Charnockitic Hypersthene-Quartz Diorite from Imala, Southwestern Nigeria

The hypidiomorphic granular texture, greenish brown colour and mineralogy of the hypersthene-quartz diorite from near Imala in southwestern Nigeria suggest charnockitic affinities. The rock differs from typical 'intermediate members' of charnockitic rocks in two respects: (i) antiperthite is only scantily developed and (ii) the orthopyroxene crystals lack the strong pleochroism characteristic of charnockitic hypersthenes. Trace clement contents and ratios as well as the Niggli c versus mg plot suggest basic (basaltic) parentage. Marked depletion in the alkali elements Rb and K in the marginal sample of the rock is considered to have resulted from loss of volatile elements to the arnphibolitic country rock. Occurrence of the Imala rock in migmatised amphibolite facies environment accords with Cooray's proposed selting for intrusive charnockites.