A VIEW INTO CHANGING MAGMATIC AND DEFORMATIONAL STYLES DURING ISLAND ARC EMPLACEMENT (AND ASSEMBLY?): HICKS BUTTE, CENTRAL CASCADES OF WASHINGTON STATE

The Hicks Butte complex, located in the central Cascades, Washington, consist of variably deformed igneous rocks, and is faulted against the Easton Metamorphic suite. However, this contact may have originally been primary as evidenced by a ~500 m wide contact aureole. The Hicks Butte complex can be separated into a 150-153 Ma island arc complex with juvenile mantle melt. This arc is intruded by ~144 Ma adakite which formed deep in the crust by melting of a mafic protolith. The thermal and structural origin of this complex was investigated to help understand its formation. A 153 Ma symplectic gabbro has a mineral assemblage which suggests rapid cooling from 1100˚C to around 700–800˚C at pressures between 6–8 kbar. Amphibole from an adakitic dacite yield temperatures of 934°–946°C and a pressure of 5.67 ± 1.42 kbar. A dacite that cuts the 153 Ma symplectic gabbro yields a complex LA-ICP-MS U-Pb zircon age of three groups: ~152 Ma; ~144.5 Ma; and, ~137 Ma.

The entire Hicks Butte complex and host rocks display similar steep NW-SE trending structures. Electron BackScatter Diffraction of plagioclase in gabbro revealed a lack of a CPO, suggesting sub-magmatic diffusion creep. Prism-a slip in quartz suggests the symplectic gabbro and intruded dacite deformed together at 550°–650°C. This mingling shows passive folding which requires high mean ductility with low contrast. The Easton displays a range from basal-a to rhomb-a to prism-a slip (~400–650˚C) and late greenschist overprint on blueschist pulse that define an areole that matches the P/T cooling path of the Hicks Butte complex and gabbroic symplectite (650˚C & > 6kbars).

The variation in zircon ages and magma generation sources could have resulted by the introduction of fluids from dewatering of metasediments in the underlying subduction channel directly to the lower-crustal root of the Jurassic island arc, possibly exposed due to subduction erosion.

The spread in the zircon data from 144-137 Ma might record incremental pluton inflation that would have produced repeated smaller thermal pulses. This mechanism would restrict the thermal influence of the pluton, explaining the small size of the contact aureole in the Easton host, regional preservation of HP- assemblages, and presence of passive folding, which a single intrusion can’t explain in the same time window.