LOW-PRESSURE GRANULITES FROM THE KETILIDIAN MOBILE BELT OF SOUTHERN GREENLAND

Early Proterozoic cordierite-garnet-orthopyroxene-K-feldspar metasedimentary gneisses are developed in the flat-lying complex (FLC) of the southern part of the Ketilidian mobile belt (1.8-1.7 Ga), Greenland. These granulites contain low-pressure assemblages and are developed in both regional metamorphic rocks and in some thermal aureoles of contemporaneous rapakivi granite plutons. Thermobarometry shows that both the regional and contact metamorphism took place at 2-4 kb and 650-800-degrees-C. The granulites were developed in an extensional tectonic regime, and apparently record the culmination of a single thermotectonic event. There is a close temporal association between the peak of high-grade metamorphism and the emplacement of synorogenic rapakivi granites, and melts from the metasedimentary pile probably contributed largely to the granites. The low pressures and high temperatures for regional metamorphism require geothermal gradients in excess of 60-degrees-C/km, and are consistent with the presence of regional extensional tectonics, synorogenic magmatism, and underplating of mafic magma in this area during the Ketilidian.