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.

Retrogression of Orthopyroxene-bearing Gneiss of Iboropa Akoko, Southwestern Nigeria

“…Orthopyroxene present could have formed during increase in grade of metamorphism from either hornblende or biotite. In Northern Nigeria, charnockite has been derived from the transformation of biotite by the addition of ferrous iron-rich juvenile solution [32]. Orthopyroxene could be formed by any of the following reactions:…”

Petrochemistry of Charnockitic Rocks of Awo-Osuntedo Area, southwestern Nigeria

“…Charnockitic rocks constitute a major petrological unit of the Precambrian Basement Complex in southwestern Nigeria (Oyawoye, 1964;Cooray, 1972;Olarewaju, 1987;Rahaman, 1988). In most places here, charnockitic rocks occur in the amphibolite facies country rocks in contrast to most worldwide occurrences in granulite facies terranes (Olarewaju, 1987;Rahaman, 1988).…”