Mineral Associations In The Boussouma Post Birimian Mafic Dyke And Their Petrogenetic Significance (Burkina Faso, West-African Craton)

Abstract

A systematic electron microprobe analysis, was carried out on constituent mineral phases (olivine, pyroxene, amphibole, plagioclase, micas, opaques) in dolerite, gabbro and pegmatitic suite associated with a mafic dyke of the Boussouma area in Burkina Faso, in order to characterize the magmatic and tectonic affinity of this intrusion. The compositions and microtextures of minerals and their paragenesis led the re- examination of the magmatic origin of this dyke. The mineralogical sequence made up of ferriferous olivine, calcic- plagioclase, orthopyroxene, augite, quartz/ micropegmatite and titano-magnetite, indicates a tholeiitic composition of the parent magma of the dyke. The composition of clinopyroxenes is in agreement with tholeiitic character of this mafic dyke and typical of rocks formed by anorogenic processes in a crustal distension zone. Enrichment in quartz/ micropegmatite and in ferro- titanic oxides from the edge to the centre of the dyke and the corresponding impoverishment of olivine, are compatible with a magmatic fractionation process. The temperature of the magma during crystallization using coexistent augite and orthopyroxene geothermometry, is estimated between 1000 and 1150°C and it progressively decreased to about 600 to 800°C, during the late crystallization stages as revealed by biotite thermometry. The post magmatic evolution of the dyke is marked by the neoformation hydrothermal minerals of dominantly low temperature hydrous phases Keywords : Dolerite, micro-analysis, minerals, tholeiite, distension, geothermometry. Global Journal of Geological Sciences Vol. 6 (2) 2008: pp. 157-174