Newly discovered late Devonian and early Carboniferous ophiolite fragments in the Diyanmiao mélange in southeastern Inner Mongolia: Implications for the late Paleozoic tectonic evolution of the southeastern Central Asian Orogenic Belt

The Late Paleozoic tectonic evolution of the southeastern Central Asian Orogenic Belt (CAOB) has long been controversial, and one of the most contentious issues has been whether there was a collisional orogenic event during the Middle Paleozoic. Here we report the discovery of a Late Devonian (373.5 ± 6.8 Ma) ophiolite fragment in the Diyanmiao mélange of southeastern Inner Mongolia. This fragment, coupled with previously reported Carboniferous ophiolites in the same region, confirm the former presence of a Late Devonian ocean basin that evolved into the Carboniferous basin. The geochemical characteristics of the Devonian and Carboniferous mafic rocks in the ophiolites are consistent with evolutionary trends from mid-ocean ridge (MOR)-like to supra-subduction zone (SSZ) compositions. The presence of a Late Devonian-Carboniferous trench-arc-basin system in the region, which is represented by the Daqing-Diyanmiao and Bayan'aobao-Hongger ophiolite belts, the Baolidao arc, and the Erenhot-Hegenshan Oceanic Basin (EHB), confirms the presence of a late Devonian-Carboniferous orogenic belt in the southeastern CAOB. The western segment of the Baolidao arc was developed on continental basement (the Xilin Gol complex), whereas the eastern segment was formed on oceanic basement. This tectonic configuration is similar to that of the modern Aleutian arc, Kamchatka-Kuril Islands and Japan-Ryukyu arc system in the northern and western Pacific Ocean. During the early Permian the oceanic basin represented by the Erenhot-Hegenshan basin closed, and the Baolidao arc was accreted onto the southern continental margin of the Southern Mongolian continental block.