Petrogenesis of the U-rich Permian Akkulen syenite intrusion, Tien Shan, Kyrgyzstan: insights into its magmatic evolution and geodynamic setting

ABSTRACT

Permian alkaline granitoids are widely distributed throughout the Tien Shan Orogen and adjacent region of the southern Central Asian Orogenic Belt. However, their petrogenesis and related tectonic setting remain equivocal. A detailed mineralogical, U-Pb zircon dating, in situ elemental and Sr-Nd-Hf isotopic studies of the uranium-rich Akkulen syenite intrusion in the Northern Tien Shan of Kyrgyzstan were undertaken to better understand its magmatic processes and geodynamic evolution. Four independent oxygen barometers have shown that the oxygen fugacity of the Akkulen syenitic magma evolved from initial low fO₂ (<NNO +1, where NNO is the nickel-nickel oxide buffer) to relatively high fO₂ (>NNO +1) during magma ascent and cooling; Apatite geochemical characteristics also indicate that the magma is relatively high F (2.9–5.1 wt.%), with low Cl (0.053–0.13 wt.%) and anhydrous (<4 wt %) contents. According to whole-rock geochemical data, the syenite samples have abnormally high U (38.5–57.1 ppm) and Th (96.2–137 ppm) concentrations, we suggest that high F content and moderate oxygen fugacity are beneficial for uranium enrichment. Zircon grains from a syenite sample yielded a weighted ²⁰⁶Pb/²³⁸U age of 283.5 ± 2.6 Ma. The intrusion has an A-type granite affinity with high alkali (Na₂O + K₂O) contents and Fe/(Fe+Mg) ratios, high LREE/HREE (~5), and low Sr, Ba, and Eu contents. They have negative εNd(t) (−5.2 to −3.6) values with Mesoproterozoic two-stage model ages (T _DM2 = 1.35–1.47 Ga), and variable εHf(t) values (−4.6 to +2.4) with Mesoproterozoic two-stage model ages (T _DM2 = 1. 2–1.59 Ga), which suggest these rock derivations from the relatively old crustal with minor contribution from juvenile crustal melts. In comparison with regional available dataset, we propose that the alkaline Permian granitoids in Kyrgyz Northern Tien Shan may have been originated from large-scale partial melting of Mesoproterozoic metamorphic basement with minor juvenile crustal material in a post-extensional setting. In contrast, the Early Permian granitoid magmatism in the Tarim Craton may have been caused by the partial melting of Neoproterozoic basement rocks associated with Permian mantle plume activities.

KEYWORDS:

• A-type granite
• syenite intrusion
• Permian
• Post-collisional setting
• Tien Shan Orogen

Highlights

• Permian granitoids in the Tien Shan were produced in post-collisional setting.

• The magmatic evolution and uranium enrichment process of Akkulen syenite are revealed.

Acknowledgments

We appreciate the science editor Robert J. Stern, and two anonymous reviewers for their constructive and insightful comments. Besides, we are grateful to Yun Zhao, Haixia Chu, and Xiaohong Mao, Junfeng Shen and Xiaowei Li for their discussions and assistance with the lab work.

Disclosure statement

No potential conflict of interest was reported by the author(s).

Supplementary material

Supplemental data for this article can be accessed online at https://doi.org/10.1080/00206814.2023.2278061

Additional information

Funding

This study was financially supported by the Natural Science Foundation of China [42372090], the Zijin Mining Group Co., Ltd. [33122022005] and the China University of Geosciences (Beijing) [YB2023YC046]. CX acknowledges funding under Department of Human Resources and Social Security of Xinjiang Uygur Autonomous Region Introduced Project “Tianchi talent”.