TECTONIC EVOLUTION OF THE CHUGACH-PRINCE WILLIAM TERRANE: GEOCHEMISTRY OF THE ORCA GROUP VOLCANIC ROCKS IN EASTERN PRINCE WILLIAM SOUND, ALASKA

Geologic evidence indicates that slab window subduction modified the forearc basin in the Chugach-Prince William terrane (CPW), producing marine basalts interbedded and structurally interleaved with the Paleocene flysch of the CPW. A similar model is proposed for the nearly coeval Crescent Formation in the Pacific Northwest. In this study we examine the geochemical composition of mafic volcanic rocks from the Orca Group in eastern Prince William Sound (PWS) to evaluate their relationship to the Knight Island ophiolite, Chenega Island volcanic rocks, and Crescent basalts in the Pacific Northwest. Maximum depositional ages from detrital zircon ages of the Orca Group indicate that the interbedded volcanic rocks formed at 50-57 Ma. The 14 basalt samples we analyzed fall into three groups based on REE abundances normalized against chondrites: light REE depleted, slightly light REE depleted, and no depletion. Trace elements normalized to MORB show N-MORB like abundances for high field strength elements with more variability in the abundances of large ion lithophiles suggesting alteration due to sediment mixing, fractional crystallization, or hydrothermal alteration. Five of the samples in eastern PWS resemble basalts on Chenega Island, and the other nine are similar to basalts from the Knight Island ophiolite in western PWS. In addition, the slightly light REE depleted N-MORB samples are similar to basalts from the Lower Crescent Formation on the Olympic peninsula in Washington State. The relationship between Orca Group volcanics in Alaska and similar sequences in the Pacific Northwest is important for evaluating the translation versus in situ hypotheses for the location of the Kula-Farallon ridge and associated rocks. One hypothesis is that the CPW formed in a similar setting to the Crescent Formation near ~48°N, and was subsequently separated and translated north to Alaska.