HICKS BUTTE COMPLEX, CENTRAL CASCADES, WASHINGTON: A RECORD OF LATE JURASSIC ISLAND ARC FORMATION AND SUBSEQUENT CRETACEOUS ADAKITE GENERATION FROM ARC ROOT MELTING
Zircon U-Pb geochronology and Hf isotope geochemistry were completed on four and two samples, respectively. A quartz diorite with low Sr/Y, and a gabbro yield U-Pb zircon ages of 150.0 ± 6.8 Ma (MSWD = 0.2) and 153.6 ± 2.0 (MSWD=1.00) respectively. This suggests the low Sr/Y ratio group is Late Jurassic. The gabbro displays strong symplectite textures of olivine mantled by pyroxene. εHf(t) values from the gabbro’s zircons range from +16.4 to +10.3. A dacite with a high Sr/Y ratio yields a U-Pb zircon age of 144.0 ± 2.4 (MSDW=1.1). This suggests the high Sr/Y ratio group is Early Cretaceous. A second dacite yields a complex U-Pb zircon age distribution of 137.3 ± 2.4 (MSWD=1.04) and 144.5 ± 2.1 (MSWD=1.5). εHf(t) values of zircons from the complex age dacite range from +16.7 to +4.8.
Using geochemistry, U-Pb ages, and zircon Hf isotopes, it is suggested the Hicks Butte complex originated as a Late Jurassic island arc complex. The Early Cretaceous adakites were then generated by the partial melting of the root of this Late Jurassic island arc. This is supported by sympletitic textures found in the gabbro, adakite geochemistry, low Mg#, Ni, and Cr, zircon Hf isotopes, and the complex ages from the dacite. The root of the Jurassic island arc could have melted as the result of arc underthrusting, or hydration from Easton Metamorphic suite due to subduction erosion. These dates mark the end of high grade the Easton Metamorphic suite tectonism.