Abstract
Preliminary petrographic observations and microthermometric measurements show that three types of fluid inclusions occur in quartz veins and late-stage carbonate veins in high- and ultrahigh-pressure (HP-UHP) rocks from the CCSD area (Donghai). Fluids include brine (NaCl-H2O) inclusions (type I), NaCl-CaCl2-H2O inclusions (type II), and N2-CH4 inclusions (type III). Type I fluids are divided into medium-high salinity (type Ia), medium salinity (type Ib), and low salinity (type Ic) inclusions. The type III inclusions are the first discovered in the CCSD area. Type Ia, Ib, and II inclusions are primary or pseudosecondary in origin and occur in vein- and matrix quartz in eclogites. Their absence in vein- and matrix quartz and in the amphibolite-facies gneisses suggests that Ia, Ib, and II inclusions were probably captured during decompression-recrystallization and retrograde metamorphism of the eclogite. In contrast, type Ic inclusions are widely distributed, and were probably captured in the last stage of UHP exhumation. N2-CH4 pure gaseous inclusions are mostly primary, and mainly occur as isolated or clustered inclusions in laminated quartz veins in eclogite together with types Ia and Ib inclusions; this implies that type III inclusions were probably captured under HP-UHP metamorphic conditions. Most quartz veins in eclogite probably formed by decompression-recrystallization and retrograde metamorphism during exhumation of the subducted plate, whereas quartz veins in the gneisses mainly formed at amphibolite-facies or late-stage retrograde metamorphism during exhumation. The distinct differences between fluid inclusions in quartz veins in eclogites versus those in gneisses, and the similarities among fluid inclusions in quartz veins and matrix quartz crystals in their respective hosting rocks suggests that metamorphic fluids expelled from the HP-UHP rocks during the exhumation migrated mainly at the grain scale.
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