The age and petrology of the Chimbadzi Hill Intrusion, NW Zimbabwe: first evidence for early Paleoproterozoic magmatism in Zimbabwe |
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Authors: | Tawanda D Manyeruke Thomas G Blenkinsop Peter Buchholz David Love Thomas Oberthür Ulrich K Vetter Donald W Davis |
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Institution: | a Department of Geology, University of Zimbabwe, P.O. Box MP167, Mount Pleasant, Harare, Zimbabwe;b School of Earth Sciences, James Cook University, Townsville, Queensland QLD 4811, Australia;c Bundesanstalt für Geowissenschaften und Rohstoffe (BGR), Stilleweg 2, D-30655 Hannover, Germany;d Earth Science Department, Royal Ontario Museum, 100 Queen ’s Park, Toronto, Ontario, Canada M5S 2C6 |
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Abstract: | The mafic-ultramafic Chimbadzi Hill intrusion in the NW of the Zimbabwe craton is a dyke with inward-dipping margins comprising magnetite peridotite, troctolite and magnetite melatroctolite. The magnetite peridotite is composed of about equal amounts of V- and Ti-bearing magnetite and olivine (Fo60). The troctolite is composed of about 50% olivine (Fo50-54), 40% plagioclase (An53-58), 7% clinopyroxene and minor apatite and magnetite with ilmenite lamellae. Geochemical trends suggest that the Chimbadzi Hill Intrusion formed by fractional crystallisation from a single initial magma. However, the more primitive magnetite peridotite overlies the more evolved troctolite in the intrusion. This ‘apparent’ inverted stratigraphy may be due to emptying of a fractionated magma chamber from the top, or to floor subsidence during intrusion.U–Pb dating on baddeleyite reveals that the age of the Chimbadzi Hill Intrusion is 2262 ± 2 Ma. This age does not correspond to any known tectono-thermal event in the Zimbabwe Craton or adjacent metamorphic belts. It is 300 Ma younger than the late Archean Great Dyke, and 230 Ma older than other Paleoproterozoic events in and around the craton. Therefore, it may represent a so far undocumented very early Proterozoic igneous event in the Zimbabwe Craton. The intrusion represents a vanadium resource for Zimbabwe, with titanium potentially being mined as by-product. |
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Keywords: | Fractional crystallisation Intrusion Magnetite peridotite Troctolite Paleoproterozoic Mafic-ultramafic complex Cumulate |
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