CONTRASTING ND ISOTOPIC CHARACTERISTICS FROM THE BAKER TERRANE OF NORTHEASTERN OREGON: IMPLICATIONS FOR TECTONIC EVOLUTION

In the Elkhorn Mountains of the Blue Mountains province of northeastern Oregon, the Bourne subterrane of the Baker terrane consists chiefly of dismembered Elkhorn Ridge Argillite and fault-bounded slabs of meta-igneous rocks. The Elkhorn Ridge Argillite is a mélange that includes widespread chert-argillite broken formation with scattered metalimestone olistoliths. Both Tethyan and McCloud Permian fauna have been reported from the metalimestone blocks, and Coward (1985) reported Early Jurassic radiolarians from the Elkhorn Ridge Argillite. The Baker terrane is commonly interpreted as an accretionary complex situated between late Paleozoic–early Mesozoic oceanic arcs, represented by the Wallowa and Old Ferry terranes. The meta-igneous rocks in the Bourne subterrane exhibit a relatively broad range in SiO2 content (~48-67 wt. %) and vary in texture from hornblende gabbro/diorite to lithic-clast volcaniclastic breccia. Siliceous argillite is locally stratigraphically above the meta-igneous rocks. Although the metaplutonic components of the meta-igneous suite locally exhibit high-temperature, solid-state foliation, these rocks are typically deformed by greenschist-facies cataclasis.

Despite their range in SiO2 content, the meta-igneous rocks are characterized by strongly positive, initial epsilon Nd values (+7.8 to +8.3), indicating derivation from depleted mantle sources. In contrast, metamorphosed argillaceous rocks from the Elkhorn Ridge Argillite are characterized by distinctly negative (-4.4 to -7.0) initial epsilon Nd values and Nd model ages of 1.64-1.45 Ga, suggesting significant input from a Proterozoic provenance. The incorporation of primitive oceanic igneous rocks into an evolved metasedimentary mélange has important implications for the evolution of this accretionary complex. One model is fore-arc tectonic erosion during subduction, whereby igneous rocks of the Wallowa fore-arc basement are imbricated within the accretionary complex. Another model involves tectonic emplacement of slices of the Wallowa terrane into the Baker terrane during late-stage amalgamation. In both scenarios, the evolved Nd signature of metamorphosed argillaceous rocks requires the proximity of a Proterozoic source during the late Paleozoic-early Mesozoic development of the Baker terrane.