Determination and geological significance of Middle Jurassic intermediate-acidic volcanic rocks in central-north region of the Great Khingan Mountains volcanic belt

By studying the geochemistry and zircon U-Pb geochronology of the intermediate-acid volcanic rocks in the Beian-Linchang area, central-north region of Great Khingan Mountains volcanic belt, the age and tectonic setting of the volcanic rocks were discussed. The LA-ICP-MS zircon U-Pb dating yielded a weighted mean age of (166±1) Ma,suggesting that the volacanic rocks were formed in Middle Jurassic, mainly composed of rhyolites, trachydacites and trachytes, with SiO₂ content of 59.58%~68.34%, Na₂O+K₂O of 7.78%~8.84%,K₂O/Na₂O of 0.73~0.82%,MgO content of 0.75%~2.67% and Mg^# of 25.5~44.4,showing that the rock is relatively rich in silicon, sodium, low in potassium, high in alkali and low in magnesium. The TAS diagram and A/CNK (0.91~0.97) showed the characteristics of quasi-aluminium alkalinity of rocks, with medium amount of rare earth elements, distinct fractionation of light and heavy rare earth elements((La/Yb)_N=6.56~11.79),and weak Eu negative anomaly(δEu=0.60~0.81).The trace elements are relatively rich in large ionophile elements Rb, Th, U, and low in high-field strong elements Nb, Ta, Hf, Ti. Geological and geochemical analyses show that the intermediate-acid volcanic rocks are the products of partial melting of crustal materials, the magma might be originated from the late orogenic tectonic environment during the closure of Mongolia-Okhotsk Ocean in Early Jurassic, and the erupting diagenesis occured in the Middle Jurassic extensional tectonic setting,providing an example of the transtemporal and transtectonic activities of the magma formed in the subduction orogenic environment in the crust.