Dating Jurassic volcanic rocks in the Western Hills of Beijing, North China: Implications for the initiation of the Yanshanian tectonism and subsequent thermal events

Abstract

The “Yanshanian movement” (tectonism) was first recognized in the Western Hills of Beijing and represents the most intense Mesozoic tectonic event in eastern China. Although many researches have been conducted since its identification in 1927, the timing for initiation of the Yanshanian tectonism has not been well constrained. The Jurassic sedimentary succession in the Western Hills of Beijing consists of two volcanic units and interbedded coal-bearing strata, which is similar to those in other regions of the Yanshan Fold-and-Thrust Belt (YFTB). As the earliest Mesozoic volcanism in the Western Hills of Beijing, the Nandaling Formation contains the earliest volcanic and sedimentary records that are associated with the initiation of the Yanshanian tectonism. Previous researches considered the Nandaling volcanic rocks as Early Jurassic in age based on the floral fossil assemblage of Coniopteris–Phoenicopsis in the Yaopo Formation that conformably overlies the Nandaling Formation. In this study we perform a systematic geological survey and present the results of zircon U–Pb dating of the Nandaling volcanic rocks to demonstrate that both the Nandaling volcanic rocks and the overlying floral fossil assemblage are Middle Jurassic in age. LA–ICP–MS zircon U–Pb dating of the Nandaling volcanic rocks in the northern part of the basin yields eruption ages of 175 ± 1 to 168 ± 2 Ma. Dioritic porphyry sills are intruded concordantly into folded Mesoproterozoic–lower Paleozoic strata of the Qingbaikou anticline, and yield a zircon U–Pb age of 177 ± 1 Ma. These ages indicate that the initiation of the Yanshanian basins, and therefore the initiation of the Yanshanian tectonism, occurred no earlier than the Middle Jurassic. Our 40Ar–39Ar dating of the Nandaling volcanic rocks yields younger ages of 138–151 Ma that show a southward-younging trend, indicating that the Nandaling volcanic rocks in the Western Hills of Beijing were overprinted by a tectono-thermal event that peaked during the Early Cretaceous.