PETROLOGICAL AND K-AR AND AR-40-AR-39 AGE CONSTRAINTS FOR THE TECTONOTHERMAL EVOLUTION OF THE HIGH-PRESSURE MELIATA UNIT, WESTERN CARPATHIANS (SLOVAKIA)

The Meliata high-pressure unit, exposed in the southern part of the We stern Carpathians, consists of four groups of rocks, which are charact erized by different lithologies and metamorphic histories. (1) The mos t common are Triassic marbles, intercalated with metabasalts and glauc ophane-bearing phyllites. Phengite from metabasalts and phyllites reco rd K-Ar and Ar-40-Ar-39 plateau ages of 152-155 Ma which are interpret ed to date the high-pressure metamorphism. (2) Blueschist-facies mafic rocks and subordinate glaucophane-bearing micaschists, devoid of marb les which contain relic low-Si muscovite and newly formed high-Si phen gite; the Ar-40-Ar-39 data obtained for the micas indicate Palaeozoic ages of greater than or equal to 375-380 Ma for the early amphibolite- facies muscovite and a less than or equal to 250 Ma age for the second ary blueschist-facies phengite. (3) Glaucophane-free phyllites and phe ngite quartzite with low-Si and high-Si micas, respectively; textural features indicate a polyphase evolution characterized by a greenschist -facies metamorphism followed by a blueschist-facies overprint; the hi gh-Si micas provide evidence for an early Middle Jurassic age (172 Ma) of the high-pressure metamorphism. (4) Amphibolite-facies slices and blocks, overprinted by blueschist-facies metamorphism, represent old b asement rocks. Regarding the differences in lithologies, metamorphic c onditions and radiometric ages of high-pressure micas (older age for c ontinental vs. younger age for oceanic rocks), subduction of a contine ntal wedge prior to that of oceanic crust can be assumed. This interpr etation is based also on palaeogeographical reconstruction of the Meso zoic Western Tethys between the Apulian promontory and the Tisza-Bihor continental block.