The evolution of the proterozoic gneisses and other metamorphites between Springbok and Vioolsdrif, South Africa

[摘要] This study covers aspects of the stratigraphy, structure andmetamorphism of part of the Proterozoic Namaqua MetamorphicComplex in northern Namaqualand, between Vioolsdrif in the north, and the area around Springbok in the south. The investigationformed part of the Namaqua Geotraverse, National GeodynamicsProject (Blignault et al., 1983). The study area is subdividedinto four thrust bounded tectonic domains. The Richtersveld Domainin the north is seperated from the centrally situated SteinkopfDomain by the Transition Zone. The Copper District occurs in thesouth while the Geselskapbank Domain represents a small easterlyextension of the study area. The latter domain was not part of theoriginal geotraverse, but a brief study of the metamorphitesprovided important insights into the tectonic development of theregion. The stratigraphy of this domain is not discussed. Themajor Steinkopf fault juxtaposes the Nama Group (+/- 600 Ma) andthe Proterozoic rocks and forms a natural western boundary tor thestudy 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 arepossibly related to the Orange River orogeny, while most of theobservable metamorphic and structural features are the products ofthe Namaqua orogeny.The supracrustal rocks of the Namaqua Metamorphic Complex areconsidered to belong to a single Group, the Bushmanland Group,subdivided into the Haib, Eenriet and Khurisberg Subgroups. TheHaib Subgroup in the study area consists of a lower felsic unit(Tsams Formation), which straddles the boundary between theRichtersveld Domain and the Transition Zone, and the more maficNous Formation, the latter being confined to the RichtersveldDomain. The Eenriet Subgroup comprises three spatially seperatedmetasedimentary units, namely the GroothoeI< Formation (mainly micaschists, confined to the Transition Zone), the Kabina Formation(metaquartzite and schistose metapelite; which builds the EenrietMountain Range) and the Besondermeid Formation (penetrativelyretrogressed metapelites and minor dark metaquartzite which occursas a xenolitic enclave in the Steinkopf Domain). The KhurisbergSubgroup comprises the supracrustal rocks of the Copper District,i.e. white metaquartzite, high grade metapelites and minormetabasites.The supracrustal rocks are engulfed in granitic batholiths and cutby smaller intrusive bodies. The most voluminous units are theVioolsdrif, Gladkop, Little Namaqualand and Spektakel Suites. TheNariams metadolerite dykes, Koperberg Suite, Blesberg (pegmatite)Suite and the Gannakouriep (dolerite) Suite complete the list otsignificant Intrusive units.The Vioolsdrif Suite (granodiorite to leucogranite) is mainly confined to the Richtersveld Domain and together with the HaibSubgroup, forms the Orange River Igneous belt, the igneousproducts of the Orange River orogeny.The Gladkop Sulle constitutes the main lithological entity of thesteinkopf 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 bandingshown to be of secondary origin through processes of mechanicalflattening of primary heterogeneities coupled with metamorphicand/or metatectic differentiation. By distinguishing primary tromsecondary features, it is shown that the precursors to the threegneiss. units were homogeneous in terms of composit.ion, texture andstructure. The gneisses do not exhibit primary banding. Intrusiverelations are established on groundS of the recognition ofxenoliths and observation of contamination and assimilation at thecontacts. It is shown that the Steinkopf Gneiss' is intrusive intometasediments of the Khurisberg and Eenriet Subgroups (i.e. it isnot a basement tor these supracrustal rocks), that theBrandewynsbank Gneiss is intrusive into the Steinkopf Gneiss andthat the Noenoemaasberg Gneiss is intrusive into both the olderunits. Both the Gladkop and Vioolsdrif Suites are intrusive intothe Eenriet Subgroup and are in turn intruded by the LittleNamaqualand Suite. Isotopic characteristics confirm the suggestedtime equivalence while chemical, stratigraphic, structural andmetamorphic evidence point to a spatial separation of the twosuites during early Proterozoic times. The Konkyp Gneiss of the Transition Zone is correlated with theLittle Namaqualand Suite augen gneisses of the Copper District ongrounds of similar structural, textural and stratigraphicrelations. A slightly more silicic and iron rich compositions ofthe Konkyp Gneiss relative to other augen gneisses in theGeotraverse is interpreted to reflect inhomogeneities of thesource area. The augen of the Little Namaqualand Suite are bestinterpreted as primary phenocrysts which are large due to growthduring very slow cooling. The Spektakel Suite includes the Concordia, Rietberg andKweekfontein Granite of the Copper District and the Eyams Graniteand Middelplaat dykes (normatively potassium-rich nephelinesyenite for most part) of the Steinkopf Domain. The Eyams andKweekfontein Granites are very similar in terms of theirmigmatitic characters and modes of emplacement.The structural development spans 1000 million years and the olderstructures are largely obliterated by the main deformationalphases of the Namaqua tectogenesis. Pre-Namaqua deformation in theGladkop Suite is inferred and correlated with pre-Namaquanortheasterly directed thrusting in the Richtersveld Domain. Themost important deformation phase involved early Namaqua, deepseated thrusting which imparted a regionally penetrative toliationto the augen gneisses and because of which lithological units weresubhorizontally displaced over distances as much as 100 km fromthe northeast to the southwest. This was followed by a secondphase of thrusting (commonly referred to as the Skelmtonteinphase) which produced zonally developed refoliation in the augengneisses and in the gneisses of the Gladkop Suite and whichproduced the foliation in the Concordia and Rietberg Granites. Thedirection of tectonic transport during Skelmfontein thrusting wasessentially the same as tor the early thrusting. The Kweekfonteinand Eyams Granites are in part syn-tectonic with the Skelmfonteinthrusting. The present distribution of lithological units in theGeotraverse is mainly the result of the two thrust phases. Latestructures include the open folds, the Ratelpoort shearing(easterly trend), the Dabbiknik shearing (northwesterly trendl andthe development of steep structures.The Richtersveld Domain comprises a core zone charcterized by greenschist facies mineral assemblages, the products of thermaloverprinting during the Namaqua orogeny, superimposed on thekinematic fabrics of the Orange River orogeny. Outside the corezone the metamorphic grade increases to lower amphibolite facies.The Transition Zone is characterized by middle amphibolite faciesmineral assemblages with prominent retrogression, involving thegrowth of andalusite at the expense of sillimanite, during latethrusting. In the Konkyp area higher grade prograde features arespatially associated with the Konkyp Gneiss. Mineral assemblagesand compositions indicate peak metamorphic conditions of 650D Cand less than 4 kb pressure for the Eenriet region. Penetrativereworking during Skelmfontein deformation tranformed doleritedykes, which transect early Namaqua fabrics, to amphibolites inthe Steinkopf Domain. Geothermometry indicates temperatures ofaround 700D C tor the central and southern parts of the SteinkopfDomain during the late Namaqua metamorphism. The Copper Districtis a granulite terrane. Peak metamorphic around 800D and 5 kbpressure during Skelmfontein deformation is indicated. Metamorphicrecrystallization of minerals in bodies of Koperberg Suiteleucodiorite suggest elevated temperatures during emplacement ofthese rocks. Temperatures around 5000 C indicated bybiotite-garnet compositions of metapelites penetratively deformedin steep structures may reflect retrogression at a much laterstage. The Geselskapbank Domain contains exotic stratigraphicunits (including early Namaqua granulites) tectonically emplacedduring late thrusting. These granulites formed at a tempet'ature ofabout 750D C. Retrogression similar to that experienced by therocks of the Transition Zone produced andalusite + chlorite incordierite-sillimanite-garnet rocks. Geothermometry andgeobarometry yield about 5000 C and less than 3 kb respectively,consistent with the observed retrograde mineralogy.The grey gneissses of the Gladkop Suite display maximummigmatization correlated with the Orange River orogeny at thenorthern extremities of Gladkop Suite outcrops, in the TransitionZone. Most significant migmatization developed during earlyNamaqua metamorphism and is located adjacent to the CopperDistrict in the south and spatially associated with the KonkypGneiss in the north.A tectonic model that fits the data includes high grademetamorphism and migmatization of the Gladkop Suite during theOrange River orogeny, culminating in the thrusting of theSteinkopf Domain over the Richtersveld Domain in a northeasterlydirection. During the Namaqua orogeny the sense of thrustingreversed and first caused the overriding of thrust sheets,rendered hot and ductile due to high volumes of the LittleNamaqualand Suite magmas, over the Richtersveld Domain. Later theRichtersveld Domain was undercut by foot wall propagation ofthrust faults. The Richtersveld Domain then started to overridethe Steinkopf Domain along the Groothoek thrust, transporting asuperjacent augen gneiss sheet. Late Namaqua thrusting causedrejuvenation of the Groothoek thrust and new movement along theSkelmfontein thrust. The distribution of thrust sheets with highvolumes of still hot augen gneisses and recently added SpektakelSuite magmas caused high grade metamorphism at the leading edge ofthe Skelmfontein thrust sheet and retrograde metamorphism at thetrailing end.

[发布机构] University of the Free State