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<p>Major, trace, and rare earth element (REE) compositions were carried out for the Red Sandstone Group of the Rukwa Basin, SW Tanzania in order to assess the nature of the source rocks, paleo-weathering intensities, paleoclimate and paleo-redox conditions. This research is the first of its kind in the basin and it is expected to broaden the database of the Red Sandstone Group whose previous investigations hinged on sedimentology. In interpreting the obtained trace element data, it was assumed that elemental ratios such as La/Sc, Th/Sc, Cr/Th, Th/Co, La/Co and REE in the detrital silicate fraction of the samples behaved as a closed system during weathering, transportation and eventual deposition in the basin. Shales from all the Red Sandstone Group show similar patterns in the primitive mantle normalized diagrams. However, notable depletions characterize the Ba and Sr abundances, while Rb and Th contents are significantly elevated relative to adjacent elements. The shales are characterized by enrichments of the light rare earth element [(La/Yb)<sub>CN</sub> range 8.18–27.761; average = 13.699; CN stands for chondrite normalized] and relatively flat heavy rare earth element (HREE; (Gb/Yb)<sub>CN</sub> = range 1.115–2.958; average = 1.723) coupled with negative Eu anomalies (Eu* average = 0.76). Such features are similar to those of the Post-Archean Australian Shale (PAAS). Overall, this points to a felsic Upper Crust Continental protolith for the Red Sandstone Group rocks. Furthermore, the trace element ratios of La/Sc, Th/Sc, Y/Ni, Cr/Th, and Th/Co indicate a more felsic source than that for the Upper Crust Compositions and PAAS. Discriminant diagrams of Co/Th versus (vs) La/Sc and Cr/V vs Y/Ni enabled to make inferences of source rocks to be the meta-anorthosites of the Ubendian Belt and Cratonic granites around and to the NE of the basin. The calculated Chemical Indices of Alteration (CIA) indicate mainly intermediate magnitudes, suggesting sporadic variations of climates. Considerations on Al/Na and CIA values point to prevalent warm-humid climates during the deposition of the Red Sandstone Group rocks. On the other hand, V/(V + Ni), V/Cr, Ni/Co, and calculated Mn* values (Mn* = log [(Mn<sub>sample</sub>/Mn<sub>shales</sub>)/(Fe<sub>sample</sub>/Fe<sub>shales</sub>)]); redox proxy parameters demonstrate an overall well oxygenated/oxic environment when the sediments were deposited in the Rukwa Basin during the geologic past. Integrated geochemical proxies are essential to improve our understanding of geological surface processes in the natural environment and can be used to constrain their effect from other factors controlling sediments composition.</p>

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Geochemical characterization of the Cenomanian–Campanian Red Sandstone Group of the Rukwa Basin, SW Tanzania: implications for provenance, paleoclimate and paleo-redox conditions

  • 期刊论文

  • 发表日期：

    2025年5月4日

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  • Charles H. Kasanzu

摘要

Major, trace, and rare earth element (REE) compositions were carried out for the Red Sandstone Group of the Rukwa Basin, SW Tanzania in order to assess the nature of the source rocks, paleo-weathering intensities, paleoclimate and paleo-redox conditions. This research is the first of its kind in the basin and it is expected to broaden the database of the Red Sandstone Group whose previous investigations hinged on sedimentology. In interpreting the obtained trace element data, it was assumed that elemental ratios such as La/Sc, Th/Sc, Cr/Th, Th/Co, La/Co and REE in the detrital silicate fraction of the samples behaved as a closed system during weathering, transportation and eventual deposition in the basin. Shales from all the Red Sandstone Group show similar patterns in the primitive mantle normalized diagrams. However, notable depletions characterize the Ba and Sr abundances, while Rb and Th contents are significantly elevated relative to adjacent elements. The shales are characterized by enrichments of the light rare earth element [(La/Yb)CN range 8.18–27.761; average = 13.699; CN stands for chondrite normalized] and relatively flat heavy rare earth element (HREE; (Gb/Yb)CN = range 1.115–2.958; average = 1.723) coupled with negative Eu anomalies (Eu* average = 0.76). Such features are similar to those of the Post-Archean Australian Shale (PAAS). Overall, this points to a felsic Upper Crust Continental protolith for the Red Sandstone Group rocks. Furthermore, the trace element ratios of La/Sc, Th/Sc, Y/Ni, Cr/Th, and Th/Co indicate a more felsic source than that for the Upper Crust Compositions and PAAS. Discriminant diagrams of Co/Th versus (vs) La/Sc and Cr/V vs Y/Ni enabled to make inferences of source rocks to be the meta-anorthosites of the Ubendian Belt and Cratonic granites around and to the NE of the basin. The calculated Chemical Indices of Alteration (CIA) indicate mainly intermediate magnitudes, suggesting sporadic variations of climates. Considerations on Al/Na and CIA values point to prevalent warm-humid climates during the deposition of the Red Sandstone Group rocks. On the other hand, V/(V + Ni), V/Cr, Ni/Co, and calculated Mn* values (Mn* = log [(Mnsample/Mnshales)/(Fesample/Feshales)]); redox proxy parameters demonstrate an overall well oxygenated/oxic environment when the sediments were deposited in the Rukwa Basin during the geologic past. Integrated geochemical proxies are essential to improve our understanding of geological surface processes in the natural environment and can be used to constrain their effect from other factors controlling sediments composition.