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A Crustal Progenitor for the Intrusive Anorthosite--Charnockite Kindred of the Cupriferous Koperberg Suite, O'okiep District, Namaqualand, South Africa; New Isotope Data for the Country Rocks and the Intrusives |
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| Authors: | CLIFFORD, TOM N. BARTON, ERIKA S. RETIEF, EDWARD A. REX, DAVID C. FANNING, C. MARK |
| Affiliation: | 1DEPARTMENT OF GEOLOGY, UNIVERSITY OF THE WITWATERSRAND PRIVATE BAG 3, WITS 2050, JOHANNESBURG, SOUTH AFRICA 2BERNARD PRICE INSTITUTE FOR GEOPHYSICAL RESEARCH, UNIVERSITY OF THE WITWATERSRAND JOHANNESBURG, SOUTH AFRICA 3DIVISION OF EARTH, MARINE AND ATMOSPHERIC SCIENCE AND TECHNOLOGY, CSIR PRETORIA, SOUTH AFRICA 4DEPARTMENT OF EARTH SCIENCES, UNIVERSITY OF LEEDS UK 5RESEARCH SCHOOL OF EARTH SCIENCES, AUSTRALIAN NATIONAL UNIVERSITY CANBERRA, ACT, AUSTRALIA |
| Abstract: | The Koperberg Suite comprises some 1700 small bodies of intrusiverocks largely composed of andesine anorthosite, biotite diorite,and leuconorite, norite and melanorite-hypersthenite; 30 mineshave been established in the O'okiep District in the cupriferousrocks of this anorthosite-charnockite kindred. The suite isintrusive into a sequence of granite gneiss and metavolcanicand metasedimentary rocks, and intrusive granite, that wereelevated to the granulite fades of regional metamorphism.TheSm-Nd model ages for the country rocks and the Koperberg Suiteare all  1700 Ma (TCHUR) and 2000 Ma (TDM) supporting a majorcrustforming event in this portion of Namaqualand at the endof Lower Proterozoic times. The granulite fades metamorphismin the O'okiep District is recorded by a Rb-Sr isochron ageof 1223 48 Ma on the Nababeep Granite Gneiss, and by (1197 15)-Ma-old inherited cores of zircons in the Koperberg Suite.The time of intrusion of the Concordia and Rietberg Granitesis believed to be reflected by their Rb-Sr whole-rock age of1105 24 Ma. The mean U-Pb age of 1029 10 Ma on individualzircon grains and zircon rims from the Koperberg Suite recordsthe time of its intrusion, and this is supported by the Sm-Ndwhole-rock age of 1022 42 Ma for the suite. Subsequent coolingand reheating events are recorded by the Ar-Ar ages of 800850Ma for the Koperberg Suite, and of 500550 Ma for thesuite and certain country rocks, respectively.An  Nd value of-7,and its volume and composition, suggest a crustal-melt sourcefor the intrusive Concordia Granite. Moreover, the age-correctedhigh lSr (07061-07272) and low Nd (-9), and the high µ2(101), that characterize the Koperberg Suite also imply a crustalsource, and a model is presented for the generation of the majorpart of the suite by partial melting of granulites of overallintermediate (diorite) composition in the lower crust. Corresponding author |
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