Browsing by Author "Scogings, Andrew John."

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Alkaline intrusives from the Tugela terrane, Natal metamorphic province.
(1991) Scogings, Andrew John.; Cooper, Mike.; Dunlevey, John N.
Three gneissose alkaline granitoid intrusives at Ngoye, Bulls Run and Wangu are described. They are located within the Nkomo Nappe of the Tugela Terrane, near the northern margin of the Natal Metamorphic Province. The Ngoye Complex comprises alkaline granites, with minor syenite and monzodiorite phases. According to modal am geochemical criteria the Ngoye granites range from peraluminous (muscovite-bearing), through metaluminous (biotite- and/or hornblende-bearing), to peralkaline (riebeckite-, aegirine- and magnetite-bearing). The granites are A-types according to their modal and geochemical characteristics. Rb-Sr isotopic data for the hornblende granites indicate an age of 1063 ± 17 Ma and the initial ratio (R๐ = 0.7025) provides evidence for derivation from a mantle source. Plotting of the Ngoye geochemistry on tectonic-discrimination diagrams suggests intrusion into rifted continental crust. It is concluded that the gneissose Ngoye granites constitute a deformed central complex, similar to anorogenic complexes in Nigeria and the Sudan. The Bulls Run Complex is situated 30 km west of the Ngoye Complex. A concentric outcrop pattern has been mapped, according to which an envelope of silica-saturated biotite-muscovite syenite surrounds a core of nepheline-bearing syenites. Minor intrusive phases include biotite-rich dykes, sovite carbonatite sheets, silica-oversaturated microsyenite dykes and feldspathic ijolite. The outer envelope of muscovite-rich syenite is interpreted as fenitised pelitic country rock. An alkali-lamprophyre origin is suggested for the biotite-rich dykes. Geochemically the syenites are predominantly miaskitic, apart from the microsyenite dykes which are mildly peralkaline. Rb-Sr isotopic data for the nepheline syenites indicate an age of 1138 ± 45 Ma (Ro = 0.70322). Carbonate separates from the carbonatites provide a similar low initial ratio (Ra = 0.70319) which supports a comagmatic mantle origin. A comparison is drawn between the Bulls Run Complex and miaskitic nepheline syenite gneisses in the mid-Proterozoic Grenville Province of canada. From this, it is suggested that the Bulls Run Complex is pretectonic and was intruded into the rifted passive margin of a continent. The Wangu Granite Gneiss is situated 3 km southwest of the Bulls Run Complex. The granites are fine grained and contain aegirine-augite and/or magnetite, and classify as alkali-feldspar granite. Peralkaline chemistry is characteristic of the Wangu granites, with trace-elenent contents indicating a distinct A-type signature. Biotite-rich mafic dykes intrude the southern part of the Wangu outcrop and, on the basis of major- and trace-element signatures, are suggested to be metamorphosed volatile-rich alkaline lamprophyres similar to those at Bulls Run. Geochemical similarities between the Wangu granites and certain comendites from the Kenya Rift are noted. It is suggested that the Wangu granites were emplaced as high-level dykes, within rifted continental crust. It is proposed that the Ngoye, Bulls Run and Wangu intrusives be united as the Nkwaleni Suite. Comparison of the Tugela Terrane with the Grenville Province reveals many similarities, particularly their mid- to late-Proterozoic age and the occurrence of pre-tectonic anorogenic continental magmatism. It is concluded that, unlike the current model which would have the Tugela Terrane as obducted ophiolite, these new data indicate that the Tugela Terrane is a metamorphosed continental rift system.
The geology of the Ngoye granite gneiss formation.
(1985) Scogings, Andrew John.
The Ngoye Granite Gneiss Formation is located in the Natal sector of the Proterozoic Namaqua-Natal Mobile Belt, about 10 km southwest of Empangeni. It forms a prominent east-west trending elongate whalebacked massif some 30 km in length, within amphibolitic gneisses and schists of the Tugela Group. A suite of twelve different, gneissic granitoids has been-recognised within the Ngoye Formation on the basis of field relationships, mineralogy and supportive geochemistry. They range in composition from peraluminous syenite to peralkaline granite. Peraluminous varieties are typically muscovite and garnet-bearing whereas metaluminous granites in the formation contain olivegreen biotite and/or hornblende and sphene. Riebeckite, aegerine and yellow-brown biotite, with accessory fluorite and zircon are characteristic of the peralkaline granites. Geochemically, the samples analysed display a range in SiO₂ from 63,79 - 78,47∞, are extremely depleted in CaO and MgO, while being enriched in Na₂O and K₂O. Depletion of CaO relative to alkalis is shown by an alkali-lime index of only 36, suggestive of an alkalic character. The agpaitic index (A. I. = mole Na₂O + K₂O/AL₂O₃) of the peralkaline samples ranges between 1,02 and 1,16; which classifies them as granites of comenditic affinity. Various chemical classification schemes have been tested and evaluated, of which the RI - R2 multicationic diagram provides results most similar to modally-derived terminology. Accordingly, the Ngoye granitoids are shown to range from minor syenites and alkali granites to predominant monzo - and syeno-granites. Trace element data indicate that the peralkaline granites are enriched in Nb, Zr and Zn relative to the other, non-peralkaline, granites in the formation. In addition, radioactive, magnetite-bearing quartz-rich rocks associated with the peralkaline granites, have extremely enhanced contents of Nb, Zr, Y, Zn, U, Th and to a lesser extent Sn and W. Peraluminous and near-peraluminous granites have the highst Rb/Sr and Rb/Ba ratios of all samples analysed, as well as enhanced Sn, U and Th contents while Zr is notably depleted. Small, muscovite-rich pods associated with muscovite-bearing granites are highly enriched in Sn. The application of certain discriminants based on modal and geochemical parameters has shown the Ngoye Formation to comprise typical "A" - type granites. "A" - type granites are characteristically intruded as ring complexes into anorogenic or post-orogenic tectonic settings in attenuated or epiorogenically-domed continental crust. Comparison of the Ngoye Formation wi th the well-known "younger granite" complexes of Nigeria and Saudi Arabia reveals marked similarities. The inference is therefore that the Ngoye Formation represents a metamorphosed "postorogenic" granite complex with most of the hallmarks of "A" type or "within-plate" magmatism. Four phases of deformation (D₁ to D₄) are recognised within the area mapped. Evidence of D₁ deformation is rare, but rootless folds within the transposed layering in the amphibolitic country rocks reflect the intensity of this prograde metamorphic event, M₁, during which upper amphibolite grades were achieved. Field evidence shows that the Ngoye granites were intruded after the D₁ event and prior to D₂. This latter event caused widespread folding about east-west F₂ axes, with the development of a pervasive S₂ planar fabric within the antiformally folded Ngoye Formation. S₂ is locally developed in the amphibolitic country rocks. The D₂ event culminated in the development of northward-directed overthrusting and retrogressive ,M₂, metamorphism of mylonitic thrust planes. Lateral shearing characterizes D₃, with development of macroscopic mylonites and mesoscopic conjugate shear zones. This was in response to a sinistral sense of movement, as indicated by prominent sub-horizontal extension lineations (L₃) and microscopic asymmetric augen structures. D₄ is deduced from stereograms and is indicated as cross-folding of F₃ fold axes.