Petrographical characteristics of the koperberg suite, South Africa — an analogy to massif-type anorthosites?

Abstract

A swarm of anorthositic/dioritic intrusives, collectively termed the Koperberg Suite, is scattered over an area of about 3000 km2 in the northwestern Cape Province (Namaqualand), South Africa. The intrusive suite is renowned for its associated magmatic copper sulphide deposits. Transgressive field relationships and the preservation of primary igneous features suggest emplacement of the Suite after the main metamorphic event, which has been dated at c. 1200 Ma. The distribution of the intrusions seems to have been controlled by vertical shearing which is manifested by antiformal disruptions (‘steep structures’) in the country rocks. Post-intrusion movement along some of these structures is deemed responsible for metamorphic textures in some mafic bodies. Igneous breccias associated with some intrusions present evidence of explosive tectonics at the time of emplacement. The rock types which compose the mafic suite include andesine-anorthosite, leucodiorite, hypersthene- and mica-diorite, hypersthenite and glimmerite. The occurrence of composite intrusions with two or more mafic varieties, and the modal gradation between the rock types suggest consanguinity. The anorthosite appears to be the oldest member of the series, followed by increasingly more mafic varieties such as leucodiorite, hypersthene-diorite (norite) and hypersthenite. Owing to the presence of virtually monomineralic rock types, various differentiation trends are recognized. Series that are dominated by hypersthene, brown mica, iron-titanium oxides and amphibole can thus be delineated. Microprobe analyses of the major silicates indicate plagioclase compositions of andesine, En58–68 for hypersthene, and brown mica compositions on the biotite/phlogopite boundary. The predominance of anorthosite and leucodiorite in the suite suggest an affinity with the massif-type anorthosite kindred. This relation is supported by other criteria such as the preponderance of hypersthene, the age of emplacement, the sequence of intrusion, En/An-ratios of plagioclase and hypersthene, and the association of syenitic rocks as well as iron-titanium oxide- and apatite-rich assemblages. It is provisionally proposed that the Okiep District represents the lower levels of an anorthosite complex of the massif-type.