U-PB SHRIMP AGES FROM DETRITAL ZIRCONS IN THE GRAND VALLEY AND WESTERN OLYMPIC LITHIC ASSEMBLAGES, OLYMPIC SUBDUCTION COMPLEX, WASHINGTON

The depositional ages of sedimentary rocks in the Olympic subduction complex are difficult to determine because age-diagnostic fossils are rare. Data from sparse fossil localities led Tabor and Cady (1978) to suggest most rocks are Tertiary, possibly early Eocene to early Oligocene, and that Miocene sediments are present adjacent to the Pacific Coast. Detrital zircon fission-track dates reported by Brandon and Vance (1992) broadly support Tabor and Cady's age assignments, and also suggest Miocene-age rocks are present in the interior of the Olympic Mountains in Tabor and Cady’s Grand Valley lithic assemblage at Wellesley Peak, and in their Western Olympic lithic assemblage at Mt. Tom, near Mt. Olympus.

New U-Pb SHRIMP analyses of 50 detrital zircons from the same sample analyzed by Brandon and Vance from Wellesley Peak range in age from 53 to 2588 Ma, with a strong peak in the late Cretaceous. Because the youngest detrital ages from Cascadia forearc strata are typically close to the depositional age, these U-Pb ages appear surprisingly old. The new U-Pb ages also contrast with the dominantly (~70%) Miocene, Oligocene and middle to late Eocene fission-track ages from this sample.

U-Pb ages of 33 detrital zircons from the sample analyzed by Brandon and Vance from Mt. Tom range in age from 22 to 2682 Ma, with a peak in the late Cretaceous and a small peak in the Miocene. These ages confirm the presence of Miocene-age rocks in the core of the Olympic subduction complex.

The differences between U-Pb dates and fission-track ages for the samples from Wellesley Peak and Mt. Tom could be the result of thermal resetting. Alternately, the differences could reflect the contrast between U-Pb crystallization ages and fission-track cooling ages. If grains in these samples are magmatic rather than volcanic, the age discrepancies could also reflect sediment sources that are near-surface portions of the Cascade arc reset by magmatic events. Differing grain proportions may reflect the difficulty in finding well-etched and countable old zircons. Our results indicate the extent of Miocene rocks in the core of the Olympic subduction complex may need to be revised, and the possible occurrence of early Tertiary and older rocks in the Olympic core should be re-evaluated.