Charnockite-granite association in SW Nigeria: rapakivi granite type and charnockitic plutonism in Nigeria?

Abstract

A plutonic complex containing both charnockitic and non-charnockitic granite rocks (Older Granites) occurs within the amplhibolite facies rocks of gneisses and migmatites in the Ado Ekiti-Akure region of southwestern Nigeria. This complex comprises three petrographic types of charnockitic rocks and three of granitic rocks. These are the coarse-grained charnockitic variety, massive fine-grained and the gneissic fine-grained types, while the granitic rocks consist of the fine-grained biotite granite, medium- to coarse-grained and the porphyritic biotite-hornblende granites. Field observation shows remarkable close association of these charnockitic and non-charnockitic components of the complex, and also geochemical evidence provides indications for a petrogenetic link between the rocks. The coarse charnockitic rock type and the granitic rocks in the area have high K 2O levels relative to SiO 2, high K 2O/Na 2O and high FeO/MgO ratios. There are striking chemical similarities which also characterize rocks of the rapakivi suite. Comparable petrogenetic processes are therefore thought to have been in operation, and the granites of the area are linked to rapakivi granite types which could be a product of charnockitic plutonism. All the above features are reminiscent of the rapakivi granite-massive anorthosite-charnockitic rock series, the close association of whic is well documented in some parts of the world. On the basis of trace element geochemistry, the charnockitic rocks are divided into two groups which are the ‘normal’ -LIL (large-ion-lithophile) and LIL-deficient types. The ‘normal’ -LIL type is represented by the coarse-grained charnockitic type while the LIL-deficient type is the massive fine-grained variety. The LIL-deficient variety has low REE while the ‘normal’ -LIL charnockitic rock type and the granites are enriched in REE and exhibit fractionated patterns. Both the LIL and REE patterns are consistent with fractionation processes involving separation of LIL-deficient phases from a basic magma emplaced under high grade conditions, and the ‘normal’ -LIL rock type with the granites represent equivalents of rapakivi granites that crystallized from the residual melt at higher structural levels.