Slab‐Mantle Interaction During Subduction Initiation: Constraints From Trace Element and Sr‐Nd‐Pb Isotope Systematics of Boninite and Other Magmas and Metamorphic Sole in the Oman Ophiolite

The Late Cretaceous Oman ophiolite includes a series of volcanic rocks generated during the transition from spreading ridge to protoarc associated with subduction initiation. We analyzed major and trace elements and Sr, Nd, and Pb isotope compositions of lavas and dikes of the protoarc stage, especially boninites. We also analyzed amphibolites and metacherts of the metamorphic sole, as subducted slab materials. Furthermore, we examined trace element patterns reconstructed based on analyses of whole rocks and relict clinopyroxene phenocrysts from volcanic rocks of both axial and protoarc stages. The compositions of protoarc tholeiites, which represent the first and most voluminous magmas generated in the protoarc stage, are consistent with flux melting of residual depleted mantle, metasomatized by aqueous fluids liberated from the amphibolite‐facies slab. On the other hand, the successively produced calc‐alkaline, low‐Si boninites show distinctly radiogenic Sr, Nd, and Pb isotope ratios, spoon‐shaped rare earth patterns, and low Nb/Ta ratios, which require addition of amphibolite slab fluids formed at higher temperatures as well as small amounts of mafic‐sedimentary hybrid slab melt to the residual highly depleted mantle. Although axial lavas lack enrichment in fluid‐mobile elements except for K, later off‐ridge lavas exhibit clear K, Sr, and Pb enrichments, suggesting decompression melting of fluid‐metasomatized mantle associated with subduction initiation near the dying spreading ridge. The resultant hot subduction zone is favorable for mantle wedge melting to generate tholeiitic and boninitic magmas in the protoarc stage.