Zircon U-Pb dating and petrogenesis of Early Cretaceous alkaline rhyolites in Tuquan Basin of middle Da Hinggan Mountains

The LA-ICP-MS zircon ²⁰⁶Pb/²³⁸U age estimates for the alkeline rhyolites yielded weighted average ages of 135±1Ma and 133±1Ma, indicating Early Cretaceous. The whole-rock geochemistry of the alkeline rhyolites shows an A-type magmatic affinity, with a typical enrichment of SiO₂, K₂O and a high ratio of TFeO/MgO, and low content of Al₂O₃, CaO and MgO, belonging to Khigh calc-alkaline series rocks; high ∑REE and fractionated REE patterns show strong negative Eu anomalies, relative enrichment of Rb, U, Zr, Hf, but depletion of Sr, Ti, Nb, Ta. In the discrimination diagrams of granites, all of the samples show the characteristics of A-type granites; in the Nb-Y-3Ga diagram, the samples show the characteristics of A₂-type granites. These elements show that the alkeline rhyolites were derived from partial melting of crust materials. Viewed from regional correlations with the volcanic rocks as well as A-type granites, it is held that the alkeline rhyolites were associated with an extensional tectonic setting, being the product of a regional extensional environment associated with the evolution of Mongol-Khotsk suture. Zircon Hf isotope analyses show that dacite of Cretaceous have ε_Hf(t) values of +4.2~+10.7 with Hf modal ages being 0.74~0.76Ga, indicating that there existed similar crustal processes between Xing'an block and Songnen block.