Geochronology and Geochemistry of Jurassic Magmatism in the Xing’an Block, Northeastern China: Implications for the Tectonic Evolution of the Paleo–Pacific Oceanic Plate

Abstract:

The Jurassic tectonic evolution and dynamic setting of the eastern part of the Central Asian Orogenic Belt (CAOB), however, remains controversial; some workers propose it was dominated by the subduction of the Paleo–Pacific oceanic plate, others suggest it experienced southwards subduction of the Mongol–Okhotsk oceanic plate. Thus, we undertook zircon U–Pb dating and geochemical analyses of the Jurassic intermediate–acidic intrusions in the Xing’an Block, with the aim of addressing the above mentioned question. Zircon U–Pb age dating demonstrates three stages of Jurassic magmatism in the study area: 180–177 Ma, 171–170 Ma, and ~151 Ma. Early Jurassic acidic rocks are dominated by calc–alkaline I–type granitoids containing high SiO₂ and total Na₂O+K₂O, and are characterized by enrichment in light rare earth elements (LREEs) and depletion in heavy rare earth elements (HREEs) and high field strength elements (HFSEs; e.g., Nb, Ta, Ti, and P), indicating they formed in a subduction–related setting. Middle Jurassic intermediate–acidic rocks comprise two suites (Tuanjie and Shijing plutons), and the Tuanjie pluton was likely derived from partial melting of a depleted mantle wedge that was subsequently metasomatized by subduction–related fluids, suggesting that subduction–related processes indeed occurred beneath the study area in the Middle Jurassic. The rocks of Shijing pluton show geochemical affinities to rocks that form in subduction zones, with enrichment in LREEs and depletion in HREEs and Ba, Sr, Eu, Nb, Ta, P, Ti. Late Jurassic acidic intrusions contain high SiO₂, Na₂O+K₂O, and Al₂O₃, and are strongly depleted in HREEs, indicating the magma was likely derived from partial melting of thickened crustal material, possibly caused by subduction–related processes. According to these findings and previous studies, we conclude that generation of the Jurassic intermediate–acidic intrusions in the study area was related to the subduction of the Paleo–Pacific oceanic plate.