Browsing by Author "Rapopo, Mafusi"

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    Petrogenesis of the syntectonic Matok Pluton in the Limpopo Belt (South Africa) and its implications of the geodynamic environment
    (Stellenbosch : Stellenbosch University, 2011-12) Rapopo, Mafusi; Stevens, Gary; Moyen, Jean-Francois; Stellenbosch University. Faculty of Sciences. Dept. of Earth Sciences.
    ENGLISH ABSTRACT: The ~2.67 Ga Matok pluton comprises calc‐alkaline pyroxene (px)‐bearing and px‐free granitoids. The pluton was constructed by means of two episodes of intrusion each of which had co‐magmatic px‐bearing and px‐free granitoid groups. All the granitoid groups (px‐bearing and px‐free) are characterised by non‐porphyritic and porphyritic varieties. The phenocrysts in both episodes of intrusion are plagioclase ± alkali feldspar and are aligned parallel to the trend of the Limpopo Belt, attesting to a syntectonic emplacement. The time gap between the first and second intrusion is insignificant and magma was most likely stored in the chamber after the first intrusion. Petrography and geochemical signature of both px‐bearing and px‐free granitoid samples have been studied and a petrogenetic model which accounts for the coeval px‐bearing and px‐free granitoids is proposed. The relevance of the syntectonic emplacement of the Matok pluton ie n the Limpopo Belt is also addrssed. Px‐bearing granitoids always have clinopyroxene but orthopyroxene is not always present. Magnetite and ilmenite are present in both px‐bearing and px‐free granitoids but are more abundant in the px‐bearing granitoids and subordinate in the px‐free granitoids. Plagioclase in both px‐bearing and px‐free granitoids is of oligoclase (An12‐30) composition but is relatively more calcic and increases in modal abundance in the px‐bearing granitoids. Alkali feldspar is more dominant in the px‐free granitoids. Hornblende is present in all the px‐bearing granitoids and the px‐free granitoids with ≤71 wt.% SiO2 but is absent in the px‐free granites with >71 wt.% SiO2. Both magmatic epidote and titanite occur exclusively in the px‐free granitoids with ≤71 wt.% SiO2 and are absent in all the px‐bearing granitoids as well as the px‐free gra nites with >71 wt.% SiO2. Px‐bearing granitoids are mainly of dioritic and granodioritic and have subordinate granitic composition while px‐free granitoids are mainly of granitic and granodioritic and have subordinate dioritic composition. All the rocks define well correlated variation of SiO2 with the rest of the major elements. However, there is always a hiatus between the granites with >71 wt.% SiO2 and all other rocks. Px‐bearing and px‐free granitoids at the same SiO2 concentrations tend to have approximately equal concentrations of MgO, CaO and TiO2, whereas K2O concentration is distinctively higher for the px‐free granitoids. The distribution of the high field strength elements (HFSE; Nb, Ta, Zr and Hf) and rare earth elements (REE) is similar in both px‐bearing and px‐free granitoids. On contrary, Th, U, Cs and Rb are characteristically higher in the px‐free granitoids. All granitoids are characterised by negative anomalies of the HFSE (Nb, Ta and Ti) and the LILE (Th, U and Sr) on primitive mantle normalised diagrams. On the one hand, concentrations of compatible elements (Cr, Ni and Mg) in the Matok pluton granitoids are rather low for a mantle source. On the other hand, all the granitoids have superchondtritic Nb/Ta ratios that overlap with those of the Ventersdorp continental flood basalts which extruded in the Kaapvaal Craton at ~2.7 Ga. The continental crust typically has subchondritic Nb/Ta ratio, and superchondtritic Nb/Ta ratios are widely accepted to resemble a mantle source. The implication is that the Matok pluton granitoids had inherited the superchondtritic Nb/Ta ratio from their source; juvenile underplated mafic magmas that had ponded owing to the impact of the Ventersdorp mantle plume. The large volumes of ponded magma s probably induced the high grade metamorphism in the Limpopo Belt. All the granitoids of the Matok pluton are probably products of one partial melting event. One possible way to account for the co‐existence of px‐bearing and px‐free granitoids in the Matok pluton is by means of, at least, two magma chambers; one which was filled with anhydrous magma and the other which was filled with hydrous magma. An alternative model would be that in which there was only one chamber. In the one chamber scenario, the magma was hydrodynamically sorted into zones that differed mostly in fH2O and concentrations of highly fluid‐mobile elements but conserved the uniformity in fluid immobile elements. Regardless of the number of chambers, magma batches intruded in the form of feeder dikes which minimally interacted, thus avoiding the hydration of pyroxene in the px‐bearing granitoids.