INTERMEDIATE ALKALI - ALUMINO-SILICATE AQUEOUS AOLUTIONS RELEASED BY DEEPLY SUBDUCTED CONTINENTAL CRUST: FLUID EVOLUTION IN UHP OH-RICH TOPAZ - KYANITE QUARTZITES FROM DONGHAI (SULU, CHINA)

Abstract

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 (2·9 GPa 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 (0·3 GPa and 400°C) on kyanite (I and II) and OH-rich topaz, respectively. These rocks may exhibit unusually low δ18O and δD 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: CaCl 2 -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 CO 2 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 3·5 GPa alkali–alumino-silicate aqueous solutions, with compositions intermediate between H 2 O fluid and melt (H 2 O > 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 % H 2 O) 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 CO 2 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.