Sr Isotope and Trace Element Studies of the Great Dyke and Bushveld Mafic Phase and their Relation to Early Proterozoic Magma Genesis in Southern Africa
New Rb-Sr and trace element data are reported for the Great Dyke and Bushveld Mafic Phase layered intrusions. It is argued that geochemical characteristics, such as 87Sr/86Sr ratios and R.E.E. distribution patterns have been little modified by crustal contamination. Rb-Sr data for whole-rocks of the Great Dyke yield an age of 2514±16 m.y. and an initial 87Sr/86Sr ratio of 0.70261±4. Mineral data are consistent with these results. The low errors on the results indicate no significant variation of 87Sr/86Sr ratios of successive magmatic influxes emplaced in different magma chambers. Earlier Great Dyke magmas were highly Mg-rich and represent extensive partial melts of the source material. One such influx is shown to have a high Rb/Sr ratio (∼0.25) and a fractionated R.E.E. pattern (CeN/YBN∼ 12). These ratios are considered to approximate those of the source region. The Bushveld Mafic Phase has been dated accurately for the first time and has a Rb-Sr age of 2095±24 m.y. Initial 87Sr/86Sr ratios increase in a stepwise manner upwards in the intrusion from 0.70563±2 to 0.70769±6. Each increase is abrupt and occurs at a horizon also characterized by a sudden irregularity in cryptic variation. The Mafic Phase was emplaced as a succession of magmatic influxes each of which had higher 87Sr/86Sr ratio than its predecessor. The first magma was both Mg-rich (MgO ≥ 21.5 per cent) and SiO2-rich (50–55 per cent SiO2) and was derived by extensive partial melting of a shallow level upper mantle source. This source was characterized by trace element abundance ratios (e.g. Rb/Sr ∼ 0.25; K/Rb ∼ 90; CeN/YbN ∼ 11), similar to those of kimberlites and some potassic lavas and comparable with those deduced for the Great Dyke source region. It is postulated that when the Rhodesian and Kaapvaal cratons stabilized, underlying refractory mantle became fixed thereto to form a proto-lithosphere. Shortly afterwards, at about 2800 m.y. ago, this proto-lithospheric mantle was enriched by passage through it of fluids with kimberlitic trace element chemistry. This sub-cratonic mantle thereafter evolved with a relatively high Rb/Sr ratio. Magmas derived from it have anomalous chemical characteristics with respect to those of ocean-floor basalts, reflecting major differences in the evolution of their respective source regions.