ZirconU-Pb age and geochemical characteristics of the rhyolite of the Yudaokou basin in Weichang, Hebei Province, and their geological implications

The SHRIMP U-Pb dating of zircon yielded a ²⁰⁶Pb/²³⁸U age of 155.2±2.0Ma for rhyolites in Yudaokou basin, located in Weichang County, northern Hebei Province, indicating that its geological age is early Late Jurassic, consistent with that of Xinmin Formation of Liaoyuan Block and Tiaojishan Formation of the Yanshan intracontinental orogenic belt. The rhyolites can be divided into gray and pinkish red colors, and both are characterized by typical enrichment of SiO₂, K₂O and Al₂O₃, low content of CaO and MgO, thus belonging to high-K calc-alkaline series rocks. Only the ratios of K₂O/Na₂O and Fe₂O₃/FeO of cinnamon rhyolite are slightly higher than the values of the gray one. Furthermore, the rhyolites exhibit high content of ∑REE, obvious fractionation of LREE and HREE, and strong negative Eu anomaly, possessing features of enrichment of LREE. These rocks are characterized by enrichment of LILE such as Rb, Th, U, Ga and Y, but depletion of Ba and Sr. In addition, HFSE, such as Zr, Hf, Nb and Ta are enriched as well. All of the samples show the trace element characteristics of A2-type granite and low Sr-Ba rhyolite, sharing the features of relatively high (⁸⁷Sr/⁸⁶Sr)_i values (0.708606~0.711171), low ε^Nd(t) values (-9.78~-7.11), and young T_DM2 (1520~1737Ma), lower (²⁰⁶Pb/²⁰⁴Pb)_t (16.93~17.06), (²⁰⁷Pb/²⁰⁴Pb)_t (15.37~15.43) and (²⁰⁸Pb/²⁰⁴Pb)_t (37.30~37.72). The projection points are located between the lower crust and the depleted mantle in the diagram of ε^Nd(t)-(⁸⁷Sr/⁸⁶Sr)_i, and between the lower crust and mantle in the diagram of (²⁰⁷Pb/²⁰⁴Pb)_t-(²⁰⁶Pb/²⁰⁴Pb)_t and (²⁰⁸Pb/²⁰⁴Pb)_t-(²⁰⁶Pb/²⁰⁴Pb)_t, indicating that the rhyolite in the Yudaokou basin was probably formed by partial melting of young lower crust material which originated from DMM mantle, EMI enriched mantle magma and old lower crustal material. The source material was different from felsic volcanic-intrusive rocks in Liaoyuan Block, but similar to felsic volcanic-intrusive rocks in the Yanshan intracontinental orogenic belt.