by Tertiary tectogeneses, the three components of the regional heat flow contribuie with 36 mWm-2, 27 mWm-2 and, respectively, 27 mWm-2 to establish the mean value of 90 mWm-2 usually encountered in terrains younger than 50 m.y. The component II, due to the cooling of the crust, diminishes ef- fectively to zero after 300—400 m.y. since the thermal transient pertur- bation associated with tectogenesis. Consequently, in the Hercynian terrains, only the radiogenic component I and “background” of heat flow JA Institutul Geologic al României 16 R/ 13 GEOTHERMICS OF THE CARPATHIAN AREA 93 ■are present. Moreover, the radiogenic component I is reduced to approxi- mately 20 mWm-2 by the erosion of the radiogenic isotopes from the uppermost crust. The two components I 4- II give only 45 mWm-2 in the Precambrian terrains. The Moldavian Platform has low heat flow values (39—55 mWm-2) which is typical for a platform with a Proterozoic basement.The lowest heat flow values are located on the contact with the Alpine orogenic zone of the Carpathians. This fact is explained by the “blanketing” ther- mal effect due to the thickening of the sediments on the epi-platformic side of the Foredeep. So, the mean of 44 mWm-2 calculated for the Mol- davian Platform (Table 5) is slightly under the heat flow mean established by Sclater et al. (1980) for the terrains affected by the Proterozoic tccto- geneses (50 mWm~2, Table 4). The values in the Moesian Platform can be grouped into two dis- tinct areas : a western part with relatively high heat flow (59—78 mWm-2) and an eastern one with lower heat flow (39—56 mWm-2). Bemarks regarding this particularity have been previously made by Neguț (1972) and Paraschiv, Cristian (1976) on the basis of geothermal gradients from oii industry boreholes as well as from geographic distribu- tion of heat flow values (Veliciu et al. 1977 ; Veliciu, Demetrescu 1979; Neguț 1982). The high heat flow in the western part of the Moesian Plat- form could be explained by the existence of the radiogenic heat sources represented by acid magmatic intrusions into its basement. Indeed, the deep drilling data (Paraschiv 1979) show that the Breton tectogenetic phase resulted iu the intrusion of gabbros, diorites, granitic porphyries and granitic aplites, which pierced the Devonian formations. Such intru- sions Avere not encountered in the eastern part of the Moesian Platform. It is interesting to notice that the mean heat floAv, calculated for the entire territory of the Moesian Platform (58 mWm'2), is in good agree- ment with the mean (61 mWm-2) established by Chapman, Furlong (1977) for the terrains affected by Hercynian tectogeneses (Table 4). The Predobrogean Depression exhibits relatively high values of heat flow (75 mWm 2 in the Danube Delta). The high values are related to the Alpine structures south of the Scythian Platform and these can be correlated Avith the structures from the North Crimea (Săndulescu 1980), where a geothermal activity has been observed. The Carpathian chain (including depressions) is characterized by heat floAA- ranging from 26 to 126 mWm-2. This broad interval is a con- sequence of various geological and tectonic features : the Alpine type structure of the Carpathians, the large development of Cretaceous and Paleogene flysch deposits, the presence of ophiolites and important masses of Neogene volcanics. These features are an expression of peculiar struc- ture of the, crust in a region where the Alpine orogenic belt coines in direct local contact with the old East-Buropean craton. The average heat flow, calculated for the Avhole Carpathian area (including depressions) is 69 mWm~2 (standard deviation : ± 16 mWm-2), that is closely to the mean heat flow from Table 4, established for the Ceno- zoic folded regions of Eurasia (71 mWm-2). This observation could be surprisingly taking into account the very low heat floAv recorded in the 4 A, Institutul Geologic al României 94 Ș. VELICIU 14 Transylvanian Basin (40—50 mWm-2); however, this low heat flowis probably compensatei! by the high heat flow from the Neogene volcanic zone (73—126 mWm-2) and from the Pannonian Basin (85—108 mWm-2). Temperatures within the crust on the Romanian territory Terrestrial heat flow values observed at the earth’s surface, radio- genic heat generation as well as heat conductivity data have been asso- ciated to the geological-structural cross-section from Fig. 6—B (modified after Rădulescu et al. 1976). As for the adopted geothermal model (Fig. 6—F), the gamma ray spectrometric measurements on the rock samples offered direct Informa- tion regarding theheat generation in the first few kilometrs of the crust. Radiogenic heat generation in the sedimentary covers ranges from 0.33 pWm-3 (carbonatic rocks) to 1.5 pWm-3 (molasse and flysch). For the metamorphic schists an average value of 2.1 uWm-3 has been considered. The deeper zone of the crust has a radiogenic heat generation value of 0.25 pWm-3 representative for intennediate-composition granulite- facies rocks. Beneath the base of the crust, a depleted ultrabasic zone with a characteristic heat production of 0.01 gWm-3 has been assumed. Both values are in good agreement with the heat generation —seismic waves velocity relationship, in the range 5.0—9.0 km s-1 (Rybach 1976). Heat conductivity values of 2.3 Wm-1 °K 1 for the upper crust and 3.3 Wm“10K-1 for the lower crust have been inserted into the calculations. Temperature vs. depth distribution has been obtained by auto- matic computation using the solution of the heat conduction equation under the following assumptions (Haenel 1979) : (1) the analytical solu- tion of the equation of heat conduction can be greatly simplified by assuming stationary conditions and an one dimensional temperature distribution, i.e. temperature depending only on depth ; (2) the model used takes into account the planeparallel stratification (within a certain portion of the geological cross-section from Fig. 6—B), where the source strength and heat conduction are uniform within each layer; (3) node- pendence of thermal properties was considered in respect of temperature and pressure. The calculated isotherms are presented in the lower part of Fig. 6—D. It is worth noticing that similar temperature-depth distributions have been obtained by Kutas (1973) for the Soviet Carpathians (in the same tectonic units of the East Carpathians) and by Bodri(1976) for the Pannonian Basin. The geoisotherms from Fig. 6—D show a possible temperature difference exceeding 500 °C at the base of the crust under the East Car- pathians Bend (Vrancea region) and a horizontal gradient of 200—300 °C/ 100 km is not to be excluded. This fact supplies new evidenee that the region is still tectonically active, the accumulated tensions in the lower crust being probably reîeased in shallow earthquakes. The mantie heat flow has been calculated according to : Qm = ffo - \ dz Jo jA Institutul Geologic al României xTgr/ 15 GEOTHERMICS OF THE CARPATHIAN AREA 95 where q0 is the heat flow observed at the surface; qM represents the heat generated at the base of the crust (Moho-discontinuity) or transferred from the deeper zones (approximately equal to the so-called “reduced heat flow"); zM is the depth at the base of the crust and A (z) describes the productivity of radiogenic heat sources within the crust. B Fig. 6. Characteristics of the geotraverse in thc Carpathian area : A-regional gravity (Ag) and magnetic (ATa ) anomalies; B-geological-structural cross-section after Rădulescu ct al. 1976). (1-upper crust; 2-lower crust; 3-oceanic crust; 4-flysch; Foredeep, 5-innerpart; 6-outer part; 7-volcanic rocks; 8-thrust and overthrust; 9-boundaries; 10-faults; data revealed by DSS and seismology, 11-Conrad discontinuity K* ; 12-Moho-discon ținui ty M; 13-hypocentres of normal and intermediate earthquakes ; 14-faulting zone); C-surface heat flow ; D-calcuIated jsotherms; E-calculated mantie heat flow (mWm-2); F-adopted geothermal model; G-identi- fying symbols for geothermal model. G Institutul Geologic al României 96 Ș. VELICIU ÎS The study performed so far indicated that the heat flow from the upper mantie increases eastwards. Below the foreland tectonic units of the Carpathians a mantie heat flow may exist being 2—4 times lower as compared to the Alpine orogenic area (11—16 mWm-2, respectively 24—61 mWm-2). On the eastern border of the Pannonian Basin the contribution of the mantie to the surface heat flow exeeeds probably 50%, whereas for the Moldavian Platform and the easthem part of the Moesian Platform this contribution may be less than 25%. It is also interesting to notice that on Romania’s territory the Moho discontinuity is clearly an unisothermal surface. Consequently, this. possibly marks a petrographic (Chemical) change but no phase transition. Geothermal regime of the East Carpathians A “classical” division of the East Carpathians (Dumitrescu, Săn- dulescu 1968; Săndulescu 1975, 1980) pointed out the following zones (from west toward east); Transcarpathian Zone, Crystalline-Mesozoic Zone, Flysch Zone and Neogene Zone. Otherwise, taking into account the age and the type of different groups of nappes, the East Carpathians exhibit the following structural units : Inner Dacidian Nappes (Pienides, Transylvanian Nappes System, Central East Carpathians Nappes System), Outer Dacidian Nappes, Moldavides and Foredeep. Post-nappe covers are also known. The eastern part of the Transylvanian Basin as well as the Neogene-Quaternary molasse depressions are superposed on parts of the deformed East Carpathians or on their post-nappe covers. The inner part of the East Carpathians is characterized by Neogene- Quaternary calc-alkaline magmatic activity. The geological structure of the East Carpathians is now viewed as a result of effects of the plate tectonics (Bleahu et al. 1973 ; Rădulescu, Săndulescu 1973; Hertz, Savu 1974; Săndulescu 1980). The present geothermal data are generally consistent with the geodynamic model suggested by Rădulescu, Săndulescu (1973). This model envisages that the Carpathian area comprises two suture zones : the Transylvanides (inner position) and the Outer Dacides (outer position) each of them corresponding to an Alpine paleoplane of „consumption” of the oceanic and/ or thinned lithosphere. The “consumption” process was connected with the main compressive tectogenetic “Carpathian phases”. The first important compressive period occurred between the Bar- remian and the Late Albian, affccting successively the Transylvanian ophiolitic realm and the East Carpathian Daeidic Zone. Consequently both the obduction in the Transylvanian area and the basement shearing nappes in the East Carpathians were generated. The Outer Daeidic megatrough was subjected to less important compressions; nevertheless it was involved in the crusta! shortening process since the end of the Upper Cretaceous. During the Early Miocene-Middle Sarmatian, the subduction was stil! active in the East Carpathians area having as a result the overthrust of the Moldavian cover nappes and simultaneously inducing the extra- heat that produced the parțial melting above the subducting plate. The Institutul Geological României (GR/' 17 GEOTHERMICS OF THE CARPATHIAN AREA 97 hot upwelling mantie material expanded laterally at the base of the continental crust and locally pierced it where a large mass of andesites extruded. Heat flow values for the East Carpathians have been reported by Veliciu et al. (1977), Demetrescu (1978), Veliciu, Visarion (1984). The geographic distribution of the surface heat flow shows that the Neogene volcanic Chain is characterized by high values ranging from 73 to 126 mWm-2, exceeding by a. factor of 1.5 or 2.0 the value accepted for the “normal” heat flow (approximately 60 mWm-2, Pollack 1982). The high heat flow anomaly overlaps also the outer Alpine paleopiane of lithospheric “eonsumption”. It is an interesting feature that the youngest andesitic rocks are located in the southein part of the chain (Harghita Mts) where values of 83—118 mWm-2 were measured; however, similar high heat flow values (85—126 mWm-2) have been recorded on the older andesitic formations from the northern part (Oaș Mts). So, despite a southward migration of the volcanic activity versus time (Rădulescu 1972), the high heat flow anomaly is almost constant along the entire neo-volcanic chain. A future review of the radiometrie age data from the northern part of the neo-volcanic chain could offer new elements for the correlation of the geological and geothermal features mentioned above. In order to study the inter-relations among the geodynamics of the East Carpathians, the building up of the Neogene volcanic chain, the inner heat sources and the lithospheric heat transfer, it has been necessary to elaborate a dynamic geothermal model (Rădulescu et al. 1981; Veliciu, Visarion 1984). This model takes into consideration not only the radiogenie heat sources and the steady-state heat conduction but also the subducting process during the Miocene time. The dynamic model is imposed by the relatively young age (less than 25 m.y.) since the last thermal-tectonic event in this region. As for the construction of the geotherm families, it has been based on the observed surface heat flow- associated with data of heat conduc- tivity and heat generation, estimated from petrologie and geophysical argumenta. The adopted model for the East Carpathians comprises an upper crusta! region enriched in radioactive sources (heat generation 2.1—2.4 gWm-3), an intermediate-composition granulite facies lower crust (0.25 gWm-3) and a depleted ultrabasic zone (0.01 gWm-3) over- lying the pyrolite mantie (details in Veliciu 1977, and a more complete discussion in Pollack, Chapman 1977). All the chosen values fit thegraph reported by Rybach (1976), according to the heat generation — seismic wave velocity relationship in the range 4.5—8.7 km s-1. Deep seismic soundings revealed depth of 16—20 km for the Conrad- discontinuity and 37—43 km for the Moho (Rădulescu 1979). The litho- spheric thickness of 150 km was inf erred from magnet otelluric measurements (Stanică, Stanică 1981). A resulting family of geotherms is presented in Fig. 7; the various heat flow values correspond to the different tectonic units. The diagrams indicate that the highest temperatures may be reached below the inner 7 - c. 231 JA Institutul Geological României igr/ 98 Ș. VELICIU 18 part of the East Carpathians: 500—600°C at the boundary between upper and lower crust and over 900°C at the base of the crust. This fact constitutes new arguments for some previous volcanologic hypotheses regarding the Neogene-Quaternary magmatic activity in the region (Rădulescu, Borcoș 1969). These hypotheses took into consideration the Fig. 7. Geotherms family calculated for the East Carpathians (steady-state conduction). Family parameter is surface heat flow (mWm"2) corresponding to the following tectonic units : 75—118, neo-volcanic zone; 57, Central East Car- pathians nappes system and Outer Dacides; 45, Moldavides and Moldavia n Platform. Phase transition for intermediate and basaltic crustal rocks after Wyllie (1971). possible existence of some phase transitions (“solidus”) in the crust, at the levei of the Conrad-discontinuity, where calc-alkaline magmas sta- tioned during their ascent. A very similar idea has been illustrated on the crustal cross sections in a work published by Socolescu et al. (1964). The calculation performed for the East Carpathians shows a value of 25—30 mWm-2 penetrating the base of the crust. Accordingly, the major part of the heat is generatei by radiogenic elements within the crust, but it may be a siguificant contribution from the young suberustal processes. The vertical temperature distribution (Fig. 6—D) suggests the presence of a positive temperature anomaly (over 1090°C) localei in The upper mantie, .The origin of this anomaly is possibly due to the Alpine subduction in the East Carpathians area. Some theoretical models for collision of two lithospheric plates generating subduction exhibit similar temperature anomalies for time constants ranging between 150 and 25 m.y. (Hasebe et al. 1970; Toksbz et al. 1971; Lubimova, Nikitina 1978). The models of the subduction mechanism at the top of a descending plate show the stretching in the plate as it bends dowawards into the underlying mantie; this implies subsequent compression as the plate Institutul Geologic al României 19 GEOTHERMICS OF THE CARPATHIAN AREA 99 experiences resistance to its motion. The compression results in an in- crease in temperature of the down-going slab that can be expressed by the general equation : ocf — +vvî) = K v2T + A \ 8t J where p is density, c is specific heat capacity, T is temperature, t is time, v is subduction rate, K is the thermal conductivity and A represents the radioactive heat generated in the crust. In Fig. 8 is presented a simplified two-dimensional geothermal model from Lubimova, Nikitina (1978), adapted to the Alpine subducting mechanism in the East Carpathians. Three distinct zones have been separated within the section from Fig. 8 : zone (I) with continental crust (lithosphere); zone (II) with oceanic crust (lithosphere); an intermediate zone (III) between zone (I) and (II) where the “consumption” process of the lithosphere (Benioff Zone) takes place. In the x-y coordinates system, the continental lithosphere is repre- sented as a semi-infinite plate (—co < a; < 0; 0 < » < Zj) with the following constant geothermal parameters : = 0 ; /kj; T^, To; lT (lT is the lithospheric thickness). For the steady-state continental plate (8T/ 8t = 0) the heat conduction equation has the form of Poisson’s equation, which can be solved without any mathematical difficulties: K^2T(x,z) +^) =0 Owing io thc possible physical effects of heating related to the displacement and evolution of a continental margin, it is necessary to consider the friction, phase tiansitions and adiabatic compression. Such effects are described by a temperature function T = f(z) on the vertical boundary of thc semi-infinite plate (I). The boundary conditions. are : Tz=Ii=Tq. T^o = T,; = fM; = 0. In this simplest formulation the continental plate (I) is represented by a “static thermal intrusion model”. The general solution to such a CONT IN t NTA L ^ZOH SU51PT SON ' LITHOSPHERE ’i ZOHE OCEANIC OR OCEANISED Fig. 8. Simplifice! two-dimensional mode) adapted from Lubimova, Nikitina (1978) for the thermal effect produced by Alpine subduction process in the East Carpathians (see explanations in text). Institutul Geological României 100 Ș. VEtiCIU 20 physical-mathematical problem is expressed by the sum of two terms: the “normal” temperature T[ (2) with uni-dimensional variance and “anomalous” temperature T'add (x, z) with a bi-dimensional variance: T = T&z) + T'^x, z) ■, ( —oo^aj^d; O^z^l,) The function fi(z) can be defined on the basis of observational data from the actual island-arc structurCs of the Pacific Ocean (Fig. 9). Fig. 9. Schematic diagrams of temperature distribution; (a) under the island arc and (b) uniler the border of the moving oceanic plate (after Liibimova, Niki- tina 1978). A similar model has beeii elaborated for the oceanic (or occanised) lithosphere of the semi-infinite plate (II), considered in a solid within limits : x0 x sg + 00 ; lr — ln z l, and parameters vtl(v„ 0, d); ^zz, ha 2'0, Tn (Fig. 8). FOr this plate, omițting the terms 81'/St and An, which do not limit the generality, and takihg into account the oceanic plate motion towards the continental one, the heat conduction equation with a convective term is obtained : ^zzV2^®» 4" ?crn « ’ 8x In this case, for the marginal part of a plate moving with a horizontal rate vn in the direction of the decreasing x, the following boundary conditions may be considered : Pj' = h = -I o j J-i = ii =i/i — J11 î J-x — x0 = fn(z); 81 /8x x^+oo = d In this formulation the oceanic plate is described in terms of a “dynamic thermal intrusion model” with v d. The general solution of the problem is presented as the sum of one-dimensional “normal” temperature T^iz) and a two-dimcnsional “anomalous” temperature T^x, z). The analytical solution of the problem for vn = constant and Ku = constant, was given by Lubimova, Nikitina (1978). There is also the possibility of defining the function f"bs(z) on the basis of the avail- able experimental data concerning heat flow over the oceanic plate border and on the ocean floor in the north-western Pacific Ocean (Fig. 9). 21 GEOTHERMICS OF THE CARPATHIAN AREA 101 For the adopted model, the intermediate “consumption” zone (i.e. the subduction zone of an oceanic or oceanised plate which descends to the marginal part of the continental plate) is located in the area : 0 < < a? < ®0; 0 z lv All the values 2 < 0 in Fig. 8 are pertinent to the mantle’s lower layer (asthenosphere). The temperature field T (x, z) in the intermediate “consumption” zone has been studied by the general heat conduction equation which takes into account both the convective heat-mass transfer vul (vx, 0, vs) and the generation of heat A 0: «) ~ z) + An,(x, z) = 0 where A'1' is the superposition of all types of heat sources in the “cons- umption” zone. Two functions of temperature distribution are the boun- dary conditions. For a rigorous mathematical solution of the formulated two-dimen- sional problem, not only geothermal parameters K,n (x, z), p, c, should be known, but also the velocity-vector vtlI (x, z) within the “consump- tion” zone. The assumption was that the material in the “consumption” zone acts, on the geological time scale, as a fluid. The vector vnI can be obtained from the solution of a problem based on Navier-Stokes equation. The problem was investigated by a computer-aided solution and the distribution rate obtained was introduced into the heat conduction equation. In this way the equation has a definite mathematical formula- tion and, consequentlv, gives a unique solution for the temperature field TUI. The result of the computation performed for the East Carpathian area is presented in Table 6. The geothermal parameters inserted into the calculation have been presented above. It is to point out that the Alpine subduction rate in the Carpathians was considered similar to the rate observed for the actual island-arc structures from the Pacific Ocean. A time scale referred to the main compressive tectogenetic “Carpathian phases” has been used. A problem that deserves consideration is whether any igneous- derived thermal anomaly may exist in connection with the Pannonian and/ or Pliocene volcanic and/ or subvolcanic formations from the East Carpathians. Mathematically the thermal calculations contain two major assumptions (Smith, Shaw Î975) : theheat transfer in the rocks surround- ing the magma is by solid-state conduction and effects of magmatic pre-heating and increases of magma after the time of the last eruption are ignored. Such estimations involve knowledge on the age and volume of magmatic manifestations. In Fig. 10 the age-volume data for seventeen volcanic systems from the East Carpathians are plotted to show the approximate present position of each System in relation to its probable cooling state. A pair of lines is drawn to represent cooling models after Carslow, Jaeger (1959), Shaw (1974), that identify igneous Systems that are now approaching the ambient temperature. jA Institutul Geological României IGRy 102 Ș. VELICIU 22 The following hypotheses have been taken into consideration. The inner part of the East Carpathians is characterized by a calc- alkaline magmatic activity manifested effusively during the Mio-Pliocene TABLE 6 Generation of extra heat due to the Alpine subducling process in the East Carpathians area Depth (km) Subduction rate (cm yr-1) Energy relcased (J m“z yr-1) Temperature (°C) 20 0.20 110 600 30 0.49 300 675 40 0.60 400 850 50 0.72 500 950 60 0.85 600 1000 70 1.00 675 1030 80 1.25 800 1050 90 1.60 1000 1080 Fig. IU. raph of theoretical cooling time vs. volume for igneous systems from the East Car- pathians. Pair of lines represents cooling models after Shaw (1974). Symbols represent youngest age and estimated volume. l-Harghita Mts. (c-Vîrghiș, e-Luci, f-Cucu); 2-Gurghiu Mts. (a- Fîncel-Lăpușna Caldera, h-Șumuleu-Fierăstrae, d-Ciumani); 3-Călimani Mts. (g-Călimani Caldera, h-Călimani Spring, i-Zebrac-Mermezeu); 4-Tibleș-Rodna zone (j-Botiza, k-Hudin, 1- Stegioara group, m-Grohot-Tomnatec, n-Țibleș-Măgura Neagră, o-Cormaia, p-South Rodna group, r-Vinului Valley-group); 5-Oaș-Gutîi Mts. (s-Coinlăușa Caldera, t-Batarci Caldera, u- Săpînța Caldera, v-Pietroasa Stratovolcano, z-Herja, y-Igniș Stratovolcano, w-Gutin). Institutul Geological României 23 GEOTHERMICS OF THE CARPATHIAN AREA 103 and Pleistocene volcanism in the Gurghiu-Harghita Mts representing the final stage of the subsequent Alpine magmatic activity (Rădulescu 1973). From a petrographic point of view, the Neogene magmatic rocks cover the whole range of calc-alkaine magmas but they exhibit a quite clear predominance of andesites in respect of rhyolites, dacites and basalts. Taking into account the petrographic composition, with the preva- lence of andesites, the following parameters have been used into the calculations : inițial temperature of magma 1000°C, mean density 2.9 g cm-3, latent heat of crvstallization 65 cal g-1, heat capacitv 0.3 cal g-1 ’C"1. As for the age of the magmatic systems, the volcanism in the Oaș- Gutîi Mts started during the Lower Badenian on a basement constituted by sedimentary formations of Senonian-Oligocene age. Numerous hypabyssal structures outcrop in the Țibleș, Hudău, Bîrgău and Rodna Mts. This region is geographically located within the volcanic andesitic chain of the East Carpathians, but no definite volcanic structure has been yet identified (Rădulescu, Dimitrescu 1982). Magmatic rocks usually come in direct contact with Paleogene formations which are metamorphosed ; so, the information on the age in this situation is of post-Paleogene time. On the graph in Fig. 10 the youngest possible age has been plotted (Săndulescu, Udubașa, personal communication). The beginning of magmatic activity in the Călimani-Gurghiu-Har- ghita Mts is considered in the Late Pannonian (Rădulescu 1973 ; Mihăilă, Peltz 1977). Here, Quaternary sediments with interbeddings of volcanic horizons have been identified. Consequently, the continuity of magmatic activity was supposed at least till the Middle Pleistocene time. The radiometric age detenninations by K-Ar method (Rădulescu 1973) offered two extreme elements foi' the Călimani-Gurghiu-Harghita Mts : 7.37 0.66 m.y. (andesite with augite and olivine from the South Călimani) and 3.92 ,± 0.2 m.y. (andesite with hypersthene from the Vlăhița pit). In respect of these age detenninations, the volcanics of the upper eompartment are situated between 7 and 3 m.y. Data regarding the magnitude of the magmatic systems are plotted in Fig. 10 on the basis of cartographic surface occupied by the systems, tectonic-magmatic characteristics, superstructure volume and geophy- sical mapping (gravity and airborn magnetic anomalies; Suceavă 1974, Cristescu 1977). The points plotted on the graph from Fig. 10 indicate that almost all igneous-systems from the East Carpathians have reached the ambient temperature or are very near to this. This fact is due to the reiatively great age of the last eruption in the region, ranging between 7 and 1.5 m. y. Considering the geological evolution of the East Carpathians area and the suggested geothermal models, it may be concluded that both the high heat flow and building up of the neo-volcanic chain are conse- quences of the lithospheric Miocene subducting process. The source of the high surface heat flow observed at the inner part of the East Car- pathians seems to have the following partition : approximately 60— 70 % of heat is released by the radiogenic generation, 25—30% is produced Institutul Geological României (GR 7 104 Ș. VELICIU 24 by the heat connected with the Alpine subduction and preserved in the upper mantie and lower crust, and a few percent is due to the heat con- tent of the upper levels in the crust where the andesitic magma accu- mulated during its ascent and the thermal equilibrium has not been reached yet. Some geothermal characteristics of the Pannonian Basin Both the geothermal gradients and heat flow data demonstrated that the Pannonian Basin is characterized by a high heat flow anomaly (mean 96 mWm"2 after Horvâth et al. 1981). Average temperatures at a depth of 1 km are of 70—80 °C. A remarkable feature of this geothermal anomaly is its uniform distribution, with high heat flow values over the whole Pannonian area. This fact has been demonstrated by the heat flow determinations perfor- med on the eastern border of the basin, where values of 94 mWm'2 (Arad) and respectivelv 85 mWm-2 (Sîniob) have been recorded (Veliciu et al. 1977). The downward continuation of the geothermal data suggested the existence of the temperatures of 800°C at the base of the crust (Fig. 6-D) and of 1000—1200°C at a depth of 60 km (Fig 13). The calculated mantie heat flow has a value of 60 mWm-2 beneath the eastern border of the Pannonian Basin (that means approximately 50% of the surface heat flow). Deep seismic soundings (DSS) indicated a thin crust (23—30 km), despite of a normally developed upper crust (17 —19 km, after Posgay et al. 1981). The low velocity zone (LVZ) is situated in an elevated po- sition. The compressional seismic wave velocity increases with depth to 9.1 km s-1 as far as 50 km depth, then it diminishes to 7.7—7.8 km s-1 at a depth of approximately 60 km. Magneto-telluric measurements (Adam 1965) revealed a rise of the high conductivity layer (HC'L). The Bouguer anomaly within the basin has an average from 4-10 to 4-15 mgali, that is surprisingly low taking into account the thinning of the crust and the rise of the upper mantie. Computed gravity models (Horvâth, Stegena 1977) argumented for a density of the upper mantie lower than normal. Geothermal data correlated with other geophysical Information (LVZ and HOL in an elevated position, low density) provide consistent arguments toward the parțial melting hypothesis of the upper mantie beneath the Pannonian Basin. The Pannonian Basin was formed during and after the Miocene tectogeneses which affected the Carpathian area, by a Badenian subsidence process continued by a rapid subsidence in the Pannonian. The Pannon- ian and post-Pannonian sediments have a maximum thickness of 5 km depending on location. The mechanism of the subsidence as well as the high heat flow anomaly can be explained using an extensional lithospheric model. Mc Kenzie (1978) noticed that in the sedimentary post-tectonic basins, where Institutul Geologic al României IC r/ GEOTHERMICS OF THE CARPATHIAN AREA 25 105 geological and geophysical evidence for an extending lithosphere exist (thinner crust and lithosphere, a certam pattern of the tensional faults, recent basic magmatism within the basin), the surface heat flow is ab- normally high (over 85 mWm-2). In order to explain this phenomenon, Lachenbruch, Sass (1978) studied two different extensional models : (1) a model with extension viewed as stretching in the homogeneous plastic sense produces a thinner lithosphere and (2) a model that represents homogeneous stretching of a lithosphere whose thickness is maintained constant by accretion of crystalline material at its base and with a ver- tical convective transport by fluid basalt in dyke-like intrusions. A horizontal extension at a constant rate “s” accounts for the reiatively uniform thickness of the lithosphere and the high heat flow on the entire basin. On the other hand, the lithospheric thinning is partly compensated by the supply (accretion) of melted basaltic material from the asthenosphere which crystallizes at the base of the lithosphere (up- doming of the asthenosphere or “mantie diapir”). The equation that describes this process is : z d2T , dT . “o Ai —— + vpc —------Aoe dt2 dz where the terms are heat conduction, heat convection and respec- tively, radiogenic heat generation; v represents the vertical migration velocity of the melted basalts. Using the error function, Carslaw, Jaeger (1959) gave an analytical solution : T = e , Lh q _r — c32 3 K where :?2 = K/spe; s = vțz. In Fig. 11 the calculated family of geotherms is presented for an extensional velocity s expressed as% per m.y., with family parameter being the surface heat flow. The temperatures within the lithosphere are remarkably increased as compared with the simple conduction in steady- state thermal regime. The adopted extensional model suggests that the high regional heat flow anomaly from the Pannonian Basin may be re- ferred to the abnorma! heat flow from the asthenosphere (approximately 60 mWm-2) plus the thermal effect of the extensional process at a rate of 1—3% per m.y. The generalized developmcnt of an extensional basin may be divided into two stages (Mc Kenzie 1978). During and immediately after extension there is a rapid subsidence. This occurs in isostatic response to net density changes resulting from the lithospheric thinning and from the heating and thermal expansion. The second stage of subsidence is a reiatively long-term process caused by cooling and contraction of the lithosphere following the extensional phase. The overall subsidence is generally am- plified by the effects of sediment loading. If original crustal thickness, elevation and temperatures arc known, a detailed analysis of subsidence history can be used to determine the magnitude of extension. Institutul Geological României loa Ș. VELICIU 26 The extension (more correctly “the distension”) of the Pannonian Basin was described as a result of large scale strike-slip faulting along a system of conjugate shears striking NE-SW and NW-SE respectively; NE trending faults exhibit sinistra! offsets whereas NW trending faults are dextral. This conjugate set implies E-W extension. Fig. 11. Temperatures vs. depth diagram. Solid curves correspond to an extending lithosphere in the Pannonian area. Dashed curves correspond to geotherms for static case fs = 0). Num- bers on solid curves are extension rates in % per m. y. Curves labeled A, B. C, D, correspond to surface heat flow values of 50, 70, 85 and respectively 110 mWm-2. Tm is the mantie solidus. Horvâth, Royden (1981) have interpreted the Pannonian Basin as the result of Badenian extension (the “thermal” phase of subsidence). The extension became conspicuous during the Sarmatian time (asthe- nospheric attenuation or “mantie diapir”) and manifested itself at a reduced rate during the Pannonian (passive cooling of the lithosphere). The cited authors assumed 50 to 100 % extension in the Pannonian Basin and a total E—W extension was estimated as 75 to 100 km. Comparing this result to approximate 120 km Miocene crustal shortening estimated from palinspastic restorations for the East Carpathians (Săndulescu 1989; Ștefănescu 1980), the magnitudes are secn in fair agreement. How- ever, it should be noticed that the palinspastic restoration of thrusts and folds provides a minimum estimate of crustal shortening. Further- Institutul Geological României PI. I S- VELICIU- Geothermis of the Carpothion area Imprim.Atei InstGeol Geof ANUARUL INSTITUTULUI DE GEOLOGIE Șl GEOFIZICĂ. VOL- 67 Institutul GedCgiC dl României VIGR/ 27 GEOTHERMICS OF THE CARPATHIAN AREA 107 more, the above figures refer to the total Miocene shorteningl in the East Carpathians whereas the extension in the Pannonian Basin occurred only in the Badenian and post-Badenian time. Geothermies of the Transylvanian Basin From a geological point of view the Transylvanian Basin, located in the inner part of the Carpathians, was defined as a structural post- tectonic element corresponding to a homogeneous and young (Neogene) area which was subjected to molasse sedimentation (Dumitrescu, Săn- dulescu 1968). The molasse deposits overlie a folded basement and its post-tectonic cover. From the measurements reported by Demetrescu (1973) and Veliciu et al. (1977) and from the Heat Flow Map (Plate I) a surprisingly low terrestrial heat flow appears typical of Transylvania. On the basis of the geothermal model elaborated by Veliciu, Visarion (1981), the present contribution tries to give a more detailed interpretation of this obser- vation. Reviewing the main geological and geophysical characteristics of the post-tectonic intermountain basins (intra- and inter-arc basins), Stegena et al. (1975) find the following common features : (1) thinner lithosphere, HOL and LVZ at higher position ; lower density in compa- rison with the average, (2) thinner crust, (3) synorogenic and particularly post-orogenic sediments affected less or not at all by tectonic movements, (4) low seismic activity for already developed basins, (5) sialic basins exhibiting andesitic volcanic activity of compressional type during the subducting process in the related areas, and “interarc-spreading” basaltic volcanism of extensional type after the subduction ceased, (6) high heat flow. Regarding the last characteristic, the average heat flow in the Transylvanian Basin is only of 45 mWm-2, indica ting that the geothermal activity of this region is low. The low heat flow is outlined by the measu- rements performed in the surrounding tectonic units (Plate I), where values of 83—126 mWm-2 for the neo-volcanic chain of the East Carpat- hians, of 80 mWm-2 for the South Apuseni Mts and of 85—lOOmWm-2 for the eastern part of the Pannonian Basin were determined (Veliciu, Demetrescu 1979). From the pre-basin evolution of the Transylvanian Basin has been concluded that the main tectogeneses responsible for the structure of its folded basement took place between the Middle Cretaceous and the Seno- nian. A complex correlation of geological, geophysical and drilling data reveals that the area occupied by the Neogene molasse of the Transyl- vanian Basin covers the junction of structural elements belonging to the major tectonic units of the Romanian Carpathians (Săndulescu, Visarion 1978). In the center of the depression both gravity and magnetic heights are partly related to the ophiolitic zone which is bilaterally overthrust on the Apuseni crystalline-bearing basement units westward and on the Central East Carpathians nappe system (the “root” of the nappes) east- Institutul Geological României 108 Ș. VELICIU 28 ward (Fig. 6-B). The folded basement is overlapped by a faulted and folded cover of the Neocretaceous, Paleogene and Lower Miocene age. The Middle Mio-Pliocene molasse, which is approximately 4 km thick, fills this intramountain depression. DSS data indicate that the crustal thickness is 34 km in the eastern part of Transylvania and decreases to 30 km in the center of the basin. Westwards the crust thickens out again to nearly 38 km. The crust is thin relative to the East Carpathians and Foredeep (40—50 km) but thick as compared to the Pannonian Basin (24—28 km). It is an interest- ing feature that the upper crust is only about 10 km thick, excluding the sedimentary complex. Compressional wave velocities show the cliaracteristics of continental type crust even under the ophiolitic zone : 5.5—6.2 km s-1 for the upper crust, 6.8—7.0 km s-1 for the lower crust and 8.2—9.0 km s-1 beneath Moho were recorded (Rădulescu 1979). The last figure may indicate that the density of the upper mantie could be greater than the average. As for the lithospheric thickness, no direct investigation jhas been carried out so far. Nevertheless, some inferences can be made from the position of HCL in the eastern part of the Pannonian Basin (Adam 1965) and in the Vrancea region (Stanică, 1981) where depths of 60 km and more than 150 km respectively were recorded. A depth of about 100 km for HCL seems to be reasonable under the Transylvanian Basin. Heat generation models were established separately for the Southern Apuseni crystalline-bearing basement units, the ophiolitic zone and the Central East Carpathians nappe System (Fig. 12). The calculated mantie heat flow varies from 20 mWm-2 to 30 % mWm-2. The mantie heat flow Qo=6OmWfe'2 I 0 1 Z^W3 Fig. 12. Heat generation models A (z) for the crustal structure of the Transylvanian Basin after Veliciu, Visarion (1982). Model I for the Southern Apuseni crystalline-bearing basement units; Model II for the ophiolitic zone; Model III for the Central East Carpathians nappes system. (1-molasse and post-tcctonic cover; 2-upper crust; 3-lower crust ; 4-ophiolitcs). Q. is surface heat flow; Qm is mantie heat flow. has two componenta : one is due to radioactive heat sources in the sub- crustal lithosphere, the other to the deeper contribution which arises from the asthenosphere and enters the lithosphere at its base. It is clear that the lower crust and upper mantie are less endowed with heat-prod- ucing radioactive sources than the upper crust; on the other hand it is Institutul Geological României 29 GEOTHERMICS OF THE CARPATHIAN AREA 109 not known whether a decrement of surface enrichment is accompanied by a proporțional decrement or increment in the lower crust (Pollack, Chapman 1977). Petrological argumente are consistent with either situa- tion. The problem is open to discussion especially for the ophiolitic zone in the central part of Transylvania, where the heat generated in the upper crust is lowered by a factor of 6 or so, because of the content of basic rocks in the folded basement (Model II in Fig. 12). Fig. 13 shows geotherms Fig. 13. Temperatures vs. depth dia- gram for the Transylvanian (1) and Pannonian (2) basins. Tm is the mantie solidus. corresponding to the average surface heat flow observed in the Transylva- nian Basin and the Pannonian area. The temperature calculated at the base of the crust in Transylvania (~ 600°C) is lower, as compared to that from the Pannonian Basin (800—900°C). : The geotherms were extended to a depth at which they intersect the mantie solidus (Tm) but were dotted to indicate provisionality above 0.85 Tm. The geotherms are characterized by nearsurface curvature due to the crusta! heat generation and a nearly linear gradient through the depleted zone. Various authors have suggested that the lithosphere-asthenosphere transition might begin at a temperature less than the solidus. Pollack, Chapman (1977) adopted as the depth of the base of the lithosphere the depth at which the geotherms reach 0.85 Tm. The geotherm for the Tran- sylvanian Basin intersects 0.85 Tm at a depth of approximately 80 km while in the Pannonian Basin it reaches the same temperature at a depth of only 40 km. Consequently, the geothermal data clearly indicate a thicker and cooler lithosphere for Transylvania. The model of Horvâth, Stegena (1977) which explains the Late Cenozoic history and geophysical features of the Pannonian Basin in terms of updoming of the asthenosphere (“Mantie diapir”)does not fit the characteristics of the Transylvanian Basin. During the Alpine sub- duction process, the Carpathian and Transylvanian area was subjected to compressions from the Middle Cretaceous to the Middle Miocene. After the subduction ceased no evidence indicates extension as occurred in the Pannonian Basin. It is shown by a thicker and cooler lithosphere, just a relatively thin crust, the scarcity of tensional faults, discontinuous subsidence history and lack of contemporaneous basic magmatic activity Institutul Geological României 110 Ș. VELICIU 30 within the basin. Conscquently, lithospheric stretching as described by Lachenbruch, Sass (1978) for extensional basins, is not very likely tobe present here. A descending convection current in the mantie could be taken into consideration under Transylvania (Constantinescu et al. 1976; Visarion, Săndulescu 1979) which was probably related to the formation of the mantie “diapir” below the Pannonian Basin. The author wishes to express his gratitude to professors Sabba Ștefănescu, Liviu Constantinescu, Dan Rădulescu and Radu Botezatu for stimulating the interest in geothermal studies, for their encourage- ment and for suggesting some improvements to this work. It is also a pleasure to acknowledge Drs. Mircea Săndulescu and Marius Visarion for valuable discussions regarding the geology and gcophysics of the Carpathians. Thanks are due to Dr. Marcian Bleahu for his kind support for this work to be published. APPENDIX I Temperature gradient, thermal conduetivity and heat floiv values for the Romanian territory Coordinates Elevation a.s.l. (m) Min. depth (m) Max. depth (m) Temp. gradient (m’Km-1) Therm. conduc. Heat îlow (tnWm-2) Ref. lat. N 1 long. E 2 3 4 5 6 7 8 9 47’28' 26’05' 424 2183 2284 18 2.20 39 1 47’15' 25’42' 823 40 320 15 3.30 45 4 47’13' 22’13' 148 2301 2502 51 1.80 94 1 47’08' 26’25' 440 2412 2915 26 1.70 43 1 47’07' 24’30' 400 100 3200 24 2.40 58 4 47’04' 24’10' 350 100 3500 20 1.70 33 4 47’03' 26’25' 409 3816 4038 26 2.60 58 1 46’56' 24’17' 374 80 980 32 1.50 45 4 46’49' 27’09 80 1731 1883 18 2.20 39 1 46’40' 24’53' 580 80 980 31 1.10 33 4 46’40' 25’47' 1018 100 530 22 3.60 57 4 46’37' 26’29' 501 1520 1610 20 2.30 47 1 46’36' 25’30' 1250 120 270 37 2.00 77 4 46’34' 24’54' 500 80 980 32 1.10 39 4 46’31' 24’45' 380 2050 2354 28 2.60 74 1 46’30' 26’40' 480 850 1012 23 2.30 54 1 46’24' 25’26' 1048 120 520 37 3.20 113 4 46’21' 25’31' 950 30 220 63 1.60 104 4 46’18' 25’44' 1240 40 200 41 1.80 73 4 46’15' 25’44' 525 314 510 50 1.70 83 1 46’12' 21’20' 120 193 377 45 1.90 85 1 46’09' 22’53' 712 215 402 35 2.20 79 1 46’09' 25’52' 600 20 540 70 1.50 118 4 45’11' 26’19' 315 5511 5653 21 2.30 48 1 45’03' 26’03' 290 1848 2100 30 2.30 67 1 45’02' 23’25' | 298 1123 1250 37 2.20 80 1 31 GEOTHERMICS OF THE CARPATHIAN AREA 111 APPENDIX I (continued) 1 2 3 4 5 6 7 8 9 44’53' 23’25' 220 2930 3440 29 2.60 75 1 44’51' 22°24' 311 610 785 40 2.30 92 1 44’48' 25’48' 168 5009 6255 18 3.00 52 1 44’47' 26’49' 64 2500 2724 24 1.80 44 1 44’31' 25’42' 144 1223 1404 37 1 .90 70 1 44’20' 24’03' 201 2092 2422 35 1.80 59 1 44’14' 23’53' 182 1546 1700 45 1 .80 78 1 45’36' 26’46' 46 2 45’17' 26’01' 59 2 45’13' 26’38' 39 2 45’06' 26’31' 53 2 45’03' 25’53' 56 2 45’03' 26’02' 45 2 44’59' 25’39' 44 2 44’53' 26’39' 58 2 44’52' 26’21' 56 2 44’52' 26’56' 38 2 44’50' 26’02' 33 2 45’16' 29’11' 75 3 48’08' 23°24' 380 45 2.80 126 5 48’38' 23’49' 410 56 1.60 90 5 47’40' 24’44' 470 35 2.40 85 5 47’02' 25’20' 750 38 2.20 82 5 Referenc.es: (1) Velieiu et al. (1977); (2) Neguț (1982); (3) Velieiu (1978); (4) Dcmctrescu (1979); (5) Velieiu, Visarion (1984). Thermal conductivity from measurements by transient .method for (1), (3) and (5), any by divided bar for (2) and (4). REFERENCES Adam A. (1965) Einige Hypothesen liber den Aufbau des oberen Erdtnantels in Ungarn. Ger- lands Beitrăge zar. Geophysik, 74 p. 20—40, Leipzig. Airinei S. (1981) Potențialul geotermic al subsolului României. Ed. Științifică și Enciclopedică, București. Birch F. (1954) Heat from radioactivity. In : II. Faul (ed) Nuclear Geology- p. 148 — 175, New York. Blcahu M., Bocaletli M.. Manclti P-, Pcitz S. (1973) Neogene Carpathian Arc: A continetal arc displaying the features of an “Island Arc”. J. Geophys. Res.. 78, p. 5025 — 5032. Bodri L. (1976) Deep temperature and heat flow in the Pannonian Basin. Ph. D. Dissertation, Eotvos University, Budapcst. Carsllaw H. S., Jaegcr .1. C. (1959) Conduction of heat in solids. 2nd edit., Calderon Press, Oxford. Chapman D. 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Wyllie P. .1. (1971) Experimental limits for melting in the earlh’s crust and upper mantie. In : The structure and phvsical properties of the earth’s crust. Am. Geophys. Union Geophys. Mon., 14, p. 279—301, Washington. REGIMUL GEOTERMIC AL ARIEI CARPATICE (Rezumat) în cadrai lucrării este prezentat regimul geotennic care caracteri- zează aria carpatică, avînd drept bază de discuție valorile observate ale fluxului geotennic, conductivității termice și generării radiogene de căldură în roci. De asemenea, au fost stabilite corelații între fluxul geo- termic din interiorul orogenului carpatic (40—120 mWm"2 incluzînd și depresiunile post-tectonice), Platforma Moldovenească (39—55 mWm-2), Platforma Moesică (39—78 mWm-2) și particularitățile geotectonice. Analiza modelelor structurale și geodinamice actuale — ce au încercat încadrarea ariei carpatice în conceptul tectonicii globale — la care s-au asociat datele obținute de autor referitoare la distribuția geo- Institutul Geologic al României 35 GEOTHERMICS OF THE CARPATHIAN AREA 115 grafică a fluxului geotermic, conductivitatea termică a rocilor și generarea radiogenă de căldură, a făcut posibilă stabilirea condițiilor impuse de observațiile geologice și geofizice variației temperaturii cu adineimea, în crustă, pe teritoriul României. Pentru Carpații Orientali, Bazinul Panouic și Bazinul Transilvaniei, s-au elaborat modele geotermice în regim dinamic, luîndu-se în considerație efectele termice ale procesului de subducție alpină presupus în Carpați. La elaborarea modelului geotermic al Carpaților Orientali s-a ținut seama de sursele radiogene de căldură, de conducția termică în regim staționar și de procesul de subducție din timpul Miocenului. Drept con- cluzii asupra surselor anomaliei pozitive de flux geotermic, localizată la partea internă a Carpaților Orientali, au rezultat următoarele: (1) 20 — 25 % din fluxul termic observat la suprafață ar putea proveni din căldura produsă de mecanica procesului de subducție miocenă, căldură conservată încă în litosfera subcrustală datorită constantei de relaxare a perturbației termice mai mare ca 25 inii, ani, pentru o crustă cu grosimea de 40 km; (2) aproximativ 70% din fluxul la suprafață ar prezenta căldura generată radiogen, din care 2/3 în crustă; (3) numai cîteva procente sînt probabil datorate conținutului în căldură din părțile superficiale ale crustei, unde magmele calcoalcaline au staționat în cursul ascensiunii lor și unde echi- librul termic nu a fost încă atins. Analiza datelor geotermice arată că anomalia de flux geotermic și punerea în loc a lanțului neo-vulcanic din Carpații Orientali, par a fi o consecință a subducției litosferice alpine produsă în zona paleo-planului extern de consum al crustei din aria carpatică. Atît variația temperaturii cu adîncimea, cît și valorile fluxului geotermic, au demonstrat că Bazinului Panonic îi este asociată o anomalie geotermică pozitivă avînd o amplitudine de 96 mWm-2. Pornindu-se de la observația că în bazinele sedimentare post-tecto- nice, unde există o seamă de evidențe clare geologice și geofizice pentru un proces de extensie a litosferei (crusta și litosfera subțiate; prezența fracturilor de tensiune distribuite într-un anumit mod; magmatism bazic recent la interiorul bazinului) fluxul geotermic la suprafață este anormal de ridicat, s-a studiat un model geotermic în regim dinamic. Modelul adoptat pentru Bazinul Panonic evaluează efectul termic al unei extensii orizontale continue, cu o viteză constantă (aproximativ 1—3% per mii. ani), care are drept rezultat o subțiere relativ uniformă a litosferei; subțierea litosferei tinde să fie compensată la suprafață de fenomenul de subsidență și sedimentare molasieă, iar la baza litosferei subțierea este compensată de adăugarea de material bazaltic topit din astenosferă (așa numitul fenomen de “atenuare astenosferică” sau diapirismul man- talei”). Pe baza modelului studiat se explică majorarea substanțială a gradientului geotermic în crustă, comparativ cu simpla conducție în regim staționar, formarea Bazinului Panonic fiind interpretată ca un rezultat al extensiei (sau mai corect „distensiei”) începută în timpul Badenianului (faza „termală” a subsidenței), cu o accelerare în timpul Sarmațianului (faza „atenuării astenosferice”) și urmată de o reducere a ratei extensiei în timpul Pannonianului (faza răcirii „pasive” a litosferei). wr- Institutul G eolog ic a I Român iei \jGRy 116 Ș. VELICIU 36 Modelul propus pentru a explica, în termenii „atenuării asteno- sferei”, particularitățile geotermice și istoria neogenă a Bazinului Panonic nu se potrivește caracteristicilor Bazinului Transilvaniei. Pentru Transil- vania, media fluxului geotermic apare — surprinzător pentru un bazin post-tectonic — ca fiind de 40—45 mWm-2. Curbele medii temperatură — adîncime calculate (geotermele), corespunzătoare mediei fluxului geo- termic măsurat la suprafață în Bazinul Transilvaniei, respectiv aria panouică, indică existența la baza crustei a unor temperaturi de cca 600°C și respectiv 800—900°C. Geoterma pentru Transilvania intersectează temperatura de tran- ziție în faza „solidus” a mantalei la o adîncime de aproximativ 80 km, în timp ce în Bazinul Panonic atinge aceeași temperatură la numai 40 km. în consecință, datele geotermice sugerează prezența unei litosfere mai reci și mai groase sub Bazinul Transilvaniei. Datorită subducției alpine din Carpați, aria transilvană a fost continuu obiectul compresiunii. După ce subducția a încetat, nu există nici un argument geologic sau geofizic în favoarea unui proces de extensie a litosferei, similar celui produs în Bazinul Panonic. L'Annuaire de l'Institut de Geologie et de Gdophysique a ete public le long des annees sous les titres suivants: Anuarul Institutului Geologic al României, t. 1-XV (1908-1930) Anuarul Institutului Geologic al României (Annuaire de l'ln- stitut Gdologique de Roumanie) t. XVl-XXl! (1931-1943) Anuarul Comitetului Geologic (Annuaire du Comitd Geologi- que) t. XXIII - XXXI V (1950-1964) Anuarul Comitetului de Stat al Geologiei (Annuaire du Co- mite d Etat pour la Geologie) t. XXXV-XXXVII (1966-1969) Anuarul Institutului Geologic (Annuaire de l'Institut Geo- logique) t. XXXVIII-XLII (1970-1974) Anuarul Institutului de Geologie și Geofizică (Annuaire de l’lnstitut de Geologie et de Geophysique) depuis le L XLIIH975 Institutul Geologic al României igr/ MINISTERE DES MINES, DU PETROLE ET DE LA GEOLOGIE' INSTITUT DE GEOLOGIE ET DE GEOPHYSIQUE H.P. HANN = Petrographic Investigation of Pegmatites Located between Teregova and Marga (Eastern Banat, South Carpathians) Ș. VELICIU ; Geothermics of the Carpathian Area TOME 67 Institutul Geologic al României