KEYNOTE ADDRESS: THE IDENTIFICATION OF MELT AND THE PATH OF MELT MOVEMENT IN CRUSTAL ROCKS

Microstructures that indicate the former presence of melt are well known from rapidly-cooled rocks (e.g. contact aureoles, plutons), but only recently have been found widely preserved in slowly-cooled, regional metamorphic terranes. A variety of microstructures are preserved, but those in melt-poor, residual rocks are different from those in melt-rich (leucosome or diatexite migmatite) rocks. Residual rocks are dominated by microstructures formed during partial melting, and those specific to the reactant phases are; 1) rounded or corroded grains and, 2) cuspate pores between corroded grains. Microstructures involving the products of melting reactions include; 1) rational faces on the solid product (peritectic) crystals which happened to grow into melt-filled pores; and for the melt; 1) thin, quartz or feldspar films along grain boundaries, 2) overgrowths on residual phases that results in a blocky outline to the melt-filled pores and, 3) an interstitial, pore-filling shape for the last-crystallised minerals. In contrast, the microstructures in melt-rich rocks formed during crystallisation; those that formed throughout the crystallisation interval include; 1) minerals with crystal faces (euhedral or subhedral) against later-crystallised phases, 2) melt inclusions, 3) magmatic foliations, 4) crystal framework-structures, whereas microstructures formed near the solidus include; 5) interstitial shapes for the last minerals to crystallise and, 6) reaction rims and symplectites. The path taken by melt as it moves through the crust can be divided into three segments. 1) A porous flow regime, in which melt moves along grain boundaries from where it formed, to low pressure sites. 2) A leucosome stage, in which melt flow in channels becomes progressively more focussed in the source. It starts with flow in arrays of narrow (mm scale), diffuse microleucosomes, but as these link together, flow is progressively through fewer, but wider, discrete channels, these are the macroscopic leucosomes in anatectic rocks. In the final, or transfer stage, the movement of melt is through large discordant fractures (veins or dykes) which enable the melt to transfer elsewhere in the crust.