METAMORPHISM, DEFORMATION, AND EXHUMATION OF BLUESCHISTS IN SELDOVIA, ALASKA

Investigating the metamorphic and deformation history of blueschists is critical to understanding the mechanical behavior and exhumation processes associated with subducted mafic oceanic crust. Here we focus on exposures of blueschists located within the Jurassic-Cretaceous Chugach accretionary complex in Seldovia, Alaska. The blueschists crop out as discontinuous lenses several hundred km along strike and are juxtaposed against inboard, upper plate Jurassic arc rocks and outboard lower grade mélange units along a reoriented subduction thrust known as the Border Ranges Fault (BRF). The proximity of the schists to the BRF suggests a potential relationship of the schist exhumation to the now near-vertical fault. Constraining the history of the blueschists will also bring insight to the reactivation of the BRF and its kinematics. Little work has been done with modern analytical techniques to constrain the P-T path and timing of peak and retrograde metamorphism of the Seldovia blueschists. Previous work provides timing of peak metamorphism based on Ar-Ar and K-Ar analysis of white micas and amphiboles, but with the assemblages present in our samples we can provide more refined data on the peak and retrograde metamorphic events allowing us to interpret the P-T path.

In this work we begin to constrain a P-T-deformation-time path for the Seldovia blueschists. We utilize quartz- and apatite-in-garnet elastic barometry to constrain pressures of garnet formation, and couple these results with major element x-ray maps of garnet. Future work will also utilize quartz-rutile oxygen isotope thermometry to constrain temperatures, and U-Pb dating of rutile and titanite. We use these data to make interpretations about the metamorphic, deformation, and exhumation history of the blueschists, and compare our results with more well-studied schist units along strike in southern Alaska.