Migmatite-Gneiss-Granite Basement of Southwestern Nigeria:Mineralogical and Geochemical Evidence for a Possible CommonAncient Protolith

Abstract

Nigeria occupies southern section of the Pan-African orogenic belt and migmatite-gneiss-granite terrain forms a major component of its basement complex. Field geology, optical mineralogy and geochemical study of this rock unit in Idanre area, SW Nigeria were undertaken with a view to characterize them, elucidate their tectonic significance, and determine their origin. Major, trace and Rare Earth Elements (REE) composition were determined by XRF and ICP-MS techniques respectively in Bureau Veritas, Vancouver, Canada. Field geology reveals the granite which occur in three textural types (fine-grained (OGf), porphyritic (OGp), and coarse-grained undifferentiated OGu) intrudes the low-lying migmatite-gneiss complex. The country rock comprises of aggregation of migmatite, biotite- ornblende gneiss and banded gneiss sub-units which bear pervasive tectonic signatures of polymetamorphic and orogenic activities. Optical results reveal the migmatite-gneiss country rock comprise of quartz, feldspar and biotite with granoblastic to porphyroblastic texture.
The granitoids contain interlocks of quartz, K-feldspar, microcline, biotite, hornblende and accessory magnetite, and sphene. Large hornblende crystals in the coarse-grained granite are poikilitic while the porphyritic type contains myrmekite. The decreasing trends of Harker diagrams with respect to SiO2 indicates crystal fractionation. FeOt/(FeOt+MgO) versus SiO2 plot classifies the gneiss as ferroan type, Al2O3 versus MgO plot classifies it as orthogneiss, total alkali (Na2O+K2O) versus SiO2 (TAS) classified the rocks as granite. K2O versus SiO2 diagram classifies the rocks as continental granophyre. AFM [(Na2O+K2O)-Fe2O3-MgO] ternary diagram suggests the rocks are calc-alkaline, while ternary plot of Ba-Rb-Sr classifies the rock as anomalous granite. Diagram of Al2O3+Na2O+K2O versus Alumina Saturation Index (ANK vs ASI) indicates ASI<1.0 showing the granite is metaluminous. R1 versus R2 diagram indicate the rocks are largely late orogenic, La/Sm versus Sm diagram suggest that partial melting play a significant role in the magma source. A consistently
low ACNK (<1.1) value, a decreasing trend on ACNK versus SiO2 diagram indicates the rocks originated from anatectic melt of igneous protoliths. The similar shape of the chondrite normalized REE distribution pattern for both the migmatite-gneiss and granite indicate similar geochemical trends which possibly connote common ancient igneous protolith.