Emeishan Basaltic Magma Eruption Cycle In Southwestern Sichuan And Its Geological Significance

The Emeishan basalt in southwestern Sichuan is located on the southwest edge of the Yangtze Platform,on the north-eastern edge of the Emeishan large igneous province(ELIP).Based on field surveys,typical profiles and drilling cores,combined with isotope dating,the Emeishan basalt in southwestern Sichuan has been identified as two eruptive cycles,temporarily named as upper and lower eruptive cycles.The lower cycle is mainly composed of trachyte and a small amount of basalt trachyte.Widely distributed,large accumulation thickness,is the main body of Mount Emei basalt.The upcycle is composed of quartz trachyte,rhyolite pyroclastic rock and some alkaline basalt,which has the characteristics of bimodal volcanic rock.Pyroclastic rocks are also distributed in a wide area and the thickness is thin,but it is relatively thicker in the basin,ranging from 100-180 m,mainly consisting of(breccia)ignimbrite.There are mainly gray-white bentonite tuffs outside the basin,with a small accumulation thickness(2-7m)and a stable distribution.Alkaline basalts on top of the tuffs are limited in distribution,and the thickness is about 20-50 m.The eruption age of the lower cycle is the Late Permian,the zircon U-Pb age is 259-268 Ma,and the peak age is 260-263 Ma.The U-Pb age of the zircon from the upper cycle tuff is about 253±1.0 Ma.There is a regional unconformity contact between the deposits of the upper and lower cycles.The iron weathering crust is generally developed on the surface of the unconformity,and the basal conglomerate is seen in some sections.The determination of unconformity has important geological significance and is also the most reliable indicator of the division of upper and lower cycles.The eruption mode of the lower cycle is fissure eruption and fissure-center eruption,overflowing a large amount of basalt,belonging to ELIP.The upper cycle is dominated by the central eruption of intermediate-acid magma,which is characterized by the accumulation of pumice flow facies(ignimbrite)and air fall facies(tuff).Pyroclastic rocks are good reservoirs with high porosity.They are the main target layers for oil and gas exploration in volcanic rock area of Sichuan Basin.In addition,the widely distributed tuff deposits on the periphery of the basin are isochronous.They are a marker for the comparison of isochronous interface regions in Southwest China and even South China.They can also be used to determine the boundary between Permian and Triassic strata in Southwest China and explain the mass extinction of organisms.Provide the basis for the cause of the incident.