Neoproterozoic magmatic complexes of the Songino block (Mongolia): A problem of formation and correlation of Precambrian terranes in the Central-Asian Orogenic Belt

An important role of the early Neoproterozoic juvenile crustal growth in the formation of the Khangai group of Precambrian terranes in the Central Asian Orogenic Belt was demonstrated by the example of the Holbo Nur Zone of the Songin Block. Magmatic complexes of this zone correspond to different settings of the Early Neoproterozoic ocean: oceanic islands, mid-ocean ridges, intraoceanic island arcs, and turbidite basins. Obtained data on volcanic rocks and associated granitoids constrain a timing of the island-arc magmatic complexes, at least within the interval of 888–859 Ma. The comparison of structures of the Songino and Tarbagatai blocks of the Khangai group of terranes showed that they share many common features in their geology and evolution and may be united into the single Songino–Tarbagatai terrane. This terrane was formed owing to the Early Neoproterozoic (~800 Ma) accretion of the ocean island, spreading, island-arc, and turbidite complexes of the oceanic plate to a stable continental massif represented by the Early Neoproterozoic Ider Complex of the Tarbagatai Block. The involvement of the Dzabkhan terrane into a Khangai collage of terranes is constrained between the formation of the volcanic rocks of the Dzabkhan Formation (~770–755 Ma), which are unknown in the Songino–Tarbagatai terrane, and the Tsagaan-Olom carbonate cover (~630 Ma), overlying both the Dzabkhan and Songino–Tarbagatai terranes. It was proposed that the formation of the Precambrian terranes of the Central Asian Orogenic Belt began from the Early Neoproterozoic accretion to the Rodinia supercontinent. The fragmentation of the latter above a mantle superplume at the end of the Early Neoproterozoic spanned also the newly formed fold area. This led to the formation of terranes, which included both fragments of the Paleoproterozoic craton and Early Neoproterozoic structures. Subsequent amalgamation of these Precambrian crustal fragments into composite terranes possibly occurred at the end of the early Baikalian tectonic phase.