Recognition of the latest Devonian volcanic rocks in Chifeng area, northern North China block, and its geological implications

LA-ICP-MS zircon U-Pb dating of two rhyolitic tuff samples from the Chaotugou Formation in Lianhuashan area of Chifeng on the northern margin of the North China block yielded weighted mean 206Pb/238U ages of 366Ma±2Ma and 364Ma±2Ma, respectively. The new geochronological results indicate that the Lianhuashan volcanic rocks were erupted during the latest Devonian instead of the Early Carboniferous as previously considered. Because the volcanic rocks are located within the Chaotugou Formation which was previously considered to be Early Carboniferous in age, the age of the Chaotugou Formation should evidently be latest Devonian. The Lianhuashan latest Devonian volcanic rocks are characterized by high content of SiO2, Na2O+K2O, Al2O3, low content of T Fe2O3（0.36%~0.70%）, MgO, CaO, TiO2 and A/CNK>1.1, quite different from the geochemical characteristics of the Late Carboniferous-Permian calc-alkaline volcanic rocks on the northern margin of the North China block.