Project Title: Evaluating the Tectonic History of the Chugach-Prince William Terrane Through Geochemistry and Petrography of Orca Group Volcanic Rocks in Eastern Prince William Sound, Alaska
Student: Elaine Young
Mentor: Dr. Karen Fryer
Geologic evidence indicates that slab window subduction modified the forearc basin of the Chugach-Prince William terrane (CPW), producing marine basalts that are anomalously close to the trench and are interbedded and structurally interleaved with the Paleocene flysch of the CPW. A similar model is proposed for the Crescent Formation in the Pacific Northwest. In this study I examine the geochemical composition of mafic volcanic rocks from the Orea Group in eastern Prince William Sound (PWS) to evaluate their relationship to similar suites of rock in the CPW and to the Crescent Formation. Maximum depositional ages from U /Pb in detrital zircons of the Orea Group turbidites indicate that the interbedded volcanic rocks formed at 50-57 Ma. The 14 basalt samples I 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. Petrographic analysis of my 14 samples shows evidence of 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; volcanic activity in the Crescent Formation occurred about 50-56 Ma. The relationship between Orea 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 the Chugach-Prince William terrane.