prograde pressure-temperature path of jadeite-bearing eclogites and associated...

Island Arc (2006) 15, 483–502

© 2006 The AuthorsJournal compilation © 2006 Blackwell Publishing Asia Pty Ltd

doi:10.1111/j.1440-1738.2006.00545.x

Blackwell Publishing AsiaMelbourne, AustraliaIARIsland Arc1038-4871© 2006 The Authors Journal compilation © 2006 Blackwell Publishing Asia Pty LtdDecember 2006154483502Research ArticleJadeite-bearing eclogites from western Tianshan, ChinaW. Lin and M. Enami

*Correspondence.

Received 13 May 2005; accepted for publication 28 April 2006.

Research ArticlePrograde pressure-temperature path of jadeite-bearing eclogites and

associated high-pressure/low-temperature rocks from western Tianshan, northwest China

WEI LIN1,2 AND MASAKI ENAMI1*1Department of Earth and Planetary Sciences, Nagoya University, Chikusa-ku, Nagoya 464-8602, Japan (email:

[email protected]) and 2State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China

Abstract Jadeite-bearing eclogites and associated blueschists locally crop out in a green-schist facies area at Kuldkourla, near the Akeyazhi River in the western Chinese Tianshanregion, northwestern China. Garnet in these metamorphic rocks shows prograde zoningwith increasing Mg and decreasing Mn from the crystal center towards the rim, and isdivided into Ca-poor/Fe-rich core and Ca-rich/Fe-poor mantle parts. The garnet coresinclude the assemblages of (i) jadeite/omphacite (Xjd = 0.34–0.96) + barroisite/taramite; and(ii) omphacite + barroisite/pargasite, with paragonite, epidote, rutile and quartz as majorphases with rare albite. The garnet mantles rarely contain inclusions of omphacite, glau-cophane, epidote, rutile and quartz. Major matrix phases of the pre-exhumation stage areomphacite, glaucophane, paragonite, rutile and quartz. These mineral parageneses givepressure (P)-temperature (T) conditions of 0.9 GPa/390°C−1.4 GPa/560°C for the stage ofthe garnet core formation, 1.8 GPa/520°C for the stage of the garnet mantle formation,and 2.2 GPa/495°C-2.4 GPa/535°C for the peak eclogite facies assemblage in the matrix.The estimated P-T conditions and continuous changes of mineral parageneses imply acounterclockwise P-T path which is a combination of (i) an early prograde stage of high-pressure/low-temperature (HP/LT) blueschist facies and/or LP/LT eclogite facies; (ii) alater prograde stage involving compression with minimal heating; and (iii) a climax-of-subduction stage characterized by a slight decrease of temperature with increasing pres-sure. The negative dP/dT of the latest subduction stage is possibly a record of the followingevents after a continuous subduction and ridge approach: (i) material migration within theupper part of the subducting slab, which has an inverse thermal gradient caused by ductileflow and/or slab break during subduction; and/or (ii) temporary cooling of the wedgemantle–slab interface by continuous subduction of a relatively cold slab following subduc-tion of a hotter ridge.

Key words: China, counterclockwise pressure-temperature path, eclogite, glaucophane,high-pressure/ low-temperature metamorphism, jadeite, western Tianshan.

INTRODUCTION

Blueschists and related eclogitic rocks are widelydistributed along many paleo-suture zonesincluding the late Precambrian-Paleozoic orogens

between pre-existing cratons (e.g. Sobolev et al.1986; Dobretsov et al. 1987; Liou et al. 1990;Dobretsov & Buslov 2004). Their metamorphicpressure (P)-temperature (T) paths play animportant role in deciphering the tectonic pro-cesses at convergent plate margins includingmaterial interactions between the crust andupper mantle.

484 W. Lin and M. Enami


The south Tianshan suture zone, as a subduc-tion-collision belt of Late Paleozoic age (Gao &Klemd 2003; Klemd et al. 2005), extends from theCentral Asian republics (Uzbekistan, Tajikistan,Kyrgyzstan and Kazakhstan) to northwest China(Volkova & Budanov 1999; Dobretsov & Buslov2004) (Fig. 1). Gao et al. (1995) first described min-eralogical and petrological characteristics of blue-schists from the Kekesu area of western ChineseTianshan (simply denoted as western Tianshanhereafter) belonging to the Tianshan orogenicbelt (Fig. 1), and discussed their metamorphicP-T path, deducing peak-metamorphic conditionsof 0.9–1.0 GPa/450°C. Subsequently, Gao (1997)reported eclogites and associated high-pressure/low-temperature (HP/LP) metamorphic rocksfrom the Changawuzhi and Keburt-Akeyazhiareas (Fig. 2) in western Tianshan, and their peak-metamorphic conditions were estimated as 1.4–2.1 GPa/480–580°C (Gao et al. 1999; Klemd et al.2002). Wei et al. (2003) recognized two groups ofeclogites showing different mineral assemblagesand equilibrium P-T conditions of 1.6–1.9 GPa/540–580°C (glaucophane eclogite) and 1.7–1.8 GPa/610–630°C (hornblende eclogite) from the Keburt-

Akeyazhi area. Zhang and his collaboratorsreported that some eclogites and associatedschists experienced ultrahigh-pressure (UHP)equilibrium based on the occurrences of exsolutionrods of possible relict coesite with quartz inomphacite (Zhang et al. 2005) and magnesite-bearing glaucophane-eclogite (Zhang et al. 2002b)from the Akeyazhi area. Subsequently, Zhang et al.(2003a) reported magnesite and calcite inclusionsin dolomite from metapelites of the Changawuzhiarea and interpreted this as evidence for an equi-librium coexistence of magnesite and aragoniteat >5 GPa/560–600°C. It is almost certain thatsome eclogites and associated HP/LT metamorphicrocks from the western Tianshan area underwentUHP metamorphism, along with those from otherregions of the Tianshan orogenic belt (e.g. Tagiriet al. 1995; Dobretsov & Buslov 2004). The varia-tions of P-T conditions of the western Tianshaneclogites, however, have yet to be revealed (Klemd2003; Zhang et al. 2003b), and it is not conclusivethat all the Tianshan eclogitic rocks recrystallizedunder UHP metamorphic conditions.

In western Tianshan, eclogites sporadicallyoccur in the Changawuzhi and Keburt-Akeyazhi

Fig. 1 Structural map of the western Chinese Tianshan region (modified from Carroll et al. 1995; Gao et al. 1998).

Jadeite-bearing eclogites from western Tianshan, China 485


areas (Fig. 2). The eclogites and associated HP/LT metamorphic rock described in this paper(simply denoted as eclogites hereafter) were col-lected from the Kuldkourla summer pasture,which is situated in the northern part of the HP/LT metamorphic belt in western Tianshan(Fig. 2). In our samples, well-preserved mineralparageneses of inclusions in porphyroblastic gar-net and zoned minerals in the matrix complementinformation on the prograde and retrogrademetamorphic stages. The estimated P-T condi-tions and P-T path will contribute to our under-standing of the metamorphic evolution of westernTianshan. In the present paper, we (i) report newmineralogical data for an eclogite including jade-ite coexisting with albite and quartz and otherHP/LT metamorphic rocks; (ii) describe a coun-terclockwise P-T path recorded in these samples;and (iii) discuss possible tectonic interpretationsof the counterclockwise P-T path. In this paper we

refer to the period of subduction with increasingpressure as the ‘prograde stage’.

Abbreviations for minerals and end-membersdescribed in the text, figures and tables followKretz (1983) and Miyashiro (1994) except for Lsp(less sodic pyroxene), Acm (acmite), Bar (bar-roisite) and Sam (subcalcic amphibole). Mineralformulae used for descriptions of reactionrelations follow Holland and Powell (1998). Abbre-viations for element-sites are: [6], octahedral M2-sites; [B], decahedral B-sites of amphibole; [A],10-coordinated A-site of amphibole.

GEOLOGICAL SETTING

In central Asia, the Tianshan region is a vast oro-genic belt which was bounded by the Kazak shieldto the northwest, the Junggar basin to the north-east and east, and the Tarim basin to the south. It

Fig. 2 Geological map of the western Tianshan HP/LT metamorphic belt (after the geological maps of 1:200 000 of XJBGMR-Moher 1978; XJBGMR-Zhaosu 1978; XJBGMR-Kayintekalasu 1979; XJBGMR-Hantenggelifeng 1982; XJBGMR-Quexiang 1982; XJBGMR-Heiyingshan 1984). Sporadic localitiesof eclogites are after Gao et al. (1999), Zhang et al. (2001), Gao and Klemd (2001, 2003), Klemd et al. (2002, 2005), Wei et al. (2003) and our field surveyin 2005.



extends broadly east–west for over 2500 km acrosscentral Asia, and exhibits some of the highest sub-aerial topographic relief on Earth. During thePaleozoic, several continental accretions to thesouthern margin of the Eurasian continent-arcsystem formed the huge orogenic collage namedthe Altaids (cf. Sengör et al. 1993). The Tianshanorogenic belt, a part of the Altaids, is consideredto have formed during tectonic amalgamation ofthe Tarim, Junggar and Yili–Central Tianshan (orYili- Kazakhstan) plates. In the western Tianshanregion near the glacier zone, HP/LT metamorphicrocks formed by the subduction and collisionbetween the Yili-Central Tianshan and Tarimplates are exposed in a late Paleozoic orogenic beltabout 200 km in length (Fig. 1). This HP/LT meta-morphic belt extends westward to connect with theAtbashy and Mailisui eclogite-blueschist belt inKazakhstan and Karategin blueschist belt ofTajikistan (Sobolev et al. 1986; Tagiri et al. 1995;Volkova & Budanov 1999; Dobretsov & Buslov

2004), and eastward to link up with the Kumishiblueschist belt of the middle segment of the southTianshan suture (Gao et al. 1995).

The HP/LT metamorphic zone of western Tian-shan consists mainly of blueschist facies and green-schist facies, metabasites and metapelites (Klemdet al. 2002). Minor late Silurian marble lenses andslices of ultramafic rocks appear as ‘exotic blocks’of mélange (Gao et al. 1999). This zone seems torepresent an accretionary wedge situated on thesouth side of the Yili-Central Tianshan plate andis involved in the late Paleozoic subduction.

The entire Tianshan HP/LT metamorphic zonehas been retrogressively metamorphosed undergreenschist facies conditions during exhumation.Blueschists occur as basic intercalations withinthese greenschist facies metamorphic rocks(Fig. 2). The eclogitic rocks commonly appear aspods, sheared boudins, thin layers or massiveblocks in the metabasites, metapelites and marblesof the blueschist facies (Fig. 3). The poor access

Fig. 3 Photographs showing the field occurrence of eclogite in western Tianshan. (a) Omphacite-rich laminae in the blueschist; (b) eclogite pods withinfolded blueschist; (c) massive eclogite in greenschist; and (d) eclogite boudin in micaschist.



and outcrop make it difficult to survey the Tian-shan region geologically, and no clear boundary isdrawn between the greenschist and blueschistunits on the available large-scale geological maps(XJBGMR-Zhaosu 1978; XJBGMR-Quexiang 1982;XJBGMR-Heiyingshan 1984). In the north, theHP/LT metamorphic belt is separated from anamphibolite facies metamorphic unit by a myloniticzone. This amphibolite facies unit is considered tobelong entirely to the basement of the Yili-CentralTianshan block in the early Proterozoic age (XJB-GMR-Zhaosu 1978; Dong 1990, 1993) or to be aPaleozoic active continental margin paired with thesubduction related to the HP/LT metamorphic belt(Gao et al. 1998; Gao & Klemd 2003). In the south,steeply dipping subvertical mylonitic marblesseparate the HP/LT belt from a Silurian sedimen-tary unit (XJBGMR-Moher 1978; XJBGMR-Hantenggelifeng 1982; XJBGMR-Quexiang 1982).Gao and Klemd (2003) obtained Sm-Nd isochronages of 343 ± 44 Ma and 345 ± 3 Ma from eclogites,and 40Ar/39Ar plateau ages of 344 ± 1 Ma for cros-site and 331 ± 2 Ma for phengite from a blueschistsample. They interpreted the Sm-Nd ages as thoseof HP-peak metamorphism and the 40Ar-39Ar agesas those related to the exhumation stage. In con-trast, Zhang et al. (2002a) considered that theUHP-HP peak age is 310 ± 5 Ma or younger basedon their SHRIMP zircon analyses. Wang et al.(1994) reported 40Ar-39Ar amphibole plateau age ofc. 229 Ma and concluded that the exhumation ageof these HP/LT metamorphic rocks was much later.Although the reported geochronological resultsshow considerable scatter, tectonic events relatedto the HP/LT metamorphism in the Tianshanregion are considered to have certainly ended bythe Triassic on the basis of stratigraphic investiga-tions (XJBGMR-Quexiang 1982).

In western Tianshan, most eclogites crop outin the Changawuzi and Keburt-Akeyazhi areas(Fig. 2). Along the northern margin of the HP/LTmetamorphic belt, the foliation trends northeast–southwest and dips to northwest with a down-dip

mineral stretching lineation. Shear criteria in thesection parallel to this lineation (e.g. sigmoidalmafic rock lenses, shear bands, asymmetric pres-sure shadows around the garnet, etc.) consistentlyindicate top-to-the-northwest shearing. Generaltrends of the regional lineations in the greenschist-blueschist-eclogite unit are parallel or subparallelto those of the amphibolite unit of the Yili–CentralTianshan plate.

The samples studied (TS191, 192 and 193) werecollected from metabasic blocks that crop out inthe Kuldkourla summer pasture of the HP/LTbelt (Fig. 2). In the Kuldkourla area northeast–southwest oriented foliation is parallel to theregional tectonic trend of the HP/LT belt and dipsto northwest with a northwest–southeast trendingweak mineral stretching lineation. Samples TS191and 192 were collected from core and marginalparts of a block of metabasite, respectively. In thefield, foliation appears to be only weakly developedin this eclogitic block, but is much clearer in thesurrounding greenschist facies rocks. Northwest–southeast oriented mineral lineation is defined byarrangements of acicular and/or prismatic parago-nite, and omphacite grains were observed aroundgarnet porphyroblasts in thin sections. SampleTS193 was collected from an outcrop 2 km north-west of the TS191 and 192 locality. At this outcrop,blueschist with eclogite (TS193) survive as a len-ticular zone in a heavily retrograded greenschistmatrix. The clear foliation in the greenschistfacies rocks demonstrates relatively strongerdeformation.

PETROGRAPHY

Mineral assemblages of the samples studied arelisted in Table 1.

Sample TS191 is a typical glaucophane-eclogitewith garnet, omphacite, glaucophane, paragonite,rutile and quartz as major matrix phases (Fig. 4a).Garnet commonly contains inclusions of jadeite/

Table 1 Mineral assemblages of eclogites from the western Tianshan HP/LT metamorphic belt, northwest China

Sample No. Grt Cpx Nam Sam Cam Ep/Zo Pg Phe Chl Ab Qtz Rt Dol Others

TS191 mr + mr i/r i/r i + i/r c/r + + Ap (Tit, Cal)TS192 mr i mr r r + + r i/r r + + mr Ap (Tit, Cal)TS193 mr +/r mt/mr i/r i/r + + mr i/r r + + mr Ap Py (Tit, Cal)

Abbreviations for minerals and their mode of occurrence: Grt, garnet; Cpx, clinopyroxene; Nam, sodic amphibole; Sam, subcalsicamphibole; Cam, calcic amphibole; Ep, epidote; Zo, zoisite; Pg, garagonite; Phe, phengite; Chl, chlorite; Ab, albite; Qtz, quartz; Rt, rutile;Dol, dolomite; Ap, apatite; Tit, titanite; Cal, calcite; Py, pyrite; +, inclusion in garnet and matrix phase; mr, matrix phase; i, inclusion ingarnet; c, inclusion in garnet core; mt, inclusion in garnet mantle; r and Minerals in parenthesis, retrograde phase during exhumation.



omphacite, barroisite/taramite, paragonite, epi-dote, chlorite, rutile and quartz. Cracks developaround these inclusions and jadeite/omphaciteinclusions are decomposed into polyphase aggre-gates of less sodic pyroxene and albite to somedegree (Fig. 5a,b). Two isolated crystals of albiteare also identified as inclusions in the core part ofthe garnet in TS191 (cf. Figs 5c,6d). This type ofalbite inclusion accompanies no secondary and less

Fig. 4 Photomicrographs (open nicol) showing the textural relation-ships of major phases in the eclogites from the western Tianshan HP/LTmetamorphic belt. (a) Euhedral garnet porphyroblast and matrix consist-ing of omphacite, glaucophane, paragonite and quartz (TS191). (b) Garnetporphyroblast and fine aggregates composed mainly of glaucophane,epidote and quartz and later barroisite/actinolite and albite. Progradeparagonite is latterly replaced by thin chlorite and/or phengite films(TS192). (c) Eclogite consisting of euhedral garnet and glaucophane, andaggregate of omphacite, paragonite, phengite, dolomite and quartz(TS193). Abbreviations for minerals are; Omp, omphacite; Gln, glau-cophane; Bar, barroisite; Act, actinolite; others are defined in Table 1.

Fig. 5 Back-scattered electron (BSE) (a, c) and sketch (b) images ofsodic phases included in garnet of TS191 (cf. Fig. 6). Jadeite inclusionin garnet is partly decomposed into less sodic pyroxene and albite (a, b)during exhumation. Prograde albite is rarely preserved as a textuallyhomogeneous single grain in garnet. Abbreviations for minerals are; Jd,jadeite; Lsp, less sodic pyroxene; others are defined in Table 1.



sodic pyroxene, and can be texturally distin-guished from albite pseudomorphs after jadeite/omphacite inclusions. The jadeite content of sodicpyroxenes systematically increases during theearly prograde stage, as will be described later (cf.Fig. 7). These textural and chemical characteris-tics indicate that the isolated albite inclusions inthe garnet were stable at an early stage of pro-grade metamorphism in equilibrium with pro-grade sodic pyroxene and quartz. In the matrix,albite forms retrogressive rims around omphacite,glaucophane and paragonite. Glaucophane isrimmed by barroisite/actinolite (Fig. 8). Apatiteoccurs as both garnet inclusions and as an acces-sory phase in the matrix.

Sample TS192 has similar inclusion assemblagesin garnet to those of TS191 but without progradeamphibole and albite. The matrix was stronglyrecrystallized during exhumation, and is com-posed of barroisite/actinolite, epidote, paragonite,phengite, chlorite, albite and quartz with acces-sory rutile, titanite, dolomite and apatite (Fig. 4b).Glaucophane occurs as cores of zoned amphiboles.Among the matrix phases, barroisite/actinolite,chlorite, albite and titanite are retrograde prod-ucts after the peak HP-metamorphic stage.

Sample TS193 has slightly different mineralassemblage from that of TS191 and 192. It consistsmainly of garnet, omphacite, glaucophane, epidote,phengite, paragonite, rutile, dolomite and quartz(Fig. 4c). Garnet commonly includes omphacite,

barroisite/taramite/pargasite/tschermakite, epi-dote, paragonite, chlorite, rutile and quartz, andno isolated albite crystal. Glaucophane inclusionsare observed only in the mantle part of the garnet.In the matrix, albite replaces paragonite, glau-cophane and omphacite, and barroisite/actinoliteand chlorite rim glaucophane. Dolomite is replacedby calcite and chlorite.

MINERAL CHEMISTRY

Quantitative analyses and X-ray mapping werecarried out using a JEOL JXA-8800R(WDS + EDS) electron-probe microanalyzer atthe Petrological Laboratory, Nagoya University.Accelerating voltage and specimen current forquantitative analyses were 15 kV and 12 nA on theFaraday cup, respectively. A beam diameter of5 µm was used for paragonite, phengite and car-bonates analyses, and 2–3 µm for analysis of allother phases. Well-characterized natural and syn-thetic phases were used as standards. The ZAFmethod was employed for matrix correction.Amphibole nomenclature follows Miyashiro (1957)and Leake et al. (1997), and Fe3+/Fe2+ values werecalculated with total cations of 13 excluding Ca,Ba, Na and K (O = 23). This is based on the fact

Fig. 6 X-ray mapping images of euhedral garnet of TS191 in terms of(a) MgKα (b) CaMα (c) FeKα and (d) MnKα. Brighter shades indicatehigher concentration of elements.

Fig. 7 Systematic change of Xjd content of sodic pyroxenes in garnetand matrix in an eclogite (TS191) from the western Tianshan HP/LTmetamorphic belt. Sodic pyroxene composition is represented by thetextually and chemically prograde part that shows jadeite-richest analysis(cf. Fig. 5). Garnet composition indicates average value of several analy-ses around individual sodic pyroxene inclusion.



that substitutions of (Fe2+ + Mn + Mg) into theoctahedral M (4) sites of sodic amphiboles are usu-ally low in HP/LT metamorphic rocks (Deer et al.1997). For garnet, all iron was assumed to beferrous and its end-member proportion (Xi) wascalculated as i/(Fe + Mn + Mg + Ca). The ferriciron content of clinopyroxene was estimated asFe3+ = Na – Al assuming a negligible amount oftschermak substitution. Thus chemical character-istics of sodic pyroxene are discussed in terms ofthe following components: Xjd = Al, Xacm = Fe3+ andXaug = 1 − (Al + Fe3+), where ‘aug’ means Ca-Mg-Fepyroxenes (Quad) component of Morimoto et al.(1988).

GARNET

Garnets belong to the almandine-pyrope seriesand are Mn-poor (less than 2.2 wt% MnO) and Ca-rich (up to 9.8 wt% CaO) (Table 2). They showprograde zoning with increasing Xprp and decreas-ing Xsps from the core towards the rim, and aredivided into relatively Ca-poor/Fe-rich core and

Ca-rich/Fe-poor mantle parts (Fig. 6). Composi-tional ranges of garnet are Xalm(0.59–0.69)Xprp(0.06–0.17) Xsps(0.00–0.04) Xgrs(0.19–0.25) inTS191, Xalm(0.62–0.70) Xprp(0.07–0.13) Xsps(0.00–0.04) Xgrs(0.18–0.24) in TS192 and Xalm(0.60–0.73)Xprp(0.05–0.13) Xsps(0.00–0.05) Xgrs(0.18–0.27) inTS193 (Fig. 9). Cr2O3 and TiO2 contents are lessthan 0.05 wt% and 0.2 wt%, respectively. No localmodification of chemical composition was observedin the garnet around sodic pyroxene and otherinclusions.

SODIC PYROXENE

The compositional range and zonal structures ofsodic pyroxenes in samples TS191 and 192 areappreciably different from those of TS193(Table 3). TiO2, Cr2O3 and MnO contents are lessthan 0.4 wt%, 0.1 wt% and 0.3 wt%, respectively.

Prograde sodic pyroxene included in the garnetof TS191 has wide compositional ranges ofXjd(0.34–0.83) Xaug(0.14–0.45) Xacm(0.01–0.20)(Fig. 10a) and shows systematic compositional

Fig. 8 X-ray mapping images of zoned glaucophane-barroisite of TS193 in terms of (a) NaKα (b) AlKα (c) CaKα and (d) FeKα. Warmer colors indicatehigher concentrations of elements. Abbreviations for minerals are defined in Fig. 4 and Table 1.



variations in position within the garnet. Jadeitecontents of prograde sodic pyroxene included inthe garnet gradually increase from 0.35 to 0.50in the garnet inner core, up to 0.80 in the outercore, and decrease discontinuously to 0.37–0.55 inthe garnet mantle (Fig. 7). Sodic pyroxene in thematrix shows no systematic chemical zoning andhas a similar compositional range [Xjd(0.37–0.56)Xaug(0.38–0.48) Xacm(0.01–0.18)] to that of sodicpyroxene included in the garnet mantle. The Mg#[= Mg/(Mg + Fe2+)] value of sodic pyroxene in-cluded in garnet (0.62–0.85) shows negative corre-lation with Xjd and tends to be lower than that inthe matrix (0.80–0.86). Sodic pyroxene included inthe garnet of TS192 also has a wide compositionalrange [Xjd(0.48–0.96) Xaug(0.04–0.37) Xacm(0.00–0.21)], but shows no systematic variation as in thecase of TS191 (Fig. 10b). The Mg# value is alsovariable from 0.22 in jadeite to 0.68 in less sodicomphacite.

Sodic pyroxene in TS193 is less sodic than thatin TS191 and 192. Sodic pyroxene in the matrix isusually chemically zoned with increasing Xjd andMg# and decreasing Xacm from the core to the rim[Xjd(0.23–0.43) Xaug(0.46–0.58) Xacm(0.08–0.23) andMg# = 0.63–0.83], though some grains are partlyrimmed by retrograde pyroxene with less sodic

composition. The compositional range of sodicpyroxene included in garnet is Xjd(0.15–0.33)Xaug(0.47–0.61) Xacm(0.18–0.27) and Mg# = 0.58–0.75, and overlaps with that of core part of matrixsodic pyroxene (Fig. 10c).

AMPHIBOLE

Amphiboles included in garnet are confirmed inTS191 and 193, and most of them are barroisite,taramite, pargasite and tschermakite withSi = 6.14–7.44 per formula unit (pfu), [B]Na = 0.26–1.19 pfu and [A](Na + K) = 0.04–0.67 pfu (Table 4and Fig. 11). In TS193, glaucophane with YAl

[= [6]Al/([6]Al + Fe3+)] = 0.75–0.87 and Mg# = 0.67–0.77 is rarely included in the mantle part of thegarnet. On the contrary, texturally progradeamphiboles in the matrix are glaucophane withYAl = 0.70–1.00 and Mg# = 0.57–0.82. These matrixglaucophanes commonly show a zonal structurewith increasing Al and decreasing Fe and Ca fromthe core towards the mantle (Fig. 8), showingan increase of glaucophane component duringcrystallization. The zoned glaucophanes arerimmed by barroisite and/or actinolite. Thesesubcalcic and calcic amphiboles around glau-cophane are more silicic than those included in

Table 2 Representative microprobe analyses of garnet in eclogites from the western Tianshan HP/LT metamorphic belt,northwest China

Sample No.Position

TS191 TS192 TS1933IC OC Mantle OR IC OC Mantle IC OC Mantle OR

SiO2 38.0 38.2 38.3 38.1 37.8 38.0 38.0 37.4 37.9 38.3 38.4TiO2 0.11 0.03 0.06 0.02 0.21 0.14 0.08 0.11 0.05 0.05 0.00Al2O3 21.6 21.4 21.6 21.8 21.2 21.4 21.6 21.0 21.3 21.3 21.7Cr2O3 0.00 0.00 0.00 0.02 0.07 0.00 0.00 0.02 0.03 0.05 0.00FeO† 30.3 30.2 27.6 27.1 30.2 29.8 28.1 32.4 30.7 30.2 27.7MnO 0.85 0.47 0.06 0.06 1.64 0.47 0.16 0.90 0.41 0.42 0.41MgO 2.54 2.96 3.94 4.15 1.77 2.74 3.22 1.34 2.78 2.87 3.24CaO 7.65 7.53 8.45 8.65 7.64 7.32 8.48 7.28 7.76 7.72 9.50Total 101.05 100.79 100.01 99.90 100.53 99.87 99.64 100.45 100.93 100.91 100.95Cations per 12 oxygensSi 2.99 3.01 3.01 2.99 3.01 3.01 3.00 3.00 2.99 3.01 3.00Ti 0.01 0.00 0.00 0.00 0.01 0.01 0.01 0.01 0.00 0.00 0.00Al 2.01 1.99 2.00 2.02 1.99 2.00 2.01 1.98 1.98 1.98 2.00Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00Fe2+† 2.00 1.99 1.81 1.78 2.01 1.98 1.86 2.17 2.03 1.99 1.81Mn 0.06 0.03 0.00 0.00 0.11 0.03 0.01 0.06 0.03 0.03 0.03Mg 0.30 0.35 0.46 0.49 0.21 0.32 0.38 0.16 0.33 0.34 0.38Ca 0.65 0.64 0.71 0.73 0.65 0.62 0.72 0.63 0.66 0.65 0.80Total 8.00 8.00 7.99 8.00 7.99 7.98 7.99 8.01 8.01 8.00 8.00Xalm 0.67 0.66 0.61 0.59 0.67 0.67 0.63 0.72 0.67 0.66 0.60Xsps 0.02 0.01 0.00 0.00 0.04 0.01 0.00 0.02 0.01 0.01 0.01Xprp 0.10 0.12 0.15 0.16 0.07 0.11 0.13 0.05 0.11 0.11 0.13Xgrs 0.22 0.21 0.24 0.24 0.22 0.21 0.24 0.21 0.22 0.22 0.26

†Total iron as FeO. IC, inner position of core; OC, outer position of core; OR, outermost rim.



garnet [Si = 6.64–7.97 pfu, [B]Na = 0.00–1.01 pfuand [A](Na + K) = 0.00–0.49 pfu]. TiO2, Cr2O3 andK2O contents are less than 0.3 wt%, 0.1 wt% and0.5 wt%, respectively.

OTHER MINERALS

Representative chemical analyses of other con-stituent minerals are shown in Table 5. Mostwhite mica is paragonite with XNa [= Na/

(Na + K + Ca + Ba)] = 0.91–0.96. K-white mica isphengitic with Si = 3.32–3.45 pfu, XNa = 0.04–0.10and BaO = 0.2–0.9 wt%. The anorthite content[XCa = Ca/(Na + K + Ca + Ba)] of albite single crys-tals included in garnet is fairly constant around0.03, while that of pseudomorphs after sodicpyroxene ranges from 0.02 to 0.09. The pistacitecontent [XFe = Fe3+/(Fe3+ + Al + Cr), with Fe3+ =total Fe] of zoisite is 0.04, and that of epidote isvariable in each sample: 0.08–0.29 in TS191, 0.10–

Fig. 9 Chemical compositions of garnet ineclogites from the western Tianshan HP/LTmetamorphic belt. Abbreviations of end-mem-bers are: alm, almandine; sps, spessartine;prp, pyrope; grs, grossular.

Table 3 Representative microprobe analyses of pyroxene in eclogites from the western Tianshan HP/LT metamorphic belt,NW China

Sample No.ModeStage

TS191 TS192Grt-inCore

TS193Garnet inclusion Matrix Grt-in

MantleMatrix

IC OC Mantle

SiO2 55.9 58.3 57.0 55.9 58.9 54.2 55.5TiO2 0.03 0.00 0.05 0.04 0.00 0.13 0.03Al2O3 9.82 19.6 13.4 11.9 23.4 7.08 9.61Cr2O3 0.00 0.00 0.00 0.06 0.00 0.05 0.00FeO† 9.73 3.83 4.03 4.22 2.09 12.9 6.99MnO 0.21 0.03 0.04 0.03 0.02 0.10 0.04MgO 5.54 2.35 6.37 7.11 0.29 6.03 7.57CaO 10.6 3.85 10.5 12.2 0.60 11.7 12.7Na2O 8.48 12.6 8.59 7.77 14.5 7.81 7.30K2O 0.01 0.01 0.00 0.00 0.00 0.00 0.01Total 100.32 100.57 99.98 99.23 99.80 100.00 99.75Cations per 6 oxygensSi 2.00 2.00 2.00 1.99 2.00 1.98 1.99Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00Al 0.41 0.79 0.56 0.50 0.94 0.30 0.41Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00Fe3+‡ 0.18 0.05 0.03 0.04 0.02 0.25 0.10Fe2+‡ 0.12 0.06 0.09 0.09 0.04 0.15 0.11Mn 0.01 0.00 0.00 0.00 0.00 0.00 0.00Mg 0.30 0.12 0.33 0.38 0.02 0.33 0.41Ca 0.41 0.14 0.40 0.47 0.02 0.46 0.49Na 0.59 0.84 0.59 0.54 0.96 0.55 0.51K 0.00 0.00 0.00 0.00 0.00 0.00 0.00Total 4.02 4.00 4.00 4.01 4.00 4.02 4.02

Xjd 0.41 0.79 0.56 0.50 0.94 0.30 0.40Xaug 0.41 0.16 0.41 0.47 0.04 0.46 0.50Xacm 0.18 0.05 0.03 0.04 0.02 0.24 0.10

†Total iron as FeO; ‡Calculated values (see text). Grt-in, garnet inclusion; IC, inner core of garnet; OC, outer core of garnet.



0.21 in TS192 and 0.14–0.29 in TS193. The calciumcontent of epidote is usually less than 1.9 pfu; thisand the wide XFe variations may be partly due toFe2+ incorporation into epidote by Fe2+REE(Fe3+,Al)-1Ca-1 substitution. Carbonates have variablechemical compositions of Xcal(0.49) Xmgs(0.36)Xsd(0.15) in TS192 and Xcal(0.49) Xmgs(0.39–0.42)Xsd(0.9–0.12) in TS193

DISCUSSION

METAMORPHIC EVOLUTION OF THE KULDKOURLA ECLOGITES

The textural relationships and compositional char-acteristics of minerals in the Kuldkourla eclogitespreserve information on the P–T path of the west-ern Tianshan HP/LT metamorphic rocks. Thesequences of mineral growth and stability aresummarized in Table 6. Mineral equilibria werecalculated using the THERMOCALC program(ver. 3.21) with the updated HP98 dataset (Holland

& Powell 1998). The activity models for mineralsused in the P-T estimations are obtained using theAX2 program (ver. 2.2) unless otherwise noted.

Prograde evolution

Early prograde stage: The presence of a singlephase of albite included in the garnet core, andsystematic increase of Xjd content of sodic pyrox-ene inclusions from the inner part towards theouter part of the garnet core (TS191: Fig. 7) sug-gest that the equilibrium coexistence of sodicpyroxene, albite and quartz at an early stage ofprograde metamorphism, and thus the major min-eral paragenesis at this stage is garnet + omphac-ite/jadeite + barroisite/taramite + paragonite +epidote + chlorite + albite + rutile + quartz inTS191. The P-T conditions of this stage can beestimated using a combination of the sodic pyrox-ene-albite-quartz geobarometer (e.g. Essene &Fyfe 1967; Newton & Smith 1967) and the follow-ing garnet-clinopyroxene Fe-Mg geothermometer:Ai (1994), Krogh Ravna (2000), and Ellis andGreen (1979) calibration with activity models ofHolland (1990) for clinopyroxene and of Bermanand Aranovich (1996), Ganguly et al. (1996) andMukhopadhyay et al. (1997) for garnet. Variationsof estimated temperatures among the employingfive garnet-clinopyroxene geothermometers are20–35°C at 1.0–1.5 GPa (Table 7); the calibration ofKrogh Ravna (2000), which gives an intermediateestimate of temperatures among the five calibra-tions at P = 1.5 GPa, is applied to the discussion inthis paper. The P-T conditions estimated progres-sively increase from 0.9 GPa/390°C in the innerpart to 1.4GPa/560°C in the outer part of garnetcores for TS191 (Fig. 12). The relatively low P/Tand T estimates explain the presence of calcic andsubcalcic amphibole inclusions in the core part ofthe garnet (also cf. Klemd et al. 2002; Wei et al.2003).

Later prograde stage: Reaction relationships atthis stage can be deduced from the mineralogicalcontrast between the formation stages of the gar-net core and mantle as follows: At the core–mantle boundary of the garnet (i) disappearanceof albite inclusions; (ii) drastic decrease of Xjd ofsodic pyroxene inclusions; (iii) increase of glau-cophane component in amphibole; and (iv) discon-tinuous increase of Xgrs of garnet. These factssuggest that the following two reactions mighthave progressed virtually simultaneously duringthis stage:

Fig. 10 Chemical compositions of sodic pyroxene in eclogites fromthe western Tianshan HP/LT metamorphic belt. Abbreviations of end-members are: jd, jadeite; aug, augite; acm, acmite.



albite = jadeite + quartz (1) 8 jadeite + 3 tremolite + 2 paragonite =

5 glaucophane + 2 grossular (2)

The combination of these two reactions suggeststhat the following reaction (3) possibly explains

compositional changes of amphibole, sodic pyrox-ene and garnet at the boundary between core andmantle parts of garnet.

8 albite + 3 tremolite + 2 paragonite =5 glaucophane + 2 grossular + 8 quartz (3)

Table 4 Representative microprobe analyses of amphibole in eclogites from the western Tianshan HP/LT metamorphicbelt, NW China

ModeStage

Ts191 TS192 TS193 Garnet inclusion Matrix Matrix Garnet inclusion MatrixCore Core PE ES ES PE ES ES Core Core Mantle PE ES

SiO2 49.0 42.4 58.3 48.5 55.4 58.0 48.5 54.4 48.2 40.4 58.7 57.40 51.2TiO2 0.14 0.22 0.04 0.22 0.00 0.04 0.18 0.05 0.23 0.18 0.03 0.00 0.08Al2O3 6.43 14.2 11.2 11.8 0.55 11.7 11.3 2.91 12.7 14.4 9.75 10.90 8.16Cr2O3 0.02 0.02 0.01 0.02 0.01 0.02 0.00 0.01 0.01 0.01 0.01 0.00 0.05FeO† 22.6 22.8 9.11 12.5 11.1 10.3 14.3 11.5 19.0 24.7 11.9 2.37 14.8MnO 0.07 0.11 0.00 0.07 0.07 0.09 0.18 0.09 0.04 0.16 0.07 6.45 0.04MgO 7.97 5.48 10.6 11.8 16.8 9.38 10.6 15.7 6.81 4.68 10.4 11.30 11.2CaO 8.80 8.39 0.34 7.48 12.6 0.21 8.34 10.2 5.37 9.85 0.46 0.79 6.95Na2O 2.51 4.25 7.32 3.98 0.19 7.36 3.57 1.56 5.29 2.96 7.40 7.17 3.83K2O 0.13 0.12 0.01 0.31 0.00 0.03 0.31 0.10 0.33 0.31 0.02 0.00 0.21Total 97.67 97.99 96.93 96.68 96.72 97.13 97.28 96.6 97.98 97.65 98.74 96.14 96.52Si 7.28 6.36 7.98 6.95 7.96 7.98 7.02 7.77 7.01 6.16 7.97 7.92 7.39Ti 0.02 0.03 0.00 0.02 0.00 0.00 0.02 0.01 0.03 0.02 0.00 0.00 0.01Al 1.13 2.51 1.81 1.99 0.09 1.90 1.93 0.49 2.18 2.59 1.56 1.77 1.39Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01Fe3+‡ 0.74 0.76 0.18 0.59 0.05 0.11 0.36 0.39 0.53 0.90 0.41 0.25 0.54Fe2+‡ 2.07 2.10 0.87 0.91 1.29 1.08 1.37 0.98 1.78 2.25 0.95 0.74 1.25Mn 0.01 0.01 0.00 0.01 0.01 0.01 0.02 0.01 0.01 0.02 0.01 0.00 0.01Mg 1.76 1.23 2.16 2.52 3.60 1.92 2.29 3.34 1.48 1.06 2.11 2.32 2.41Ca 1.40 1.35 0.05 1.15 1.94 0.03 1.29 1.56 0.84 1.61 0.07 0.12 1.08Na 0.72 1.24 1.94 1.11 0.05 1.96 1.00 0.43 1.49 0.88 1.95 1.92 1.07K 0.03 0.02 0.00 0.06 0.00 0.01 0.06 0.02 0.06 0.06 0.00 0.00 0.04

†Total iron as FeO; ‡Calculated values (see text). ES, exhumation stage; PE, peak eclogite facies stage.

Fig. 11 Chemical compositions of amphibole in eclogites from the western Tianshan HP/LT metamorphic belt.



The peak metamorphism stage: The presence ofglaucophane inclusions in the garnet mantle andthe zonal structure of matrix glaucophane indicatethat glaucophane was stable at the eclogite facies,

the peak metamorphism stage. Thus P-T condi-tions of this stage may be retrieved with referenceto the garnet + omphacite + glaucophane + para-gonite + epidote + rutile + quartz assemblage inTS191 and TS192. In TS193 phengite joins themineral paragenesis as a prograde phase. Theseparageneses lead the mineral equilibria describedby the following two reactions:

3 jadeite + pyrope + 2 quartz + 2 H2O =glaucophane + paragonite (4)

(Ota et al. 2004)

3 celadonite + pyrope + 2 grossular = 6 diopside + 3 muscovite (5)

(Waters & Martin 1993)A combination of garnet-clinopyroxene geother-

mometers and the reactions (4) and (5) yield esti-mates of P-T conditions of 2.4 GPa/535°C for the

Table 5 Representative microprobe analyses of mica andepidote in eclogites from the western Tianshan HP/LTmetamorphic belt, NW China

MineralMode

TS191Pg

Matrix

TS193Pg

Grt-inPhe

MatrixEp

Grt-in

SiO2 47.3 47.0 50.7 37.6TiO2 0.00 0.01 0.31 0.10Al2O3 39.6 39.4 25.8 22.5Cr2O3 0.00 0.01 0.00 0.00FeO† 0.17 0.86 2.96 14.6‡

MnO 0.00 0.00 0.01 0.22MgO 0.18 0.25 3.63 0.01BaO 0.00 0.00 0.52CaO 0.18 0.92 0.05 22.7Na2O 7.05 7.03 0.35 0.02K2O 0.56 0.12 10.40 0.00Total 95.04 95.60 94.73 97.75O 11 11 11 12.5Si 3.02 3.00 3.43 3.01Ti 0.00 0.00 0.02 0.01Al 2.98 2.96 2.06 2.12Cr 0.00 0.00 0.00 0.00Fe2+† 0.01 0.05 0.17 0.88‡

Mn 0.00 0.00 0.00 0.02Mg 0.02 0.02 0.37 0.00Ba 0.00 0.00 0.01 0.00Ca 0.01 0.06 0.00 1.95Na 0.87 0.87 0.05 0.00K 0.05 0.01 0.90 0.00Total 6.96 6.97 7.01 7.99

†Total iron as FeO, ‡Total iron as Fe2O3. Abbreviations aredefined in Table 1 and Fig. 4.

Table 6 Mineral parageneses for the different stages of evolution of eclogites from the western Tianshan HP/LT metamor-phic belt, northwest China.

Mineral Prograde (subduction) stage Retrograde(exhumation) stage

Inclusion in garnet Matrix

Core Mantle

Garnet ?ClinopyroxeneGlaucophaneBarroisite/TaramiteActinoliteEpidote/ZoisiteParagonitePhengite ?Chlorite ? ? ?AlbiteQuartzRutileTitaniteCarbonates

Table 7 Comparison of temperatures (°C) estimated usingvarious garnet-clinopyroxene geothermometers

Inner part(P = 1.0 GPa)

Outer part(P = 1.5 GPa)

BA96 + H90 465 565GCT96 + H90 475 570MHK97 + H90 460 550A94 440 570RK00 440 565

A94, Ai (1994); BA96, Berman and Aranovich (1996); GCT96,Ganguly et al. (1996); H90, Holland (1990); MHH97, Mukho-padhyay et al. (1997); RK00, Krogh Ravna (2000).



matrix paragenesis of TS191, and 1.8GPa/520°Cand 2.2–2.4 GPa/495–520°C for the garnet mantleand matrix parageneses of TS193, respectively(Fig. 12). The P-T estimations using equilibria inthe Fe2+-system gives 0.5–0.8 GPa lower pressureconditions than those in the Mg-system: 1.6 GPa/505°C for matrix paragenesis of TS191, and1.3 GPa/500°C and 1.8GPa/470°C for garnet man-tle and matrix parageneses of TS193, respectively.The considerable differences in pressure esti-mates are possibly due to uncertainties in the cal-culated Fe3+/Fe2+ values especially for amphibole.The reaction (5) has been used extensively asgeobarometer for eclogites (Carswell et al. 1997,2000; Nowlan et al. 2000; Krogh Ravna & Terry2004). Thus, the similarity of pressure estimatesbetween the reactions (4) and (5) in the Mg-systemfor TS193 suggest that the P-T conditions at thepeak eclogite facies stage might close those esti-mated by calibration in the Mg-system also forTS191 and 192.

The estimated HP/LT conditions of the Kuldk-ourla samples for the peak metamorphism stageapparently suggest the stable occurrence of law-sonite + sodic pyroxene assemblage, which is ahigh P/T equivalent of paragonite + epidote +quartz assemblage, in common basaltic bulk rock

compositions, as defined by the following reaction(Evans 1990; Wei et al. 2003; Hoschek 2004):

jadeite + 4 lawsonite = paragonite + 2 clinozoisite/zoisite + quartz + 6H2O (6)

However, paragonite, epidote, quartz andomphacite coexist in the matrices of TS192 and193, and no lawsonite is observed. The matrix epi-dote in TS193 has high XFe of 0.29–0.30, and thusone of the most possible reasons for the absenceof lawsonite in TS193 is that the stability field ofparagonite + epidote + quartz assemblage enlargestowards higher P/T side and that of omphacite +lawsonite assemblage relatively contracts due tothe Fe3+-rich composition of epidote: The maxi-mum pressure of paragonite + epidote + quartzstability defined by the reaction (6) is 2.6 GPa atT = 550°C for such a Fe3+-rich epidote-bearingassemblage. The absence of lawsonite in TS192 ispossibly due to strong retrograde metamorphismduring the exhumation stage.

Retrograde stage

The early retrograde stage is represented by themineral assemblage of omphacite/less sodic pyrox-ene + barroisite + paragonite + epidote + albite +rutile + quartz in the matrix. The sodic pyroxeneand barroisite become unstable and are rimmedby actinolite at a later stage. Thus the mineralparageneses at this later stage metamorphism isrepresented by actinolite-bearing assemblage ofthe greenschist facies.

The initial replacement of glaucophane by bar-roisite and subsequently actinolite indicate epi-dote-amphibolite facies and/or greenschist faciesoverprinting. Barroisite rims around glaucophaneoccur commonly in HP metamorphic rocks thathave undergone nearly isothermal decompressionto epidote-amphibolite facies conditions (e.g.Krogh 1980; De Sigoyer et al. 1997; Gao et al. 1999;Ota et al. 2004). This replacement has beenexplained by the reactions:

glaucophane + omphacite + garnet + H2O =barroisite + albite + chlorite (7)

(Gao et al. 1999)and

glaucophane + jadeite + garnet + quartz +H2O = barroisite (8)

(Ota et al. 2004).

Fig. 12 P-T estimates of the eclogites from the western Tianshan HP/LT metamorphic belt. Mineral equilibria were calculated using the THER-MOCALC program (ver. 3.21) with the updated HP98 dataset (Holland &Powell 1998). The activity models for minerals used in the P-T estimationsare obtained using the AX2 program (ver. 2.2). The uncertainties are about0.1 GPa/25–35°C for the estimated P-T conditions. Abbreviations forminerals are: Coe, coesite; Ky, kyanite; others are defined in Table 1 andFig. 5.



The inferred P-T path for this retrograde stageis similar to that discussed in the Keburt area byKlemd et al. (2002).

COMPARISON OF P-T PATHS OF TIANSHAN ECLOGITES

The P-T path of the Kuldkourla eclogites describedin this paper can be deduced from the sequentialchange of mineral parageneses and the P-T esti-mations discussed above (Fig. 13a). The eclogitesrecord relatively lower P/T paths (0.3 GPa/100°C)with the estimated P-T conditions of 0.9 GPa/390°C−1.4 GPa/560°C at the early prograde meta-morphism stage, implying the recrystallizationunder the LP/HT blueschist and/or LP/LT eclogitefacies conditions. The estimated P-T path at thisstage is similar to that of eclogites discussed byGao et al. (1999) and of glaucophane eclogites pro-posed by the P-T pseudosection analyses (Weiet al. 2003), but shows distinctly lower-pressureconditions compared to those discussed by Klemdet al. (2002) who presume prograde recrystalliza-tion from the lawsonite blueschist/lawsonite eclog-ite to epidote-glaucophane eclogite.

The P-T conditions of the peak metamorphismstage, for the eclogites from Kuldkourla, rangefrom 2.2 GPa/495°C to 2.4 GPa/535°C. These esti-mations are distinctly higher in pressure andslightly lower in temperature than the highestestimates for the early prograde metamorphism(1.4 GPa/560°C), indicating a very steep P-T pathduring the later prograde metamorphism stage.The compositional change of amphibole from bar-roisite to glaucophane during the prograde stageis well documented in glaucophane-eclogites fromsome HP metamorphic belts such as the westernNorway (Krogh 1980), northwest Himalaya (deSigoyer et al. 1997) and Sanbagawa (Aoya et al.2003). The Sanbagawa examples have recorded avery steep subduction-stage P-T path with dP/dT>0.7 GPa/100°C. The inferred steep prograde P-Tpath of the western Tianshan eclogite is consistentwith the observed progressive change of amphib-ole composition from barroisite/taramite to glau-cophane during prograde metamorphism (Fig. 11).The peak temperatures at the prograde stage forthe Kuldkourla eclogites are similar to those ofeclogites and blueschists from the Keburt-Akeyazhi area (Fig. 13b, 480–580°C: Gao et al.1999; Klemd et al. 2002). The peak pressureconditions of the Keburt-Akeyazhi samples are,however, estimated as 1.4–2.1 GPa (Fig. 13b, 480–580°C: Gao et al. 1999; Klemd et al. 2002), i.e.slightly lower than those of the Kuldkourla

Fig. 13 Comparison of P-T paths of the Tianshan HP/LT metamorphicrocks proposed in the present study (a) and literature (b and c). Abbrevi-ations of literature: G99, Gao et al. (Gao et al. 1999); K02, Klemd et al.(2002); W03, Wei et al. (2003); Z02a (Zhang et al. 2002a); Z02b (Zhanget al. 2002b); Z03a (Zhang et al. 2003a). Abbreviations for minerals are:Mgs, magnesite; Arg, aragonite; Dia, diamond; Gr, graphite; Cz; clinozo-isite; others are defined in Table 1 and Figs 3,4,6 and 12.



samples. The upper pressure limit of the Keburt-Akeyazhi samples described in the literaturewas defined by a net-transfer reaction ofparagonite = sodic pyroxene (Xjd = 0.5) + kyanite +H2O. The Keburt-Akeyazhi and Kuldkourla sam-ples, however, contain no kyanite, and the pro-posed equilibrium does not exactly define theupper pressure limit of the peak prograde stagefor the western Tianshan eclogites. The upperlimit of equilibrium pressure of paragonite-bear-ing and kyanite-free eclogite should be discussedby the stability of a single paragonite defined bythe following reaction:

paragonite = jadeite + kyanite + H2O (9)

(Holland 1979).Thus the pressure conditions for eclogites from

the Keburt-Akeyazhi area are possibly within theranges of 1.4–2.6 GPa at T = 550°C (cf. Fig. 12),and are not inconsistent with those estimated forthe Kuldkourla samples. The Kuldkourla samplesrecord no UHP equilibrium, in contrast to thosereported from the Changawuzhi and Keburt-Akeyazhi areas by Zhang et al. (2002a,b, 2003a)(Fig. 13c).

COUTERCLOCKWISE P-T PATH

The estimated P-T path of the Kuldkourla eclog-ites implies a slight temperature decrease at somepoint during subduction, which was expressed bythe prograde evolution of metamorphic rocks.Several types of counterclockwise P-T paths havebeen rarely reported from some eclogites-associated HP/LT metamorphic rocks (Fig. 14).The counterclockwise P-T path of the typerecorded by the Kuldkourla eclogites (Fig. 13a)was described from obduction-related HP/LTmetamorphic rocks in the Arabian continentalmargin of Oman, which was attributed to theobduction of already cold oceanic lithosphere(25 km in the maximum thickness) upon a rela-tively hot thinned passive margin (Goffé et al.1988). In the case of Kuldkourla eclogites, thecounterclockwise P-T path was obtained at the fardeeper position than the case of Oman, and thelarge-scale thrusting of colder materials on thecontinuously subducted slab is an unlikely eventnear the wedge mantle–slab interface. Therefore,the obduction model proposed in the case of OmanHP/LT metamorphic rocks does not sufficientlyexplain the counterclockwise P-T path recorded inthe Kuldkourla eclogites.

A temperature decrease event with increasingpressure (negative dP/dT path) during subductionis hard to interpret in terms of continuous coolingof a wedge mantle–slab interface due to a steadysubduction of a cold oceanic slab, and may provideinsights into the tectonic process of the Tianshansubduction system. In a subduction zone, aninverse thermal gradient is locally maintained inthe upper section of a slab; thus the deduced neg-ative dP/dT path suggests that the Kuldkourlasamples may have undergone migration from nearthe wedge mantle–slab interface into the slab tra-versing the inverse thermal structure during sub-duction (Fig. 15a). Ductile and complex flowswithin the slab may have been responsible for themigration of slab materials (Fig. 15b). Thrustfaulting (Fig. 15c) may also cause slab materials tobe brought into contact with relatively cooler anddeeper parts of a slab, and this could also explainthe deduced negative dP/dT section of the P-Tpath. The inverse thermal gradient part of the slabis, however, restricted to several- to 10-km thick(Peacock 1996; Hacker et al. 2003). Thus, the pro-posed material migration and thrust models can-not account for the continuous cooling proposedhere, which occurs over a vertical distance of about30 km (from 1.4 to 2.4 GPa depth: Fig. 12). A verysteep dP/dT path segment immediately before thepeak metamorphic stage is commonly documentedin eclogites (Enami & Nagasaki 1999; Katayamaet al. 2000; Aoya et al. 2003), and are numerically

Fig. 14 Inferred counterclockwise P-T paths for eclogite and HP/LTmetamorphic rocks. Abbreviations: GMKL88, Goffé et al. (1988); KOL94,Krogh et al. (1994); POEF99, Perchuk et al. (1999).



simulated for subducting oceanic slabs (Peacock1992; Bousquet et al. 1997). The combination of thekinked and concave-upwards P-T path and subse-quent cooling stage for a short period during sub-duction induced by the material migration and/orslab break may be a possible factor causing thecounterclockwise P-T path recorded in the Tian-shan eclogites (Fig. 13a).

Recently Uehara and Aoya (2005) numericallysimulated the thermal effect of relative motionbetween an oceanic ridge and a trench on the sub-duction system. They emphasized that the contin-uous approach of a ridge is likely to producestrongly curved subduction P-T paths with dP/dTprogressively increasing with pressure. If thekinked and concave-upwards P-T path of the Tian-shan eclogites is the record of a ridge approachevent, the slight decrease of temperature at theclimax of subduction could also reflect the coolingof the wedge mantle–slab interface by subductionof a relatively cold portion of slab following thepassage of a hot ridge, and the combination ofthese two events may explain the counterclockwiseP-T path.

ACKNOWLEDGEMENTS

The results presented in this study are issued fromgrants from the Chinese National 973 Project no.2001CB409801, the NSFC (40472116) and Grant-in-Aid for Scientific Research from the JSPS(14540448 to M.E). JSPS is acknowledged forproviding a Grant of Postdoctoral Fellowship forForeign Researchers (14-02060 to W.L). W. Linexpresses sincere thanks to J. Gao and L. Zhangfor their frank discussion and useful informationon the Tianshan HP/LT-metamorphic belt. We aremost grateful to S. R. Wallis and T. Ikeda for theirconstructive scientific and careful comments onthis manuscript.

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