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Some of the best developed calc-alkaline plutons in an oceanic island arc are found in the Aleutian arc, Alaska. These Tertiary plutons are similar in many ways to continental granodioritic I-type plutons, but are smaller and have isotopic ratios and Rb contents that reflect their oceanic setting. Some differences between Aleutian plutons occur, as exemplified by the higher K content and more oxidized, lower-temperature mineralogy in the Hidden Bay pluton (Adak Island) compared to the Captains Bay pluton (Unalaska Island). This variability implies regional differences in arc crust that may reflect the 20 m.y. of arc history, including thickening of the crust to a critical value, that preceded the onset of calc-alkaline magmatism. The plutons appear to have formed during the waning stages of volcanic periods, when the magmas were too cool and crystalline to erupt. Differentiation of the gabbroic to leucogranodioritic units occurred in the lower to mid-crust, followed by buoyant rise to mid- to upper-crustal magma chambers where final crystallization, and segregation of aplites occurred. Mafic dikes that intrude the plutons represent magmas that were able to fracture their way through the largely solidified pluton from below. Many gabbros and mafic diorite units in the plutons are crystal cumulates, and have no equivalents among volcanic rocks and dikes. In contrast, the predominant silicic plutonic units (58 to 63 percent SiO2) and silicic volcanic rocks (or, in hybrid magmas, mixing end members) are similar. Whether these silicic magmas are deep-crustal differentiates of a high-Al basalt or high-temperature partial melts of older mafic arc volcanic rocks is difficult to discern because of the small chemical contrast between Aleutian magmas and country rock. Either model implies a large volume of lower-crustal mafic and ultramafic rock complementary to the observed upper-crustal plutons.

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