NEW CA-ID-TIMS U-PB ZIRCON GEOCHRONOLOGY FROM THE CRETACEOUS ENTIAT PLUTON, NORTH CASCADES, WASHINGTON: VARIATIONS IN MAGMA EMPLACEMENT RATE IN A MID-CRUSTAL, INTERMEDIATE, SHEETED PLUTON

The Entiat Pluton is a Late Cretaceous, dominantly tonalitic, pluton within the North Cascades Washington. It is one of several elongate plutons that span 50 Myr and intrude the area east of the Entiat fault. Previous mapping, structural, and geobarometric data show that the pluton was emplaced as a series of sheets at 0.6-0.7 GPa during a period of regional shortening and high rates of magma addition across the region. However, our understanding of how the pluton was emplaced and the magmatic processes that occurred within it has been limited by a lack of published geochronologic data. Here we report new high-precision U-Pb zircon geochronology that delineates three main pulses of pluton construction over the course of 8 Myr (76, 72, and 68 Ma). All dated samples contain >500 kyr dispersion in 206Pb/238U dates, indicating that there was either extensive recycling of zircon between individual increments of magma or protracted magmatic residence. We combine our new geochronologic data with existing and new structural data to better understand how the pluton was emplaced and with new whole rock geochemical measurements to assess how crustal thickness may have varied during pluton construction. We also report a newly recognized Eocene intrusion that was previously mapped within the Entiat pluton. It represents one of the earliest manifestations of plutonism during the Eocene magmatic flare-up in the North Cascades and we discuss its geochemistry in relation to this event.