Element mobility in mafic and felsic ultrahigh-pressure metamorphic rocks from the Dabie UHP Orogen, China: insights into supercritical liquids in continental subduction zones

摘要

Geochemical analyses of minerals and whole rocks for major and trace elements as well as Sr-Nd-O isotopes on samples along a profile across the boundary between amphibolite retrogressed from ultrahigh pressure (UHP) eclogite and its country rock granitic gneiss from the South Dabie low-T/UHP zone, China, are studied to assess the process that controls element mobility under extreme metamorphic conditions. Directly at the contact with the granitic gneiss, the amphibolite has higher concentrations of K, Al, LILEs, REEs, HFSEs, Th, and U, slightly lower concentrations of SiO2, MgO, and CaO, but very similar FeOt and transitional metal element contents relative to the other amphibolites further away from the boundary. Consistently, delta O-18 values of the amphibolite display a progressive increase towards the boundary, indicating fluid-assisted O-isotope exchange across the contacts of different lithologies at local scales. These can neither be attributed to amphibolite-facies retrogression of eclogite, which is known to have no significant effect on their major and trace elements, nor to Si-rich metasomatism from partially melted granitic gneiss, which should increase the silica content of the amphibolite at the contact. Therefore, neither of the two processes is viable here. We thus propose that the variations observed here were caused by metasomatism of supercritical liquids that were generated at pressure higher than that of the second critical endpoint in the granitic gneiss-H2O system. This interpretation is supported by multiphase solid inclusions of K-feldspar+quartz+ calcite+zircon +/- amphibole +/- clinozoisite +/- garnet +/- apatite within garnet in the amphibolite.