GENESIS OF CONTINENTAL-CRUST UNDER ISLAND-ARC CONDITIONS

In the Japan context, the following characteristics of arc volcanic rocks are important for crustal growth: (1) intimate association of volcanic rocks of tholeiitic and calc-alkaline series; (2) repeated eruption of basaltic and dacitic lavas in a volcanic succession; (3) gradual change in chemistry from tholeiitic to calc-alkaline volcanic rocks; (4) crystallization differentiation which includes the crystallization of plagioclase along with other mafic minerals; (5) marked enrichment of K2O associated with slight depletion of compatible elements; (6) an evolution limit represented by, for instance, Rb/Sr ratios of 0.23-0.28 and SiO2 contents of 60-66 wt.% and (7) repetitive magma mixing. Batch fractionation in refilled magma chambers best accounts for these characteristics, transforming mantle-derived magmas through calc-alkaline magmas to magmas of composition very close to the bulk composition of the upper continental crust. Systems of periodically refilled magma chambers are general in the Japanese arcs. Differentiates and accumulated crystals may form the compositional stratification of the continental crust.