Transition from gneiss to migmatite and the relationship of leucosome to peraluminous granodiorite, Cooma Complex

R. Vernon, and S. Johnson

In the Cooma Complex, SE Australia, leucosomes first appear as small patches and veinlets in high-grade, muscovite-free gneisses containing cordierite, andalusite and K-feldspar. Simultaneously, fibrous sillimanite appears in discontinuous folia. The leucosomes consist of quartz, microperthitic K-feldspar and cordierite, rarely with minor andalusite or biotite. Plagioclase is absent, apart from exsolution lamellae in the K-feldspar. Breakdown of biotite probably produced the leucosomes. The leucosomes are largely confined to the metapelitic beds, which are plagioclase-poor; this explains the calcium-poor composition of the leucosomes. Most of the melting occurred during D3, which is responsible for most of the macroscopic folding in the area, though some leucosomes may predate D3. The metapelite leucosome is compositionally unsuitable as a source of the Cooma Granodiorite magma. Leucosome rich in plagioclase, which could be a source for the Cooma Granodiorite magma, was produced later (probably early during D5) by partial melting of quartzofeldspathic metapsammitic rocks. It intrudes and disaggregates the metapelitic leucosomes, confirming that melting of the local metapelites did not produce the Cooma Granodiorite.