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Origin and evolution of Ngaye River alluvial sediments, Northern Cameroon: Geochemical constraints

Abstract : The origin of Ngaye River alluvial sediments and the evaluation of the weathering degree of their source rocks are assessed using trace and rare-earth element geochemistry in three bulk sediments and their different size fractions (2000–200 lm, 200–50 lm, 50–2 lm and <2 lm). The alluvial sediments consist of two sandy clay layers at the bottom and one sandy heavy clay layer at the top. Quartz and feldspars are the main minerals in the sand fractions while kaolinite and smectite are dominant in the finest sediments. The relatively low Chemical Index of Alteration (CIA) indicates that the sediments and their potential source rocks are moderately weathered. Highest trace element contents are observed in the fine sands, which are the richest in Zr, Th, U, Sc and REE. La, Ce and Nd are the most abundant REE in this fraction. The coarse fractions are characterized by LREE-enrichment relative to HREE. The PAAS-normalized REE patterns exhibit large positive Eu anomalies (Eu/Eu à $3.1 to 3.9) in the coarse sand fraction of the sandy clay layers and strong negative Eu anomalies (Eu/Eu à $0.35 to 0.70) in the two sand fractions of the sandy heavy clay layer. Our results document the immaturity of the Ngaye River sediments, which derive mainly from the erosion of moderately weathered granitoids of the surrounding reliefs, and in some extent from greenstones and/or basic volcanics. The results also suggest an obvious dependence of trace and rare-earth element fraction-ation on mineral sorting and weathering in the different grain-sized fractions of the alluvial sediments.
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Submitted on : Tuesday, December 20, 2016 - 2:47:57 PM
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Paul-Désiré Ndjigui, Anicet Beauvais, Soureiyatou Fadil-Djenabou, Jean-Paul Ambrosi. Origin and evolution of Ngaye River alluvial sediments, Northern Cameroon: Geochemical constraints. Journal of African Earth Sciences, 2014, 100, pp.164 - 178. ⟨10.1016/j.jafrearsci.2014.06.005⟩. ⟨ird-01419945⟩



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