Vörös A. szerk.: Fragmenta Mineralogica Et Palaentologica 13. 1987. (Budapest, 1987)

FRAGMENTA MINERALOGICA et PALAEONTOLOGICA 13. Budapest, 1987 7

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o Contribution to the knowledge of the polymetamorphic basement of the Great Plain (Pannonian Basin, East Hungary): the environment of the Derecske Depression By P. ÁRKAI (Received December 16, 1985) Abstract. In the basement the first, medium-grade, medium-pressure (Barrovian, 18-26°C/km thermal gradient) event was overprinted by a low pressure type (29-43°C/km), diverse, partly prograde, mainly retrograde recrystallization preceded locally by mylonitization. The age relations are not proved at present. However the basement may be correlated with the Middle Proterozoic (? ) Someç series (Apuseni Mts.); a Hercynian or Caledono-Hercynian plurifacial model based on the striking similarities to the other parts of the Great Plain, Transdanubia, and the Eastern Alps seems to be more probable. INTRODUCTION The crystalline basement of the Great Plain covered by several thousand meter thick Neogene sediments represents an important connection between the Eastern Alps, the Transylvanian Central (Apuseni) Mts, the Carpathians and the Serbo-Macedonian Massif. Knowledge of its metamorphic history may promote the correlation of the crystalline complexes in different tectonic units (microplates), the final features of which were shaped mostly in the Alpine tectonocycle (folding, thrusting, faulting, horizontal movements, etc. ). The aim of the present paper is to characterize the premetamorphic lithotypes, the different metamorphic events, to determine their physical conditions in a given part of the basement (Fig. 1) by textural, mineral paragenetic, petro- and mineral-chemical, and thermobarometric investigations carried out on core samples derived from hydrocarbon exploratory wells. This work forms part of a complex petrogenetic study on basement rocks, initiated and sponsored by the National Oil and Gas Trust (Budapest). GEOLOGY, PREVIOUS PETROLOGICAL DATA The geological and tectonic situation of the surroundings of the Derecske Depression is shown on a simplified metamorphic sketch map of Hungary (Fig. 1). The basement topography and the locations of the investigated bore-holes are presented in Fig. 2 constructed by the Oil Exploration Enterprise (Szolnok). As the drilling activity started in the second half of the last decade, the first comprehensive research reports on the metamorphic basement have been submitted only recently (ÁRKAI 1979, 1980, 1981), the results of which form the basis fof the relating parts of the publication of SZILI (1985) dealing with the basement of a larger part of the Great Plain. Nevertheless, it is worth mentioning the earlier extrapolations for the given area made on the basis of the surrounding territories. According to the "Map of Metamorphites of the Carpatho-Balkan-Dinaride Area" (SZÁDFCZKY-KARDOSS et al. 1969, 1976), the basement of the Great Plain is built up of Pre-