NEW DETRITAL ZIRCON AGE CONSTRAINTS FOR THE DARRINGTON PHYLLITE EAST OF THE STRAIGHT CREEK-FRASER RIVER FAULT

The Northern Cascade Mountains are characterized by Mesozoic orogenies involving island arc and oceanic basin accretions that were metamorphosed by oceanic-continental subduction. The Northern Cascades include the Strait Creek Fault system (SCF) that creates a divide between the Coast Plutonic Complex from the western North Cascade units. The magnitude of dextral strike-slip motion along the SCF appears to be approximately 90 km via correlation of the subduction related Easton suite at Easton WA to lithologically similar units to the northwest. The Easton suite contains the late Jurassic oceanic and trench deposits of the Shuksan greenschist-blueschist Ep+Chl+Ab+/-Gl+Q+Mt and Darrington phyllite Qtz+Ab+Mu+Gr+Ep+Chl, with lawsonite in certain areas west of the SCF. The Hicks Butte near Cle Elum WA found only east of the SCF tonalite is part of a larger volcanic arc complex that intruded from 136-157 Ma. While a lithologic correlation has been made, this project presents the first age correlation of the Darrington phyllite across the SCF.

Outcrops of Phyllite between the Kaches and Cle Elum lakes and near the Hicks Butte inlier were sampled for thin section, bulk compositional analysis, and detrital U-TH-Pb zircon Laser Ablation ICP Mass Spectrometry at the Arizona LaserChron Center at the University of Arizona. While thin-section and bulk compositional analyses were roughly comparable across the SCF, the detrital zircons yielded a surprisingly wide range of ages compared published ages (141-155Ma) for Darrington Phyllite west of the SCF. This new data shows a significant Precambrian input, with one Archean grain, several spanning the Mesoproterozoic, and significant Neoproterozoic input. Phanerozoic ages show a spike ~ 520-560 Ma and continuous ages from ~130-175 Ma. Compared to the age dates from the northern cascades, the ages that trend 10 Ma younger than previous 141 Ma ages can be explained by later closure of the oceanic basins, interconnecting arc basins, or a separate basin from a similar tectonic setting. The Precambrian age distribution suggest that there was a cratonic component to the source region that otherwise were probably locally derived from volcanic arcs. These findings suggest a lack of correlation of this Kaches-Cle Elum lake area phyllite from the Darrington phyllite across the SCF.