MAIN FEATURES OF THE REGIONAL METAMORPHIC EVENTS IN HUNGARY - A REVIEW

The present paper reviews data concerning the crystalline basement of all tectonic units in Hungary. Their lithologic content, biostratigrap hic data, metamorphic features and radiometric age data are presented and critically discussed. The effects of at least two regional metamor phic cycles have been definitely recognized, Alpine and Variscan, and some polymetamorphic situations suggest the possible existence of a th ird, older event. The following main events are distinguished: the Alp ine, the Variscan and an early Variscan or pre-Variscan stage or event . Alpine metamorphism turns out to be more widespread than previously reported. The Alpine metamorphic grade ranges from subgreenschist to g reenschist facies conditions, locally reaching the lower amphibolite f acies. Low to medium pressure conditions have been generally ascertain ed for it, with local exceptions of high and transitional medium to hi gh pressure conditions. The Variscan metamorphism has been detected in the majority of the pre-Permian rock sequences in Hungary. Its metamo rphic grade covers the whole temperature field of metamorphism, up to anatectic conditions. The pressure conditions in the monometamorphi. V ariscan low grade metapelites turn out to be low, related to a thermal gradient close to 40 degrees C/km. A pre-Alpine polymetamorphic chara cter is displayed by some Variscan amphibolite facies metapelites. The youngest mineral assemblages in them are characterized by the occurre nce of andalusite in the rocks of suitable bulk composition, displayin g a low-pressure character of the Variscan event, consistently with th e above reported pressure estimate inferred from the greenschist facie s monometamorphic metapelites. The oldest mineral assemblage is charac terized by the occurrence of kyanite in rocks of suitable bulk composi tion, which may be either referred to an early Variscan stage or to a pre-Variscan event.