Rare-earth and trace elements and hydrogen and oxygen isotopic compositions of Cretaceous kaolinitic sediments from the Lower Benue Trough, Nigeria: provenance and paleoclimatic significance

摘要

This study evaluated the Cretaceous (Campanian-Maastrichtian) kaolinitic sediments of the Ajali/Mamu and Enugu/Nkporo Formations from the Lower Benue Trough of Nigeria.A combined method of inductively coupled plasma-mass spectrometry and isotope ratio mass spectrometry was used to investigate trace and rareearth element geochemistry and hydrogen and oxygen isotopic compositions.These data were then used to infer the sediments' provenance and paleoclimatic conditions during their deposition.The sediments contained low concentrations of most trace elements,with the exceptions of Zr (651-1352 ppm),Ba (56-157 ppm),V (38-90 ppm),and Sr (15.1-59.6 ppm).Average values of Co and Ni were 1.5 and 0.7 ppm,respectively.Trace and rare earth element values were lower than corresponding values for upper continental crust and Post-Archean Australian Shale,with the exception of Zr.The samples showed only slight light rare-earth enrichment and nearly flat heavy rare-earth depletion patterns,with negative Eu and Tm anomalies,typical of felsic sources.Geochemical parameters such as La/Sc,Th/Sc,and Th/Co ratios support that the kaolinitic sediments were derived from a felsic rock source,likely deposited in an oxic environment.18O values ranged from + 15.4 to + 21.2‰ for the investigated samples,consistent with a residual material derived from chemical weathering of felsic rock and redeposited in a sedimentary basin (typical values of + 19 to + 21.2‰).While in the basin,the sediments experienced extended interactions with meteoric water enriched in δD and δ6O.However,the variation in δD and δ16O values for the investigated samples is attributed to the high temperature of formation (54-91 ℃).The δD and δ8O values suggest that the sediments,although obtained from different localities within the Lower Benue Trough,formed under similar hot,tropical climatic conditions.