Journal of Petrology Advance Access originally published online on April 17, 2007
Journal of Petrology 2007 48(6):1219-1241; doi:10.1093/petrology/egm015
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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)
1Dipartimento Di Scienze Della Terra, Università Degli Studi Di Siena, Via Laterina 8, I-53100 Siena, Italy
2Dipartimento Di Scienze Mineralogiche E Petrologiche, UniversitÀ degli studi di torino, Via Valperga Caluso 35, I-10125 Torino, Italy
3CNR–IGG, Istituto Di Geoscienze E Georisorse, Via G. Moruzzi 1, I-56124 Pisa, Italy
RECEIVED APRIL 7, 2006; ACCEPTED MARCH 8, 2007
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Abstract |
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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 
18O and D 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 3·5 GPa 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.
KEY WORDS: ultrahigh-pressure metamorphism; OH-rich topaz; fluid inclusions; stable isotopes; supercritical liquids
*Corresponding author. Telephone: (+39)0577 233929. Fax: (+39)0577 233938. E-mail: frezzottiml{at}unisi.it
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