P-T paths and migmatite formation: An example from Deep Freeze Range, northern Victoria Land, Antarctica

Abstract

Migmatites exposed in the Deep Freeze Range, northern Victoria Land, Antarctica contain the mineral assemblage biotite + quartz + plagioclase + K-feldspar ± garnet ± cordierite ± sillimanite in mesosomes, with concordant and discordant leucosomes. Partial melting is considered the main migmatite-forming process at approximately 700–750 °C and 4–5 kbar. In mesosomes, garnet displays contrasting microstructural and compositional features. In some samples, spessartine-rich almandines record growth zoning in core regions. These garnets, together with cordierite and K-feldspar, grew along the main foliation at the expense of biotite and sillimanite. This textural evidence suggests a PT heating path due to regional low P-high T metamorphism linked to the Ross Orogeny. In other samples, spessartine-poor almandines are resorbed and overgrown with coronas consisting mainly of cordierite aligned along the main foliation. In these rocks, the matrix is defined by the association biotite + cordierite; relics of sillimanite are present in cordierite. These textural features are indicative of an early higher pressure assemblage (garnet + sillimanite) that was overprinted by a lower pressure assemblage containing cordierite as a stable phase. These samples follow an isothermal decompression path in P-T space. Data suggest two contrasting P-T paths which are interpreted as P-T trajectories of different crustal levels in the same tectonic setting that developed during the Cambro-Ordovician Ross Orogeny.