RADIOGENIC ISOTOPIC CHARACTERISTICS OF INTRUSIVE ROCKS OF THE COUGAR CREEK COMPLEX, BLUE MOUNTAINS PROVINCE, NORTHEASTERN OREGON AND WEST-CENTRAL IDAHO: IMPLICATIONS FOR THE PALEOGEOGRAPHY AND TECTONIC EVOLUTION OF THE WALLOWA TERRANE

The Cougar Creek Complex (CCC) is interpreted as the mid crustal levels of the Wallowa island arc terrane of the Blue Mountains Province of northeastern Oregon and west-central Idaho. The CCC is comprised entirely of dikes and small stocks of a wide range of bulk compositions, including gabbro, diorite, quartz diorite, tonalite, trondhjemite, and their metamorphosed and deformed equivalents. Plutonic complexes such as the CCC provide windows into the middle and deep crustal roots of volcanic arc systems. Isotopic analyses of rocks from this tectonic environment supply valuable information pertaining to the petrologic nature of the foundation(s) upon which arcs are built.

Preliminary ⁸⁷Sr/⁸⁶Sr isotopic data are interpreted to indicate that the Wallowa island arc developed within an intra-oceanic environment from a depleted mantle source with little or no interaction with evolved continental crust. These new data provide a basis for exploring likely petrologic relationships between lithologies internally within the CCC as well as with those of other Late Paleozoic and Mesozoic plutonic and volcanic basement rocks within the BMP. Further development of such isotopic databases for other basement complexes within the BMP will help to: 1) establish clearer and more accurate terrane characterization and discrimination; and 2) develop comprehensive models for the tectonic evolution of individual terranes as well as for the BMP as a regional geologic province.