Geochemistry of the Archean Bulawayan Group, Midlands greenstone belt, Rhodesia

Kent C. Condie*, N. M. Harrison

*Corresponding author for this work

Research output: Contribution to journalArticle

65 Citations (Scopus)

Abstract

The Bulawayan Group in the Midlands greenstone belt can be divided into three formations. The Mafic Formation is composed principally of pillowed, low-K tholeiites and minor bedded chert. The Maliyami Formation and conformably overlying Felsic Formation are composed of calc-alkaline tholeiites, andesites, and dacites with andesites dominating in the Felsic Formation. Minor rhyolite quartz porphyries and ultramafic bodies also occur in the section. The Bulawayan Group near Que Que is perhaps the least altered and metamorphosed Archean greenstone succession known. The absence of andesite and related rocks, the association of bedded chert, and the consistently low K2O, Rb, and Sr contents of Mafic Formation tholeiites suggest that they represent Archean oceanic rise tholeiites. The compositions of tholeiites and andesites of the Maliyami Formation, however, suggest that they represent an emerging arc system. The Felsic Formation is interpreted as a more advanced stage in the evolution of this arc system. Trace-element model calculations favor an origin for Mafic Formation tholeiites involving about 30% partial melting of a lherzolite source. Similar calculations are consistent with an origin for Maliyami Formation tholeiites, Maliyami and Felsic Formation andesites, and Midlands rhyolites involving, respectively, 50, 20-30, and 10% equilibrium melting of eclogite or garnet amphibolite (of Mafic Formation tholeiite composition). The low K2O, Rb, and Sr contents of Mafic Formation tholeiites suggest that they were derived from an upper mantle source as depleted in these elements as the oceanic upper mantle is today. A plate tectonic model is proposed for the Bulawayan Group in which the Mafic Formation is derived from a depleted lherzolite source beneath a spreading center in a marginalsea basin and the Maliyami and Felsic Formations and associated rhyolites are produced by partial melting of eclogite in a descending slab located west of the basin.

Original languageEnglish
Pages (from-to)253-271
Number of pages19
JournalPrecambrian Research
Volume3
Issue number3
DOIs
Publication statusPublished - 1976
Externally publishedYes

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