EPISODIC GENERATION OF HIGH SR/Y PLUTONS IN RESPONSE TO LATE JURASSIC OROGENESIS IN THE BLUE MOUNTAINS PROVINCE, NE OREGON: IMPLICATIONS FOR TEMPORAL AND SPATIAL CRUSTAL THICKNESS VARIATIONS ALONG THE WESTERN NORTH AMERICAN CORDILLERA

High 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 relationship to contractional deformation through time. In the Blue Mountains province (NE Oregon), Late Jurassic, high Sr/Y plutons (148-145 Ma) intrude Middle Jurassic, low Sr/Y arc-related rocks (162-157 Ma) in the Greenhorn subterrane of the Baker terrane immediately after widespread Late Jurassic arc-arc collision (159-154 Ma). Pre- to syn-kinematic, low Sr/Y lavas and plutons (162-157 Ma) have flat to slightly enriched rare earth element (REE) patterns (La/Yb <10), low Sr (<400 ppm) and Sr/Y values (<40), and strongly positive initial epsilon ¹⁷⁶Hf/¹⁷⁷Hf values (+10.1 to +12.3). Ce/Y values indicate a maximum crustal thickness of ~23 km. These features are consistent with derivation from a depleted-mantle source and/or shallow-level (<<40 km) melting of pre-existing island arc crust with little to no evolved crustal input. In contrast, post-kinematic high Sr/Y plutons (148-145 Ma) are more compositionally restricted consisting of tonalite, trondhjemite and granodiorite, and display steeply fractionated REE patterns (La/Yb > 10), an absence of Eu anomalies, elevated Sr (>600 ppm) and Sr/Y values (>40), and positive initial epsilon ¹⁷⁶Hf/¹⁷⁷Hf values (+10.5 to +7.8). These results are consistent with geochemical models involving partial melting of pre-existing island arc crust in the presence of a plagioclase-poor to absent, clinopyroxene + hornblende + garnet-bearing source (depths >35-40 km).

We propose that the transition from low to high Sr/Y magmatism resulted from orogenic thickening of island arc crust during Late Jurassic arc-arc collision. This change in crustal structure influenced subsequent post-orogenic magmatism and resulted in a brief (~3 myr: 148-145 Ma) episode of high Sr/Y magmatism. High Sr/Y plutons occur throughout the western North American Cordillera (Salmon River suture zone, Klamath Mountains, Peninsular Ranges) and closely follow arc-arc and arc-continent collisions. These plutons may signify episodic generation of post-tectonic high Sr/Y magmatism along the western North American Cordillera following Late Jurassic to Early Cretaceous collisional events.