Oxygen and Hydrogen Isotope Stratigraphy of the Rustenburg Layered Suite, Bushveld Complex: Constraints on Crustal Contamination

Abstract

New δ18O values for plagioclase, pyroxene and olivine, and limited whole-rock δD values are presented for samples from the Rustenburg Layered Suite of the Bushveld Complex, South Africa. In combination with existing data, these provide a much more complete composite O-isotope stratigraphy for the intrusion. Throughout the layered suite, mineral δ18O values indicate that the magmas from which they crystallized had δ18O values that were about 7·1‰, that is, 1·4‰ higher than expected for mantle-derived magmas, suggesting extensive crustal contamination. More limited H-isotope data suggest that the OH present within whole rocks, regardless of the degree of alteration, is of magmatic origin and not an alteration phenomenon. There appears to be no systematic change in δ18O value with stratigraphic height and this requires the contamination to have taken place in a ‘staging chamber’ before emplacement of the magma(s) into the present chamber. Large amounts (30–40%) of contamination by the lower to middle crust are needed to explain these δ18O values, which is in general agreement with previous estimates based on Sr- and Nd-isotope data. Alternatively, smaller amounts of contamination (∼20%) by sedimentary rocks, or their partial melts, represented by the country rock can explain the data, but it is not apparent how such material could have been present at the depth of the ‘staging chamber’ in the lower to middle crust.