Geochronology, geochemistry, and Sr–Nd isotopes of Permian dike swarms in the Beishan orogenic belt, southernmost Central Asian Orogenic Belt: Petrogenesis and implications for geodynamic setting

ABSTRACT

Permian mafic dikes crop out widely within Beishan area in the southernmost Central Asian Orogenic Belt (CAOB). These mantle-derived products provide important clues for elucidating the genesis and geodynamic setting of Permian magmatism. The mafic dikes within the Beishan area display zircons U–Pb ages of 281 ± 4 Ma. Characterized by low SiO₂ (44.88–47.73 wt.%) and high Mg^# values (62.2–68.6), the type 1 dikes are relatively depleted in light rare earth elements (LREEs; (La/Yb)_n = 0.72–1.43) but enriched in large ion lithophile elements (LILEs). They show high positive ε_Nd(t) values (6.6–8.5) with a range of (⁸⁷Sr/⁸⁶Sr)_i (0.703477–0.705789) ratios. In contrast, the type 2 dikes have higher SiO₂ content (47.47–51.13 wt.%) and lower Mg^# (40.9–63.0). They are enriched in LREEs ((La/Yb)_n = 1.79–5.02) and LILEs, and have lower ε_Nd(t) values (3.8–7.4) with a variable range of (⁸⁷Sr/⁸⁶Sr)_i (0.702873–0.706949) ratios. Thus, the type 1 dikes resulted from lower degrees of fractional crystallization and contamination than the type 2 dikes. Considering the LILEs and Pb enrichments, high field strength elements depletions, and high Ba/Th (86–460) ratios, the mantle source was metasomatized by subduction-related fluids. These dikes should be derived from high degrees (>20%) of partial melting of a Sr–Nd isotope-depleted, phlogopite-bearing spinel lherzolite lithospheric mantle according to their high Rb/Sr (0.04–0.24), low Ba/Rb (0.9–3.3), low La/Sm, and constant Sm/Yb ratios in addition to Sr–Nd depletion. Combined with the coeval evidence of regional geology and novel tectonic discriminate diagrams, we suggest that the mafic dikes in Beishan area formed in arc-related setting rather than intracontinental setting during the Permian. The metasomatized lithospheric mantle by subduction-related fluids were heated by upwelling asthenosphere mantle triggered by the slab roll-back, resulting in the formation of the parental magma of the mafic dikes.

KEYWORDS:

• Mafic dikes
• geodynamic setting
• Beishan
• Permian
• CAOB

Highlights

• Two types Permian (~281 Ma) mafic dikes have been identified.

• Dikes derived from a lithospheric mantle was metasomatized by subduction-related fluids.

• Permian Mantle plume and slab break-up models cannot account for the formation of dikes in the Beishan area.

• Dikes in the Beishan area formed as the result of slab roll-back.

Acknowledgments

We appreciate Editor in-Chief Dr. Robert J. Stern, Wenjiao Xiao and one anonymous reviewer for critical comments that improved the manuscript significantly. The authors give thanks to Chunrong Diwu for providing assistance with the zircon U–Pb analyses and are grateful for the critical discussions with Gaoxue Yang, Changzhi Wu, Junwen Zhang, and Zhen Li, which improved the manuscript.

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/10.1080/00206814.2023.2233091

Additional information

Funding

This work was financially supported by the National Natural Science Foundation of China (Grant Number 41073027), Fundamental Research Funds for the Central Universities (Grant Number 310827172003 and 300102278302), and the Basic Research Project of Nature Science of Shaanxi Province (Grant Number 2019JM-161).