Migmatite-granite relationships: origin of the Cooma Granodiorite magma, Lachlan Fold Belt, Australia

Most published interpretations infer that the Cooma Granodiorite (southeastern Australia) was formed by more or less in situ melting of metasedimentary rocks of the Cooma Complex. Detailed work has shown that melting of metapelites, which occurred by biotite breakdown during D3 (after muscovite had disappeared), produced relatively immobile, plagioclase-poor or plagioclase-free leucosomes that are compositionally unsuitable as the source magma for the granodiorite. However, melting of feldspathic metapsammitic rocks, which occurred during D5, as P-T conditions followed an anticlockwise path, produced mobile, plagioclase-rich leucosomes that are more appropriate for the granodiorite source magma. Though gradations from metapsammite-derived migmatite to Cooma Granodiorite are present, accumulation of magma derived locally from metapsammite melting appears to be unable to account for all of the exposed body of Cooma Granodiorite, implying some ascent of similar magma from deeper levels of the source rocks.