Geochemical constraints on the provenance, paleoweathering and maturity of the Mamfe black shales, West Africa

摘要

Whole rock geochemistry was conducted on shales of the Mamfe Basin (West Africa) to identify their source rock, tectonic environment, paleoweathering and maturity. Binary diagrams of Th/Cr, La/Sc, Th/Sc, Zr vs TiO2 AL(2)O(3) vs TiO2, and, Cr/Th, designate that the shales originated from a felsic rock source. The felsic composition of the shale is also shown by its abundance LREE, flatter HREE model, and a negative Eu anomaly. Ternary diagrams of Th-Sc-Zr/10, and La-Th-Sc, plus binary diagrams using DF(A-P)M, DF(A-P)MT designate an active margin rock source for shales, which during deposition had passive margin characteristics. The indexes of weathering (CIX and CIA) of the studied samples suggest that the strength of chemical weathering was low to moderate. The Plagioclase Index of Alteration (PIA) suggests that the strength of plagioclase lexiviation was low to intense. The discrepancy was probably caused by K+, freed during weathering from K-feldspars, forming and adhering to clays, whereas Na+ and Ca2+ left the sediment in solution. This caused the CIA, with K in its formula, to incorrectly indicate low amounts of weathering, whereas the PIA, without K influencing the outcome, provided evidence for more intense weathering. Binary plots of Rb/Sr, Th/U, Zr/Sc and Th/Sc, and (Gd/Yb)(N) ratios, plus correlation of Zr and (La/Yb)(N) and low ICV values shows the Mamfe shales are immature first cyclesediments, which have not undergone heavy mineral enrichment.