Geochemistry of mafic and ultramafic igneous rocks of the Matsitama supracrustal belt, northeastern Botswana; provenance implications

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Authors:Majaule, T.; Hall, R. P.; Hughes, D. J.
Author Affiliations:Primary:
Botswana Geological Survey, Lobatse, Botswana
University of Portsmouth, United Kingdom
Volume Title:South African Journal of Geology
Source:South African Journal of Geology, 100(2), p.169-179. Publisher: Bureau for Scientific Publications at the Foundation for Education, Science and Technology, Pretoria, South Africa. ISSN: 1012-0750
Publication Date:1997
Note:In English with Afrikaans summary. 52 refs.; illus., incl. 1 table, geol. sketch map
Summary:The Matsitama supracrustal belt (MSB) of northeastern Botswana occurs at the southwestern extremity of the Zimbabwe Craton. It comprises four composite lithostratigraphic associations which are dominated by shallow-water clastic and chemical metasediments. Mafic metavolcanics and hypabyssal intrusive are subordinate and are mainly restricted to one association. Ultramafic metavolcanic schists, serpentinized peridotites, and intermediate lithologies are minor components. However, acid volcanics are absent and the belt is thus lithologically anomalous with respect to the majority of the greenstone belts in the Zimbabwe Craton. The whole supracrustal assemblage has been metamorphosed at greenschist facies, locally attaining low amphibolite grade. It is associated with an originally intrusive tonalite-trondhjemite-granodiorite orthogneiss suite (the Jankie gneisses) which locally contains intercalated amphibolite facies paragneiss horizons and is intruded by K-feldspar-rich equigranular to megacrystic granitoids. The MSB metabasites are tholeiitic and show many geochemical affinities with modern mid-ocean ridge and island arc basalts. However, they exhibit a minor negative Nb anomaly, and large-ion lithophile and slight light rare-earth element enrichment, suggestive of a crustal contamination component, such that in some respects they resemble a continental tholeiite suite. Minor geochemical differences, mainly among the (mobile) large-ion lithophile elements, indicate that the metavolcanic and metadolerite suites are possibly not simply petrogenetically related; otherwise their geochemical signatures are very similar. The ultramafic schists are interpreted as basaltic komatiites which appear not to be related petrogenetically to the metabasites. The MSB probably formed in a back-arc basin during extensional attenuation and rifting of Archaean continental crust at the margin of the Zimbabwe Craton.
Subjects:Back-arc basins; Basalts; Basins; Chemical composition; Facies; Gneisses; Greenschist facies; Greenstone; Igneous rocks; Mafic composition; Major elements; Metamorphic rocks; Metamorphism; Mid-ocean ridge basalts; Mineral assemblages; Provenance; Schists; Supracrustals; Trace elements; Ultramafic composition; Volcanic rocks; Africa; Botswana; Southern Africa; Zimbabwe Craton; Matsitama Belt
Coordinates:S211000 S205000 E0270000 E0263000
Abstract Numbers:98M/544
Record ID:1997068293
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute.
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