ABSTRACT

On the basis of Nd isotopic data on volcanic rocks, island arcs are classified into two types; type A, having high average ε_Nd (8.1) and type B, having variable ε_Nd. A-type island arc volcanics are probably derived from subducted oceanic crust with possibly a small amount of oceanic sediment and seawater. They correspond to ‘immature’ island arcs with active back-arc spreading. B-type island arcs are subdivided into two groups, B-1 and B-2, from the data plot on ε_Nd–⁸⁷Sr/⁸⁶Sr correlation diagram. Volcanics of B-1 type, having Nd and Sr isotopic compositions plotted close to the mantle array, suggest involvement of a small amount of crustal component to the magma that had isotopic composition originally similar to that of A-type arc magma. This type of volcanics may be characteristics for the well-developed island arc under tensional stress field conditions of a back-arc basin. Lavas of B-2 type island arcs involve a considerable amount of continental crustal component and are found in the well-developed continental arcs where the compressional stress conditions are observed in the back-arc basins.