ISOTOPIC CHARACTERISTICS OF THE BAKER TERRANE, NE OREGON: IMPLICATIONS FOR THE RELATIONSHIP BETWEEN THE BOURNE AND GREENHORN SUBTERRANES

The Baker terrane of northeastern Oregon consists of two distinctive subterranes, the Bourne and Greenhorn subterranes (Ferns and Brooks, 1995). These distinct lithotectonic units preserve important depositional and deformational features that provide clues to the geodynamic evolution of the Baker terrane. The Bourne subterrane consists of dismembered Permian–Early Jurassic Elkhorn Ridge Argillite with tectonically intercalated 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. In contrast, the Greenhorn subterrane is chiefly characterized by serpentinite-matrix mélange containing large blocks of metaplutonic, metavolcanic, metavolcaniclastic, and chert-argillite breccia. Associated with the Greenhorn subterrane are Permian–Triassic conglomerate, sandstone, argillite, and limestone of the Badger Creek unit. Fusulinids in the Greenhorn subterrane are of McCloud affinity and are distinctive from similar-aged fusulinids in the Elkhorn Ridge Argillite in the Bourne subterrane. Ferns and Brooks (1995) interpreted the characteristic features of the Bourne subterrane as representing an accretionary prism developed marginal to the Wallowa arc, whereas the Greenhorn terrane is interpreted as the dismembered fore-arc of the pericratonic Olds Ferry arc.

We present new Sm-Nd and Rb-Sr isotopic data that indicate that both the Elkhorn Ridge Argillite and Badger Creek unit of the Bourne and Greenhorn subterranes, respectively are characterized by strongly evolved isotopic signatures. The Elkhorn Ridge Argillite has initial eNd values ranging from -4.7 to -7.8 and Nd model ages of 1.4–1.6 Ga, whereas the Badger Creek unit has initial eNd values ranging from -2.4 to -3.8 and Nd model ages of 1.3–1.6 Ga. These evolved isotopic signatures demonstrate the involvement of Proterozoic sources in the depositional histories of both the Bourne and Greenhorn subterranes and offer new insights into their origin. We propose a model whereby the Bourne and Greenhorn subterranes evolve together as distinct parts of a long-lived, accretionary complex/fore-arc, which was adjacent to the Olds Ferry arc and the North American margin.