Discovery of A-type rhyolite in the Lower Cretaceous Zhangjiakou Formation at the northern margin of the North China Craton and its geological significance

Previous studies have shown divergent views on the tectonic setting of the volcanic rocks in the Zhangjiakou Formation, and detailed research on the relationship between these volcanic rocks and the destruction of the North China Craton (NCC) has not been conducted. In this study, we conducted comprehensive geological, petrological, geochronological, and geochemical investigations on the A-type rhyolites discovered for the first time in the upper Zhangjiakou Formation in the Zhangjiakou area, and further analyzed their petrogenesis and formation environment. LA-ICP-MS zircon U-Pb dating yielded an age of 138.4±1.3 Ma (n = 30, MSWD = 0.84), indicating an Early Cretaceous formation. The rocks are characterized by high silica (SiO2 = 69.94%~73.35%), high alkalinity (Na2O+K2O = 10.13%~10.71%), low contents of MgO, CaO, TiO2, and high Ga/Al ratios. The rhyolites at the top of the Zhangjiakou Formation belong to the A1-type granites. They were formed in a high-temperature, low-pressure intraplate extensional environment, and originated from the partial melting of a thinned felsic middle to lower crust. Based on previous research findings, it is proposed that the A-type rhyolite at the top of the Zhangjiakou Formation represents a geological response to the tectonic regime transition of the North China Craton in northern Hebei, marking the onset of the destruction of the North China Craton.