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Physiographic configurations of island arcs indicate that the growth of a volcanic arc and an outer arc is alternative. The Miocene igneous activities in Japan indicate that the emplacement of graniti...c plutons occurs alternatively in the volcanic arc or the outer arc. The volcanic arc is recorded geologically as a belt of an intimate association of volcanic rocks and subsequent granitic plutons, and the outer arc as a belt of granitic plutons with no or few contemporaneous volcanic rocks. To explain these relationships, two alternative fractionation models proposed by YANAGI (1975) were developed under possible geophysical conditions under the island arc. The volcanic arc may owe its growth to batch fractionation of a series of parental magmas. Thick overlying crust and a sufficient rate of magma supply to the chamber are indispensable conditions to keep the batch fractionation working. In case the conditions are not fulfilled, magmas may migrate horizontally along the crust-mantle boundary surface and then accumulate behind the aseismic front, resulting in the formation of the outer arc behind the aseismic front. These two alternative magmatic processes give a good explanation to the geological observations.show more
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