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The Bushmanland Group supracrustal succession, Aggeneys, Bushmanland, South Africa: Provenance, age of deposition and metamorphism

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 portion of the structurally complex and poly-metamorphosed Namaqua Province of western South Africa is underlain by a series of pelitic and psammitic metasediments, termed the Bushmanland Group, which is host to world class Broken Hill-type Pb-Zn-Cu-Ag deposits at Aggeneys. Geochemical, U-Pb SHRIMP zircon geochronology and Sm-Nd isotopic determinations of the metasediments at Aggeneys were undertaken to determine the age and nature of the provenance, as well as to confine the age of deposition of the Bushmanland Group in this area.

The metasediments were derived from moderately-weathered granodiorites to granites, or their volcanic equivalents, with ~2.65 to ~2.25 Ga Sm-Nd T_DM model ages. Mixed sources are likely from the geochemical signatures of the metasediments as well as the varied detrital zircon ages, with two main age groupings present of ~2.12 to ~1.97 Ga and ~1.88 Ga to ~1.64 Ga. The upper part of the succession has a narrower compositional range and was derived from a more mafic provenance than the basal portion. The provenance is likely to have been a combination of ~1.8 Ga Gladkop Suite granitic gneisses found to the west and the ~2.1 to ~1.9 Ga intrusive and volcanosedimentary rocks of the Richtersveld Subprovince to the northwest. The metasediments were deposited in a passive margin setting and underwent extensive reworking in addition to recycling.

The time of deposition of the Bushmanland Group is constrained between the age of the youngest detrital zircon (~1.64 Ga) and the onset of the regionally extensive Kibaran Orogeny at ~1.2 Ga. Metamorphic overgrowths and rims on detrital zircons record two distinct events, the ~1.21 to ~1.18 Ga Kibaran Orogeny, characterised by extensive granitic magmatism, peak D₂ deformation and peak upper amphibolite facies grade M₂ metamorphism, and the more ubiquitous ~1.04 to ~1.01 Ga Namaquan Orogeny characterised by weaker M₃ metamorphism.

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]