Probable anticlockwise P,–T evolution in extending crust: Hlinsko region, Bohemian Massif

Abstract

ABSTRACT In the Hlinsko region (Variscan Bohemian Massif, Czech Republic) a major extensional shear zone separates low‐grade metasedimentary series (Hlinsko schists) and high‐grade rocks of the Moldanubian terrane (Svratka Crystalline Unit). During late‐Variscan extension, a tonalite intruded syntectonically into the normal ductile shear zone, and caused contact metamorphism of the overlying schists. Concurrent syntectonic sedimentation of a flysch series took place at the top of the hangingwall schists. In order to decipher the detailed petrological evolution of the Hlinsko unit situated in the hangingwall of this tectonic contact, a phase diagram approach and petrogenetic grids, calculated with the thermocalc computer program, were used.The crystallization/deformation relationships and the paragenetic analysis of the Hlinsko schists define a P–T path with an initial minor increase in pressure followed by cooling. Calculated pseudosections constrain this anticlockwise P‐T evolution to the upper part of the andalusite field between 0.36 and 0.40 GPa for temperatures ranging from 570 to 530°C. A low aH2O is required to explain the presence of andalusite‐biotite‐bearing assemblages, and could be related to the presence of abundant graphite.In contrast, the footwall rocks of the Svratka Crystalline Unit record decompression from around 0.8 GPa at a relatively constant temperature, followed by cooling. Thus, the footwall and the hangingwall units display opposite, but convergent P–T histories. Decompression in the footwall rocks is related to a rapid exhumation. We propose that the inverse, anticlockwise P–T path recorded in the hangingwall pelites is related to the rapid, extension‐controlled sedimentation of the overlying flysch series.