The framework of the Triassic arc and backarc basin system is clear i n the Changdu area of eastern Tibet.The activity of the arc tended to reach its summit in the Tiassic.From Early Triassic to Late Triassic,volcanism p rogresses successively and types of volcanic rocks vary from predominant rhyolit ic or dacite to andesitic volcanic rocks,the indices of volcanic explosion enlar ge,quantities of volcanic and sedimentary cycles also grow.It turns out that the r e are welldeveloped arcs and collided volcanic rocks and the source rocks of a rc volcanic rocks are a mi x ture of the wedge upper mantle and overlying assimilatory and hybridized crust ( 40%～60%) with minor oceanic crust (8%～20%).