The Shere Igneous Complex, Central Nigeria, geochemical constraints on the origin of peralkaline and associated granites

Abstract

The Shere Igneous Complex comprises a sequence of metaluminous-peraluminous and peralkaline granites in the following order: (1) a central amphibole-fayalite granite, metaluminous in the core and peralkaline towards the margin; (2) a mildly peraluminous biotite granite in the east of the complex; (3) a peralkaline arfvedsonite-aegirine granite forming a peripheral ring around the central amphibole fayalite granite; (4) a peralkaline arfvedsonite-biotite granite forming a semi-circular body between (1) and (3); and (5) a peralkaline albite-arfvedsonite granite forming an outer semi-circular intrusion in the west of the complex. The peralkaline and metaluminous/peraluminous granites show some overlap of agpaitic indices, but marked dissimilarities in trace element characteristics. The amphibole-fayalite granite has high contents of Ba, Sr, Zr and low contents of Rb, Th, Y and Li. The biotite granite is enriched in Rb, Y, Th, Li, F relative to the amphibole-fayalite granite and is presumably crystallized from a differentiated magma produced by fractional crystallization of the amphibole-fayalite granite magma. The arfvedsonite-aegirine granite, the arfvedsonite-biotite granite and the albite-arfvedsonite granite are enriched in HFS elements and strongly depleted in Sr and Ba; these granites are presumably associated with a fluorine-bearing phase carrying high concentrations of HFS and other trace elements. This volatile phase has caused metasomatic alterations in surrounding rocks. The part of the arfvedsonite-biotite granite that is characterized by extreme levels of HFS elements is believed to have formed by metasomatic alteration of amphibole-fayalite granite along the contacts with arfvedsonite-biotite granite. The granitic magmas were presumably formed by partial melting of anhydrous residual crust enriched in refractory ferromagnesian and accessory minerals during a previous cycle of anatexis. The high temperatures necessary for melting this granulitic crust were provided by the emplacement of mantle derived mafic magmas into the lower crust. The extensional tectonic setting allowed small batches of granite magma to rise without substantial mixing.