Abstract: Jurassic stratigraphy of the eastern Daxinganling region, which covers the western margin of Heilongjiang Province, China, has not been a high priority research. For many years, coalbearing strata determination of the Middle Jurassic relies on regional correlation with the typical Middle Jurassic coalbearing Wanbao Formation of western Jilin Province. Now for the first time, we discovered a set of Middle Jurassic coalbearing strata in a new round of geological survey in the region. In this paper, we determined the accurate forming age of 2 tuffs from the Wanbao Formation of the Longjiang Basin to be 165.2±1.7 Ma and 162.1±1.6 Ma. In addition, abundant megaplant fossils were collected from these strata, which were found to represent the NeocalamitesConiopterisRaphaelia assemblage, indicating EarlyMiddle Jurassic age. Sporepollen study on the megafossils reached similar assessment. We conclude that the Wanbao Formation formed in the Middle Jurassic, most probably in the late Middle Jurassic. Geochemical analyses show that the 2 tuffs were made of peraluminous calcalkaline volcanic rocks containing high Si, Al, low Ca, P, and enriched light rare earth elements with REE pattern gently tilting to the right and negative Eu anomaly. The enriched large ion lithophile elements Cs, Th, U, and depleted high field strength elements Nb, Ti, Zr, imply that the source rocks are crust genetic types. Volcanic activity of Wanbao period may be related to the dual role of the evolution of the ancient Pacific Plate and the evolution of the MongolianOkhotsk suture zone, and is more closely related to the evolution of the MongolianOkhotsk suture zone. Currently the Wanbao Formation of the Middle Jurassic represents an important newly discovered strata for the oil and gas exploration. Here, we provide reliable geochronological and biostratigraphic evidence not only for the classification and comparison of the Middle Jurassic coalbearing strata in the eastern Daxinganling Region, but also for the formation and evolution history of the Longjiang Basin.

Key words:  megaplant fossils, sporopollen, U-Pb dating, geological significance, Middle Jurassic, Longjiang Basin, Wanbao Formation