Relict chromian spinels in Tulu Dimtu serpentinites and listvenite,Western Ethiopia: implications for the timing of listvenite formation |
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Authors: | Ayano Sofiya Akira Ishiwatari Naoto Hirano Tatsuki Tsujimori |
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Institution: | 1. Department of Earth Science, Graduate School of Science, Tohoku University, Sendai, Japan;2. Center for Northeast Asian Studies, Tohoku University, Sendai, Japan;3. Department of Mineral Exploration, Geological Survey of Ethiopia, Addis Ababa, Ethiopia;4. Nuclear Regulation Authority, Tokyo, Japan;5. Center for Northeast Asian Studies, Tohoku University, Sendai, Japan |
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Abstract: | Serpentinites (massive and schistose) and listvenite occur as tectonic sheets and lenses within a calcareous metasedimentary mélange of the Tulu Dimtu, western Ethiopia. The massive serpentinite contains high-magnesian metamorphic olivine (forsterite fo] ~96 mol%) and rare relict primary mantle olivine (Fo90–93). Both massive and schistose serpentinites contain zoned chromian spinel; the cores with the ferritchromite rims preserve a pristine Cr/(Cr+Al) atomic ratio (Cr# = 0.79–0.87), suggesting a highly depleted residual mantle peridotite, likely formed in a suprasubduction zone setting. Listvenite associated with serpentinites of smaller ultramafic lenses also contain relict chromian spinel having identical Cr# to those observed in serpentinites. However, the relict chromian spinel in listvenite has significantly higher Mg/(Mg+Fe2+) atomic ratios. This suggests that a nearly complete metasomatic replacement of ultramafic rocks by magnesite, talc, and quartz to prevent Mg–Fe2+ redistribution between relict chromian spinel and the host, that is, listvenite formation, took place prior to re-equilibration between chromian spinel and the surrounding mafic minerals in serpentinites. Considering together with the regional geological context, low-temperature CO2-rich hydrothermal fluids would have infiltrated into ultramafic rocks from host calcareous sedimentary rocks at a shallow level of accretionary prism before a continental collision to form the East African Orogen (EAO). |
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Keywords: | Listvenite carbonate metasomatism serpentinization East African Orogen Ethiopia |
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