Department of Earth Sciences

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Browsing Department of Earth Sciences by browse.metadata.advisor "Clemens, John"

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    The petrogenesis of the ignimbrites and quartz porphyritic granites exposed along the coast at Saldahna, South Africa
    (Stellenbosch : Stellenbosch University, 2013-12) Joseph, Cedric S. A.; Stevens, Gary; Farina, Federico; Clemens, John; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: To date, the only volcanic rocks described from the Cape Granite Suite are ignimbrites that crop out along the western margin of Langebaan lagoon, to the south of Saldahna. These ignimbrites, with an age of 515 ± 3 Ma, represent the youngest rocks within the Suite. This study aims to investigate the petrogenesis of fine grained granitic rocks exposed to the north of Langebaan lagoon, as well as to reassess the classification of these rocks as a sub-volcanic quartz porphyritic intrusive. These rocks exhibit a dark grey to brown micro- to crypto-crystalline matrix containing prominent feldspar and quartz phenocrysts that are often embayed and broken. Phenocrysts of biotite and orthopyroxene (Fs50-70) can be identified microscopically, as can replacement of both phases by chlorite. Ilmenite commonly occurs in close association with the orthopyroxene phenocrysts. A second generation of poikiloblastic biotite overgrows the matrix and is clearly formed through sub-solidus reaction. The feldspar phenocrysts are commonly microcline microperthite. Contact exposures can be observed in the study area which indicates that the finer grained rock intruded older coarser grained granites. The predominance of broken phenocrysts as well as the presence of fiamme present in outcrop in rocks with a microcrystalline matrix is indicative of a volcanic origin, suggesting that these rocks be classified as ignimbrite as opposed to quartz porphyry. The foregoing observations and features could be interpreted to represent a welded ignimbrite deposit which is underlain by coarse grained granite. The finer grained ignimbrite would then represent a subsequent intrusion by a later pulse of similar magma along the contact with the coarser grained granite. The ignimbrites are silicic with SiO2 ranging between 69 and 76 wt. %; they are mildly peraluminous with values for ASI (ASI = mol. Al2O3/ (CaO+Na2O+K2O)) ranging from 1.02 to 1.09; and ASI is negatively correlated with Mg + Fe (hereafter maficity). Tight to very tight inter-element correlations exist for several major elements as well as trace elements when plotted against maficity. The following R² values apply: Al =0.94; Ca = 0.98; Si = 0.97; Ti = 1.00; Na =0.90; Zr =0.95; La = 0.87. These elements are all positively correlated with maficity, except for Si which is negatively correlated. Orthopyroxene and ilmenite represent early formed, high temperature minerals in the magma. In the biotite-poor rocks, ilmenite represents the main reservoir of titanium whilst orthopyroxene represents the main MgO and FeO reservoir. The exceptionally tight Ti: maficity correlation requires that both these minerals always be present at the same molecular ratio in the magma, despite the significant range in maficity portrayed by the rocks and despite the fact that these minerals have different size-density relationships. This exceptionally tight correlation can be readily interpreted to reflect entrainment of a peritectic assemblage consisting of ilmenite and orthopyroxene. The Al, Ca and Na correlations require the entrainment of peritectic plagioclase. The decreasing trend for ASI requires the entrainment of peritectic clionopyroxene. A near perfect match with the concentrations of these elements in the ignimbrites is produced by modelling entrainment of a peritectic assemblage consisting of plagioclase, ilmenite, orthopyroxene and clionopyroxene in stoichiometric proportions dictated by the melting reaction. A peritectic assemblage formed by these phases’ points to the partial melting of a source undergoing coupled biotite and hornblende fluid-absent melting, with hornblende being subordinate. The opx- and ilmenite-rich micro-domains in the rocks represent zones in the magma rich in original peritectic orthopyroxene and ilmenite. In contrast, the peritectic plagioclase demanded by the chemistry of the rocks has melted during ascent due to overheating and decreasing water solubility in the melt. The phenocrystic potassium feldspar observed in the rocks crystallised after significant cooling and the physical behaviour of these crystals does not shape the chemistry of the magma. K contents of the ignimbrites are however not well replicated by this modelling, which predicts a significant K decrease due to dilution. K in the rocks is not correlated with maficity. This may reflect the fact that the K behaviour represents two slightly different source protoliths with differing K contents.