| Summary: | An area south of Warmbad, South West Africa (Namibia) consisting of medium-to high-grade rocks of the Namaqua Metamorphic Complex has been mapped. It is established that this original dominantly sedimentary region has undergone polyphase metamorphism and deformation and has been intruded by a variety of gabbroic rocks. These are dominated by the Tantalite Valley and Kumkum Complexes which are associated with the major Tantalite Valley Shear Zone.
Metamorphism took piace under low-P facies series conditions characterized by high geothermal gradients (25-50°C/km). The Tantalite Valley Shear Zone axis defines the boundary between high-and medium-grade metamorphic rocks which occur to the north and south of this line respectively. Pelitic gneiss (Black Gneiss) and granite gneiss are restricted to the high grade domain where muscovite is not found and migmatites are common. The Black Gneiss is interpreted as a restite after subtracting a granitic liquid. Black Gneiss parageneses formed during MI (P = 5-6 kb (P tot >PH2O), T = 750 - 860°C, low f02).
During this event the Inequigranular granite gneiss and the Kumkum and Eselruh gabbronorites were emplaced 'in that order. Partial remobilization of granite gneiss by these gabbronorites gave rise to the hybrid Contaminated gabbronorite. The Tantalite Valley Shear Zone was initiated during M2, which was a retrogressive metamorphic event with P = 4-5,5 kb (Ptot >) PH20), T = 5 - 0 – 7600c (from south to north of the Lineament) and low f02. Further Migmatites and conformable pegmatites were produced.
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 - 8200 c, pressure was constant at about 5,5 kb (Ptot> PH2O) and fo2 low. The proto-mottled metagabbro and proto-metagabbro then underwent thorough hydration metamorphism to metagabbros. Gabbronorite was then intruded into metagabbro at temperatures of - 1200OC as plugs, sheets and dykes and cooled slowly.
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.
|
|---|
