GENESIS OF CONTINENTAL-CRUST UNDER ISLAND-ARC CONDITIONS

In the Japan context, the following characteristics of arc volcanic ro cks are important for crustal growth: (1) intimate association of volc anic rocks of tholeiitic and calc-alkaline series; (2) repeated erupti on 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) a n 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. B atch fractionation in refilled magma chambers best accounts for these characteristics, transforming mantle-derived magmas through calc-alkal ine magmas to magmas of composition very close to the bulk composition of the upper continental crust. Systems of periodically refilled magm a chambers are general in the Japanese arcs. Differentiates and accumu lated crystals may form the compositional stratification of the contin ental crust.