Petrochemistry of the late Archaean granites of the Chinamora batholith, Zimbabwe

Abstract

At least the lowermost successions of the main greenstone belts which surround the Chinamora batholith were intruded by several discrete granite plutons prior to the intrusion of the Great Dyke into the Rhodesian craton (ca. 2500 Ma-ago) but after ca. 2800 Ma-ago. The intruded granites constitute the Chinamora Igneous Granite Suite (C.I.G.S.). The C.I.G.S. was emplaced over a time period spanning two tectonic events, and the granites display considerable variation in both field and petrochemical characteristics. The oldest plutons contain rather higher contents of mafic minerals than the younger ones and have, on the whole, undergone more intense deformation, hence the recognition of two groups of plutons, the older ‘Gneissic Granites’ (mainly trondhjemites) and the younger ‘Late Granites’ (mainly granites). Samples from each of 22 plutons were analysed for both major and trace elements. Consideration of the major-element data indicates that the C.I.G.S. plutons show characteristics of a calc-alkaline suite. The trace-element data suggest that the C.I.G.S. has evolved by fractional crystallization. In order to explain the origin of the oldest of the Gneissic Granite plutons, a model deriving these plutons from a magma formed by a major addition of basaltic magma to the lower crust and its mixing with a partial melt derived from sialic crust is postulated. The ultimate source material for the Late Granites is not known, but simple modelling does show that their origin can be explained by fractionation along a liquid line of descent.