Geochemistry and zircon LA-ICP-MS U-Pb age of volcanic rocks in Longjiang Formation of Longjiang basin

The Longjiang Formation in Longjiang basin is composed of intermediate-acidic volcanic rocks, pyroclastic rocks and tuffaceous sedimentary rocks. The petrochemical compositions of most volcanic rocks belong to the calc-alkaline island arc series. These volcanic rocks are characterized by high SiO₂, Na₂O, Al₂O₃ and Sr content, low MgO, Y and Yb content and clear depletion of high field strength elements (Ta, Nb), similar to the characteristics of the classical adakite. These Sr values and Sr/Y ratios are also similar or close to features of most subduction-type adakites. Geochemical characteristics indicate that the magma experienced crust contamination and fractional crystallization of clinipyroxene, plagioclase, Ti-Fe-oxides and apatite during the early evolution process. The zircon LA-ICP-MS U-Pb age is 125.9±1.5Ma, consistent with the fossils data, representing volcanics formation age. Combined with the spatial variation of ages of the Mesozoic volcanic rocks in Northeast China, the authors hold that the formation of the volcanic rocks was related to the Circum-Pacific tectonic system as well as the Mongolia-Okhotsk structural belt.