A Clockwise P-T Path from the Variscan Basement of the Tisza Unit, Pannonian Basin, Hungary

Abstract

The polymetamorphic basement of the Tisza Unit forms a detachedfragment of the Variscan European foreland of the Neotethyanrealm. A clockwise evolution path of a gneiss-amphibolite complexof the Tisza Unit was reconstructed, investigating the polymetamorphicrocks of the borehole Baksa-2, SE Transdanubia, Hungary.The results obtained by microstructural and mineral parageneticobservations, mineral chemical analyses, and thermobarometric calculations define a P-T loop which suggests a complex Variscanpolyphase model rather than a pre-Variscan - Variscan polycyclicone. The early part of the prograde path with kyanite is characterizedby T-P conditions of 480±50¡C and 470±70 MPa, respectively. Themetamorphism reached its peak at 660±25 ¡C and 750±50 MPa,when both kyanite and staurolite were stable. This metamorphic climaxwas followed by a nearly isothermal decompression to 440±2 0MPa at 650±40¡C. This event is marked by the presence of sillimaniteand a second generation of garnet, and is closely related to the collisional Variscan granitoid magmatism observed in considerable partsof the Tisza Unit. In amphibolites intercalated with gneisses, only thislast event was preserved, providing T-P estimates of ca. 650-690¡C /400-500 MPa. The present paper provides the first demonstration of acontinuous, clockwise P-T path from the metamorphic basement ofthe Hungarian part of the Tisza Unit.