Abstract
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•We discovered unique meta-ultramafics in the Heiani ophiolite, SED, Egypt.•Protolith of the Heiani carbonate-orthopyroxenites is a highly depleted peridotite.•The Heiani carbonate-orthopyroxenites are results of regional metamorphism.•Carbonate-orthopyroxenite is considered a by product of natural carbon sequestration.
Among a set of peculiar meta-ultramafics, carbonate-orthopyroxenites are observed for the first time in the Heiani ophiolite belt, South Eastern Desert, Egypt. They form massive lensoidal masses up to 50m long and 20m wide. The lenses show a marked structural concordance with their neighboring country rocks. The typical country rocks are represented by the following high-grade metamorphic rocks: kyanite–muscovite schists, amphibolites, kyanite-bearing biotite gneisses, migmatites, granite gneisses and mobilizates. The studied carbonate-orthopyroxenites consist mainly of metamorphic orthopyroxene+magnesite, among other metamorphic, relict primary and retrograde secondary minerals. According to primary chromian spinel (Cr#, 0.7–0.84) chemistry and morphology, absence of clinopyroxene and presence of primary mantle olivine (Fo89–91) as relicts in the metamorphic orthopyroxene, the Heiani carbonate-orthopyroxenites seem to have formed from a highly depleted mantle peridotite precursor. At a late collisional stage during the Pan-Africa terrane accretion and the E–W crustal shortening (ca. 650–620Ma), high-grade (upper amphibolite facies) low-P/high-T regional metamorphism (ca. 660Ma) accompanied by CO2-metasomatism resulted in formation of the Heiani carbonate-orthopyroxenites. Mostly the carbonate-bearing shelf sediments beneath and/or in juxtaposition with the Heiani ophiolite are considered to be the proven source of the CO2-rich fluids. Although, a mixed sedimentary-mantle C source is not unlikely. A mineral paragenetic correlation with experimental data for the system MgO–SiO2–H2O–CO2 suggests metamorphic conditions consistent with those of the high-grade country rocks; i.e. 630–650°C, 6–7kbar (20–23km depth) and high-XCO2 (0.6–0.7). The CO2-bearing fluids discharging along faults gave rise to regionally widespread carbonate-bearing assemblages. Accordingly, the Heiani carbonate-orthopyroxenites are considered the by-products of natural carbon sequestration by an ultramafic rock.