peraluminous

Geochemistry, Classification Characteristics of Pegmatites From Ijero-Ekiti, Ekiti-State, Southwest Nigeria. (Published)

Geochemical analysis of ten (10) representative pegmatite samples  reveal high silica (SiO₂) 66-78 wt %  and alumina (Al₂O₃) 10.08-19.57 wt % contents. The alumina (Al₂O₃) is greater than the alkali Na₂O + K₂O +CaO in all the rock samples by values ranging between 1.43 and 4.90 wt % implying that the rocks are peraluminous. MgO (0.02-1.60wt%), CaO (-0.01-1.39wt %) and Fe₂O_{3 T}  (0.77–6-99 wt %) have low contents. Rocks that are characterised by low Mg, Ca, and Fe as well as low ratio of Na₂O/K₂O are termed “fertile” and  peraluminous. A plot of Al₂O₃/(CaO + Na₂O + K2O) versus Al₂O₃/Na₂O + K₂O shows and confirms the peraluminous character of the  pegmatite from Ijero while the  molecular Al₂O₃/Cao+Na₂O+K₂O versus SiO₂ shows the plottings on the S-type granitoids . Rocks that are fertile and peraluminous are  rich in albite (NaAlSi₃O₈), potassium feldspar (KAlSi₃O₈) and quartz (SiO₂). The abundance of feldspar and mica are geochemical indicators for Sn-Ta mineralisation. Industrial minerals (Kaolin, feldspar and gemstones (tourmaline, beryl and topaz)) occur in abundance considered significant and exploitable. The crossplots of MgO, Fe₂O₃, Na₂O Al₂O₃ and TiO₂ against SiO₂.respectively indicate negative correlation that suggest non association with SiO₂. However, some oxides like Fe₂O₃, Na₂O Al₂O₃ trend negatively downward but cluster along the trend showing  their relative abundance with silica .  The plotting of samples in both the tholeiitic and calc-alkaline fields shows that the magma from which the pegmatite was formed was not totally restricted in occurrence only to subduction-related environment but also from the oceanic crust of calc-alkaline environments.    Samples plottings  on both the igneous, sedimentary and metasedimentary origins as seen in   Na₂O/Al₂O₃ versus K₂O/Al₂O₃   support  the derivation of materials that made up the rock from mixed sources. Further  classification using various parameters show that pegmatite samples  plot on the granodiorite, syenogranite, alkali granite and quartz syenite as well as granitic groups, all having the attributes of  granites, and  supporting  the granitic character of pegmatites .

Keywords: Geochemistry, S-type granitoid, continental/oceanic., mixed sources, peraluminous