DISTINCT MAGMATIC STAGES ASSOCIATED WITH CRETACEOUS THROUGH PALEOGENE CRUSTAL FORMATION AND EVOLUTION OF THE SOUTHERN ALASKA MARGIN

Mid-Cretaceous to Oligocene intrusive rocks of the western Alaska and northern Aleutian Ranges preserve a crystallization history documenting at least five magmatic flare-up episodes, each lasting ~10 My, during and following accretion of the Wrangellia composite terrane (WCT) to the North American margin in Alaska (locally, Farewell terrane, FT). Isotopic and chemical variance between the pulses, spanning ca. 105–28 Ma, contribute to the understanding of tectonic evolution of the southern Alaska margin. Calc-alkaline diorite to granite plutons formed in two ~10 My pulses during progressive retro-arc basin closure (ca. 95-76 Ma), emplaced in WCT basement or proximal to the WCT–FT margin, and are interpreted as products of the migrating arc associated with final suturing of the outboard WCT. After 76 Ma, plutons define linear trends and cut across the suture zone, suggesting structurally controlled emplacement during regional transpression. These Late Cretaceous plutons are gabbro to granodiorite and have arc to collisional chemical affinity; some are adakite-like, possibly due to crustal thickening associated with WCT collision. In contrast, Paleocene plutons are fractionated granites with scattered spatial distribution, and are frequently intruded by mafic dikes that lack a subduction affinity. A maximum contribution of upper-plate components to intrusive rocks from Late Cretaceous to Paleocene time, likely due to crustal thickening, is suggested by anomalous Hf isotope ratios. A ~10 My magmatic hiatus began ca. 56 Ma, coincident with models invoking spreading ridge subduction along the southern Alaska margin. An Eocene magmatic stage produced more linearly distributed gabbro to granite plutons that intrude voluminous and thick (~1 km, locally) cogenetic rhyodacite deposits. Eocene circum-Pacific plate reorganization, resulting in arc initiation in the western Pacific, may have also established a rejuvenated subduction geometry in southern Alaska. Oligocene plutons are compositionally bimodal, trending to within-plate chemistry and represent melting away from the trench as the result of either 1) exhumation and extension due to slab rollback or 2) localized arc geometry disruption associated with incipient Yakutat plate subduction. The latter is suggested by magmatic quiescence after 28 Ma.