The generation of high Sr/Y plutons following Late Jurassic arc–arc collision, Blue Mountains province, NE Oregon

Abstract

High Sr/Y plutons (Sr/Y > 40) 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), a suite of Late Jurassic (148–145 Ma), high Sr/Y plutons intrude Middle Jurassic (162–157 Ma), low Sr/Y (< 40) arc-related lavas and plutons in the Dixie Butte area immediately after widespread Late Jurassic arc–arc collision (159–154 Ma). Early, pre- to syn-kinematic low Sr/Y lavas and plutons (162–157 Ma) have flat to slightly enriched light rare earth element (REE) abundances, low Sr (< 400 ppm) and Sr/Y values (< 40), and strongly positive initial epsilon Hf values (+ 10.1 to + 12.3: 2σ weighted average). Ce/Y values from basalts and gabbros yield a maximum crustal thickness of ~ 23 km. These geochemical and isotopic features suggest 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 (tonalite–trondhjemite–granodiorite) and display depleted heavy REE abundances, an absence of Eu anomalies, elevated Sr (> 600 ppm) and Sr/Y values (> 40), and positive initial epsilon Hf values (+ 10.5 to + 7.8: 2σ weighted average). These geochemical and isotopic results are consistent with geochemical models suggesting derivation from partial melting of 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 Sr/Y to high Sr/Y magmatism resulted from orogenic thickening of island arc crust in the Dixie Butte area during Late Jurassic arc–arc collision between the Olds Ferry and Wallowa island arcs at 159–154 Ma. This fundamental change in crustal structure influenced post-orogenic magmatism and resulted in a relatively brief (~ 3 myr: 148–145 Ma) episode of high Sr/Y magmatism. Other high Sr/Y plutons occur throughout the US sector of the western North American Cordillera (e.g., Salmon River suture zone, Klamath Mountains, Peninsular Ranges) and closely follow major contractional events involving arc–arc and arc–continent collisions.