Relict chromian spinels in Tulu Dimtu serpentinites and listvenite, Western Ethiopia: implications for the timing of listvenite formation. Issue 13 (3rd October 2017)

• Record Type: Journal Article
• Authors: Sofiya, Ayano; Ishiwatari, Akira; Hirano, Naoto; Tsujimori, Tatsuki
• Is Part Of: International geology review. Volume 59:Issue 13(2017)
• Page Start: 1621
• Page End: 1631
• Publication Date: 2017-10-03
• Subjects: Listvenite -- carbonate metasomatism -- serpentinization -- East African Orogen -- Ethiopia; Geology -- Periodicals; Electronic journals; 551.05
• DOI: 10.1080/00206814.2016.1213142
• Languages: English
• ISSNs: 0020-6814

Abstract

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+Fe 2+ ) atomic ratios. This suggests that a nearly complete metasomatic replacement of ultramafic rocks by magnesite, talc, and quartz to prevent Mg–Fe 2+ 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).