COOLING HISTORY OF THE SANAK-BARANOF PLUTONS, ALASKA, USING ZIRCON AND APATITE (U-TH)/HE THERMOCHRONOLOGY

The Sanak-Baranof belt plutons intruded the Late Cretaceous to Paleocene Chugach-Prince William terrane in southern Alaska between 63-48 Ma and young from west to east. The plutons are unusual due to their emplacement within an accretionary prism, and are typically attributed to forearc magmatism associated with the subduction of the Kula-Farallon or Kula-Resurrection ridge. In this study, we present new zircon and apatite (U-Th)/He dates from seven widely distributed plutons of the Sanak-Baranof belt from Sanak Island in the west to Baranof Island in Southeast Alaska. Apatite (U-Th)/He data from the Sheep Bay pluton in Prince William Sound, and the Krestof Island and Crawfish Inlet Plutons on Baranof Island yield average ages of 3.4, 6.1, and 4.0 Ma, respecively. Zircon (U-Th)/He cooling ages decrease systematically from west to east, with cooling ages of ~37 and ~38 Ma for the Sanak Island and Nagai Island plutons, respectively, in the west, a ~33 Ma cooling age for the Eagle Harbor pluton on Kodiak Island, a ~29 Ma cooling age for the Aialik pluton near Seward, and a ~16 Ma cooling age for the Crawfish Inlet pluton on Baranof Island. Preliminary thermal modeling using HeFTy indicates rapid cooling of the Sanak-Baranof plutons in the west (Sanak and Nagai) through the zircon (U-Th)/He closure temperature followed by relatively slow cooling. In the east (Baranof Island), cooling histories appear more complicated with initial rapid cooling through the hornblende and biotite K-Ar closure temperatures, followed by slow cooling through the zircon (U-Th)/He closure temperature and a relatively recent rapid cooling event.