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Origin of the late Eocene Eshamy suite granitoids of western Prince William Sound, Alaska
Emily P. Johnson
May 9, 2012
Department or Program
The geochronology and geochemistry of five Eshamy suite plutons in Prince William Sound, Alaska, tests the applicability of tectonic models explaining late Eocene magmatism in Prince William Sound with respect to the Sanak-Baranof plutons. Based on zircon U/Pb ages, the Sheep Bay pluton in eastern Prince William Sound (54.5 ±1.8 Ma) correlates with Sanak-Baranof plutonism (62-48 Ma); in contrast, Eshamy suite pluton ages range from 37.56 ±0.37 Ma to 39.91 ±0.66, requiring evaluation of a tectonic model to explain younger magmatism in the region. Subduction of a mid-ocean spreading center and subsequent formation of a slab window beneath the accretionary wedge and plate margin predicts anomalously high heat flow that will melt accretionary wedge sediments. These sediment melts may then mix with mantle-derived basalts to create the Eshamy suite plutons. The geochemistry of two Eshamy suite plutons, the Eshamy Bay and the Nellie Juan, shows three main results: 1) The Eshamy suite granites are heterogeneous and look similar to the Orca Group sediments in trace element chemistry but not in major element chemistry; 2) Xenoliths within Eshamy suite plutons have major element compositions between Pacific NMORB and the felsic granite end-member; 3) The Eshamy suite correlates with rhyolites and granitoids in the Caribou Creek Volcanic Field (CCVF) in trace element chemistry. The model of MOR subduction and slab window formation in response to Eocene plate reorganization as suggested by Cole et al. (2006) and Kusky et al. (2003) to explain the genesis of the CCVF rhyolites is a reasonable hypothesis to explain genesis of PWS pluton.