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 detached fragment of the Variscan European foreland of the Neotethyan realm. A clockwise evolution path of a gneiss-amphibolite complex of the Tisza Unit was reconstructed, investigating the polymetamorphic rocks of the borehole Baksa-2, SE Transdanubia, Hungary. The results obtained by microstructural and mineral paragenetic observations, mineral chemical analyses, and thermobarometric calculations define a P-T loop which suggests a complex Variscan polyphase model rather than a pre-Variscan - Variscan polycyclic one. The early part of the prograde path with kyanite is characterized by T-P conditions of 480±50iC and 470±70 MPa, respectively. The metamorphism reached its peak at 660±25 iC and 750±50 MPa, when both kyanite and staurolite were stable. This metamorphic climax was followed by a nearly isothermal decompression to 440±2 0 MPa at 650±40iC. This event is marked by the presence of sillimanite and a second generation of garnet, and is closely related to the collisional Variscan granitoid magmatism observed in considerable parts of the Tisza Unit. In amphibolites intercalated with gneisses, only this last event was preserved, providing T-P estimates of ca. 650-690iC / 400-500 MPa. The present paper provides the first demonstration of a continuous, clockwise P-T path from the metamorphic basement of the Hungarian part of the Tisza Unit.