Browsing by Author "Janney, Philip"

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    Kimberlites, megacrysts and related xenocrysts from southern and Central Africa: geochemistry and petrology
    (2019) Nkere, Buliba; Janney, Philip
    Kimberlites, megacrysts and related xenocrysts from twelve localities in central and southern Africa have been investigated with particular emphasis on megacryst petrogenesis, the evolution of the subcontinental lithospheric mantle (SCLM) beneath the northeastern Kasai Craton and the petrogenesis and sources of central African kimberlites. The geochemistry of southern and central African megacryst suites is generally consistent with their derivation from protokimberlitic magmas by a combined fractional crystallization-assimilation process. Through the comparison of the geochemistry of Group 1 and Group 2 kimberlite megacrysts, evidence is provided for a genetic link between megacrysts and their host kimberlites. Major element, trace element and Sr isotope similarities of Group 1 and Group 2 kimberlites and their respective megacrysts strongly suggest a close genetic relationship. The geochemistry and single mineral thermobarometry of peridotitic clinopyroxene xenocrysts suggests that the SCLM is cooler and more depleted beneath Tshibwe compared to that below Mbuji-Mayi, and this is consistent with the finding from this work that the age of Tshibwe is 84 Ma, approximately 14 Myr older than Mbuji-Mayi. A metasomatic event in this region between 84 and 70 Ma is the simplest explanation for the differences in geochemistry and geothermal gradient inferred for the SCLM sampled by the two kimberlites. The Tshibwe and Mbuji-Mayi kimberlites belong to two clusters that together constitute the Mbuji-Mayi kimberlite field in the northeastern Kasai craton, and this field, possibly combined with the kimberlites of the Kabinda field further to the east in the DRC was formed by a pulse of kimberlite magmatism, from roughly 85 to 70 Ma, that formed a lineament with a roughly east-west orientation. The apparently similar age and orientation of these kimberlites suggests that they were emplaced along an eastwest oriented zone of lithospheric weakness in the northeastern Kasai craton, tentatively termed the "Kasai corridor". This may connect with the "Lucapa corridor", a linear zone of concentrated kimberlite magmatism with a northeast-southwest orientation cutting across much of Angola. The whole-rock composition of central African kimberlites is fairly similar to southern African kimberlites except for having generally lower MREE/HREE ratios, lower overall concentrations of incompatible elements and isotopic compositions ranging from those typical of southern African Group 1 kimberlites to extreme DUPAL signatures with high 207Pb/204Pbi and 208Pb/204Pbi for moderate 206Pb/204Pbi values. Published data for Sr and Pb isotope compositions of southern and southwestern African peridotite xenoliths indicate that this DUPAL signature most likely does not originate from the lithospheric mantle, but rather is present in the underlying convecting mantle, as appears to be the case for the source for Brazilian kimberlites with similar extreme DUPAL isotope signatures. Data from this study, therefore, require a greater lateral distribution for the extreme DUPAL signatures than has been previously recognised, extending from southwestern Africa to the far southern Mid-Atlantic Ridge to Brazil. The prior, exclusive attribution of this signature, to the "African large low shear velocity province" may be premature.
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    The geochemistry and petrogenesis of the Saltpeterkop carbonatite complex near Sutherland, Northern Cape, South Africa
    (2018) Marageni, Manoka; Janney, Philip; Greyling, Lynnette
    The Saltpeterkop Carbonatite Complex is a Late Cretaceous (≈76 Ma) volcanic and shallow intrusive magmatic feature located approximately 20 km southeast of Sutherland in the Northern Cape. It is unusual among southern African carbonatite complexes in that it has not been deeply eroded, and retains a significant vestige of its original volcanic features. The main geologic expression of the Complex is a ≈1.5 km diameter tuff ring, located on top of prominently updomed and fractured Beaufort Group (Karoo) sediments, that appears to have formed as the result of a major diatreme-type eruption. The volcaniclastic breccias making up the tuff ring have been heavily altered and silicified by hydrothermal activity, and thick (mm to tens of cm) Fe oxide-rich crusts, which appear to represent the alteration products of Fe-rich carbonatites, are common in this area. Outside of the central ring structure are numerous shallow intrusions (dykes, sills and irregular shapes), satellite breccia pipes and pipe-shaped intrusions that host fresh to only moderately altered igneous rocks. The main igneous rock types include (in decreasing order of abundance): carbonatite, potassic trachyte, olivine melilitite and ultramafic lamprophyre. This thesis provides the first detailed petrographic and geochemical description of these rocks (e.g., major and trace elements) and attempts to explain several aspects of their petrogenesis. The olivine melilitites and ultramafic lamprophyres are the most primitive igneous rocks in the complex and have experienced only minor to moderate extents of differentiation, respectively. They apppear to have been derived by low-degrees of partial melting of a carbonated, likely phlogopite-bearing mantle source. The lamprophyres appear to have been derived by melting at shallower depths than the melilitites based on REE constraints. The carbonatites range from relatively primitive to highly differentiated and they form a nearly continuous compositional range with the ultramafic lamprophyres and melilitites. This seems to argue against a major role for liquid immiscibility in their origin. Their REE content (up to 2 wt.% total REE oxides) correlates with their extent of differentiation. The potassic trachytes are plausibly linked to melts of mafic lower continental crust that has been metasomatised by hydrous potassium-rich carbonatitic melts and which have experienced significant fractional crystallization and assimilation of upper crustal sedimentary rocks during emplacement.