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Since the Paleozoic, the tectonic evolutionary process of eastern Eurasia has been affected by at least three paleo-ocean regimes: the Paleo-Asian Ocean, the Mongol-Okhotsk Ocean, and the Paleo-Pacific Ocean. However, the tectonic transition among these ocean regimes is not well understood. Recently, the Heilongjiang Ocean was proposed to play an important role during the transition from the Paleo-Asian Ocean regime to the Mongol-Okhotsk Ocean (and later the Paleo-Pacific Ocean) regime. Here we report on a new combined study of petrological, geochemical, and geochronological data of biotite–plagioclase gneisses from the Qinglongcun Complex in the west Jiamusi-Khanka Block to better understand the tectonic evolution during this transition period. The results show that the protoliths of these biotite–plagioclase gneisses are medium-K calcalkaline rhyolites that were derived from a volcanic island arc environment related to paleo-oceanic subduction that occurred at ca. 260 Ma. Integrated with previous studies of the Permian arc-affinity rocks within the Jiamusi-Khanka Block, we identified two island arc magmatic belts with ages of ca. 290 Ma and ca. 260 Ma at the east and west margins of Jiamusi-Khanka Block, respectively. The east island arc magmatic belt was related to subduction of the Mongol-Okhotsk Ocean during the Early–middle Permian, which provided a driving force for the opening of the Heilongjiang Ocean. Subduction of the Heilongjiang Ocean during the middle–Late Permian resulted in the west arc island magmatic belt. These two belts provide key evidence for understanding the tectonic transition from the Paleo-Asian Ocean to the Mongol-Okhotsk Ocean to the Paleo-Pacific Ocean during the late Paleozoic to early Mesozoic.

Divergent double-subduction is an important component of plate tectonics and continental growth at modern active convergent margins. However, such a process is rarely identified by deep seismic reflection (DSR) profiles, which casts doubt on its past significance. We report here a 416 km DSR profile of large dynamite shots across the Great Xing’an Range (NE China), which shows multiple arc-shaped reflections in the upper-middle crust of the Xing’an terrane, and densely layered reflection structures suggesting tectonic nappes in the lower crust. This new evidence, together with recent geological studies, indicates that the Xing’an terrane may not be a micro-continental block as previously thought, but a Paleozoic accretionary complex. In addition, there is a strong west-dipping reflection at the boundary between the Xing’an and Erguna terranes in the western DSR profile extending from the lower crust to the lithospheric mantle, suggesting that this fossil subduction zone is most likely the Toudaoqiao-Xinlin suture, which separates the Xing’an and Erguna terranes. In addition, an east-dipping strong reflection was also found in the eastern profile between the Xing’an and Songliao terranes, corresponding to the Hegenshan-Nenjiang suture at the surface. Given the newly identified crust-mantle structure, the opposite subduction zone, and other geological evidence, we propose a new model that the Xing’an terrane was developed by divergent double-subduction of the Paleo-Xing’an Ocean (part of the Paleo-Asian Ocean in the area) from ca. 500 Ma to ca. 300 Ma. Our study provides the first DSR evidence for a divergent double-subduction in the Paleozoic, which emphasizes its importance in past accretionary orogens.

Detailed analytical methods. Table S1: Major, REE and trace element analyses for the selected samples from the study area. Table S2: U–Pb isotopic data for zircons from selected samples in the study area.