The crust in the central oceanic Aleutian arc has been formed by intrusion and extrusion of subduction-related magmas into and onto pre-existing oceanic crust over at least the past 50 m.y. Upper crustal rocks that have differentiated from a parental magma of olivine tholeiite composition, and residual crystals complementary to them, along with the pre-existing oceanic crust and some early extrusive lavas, form the lower crust. The volumes of upper and lower crust calculated from crystal fractionation models are consistent with Aleutian crustal structure, velocity, and density. Xenoliths of ultramafic to intermediate composition igneous cumulate rocks and garnet-free basaltic pyroxene granulites in Aleutian volcanic rocks confirm the existence of the predicted rock types in lower crust. Residues complementary to the Fe-rich basalts from tholeiitic volcanoes that have differentiated at low pressure should form part of the upper to middle crust. The model implies that continental crust formed by the accretion of island-arc terrains, which comprise most new Phanerozoic crust, has a basaltic lower crust.