The petrology of the older granites around Bauchi, Nigeria

Abstract

The area studied is situated in the North East of Nigeria and consists of a rock complex known as the Older Granites to differentiate it from the granites of the ring - dyke complexes, termed the Younger Granites. The rocks in the district are classified as metasediments, migmatites, Biotite Granite, Biotite Hornblende Granite, Fayalite Quartz Monsonite and Quartz Diorite. In the field the rocks were examined in detail and a geological map of the area constructed. The contant relationships of the members of the OlderGranites are described and results of detailed petrographical examination given. In particular, a study has been made of the microstructures, including perthite and myrmekite; from various considerations the microstructure appear to indicate a metasomatic origin for large microcline crystals found in these rocks. Various lines of evidence for and against both magmatic and metasomatic origins for the Bauchi rocks are examined; the equivocal nature of much of this evidence is stressed. The weight of petrographic and field evidence, backed by the plutonic setting and spatial distribution of the different rocks, is thought to indicate a metasomatic origin for the migmatites, the Biotite Granite and the Biotite Hornblende Granite. The Quartz Diorite and the Fayalite Quartz Monzonite are demonstrated to be related the Charnockite series; they originated by a soaking of the Biotite Granite and the Biotite Hornblende Granite respectively with dilute basic magma or solutions under deep seated conditions. It is suggested that the basic solution could have originated from the basic material displaced in the zones of granite and migmatites. The petrology of the Bauchi rocks is considered in the light of problems relating to granites and charnockites in general. The account includes ten chemical analyses of' rocks from the Bauchi area, as well as optical data on the minerals and numerous modal analyses.