Application of Raman spectroscopy of quartz inclusions in garnet for pressure assessment (case study of the Belomorian Province, Fennoscandian Shield)

This paper presents an example of using a quartz-in-garnet Raman geobarometer to calculate the pressure during the formation of metamorphic rocks in the Boyarsky Massif of the Belomorian Province. The method is based on determining residual strains in inclusions that arise during rock ascent from depth to the surface. These strains arise due to differences in the elastic properties of the host mineral and the inclusion. To calculate residual stress using the elastic strain method, Raman spectra of quartz inclusions and free quartz in the matrix were obtained, and the shifts of the main quartz peaks (464, 206, and 128 cm-1) were calculated. After determining the strains, their values were used to calculate the pressure at which the inclusions were trapped by the host mineral. The studied gabbro-pegmatite sample contains garnet porphyroblasts up to 1 mm in size, saturated with quartz inclusions ranging from 5 to 65 μm in size. Fifteen quartz inclusions, 20–65 μm in diameter, and five free quartz grains in the matrix were analyzed. The trapping pressures were calculated over a specified temperature range of 200–1300 °C with a 10 °C increment. The temperature of quartz crystallization in garnet, determined using a Ti-in-Qz thermometer, was 725–902 °C, corresponding to a pressure of 8.9–13.6 kbar calculated using quartz-in-garnet Raman geobarometry. The crystallization temperature range, obtained using a classic Grt-Amph geothermometer, was 530–580 °C, corresponding to an estimated pressure of 5.2–6.0 kbar. These pressure values are consistent with previously conducted thermobarometric assessments of the conditions of metamorphism of gabbro-pegmatites and the metagabbros enclosing them.