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Abstract
Western Aleutian volcanoes have been characterized as erupting predominantly high Mg# [(MgO/(MgO+FeO)*100) >60)] calc-alkaline andesites that distinguish them from all other volcanic centers within the Aleutian arc. By using a combination of whole-rock chemistry and mineral chemistry analysis, thermobarometry, and thermodynamic modeling, a gradational shift in both primary magmatic composition and subsequent differentiation trends is present within the distal Aleutians. Evidence is presented for classifying Gareloi (179°W) as a high-K tholeiitic volcanic center with magmas predominantly differentiated in a shallow crustal reservoir (1–4 kbar) and Kiska (182°W) as a calc-alkaline center with magmas that have undergone polybaric differentiation at 0.8–3.9 kbar and 4.6–7.5 kbar. A common high-Al basalt parent is proposed for both volcanoes. West of Kiska, magmas have high Mg# (>60), Sr/Y (>50), and La/Yb (>9) suggesting a transitional shift to magmas influenced by a subducted slab melt of mid-ocean ridge basalt (MORB) in eclogite facies.