GEOCHRONOLOGIC CONSTRAINTS ON THE HISTORY OF MAGMATISM, METAMORPHISM, AND DEFORMATION IN THE CHELAN MOUNTAINS TERRANE, NW CASCADES CRYSTALLINE CORE, WA

New structural and geochronologic data from the NW Cascades crystalline core constrain the evolution of this complexly deformed region and suggest new terrane correlations. Cascade River Arc (CRA) includes island arc plutonic, volcanic and sedimentary rocks, and an injection complex of sills into the supracrustal rocks (Magic Mountain intrusive Complex, MMC). CRA is thrust above an ocean floor unit along the Razorback thrust (RT). Mapping indicates a flattening fabric (S1) formed at greenschist (3-4 kb, <400C) to amphibolite (9-10 kb, ~650C) conditions and was later folded into km-scale recumbent SW-vergent fold nappes (D2). Nappes are cut by the SW-vergent RT, refolded by NW-SE trending upright folds, and cut by NW-striking dextral shear zones. Transpression caused alternating and coeval orogen-normal and orogen-parallel shortening.

U-Pb zircon dating constrains protolith and deformation ages. Pre-tectonic orthogneiss dated at 263.5±2.3 Ma suggests island-arc magmatism is older than previously thought. Pretectonic orthogneiss sills in MMC intruded volcanic and clastic rocks at ~164 and ~155 Ma. Pegmatite intruded between D1 and D2 is ~137 Ma. Cyclone Lakes pluton, 81.0±0.7 Ma, cuts S1 and is weakly foliated parallel to the RT. Gneissic Marble Creek pluton, emplaced at ~8 kb, is 76.2±1.2 Ma, while the unfoliated, lower pressure (~4 kb) Hidden Lake Peak pluton is 73.5±0.7 Ma. Ar/Ar dates on CRA schists are 95±3 to 83±2 Ma (hornblende), and 88.9±1.6 to 85.4 ±1.4 Ma (muscovite). Ar/Ar ages of white mica and biotite adjacent to the RT are 73-52 Ma, likely due to resetting by younger plutons and later low-grade deformation and exhumation.

Together the data suggest accretion of terranes in Late Jurassic-earliest Cretaceous, followed by tectonic loading. Peak temperature and pressure were reached by 95 Ma, and lasted until <76 Ma in the north, but cooled to <350C by 85 Ma in the south. Kinematics and the cause of the D1 loading event are uncertain, but cooling occurred before lower grade recumbent folding and thrusting. Uplift and decompression began by 73 Ma in the study area, but was later in areas further to the NE. Protolith and D1 deformation ages in the study area are older than previously recognized in the Cascades core and suggest correlation with the Intermontane and/or Wallowa terrane.