Metamorphism and fluid evolution of the Sumdo eclogite, Tibet: Constraints from mineral chemistry, fluid inclusions and oxygen isotopes

The Sumdo metamorphic belt, located in the Lhasa terrane of Tibet, is a key area for unraveling the Paleo-Tethys evolution. In order to identify the fluid evolution in oceanic subduction zones, eclogite from the Sumdo metamorphic belt, has been investigated in a combined study of petrography, mineral chemistry, fluid inclusions and oxygen isotopes. On the basis of petrography and mineral chemistry, two kinds of garnet were distinguished. The core part of Grt-I shows increasing grossular content and contains prograde mineral inclusions, suggesting that it grew during prograde metamorphism, while the rim of Grt-I shows varying grossular and pyrope contents, representing subsequent overgrowth. Grt-II displays a composition comparable with the outermost part of Grt-I, indicating that Grt-II grew at the peak or initial retrograde stage. Garnet trace element zoning patterns and P-T estimates suggest that the Sumdo eclogite experienced a heating stage during an initial exhumation after the peak metamorphic event. Based on textural criteria and host minerals, three types of fluid inclusions were distinguished: Type-I are primary NaCl dominated intermediate to high-salinity (10-22 wt% NaCl) fluid inclusions hosted by omphacite, which were most likely released during dehydration of the subducting oceanic crust and reflect the composition of eclogite-facies fluids; Type-II are defined as primary low-salinity (1-6 wt% NaCl) fluid inclusions hosted by epidote/clinozoisite, representing fluids that evolved towards low-salinity during uplift; Type-III are secondary low-salinity (0-7 wt% NaCl) fluid inclusions hosted by matrix quartz (i.e., minerals other than the mineral inclusions) and rarely by omphacite and must have formed at a very late stage. In addition, the eclogite displays delta O-18 values (delta O-18 = +5.0 to +8.9%o) similar to its protolith, low-temperature altered oceanic crust (delta O-18 = +4.9 to +12.7960). The equilibrium oxygen isotope fractionations (Delta O-18(Qtz-Min)) between quartz and garnet/omphacite suggest isotopic equilibrium and a closed fluid system during eclogite facies metamorphism. Our data indicate that the Sumdo eclogite experienced "hot" exhumation, during which large amounts of fluids were released by dehydration. Fluids in the Sumdo eclogite were typically aqueous fluids of varying salinities and now preserved as fluid inclusions in HP metamorphic minerals. In comparison with continental subduction zones, we confirm previous conclusion that the fluid regime in oceanic subduction zones is relatively simple (i.e. mainly NaCl bearing brines) with seawater-altered slabs as the dominant source.