3:45 PM - 4:00 PM
[SCG48-13] Residual pressure of quartz inclusion in garnet of high- to ultrahigh-temperature metamorphic rock
Keywords:quartz Raman barometry, β-quartz, garnet, residual pressure, HT/UHT metamorphic rocks
Quartz inclusions in garnet porphyroblasts grown during prograde metamorphism retain residual pressure. The residual pressure retained by quartz inclusions can be estimated from the peak shift of Raman spectra, and methods have been developed to constrain the metamorphic conditions from the measured residual pressure values (called “quartz Raman barometry”; e.g., Enami et al. 2007; Kouketsu et al., 2014). These methods are limited to the stable region of low-temperature quartz (α-quartz), and there are no reports of their application to metamorphic rocks that have undergone a phase transition to high-temperature quartz (β-quartz) region under high- to ultrahigh-temperature conditions. In this study, we focused on quartz inclusions in garnet of high-temperature to ultrahigh-temperature metamorphic rocks grown under the stable region of high-temperature quartz, and examined whether the quartz Raman barometry can be applied in the same way as for low-temperature quartz.
Pelitic schist from the garnet-cordierite zone of the Yanai Ryoke metamorphic belt (Grt-Crd schist, Ryoke belt) and garnet-sillimanite gneiss from the Rundvågshetta, East Antarctica (Grt-Sill gneiss, East Antarctica) were analyzed. In the previous studies, the garnet-cordierite zone in the Yanai Ryoke metamorphic belt was estimated to be 0.47±0.1 GPa/852±35℃ (Ikeda 2004), and the garnet-sillimanite gneiss in the Rundvågshetta was constrained to be 1.40 GPa±0.1 GPa/1040℃( Kawasaki et al. 2010), both of which are stable regions of high-temperature quartz. The quartz inclusions in garnet crystals show negative crystalline morphology reflecting the crystal system of garnet. The Raman spectra of quartz inclusions in the Ryoke belt and East Antarctica samples both show a peak shift to the low wavenumber side, and the residual pressure values of -0.3 GPa for the Ryoke belt Grt-Crd schist and -0.7 GPa for East Antarctica Grt-Sill gneiss are negative, indicating tensile stress in both cases. Using the GUI software EoSFit-Pinc (Angel et al, 2017), which theoretically calculates residual pressure values using the equation of state for quartz and garnet, we performed numerical calculations extending to the stable region of high-temperature quartz, and found that -0.6 GPa under metamorphic conditions of Grt-Crd schist in Ryoke belt, and -0.3 GPa under metamorphic conditions of Grt-Sill gneiss in East Antarctica, respectively, indicating that the theoretical calculations are inconsistent with the measured results.
To interpret the discrepancy between the theoretical calculations and the measured results, the plastic deformation of garnet under high temperature condition was examined. It is generally known that garnet is plastically deformed at high temperatures above 700–800℃ (e.g., Storey & Prior, 2005). Theoretical calculations assume that the host garnet and quartz inclusions are elastically deformed, but considering the case of plastic deformation of the host garnet in the high temperature region, it is suggested that the host garnet is pushed by the expanded quartz inclusions due to the temperature increase and the residual pressure is not maintained. In this case, the residual pressure will be recorded at temperature conditions below 700–800℃, when the garnet transitions from plastic to elastic deformation. Using the measured residual pressure values, the metamorphic pressure conditions at 700℃ were calculated to be 0.6 GPa for Grt-Crd schist in Ryoke belt and 0.4 GPa for Grt-Sill gneiss in East Antarctica. It is reasonable to interpret that the East Antarctic Grt-Sill gneiss experienced a peak metamorphic condition of 1.4 GPa/>1000℃ and then passed through of 0.4 GPa/700℃ in the retrograde metamorphism during the exhumation stage. On the other hand, the path of 0.47 GPa/852℃ during the peak metamorphism and the subsequent pressure increase to 0.6 GPa during the retrograde metamorphism does not seem to be consistent in the Ryoke belt. Another interpretation is that after quartz was trapped during prograde metamorphism, the garnet was not sufficiently plastically deformed under peak metamorphic conditions due to the short heating time, and may have been exposed to the surface with retaining original residual pressure. This is consistent with the fact that the cooling rate of the Ryoke granite was fast (>40℃/myr) in the high-temperature region (Okudaira et al., 2001). The above results suggest that the metamorphic conditions constrained by quartz Raman barometry do not necessarily reflect the peak metamorphic conditions in the high-temperature and ultra-high-temperature metamorphic rocks.
Pelitic schist from the garnet-cordierite zone of the Yanai Ryoke metamorphic belt (Grt-Crd schist, Ryoke belt) and garnet-sillimanite gneiss from the Rundvågshetta, East Antarctica (Grt-Sill gneiss, East Antarctica) were analyzed. In the previous studies, the garnet-cordierite zone in the Yanai Ryoke metamorphic belt was estimated to be 0.47±0.1 GPa/852±35℃ (Ikeda 2004), and the garnet-sillimanite gneiss in the Rundvågshetta was constrained to be 1.40 GPa±0.1 GPa/1040℃( Kawasaki et al. 2010), both of which are stable regions of high-temperature quartz. The quartz inclusions in garnet crystals show negative crystalline morphology reflecting the crystal system of garnet. The Raman spectra of quartz inclusions in the Ryoke belt and East Antarctica samples both show a peak shift to the low wavenumber side, and the residual pressure values of -0.3 GPa for the Ryoke belt Grt-Crd schist and -0.7 GPa for East Antarctica Grt-Sill gneiss are negative, indicating tensile stress in both cases. Using the GUI software EoSFit-Pinc (Angel et al, 2017), which theoretically calculates residual pressure values using the equation of state for quartz and garnet, we performed numerical calculations extending to the stable region of high-temperature quartz, and found that -0.6 GPa under metamorphic conditions of Grt-Crd schist in Ryoke belt, and -0.3 GPa under metamorphic conditions of Grt-Sill gneiss in East Antarctica, respectively, indicating that the theoretical calculations are inconsistent with the measured results.
To interpret the discrepancy between the theoretical calculations and the measured results, the plastic deformation of garnet under high temperature condition was examined. It is generally known that garnet is plastically deformed at high temperatures above 700–800℃ (e.g., Storey & Prior, 2005). Theoretical calculations assume that the host garnet and quartz inclusions are elastically deformed, but considering the case of plastic deformation of the host garnet in the high temperature region, it is suggested that the host garnet is pushed by the expanded quartz inclusions due to the temperature increase and the residual pressure is not maintained. In this case, the residual pressure will be recorded at temperature conditions below 700–800℃, when the garnet transitions from plastic to elastic deformation. Using the measured residual pressure values, the metamorphic pressure conditions at 700℃ were calculated to be 0.6 GPa for Grt-Crd schist in Ryoke belt and 0.4 GPa for Grt-Sill gneiss in East Antarctica. It is reasonable to interpret that the East Antarctic Grt-Sill gneiss experienced a peak metamorphic condition of 1.4 GPa/>1000℃ and then passed through of 0.4 GPa/700℃ in the retrograde metamorphism during the exhumation stage. On the other hand, the path of 0.47 GPa/852℃ during the peak metamorphism and the subsequent pressure increase to 0.6 GPa during the retrograde metamorphism does not seem to be consistent in the Ryoke belt. Another interpretation is that after quartz was trapped during prograde metamorphism, the garnet was not sufficiently plastically deformed under peak metamorphic conditions due to the short heating time, and may have been exposed to the surface with retaining original residual pressure. This is consistent with the fact that the cooling rate of the Ryoke granite was fast (>40℃/myr) in the high-temperature region (Okudaira et al., 2001). The above results suggest that the metamorphic conditions constrained by quartz Raman barometry do not necessarily reflect the peak metamorphic conditions in the high-temperature and ultra-high-temperature metamorphic rocks.