Geological characteristics, formation epoch and geotectonic significance of the Yueyashan ophiolitic tectonic mélange in Beishan area, Inner Mongolia.

The Yueyashan ophiolite tectonic melange belt is located in the Hongliuhe-Xichangjing ophiolite belt of eastern Yueyashan area. Yueyashan ophiolite suite is exposed completely, composed from the bottom upward of ultrabasic complex, gabbro, layered basalt and pillow basalt as well as radiolarian siliceous rocks. On the northern side of the ophiolite suite there happened intense tectonic melange activity, forming the ophiolite tectonic melange belt composed mainly of strongly mylonitized and serpentinized pyroxene peridotite, basalt matrix, pyroxenite, cumulate gabbro, plagioclase granite, peridotite, olivine gabbro, hornblendite, dolomite, radiolarian siliceous rocks, and altered basalt. The authors carried out zircon U-Pb isotopic dating of ophiolite suite pyroxenite, gabbro, plagioclase granite and gabbro diorite. In addition, the authors conducted U-Pb isotope experiments on andesite and granodiorite derived from the Xieshan-Dongqiyishan volcanic arc on the northern side of the ophiolite suite. The isotope dating confirmed that the Yueyashan ophiolite suite was formed at about 530Ma, corresponding to early Cambrian. It is also concluded that the large-scale ocean basin subduction from the south to the north occurred in the period of 421.0±15-442.4±1.5Ma, suggesting Silurian. Based on the study of stratigraphic formation environment on both sides of the ophiolite belt and its significance for the limit of stratigraphic unit, the authors hold that the Hongliuhe-Xichangjing ophiolite belt represents the ancient ocean closed plate suture zone. With this zone as the boundary, the northern side is the Kazakhstan plate and the south side is the Tarim plate.