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

A growing dataset of precise U-Pb zircon ages coupled with whole-rock and trace-element geochemistry for Late Cretaceous to Oligocene igneous rocks in the western Alaska Range provides critical constraints on the spatial distribution, petrogenesis, and tectonic setting of magmatism through time. These igneous rocks were emplaced across multiple basement domains that include Paleozoic to early Mesozoic carbonate and siliciclastic strata of the Farewell terrane, the Mesozoic Peninsular terrane, and late Mesozoic turbiditic strata of the Kahiltna basin. The oldest intrusive suite ranges from ca. 99 to 75 Ma and include the Shell Hills pluton and igneous exposures along the shores of Lake Chakachamna. The youngest plutons of this relatively localized suite were broadly coeval with but also cross-cut deformational features in the Kahiltna overlap succession. Deformation during this time 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 was generally diachronous with contrasting geochemistry. The first pulse was dominantly magnesian ca. 71-66 Ma (e.g., Mt. Estelle, Kohlsaat plutons), and the second was dominantly ferroan ca. 63–57 Ma (e.g., Tordrillo Mountains and Tired Pup plutons). The latter phase is associated with widespread andesitic to rhyolitic volcanic rocks and overlaps the onset of extensive intermediate to mafic dike emplacement ca. 59–51 Ma. Circa 63–57 Ma magmatism was also coeval with dextral-oblique transcurrent deformation along the 1-km-wide, subvertical Chakachamna shear zone and localized deformation at pluton margins. Magmatism generally 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 Meshik/Aleutian arc. Dominantly subalkaline Eocene magmatism included emplacement of the elongate Merrill Pass pluton and large volumes of associated ca. 44–37 Ma andesitic flows, tuffs, and lahars. An Oligocene magmatic pulse involved emplacement of a compositionally variable suite ranging from gabbro to peralkaline granite ca. 31–25 Ma. Magmatism declined after ca. 25 Ma, coinciding with initiation of Yakutat slab subduction.