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.

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