Transition to Migmatite and Peraluminous Granite in the Cooma Complex, SE Australia

Vernon, R.H. and Johnson, S.E.

School of Earth Sciences, Macquarie University, Sydney NSW 2109, Australia

In the low-P Cooma Complex, SE Australia, the highest-grade, non-migmatitic rocks are gneisses and granofelses of the andalusite-K-feldspar zone. The first indication of partial melting is patchy leucosome in metapelites. At slightly higher grade, these grade into veinlets, which are crenulated by, and so presumably pre-date, the main foliation, S3. Some leucosome is localized on metapelite-metapsammite contacts. In the migmatite zone, prominent lenses and veins of white leucosome are oblique to S3, and we interpret them tentatively as having formed by fracturing, together with incipient boudinage of the strong metapelitic beds (in contrast with ductile metpsammites), providing low-pressure sites for leucosome segregation. Fracturing may have been necessitated by difficulty of slip on folia anastomosing between large, strong grains of K-feldspar, andalusite and cordierite.

As the amount of partial melting increases, the migmatites become coarser-grained and more stromatic, though remaining essentially "bedded migmatites", with melting largely confined to metapelitic beds. They are commonly folded and boudinaged by D5, and leucosomes occur in several S surfaces, suggesting that melting temperatures were reached relatively early in the deformation history. Locally, granite patches occur in the highest-grade migmatites, typically with scattered metasedimentary xenoliths and migmatite remnants, resembling the Cooma Granodiorite. They show only the latest foliation (S5), as does the Cooma Granodiorite. These observations suggest that the Cooma Granodiorite was formed in a similar situation, involving more extensive coalescence of leucosome and migration of parts of the resulting magma, locally preserving gradational contacts into migmatite.

Most of the leucosomes consist only of quartz and feldspar, though porphyroblasts of cordierite occur locally. Melanosomes are rare, suggesting that the leucosomes have moved away from sites of initial segregation. In contrast, the patchy leucosomes are probably in contact with residual mafic minerals. The absence of muscovite from non-migmatitic rocks of the cordierite-K-feldspar and andalusite-K-feldspar zones, indicates that muscovite dehydration did not contribute to the leucosomes. Alternative sources of melt are biotite breakdown and water-saturated melting. Abundant water in the high-grade cordierite at Cooma (S.L. Harley, pers. comm.) suggests that the associated melt was nearly water-saturated (5.5. wt % water). This is consistent with introduction of external water, promoting melting. Reduction in the melting temperature could have been assisted by the presence of boron in the fluids, which is supported by the local abundance of tourmaline in the leucosomes and pegmatitic patches. This could explain the presence of andalusite in some leucosomes. However, biotite breakdown melting is also a possibility at the prevailing temperature and pressure, and is supported by the local occurrence of overgrowths of inclusion-free cordierite in some leucosomes. Because water-saturated melting and biotite breakdown, as well as the transformation of andalusite to sillimanite, may occur at practically the same temperature and pressure, a prograde history is difficult to infer with confidence.

Back to the GEMOC Abstract Titles Page