Abstract
The Western Ethiopian Shield is an exposed Neoproterozoic metamorphic belt and forms part of the Arabian-Nubian Shield. The metamorphic belt consists of high-grade biotite gneisses, low-grade volcanogenic sediments, and mafic-ultramafic complexes. The Bikilal-Ghimbi gabbro is a mafic body surrounded by these gneissic rocks, and is located 440 km west of Addis Ababa. The gabbro is elliptical in shape and covers an area of 350 km2. It consists of olivine gabbro in its center and hornblende gabbro and hornblendite at the perimeter. The olivine gabbros are very fresh and undeformed, but hornblende-bearing suites have deformational textures. Each rock type can be divided into apatite-bearing and apatite-free subtypes. The major element geochemistry shows that despite the differences between the olivine and the hornblende gabbros, there is no systematic chemical contrast between the lithotypes except for the fluid mobile elements, suggesting an origin from a common parental magma. Only the perimeter is affected by metasomatism. An estimation of the parental magma composition using the trace element abundance in fresh clinopyroxenes and fresh olivine gabbro bulk rock suggests an intraplate-type tholeiite. Crystallization model calculations using a tholeiitic parental magma suggest that the gabbros crystallized in a manner where small amounts of interstitial melt were retained. The apatite-bearing varieties are always associated with Mg-rich mineral phases, suggesting an origin from the supercooling of replenished basalt into an evolved low temperature magma chamber. The supercooling caused saturation of the apatite in the basalt melt, along with Mg-rich crystals, and these later mixed together with the more evolved crystals that had precipitated previously. The intraplate-type tholeiitic parental magma suggests plume-type magmatism for the origin of the Bikilal-Ghimbi gabbro body.
