CRETACEOUS VS. EOCENE MAGMATISM IN NE WASHINGTON AND IDAHO: PETROLOGIC COMPARISONS, TEMPORAL TRENDS, AND TECTONIC INFERENCES

Distributed across NE Washington and N Idaho are >120 volcanic and plutonic units of Cretaceous to Eocene age that preserve a record of magmatism spanning >80 Ma. The Cretaceous rocks are likely associated with eastward-directed subduction of the Farallon slab, but the tectonic setting of the Eocene rocks is uncertain. They are part of the Challis-Kamloops belt, a broad region of volcanism, plutonism, and extension that runs from central BC to Idaho (Ickert et al., 2009). Explanations for Challis magmatism include intra-arc rifting (Dostal et al., 2005), decompression melting (Hooper et al., 1995), and passage of a slab window (Madsen et al., 2006). To better understand this Eocene event we have analyzed and/or U-Pb dated 20 intrusions (15 Eocene, 5 Cretaceous) from central WA to NW Idaho.

All sampled units have arc HFSE signatures and Sr-Nd isotopic compositions indicative of large-scale crustal melting but there are consistent differences between the Eocene and Cretaceous rocks. The former were emplaced at shallow depth (< ~5 km), rarely contain inherited zircon cores, were relatively dry (no evidence of amphibole fractionation, B/Be < 5), and have REE patterns consistent with a garnet bearing lower crustal source. Conversely, the Cretaceous intrusions were generally emplaced at mid-crustal depths (magmatic epidote is common), usually contain inherited zircon cores, and tend to have bulk compositions (peraluminous, Si-rich) suggestive of a garnet-free metasedimentary source.

New U-Pb ages of five Eocene plutons in WA range from 50.7 – 46.7 Ma and young to the south. These ages overlap closely with those of Challis-age units in the Idaho batholith (51 – 43 Ma; Gaschnig et al., 2009), which also young to the south. Additional dates would better define these space-time trends but Challis activity in WA and ID appears to have been synchronous and migrated in the same direction over an east-west distance of > 300 km.

Plate motion reconstructions (e.g., Madsen et al., 2006) locate the Kula-Farallon slab window beneath NE WA during the Eocene. Elevated heat flow associated with this NW-migrating slab window may account for widespread crustal melting during this interval, but some other large-scale mantle process is probably required to account for the southward migration of Challis activity.