ANNUAL REPORT OF THE GEOLOGICAL INSTITUTE OF HUNGARY, 1999 (2000) MINERALOGICAL, PETROLOGICAL AND GEOCHEMICAL CHARACTERISTICS OF CRYSTALLINE ROCKS OF THE ÜVEGHUTA BOREHOLES (MÓRÁGY HILLS, SOUTH HUNGARY) GYÖRGY BUDA*, ZUÁRD PusKÁs**, KAMILLA GÁL-SÓLYMOS**, URS KLÖTZLI*** and BRIAN L. COUSENS**** *Department of Mineralogy, Eötvös L. University, H -1088 Budapest, Múzeum krt. 41A. **Department ofPetrology and Geochemistry, Eötvös L. University, H-1088 Budapest, Múzeum krt. 4/A. ***Laboratory for Geochronology, University of Vienna, Geocentrum, Department of Geology, Althanstrasse l4,A-1090 Vienna ****Earth Sciences, Carleton University, 1125 Colonel By Drive, Ottawa, Ontario, K IS 5B6 ~ ... -..' Keywords: cataclasites, chromite, granites, Hungary, isotope, lamprophyre, microc1ine, microgranite, mylonites Four types of crystalline rocks can be distinguished in the Üveghuta boreholes: 1. Microcline megacryst-bearing granitoids. 2. Amphibole-rich enc1aves. 3. Microgranites. 4. Pegmatites. In the Mórágy Hills these rock types can be found in outcrops as weIl. The amphibole-rich enc1aves are K-Mg-rich calc-alkaline vaugnerite-durbachite with lamprophyric character. The enc10sing granitoids have also K-Mg-rich calc-alkaline character. The two rock types are mineralogically and petrologically different, however, as a result of the interaction between the basic and acidic melts they show many geochemical similarities, e.g. normalised REE patterns and isotope ratios. Partial melts were formed in the collision zone oftwo continental crustal blocks during the Variscan orogeny (340-350 Ma). The more basic melts were formed as a result of partial fusion of a K-, Ba-, Rb-, Sr-rich upper mantle) wedge situated above an older subduc- tion zone, whereas the granitoid melts inc1ude both mantle and continental crustal contributions. The continental crust is presumed to .,.. be of Pan-African origin with Cadomian age (- 620 Ma). Peraluminous microgranitic melts of crustal origin intruded subsequently. Their volatile-rich fractions crystallised as pegmatites. The outer zones of microcline megacrysts with younger age, the widespread micro- cline replacement textures and the biotitisation of amphibole point to K-metasomatism. The crystalline massif has been subjected to vari- ous tectonic events causing frequent mylonitisation, catac1asis and hydrothermal mineralisation (carbonate, pyrite, chalcopyrite etc.). 1. Introduction 2. amphibole-rich enc1aves (syenite, monzonite, diorite), 3. microgranite, The Department of Mineralogy,and the Department of 4. pegmatite. Petrology and Geochemistry of the Eötvös Loránd These rock types also occur in outcrops and quarries University were requested by the Hungarian Geological where their geological settings can be studied better than Institute to carry out mineralogical-petrologicaland geo- on borehole samples (BUDAand PusKÁs 1997). The major- chemical investigationson the crystallinerocks of ten bore- ity of the samples are strongly sheared; mylonitisation and holes drilled in the Mórágy Hills, South Hungary. The aim catac1asis are common. The vein fillings are quartz, car- of the project, co-ordinated by the Institute, was to delin- bonate and small amount of sulphides. ~ eate the most favourablearea for disposal of low-and inter- mediate-levelradioactivewaste. 2.1. Microcline megacryst-bearing granitoiis -.%----- In this paper we described the mineralogy and petro- -~ logyof the main rock types, inc1udingthe alteration of min- This rock type is the dominant one in the borehole sam- erals as well as the deformation of rocks. Tectonic settings ples. The occurrences of white or pinkish microc1ines are have been identified based on major and trace element random. The rocks occurring in the near-surface, mylonitic compositions.Trace-elements,Sr,Nd and Pb isotope ratios zones or near to the microgranite dykes contain pinkish were used in order to determine the origin of melts these microc1ine. The white microc1ine-bearing granitoids usual- rocks crystallised from. Finally we outline the genesis of ly contain unaltered amphibole and biotite. the crystalline rocks occurring in the investigatedarea. Microcline megacrysts are mostly euhedral, attaining sizes up to 5x3 cm. The white microc1ine, which also occurs in basic amphibole-rich enc1aves, has a slightly dis- 2. Mineralogy and petrology of the main rock types ordered structure (tjO = 0.86) compared with the pinkish microc1ine (tjO = 0.96 BUDAet al. 1999). Four main rock types havebeen distinguishedbased on lnc1usions of groundmass in microc1ine megacrysts are their textural characters, mineralogical and chemical com- common, inc1uding apatite, sphene, allanite, amphibole, positions : biotite, quartz, plagioc1ase.These inc1usions are not deformed 1. microc1ine megacryst-bearing granitoids (quartz whereas the groundmass of the rock is strongly deformed monzonite, monzogranite), which suggests that the megacrysts crystallised before the " 232 Gy. BUDA, Z. PusKÁs, K. GÁL-SÓLYMOS, U. KLÖTZLI and B. L. COUSENS deformation. The most common twin law is Carlsbad but Apatite is a common accessory mineral, occurring as cross-hatched twinned or untwinned megacrysts are also short, stubby prismatic crystals. observed. String perthite is common. Microc1ineoccurring Pyrite is disseminated throughout the granitoids, and is in ground mass has undulose extinction or shows cross- commonly altered to limonite. hatched twins and has disordered structure (~= 0.5), indi- Chalcopyrite occurs as an inc1usion in pyrite in a form cating crystallisation under the higher temperature and of droplet or rarely as a vein-filling. faster rate of cooling.Manyplagioc1asesare partly or entirely replaced by cross-hatchedmicroc1ines.These replacement 2.2. Amphibole-rich enclaves (diorite, monzonite, syenite) textures can be observed in amphibole-richenc1avesas weIl as in granitoids. The microc1ine megacryst-bearing granitoids contain Plagioclaseis very common. They are mostly zoned dark green, fine grained amphibole-rich enc1aves with few " with lamellar twinning and commonly altered to sericite. microcline and/or plagioc1ase megacrysts. The thickness of The core of the zoned crystals are enriched in biotite-inc1u- these enc1aves are variable between few cms and more than sions. Sometimesthey are strongly deformed and brecciat- 50 metres. The contact between enc1aves and host grani- -/ ed. The vein-fillingsare mostly quartz and epidote-zoisite. toids is not sharp, and the microc1ine megacrysts common- Microc1ine replacement is common. Compositions are ly have "grown" into the enc1aves. These amphibole-rich oligoc1aseto andesine (An28-3S)' enc1aves are slightly strained but porphyritic textures are Quartz forms knots, mostly deformed with undulose weIl preserved. extinction. Their mineralogical and chemical compositions are Biotiteis the most common mafic constituentwith tabul- highly variable. The total alkali content of enc1aves is usu- ar or lamellar shapes. It has reddish-brownand yellowish- ally higher (Na20+K2O -9 Wt%) than that of the enc1os- brown pleochroism in the pinkish microc1inemegacryst- ing granitoids suggesting that the basic and acidic melts bearing granitoids whereas the biotite in the white micro- coexisted. The higher temperature basic melt started to c1ine-bearing rock has greenish-brown, yellowish-brown crystallise at first (e.g. biotite) resulting in a deficiency in pleochroism indicating a slightlyhigher amount of Mg. alkalies which was equilibrated continuously from the SUf- Both are Mg-biotite(ca1c-alkalinetype). They are com- rounding alkali rich granitoid melt by K diffusion. monly strongly deformed with undulose extinction. They Plagioclase is an import ant constituent, commonly altered to chlorite in mylonitic zones. In microc1ine occurring as large porphyritic euhedral crystals. The crys- megacrysts the biotite is aligned parallel to the rim of tal are zoned from andesine (An41-43) cores through megacryst. These inc1usionsare not deformed. labradorite (And back to andesine rims (An36-3S)' This Amphiboleis not common. Mostlyit occurs in the white complex zonation also suggests the coexistence of acidic microc1inemegacryst-bearinggranitoid. It forms tabular or and basic melts before and during the crystallisation. prismatic euhedral crystals. Twins are common according Plagioc1ase crystals are saussuritised and sericitised, and to the (100) law. Rarely it forms knots. Alteration to chlo- microc1ine-replacement texture also occurs. Rarely crystals rite and ca1cite is also common. Compositions are Mg- are strained and exhibit undulose extinction. hornblende and actinolitic hornblende. Except for the euhedral megacrysts, microcline is main- Accessory minerals: ly an anhedral groundmass phase, containing inc1usions of AIIaniteis euhedral and zoned. Metamictisation is com- plagioc1ase, amphibole, biotite and abundant acicular mon, with most crystals altered to REE-fluorcarbonates, apatite. They are either untwinned or have cross-hatched -~ c1ayand/or opaque minerals. It contains biotite, quartz, twins, and also OCcUfwith or without perthite exsolu- feldspar inc1usionssuggestinglate crystallisation. Epidote- tion. zoisite overgrowth is occasionally observed. Sometimes Quartz is rare and mostly forms knots in the grtmnd- ,.-;.-- crystals are fractured, and cracks are filled with quartz. mass. They are mylonitised with undulose extinction. The Amphibole is a very common mostly euhedral, prismatic high LREE content of the rock originates from abundant phase. Sometimes they are