Postmagmatic arfvedsonite-aenigmatite paragenesis in the ring-dyke of the Burra Centre, Ningi-Burra complex, Nigeria

Abstract

The SW segment of the peripheral ring-dyke of the Burra Centre contains a ferrohedenbergite quartz porphyry in which fayalite has destabilized and arfvedsonite has partially replaced the pyroxene. Arfvedsonite coexists with a later formed aenigmatite, this pair is thus considered as a new subsolidus assemblage developing at the expense of fayalite and clinopyroxene. The pair also crystallizes in textures considered to be typical vapour phase postmagmatic products. This is the first recorded occurrence of aenigmatite in the porphyry ring-dykes of the Nigerian Younger Granite province. Some representative chemical analyses of the rocks and minerals are presented. While other factors such as oxygen fugacity, temperature and volatile content must have contributed to the occurrence of aenigmatite, the single most important factor discriminating between host rocks is peralkalinity, developed at the magmatic stage but initiated mineralogically here only at the postmagmatic stage. It is suggested that the arfvedsonite-aenigmatite paragenesis marks a continuation of the postmagmatic development of peralkalinity in the residual fluids. Similar pathways were initiated by the sodian hedenbergite-ferrorichterite trend found in similar porphyritic facies in the adjoining Tibchi complex.