Protolith interpretation in metamorphic terranes: a back-arc environment with Besshi-type base metal potential for the Quha Formation, Natal Province, South Africa

Abstract

A scheme for protolith interpretation of high-grade gneisses is presented which integrates field, petrographic and geochemical criteria to interpret not only the nature of the original rock types, but also the geological setting in which they formed. The amphibolite to granulite-grade supracrustal gneisses of the ∼ 1.2 Ga Quha Formation from the Mzumbe Terrane, Natal Metamorphic Province, South Africa, represent a metamorphosed sequence of predominantly volcanic and volcaniclastic rocks of K-feldspar-absent basaltic to andesitic compositions, along with penecontemporaneous (greywacke) sedimentary rocks derived from their erosion. Interlayered, finely laminated quartz-garnet-pyrite rocks (coticules) point to localised volcanogenic exhalative activity, which may indicate the existence of Besshi-type base metal sulphide deposits. Two types of nearly concordant amphibolite layers represent metamorphosed dykes of pre- and late-tectonic age. Geochemical parameters including ACF, TAS and REE plots show strong calc-alkaline volcanic-arc signatures, while bulk compositional data can be successfully matched with proposed protolith mineral compositions. UPb dating of selected zircon separates from Quha metavolcanic volcanic rocks suggest an age of crystallisation of ∼ 1200 Ma, indistinguishable from that of the pre-tectonic, arc-related Mzumbe tonalite-trondhjemite with which the Quha Formation is intimately associated. However, none of the Quha rocks are cogenetic with the Mzumbe Suite.