The Early Permian adakite in the Meilaotewula ophiolite of Inner Mongolia and intra-oceanic subduction in eastern Palaeo-Asian Ocean

This paper reports the Early Permian adakite in the Meilaotewula SSZ-type ophiolite, Inner Mongolia.The Meilaotewula adakitic rocks are mainly composed of andesite and dacite, which were emplaced into the Late Carboniferous Meilaotewula suprasubduction zone(SSZ)ophiolite and were developed along the Hegenshan suture zone.The zircon U-Pb LA-ICP-MS dating result reveals that the Meilaotewula adakite is dated at 294.1±2.2 Ma, suggesting that it was formed during Early Permian.The adakite belongs to the low-K tholeiitic and medium-K calc-alkaline series.The rocks show such geochemical characteristics as high SiO₂(64.12%~69.12%)and Al₂O₃(16.05%~18.59%)content with rich sodium and poor potassium(Na₂O=5.08%~6.80%, K₂O=0.70%~1.22%, Na₂O/K₂O=4.50~7.26).For trace elements, they show high content of Sr(291.22×10^-6~762.20×10^-6)and low content of Yb(0.74×10^-6~1.28×10^-6)and Y(7.33×10^-6~12.74×10^-6).In addition, they are relatively enriched in large-ion lithophile elements such as K, Rb, and Sr and depleted in high-field-strength elements such as Nb, Ta, Zr, Ti and P. Furthermore, they have low total rare-earth element(REE)content(40.97×10^-6~108.69×10^-6), with low heavy rare-earth elements(HREE)without obviously negative Eu anomaly.These geochemical characteristics indicate that the Meilaotewula andesite and dacite belong to adakite.The adakite was formed in the island arc environment of subduction zone, which might have originated from partial melting of subducted oceanic crust and then interacted with mantle wedge peridotite during the rise of subduction zone.The adakite and the Meilaotewula ophiolite(308 Ma)made up one abundant and complete rock assemblage, which was formed by the initial intraoceanic subduction.Therefore, Meilaotewula ophiolite consists of serpentined augite peridotite, beded-massive gabbro, pillow basalt, boninite, Nb-enriched basalt and high-Mg andesite.The results show that the initial intraoceanic subduction occurred in Late Carboniferous to Early Permian in southeastern Palaeo-Asian Ocean.