Basement studies in a continental suture zone - Xenoliths from the mid-German crystalline rise (Rhon area, mid-European Variscides)

A suite of crustal xenoliths from the Mid-German Crystalline Rise (Rhon area, Mid-European Variscides) reveals the thermal and deformational history of a Late Paleozoic continental collision zone. Beside sandstones and rare claystones from the Mesozoic cover, two different suites of crustal xenoliths were detected in Tertiary basalt vents and lava flows: - Garnet-chlorite micaschists, garnet-staurolite micaschists, amphibolites and muscovite bearing gneisses represent a mid-crustal level, which underwent an upper greenschist- to amphibolite-facies metamorphism. Peak metamorphic P-T conditions of 500-600 degrees C at 4-7 kbar were attained in a static, posttectonic phase while mineral zonation and inclusions in garnet reveal a prograde, clockwise P-T path. - The present lower crust is mainly represented by granulite facies metabasites (pyroxene gneisses) and rare sillimanite bearing gneisses. P-T conditions are estimated at 730-850 degrees C at maximum pressures of 9-11 kbar for the metabasites and at 750-860 degrees C at 7-12 kbar for the gneisses. These data as well as the seismic data of the EGT and DEKORP 3 profiles reveal a relatively thin continental crust in this collision zone with no indications for any high pressure event. The reduced lithospheric thickness in the Rhon may be explained by 1. late to post-Variscan gravitational forces and additional tensile plate boundary forces or 2. the replacement of originally upper plate rocks by syncollisional transfer of material from the orogenic root into the upper crust by basal accretion.