ORIGIN OF THE REGIONALLY DISTINCTIVE DACITIC TO RHYOLITIC MAGMA SERIES ERUPTED AT HAYES VOLCANO, THE EASTERNMOST VOLCANO OF THE ALEUTIAN-ALASKA ARC

Hayes Volcano, the northeastern-most extent of the modern continental Alaskan arc, erupted regionally distinctive magmas over a 30 kyr time span from Late Pleistocene to Holocene. Individual glass shard and whole rock geochemical data delineate two principal magmatic groups. Group 1 dacitic and rhyolitic eruptive products are characterized by higher CaO and FeO* with adakite-like geochemical characteristics (e.g., Sr/Y = 25-118 and middle-heavy REE depletions). Group 2 materials are more evolved (up to high-Si rhyolite) with non-adakitic calc-alkaline arc signatures (Sr/Y =1.8-13) and lower CaO and FeO*. Despite contrasting major and trace element compositions, these groups are isotopically indistinguishable and possess isotopic compositions consistent with magmas derived from melts of a sediment-modified mantle wedge. Though adakitic characteristics could be attributed to slab melting, Hayes Group 1 geochemical traits, including low compatible elements (Cr, Ni) and middle REE depletions, are inconsistent with this origin. Rather, the Group 1 adakite-like features are interpreted and modeled to be products of fractional crystallization involving amphibole > garnet from an andesitic parent at lower crustal depths (~0.8 GPa). Highly evolved rhyolites of Group 2 are proposed to have originated from the same parental andesite but with primary differentiation at shallower initial levels via an AFC process involving granodioritic melts. The regionally distinct character of Hayes eruptive products is hypothesized to be controlled by the shallowing trajectory of the Pacific Plate beneath Hayes, yielding more hydrous mantle source lithologies and derivative magmas relative to other eastern Alaskan arc volcanoes. Additionally, shared isotope and trace element characteristics between Hayes and the closest volcano to the south, Mount Spurr, suggest a common mantle source. This study provides valuable insights into the production of diverse magmas along the continental Alaskan-Aleutian arc and the control of the subducting plate, crustal lithologies, and the crustal plumbing system on the ultimate compositions of erupted materials.