CONTROLS ON MELT SEGREGATION AND MIGRATION: EVIDENCE FROM MIGMATITES BENEATH THE BUSHVELD COMPLEX, SOUTH AFRICA

Anatectic migmatites are developed within pelites at the base of the ~8 km-thick Rustenburg Layered Suite (RLS) in the Eastern Bushveld Complex. Outcrops preserving the earliest stages of partial-melting contain stromatic and discordant leucosomes. Laterally continuous stromatic veins formed by H₂O-rich volatile phase-present melting at ~650 ^oC and ~3 kbar. Subsidence of the central part of the RLS gave rise to syn-anatectic deformation that combined pure-shear flattening with lateral expulsion of the floor rocks. Strain was strongly partitioned into the ductile melt-bearing pelites. This is shown by deformed stromatic veins and the location of leucosome in bedding-discordant dilational sites, principally around anticlinal closures and within a developing schistosity, which is inferred to record melt flow down gradients in melt pressure generated by the deformation. Continued deformation caused ptygmatic folding of residual stromatic leucosomes and the development of a strong fibrolite schistosity. Discordant veins are discontinuous, with a coarse grain-size and no observed plastic strain. They have a tourmaline-rich granitic composition, which supports the conclusion that they represent the frozen melt-fraction expelled from residual stromatic leucosomes. These melts are inferred to have remained largely immobile due to the negative dP/dT of the melting equilibria. Closer to the RLS contact, larger volumes of garnet-bearing leucosome reflect melting at T>720 ^oC by volatile phase-absent reactions consuming biotite. The positive dP/dT of these equilibria allowed effective segregation and migration of melts to higher levels. Calculations using a conductive model suggest that H₂O-undersaturated melts had a maximum residence time of ~0.2 Ma before crystallising. Garnet-bearing leucosomes are undeformed, placing a similar time constraint on the duration of major subsidence of the RLS. Microstructures inferred to record reactions suggest pervasive high-T rehydration of the migmatites by incursion of H₂O derived from subsolidus dehydration of successively deeper metasedimentary rocks. The influx of H₂O potentially re-mobilised low-T leucosomes. Such fluid-saturated melts may have remained largely in situ for up to ~0.5 Ma before solidification.