The recognition of Baiyunshan ophiolitic mélanges belt in Beishan orogenic belt, Inner Mongolia and its indication for the subduction of the Beishan ocean

The NWW-striking Baiyunshan ophiolitic mélanges belt is exposed in the central part of Beishan orogenic belt, which extends from Hongliuhe-Niujuanzi in the west to Xichangjing in the east.There are three stages of structural deformation in the Baiyunshan ophiolitic mélanges belt, i.e., mylonite fold in subduction period, thrust fault system in collage period and strike-slip fault system in uplift period.The Baiyunshan ophiolitic mélanges are composed of different types of blocks and matrix.The ophiolitic blocks include rocks of chassignites, pyroxene peridotites, olivine pyroxenolites, pyroxenites, carbonated ultramafic rocks, gabbros, basalts, plagiogranites, and the blocks include rocks of siliceous rocks, limestones and sandstones.The matrix is composed of serpentine matrix, green-schist matrix and sand-slate matrix.In the center of the ophiolitic mélanges belt, the fragments of intact oceanic crust from south to north were found, which are composed of cumulate ultramafic rocks, cumulate gabbros, basalts.Based on the identification of MORBs, OIBs and Late Cambrian island arc-calc alkaline gabbros, the authors believe that the ophiolite blocks of MOR type, OIB type and SSZ type were developed in the Cambrian in the Baiyunshan ophiolitic mélanges belt, and the subduction of the Hongliuhe-Xichangjiang Ocean lasted until late Silurian.