Paper No. 2
Presentation Time: 8:00 AM-6:00 PM
U-PB GEOCHRONOLOGY AND HF ISOTOPE GEOCHEMISTRY OF DETRITAL ZIRCONS FROM LATE CRETACEOUS SEDIMENTARY ROCKS IN EASTERN OREGON
GASCHNIG, Richard M., School of Earth and Environmental Sciences, Washington State University, Webster Physical Science Building 1228, Pullman, WA 99164-2812, TIKOFF, Basil, Department of Geoscience, University of Wisconsin Madison, 1215 W. Dayton St, Madison, WI 53706, VERVOORT, Jeffrey D., School of Earth and Environmental Sciences, Washington State University, Pullman, WA 99164, HOUSEN, Bernard A., Geology Department, Western Washington University, 516 High St, Bellingham, WA 98225-9080 and DORSEY, Rebecca J., Dept. of Geological Sciences, 1272 University of Oregon, Eugene, OR 97403-1272, gaschnig@wsu.edu
Late Cretaceous basins along the Cordilleran margin should record the signal of adjacent terranes and provide important constraints on problems of margin-parallel translation exemplified by the “Baja-BC” controversy. Here, we present new detrital zircon U-Pb geochronology and Hf isotopic data for ~93-Ma sediments in the Blue Mountains Province of east-central Oregon. These sediments consist of immature sandstones that rest on basement of the Baker terrane in the Dixie Creek drainage north of Prairie City. The detrital zircon age spectrum from one sandstone sample is characterized by four distinct age peaks at ~110, ~165, ~190, and ~310 Ma. No Precambrian grains were found, and prominent age gaps occur, with no ages between 124 and 147 Ma and only one between 202 and 273 Ma. Hf isotopic analyses of zircons from all of the age populations are consistently juvenile, with no initial epsilon Hf values less than +5.5, suggesting that they are sourced by outboard island arc terranes.
Both the Blue Mountains and Klamath Mountainss experienced episodes of isotopically juvenile magmatism at 165 and 190 Ma. However, rocks in the Blue Mountains contain only trace amounts of the requisite 310 Ma source, whereas the necessary 110 Ma igneous rocks are absent from the Klamaths. The sandstone from Dixie Creek may represent a mixture of input from both regions.
Late Cretaceous sedimentary rocks of the Ochoco basin in central Oregon are about the same age as the Dixie Creek deposits but show significant petrographic differences. This, combined with subtle but important differences in the detrital zircon spectra, such as the presence of ages in the 124 to 147 Ma interval, suggests a different source for the Ochoco basin sediments. This difference could be explained if the two rock packages originated at different latitudes and were later juxtaposed during margin parallel translation of outboard terrenes.
