Study of fluid inclusions in zircons of UHP metamorphic rocks from the main drillhole of the Chinese Continental Scientific Drilling Project (CCSD).

The main drillhole of the Chinese Continental Scientific Drilling Project ( CCSD) is located in the south of the Sulu ultrahigh-pressure (UHP) metamorphic belt in central-east China. The UHP metamorphic rocks cut through by the drillhole consist mainly of eclogite, ortho- and para-gneisses, garnet peridotite, amphibolite with minor schist and quartzite. Zircon is a common accessory mineral in the UHP metamorphic rocks. Genetically, the zircon can be classified into three types, i. e. , protolith zircon, metamorphic (overgrowth) zircon and newly-formed zircon. The three types of zircons usually contain various amounts of mineral and fluid inclusions, which record the information of the role played by fluids during the prograde-, peak- and retrograde metamorphism. Fluid inclusions in zircons show the following characteristics: (1) primary H2O and H2O-CO2 inclusions can often be seen in the cores of protolith zircon grains and the composition and salinity of the H2 O inclusions vary considerably whereas secondary H2 O-CO2 and/or CO2 inclusions occur along the cracks or microfractures of the protoloth zircon sometimes; (2) in the metamorphic zircon or metamorphic overgrowths (mantles or rims) on the protolith zircon H2O-CO2 inclusions are rarely found to coexist with UHP mineral inclusions; (3) some eclogite, especially gneisses contain abundant newly-formed zircon crystals of minute size which bear little or no mineral inclusions but occasionally H2 O-CO2 inclusions of low salinity; (4 ) there is a correlation between the abundance of fluid inclusions in zircon and the oxygen isotope values of the host rocks, i. e. , fluid inclusions in zircon in the rocks with very lowδ18O values are particularly abundant but are lack or even absent in the rocks with normal oxygen compositions. Considering that the protolith zircon, metamorphic zircon and new zircon all contain aqueous fluid inclusions of low salinity it is suggested that the metamorphic fluids were inherited during the continental subduction-exhumation processes. The isochores of H2O and H2O-CO2 inclusions all pass through the P-T box below the peak metamorphic pressures and temperatures calculated from the mineral thermobarometers. Fluid inclusions trapped in the prograde- and peak UHP metamorphic stages may have been modified. The presence of metamorphic fluids during the UHP metamorphism and subsequent retrogression promoted dissolution, metaporphic overgrowth of protolith zircon and formation of metamorphic zircon and new zircon.