Areal extent of eclogite facies metamorphism in the Sanbagawa belt, Japan: New evidence from a Raman microprobe study of quartz residual pressure

摘要

Application of Raman spectrometry to determine residual pressure retained by quartz grains sealed in garnet reveals significant differences between eclogite facies rocks and lower-grade schistose rocks of the Sanbagawa metamorphic belt in central Shikoku, Japan. Garnet in the eclogitic lithologies commonly exhibits two chemically distinct growth stages. The inner segment commonly includes quartz grains with higher residual pressures than those in the outer segment, suggesting that the former represents prograde eclogite facies and the latter formed during exhumation. Regional variation of the quartz residual pressure suggests that the eclogite unit has a far greater extent than previously recognized, and reveals a large pressure gap between the newly proposed eclogite and noneclogite units.