Electron probe petrochronology of polymetamorphic garnet micaschists in the lower nappe units of the Austroalpine Saualpe basement (Carinthia, Austria)

Garnet micaschists from the parautochthonous Kliening and the overlying Preims complexes in the lower part of the Austroalpine Saualpe basement in Carinthia (Austria, Eastern Alps) display a polymetamorphic microstructural and mineral-chemical evolution. SEM automated mineralogy methods and analysis by EPMA were applied for garnet mineralchemical characterisation and monazite dating. Two garnet generations with monophase and polyphase porphyroblasts and two monazite age populations have been revealed in low-Ca and high-Al-metapelites. Garnet 1 in both Kliening and Preims complexes has decreasing Mn, low Ca and significantly increasing Mg from cores to rims. Geothermobarometry of garnet 1 assemblages signals a crystallisation along a M1 prograde metamorphic P-T path culminating at 600-650 degrees C/7 kbar, followed by decompression. In the Preims Complex a subsequent crystallisation of a similar to 265 Ma monazite 1 population with high-Y and high-Gd indicates a Permian minimum age for the M1 metamorphic event. Monazite 1 crystallisation coincides with the intrusion of Permian and Early Triassic pegmatites. In the Kliening Complex where pegmatites are lacking, no Permian monazite is observed. Resorption of garnet 1 rims and corona structures of apatite and allanite around Permian monazite in the Preims Complex signal a retrogressive stage. An M2 metamorphic event with garnet 2 postdates the monazite corona structures. Garnet 2 displays always low Mn at high Ca and Mg. The XCa is increasing and then decreasing, while XMg is decreasing and then increasing toward the rims. The garnet 2 assemblages record a prograde P-T path from 530 degrees C/8 kbar to 700 degrees C/10-13 kbar in the Kliening Complex, and from 620 degrees C/10-12 kbar to 700 degrees C/14-15 kbar in the Preims Complex. Monazite 2 populations with ages between 95 and 81 Ma and with lower Y and Gd contents crystallised at decreasing pressure subsequent to garnet 2 and indicate a Cretaceous (Eo-Alpine) age for the metamorphic event M2. In the histogram view, the monazite age pattern in the Preims Complex and the overlying Saualpe Eclogite Complex are quite similar with Permian and Cretaceous age maxima. These patterns differ from other Austroalpine basement areas with unimodal Carboniferous or bimodal Carboniferous and Permian monazite age populations.

Automated summary

This study investigates the polymetamorphic evolution of garnet micaschists from the Austroalpine Saualpe basement in Carinthia, Austria, using mineralchemical characterization and monazite dating. Two garnet generations were identified, indicating two metamorphic events. The different complexes exhibit distinct metamorphic paths, with Permian and Cretaceous ages being the main crystallization periods.