184 Tectonics II

Wednesday, 25 October 2006
8:00 AM - 12:00 PM

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

Paper number: 184-9
Program: Technical Sessions
Time: 10:00 AM – 10:15 AM

Authors

Abstract

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.