Fluid Evolution during HP and UHP Metamorphism in Dabie Shan, China: Constraints from Mineral Chemistry, Fluid Inclusions and Stable Isotopes

JOURNAL OF PETROLOGY VOLUME 43 NUMBER 8 PAGES 1505–1527 2002 Fluid Evolution during HP and UHP Metamorphism in Dabie Shan, China: Constraints from Mineral Chemistry, Fluid Inclusions and Stable Isotopes YILIN XIAO1, JOCHEN HOEFS1∗, ALFONS M. VAN DEN KERKHOF1, KLAUS SIMON1, JENS FIEBIG1,2 AND YONG-FEI ZHENG3 1 GÖTTINGEN ZENTRUM GEOWISSENSCHAFTEN, GOLDSCHMIDTSTRASSE, D-37077 GÖTTINGEN, GERMANY 2 INSTITUT DE MINÉRALOGIE, BFSH 2, CH-1015 LAUSANNE, SWITZERLAND 3 DEPARTMENT OF EARTH AND SPACE SCIENCES, UNIVERSITY OF SCIENCE AND TECHNOLOGY OF CHINA, HEFEI, ANHUI 230026, P.R. CHINA RECEIVED APRIL 23, 2001; REVISED TYPESCRIPT ACCEPTED JANUARY 29, 2002 The Dabie Shan ultrahigh-pressure (UHP) metamorphic terrane is located in the eastern part of the east–west-striking Qinling–Dabie orogenic belt in China. A major mylonitized contact zone of 200–300 m width divides Dabie Shan into the South Dabie Terrane (SDT) and the North Dabie Complex (NDC). Combined investigation of major and trace element geochemistry, fluid inclusions, and oxygen and hydrogen isotopes constrains the fluid history during the metamorphic evolution of the two metamorphic belts, which differ in their fluid and metamorphic evolution. Fluid inclusions in rocks from the SDT are mainly aqueous with varying salinities, whereas those from the NDC are dominated by CO2. Low
18O values in the SDT rocks (−2·8 to 8·6‰) indicate meteoric water–rock interactions before UHP metamorphism, whereas rocks from the NDC show ‘normal’
18O values (6·7–9·0‰) with no obvious meteoric water–rock signature. Whole-rock rare earth element (REE) contents correlate with oxygen isotope compositions: samples from the SDT have higher REE contents and lower
18O values, whereas samples from the NDC have lower REE contents and higher
18O values. During retrograde metamorphism fluids with different hydrogen isotope compositions interacted with the rocks from the SDT. INTRODUCTION metamorphism; fluid inclusions; oxygen and hydrogen isotopes; REE elements; Dabie Shan The discovery of coesite and micro-diamond as phases produced by deep metamorphism of continental crust revolutionized our understanding of continental collision zones and mantle dynamics attending subduction of continental lithosphere. In general, we define metamorphic rocks with coesite and micro-diamond as ultrahigh-pressure (UHP) rocks, which have been increasingly recognized. So far more than a dozen UHP terranes have been documented within the major continental collision belts in the Eurasian Plate. The Dabie–Sulu belt in east–central China is the largest among the UHP metamorphic terranes found worldwide. Research activities over the past decade have documented a number of characteristic features of this area, including rapid subduction followed by rapid initial uplift (e.g. Li et al., 1993; Xiao & Li, 1993; Eide et al., 1994), the abundance of hydroxyl-bearing UHP mineral phases (Okay, 1994; Zhang et al., 1995), very low
18O values ranging from −15 to 10‰ (Yui et al., 1995, 1997; Zheng et al., 1996, 1998, 1999; Xiao et al., 1997; Rumble & Yui, 1998), and the world-record highest Nd (0) values ever measured for eclogite ( Jahn et al., 1996). The nature and mobility of fluids in high-pressure (HP) and UHP metamorphic terranes is a subject of discussion ∗Corresponding author. Telephone: 49 551 393986. Fax: 49 551 393982. E-mail:  Oxford University Press 2002 KEY WORDS: UHP JOURNAL OF PETROLOGY VOLUME 43 (for review, see e.g. Philippot & Rumble, 2000). Thompson (1992) postulated that substantial quantities of H2O can be transported to depths >100 km in the form of hydrous minerals, including phengite, clinohumite and epidote–zoisite. Experimental studies and thermodynamic calculations have demonstrated that lawsonite and phengite are able to store H2O below 200 km in cold subduction zones (Poli & Schmidt, 1995). These studies show that H2O may play an important role during UHP metamorphism. In this study, we combined petrological observations, major and trace element analysis, fluid inclusion study, and stable isotope measurements to characterize the metamorphic fluid architecture of the Dabie Shan area. Specifically, we sought to: (1) characterize fluid compositions generated in rocks that have been formed at great depths in the Earth; (2) determine the nature and extent of the pre-, syn- and post-peak metamorphic fluid–rock interactions during HP and UHP metamorphism in Dabie Shan; (3) understand the regionaland submillimetre-scale isotope systematics of the metamorphic rocks; (4) compare the fluid histories between the South Dabie Terrane and the North Dabie Complex. GEOLOGICAL SETTING AND SAMPLE DESCRIPTIONS The Dabie–Sulu UHP metamorphic terrane is located in the eastern part of the east–west-striking Qinling–Dabie orogenic belt. It represents deep parts of a collision zone between the North China Block and the Yangtze Block. Most geologists assume that Dabie Shan and the Sulu region were linked before the Mesozoic, and subsequently the Sulu region was displaced northward >500 km by the NE-trending left-lateral Tan-Lu fault (Fig. 1a). On the basis of petrotectonic assemblages and a major mylonitized contact zone of 200–300 m width, the Dabie Shan terrane has been subdivided into the South Dabie Collision Terrane (SDT) and the North Dabie Complex (NDC) (Fig. 1b). The SDT is characterized by the occurrence of UHP metamorphic rocks. It consists mainly of quartzofeldspathic gneisses; eclogites, marbles, jadeite quartzite and ultramafic rocks occur as lenses, blocks and/or layers in the gneisses. Coesite and micro-diamond have been recognized as inclusions in minerals from eclogites and surrounding gneisses (Okay et al., 1989; Wang et al., 1989; Wang & Liou, 1991; Xu et al., 1992). On the basis of Sm–Nd analyses of eclogitic minerals, it has been proposed that UHP metamorphism and collisional events took place at 210–230 Ma (e.g. Li et al., 1993; Chavagnac & Jahn, 1996). This hypothesis has been supported by U–Pb zircon analyses (Ames et al., 1993; Rowley et al., 1997; Hacker et al., 1998). NUMBER 8 AUGUST 2002 The NDC consists mainly of granitic orthogneiss, migmatite, metasediments, and minor amphibolite, granulite and ultramafic rocks (Okay, 1993; Zhang et al., 1996). The occurrence of granulites and granulite-facies rocks in the NDC is a distinguishing feature compared with the SDT (e.g. Zhang et al., 1996). Biotite and hornblende from the orthogneiss yield Ar–Ar ages of 120–130 Ma (Hacker & Wang, 1995). Recent zircon studies for the orthogneiss gave ages of 125–138 Ma (Xue et al., 1997; Hacker et al., 1998). The metamorphic ages of the granulite or granulite-facies rocks, however, are still controversial. Caledonian ages, about 200 Myr prior to the UHP metamorphism of the SDT, have been suggested by Kröner et al. (1993), Yang & Jian (1998) and Zhai et al. (1998), whereas Pb–Pb isochron ages of 1998–2456 Ma have been also interpreted as the t (...truncated)