In Situ Sr isotopes in Plagioclase and Trace Element Systematics in the Lowest Part of the Eastern Bushveld Complex: Dynamic Processes in an Evolving Magma Chamber

The Bushveld Complex is the largest mafic layered intrusion on Earth, containing immense mineral resources. Despite its importance there remains uncertainty as to its origin and the source of the magmas that formed it. The lower Bushveld series, made up of the Basal Ultramafic Sequence (BUS), the Marginal Zone and Lower Zone, is of particular importance because it allows detailed insight into the reservoirs that multiply contributed to the formation and modification of the initial magmas. This sequence is represented by three overlapping drill cores (2100m of stratigraphy) in the eastern Bushveld Complex for which we present the first comprehensive study on intercumulus mineral assemblages, incompatible trace elements and in situ Sr isotopes in plagioclase from over 130 core samples for this section (Clapham area). The intercumulus mineral assemblage that crystallized from the trapped melt comprises plagioclase, K-feldspar, biotite and quartz and a wide variety of accessory phases, including zircon, loveringite and primary magmatic anhydrite, the first time the last mineral has been reported in the layered sequence of the Bushveld Complex. The incompatible trace elements (ITE) show a gradual decrease in concentration upwards through the stratigraphic section, reflecting mainly the decreasing trapped melt component in the cumulates as the chamber became established. ITE ratios and changes in initial Sr-87/Sr-86 [referred to as (Sr-87/Sr-86)(i) at 2055 Ma] give insight into the various mantle sources and the contaminants, which were derived from both the lower crust and the country-rock sediments of the Pretoria Group (upper crust). (Sr-87/Sr-86)(i) ranges from 0.7042 to 0.7076, with the lowest values associated with the most ultramafic rock units in the BUS and the highest in Marginal Zone norites that formed from evolved magma at the top of the early chamber prior to ingression of Lower Zone magma. The evolved magma was affected by significant assimilation of sediments as evidenced by partly digested (now restitic) metapelite xenoliths, particularly in the upper part of the Marginal Zone. Rare earth element (REE) patterns combined with modelling of trace elements and (Sr-87/Sr-86)(i) confirm the strong crustal contamination derived from both the lower crust of the Kaapvaal Craton and the enclosing sediments. The Lower Zone shows a progressively increasing mantle signature upwards in the sequence with both flat and steep light REE (LREE) patterns indicating primitive (but still crustally contaminated) and more highly contaminated melts. However, the relatively low Mg# for orthopyroxene and olivine (maximum Mg# 0.88) in the Lower Zone requires a melt contribution of lower Mg# than would be associated with an asthenosphere mantle source and at the same time having an LREE-depleted composition. Such a source is indicative of subcontinental lithospheric mantle (SCLM) and a progressively greater melt component of an eclogitic protolith contributing to the parental magmas. The source of heat for this massive, but short-lived, melting event is likely to have been a mantle plume, which gave rise to the initial and most primitive (asthenospheric origin) magmas of the BUS but with subsequent melting of an SCLM source. Both magmas were subject to crustal contamination from the lower crust and the enclosing sediments. The results of this study are used to quantify the roles of the various Bushveld reservoirs including the previously proposed metasomatized SCLM, most probably the source of the PGE and Cr that must have been present.