Geochronology and geochemistry of the Middle Triassic magmatic rocks in the southeastern part of the Da Hinggan Mountains and their constraints on the tectonic evolution of Paleo-Asian Ocean

Middle Triassici ntermediate-basic volcanic rocks and contemporaneous granite in the Laoshanmen area on the southeastern margin of the Da Hinggan Mountains were identified.Zircon U-Pb dating results show an eruption age of 238.8±1.8 Ma for the andesite and a crystallization age of 242.2±2.1 Ma for the granite.The intermediate-basic volcanic rock samples belong to the high potassium calc-alkaline series, and are aluminous, with A/CNK being 0.92~0.97 and Mg^# being 41~45.Their chondrite and primitive mantle normalized REE patterns and spidergrams show no negative Eu anomaly but significant depletion of high field strength elements, such as Nb, P and Ti (Nb*= 0.19~0.39;Eu*=0.89~0.99).These are typical characteristics of continental lithospheric mantle modified by plate subduction action.The granite samples are weakly peraluminous I to A type transitional granite, and belong to high potassium calc-alkaline series, with A/CNK being 0.91~1.17 and Eu* being 0.06 ~ 0.66.These observations indicate that the Laoshanmen intermediate-basic volcanic rocks originated from the deep primitive mantle source, which underwent strong fractional crystallization during upwelling and was contaminated by the crust.The granites were formed by the lower crust which was heated by the upwelling intermediate-basic magma.They were formed in the transitional period of collision-post collision-extension in the Paleo-Asian Ocean, which lasted from 240~220 Ma and belonged to the tectonic environment of regional compression in the late evolution of collision orogeny and gradually turned into back-arc extension.