Paper No. 11
Presentation Time: 11:00 AM
AGE AND ORIGIN OF EOCENE PLUTONS IN WESTERN PRINCE WILLIAM SOUND, ALASKA: LOW PRESSURE MELTING OF THE CHUGACH-PRINCE WILLIAM ACCRETIONARY COMPLEX
DAVIDSON, Cameron, Department of Geology, Carleton College, 1 N College St, Northfield, MN 55057, JOHNSON, Emily, Department of Geology, Whitman College, 345 Boyer Avenue, Walla Walla, WA 99362, NICOLAYSEN, Kirsten P., Department of Geology, Whitman College, 345 Boyer AVE, Walla Walla, WA 99362 and GARVER, John I., Geology Department, Union College, 807 Union ST, Schenectady, NY 12308-2311, cdavidso@carleton.edu
The Chugach-Prince William (CPW) composite terrane is a Late Cretaceous-Paleogene accretionary complex exposed for ~2200 km in southern Alaska, which is principally composed of deep-water turbidites with abundant quartzofeldspathic and volcanic-lithic sandstones and basaltic rocks of the Valdez and Orca Groups. In western Prince William Sound, Eocene age granitic plutons and associated gabbroic rocks of the Eshamy Suite intrude into the accretionary wedge rocks. Contact metamorphic aureoles developed in the flysch adjacent to the plutons contain biotite and cordierite. These plutons are anomalous because they intrude the accretionary wedge, and are ~15 million years younger than the well-documented near-trench plutons of the Sanak-Baranof belt that intrude the CPW terrane from Sanak Island in the west to Baranof Island in Southeast Alaska. The tectonic setting, heat source, and mechanism for the generation of melt in the wedge during the Eocene are poorly understood.
In this contribution we report five new U/Pb zircon dates for the Eshamy Suite plutons from western Prince William Sound, and show that these plutons formed from the low pressure melting of the flysch of the CPW accretionary wedge. U/Pb dates for five of the plutons range from 37.56 ± 0.37 Ma to 39.91 ± 0.66 Ma. Andalusite xenocrysts from the Hidden Bay pluton have spectacular reaction rims of hercynite + cordierite ± sillimanite ± corundum symplectites. Geothermobarometry and pseudosection modeling suggests that the melting reactions responsible for the textures preserved in these rocks occurred at ~3 kbar and over 700°C. Major and trace element geochemistry indicate that a Pacific NMORB source combined with melts derived from the wedge can explain the compositional variation found in the Eshamy Suite plutons. Taken together these data suggest that a mid-ocean ridge (MOR), or some other type of asthenospheric upwelling and partial melting must have interacted with the CPW accretionary wedge in the Middle Eocene.
