High-Mg andesite/diorite (HMA) is useful for identifying subduction-related processes in orogenic belts, including the identification of ophiolites formed in suprasubduction zone (SSZ) environments. The E'rentaolegai high-Mg diorite from the Diyanmiao ophiolite in central Inner Mongolia, North China, has been investigated revealing low-K tholeiitic–calc-alkaline characteristics and have SiO2 contents of 53.44–54.92?wt%, MgO contents of 8.44–9.54?wt%, and Mg# of 54.35–57.60, with variable Fe2O3 (7.51–8.61?wt%), Al2O3 (11.95–15.09?wt%), and Na2O (3.42–3.94?wt%) contents, low K2O (0.34–0.97?wt%), TiO2 (0.35–0.67?wt%), and P2O5 (0.12–0.15?wt%) contents, and high Ni (43–193?ppm) and Cr (189–556?ppm) contents. Samples collected have low total rare earth element (REE) contents (30.58–77.80 ppm), with flat or slightly right-dipping REE patterns (LaN/YbN = 2.19–3.11) and a lack of pronounced Eu anomalies. The samples are also enriched in large-ion lithophile elements (LILEs, e.g., K, Rb, Ba, U, and Sr) and depleted in high field strength elements (e.g., Ta, Nb, Ti, and P). The E'rentaolegai high-Mg diorite has characteristics typical of HMA, and are similar to those of sanukites from the Setouchi Arc in SW Japan. They also display high positive εNd(t) values (+6.32 to +7.80), comparable to the values of their host rocks. Petrogenetic analyses suggest that the E'rentaolegai HMA was probably formed by the interaction of partial melts and aqueous fluids from subducted sediments with mantle peridotite. Zircon U-Pb dating reveals that the high-Mg diorite crystallized at 313.6?±?2.4 Ma, i.e., late Carboniferous. Combining our data with the temporal and spatial distribution of the Diyanmiao SSZ-type ophiolite, we propose that the eastern Paleo-Asian Ocean had not closed by the late Carboniferous, but intra-oceanic subduction was ongoing. A new model of the initiation of subduction in the eastern PAO during the late Paleozoic.