10:45 AM - 11:00 AM
[SMP24-06] Ultrahigh-pressure metamorphic records in whiteschists from Dora-Maira massif: verification from anisotropy of residual stress in coesite inclusions within kyanite
Keywords:coesite, Raman spectroscopy, UHP metamorphism, whiteschist
Some mineral inclusions entrapped in host minerals of metamorphic rocks, such as quartz in garnet, retain residual stresses due to differences in thermal expansion and bulk modulus between the inclusions and the host minerals. This study examines on "coesite", a high-pressure polymorph of quartz, from the ultra-high pressure (UHP) whiteschists of the Dora-Maira Massif (Brossasco-Isasca Unit, Western Alps), which are known for the preservation of coarse-grained coesite inclusions and its pseudomorphs within the matrix. By analyzing up to 120 coesite inclusions within five host minerals such as pyrope garnet, kyanite, zircon, and rutile selected from crushed samples of the whiteschist, we investigated Raman shifts to provide new insights into the anisotropy of residual stresses particularly in host kyanite, and the potential studies of coesite inclusions in UHP metamorphic rocks. The Raman shift of coesite inclusions within kyanite, with significant residual stresses is inferred from shifts in the Si-O-Si symmetric stretching vibration peaks. The maximum estimated residual stress of 1.74 GPa yields apparent metamorphic pressures of up to ~9 GPa, highlighting the high residual stresses retained by coesite inclusions in kyanite. The coesite inclusion has a quartz rim, which accounts for the excess residual pressure. Our study suggests that consideration of the kyanite anisotropy and phase transitions at the host-inclusion interface allows the interpretation of pressure records from coesite inclusions. This provides new insights into the temperature and pressure history of UHP metamorphic rocks within the coesite stability field, where conventional geobarometry does not yield reliable results.