DETRITAL MINERAL CONSTRAINTS REFLECTING MODERATE NORTHWARD TRANSLATION OF THE GOLD BEACH TERRANE, OREGON

Evolution of the western North American Cordillera during the Mesozoic included the progressive accretion of allochthonous terranes, some of which may have experienced large-scale northward translation as described in the Baja-BC hypothesis. We investigate evidence for northward translation through isotopic analysis of detrital minerals gathered from Late Cretaceous sandstones in the Gold Beach terrane of southwestern Oregon. Mineral ages are primarily Jurassic-Cretaceous with only limited Paleozoic and Precambrian populations; Mesozoic zircon and titanite isotope and trace element geochemistry record transitions over time from juvenile signatures to evolved crustal signatures. Zircon Hf isotopes and titanite Nd isotopes in Jurassic grains (220-140 Ma) yield positive ϵHf and ϵNd values, respectively, while Late Cretaceous grains (107-90 Ma) have negative values. Rutiles, which are mostly Late Jurassic, yield Zr-in-rutile temperature consistent with medium- to high-grade metamorphic conditions in the source regions. Monazites include a prominent mid-Cretaceous age population that is not well-recorded by zircons and titanite. Garnets include a Mn-rich population of probable magmatic origin and a Ca-rich population derived from calc-silicates and/or skarns.

Age and geochemical data from these multiple detrital minerals suggest a southern California provenance. These results offer a comprehensive geochemical record of tectonic and depositional processes in the Gold Beach terrane. While Jurassic grains can be supplied by the Klamaths or Northern Sierras, the evolved Late Cretaceous zircon and titanite requires a central or southern Sierra Nevada source. Peraluminous plutons in the southern Sierra may also be the source of the igneous garnets and mid-Cretaceous monazites. All of these data support moderate northward translation of the Gold Beach terrane.