Fluid Composition and Evolution Attending UHP Metamorphism: Study of Fluid Inclusions from Drill Cores, Southern Sulu Belt, Eastern China

Abstract

Rocks from the first pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSDPPH1, 432 m), located in the eastern part of the Dabie-Sulu ultrahigh-pressure (UHP) metamorphic belt, have been subjected to a coesite-eclogite-facies metamorphic event, followed by an amphibolite-facies overprint. Primary fluid inclusions occur in garnet, omphacite, and apatite from eclogite; in kyanite and in topaz from quartzite; and in garnet, epidote, and apatite from paragneiss. Secondary fluid inclusions are present in all lithologies. Fluid inclusions are absent from ultramafic rocks. Based on fluid compositions and textural criteria we distinguished: (1) low-salinity aqueouscarbonic inclusions in topaz from quartzite, which may have originated from a supracrustal protolith; (2) primary CaCl2-NaCl-rich brine inclusions in garnet and in omphacite from eclogite and in kyanite from quartzite, representing UHP metamorphic fluids; (3) high-salinity aqueous-carbonic inclusions in quartz from eclogite and quartzite, representing amphibolite-facies fluids; (4) aqueous fluids of low- and intermediate salinity trapped as primary inclusions in garnet, epidote (or allanite) and apatite from gneiss, or as secondary inclusions, representing amphibolite-facies and later retrograde fluids; (5) carbonic inclusions are distributed along transgranular fractures in quartz from quartzite, and probably represent the latest retrograde fluid. The diversity in fluid inclusion populations and compositions from different vertical depths suggests a closed fluid system without largescale fluid migration during UHP metamorphism. However, the common low- and medium-salinity inclusions in most rock types suggests that a water-dominated fluid from an external source infiltrated into the rock system during amphibolite-facies metamorphism, resulting in extensive retrogression of the UHP rocks.