AGE AND TECTONIC SETTING OF THE PALEOCENE GLACIER ISLAND VOLCANIC SEQUENCE OF THE ORCA GROUP IN PRINCE WILLIAM SOUND, ALASKA

The Prince William terrane includes the Paleocene-Eocene Orca Group in northern Prince William Sound (NPWS), Alaska. The Orca Group is dominated by turbidites but contains volcanic rocks and several ophiolite sequences. Understanding the age and origin of the ophiolites as well as the other mafic volcanic rocks will contribute to a greater understanding of the tectonic history of the terrane. Seventeen samples of mafic volcanic rocks including sheeted dikes, pillow basalt, and one gabbro were collected from the Glacier Island ophiolite and other locations in NPWS. Detrital zircons from sandstones from Glacier Island and from NPWS were dated by LA-ICPMS. A sandstone sample cross-cut by a mafic dike on Glacier Island yields a maximum depositional age (MDA) of 57 Ma and is similar to the MDA’s of the surrounding Orca Group turbidites. Major and trace element data show that the mafic volcanic rocks on Glacier Island are similar to the Knight Island and Resurrection Peninsula ophiolites exposed in a discontinuous belt 60 and 150 km to the southwest. Recent work has shown that the Resurrection ophiolite was erupted into the turbidites of the Orca Group at 57 Ma, and based on similar field relationships and geochemistry, it is likely that the Knight Island ophiolite formed at the same time. This study shows that the Glacier Island ophiolite also formed at 57 Ma and was erupted into a thick sequence of turbidites that were rapidly accumulating along the continental margin at this time. The similarity in the age and composition of the mafic igneous rocks and interbedded turbidites of the Orca Group in Prince William Sound and on the Resurrection Peninsula suggest that these rocks formed in a similar geologic setting and time. Previous workers have suggested that the ophiolites are fragments of the Kula-Farallon ridge that were tectonically emplaced into the Prince William accretionary wedge; our new data support an alternative explanation that these are upper plate rocks related to transtension in an extended marginal basin.