PROVENANCE ANALYSIS OF CONGLOMERATE COBBLES AND ASSOCIATED SANDSTONE IN THE MITCHELL INLIER, CENTRAL OREGON

The North American Cordillera developed through accretion and translation of terranes, but the timing and extent of translation remains debated. Previous paleomagnetic analysis of Cretaceous sediment in the Mitchell Inlier within the Ochoco Basin of central Oregon indicates that Cretaceous deposition occurred more than 1000 km south of its present location, but geologic mapping and provenance analysis suggest northern sediment sources within the Blue Mountains province, early Idaho Batholith, and/or northern Sierra Nevada. We collected two sandstone and five plutonic cobbles from the intertonguing Gable Creek and Hudspeth Formations of the middle Mitchell Inlier for petrography and U-Pb age and hafnium isotope analysis of zircon. Sandstone samples are texturally and compositionally immature, and plot in the recycled orogenic or dissected arc tectonic provenance fields. The normalized QFL values are 45% total quartz, 27% lithic grains and 27 % feldspar. Potassium feldspar is 13% and 56% of total feldspar in the two sandstone samples, and volcanic lithic grains make up ~85% of total lithic grains for both samples. The five felsic plutonic cobbles are all Late Jurassic age (153.5 ± 1.3 to 151.1 ± 1.1 Ma), and detrital zircon age spectra from the sandstone samples include Late Jurassic ages, as well as age peaks in the middle Cretaceous and Early Jurassic. Five detrital zircon grains are older than Jurassic age (2.6%; ~211, 232, 302, 325, and 1018 Ma). Epsilon Hf values of zircon from the Jurassic cobbles and selected Jurassic through middle Cretaceous detrital zircon are consistently positive (+5.9 to +12.8), while detrital zircon from grains younger than 100 Ma have negative epsilon Hf values that range from -10.5 to -0.1. These results suggest that the sediment sources for the Mitchell Inlier included a Jurassic-Cretaceous magmatic arc characterized by juvenile Jurassic and Early Cretaceous magmatism and more evolved Late Cretaceous magmatism. Further work will focus on better discriminating between potential source regions, reconciling paleomagnetic and provenance interpretations, and assessing the extent of the Ochoco Basin by comparing our results with provenance data from smaller inliers southeast of Mitchell and from the Hornbrook Formation southwest of Mitchell.