COMPARISON OF CRETACEOUS VS. EOCENE GRANITOID INTRUSIONS IN NE WASHINGTON: PETROLOGY AND AL-IN-HORNBLENDE BAROMETRY

Granitoid intrusions in NE WA range from Paleozoic to Eocene in age and represent diverse tectonic environments. We have examined four closely-spaced plutons in the Eastern Twisp 1:250000 Quadrangle to ascertain whether there are systematic petrologic differences between Cretaceous and Eocene intrusions that might shed light on tectonic changes during this interval. The four plutons studied are the Eocene No Name (NN) and Hungry Mountain (HM) stocks and the Cretaceous Texas Creek (TC) and Carlton (CS) stocks. Using geochemistry, petrography, and Al-in-hornblende barometry we focus on two questions: (1) Were the magma sources and/or tectonic setting(s) of these two episodes different? (2) Can we use age-barometry relations of these plutons to constrain the timing of erosion / crustal thinning that occurred in this region between ~90 and ~50 Ma?

SiO₂ contents overlap considerably between the Eocene (NN = 65.7 – 67.2 wt. %; HM = 55.9 – 68.6 wt. %) and Cretaceous plutons (TC =57.8 –69.6 wt. %; CS ~74 wt. %). Spidergrams for all four plutons show Ta and Nb depletions and LILE enrichments characteristic of arc magmas. REE plots, although variable, show the Eocene plutons have steeper REE slopes (La/Yb_N = 4.9-17.2) and smaller Eu anomalies (Eu/Eu* =1.0-0.81) than the Cretaceous plutons (La/Yb_N = 2.9-4.1; Eu/Eu* = 0.62-0.57). The Eocene plutons also have slightly higher Na at equivalent SiO₂ contents. Al-in-hornblende barometry reveals a slightly deeper emplacement depth (1.5±0.7 Kb) for TC (~88 Ma; Todd, 1989) compared to 0.64±0.4 Kb for the NN (63.9±0.7 Ma U-Pb; Tepper unpublished data) and HM (44.5±0.9 Ma K-Ar; Barksdale, 1975). These results indicate that <~5 km of uplift have occurred in this area since ~88 Ma. We conclude that the Cretaceous and Eocene plutons were derived from similar source(s) and that any thickened crust that may have developed during the Cretaceous was in this area largely eroded by ~88 Ma.