Stratigraphy, igneous petrology and evolution of the Sinclair Group in southern South West Africa

Doctoral Thesis


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University of Cape Town

The investigation of an area fringing the Namib Desert in southern South West Africa has led to the clarification of relationships between several geological units of the Late Precambrian Sinclair Group, and to the formulation of a petrogenetic scheme and geotectonic model of evolution. In the area studied, the oldest exposed unit of the Group is the hybrid and heterogeneous Haremub granite which is overlain by the predominantly arkosic Kunjas Formation. This is followed conformably by the Barby Formation, consisting predominantly of basic and felsic lava flows and various volcaniclastic beds including ash-flow tuff deposits. Basic lavas constitute the bulk of the succession and are typically highly porphyritic. A preliminary 'member' subdivision of the Barby sequence is suggested, based mainly on easily-recognisable field and petrographic characteristics. Gabbroic and noritic intrusives in the southwestern part of the area relate to the Barby basic magmatic phase. The following en bloc emplacement of the northwesterly elongated body of Spes Bona syenite was accompanied by intrusion of monzonitic and dioritic magma along fracture zones marginal to the syenite. The emplacement into high crustal levels of large volumes of the porphyritic/granophyric Nubib granite (1360±50 Ma) took place within a major northwest trending zone of crustal weakness which also marked the site of the developing Nam Shear Belt. This shearing imposed a strong but localised mylonitic imprint on all but the youngest intrusions of Nubib granite and on the Barby basic lavas invaded by the granite. Post-dating the Nubib granite and the main phase of shearing is the mixed sedimentary-volcanic Guperas Formation. Deposition took place mainly within a prominent north-south trending graben structure, and volcanic activity was apparently most intense in three fairly distinct centres. Ashflows and felsic lavas were extruded from vents, now preserved as plug-like bodies. The intrusion of dense composite swarms of .basic and felsic dykes took place toward the end of the Guperas phase. The Rooiberg granite (±1270 Ma) is intrusive into the Guperas. The Auborus Formation, comprising about 2,600 m of red felspathic sandstone and conglomerate represents the youngest unit in the evolution of the Sinclair Group. It is preserved in prominent north-south trending graben structures. It has been possible to postulate distinct parent magma types on the basis of major and trace element analyses carried out on sixty-seven samples from the Sinclair Group. The felsic rock-types can be differentiated into a relatively high-Ca group and a relatively low-Ca group, the latter being domi- .nant. The basic rock-types comprise both tholeiitic and calc-alkaline types and the latter are characteristically rich in potassium and associated elements, and have been classed as 'shoshonites'. There exists no direct genetic relationship between the basic and felsic rock-types.