MINERALOGY AND METAMORPHISM OF SHUKSAN BLUESCHIST, EASTON METAMORPHIC SUITE, KACHESS LAKE INLIER, CENTRAL CASCADES, WASHINGTON

The Easton Metamorphic Suite occurs throughout the central and northwest Cascades, Washington. It is composed of the Darrington Phyllite and Shuksan Greenschist. The Shuksan Greenschist consists of both greenschist and blueschist. Regional blueschist ages from the Easton Metamorphic Suite range from 120 to 130 Ma. This suite was emplaced along the Shuksan thrust during the Late Cretaceous. We conducted mineral geochemistry of amphibole and epidote from a fine-grained epidote-glaucophane schist from the Kachess Lake inlier occurrence of the Shuksan. The mineral assemblage of this samples is epidote-amphibole-quartz-albite-oxide with chlorite, carbonate and epidote overgrowths replacing original minerals. Amphibole and epidote were chemically analyzed by an electron probe micro-analyzer from the Florida Center for Analytical Electron Microscopy. 6 amphiboles from one sample were analyzed. They are Na-amphibole using the classification of Hawthorne et al. (2012). 4 are ferro-glaucophane, 1 is glaucophane, and 1 is winchite. Minor substitution of Ca occurred for Na. Little ^IVAl substituted for Si, and there is little Fe³⁺ stoichiometrically estimated in the M4 site. The amphiboles have moderate Al₂O₃, low TiO₂, high ^M4Na (between 1.59 to 1.73 for glaucophane and 1.20 for the winchite), and average X_Mg = 0.52. 7 epidotes, with one core-rim overgrowth pair, were also analyzed from the same sample. There is little Cr and Al substitution for Fe³⁺ in the M-position. X_Ep ranges from 0.83 to 0.9 (0.71 in the overgrowth), X_Czo ranges from 0.01 to 0.17 (0.29 in the overgrowth), and X_Taw is very low (< 0.003). The average X_Fe for epidote is 0.28 (0.22 for the overgrowth). Based on stoichiometry, there is little to no Fe³⁺-Al partitioning between coexisting Na-amphibole and epidote. This may be due to the oxygen fugacity conditions deviating from the hematite-magnetite buffer. Mineral assemblage and amphibole geochemistry suggests pressures between 6 and 7 kbar, and temperatures between 300 and 400°C. This is within published P-T ranges for the Shuksan. The X_Fe of the epidote and TiO₂ of the amphibole supports lower blueschist metamorphic grade. The sample experienced a lower greenschist facies overprint.