Melt Generation and Fluid Flow in the Thermal Aureole of the Bushveld Complex

Abstract

Granite sheets emplaced into the migmatite zone of the eastern contact aureole of the Bushveld Complex resulted from fluid-enhanced, incongruent biotite melting of the underlying Silverton Formation shales during prograde metamorphism. Ba concentrations are extreme in both the sheets (>1000 ppm) and the hornfels (>800 ppm) into which they have been emplaced. We conclude that a Ba-rich, hydrothermal fluid induced melting in the aureole, and that fluid transport of Ba2+, and to a lesser extent, Sr2+ and Eu2+, persisted in the melt zones under subsolidus conditions. Sr-isotope systematics from high-Ba localities define an errorchron of 2161 ± 106 Ma with an initial (87Sr/86Sr) ratio of 0·705 ± 0·001. Metasedimentary rocks unaffected by fluid infiltration were homogenized at the same time but with an increased initial ratio, suggesting that whereas isotope homogenization was achieved between outcrops permeated by fluids, there is no evidence of regional homogenization. Oxygen-isotope compositions of psammitic metasediments in the aureole are uncorrelated with distance from the contact, suggesting the infiltrating fluid equilibrated isotopically with the metasediments. Their elevated δ18O values (11·3–12·1‰) are consistent with a fluid source from devolatilization of the sedimentary lithologies. Textural analysis at both outcrop and thin-section scale of arkose and psammites in the aureole (Lakenvalei and Magaliesberg Formations) shows that the extent of melting was highly heterogeneous, even on the grain scale, and resulted from heterogeneously distributed infiltration of aqueous fluid on dilatant cracks and grain boundaries. Cathodoluminescence imaging of quartz shows a marked difference in the amount of fine structure, with that in the melted rocks having uniform luminescence in contrast to that in rocks containing little or no melt, which preserve textures inherited from the regionally metamorphosed protolith. Simple finite-difference thermal modelling of the aureole suggests that the width of the melt zone (>500 m) is inconsistent with conductive heat transfer, and hence that the thermal structure has been modified by fluid advection.