An evolving tectonic environment of Early to Middle Permian granitic plutons in the Barkol area, East Junggar: insights from integrated zircon U–Pb ages, geochemistry, and Nd–Sr–Hf isotopes

ABSTRACT

Located in the central part of the Central Asian Orogenic Belt, East Junggar represents one of the most important sites of juvenile crustal growth during the Phanerozoic. However, the mechanism of juvenile crustal growth during the Permian remains controversial. The Daheishan granites in the eastern segment of East Junggar are dated at ~292 Ma (Early Permian) and 269 Ma–267 Ma (Middle Permian). The Early Permian granites are coarse-grained with very high SiO₂ (76.64 wt%–80.02 wt%), Al₂O₃ (10.72 wt%–12.96 wt%), and K₂O/Na₂O (1.27‒2.14), and low TiO₂ (0.12 wt%–0.18 wt%), CaO (0.27 wt%–0.90 wt%), Mg# (25–32), and Sr/Yb (16–25). The Middle Permian granites are medium‒fine-grained with relatively high SiO₂ (71.84 wt%–76.26 wt%), Al₂O₃ (11.26 wt%–13.00 wt%), and K₂O/Na₂O (0.53–1.54), and low TiO₂ (0.17 wt%–0.21 wt%), CaO (0.37 wt%–3.20 wt%), Mg# (33–43), and Sr/Yb (5–9). These Permian granites display slightly positive whole-rock εNd(t) (+0.77 to + 1.04) and highly positive zircon εHf(t) (+7.3 to + 8.7) values, and low Ga/Al (2.04–2.55 < 2.6), P₂O₅ (0.02%–0.05% <0.14%), and zircon saturation temperature (690°C–749°C, take the median for each sample). Moreover, they display characteristic tetrad rare earth element patterns and non-CHARAC (CHArge-and-RAdius-Controlled) trace element behaviour, suggesting that the Daheishan granites are I-type granites that underwent both fractional crystallization (K-feldspar and plagioclase, e.g.) and crustal contamination prior to their emplacement. In combination with data from volcanic rocks in the Santanghu Basin, we infer that the Early Permian granitic magmas were derived from delamination of a thickened lithospheric mantle base and partial melting of pre-Permian arc components, while the Middle Permian granites formed during the subsequent stretching and thinning of the lithosphere.

KEYWORDS:

• Highly differentiated granite
• I-type granite
• Permian
• delamination
• East Junggar

Acknowledgments

We thank the Editor, two reviewers and Prof. Tong Ying for their constructive comments that have greatly 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/10.1080/00206814.2023.2243476

Additional information

Funding

This work was supported by the National Natural Science Foundation of China (42072265, 41572206); the Geological Survey Program of China (grant number DD20221646).