Mesoarchean TTG magmatism from the northeastern margin of the Kaapvaal Craton, southern Africa: Arguments for an exotic terrane (remnant of Pilbara Craton?)

Abstract

High-grade terranes along Archean cratonic margins either represent remobilized equivalents of the adjacent low-grade domain or an exotic entity which accreted to the developing craton. This study addresses this debate for the Southern Marginal Zone high-grade terrane along the northeastern margin of the Kaapvaal Craton. The Hout River shear zone separates the high-grade terrane from the adjacent low-grade Pietersburg block of the Kaapvaal Craton. Tonalite-trondhjemites of the Baviaanskloof gneiss unit constitute the dominant rock in the Southern Marginal Zone. Both pyroxene-bearing and pyroxene-absent variants occur. Locally, the granitoids are affected by anatectic and metasomatic overprints. Magmatic textures are better preserved in the massive granitoids. U-Pb zircon analyses of two orthopyroxene-bearing granitoids gave Mesoarchean crystallization ages [3088 ± 11 Ma (massive), 3121 ± 9 Ma (banded)]. However, overprint ages differ, with 2706 + 50/−34 Ma from the massive variety, and 1925 ± 19 Ma, 1976 ± 11 Ma from the banded variety. The syn-tectonic ~2.71 Ga is interpreted as metamorphic overprint age, while the post-tectonic ~2 Ga represent age of local overprint. Available geochemical data on the Baviaanskloof granitoids are similar to Archean TTGs. A possible spatial-temporal transition from ~3.13 Ga low-pressure through ~3.12–2.98 Ga medium-pressure to ~2.92 Ga high-pressure TTG is delineated from the northwest towards the southeastern Southern Marginal Zone. This trend is compatible with the southward directed polarity of accretion events recognised in the adjacent low-grade Pietersburg block. We argue that the northeastern margin of the Kaapvaal Craton, north of the Hout River shear zone, is an exotic terrane because 1) the Baviaanskloof granitoid event (~3.1–2.9 Ga with peak at ~3.1 Ga) is older than the accretion episodes (~2.97–2.92 Ga with peak at ~2.95 Ga, and ~2.87–2.84 Ga with peak at ~2.86 Ga) in the adjacent Pietersburg block, 2) the younger ~2.86 Ga event occurs closer to the Hout River shear zone, while the older ~2.95 Ga event was away from it, and 3) contemporaneous (~3.1–3.0 Ga) detrital zircons are absent in greenstone belts of the Pietersburg block that are proximal to the Hout River shear zone. Possible connection to rocks of similar age from the Pilbara Craton as part of the Vaalbara supercraton is explored.