EXPANDING THE RAMAN SPECTROSCOPIC QUARTZ-IN-GARNET ELASTIC GEOBAROMETER INTO THE TENSIONAL STRESS REGIME: UNLOCKING QUIG IN LOW-TO-MODERATE PRESSURE BARROVIAN METAMORPHISM

This study presents an extensive quartz-in-garnet elastic geobarometry (QuiG) dataset on quartz inclusions that exhibit tensional stresses and tests procedures that can be used to analyze inclusions that were exhumed from low-to-moderate pressure and moderate-to-high temperature Barrovian metamorphic conditions. The QuiG technique utilizes experimentally calibrated shifts of Raman spectra, which can indicate the strain caused by the differential expansion of a quartz inclusion and a garnet host. The strains and compressibilities of garnet and quartz can be used to infer the pressures of inclusion entrapment in garnet. Typically, QuiG studies focus on high-P rocks in which the exhumed quartz has expanded more than the garnet host, causing compression of the quartz. This study focuses on the much less studied moderate-P scenario where quartz expands less than garnet upon exhumation, causing a negative Raman wavenumber shift that records tension. QuiG analysis of 395 quartz inclusions from mid-crustal rocks from the Grand Canyon reveals differences in entrapment P calculated using the Grüneisen tensor approach and the hydrostatic 464-mode simplification. Our analysis reveals that the Grüneisen tensor approach yields different (often higher) P from the hydrostatic simplification in some inclusions, particularly those with high anisotropic strain. Thus, one important finding is that quartz inclusions in tension can be compromised to yield either spuriously higher or lower P, potentially due to anisotropic elasticity. Compromised data can be identified and culled by filtering and setting thresholds for acceptable levels of deviation from isotropic strain. Once filtered, QuiG data yield precise, reasonable P estimates of 5-7 kbar for the Grand Canyon suite. One zoned garnet preserves core and rim QuiG P of ~5 kbar and ~6.5 kbar, respectively, despite evidence for peak T >700°C. Thus, another important finding is that QuiG P can be preserved up to granulite facies. By applying filters, collecting large data sets, and using statistical approaches, QuiG on quartz inclusions experiencing tensional strains can yield robust P estimates, unlocking the ability to obtain metamorphic P information in rocks from the crucial metamorphic environment of midcrustal, Barrovian metamorphic rocks.