U-PB DATING OF THE WENATCHEE RIDGE ORTHOGNEISS AND RELATED ROCKS WITH IMPLICATIONS FOR THE TECTONIC HISTORY OF THE NASON TERRANE

The Wenatchee Ridge Orthogneiss (WRO, Magloughlin & Evans, 1987) is a stock-scale, weakly to moderately metamorphosed, banded gneiss pluton composed dominantly of tonalite and leucotrondhjemite within the Nason Terrane (NT) of the North Cascade Mountains. It was emplaced into the Chiwaukum Schist (CS) and is cut by common muscovite tonalite pegmatite, some magmatic epidote-bearing. WRO is very similar to Archean TTGs, based on a variety of geochemical parameters. The body ranges from 56.3-76.8 wt% SiO₂. The HREEs are strongly depleted, possibly by fractionation of hornblende, epidote, and/or titanite, and it may have originated by partial melting of overthickened eclogitic crust.

Structurally, subhorizontal mineral lineations, oblate ultramafic bodies and metasomatic material, and dismembered pegmatites indicate the pluton was emplaced during a high-strain event that produced a well-developed foliation post-dating regional metamorphism.

We applied zircon U-Pb depth profiling to ~30 zircons each from 9 samples to understand the emplacement and metamorphic history of the WRO and associated lithologies. One CS sample yielded core-and-rim relationships with metamorphic overgrowth ages of 96-133 Ma on cores ranging from 146-1624 Ma, whereas single-grain ages ranged from 96-2936 Ma.

Four samples of WRO yielded ages of 93.0-95 Ma, with the nearby Dirty Face Pluton at 97.5±0.9 and a garnet diorite body from the WRSZ is 93.2±1.7 Ma. Among the undeformed, cross-cutting pegmatites, one yielded only highly discordant zircons (lower intercept 83.8 Ma), and an epidote pegmatite yielded an age of 90.1±1.7 Ma. WRO and WRSZ samples also yielded a few 76-79 Ma zircons and antecrystic mid-K and Triassic grains.

Data from the CS indicate metamorphism pre~96 Ma, and WRO and related magmatism at 93-97 Ma. Younger zircons may indicate a late K metamorphic or metasomatic event. Age similarity between the WRO and Mount Stuart batholith implies the northern and southern NT were at very different crustal levels with the deeper north undergoing extreme plastic strain, whereas the shallower southern part of the terrane largely escaped post-MSB plastic strain.