GEOCHEMISTRY AND AGE OF THE DARRINGTON PHYLLITE WITHIN THE HICKS BUTTE AND KACHESS LAKE INLIERS, CENTRAL CASCADES, WASHINGTON. TECTONIC SETTING AND REGIONAL CORRELATIONS SUGGESTING A POLYGENETIC ORIGIN

Easton Metamorphic Suite (EMS) consists of the Shuksan blueschist and greenschists, Darrington Phyllite, and semischist of Mount Josephine. Most rocks achieved blueschist facies metamorphic grade. West of the Straight Creek-Fraser River Fault (SCFRF) the EMS has all lithologies present. East of the SCFRF the EMS only consists of the Shuksan and Darrington Phyllite within two Mesozoic inliers. The Darrington Phyllite in these inliers are dark to light colored fine-grained graphitic phyllite. These phyllites consist of muscovite–chlorite–albite–quartz. A maximum depositional age (MDA) for a phyllite within the Kachess Lake inlier is ca. 133 Ma. An Ar/Ar age from white mica from the same outcrop yields an age of 127.6 ± 0.1 Ma. A second white mice Ar/Ar age from a phyllite in the Hicks Butte inlier yields an age of 125.3 ± 0.3 Ma. New whole-rock geochemistry of 12 phyllites located in these inliers constrain their sedimentary provenance and original tectonic setting. The index of compositional variability, major element discrimination function, and Ni-V-Th suggests these phyllites were derived from intermediate to mafic igneous sources. Th/Sc, Zr/Sc, and La/Th ratios plot in fields defined by modern back-arc basins, and away from a field defined by Izu-Bonin forearc turbidites. These ages and sedimentary geochemistry suggest the Darrington Phyllite east of the SCFRF formed in an Early Cretaceous back-arc setting and were then subducted and cooled to muscovite closure temperatures <10 Ma after their deposition.

Our new sedimentary geochemistry is similar to the Mount Josephine and phyllite geochemistry west of the SCFRF. Published detrital zircon MDA for the Darrington Phyllite west of the SCFRF is ca. 152 Ma. Jurassic protolith ages from the Darrington are also suggested by 163-164 Ma igneous ages from blocks that were either intruded or faulted into the Darrington Phyllite prior to metamorphism. Five published Mount Josephine MDA range from 121 Ma to 157 Ma. Metamorphic ages for the Darrington Phyllite west of the SCFRF range from 120 Ma to 148 Ma. Differing protolith ages for the Darrington Phyllite suggest it may have originated in multiple mid- to late Mesozoic marginal basins which closed, buried, and exhumed at different rates ̶ slower for rocks now located west of the SCFRF and faster for rocks now located to the east.