Fluid Inclusions in Ultra High-Pressure Eclogites from the Dabie Shan, Eastern China

Abstract

Although a lot of work has been done on the ultrahigh-pressure (UHP) eclogites from the Dabie Shan, eastern China, the role of the fluid phase during eclogite-facies metamorphism is not answered yet. In order to obtain information on the physical-chemical conditions of the fluid phase during UHP metamorphism, we studied fluid inclusions in coesitebearing eclogites from the Bixiling complex in the southern Dabie Shan. These rocks outcrop in an area of about 1 kn'l 2 within foliated quartzofeldspathic gneiss. The boundary between the eclogites and country rock shows tectonic contact relations. The complex mainly consists of eclogites and elongated (1-40m) lenses of garnet peridotite. The field observations suggest that both eclogites and garnet peridotite underwent coesite-eclogite-facies metamorphism. The eclogites from the Bixiling complex typically contain garnet, omphacite, phengite, rutile, quartz, kyanite, with minor zoisite, amphibole, plagioclase, epidote, biotite, zircon and coesite. The peak mineral assemblage for the eclogites is characterized by garnet + omphacite + phengite + futile • kyanite • coesite _+ zoisite +_ zircon. The eclogites underwent a complex metamorphic evolution of at least five metamorphic stages (Xiao et al., 1995): (a) prograde pre-eclogite-facies metamorphism at ~ 27 kb and /> 700 ~ as documented by coesite relics preserved as inclusions in garnet and omphacite; (c) quartz-eclogite-facies metamorphism at 15-20 kb and -800 ~ characterized by coarsegrained garnet + omphacite + kyanite + quartz; (d) a retrograde symplectite stage at 9 -13 kbar and 600-700 ~ represented by various symplectitic replacement assemblages of clinopyroxene, amphibole and plagioclase; and (e) a post-symplectitic stage corresponding to the amphibolite-facies metamorphism. In the coesite-bearing eclogites from Bixiling, fluid inclusions normally are < 20 gm in diameter, rarely up to 40 p.m. Four types of fluid inclusions were distinguished according to textural criteria and fluid compositions: (1) High-salinity brines in quartz blebs in kyanite represent the oldest generation of fluid inclusions. Sometimes they occur also in kyanite itself. They are isolated or randomly distributed inclusions. They are mostly 3-phase (LVShalite) and show rounded or negative crystal morphologies. They have filling degrees of about 80 vol%. These fluid inclusions do not show any phase transition during cooling; on subsequent warming, they show a granular texture between -65 ~ and -75 ~ interpreted as a devitrification phenomenon (metastable non-crystalline to crystalline state) and hides a clear observation of the eutectic point. We asumme that the Te is slightly higher than the devitrification temperature and indicates the presence of Ca 2+ (and/or Mg2+). Rare clathrate melting between 10 and 15 ~ indicates the presence of CO2. (2) High-salinity aqueous inclusions (_+ halite) in omphacite and kyanite occur as tubes that are oriented parallel to the growth zones of the host mineral suggesting primary origin. They probably contain relic metamorphic fluids of the eclogitefacies. Unlike the isolated fluid inclusions in quartz blebs, this type of high-salinity inclusions show clear freezing between 6 0 and 7 0 ~ and display Tm ico between -24~ and -16~ They have eutectic temperatures between 3 0 and -21 ~ indicating a dominantly NaCl-bearing fluid. (3) Carbonic inclusions were found both in omphacite and in matrix quartz. They are monophase at room temperature. They occur isolated and as clusters, and have rounded or negative crystal shapes. The CO2 inclusions in omphacite display final Tmco2 around -58.5~ and Th (to liquid) between -31 and