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 “normalâ€, “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.  Â
Highlights
Precambrian Basement Rocks of Birnin Gwari schist belt consists of Migmatite-Gneiss-Quartzite suite represented by dioritic, granodioritic, granitic gneisses, biotite-staurolite quartz schist and syn-collisional granite
Earliest work on the Birnin Gwari Schist belt was by Truswell and Cope (1963), Turner (1983), Ajibade (1980) and Ajibade et al (2008); they reported a gneissic and migmatitic assemblage with intercalated quartzites and amphibolites; the metasedimentary greenschist-facies which consists of metapelites, meta-semipelites and metamorphosed mudstone conglomerates member preserved in the synclinal core of the belt
The basement sensu stricto comprises of banded gneiss, granite and granitic gneisses; the metasedimentary schist belt is represented by biotite-staurolite quartz schist; while syn-collision granite is represented by biotite granite (BG) and biotite-muscovite granite (BMG)
Summary
Precambrian Basement Rocks of Birnin Gwari schist belt consists of Migmatite-Gneiss-Quartzite suite represented by dioritic, granodioritic, granitic gneisses, biotite-staurolite quartz schist and syn-collisional granite. Earliest work on the Birnin Gwari Schist belt was by Truswell and Cope (1963), Turner (1983), Ajibade (1980) and Ajibade et al (2008); they reported a gneissic and migmatitic assemblage with intercalated quartzites and amphibolites; the metasedimentary greenschist-facies which consists of metapelites, meta-semipelites and metamorphosed mudstone conglomerates member (the Durumi Pebbly Schist Member) preserved in the synclinal core of the belt They reported a suite of granitic rocks ranging from diorite to coarsegrained porphyritic granite (the Older Granites). Radiometric age measurement of Rb – Sr data from nearby Kusheriki granite emplaced across the gneissic envelope gives an age of 500 + 4 Ma. which suggests Pan-African deformation and emplacement in the region (Ajibade et al, 1979; 2008) They described a complex structural sequence from parts of the neighbouring Birnin-Gwari Schist and Ushama Schist Formations and identified four deformation episodes that have affected the rocks in these areas and later define the Zungeru Mylonites. Ages of rock in the two belts ranges from Archaean to Pan-African; rocks of Birnin Gwari schist belt have been assigned Pan-African age
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