Eclogite and carpholite-bearing metasedimentary rocks in the North Qilian suture zone, NW China: implications for early palaeozoic cold oceanic subduction and water transport into mantle

Low-temperature eclogite and eclogite facies metapelite together with serpentinite and marble occur as blocks within foliated blueschist that was originated from greywacke matrix; they formed a high-pressure low-temperature (HPLT) subduction complex (melange) in the North Qilian oceanic-type suture zone, NW China. Phengite-eclogite (type I) and epidote-eclogite (type II) were recognized on the basis of mineral assemblage. Relic lawsonite and lawsonite pseudomorphs occur as inclusions in garnet from both types of eclogite. Garnet-omphacite-phengite geothermobarometry yields metamorphic conditions of 460-510 degrees C and 2.20-2.60 GPa for weakly deformed eclogite, and 475-500 degrees C and 1.75-1.95 GPa for strongly foliated eclogite. Eclogite facies metasediments include garnet-omphacitephengite-glaucophane schist and various chloritoid-bearing schists. Mg-carpholite was identified in some high-Mg chloritoid schists. P-T estimates yield 2.60-2.15 GPa and 495-540 degrees C for Grt-OmpPhn-Gln schist, and 2.45-2.50 GPa and 525-530 degrees C for the Mg-carpholite schist. Mineral assemblages and P-T estimates, together with isotopic ages, suggest that the oceanic lithosphere as well as pelagic to semi-pelagic sediments have been subducted to the mantle depths ( >= 75 km) before 460 Ma. Blueschist facies retrogression occurred at c. 454-446 Ma and led to eclogite deformation and dehydration of lawsonite during exhumation. The peak P-Tconditions for eclogite and metapelite in the North Qilian suture zone demonstrate the existence of cold subduction-zone gradients (6-7 degrees C km(-1)), and this cold subduction brought a large amount of H2O to the deep mantle in the Early Palaeozoic times.