Petrology and Age of Dixie Butte Plutonic Rocks, Blue Mountains, NE Oregon

The Blue Mountains province of northeastern Oregon contains Late Jurassic to Early Cretaceous plutons that occur along a NE-SW-trending belt extending from the Wallowa island arc terrane through the Bourne and Greenhorn subterranes of the Baker terrane. Previous studies suggest that this Late Jurassic magmatic belt represents renewed magmatism after arc-arc collision in the region; however, the timing and duration of this event is poorly understood. Some of the oldest plutonic rocks in this belt intrude the Greenhorn subterrane, a Late Permian-Triassic forearc complex characterized by serpentinite-matrix mélange. In the Dixie Butte area, plutonic rocks intrude basaltic to andesitic metavolcanic flows and associated metavolcaniclastic breccia, serpentinite, and Late Permian radiolarian-bearing siliceous argillite. These plutons consist of two distinct suites: Group 1 plutons are weakly altered diorite and quartz-hornblende diorite; whereas, Group 2 plutons are characterized as biotite-hornblende tonalite. Diorites of Group 1 contain variable mineral assemblages of plagioclase ± clinopyroxene ± hornblende ± alkali feldspar ± biotite ± quartz. Group 2 tonalites consist of plagioclase + hornblende + biotite + alkali feldspar + quartz.

We present new U-Pb SHRIMP zircon ages for both suites which constrain the timing and duration of Late Jurassic magmatism in the Dixie Butte area. Two samples from Group 1 plutons yielded weighted average ²⁰⁶Pb/²³⁸U ages of 156.9 ± 1.7 Ma for a diorite and 157.7 ± 1.5 Ma for a quartz-hornblende diorite, respectively. A biotite-hornblende tonalite from Group 2 yielded a weighted average ²⁰⁶Pb/²³⁸U age of 146.3 ± 1.6 Ma. Rare earth element patterns for Group 1 zircons show pronounced negative europium anomalies (Eu/Eu* <0.2); whereas Group 2 zircons show very slightly negative europium anomalies (Eu/Eu*> 0.5). These new ages suggest that renewed magmatism in the Dixie Butte area was established by ca. 158 Ma and continued either episodically or continuously for ~10 m.y.