Tectonomagmatic origin of some volcanic and sub-volcanic rocks from the Lower Benue rift, Nigeria

Abstract

Petrological and geochemical studies on some volcanic and sub-volcanic rocks from the Lower Benue rift indicate that they are basalts, basaltic and doleritic sills, trachybasalt and trachyte which generally belong to the alkali basalt series. The alkaline affinity is clearly evident in both their normative and modal mineral compositions, as well as their chemical compositions. The generally high fractionation indices [(La/Yb) N ] are 7.06 to 17.65 for the basaltic rocks and 23.59 to 135. 35 for the trachytic rocks, against low values commonly seen in subalkaline (tholeiitic) series, with strong enrichments in the incompatible elements. All this strongly supports their alkaline affinity. The basaltic rocks are generally fine-grained and porphyritic, consisting of phenocrysts of clinopyroxene and olivine in the groundmass of the same minerals together with plagioclase. The clinopyroxene is either diopside or clinoenstatite. The trachyte consists of oligoclase, orthoclase, biotite, quartz and exhibits typical trachytic, flow structure. The basaltic and doleritic sills are commonly altered, with calcite and epidote as common alteration products. This alteration, which is reflected in the erratic behaviour of K 2 O, MnO and P 2 O 5 on Harker variation diagrams, high values of LOI, strong depletions in the more mobile LILE (Rb, K, Ba and Sr) and high Th/Ta ratios, is attributed to the effects of an aqueous fluid phase and crustal contamination. On the whole, the mineralogical, as well as major-, trace-elements and REE data suggest that the rocks are co-genetic and most likely derived from differentiation of an alkali olivine-basalt magma, generating through variable low degrees of partial melting of probably an enriched lithospheric (upper) mantle following an asthenospheric uplift (mantle plume or intumescence) with HIMU signatures in a within-plate continental rift tectonic setting. This corroborates earlier results obtained for the intrusive rocks in the region.

Petrological characteristics of some cretaceous igneous rocks, in Southwest of Gboko, Southern Benue Trough, Nigeria

“…The observed alkaline affinity of these rocks suggests that they are products of extensional tectonism. It is consistent with an active rifting model (Wilson and Guiraud, 1992;Obiora and Charan, 2011;Najime, 2014) and corroborates the results obtained by previous workers in the area and that of the Benue Trough as an aulacogen(a failed rift).…”

“…nepheline) in the dioritic rocks, and the presence of clinopyroxene and olivine in the groundmass of the doleritic rocks suggest an alkaline affinity for these rocks. This mineralogical assemblage suggests that they are co-genetic and likely derived from differentiation of an alkaline olivine-basalt magma, generating through variable low degrees of partial melting of an enriched upper mantle following asthenospheric uplift (mantle plumes) (Obiora and Charan, 2011). General alteration into calcite in these rocks can be attributed to the effects of a -rich hydrous phase possibly related to calcium-rich saline groundwater pressed out of calcareous marine sediments in the area during burial (Obiora and Charan, 2010).…”

Whole-rock geochemistry of basic and intermediate intrusive rocks in the Ishiagu area: further evidence of anorogenic setting of the Lower Benue rift, southeastern Nigeria

“…The Benue rift, the failed arm of the triple junction, was formed during the separation of the South American plate from the African plate in the Mesozoic times through the Gulf of Guinea-South Atlantic-Benue triple junction (Burke et al, 1971;Grant 1971), as cited by Obiora and Charan (2011a) (Figure 1). The Benue rift is subdivided into Upper, Middle, and Lower parts based on geographic locations (Petters, 1978;Nwajide, 1990;Idowu and Ekweozor, 1993;Obaje et al, 1999) Aptian to Coniacian) sedimentary rocks that have been compressionally folded, faulted, and uplifted in several places and have undergone regional burial metamorphism at the anchizonal to lower greenschist grade (Benkhelil, 1986(Benkhelil, , 1987Obiora, 2002;Obiora and Umeji, 2004;Obiora and Charan, 2011b).…”

“…The mechanism of triple junction rifting involves mantle plume, crustal stretching and thinning, block faulting, and igneous activities (Olade, 1975;Bott, 1976). However, other theories were proposed, such as the transform fault mechanism along the northern margin of the Gulf of Guinea (Benkehlil, 1986) and the membrane stress model, which explains the presence of a series of anticlinorium and synclinorium suggesting a deformational stage in the evolution of the rift (Obiora and Charan, 2011a). Farrington (1952) and Burke et al (1971Burke et al ( , 1972 proposed a subduction origin for the Benue rift igneous suite due to existence of folds and a wrong interpretation of the pyroclastic rocks in the Abakaliki area as andesite.…”

“…The Benue rift, the failed arm of the triple junction, was formed during the separation of the South American plate from the African plate in the Mesozoic times through the Gulf of Guinea-South Atlantic-Benue triple junction (Burke et al, 1971;Grant 1971), as cited by Obiora and Charan (2011a) (Figure 1). The Benue rift is subdivided into Upper, Middle, and Lower parts based on geographic locations (Petters, 1978;Nwajide, 1990;Idowu and Ekweozor, 1993;Obaje et al, 1999) (Figure 1).…”