THE ROLE OF MINERAL TEXTURES ON MIGMATITE FORMATION

Migmatites are generally accepted as the products of partial melting in the crust. The most commonly inferred melting steps are dehydration melting reactions in which a reaction involving a hydrous reactant is crossed resulting in a number of anhydrous solid products and hydrous silicate melt. The mineral textures will influence and reflect the melting process involved. Although most leucosomes have been inferred to have developed via the intergranular migration of melt to local low mean pressure sites, many others show melt-mineral relationships consistent with focussed melt formation around either a specific reactant or product of a melting reaction. In this contribution the role of mineral textures on the spatial development of leucosomes in two rocks is discussed. These complex mineral-melt relationships have implications for the segregation of melt, and its subsequent migration, in crustal rocks.

The first example involves the preferential development of leucosomes around aluminosilicate porphyroblasts in low-pressure rocks from Mt Stafford in Central Australia. Melt production was primarily via two narrow trivariant reactions in NCKMASHTO. The melt-producing reaction is spatially focussed around large aluminosilicate porphyroblasts that grew during the subsolidus evolution of the rock. The second example involves the development of leucosome networks around large garnet porphyroblasts in a felsic gneiss from Broken Hill, Australia. The garnet porphyroblasts are the product of a high variance biotite-breakdown melting reaction that extends as a field close to 50°C wide on calculated pseudosections. The focussing of the reaction around the garnet porphyroblasts reflects the poor ability for the garnet to nucleate. If diffusion rates were sufficiently fast, the strongly partitioned nature of the rock may still represent chemical equilibrium between the leucosome and host

In both the examples the melt formed in cm-scale segregations negating the need for migration of diffuse melt along intergranular boundaries to leucosome sites. These segregations formed a complex interconnected melt network. This network allowed the efficient draining of melt from the area and the preservation of high-grade mineral assemblages with little retrogression.