COMPARISON OF QUIG BAROMETRY WITH CONVENTIONAL THERMOBAROMETRY IN THE TOWNSHEND–BROWNINGTON SYNCLINE, VERMONT

Quartz in garnet (QuiG) barometry has been applied to metapelite samples from the Townshend–Brownington Syncline, central Vermont, in order to compare published conventional thermobarometric methods with QuiG measurements. QuiG involves determination of the internal pressure of a quartz inclusion in garnet by measuring the shift of the 464 cm^-1Raman band. The internal pressure is used to calculate the isomeke of inclusion entrapment by applying a thermoelastic model. Data from QuiG barometry provides a physical measurement of P–T conditions of quartz entrapment, reflecting the growth of garnet. This provides a second method of determining the P–T conditions of metamorphic rocks that can be compared with conventional thermobarometry.

Metapelites from the Townshend–Brownington syncline are compositionally zoned and contain abundant garnet with plentiful quartz inclusions for use in Raman spectroscopy. Sample TM-916C, which was studied in detail, is a foliated, compositionally layered metapelite containing garnet, quartz, muscovite, biotite, chlorite, plagioclase, and ilmenite. Garnet is compositionally zoned with decreasing Mn and depletion of Ca at the rim. Minor enrichment of Mn at the rim indicates resorption and back diffusion of Mn. Quartz inclusions in garnet within the mica-rich layer, display high waveshifts of 2.7 cm^-1, 2.2 cm^-1, and 1.9 cm^-1, indicating pressures of 10.7, 9.7, and 9.3 kbar at 550 °C, respectively. Within the biotite-rich layer, a high waveshift value of 2.1 cm^-1 in quartz inclusions was observed, with a corresponding entrapment pressure of 9.7 kbar at 550 °C. From this single sample there is not enough evidence to state that there is a difference in maximum waveshifts of the quartz peaks across compositional layers, as other factors such as cracks or deformation can lower these values. Within these crystals, no systematic variation of internal pressure of quartz inclusions was observed from core to rim in garnet. Assuming the highest waveshift reflects the conditions of entrapment, these quartz inclusions imply an entrapment pressure of 10.6 ± 0.1 kbar at an assumed temperature of 550 °C. These values are significantly higher pressure than previous data showing 8.5 kbar at 550 °C, suggesting previously used methods are underestimating the depth of metamorphism at this locality.