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

Polymetamorphic garnet micaschists from the parautochthonous Kliening and the overlying Preims units in the lower part of the Austroalpine Saualpe basement in Carinthia (Austria, Eastern Alps) display complex microstructural and mineral-chemical relationships. Electron-beam based automated routines with EDX spectral mapping, WDS element mapping and manual WDS element analysis were applied for garnet mineral-chemical characterisation and monazite chemical dating. Two garnet porphyroblast generations and diverse monazite age populations have been revealed in low-Ca and high-Al-metapelites. Garnet 1 in both Kliening and Preims units has decreasing Mn, constant Ca and significantly increasing Mg from cores to rims. Geothermobarometry of garnet 1 assemblages signals a crystallization along a M1 prograde metamorphism culminating at 550 - 670 °C/7 - 8 kbar. In the Preims Unit a subsequent pervasive 300 - 250 Ma high-Y and high-Gd monazite 1 crystallisation coincides with the intrusion of Permian and Early Triassic pegmatites. In the Kliening Unit where pegmatites are lacking, no Permian monazites are observed. Coronas of apatite and allanite around the large Permian monazites in the Preims Unit signal a retrogressive stage. A M2 event with garnet 2 postdates the corona formation around Permian monazites. Garnet 2 displays always low Mn at high Ca and Mg, with increasing and then decreasing XCa at decreasing and then increasing XMg. The garnet 2 assemblages recorded a prograde P-T evolution at a high P/Tgradient, from 550 °C/8 kbar to 700 °C/10 - 13 kbar in the Kliening Unit, and from 600 °C/6 - 10 kbar to 700 °C/14 - 15 kbar in the Preims Unit. Monazite 2 populations with ages from 100 - 85 Ma and with lower Y and Gd contents crystallized at decreasing pressure subsequent to garnet 2, indicating a Cretaceous (Eo-Alpine) age for the metamorphic event M2. In the histogram view, the monazite age pattern in the Preims Unit and the overlying Saualpe Eclogite Unit are quite similar with Permian and Cretaceous age maxima. When regarding the considerable differences of peak pressures during the M2 event, a major tectonic discontinuity can be stated between these units. The Saualpe nappe units underwent two distinct clockwise metamorphic cycles at Carboniferous - Early Permian and then Cretaceous periods, related to continental collisions under different P-T regimes. This led to a characteristic distribution pattern of monazite ages in these units which is different from other Austroalpine basement areas.