Fluid inclusions hidden in coesite-bearing zircons in ultrahigh-pressure metamorphic rocks from southwestern Sulu terrane in eastern China

Primary fluid inclusions, together with coesite mineral inclusions, are identified in the same zircon domains by laser Raman spectroscopy, cathodoluminescence (CL) image and micro-texture analysis in paragneiss and eclogite from the main drilling hole of Chinese Continental Scientific Drilling Project in southwestern Sulu terrane. Most fluid inclusions are characterized by CO₂ (gas)-H₂O (liquid) two-phase, a few by H₂O one-phase liquid inclusions. These features indicate that the eclogite and its country-rocks may be located in the “wet system” rather than in the “dry system” during UHP metamorphism. SHRIMP U-Pb dating indicates that the timing of trapping the fluid and coesite inclusions in metamorphic zircon domains is about 233.7 ±4.3 Ma, which may represent the age of zircon growth in the stage of pressure decrease but temperature increase during the retrograde period of UHP metamorphism thus indicating the fluid activity still under the UHP conditions. The zircons further overgrew at about 213.2 ±5.2 Ma in response to amphibolite-facies retrogression. Therefore, fluid activity in the Sulu UHP metamorphic rocks principally occurred during the exhumation of UHP slab in the Middle to Late Triassic. The present results not only provide insight into the fluid property and fluid-rock interaction mechanism in the Sulu-Dabie UHP terrane, but also present a new means to exactly identify the primary fluid inclusions preserved in zircons from the UHP metamorphic rocks.