Depositional Geochemistry of Clastic Rocks and Siliceous Deposits of the Upper Permian on the Northern Margin of the Youjiang Basin and Its Geological Significance

The Upper permian of Ziyun,Tiane and Jiuxu areas on the northern margin of the Youjiang basin mainly consist of basinal successions including clastic rocks intercalated with cherts and limestone,which are associated with the outer zone based on the estimated boundaries of the eroded zone.The geochemistry of these clastic rocks and siliceous deposits indicate that the clastic rocks and siliceous deposits of the middle-lower Upper Permian have a geochemical affinity to Emeishan high-Ti basalt which show lower Al2O3/TiO2values(1.67to 14.42),higher Eu/Eu*values(0.57to 1.10)and no negative anomalies of Nb,Ta and Ti.The tuffs and tuffaceous siliceous deposits of the top Upper Permian show higher Al2O3/TiO2values(22.78to 144.25),lower Eu/Eu*values(0.24to 0.57)with large negative anomalies of Nb,Ta and Ti,which is similar with the geochemistry of arc volcanisms,including the similar Zr/Hf values(25.06to 36.10)and(La/Yb)Nvalues(3.49to 11.56).It is suggested that the northern margin of the Youjiang basin was close to the Yangtze block as a deep-water basin during the Late Paleozoic and evolved into active continental margin during the end of Late Permian.