CHARACTERISTICS OF PYROXENES IN THE MAFIC-ULTRAMAFIC ROCKS OF THE UBO MARBLE AREA

Abstract

Pyroxene crystals of a dolerite, gabbro and pyroxenite that intruded in the Ubo marble area have been analysed on the basis of 4 cations and 6 oxygen anions using conventional electron microprobe coupled with a scanning electron microscope. The results show that SiO2 in the pyroxenite (51.43–52.52%) was slightly higher than that of the gabbro (48.32-50.83%) and that of the dolerite (47.83-48.95%) due to disilication of dolerite and gabbro liquids. The magnesium content of the pyroxenite (12.29-12.95%) is relatively higher than that of the gabbro (9.95.12.31) and dolerite (4.72-6.33%)because of the more primitive nature of the pyroxenite. ‘Others' in the pyroxenites (Cr. Ni Ba, Zr, Ti, Al) were more abundant in the gabbro (8.38-13.92) and dolerite (5.96-6.98) than that of the pyroxenite (2.45-4.51) which was probably due to crystallization modes of the different rocks. The CaO content of all pyroxenes was (21.99-24.43) which appears to be on the high side when compared to calcic pyroxenes from several diverse bulk compositions. The high abundant CaO enhanced a Wo-content of (41.43-49.36) and Xca=Ca/(Ca+Mg) (0.57-0.78) suggesting an excess of Xca = (0.02-0.23). The intergrowth of amphiboles with the pyroxenes is thought either due to exsolution-hydration process of genetic solid solution or due to both. When cation exchange and site occupation characteristics of the pyroxenes are investigated, high concentration of Alvi,. AI/Si, and Ti/AL implied high pressure/temperature crystallization conditions under bimodal or tholeiitic-alkaline transition magmatism in an island arc environment. Key words: Pyroxenite Ultramafic amphibole Island arcs. [Global Jnl Geol. Sci. Vol.1(1) 2003: 13-28]