GEOCHEMISTRY AND PETROLOGY OF EOCENE TO MIOCENE ROCKS IN A REAR-ARC SETTING, CENTRAL CASCADES, WASHINGTON

Shallow intrusions and/or lavas near Wenatchee, WA, provide a unique opportunity to study a diverse array of samples that erupted over a 30+ Ma interval in the Cascadia rear arc. Included are two Eocene units, the Wenatchee Pinnacles and Sugarloaf Peak (SP), the Oligocene Horse Lake Complex, two Miocene units, Eagle Rock (ER) and Burch Mountain (BM), and an undated andesite at Peoh Point (PP). Goals of this study are to use trace and major element data to determine the tectonic setting and petrogenesis of these rocks and identify any compositional differences between these rear-arc sites and the Cascade arc as a whole.

The Wenatchee Pinnacles (Gresens, 1983), an alignment of shallow intrusions, include Castle Rock (CR), Saddle Rock (SR) and the Wenatchee Dome (WD). U-Pb ages (44.9 – 44.4 Ma) overlap but WD, site of the former Cannon gold mine, is more felsic (76% SiO₂ vs. 67-69% SiO₂ for CR and SR) and is the only body associated with economic mineralization. REE modelling suggests WD can be related to the other intrusions by 50-55% plag + hbl crystallization. All Pinnacles samples have arc affinities (HFSE depletions, Ba/Nb > 100) and concave-upward REE profiles. These data support derivation of Pinnacles magmas from a lower crustal mafic (amphibole-bearing) arc source. Associated with the Pinnacles are tholeiitic dikes that may be representative of basaltic magmas that drove melting. Based on their age and location these magmas could represent the end of the Challis Event or the inception of the modern arc. If related to the modern arc, the Pinnacles are one of the oldest dated manifestations of renewed Cascadia subduction.

The Miocene units occur as a cluster of three small stocks, with BM dated at 11 Ma (Tabor, 1987). All are low silica adakites (LSA) that exhibit Sr > 1400 ppm, La/Yb > 15, and Sr/Y > 130. Mineralogical and geochemical differences suggest each stock was a separate magma. Andesites from Sugarloaf Peak (36.9 Ma; Gilmour, 2012) and Peoh Point (undated) also exhibit adakite characteristics (Sr > 500 ppm, La/Yb > 8, Sr/Y > 38). The presence of LSAs implies melting of the mantle wedge after interaction with slab-derived melts. Slab melting could result from a subducting slab window or a slab breakoff event, but this need not have been a Miocene event.