The discovery of the Early Permian high-Mg diorite in Meilaotewula SSZ ophiolite of Inner Mongolia and and its Intra-oceanic Subduction

This paper reports a new discovery of the Early Permian Bagahaer high-Mg diorite in the Meilaotewula SSZ ophiolite in Xi Ujimqin Banner of Inner Mongolia. The Bagahaer diorite occurs along the Hegenshan suture zone. The zircon U-Pb LA-ICPMS dating shows that the age of the diorite is 282±2Ma, suggesting Early Permian. The rock has high MgO (5.00%-10.94%, Mg^# 54-74), Cr (75.00×10^-6-555.90×10^-6) and Ni (26.20×10^-6-228.40×10^-6) values and low Al₂O₃ (13.48%-14.98%), K₂O (0.48%-1.61%), TiO₂ (0.28-0.76%) and P₂O₅ (0.080%-0.160%) values. The diorite is enriched in K, Rb and Sr large ion lithophile elements, and depleted in Nb, Ta, Zr, Ti and P high field strength elements. The total REE is low (25.16×10^-6-83.57×10^-6). Rare earth curve is of flat slightly right pattern. All of the geochemical characteristics indicate that the Bagahaer pluton belongs to high-Mg dioritoids and island-arc magmatite generated along a subduction zone, similar to features of sanukitoid. Combined with the temporal and spatial distribution characteristics of the boninite and adakite within the Meilaotewula SSZ ophiolite, it is suggested that, in Early Permian, the Paleo-Asian Ocean was not closed in the study area but was at its subduction stage. It is inferred that the high-Mg diorite might have been derived from partial melting of the mantle wedge metasomatised by subducted oceanic crust+sedimentderived siliceous melts during the Paleo-Asian Ocean subduction.