BALTICA IN THE BLUE MOUNTAINS: DETRITAL ZIRCON GEOCHRONOLOGY OF THE LATE TRIASSIC–EARLY JURASSIC ALDRICH MOUNTAINS GROUP, EASTERN OREGON

The sedimentary provenance of late Paleozoic–early Mesozoic basins in the Blue Mountains Province (BMP) of eastern Oregon changed from (1) Permian and Late Triassic chert-rich, subduction-related sources (megasequence 1), to (2) Early and Middle Jurassic volcanic and syn-orogenic sources (megasequence 2). Intervening rocks of the Late Triassic–Early Jurassic Aldrich Mountains Group (AMG) represent a series of fault-bounded successor basins which provide a window into plate-margin tectonics. Two detrital zircon samples from the basal Fields Creek Formation have distinct Paleozoic age modes (ca. 477–437, 373–253 Ma); the remaining 72% of grains are Precambrian and 7% of all ages fall within the North American Magmatic Gap (NAMG; 1.61–1.49 Ga). Petrofacies analyses indicate a recycled orogenic provenance. One detrital sample from the overlying Murderers Creek Formation contains 48% Mesozoic ages (ca. 412–386, 468–429, 356–319, 203 Ma) and distinct Neoproterozoic ages (ca. 936, 789–580 Ma); 3% of grains fall within the NAMG. Both the Murderers Creek and Fields Creek Formations contain similar Mesoproterozoic (1.6–1.1 Ga), Paleoproterozoic (2.1–1.6, 2.5–2.3 Ga) and Archean (2.9–2.6 Ga) age modes. Petrofacies analysis of the Murderers Creek Formation indicates an arc-orogen provenance interpreted to reflect mixing between young arc detritus and recycled orogenic sources.

Detrital zircon U-Pb ages in the AMG are distinct from older subduction-related sources and from younger volcanic and syn-orogenic sources. Age distributions in the AMG match the ages of igneous rocks and detrital age distributions of Baltican-affinity terranes in the Shoo-Fly (Northern Sierra terrane), the Yreka subterrane (Eastern Klamath terrane), and the Okanagan subterrane (southern British Colombia). Petrographic and detrital U-Pb data are consistent with derivation of Baltican-derived orogenic sediment in AMG basins beginning as early as Late Triassic time. Sediment was either eroded directly from the Northern Sierra-Eastern Klamath-Okanagan terranes, or a similar source terrane that is no longer present in the BMP. This interpretation suggests that recycling of collided Baltican crust played a significant role in Cordilleran tectonics well into early Mesozoic time.