SIGNIFICANT TRIASSIC VOLCANIC-MAGMATIC EVENT IN THE NORTHERN SONGLIAO BASIN: Constraints on the Late Paleozoic Stratigraphic Timing

The isotopic U-Pb chronology of intrusive rocks in HFD-3 well in northern Songliao Basin is analyzed to study the formation age and tectonic evolution and determine the timing of invaded strata. The results show that the weighted average of zircon ²⁰⁶Pb/²³⁸U age is 245.5±1.9 Ma, which reflects the crystallization age of intrusive rock is Early-Middle Triassic, indicating the development of Early-Middle Triassic magmatic thermal event in northern Songliao Basin. There are also two groups of zircon ages: 269-295 Ma and 313-353 Ma, with the weighted averages of 289.0±6 Ma and 338.0±3 Ma respectively, indicating the basement or deep part of northern Songliao Basin underwent at least once Carboniferous and Early-Middle Permian volcanic activity and magma emplacement event. The tectonic mechanism leading to the magmatic thermal event should be related to the collision and post-collision extension of Paleo-Asian Ocean. The crystallization emplacement age of dike constrains the upper limit of sedimentary age of strata; that means, the clastic formation was consolidated before 245.5±1.9 Ma. Combined with other geological evidence, it is proved that the clastic rocks belong to the Late Permian Linxi Formation.