Zircon U-Pb ages and geochemical characteristics of Late Mesozoic volcanic rocks from Shibazhan-Hanjiayanzi area of Da Hinggan Mountains and their tectonic significance

There exist a set of basic to intermediate-acid volcanic rocks in Shibazhan-Hanjiayuan area of Da Hinggan Mountains. In this paper, chronology and petrogeochemistry of trachyandensite, trachyte and rhyolite were discussed. LA-ICP-MS zircon U-Pb dating of trachyandensite yielded an age of 125.2±0.9Ma, revealing that it was a product of the Early Cretaceous volcanism. Geochemical characteristics suggest this set of volcanic rocks should belong to high-K calc-alkaline to shoshonite series. Volcanic rocks possess characteristics of REE composition as follows:∑REE are 121.42×10^-6~154.43×10^-6, the fractionation between LREE and HREE is obvious with (La/Yb)_N ranging from 19.25 to 31.98, the REE patterns show rightward inclining, and most samples present a weak Eu anomaly(δEu=0.45~0.90) except one rhyolite. The trace elements geochemistry is characterized by weak enrichment of LILE (Ba, Sr), accompanied by prominent negative HFSE(Nb, Ta, Ti). Combining previous regional achievements with this study, the authors have reached the conclusion that the Early Cretaceous intermediate-acid volcanic rocks were formed in a post-collision extension environment, which was related to closure of Mongolia-Okhotsk Ocean. Trachyandensite, trachyte and rhyolite were formed by partial melting and fractional crystallization of comagma, which was derived from an enrichment of lithospheric mantle and probably experienced metasomatism of subduction fluid.