Petrology and Geochemistry of the Neoproterozoic Tays Serpentinite, Arabian Shield: Insights into the Composition and Nature of Forearc Mantle

Abstract

The Tays ophiolite is exposed near the easternmost limit of the Neoproterozoic Arabian Shield. It is dismembered, strongly deformed, metamorphosed at greenschist to amphibolite grade, and shows various styles of CO₂ metasomatism and alteration to talc-carbonate, magnesite, and listvenite. The ultramafic mantle sequence contains Fe-rich garnierite, the first report of this phase in Arabian Shield serpentinite; it is attributed to Mg-Ni cation exchange during serpentinization. The abundance of bastite and mesh textures in Tays serpentinite samples indicates harzburgite and dunite protoliths. Highly strained primary olivine relics have high forsterite (Fo_89–92) and NiO (0.31–0.47 wt%) contents. We also observe unstrained thermally metamorphosed olivine with low forsterite (Fo_84–88) and NiO (0.21–0.29 wt%) contents. The latter is associated with monticellite and talc and is formed by deserpentinization in aureoles around adjacent granitoid intrusions. Fresh cores of Cr-spinel are always surrounded by ferritchromite and Cr-magnetite rims; typically Cr₂O₃, Al₂O₃, and MgO show a systematic decrease from cores to rims, whereas FeO_T and MnO increase outward. Fresh Cr-spinel crystals have high Cr# (0.60–0.74) and low TiO₂ (0.03–0.18 wt%). The primary mineral compositions and aspects of the whole-rock compositions (elevated Mg#, Cr, Ni, and Co; depletions in Al₂O₃, TiO₂, and CaO) show that the serpentinite suite represents the remnants of depleted to highly depleted forearc mantle.

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