ORIGIN OF PAIRED LATE JURASSIC HIGH AND LOW SR/Y MAGMATIC BELTS IN THE BLUE MOUNTAINS PROVINCE, NE OREGON

High and low Sr/Y plutons occupy large areas in ancient and modern orogenic belts, yet considerable controversy exists regarding mechanisms of their generation, the tectonic settings in which they form, and their role in the evolution of continental crust through time. In the Blue Mountains province (northeastern Oregon), Late Jurassic plutons and batholiths define two coeval yet geochemically and spatially distinct high and low Sr/Y belts that intrude pre-existing island arc crust. These belts trend NE-SW and occur within the Wallowa island arc terrane and Bourne subterrane of the Baker terrane (low Sr/Y belt), and the Greenhorn subterrane of the Baker terrane (high Sr/Y belt), respectively. The low Sr/Y belt (148-141 Ma) consists of predominantly tonalite and granodiorite (± diorite, gabbro), and has flat to slightly LREE-enriched patterns (La/Lu ≤60), low Sr (<400 ppm), and low Sr/Y values (<40). The compositions of these plutons are consistent with derivation from a mantle source and/or shallow-level (<30 km) melting of pre-existing island arc crust. In contrast, the high Sr/Y belt (147-145 Ma) is more compositionally restricted, consisting of tonalite and granodiorite, and displays steeply fractionated REE patterns (La/Lu >120), a lack of Eu anomalies, elevated Sr concentrations (>600 ppm) and high Sr/Y values (>40). The composition of these plutons is consistent with partial melting of pre-existing island arc crust in the presence of a plagioclase-poor to absent, hornblende + garnet-bearing source (depths >30 km). Notably, the occurrence of the Late Jurassic high and low Sr/Y magmatic rocks post-dates an earlier episode of low Sr/Y magmatism (162-154 Ma) in the Baker and Wallowa terranes, and a short-lived regional contractional event at 159-157 Ma.

We propose that the geochemically and spatially distinct high and low Sr/Y magmatic belts in the Blue Mountains province formed as a result of Late Jurassic regional contraction. In this scenario, collision between the Olds Ferry and Wallowa island arc terranes at 159-157 Ma resulted in crustal thickening beneath the Greenhorn subterrane of the Baker terrane. This fundamental change in crustal structure influenced subsequent post-orogenic subduction-related magmatism, and led to the establishment of the two coeval high and low Sr/Y magmatic belts.