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The Aleutian arc provided the setting for a proposal by Coats (1962) that arc magmas are related to a subducted oceanic plate in a convergent tectonic setting—a proposal that predated the theory of plate tectonics. Since that time, the study of Aleutian arc magmas has generated controversies over their origin and evolution that are fundamental to understanding the origin of magmas in all island arcs.

The first and longest controversy has been over the composition of the most important parental lava (precursor to most arc magmas) derived from the mantle. The question is whether this magma is a high-Al basalt generated by partial melting of the subducting plate (e.g., Marsh, 1982) or a more Mg-rich basalt derived from the peridotite overlying the plate that has been fluxed by a component from the plate (Kay, 1977; Perfit and others, 1980b).

The second major controversy is over the origin of the arctype trace-element and isotopic characteristics of Aleutian magmas. The models considered for the magmatic source include (1) a mixture of subducted oceanic crustal and sedimentary components with a depleted mid-oceanic ridge-type mantle (Kay and others, 1978; Kay, 1980); (2) a subducted oceanic crustal source (Marsh, 1976) that includes a subducted sedimentary component (Brophy and Marsh, 1986); and (3) an upper mantle source composed of mid-oceanic ridge and oceanic island-type components (Morris and Hart, 1983). The third major controversy is over the origin of the petrologic and geochemical diversity along the arc and within individual centers, and the relation of this diversity

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