Geochemistry and Geochronology of Eocene Plutons in Northeastern Washington: A Test of Farallon Slab Rollback as a Cause of the Challis Event

Abstract:

The causes of widespread magmatism and extension that affected the Pacific Northwest during the Eocene “Challis Event” are poorly understood. Two models that have been advanced to explain this activity are passage of a slab window (e.g., Haeussler et al., 2011) and rollback of the subducting Farallon slab following accretion of Siletzia (Schmandt and Humphreys, 2011). Both scenarios would have resulted in widespread magmatism but with different temporal patterns. Based on reconstructed plate motion vectors magmatism related to a slab window should produce a younging-to-the-NW pattern whereas magmatism associated with slab rollback should young to the S or SW. Existing dates on Eocene igneous units in NE Washington appear to show an overall younging to the SW, consistent with the slab rollback model. However, many of these dates (mainly K-Ar) have large uncertainties so we are conducting a U-Pb dating and geochemical survey of Eocene plutons across the region.

An initial set of zircon U-Pb ages (by LA-MC-ICP-MS) from five intrusions in east-central WA range from 50.7 – 46.7 Ma and young to the SW, a trend similar to that observed among Eocene rocks in the Idaho Batholith (Gaschnig et al., 2013). To further investigate this pattern we are dating an additional ten plutonic units that define a ~100 km SW-NE transect through NE WA. From NE to SW the units in this transect are (with dates from WA DNR mapping) as follows: Sheppard granite (undated), Herron Creek intrusion (51.4 +/- 1.9 Ma), Mt. Bonaparte pluton (52.8 +/- 2.6 Ma), Daisy Trail granite (49.9 +/- 0.3 Ma), Swimptkin Creek pluton (48.2 +/- 1.2 Ma), Moses pluton (48.6 +/- 1.2 Ma), Keller Butte granite (52.9 +/- 0.4 Ma), Johnny George plutonic complex (49.9 +/- 0.45 Ma), Manilla Creek (undated), and Swawilla Basin pluton (58.8 +/- 2.2 Ma). Results of this study should lead to a better understanding of the cause(s) of Challis magmatism and specifically its relationship to the ~50-48 Ma accretion of Siletzia.