Geochronologic and isotopic constraints on the magmatic evolution of Pleistocene-recent Aleutian and Andean volcanic complexes

The long-term evolution of volcanic and magmatic processes at basaltic to rhyolitic volcanoes in the Aleutian Island Are and Southern Volcanic Zone of Chile (SVZ) has been constrained using 40Ar/39Ar and U-Th isotope geochronology. At Seguam Island, Alaska, sixty new 40Ar/39Ar ages indicate that subaerial volcanic activity began as early as 318 ka. The time-averaged eruptive rate at Seguam varied from 0.07 to 1.18 kin3/k.y., which is similar to growth rates of other well-dated arc volcanoes.; Twenty eight new 40Ar/39Ar age determinations on subaerial and submarine volcanic and plutonic rocks from the Aleutian Island Arc provide insight into the timing of are formation and subsequent growth since the middle Eocene. 40Ar/39Ar geochronology coupled with recent seismic data indicates that magma production rates for the entire arc were extraordinarily high and occurred in three main pulses at 38-29, 16-11, and 6-0 Ma. These high rates of magma production coincide with periods of intense magmatism in other western Pacific island arcs, which likely implies increased plate velocities and subduction fluxes into the mantle beneath all arcs in the circum-Pacific region.; U-Th isotope data from minerals, glass, and whole-rocks from basaltic to rhyolitic lavas and tuffs at Seguam, interpreted in light of 40 Ar/39Ar ages, geochemical variations and Sr isotope compositions, indicate that phenocrysts crystallized shortly before the eruption during the past 142 ka. Crystallization and differentiation were probably driven by decompression as magmas ascended through the upper crust. U-Th isotope data also suggest a model involving erupted magmas being derived from a single, deep seated reservoir for ∼130 kyrs until the collapse of the Wilcox volcano and injection of isotopically distinctive basalt from the mantle at 9 ka. In contrast, basaltic to rhyolitec lavas erupted during the past 70 kyrs at Puyehue-Cordon Caulle, Chile vary little in (230Th/ 232Th), which implies recent U-Th fractionation, rapid ascent from the mantle, and differentiation to produce rhyolites in 1,000 years or less. Seguam and Puyehue-Cordon Caulle sit atop crustal columns of similar thickness and composition, however, the crust in the SVZ is strongly faulted, thereby possibly allowing magma transport to occur more rapidly than in the Aleutians.