ESTIMATES OF GEOLOGIC PRESSURES IN THE FUNERAL MOUNTAINS AND THE WOOD HILLS METAMORPHIC CORE COMPLEXES: QUIG VERSES THERMODYNAMIC MODELING

Pressures of metamorphism for Barrovian rocks from the Funeral Mountains, CA and Wood Hills, NV in the western US Cordilleran hinterland are determined using equilibrium thermodynamic modeling and quartz-in-garnet elastic geobarometry (QuiG). In the Funeral Mountains, QuiG was compared to previously published Gibbs free energy minimization garnet P-T paths, garnet core composition isopleth intersections, and isochemical plots, which yeild pressures of 5.2, 5.6, and 6.0 kbar at peak temperatures for three locations across a Barrovian metamorphic field gradient. QuiG pressures reproduce these pressure changes but are higher in magnitude (8.3, 8.8, and 9.2 kbar). New GASP geobarometry, garnet core composition isopleth intersections, and isochemical plots in the Wood Hills yield pressures of 6-8 kbar and QuiG pressures of 9-11 kbar based on data collected from 14 garnets from a single rock sample. Potential causes for the pressure differences between QuiG and equilibrium-based barometers include overstepping of the garnet nucleation reaction causing thermodynamic estimates of garnet growth pressures to be underestimated or an unknown systematic issue causing QuiG to overestimate pressures of entrapment. To evaluate the internal consistency of various approaches to QuiG, this method was applied using (1) Raman wavenumber shifts in quartz inclusions assuming hydrostatic elastic strain, (2) Raman wavenumber shifts assuming anisotropic elastic strain, and (3) synchrotron X-ray diffraction.