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

The increasing recognition of high P-T metamorphic conditions in crustal rocks has lead to a major reappraisal of the processes inherent to continental collision. Within high-grade regions in orogenic belts, a particular problem is that bodies of high P-T rocks are often found preserved within an apparently lower grade gneissose matrix. Resolving the true extent of high P-T conditions in such situations requires a detailed examination of the mineral 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 assemblage garnet + clinopyroxene + plagioclase + quartz are found intercalated within the voluminous Gfohl gneiss. Calculated P-T conditions for the granulites 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 C. In the host felsic gneiss, an early high-P assemblage 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 estimates for a widespread biotite + sillimanite retrograde overprint range from 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 indicates that the granulite-gneiss association represents a coherent high P-T unit, with the felsic gneisses being a retrogressive product of former leucocratic 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-deformational migmatisation along basal parts of the unit during consolidation of the Moldanubian nappe pile. (C) 2001 Elsevier Science B.V. All rights reserved.