Geochemical and geochronological investigations of the vumba granite-greenstone terrain of NE Botswana

Abstract

U-Pb SHRIMP ages were determined for five samples representative of various granitoid groups from the Vumba granite-greenstone terrain, south western margin of the Zimbabwe craton in NE Botswana. These age data range from 2733 ± 5 Ma or 2696 ± 3.5 Ma (if the age for the protolith of an orthogenesis xenolith is excluded) to 2647 ± 4 Ma; they are in the range of published U-Pb zircon ages of 2710 ± 19 to 2639 ± 2 Ma for the Matsitama granite-greenstone terrain in NE Botswana. Precise crystallisation ages for granitoids from these two granite-greenstone terrains are compared with published U-Pb zircon ages of Archaean rocks exposed in the Zimbabwe craton. This comparison suggests that the Neoarchaean granitoids from NE Botswana are correlatives of the Sesombi granitoids/upper Bulawayan volcanic sequence of the Zimbabwe craton. Data from this study support diachronous growth of the craton. The granitoids and spatially associated felsic metavolcanics, amphibolites and ultramafic intrusives all possess volcanic arc geochemical signatures. There are three granitoid types recognised based on petrographic and geochemical analyses: (i) granodiorite-tonalite (gneissic), (ii) biotite-homblende quartz monzonite (gneissic) and (iii) monzogranite (post-kinematic). All the granitoids are medium- to high-K calc- alkaline and they possess I-type characteristics. The felsic metavolcanics are rhyodacite/dacite in composition with variable contents of biotite, muscovite and hornblende. They are calc-alkaline and metaluminous to weakly peraluminous. They are interpreted as a volcanic arc sequence that formed from and melting of hydrated oceanic crust during subduction probably along a volcanic arc which represented the nucleus of felsic continental crust. Some of the associated granitoids are plutonic equivalents of these felsic rocks. There are two types of amphibolite, namely metabasaltic and metagabbroic. They are characterised by flat REE pattern, high Th/Nb ratio and calc-alkaline affinity. The amphibolites are interpreted as volcanic arc basalts formed from mantle wedge as a result of subducting oceanic slab. The ultramafic intrusives of the Vumba granite-greenstone terrain are characterised by Nb depletion and high Th/Nb ratios. A subduction-related genesis is preferred for these rocks.