INVESTIGATING THE MINERALOGY OF SYMPLECTIC GABBRO FROM THE HICKS BUTTE COMPLEX, CENTRAL CASCADES, WASHINGTON, SUPPORTS A FORMATION IN AN ISLAND ARC SETTING

The Hicks Butte complex consists of variably deformed Late Jurassic (150–154 Ma) intrusive rocks with rare Early Cretaceous (ca. 144 Ma) dacite. Previous whole-rock and trace element geochemistry work suggest the Late Jurassic age intrusive rocks formed in an island arc setting; while the Early Cretaceous dacite are adakites which formed from melting of mafic lower crust. Mineral geochemistry determined by electron probe micro-analyzer, Florida Center for Analytical Electron Microscopy, from two hornblende olivine gabbro were studied to help constrain the origin of this complex. Plagioclase (n=24), olivine (n=8), pyroxene (n=7), and amphibole (n=7) were analyzed. The hornblende olivine gabbro have symplectite and corona textures. The corona textures are pyroxene surrounding olivine; while the symplectic textures occur as fine-grained pyroxene, amphibole and possibly quartz in contact with the pyroxene, olivine and plagioclase.

Plagioclase are anorthite and range from An₉₂ to An₉₈. Olivine is forsterite and range from Fo₆₇ to Fo₇₄. Pyroxene are enstatite and range from En₇₄ to En₇₈. Amphiboles are Ca-rich, magnesio-ferri-hornblende with one ferri-tschermakite. Hawthorne’s et al. (2012) classification for amphiboles was used. Al₂O₃ and Mg^# of orthopyroxene plot along modern trends for low-pressure magma differentiation. Enstatite and anorthite percentages plot within fields defined by gabbroic samples from modern island arcs. Forsterite and anorthite percentages plot in the field defined by island arc gabbroic and ultramafic rocks. Amphibole thermobarometry of Ridolfi et al. (2010) resulted in temperature and pressure estimates of 896⁰C ± 29⁰ and 2.61 kbar ± 0.12. Our new mineral geochemical data supports the interpretation of the gabbro initially forming in a Late Jurassic island arc setting. The temperature and low pressure of the symplectic amphibole suggests it formed as a late phase in the gabbro during structural emplacement of the complex. The symplectite textures, along with the pressure and temperature estimates from the amphibole, suggest the gabbro was the mafic crust which melted to produce the Early Cretaceous adakitic dacite.