Migmatization process and the nature of transition from amphibolite to granulite facies metamorphism in Ikare area south western Nigeria

摘要

The Ikare area is underlain by rocks of the migmatite-gneiss-quartzite complex and older granite lithologic groups and charnockites. Categorization of these rocks in the literature and published map is generalized and the rocks are undifferentiated. Though reported in few literature as an area with rocks composed of granulite facie grade, the nature of transition is not well documented. Field and petrographic studies were carried out to properly characterize the different rock types, evaluate the migmatization processes and elucidate the nature of amphibolite to granulite facie transition. Major rock types recognized and mapped includes grey gneiss, charnockitic gneiss, granite gneiss, pelitic gneiss, porphyritic granite and quartzite. Minor rock types are the mafic and ultramafic components which are made up of basic schist and amphibolites. The area suffered at least four episodes of deformation (D1, D2, D3 and D4) and at least three episodes of metamorphism (M1, M2 and M3). Petrographic studies further show that the mineral assemblage within the quartzo-feldspathic rocks changed from hydrous to anhydrous phases as one traverses from the southern part of the study area to the northern part. Reaction textures indicate that the transition zone between the amphibolite and granulite facie rocks is achieved by a prograde dehydration reaction while rehydration reactions took place during the waning retrograde stage. Anatexis and metamorphic differentiation were mainly responsible for the migmatization process in Ikare area while the charnockitization process was a product of dehydration reactions aided by reduction in water fugacity due to influx of CO2.