Nature and multi-stage evolution of the Mongol-Okhotsk Ocean: New insights from the sedimentary record in the Mohe Basin

The Mongol-Okhotsk suture belt represents the result of the final closure of the Mongol-Okhotsk Ocean between the Siberian Craton and Erguna Block (also called Amuria), which played an essential role in the formation and tectonic evolution of eastern Eurasia during Mesozoic times. However, the timing of the final closure and subduction polarity of the Mongol-Okhotsk Ocean are still debated. In this study, provenance analysis of Multidimensional scaling and crustal thickness variation based on zircon Eu anomaly were used for the first time to reveal the tectonic evolution of the Mongol-Okhotsk Ocean during Mesozoic times. The results show that the evolution of the Mongol-Okhotsk Ocean can be divided into several stages. From Early Triassic (∼245 Ma) onwards, the Mongol-Okhotsk Ocean started to shrink due to southward subduction beneath the Erguna Block. The closure of western segments of the Mongol-Okhotsk Ocean occurred in the Late Triassic, leading to a weakening or cessation of oceanic plate subduction, and the prevalence of an extensional environment during 225–215 Ma. Subsequently, after the Late Triassic (starting at ca. 205 Ma), subduction of the Mongol-Okhotsk Ocean continued until the middle segments closed during Middle Jurassic (∼165 Ma). From 165 to 155 Ma, many magmatic arc rocks and metamorphic core complexes formed in the central and western segments of the Mongol-Okhotsk suture belt within a post-orogenic extensional setting. The eastern part of the Mongol-Okhotsk Ocean rapidly closed in a scissor-like manner, resulting in the collision between the Erguna Block and the Siberian Craton during the Late Jurassic (∼150–145 Ma). Collision also caused uplift of the southern margin of the Siberian Craton and the sediment deposition in the Mohe Basin. The subsequent crustal shortening associated with the collision led to the formation of nappes and the emplacement of adakites along the northern margin of Greater Xing'an Mountains. Since the Early Cretaceous, the compressional environment transitioned into a post-orogenic extensional tectonic environment influenced by the west-directed slab rollback of the Paleo-Pacific Plate and detachment of the subducted Mongol-Okhotsk oceanic plate after closure of the Mongol-Okhotsk Ocean.