The geology, petrology, and geochemistry of the Kanye volcanics and Nnywane Formation, Ramotswa area, southeast Botswana

Chaoka, Thebeyame R. (1988) The geology, petrology, and geochemistry of the Kanye volcanics and Nnywane Formation, Ramotswa area, southeast Botswana. Masters thesis, Memorial University of Newfoundland.

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Abstract

Two major felsic volcanic sequences of late Archean to early Proterozoic age are distinguished in southeast Botswana: the Kanye Volcanics and the Nnywane Formation. The Kanye Volcanics are red, purple and grey, dense, porphyritic rocks with pink and white feldspar phenocrysts in a felsitic to aphanitic groundmass. Presently, these rocks are regarded as part of the Gaborone Granite Complex. In addition to the volcanic rocks, the complex contains rapakivi and alaskitic granites, leucocratic microgranites, porphyritic granophyres, and variable amounts of mafic and ultramafic rocks. -- The Nnywane Formation is an interbeded sequence of felsic flows, porphyries, tuffs and agglomerates. It is part of the Lobatse Volcanic Group which is correlated with the early Proterozoic Ventersdorp Supergroup of South Africa. -- Despite the current correlation of the Kanye Volcanics and the Nnywane Formation with different stratigraphic units and the suggestion that these rocks are products of two distinct magmatic events, the evidence is not irrefutable. In addition, previous explanations for the origin of these volcanic rocks are inconsistent with available petrological, geochemical and geochronological data. -- This study examines the relationship between the Kanye Volcanics and the Nnywane Formation in the Ramotswa area. The geology of the Ramotswa area is described. Major, minor and rare-earth element analyses of numerous samples from the Kanye Volcanics, porphyritic granophyres, leucocratic microgranites and the Nnywane Formation are discussed. Chemically, the Kanye Volcanics, porhyritic granophyres and Nnywane Formation have alkaline affinities, while the intrusive microgranites have major and trace element characteristics of within-plate type granites. Further, comparison of whole-rock major, trace and rare-earth element abundances shows that the Kanye Volcanics and porphyritic granophyres are virtually identical. In contrast, the microgranites have higher TiO₂, Al₂O₃, total iron, CaO, Sr, Zr and Ba and lower SiO₂, average K₂O, Rb, Y and Nb relative to the Kanye Volcanics and porphyritic granophyres. The Nnywane Formation has anomalously high SiO₂ and K₂O and low Na₂O and CaO and more variable Rb, Sr and Ba values relative to the Kanye Volcanics, but relatively the same range TiO₂, Al₂O₃, MgO, total iron as Fe₂O₃, Y, Nb, Ga and Zr. Rare-earth element abundances of these four units are very similar, but the microgranite has a more pronounced negative Eu anomaly. -- Contrary to previous interpretations, these results suggest that the granophyres, Kanye Volcanics and Nnywane Formation (excluding mafic units) crystallized from essentially the same magma. The anomalous SiO₂, K₂O, Na₂O, CaO, Rb, Sr and Ba concentrations of the Nnywane Formation are probably due to subsequent hydrothermal and groundwater alteration. -- However, the results of this study are inconclusive regarding the relationship of the leucocratic microgranites to the other units.

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