The reworked Pre-Neoproterozoic and juvenile Neoproterozoic terrane of the Western Ethiopian Shield (WES) consists of three N-S trending terranes. These are the western migmatitic gneissic terrane, the central metavolcano sedimentary terrane (CVST) and the eastern migmatitic gneissic terrane. The eastern part of the CVST mostly consists of suture-related ultramafic-metasedimentary complexes, whereas metavolcanics predominate in the western part. Gabbroic to granitic intrusions frequently occur in the CVST and in adjacent areas. New zircon SHRIMP U-Pb ages for two gabbros and three diorites in the Ghimbi-Nedjo region of the WES indicate magmatic crystallization ages. Two pulses of magmatism, at 860-850 and 795-785 Ma, are documented with the former for the first time. The tholeiitic Kemashi diorite and BikilalGhimbi gabbros have oceanic affinities and yield U/Pb zircon ages of 856.3 ± 9.8 and 846.0 ± 7.6 Ma, respectively. The calc-alkaline Gebeya Kemisa pyroxene diorite, and the Senbet Dura hornblende diorite plus the tholeiitic Wayu Meni gabbro, which collectively have arc-back arc characteristics are indistinguishable at ages of 794.3 ± 9.4, 787.7 ± 8.8 and 778.1 ± 6.3 Ma, respectively. Positive eNd (4.5-7.0) and low initial 87 Sr/ 86 Sr (0.7029 ± 0.0002) and a mean T DM model age of 0.95 Ga for the GhimbiNedjo region (mean T DM model age of 0.95 Ga for the WES overall) indicate that the magmas were generated from juvenile Neoproterozoic depleted mantle sources, with no discernable involvement of pre-Neoproterozoic continental crust. The occurrence of gabbros and diorites with oceanic tholeiite affinities combined with the new ages suggests that the intrusions were emplaced in the earliest stages of the rifting of Rodinia. This event in the WES led to the development of a passive margin and associated plume-type magmatism at *855 Ma. The two intrusive groups with differing magma chemistry and ages suggest that the earliest magmatism was tholeiitic and associated with the passive margin system followed by continental breakup to form the Mozambique Ocean. The combination of tholeiitic and calc-alkaline magmatism was related to arc and back-arc basin formation and later terrane accretion (*830-690 Ma).

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 km 2 . 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.