Abstract

The central Bushmanland portion of the Namaqua Province of South Africa is underlain by three voluminous suites of granitic gneisses, termed the Achab, Hoogoor and Aroams Gneisses, in the Aggeneys District, the site of major world-class Broken Hill-type Pb-Zn-Cu-Ag deposits. Geochemically the granitic gneisses have similar characteristics with only slight differences in the degree of fractionation present. The gneisses exhibit a predominance of Kibaran magmatic zircon ~1.21 to ~1.17 Ga ages, which correspond closely to a similar tectono-magmatic event in the Okiep District in Namaqualand to the west and to similar aged granitic gneisses in the Natal Province of the Namaqua-Natal Belt. During this period the Aggeneys District was subjected to peak prograde M2 amphibolite facies grade metamorphism associated with peak D2 deformation. This period was characterised by continental collision and accretion throughout the belt. After a 140 m.y. period of tectonic quiescence a discrete tectono-magmatic episode is recorded in U-Pb ages of ~1.04 to ~1.01 Ga Namaquan Orogeny age from low Th/U metamorphic overgrowths also found in the Okiep District in Namaqualand to the west. Unlike Namaqualand and Natal, however, the Aggeneys District was not subjected to extensive magmatism at this time, and metamorphic conditions did not reach granulite facies grade as it did in Namaqualand. All of the granitic gneisses were intruded syn- to post-collision over a short time period. As the granitic gneisses are texturally, petrographically and geochronologically similar it is argued that they were coeval. The generally I- to S-type metaluminous to peraluminous nature of all of the granitic gneisses, along with previously published depleted mantle (TDM) Sm-Nd model ages of between ~2.42 and ~2.05 Ga, suggests derivation by partial melting of pre-existing ~2.0 Ga crust at ~1.2 Ga in a continental collision tectonic setting.

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