Eocene to P leistocene magmatic evolution of the D elarof I slands, A leutian A rc

The Delarof Islands in the Aleutian Arc near 179º W record ∼37 million years of discontinuous arc magmatism along a SW‐NE cross‐arc transect from near the trench to the active volcanic front. Geochemical and geochronologic data from the pre‐Pleistocene volcanic record in this region are limited and the 40 Ar/ 39 Ar, isotopic, and trace element data presented here are the first from units older than the Pleistocene‐Holocene volcanoes (Tanaga, Gareloi). Twenty‐two new 40 Ar/ 39 Ar ages establish a temporal framework for geochemical data and reveal that magmatism in the Delarof region was coincident with two arc‐wide magmatic flare ups in the late Eocene/early Oligocene and latest Miocene/Pliocene. Mafic lavas and plutons in the southern Delarofs give 40 Ar/ 39 Ar plateau ages ranging from 36.8 ± 0.2 to 26.9 ± 0.6 Ma on Amatignak Island and 37.0 ± 0.2 to 29.3 ± 1.0 Ma on Ulak Island. To the north 25 km, 40 Ar/ 39 Ar ages from the central Delarof Islands, Kavalga, Ogliuga, and Skagul are late Miocene (6.28 ± 0.04 Ma) to Pliocene (4.77 ± 0.18 Ma) with younger ages to the northeast. A significant transition in arc chemistry occurs in the Pleistocene where lavas from active volcanoes Gareloi and Tanaga exhibit higher sediment and hydrous fluid signatures (Th/La, Cs/Ta, La/Sm, LILE abundances) and lower 143 Nd/ 144 Nd than older Delarof Island units closer to the trench. Similar findings from Eocene‐Miocene lavas from Amchitka to Adak suggest that a previously minor sediment melt component became more pronounced in the Quaternary.