FLUID-ROCK INTERACTION IN THE UHP KY-EP-PHE-ECLOGITE FROM DONGHAI AREA, SU LU TERRANE, EASTERN CHINA

The Ky-Ep-Phe-eclogite from Qinglongshan and Hushan, Donghai area, Su-Lu terrane, is hosted in granitoid gneiss and consists of garnet, omphacite, epidote, amphibole, kyanite, quartz (coesite), paragonite, phengite, and accessory rutile, OH-apatite, zircon, and opaque ores. On microstructural and minerochemical base, different generations of garnet, omphacite, epidote, amphibole, kyanite, and phengite have been recognised, which suggest a polyphase metamorphic evolution.

Three main fluid/multisolid inclusion populations are observed, which are characteristic of different metamorphic events: 1) early multiphase solid inclusions in garnet and kyanite (Type I); 2) early brines ± CO2 within epidote and kyanite (Type II); 3) low-salinity aqueous inclusions in quartz and epidote (Type III).

At the UHPM peak (T=880 ± 45 °C and P=4.0 GPa) the mineral assemblage is Grt + Cpx + Ky + Coe + Rt + OH-Ap and the fluid phase is a dense supercritical brine (Type I fluid) extremely enriched in Si, Al, Na, Fe, Mg, Ca, Ti, P, Zn and Zr ± S ± Cl. The postclimactic evolution, including by an earlier isothermal decompression still at UHP conditions, and a later decompression from UHP to HP conditions coupled with a moderate T decrease, is characterised by growth of zoned Grt, Cpx, together with Phe, Ep, and Amp in presence of brines with variable salinity and type of dissolved cations ± CO2. The final evolution, where moderate decompression is coupled with strong cooling, is mainly characterised by the growth of Amp + Pl symplectites in the presence of a low salinity aqueous fluid.

The comparison with Ky-quartzite from the same area suggests that in part at least the composition of the fluid inclusions is closely related to the rock bulk chemistry.