OVERVIEW OF JURASSIC-CRETACEOUS MAGMATISM IN THE BLUE MOUNTAINS PROVINCE (NE OREGON & W IDAHO): INSIGHTS FROM NEW PB/U (SHRIMP-RG) AGE DETERMINATIONS

The Blue Mountains Province (BMP) comprises, from NW to SE, the Wallowa (island arc), the Baker (accretionary complex and forearc), and the Olds Ferry (island arc) terranes. New Pb/U age determinations from Mesozoic plutons have allowed us to refine the age and duration of distinct magmatic episodes in the BMP associated with terrane amalgamation and accretion to the North American continental margin. Cessation of Wallowa and Olds Ferry volcanism in the Early Jurassic was followed by renewed arc activity across the Wallowa and Baker terranes, from ~162 to 154 Ma. This period coincides with regional contractile deformation, and resulted in the formation of small, low Sr/Y pyroxene-bearing gabbroic and dioritic plutons. A distinct pulse of voluminous low Sr/Y magmatism followed from 148 to 141 Ma, and led to the emplacement of plutons and batholiths of predominantly tonalite and granodiorite (± gabbro and diorite) across the Wallowa and Baker terranes, including the Wallowa and Bald Mountain batholiths. A localized comagmatic suite of high Sr/Y granodioritic plutons in the Baker terrane (Greenhorn subterrane) resulted from partial melting of mafic crust overthickened by contractile deformation (collision of Wallowa and Olds Ferry arcs?). The final stage of magmatism (pre-Idaho batholith) occurred between ~124 and 111 Ma, producing small plutons of both metaluminous and strongly peraluminous tonalite and trondhjemite. Most of the strongly peraluminous plutons coincide with the 0.706 line, and resulted from partial melting of island arc terrane crust overthickened by collision of the BMP with the continental margin at ~128 ± 3 Ma. When ~60° of Cretaceous-Cenozoic clockwise rotation is removed, the three distinct stages of magmatism define roughly parallel N-S striking belts (with the exception of the strongly peraluminous trondhjemite belt), similar to what is observed in other segments of the North American Cordillera.