PETROLOGY, GEOCHRONOLOGY, AND TECTONIC IMPLICATIONS OF THE EOCENE NACHES FORMATION, CENTRAL CASCADES, WASHINGTON

The ~50 Ma Naches Formation (NF) is a thick unit of interbedded volcanic and sedimentary rock located in the central Washington Cascade Mountains. It formed during the Challis Event (~52-45 Ma), a time of widespread magmatic activity, faulting, and crustal uplift in the Pacific Northwest. Two major theories have been proposed to explain this largescale tectonic activity: (1) subduction of a ridge-trench triple junction that then opened a slab window under Idaho, Montana, Washington, and Wyoming, or (2) breakoff of the Farallon plate underneath the Pacific Northwest in response to the accretion of Siletzia, the landmass that now makes up the Olympic Peninsula. The age, location, and distinct composition of the NF makes it a good place to look for more information about the cause of the Challis event.

The lower portion of the NF, primarily sandstone interbedded with rhyolite flows and tuffs, grades upward into a sequence dominated by basalt and andesite. Geochemical analysis of 41 samples reveals that the NF is bimodal, with 85% of samples having either <60 wt.% or >70 wt.% SiO₂. On spidergrams the more silicic samples have modest Ta-Nb depletions indicative of an arc setting but the basaltic samples do not. The NF also shows compositional differences compared to the modern Cascade arc: it contains a higher proportion of rhyolites and mafic NF lavas have lower Al₂O₃ and K₂O, and higher TiO₂.

Bordering the NF to the east are the slightly older Taneum Formation, dated at 51.3 Ma (Eddy et al., 2016), which has arc affinities, and the poorly dated Basalt of Frost Mountain (BFM) (Randall & Tepper, this meeting), which has a mix of MORB and arc traits. We suggest that the NF and BFM record the transition from arc magmatism to slab breakoff magmatism, the latter being a response to the accretion of Siletzia. Ongoing LA-ICP-MS U-Pb zircon dating and Sr-Nd isotopic studies will help to further test this hypothesis.