Geochemical constraints of the petrogenesis of the O’okiep Koperberg Suite and granitic plutons in Namaqualand, South Africa: A crustal source in Namaquan (Grenville) times

Abstract

The Namaquan (Grenville) Orogeny (late Mesoproterozoic) in the O’okiep District is characterized by two tectono-magmatic episodes: the O’okiepian Episode (1210–1180 Ma) with the intrusion of batholitic granites, and the Klondikean Episode (1040–1020 Ma), which includes the intrusion of the copper-bearing Koperberg Suite and the Rietberg Granite. This study focuses on the geochemistry (major and trace elements, Sr and Nd isotopes) of intrusive rocks of the O’okiep terrane to provide better constraints on their source rock characteristics and on their petrogenesis. The O’okiepian Granites belong to the K-rich granite kindred, with shoshonitic affinities. The Concordia Granite results from dehydration melting of a pelitic source. The Rietberg Granite shows geochemical similarities with post-collisional magmatic series. The anorthosites and related rocks of the Koperberg Suite are cumulates; their REE distribution is controlled by their apatite content. A new rock type, jotunite, has been identified in the Koperberg Suite; it is analogous to the Rogaland chilled jotunite, a characteristic which gives strong evidence that the Koperberg rocks belong to the massif-type anorthosite suite. Inversion modelling of plagioclase REE compositions from anorthosite permits the reconstruction of melt compositions and places constraints on the melting process and on the characteristics of the source rocks. The occurrence of jotunite in the Koperberg Suite is strong additional evidence for a crustal source, because jotunite is produced by remelting of gabbronorite under dry conditions. The various intrusions in the Koperberg Suite show distinct isotopic signatures, which resulted from isotopic heterogeneities of the crustal source and from minor contamination with the country rocks. The characteristic negative ɛ Nd(1030 Ma) values (−5 to −11) can be explained by remelting at 1030 Ma of a 1900-Ma-old oceanic crust protolith with an enriched MORB REE distribution. The large range in Sr initial ratios (0.709–0.748) may reflect hydrothermal alteration of the oceanic crust, a process which may also explain the Cu enrichment. The Koperberg intrusions were produced by forceful injection of cumulate crystal mush, which were differentiated in deeper magma chambers or in conduits.