Geochemical constraints on the evolution and tectonic setting of United Arab Emirates Ophiolitic Serpentinites

Abstract

Serpentinites in fore-arc settings provide critical constraints on mantle depletion, redox evolution, and slab–mantle fluid transfer during subduction initiation. In the northern Semail Ophiolite of the United Arab Emirates, serpentinites of the Jabal Mundassah–Malaqet (JMM) are studied using mineral chemistry, whole-rock geochemitry, and platinum-group element (PGEs) compositions to constrain protolith characteristics, melting history, and tectono-magmatic evolution. The rocks comprise massive and foliated serpentinites dominated by intergrown lizardite and antigorite with relict Cr-spinel and altered olivine. The JMM serpentinites are characterized by high MgO (34.9–39.2 wt.%), extremely low Al₂O₃ (0.21–0.92 wt.%) and TiO₂ (0.01–0.03 wt.%), elevated Ni (up to 2462 ppm) and Cr (up to 3143 ppm), and very low high-field-strength element (HFSE) and Th–U contents. These features indicate a highly refractory dunite–harzburgite protolith formed by extensive melt extraction. This interpretation is reinforced by Cr-spinel compositions (Cr# = 0.42–0.48; Mg# = 0.58–0.61) and low total PGE abundances (ΣPGE ≈ 15–35 ppb), which are diagnostic of a strongly depleted, sulfide-exhausted mantle residue. Pronounced depletion in HFSE coupled with diagnostic Nb–Th systematics definitively fingerprints a supra-subduction zone (SSZ) fore-arc tectonic setting. PGE systematics (ΣPGE ≈ 8–26 ppb; modest Pt–Pd enrichment over Os–Ir–Ru; low Pd/Ir=1.18-1.61) record moderate partial melting followed by selective mobilization of PPGEs during serpentinization, preserving a dominantly magmatic signature with a limited metasomatic overprint. We propose the JMM serpentinites originated as a highly depleted mantle residue during intra-oceanic subduction initiation. It was subsequently serpentinized by slab-derived fluids under fore-arc conditions before being tectonically emplaced onto the Arabian margin during Late Cretaceous ophiolite obduction. Relative to the main Semail mantle section, the JMM captures an earlier, less-evolved snapshot of fore-arc development during Late Cretaceous obduction, refining models for Neo-Tethyan subduction initiation and ophiolite assembly.