The Contact Relationship of Charnockite and Biotite Gneiss at Bauchi, Northern Nigeria

Abstract Petrographic and chemical study of biotite gneiss and charnockite and the gradational transition zone between them suggests three probable modes of origin: (i) that charnockite was regionally widespread and the biotite gneiss was derived from it by retrograde metamorphism; (ii) that the charnockite represents a conformable intrusive igneous rock which with biotite gneiss was subjected to high-grade metamorphism; (iii) that the charnockite was derived from the biotite gneiss, by the introduction of some ferrous-iron rich juvenile solution. The latter is preferred by the author because it explains better the physical, chemical, and mineralogical characteristics of the charnockite. Evidence is presented to show that the hornblende in the charnockite was not due to retrograde metamorphism and the conclusion is drawn that the crystallization of hypersthene is not evidence of dry conditions of metamorphism.