AN ISOTOPIC STUDY OF THE BURNT RIVER SCHIST: IMPLICATIONS FOR THE DEVELOPMENT OF THE COMPOSITE BAKER TERRANE, BLUE MOUNTAINS PROVINCE, NE OREGON

The Burnt River Schist (BRS) is an assemblage of diverse rock types within the composite Baker terrane of the Blue Mountains province of NE Oregon. The Baker terrane is interpreted as a long-lived accretionary complex consisting of the Elkhorn Ridge Argillite (ERA), serpentinite-matrix mélange, and the BRS. One hypothesis is that the BRS is a higher grade metamorphic equivalent of the ERA and associated rocks. A major goal of this isotopic-geochemical study is to test this hypothesis.

The BRS consists chiefly of fine-grained metasedimentary rocks (e.g., slaty argillite and siliceous phyllite), but also includes mappable belts of greenschist-facies metvolcanic rocks and discontinuous bodies of impure calcite marble (interpreted as olistoliths of shallow-water limestone). The BRS exhibits a complex, polyphase deformational history; a distinctive characteristic is disrupted bedding typical of broken formation.

Rb-Sr and Sm-Nd isotopic analyses were used to characterize the BRS. Bulk geochemical characteristics of the pelitic rocks of the ERA and BRS are strikingly similar. Initial analyses indicate that the BRS is isotopically distinct from the ERA—⁸⁷Sr/⁸⁶Sr = 0.703 to 0.705 and 0.707 to 0.709; εNd = +0.04 to +5.99 and -4.7 to -8.8, respectively. These data suggest that the BRS is not simply a higher-grade portion of the ERA, but had a different provenance. Paleontological data from the ERA and BRS also support significant differences between these rock assemblages. Middle to Late Triassic conodonts have been recovered from the BRS, whereas the ERA has yielded a more diverse faunal assemblage that spans a longer time frame. If these preliminary conclusions are supported by future studies (e.g., detrital zircon analyses) the BRS may be a discrete subterrane(?) in the composite Baker accretionary complex. The Connor Creek reverse fault emplaced the southern part of the Baker terrane structurally above Jurassic metasedimentary rocks of the Izee basin terrane, providing an oblique section from a structurally deep part of the Baker terrane (i.e., BRS and associated rocks) to a shallower part exposed in the Elkhorn Mountains to the northwest.