In-situ U-Pb dating, Hf isotope analyses of zircons, and geochemical characteristics in the volcanic rocks of the Tiaojishan Formation in the Jinyang Basin, western Liaoning province

This paper is the result of geochemical research.ObjectiveThe volcanic rocks of the Tiaojishan Formation in Western Liaoning not only represent the beginning of the Yanshanian large-scale volcanic eruption, but also are closely related to the time, scope and mechanism of the Craton weakening in North China.In recent years, a lot of new understanding has been obtained on the petrogenesis of the Tiaojishan Formation volcanic rocks, but its tectonic background, magmatic source area are still controversial. MethodsZircon U-Pb dating, Hf isotope composition and geochemical characteristics were analyzed in the core samples from the continuous drilling of borehole Yang D1. ResultsThe results of zircon U-Pb dating show that the zircon U-Pb age of trachyte in the lower part of Tiaojishan Formation is 162.8±1.7 Ma, and the U-Pb age of tuff at the bottom of Tuchengzi Formation is 153.8 ±1.7 Ma. We believe that the volcanic rocks of Tiaojishan Formation formed in the Late Jurassic. In addition, the trachyte of Tiaojishan Formation is characterized by light rare earth enrichment, heavy rare earth deficit, and low Sr and Yb-type magmatic rocks, which indicates that the trachyte of Tiaojishan Formation is produced by partial melting of crustal magma under high pressure and accompanied by insignificant crystallization differentiation.The εHf(t)values of the zircons range from-22.1 to-15.4, and the two-stage model ages(TDMC)range from 2.182~2.604 Ga, indicating that the Tiaojishan Formation is mainly derived from the late Archean magma source, which indicates that the Tiaojishan formation was formed by the partial melting of the basic rocks in the lower crust under the background of the Pacific plate subduction to Eurasia continent. ConclusionsThe research results define the formation age and magma source area of the Tiaojishan Formation volcanic rocks, and provide a scientific basis for exploring the Mesozoic magmatic and regional tectonic evolution of the Yanshan orogenic belt.