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[SMP13-04] Ophiolite in the Western Ethiopia: A fossil mantle wedge of the East African Orogenic Belt
Keywords:listvenite, metasomatism, serpentinization, East African orogeny, Ethiopia
We have studied metamorphosed mafic–ultramafic bodies in Tulu Dimtu area (western Ethiopia) of the Arabian–Nubian Shield. The ophiolitic bodies exposed within a NNE–SSW trending metavolcanics and quartzite complex; they consist mainly of serpentinite (antigorite schist), serpentinized harzburgite with minor metagabbro/metadolerite. The ultramafic bodies bear abundant Cr–muscovite-bearing silica–carbonate rocks, so called “listvenite”. The serpentinized harzburgite contains high-magnesian metamorphic olivine (forsterite [fo]93–96) with magnetite and rare relict primary mantle olivine (fo90–91). Both serpentinite schists and serpentinized harzburgite contain zoned chromite; the cores with the ferritchromite rims preserve a pristine Cr/(Cr+Al) atomic ratio (Cr#= 0.79–0.87), suggesting a highly-depleted residual peridotite likely formed a supra-subduction zone wedge mantle. Metagabbros have a MORB/OIB-like affinity; they contain epidote-amphibolite facies mineral assemblages but rich in carbonate minerals. Listvenites in Tulu Dimtu contain relict chromites that overlap with Cr# of those in serpentinite and serpentinized harzburgite, excepting one sample (Cr#=0.57). Noteworthy chromites in listvenite has a significantly higher Mg/(Mg+Fe) ratio. This indicates that a complete metasomatic replacement of serpentinized peridotite to form listvenite took place prior to re-equilibration between chromite and surrounding mafic minerals; in other words, listvenite-forming metasomatism have occurred before the serpentinization of harzburgite. The CO2-rich hydrothermal fluids infiltration into wedge mantle might have occurred prior to regional metamorphism/deformation of the EAO.