Geochemistry and Petrogenesis of Granitic Rocks in Bishini Sheet 165 SW, North-Central Nigeria

Abstract

The geology and whole rock geochemistry of granitic rocks within Bishini sheet 165 SW, North Central Nigeria have been studied in order to determine the petrographic characteristics of the rock assemblages, their original protoliths and petrogenesis. Field mapping and laboratory work revealed that the study area is underlain by crystalline Precambrian Basement Complex which comprises of migmatite, granite, schist and granodiorite. The rocks have a major structural trend of NNE - SSW which was determined by measurement of joints and faults on them. Plagioclase, quartz and K-feldspar occur as the major minerals associated with the rocks with biotite and hornblende occurring as the major ferromagnesian mineral. A total of thirty-five (35) rock samples were collected from the study area which were described based on their field relationships. Twenty-nine (19) rock samples were later selected and analysed for major and trace elements using an X-Ray fluorescence (XRF) spectrometer. The results show that the granite contains, on average, 70.33 % SiO , 14.90 % Al O , 2.95 % Fe O , 1.36 % CaO, 0.52 % MgO, 3.58 % Na O, 2 2 3 2 3 2 4.52 % K O, 0.035 % MnO, and the granodiorite contains, on average, 67.06 % SiO , 15.81 % Al O , 2.89 % Fe O , 2 2 2 3 2 3 2.73 % CaO, 1.27 % MgO, 3.99 % Na O, 3.19 % K O, 0.08 % MnO. The negative linear trend between SiO and CaO, 2 2 2 MgO, Na O and Fe O on Harker plots for the granitoids of the study area suggests that the primary mineral 2 2 3 assemblage may have undergone changes during fractionation. Trace element analyses show that Barium (Ba) is the most abundant with an average of 42.67 ppm for migmatite, 428.67 ppm for granite, 419.5 ppm for schist and 440.62 ppm for granodiorite compared to other trace elements present in the rock samples. The depletion of Strotium (Sr) suggests plagioclase fractionation from magmatitic melt. Variation diagrams show that the granitoids are S-type granites, generally peraluminous though few samples displayed metaluminous characteristics of calc-alkaline affinity in a plate tectonic setting related to syn-to-within plate collision environment.