THE HICKS BUTTE COMPLEX, CENTRAL CASCADES, WASHINGTON: A RECORD OF A LATE JURASSIC ISLAND ARC AND EARLY CRETACEOUS TECTONISM

The Hicks Butte complex consists of variably deformed hornblende tonalite and hornblende quartz diorite with lesser gabbro and dacite. The dacite intrudes the other lithologies. Penetrative deformational fabrics in the complex increase toward the northern margin and across the contact with the similarly deformed blueschist-greenschist of the Easton Metamorphic Suite. Gabbro can have symplectic textures. The symplectite intergrowth consists of enstatite, Mg-amphibole, spinel ± quartz. U-Pb and Lu-Hf isotopic data for zircons was measured using LA-ICP-MS. Zircons (n = 11) from a quartz diorite yielded a mean age of 150.0 ± 6 Ma. Zircons (n = 41) from a symplectic gabbro yield a mean age of 153.6 ± 2.0 Ma. These ages corroborate previous age data for the complex (153 ± 3 Ma). Zircons (n = 21) from a dacite dike intruding a tonalite yield a mean age of 144.0 ± 2.4 Ma. A second dacite dike intruding symplectic gabbro has a complex age distribution. The 9 youngest zircons from this dacite yield a mean age of 137.3 ± 2.4 Ma. 21 zircons from this dacite yield a mean age of 144.5 ± 2.1 Ma. Finally, the 11 oldest zircons from this sample yield a mean age of 152.07 ± 1.0 Ma. εHf_(t) values of zircons from the 153.6 Ma symplectic gabbro range between 8 and 16.4 and are primarily juvenile. εHf_(t) values of zircons from the complex age dacite dike range from 0.9 to 16.7, are primarily juvenile but range up to evolved. Whole-rock and trace element geochemistry suggests the Late Jurassic age intrusive rocks formed in an island arc setting; while the Early Cretaceous dacite are adakites and formed from melting of mafic lower crust. The symplectic textures, geochemistry, U-Pb ages, and εHf_(t) values suggest the Early Cretaceous dacites form from the partial melting of the Late Jurassic gabbro.

The relationship between the Hicks Butte complex and the blueschist-greenschist Easton Metamorphic Suite helps constrain the complexes formation. Protolith ages for the Easton range from 163–131 Ma, while regional blueschist ages range from 140–120 Ma. The juxtaposition of subducted HP-LT water-rich rocks against the mafic lower arc, possibly by underplating or subduction erosion, likely caused the partial melting of the 153.6 Ma gabbro to produce the 144–137 Ma dacite. Both processes require the Hicks Butte complex to have formed as an island arc adjacent to the subduction zone setting for the Easton Metamorphic Suite.