Contrasting petrogenetic attributes of associated pegmatitesaplites in Iwo Area, Southwestern Nigeria

Abstract

Field, petrographical and geochemical data were used to constrain the petrogenetic attributes of the associated pegmatites and aplites in Iwo area, southwestern Nigeria. The pegmatites and aplites, which occur as veins and dykes are hosted by the 642 ± 6 Ma quartz potassic syenite. The pegmatites consist of quartz, feldspar with rare biotite and occasional beryl while in addition the aplites contain hornblende instead of biotite and totally lack beryl. Field evidences indicate that the pegmatites are older than the aplites. The two rock varieties show contrasting petrogenetic attributes in their chemical signatures. This is shown by the lack of any correlation or lines of descent in Harker's diagram, non-similarity of the K/Rb and Nb/Ta ratios and the contrasting patterns displayed by some elements in the mantle normalized diagrams (spidergrams) for the associated pegmatites and aplites. The elemental ratios do not support any rare element mineralization potentials for the two rock varieties. The two rock varieties lack significant tetrad effects which indicate crystallization from less evolved magma. This corroborates the negligible Eu (europium) anomalies for the two rocks (pegmatite: Eu/Eu*= 1.02, aplite: Eu/Eu*= 1.09). The parental magmas of these rocks though less evolved, displayed contrasting geochemical attributes which indicate different origins. This observation contrasts the general view that associated pegmatite-aplite have a common magma source.

Keywords: aplite, pegmatites, Neoproterozoic, magma, Iwo, southwestern Nigeria