The evolution of the proterozoic gneisses and other metamorphites between Springbok and Vioolsdrif, South Africa
Van Aswegen, Gerhard
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This study covers aspects of the stratigraphy, structure and metamorphism of part of the Proterozoic Namaqua Metamorphic Complex in northern Namaqualand, between Vioolsdrif in the north, and the area around Springbok in the south. The investigation formed part of the Namaqua Geotraverse, National Geodynamics Project (Blignault et al., 1983). The study area is subdivided into four thrust bounded tectonic domains. The Richtersveld Domain in the north is seperated from the centrally situated Steinkopf Domain by the Transition Zone. The Copper District occurs in the south while the Geselskapbank Domain represents a small easterly extension of the study area. The latter domain was not part of the original geotraverse, but a brief study of the metamorphites provided important insights into the tectonic development of the region. The stratigraphy of this domain is not discussed. The major Steinkopf fault juxtaposes the Nama Group (+/- 600 Ma) and the Proterozoic rocks and forms a natural western boundary tor the study area. The Namaqua Metamorphic Complex is the product of two orogenies, the Orange River orogeny (1730 - 2000 Ma) and the Namaqua orogeny (1000 1300 Ma). The stratigraphy of the supracrustal rocks are possibly related to the Orange River orogeny, while most of the observable metamorphic and structural features are the products of the Namaqua orogeny. The supracrustal rocks of the Namaqua Metamorphic Complex are considered to belong to a single Group, the Bushmanland Group, subdivided into the Haib, Eenriet and Khurisberg Subgroups. The Haib Subgroup in the study area consists of a lower felsic unit (Tsams Formation), which straddles the boundary between the Richtersveld Domain and the Transition Zone, and the more mafic Nous Formation, the latter being confined to the Richtersveld Domain. The Eenriet Subgroup comprises three spatially seperated metasedimentary units, namely the GroothoeI< Formation (mainly mica schists, confined to the Transition Zone), the Kabina Formation (metaquartzite and schistose metapelite; which builds the Eenriet Mountain Range) and the Besondermeid Formation (penetratively retrogressed metapelites and minor dark metaquartzite which occurs as a xenolitic enclave in the Steinkopf Domain). The Khurisberg Subgroup comprises the supracrustal rocks of the Copper District, i.e. white metaquartzite, high grade metapelites and minor metabasites. The supracrustal rocks are engulfed in granitic batholiths and cut by smaller intrusive bodies. The most voluminous units are the Vioolsdrif, Gladkop, Little Namaqualand and Spektakel Suites. The Nariams metadolerite dykes, Koperberg Suite, Blesberg (pegmatite) Suite and the Gannakouriep (dolerite) Suite complete the list ot significant Intrusive units. The Vioolsdrif Suite (granodiorite to leucogranite) is mainly confined to the Richtersveld Domain and together with the Haib Subgroup, forms the Orange River Igneous belt, the igneous products of the Orange River orogeny. The Gladkop Sulle constitutes the main lithological entity of the steinkopf Domain. Three major units are distinguished, namely the Steinkopf Gneiss (granodioritic to granitic), the Brandewynsbank Gneiss (granitic) and the Noenoemaasberg Gneiss (leucogranilic). Together they forma grey gneiss complex similar to basement complexes elsehwere. The gneisses exhibit well-developed banding shown to be of secondary origin through processes of mechanical flattening of primary heterogeneities coupled with metamorphic and/or metatectic differentiation. By distinguishing primary trom secondary features, it is shown that the precursors to the three gneiss. units were homogeneous in terms of composit.ion, texture and structure. The gneisses do not exhibit primary banding. Intrusive relations are established on groundS of the recognition of xenoliths and observation of contamination and assimilation at the contacts. It is shown that the Steinkopf Gneiss' is intrusive into metasediments of the Khurisberg and Eenriet Subgroups (i.e. it is not a basement tor these supracrustal rocks), that the Brandewynsbank Gneiss is intrusive into the Steinkopf Gneiss and that the Noenoemaasberg Gneiss is intrusive into both the older units. Both the Gladkop and Vioolsdrif Suites are intrusive into the Eenriet Subgroup and are in turn intruded by the Little Namaqualand Suite. Isotopic characteristics confirm the suggested time equivalence while chemical, stratigraphic, structural and metamorphic evidence point to a spatial separation of the two suites during early Proterozoic times. The Konkyp Gneiss of the Transition Zone is correlated with the Little Namaqualand Suite augen gneisses of the Copper District on grounds of similar structural, textural and stratigraphic relations. A slightly more silicic and iron rich compositions of the Konkyp Gneiss relative to other augen gneisses in the Geotraverse is interpreted to reflect inhomogeneities of the source area. The augen of the Little Namaqualand Suite are best interpreted as primary phenocrysts which are large due to growth during very slow cooling. The Spektakel Suite includes the Concordia, Rietberg and Kweekfontein Granite of the Copper District and the Eyams Granite and Middelplaat dykes (normatively potassium-rich nepheline syenite for most part) of the Steinkopf Domain. The Eyams and Kweekfontein Granites are very similar in terms of their migmatitic characters and modes of emplacement. The structural development spans 1000 million years and the older structures are largely obliterated by the main deformational phases of the Namaqua tectogenesis. Pre-Namaqua deformation in the Gladkop Suite is inferred and correlated with pre-Namaqua northeasterly directed thrusting in the Richtersveld Domain. The most important deformation phase involved early Namaqua, deep seated thrusting which imparted a regionally penetrative toliation to the augen gneisses and because of which lithological units were subhorizontally displaced over distances as much as 100 km from the northeast to the southwest. This was followed by a second phase of thrusting (commonly referred to as the Skelmtontein phase) which produced zonally developed refoliation in the augen gneisses and in the gneisses of the Gladkop Suite and which produced the foliation in the Concordia and Rietberg Granites. The direction of tectonic transport during Skelmfontein thrusting was essentially the same as tor the early thrusting. The Kweekfontein and Eyams Granites are in part syn-tectonic with the Skelmfontein thrusting. The present distribution of lithological units in the Geotraverse is mainly the result of the two thrust phases. Late structures include the open folds, the Ratelpoort shearing (easterly trend), the Dabbiknik shearing (northwesterly trendl and the development of steep structures. The Richtersveld Domain comprises a core zone charcterized by greenschist facies mineral assemblages, the products of thermal overprinting during the Namaqua orogeny, superimposed on the kinematic fabrics of the Orange River orogeny. Outside the core zone the metamorphic grade increases to lower amphibolite facies. The Transition Zone is characterized by middle amphibolite facies mineral assemblages with prominent retrogression, involving the growth of andalusite at the expense of sillimanite, during late thrusting. In the Konkyp area higher grade prograde features are spatially associated with the Konkyp Gneiss. Mineral assemblages and compositions indicate peak metamorphic conditions of 650D C and less than 4 kb pressure for the Eenriet region. Penetrative reworking during Skelmfontein deformation tranformed dolerite dykes, which transect early Namaqua fabrics, to amphibolites in the Steinkopf Domain. Geothermometry indicates temperatures of around 700D C tor the central and southern parts of the Steinkopf Domain during the late Namaqua metamorphism. The Copper District is a granulite terrane. Peak metamorphic around 800D and 5 kb pressure during Skelmfontein deformation is indicated. Metamorphic recrystallization of minerals in bodies of Koperberg Suite leucodiorite suggest elevated temperatures during emplacement of these rocks. Temperatures around 5000 C indicated by biotite-garnet compositions of metapelites penetratively deformed in steep structures may reflect retrogression at a much later stage. The Geselskapbank Domain contains exotic stratigraphic units (including early Namaqua granulites) tectonically emplaced during late thrusting. These granulites formed at a tempet'ature of about 750D C. Retrogression similar to that experienced by the rocks of the Transition Zone produced andalusite + chlorite in cordierite-sillimanite-garnet rocks. Geothermometry and geobarometry yield about 5000 C and less than 3 kb respectively, consistent with the observed retrograde mineralogy. The grey gneissses of the Gladkop Suite display maximum migmatization correlated with the Orange River orogeny at the northern extremities of Gladkop Suite outcrops, in the Transition Zone. Most significant migmatization developed during early Namaqua metamorphism and is located adjacent to the Copper District in the south and spatially associated with the Konkyp Gneiss in the north. A tectonic model that fits the data includes high grade metamorphism and migmatization of the Gladkop Suite during the Orange River orogeny, culminating in the thrusting of the Steinkopf Domain over the Richtersveld Domain in a northeasterly direction. During the Namaqua orogeny the sense of thrusting reversed and first caused the overriding of thrust sheets, rendered hot and ductile due to high volumes of the Little Namaqualand Suite magmas, over the Richtersveld Domain. Later the Richtersveld Domain was undercut by foot wall propagation of thrust faults. The Richtersveld Domain then started to override the Steinkopf Domain along the Groothoek thrust, transporting a superjacent augen gneiss sheet. Late Namaqua thrusting caused rejuvenation of the Groothoek thrust and new movement along the Skelmfontein thrust. The distribution of thrust sheets with high volumes of still hot augen gneisses and recently added Spektakel Suite magmas caused high grade metamorphism at the leading edge of the Skelmfontein thrust sheet and retrograde metamorphism at the trailing end.