TONALITIC MAGMATISM, EXHUMATION, AND ROTATION ALONG THE BAKER-OLDS FERRY TERRANE BOUNDARY, BLUE MOUNTAINS, EASTERN OREGON

A series of four tonalitic plutons intrude on or near the Baker-Olds Ferry terrane boundary in the Blue Mountains of eastern Oregon. We studied these plutons to help constrain the geological history of the Blue Mountains within the footprint of the IDOR EarthScope project. These plutons have been inferred to be similar in both composition and age to plutons affected by the Salmon River suture zone along the arc-continent boundary; here we explore the different exhumation history recorded in these two areas.

CA-ID-TIMS U-Pb zircon dating of pluton emplacement reveals a younging trend from SW to NE: Tureman Ranch (129.36 ± 0.06 Ma), Amelia (125.50 ± 0.08 Ma), Pedro Mountain (124.84 ± 0.07 Ma), and Lookout Mountain (123.84 ± 0.04 Ma). Geochemical analyses indicate that the plutons were sourced from mantle-derived and crustal melts of the juvenile accreted arc terranes. Hornblende geobarometry data are consistent with relatively shallow (< 3 kbar) emplacement depths. Ongoing thermochronological studies (⁴⁰Ar/³⁹Ar on hornblende and biotite and (U-Th)/He on zircon) will provide timing constraints on the exhumation of these plutons relative to similarly aged plutons associated with the Salmon River suture zone. Preliminary paleomagnetic results from two of these plutons indicate that the Blue Mountain terranes may have undergone significant vertical axis rotation since ~120 Ma. Overall, the age of plutonism in eastern Oregon is coeval with high-grade metamorphism and plutonism in the Salmon River suture zone, although the subsequent history appears to differ from adjacent cratonic North America.