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A chilled margin of komatiite and Mg-rich basaltic andesite in the western Bushveld Complex, South Africa

Maier, Wolfgang D., Barnes, Sarah-Jane and Karykowski, Bartosz 2016. A chilled margin of komatiite and Mg-rich basaltic andesite in the western Bushveld Complex, South Africa. Contributions to Mineralogy and Petrology 171 (6) , 57. 10.1007/s00410-016-1257-5

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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.

Item Type: Article
Date Type: Publication
Status: Published
Schools: Earth and Ocean Sciences
Subjects: Q Science > QE Geology
Uncontrolled Keywords: Komatiite · Basaltic andesite · Bushveld Complex · Chilled margin · Subcontinental lithospheric mantle
Publisher: SpringerLink
ISSN: 0010-7999
Date of First Compliant Deposit: 14 June 2016
Date of Acceptance: 28 March 2016
Last Modified: 22 Feb 2019 22:36
URI: http://orca.cf.ac.uk/id/eprint/91747

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