Structural and Geochronological Analyses on the Hulan Metamorphic Complex in Central Jilin Province, NE China: New Constraints on the Final Closure of the Eastern Paleo-Asian Ocean

Abstract

The final closure time of the Paleo-Asian Ocean (PAO) is key to understanding the evolution of the Central Asian Orogenic Belt, and it has remained highly controversial for a long time. Here, we conduct structural investigations and zircon U-Pb and ⁴⁰Ar/³⁹Ar dating on the Hulan Metamorphic Complex (HMC) in central Jilin Province to determine the final closure time and direction of the eastern PAO. The metamorphic complexes are composed mainly of schists, marbles, gneisses, quartzites, metamorphosed plutons and dikes, which were involved in a nearly NEE-directed ductile thrusting and deformed to mylonites at a temperature of 550–450°C. Protolith ages of 262–252 Ma from three metamorphosed igneous rocks, emplacement ages of 229–221 Ma from two unmetamorphosed igneous intrusions and metamorphic ages of 243–238 Ma from three gneisses are yielded based on the zircon U-Pb dating. Three muscovite samples and one biotite sample yield ⁴⁰Ar/³⁹Ar plateau ages of 230–228 and 222 Ma, respectively. By integrating the dating results from this study and previous studies, the timing of metamorphism of the HMC is constrained to an interval between 248 and 229 Ma, and its peak metamorphism is reached at 243–238 Ma (Middle Triassic). During the period of 230–220 Ma, the HMC has already been uplifted to a geothermal level of 350–300°C. We consider that the metamorphism and shortening ductile deformation of the HMC is related to the final closure of the eastern PAO along the eastern Solonker Suture in an NEE–SWW closure direction. The Middle Triassic closure time of the eastern segment suggests a west-to-east scissor-style closure model for the PAO.

Keywords

Hulan Metamorphic Complex / zircon U-Pb dating / ⁴⁰Ar/³⁹Ar dating / final closure time / eastern Paleo-Asian Ocean

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2026 Geological Society of China.