CHARACTERIZING MELT TEXTURES AND ACCESSORY MINERAL DISTRIBUTION IN MIGMATITES FROM THE LARSEMANN HILLS, EAST ANTARCTICA

The extraction of melts from the lower crust and their subsequent emplacement at higher structural levels is an important differentiation mechanism that leads to the transfer of major and trace elements, volatiles and heat through the crust. Residual migmatites and granulites exposed at the Earth’s surface represent former melt-bearing crust and preserve evidence for melt transport on the micro- and meso-scale. The study of accessory phases, in particular their stabilities in melt-bearing systems and interactions with major minerals, is key to understanding the fluxes of trace and heat-producing elements during crustal anatexis.

The Larsemann Hills of Prydz Bay, east Antarctica provide excellent 2-D exposure of granulite facies (c. 7 kbar and 800 °C) metasedimentary rocks and leucognesisses that preserve evidence for in situ partial melting, melt mobilization and connectivity. Leucosome found here contains K-feldspar, quartz, plagioclase, and minor garnet and biotite. Domains of residuum include: a) garnet (with aligned acicular sillimanite inclusions)-cordierite-spinel-ilmenite melanosome, the inferred assemblage remaining after melting and the extraction of some or all of the melt fraction; and b) coarse-grained garnet-sillimanite-cordierite ± biotite selvage zones that formed during melt-wall rock interaction.

Petrographic analysis of melanosome, selvage, and leucosome domains has characterized mineral assemblages, structural fabrics and/or reaction textures, in particular the textural context of accessory phases (zircon, monazite, apatite) that are found as inclusions in and along grain boundaries of major minerals (e.g. garnet). Detailed trace element characterization of major and accessory phases will be integrated with in situ U-Pb geochronology to provide petrologic and age constraints on the breakdown, growth or transport (as xenocrysts) of trace-element carriers in the melt system. The results from this study will allow quantification of trace element budgets during partial melting, importantly those still residing in residual granulites, and thereby contribute further to our understanding of chemical differentiation of the continental crust.