Origin and Geological Setting of Magmatic Rocks within the Kushaka and Birnin Gwari Schist Belts, Northwest Nigeria

• Published in: Geology and Natural Resources of Nigeria pp. 124 - 159
• Main Authors: Oluyede, Kehinde, Garba, Ibrahim, Danbatta, Umar, Ogunleye, Paul, Klötzli, Urs
• Format: Book Chapter
• Language: English
• Published: United States CRC Press 2025; Taylor & Francis Group
• Edition: 1
• Subjects: Birnin Gwari; Granodiorite; Kushaka; Granitoid; Island Arc; Dolerite
• ISBN: 9781032594866; 1032594861; 9781032594859; 1032594853
• DOI: 10.1201/9781003454908-8

The magmatic rocks within the Kushaka and Birnin Gwari schist belts were emplaced in the ca. 3.5-1. 0 Ga remobilized basement complex terrain that later underwent medium- to high-grade metamorphism during the Pan-African thermo-tectonic event. They show a wide range of compositions, from dolerite, diorite, granodiorite to tonalite and granite, syenite to leucogranite, and are characterized by the development of high-temperature minerals, locally attaining granulite facie metamorphism. Geochemical characteristics have revealed the magmatic rocks as tholeiitic to calc-alkaline, ferroan and magnesian, metaluminous and peraluminous in character. They also exhibit I- and S-type signatures with enrichment in LILE, radioelements (Th and U), depletion in Nb, Sr, P, and Ti, high LREE fractionation factors (La/Yb) (1.05-77.20), and pronounced negative Eu anomalies (Eu/Eu* = 0.34-1.10). A similar pattern of spidergrams shows that the magmatic rocks are genetically related and were emplaced in a volcanic arc and syn-collisional setting. The dolerite has revealed different chemical trends from the granitoids and is tholeiitic, metaluminous, and high in Fe and Mg, with relative enrichment in LILE, HFSE, low and near-flat LREE and HREE, and low fractionation [(La/Yb)N = 1.4] with an Eu/Eu* value of 1.10. The magmatic intrusion is believed to have resulted from aggregation of crustal blocks and micro-continent collisions at the eastern margin of the West Africa craton, which have led to subduction and detachment of continental keel into the asthenosphere, opening up the lower crust and generating melt, which exploited the fracture zones as pathways for the intrusion in an arc setting. The end of collision was later accompanied by uplift, major fracturing, and reactivation of the Kalangai transcurrent fault, which resulted in fracture-controlled syenite, dolerite, and pegmatite intrusions in an extensional post-collision setting.