NEW UNDERSTANDING OF THE LOWER CRETACEOUS LONGJIANG FORMATION IN THE EASTERN DAXINGANLING REGION

On the basis of field survey, the stratotype section of Longjiang Formation is researched particularly on the lithostratigraphy, biostratigraphy, isotope chronology and lithogeochemistry. The meaning of Longjiang Formation is redefined. The neologism of Longjiang Formation consists of andesite and dacite with its pyroclastic rocks, rhyolitic pyroclastic rocks and sedimentary clastic rocks. The paleontological fossils belong to Eosestheria-Lycoptera-Ephemeropsis trisetalis fossil assemblage of the Middle Jehol Biota. Among them, Conchostracans are represented by the Eosestheria opima-E. (Dongbeiestheria) guanghuaensis assemblage. The volcanic lithogeochemistry is characterized by SiO₂ contents between 63.88% and 75.65%, high Al₂O₃ (12.19% to 19.39%), high Na₂O (1.04% to 5.24%), rich alkalis (Na₂O+K₂O 3.65% to 6.25%)and strong enrichment in light rare earth elements(LREE). The isotopic age of ⁴⁰Ar/³⁹Ar is 125.4±1.8 Ma.