Zircon U-Pb chronology and geochemistry of the Kalagang Formation volcanic rocks in Bayindala area, West Junggar, and their geological significance

Silicic volcanic rocks are developed in Bayindala area of West Junggar. The Kalagang Formation volcanic rocks are mainly composed of rhyolite, dacite, rhyolitic tuff and ignimbrite. This paper reports the results of high-precision zircon LA-ICPMS U-Pb dating of the Kalagang Formation volcanic rocks, which yielded weighted mean ²⁰⁶Pb/²³⁸U ages of(294.4±1.3) Ma (n=9, MSWD=0.8, dacite) and(298.4±2.5) Ma (n=9, MSWD=3.4, rhyolite), corresponding to the Early Permian. Petrogeochemical analyses suggest that the Kalagang Formation volcanic rocks are characterized by high silica (SiO₂:66.69%-75.21%) and alkali (Na₂O + K₂O:6.98%-8.35%), and low magnesium and calcium (MgO 0.18%-1.32%, CaO 0.24%-2.49%). They belong to peraluminous (A/CNK:1.14-1.33) and high-K calc-alkaline series. The rocks are poor in rare earth elements with ∑REE being 83.82×10^-6-197.61×10^-6, enriched in LREE(La/Yb)_N=3.15-12.50 and LILEs (Rb, Th, U, K) and depleted in HFSE, especially Nb, Ta, P and Ti. In combination with previous studies, the authors hold that the Kalagang Formation volcanic rocks in a postcollisional extensional setting were formed by varying degrees of crust partial melting and the residua had a little plagioclase and hornblende.