ANATEXIS OF METAPELITIC ROCKS UNDER THE BUSHVELD COMPLEX, SOUTH AFRICA

The aureole beneath the Bushveld Complex is characterised by an inverted thermal structure that enables investigation of volatile-phase influx into a zone of melting. Migmatised metapelitic rocks contain leucosomes that display a range of spatial and temporal relationships with penecontemporaneous deformation structures. At the outer edge of the migmatite front, metatexites contain two discrete leucosome types: thin (~10 mm wide) subhorizontal stroma and larger, coarser-grained subvertical veins that are commonly parallel to a variably inclined foliation. Phase relations modelled in the NCKFMASH system are consistent with field and petrographic observations. In situ volatile phase-absent melting produced small quantities (< 1 mol. %) of peritectic cordierite-bearing melts that remained trapped within the matrix. A larger volume of melt (3-4 mol. %) formed locally following a structurally-controlled influx of a small quantity (< 1 mol %) of H₂O-rich volatile phase derived from subsolidus dehydration of underlying rocks. The increased melt fraction enabled limited segregation. Thin fibrolite seams record passage of melt (± volatiles) through the system as it flowed down gradients in melt pressure. Seams at a low angle to bedding provide evidence for compaction-driven subhorizontal melt flow from metapelite to metapsammite layers and stromatic leucosome formation. Locally, buoyancy-driven upward movement of melt, recorded by subvertical seams, occurred along an axial planar schistosity. Coarse discordant leucosomes record the sites of melt crystallisation. Softening due to the presence of melt resulted in an intensification of subvertical structures and feedback between volatile influx, melting and deformation. Granulite-facies conditions (> 725 ^oC, ~3 kbar) are recorded closer to the contact with the ultramafic rocks, where coarse-grained leucosome-poor garnet-cordierite granofels occurs. The equant form and modal abundance of garnet imply that most (> 90 %) of the 10-15 mol. % melt was lost from the equilibration volume. At the contact, leuco-diatexites are largely devoid of peritectic phases, suggesting effective segregation of melt from an underlying source.