Geochemistry and geochronology of Upper Permian–Upper Triassic volcanic rocks in eastern Jilin Province, NE China: implications for the tectonic evolution of the Palaeo-Asian Ocean

ABSTRACT

We present zircon U–Pb dating, whole-rock geochemistry, and Sr–Nd isotope results for the Upper Permian–Upper Triassic volcanic rocks to constrain the timing of the final closure of the eastern segment of the Palaeo-Asian Ocean. The volcanic rocks were mainly collected from the Yanbian area in eastern Jilin Province, northeastern China. The zircon U–Pb dating results indicate that the samples can be classified as Upper Permian–Lower Triassic basalts (ca. 262–244 Ma) and Upper Triassic dacites (ca. 216 Ma). The whole-rock geochemical results indicate that the rocks predominately belong to the medium-K and high-K calc-alkaline series. The basalts are enriched in large ion lithophile elements (LILEs, e.g. Ba and K) and depleted in high field strength elements (HFSEs, e.g. Nb and Ta), with weak positive Eu anomalies. The dacites are enriched in LILEs (e.g. Rb, Ba, Th, and K) and light rare earth elements (LREEs) and marked depletion in some HFSEs (e.g. Nb, Ta, and Ti), with significant negative Sr, P, and Eu anomalies. Moreover, the Upper Permian–Lower Triassic basalts have low initial ⁸⁷Sr/⁸⁶Sr ratios (0.7037–0.7048) and high ε_Nd values (4.4–5.4). In contrast, the Upper Triassic dacites possess relatively high initial ⁸⁷Sr/⁸⁶Sr ratios (0.7052) compared with their low ε_Nd values (1.4). The basaltic magma likely originated from the partial melting of a depleted mantle wedge metasomatized by subduction-related fluids, and the felsic magmas likely originated from the partial melting of a dominantly juvenile source with a minor component of ancient crust. Taken together, the Upper Permian–Lower Triassic basalts (ca. 262–244 Ma) are arc basalts that formed in an active continental margin setting, and the Upper Triassic dacites (ca. 216 Ma) are A-type granitic rocks that formed in an extensional setting. Therefore, the final closure of the Palaeo-Asian Ocean occurred during the Middle–Late Triassic.

KEYWORDS:

• Tectonic evolution
• Palaeo-Asian Ocean
• Volcanic rocks
• Zircons U–Pb dating
• Sr–Nd isotopes
• Geochemistry

Acknowledgements

The authors appreciate Yujing Peng’s help with geological field sampling. We also thank the staff of the State Key Laboratory of Continental Dynamics, Northwest University, Xi’an, China, for their instruction and help with zircon U–Pb dating. We appreciate the Jinnan Supervision and Inspection Center of Mineral Resources, the Ministry of Land and Resources, China, for their assistance in the major and trace element analysis. We are also grateful to the State Key Laboratory of Isotope Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences for help with collecting the whole-rock Sr–Nd isotopic data.

Disclosure statement

No potential conflict of interest was reported by the authors.

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Funding

This study was financially supported by the National Natural Science Foundation of China [41372108 and 41602110], the Research Fund for the Doctoral Program of Higher Education of China [20133718130001], and the SDUST Research Fund [2015TDJH101].