GEOCHEMISTRY OF THE HICKS BUTTE COMPLEX, CENTRAL CASCADES, WASHINGTON: REMNANTS OF A LATE JURASSIC VOLCANIC ISLAND ARC WITH INTRUDING EARLY CRETACEOUS ADAKITES

The Hicks Butte complex, central Cascades, Washington, represents a Late Jurassic to Early Cretaceous occurrence of igneous rocks that have been variably deformed. The geochemistry of these igneous rocks is poorly understood. Knowledge of their geochemistry would aid in constraining their original plate tectonic setting. Whole-rock and trace element geochemistry was conducted on quartz diorite, diorite, tonalite, dacite, rhyolite, and gabbro to constrain and classify these Mesozoic Era rocks. Laser ablation ICP-MS U-Pb age dating of zircons, combined with published ages, reveals a Late Jurassic phase is ca. 150-153 Ma and an Early Cretaceous phase is ca. 144 Ma. The Early Cretaceous sample contains Middle to Late Jurassic age zircon xenocrysts that may have been entrained in the magma.

The major element geochemistry of the Late Jurassic phase is primarily: mafic to intermediate; metaluminous; magnesian; calcic; and low-K. Their Mg# ranges from 16 to 46 and their Eu/Eu* ranges from 0.80 to 1.25. They have low Sr/Y (2-37) and Nb/Yb trace element ratios. The major element geochemistry of the Early Cretaceous phase is primarily: felsic; peraluminous; magnesian; calcic; and low-K. Their Mg# range from 18-32 and their Eu/Eu* ranges from 1.25 to 5.34. They have high Sr/Y (89-607), low Cr and Ni, and moderate Nb/Yb trace element ratios.

The major element geochemistry, low Sr/Y, and Nb/Yb ratios of the Late Jurassic phase suggests an island arc tectonic setting with magma derived from a depleted mantle source. The major element geochemistry of the Early Cretaceous phase also suggests an island arc however, their Nb/Yb ratios suggest they were derived from a more enriched source than the Late Jurassic phase. The Early Cretaceous phase has very high Sr/Y ratios suggesting that these magmas are adakites as well as meeting all other geochemical classifications for adakites. Their high Eu/Eu* suggests little plagioclase fractionation. The low Mg#, Cr, and Ni for the adakites suggest they were derived from melting of lower arc crust, however, partial melting of subducting oceanic crust could have also resulted in adakite petrogenises.