Applications of Quartz-in-Garnet Elastic Geobarometry to Mid-Crustal Barrovian Metamorphic Rocks Demonstrated Using Tensionally Stressed Inclusions From the Grand Canyon

Abstract

Quartz-in-garnet elastic geobarometry (QuiG) pressures were obtained for inclusions exhumed from low-to-moderate pressure and moderate-to-high temperature Barrovian metamorphic conditions. Exhumation from these conditions causes quartz inclusions to expand less than their garnet hosts and experience tensional stresses. Most of the inclusions analysed in this study had near zero strains and stresses, which can preclude the ability to identify entrapped inclusions. New methods for identifying inclusion strains related to entrapment when the results cluster near zero were applied, revealing a population of tensile inclusions related to entrapment during metamorphism. Three hundred twenty-three inclusions were analysed from rock samples from the Upper Granite Gorge of the Grand Canyon. Inclusion remnant pressures were calculated using (1) shifted Raman modes assuming hydrostatic stress and (2) strains and the phonon-mode Grüneisen tensor of quartz. A range of entrapment pressures was determined for each garnet, but careful inspection of the inclusions and statistical analysis of the data sets successfully identified and removed inclusions that returned spurious results. QuiG yielded robust entrapment pressure estimates using the two different elastic geobarometry approaches. In this study, the refined QuiG technique yielded entrapment pressures that are consistent with the known geology of upper greenschist facies and upper amphibolite facies schists from the base of the Grand Canyon. Results from a zoned garnet grain show that multiple pressure populations can be preserved in a single garnet. The new QuiG results have implications for the robustness of QuiG in Barrovian rocks, where strains in inclusions can be near zero, and the ability of inclusions to retain early pressure information after being subjected to subsequent high-grade metamorphism.