Alkaline rocks of the northern part of Birnin Gwari schist belts, northwestern Nigeria: provenance and evolution

Abstract

Rocks of the northern part of Birnin Gwari schist belt is underlain predominantly by (i) banded gneiss of dioritic and granodioritic composition and granitic gneisses; (ii) biotite-staurolite quartz schist; and (iii) syn-tectonic biotite hornblende (quartzolite - BHG) granite, biotite granite (BG), and biotite-muscovite granite (BMG). Banded gneiss rocks are of hybrid sedimentary–igneous protoliths; their pelitic and mafic protoliths were derived essentially from a quartz-diorite, granodiorite and granite-quartz monzonite source. Metasediments are enriched in SiO2 (63.03 to 65.13 wt %), with moderately elevated Al2O3 (15.4 – 15.16 wt %) values and depleted in Ba, V, W, La, Nb, Nd, Rb, Th and Zr trace elements; inherited from shale-greywacke sedimentary protoliths. Cogenetic syn-tectonic granites display similar trace elements and REE patterns, with diverse trends such as “normal”, “anomalous” and “strongly differentiated” and characterized by LILE enrichment, high LREE fractionation factor (La/Yb of 6.74 to 45.14) with weak to moderate negative Eu (Eu/Eu* = 0.38 to 0.62) and strong negative Nb, P and Ti anomalies. The belt consists of rocks with alkaline affinity and evolved as back arc behind subducted Pan-African plate due effect of compressional forces and differentiation of quartz diorite, granodioritic and granite-quartz monzonite magma and partial melting of crustal components inherited from shale-greywacke sedimentary protoliths in volcanic arc and post collisional settings. The precursor of these rocks originated from basalt of depleted mantle that differentiated progressively to the granite.