Residual melt extraction and out-of-sequence differentiation in the Bushveld Complex, South Africa

Abstract

There is strong evidence (e.g. presence of adcumulates) that the Rustenburg Layered Suite of the 2·06 Ga Bushveld Complex lost significant quantities of residual melt. However, the mechanisms of residual melt extraction during the solidification of its crystal mushes are unclear. Here, we present a petrological study of the Marikana dykes, a suite of ESE-WNW-trending dioritic pegmatites that crosscut gabbronorites of the Main Zone in the western Bushveld. The dykes comprise an evolved (and hydrous) mineral assemblage of zoned plagioclase laths (An60-25) and poikilitic Fe-rich hornblende (Mg# 40-18), mantled by biotite (Mg# 49), with magnetite and quartz inclusions. Primocryst margins are intergrown with a granitic (K-feldspar and quartz) groundmass that exhibits graphic textures. The Marikana dykes are cogenetic with the Rustenburg Layered Suite, because (1) the dykes lack chilled margins, (2) host Main Zone rocks are not thermally metamorphosed, (3) meterscale rounded blocks of gabbronorite in the dykes indicate entrainment of hot Main Zone cumulate material in the dykes, (4) U-Pb zircon ages (between 2040 and 2060 Ma) overlap with previously published Bushveld ages, corroborating the field relationships and indicating that the dykes and layered sequence are coeval, and (v) dyke zircon EHf values (-7·6 ± 0·9 at 2·06 Ga) demonstrate that the melt Hf isotopic signature is identical to that of the melts that fed the layered sequence. Primocrysts from the host Main Zone were locally entrained into the dykes [e.g. relict clinopyroxene (Mg# 57) in poikilitic hornblende and relict plagioclase cores (c. An70)]. High-An plagioclase xenocrystic cores in the dykes, and plagioclase laths in the host Main Zone, share a similar 87Sr/86Sr isotopic composition (0·7075-0·7076 at 2·06 Ga) consistent with recycling of local Main Zone primocrysts into the dykes. In contrast, low-An margins of plagioclase in the dykes record greater 87Sr/86Sr isotopic compositional variability (0·7073-0·7078 at 2·06 Ga) consistent with crystallization from an evolved and isotopically heterogeneous melt. Based on the occurrence of similar pegmatite veins, bulk REE compositions, and plagioclase 87Sr/86Sr isotope compositions, the evolved melts that are recorded in the Marikana dykes are residual liquids extracted from the underlying Upper Critical Zone. We further postulate, because the Main Zone was rigid enough to allow dyking, that parts of the Upper Critical Zone were either (1) emplaced and crystallized after consolidation of the Main Zone or (2) remained above its solidus (i.e. allowing retention of residual pore melt) for timescales longer than the emplacement and consolidation of the Main Zone. The Marikana dykes represent fossilized conduits that facilitated, during the solidification of crystal mushes in the Bushveld Complex, the extraction of differentiated residual melts. Our model is consistent with an out-of-sequence differentiation history in the Rustenburg Layered Suite.