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