Objectives: The Zagayin high- Mg andesite is newly recognized in the Meilaotewula SSZ(Supra Subduction Zone)- type ophiolite of the Hegenshan suture, Inner Mongolia. High- Mg andesites are usually believed to be resulted from the equilibrium interaction between overlying mantle peridotite and siliceous melt derived from dehydrated fluid of subducted oceanic crust + sediments, representing intraoceanic arc and initial subduction. However, whether the Late Carboniferous Paleo- Asian Ocean was still in subduction stage is lacks of further definitely petrological and chronological evidences and constraints. Therefore, we carried out zircon LA- ICP- MS U- Pb geochronological and geochemical study on the Zagayin high- Mg andesite to discuss its origin, and to provide new evidences for the tectonic evolution of the eastern Central Asian Orogenic Belt. Methods: Based on field geological surveying, petrological, geochemical and LA- ICP- MS zircon U- Pb geochronological study, this paper discusses the petrogenesis and tectonic setting of the Zagayin high- Mg andesite, and the subduction of the Paleo- Asian Ocean. Results: The Zagayin high- Mg andesite is mainly composed of basaltic andesite and andesite, which has relatively high contents of MgO(4. 33%~9. 82%, Mg#57~72) and SiO2(55. 34%~64. 12%), low those of TiO2 (0. 30%~0. 60%), K2O(0. 06%~0. 73%), Al2O3(12. 18%~17. 32%), low FeOT/MgO values (0. 70~1. 34), and high Na2O/K2O values (2. 23%~40. 88%). The total REE is low(14. 29×10-6~60. 20×10-6). Rare earth curve is of flat slightly right pattern (LaN/YbN=0. 57~2. 58), without pronounced Eu anomaly. The rocks are enriched in Rb, Ba, Th, U and Sr large ion lithophile elements, depleted in Nb, Ta, Ti and P high field strength elements, and with low Ti/V. It has typical characteristics of high- Mg andesite(HMA) and is similar to sanukite of the Setouchi arc volcanic belt in the SW Japan. The zircon U- Pb LA- ICP- MS dating shows that the age of the Zagayin basaltic andesite is 315. 0 ±2. 3 Ma, reflecting the Late Carboniferous high- Mg andesitic magmatism event. Combined with the crust—upper mantle electrical structure and the temporal—spatial distribution of the Late Paleozoic ophiolites—arc magmatic rocks in the Hegenshan suture, the Late Carboniferous intraoceanic subduction model of the Paleo- Asian Ocean for the Zagayin high- Mg andesite has been preliminarily proposed. Conclusions: This study determines the Late Carboniferous (315. 0 ±2. 3 Ma) Zagayin high- Mg andesite, which was probably generated by equilibrium interaction between overlying mantle peridotite and siliceous melt derived from dehydrated fluid of subducted oceanic crust + sediments and partial melting of subducting sediments. The Zagayin high- Mg andesite was probably formed in the fore- arc tectonic environment in the Late Carboniferous. The discovery and confirmation of the Zagayin high- Mg andesite in the Late Carboniferous may provide new evidence for the Late Carboniferous intraoceanic subduction of the Paleo- Asian Ocean.