HICKS BUTTE COMPLEX, CENTRAL CASCADES, WASHINGTON: A RECORD OF LATE JURASSIC ISLAND ARC FORMATION AND SUBSEQUENT CRETACEOUS ADAKITE GENERATION FROM ARC ROOT MELTING

The Hicks Butte complex, central Cascades, Washington, represents an occurrence of intrusive igneous rocks that have been variably deformed. This complex is faulted against high P/T rocks of the Easton Metamorphic suite. The geochemistry of the Hicks Butte complex can be group by low Sr/Y (2-37) and high Sr/Y (89-607) ratios. The low Sr/Y samples consist of variably deformed quartz diorite, diorite, tonalite, gabbro, and rare hornblendite. The geochemistry of the low Sr/Y group is: mafic to intermediate; metaluminous; magnesian; calcic; and low-K. Mg# range from 16 to 46. The high Sr/Y samples consist of variably deformed dacite, with lesser rhyolite, tonalite and trondhjemite. The geochemistry of the high Sr/Y groups is: felsic; peraluminous; magnesian; calcic; and low-K. Mg# range from 18-32, and they have low Cr (2.62-13.35 ppm) and Ni (0.2-7.12 ppm). The high Sr/Y samples have adakite geochemical affinities.

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.