Geochemistry and origin of low and high TiO 2 mafic rocks in the Barkasan complex: a comparison with common Neoproterozoic gabbros of northern Somali crystalline basement

Abstract

The Barkasan complex comprises numerous metagabbroic little bodies intruded along a Neoproterozoic shear zone in the crystalline basement of northern Somalia. The constituting rocks can be subdivided into a low TiO 2 (0.41–1.39 wt.%) group, ( e.g. the lower portion of the Ago Marodi intrusion and the Wahmadhigato body) and a high TiO 2 (2.13–6.75 wt.%) group, typical of the upper portion of the Ago Marodi intrusion. The compositional and mineralogical changes result from fractionation and differentiation in a cooling tholentic magma. The low titanium rocks are related to a first stage of crystallization and the high titanium rocks represent a relatively later stage. By comparison, the Sheikh gabbro (one of the principal Neoproterozoic gabbroic intrusions in northern Somalia), despite its common contents in phases of the first stage of fractional crystallization (olivine and pyroxene), has lower nickel, chromium and Mg#. Significant differences also exist in terms of Zr Y , Zr Rb and Zr Nb ratios, reflecting a different parental magma composition.