CRYSTALLIZATION AGES AND GEOCHEMISTRY OF THE MINERS BAY AND CEDAR BAY PLUTONS, PRINCE WILLIAM SOUND, ALASKA

The Late Eocene Eshamy Suite plutons (ESP) intrude turbidites of the Chugach-Prince William (CPW) terrane in western Prince William Sound. The ESP is bimodal dominated by biotite granites and leucogranites with subordinate gabbro, and their tectonic setting is enigmatic. The CPW has been traditionally interpreted as a Late Cretaceous to Paleocene accretionary wedge complex that was either formed in place or was transported a significant distance along the continental margin. Whether translated or not, most workers agree that the CPW was more or less in place by ~30 Ma. Outstanding questions for the ESP include the crystallization ages for the plutons, their relationship to igneous rocks found farther inboard, and the nature of the tectonic setting of these rocks. For this study, we measured major and trace element compositions using XRF and LA-ICP-MS at the Hamilton Analytical Laboratory for fourteen samples from the Miners Bay and Cedar Bay plutons. For five of the samples, U/Pb ratios of zircon were measured at the Arizona Laserchron Center with LA-ICP-MS. A gabbroic diorite from the Miners Bay pluton (MBP) yields a crystallization age of 40.4±0.3 Ma and a small stock of leucogranite immediately north of the MBP yields a crystallization age of 40.5±0.4 Ma. The Cedar Bay pluton (CBP) is primarily composed of leucogranite with rare mafic enclaves. Three leucogranite samples from the CPB yield a crystallization ages of 41.5 to 41.1 Ma. These U/Pb zircon dates are slightly older than those reported for ESP intrusions to the SW, which range in age from 39.9 to 37.6 Ma, thus suggesting a NE to SW age progression. The Miners Bay pluton is cut by the Contact fault (aka Eaglek fault) in the Unakwik Inlet region showing that reverse dip-slip motion on this fault postdates ~40 Ma. The ESP fall within the age range of the Caribou Creek Volcanics (CCV) that crop out in the Matanuska area about 90 km inland. However, preliminary geochemical data suggest that the ESP may not be directly related to the CCV. Granites from ESP and granites and rhyolites from the CCV are both ferron and magnesian, but the CCV appears to have much stronger Fe-enrichment than the ESP. Using the modified alkali-lime index, the ESP is mostly calc-alkalic, and the CCV is predominately calcic, and on discrimination diagrams, the ESP typically plot in different fields from the CCV.