Resolving the relationship between high P-T rocks and gneisses in collisional terranes: an example from the Gfohl gneiss-granulite association in theMoldanubian Zone, Austria

The increasing recognition of high P-T metamorphic conditions in crustal ro cks has lead to a major reappraisal of the processes inherent to continenta l collision. Within high-grade regions in orogenic belts, a particular prob lem is that bodies of high P-T rocks are often found preserved within an ap parently lower grade gneissose matrix. Resolving the true extent of high P- T conditions in such situations requires a detailed examination of the mine ral assemblage and compositional evolution in spatially related lithologies , so that realistic geothermobarometric comparisons can be made. Within the Gfohl Unit in Austria, an integral part of the Moldanubian Zone of the Variscan orogen of Europe, granulites containing the high-P assembla ge garnet + clinopyroxene + plagioclase + quartz are found intercalated wit hin the voluminous Gfohl gneiss. Calculated P-T conditions for the granulit es indicate peak metamorphic equilibration at 14-16 kbar, 950-1050 degreesC . Post-peak, close to isothermal decompression to a medium-P orthopyroxene- bearing overprint at 7-8 kbar, 800-870 degreesC, was followed by cooling at < 6 kbar, 800-500 <degrees>C. In the host felsic gneiss, an early high-P a ssemblage of garnet + kyanite + ternary feldspar + quartz can be recognised in rarely preserved, less deformed samples. Calculated peak P-T conditions for this assemblage are 14-16 kbar, > 950 degreesC, whilst temperature est imates for a widespread biotite + sillimanite retrograde overprint range fr om 700 to 500 degreesC, at a minimum pressure of ca. 4 kbar. The coherence in metamorphic conditions and derived P-T paths from both rock types indica tes that the granulite-gneiss association represents a coherent high P-T un it, with the felsic gneisses being a retrogressive product of former leucoc ratic granulites. The lack of preservation of high P-T assemblages in many parts of the gneiss is considered to be a function of pervasive deformation and associated fluid influx, together with the effects of syn-deformationa l migmatisation along basal parts of the unit during consolidation of the M oldanubian nappe pile. (C) 2001 Elsevier Science B.V. All rights reserved.