Abstract
A chill sequence at the base of the Lower Zone of the western Bushveld Complex at Union Section, South Africa, contains aphanitic Mg-rich basaltic andesite and spinifex-textured komatiite. The basaltic andesite has an average composition of 15.2 % MgO, 52.8 % SiO2, 1205 ppm Cr, and 361 ppm Ni, whereas the komatiite has 18.7 % MgO, 1515 ppm Cr, and 410 ppm Ni. Both rock types have very low concentrations of immobile incompatible elements (0.14–0.72 ppm Nb, 7–31 ppm Zr, 0.34–0.69 ppm Th, 0.23–0.27 wt% TiO2), but high PGE contents (19–23 ppb Pt, 15–16 ppb Pd) and Pt/Pd ratios (Pt/Pd 1.4). Strontium and S isotopes show enriched signatures relative to most other Lower Zone rocks. The rocks could represent a ~20 % partial melt of subcontinental lithospheric mantle. This would match the PGE content of the rocks. However, this model is inconsistent with the high SiO2, Fe, and Na2O contents and, in particular, the low K2O, Zr, Hf, Nb, Ta, Th, LREE, Rb, and Ba contents of the rocks. Alternatively, the chills could represent a komatiitic magma derived from the asthenosphere that underwent assimilation of the quartzitic floor accompanied by crystallization of olivine and chromite. This model is consistent with the lithophile elements and the elevated Sr and S isotopic signatures of the rocks. However, in order to account for the high Pt and Pd contents of the magma, the mantle must have been twice as rich in PGE as the current estimate for PUM, possibly due to a component of incompletely equilibrated late veneer.
Highlights
The Bushveld Complex is the product of what was one of the world’s largest magmatic events, comprising the largest layered mafic–ultramafic intrusion globally plus the ~150 × 100 km Molopo Farms Complex in Botswana, a number of mafic–ultramafic complexes located up to >100 km to the S and SW of the Bushveld Complex (de Waal et al 2006), the Phalaborwa carbonatite located >100 km to the east of the Bushveld (Reischmann 1995), and several other small intrusions (Rajesh et al 2013, and references therein)
We present compositional data from a relatively unevolved chill sequence which overlies quartzite at Union Section in the western Bushveld Complex (Fig. 1). We evaluate how these magmas are related to other examples of Bushveld marginal rocks and what their compositions tell us about the mantle source of the Bushveld magmatism
The base of the chill sequence has been injected by a microsill and small Ca–Al veins, demonstrating that the interpretation of chilled margins of intrusions requires detailed examination
Summary
The Bushveld Complex is the product of what was one of the world’s largest magmatic events, comprising the largest layered mafic–ultramafic intrusion globally (the 400 × 300 km Bushveld Complex) plus the ~150 × 100 km Molopo Farms Complex in Botswana (von Gruenewaldt et al 1989; Reichhardt 1994), a number of mafic–ultramafic complexes located up to >100 km to the S and SW of the Bushveld Complex (de Waal et al 2006), the Phalaborwa carbonatite located >100 km to the east of the Bushveld (Reischmann 1995), and several other small intrusions (Rajesh et al 2013, and references therein) Associated with these mafic rocks is the largest A-type granite on Earth (the Lebowa Granite Suite, Walraven and Hattingh 1993) and a vast rhyolitic lava flow field (the Rooiberg felsites) whose remnants are preserved over 800 km in EW direction and 300 km in NS direction (Twist and French 1983). We present compositional data from a relatively unevolved chill sequence which overlies quartzite at Union Section in the western Bushveld Complex (Fig. 1) We evaluate how these magmas are related to other examples of Bushveld marginal rocks and what their compositions tell us about the mantle source of the Bushveld magmatism
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