O-isotope Study of the Bushveld Complex Granites and Granophyres: Constraints on Source Composition, and Assimilation

The 2·05 Ma Bushveld magmatic event culminated in the production of >90 000 km3 of granite and granophyres. In these granitic rocks, high-temperature equilibrium O-isotope fractionations are generally preserved between quartz and zircon, but not between quartz and feldspar, or between biotite and amphibole. Quartz separated from four granite samples shows no significant difference in core and rim δ18O values, which indicates that quartz is not significantly zoned, and provides further evidence that it is unaffected by alteration. Quartz can, therefore, be used as a proxy for the magma δ18O value, leading to estimates of 6·9‰ for both the granites (assuming Δquartz–magma = 1·11‰) and granophyres (assuming Δquartz–magma = 0·62‰). Similar magma δ18O values (6·6‰) were obtained using zircon δ18O values, assuming Δzircon–magma = −1·3‰. The initial Nd-isotope ratio of the granitic rocks ranges from 0·509676 to 0·509822, with an average value of 0·509655 (n = 12). This corresponds to average εNd values of −5·9 and −4·8 for the granites and granophyres, respectively. The similarity in isotope composition between the granites and granophyres, and between the granitic rocks from each of the three major lobes of the Bushveld complex, is consistent with a common origin. The δ18O values of the granitic rocks suggest derivation from mantle-derived magmas by either fractional crystallization or partial melting, but this hypothesis is incompatible with their crustal εNd values (average −5·5). The associated Rustenburg Layered Suite (RLS) rocks have average δ18O values of 7·1‰, which is within error of the average estimate for the Bushveld granitic rocks, and similar εNd values. However, granitic magma derived from the same paretal magmas that produced the RLS would have had an average magma δ18O of about 7·9‰, 1‰ higher than observed. We therefore suggest that the granitic magmas were produced by fractional crystallization of RLS magma (or by partial melting of solidified RLS magma at depth) followed by assimilation, at a shallower level, of a significant quantity of hydrothermally altered low δ18O material from the since eroded volcanic edifice.