The Calc-Alkaline Hidden Bay and Kagalaska Plutons and the Construction of the Central Aleutian Oceanic Arc Crust

Abstract

Calc-Alkaline plutons are the major crustal building blocks of continental margins, but are rarely exposed in oceanic island arcs. Two of the best examples are the â 1/410 km wide Hidden Bay and Kagalaska plutons that intrude Eocene mafic volcanic-sedimentary rocks on Adak and Kagalaska islands in the central Aleutian arc. Twenty new Ar/Ar and U/Pb ages, coupled with published ages, show that the Hidden Bay pluton was intruded in multiple stages from â 1/434·6 to 30·9 Ma, whereas the Kagalaska pluton was intruded at â 1/414 Ma. The plutons largely consist of medium-to high-K 2 O hornblende-bearing cumulate diorite (53-55 wt % SiO 2) and hornblende-biotite granodiorite (57-64 wt %), with lesser amounts of gabbro (50-52 wt % SiO 2), leucogranodiorite (67-69 wt % SiO 2) and aplite (76-77 wt % SiO 2) that can generally be linked to each other by crystal fractionation. The compositions of these plutons are generally similar to those of continental plutons, except for more oceanic-like large ion lithophile element and isotopic signatures (87 Sr/ 86 Sr = 0·703-0·7033; I Nd = 9·4-7·7) that reflect oceanic-rather than continental-Type crustal contaminants. Chemical similarities between the Hidden Bay homogeneous gabbros and high-Al basalts in Adak Pleistocene-Holocene volcanoes indicate little temporal evolution in the general character of the mantle-derived basalts. Rather than a unique arc setting and distinctive magmas, formation of the Aleutian calc-Alkaline plutons seems to require a sufficient crustal thickness (â 1/437 km) and a high enough water content to stabilize pargasitic hornblende amphibole in a relatively closed magma system that favors increasing K, Ti and H 2 O at the end of a magmatic cycle. This termination of magmatism coincides with a northward migration of the magmatic front that is inferred to be associated with fore-Arc subduction erosion. In accord with Adak region crustal architecture based on seismic data, crystallization models for the plutons suggest that mantle-generated hydrous arc basalts fractionated olivine and clinopyroxene in the lower crust to form high-Al basaltic composition magmas that rose into the mid-crust, where gabbro and diorite crystallized to form the magmas that buoyantly rose into the upper crust and crystallized to form the volumetrically dominant granodiorite (58-63 wt % SiO 2). The most important temporal changes in chemistry can be explained by fore-Arc crust incorporated into the mantle wedge by fore-Arc subduction erosion creating â € adakitic' signatures at times of northward arc migration and a change to a more continental subducted sediment component at the time of Plio-Pleistocene glaciation.