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Composition and single zircon U-Pb emplacement and metamorphic ages of the Aggeneys Granite Suite, Bushmanland, South Africa

Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa, Present address: Paleoproterozoic Mineralisation Research Group, Dept. of Geology, University of Johannesburg, P.O. Box 524, Auckland Park, Johannesburg, 2006, South Africa e-mail: russellb{at}postgrad.uj.ac.za

Richard Armstrong

Research School of Earth Sciences, The Australian National University, Canberra, 0200, A.C.T., Australia e-mail: richard.armstrong{at}anu.edu.au

David Reid

Department of Geological Sciences, University of Cape Town, Private Bag, Rondebosch, 7700, South Africa e-mail: dlr{at}geology.uct.ac.za

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 (T_DM) 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.

This article has been cited by other articles:

				R. Bailie, R. Armstrong, and D. Reid Response to the papers by Bailie et al. concerning the age and deposition of the Bushmanland Group (South African Journal of Geology, 110, 59-86) and single zircon ages of the Aggeneys Granite Suite (South African Journal of Geology, 110, 87-110) South African Journal of Geology, March 1, 2008; 111(1): 112 - 113. [Full Text] [PDF]