International Journal of Science and Technology Volume 1 No. 4, April, 2012 IJST © 2012 – IJST Publications UK. All rights reserved. 164 Application of Biotite Composition in Determination of Petrogenesis of Diorites from Toro and Dass, North Central, Nigeria Dada, O.A 1 , Ashano, E.C 2 1 Department of Marine Science and Technology, Federal University of Technology, Akure, Nigeria 2 Department of Geology and Mining, University of Jos, Nigeria ABSTRACT The compositional systematics of biotites from diorites of Toro and Dass, north Central Nigeria have been examined for the purpose of describing the nature of the granitic magma. Based on chemistry of biotites, Toro and Dass diorites are formed from the transition between peraluminous and calc-alkaline magmas. This type of magma is typically produced in subduction environments. It means that the diorites could have formed in an orogenic suit from calc-alkaline magma derived from melting in a subduction zone slab. There is little evidence of either magma mixing or large-scale crustal contamination. The petrographic studies of the representative samples of diorite from Toro show biotite replacing pyroxenes, which necessarily produce a biotite-pyroxene-plagioclase paragenesis from pre-existing assemblage. The Dass diorite samples show biotite overgrowing amphiboles and also replacement of biotite by chlorite. Keywords: Biotite composition, petrogenesis, diorites, Toro and Dass, North-central Nigeria. I. INTRODUCTION The composition of minerals provides a means of evaluating the nature and conditions that existed at the time of magma emplacement. Biotite is a significant ferromagnesian mineral in most intermediate and felsic igneous rocks (Masoudi and Jamshidi Badr, 2008). Biotite compositions depend largely upon the nature of magmas from which they have crystallized (Abdel-Rahman, 1994; Moazamy, 2006; Shabbani and Lalonde, 2003). Its potential to reflect both the nature and the physicochemical conditions of magmas from which it formed is high (Masoudi and Jamshidi Badr, 2008). In this study, we present electron microprobe data of biotites from diorites of Toro and Dass, which constitute magmatites intruded into the Basement at the closing phases of the Pan African Orogeny, to investigate the nature and conditions of the magma vis- à-vis their affliations. II. GEOLOGICAL SETTING The Toro and Dass study areas are situated in north central Nigeria. Toro diorite complex occupies a low hilly area above the surrounding Basement about 28 km NE of the Jos Younger Granite Magmatic Centre. Dada et al. (1989) said that Toro diorite forms a near Annular Complex, which constituted into a circular mass of about 9km across and is composed of three types of granites and a hypersthene-diorite (Fig. 1). The inner ring shows a sharp contact with the diorite, but marginal exposures against the diorite are dark and mottled due probable to contamination (granodioritic) (Ashano, 2008). The outer ring differs from the other granite types by the absence of hornblende and its characteristic foliation is parallel to its margins. This informs its classification as an anatectic granites (Ashano, 2008). Its contact with the central granite is sharp, but where it abuts against the innermost granite in the north-western portion of the Complex, the degree of exposure precludes adequate evaluation of the relationship. The diorite outcrops in four main areas. It is typically equigranular, fine to medium grained and sometimes porphyritic. It is dark green in colour. It is suggested by Dada et al. (1989) that the Complex was formed by the initial intrusion of the medium- grained porphyritic granite (inner ring) whose outer sheared margin is now represented by the outer ring, with the subsequent intrusion of the coarse grained, equigranular granite between the outer sheared portion and the inner more massive portion. Dass study area forms part of the Nigerian Basement Complex, which is underlain by gneisses, migmatites and metasediments of Pre-Cambrian age. The major rock units of the area include granite gneiss, migmatite gneisses and diorite. Others include pegmatites and dolerites (Haruna et. al, 2008). Migmatite gneiss occurs in the northcentral and northeastern parts of the area. Texturally, they are medium to coarse grained, dark to light coloured and occur as low-lying outcrops.