Rb-Sr ages for Archaean granitoids and tin-bearing pegmatites in Swaziland, southern Africa

Late Archaean cratonization in the Kaapvaal craton of southern Africa was associated with voluminous granitoid emplacement, and we report on the Rb-Sr isotopic systematics of the Lochiel suite and associated tin-bearing pegmatites in northwestern Swaziland, one of the largest composite batholiths of this region. Thirty-nine samples of the various granitoid phases of the Lochiel batholith and related pegmatites were collected within the "tin-belt" of Swaziland and that extends in a SE direction from Makwane beacon (South African border) to Nyonyane hill some 20 km SW of Mbabane. Rb-Sr whole-rock dating yielded an isochron age of 2979 ± 33 M.a. for the Granodiorite phase (GD) with an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7017 ± 0.0005 while the Alkali-Feldspar Granite (AFG), that intrudes the GD, has an apparent age of 3000±125 M.a. and a ⁸⁷Sr/⁸⁶Sr initial ratio of 0.7047 ± 0.0103. Two pegmatite suites from widely separated localities have virtually identical isotopic systematics and, if regressed together, define an age of 2613 ± 35 M.a. The age of ca. 3000 M.a. for the GD and AFG is in good agreement with published ages for the Lochiel Batholith from other localities. The previously held contention that the various phases of the Lochiel Batholith are indistinguishable with respect to their Sr isotopes is supported by our new data. The low initial ratios for both granitoid suites makes it unlikely that the Lochiel batholith was produced by remelting of the 3.5-3.6 G.a. old Ancient Gneiss Complex, and we favour an underplating model with a significant juvenile magma component. The high Sr initial ratio of the pegmatite phase is in line with reported initial ratios of younger granitoids in Swaziland and suggests that these rocks are remelting products of older sialic crust.