NEW U/PB SHRIMP-RG AGES FROM THE MANASTASH INLIER, CENTRAL CASCADES, WASHINGTON

Zircons from two units, the Quartz Mountain stock and Lookout Mountain Formation, located within the Manastash inlier, central Cascades, Washington, were dated using an ion probe (SHRIMP-RG). The Quartz Mountain stock consists predominantly of tonalite and granodiorite, is weakly zoned, has numerous cupolas, and contains mafic enclaves. A previously published U/Pb age of 157 ± 2 Ma for the stock was reported by Miller et al. (1993). This date was not from the main body of the stock and might have experienced Pb loss (Pb/Pb ≈ 165 Ma). A new U/Pb SHRIMP-RG age is from the main phase of the stock and consists of 10 homogenous single zircons. A ²⁰⁶Pb/²³⁸U age of 157.4 ± 1.2 Ma (2σ; MSWD = 2.5), which includes the 2σ error of the standard, was obtained. This ~157 Ma age corroborates the age of Miller et al. (1993).

The Lookout Mountain Formation consists of siltstone, shale, and lesser sandstone that have been deformed into semi-schists and biotite schists and is intruded by the Quartz Mountain stock. The sandstones are interpreted to have been volcanically derived, and consist of the middle part of a Bouma sequence. 66 detrital zircons from moderately sorted, fine-grained sandstone were dated using the SHRIMP-RG. The greatest (n = 24), and youngest age population of detrital zircons is ~160 Ma (Late Jurassic). This ~160 Ma peak and the intruding ~157 Ma stock tightly bracket the Lookout Mountain age as Late Jurassic. The similarities in age between volcanically derived sandstone and the large ~160 Ma peak indicates active magmatism occurred in the source area, probably an arc. Other statistically significant age populations include: Middle Jurassic; Early Jurassic; Permian-Triassic; Devonian; Neoproterozoic; Mesoproterozoic; and Paleoproterzoic. Of all possible source areas for the Lookout Mountain Formation that contain abundant plutonic rocks of suitable ages, the Klamath Mountains, OR-CA, is most favorable.