Carboniferous Polymetamorphism Recorded in Paragneiss-Migmatites from the Bavarian Unit (Moldanubian Superunit, Upper Austria): Implications for the Tectonothermal Evolution at the End of the Variscan Orogeny

The Bavarian Unit in the Moldanubian Superunit exposes a distinct segment of the central European Variscan orogen, which is characterized by strong, late Variscan, low pressure, high temperature (LP–HT) metamorphism. The predominant lithology of the Bavarian Unit is a paragneissderived migmatite with abundant cordierite and K-feldspar. Rare paragneiss varieties with large garnets from the Lichtenberg complex near Linz (Upper Austria) record detailed information regarding the regional P–T–t evolution. The large garnet porphyroblasts of this exceptional rock preserve complex three-phase growth zoning indicative of a polymetamorphic history. Garnet cores with uniformly elevated grossular contents are discontinuously mantled by lower grossular garnet, and this calcium-poor central garnet zone is further overgrown by a garnet rim zone with elevated grossular content. Garnet zones also display different mineral inclusions, i.e. sillimanite, plagioclase and spinel in the core, and staurolite, biotite, plagioclase, sillimanite and muscovite in the mantle. Cordierite, sillimanite, K-feldspar and spinel in the matrix are equilibrated with the garnet rim. Geothermobarometric calculations coupled with thermodynamic modelling constrain the P–T peak of the first prograde metamorphism at 0 85–1 10 GPa and 720–780 C (garnet core). Subsequently there was a stage of decompression and cooling during which the first garnet generation became partly resorbed. A second prograde metamorphic stage resulted in a new growth phase of garnet. This started at 0 45–0 60 GPa and 580–630 C (garnet mantle) and progressed to granulite facies peak conditions of 0 55–0 65 GPa and 830–900 C (garnet rim). Th–U–total Pb dating of monazite inclusions in the garnet cores indicate a Visean age for the first medium pressure, medium temperature (MP–MT) metamorphic event (340 6 7 Ma), relating it to the Variscan collision stage. Dating of matrix monazite yields a Pennsylvanian age (312 6 5 Ma) for the LP–HT overprint, consistent with existing geochronological data from the Bavarian Unit. Our study documents that deeply buried Variscan collisional crust was exhumed to mid-crustal levels in the Visean, before being near-isobarically heated to LP–HT granulite facies conditions in the Pennsylvanian (Bavarian event of the Variscan orogeny). This P–T–t evolution implies a significant external heat influx into mid-crustal levels at the end of the Variscan orogeny. VC The Author(s) 2018. Published by Oxford University Press. All rights reserved. For permissions, please e-mail: journals.permissions@oup.com 1359 J O U R N A L O F P E T R O L O G Y Journal of Petrology, 2018, Vol. 59, No. 7, 1359–1382 doi: 10.1093/petrology/egy063 Advance Access Publication Date: 22 June 2018