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Journal of Petrology | Volume 43 | Number 8 | Pages 1505-1527 | 2002
© Oxford University Press 2002
Fluid Evolution during HP and UHP Metamorphism in Dabie Shan, China: Constraints from Mineral Chemistry, Fluid Inclusions and Stable Isotopes
1GÖTTINGEN ZENTRUM GEOWISSENSCHAFTEN, GOLDSCHMIDTSTRASSE, D-37077 GÖTTINGEN, GERMANY
2INSTITUT DE MINÉRALOGIE, BFSH 2, CH-1015 LAUSANNE, SWITZERLAND
3DEPARTMENT 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
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONThe 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. KEY WORDS: UHP metamorphism; fluid inclusions; oxygen and hydrogen isotopes; REE elements; Dabie Shan
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INTRODUCTION |
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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 (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 regional- and submillimetre-scale isotope systematics of the metamorphic rocks; (4) compare the fluid histories between the South Dabie Terrane and the North Dabie Complex.
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GEOLOGICAL SETTING AND SAMPLE DESCRIPTIONS |
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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).
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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).
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 time of peak metamorphism for these granulites (e.g. Jian et al., 1999). Eclogite-facies metamorphism (240 Ma, Li et al., 1993) and a subsequent granulite-facies overprint have been demonstrated by Xiao et al. (2001) for some rocks from the NDC (see also Tsai & Liou, 2000). The ages of the granulites suggest that there might be different episodes of granulite-facies metamorphism, respectively before and after the eclogite-facies metamorphism in the NDC. In this study we infer that the term ‘granulite-facies metamorphism in the NDC’ indicates only an overprint process after the eclogite-facies metamorphism.
To compare the fluid histories of the NDC and the SDT, six representative localities were selected. Samples from the SDT include coesite and diamond-bearing eclogite from Bixiling (1), coesite-bearing eclogite from Shima (2), UHP jadeite quartzite and eclogite from Shuanghe (3), and ‘cold’ eclogite from Lidu (4). Samples from the NDC include eclogite from Raobazhai (5) and granulite from Yanzihe (6). The locations are numbered in Fig. 1b. The modes of samples that have been studied in detail are listed in Table 1.
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Samples from the SDT
The Bixiling UHP eclogites (samples DB02, DB05, DB07 and DB10)
The Bixiling complex is the largest coesite-bearing eclogitic body (1·5 km2) in the Dabie–Sulu area. It occurs as a tectonic block in foliated quartzofeldspathic gneiss and mainly consists of ‘fresh’ eclogite, kyanite-rich eclogite, retrograded eclogite and elongated lenses of meta-ultramafic rocks. The contacts between different types of eclogites, including the ultramafic rocks, are gradual.
Sampling was carried out along a north–south road section across the eclogitic rock body and four samples were selected for detailed studies. Samples DB05 and DB07 were collected from the central part of the rock body; the former is a ‘fresh’ eclogite without kyanite, whereas the latter contains coarse-grained kyanite. Sample DB02 was collected at the margin of the rock body and shows strong retrograde signs. Sample DB10 is a garnet peridotite from an elongated lens of meta-ultramafic rock. Petrographic characteristics have been described in detail by Xiao et al. (2000).
The coesite-bearing eclogite from Shima (sample DB63)
Coesite-bearing eclogitic rocks are well developed in the Shima region and occur as aggregate layers or boudins in garnet-bearing biotite gneisses and as boudins in impure marbles. Most coesite-bearing eclogites in this area range from centimetres to >1 m in size. The investigated samples from Shima (DB63) are medium grained and granular in texture, and contain a typical eclogite assemblage of garnet, omphacite and rutile. Garnet is most abundant, representing >50 vol. %. Most garnet grains range from 1 to 2 mm in diameter and sometimes contain a few inclusions of omphacite and coesite. Omphacite is very fresh without visible retrogression.
The Shuanghe UHP metamorphic slab (samples DB29, DB31 and DB32)
The UHP metamorphic rocks from Shuanghe occur as a thrust slab, which was offset by a dextral strike-slip fault (Cong et al., 1995). Three representative samples were selected for the study, including coesite-bearing jadeite quartzite (DB31), eclogite boudin in marble (DB29) and massive eclogite in gneiss (DB32). The jadeite quartzite consists of 30–50 vol. % jadeite, 40–55% quartz, 5% garnet and minor rutile. Jadeite and quartz range from 1 to 4 mm in size, whereas garnets are finer grained (0·5–1·5 mm). Jadeite crystals are successively rimmed by thin coronas (0·1–0·2 mm wide) composed of fibrous oligoclase (inner rim), albite (outer rim) and minor Ca-clinopyroxene. Inclusions of quartz pseudomorphs after coesite can be observed both in garnet and jadeite. Sample DB29 was collected from an eclogite boudin 30 cm in diameter enclosed in relatively pure white marble. It consists of garnet + omphacite + quartz + rutile + minor carbonate. Omphacite is usually replaced by amphibole, plagioclase and Ca-Cpx. Sample DB32 is garnet and quartz rich (up to 52 and 26%, respectively); later-stage amphibole is well developed; inclusions of quartz pseudomorphs after coesite with radial fractures occur in garnet.
‘Cold’ eclogite from Lidu (samples DB44, DB45 and DB48)
Okay (1993) found that eclogites from Huangzhen are different from eclogites from other localities, and suggested two contrasting eclogite terranes in the Dabie Shan: ‘hot eclogite’ and ‘cold eclogite’ terranes. The most important argument for this division is the absence of coesite and micro-diamond inclusions in ‘cold’ eclogite.
The largest ‘cold’ eclogite lens is exposed on the south side of Lidu village near Huangzhen. The eclogite crops out in an area of about 500 m x 150 m. The host rock is a two-mica–epidote–garnet–plagioclase gneiss. Megascopic isoclinal and open folds are found in the gneiss, showing subhorizontal NW–SE-trending axes (Castelli et al., 1998). The Lidu eclogite contains garnet + omphacite + kyanite + phengite + quartz + paragonite + rutile ± epidote ± amphibole ± feldspar. Garnets are poikiloblastic and range from 2 to 10 mm in size. Omphacite in Lidu eclogite is relatively fine grained (<1 mm) compared with garnet. Most omphacite is rimmed by symplectic clinopyroxene, amphibole and albite as a result of retrograde metamorphism. Kyanite is stable with garnet, omphacite and phengite; coarse-grained kyanite is common in the matrix and sometimes replaced by coarse-grained aggregate of paragonite, which, however, is locally replaced by a fine-grained corundum + albite symplectite. Phengite is found as inclusions in garnet and in the matrix. Other phases in Lidu eclogite include epidote, amphibole, rutile, apatite, quartz, talc and corundum.
Samples from the NDC
The eclogite from Raobazhai, NDC (sample R-14)
The Raobazhai meta-ultramafic complex is located at the southern Foziling Reservoir near Huoshan in the NDC, with an outcrop area of 3 km in length and 0·2–0·9 km in width. It consists mainly of Cr-spinel harzburgite, dunite and lenses of eclogite (garnet pyroxenite). On the basis of the compositional zonations in pyroxene and garnet, Xiao et al. (2001) concluded that the Raobazhai complex has been subjected to eclogite-facies metamorphism followed by a granulite-facies overprint during subsequent uplift. Samples from the Raobazhai eclogite have been described in detail by Xiao et al. (2001).
The granulite from Yanzihe, NDC (sample Y01)
The felsic granulite from the Yanzihe region is hosted by gneiss of upper amphibolite-facies metamorphism. It is principally composed of garnet (34%), quartz (36%), hypersthene (15%), amphibole (7%), magnetite (5%) and chlorite (2%). Accessory minerals include apatite and minor ilmenite. Garnets are porphyroblasts (2–8 mm in diameter) with a few inclusions of quartz that occur in cores of garnets. Hypersthene is usually smaller than 2 mm and very rarely includes some other minerals.
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ANALYTICAL METHODS |
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Major elements in the various mineral phases were determined by electron-microprobe analysis using a JXA-8900RL Jeol Superprobe. The analyses were performed at 15·0 kV accelerating voltage, 12 nA beam current, 1–5 µm probe diameter and 15–30 s counting time on the peak of all measured elements. Standards included silicates and pure oxides. Raw data were corrected by the CITZAF method of Armstrong (1991). Electron-microprobe analysis was also performed after UV-laser and laser-inductively coupled plasma mass spectrometry (laser-ICP-MS) measurements, to ensure oxygen isotope and rare earth element (REE) analyses of representative mineral compositions.
To reveal and image two-dimensional compositional inhomogeneities of major elements (Fe, Mn, Mg, Ca, Si, Al, etc.) in garnet, pyroxene and amphibole, digital element distribution maps (DEDMs) were produced by means of electron microprobe and subsequent digital image processing. Step lengths of the scans were set at 1–35 µm, depending on the desired spatial resolution and size of the investigated domains.
REE in single minerals were determined using a 266 mm Nd:YAG laser ablation ICP-MS system (Fisons UV Microprobe). The ablated particles were transported by an argon gas stream into the VG PlasmaQuad 2 + ICP-MS system. Electron microprobe analysis of Mn for garnet and clinopyroxene was used as an internal standard for laser ablation ICP-MS. The experimental procedure has been described by Simon et al. (1997). The results are considered accurate to within 15–20%.
Fluid inclusions were investigated by using a Linkam heating–freezing stage, provided with a video system for ease of observation. Measurements were made on doubly polished unmounted sections of 200 µm thickness. The stage was calibrated by a set of synthetic fluid inclusion standards.
Samples for oxygen isotope analysis were prepared as polished thick sections [15 mm x 10 mm x 2 mm]. The sections were cleaned with distilled water using an ultrasonic device and then vacuum dried overnight at 300°C. In situ oxygen isotope analysis was performed by ArF-laser fluorination (Fiebig et al., 1999; Wiechert et al., 2002). Spatial resolution varied between 250 and 350 µm, mostly depending on focal properties. As a result of explosive ablation, spatial resolution for the measurements in quartz was 500 µm. The analytical error is better than 0·2 (Fiebig et al., 1999).
Hydrogen isotope analyses were performed by vacuum fusion with inductive heating to 1300°C, conversion of liberated H2O to H2 over uranium at 800°C, and adsorption of the H2 on activated charcoal. Hydrogen isotope ratios are normalized to the V-SMOW-SLAP scale, and precision is about ±3.
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MINERAL CHEMISTRY |
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On the basis of whole-rock data, eclogites are mafic (basalt, high-Al basalt and gabbros), ultramafic rocks are cumulates or mantle slices, jadeite quartzite is a metasediment, and protolith of granulite from Yanzihe may be a dacite (geochemical data are available on request). In the following we present data for garnet, clinopyroxene and amphibole, the three most common minerals in the investigated samples. Detailed data for the three minerals and for other mineral phases are available on request.
Garnet
Garnets show a large compositional range of Alm24–66, Prp10–57, Grs10–39 and Sps1–5. Systematic differences have not been observed between the samples from the SDT and the NDC (Fig. 2). The DEDMs (Fig. 3) display different zoning patterns. For garnet from Lidu (Fig. 3a), Mg shows a distinct increase in concentration, whereas Fe decreases towards the rim of the crystal; Mn shows a peak in the centre of the crystal, a steady decrease towards the rim and an enrichment just at the rim; variations of Ca are very pronounced, and sometimes irregular—this feature probably indicates disturbances from fluids during garnet growth. DEDMs for a twin-garnet in the same section show a different compositional pattern: the rim and the core have similar MgO and MnO contents, which are clearly higher than those of the relics of the original central portion (Fig. 3b).
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A 9 mm garnet porphyroblast from Lidu shows REE patterns that can be related to major element and oxygen isotope zonings (see Fig. 9, below). In contrast to garnet in the other samples, REE concentrations of garnet from Lidu are higher in the rim than in the core; this probably resulted from fluid–mineral interaction during garnet growth (see Discussion).
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Texturally different garnets from the Yanzihe granulite show variable REE patterns: garnet with homogeneous major element compositions shows more or less the same light REE (LREE) concentrations and a slight decrease of heavy REE (HREE) from core to rim, whereas garnet with secondary amphibole veins shows much higher LREE but similar HREE concentrations in portions close to the amphibole veins; this agrees with the oxygen isotope mapping of the same grain, which revealed lower 18O values for portions near the veins (see Fig. 10, below).
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Clinopyroxene
Representative compositions of clinopyroxene are plotted in the Jd–Ac–(Di + Hd) diagram (Fig. 4). The clinopyroxenes are impure jadeite in the jadeite quartzite from Shuanghe, and omphacite in eclogites from Shuanghe and Shima. In retrograded eclogite from Bixiling, the omphacite inclusions in garnet have higher jadeite contents (>Jd60) than coarse-grained omphacite coexisting with garnet (Jd55–60). A few omphacite inclusions have significant K2O contents, indicating very high formation pressures (Xiao et al., 2000). Clinopyroxene grains in the eclogite from Raobazhai show strong compositional zoning with an omphacitic core and a diopsidic rim (Xiao et al., 2001). All analysed clinopyroxenes are poor in TiO2 (<0·2%), Cr2O3 (<0·2%) and MnO (<0·1%).
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Amphibole
All samples contain amphiboles in variable proportions with a wide range in composition corresponding to edenite, hornblende and pargasite fields according to the classification of Leake et al. (1997). Compositions of amphiboles in HP and UHP metamorphic rocks may vary with metamorphic conditions (e.g. Tropper et al., 2000). As shown in Fig. 5, amphiboles from the SDT and the NDC are distinctly different in the AlVI and AlIV contents, probably reflecting formation conditions during eclogite–amphibolite- and granulite-facies metamorphism.
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FLUID INCLUSION STUDIES |
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Fluid inclusions preferentially formed in quartz, garnet, omphacite and kyanite. Six types of fluid inclusions were identified and include low-salinity aqueous inclusions (in all investigated samples), high-salinity aqueous inclusions (DB02, DB05, DB07, DB63, R14), Ca2+ (Mg2+)-rich brines (DB07), N2-bearing aqueous inclusions (R14), carbonic inclusions (DB02, DB31, R14, Y01) and H2O–CO2 inclusions (DB31, R14, Y01). These fluid inclusion types can be clearly distinguished by textural criteria (see below). The abundances of the various fluid inclusion types for the studied rocks from the SDT and the NDC are summarized in Table 2. Examples of typical occurrences and textural settings of fluid inclusions are shown in Fig. 6. Microthermometric results are shown in Fig. 7.
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Fluid inclusions in rocks from the SDT
In Bixiling, four types of fluid inclusions were distinguished: (1) Ca-rich brines; (2) high-salinity NaCl-dominated aqueous inclusions; (3) low-salinity aqueous (or almost pure water) inclusions; (4) carbonic inclusions. Ca-rich brines (L + V or sometimes S + L + V; where L indicates liquid, V indicates vapour and S indicates solid) were found mainly in quartz blebs in kyanite, and sometimes in kyanite itself (DB07). Eutectic melting temperatures are lower than -40°C and indicate additional CaCl2 and/or MgCl2. Final ice melting in L + V inclusions occurs between -38 and -21°C, and homogenization between 132 and 243°C. High-salinity NaCl-dominated aqueous inclusions (L + V or sometimes S + L + V) in omphacite and kyanite (DB02, DB05, DB07) occur as tubes that are oriented parallel to the growth zones of the host mineral, suggesting a primary origin. Final ice melting in L + V inclusions occurs between -27 and -17°C, and homogenization between 97 and 440°C (Fig. 7a). Low-salinity aqueous inclusions (sometimes almost pure water) occur in matrix quartz and were observed only in the retrograded eclogite (DB02); most of the inclusions formed in healed fractures, indicating their secondary origin; the ice melting temperatures of -6 to 0°C correspond to low salinity. Carbonic inclusions were rarely found in omphacite and in matrix quartz of retrograded eclogite; the few measured carbonic inclusions in omphacite display CO2 melting temperatures around -58·5°C and homogenized to liquid at temperatures between -31 to -25°C, whereas those in matrix quartz have melting temperatures of -59·5 to -58·0°C and homogenization temperatures between -17 and -4°C.
In Shima, only high-salinity NaCl-dominated aqueous inclusions have been observed in quartz. They show in part regular morphologies, suggesting primary origin, and in part irregular morphologies, which are assumed to be the result of post-trapping modification (Fig. 6a). Final ice melting and homogenization temperatures are -22 to -11°C and 159–297°C, respectively.
Jadeite and garnet in jadeite quartzite from Shuanghe are generally devoid of fluid inclusions, which are abundant, however, in quartz (Fig. 6b and c). On the basis of textural criteria, three generations of aqueous inclusions can be distinguished:
- low-salinity aqueous inclusions in quartz blebs in jadeite occur isolated and as clusters, and have rounded or negative crystal shapes. They are monophase or two-phase (L + V) inclusions with fill degrees of 80–95 vol. %.
- Low-salinity inclusions in coarse-grained quartz occur isolated or scattered in groups and clusters, and are interpreted as primary. The groups and clusters are often cut by trails of fluid inclusions. These inclusions are either monophase or two-phase with fill degrees of 80–90 vol. % at room temperature.
- Secondary aqueous inclusions in coarse-grained matrix quartz mostly occur in healed fractures. In the same quartz grains inclusions with CO2 contents of 10 to >90 vol. % have been observed. These inclusions occur in microfractures, suggesting relatively late trapping.
Fluid inclusions in the ‘cold’ eclogites from Lidu are not as abundant as in the UHP eclogites. Low-salinity fluid inclusions in matrix quartz have oblate, roundish, negative crystal shapes, or irregular morphologies. They mostly occur in healed fractures. Other minerals are commonly devoid of fluid inclusions. The inclusions in the Lidu eclogite show ice melting temperatures between -6 and 0°C, and homogenization temperatures of 110–234°C (Fig. 7).
Fluid inclusions in the rocks from NDC
For eclogite from Raobazhai (R14), four types of fluid inclusions were found in garnet porphyroblasts by Xiao et al. (2001). These inclusions included: (1) high-salinity aqueous inclusions with or without N2, occurring in the cores of large garnet crystals; (2) carbonic inclusions, which represent the majority of fluid inclusions, found in rims of large garnet grains; (3) H2O–CO2 inclusions (fill degrees of water vary from 20 to 80%), which were rarely observed; (4) secondary low-salinity aqueous inclusions occurring in healed fractures.
In granulite from Yanzihe (Y01) fluid inclusions are moderately abundant in garnet and quartz blebs in garnet, and highly abundant in matrix quartz. Three generations of fluid inclusions were distinguished:
- H2O–CO2 inclusions with fill degrees of 10–80% in quartz blebs in garnet occur isolated or scattered in groups and clusters, and have rounded or negative crystal shapes, suggesting early primary formation.
- Carbonic inclusions (no visible water) in garnet and in matrix quartz (Fig. 6d). They are isolated or randomly distributed and assumed to be primary. Most inclusions contain carbonate crystals in addition to the fluid, indicating reaction between garnet and the inclusion fluids.
- Secondary H2O–CO2 inclusions with fill degrees of water from 20 to 90% in volume in matrix quartz (Fig. 6e). These fluid inclusions are bound to healed trans-granular microfractures. They show roundish or negative crystal morphologies.
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STABLE ISOTOPE ANALYSIS |
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Oxygen isotope compositions of rocks from the SDT
In Bixiling, garnets and clinopyroxenes from the ‘fresh’ eclogite (DB05), kyanite-rich eclogite (DB07) and garnet peridotite (DB10) have narrow
18O values ranging from 3·0 to 3·9; on the other hand, garnet and omphacite in retrograded eclogite (DB02) have 18O values of -1·8 to -1·2 and -1·1 to -0·6, respectively (Fig. 8). These 18O values are considerably lower than those of normal metamorphic rocks (Hoefs, 1997).
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18O values of minerals in the eclogite from Shima (DB63) fall within a narrow range from +6·4 to 6·6 for garnet and from +6·4 to 6·8 for omphacite. In the Shuanghe area, 18O values of minerals in the jadeite quartzite (DB31) vary from +9·2 to +9·7 for quartz, from +7·6 to 8·5 for jadeite and +7·3 to +7·8 for garnet, corresponding to whole-18O values of +8·8 (Fig. 8). Five points on the same jadeite grain show homogeneous 18O values between 8·2 and 8·5. 18O values of minerals in the eclogite in marble (DB29) are 11·0–11·5 for quartz, 8–8·5 for garnet and 8·2 for retrograde amphibole. The calculated whole-rock 18O value is 8·6. Quartz, garnet, omphacite and rutile in massive eclogites in biotite gneiss (DB32) have 18O values of 8·1–8·4, 4·5–5·2, 5·2–5·9 and 2·5–3·1, respectively, with a calculated whole-rock 18O value of 5·2.
Garnets from Lidu are concentrically zoned in oxygen isotopes. They have an 18O-depleted core and rim but an 18O-enriched central zone (Fig. 9a). Because mineral inclusions have been carefully avoided during laser ablation, and 18O variations in all three garnets are reproducible and remarkably systematic, isotopic zoning should reflect variations of O-isotope compositions during garnet growth. 18O values of the other minerals in the eclogite vary from -0·6 to -0·2 for quartz, from -3·1 to -3·6 for muscovite and from -3·5 to -3·3 for small garnet crystals in the matrix, and are -2·3 for kyanite, -1·6 for plagioclase and -1·7 for symplectite retrograded after omphacite. The calculated whole-rock 18O value is -2·8.
Oxygen isotope compositions of rocks from the NDC
In situ UV-laser oxygen isotope analysis was undertaken on garnet, pyroxene, amphibole and plagioclase from Raobazhai. The nearly homogeneous oxygen isotopic composition (18O 6·7, Fig. 8) in a garnet porphyroblast indicates closed fluid system conditions during garnet growth; isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet, however, show an oxygen isotopic disequilibrium, indicating retrograde fluid–rock interactions [see Xiao et al. (2001) for details].
The 31 analyses of garnets in the granulite from Yanzihe indicate that most garnets are homogeneous, with an average isotopic value of 9·1 ± 0·2 (1
), except five analyses that have 18O values from 6·8 to 8·8. These five analytical outliers are close to a healed fracture with Cl-rich amphibole within the garnet (Fig. 9b). REE laser-ICP-MS analyses indicate an LREE enrichment for the lower 18O spots. From microprobe and microscope data it can be concluded that these analytical spots with lighter oxygen isotopic compositions are definitely garnet rather than other minerals. One analysis of the amphibole vein indicates a 18O value of 7·5.
Hydrogen isotope analysis of eclogites from the SDT
Hydroxyl-bearing mineral separates from eclogites in Dabie Shan, including phengite, epidote (zoisite) and amphibole, were analysed by the conventional (U-reduction) method. Analysed samples are all from the localities in the SDT; suitable mineral separates from the NDC could not be obtained. As shown in Table 3, the total range of D values is between -100 and -53. Amphiboles show a narrow range between -100 and -92. Phengites have a range from -91 to -64. Epidote, with a D range between -62 and -53, is the most enriched phase in D.
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METAMORPHIC EVOLUTION AND P–T ESTIMATES |
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For the samples from the SDT, coesite inclusions in the eclogites from Bixiling and Shima, as well as in the jadeite quartzite and eclogites from Shuanghe (Cong et al., 1995; Zhang et al., 1995; Liou et al., 1997; own observations) indicate that these rocks have been subjected to peak metamorphic pressures of at least 30 kbar or even up to 50–70 kbar (Zhang et al., 1995; Xiao et al., 2000; Ye et al., 2000). On the basis of petrographic observations, a five-stage metamorphic evolution has been deduced for these rocks: (1) prograde eclogite-facies metamorphism, represented by minerals such as phengite and omphacite inclusions in garnet; (2) coesite eclogite-facies or UHP (peak) metamorphism characterized by the formation of coesite or other UHP minerals; (3) recrystallized eclogite-facies metamorphism, indicated by the formation of coarse-grained garnet and omphacite; (4) retrograde symplectite stage, represented by the symplectitic replacement assemblage; (5) retrograde amphibolite-facies stage demonstrated by coarse-grained amphiboles that occur as kelyphitic rims of garnet and symplectite. The last two retrograde metamorphic stages, however, are not developed in the Shima eclogite.
The ‘cold’ eclogite from Lidu shows a four-stage metamorphic evolution: (1) early prograde metamorphism, represented by epidote, rutile and micas as inclusions in garnet; (2) eclogite-facies metamorphism, demonstrated by the assemblage of garnet rim + omphacite + kyanite; the absence of coesite and the coexistence of kyanite + omphacite in the matrix indicate a peak metamorphic pressure of 20 kbar for the eclogite (Newton, 1986); (3) retrograde symplectite stage, well demonstrated by the hydrothermal breakdown of paragonite 
albite + corundum + H2O and by symplectites after omphacite and garnet; (4) retrograde amphibolite–greenschist-facies metamorphism stage, documented by secondary epidote, amphibole and sodic plagioclase.
Evidence of pressures >30 kbar for the samples from Raobazhai and Yanzihe in the NDC has not been found; this is consistent with the conclusion of most workers that the NDC was subjected to peak pressure <25 kbar (e.g. Wang & Liou, 1991; Li et al., 1993; Zhang et al., 1996). UHP indicator minerals in the NDC might have been obliterated by the high-temperature granulite-facies retrograde event; this, however, seems to be unlikely because coesite inclusions were also found in the strongly granulitized eclogites from Sulu (e.g. Wang et al., 1993; Nakamura & Hirajima, 2000), even though some of them have been subjected to significant fluid–rock interactions during metamorphism (e.g. Jahn et al., 1996). We thus assume that the rocks from the NDC have not been subjected to UHP metamorphism. Four stages were suggested for the metamorphic evolution of the Raobazhai eclogite (Xiao et al., 2001): (1) peak pressure eclogite-facies metamorphism; (2) recrystallized eclogite-facies metamorphism; (3) retrograde granulite-facies metamorphism; (4) retrograde amphibolite-facies metamorphism. On the other hand, three metamorphic stages can be assumed for the granulite from Yanzihe: garnet porphyroblasts, matrix hypersthene and quartz record granulite-facies conditions, whereas secondary amphibole is assumed to represent an early retrograde amphibolite-facies metamorphism; chlorite retrograded from amphibole is characteristic of the late retrograde metamorphism.
The P–T conditions for the multi-stage metamorphic evolution were estimated using various petrographic thermobarometers; the results are listed in Table 4 and shown in Fig. 12 (below). Temperature estimates based on oxygen isotopic fractionations between various phases are given also in Table 4 for comparison.
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Two important observations emerge from an inspection of the metamorphic P–T estimates:
- the peakP–T conditions for the four localities in the SDT show a consistent trend from 700°C, 20 kbar in the Lidu area in the south of the SDT, to 800–850°C, >29 kbar in the Shuanghe and the Shima areas, and to 1000°C, >40 kbar in the Bixiling area in the northern part of the SDT (Table 5). Wang et al. (1992) presented similar trends for other eclogite localities in the SDT. Thus metamorphic rocks in the northern margin of the SDT might have been subjected to metamorphic conditions involving much higher P and T than those in the southern margin, or, in other words, rocks from the north (e.g. Bixiling) might have been subducted to much greater depth than those in the south of the SDT (e.g. Lidu). This trend may indicate the subduction direction of the Yangtze craton.
- During the stage of initial uplift of the metamorphic rock in the NDC, especially in Raobazhai, there is a significant temperature rise. The P–T paths in the NDC are therefore very different from those of the UHP eclogite in the SDT, in which all the localities show nearly isothermal decompression paths following the maximum pressure (Fig. 12a, below) (see also Wang et al., 1992; Okay, 1993; Xiao & Li, 1993; Xiao et al., 1995). Thus the initial retrograde P–T path indicates that the NDC might be related to another tectonic environment.
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THE ROLE OF THE FLUID DURING METAMORPHIC EVOLUTION OF DABIE SHAN |
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Meteoric water–rock interaction before metamorphism?
Previous oxygen isotope studies in the Dabie–Sulu UHP metamorphic belt (e.g. Yui et al., 1997; Rumble & Yui, 1998; Zheng et al., 1998, 1999) have already indicated the well-known and sometimes enormous depletion in 18O. This is supported by our data. The very low
18O values of eclogites from Bixiling and Lidu can be explained only by isotope exchange with low 18O meteoric fluids. The 18O values of the Shima eclogite are slightly higher than primary mantle values and overlap with the low end of normal metamorphic rocks (Hoefs, 1997). This implies either that its protolith has not been affected by the meteoric water alteration before subduction (i.e. the 18O value of 6·6 represents the oxygen isotope composition of a basaltic protolith), or alternatively the protolith had primarily higher 18O values, which, as a result of a meteoric water–rock interaction, have been lowered to the values observed today. Zhang et al. (1998) measured lower 18O values of 1·5–4·5 and suggested fluid–rock interactions before the Triassic UHP metamorphism for a mafic–ultramafic complex that is very close to our investigated eclogite from Shima. Thus, the explanation that the protolith of the eclogite originally had higher 18O values appears reasonable. The jadeite quartzite in Shuanghe is a metasedimentary rock (Liou et al., 1997); its protolith would probably have primarily higher 18O values than 10 (e.g. Hoefs, 1997). We thus conclude that there was a meteoric water–rock interaction(s) before subduction in the SDT; such meteoric water–rock interactions, however, must have been variable from locality to locality. In contrast, the oxygen isotope compositions for the rocks from Raobazhai and Yanzihe in the NDC do not show obvious indications of water–rock interaction before subduction.
The conclusion that metamorphic rocks in the SDT underwent meteoric water–rock interactions before subduction but those in the NDC did not, is supported also by the present REE data. As shown in Fig. 11a, a negative correlation can be recognized between 18O values and whole-rock total REE (TREE) concentrations (the meta-sediment sample from Shuanghe has been excluded). Samples from Lidu and Bixiling, which have 18O values of -2·8 to 3·7, showing obvious indications for meteoric water–rock interaction, have higher TREE concentrations than the other samples. Although the eclogites from Shima show ‘normal’ 18O values, the higher REE content may indicate interactions with an REE-rich fluid before UHP metamorphism (no obvious retrograde metamorphic indicator exists in the studied sample). Such a negative correlation is also observed between O isotope and La/Yb ratios (Fig. 11b), indicating that this correlation is a primary feature. It is suggested that the meteoric water–rock interactions before subduction resulted in both depletion in the 18O content and enrichment of the LREE for the SDT. By contrast, eclogite (Raobazhai) and granulite (Yanzihe) from the NDC have much lower TREE concentrations and ‘normal’ 18O values, which do not support meteoric water–rock interaction before their metamorphism.
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Fluid system during peak metamorphism
Fluid inclusions in different textural settings reflect trapping and remobilization of fluids at different metamorphic stages. Compared with omphacite, which is a typical primary phase during UHP eclogite-facies metamorphism, kyanite is not irrefutably a primary mineral as it sometimes occurs in veins as well. However, in the investigated samples, kyanite occurs in the matrix together with omphacite and garnet without any chronological difference based on petrographic observations. In Dabie Shan, kyanite with inclusions of coesite is assumed to be a primary phase during UHP metamorphism (Zhang et al., 1995). Therefore, high-salinity NaCl-dominated fluid inclusions that occur as tubes in omphacite and kyanite are likely to be trapped during UHP conditions and hence represent the fluid composition during peak metamorphism, although fluid densities might have been modified during decompression.
The Ca-rich brines in quartz blebs in kyanite are assumed to represent the earliest recognizable fluid originating from prograde metamorphism. Radial fractures around the quartz blebs in the host kyanite indicate that these quartz blebs might once have been coesite, included by kyanite before or during UHP metamorphism. Recrystallization during the transformation of coesite to quartz might have destroyed early trapped fluid inclusions and the present inclusions would post-date the coesite to quartz transformation. However, because kyanite is an anhydrous mineral, an enclosed quartz inclusion is considered not to be in contact with external fluids. The coesite inclusions in kyanite (Zhang et al., 1995) support this assumption. The compositions of the retrapped fluid inclusions therefore may very well reflect the early prograde fluid.
Garnet porphyroblasts from the Raobazhai eclogite and Yanzihe granulite in the NDC have more or less homogeneous 18O values. The absence of 18O zoning in these garnets reflects either that diffusional processes were effective as a result of the high-temperature conditions experienced by the NDC rocks on the retrograde paths, or alternatively, the absence of infiltrating fluids during garnet growth. It is well known that diffusion rates in garnet are usually very low at metamorphic temperatures. Experimentally determined oxygen diffusion rates in garnet are 10–100 times lower than REE diffusion rates (Burton et al., 1995), which in turn are more than two orders of magnitude lower than Ca, Fe, Mg and Mn diffusion rates (Van Orman et al., 2002). Major element zoning has been preserved in the garnet from Raobazhai, which was subjected to temperatures of up to 900°C (Tsai & Liou, 2000; Xiao et al., 2001). It is unlikely that the high temperatures would destroy any possible oxygen zoning. On the other hand, infiltrating fluids present during garnet growth should cause an oxygen isotope gradient from the centre outwards to the rim (Chamberlain & Conrad, 1993). Given the homogeneity of 18O, external fluids infiltrating the eclogite and granulite during garnet growth can be excluded. In other words, fluid phases in the rocks of the NDC should be internally equilibrated during eclogite- and subsequent granulite-facies metamorphism, and hence indicate a closed fluid system during garnet growth.
By contrast, garnet in the ‘cold’ eclogite from Lidu shows systematic major element, oxygen isotope and REE zoning. Mg shows a distinct increase in concentrations, whereas Ca decreases from the core towards the rim of the garnet (Fig. 3). Meanwhile, the core and the rim are characterized by an enrichment in REE and a depletion in 18O, compared with the intermediate zone of the grain (Fig. 9).
It has been suggested that garnet growth usually begins at 400°C (e.g. Christensen et al., 1989). Temperatures calculated from quartz–garnet 18O data agree with temperatures calculated from cation thermometry (Table 4), indicating peak metamorphic temperatures of 700°C for the Lidu eclogite. Zheng (1993a) revealed a relatively small oxygen isotope fractionation of <0·2 between garnet and fluid at temperatures of 400–700°C, which cannot match the observed shifts in 18O values from the core to the central zone and from the central zone to the rim of the garnet. Cation substitution may affect the oxygen isotope compositions of garnet up to a maximum of 0·5 for the end-members (Kohn & Valley, 1998). On the basis of our microprobe data, the major element zoning pattern is characterized by compositional variation from core to rim with ranges of 62–50% for almandine, 16–37% for pyrope and 22–13% for grossular contents. These compositional variations, therefore, are too small to explain the observed 18O variations.
REE analyses of the same garnet indicate that REE concentrations in the rim are about three orders of magnitude higher than those in the core (Fig. 9b). It is possible that at lower grades the Ca for garnet growth is supplied by breakdown of epidote phases (as seen in numerous inclusions in the garnet). The Ca of the second-stage garnet overgrowth might result from breakdown of amphibole. These two phases show differences in REE contents that are inherited by the garnet.
The fact that most fluid inclusions in the Lidu eclogite are almost pure water inclusions within healed fractures indicates high water activity during its metamorphism. The Ca-rich cores of the atoll garnets must have been partly replaced at higher pressures during the influx of a fluid phase, which also produced new hydrous mineral phases such as talc, amphibole and kyanite (Fig. 3b), suggesting an active role of the fluid during metamorphism. The area around the Lidu eclogite is cross-cut by kyanite-rich quartz veins (e.g. Castelli et al., 1998), indicating a high-pressure fluid-rich overprint. Infiltration of fluids with different 18O values and REE contents into the Lidu eclogite during garnet growth, and/or the breakdown of pre-existing minerals (epidote, amphibole, etc.), are the most likely causes for the observed zoning patterns in garnet.
Indicators of retrograde fluid–rock interaction
Evidence for retrograde fluid–rock interactions comes from fluid inclusions, and O- and H-isotopes. As demonstrated above, oxygen isotope equilibrium has been deduced in the investigated samples from the SDT. In the NDC, however, negative fractionations of -1·7 to -0·5 between secondary minerals (amphibole and plagioclase) and garnet in Raobazhai, and of -1·9 between orthopyroxene and garnet in Yanzihe indicate isotope disequilibrium. This disequilibrium is interpreted as a result of fluid–rock interactions during the retrograde formation of these minerals. Because different minerals have different closure temperatures for oxygen isotope exchange (garnet being the most resistant, whereas minerals such as amphibole and plagioclase are more susceptible), differential isotopic exchange has taken place between the minerals and coexisting fluid. In addition, the fact that garnet from the Yanzihe granulite shows an 18O depletion of 2 and higher LREE, but similar HREE concentrations close to retrograde veins compared with the vein-free portions (Fig. 10), strongly supports fluid–rock interactions during retrograde metamorphism.
In the SDT, significantly lower 18O values and lower salinities of secondary fluid inclusions in matrix quartz of retrograded eclogite compared with fresh eclogite indicate a retrograde fluid–rock interaction at the margin of the Bixiling eclogites (Xiao et al., 2000). Secondary CO2-rich inclusions in jadeite quartzite from Shuanghe may be related to retrograde fluids from coexisting marbles (see also Fu et al., 2001). It is unlikely that the CO2 component resulted from metasedimentary protolith, because no primary carbonic inclusions have been observed in the rock. Secondary low-salinity aqueous inclusions in the Lidu eclogite indicate a low-salinity fluid during retrograde metamorphism. The source of this fluid is unclear so far. Retrograde fluid–rock interactions in eclogites from other UHP localities in the SDT have been also described by Yui et al. (1997) and Zheng et al. (1999).
The difference in D values among phengite, amphibole and epidote from the SDT probably originated from exchange with fluids having different D values. Petrological observations indicate that coarse-grained phengite in eclogite occurs as a primary phase (formation temperatures >700°C), whereas retrograde amphibole and epidote formed around 450°C. If the fractionation behaviour of phengite is assumed to be close to that of muscovite, hydrogen isotope fractionation at 700°C between phengite and water is about -10; at temperatures around 450°C, hydrogen isotope fractionation between epidote and water is about -35 (Sheppard, 1986). Thus, fluids responsible for the H-isotope compositions of phengite are expected to have D values of about -80, whereas those responsible for the epidote would have D values of about -25. D values of about -80 indicate the involvement of either meteoric water before subduction, or alternatively primary magmatic water (Hoefs, 1997). D values of about -25 suggest the presence of a metamorphic fluid during the retrograde formation of epidote. Experimental studies of hydrogen isotope exchange kinetics under hydrothermal conditions (Graham, 1981) imply that muscovite (phengite) has higher closure temperatures than zoisite (epidote). Therefore, phengite has probably preserved its hydrogen isotope composition of the peak metamorphic conditions, whereas epidote has preserved the hydrogen isotope composition of fluids present during retrograde metamorphism. In summary, various kinds of evidence strongly indicate a fluid–rock interaction on a regional scale during the retrograde metamorphism of Dabie Shan.
P–T–t–FLUID PATHS AND GEOLOGICAL IMPLICATIONS
On the basis of currently observed textures, fluid inclusion data, oxygen isotope compositions and P–T estimates, together with metamorphic ages from the literature (Ames et al., 1993; Li et al., 1993; Xiao et al., 1995; Chavagnac & Jahn, 1996; Xue et al., 1997; Hacker et al., 1998), P–T–t–fluid paths for the metamorphic rocks from the SDT and NDC are summarized in Fig. 12a and b, respectively. The present data clearly demonstrate different fluid histories for the SDT and the NDC, which require that the two tectonic units had separate histories. This contrasting feature can be interpreted by assuming that the SDT and the NDC belonged to the Yangtze and North China cratons, respectively, before the Triassic continent–continent collision. High-temperature and low-pressure conditions in the NDC in comparison with the SDT suggest that the NDC is not a thrust plane in the subducted continental crust of the Yangtze craton as proposed by Okay & Sengör (1992). Two-stage models suggest initial rapid exhumation to 40 km depth and a much slower uplift above 40 km for the UHP metamorphic rocks in the SDT (Xiao, 1991; Li et al., 1993; Chavagnac & Jahn, 1996). Li et al. (2000) showed that the UHP metamorphic rocks in Shuanghe uplifted rapidly to a depth of 30 km at 219 ± 7 Ma. It is possible that the SDT containing the UHP metamorphic rocks reached greater depths (e.g. >120 km?) during subduction and exhumed rapidly to shallow levels, whereas the NDC rocks, containing the hotter eclogites and granulites, reached lesser depths (e.g. <80 km?) but stayed longer there. Such an evolution would be consistent with a thermal event leading to the granulite-facies overprint in the NDC. However, because of the lack of a complete P–T–t path for the NDC rocks, the possibility that these rocks have experienced UHP metamorphism together with the SDT cannot be excluded; in this case, the NDC rocks would have been subjected to the same Triassic UHP metamorphism but different uplift processes after their peak metamorphism.
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CONCLUSIONS |
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- Fluids in the UHP metamorphic rocks of the SDT evolved from highly concentrated Ca-rich brines (prograde metamorphism) towards NaCl-dominated fluids (peak metamorphism), followed by low-salinity aqueous fluids during retrograde metamorphism. CO2 does not appear to be important during the metamorphic evolution of the SDT. By contrast, the fluid phase of the rocks from the NDC was dominated by CO2, being typical for the granulite facies. Although previous studies have shown that N2-rich solutions are typical peak metamorphic fluids in many eclogite localities worldwide, the present data indicate that nitrogen is not a major fluid phase in the rocks from Dabie Shan. This could be due to their continent–continent collision setting.
- Investigated samples in the Dabie Shan area show a negative correlation between 18O values and REE contents. The protoliths of the metamorphic rocks from the SDT have interacted with meteoric water on a regional scale before subduction, whereas those from the NDC might not have been affected by such a meteoric water–rock interaction. O and H isotope and fluid inclusion data imply retrograde fluid–rock interaction in both the SDT and the NDC, which was, however, limited and heterogeneous in the SDT, and perhaps regional in the NDC.
- Oxygen isotope distribution in garnet and fluid inclusion data indicate that fluids in both the SDT and the NDC reflect more or less closed-system conditions during peak metamorphism. Oxygen isotope and REE zoning in garnet from Lidu indicates fluid infiltration during garnet growth, thus the ‘cold’ eclogite may have had a different fluid evolution compared with the UHP eclogites in the SDT.
- Pressure and temperature calculations suggest different P–T paths for the rocks from the SDT and the NDC: rocks in the SDT have metamorphic peak temperatures that are synchronous with the maximum pressure along a nearly isothermal decompression path during early uplift, whereas those from the NDC show the maximum pressure that is followed by a decompression path with substantial heating. A consistent trend towards higher P–T conditions in the northern margin compared with the southern margin of the SDT may indicate the subduction direction of the Yangtze craton.
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ACKNOWLEDGEMENTS |
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This project was supported by grants from the DFG (HO 375/19). Y.X. thanks the Max-Planck-Society for a stipend during the period of 1997–1998. Many thanks are due to Drs G. Hartmann, A. Kronz and R. Przybilla, and Mrs. A. Reitz for analytical assistance. Constructive reviews by Professors P. J. O’Brien, P. Philippot and J. L. R. Touret, and editorial comments by Professor S. Harley, have significantly improved the clarity of presentation.
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FOOTNOTES |
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*Corresponding author. Telephone: 49 551 393986. Fax: 49 551 393982. E-mail: jhoefs{at}gwdg.de
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REFERENCES |
|---|
Ames, L., Tilton, G. R. & Zhou, G. (1993). Timing of collision of the Sino-Korean and Yangtze cratons: U–Pb zircon dating of coesite-bearing eclogites. Geology 21, 339–342.
Armstrong, J. T. (1991). Quantitative elemental analysis of individual microparticles with electron beam instruments. In: Heinrich, K. F. J. & Newbury, D. E. (eds) Electron Probe Quantitation. New York: Plenum, pp. 261–285.
Blundy, J. D. & Holland, T. J. B. (1990). Calcic amphibole equilibria and a new amphibole plagioclase geothermometer. Contributions to Mineralogy and Petrology 104, 208–224.[Web of Science]
Bohlen, S. R. & Boettcher, A. L. (1982). The quartz–coesite transformation: a pressure determination and the effects of other components. Journal of Geophysical Research 87, 7073–7078.
Burton, K. W., Kohn, M. J., Cohen, A. S. & O’Nions, R. K. (1995). The relative diffusion of Pb, Nd, Sr and O in garnet. Earth and Planetary Science Letters 133, 199–211.[Web of Science]
Castelli, D., Rolfo, F., Compagnoni, R. & Xu, S. (1998). Metamorphic veins with kyanite, zoisite and quartz in the Zhu-Jia-Chong eclogite, Dabie Shan, China. Island Arc 7, 159–173.
Chamberlain, C. P. & Conrad, M. E. (1993). Oxygen-isotope zoning in garnet: a record of volatile transport. Geochimica et Cosmochimica Acta 57, 2613–2629.[Web of Science]
Chavagnac, V. & Jahn, B.-M. (1996). Coesite-bearing eclogites from the Bixiling Complex, Dabie Mountains, China: Sm–Nd ages, geochemical characteristics and tectonic implications. Chemical Geology 133, 29–51.[Web of Science]
Christensen, J. M., Rosenfeld, J. L. & DePaolo, D. J. (1989). Rates of tectonometamorphic processes from rubidium and strontium isotopes in garnet. Science 244, 1465–1469.[Web of Science]
Cong, B. L., Zhai, M. G., Carswell, D. A., Wilson, R. N., Wang, Q. C., Zhao, Z. Y. & Windley, B. F. (1995). Petrogenesis of ultrahigh-pressure rocks and their country rocks at Shuanghe in Dabieshan, Central China. European Journal of Mineralogy 7, 119–138.
Deer, W. A., Howie, R. A. & Zussman, J. (1992). An Introduction to the Rock-forming Minerals, 2nd edn. Harlow, UK: Longman.
Eide, E. A., McWilliams, M. Q. & Liou, J. G. (1994). 40Ar/39Ar geochronology and exhumation of high- pressure to ultrahigh pressure metamorphic rocks in eastern–central China. Geology 22, 601–604.
Ellis, D. J. & Green, D. H. (1979). An experimental study of the effect of Ca upon garnet–clinopyroxene Fe–Mg exchange equilibria. Contributions to Mineralogy and Petrology 71, 13–22.[Web of Science]
Essene, E. J. & Fyfe, W. S. (1967). Omphacite in Californian metamorphic rocks. Contributions to Mineralogy and Petrology 15, 1–23.
Fiebig, J., Wiechert, U., Rumble, D. & Hoefs, J. (1999). High-precision in situ oxygen isotope analysis of quartz using an ArF laser. Geochimica et Cosmochimica Acta 63, 687–702.[Web of Science]
Fu, B., Touret, J. L. R. & Zheng, Y.-F. (2001). Fluid inclusions in coesite-bearing eclogites and jadeite quartzites at Schuanghe, Dabie Shan, China. Journal of Metamorphic Geology 19, 531–548.
Graham, C. M. (1981). Experimental hydrogen isotope studies. III. Diffusion of hydrogen in hydrous minerals and stable isotope exchange in metamorphic rocks. Contributions to Mineralogy and Petrology 76, 216–228.[Web of Science]
Graham, C. M. & Powell, R. (1984). A garnet–hornblende geothermometer; calibration, testing, and application to the Pelona Schist, Southern California. Journal of Metamorphic Geology 2, 13–31.[Web of Science]
Hacker, B. R & Wang, Q. C. (1995). 40Ar/39Ar geochronology of ultrahigh-pressure metamorphism in central China. Tectonics 14, 994–1006.[Web of Science]
Hacker, B. R., Ratschbacher, L., Webb, L., Ireland, T., Walker, D. & Dong, S. W. (1998). U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling–Dabie Orogen, China. Earth and Planetary Science Letters 161, 215–230.[Web of Science]
Harley, S. L. (1984). The solubility of alumina in orthopyroxene coexisting with garnet in FeO–MgO–Al2O3–SiO2. Journal of Petrology 25, 665–696.
Hoefs, J. (1997). Stable Isotope Geochemistry, 4th edn. Berlin: Springer-Verlag.
Holland, T. J. B. (1979). High water activities in the generation of high-pressure kyanite eclogites of the Tauern window, Austria. Journal of Geology 87, 1–27.[Web of Science]
Holland, T. J. B. (1980). The reaction albite = jadeite + quartz determined experimentally in the range 600–1200°C. American Mineralogist 65, 125–134.
Jahn, B. M., Cornichert, J., Cong, B. L. & Yui, T. F. (1996). Ultrahigh-Nd eclogites from an ultrahigh-pressure metamorphic terrane of China. Chemical Geology 127, 61–79.[Web of Science]
Jian, P., Yang, W. & Zhang, Z. (1999). 207Pb/206Pb zircon dating of the Huangtuling hypersthene–garnet–biotite gneiss from the Dabie Mountains, Luotian County, Hubei Province, China; new evidence for early Precambrian evolution. Acta Geologica Sinica (English Edition) 73, 78–83.
Kohn, M. J. & Spear, F. S. (1990). Two new barometers for garnet amphibolites with applications to southeastern Vermont. American Mineralogist 75, 89–96.[Abstract]
Kohn, M. J. & Valley, J. W. (1998). Effects of cation substitutions in garnet and pyroxene on equilibrium oxygen isotope fractionations. Journal of Metamorphic Geology 16, 625–639.[Web of Science]
Kretz, R. (1983). Symbols for rock-forming minerals. American Mineralogist 68, 277–279.[Abstract]
Kröner, A., Zhang, G. W. & Sun, Y. (1993) Granulites in the Tongbai area, Qinling belt, China: geochemistry, petrology, single zircon geochronology, and implications for the tectonic evolution of eastern Asia. Tectonics 12, 245–255.[Web of Science]
Leake, B. E., Woolley, A. R., Arps, C. E. S., et al. (1997). Nomenclature of amphiboles: Report from the Subcommittee on Amphiboles of the International Mineralogical Association, Commission on New Minerals and Mineral Names. American Mineralogist 82, 1019–1037.[Abstract]
Lee, H. Y. & Ganguly, J. (1988). Equilibrium compositions of coexisting garnet and orthopyroxene: experimental determinations in the system FeO–MgO–Al2O3–SiO2, and applications. Journal of Petrology 29, 93–113.
Li, S. G., Xiao, Y. L., Liu, D. L., Chen, Y. Z., Ge, N. G., Zhang, Z. Q., Sun, S. S., Cong, B. L., Zhang, R. Y., Hart, S. R. & Wang, S. S. (1993). Collision of the North China and Yangtse Blocks and formation of coesite-bearing eclogites: timing and processes. Chemical Geology 109, 89–111.[Web of Science]
Li, S. G., Jagoutz, E., Chen, Y. Z. & Li, Q. L. (2000). Sm–Nd and Rb–Sr isotopic chronology and cooling history of ultrahigh-pressure metamorphic rocks and their country rocks at Shuanghe in the Dabie Mountains, Central China. Geochimica et Cosmochimica Acta 64, 1077–1093.[Web of Science]
Liou, J. G., Zhang, R. Y. & Jahn, B. M. (1997). Petrology, geochemistry and isotope data on an ultrahigh-pressure jadeite quartzite from Shuanghe, Dabie Mountains, east–central China. Lithos 41, 59–78.[Web of Science]
Nakamura, D. & Hirajima, T. (2000). Granulite-facies overprinting of ultrahigh-pressure metamorphic rocks, northern Su-Lu region, eastern China. Journal of Petrology 41, 563–582.
Newton, R. C. (1986). Metamorphic temperature and pressure of group B and C eclogites. In: Evans, B. & Brown, E. (eds) Blueschists and Eclogites. Geological Society of America, Memoir 164, 17–30.
Okay, A. I. (1993). Petrology of a diamond and coesite-bearing metamorphic terrain: Dabie Shan, China. European Journal of Mineralogy 5, 659–675.
Okay, A. I. (1994). Sapphirine and Ti-clinohumite in ultra-high-pressure garnet-pyroxenite and eclogite from Dabie Shan, China. Contributions to Mineralogy and Petrology 116, 145–155.[Web of Science]
Okay, A. I. & Sengör, A. M. C. (1992). Evidence for intracontinental thrust-related exhumation of the ultrahigh-pressure rocks in China. Geology 20, 411–414.
Okay, A. I., Xu, S. T. & Sengor, A. M. C. (1989). Coesite from the Dabie Shan eclogites, central China. European Journal of Mineralogy 1, 595–598.
Perchuk, L. L. (1969). The effect of temperature and pressure on the equilibrium of natural iron–magnesium minerals. International Geology Review 11, 875–901.
Philippot, P. & Rumble, D. (2000). Fluid–rock interactions during high-pressure and ultrahigh-pressure metamorphism. International Geology Review 42, 312–327.[Web of Science]
Poli, S. & Schmidt, M. W. (1995). H2O transport and release in subduction zones: experimental constraints on basaltic and andesitic systems. Journal of Geophysical Research 100, 22299–22314.
Powell, R. (1985). Regression diagnostic and robust regression in geothermometer/geobarometer calibration: the garnet–clinopyroxene geothermometer revisited. Journal of Metamorphic Geology 3, 231–243.[Web of Science]
Raase, P. (1974). Al and Ti contents of hornblende, indicators of pressure and temperature of regional metamorphism. Contributions to Mineralogy and Petrology 45, 231–236.[Web of Science]
Råheim, A. & Green, D. H. (1975). P–T paths of natural eclogites during metamorphism—a record of subduction. Lithos 8, 317–328.[Web of Science]
Rowley, D., Xue, F., Tucker, R., Peng, Z. X., Baker, J. & Davis, A. (1997). Ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern Dabie Shan: U/Pb zircon geochronology. Earth and Planetary Science Letters 151, 191–203.[Web of Science]
Rumble, D. & Yui, T. F. (1998). The Qinglongshan oxygen and hydrogen isotope anomaly near Donghai in Jiangsu Province, China. Geochimica et Cosmochimica Acta 62, 3307–3321.[Web of Science]
Sheppard, S. M. F. (1986). Characterization and isotopic variations in natural waters. In: Valley, J. W., Taylor, H. P. & O’Neil, J. R. (eds) Stable Isotopes in High Temperature Geological Processes. Mineralogical Society of America, Reviews in Mineralogy, 16, 165–183.
Simon, K., Wiechert, U., Hoefs, J. & Grote, B. (1997). Microanalysis of minerals by laser ablation ICPMS and SIRMS. Fresenius Journal of Analytical Chemistry 359, 458–461.
Sun, S. S. & McDonough, W. F. (1989). Chemical and isotopic systematics of oceanic basalts: implications for mantle composition and processes. In: Saunders, A. D. & Norry, M. J. (eds) Magmatism in the Ocean Basins. Geological Society, London, Special Publications 42, 313–345.
Thompson, A. B. (1992). Water in the Earth’s upper mantle. Nature 358, 295–302.
Tropper, P., Manning, C. E., Essene, E. J. & Kao, L.-S. (2000). The compositional variation of synthetic sodic amphiboles at high and ultrahigh pressures. Contributions to Mineralogy and Petrology 139, 146–162.[Web of Science]
Tsai, C. H. & Liou, J. G. (2000). Eclogite-facies relics and inferred ultrahigh-pressure metamorphism in the North Dabie complex, central–eastern China. American Mineralogist 85, 1–8.
Van Orman, J. A., Grove, T. L., Shimizu, N. & Layne, G. D. (2002). Rare earth element diffusion in a natural pyrope single crystal at 2.8 GPa. Contributions to Mineralogy and Petrology 142, 416–424.[Web of Science]
Wang, Q. C., Ishiwatari, A., Zhao, Z., Hirajima, T., Enami, M., Zhai, M., Li, J. & Cong, B. L. (1993). Coesite-bearing granulite retrograded from eclogite in Weihai, eastern China. European Journal of Mineralogy 5, 141–152.
Wang, W. & Takahashi, E. (1999). Subsolidus and melting experiments of a K-rich basaltic composition to 27 GPa: implication for the behaviour of potassium in the mantle. American Mineralogist 84, 357–361.[Abstract]
Wang, X. & Liou, J. G. (1991). Regional ultrahigh-pressure coesite-bearing eclogitic terrain in central China: evidence from country rocks, gneiss, marble and metapelite. Geology 19, 933–936.
Wang, X., Liou, J. G. & Mao, H. K. (1989). Coesite-bearing eclogites from the Dabie Mountains, central China. Geology 17, 1085–1088.
Wang, X., Liou, J. G. & Maruyama, S. (1992). Coesite-bearing eclogites from the Dabie Mountains, Central China: petrology and P–T path. Journal of Geology 100, 231–250.[Web of Science]
Wiechert, U., Fiebig, J., Przybilla, R., Xiao Y. L. & Hoefs, J. (2002). Excimer laser isotope-ratio-monitoring mass spectrometry for in situ oxygen isotope analysis. Chemical Geology 182, 179–194.[Web of Science]
Wood, B. J. & Banno, S. (1973). Garnet–orthopyroxene and orthopyroxene–clinopyroxene relationships in simple and complex systems. Contributions to Mineralogy and Petrology 42, 109–124.[Web of Science]
Xiao, Y. L. (1991). P–T–t paths of eclogites from Dabie Shan, eastern China. Master’s thesis, University of Science and Technology of China, Hefei (in Chinese with English abstract).
Xiao, Y. L. & Li, S. G. (1993). P–T–t path and its tectonic implication for eclogite from the Dabie Mountains. Geotectonia et Metallogenia 17, 239–250 (in Chinese with English abstract).
Xiao, Y. L., Li, S. G., Jagoutz, E. & Cheng, W. (1995). P–T–t path for coesite-bearing peridotite–eclogite association in the Bixiling, Dabie Mountains. Chinese Science Bulletin 40, 156–158.
Xiao, Y. L., Fu, B., Li, S. G. & Zheng, Y. F. (1997). Metamorphic P–T and oxygen isotope studies of coesite-bearing eclogite from the Dabie Mountains. Acta Geoscientia Sinica 18, 318–323 (in Chinese with English abstract).
Xiao, Y. L., Hoefs, J., van den Kerkhof, A. M., Fiebig, J. & Zheng, Y. F. (2000). Fluid history of UHP metamorphism in the Dabie Shan, China: a fluid inclusion and oxygen isotope study on the coesite-bearing eclogite from Bixiling. Contributions to Mineralogy and Petrology 139, 1–16.[Web of Science]
Xiao, Y. L., Hoefs, J., van den Kerkhof, A. M. & Li, S. G. (2001). Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan, China. Journal of Metamorphic Geology 19, 3–19.[Web of Science]
Xu, S. T., Okay, A. I. & Ji, S. (1992). Diamond from the Dabie Shan metamorphic rocks and its implication for tectonic setting. Science 256, 80–82.
Xue, F., Rowley, D. B., Tucker, R. D. & Peng, X. Z. (1997). U–Pb zircon ages of granitoid rocks in the North Dabie Complex, eastern Dabie Shan, China. Journal of Geology 105, 744–753.[Web of Science]
Yang, W. & Jian, P (1998). Geochronological study of Caledonian granulite and high-pressure gneiss in the Dabie Mountains. Acta Geologica Sinica 72, 264–270.
Ye, K., Cong, B. L. & Ye, D. (2000). The possible subduction of continental material to depths greater than 200 km. Nature 407, 734–736.[Medline]
Yui, T. F., Rumble, D. & Lo, C. H. (1995). Unusually low 18O ultra-high-pressure metamorphic rocks from the Sulu Terrain, eastern China. Geochimica et Cosmochimica Acta 59, 2859–2864.[Web of Science]
Yui, T. F., Rumble, D., Chen, C. H. & Lo, C. H. (1997). Stable isotope characteristics of eclogites from the ultra-high-pressure metamorphic terrain, east–central China. Chemical Geology 137, 135–147.[Web of Science]
Zhai, X., Day, H. W., Hacker, B. R. & You, Z. (1998). Paleozoic metamorphism in the Qinling Orogen, Tongbai Mountains, central China. Geology 26, 371–374.
Zhang, R. Y., Liou, J. G. & Cong, B. L. (1995). Talc-, magnesite- and Ti-clinohumite-bearing UHP meta-mafic and ultramafic complex in the Dabie Mountains, China. Journal of Petrology 36, 1011–1037.
Zhang, R. Y., Liou, J. G. & Tsai, C. H. (1996). Petrogenesis of a high-temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China. Journal of Metamorphic Geology 3, 231–243.
Zhang, R. Y., Rumble, D., Liou, J. G. & Wang, Q. C. (1998). Low-18O, ultrahigh-P garnet-bearing mafic and ultramafic rocks from Dabie Shan, China. Chemical Geology 150, 161–170.[Web of Science]
Zheng, Y. F. (1993a). Calculation of oxygen isotope fractionation in anhydrous silicate minerals. Geochimica et Cosmochimica Acta 57, 1079–1091.[Web of Science]
Zheng, Y. F. (1993b). Calculation of oxygen isotope fractionation in hydroxyl-bearing silicates. Earth and Planetary Science Letters 120, 247–263.[Web of Science]
Zheng, Y. F., Fu, B. & Gong, B. (1996). Extreme 18O depletion in eclogite from the Su-Lu terrain in East China. European Journal of Mineralogy 8, 317–323.
Zheng, Y. F., Fu, B., Li, Y. L., Xiao, Y. L. & Li, S. G. (1998). Oxygen and hydrogen isotope geochemistry of ultrahigh-pressure eclogites from the Dabie Mountains and the Sulu terrane. Earth and Planetary Science Letters 155, 113–129.[Web of Science]
Zheng, Y. F., Fu, B., Xiao, Y. L., Li, Y. L. & Gong, B. (1999). Hydrogen and oxygen isotope evidence for fluid–rock interactions in the stages of pre- and post-UHP metamorphism in the Dabie Mountains. Lithos 46, 677–693.[Web of Science]
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