HIGH-GRADE METAMORPHISM AND RETROGRESSION OF MOLDANUBIAN GRANULITES, AUSTRIA

Charnockitic granulites from the Dunkelsteiner Wald, Moldanubian zone in Austria, bear the assemblage Grt(1) + Opx + Bt + Kfs + Pig + Qz + Pt + Ilm. Textural and compositional relations suggest a near-equilibrium state among these minerals, thus allowing the application of thermobarometry. Estimated conditions for the latest high-temperature overprint are 770 degrees C, 10.7 kbar and a(H2O) approximate to 0.1. Green spinel inclusions occasionally intergrown with corundum occur only in garnet core. The latter may be replaced by optically ''dirty'' aggregates of spinel, diasporized corundum, zoisite and white mica. Electron micro-probe investigations revealed that the largest part of the aggregates is composed of intergrowths of zoisite with Na-bearing margarite. Margarite is commonly intergrown with muscovite. Textures and phase relations suggest that zoisite and margarite were formed during the early stages of retrogression as the PT-path entered the stability field of the assemblage zoisite + margarite. Break-down of garnet and inclusions of corundum and plagioclase therein was initiated by infiltrating fluids; their composition (X(H2O) approximate to 0.7) was estimated from early fluid inclusions in quartz. Late stages of retrogression are documented by the diasporization of corundum. The studied rock show a clockwise PT-path characterized by (a), isothermal decompression during and after attainment of peak metamorphic conditions, (b), nearly isobaric cooling in the range 500-600 degrees C and 5-6 kbar and (c), further decompression that may be related to thrusting of Moldanubia over Moravia.