U-Pb Zircon Calendar for Namaquan (Grenville) Crustal Events in the Granulite-facies Terrane of the O'okiep Copper District of South Africa

Abstract

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 highresolution 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 granulitefacies 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 (510 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.