GRANITE FALLS STOCK AND THE HANSEN LAKE RHYOLITE—A HISTORY OF SYN-TECTONIC EOCENE MAGMATISM AND UPLIFT IN THE PILCHUCK RIVER VALLEY DURING REGIONAL TRANSTENSION, SNOHOMISH COUNTY, WASHINGTON

Our new mapping, U-Pb zircon dating, geochemistry, and isotope information shows that ~43–50 Ma igneous bodies in the Pilchuck Valley preserve a history of Eocene syn-tectonic intrusion and uplift during regional transtension. The Bald Mountain Pluton (BMP), Mount Pilchuck Stock (MPS), and Granite Falls Stock (GFS) are mesozonal to epizonal to intrusions in the Pilchuck River valley whose emplacement was controlled by bounding faults. The ~43 Ma Granite Falls rhyolite (GFR) exposed directly south of Granite Falls is an extrusive equivalent of the metaluminous ~44–45 Ma GFS. The Hansen Lake rhyolite and S-type BMP and MPS constitute a slightly older ~49 Ma comagmatic package preserved to the east in the upper Pilchuck River valley.

The contact complex (CC) bordering the GFS main phase consists of intermediate to felsic aphanitic to porphyritic dikes, and mafic to intermediate, medium-grained intrusive bodies. Early dikes are contact metamorphosed by later intrusions. The CC and GFS syn-tectonically intruded into conjugate ENE and NNW trending fault zones that bound the GFS and controlled magmatic emplacement paths. In this the “pull-apart basin” intrusive model space was created by dilation across conjugate NE-trending transtensional faults and NW-trending transpressional to transtensional faults. This model explains four key observations: 1) similar 43-45 Ma ages of the andesite dikes, GFR, and GFS main phase, 2) dikes intrude faults but are cut again by later faults of the Granite Falls and Iron Mountain fault zones, 3) the GFS intruded the CC dikes, but dikes also cut the main stock, and 4) the more-felsic andesite dikes are geochemically and isotopically similar to the GFS. An extensional tectonic model fits with the observation of contemporaneous regional sedimentary basins.

Geothermobarometry and mapping indicate the BMP and MPS crystalized at depth (4.5–5.4 kb) whereas the GFS was shallowly emplaced (0.7 kb) and fed the GFR and CC. This suggests >5 km to perhaps as much as 10 km of mid-Eocene uplift along the Pilchuck River Fault, a regional fault mapped north of the GFS, HLR and GFR. This combination of mid-Eocene extension, uplift, and crustal melting in a forearc setting support Farallon Slab breakoff following the Siletzia accretion, all being responses to hot asthenosphere upwelling through a gap in the slab.