Zircon chemistry of the late cretaceous-eocene granites from Tengchong block: insights into the genesis of host magmas

The Tengchong Block is notable for its diverse granitic intrusions and associated tin deposits. This study investigates the chemical compositions of zircons from the Late Cretaceous Guyong pluton and Eocene Lailishan pluton in the Tengchong Block to elucidate magmatic processes and sources, and their implications for tin mineralization. The zircons are characterized by long-prismatic shapes and oscillatory zoning, relatively high Hf concentrations (> 5000 ppm) and Th/U ratios ranging from 0.20 to 1.97, and low Eu/Eu* [= Eu_N/(Sm_N×Gd_N)^0.5; where the subscript N denotes chondrite-normalised mass fractions] ratios of 0.02–0.40. Both the Guyong and Lailishan granites underwent significant differentiation, as evidenced by variations in zircon chemistry with increasing Hf concentrations. The wide range of Ti-in-zircon temperatures (870–650 ℃), along with decreasing Ti contents and Th/U ratios, suggests progressive cooling and fractional crystallization from the point of zircon saturation to the final solidification of the magma. The declining Dy/Yb ratios point to the fractionation of hornblende or titanite, while the reduction in Eu/Eu* ratios implies plagioclase fractionation. The relatively high U/Yb ratios (0.23–6.40) confirm a crustal magma source for these granites. Additionally, the pronounced negative Eu anomaly and the varying Ce/Ce* [= Ce_N/(La_N×Pr_N)^0.5] ratios of 6–400 without a clear correlation with ΔFMQ [fayalite-magnetite-quartz buffer] indicate that high magmatic differentiation, rather than a reducing environment, may have been the primary factor influencing in tin mineralization.