Geological Characteristics Study Of The Qugasi Formation In Baisong-bendu Area,Sichuan Province

The Baisong-Bendu area is located in the south of the Sanjiang area,Southwest China.It's in the west of the Jinsha River combined belt,across the Zhongzha-Zhongdian landmass,and into the Yidun Island arc belt of the Ganzi-Litang arc-basin system in the east.It experienced a complex and special geological structure evolution history.The Upper Triassic Qugasi Formation in the area is widely distributed,with the development of large-scale mafic volcanic rocks.However,there are still some problems in the division and stratigraphic characteristics study of the Qugasi Formation in the area,and there are much less studies on the volcanic rock type and petrogenic tectonic environment of it.Through the field observation and profile measurement,rock-mineral identification and rock geochemical data analysis work,do more systematic studies in the petrography,rock geochemistry and chronology aspects,then get the following understanding.1,There is obvious phase change in the lithology of the Qugasi Formation,from north to south,from west to east,showing the trend of decreasing grain size,thinning thickness and reducing the volcanic rock to disappear.As a whole displaying a regressive sedimentary sequences from a fine to coarse.2,The Qugasi Formation volcanic rocks are classified as alkali basalts riching in Ti and a few tholeiites.Rare Earth elements are characterized by LREE enrichment,HREE relative loss,and significant REE fractionation,showing a mixture of EM-1 OIB and EM-2 OIB.The lava eruptions presented the character of almond-shaped basalt composition-porphyritic basalt-crystalline basalt.And the erupted performance of pyroclastic rocks are melting of basaltic volcanic breccia or conglomerate-basaltic volcanic breccia crystalline basalt-volcanic breccia.3,Dating results of zircon LA-ICP-MS U-Th-Pb of olivine basalt is 217.0±2.5Ma(n=13,MSWD=0.47),it shows that the basic volcanic rocks in study area should be formed in the Early Late Triassic.In addition,combined with the fossils of bivalves,gastropods,corals and conodont,it is found that the sedimentary age is the Early Late Triassic and may last until the middle stage.4,The sedimentary environment of the Qugasi Formation showed early neritic shelf,advanced fan delta and carbonate platform interactive combination.