COMPARING RAMAN-INCLUSION BAROMETRY AND THERMODYNAMIC PT MODELING IN BARROVIAN METAMORPHIC ENVIRONMENTS

Raman-inclusion barometry, with the potential to be an essential tool in petrotectonic analysis, has come under scrutiny in recent studies. Here we compare quartz-in-garnet (QuiG) Raman-inclusion barometry with G-minimization modeling across the metamorphic field gradient in the Funeral Mountains and thermobarometry in the Wood Hills. In the Funeral Mountains, metamorphic grade increases across the range, from upper greenschist facies (at Indian Pass, IP), lower amphibolite facies (at East Chloride, EC), and middle amphibolite facies (at Chloride Cliff, CC). Across the field gradient, both methods show grades of metamorphism increasing to the northwest, with PT modeling giving peak garnet growth PT conditions of 5.2 kb at 530ºC for IP, 5.6 kb at 553ºC for EC, and 6.2 kb at 588ºC for CC. QuiG gave pressures from these three locations of 8.3, 8.8, and 9.2 kb, respectively. In the Wood Hills, at an intermediate location in the metamorphic field gradient between the Pequop Mountains and the East Humboldt Range, thermobarometry gave garnet growth PT conditions of 6.6-6.4 kb and 625-638ºC. Four garnets gave maximum QuiG pressures between 10.5-11 kb. Uncertainties are taken as ±1 kb and ±50ºC for PT and ±0.59 kb for Raman.

Different results obtained from the two methods have been reported in the literature. QuiG pressures reported by Wolfe and Spear (2017) were ~2-4 kb greater than the chemical thermodynamic garnet-in pressures. Although these two methods tend to overlap in high dP/dT metamorphic environments (i.e. eclogite, blueschist, etc.), differences between pressures obtained using PT modeling and QuiG barometry seem ubiquitous in rocks that experienced moderate dP/dT (i.e. Barrovian) metamorphism, with QuiG universally producing higher pressures. Potential causes for these differences include reaction overstepping or retrograde re-equilibration causing PT modeling to underestimate pressure, or a yet unknown factor causing QuiG to overestimate pressure such as: a deviatoric stress effect caused by inclusion anisotropy, an entrapment temperature effect, or an inclusion crowding effect. Future work using experimental and observational techniques aims to address this ongoing question.