Triassic Magmatism in Central Great Xing’an Range, NE China: Constraints on the Tectonic Evolution of Mongol–Okhotsk and Paleo–Asian Oceans

Abstract:

The Triassic tectonic evolution of NE China was dominated by post–orogenic extension due to lithospheric delamination after the final closure of Paleo–Asian Ocean. Nevertheless, some workers recently proposed that the southward subduction Mongol–Okhotsk Ocean played an important role in the tectonic development of the Erguna Massif during Triassic. Thus, what deep geodynamic processes controlled the early Mesozoic evolution of the Xing’an Terrane remains debated. In this contribution, we present zircon U–Pb ages and geochemical data of Triassic granitic intrusions in central Great Xing’an Range, aiming to provide constraints on the question. Zircon U–Pb dating demonstrates a Triassic granitic belt with NNE orientation in the Xing’an Terrane, yielding ages between 244 and 206 Ma. The intrusive rocks consist mainly of monzogranite and syenogranite, with minor granodiorite. Geochemically, they are enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs), and depleted in heavy rare earth elements (HREEs) and high field strength elements (HFSEs) such as Nb, Ta, and Ti, similar to those from an active continental margin or an island arc setting. However, they contain high SiO₂ and total K₂O+Na₂O contents, and high K₂O/Na₂O, Sr/Y and Zr/Y ratios, indicating that the Triassic rocks are actually post–orogenic high–K calc–alkaline I–type granites, and the arc–affinity features could be derived from the previous subducted oceanic fragments. The rock association, geochemical data and spatial distribution of the Triassic intrusions, along with regional sediment and structure information, make us conclude that the Triassic magmatism in central Great Xing’an Range could be generated under a post–orogenic extension setting related to closure of Paleo–Asian oceanic basin between the Xing’an and Songliao terranes, and the influence of the southward subduction of the Mongol–Okhotsk Ocean beneath the Xing’an Terrane might not be as great as expected.