Intermediate alkali-alumino-silicate aqueous solutions released by deeply subducted continental crust: fluid evolution in UHP OH-rich topaz kyanite quartzites from Donghai, southern Sulu (China)

Minerals, fluid inclusions and stable isotopes have been studied in ultrahigh-pressure (UHP) OH-rich topaz-kyanite quartzites from Hushan (west of Dongai), in southern Sulu (China). The quartzites underwent a metamorphic evolution characterized by a peak stage (3,5 GPa and 730-820 °C) with the anhydrous assemblage coesite + kyanite I, followed by an early near-isothermal decompression stage (2,9GPa and 705-780 °C) with growth of kyanite II, muscovite, and OH-rich topaz, and by decompression-cooling stages, represented by paragonite (1,9 GPa and 700-780 °C) and pyrophyllite (0,3GPa and 400 °C) on kyanite (I and II) and OH-rich topaz, respectively.These rocks may exhibit unusually low d18O and dD values acquired before undergoing UHP metamorphism. Five distinct fluid generations are recognized.Type I: concentrated peak solutions rich in Si, Al, and alkalis, present within multiphase inclusions in kyanite I. Type II: CaCl2-rich brines present during the growth of early retrograde OH-rich topaz. Type III, IV, and V: late aqueous fluids of variable salinity, and rare CO2 present during amphibolite- and late greenschist-facies conditions. A number of conclusions may be drawn from these relationships that have an effect on fluid evolution in deeply subducted continental rocks. (1) At a pressure of about 3,5GPa alkali-alumino-silicate aqueous solutions, with compositions intermediate between H2O fluid and melt (H2O > 25 and < 50 wt %) evolved from quartzites, probably generated by dehydration reactions. (2) During early decompression stages, at the transition from UHP to high-pressure (2,9 GPa) conditions, brines of external origin with higher water contents (82 wt % H2O) initiated the growth of OH-rich topaz and muscovite. (3) The subsequent decompression, at P < 2 GPa, was defined by a limited circulation of NaCl aqueous fluids, and CO2 infiltration. Overall, fluid inclusions and stable isotopes highlight a metamorphic fluid-rock interaction characterized by internally derived intermediate aqueous solutions at UHP, followed by infiltration of Cl-rich brines with higher water activities.