Polymetamorphism in garnet micaschists of the Saualpe Eclogite Unit (Eastern Alps, Austria), resolved by automated SEM methods and EMP-Th-U-Pb monazite dating

Polymetamorphic garnet micaschists from the Austroalpine Saualpe Eclogite Unit (Karnten, Austria, Eastern Alps) display complex microstructural and mineral-chemical relationships. Automated scanning electron microscopy routines with energy dispersive X-ray (EDX) spectral mapping were applied for monazite detection and garnet mineral-chemical characterization. When the Fe, Mg, Mn and Ca element wt% compositions are used as generic labels for garnet EDX spectra, complex zonations and porphyroblast generations can be resolved in complete thin sections for selective electron-microprobe analyses. Two garnet porphyroblast generations and diverse monazite age populations have been revealed in low-Ca and high-Al-metapelites. Garnet 1 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 at similar to 650 degrees C/6-8kbar. Sporadic monazite 1 crystallization started at c. 320Ma. Subsequent pervasive 300-250Ma high-Y and high-Gd monazite 1 formation during decompression coincided with the intrusion of Permian and Early Triassic pegmatites. Monazite 1 crystallized along the margin of garnet 1. Coronas of apatite and allanite around the large 320-250Ma monazite signal a retrogressive stage. These microstructures suggest a Carboniferous-to-Early-Permian age for the prograde M1 event with garnet 1. Such a M1 event at an intermediate-P/T gradient has not yet been described from the Saualpe, and preceded a Permo-Triassic low-P stage. The M2 event with garnet 2 postdates the corona formation around Permian monazite. Garnet 2 displays first increasing X-Ca at decreasing X-Mg, then increasing X-Ca and X-Mg, and finally decreasing X-Ca with increasing X-Mg, always at high Ca and Mg, and low Mn. This records a P-T evolution which passed through eclogite facies conditions and reached maximum temperatures at similar to 750 degrees C/14kbar during decompression-heating. A monazite 2 population (94-86Ma) with lower Y and Gd contents crystallized at decreasing pressure during the Cretaceous (Eo-Alpine) metamorphism M2 at a high-P/T gradient. The Saualpe Eclogite Unit underwent two distinct clockwise metamorphic cycles at different P-T conditions, related to continental collisions under different thermal regimes. This led to a characteristic distribution pattern of monazite ages in this unit which is different from other Austroalpine basement areas.