Two groups of fluid inclusions in the Yunotani eclogite from the Hida-Gaien Belt: Implications for changes of fluid salinity during exhumation

摘要

Late Paleozoic Yunotani eclogites in the Itoigawa-Omi area of the Hida-Gaien Belt were subjected to a blues-chist-facies recrystallization and deformation after the peak eclogite-facies metamorphism. We studied fluid inclusions in quartz domains of different metamorphic stages in the retrograde eclogite. Coarse-grained quartz filling pressure shadow of porphyroblastic garnets contains transgranular fluid inclusions (Group-Ⅰ) with high salinity [6.0-8.0 eq mass% NaCl, with a mean value 7.2 ± 0.6 eq mass% NaCl (n = 12)]. In contrast, trans-granular fluid inclusions in later stage quartz-rich band (Group-Ⅱ) in parallel to a penetrative foliation developed in matrix are characterized by lower salinity [1.1-5.8 eq mass% NaCl, with a mean value 3.4 ± 1.2 eq mass% NaCl (n = 12)]. The occurrences of fluid inclusion trails suggest that each group corresponds to the fluids entrapped prior to the blueschist-facies recrystallization and the fluids entrapped during the blueschist-facies stage, respectively. The Group-II fluid inclusions formed at blueschist-facies conditions support the idea that aqueous fluids in the subduction channel have a salinity similar to seawater (～3.5 eq mass% NaCl). Moreover, the Group-I fluid inclusions indicate a possibility that aqueous fluids at the eclogite-facies depth in subduction channels possess a salinity higher than seawater.