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

Most published interpretations infer that the Cooma Granodiorite (southeast ern Australia:) was formed by more or less in situ melting of metasedimenta ry rocks of the Cooma Complex. Detailed work has shown that melting of meta pelites, which occurred by biotite breakdown during D3 (after muscovite had disappeared),, produced relatively immobile, plagioclase-poor or plagiocla se-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. (C) 2001 Elsevier Science Ltd . Al rights reserved.