Based on field geological surveying, petrology, geochemistry and LA- ICP- MS zircon U- Pb geochronology of the Wulanbaiqi rhyolite and trachyte of the Baiyingaolao Formation exposed in the northern part of the Diyanmiao ophiolitic melange zone of the Hegenshan suture, this paper discusses the petrogenesis and tectonic setting of the volcanic rocks, and the slab breakoff- post orogenic extension of the Paleo- Asian Ocean. Petrogeochemical studies show that the rhyolite of the Baiyingaolao Formation in Wulanbaiqi area has high SiO 2, Al 2O 3, K 2O and Na 2O+K 2O contents, higher Ga/Al ratios but poor in CaO, MgO, Sr, Ba, Eu, Ti and P contents. The chondrite normalized REE distribution patterns in rhyolites are of right- inclined Gull- wing shape with remarkably negative Eu anomalies. The trachyte belong to shoshonite series, which is high in Na 2O+K 2O, K 2O and Al 2O 3 and low in TiO 2, obviously enriched in Rb, Ba, U and LREEs, and depleted in Nb, Ta and Ti. The chondrite normalized REE distribution patterns in the trachytes are of right- inclined shape. Petrological and geochemical characteristics show that the rhyolites were the extrusive igneous rocks(A- type rhyolites) of the Early Cretaceous post orogenic A- type granite magma which occur in the Diyanmiao- Baiyinbulage ophiolitic melange zone of the Hegenshan suture, formed in post- orogenic extensional tectonic setting. The trachytes are the post orogenic shoshonitic rocks, were related to the previous oceanic subduction, and formed in the subsequent subducted slab breakoff- post orogenic extension tectonic setting. The zircon LA- ICP- MS U- Pb dating for the rhyolite and trachyte yielded a weighted mean 206Pb/238U age of 140. 4±1. 2 Ma and 134. 9±1. 1 Ma respectively, indicating that the rhyolite and trachyte of the Baiyingaolao Formation erupted in the Early Cretaceous, reflecting the Early Cretaceous post orogenic A- type and shoshonitic magmatism event in the Hegenshan suture zone. Combined with the crust- upper mantle electrical structure in the Hegenshan suture and the time- space evloution of the Carboniferous ophiolites, Carboniferous- Permian island arc rocks and Triassic- Early Cretaceous post orogenic A- type magmatic rocks in the Erenhot- Hegenshan suture, it is considered that post orogenic extensional tectonic- magmatism of the Erenhot- Hegenshan suture zone was related to the subducted slab break off of the Paleo- Asian Ocean during the Triassic- Early Cretaceous. The geodynamic model of the subducted slab break off- post orogenic extension of the post orogenic A- type granitic and shoshonitic magmatism is proposed.