Summary
Megacrystic granites which form the Oribi Gorge Suite of southern Natal show many features that closely resemble the rapakivi granite—charnockite associations of the northern hemisphere. Detailed studies of the late-kinematic Mgeni batholith indicate that the granitic rocks can be divided into three major groups, characterised by distinctive mafic mineralogy, chemistry and isotopic ratios. The batholith consists of biotite ±garnet granites and of hornblende ± pyroxene granitoids. A high silica, coarse-grained leucogranite, the Nqwadolo granite, intrudes the core of the batholith. Most rocks are garnetiferous and appear to belong to the ilmenite-series of granites. The granites are classified as rapakivi-like granites.
Zusammenfassung
Megakrystische Granite aus der Oribi Gorge Suite des südlichen Natal zeigen vielerlei Ähnlichkeiten mit den Rapakivigranit-Charnokitabfolgen der nördlichen Hemisphäre. Detailstudien des spätkinematisch intrudierten Mgeni-Batholiten zeigen, daß diese granitischen Gesteine in drei Hauptgruppen, die durch unterschiedliche mafische Mineralzusammensetzung, Chemismus und Isotopenverhältnissen charakterisiert werden, untergliedert werden können. Der Batholith wird aus Biotit± Granat-Graniten und aus Hornblende ± Pyroxen-Granitoiden aufgebaut. Ein siliziumreicher, grobkörniger Leukogranit, der Nqwaldolo Granit, intrudierte in den Kern des Batholithen. Die meisten Gesteine sind granatführend und scheinen den Graniten der “ilmenite series” zuzurechnen zu sein. Die Granite sind als Rapakivi-Granite zu klassifizieren.
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References
Anderson JL (1983) Proterozoic anorogenic granite plutonism of North America. In:Medaris LC, Byers CW Mickelson DM, Shanks WC (eds) Proterozoic geology. Geol Soc Am Mem 161: 133–154
Anderson JL, Cullers RL (1978) Geochemistry and evolution of the Wolf River batholith, a late Precambrian rapakivi massif in North Wisconsin, USA. Precambian Res 7:287–324
Barker F, Wones DR, Sharp WN, Desborough GA (1975) The Pikes Peak batholith, Colorado Front Range, and a model for the origin of the gabbro-anorthosite-syenite-potassic granite suite. Precambrian Res 2: 97–160
Batchelor RA, Bowden P (1985) Petrogenetic interpretation of granitoid rock series using multicationic parameters. Chem Geol 48: 43–55
Brown GC (1982) Calc-alkaline intrusive rocks: their diversity, evolution and relation to volcanic arcs. In:Thorpe RS (ed) Andesites. Wiley, New York, pp 437–461
Bulley BG (1981) Geology of the Tafamasi area, Natal. Thesis, University of Natal, Durban, 126 pp (unpublished)
Collins WJ, Beams SD, White AJR, Chappell BW (1982) Nature and origin of A-type granites with particular reference to southeastern Australia. Contrib Mineral Petrol 80:189–200
Creaser RA, Price RC, Wormald RJ (1991) A-type granites revisited: assessment of a residual-source model. Geology 19: 163–166
Dall'Agnol R, Macambira M, Lafon J (1991) Petrological and geochemical characteristics of the lower and middle Proterozoic anorogenic granites of the Central Amazonian Province, Amazonian Craton. In:Haapala I, Rämö OT (eds) Symposium on Rapakivi Granites and Related Rocks. Geol Surv Finland Guide 34: 12–13
De la Roche H, Leterrier J, Grand Claude P, Marchal M (1980) A classification of volcanic and plutonic rocks using R1–R2 diagrams and major element analyses—its relationships with current nomenclature. Chem Geol 29: 183–210
Du Toit P (1979) Geology of the Nanda area, Natal. Thesis, University of Natal, Durban, 132 pp (unpublished)
Eglington BM (1987) Field, geochemical and isotopic studies of selected areas of Proterozoic crust in south-central Natal. Thesis, University of Natal, Durban, 373 pp (unpublished)
Eglington BE, Harmer RE, Kerr A (1986) Petrographic, Rb-Sr isotope and geochemical characteristics of intrusive granitoids from the Port Edward—Port Shepstone area, Natal. Trans Geol Soc S Afr 86: 199–213
Eglington BM, Harmer RE, Kerr A (1989) Rb-Sr isotopic constraints on the age of the Mgeni and Nqwadolo granites, Valley of a Thousand Hills, Natal. S A J Geol 92: 393–399
Emshe RF (1973) Some chemical characteristics of anorthosite suites and their significance. Can J Earth Sci 10: 54–71
Emslie RF (1978) Anorthosite massifs, rapakivi granites, and late Proterozoic rifting of North America. Precambrian Res 7: 61–98
Haapala I, Rämö OT (1990) Petrogenesis of the Proterozoic rapakivi granites of Finland. In:Stein HJ, Hannah JL (eds) Ore-bearing granite systems. Petrogenesis and mineralizing processes. Geol Soc Am Spec Pap 246: 275–286
Jackson NJ, Walsh JN, Pegran E (1984) Geology, geochemistry and petrogenesis of the late Precambrian granitoids in the Central Hijaz Region of the Arabian Shield. Contrib Mineral Petrol 87: 205–219
Kerr A (1985) Characterisation of the granitic rocks from the Valley of a Thousand Hills area, Natal. S Aft J Sci 81: 475–479
Kuyper JL (1979) Geology of an area south-east of Nagle Dam, Natal. Thesis, University of Natal, Durban, 166 pp (unpublished)
Le Maitre R W (1976) The chemical variability of some common igneous rocks. J Petrol 17: 589–637
Matthews PE (1981) Eastern or Natal sector of the Namaqua-Natal Mobile Belt in southern Africa. In:Hunter DR (ed) Precambrian of the southern hemisphere. Elsevier, Amsterdam, pp 705–715
Pearce JA, Harris NBW, Tindle AG (1984) Trace element discrimination diagrams for the tectonic interpretation of granitic rocks. J Petrol 25: 956–983
Thomas RJ (1988) The petrology of the Oribi Gorge Suite: Kibaran charnockitic granitoids from southern Natal. S Afr J Geol 91: 275–291
Thomas RJ (1989) A tale of two tectonic terranes. S Aft J Geol 92: 306–321
Vorma A (1976) On the petrochemistry of rapakivi granites with special reference to the Laitila massif, southeastern Finland}. Geol Surv Finland Bull 272, 98 pp
Whalen JB, Currie LC, Chappell BW (1987) A-type granites: geochemical characteristics, discrimination and petrogenesis. Contrib Mineral Petrol 95: 407–419
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Kerr, A., Milne, G.C. The Mgeni granite: an example of the Megacrystic mantled-feldspar granite-charnockite Intrusives in southeastern Africa. Mineralogy and Petrology 50, 139–155 (1994). https://doi.org/10.1007/BF01160144
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DOI: https://doi.org/10.1007/BF01160144