AN EXAMPLE OF HIGHLY MOBILE AQUEOUS FLUID DURING CONTINENTAL UHP METAMORPHISM

Petrographic, stable and radiogenic isotope, and fluid-inclusion studies of continental crust metamorphosed at ultra-high pressure (UHP) conditions have generally concluded that water activity is low at peak pressure and that fluid transport length scales are limited. In contrast, there is no doubt that extensive migration of metasomatic fluids and melts occurs over 10’s of kilometers during subduction of oceanic crust and overlying sediment to hydrate and flux arc magmatism in the overlying lithospheric mantle. It thus seems inescapable that environments exist where large masses of continental crust subducted to UHP depths interact with highly mobile aqueous fluids. This was the case for the Luliang Shan massif, a 100 km² exposure of continental crust exposed within the early Paleozoic North Qaidam HP-UHP terrane. Inasmuch as the Luliang Shan is the oldest and coolest UHP exposures within the North Qaidam region, it is the most likely to have been affected by oceanic subduction processes. Abundant lensoidal blocks of mafic eclogite hosted within felsic gneiss of the Luliang Shan are mantled by phengite-garnet selvages formed at near peak-UHP conditions at ca. 490 Ma. Whole rock compositions, O and Ar isotopic data, and high concentrations of fluid mobile elements (notably Li, Rb, Cs, and Ba) in phengite cannot be explained by metasomatic redistribution within a local closed system involving metabasalt and gneiss. Given the high density of selvage-mantled eclogite, we conclude that aqueous fluids were effectively mobile throughout the entire gneiss terrane when it reached ca. 600°C at 75 km depth. Regardless of whether the peak grade fluids were externally derived, they were compositionally distinct from later fluids that infiltrated the gneiss during exhumation. These retrograde fluids triggered muscovite growth through the gneiss to impart a regionally extensive epidote-amphibolite facies foliation. We interpret the high concentrations of B and Be (to 3800 ppm and 40 ppm respectively) exhibited by retrograde muscovite within the gneiss and shear zones cutting the selvage as having been supplied by fluids devolatilized from low-grade metasediments in more shallow regions of the subduction channel during exhumation at 460 Ma.