The geology of the Tantalite Valley mafic-ultramafic complex and the Kumkum metamorphic-igneous massif near Warmbad, South West Africa, Namibia

Abstract

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Following this, ultramafic plugs were emplaced into the Complex and in the enclosing gneisses. A drop in pressure, then occurred, probably as a result of rapid uplift, reactivating the Shear Zone in the process. Shearing accompanied by the introduction of water led to some low-grade metamorphism and partial melting to produce discordant pegmatites. The Tantalite Valley Complex is regarded here as a variety of layered complex. Successive tapping of a magma chamber in which ultramafic cumulates sank to the floor leaving cumulus-enriched basic liquids gave rise to the emplacement sequence observed in the Complex. The magmas from which these rocks were derived, were evolved, aluminous tholeiites depleted in some lithophile elements. Element concentrations in mafic rocks from the Tantalite Valley and Kumkum areas rules out the possibility that these suites can be related by crystal fractionation but rather that lateral inhomogeneity in the source area is involved. All gabbroic rocks in these areas were derived from unusual magma-types which suggest an origin from a depleted source area, as a result of tectonism associated with the Tantalite Valley Shear Zone

The charnockite hypothesis for the mafic-ultramafic rocks of Jackson (1976) and Toogood (1976) is rejected

In Tantalite Valley the proto-mottled metagabbro was emplaced along the axis of the Lineament. This led to partial remobilization of granite gneiss and metasomatic alteration by the latter. Proto-metagabbro, probably a gabbronorite, was then intruded into relatively dry rocks producing a narrow contact aureole and generating granitic partial melts and cordierite-anthophyllite/pelitic hornfels restites. Temperatures in the aureole were from 575 - 82O°C, pressure was constant at about 5, 5 kb (P tot P H20), and f02 low. The proto-mottled metagabbro and proto-metagabbro then underwent thorough hydration metamorphism to metagabbros. Gabbronorite was then intruded into metagabbro at temperatures of ~1200 °C as plugs, sheets and dykes and cooled slowly