Reconstruction of Pliocene-Pleistocene sediment sources and weathering intensity in the paleo-life rich Olduvai and Laetoli basins of northern Tanzania using major and trace element geochemistry and Sr isotopic data

摘要

Major and trace elements and Sr isotopic compositions were analyzed for samples from the Pliocene-Pleistocene to Recent Olduvai and Laetoli basins of northern Tanzania with the aim of constraining their provenance and paleo-climatic conditions. The Olduvai samples are characterised by La/Cr ratios of 0.69-3.73, Th/Sc = 1.12-24.6 and Sr-87/Sr-86 = 0.70448-0.70563; features which suggest the volcanic facies of the nearby Olmoti as their possible source. The relatively lower values for the Laetoli basin (Th/Sc = 0.4-0.9 and Sr-87/Sr-86 = 0.70412-0.70508) coupled with Zr/Nb, Nb/Ta ans Zr/Hf ratios are suggestive of a mafic protolith inferred to be the Sadiman lavas. The calculated Chemical Index of Alteration (CIA) values for the Laetoli samples are 58-78% (mean = 65%) indicating higher weathering intensities of the source compared to 45% in the Olduvai samples. The calculated Mean Annual Precipitation (MAP) values are higher at Laetoli (mean = 950 +/- 181 mm/yr) relative to Olduvai (mean = 694 +/- 181 mm/yr) consistent with increased rainfall in the Pliocene during the deposition of Laetoli strata when compared to the Pleistocene times when the Olduvai sediments were deposited.