长江中下游地区白垩纪富碱火山岩浆作用

According to previous research achievements of Mesozoic volcanics and mineralizations in the middle and lower reaches of Yangtze River, the less studied volcanics in Lishui basin have been focused on their geochronology, and detailed U-Pb chronological study of zircons in trachyte has been carried out. Combining with geochemical data and lithostratigraphy for the four separated Mesozoic volcanic basins (i.e., Luzong basin, Fanchang basin, Ningwu basin and Lishui basin), volcanics from each basin with four volcanic cycles has been studied comparatively. It shows that, trachytes from the Longwangshan Formation of Lishui basin were formed in the Early Cretaceous with the crystallization age of about 128 Ma, which implies the incipience of extensive eruption of volcanics along the middle and lower reaches of Yangtze River. The ages of four volcanic cycles are: before 125 Ma, during 125-120 Ma, ranging from 120 to 110 Ma and from 110 to 90 Ma, respectively. It is more reasonable that the Fushan Formation of Luzong basin should be considered as the product of third volcanic cycle, and corresponding to the Gushan and Niangniangshan Formation of Ningwu basin (i.e., Guanshan and Jiashan formations of Lishui basin). The geochemical characteristics of Cretaceous volcanics in this area suggest that the mantle derived alkali magma had been contaminated by crustal materials during its eruption, and early volcanics (e.g., the Longwangshan Formation of Lishui basin) could be the source rock of Sr deposit.