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)

JOURNAL OF PETROLOGY VOLUME 48 NUMBER 6 PAGES 1219^1241 2007 doi:10.1093/petrology/egm015 Intermediate Alkali^Alumino-silicate Aqueous Solutions Released by Deeply Subducted Continental Crust: Fluid Evolution in UHP OH-richTopaz^Kyanite Quartzites from Donghai (Sulu, China) MARIA LUCE FREZZOTTI1*, SIMONA FERRANDO2, LUIGI DALLAI3 AND ROBERTO COMPAGNONI2 DIPARTIMENTO DI SCIENZE DELLA TERRA, UNIVERSITA' DEGLI STUDI DI SIENA, VIA LATERINA 8, I-53100 SIENA, 1 ITALY 2 DIPARTIMENTO DI SCIENZE MINERALOGICHE E PETROLOGICHE, UNIVERSITA' DEGLI STUDI DI TORINO, VIA VALPERGA CALUSO 35, I-10125 TORINO, ITALY 3 CNR^IGG, ISTITUTO DI GEOSCIENZE E GEORISORSE, VIA G. MORUZZI 1, I-56124 PISA, ITALY RECEIVED APRIL 7, 2006; ACCEPTED MARCH 8, 2007 ADVANCE ACCESS PUBLICATION APRIL 17, 2007 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 730^8208C) with the anhydrous assemblage coesite þ kyanite I, followed by an early near-isothermal decompression stage (2
9 GPa and 705^7808C) with growth of kyanite II, muscovite, and OH-rich topaz, and by decompressioncooling stages, represented by paragonite (1
9 GPa and 700^7808C) and pyrophyllite (0
3 GPa 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 3
5 GPa alkali^alumino-silicate aqueous solutions, with compositions *Corresponding author. Telephone: (þ39)0577 233929. Fax: (þ39)0577 233938. E-mail: intermediate between H2O fluid and melt (H2O425 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 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. KEY WORDS: ultrahigh-pressure metamorphism; OH-rich topaz; fluid inclusions; stable isotopes; supercritical liquids I N T RO D U C T I O N Numerous discoveries of coesite and diamond in regional ultrahigh-pressure (UHP) rocks have demonstrated that crustal material can be subducted to mantle depths
The Author 2007. Published by Oxford University Press. All rights reserved. For Permissions, please e-mail: journals.permissions@ oxfordjournals.org JOURNAL OF PETROLOGY VOLUME 48 (Chopin, 1984; Smith, 1984; Sobolev & Shatsky, 1990), and provided new understanding of subduction and continental collision processes. The presence of aqueous fluids ( CO2, N2, etc.) at high-pressure (HP) and UHP conditions was recognized as the driving mechanism for metamorphic reactions, and ultimately for melting in crustal lithologies subducted to mantle depth (e.g. Poli & Schmidt, 2002). Our knowledge of fluid chemical properties is poor, yet is critical for understanding the potential concentration and transport of elements in the mantle wedge during subduction. One of the open questions concerns the nature and the amount of the chemical species in solution: moderate- to low-salinity aqueous fluids vs concentrated solutions, or hydrous melts. One approach to obtaining information on the nature of fluids evolving from deep-subducted rocks is provided by fluid inclusion analysis combined with stable isotope geochemistry of natural UHP rocks, although the study of inclusions in these rocks is not an easy task. Peak metamorphic conditions are largely outside the isochore fields even for the densest fluids, and the exhumation P^T paths of the host rocks strongly favor decrepitation of the early trapped fluid inclusions (Touret, 2001). Despite these potential problems, a number of documented examples show that fluid inclusions can be preserved, providing valuable information on the composition of UHP fluids, and, to some extent, on their evolution (for reviews, see Scambelluri & Philippot, 2001; Touret & Frezzotti, 2003; Ferrando et al., 2005a). High-salinity aqueous fluid inclusions are often observed in UHP minerals. For example, in the UHP rocks from the Alps, fluid inclusions are characterized by high amounts of NaCl and MgCl2, and subordinate concentrations of CaCl2 and KCl (up to 50 wt % NaCl equiv.; Philippot & Selverstone, 1991; Selverstone et al., 1992; Philippot et al., 1995; Scambelluri et al., 2001). To explain the NaCl-dominated nature of such HP solutions, Scambelluri et al. (1997) advocated recycled sea-water, Cl and alkalis, whereas Philippot et al. (1998) suggested that Cl-rich inclusions are derived from hydrothermal alteration of the oceanic lithosphere. More recently, Sharp & Barnes (2004) presented a model for the generation of brines, via breakdown of subducted serpentinites, forming mobile high-salinity aqueous plumes at mantle depths. In the Dabie-Shan and Sulu UHP continental metamorphic rocks, fluids preserved within inclusions are also aqueous and salt-rich, but generally CaCl2-dominated, and not NaCl-rich as would be expected if their ultimate origin was from past sea-water (Xiao et al., 2000, 2001; Fu et al., 2001, 2002, 2003; Zhang et al., 2005b). Xiao et al. (2000) and Fu et al. (2001, 2003) described Ca-rich brines (N2), which may have originated during prograde and peak metamorphism. Zhang et al. (2005b) reported a spatial and temporal reconstruction of fluid composition NUMBER 6 JUNE 2007 within a vertical sequence of UHP rocks of different composition, and recognized primary CaCl2^NaCl-rich brines as peak fluids in both eclogite and quartzite lithologies. As stable isotope data showed that brines are internally derived, Fu et al. (2003) proposed that they represent significant amounts of meteoric water brought to mantle depths through continental collision. Fluids reveal UHP metamorphism with limited fluid mobility during subduction, peak metamorphism, and exhumation. Whereas previous studies indicated a substantial e (...truncated)