Abstract

Layered gneisses of the Quha Formation make up the oldest component of the Mzumbe terrane, Natal Metamorphic Province. They have been interpreted as predominantly metamorphosed andesitic volcanic rocks and their sedimentary derivatives, formed in a juvenile volcanic are that represents the earliest stage in the evolution of the Province. Zircons extracted from a sample of quartz-plagioclase-biotite-garnet gneiss (metagreywacke) of the Quha Formation with thin albite pegmatite veins have been subjected to cathodoluminescence (CL) and U-Th-Ph SHRIMP analysis. CL-bright zircon cores with igneous, oscillatory zoning gave a U-Pb SHRIMP date of 1235+ or -9 Ma. This is the age of volcanic andesites from which the sample was derived, and is the oldest date yet obtained from the province. Deposition of the Quha Formation in the Type Area is thus constrained between 1235 Ma and the published U-Pb zircon age of 1207+ or -10 Ma for the Mzumbe Suite which intrudes it. The 1235 Ma date is consistent with isotopic data from the oldest metavolcanic rocks of the Tugela terrane to the north and similar to that of the juvenile Areachap terrane of eastern Namaqualand. CL-dark rims to the igneous zircons, interpreted as metamorphic overgrowths, along with similar discrete zircon grains, gave a mean U-Pb SHRIMP date of 1065+ or -15 Ma. This is considered to date the high-grade M2 metamorphism associated with the emplacement of the large rapakivi granite-charnockite plutons of the Oribi Gorge Suite nearby.

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