An oxygen isotope study of quartz veins within eclogites from the Dabie terrane

The oxygen isotope composition of minerals from quartz veins and host eclog ites in the Dabie terrane was measured in order to place geochemical constr aints on the origin and transport of metamorphic fluid. The results are dis cussed together with structural and petrological relationships between quar tz vein and wallrock. The quartz veins can be temporally classified into th ree groups: (1) synmetamorphic vein which would be formed prior to eclogite -facies recrystallization when they were exhumated from mantle depths to de ep crustal levels; (2) early retrogressive vein which was formed in the ear ly stage of eclogite exhumation subsequent to the recrystallization, the ve in-forming fluid is still relevant to the eclogites; (3) late retrogressive vein which was formed in the late stage of eclogite exhumation from deep c rustal to upper crustal levels, oxygen isotope fractionation between vein q uartz and host eclogite significantly deviates from equilibrium values and the vein-forming fluid was principally derived from granitic gneiss hosting the eclogites. For the synmetamorphic vein, it appears that local advectiv e transport of fluid is the predominant mechanism in the processes of vein precipitation; the scale of oxygen isotope homogenization within the veins is much larger than that within the associated eclogites. The vein-forming fluid would be derived from the exsolution of dissolved hydroxyls within ec logite minerals due to significant pressure decrease. Fluid flow prior to t he eclogite-facies recrystallization and the early retrogression may occur mainly along pressure gradients.