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

The Marsitama 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 intrusives 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 are 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-are basin during extensional attenuation and rifting of Archaean continental crust at the margin of the Zimbabwe Craton.