Journal of Petrology | Volume 45 | Number 4 | Pages 669-691 | 2004
Journal of Petrology 45(4) © Oxford University Press 2004; all rights reserved.
U–Pb Zircon Calendar for Namaquan (Grenville) Crustal Events in the Granulite-facies Terrane of the O'okiep Copper District of South Africa
1 SCHOOL OF GEOSCIENCES, UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG, SOUTH AFRICA
2 HUGH ALLSOPP LABORATORY, ECONOMIC GEOLOGY RESEARCH INSTITUTE, SCHOOL OF GEOSCIENCES, UNIVERSITY OF THE WITWATERSRAND, JOHANNESBURG, SOUTH AFRICA
3 J. C. RODDICK LABORATORY, GEOLOGICAL SURVEY OF CANADA, OTTAWA, CANADA K1A 0E8
4 DÉPARTEMENT DE GÉOLOGIE, UNIVERSITÉ DE LIÈGE, LIÈGE, BELGIUM
* Corresponding author. E-mail: school{at}geosciences.wits.ac.za
The O'okiep Copper District is underlain by voluminous 1035–1210 Ma granite gneiss and granite with remnants of metamorphosed supracrustal rocks. This assemblage was intruded by the 1030 Ma copper-bearing Koperberg Suite that includes jotunite, anorthosite, biotite diorite and hypersthene-bearing rocks ranging from leuconorite to hypersthenite. New sensitive high-resolution ion microprobe age data demonstrate the presence of 1700–2000 Ma zircon as xenocrysts in all of the intrusive rocks, and as detrital zircon in the metasediments of the Khurisberg Subgroup. These data are consistent with published Sm–Nd model ages of c. 1700 Ma (TCHUR) and c. 2000 Ma (TDM) of many of the intrusives that support a major crust-forming event in Eburnian (Hudsonian) times. In addition, U–Th–Pb analyses of zircons from all major rock units define two tectono-magmatic episodes of the Namaquan Orogeny: (1) the O'okiepian Episode (1180–1210 Ma), represented by regional granite plutonism, notably the Nababeep and Modderfontein Granite Gneisses and the Concordia and Kweekfontein Granites that accompanied and outlasted (e.g. Kweekfontein Granite) regional tectonism [F2(D2)] and granulite-facies metamorphism (M2); (2) the Klondikean Episode (1020–1040 Ma), which includes the intrusion of the porphyritic Rietberg Granite and of the Koperberg Suite that are devoid of regional planar or linear fabrics. Klondikean tectonism (D3) is reflected by major east–west-trending open folds [F3(D3a)], and by localized east–west-trending near-vertical ductile folds [‘steep structures’; F4(D3b)] whose formation was broadly coeval with the intrusion of the Koperberg Suite. A regional, largely thermal, amphibolite- to granulite-facies metamorphism (M3) accompanied D3. This study demonstrates, inter alia, that the complete spectrum of rock-types of the Koperberg Suite, together with the Rietberg Granite, was intruded in a short time-interval (<10 Myr) at c. 1030 Ma, and that there were lengthy periods of about 150 Myr of tectonic quiescence within the Namaquan Orogeny: (1) between the O'okiepian and Klondikean Episodes; (2) from the end of the latter to the formal end of Namaquan Orogenesis 800–850 Ma ago.
KEY WORDS: U–Pb, zircon; O'okiep, Namaqualand; granite plutonism; granulite facies; Koperberg Suite; Namaquan (Grenville) Orogeny
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