LATE JURASSIC EVOLUTION OF THE GALICE BASIN, KLAMATH MOUNTAINS, CA AND OR, FROM DETRITAL ZIRCON AGES

The metasedimentary Galice Formation of the Western Klamath terrane formed before and during a protracted Late Jurassic Nevadan Orogeny. The 164-152 Ma depositional age of the Galice Fm is based on the age of the underlying Josephine ophiolite, Oxfordian-Kimmeridgian fossils within the formation, and cross-cutting intrusions. Detrital zircon U-Pb age data from 11 samples (n=2976) better constrain the depositional age and provenance of the upper turbiditic section of the Galice Fm. The proximity of the Galice basin to active arc sources during deposition means that maximum depositional ages (MDAs) calculated from detrital zircon age populations closely reflect true depositional ages. Maximum likelihood ages approximate MDAs in samples with numerous near-depositional age grains (N=9), whereas the Isoplot unmix function was used in samples with fewer near-depositional age grains (N=2). The MDAs range from 159.6±0.6 to 151.4±0.3 Ma, with possible younging of MDAs across strike to the west. The oldest MDA is close to the minimum 162 Ma age of the underlying Josephine Ophiolite, indicating that turbidite deposition began soon after ophiolite formation, which implies a shorter duration of deposition of the underlying hemipelagic sequence. The nearly 10 My span of MDAs suggests an extended period of turbidite deposition that continued even as deformation and metamorphism associated with the Nevadan orogeny began at ca. 155 Ma. Multidimensional scaling of detrital zircon age spectra reveals that the 5 southernmost samples cluster close together in MDS space, with the other 6 samples widely separated, both from the southern group and from each other. Precambrian-age zircons present in all samples (10-73%) are characteristic of the North American craton. Paleozoic and Mesozoic zircon likely were sourced by rocks within the Klamath terranes and more distal terranes in the Blue Mountains, northwest Nevada, and the northern Sierra Nevada.