Geochronology of the Hout River Shear Zone and the metamorphism in the Southern Marginal Zone of the Limpopo Belt, Southern Africa

Abstract

In this paper monazite U–Pb and zircon evaporation dates, stepleaching Pb/Pb results on garnet, staurolite and kyanite, and hornblende Ar/Ar data are presented which constrain the timing of granulite facies metamorphism in the Southern Marginal Zone of the Limpopo Belt and its thrusting onto the Kaapvaal Craton. The Southern Marginal Zone of the Limpopo Belt is considered to be a lower crustal equivalent of the northern Kaapvaal Craton. Granulite exhumation is associated with southward thrusting along the Hout River Shear Zone which is a set of thrust and strike slip shear zones. Zircon ages for the Matok Intrusive Complex which was emplaced within the zone during this thrusting (charno-enderbites: 2671±4 Ma; granodiorites: between 2667 and 2664 Ma) have previously been interpreted as evidence for rapid exhumation of the Southern Marginal Zone within only ∼7 Ma. We have obtained a U/Pb date of 2691±7 Ma for monazite from the Bandelierkop Quarry in the Southern Marginal Zone, interpreted as the age of high grade metamorphism. A single zircon evaporation Pb/Pb date of 2643±1 Ma from a leucosome band at the same locality may indicate longer lasting metamorphism or decompression melting during exhumation. Anatexis of metapelitic xenoliths within the Matok Intrusive Complex was dated at 2663±4 Ma by U/Pb on monazite, indistinguishable from existing zircon ages for this complex. Pb/Pb step leaching dates obtained on synkinematically grown garnet (2691±20 Ma), staurolite (2712±37 Ma) and kyanite (2672±51 Ma) from the Khavagari Hills in the Giyani Greenstone Belt, in the immediate footwall of the Hout River Shear Zone, indicate that early thrusting was contemporaneous with peak metamorphism in the Southern Marginal Zone. Ar/Ar dating and geochemistry on syntectonic hornblende separates from the same shear zone system yielded disturbed spectra and indicated multiple populations, probably reflecting repeated or continuous tectonic activity of the Hout River Shear Zone up to about 2600 Ma.