Research On The Petrology,Geochemistry And Chronology Of Triassic Mungbean Rock In The Wenquan Town,North Chongqing

There is a wide range of mungbean deposits with stable thickness and obvious characteristics in Sichuan Basin and its surrounding areas.Most of them are hydromica claystone and feldspathic tuff,which often represent the boundary between the lower Triassic Jialingjiang Formation(T1j)and the middle Triassic Leikoupo Formation(T2l).As early as the 1980 s,predecessors realized the importance of the study of mungbean rock,and did a lot of work on its distribution and mineral composition.In recent years,the whole-rock geochemistry and chronology of mungbean rocks have been paid more and more attention,but the work is mainly concentrated in the southwest and south of Sichuan Basin.Taking the "mungbean rock" samples from Pinghongcun section and Xiejiagou section of Hot Spring Town in Chongqing as research objects,this paper analyzed their petromineralogy,element geochemistry,zircon U-Pb chronology and zircon trace element characteristics,combined with the existing data,preliminarily explained the original magmatic composition and genesis of "mungbean rock" and further discussed the Triassic volcanic events.The impact of the event on the co-evolution of environment and biology.The main conclusions are as follows:(1)Powder diffraction reveals that the mineral components of mungbean rocks in Pinghongcun and Xiejiagou profiles are mainly quartz and alkaline feldspar,while clay minerals are mainly montmorillonite and illite,but the samples in Xiejiagou profiles contain more quartz and illite,reflecting that the weathering degree of the profiles is higher than that in Pinghongcun profiles;lithofacies,Th/U ratio and zircon REE distribution pattern of zircon crystals indicate that Pinghongcun is a good example.Magmatic origin of zircons in Mungbean rocks in Hongcun and Xiejiagou profiles,both of which occur in oxygen-rich environments.(2)Zircon U-Pb dating reveals that the sedimentary emplacement ages of mungbean rocks in Pinghongcun and Xiejiagou profiles of Hot Spring Town in Chongqing are 247.2 ±0.59 Ma and 225 ±0.64 Ma,respectively,indicating that they are volcanic eruptions of different periods.(3)Comprehensive studies of regional geology,zircon age and zircon geochemistry indicate that the "mungbean rock" in Xiejiagou may originate from the Andean-type continental margin volcanic arc associated with the North China-South China subduction collision,while the "mungbean rock" in Pinghongcun may be associated with the Emeishan large igneous province,which is the product of magmatic eruption events caused by intraplate tectonic compression in the magma chamber after the main active period.(4)A series of volcanic activities represented by "mungbean rock" have a great impact on the environment,which is likely to be the restraining factor of early Triassic biological recovery activities.This series of volcanic activities gradually stopped at the end of early Triassic,and the volcanic debris material erupted transformed the shallow sea environment,providing sufficient material basis for the prosperity of primary productivity and becoming a matter of biological radiation.Hotbeds for piece production.