Abstract: The Qinhuangdao area of eastern Hebei is located in the eastern part of the North China plate, where Mesozoic volcanic rocks are widely developed. In order to further study the tectonic setting and movement mechanism of Mesozoic magmatism in this area, the LA-ICP-MS zircon U-Pb geochronology, Hf isotopic compositions, major and trace elements of the rhyolitic crystal tuffs in Rugezhuang were analyzed. The results show that zircons have a clear oscillatory zoning, and the Th/U value of zircons is 0.37-0.83, indicating they are magmatic zircons with the weighted average age of (113±1) Ma. The rocks are characterized by high silicon (75.95%), high alkali (8.47%) and low calcium (0.39%). HFSE (Th, U, Ce, Nd, Zr, Hf) are enriched and LILE (Ba, Sr, P, Ti) are depleted in analyzed samples. The REE distribution patterns dip slightly to the right, andδEu (0.02-0.30) is obviously negative anomaly, suggesting that a large amount of plagioclase was crystallized. During the evolution of the magma, along with the crystallization of apatite, ilmenite and other minerals, the value of Nb/Ta is close to that of the continental crust, suggesting that analyzed rocks have the characteristics of crustal rocks. Zircon Hf isotopic compositions and temporal variations show that the magma source of the rocks is the product of partial melting of the ancient lower crust of the North China plate in the Late Early Proterozoic to Middle Mesoproterozoic due to the heating from mantle-derived magma. The results show that the rhyolitic crystal tuffs in Rugezhuang are the products of magmatic activities in the Early Cretaceous, and was formed in the intraplate extensional environment driven by the subduction and roll-back of the Paleo-Pacific plate.

Key words: calc-alkaline pyroclastic rock, zircon U-Pb geochronology, zircon Hf isotopic compositions, petrogeochemistry