Finding Crystallization Pressures and Temperatures for Plutons

In this study we determined the crystallization temperature and pressure of a tonalite from the Triangle Mountain pluton, located in the Cougar Creek complex of the Wallowa island arc terrane, in the Blue Mountain Province of the northern U.S. Cordillera. This tonalite contains coexisting zircon, titanite, and quartz, from which we can use Ti- and Zr- trace element thermobarometry to find crystallization pressure and temperature. We obtained in-situ chemical analyses for zircon and titanite using laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), From that analysis we were able to assess equilibrium between the two minerals based upon rare earth element partitioning. Using these Ti and Zr concentrations, a temperature range of 690-730 ⁰C was obtained for the Triangle Mountain pluton based on the assumption of an activity of TiO ₂ (aTiO ₂ ) between (0.6-0.9) and a pressure range of (6-8) kbar. Alternatively, using aTiO ₂ = 0.8, and T = 720 ⁰C from the Ti-in-zircon and quartz thermometers, the titanite thermobarometer yields a pressure estimate of 7.5 ± 0.7 kbar. The combination of zircon and titanite thermobarometry holds significant promise for the thermobarometry of intermediate to felsic plutons, and will complement other more traditional ion exchange and net transfer thermobarometers.