Intermediate Alkali–Alumino-silicate Aqueous Solutions Released by Deeply Subducted Continental Crust: Fluid Evolution in UHP OH-rich Topaz–Kyanite Quartzites from Donghai (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 (35 GPa and 730^8208C) with the anhydrous assemblage coesiteþkyanite I, followed by an early near-isothermal decompression stage (29GPa and 705^7808C) with growth of kyanite II, muscovite, and OH-rich topaz, and by decompressioncooling stages, represented by paragonite (19 GPa and 700^7808C) and pyrophyllite (03GPa and 4008C) 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 greenschistfacies 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 35GPa alkali^alumino-silicate aqueous solutions, with compositions intermediate between H2O fluid and melt (H2O425 and50 wt %) evolved from quartzites, probably generated by dehydration reactions. (2) During early decompression stages, at the transition from UHP to high-pressure (29 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 P52 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.