Thermobarometry and in situ U–Pb dating of zircon in texturally and petrologically defined positions are used to assess exhumation accompanied by fluid-enhanced retrogression of ultra high pressure (UHP) eclogite core samples from the Chinese Continental Scientific Drilling (CCSD) project.
Eclogites from the CCSD project show discrete retrograde zones that can be attributed to exhumation processes. The retrogression from UHP eclogite to HP eclogite, amphibolite and finally greenschist facies conditions is fluid-induced. The occurrence of significant barium concentrations in two mica generations points to an external fluid source during retrogression. This Ba and F-bearing fluid mobilized zirconium from the eclogitic host, which was preferentially precipitated as overgrowths on pre-existing grains in the retrograde zone.
Thermodynamic modelling with progressive addition of H2O to the bulk composition simulates the petrological development of the fluid-influenced retrograde zone. Growth of zircon precipitated from the fluid occurred between 730°–740 °C at 3.46 GPa and 700 °C at 1.4 GPa.
LA-SF-ICPMS analyses point to a local onset of metasomatic zircon growth at 233.1 ± 5.3 Ma ongoing to 207.4 ± 1.6 Ma that is spatially limited to the retrograde zone.