LATE CRETACEOUS THROUGH OLIGOCENE MAGMATIC AND TECTONIC EVOLUTION OF THE WESTERN ALASKA RANGE

New U-Pb zircon ages coupled with whole-rock and trace-element geochemistry for Late Cretaceous to Oligocene igneous rocks in the western Alaska Range constrain the spatial distribution, petrogenesis, and tectonic setting of magmatism through time. These igneous rocks were emplaced across multiple basement domains that include Neoproterozoic to Jurassic carbonate and siliciclastic strata of the Farewell terrane, Mesozoic plutonic and volcanic rocks of the Peninsular terrane, and Cretaceous turbiditic strata of the Kahiltna basin. They also host multiple types of mineralization including intrusion-related Au, porphyry Cu-Mo-Au, polymetallic veins and skarns, and peralkaline intrusion-related REE. The oldest intrusive suite is ca. 104 to 78 Ma; the youngest plutons of this relatively localized suite were intruded during folding of the Kahiltna succession. This deformation is interpreted to represent closure of the retroarc Kahiltna basin and a transition to transpression-dominated tectonics along the southern Alaska margin. More widespread magmatism ca. 75– 55 Ma occurred in two general pulses, each with contrasting geochemistry. The first pulse was dominantly magnesian ca. 75–66 Ma, and the second pulse was dominantly ferroan ca. 63–55 Ma. The latter pulse is associated with widespread andesitic to rhyolitic volcanic rocks and was coeval with dextral transpressional deformation. Emplacement of widespread and voluminous intermediate to mafic dikes ca. 59–51 Ma overlaps the final phase of the second pulse. Magmatism waned during inferred late Paleocene to early Eocene ridge subduction along the southern Alaska margin but then resumed ca. 45 Ma, around the same time as the initiation of the Aleutian/Meshik arc. In the western Alaska Range, this dominantly subalkaline Eocene magmatism included emplacement of the elongate N–S Merrill Pass pluton and large volumes of associated ca. 44–37 Ma andesitic flows, tuffs, and lahar deposits. Finally, an Oligocene magmatic pulse involved emplacement of a compositionally variable suite of subduction-related magmas ranging from gabbro to peralkaline granite ca. 31–25 Ma, followed by waning magmatism that coincided with initiation of Yakutat slab subduction.