T270 Tectonic Evolution of Eastern Oregon and Western Idaho

Wednesday, 12 October 2011
1:30 PM - 5:30 PM
Room 200A-C

MAGMATIC CONSTRUCTION OF THE BALD MOUNTAIN BATHOLITH, NE OREGON: INSIGHTS FROM U-PB ZIRCON GEOCHRONOLOGY AND HAFNIUM-OXYGEN ISOTOPE GEOCHEMISTRY

Paper number: T270-7
Program: Technical Sessions
Time: 2:55 PM – 3:10 PM

Authors

Abstract

The Late Jurassic to Early Cretaceous Bald Mountain batholith is a large (~450 km2), composite, silicic batholith located in the Blue Mountains province, NE Oregon. It intrudes a complex structural boundary containing imbricated slabs of hornblende metagabbro (likely derived from the Wallowa island-arc terrane), which are tectonically interleaved within the Permian to Early Jurassic Elkhorn Ridge Argillite of the Baker terrane. The batholith itself contains a number of smaller plutons including an inner core of granodiorite (Granodiorite of Anthony Lake) surrounded by a large tonalite unit (Tonalite of Bald Mountain), and smaller peripheral bodies of norite (e.g., Norite of Badger Butte) and granite (e.g., Granite of Anthony Butte). Despite detailed field mapping by Taubeneck (1957, 1995), relatively little is known about the timing of magmatism and the petrogenesis of the Bald Mountain batholith.

Here, we present new U-Pb and combined Hf-O isotopic data for zircon from six samples representing the main-stage units of the Bald Mountain batholith. Our 206Pb/238U zircon ages (2σ) indicate that magmatic construction of the batholith commenced with the emplacement of a mafic-felsic suite including the Norite of Badger Butte (156.9 ± 1.4 Ma) and the Granite of Anthony Butte (154.7 ± 1.3 Ma). A second phase of tonalite-granodiorite magmatism resulted in the emplacement of the inner core of the batholith (Granodiorite of Anthony Lake: 146.7 ± 2.1 Ma) followed by intrusion of the largest unit, the Tonalite of Bald Mountain; three samples of tonalite yielded ages of 142.6 ± 1.5 Ma, 141.2 ± 1.9 Ma, and 140.9 ± 2.0 Ma. Plutons of the norite-granite suite display a narrow range in initial εHf of 7.2—7.7 and elevated d18O (zircon) ranging from 8.2 to 11.9‰. Zircons from the granodiorite-tonalite suite show a similar range of initial εHf values (6.3—8.9) and δ18O (7.1—10.0‰) indicating a shared history of assimilation of evolved crustal material. Xenoliths of metagabbro [δ18O(WR) = 13.4 ± 2.3‰; 2σ] and Elkhorn Ridge Argillite [δ18O(WR) = 23.4 ± 3.1‰] occur throughout the batholith and are possible sources of assimilation. Best-fit assimilation-fractional-crystallization geochemical models are most consistent with significant assimilation of Elkhorn Ridge Argillite during the construction of the Bald Mountain batholith.