Late-Permian subduction-to-collision transition and closure of Paleo-Asian Ocean in eastern Central Asian Orogenic Belt: Evidence from borehole cores in the Songliao Basin, Northeast China

Abstract

The process and timing of the closure of the Paleo-Asian Ocean remain uncertain. The upper Permian Linxi Formation in the Songliao–Xilinhot block provides a key sedimentary record of the closure of the eastern Paleo-Asian Ocean and the associated orogenesis. Here we present a seismic reflection profile and geochronological, paleontological, and geochemical data from 600 m of borehole cores (2022–1394 m depth) from the Songliao Basin to constrain the timing, provenance, and tectonic background of the Linxi Formation. Detrital zircons from the Linxi Formation yield a maximum depositional age of 257.4 ± 4.1 Ma. The formation was intruded by a syenite porphyry dike that crystallized at 245.6 ± 2.7 Ma. Furthermore, the paleontological assemblage indicates an upper Permian stratigraphic age. Borehole samples yield fingerprints of major oxides that are similar to clastic rocks in continental arc basins. Their Rb contents are relatively high (92 ppm on average), Rb/Sr (0.02–2.86) and Th/U (1.12–6.30) ratios are low, and (La/Yb)N and Eu/Eu* ratios are 3.85–8.13 and 0.54–0.98, respectively. These geochemical characteristics indicate a source of continental magma arc. Aided by discrimination diagrams, we propose that the Linxi Formation was deposited within a continental arc or on a continental margin during the closure of the Paleo-Asian Ocean. The Linxi Formation was deposited in a water body that experienced a gradual reduction in salinity, transitioning from a marine environment at the bottom to a transitional marine-terrestrial environment at the top. The water body also became gradually shallower and deposited sediment in the tectonic setting of continental arc. These changes indicate that there was a transition from subduction to collision during the Late Permian.