The Petrology of the District around Bauchi, Northern Nigeria

Abstract

The area studied lies around Bauchi in the northeast of Nigeria and within the basement complex of Nigeria. The rocks here consist of metasedimentary rocks, migmatites, granites, quartz monzonite, and diorite. These are disposed in an aureole with the diorite and monzonite forming a core which is surrounded successively by zones of a biotite-hornblende granite, a finer grained biotite granite, and migmatites. The rocks are described with some petrographic detail. An account of the study made of the myrmekite structure commonly associated with the feldspars of the granites and the monzonite is given; and the structure is suggested as evidence of microclinization by metasomatism. Some features suggested as evidence of probable origin by plutonic process are: the spatial distribution of the rocks, the gradational nature of the boundaries between the principal rocks, the presence of a northeast-southwest lamination common to all the rocks, the crystalloblastic outline of the microcline crystals in the different rocks and the presence of schlieren and such petrographic structure as the myrmekite. This plutonic process probably involves an early introduction of metasomatic microcline and quartz, and a later soaking of the granites by a ferrous-iron-rich migma to produce diorite and monzonite. The migma probably originated partly from the unstable material displaced from the zone of granitization above and partly from the zone of transformation to eclogite below. A similarity of the quartz diorite and monzonite to the Indian charnockite is suggested, and the problem of charnockite genesis is discussed in the light of the present investigation.