Metabasites along the amphibolite-granulite facies transition in the Okiep Copper District, South Africa

Abstract

At least two types of metabasites occur along the granulite amphibolite facies transition in the Okiep Copper District in western Namaqualand. Low-pressure mafic granulites are mainly found in metagranitoids (Nababeep Gneiss) and supracrustal sequences (Khurisberg Subgroup) in the southern granulite facies Okiep domain; they include low-MgO as well as Mg-rich ol-normative types. Prograde amphibolites and rare relics of mafic granulite occur in orthogneisses of the −1.8 Ga Gladkop Suite and associated volcano-sedimentary rocks of the Lammerhoek Subgroup in the northern Steinkopf domain. All metabasites were metamorphosed during the late Narnaqua (1.02 to 1.03 Ga) high-grade event and are of orthomagmatic origin: precursors of mafic granulites were of subalkaline tholeiitic, while those of amphibolites of transitional alkali olivine basaltic to tholeiitic composition. MORB-normalised spidergrams and REE patterns are vely similar for the two groups and indicate prominent large ion lithophile (LIL) element and LREE enrichment compared to MORB ((La/Lu)N mafic granulites, 1.2–16.0; amphibolites, 0.6–5.1). Geochemical differences were, however, confirmed for LIL elements; the lower and more erratic concentrations of K2O, Na2O, Rb, and Pb in the mafic granulites may be the result of mobile behaviour of LIL elements during high-grade metamorphism. However, observed increased LREE concentrations cannot be explained with post-magmatic mobilisation alone. REE modelling demonstrates that some REE patterns of mafic granulites may be explained with about 20 % of partial (batch) melting of a fertile mantle source with subsequent variable degrees of fractional crystallisation of clinopyroxene, olivine, plagioclase ± hornblende; higher melting rates up to 40 % need to be assumed for successful modelling of the Mg-rich granulite. 10–15 % partial melting of fertile mantle can reproduce the most primitive amphibolite REE patterns. However, partial melting of a primitive mantle source and subsequent fractionation could not reproduce LREE-rich patterns documented for most amphibolites; an enriched mantle source is indicated. Geochemical data exclude an intra-plate or a depleted MORB-type mantle source for the various types of mafic granulites as well as for amphibolites but do not allow discrimination between extensional (e.g. continental flood basalt) and subduction-related (e.g. arc type) geotectonic settings. Our data establish strong geochemical similarities to older 1.65 Ga amphibolites from central Bushmanland.