GEOCHEMISTRY AND GEOTHERMOBAROMETRY OF PLAGIOCLASE AND AMPHIBOLE FROM THE HICKS BUTTE COMPLEX, CENTRAL CASCADES, WASHINGTON

The Hicks Butte complex, central Cascades, Washington, consists of Late Jurassic (150 – 153 Ma) intrusive rocks, and Early Cretaceous (144 Ma) extrusive rocks. We analyzed plagioclase (PL) and amphibole (AMP) from three samples by EPMA, at the Florida Center for Electron Microscopy, Florida International University, to better constrain the magmatic origin of this complex. PL from two mafic 150 – 153 Ma samples are bytownite (An₇₆ to An₉₀). PL from the 144 Ma felsic sample are labradorite and one andesine (An₄₈ to An₅₃). PL samples do not display chemical zoning and have little K and Sr substitution. No K-feldspar was identified for analyses. Hawthorne et al. (2012) classification for AMP was utilized for this study. All AMP are calcium amphiboles. AMP from two 150 – 153 Ma samples are magnesio-ferri-hornblende, signified by Mg>Fe²⁺ and Fe³⁺>Al in the C position. The Mg# range from 0.47 – 0.49, Fe# 0.51 – 0.53, Fe³⁺/ΣFe 0.34-0.52, and Al# ≤ 0.18. AMP from the 144 Ma sample is ferri-tschermakite. Mg# range from 0.35 – 0.43, Fe# 0.57 – 0.65, Fe³⁺/ΣFe 0.47 – 0.68, and Al# ≤ 0.23. AMP thermometry of Ridolfi et al. (2010) resulted in temperature ranges of 828 – 864°C (± 22°C) for the 150 – 153 Ma samples and 934°C (± 22°C) – 945°C (± 56°C) for the 144 Ma samples. ƒO₂ calculated from Ridolfi et al. (2010) range from -11.0 – -11.6 (± 0.4) for the 150 – 153 Ma samples and -10.2 (± 0.4) for one 144 Ma sample. AMP barometry of Anderson and Smith (1995) resulted in pressure ranges 0.66 – 1.04 kbar (± 0.16 kbar) for the 150 – 153 Ma samples and 0.21 – 0.26 kbar (± 0.05 kbar) for the 144 Ma samples. All error reported above are 1σ. The AMP temperature and pressure suggest hypersolidus crystallization. The An%, Ca amphibole composition, temperature, and ƒO₂ from the 150 – 153 Ma mafic samples are in the range for a H₂O-rich magma that formed in an island arc setting. The Ca amphibole composition, temperature, and ƒO₂ of the 144 Ma felsic sample are consistent with an island arc setting; while the lower An% may suggest that the magma source was water-poor. These interpretations are supported by the whole-rock geochemistry of these samples.