Hf Isotope Constraints on the Origin of the Bushveld Magmatic Province

English Abstract:

The Bushveld Magmatic Province (BMP) contains >30 individual mafic-ulramafic to felsic bodies that were intruded into the northern half of the Kaapvaal craton of South Africa and Zimbabwe, between 2.05 and 2.06 Ga. The initial Hf isotope compositions of zircon from the Bushveld Igneous Complex (BIC) and Phalaborwa Carbonatite Complex (PCC), two large bodies in the BMP, are unradiogenic, internally homogeneous, and similar to one another (εHf _{(2.06 Ga)} = -8.6 ± 2.6 and -7.5 ± 2.4 for the BIC and PCC, respectively). We have previously interpreted this to mean that the BIC and PCC parent magmas were produced by melting of a geochemically enriched source within the ancient subcontinental lithospheric mantle (SCLM). We are now determining the Hf isotope compositions of zircon by LA-ICP-MS from related rocks in the region to constrain the geographic distribution of this SCLM source and the extent to which it was involved in the generation of other bodies in the BMP. The Marble Hall diorite, a small intrusion situated near the BIC’s geographic center but whose structural relationship to the BIC remains unclear, yields a range of εHf _{(2.06 Ga)} from -1.3 ± 0.9 to -6.2 ± 1.9 (Avg. = -3.7 ± 2.8; n = 15). This more radiogenic composition relative to the BIC and PCC may indicate that the SCLM source of these intrusions is more heterogeneous than initially thought, or that the Marble Hall diorite was derived from a different source altogether. In the southeastern part of the BMP, the Schurwedraai alkali granite, a ~2.05 Ga peralkaline to peraluminous intrusion exposed in the northern collar of the Vredefort Dome, yields εHf _{(2.06 Ga)} from -18.6 ± 2.0 to -30.4 ± 3.0, (Avg. = -22.6 ± 6.4; n = 29). This unradiogenic composition is consistent with melting of Archean continental crust, suggesting that the heat source that induced melting of the SCLM to produce the BIC and PCC parent magmas also resulted in anatexis.