An Intra-Oceanic Subduction System Influenced by Ridge Subduction in the Diyanmiao Subduction Accretionary Complex of the Xar Moron Area, Eastern Margin of the Central Asian Orogenic Belt

This study focuses on the geology, geochemistry, Sr-Nd isotopes and their tectonic settings of three types of basalts in Diyanmiao ophiolite in the Xar Moron area located on the eastern margin of the Central Asian Orogenic Belt. Type Ⅰ basalts are oceanic tholeiites w ith a depleted light rare earth elem ent(LREE) pattern, which are similar to the typical N-mid-oceanic ridge basalt(MORB) and suggests that they were formed at a mid-oceanic ridge. The initial87Sr/86Sr ratios of Type Ⅰ basalts range from 0.703 966 to 0.705 276 and the εNd(t) values are from 16.49 to 17.15, indicating that they were derived from a depleted mantle source. Type Ⅱ basalts belong to the m edium-potassium calc-akaline series and have the geochemical characteristics of Nb-enriched basalt(NEB) with high Nb content(14.5 ppm) and strong enrichment in LREEs, implying that they were created by the partial melting of mantle wedge peridotite that previously metasomatized by slab melts. Type Ⅲ basalts are high-Al basalt(HAB) with high-Al contents(Al2O3=16.75 wt.%–18.00 wt.%), distinct Nb depletion and high Th/Yb ratios. Thus they were likely generated in a normal island-arc setting. Therefore, the association of MORB, NEB, and HAB in the study area may be due to the subduction of a mid-oceanic ridge, and the Diyanmiao ophiolite is proposed to be formed in the forearc setting of a mid-oceanic ridge subduction system.