Geochronology of the Hout River Shear Zone and the metamorphism in the southern marginal zone of the Limpopo Belt, Southern Africa

In this paper monazite U-Pb and zircon evaporation dates, stepleaching Pb/P b results on garnet, staurolite and kyanite, and hornblende Ar/Ar data are presented which constrain the timing of granulite facies metamorphism in th e Southern Marginal Zone of the Limpopo Belt and its thrusting onto the Kaa pvaal 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 Hour River She ar Zone which is a set of thrust and strike slip shear zones. Zircon ages f or the Matok Intrusive Complex which was emplaced within the zone during th is thrusting (charno-enderbites: 2671 +/- 4 Ma; granodiorites: between 2667 and 2664 Ma) have previously been interpreted as evidence for rapid exhuma tion of the Southern Marginal Zone within only similar to 7 Ma. We have obt ained a U;Pb date of 2691 +/- 7 Ma for monazite from the Bandelierkop Quarr y in the Southern Marginal Zone. interpreted as the age of high grade metam orphism. A single zircon evaporation Pb/Pb date of 2643 +/- 1 Ma from a leu cosome band at the same locality may indicate longer lasting metamorphism o r decompression melting during exhumation. Anatexis of metapelitic xenolith s 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 K havagari Hills in the Giyani Greenstone Belt, in the immediate footwall of the Hout River Shear Zone, indicate that early thrusting was contemporaneou s with peak metamorphism in the Southern Marginal Zone. Ar/Ar dating and ge ochemistry on syntectonic hornblende separates from the same sheer zone sys tem 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. (C) 2001 Elsevier Science B.V. All rights reserv ed.