U-Pb zircon calendar for Namaquan (Grenville) crustal events in the granulite-facies terrane of the O'okiep Copper District of South Africa

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 (T-CHUR) and c. 2000 Ma (T-DM) 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 [F-2(D-2)] and granulite-facies metamorphism (M-2); (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 (D-3) is reflected by major east-west-trending open folds [F-3(D-3a)], and by localized east-west-trending near-vertical ductile folds ['steep structures'; F-4(D-3b)] whose formation was broadly coeval with the intrusion of the Koperberg Suite. A regional, largely thermal, amphibolite- to granulite-facies metamorphism (M-3) accompanied D-3. 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.