DETRITAL ZIRCON AGE POPULATIONS AND GEOCHEMISTRY OF SELECTED LATE JURASSIC SEDIMENTARY BASINS IN THE NORTH AMERICAN CORDILLERA: LINKS BETWEEN THE KLAMATH AND CASCADE MOUNTAINS

Late Jurassic sedimentary rocks occur throughout the North American Cordillera. Their tectonic setting and provenance are critical to understanding the evolution of the Cordilleran margin during this time. We review new and published detrital zircon ages and whole rock geochemistry of the Galice Fm., in CA-OR, and the Peshastin Fm. and Darrington Phyllite, of WA, with the goal of better constraining the tectonic development of the Cordillera and strengthening regional correlations of these sedimentary units. The Galice Fm. conformably overlies the 162 Ma Josephine ophiolite. SHRIMP-RG dating of detrital zircons (n=74) from a Galice sandstone have a youngest U-Pb age population of ~157 Ma and a significant U-Pb age population of ~233 Ma. Early Paleozoic, Mesoproterozoic and Neoproterozoic detrital zircons were also identified. Zircon multi-grain U-Pb ages by Miller & Saleeby (1995) also revealed Precambrian detrital zircons in the Galice. The Peshastin Fm. conformably overlies the 161 Ma Ingalls ophiolite complex. SHRIMP-RG dating of detrital zircons (n=76) from a Peshastin sandstone have a youngest U-Pb age population of ~152 Ma and a significant U-Pb age population of ~232 Ma; no Paleozoic or Precambrian zircons were identified. The Darrington Phyllite structurally overlies the Shuksan Greenschist; and, these two units have been grouped into the Easton Metamorphic Suite. Detrital zircons from a Darrington metasandstone have a youngest LA-ICP-MS U-Pb age population of ~155 Ma and a significant U-Pb age population of ~238 Ma (Brown & Gehrels, 2007). A discordant zircon multiple grain fraction Pb-Pb age indicates the Darrington also contains Precambrian detrital zircons (Tabor et al., 1993).

The trace element geochemistry of the all three units is transitional between felsic and intermediate volcanic sources with a minor mafic component. These units consistently plot in fields defined by modern back-arc basin turbidites on geochemical diagrams. The youngest detrital zircon age populations and geochemistry of these units suggest they formed in voluminous Late Jurassic arc-fed basins, similar to the model of Saleeby & Busby-Spera (1992). We suggest that these units are age correlative and formed in arc proximal basins (back-arc and/or rifted fore-arc).