Fluid inclusions in granulites,granulitized eclogites and garnet clinopyroxenites from the Dabie-Sulu terranes,eastern Chian

摘要

Minor granulites (believed to be pre-Triassic),surrounded by abundant amphibolite-facies orthogneiss,occur in the same region as the well-documentaed Triassic high- adn ultrahigh-pressure(HP and UHP) eclogites in the Dabie-Sulu terranes,eastern China.Moreoer,some eclogites and garnet clinopyroxenites have been metamorphosed at granulite- to amphibolite-facies conditions during exhumation.Granulitized HP eclogites/garnet clinopyoxenites at Huangweihe and Baizhangyan record estimated eclogite-facies metamorphic conditions of 775-805 deg C and >=15 kbar,followed by granulite-to amphibolite-facies overprint of ca.750-800 deg C and 6-11 kbar.The presence of(Na,Ca,Ba,Sr)-feldspars in garnet and omphacite corresponds to amphibolite-facies conditions.Metamorphic mineral assemblages and P-T estimates for felsic granulite at Huangtuling an dmafic granulite at Huilanshan indicate peak conditions of 850 deg C and 12 kbar for the granulite-facies metamorphism and 700 deg C and 6 kbar for amphiloblite-facies retrograde metamorphism.Cordierite-orthopyroxene and ferropargasite-plagioclase coronas and symplectites around garnet record a strong,rapid decompression,possibly contemporaneous with the uplift of neighbouring HP/UHP eclogites.Carbonic fluid (CO_2-rich)inclusions are predominant in both HP granulites and ranulitized HP/UHP eclogites/garnet clinopyroxenites.They have ow densities,having been reset during decompression.Minor amounts of CH_4 and/or N_2 as well monor N_2,whereas low-salinity fluids formed during retrogression.Syn-granulite-facies carbonic fluid inclusions occur either in quartz rods in clinopyroxene (granulitized HP garnet clinopyxeronite) or in quartz blebs in garnet and quartz materices (UHP eclogite).For HP granulites,a limited number of primary CO_2 and mixed H_2O-CO_2(liquid) inclusins have also been observed in undeformed quartz inclusins within garnet,orthopyroxene,and plagioclase which contain abundant,low-density CO_2+- carbonate inclusions.It is suggested that the primary fluid in the HP granulites was high-density CO_2,mixed with a significant quantity of water.The water was consumed by retrograde metamorphic mineral reactions and may also have been responsible for metaqsomatic reacions("giant myrmekites") occurring at quartz-feldspar boundaries.compared with the UHP eclogites in this region,the granulites were exhumed in the presence of massive,externally derived carbonic fluids and subsequently limited low-salinity aqueous fluids,probably derived from the surrounding gneisses.