INSIGHTS INTO THE MESOZOIC TECTONIC HISTORY OF THE BLUE MOUNTAINS PROVINCE, EASTERN OREGON, FROM INITIAL RESULTS OF DETRITAL ZIRCON DATING

Controversy exists regarding models for the tectonic development of the Blue Mountains Province (BMP).  We use new U-Pb LA-ICPMS ages from detrital zircons (DZ) in Triassic and Jurassic sedimentary rocks of the BMP to assess the timing of arc amalgamation and accretion, tectonic controls on basin formation, and relation to adjacent Cordilleran terranes.  In the Wallowa terrane, DZ ages from the Triassic Doyle Creek Fm. are unimodal with a peak age of 231 Ma.  The overlying Triassic-Jurassic Hurwal Fm. includes peak ages of 210, 265 and 326 Ma.  Lack of Pє zircons in both units supports an intra-oceanic origin for the pre-Jurassic Wallowa terrane.  Overlying these units along a regional angular unconformity, the Jurassic lower Coon Hollow Fm. includes Mz (62%) and Pz (31%) grains (multi-grain modal age peaks at 160, 186, 232, 256, 271 Ma) as well as Pє (7%) grains (840, 1100, 1430 Ma). Precambrian grains in the Coon Hollow Fm. support models for a regionally integrated marine basin across the BMP during the Jurassic.  In the Izee-Suplee area, DZ ages from the Late Triassic Vester Fm. include 31% Pz grains (273, 305, 374, 428 Ma) and 68% Pє grains (1383, 1675, 1872, 1996, 2320, 2560, 2595, 2700 Ma); only 1% of grains are Mz.  DZ ages in the overlying Triassic Aldrich Mountains Group include 28% Mz grains (195 Ma), 36% Pz grains (267 and 420 Ma), and 36% Pє grains (550, 1380, 1650, 1875, 2360, 2600, 2705, 2850 Ma).  Abundance of Pє and Pz grains in deposits known to be sourced from the Baker terrane provides evidence for sediment recycling through the accretionary complex.  DZ ages in the Jurassic Mowich Group and Snowshoe Fm. include 73% Mz grains (180 Ma), 2% Pz grains and 25% Pє grains (1300, 1845 Ma).  The DZ age spectrum for the Jurassic Weatherby Fm. matches that of the Snowshoe Fm.  These data support a fringing-arc origin for the Triassic-Jurassic Olds Ferry arc system and suggest that the Olds Ferry arc may have been active during Early and Middle Jurassic time, consistent with models for a Jurassic forearc basin in the BMP.  However, DZ ages in the Izee-Suplee area correlate well with ages of igneous rocks in the Klamath and northern Sierra terranes, suggesting longitudinal input from the south.  Thus, the 180-Ma zircons may have been sourced in the Klamath Mts. during volcanic quiescence in the BMP, consistent with a collisional-basin model for the BMP.