Geochemical Characteristic and Petrogenesis of Malumfashi Schist around Tandama Area, North-Western Nigeria

摘要

Malumfashi schists which occur in association with migmatite-gneisses and biotite granite, occupy about 60% of Tandama area, north-western Nigeria. These rocks were studied with a view to evaluate their compositional characteristics and their evolution.The lithology revealed from systematic mapping and petrographic examinations shows that the schist is mica schist, which is part of the Malumfashi Schist Belt of Nigeria. The mica schist consists of greater than 50% micaceous minerals and less than 50% quartz and feldspar. It is light grey to silver grey in colour, well foliated and mostly weathered to rusty brown or reddish brown. Mineralogical determinations from optical studies show a high proportion of biotite and minor amounts of plagioclase and quartz. Chemical analysis of the samples obtained by Inductively Coupled Plasma Mass Spectrometry (ICPMS) instrumentation method, involving major and trace elements reveals the nature of the mica schist. The A-CN-K diagram reveals a shale protolith which suffered moderate to high chemical weathering, prior to being metamorphosed. Obtained Rb/Sr ratio (>0.4%) and SiO2/Al2O3 values support the shale protolith hypothesis for the schist, diagnostic geochemical features like the Na2O/Al2O3 values, trace element ratios such as Th/U and REE patterns suggest that the protolith are predominantly derived from felsic igneous sources. Tectonically, SiO2/Al2O3 versus K2O/Na2O plot revealed the protolith to have evolved within passive margin environment.