The identification and subduction polarity of the Baiheshan ophiolite mélanges belt in the Beishan area, Inner Mongolia-New understanding based on the geological map of Qinghegou Sheet (1: 50000)

Detailed studies of the texture and composition of ophiolite mélanges belt can provide important information for the reconstruction of ancient plate tectonics and the evolution of ancient ocean basins. The Beishan orogenic belt contains multiple belts of ophiolite mélanges and records Paleozoic subduction and closure processes of the Paleo-Asian Oceans. However, the tectonic evolution of the North Beishan orogenic belt (NBOB) is still hotly debated. Hongshishan-Baiheshan ophiolite mélanges belt which consists of ophiolite and accretionary complex is located in the north of the Beishan orogenic belt. The ophiolite mélanges show that typical block-in-matrix fabric is characterized by intense deformation recorded by tight fold, rootless intrafolial folds, penetrative foliations and duplex thrust faults. The mélange is composed of various exotic or native blocks, including ultramafic rock (metamorphic peridotite, pyroxene peridotite, peel pyroxenite and serpentinite), gabbro (344Ma), basalt, plagiogranite (297Ma), chert, limestone and Ordovician volcanic rocks. The matrix consists of serpentinite, sandstone and a small quantity of epidotechlorite schist. The sedimentary blocks composed of limestone and turbidite suggest a characteristic of slump deposit. The ophiolite mélanges belt developed three distinct foliations which showed middle and deep tectonic stratigraphy in the first two phase and nonpenetrative foliation in the third phase. Spatially, the Baiheshan ophiolite mélanges belt composed of ophiolite and accretionary complex was thrust over turbidite of Lvtiaoshan Formation and shared similar compositions, deformations, spatial and temporal distributions to that in Hongshishan area. The Mingshui magmatic arc consisting of quartz diorite-granodiorite-monzonitic granite and Baishan Formation arc volcanic rocks is located in the north of the Baiheshan ophiolite mélanges belt. Baiheshan ophiolite mélanges belt and the Mingshui magmatic arc constitute a Carboniferous-Permian trench-arc system within the North Beishan orogenic belt (NBOB) during the Carboniferous-Permian southward subduction of the Hongshishan-Baiheshan Ocean.