GEOCHEMISTRY OF REGIONALLY METAMORPHOSED SEDIMENTARY ROCKS FROM THE LOWER BENUE RIFT, NIGERIA:IMPLICATIONS FOR PROVENANCE AND TECTONIC SETTING OF THE BENUE RIFT SEDIMENTARY SUITE

摘要

Major-elements oxide, trace- and rare-earth-elements data on very- to low-grade regionally metamorphosed pelites in the LowerBenue rift reveal that the rocks have dominant upper continental crust (UCC) geochemical signature, but without a reworking trend.There was significant weathering of the detritus in the source area as indicated by values of the CIA, the Th/U versus Th diagram and the generally high Cs concentrations in the rocks, which is reflected in their low K/Cs ratios. They are characterized by LREE enrichment and almost flat HREE, coupled with negative Eu-anomalies (Eu/Eu~* = 0.58 to 0.74) and chondrite-normalized REE patterns which are similar to those of PAAS and NASC; these are strong support for cratonic and crustal (UCC) derivation.Their protoliths were derived mainly from intermediate igneous provenance constituted by volcanogenic greywacke and argillite, containing volcanic lithic fragments which are predominantly andesitic in composition. This provenance is most likely the metamorphic preCambrian Basement Complex of southeastern Nigeria in the Oban and Bamenda Massifs/cratons, located at the eastern border of the rift. Basic input from the source terrain is suggested by enrichments in V, Cr, Ni, Sc and Ti. Their abnormally high La/Y ratios may have been introduced from the predominantly alkaline basic to intermediate igneous activity,which is coeval with the sedimentation spanning Late Aptian to Coniacian times. The sediments in the Lower Benue rift were deposited in a passive margin tectonic setting in a reducing environment as suggested by Cerium anomalies in the range of 0.01 to 0.07. This passive margin setting corroborates the extensional, within-plate, intra-continental setting obtained for the Benue rift using the igneous rock suite.