Thematic Article

Metamorphic veins with kyanite, zoisite and quartzin the Zhu-Jia-Chong eclogite, Dabie Shan, China

DANIELEANIELE CASTELLIASTELLI,1 FRANCORANCO ROLFOOLFO,1 ROBERTOOBERTO COMPAGNONIOMPAGNONI1

ANDAND SHUTONGHUTONG XUU2

1Dipartimento di Scienze Mineralogiche e Petrologiche, Via Valperga Caluso 35, I-10125 Torino, Italy and2Anhui Institute of Geology, Ningguo Road 1, Hefei, Anhui Province 230001, China

Abstract The Zhu-Jia-Chong quartz eclogites occur within two mica±epidote±garnet±plagioclase gneiss as lenses up to 200 m thick and more than 1 km long. The eclogite ismedium-grained and consists of omphacite, garnet, clinozoisite, minor kyanite andquartz. No evidence for the former presence of coesite was found. Locally, poikiloblasticamphibole occurs, which is zoned from glaucophane in the core to Mg±cummingtonite inthe rim. In some localities, clinozoisite occurs as polycrystalline aggregates, interpretedas pseudomorphs after former porphyroblasts of lawsonite. The eclogite is cut byirregular metamorphic veins up to 20 cm thick and several meters long, consisting ofkyanite � zoisite � omphacite � rutile � apatite � clinozoisite in a quartz matrix. Lo-cally, vein minerals attain pegmatite-like grain size. Geothermobarometric calculationsand mineral compatibilities of peak and retrograde minerals of both eclogite and veinsindicate a clockwise P±T path, characterized by initial eclogitic conditions progradingfrom the lawsonite to the epidote stability ®eld. The veins are considered to have formedbefore peak metamorphic conditions by prograde dehydration of lawsonite to kya-nite + zoisite/clinozoisite + quartz + ¯uid; the ¯uid released from the reaction favoredthe growth of coarse-grained vein minerals. The eclogitic peak, estimated at � 24 kbarand 700 °C, is followed by a two-stage retrograde evolution: the earlier stage,characterized by a quasi-adiabatic decompression from eclogite- to the amphibolite-facies conditions, implies a rapid exhumation from depths of � 90 km to mid-crustallevels; the later stage, characterized by slower exhumation rates, led to the ®nalexposure of eclogites and country rocks. The role of lawsonite as storage of ¯uid insubducting plates is therefore discussed.

Key words: China, clinozoisite pseudomorphous after lawsonite, Dabie Shan, eclogite,¯uid in subduction zones, HP-metamorphism, kyanite±zoisite±quartz vein, P±T-path.

INTRODUCTION

The Dabie Mountains have become internation-ally well-known since the discovery of coesite(Okay et al. 1989; Wang et al. 1989; Wang & Liou1991, 1993) and microdiamond in eclogite (Xuet al. 1991, 1992; Okay 1993). A number of articleshave been devoted to the petrology of a variety ofeclogite-facies rocks, but little importance hasbeen given to metamorphic veins within the

eclogite. In the present paper kyanite±zoisite±quartz veins and host eclogites, exposed alongthe Taihu±Jiutian road in the Zhu-Jia-Chongvalley, are described. The presence of metamor-phic veins in eclogites, usually still considered as`dry' rocks, is especially interesting, since it mayshed light on the nature and amount of ¯uidpresent during the high-pressure (HP) and ul-trahigh-pressure (UHP) metamorphism. The in-vestigation of the Zhu-Jia-Chong eclogites andveins is also useful for reconstructing the meta-morphic P±T path, since the eclogite and vein

The Island Arc (1998) 7, 159±173

Accepted for publication July 1997.

minerals exhibit polystage retrograde recrystal-lization, which marks the different stages of thepostclimactic exhumation.

REGIONAL GEOLOGY

The Dabie Mountains, located in central-easternChina, belongs to an � 2000 km long orogenicbelt produced by the collision between the Sino-Korean and Yangtze plates (Liou et al. 1995).This orogen, which extends from Qingling to theDabie Mountains, is cut by the northeast±south-west-trending left-lateral Tan±Lu Fault (TLF;Wang et al. 1989; Okay et al. 1993) with an offsetof � 500 km. The Su±Lu orogen, exposed north-east of this fault, is believed to be the displacedcontinuation of the Dabie Mountains (Fig. 1).

The Dabie Mountains are bounded by twomain faults: the Xiaotian±Mozitang Fault (XMF)to the north, and the Xiangfan±Guangji Fault(XGF) to the south. The XMF separates theDabie Mountains from the Palaeozoic Foziling

Group (Gao & Liu 1988; Ye et al. 1991), a meta-¯ysch which is mainly composed of low green-schist-facies slate, phyllite and meta-sandstone.The XGF separates the Dabie Mountains fromthe foreland of the Yangtze craton (Fig. 1; Xuet al. 1992, 1994). In the Dabie Mountains, atectono-metamorphic zonation can be traced,which includes from north to south: the mig-matitic North Dabie Complex (NDC), the UHPand HP Eclogite Complex (EC), and the SusongGroup (SG) (Fig. 1). In the western DabieMountains, a thermal high occurs, where am-phibolite- to granulite-facies rocks are exposed.These metamorphic units were extensively in-truded by post-metamorphic Mesozoic granitoids(Fig. 1), which signi®cantly obliterate the con-tacts between the different lithologies and/ortectonic units.

The EC mainly consists of felsic gneiss withintercalations of eclogite-facies rocks, includingeclogite, garnet±pyroxenite, garnet±peridotite,marble, and quartz/coesite±jadeite±garnet gran-ofels. Eclogites occur as abundant lenses, dis-

Fig. 1 Tectono-metamorphic sketch-map of the Dabie±Tongbai belt in central-eastern China (modi®ed after Xu et al. 1994; You et al. 1996;Zhou et al. 1996). The inset shows the location of the Dabie±Tongbai Belt in the Qinling±Dabie±Su-Lu Orogen. DGF, Dawu±Geshi Fault; SMF,Shancheng±Macheng Fault; SWS, Shuihe±Wuhe Shear Zone; TLF, Tan±Lu Fault; TMS, Taihu±Mamiao Shear Zone; TTF, Tongbai±TongchengFault; XMF, Xiaotian±Mozitang Fault; XGF, Xiangfan±Guangji Fault; UHP, ultrahigh-pressure; HP, high pressure.

160 D. Castelli et al.

continuous bands and nodules within felsicgneiss, marble and ultrama®c rocks (Xu et al.1991, 1994; Wang et al. 1992; Okay 1993). Inclu-sions of coesite and its polycrystalline quartzpseudomorphs have been reported from eclogite(Okay et al. 1989; Wang et al. 1989), marble(Wang & Liou 1991; Schertl & Okay 1994; Zhang& Liou 1996), felsic gneiss, phengite schist(Wang et al. 1992) and quartz/coesite±jadeite±garnet granofels (Cong et al. 1995; Su et al. 1996).Very rare microdiamond inclusions have beenfound in eclogite and garnet±pyroxenite garnets(Xu et al. 1991, 1992; Okay 1993).

The EC consists of a northern zone, wherecoesite±eclogite is found, and a southern zone,where only quartz±eclogite occurs (Okay 1993);these two zones are equivalent to the `hot eclo-gite terrain' and the `cold eclogite terrain' (Wanget al. 1992; Carswell et al. 1993), respectively.The peak metamorphic conditions of the coesite±eclogite were estimated to be P > 4.0 GPa andT > 900 °C (Xu et al. 1991, 1992) orP � 2.8 � 0.3 GPa and T � 850 � 50 °C (Okay1993), whereas those of the quartz±eclogite wereestimated to be P � 2.4 � 0.3 GPa and T �635 � 40 °C (Okay 1993). The nature of the con-tact between the two parts is still under debate:for some researchers it is tectonic (Okay 1993),whereas for others it is transitional (X. Wanget al. 1995). Northward, the Shuihou±WuheShear Zone (SWS; Q. Wang et al. 1995) separatesthe EC from the NDC.

The NDC consists of a variety of metamorphicrocks hosting more than 100 ma®c±ultrama®cblocks (Xu et al. 1994; Cong et al. 1995). Themetamorphic rocks include orthogneisses, am-phibolite, migmatitic gneiss with minor granulite(Zhang et al. 1996). The EC is juxtaposed to theSG along the Taihu±Mamiao Shear Zone (TMS).The SG is composed of epidote±amphibolite fa-cies metamorphic phosphorous-bearing sequenc-es including micaschist, graphite-schist, marbleand kyanite±quartzite, together with some felsicgneiss and amphibolite, and minor blueschist (Xuet al. 1994).

Sm±Nd and U±Pb radiometric data of ca 220±230 Ma indicate that the HP and UHP meta-morphism in the Dabie Mountains is related tothe Triassic Indosinian orogeny (Ames et al.1993, 1996; Li et al. 1993; Hacker & Wang 1995;Hacker et al. 1996). However, some researchersstill consider the eclogitic metamorphism eitherpartially Caledonian (You et al. 1996) or entirelyPrecambrian in age (Cao & Zhu 1996).

GEOLOGICAL SETTING

Lenses of eclogite, up to 100 m thick and morethan 1 km long, are exposed in the Zhu-Jia-Chong area, west of the town of Taihu. This areais located in the quartz±eclogite zone of theDabie EC, 2±3 km to the south of the boundarywith the coesite±eclogite zone (Fig. 1). Theeclogites occur in two localities of the Zhu-Jia-Chong Valley, along the Taihu±Jutian road,� 500 m to the north of the Zhu-Jia-Chongvillage, close to the bridge on the Zhucong river(Fig. 2), and � 2 km northward, close to thevillage of Lidu near Huangzhen.

The Zhu-Jia-Chong eclogite occurs as elon-gated lenses nearly parallel to the regionalfoliation within two mica±epidote±garnet±pla-gioclase gneiss (Fig. 2). Megascopic isoclinal andopen folds are also found in the gneiss, showingsubhorizontal northwest±southeast-trending ax-es. The eclogite exhibits both weak foliation (Se),consistent with the trend of the country gneiss

Fig. 2 Simpli®ed geological map of the eclogite and country gneissclose to the Zhu-Jia-Chong village. Black star shows the location ofthe main kyanite±zoisite±quartz vein.

Zhu-Jia-Chong eclogite, Dabie Shan, China 161

foliation, and locally well developed omphacitelineation, steeply plunging towards south±southwest.

The Lidu eclogite occurs within deeplyweathered gneiss as smaller poorly exposedlenses. The country gneiss is similar in compo-sition and mineralogy to that at the Zhu-Jia-Chong area, but includes cm- to dm-thick inter-layers of a peculiar magnetite-bearing leu-cogneiss, a widespread eclogite-hosting lithologyelsewhere in the EC. The Lidu eclogites aredistinguished by the occurrence of mm-long yel-lowish aggregates of epidote; some of them pos-sess short prismatic habit, interpreted aspseudomorphs after former lawsonite (see nextsections).

In both localities, irregular metamorphic kya-nite±zoisite±quartz � omphacite veins and pock-ets, from a few mm to several dm wide and froma few cm to several m long, are present, whichcut across the eclogite foliation. Most vein min-erals are coarser-grained than in the host eclo-gite; some may attain pegmatite-like grain size.The contact between the vein and the countryeclogite is relatively sharp, but without apparentreactions. A schematic relationship betweeneclogite and vein minerals of a distinctive vein,exposed in the roadcut near the Zhu-Jia-Chong

bridge, is shown in Fig. 3. The vein is composedof pale yellowish-green zoisite and bluish kyaniteup to 20 cm long, together with accessory cm-long rutile and apatite enclosed in a quartz ma-trix. Locally, idioblastic omphacite occurs closeto the contact with the host eclogite. Kyanite ispartially pseudomorphed by coarse-grained palebluish-green paragonite.

PETROGRAPHY

COUNTRY TWO MICA±EPIDOTE±GARNET±PLAGIOCLASE-GNEISS

The eclogite-hosting gneiss is composed ofquartz, plagioclase, biotite, garnet, white mica,epidote, and rare stilpnomelane, titanite, opaqueminerals, apatite and carbonates. The foliation isde®ned by the preferred orientation of mica¯akes and elongated plagioclase. Plagioclase iszoned from oligoclase in the core to albite in therim. A second generation of white mica developsafter Na±plagioclase. Garnet porphyroblasts con-tain small inclusions of rutile, opaque mineralsand epidote. Most biotite ¯akes are chloritizedand white mica is rimmed by a very ®ne-grainedworm-like feldspar. The magnetite-bearingleucocratic interlayers of the Lidu eclogites aretonalitic gneisses consisting of plagioclase, mag-netite, epidote with allanitic core and minorK-feldspar.

ECLOGITE

The fresh Zhu-Jia-Chong eclogite (Fig. 4) con-sists of omphacite, garnet, clinozoisite, minorquartz, kyanite and porphyroblasts of zoisite,blue amphibole and paragonite. Accessory min-erals are rutile, apatite, zircon and opaque phas-es. No inclusion of coesite or its quartzpseudomorph was found. Especially along themargins, the eclogite is strongly retrogressedand the eclogitic minerals are replaced bysymplectites.

The garnet core is generally crowded with in-clusions of clinozoisite, rutile, quartz, omphacite,glaucophane, rare paragonite and apatite, to-gether de®ning a foliation (Si) discordant to re-gional foliation (Se). The garnet rim includes onlyrutile. Locally, the garnet is atoll-like. Most gar-nets are rimmed with a retrogression coronaconsisting of blue±green pargasitic amphi-bole + Na±plagioclase + magnetite.

Fig. 3 Relationships between vein and host eclogite at Zhu-Jia-Chong. Signi®cant retrograde mineral compositions are given inbrackets. Boxes show locations of Figs 4 and 7, respectively.

162 D. Castelli et al.

Omphacite is extensively replaced by a very®ne-grained symplectite consisting of an earlieralbite/oligoclase + diopside (Fig. 5) and a lateralbite + edenitic amphibole intergrowth. At thecontact with the quartz grains, the symplectiteafter omphacite is rimmed by a thin corona ofaegirine±augite. A zoned porphyroblastic am-phibole locally occurs, which is glaucophane inthe core, Mg±taramite and ferroan pargasite inthe middle, and Mg±cummingtonite in the rim.

Locally glaucophane contains corroded crystalsof garnet surrounded by a opaque magnetitecorona. The porphyroblastic amphibole includesidioblastic clinozoisite and peculiar strings orrods of zoned plagioclase similar to, althoughcoarser grained than, that of the symplectitesformed from the omphacite breakdown. Close toamphibole or zoisite, kyanite is usually replacedby a hercynite + andesine symplectite.

In the Lidu area, the eclogite is locally char-acterized by the occurrence of abundant mm-sized yellowish mineral aggregates, which ap-pear to be pseudomorphous after an earlierporphyroblastic mineral with a short prismaticor lozenge-shaped habit. The aggregates consistof medium-grained clinozoisite with a randomorientation, in apparent equilibrium with theeclogitic minerals. Based on both mineral com-position and shape, these clinozoisite aggregatesare considered to be pseudomorphs after formerporphyroblastic lawsonite (Fig. 6). The occur-rence of lawsonite in eclogitic rocks has beendescribed from a number of localities (Deer et al.1986). However, fresh lawsonite is rare, beingusually replaced by epidote-bearing pseudo-morphs. For example, similar pseudomorphs arefrequently found in quartz±eclogites from thewestern Alps, and have long been interpreted asa breakdown product of early formed lawsonite(Fry & Fyfe 1969; Bearth 1973; Fry 1973).

Fig. 4 Contact between the main Zo±Ky±Qtzvein and the host eclogite at the Zhu-Jia-Chong bridge. Note the occurrence in theeclogite of both idioblastic and atoll-likegarnet, and the presence of amphibole poi-kiloblasts. Box shows location of the portionenlarged in Fig. 5.

Fig. 5 Back-scattered image of the symplectite corona afteromphacite, consisting of aegirine±augite/diopside (light-grayworms) intergrown with albite/oligoclase (black). The omphacite isincluded in a poikiloblastic zoned amphibole. The white mineral(bottom-right) is garnet. Width of ®eld is 3 mm. Sample RPC106/1,Zhu-Jia-Chong eclogite.

Zhu-Jia-Chong eclogite, Dabie Shan, China 163

Pseudomorphs of similar grain size, shape andmineralogy, interpreted as evidence of the pre-vious presence of lawsonite, have also been re-ported from the coesite-bearing ophioliticeclogites (Tamagno 1997) and related siliceousmetasediments (van der Klauw et al. 1997) ofLago di Cignana, Zermatt±Saas Zone, WesternAlps.

VEINS

The veins are heterogeneous in texture andmineral composition; different portions consist ofquartz + omphacite, quartz + kyanite, quartz +kyanite + omphacite, and zoisite + kyanite. Lo-cally, porphyroblastic accessory apatite andsmaller rutile crystals are observed. Both zoisiteand clinozoisite occur, the former being earlierthan the latter. However, both zoisite and clin-ozoisite appear to be stable with kyanite andclinopyroxene. Most omphacite inclusions withinquartz are idioblastic and sometimes are skeletalwith an atoll-like habit. Kyanite includes idiob-lastic clinozoisite and omphacite. Omphaciteshows the same retrogression as the omphacitein the host eclogite, that is, an intergrowth ofalbite + diopside or amphibole + albite; each ret-rograde phase has the same crystallographicorientation all over the pseudomorph. Kyanite ispseudomorphically replaced by coarse-grainedaggregates of paragonite or, more rarely, it issurrounded by a thin corona of muscovite. Kya-nite and retrogression paragonite are locallyreplaced by an extremely ®ne-grained symplec-tite, which under the electron microscopeappears to be an intergrowth of coru-ndum + oligoclase + scarce magnetite (Fig. 7).

Most kyanite blades are rimmed by a thin coronaof polycrystalline albite, with palisade crystalsgrown perpendicularly to the original kyanitegrain boundaries.

MINERAL CHEMISTRY

Electron microprobe analyses of both primaryand retrograde minerals from selected sampleswere performed employing an automated Cam-bride electron-microprobe, STEREOSCAN 360energy dispersive (EDS) system. Operating con-ditions were 15 kV accelerating voltage and 50 scounting time. Structural formulae of mineralswere processed using the programs of Petra-kakis and Dietrich (1985) and Ulmer (1986). Foramphiboles, the Fe3+/Fe2+ ratio was calculatedassuming total cations � 13 + (Ca + Na + K).For pyroxenes, the Fe3+/Fe2+ ratio was calculat-ed following the method of Lindsley and Ander-son (1983). Representative analyses of primaryand retrograde minerals from both vein andeclogite are listed in Table 1. Mineral abbreviat-ions are after Kretz (1983).

GARNET

Eclogitic garnet is rich in almandine with lowerpyrope and grossular components (Fig. 8). Aweak zoning is rarely observed, which is charac-terized by Mn- and Ca-decrease, and Mg-in-crease, from core to rim. The averagecomposition is Alm53Prp21Grs22Sps03Adr01 inthe core, and Alm54Prp30Grs11Sps01Adr04 in therim.

Fig. 7 Back-scattered image of the retrograde symplectite afterkyanite (ky), consisting of corundum lamellae (crn) and oligoclase(olg). Width of ®eld is 0.45 mm. Sample RPC 106/1, vein in theZhu-Jia-Chong eclogite.

Fig. 6 Clinozoisite aggregate pseudomorphous after porphyrob-lastic lawsonite. Partly retrogressed Lidu eclogite, sample RPC173.Scale bar is 0.2 mm.

164 D. Castelli et al.

CLINOPYROXENE

The clinopyroxene compositions are plotted inthe diagram from Morimoto (1988) (Fig. 9).Omphacite in both eclogite and veins is homoge-neous with average composition (Na0.64

Ca0.33Fe2+0.03) (Fe3+

0.16Mg0.34Al0.50) Si2O6; thejadeite content ranges between XJd � 0.41 and0.49 and aegirine ranges between XAeg � 0.07and 0.16. The omphacite included in garnet showsthe highest jadeite content.

A wider range of compositions is shown bysecondary clinopyroxenes, formed during retro-grade evolution. In symplectite after eclogiteomphacite, clinopyroxene in equilibrium withalbite is a diopside with a maximum jadeitecontent XJd � 0.13 and XAeg � 0.04, whereascoronitic clinopyroxene at the contact withquartz is an aegirine±augite, with a lower jade-ite- (XJd � 0.03±0.05) and a higher aegirine-content (XAeg � 0.29±0.32). In the albite±clino-pyroxene symplectite after vein omphacite,clinopyroxene is an aegirine±augite with a max-imum jadeite content XJd � 0.12 and aegirinecontent XAeg � 0.23.

AMPHIBOLE

Both glaucophane and Mg±taramite (nomencla-ture following Leake 1978) were found as inclu-sions in eclogite garnet porphyroblasts. Theeclogite porphyroblastic amphibole is zoned fromglaucophane in the core, through Mg±taramiteand ferroan±pargasite in the middle, to Mg±cummingtonite in the rim. From core to rim, XMg

ranges from 0.79 to 1.00 and the Si contentranges from 7.42 to 5.27 atoms per 13 cations.The Ti content is very low (0.00±0.03 atomsp.f.u.), while AlIV and AlVI range between 0.58and 2.74, and 1.32 and 0.66 atoms per 13 cations,respectively. Retrograde coronitic amphibole af-ter garnet is a ferroan pargasite withXMg � 0.59±0.69, AlIV � 2.09±2.25 and AlVI �0.93±1.49, whereas the amphibole of the albite-bearing symplectite after omphacite is a silicicedenite richer in Si and Ca and poorer in Al.

PLAGIOCLASE

The plagioclase composition ranges from ande-sine to nearly pure albite. The more calcicplagioclase (XAn � 0.43) occurs together withhercynite in the symplectite after eclogitic kya-nite. A less calcic plagioclase (XAn � 0.23) oc-curs together with corundum in the symplectiteafter the vein kyanite. The plagioclase from theamphibole-bearing coronas around garnetporphyroblasts ranges in composition from An34

close to garnet to An2 in the outermost corona.Pure albite (An1±3) occurs in the symplectiteafter omphacite in both eclogite and veins.

EPIDOTE

In the eclogite, epidote is weakly zoned withpistacite component 44±60% and Fe3+ ranging

Fig. 8 Garnet compositions of the Zhu-Jia-Chong eclogite in the(grossular + andradite + spessartine)±almandine±pyrope triangle.Dashed lines link core to rim compositions. (n), core; (h), inter-mediate; (s), rim.

Fig. 9 Compositions of peak and retrograde clinopyroxenes fromeclogite and veins in the jadeite±aegirine±(wollastonite + enstatite+ ferrosilite) diagram of Morimoto (1988). (e), omphacite inclu-sions in garnet; (h), omphacite in eclogite; (n), omphacite in vein;(d), coronitic aegirine±augite; (r), diopside in symplectite afteromphacite (in eclogite); (j), aegirine±augite in symplectite afteromphacite (in vein).

Zhu-Jia-Chong eclogite, Dabie Shan, China 165

Table 1 Representative analyses of minerals in the Zhu Jia Chong eclogite. Mineral abbreviations are after Kretz (1983)

Sample 9Grt62rim 13Grt63core 19Px137 16Px147 11Px76 15Px129 18Anf144 16Anf106C 16Anf109R 17Anf114 17Anf119

MineralGarnet Garnet Pyroxene Pyroxene Pyroxene Pyroxene Amphibole Amphibole Amphibole Amphibole Amphibole

SiO2 39.39 39.33 57.66 56.88 54.44 55.19 44.94 53.45 37.62 39.43 50.21TiO2 Ð Ð 0.03 0.00 0.23 0.00 0.57 0.06 0.00 0.00 0.17Al2O3 21.28 21.71 12.37 11.87 2.95 1.25 16.57 11.57 20.58 21.31 8.01Fe2O3 1.23 0.30 1.55 5.81 7.89 10.34 5.01 4.70 26.46 2.38 0.00FeO 25.01 24.29 3.03 1.43 2.35 2.39 7.88 5.65 0.00 10.85 10.05MnO 0.61 1.09 0.00 0.06 0.01 0.23 0.03 0.38 0.90 0.00 0.13MgO 7.75 5.38 6.97 6.19 10.56 9.70 10.34 12.00 6.95 8.70 11.27CaO 5.28 8.26 10.65 8.80 16.51 15.55 7.09 4.06 5.78 10.16 13.97Na2O 0.00 0.00 8.15 9.47 4.93 4.77 5.20 5.64 0.00 4.36 3.70K2O 0.00 0.00 0.00 0.04 0.15 0.00 0.15 0.18 0.00 0.00 0.00H2O Ð Ð Ð Ð Ð Ð 2.09 2.15 2.14 2.04 1.96Total 100.56 100.35 100.41 100.55 100.02 99.42 99.87 99.85 100.42 99.22 99.47

Si 3.028 3.047 2.021 2.003 1.996 2.042 6.442 7.423 5.265 5.799 7.671Ti Ð Ð 0.001 0.000 0.006 0.000 0.062 0.006 0.000 0.000 0.020Al 1.929 1.983 0.511 0.493 0.128 0.054 2.799 1.894 3.394 3.695 1.443Fe3+ 0.072 0.017 0.041 0.154 0.218 0.288 0.540 0.491 2.786 0.263 0.000Fe2+ 1.608 1.574 0.089 0.042 0.072 0.074 0.944 0.656 0.000 1.334 1.284Mn 0.040 0.072 0.000 0.002 0.000 0.007 0.004 0.045 0.106 0.000 0.016Mg 0.889 0.621 0.364 0.325 0.577 0.535 2.209 2.485 1.449 1.909 2.566Ca 0.435 0.686 0.400 0.332 0.649 0.616 1.090 0.604 0.866 1.601 2.286Na 0.000 0.000 0.554 0.647 0.351 0.342 1.446 1.518 0.000 1.243 1.096K 0.000 0.000 0.000 0.002 0.007 0.000 0.028 0.032 0.000 0.000 0.000OH Ð Ð Ð Ð Ð Ð 2 2 2 2 2

Sample 11PI77 11PI83 9PI94 17PI116 17PI118 15Ep131c 15Ep132r 106Ep111 106Ep409 11Ep71 8Mb70 106Mb231 18Mb143

MineralPlagioclase Plagioclase Plagioclase Plagioclase Plagioclase Epidote Epidote Epidote Epidote Epidote White

micaWhitemica

Whitemica

SiO2 68.49 61.52 53.44 61.31 68.98 40.51 39.50 40.01 39.39 39.27 47.62 47.51 46.98TiO2 Ð Ð Ð Ð Ð 0.00 0.09 0.00 0.17 0.15 0.19 0.05 0.14Al2O3 18.99 23.66 31.15 25.37 19.65 26.33 28.12 32.02 29.27 27.08 34.96 38.89 38.68Fe2O3 Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð 0.87 0.25 1.30FeO Ð Ð Ð Ð Ð 9.18 7.71 1.70 4.85 7.60 0.00 0.36 0.00MnO Ð Ð Ð Ð Ð 0.00 0.00 0.00 0.00 0.25 0.00 0.00 0.05MgO Ð Ð Ð Ð Ð 0.57 0.37 0.00 0.27 0.20 0.84 0.28 0.27CaO 0.31 5.05 8.58 6.58 0.38 21.78 22.31 24.36 24.18 23.51 0.09 0.25 0.34Na2O 11.9 9.4 6.36 7.18 11.59 Ð Ð Ð Ð Ð 0.08 7.58 7.24K2O 0 0.1 0.03 0.01 0.02 Ð Ð Ð Ð Ð 10.44 0.63 0.00H2O Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð 4.54 4.71 4.70Total 99.69 99.73 99.56 100.45 100.62 98.37 98.10 98.10 98.13 98.06 99.65 100.51 99.72

Si 2.994 2.716 2.372 2.733 2.998 6.203 6.059 6.066 6.035 6.060 3.144 3.023 2.996Ti Ð Ð Ð Ð Ð 0.000 0.010 0.000 0.020 0.018 0.010 0.002 0.007Al 0.978 1.231 1.629 1.333 1.007 4.753 5.085 5.723 5.283 4.927 2.720 2.916 2.907Fe3+ Ð Ð Ð Ð Ð 1.175 0.906 0.216 0.622 0.982 0.043 0.012 0.062Fe2+ Ð Ð Ð Ð Ð 0.000 0.083 0.000 0.000 0.000 0.000 0.019 0.000Mn Ð Ð Ð Ð Ð 0.000 0.000 0.000 0.000 0.032 0.000 0.000 0.003Mg Ð Ð Ð Ð Ð 0.130 0.085 0.000 0.063 0.045 0.083 0.027 0.026Ca 0.015 0.239 0.408 0.314 0.018 3.573 3.667 3.958 3.968 3.888 0.007 0.017 0.024Na 1.009 0.805 0.547 0.620 0.976 Ð Ð Ð Ð Ð 0.010 0.935 0.896K 0.000 0.006 0.002 0.001 0.001 Ð Ð Ð Ð Ð 0.879 0.051 0.000OH Ð Ð Ð Ð Ð Ð Ð Ð Ð Ð 2 2 2

166

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from 1.18±0.84 atoms p.f.u., from core to rim,respectively. In the veins, zoisite has Fe3+ lowerthan 0.22 atoms p.f.u., whereas clinozoisite has anaverage Fe3+ content of 0.6 atoms p.f.u. and 30±35% of pistacite component.

WHITE MICA

The mica included in the garnet porphyroblastsis paragonite, with chemical composition verysimilar to that of the coarse-grained paragonitedeveloped at the expense of the vein kyanite(average Na � 0.90 atoms p.f.u.; average AlIV

and AlVI � 1.00 and 1.90 atoms p.f.u., respec-tively). The small mica ¯akes locally rimmingvein kyanite have compositions close to puremuscovite (Si � 3.14; AlIV � 0.86; AlVI � 1.86;K � 0.88 atoms p.f.u., on average).

METAMORPHIC EVOLUTION

The metamorphic evolution of both eclogite andthe Ky±Zo±Qtz veins, as inferred from petro-graphic observations and petrological consid-erations, is summarized in Table 2. Sixmetamorphic stages have been distinguishedwhich de®ne a clockwise P±T path (Fig. 10). Thistrajectory was based on mineral compatibilities,geothermometric calculations and theoreticalphase relationships in the CaO±Na2O±MgO±Al2O3±SiO2±H2O (CNMASH) system. Petrologi-cal modelling of this system was performed usingthe thermodynamic approach and the pseudo-compound approximation of Connolly (1990), andthe equation of state of Holland and Powell(1990). Thermodynamic mineral data were takenfrom a revised version (J. Connolly pers. comm.,1994) of the database of Holland and Powell(1990). Lawsonite thermodynamic data of Sch-midt and Poli (1994) were used, which are con-sistent with the database of Holland & Powell(1990) and the equation of state for pressureslower than � 30±40 kbar (S. Poli pers. comm.,1997).

STAGE 1

The earliest recognized stage 1 corresponds toeclogite-facies conditions in the lawsonite stabil-ity ®eld (Table 2). The original occurrence oflawsonite in the Lidu eclogite is inferred fromthe discovery of mm-sized clinozoisite aggre-gates, as discussed in the previous petrographic

section. However, lawsonite porphyroblastsshould have developed after the pervasive eclo-gite-facies deformation which produced theomphacite lineation. This is suggested by shapepreservation of the epidote-bearing pseudo-morphs after lawsonite. Pressure±temperatureconditions of stage 1 cannot be accurately deter-mined, since the only available constraint is thelawsonite stability ®eld in the eclogite-facies(Fig. 10). Therefore, this stage was locatedsomewhere along the most probable geothermalgradient, leading to the peak stage 3.

STAGE 2

This stage, which corresponds to the vein for-mation, is located along the prograde pathbetween stages 1 and 3, just past the upperthermal stability of lawsonite. In Fig. 10, thecurve of the most likely reaction lawsonite� zoisite + kyanite + quartz + ¯uid is shown,which may account both for the development ofthe vein minerals and their coarse grain-size. AtP � 20 kbar the same lawsonite breakdownreaction is located at a temperature only 15 °Chigher, if clinozoisite instead of zoisite is consid-ered. During this event, eclogite must havesuffered mineral re-equilibration and partialrecrystallization without pervasive deformation,since pseudomorphs after lawsonite still pre-serve the original euhedral shape and the veinscut across the omphacite lineation.

STAGE 3

The eclogite continued re-equilibrating up to themetamorphic peak. For the assumed pressure of24 kbar, peak temperatures of 700 � 18 °C havebeen estimated from the garnet±omphacite Fe±Mg exchange geothermometer, using the cali-bration of Powell (1985). Using the calibration ofEllis and Green (1979), temperatures � 20 °Chigher have been obtained. The peak pressure isconstrained by the presence of quartz instead ofcoesite and by the coexistence of omphacite(Jd43±50) in equilibrium with kyanite (Fig. 10).

STAGE 4

The early decompression is marked by the growthof porphyroblastic glaucophane mostly at theexpense of omphacite. The upper stability of thisglaucophane (Gln70±80), seemingly in equilibriumwith kyanite, may be modelled by the reaction

Zhu-Jia-Chong eclogite, Dabie Shan, China 167

Table 2 Metamorphic evolution of eclogite and Ky±Zo±Qtz veins

P±T Path Progrademetamorphism

Veinformation

Peakmetamorphism

Retrogrademetamorphism

Stages 1 2 3 4 5 6

Quartz jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjOmphacite j jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjGarnet j jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjKyanite jj jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjRutile jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjZoisite jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjClinozoisite/Epidote jjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjjParagonite jjjjjjjjjjjjj jjjjjjjjj(Lawsonite) jjjjjjjjjjjjjGlaucophane jjjjjjjjjjjjj jjjjjCa±Na amphibole jjjjjjjjjjjjjjjjjjjjjjjjjjDi±Pl symplectite jjjjjjjjjjjjjjjCrn±Pl symplectite jjjjjjjjjjjjjjjSpl±Pl symplectite jjjjjjjjjjjjjjjAmph±Ab symplectite jjjjjjjjjjjjjjjAlbite jjjjjjjjjjjjjjjChlorite jjjjjjjjjjjjjjjTitanite jjjjjjjjjjjjjjj

1, prograde quartz±eclogite stage in the lawsonite stability ®eld; 2, development of the Ky±Zo±Qtz veins; 3, peak eclogitic stage in the epidote stability ®eld; 4, post-eclogitic

retrograde decompressional stage; 5, amphibolite facies stage; 6 greenschist-facies stage.The mineral in parentheses is inferred on a microtextural basis.

jjjj jjjj jjjj jjjjjj jjjjj jjjj jjjj

168

D.

Ca

stelliet

al.

kyanite + glaucophane + H2O � talc + parago-nite. This reaction must have occurred at a pres-sure lower than the stability ®eld of omphacite(Jd43±50) in equilibrium with albite and quartz(Fig. 10). The best pressure estimate for stage 4 is� 14 kbar, which is also constrained by the pres-ence of quartz + paragonite instead of kyanite +albite. The temperature of stage 4 is constrainedat� 700 °C by the close similarities of stage 3 and5 temperatures, which suggest a post-climacticquasi-adiabatic decompression. During this de-compression, vein kyanite close to omphacite wasextensively replaced by paragonite.

STAGE 5

This high-T amphibolite-facies stage is wellconstrained at P� 8 kbar and T £ 700 °C by thesymplectitic breakdown reactions of paragoniteto produce corundum + albite + H2O, of kyanite

close to zoisite to produce corundum + plagiocla-se + H2O, and of omphacite to produce diopside(XJd � 0.13) in equilibrium with albite (Fig. 10).It is interesting to note that the presence ofcorundum, a mineral restricted to silica-under-saturated environments, indicates that the twodehydration reactions occurred pseudomorphic-ally without the involvement of the surroundingphases, especially quartz. This stage is responsi-ble for the amphibolite-facies eclogite retrogres-sion and the widespread country gneissrecrystallization.

STAGE 6

The latest metamorphic stage took place undergreenschist-facies conditions, as suggested bythe local retrograde growth of albite aroundplagioclase, the chloritization of biotite and gar-net, and the development of late stilpnomelane.

The presence of relict rutile armoured in gar-net in the gneiss surrounding the eclogite sug-gests that the felsic eclogite-hosting rocks alsosuffered an early high-pressure and/or high-temperature recrystallization, predating theamphibolite-facies event. Since there is no evi-dence in the area of an early granulite-faciesimprint, it is likely that these gneisses did sufferthe eclogite-facies recrystallization. Eclogiticmineral assemblages were later extensivelyobliterated during the amphibolite-facies stage.This stage, the main metamorphic event re-corded in the gneiss, is followed by a greenschist-facies recrystallization.

DISCUSSION

THE DEVELOPMENT OF THE KYANITE±ZOISITE±QUARTZVEINS

The widespread occurrence of kyanite±zoisite±quartz veins and the coarse to pegmatitic size ofvein minerals require the presence of abundant¯uid during the eclogite-facies metamorphism.This ¯uid may have been either introduced fromoutside into the ma®c system or stored in meta-morphic mineral phases, stable only at the initialeclogitic conditions. The discovery of pseudo-morphs after earlier lawsonite in the Lidueclogite supports the second possibility and wesuggest the following scenario. During the pro-grade transition from the lawsonite to the epi-dote stability ®elds (Fig. 10), lawsonite brokedown according to the reaction:

Fig. 10 Selected phase relationships in the CNMASH system,calculated with the thermodynamic approach of Connolly (1990).Phase diagram analysis according to Castelli et al. (1997). Dashedcurves are isopleths modelling the jadeite content in omphacite(diopside±jadeite solution model of Gasparik 1985) and the glau-cophane content in blue amphibole (tremolite±glaucophane idealsolution model), according to the pseudocompound approximationof Connolly (1990). The jadeite-isopleths pairs bracket the stability®elds of omphacite, with composition between Jd43 and Jd50, inequilibrium with Ab + Qtz, and Pg + Ky, respectively. The glau-cophane-isopleths pairs bracket the stability ®eld of glaucophane,with composition between Gln70 and Gln80, in equilibrium withTlc + Pg + Ky. Fields labelled from 1 to 6 refer to the main meta-morphic stages described in Table 2. Dashed±dotted lines bracketthe climax temperatures (stage 3) estimated from the Grt±Cpx Fe±Mg exchange geothermometer, the calibration from Powell (1985).See text for discussion.

Zhu-Jia-Chong eclogite, Dabie Shan, China 169

1Law � 1Zo=Czo� 1Ky� 2Qtz� 3:5H2O

Due to the low eclogite permeability, the waterreleased from this reaction opened a series ofdiscrete fractures, healed by coarse-to-pegmatiticquartz + kyanite + zoisite/clinozoisite � ompha-cite � rutile assemblage. After mineral growth,the ¯uid phase should have passed through theeclogite, favouring mineral re-equilibration.

THE FLUID PHASE DURING SUBDUCTION

Lawsonite, a major rock-forming mineral inwhich more than 10 wt% water is stored, mayoccur not only in ma®c and intermediate meta-igneous rocks, but also in carbonate-bearingmetasediments, such as the `schistes lustreÂs'(calcschist) of the Piemonte zone, western Alps(Caron 1977). This mineral, formerly consideredtypical of blueschist facies, is now experimentallyproved to be stable also at very high tempera-tures (up to � 1000 °C), provided that pressuresare high enough to prevent its thermal break-down (Pawley & Holloway 1993; Pawley 1994;Schmidt & Poli 1994; Schmidt 1995).

Therefore, in subducting slabs, lawsonite-bearing eclogites may carry water to a depthwhich depends on the local active geotherm. Inthe Zhou-Jia-Chong eclogite, lawsonite was sta-ble down to � 70 km (Fig. 10). At this depth, only80% of the water stored in lawsonite is releasedfrom the rock system, since epidote still contains� 2 wt% water. Therefore, the eclogite will be-come a really dry rock at much greater depth.From the number and average size of epidotepseudomorphs in the Zhu-Jia-Chong eclogite, amaximum lawsonite content of � 20 vol% may beestimated, which corresponds to a water contentof � 2 wt%. This indicates that the eclogiteprotolith must have been a ma®c rock more hy-drated, that is, of lower metamorphic grade, thana hornblende-bearing amphibolite. The not un-common presence of clinozoisite aggregates ineclogites from a variety of metamorphic belts(Deer et al. 1986) suggests that lawsonite mayhave formed in hydrated ma®c systems of suit-able composition at least during early stages ofsubduction. Therefore, lawsonite in ma®c sys-tems may be analogous to antigorite in ultra-ma®cs (Ulmer & Trommsdorff 1995) and be veryimportant for recycling of water in subductionzones (Pawley 1994; Schmidt & Poli 1994; Sch-midt 1995).

P±T PATH AND TECTONICS

The earliest part of the inferred P±T path isprograde and developed under eclogite-faciesconditions from the lawsonite- (stage 1) to theepidote-stability (stage 3) ®elds. The transitionfrom the lawsonite- to the epidote-stability ®eldsis marked by the development of the kyanite±zoisite±quartz-veins. The metamorphic peak ofthe Zhu-Jia-Chong eclogite, estimated at� 700 °C and � 24 kbar, is higher than the esti-mates of 600 � 25 °C and 19 � 1 kbar given byCarswell et al. (1994) for the Dabie Shan `cold'eclogites. However, the higher values from thepresent study may be explained by the proximityof the Zhu-Jia-Chong eclogite to the boundarywith the coesite±eclogite zone.

A detailed petrographic investigation includ-ing various retrograde reaction textures alloweda well-constrained decompressional P±T path forthe kyanite±zoisite±quartz-veins and host Zhu-Jia-Chong eclogites to be established. This ret-rograde path (Fig. 10) is characterized by aninitial quasi-adiabatic decompression from eclo-gite- to amphibolite-facies conditions followed bya cooling to low greenschist-facies. This two-stage decompressional path, already recognizedby Liou et al. (1996) in other areas of the Dabie±Su-Lu Orogen, is steeper, but consistent withthat suggested by You et al. (1995) and Conget al. (1995) for coesite±eclogites; on the contraryit is signi®cantly different from that of Liu et al.(1995) and X. Wang et al. (1995), which envisagea single retrograde stage, characterized by apost-climactic decompression coupled with asubstantial cooling.

The earlier stage of the established retrogradeP±T path indicates a subduction of continentalcrust to a depth of � 90 km, followed by a rapidexhumation up to � 25 km. This early post-cli-mactic history is consistent with the geochrono-logical SHRIMP data of Maruyama et al. (in Liouet al. 1995, Fig. 29), which suggest a very rapidexhumation. The later stage of the retrograde P±T path (Fig. 10) is compatible with a slower ex-humation due to isostatic doming and erosion.

The consistent retrograde metamorphic evo-lution of eclogite and country gneiss indicatesthat both lithologies jointly experienced thesame evolution, at least from amphibolite-faciesconditions. However, considering that felsicrocks are more susceptible to retrogression, it ispossible that some gneissic rocks of southernDabie Shan, presently exhibiting amphibolite-

170 D. Castelli et al.

facies mineralogy, did experience an early HP orUHP recrystallization. A similar conclusion hasrecently been suggested by Compagnoni et al.(1996) for amphibolite-facies gneiss bearing rel-ics of aegirine-rich clinopyroxenes, exposed inthe southernmost part of the EC within thequartz±eclogite belt at Caochong, � 10 km westof Taihu. Similar conclusions were drawn byZhang et al. (1996) on eclogites and country rocksfrom the Weihai area, northern Su±Lu Orogen.

ACKNOWLEDGEMENTS

Fieldwork in the Dabie Shan was ®nanciallysupported by the National Natural ScienceFoundation of China and the Anhui Institute ofGeology, Hefei (People's Republic of China). Thetravel expenses and laboratory work of DC, RCand FR were supported by a special grant of theUniversity of Turin (Italy) and by funds of CNR,Centro di Studio sulla Geodinamica delle CateneCollisionali, Torino. Minerals were analysedusing the EDS/SEM Cambridge microprobe in-stalled at the Department of Mineralogical andPetrological Sciences, the University of Turin.Jiang Laili is acknowledged for help in editingthe tectono-metamorphic sketch-map of Fig. 1and for fruitful discussions on regional geology ofDabie Shan. Reviews by Ye Kai and Zhai Ming-guo were greatly appreciated. We are especiallyindebted to J. G. `Louie' Liou for careful readingand constructive suggestions. Simon R. Wallis isacknowledged for critical reading.

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