EVALUATION OF ELASTIC BAROMETRY BY RAMAN SPECTROSCOPY OF QUARTZ IN GARNET FORMATION PRESSURES IN THE DUTCHESS COUNTY, NEW YORK BARROVIAN METAMORPHIC SEQUENCE

Determination of the pressure (P) and temperature (T) formation conditions of metamorphic rocks provides insight into tectonic-scale processes. While qualitative methods using index minerals allow for some constraint of P and T, quantitative determination relies on the use of thermobarometers dependent on thermodynamic models and chemical equilibrium assumptions. Estimating P is especially challenging, with a limited arsenal of reliable mineral barometers because the minerals used in geobarometry undergo relatively large volume changes in chemical reactions that are much less abundant than thermometry reactions. This project aims to evaluate the reliability of elastic barometry as an alternative geobarometric tool to investigate PT conditions of metamorphic rocks where other thermobarometry tools are not applicable, such as in chemically simple systems.

Raman spectroscopy has been identified as a tool to estimate entrapment pressure of mineral inclusion-host systems. Metamorphic quartz in garnet (QuiG) inclusions systems barometrically “freeze” quartz grains at entrapment pressure. During exhumation, the volume of garnet and quartz inclusions often change at different rates, causing entrapment stress from differential pressures. This is observed as shifts in the quartz vibrational frequencies compared to unstrained samples. Thus, the purpose of this project is to evaluate potential correlation between inclusion entrapment pressure of QuiG, and P estimates based on cation exchange (e.g. garnet-plagioclase-muscovite-biotite) in a classic prograde metamorphic sequence associated with a single metamorphic event. Raman spectroscopy was conducted on representative QuiG pairs in five samples from a well-studied Barrovian metamorphic sequence in Dutchess County, New York. Approximately 25 quartz inclusions of length ≥8μm were spatially mapped in each sample, with Raman spectra taken at central and edge points within the inclusion. The measured Raman shift at each location were modeled against P using the stRAinMAN program. Calculated P from QuiG inclusions was compared with existing PT estimates from the Dutchess County sequence to assess P derived from stress in QuiG inclusion-host systems.