MICROSTRUCTURAL ANALYSIS OF THE MIGMATITES FROM THE FOSDICK DOME, WEST ANTARCTICA: A QUEST FOR ANATEXIS

Partial melting and migmatization are recognized as a primary means of physical and chemical differentiation of continental crust, leading to formation of granitoids. The physical mechanisms of melt formation and extraction are poorly understood, compared to the geochemical processes. Although it is generally recognized that migmatitic leucosome occurs in mesoscopic structural sites such as fold hinges, shear zones, and interboudin necks, there is less information about microscopic-scale textures in natural rocks representing the migmatite-to-granite transition.

This study examines 10 samples from the Fosdick gneiss dome, Antarctica, selected from the diverse migmatite domains that constitute the dome: depleted gneiss, metatexite, diatexite, and leucogranite networks. Thin sections are evaluated in terms of peak mineral assemblage, reaction textures, and microstructure. The aim of this study is to establish whether there are textures indicative of melt presence in each setting, to identify textural differences between domains, and to gain insight into the processes of melt formation, coalescence, and migration.

In depleted gneisses and metatexites, lobate quartz and subhedral cordierite overprint foliations defined by aligned biotite, and quartz microvein arrays crosscut compositional layering. Coarse-grained diatexites and leucogranites contain euhedral Kspar within an optically uniform quartz groundmass, and along Kspar-Kspar contacts, quartz is observed to form a low energy “string of beads” configuration. In some instances the optically uniform quartz, and coeval muscovite, extends into a honeycomb-like network overprinting the preexisting Kspar grain. We interpret biotite breakdown to be a consequence of melt forming reactions, and the well-formed euhedral Kspar grains to be a crystallization product from melt. The optically uniform quartz and muscovite overprinting the euhedral grains is in turn a melt texture, suggesting that some materials formed from partial melt were re-melted, possibly due to rapid decompression. Textures like the string of beads suggest that grain boundaries are important sites for the accumulation and migration of melt. These microstructures and melt textures present a means to understand the processes in effect at the migmatite/granite transition.