Browsing by Author "Kroner, A"

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    The geological evolution of the eastern Richtersveld
    (1978) Ritter, Ulrich; Kroner, A; Joubert, P
    The lithology of the early Proterozoic (-1950-1750 my) Richtersveld is characterised by predominantly calc-alkaline volcanics, (De Hoop Subgroup) overlain by and intercalated with quartzitic metasediments into which the mainly granodioritic rocks of the Vioolsdrif Intrusive Suite (VIS) were emplaced. Metavolcanics and sediments of the De Hoop Subgroup have been subdivided into formations, and their paleogeographic environments have been deduced from lithological criteria. In the north eastern Richtersveld systematic reconstruction permitted the recognition of four distinct depositional domains: (a) a high altitude volcanic terrain in the northeast, characterised by a great thickness of an entirely volcanic succession. (b) a synvolcanic graben structure in the southeast; infilled by melanocratic volcanics and immature sediments. (c) a low-lying volcanic terrain in the southwest, possibly representinga coastal plain, lithologically characterised by volcanic debris and redeposited volcanics of domain (a).d) marine quartzites in the south, indicating a marine ingression from the southwest towards the end of the volcanic cycle ,In the southeastern Richtersveld more strongly metamorphosed acid to intermediate volcanics overlie metasediments of the Namaqua Metamorphic Complex(NMC).Within the Vioolsdrif Intrusive Suite four intrusive phases can be distinguished. The oldest phase, a fine-grained granodiorite, is only tentatively assigned to the Vioolsdrif Intrusive Suite and probably constitutes a subvolcanic member of intermediate metavolcanics of the Kookrivier Formation. It was followed by syntectonic even-grained granodiorite and by posttectonic granite and leucogranite. Textural analysis of the undeformed granite phase provided information as to its intrusion and crystallisation history and the sequence of crystallisation. Since the latter allows the estimate of the water content of the initial magma, and as water content of the magma determines - according to some models -geological position and extent of porphyry ore deposits, predictions could be made as to the probable site and extent of possible porphyry ore deposits, which are in agreement with preliminary results obtained during field work. The metamorphic and tectonic development probably started soon after or even during deposition of the supracrustal rocks. Low grade metamorphism of low pressure type (andalusite-sillimanite facies series sensu Miyashiro, 1973) in the central northeastern Richtersveld (associated with tight to isoclinal Fi-folds of different magnitudes) and migmatisation in the south and northeast (mainly postdating F1) suggest an essentially zoned metamorphic complex with increasing grade of metamorphism towards the margins of the Richtersveld.
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    High-grade metamorphism and migmatizaof the Namaqua metamorphic complex around Aus in the Southern Namib Desert, South West Africa
    (1976) Jackson, MPA; De Villiers, John; Kroner, A
    Rocks of the Namaqua Metamorphic Complex are exposed in an area of 10 000 km² in the northwestern part of the Namaqua Mobile Belt east of Liideritz. The Garub sequence represents the oldest rocks in the Aus area and comprises a diverse group of layered rocks of mainly semi-pelitic, pelitic, mafic, calcareous and quartzose composition. These rocks have been metamorphosed to form marbles, calcitic gneisses, metaquartzites, biotite schists, sillimanitecordierite garnet gneisses, amphibolites and granolites with minor amounts of iron formation and magnesian rocks. The principal metasediments are concentrated in west-trending zones. A central zone of calcareous rocks broadens westwards and contains the largest known bodies of carbonate rocks in the Namaqua Metamorphic Complex. The calcareous zone is bounded in the north by a narrow beit of quartzose rocks and in the south, by a broad zone of aluminous rocks. Gradational rock types between these zones are compatible with original sedimentary lithofacies changes. Layered biotite gneiss of psammitic composition has been interfolded with units of the Garub sequence. Both these rock units are present as inclusions within a tonalitic augen gneiss.
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    Structural and metamorphic evolution of a gneiss terrain in the Namaqua Belt near Onseepkans, South West Africa
    (1976) Toogood, David J; Kroner, A
    A 4000 km² area of Precambrian rocks in the Namaqua belt has been examined and it is concluded the present structural and metamorphic framework is the result of a complex polydeformational and polymetamorphic evolution. A major crustal break is present in the area and is represented by a northwest trending dextral shear zone - the Pofadder ZAHNCAFS. The zone of reorientation associated with this shear zone controls the geometry of the western part of the area. The shear zone varies in width from 20 to 40 km and the core contains a 2 km - 7 km wide belt of mylonites. Two sets of folds (D₅ and D₆) have been formed in the zone of reorientation in the northern block. The D₅ folds are northeast trending en-echelon structures up to 30 km long and the interference pattern produced by superimposed northwest trending D₆ folds has resulted in a series of crescent-shaped antiforms. These folds are not present in the southern block and this is thought to be due to a pressure shadow effect connected with the nearby Vioolsdrif complex. Components of both pure shear and simple shear were involved in the shear zone development and a minimum displacement of 85 km is indicated by the strain analysis. The shear zone developed under medium to high-grade metamorphic conditions and the mylonites were formed by a process of dislocation and recovery.