The Yidun Island Arc is a Triassic tensional island arc developed at the bese of a thin continental crust, which was tensional in the Permian and tbe eaarlier Triassic.it is characterized by .development of infra-arc rifting zone and inner-and outer- arcs. Arc volcanic rocks are mainly calc-alkaline volcanic rocks, consisting of basalt,basaltic andesite,andesite, dacite and pyioclastic rocks. Studies on their petrography, major and trace element geochemistry and phase equilibrium have shown that the calc-alkaline volcanic rocks were fomed by fractional crystallization of a calc-alkaline basaltic magma and that the andesite was formed by multi-stage crystalli- zation of calc-alkaline basaltic magma under different pressures. Model calculations show that after Ol+Cpx+Plag士Mt士Ap crystallization from calcalkaline basaltic magma under higher pressure, fractional crystallization of Hb+Cpx+Plag+Mt tooh place in derived magma, thus producing andesite magma. So, separation of a large quantity of Hb mineral is an important factor controlling the formation of andesite. nd the