PROTOLITH INTERPRETATION IN METAMORPHIC TERRANES - A BACK-ARC ENVIRONMENT WITH BESSHI-TYPE BASE-METAL POTENTIAL FOR THE QUHA FORMATION, NATAL PROVINCE, SOUTH-AFRICA

A scheme for protolith interpretation of high-grade gneisses is presen ted which integrates field, petrographic and geochemical criteria to i nterpret 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 similar to 1.2 Ga Quha Formation fr om the Mzumbe Terrane, Natal Metamorphic Province, South Africa, repre sent a metamorphosed sequence of predominantly volcanic and volcanicla stic rocks of K-feldspar-absent basaltic to andesitic compositions, al ong with penecontemporaneous (greywacke) sedimentary rocks derived fro m their erosion. Interlayered, finely laminated quartz-garnet-pyrite r ocks (coticules) point to localised volcanogenic exhalative activity, which may indicate the existence of Besshi-type base metal sulphide de posits. Two types of nearly concordant amphibolite layers represent me tamorphosed dykes of pre- and late-tectonic age. Geochemical parameter s including ACF, TAS and REE plots show strong calc-alkaline volcanic- are signatures, while bulk compositional data can be successfully matc hed with proposed protolith mineral compositions. U-Pb dating of selec ted zircon separates from Quha metavolcanic volcanic rocks suggest an age of crystallisation of similar to 1200 Ma, indistinguishable from t hat of the pre-tectonic, are-related Mzumbe tonalite-trondhjemite with which the Quha Formation is intimately associated. However, none of t he Quha rocks are cogenetic with the Mzumbe Suite.