Geochemistry of the Zhanbeicun Formation volcanic rocks in Xinlin area of northern Great Xing'an Range and its tectonic environment

The Lower Jurassic volcanic rocks of the Zhanbeicun Formation in Xinlin area of the northern Great Xing'an Range are petrographically composed of rhyolite and rhyolitic pyroclastic rocks.Chemically, these rhyolites belong to high-K calc-alkaline series characterized by high contents of silica, alkalis, low abundances of calcium and magnesium.The volcanic rocks have the characteristics of low ΣREE content(ΣREE=81.84×10^-6~110.32×10^-6), obvious differentiation between light rare earth and heavy rare earth elements(La/Yb)_N=21.57~40.21, with moderate negative Eu anomaly(δEu=0.42~0.62).These rocks also have characteristics of crust-derived magma with enrichment of large ion lithophile elements(LILEs, such as Rb, K, Th and U), depletion of high field strength elements(HFSEs, such as Nb, Ta, Ti and P)and lower content of basic compatible elements(such as Cr, Co and Ni)and Mg^#.The low Sr and Yb content of rhyolites indicates that it is similar to Himalayan type.Combined with tectonic characteristics of the regional Early Jurassic volcanic rocks, it is suggested that the Zhanbeicun Formation volcanic rocks were formed in the tectonic background of the southeastward subduction of the Mongolia-Okhotsk ocean.