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 fou r composite lithostratigraphic associations which are dominated by sha llow-water clastic and chemical metasediments. Mafic metavolcanics and hypabyssal intrusives are subordinate and are mainly restricted to on e association. Ultramafic metavolcanic schists, serpentinized peridoti tes, and intermediate lithologies are minor components. However, acid volcanics are absent and the belt is thus lithologically anomalous wit h respect to the majority of the greenstone belts in the Zimbabwe Crat on. The whole supracrustal assemblage has been metamorphosed at greens chist facies, locally attaining low amphibolite grade. it is associate d with an originally intrusive tonalite-trondhjemite-granodiorite orth ogneiss suite (the Jankie gneisses) which locally contains intercalate d 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 N b anomaly, and large-ion lithophile and slight light rare-earth elemen t enrichment, suggestive of a crustal contamination component, such th at in some respects they resemble a continental tholeiite suite. Minor geochemical differences, mainly among the (mobile) large-ion lithophi le elements, indicate that the metavolcanic and metadolerite suites ar e possibly not simply petrogenetically related; otherwise their geoche mical signatures are very similar. The ultramafic schists are interpre ted as basaltic komatiites which appear not to be related petrogenetic ally to the metabasites. The MSB probably formed in a back-are basin d uring extensional attenuation and rifting of Archaean continental crus t at the margin of the Zimbabwe Craton.