Geochemical and genetic implications of adakites associated with the suprasubduction-type al-ays ophiolite, Northwestern Arabian shield

The Yanbu suture in the Arabian shield is defined by the southeastern boundary of the Midyan terrane against the Hijaz terrane; its southwestern part harbors the Neoproterozoic Al-Ays ophiolite, generated in a suprasubduction-zone tectonic setting. Whole-rock geochemistry of the ophiolite, especially that of the volcanic suite, exhibits REE geochemistry characteristic of E-MORB as well as calc-alkaline affinities. This study reports the new recognition of adakites in serpentinized dunite close to gabbro. Adakite petrographic and chemical features resemble those of typical dacitic adakites, strongly enriched in LREE and Al, Na, Sr, and Eu, but poor in Mg, Ti, and HREE. The geochemistry indicates derivation of partial melts directly from hydrated basaltic oceanic crust of a subducting slab at the depth regime characterized by residual garnet. A mesocratic adakitic rock with elevated Mg# occurs in the same ophiolite, indicating interaction with the overlying mantle wedge. Adakites imply formation of the Al-Ays ophiolite by subduction of young, hot oceanic crust, probably representing a small ocean basin with its mid-oceanic generation located relatively close to the subduction zone.