Geological and tectonic significance of rodingite in the Ess ophiolite, Arabian Shield, Saudi Arabia

Abstract

We report the first study of rodingite in the Ess ophiolite, exposed at the northeast end of the Yanbu suture of the Arabian Shield. The rodingite forms thin cross-cutting dykes in serpentinite and irregular blocks in ophiolitic mélange. Both dykes and blocks of rodingite are bounded by greenish chloritite blackwall zones and may contain relics of ophiolitic metagabbro. The mineral assemblage in rodingite is (hydro)garnet+vesuvianite+diopside+chlorite+Mn-ilmenite±titanite. The bulk chemical change from gabbro to rodingite involves loss of SiO₂, K₂O, Na₂O, Al₂O₃, Fe₂O₃, and MgO compensated by a strong increase in CaO contents. Petrographic evidence indicates a two-stage process that formed diopside, hydrogarnet and chlorite first, followed by vesuvianite. We show that the major episode of rodingitization was contemporaneous with serpentinization, proceeding from temperatures near 400 °C down to about 250 °C, and predated the obduction of the Ess ophiolite section onto the Arabian Shield. This sequence of events is indicated by formation of chloritite blackwall, by shear planes cutting rodingite dikes, and by fragments of rodingite in fault breccias. We conclude that gabbroic protoliths were transformed to rodingite by Ca-metasomatism related to serpentinization of adjacent ultramafic rocks in an intraoceanic supra-subduction spreading environment. The calcium-rich hydrothermal solutions that drove rodingitization may have been derived from CO₂-bearing slab fluids released by subducted carbonate sediments, modified by reaction with and serpentinization of ultramafic mantle lithologies. The presence of rodingite and associated chloritite blackwall therefore indicates a distinctive feature of serpentinization in suprasubduction zone oceanic lithosphere, namely Ca-rich fluid fluxes during hot serpentinization on the seafloor. Because such features predate and are not associated with obduction, it is likely that subducted suprasubduction zone oceanic lithosphere would contain similar assemblages, able to modulate the transport of volatiles, Ca, and trace elements into the deep mantle.

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