PARTIAL MELTING DURING TECTONIC EXHUMATION OF A GRANULITE TERRANE - AN EXAMPLE FROM THE LARSEMANN HILLS, EAST ANTARCTICA

Anatectic migmatites in medium- to low-pressure granulite facies metas ediments exposed in the Larsemann Hills, East Antarctica, contain leuc osomes with abundant quartz and plagioclase and minor interstitial K-f eldspar, and assemblages of garnet-cordierite-spinel-ilmenite-silliman ite. Qualitative modelling in the system K2O-FeO-MgO-Al2O3-SiO2-H2O-Ti O2-O-2, in conjunction with various P-T calculations indicate that the high-grade retrograde evolution of the terrane was dominated by decom pression from peak conditions of c. 7 kbar at c. 800 degrees C to 4-5 kbar at c. 750 degrees C. Extensive partial melting during decompressi on involved the replacement of biotite by the assemblage cordierite-ga rnet-spinel within the leucosomes. These leucosomes represent the site of partial melt generation, the cordierite-garnet-spinel-ilmenite ass emblage representing the solid products and excess reactants from the melting reaction. The extraction and accumulation of this decompressio n-generated melt led to the formation of syntectonic pegmatites and ex tensive granitic plutons. Leucosome development and terrane decompress ion proceeded during crustal transpression, synchronous with upper cru stal extension, during a progressive Early Palaeozoic collisional even t. Subsequent retrograde evolution was characterized by cooling, as in dicated by the growth of biotite replacing spinel and garnet, thin man tles of cordierite replacing spinel and quartz within metapelites, and garnet replacing orthopyroxene and hornblende within metabasites. P-T calculations on late mylonites indicate lower grade conditions of for mation of c. 3.5 kbar at c. 650 degrees C, consistent with the develop ment of late cooling textures.