THE CAUSE(S) OF WIDESPREAD EOCENE MAGMATISM IN THE PACIFIC NORTHWEST: INSIGHTS FROM GEOCHEMISTRY AND GEOCHRONOLOGY OF IGNEOUS ROCKS IN WASHINGTON

Widespread igneous activity across the Pacific Northwest during the middle Eocene (~52 – 46 Ma) is part of the poorly understood “Challis Event” and has been attributed to passage of a slab window (e.g., Haeussler et al., 2003) or to slab rollback and breakoff (Schmandt & Humphreys, 2011). These explanations can be evaluated using petrologic traits and temporal patterns of three groups of igneous rocks emplaced during this interval. From west to east these are: (1) granitoid rocks in NE WA, (2) a diverse N-S trending belt of volcanic and plutonic rocks in the Western Cascades, and (3) adakites in the Cascadia forearc.

Eocene granitoid rocks in NE WA have traits of breakoff magmas (e.g., Sr/Y>10, La/YbN>10; Hildebrandt, 2009) and display a younger-to-the-SW age progression from 52 – 46 Ma. Sr-Nd isotopic data indicate a significant component of older crustal material in these rocks, probably lower crust of accreted arc terranes. Overlapping in age with these granitoids but farther west is a discontinuous ~160 km long belt of volcanic and plutonic rocks that is defined by seven map units including the Basalt of Summit Creek, Basalt of Frost Mountain, Teanaway Basalt, Naches Formation, Barlow Pass Volcanics, and Bald Mountain and Mount Pilchuck plutons. Both plutons are S-types indicative of crustal melting whereas the volcanic formations are bimodal and contain rhyolites plus some mafic lavas with MORB chemical traits and others with arc affinities. High precision U-Pb ages of these units cluster between 49.93 – 48.09 Ma and restoration of offset on the Straight Creek Fault locates them atop the inferred edge of Siletzia in the subsurface. Roughly 70 km west of this belt are the adakites, represented by dikes and tuffs with ages between 48.4 – 46.7 Ma.

The compositions, ages, and locations of these rocks are consistent with a model in which accretion of Siletzia caused breakoff of the shallowly dipping Farallon slab. Initial breakoff under Idaho or Montana >52 Ma led to SW-ward rollback beneath WA and was followed by a second rupture under western WA at ~50 Ma. Upwelling hot mantle generated both MORB-like and arc magmas that drove crustal melting. The adakites may represent melting at edges of a slab, either at a tear in the old Farallon slab or at the leading edge of the “new slab” as subduction was reestablished.