Fluid flow during exhumation of deeply subducted continental crust: zircon U-Pb age and O-isotope studies of a quartz vein within ultrahigh-pressure eclogite

Quartz veins in high-pressure to ultrahigh-pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis. This flow can occur in the direction of changing temperature/pressure during subduction or exhumation. SHRIMP U-Pb dating of zircon from a kyanite-quartz vein within ultrahigh-pressure eclogite in the Dabie continental collision orogen yields two age groups at 212 +/- 7 and 181 +/- 13 Ma, which are similar to two groups of LA-ICPMS age at 210 +/- 4 and 180 +/- 5 Ma for the same sample. These ages are significantly younger than zircon U-Pb ages of 224 +/- 2 Ma from the host eclogite. Thus the two age groups from the vein date two episodes of fluid flow involving zircon growth: the first due to decompression dehydration during exhumation, and the second due to heating dehydration in response to a cryptic thermal event after continental collision. Laser fluorination O-isotope analyses gave similar delta O-18 values for minerals from both vein and eclogite, indicating that the vein-forming fluid was internally derived. Synchronous cooling between the vein and eclogite is suggested by almost the same quartz-mineral fractionation values, with regularly decreasing temperatures that are in concordance with rates of O diffusion in the minerals. While the quartz veining was caused by decompression dehydration at 700-650 degrees C in a transition from ultrahigh-pressure to high-pressure eclogite-facies retrogression, the postcollisional fluid flow was retriggered by heating dehydration at similar to 500 degrees C without corresponding metamorphism. In either case, the kyanite-quartz vein formed later than the peak ultrahigh-pressure metamorphic event at the Middle Triassic, pointing to focused fluid flow during exhumation rather than subduction. The growth of metamorphic zircon in the eclogite appears to have depended on fluid availability, so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles.