Magmatic Response to Closure of the Hegenshan Ocean and Subsequent Post‐Orogenic Extension: Insights From Middle Triassic A‐Type Granites in the Meilaotewula Ophiolitic Belt, Central Inner Mongolia, China

ABSTRACT The timing of closure of the Hegenshan Ocean and the evolution of post‐orogenic extension in the southeastern Central Asian Orogenic Belt (CAOB) are debated. To address this, we investigated the petrography, whole‐rock geochemistry and zircon U–Pb geochronology and Hf isotopic composition of the Harigentai granites that intrude the Meilaotewula ophiolitic belt in the Hegenshan ophiolite‐arc‐accretionary complex, central Inner Mongolia. These granites are monzogranites with Middle Triassic ages of 239.3 ± 1.4 and 238.7 ± 1.3 Ma. The monzogranites are weakly peraluminous and highly alkaline, and are enriched in Rb, Th, U, K and Ta, and depleted in Ba, Sr, P and Ti. They have flat chondrite‐normalised rare earth element (REE) patterns with negative Eu anomalies ( δ Eu = 0.15–0.38) and high Y/Nb ratios (1.42–2.72). They have positive ε Hf ( t ) values (7.5–12.0) and young two‐stage Hf isotopic model ages (788–500 Ma), with high zircon saturation temperatures (792°C–817°C). These characteristics suggest the monzogranites are A 2 ‐type granites that formed under high‐temperature and low‐pressure conditions by partial melting of juvenile mafic‐intermediate crust. The results of this study, combined with the spatial distribution of Carboniferous‐Triassic ophiolites and igneous rocks within the Hegenshan ophiolite‐arc‐accretionary complex, indicate that the Hegenshan Ocean closed at the end‐Permian, followed by a post‐orogenic extensional setting during the Triassic.