Partial melting of metapelitic rocks beneath the Bushveld Complex, South Africa

Abstract

Metapelitic rocks in the aureole beneath the Bushveld Complex preserve evidence for both high- and low-aH2O anatexis. The aureole is characterized by an inverted thermal structure in which suprasolidus rocks potentially interacted with an H2O-rich volatile phase derived from underlying, dehydrating rocks. At lower grade (T < 700°C) the rocks contain fibrolite mats and seams that record local redistribution of volatiles. Incongruent reactions consuming biotite produced small quantities (< 1 mol %) of liquid and peritectic cordierite that remained trapped within the mesosome. Larger volumes of melt (3-4%); preserved as coarse-grained discordant leucosomes, were produced by congruent melting following a structurally focused influx of H2O. Subhorizontal volatile-phase flow was concentrated within thin (∼ 10 mm) metapsammite horizons that are preserved as stromatic quartz-sillimanite veins. Upward migration occurred along steep fibrolite seams that are subparallel to a variably inclined foliation. Discordant leucosomes are concentrated within antiformal fold closures of quartz-sillimanite veins and along the axial planar schistosily. Closer to the contact (T > 725°C), volatile-phase-absent, biolite-consuming melting and melt extraction produced coarse-grained garnet-cordierite granofels. At the contact, leucodiatexites devoid of peritectic phases suggest effective segregation of melt from an underlying source. Migmatitic metapelites and their lower-grade stratigraphic equivalents have similar bulk-rock oxygen isotope values, consistent with very limited volatile-phase infiltration and precluding the Bushveld Complex magmas as the source of the volatiles.