Metamorphic Fluids and Its Evolution in the UHP Rocks from the Pre-pilot Hole of Jiangsu Province, China

Drill cores from the first pre-pilot hole of the Chinese Continental Scientific Drilling Project (CCSD-PPH1) located in the eastern part of the Dabie-Sulu UHP metamorphic belt consist of a rock association of metamorphic supracrustal rocks, granitic gneiss, and mafic and ultramafic intrusives. These rocks, except the garnet peridotite, have been subjected to an eclogite-facies UHP metamorphic event followed by a later amphibolite-facies retrometamorphism. The metamorphic p-t conditions for the coesite-bearing eclogite and eclogitic gneiss were 785~820℃ and 2. 7 GPa. Four types of fluid inclusions have been distinguished: (1) High-salinity CaCl2-NaCl-H2O inclusions (type Ⅰ) in garnet and omphacite from eclogite, and in kyanite from kyanite quartzite, probably representing the peak metamorphic fluids. (2) Inclusions containing mixed H2O-CO2(±N2) -NaC1(other solids (typeⅡ) , which occur in quartz from eclogite and in kyanite. They may reflect the fluids which were introduced during the exhumation and retrometamorphism. (3) Inclusions containing aqueous solutions of low salinity in the kyanite quartzite (type Ⅲ) have partly inherited the fluids released from dehydration-devolatiliztion of metasediments during the subduction, whereas those containing aqueous solutions of medium-low salinity (type Ⅳ) in eclogites and gneisses have been trapped during the amphibolite-facies retrometamorphism or a later stage. (4) CO2-rich inclusions (type Ⅴ) are distributed along the transgranular fractures of quartz in kyanite quartzite and may have formed by retrapping of leaked H2O-CO2 fluids. The pressure derived from the isochores of type Ⅰ inclusions is much lower than that obtained from the mineral thermobarometer. This implies that most of type Ⅰ inclusions have undergone changes in density after the trapping. In a rare case, high-density type Ⅰ inclusions may produce isochores that correspond to the eclogite-facies metamorphism. Fluid inclusion data also suggest that the fluid system was more or less closed and there was no large-scale fluid flow during the peak of UHP metamorphism.