Composite Sunrise Butte pluton: Insights into Jurassic-Cretaceous collisional tectonics and magmatism in the Blue Mountains Province, northeastern Oregon
Abstract
The composite Sunrise Butte pluton, northeastern Oregon, preserves a record of subduction-related magmatism, arc-arc collision, crustal thickening, and deep-crustal anatexis. The earliest phase of the pluton was generated in a subduction zone -environment, as the oceanic lithosphere between the Wallowa and Olds Ferry island arcs was consumed.
Zircons from this unit yielded a 206Pb/238U age of 160.2 +- 2.1 Ma. A magmatic lull ensued during arc-arc collision, after which partial melting at the base of the thickened Wallowa arc crust produced siliceous magma that was emplaced into metasedimentary rocks and serpentinite of the overthrust forearc complex.
This magma crystallized to form the bulk of the Sunrise Butte composite pluton. The heat necessary for crustal anatexis was supplied by coeval mantle-derived magma.
The lull in magmatic activity between 160 and 148 Ma encompasses the timing of arc-arc collision. Previous researchers have proposed a tectonic link between the Blue Mountains Province and the Klamath Mountains and northern Sierra Nevada Provinces farther to the south; however, timing of Late Jurassic deformation in the Blue Mountains Province predates the timing of the so-called Nevadan orogeny in the Klamath Mountains.
In both the Blue Mountains Province and Klamath Mountains, the onset of deep-crustal partial melting initiated at ca. 148 Ma. One possibility is that the Late Jurassic shortening event recorded in the Blue Mountains Province may be a northerly extension of the Nevadan orogeny.
Differences in the timing of these events in the Blue Mountains Province and the Klamath-Sierra Nevada Provinces suggest that shortening and deformation were diachronous, progressing from north to south. We envision that Late Jurassic deformation may have collapsed a Gulf of California-style oceanic extensional basin that extended from the Klamath Mountains to the central Blue Mountains Province, and possibly as far north as the North Cascades.