Accepted Manuscript

Metamorphic evolution of ultrahigh-pressure rocks from Chinese southwestern

Tianshan and a possible indicator of UHP metamorphism using garnet compo-

sition in low-T eclogites

Jin-Xue Du, Li-Fei Zhang, Thomas Bader, Ting-Ting Shen

PII: S1367-9120(14)00170-9

DOI: http://dx.doi.org/10.1016/j.jseaes.2014.04.010

Reference: JAES 1926

To appear in: Journal of Asian Earth Sciences

Received Date: 5 November 2013

Revised Date: 4 March 2014

Accepted Date: 13 April 2014

Please cite this article as: Du, J-X., Zhang, L-F., Bader, T., Shen, T-T., Metamorphic evolution of ultrahigh-pressure

rocks from Chinese southwestern Tianshan and a possible indicator of UHP metamorphism using garnet composition

in low-T eclogites, Journal of Asian Earth Sciences (2014), doi: http://dx.doi.org/10.1016/j.jseaes.2014.04.010

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1

Metamorphic evolution of ultrahigh-pressure rocks from Chinese southwestern

Tianshan and a possible indicator of UHP metamorphism using garnet

composition in low-T eclogites

Jin-Xue Du *, Li-Fei Zhang, Thomas Bader, Ting-Ting Shen

MOE Key Laboratory of Orogenic Belts and Crustal Evolution, School of Earth and

Space Sciences, Peking University, Beijing, China

* Corresponding author.

E-mail addresses: Dujinxue1@163.com.

Abstract

How to identify ultrahigh-pressure metamorphism in the absence of coesite is a key

problem to gain the correct P-T history for an orogenic belt. In this study, garnet

composition combined with the pseudosection approach was used to identify

ultrahigh-pressure metamorphism and to determine P-T paths for eclogites and

metapelites from Chinese southwestern Tianshan. Porphyroblastic garnets from both

eclogites and metapelites develop pronounced chemical core-rim textures: the

relatively homogeneous core with low pyrope [Prp; Mg/(Ca+Mn+Mg+Fe2+)×100] and

grossular [Grs; Ca/(Ca+Mn+Mg+Fe2+)×100] content is overgrown by a thin rim with

sharply increased Prp and Grs. Phase equilibria modeling indicates that the

ultrahigh-pressure rocks have undergone a clockwise P-T path characterized by

heating during early exhumation with peak P-T at 31-33 kbar and 490-520 ºC. The

P-T pseudosections for eclogites show that isopleths of Prp and Grs strongly depend

2

on temperature and pressure, respectively, especially in the stability fields of

glaucophane-lawsonite-bearing eclogite facies assemblages. This indicates that garnet

composition provides robust thermobarometric constraints. Consequently, we propose

a Prp-Grs diagram which is subdivided into a high-pressure region and an

ultrahigh-pressure region by the quartz-coesite-transition curve. Those garnet

compositions which fall into the ultrahigh-pressure region are regarded to have

experienced ultrahigh-pressure metamorphism. This approach is expected to be a

useful tool to qualitatively identify ultrahigh-pressure metamorphism for

glaucophane-lawsonite-bearing eclogites and its particular strength is the quick

examination of large datasets comprising samples with similar bulk composition.

Using this method, garnet compositions of eclogites and mafic blueschists from

Chinese southwestern Tianshan and lawsonite eclogites worldwide are plotted in the

Prp-Grs diagram and several possible ultrahigh-pressure eclogite occurrences are

newly identified.

Key words: Ultrahigh-pressure metamorphism; Garnet composition; Lawsonite

eclogite; Phase equilibrium; Chinese southwestern Tianshan

3

1 Introduction

Discoveries of coesite and magnesite + aragonite in eclogites and metapelites from the

Chinese southwestern Tianshan eclogite-blueschist belt (Lü et al., 2008, 2009, 2012a;

Lü and Zhang, 2012; Zhang et al., 2003a) have demonstrated that some of the

eclogites and metapelites in this belt have undergone UHP metamorphism. However,

prior to these discoveries, most eclogites have been recognized as high-pressure (HP)

rather than UHP rocks due to the absence of coesite (e.g., Gao et al., 1999; Lü et al.,

2007; Wei et al., 2003). It is still ambiguous whether there are any UHP rocks, which

have not been identified in the literature and whether UHP eclogites and the

surrounding metapelites have undergone the same UHP metamorphism or not.

Moreover, among the ~20 lawsonite-bearing eclogite terranes worldwide (Tsujimori

et al., 2006; Wei and Clarke, 2011), only a few of them have been proposed to have

undergone UHP metamorphism (e.g., Type-U in Wei and Clarke (2011)). Are there

any other potential UHP lawsonite-bearing eclogite terranes?

The presence of diagnostic minerals and mineral assemblages, such as metamorphic

coesite, diamond and magnesite + aragonite, or their pseudomorphs, is an essential

standard to determine ultrahigh-pressure (UHP) metamorphism (see review by Liou et

al. (2009)). However, the formation of coesite and diamond is not only controlled by

pressure, but also influenced by chemical factors, e.g., the bulk-rock SiO2- and

CO2-contents and the oxygen fugacity (Yang, 1998). Moreover, mineralogical UHP

record can be partially or completely obliterated by back-transformation of coesite

into quartz and diamond into graphite during exhumation. This raises the question of

4

how to identify UHP metamorphism in the absence of diagnostic minerals and

mineral assemblages.

Mineral compositions can furthermore be potential indicators of UHP metamorphism,

e.g. majoritic garnet in peridotites (van Roermund and Drury, 1998; Scambelluri et al.,

2008). In eclogites, garnet has been widely used to estimate P and T, e.g., by

conventional thermobarometry (Ravna, 2000; Ravna and Terry, 2004) and the

“pseudosection thermobarometer” (Powell and Holland, 2008). Liou, et al. (1997)

even made a penetrating exposition of the significance of garnet to study of orogenic

belts “Seeing a Mountain in a Grain of Garnet”. Garnet’s composition is even a

potential indicator of UHP metamorphism for low-T eclogites, because, in P-T

pseudosections for eclogites, its grossular content is strongly correlated with P,

especially in the stability field of the glaucophane-lawsonite-bearing eclogite facies

(Brovarone et al., 2011; Endo et al., 2012; Wei et al., 2010; Wei and Clarke, 2011).

Hence, as a case study, the present paper first deciphers phase relations and

determines metamorphic evolution of a typical UHP eclogite and its host metapelite

from Chinese southwestern Tianshan based on P-T pseudosections. Afterwards,

compositional characteristics of low-T UHP garnet is explored to detect whether

garnet composition is an indicator of UHP metamorphism in low-T lawsonite

eclogites in the absence of UHP index minerals. Using this method, several UHP

eclogite occurrences from Chinese southwestern Tianshan and a possible UHP

lawsonite eclogite terrane are newly identified based on the analysis of previously

published garnet compositions.

5

Mineral abbreviations follow Whitney and Evans (2010).

2 Geological overview

The long and narrow LT-HP/UHP metamorphic belt in Chinese southwestern

Tianshan (Fig. 1) is sandwiched between the Yili-Central Tianshan and the Tarim

plates along the South Tianshan Fault. To the north, it is separated from a LP/HT belt

composed of cordierite-bearing garnet-sillimanite gneiss and two-pyroxene granulite

by the South Tianshan Fault; both build up a paired metamorphic belt (Zhang et al.,

2007a; and references therein). To the south, it is fault juxtaposed against a unit of

interlayered marble and chlorite-white mica schist. Metapelites and mafic blueschists

constitute the main rock types of the HP-UHP metamorphic belt, in which eclogites,

characterized by N-MORB, E-MORB and OIB signatures (Ai et al., 2006; Gao et al.,

1999; Zhang et al., 2008b), serpentinites and marbles sporadically occur as interlayers,

lenticular bodies and massive blocks (Zhang et al., 2001). This HP-UHP metamorphic

belt has been subdivided into a northern UHP sub-belt and a southern HP sub-belt (Lü

et al., 2012a). Coesite and its pseudomorphs occur as inclusions in garnet or as

exsolution lamellae in omphacite from UHP eclogites and metapelites in the UHP

sub-belt (Lü et al., 2008, 2009; Lü and Zhang, 2012; Wei et al., 2009; Zhang et al.,

2002a, 2005). In addition, lawsonite, an index mineral for LT-(U)HP conditions, has

been discovered in garnet-free metapelites (Du et al., 2011) and eclogites (Li et al.,

2013). There is growing evidence that eclogites as well as metapelites in both HP and

UHP sub-belts experienced peak lawsonite eclogite facies metamorphism (Lü et al.,

6

2009; Wei et al., 2009). Recent geochronological studies indicate that this belt formed

in the Late Carboniferous (~310-320 Ma; Gao et al., 2011; Gou et al., 2012; Li et al.,

2011; Su et al., 2010) rather than in the Triassic (~220-240 Ma) as suggested by

Zhang et al. (2007c).

Our sampling location (white star with green outline in Fig. 1) is located in the UHP

sub-belt in the Habutengsu valley in the northern part of the Chinese southwestern

Tianshan HP-UHP belt. Eclogites commonly occur as lenses with diameters of ~2-4

m and thin layers within garnet-glaucophane-phengite schist (Fig. 2a; Lü et al., 2008,

2009; Lü and Zhang, 2012). The coesite-bearing UHP eclogites, e.g., represented by

sample H607-5 of Lü et al. (2009), are dominated by garnet, omphacite,

glaucophane/barroisite and paragonite and contain minor rutile. Quartz/coesite,

omphacite and lawsonite pseudomorphs (czo + pg) occur as inclusions in garnet. For

this study, a pristine lenticular eclogite (H608-14) and its host, a

jadeite-lawsonite-bearing garnet-glaucophane-phengite schist (H71-4), were selected

for detailed petrographic and mineralogical investigations.

3 Analytical methods

Mineral compositions have been determined using a JXA-8100 electron microprobe

(EMP) at the MOE Key Laboratory of Orogenic Belts and Crustal Evolution, Peking

University operated in wave length-dispersive mode with 15kV accelerating voltage,

10nA beam current and, except for phengite and paragonite (5 µm), 1 µm beam

diameter. Raw counts have been converted into oxide wt.-% using the PRZ routine.

7

For calibration, natural and synthetic mineral standards have been used. Relative

analytical uncertainties are <2% for major elements. The formulae as well as the

Fe2O3 contents of minerals have been calculated using the AX software (Holland;

available at http://www.esc.cam.ac.uk/research/research-groups/holland/ax). The

uncertainties of garnet end-members Sps [Mn / (Ca + Mn + Mg + Fe2+) × 100], Prp

[Mg / (Ca + Mn + Mg + Fe2+) × 100], Alm [Fe2+ / (Ca + Mn + Mg + Fe2+) × 100] and

Grs [Ca / (Ca + Mn + Mg + Fe2+) × 100] propagated from the EMPA through cation

calculation are 0.2-0.5, 0.6-1.2, 0.9-1.9 and 0.5-1.7, respectively (2σ; calculated using

standard deviations).

Raman spectroscopy of minerals was conducted on a Renishaw-RM1000 Laser

Raman probe at the MOE Key Laboratory of Orogenic Belts and Crustal Evolution,

Peking University; for detailed operating conditions, see Zhang et al. (2005).

4 Petrography

4.1 Glaucophane eclogite (sample H608-14)

Glaucophane eclogite H608-14 displays a porphyroblastic texture and a massive

structure. It consists mainly of garnet (20 vol.-%, % for short in what follows),

omphacite (50 %), glaucophane/barroisite (15 %), paragonite (3 %) and quartz (3 %)

with minor plagioclase, carbonates, rutile and sphene.

Garnet (labeled GI, Fig. 2b) occurs as coarse-grained (0.8-1.5 mm in diameter)

euhedral to subhedral porphyroblasts. It is commonly fractured (Fig. 2b) and

occasionally overprinted by late-stage chlorite or barroisite grown on grain boundaries

8

and fractures. It exhibits a well-developed core-rim texture (Fig. 2b) with cores being

brighter at BSE images. The core contains lots of primary inclusions of omphacite,

quartz (occasionally with radial fractures, e.g., upper left in Fig. 2b), rutile and

secondary aggregates of carbonates + paragonite (Fig. 2b and c), barroisite + quartz

(Fig. 2b), Ca-rich garnet (GII, Fig. 2b, d and e) ± paragonite ± quartz and paragonite +

clinozoisite/epidote ± chlorite (Fig. 2e and f). The rim has few inclusions of

omphacite, quartz and rutile.Omphacite is fine-grained, anhedral and occurs primarily

as matrix aggregates (Fig. 2f) as well as inclusions in paragonite and GI (Fig. 2d). It

commonly shows a heterogeneous appearance at BSE images and under the

microscope. Subhedral to euhedral Na-amphibole with a length of 0.3-1.5 mm is

commonly overprinted by anhedral bluish-green Na-Ca-amphibole (Fig. 2f).

Paragonite is subhedral to anhedral and coarse-grained (0.6-2.0 mm in length). It

contains inclusions of rutile and omphacite (Fig. 2f).

Plagioclase occurs as secondary coronas around omphacite and paragonite. Dolomite

forms coarse-grained porphyroblasts commonly mantled by late-stage calcite.

4.2 Jadeite-lawsonite-bearing garnet-glaucophane-phengite schist (sample H71-4)

Jadeite-lawsonite-bearing garnet-glaucophane-phengite schist H71-4 is mainly

composed of garnet (30 %), glaucophane (10 %), phengite (20 %), paragonite (5 %),

plagioclase (10 %) and quartz (25 %), with a small amount of carbonates, rutile,

sphene, zircon and secondary chlorite.

9

Three types of garnet (denoted as GIA, GIB and GII) are discriminated. GIA (Fig. 3a)

occurs as subhedral to euhedral porphyroblasts measuring 0.1-0.3 mm across. It

contains inclusions of jadeite, rutile, graphite and quartz. Around quartz inclusions,

garnet occasionally shows radial fractures (Fig. 3a), perhaps an indication of former

coesite presence and thus formation of GIA under UHP conditions. GIB (Fig. 3b) is

subhedral to euhedral with a much larger grain size (1.5-3 mm) than GIA. It contains

numerous primary inclusions of jadeite, omphacite, glaucophane, quartz (Fig. 3b, c

and d) and secondary aggregates of clinozoisite ± paragonite/phengite (Fig. 3b, e and

f). GII occurs as box-shaped inclusions in GIA and GIB, often together with paragonite

(Fig. 3f), which shows a texture comparable to garnet GII in the glaucophane eclogite

H608-14.

A minute lawsonite grain (~10 µm in length and ~5 µm in width; Fig. 3g), which

occurs as inclusion in zircon, was identified using the Raman spectroscopy (Fig. 3h).

It shows the strongest diagnostic peaks of lawsonite at 567.9, 696.4 and 939.3 cm-1,

similar to those of lawsonite in a lawsonite-bearing chloritoid-glaucophane schist

described by Du et al. (2011).

As mentioned above, jadeite occurs as inclusions in GIA and GIB (Fig. 3a and d) while

rare omphacite has only been found as inclusions in quartz, which, on its part, is

included in GIB (Fig. 3c). Jadeite is commonly enveloped or partially replaced by

late-stage chlorite and plagioclase. Glaucophane is subhedral to anhedral and

fine-grained (0.1-0.4 mm long). It contains a few rutile inclusions and is occasionally

enveloped by late-stage Na-Ca-amphibole.

10

White mica (phengite and paragonite) occurs as euhedral and fine- to medium-grained

flakes measuring 0.2-2 mm in length. Its alignment highlights the rock’s weak

foliation. It does not contain inclusions. Plagioclase occurs as coarse-grained

aggregates (>3mm) in the matrix and contains numerous inclusions of garnet, white

mica and glaucophane.

5 Mineral chemistry

Selected compositions of minerals from samples H608-14 and H71-4 are presented in

Tables 1 and 2, respectively.

5.1 Garnet

Garnet GI in the glaucophane eclogite H608-14 displays a conspicuous two-stage

compositional zoning (Fig. 4a and b) with a bell-shaped decrease of Sps, from 6.0 in

the core to 0.9 at the rim, suggesting a well preserved growth zonation (Spear, 1993).

The pyrope- and grossular-poor core (Alm68.2-71.6Grs18.0-15.7Prp7.2-10.3Sps6.0-1.2) is

characterized by subtly outward increasing Prp and Alm and decreasing Grs, while

the rim shows a sharp increase in Prp and Grs and a decrease in Alm

(Alm70.3-53.9Grs16.3-21.7Prp11.8-23.9Sps1.6-0.9). Garnet GII shows much higher Grs

(Alm50.4-52.9Grs35.9-41.2Prp7.3-9.8Sps1.1-1.4) than the host GI. Similar Ca-rich garnet

inclusions in eclogitic garnet are common in metapelites and eclogites from Chinese

southwestern Tianshan (Lü et al., 2009; Wei et al., 2009).

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Garnet GIA in the jadeite-lawsonite-bearing garnet-glaucophane-phengite schist

H71-4 displays a gradual growth zonation (Fig. 4a and c) with a spessartine-rich core

(Alm75.6-77.1Grs6.3-8.1Prp10.5-12.1Sps4.2-5.4) and a grossular-rich rim

(Alm60.0-75.9Grs10.0-20.4Prp11.1-14.1Sps0.4-3.9). It shows a subtle increase of Prp and an

obvious increase of Grs. Garnet GIB shows similar compositions as the rims of GIA

and a weak zonation (Alm63.6-71.7 Grs14.6-24.8 Prp11.3-14.7 Sps0.4-2.5) with a gradual

rim-ward decrease of Alm and a step-like increase of Grs at the outer rim (Fig. 4d). In

contrast, garnet GII has a distinctly higher grossular content than GIA and GIB (Fig. 4a;

Table 2).

5.2 Clinopyroxene

According to the WEF-Jd-Aeg diagram (Morimoto et al., 1988), clinopyroxene in

sample H608-14 is omphacite. Both omphacite in the matrix and included in garnet

have compositions scattering in a large range of WEF40-59Jd32-50Aeg3-21 (Fig. 3e).

Omphacite in H71-4 (WEF43.9Jd47.1Aeg9.0) is compositionally similar to that in

H608-14 (Fig. 3e). The composition of jadeite (Jd = 85.3-87.5 mole-%; Aeg =

10.6-12.4 mole-%) varies little across the thin section (Fig. 3e).

5.3 Other minerals

According to (Leake et al., 1997), the Na-amphibole in sample H608-14 is

glaucophane and has NaM4 = 1.71-1.87 p.f.u., (Na+K)A = 0-0.18 p.f.u. and XFe [Fe2+ /

(Fe2+ + Mg)] = 0.20-0.36; the Na-Ca-amphibole in H608-14 is barroisite, hornblende,

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edenite and pargasite and shows a wide range of NaM4-, (Na+K)A- and XFe-values,

namely 0.39-1.05, 0.20-0.67 and 0.20-0.68, respectively. The composition of

H71-4glaucophane is very similar: NaM4 = 1.81-1.98 p.f.u., (Na+K)A = 0-0.16 p.f.u.

and XFe = 0.32-0.42; in contrast, H71-4 Na-Ca-amphibole is barroisite and actinolite

with little varying NaM4-, (Na+K)A- and XFe-values, these being 0.72-0.85, 0.30-0.37

and 0.35-0.37, respectively.

Paragonite from both samples has compositions (Tables 1 and 2) very close to the one

of the pure end-member, while plagioclase is nearly pure albite. Phengite from H71-4

has Si contents of 3.28-3.37 p.f.u. (Table 2).

6 Phase equilibria modeling

Phase equilibria modeling has been performed for both samples described aboveand a

representative MORB composition taken from Sun and McDonough (1989) using

Thermocalc 3.33 (Powell et al., 1998) and the internally consistent dataset of Holland

and Powell (1998), updated in November 2003 (file tc-ds55.txt). Solid solution

models used are: garnet: White et al. (2005), clinopyroxene: Green et al. (2007),

amphibole: Diener et al. (2007), chloritoid: Mahar et al. (1997), chlorite: Holland et

al. (1998), phengite: Coggon and Holland (2002), plagioclase: Holland and Powell

(2003), talc and epidote: Holland and Powell (1998), carpholite: binary ideal mixing

model. Pure end-members used here include quartz/coesite, lawsonite, kyanite,

paragonite, clinozoisite and H2O, except for P-X[CO2/(CO2 + H2O)]-pseudosections,

where ideal CO2-H2O mixing (Holland and Powell, 1998) is applied. For comparison,

13

P-T pseudosections countered with Prp and Grs isopleths were calculated for the

glaucophane eclogite H608-14 using Domino/Theriak (de Capitani and Brown, 1987)

and Perple_X (Connolly, 1990) with the same database and solid solution models as

above (Figs. B.1-2).

The MnNCFMASH(O) and MnNCKFMASH systems were selected to model phase

equilibria, depending on characteristics of dominating mineral assemblages and their

compositions in each sample. Mn-bearing garnet, chlorite and chloritoid are taken into

account. P2O5 and TiO2 are ignored in our calculations, as they are mainly

incorporated in minor apatite and rutile/sphene.

To obtain the effective bulk compositions for glaucophane eclogite H608-14 and

garnet-glaucophane-phengite H71-4, the modal abundance of all relevant phases in

the model system have been integrated with the EPM-data of minerals (Carson et al.,

1999; Warren and Waters, 2006). This approach is proven to be valid for estimating

peak P-T conditions (Warren and Waters, 2006; Wei and Song, 2008). For phase

equilibria modeling of H607-5 (Lü et al., 2009) and MORB (Sun and McDonough,

1989), however, the bulk compositions given in the respective publication have been

used; all are listed in Table 3.

The fluid phase was assumed to be pure H2O ( ) and in excess, because

experiments and investigations on natural samples revealed that especially in cold

subduction zones, as is the case in Chinese southwestern Tianshan, even

carbonate-bearing metabasalts coexist with low-X(CO2)-fluids (Castelli et al., 2007;

Gao and Klemd, 2001; Molina and Poli, 2000).

14

6.1 P-T pseudosection for glaucophane eclogite (sample H608-14)

The MnNCFMASHO P-T pseudosection for glaucophane eclogite H608-14 contoured

with Prp and Grs isopleths and garnet’s modal proportion (based on one-oxide) is

presented in Fig. 5a and b. Lawsonite has a relatively narrow stability field at low

temperature conditions (T < 590°C) with respect to predictions of Wei and Clarke

(2011). Garnet is predicted to be stable across the whole calculated P-T range

(400-650°C and 15-35 kbar). Its stability toward lower P and T is, however,

significantly overestimated, because the bulk MnO-content and the incorporation of

Mn into other phases decisively influence the location of the garnet-in isograde

(Mahar et al., 1997), yet thermodynamic models for Mn-amphibole and

-clinopyroxene are lacking. Furthermore, stilpnomelane, which affects garnet stability

in HP metamorphic rocks too (Endo et al., 2012), has not been taken into account here,

because available models are uncertain (Endo et al., 2012). Consequently, in the

MnO-free sub-system garnet is predicted to be stable only above 450-490 °C (Fig.

5b).

The observed matrix mineral assemblage of garnet + omphacite + glaucophane +

paragonite + quartz corresponds to a wide penta-variant field at 15-21 kbar and

550-650°C.

In Fig. 5a, the uncertainties of the Grs and Prp isopleths propagated from the

thermodynamic data are ~2 kbar and ~10 °C (2σ), which should be regarded as

minimum value, because other sources of uncertainties (e.g. solution models) have not

been taken into account. Grs isopleths strongly depend on and negatively correlate

15

with pressure, especially in those fields, where garnet, omphacite, lawsonite and

glaucophane coexist. On the contrary, Prp isopleths are subparallel to the P-axis with

a negative slope in most mineral assemblages. Due to the resulting high intersection

angle, both compositional garnet isopleths provide a robust method for estimating P

and T (Wei and Clarke, 2011; Endo et al., 2012). As shown in Figs. 5a and B.1-2,

there are no substantial differences between phase relations and Prp and Grs isopleths

calculated for sample H608-14 using Thermocalc, Domino/Theriak and Perple_X.

The garnet (GI) core-rim zoning profile (points 1-10 and 12 in Fig. 5a; numbers refer

to those in Fig. 3b) corresponds to a P-T vector (the hollow arrow with solid outline in

Fig. 5a) from peak-P at 29-32 kbar and ~490°C to peak-T at ~580°C and 27 kbar. The

garnet core composition (points 1-9 and 12) plots above the quartz-coesite transition

in consistency with coesite pseudomorphs in garnet’s outer core (Fig. 2b), whereas its

rim composition (point 10) plots below the quartz-coesite transition. The uncertainties

of Prp and Grs in the measured garnet compositions used to derive P-T conditions are

in the range of 0.6-1.2 and 0.5-1.1 (2σ), respectively, leading to a P-T deviation of ~1

kbar and ~5°C. This P-T-vector is located in the glaucophane-lawsonite-bearing

eclogite facies and the mineral assemblage evolves by the breakdown of chlorite and

lawsonite and the appearance of coesite/quartz. This is in agreement with the presence

of lawsonite pseudomorphs and coesite/quartz in garnet. However, no intersection

corresponding to the outermost garnet rim (point 11) is obtained with the original bulk

composition, probably due to garnet fractionation. Thus, a supplementary P-T

pseudosection (not shown) was calculated using an effective bulk composition

16

obtained by subtracting the garnet core composition from the original one. It suggests

that the outermost garnet rim (point 11) formed at ~600 °C and 25 kbar, which is still

within the glaucophane-lawsonite-bearing eclogite facies assemblage (see point 11 in

Fig. 5a and the lws-out isograd).

The resulting P-T vector indicates that garnet has grown during decompression (32 →

25 kbar) and apparent heating (~490 → 600 °C). Based on the matrix mineral

assemblage garnet + glaucophane + omphacite + paragonite + quartz, a

post-garnet-growth part of the P-T path characterized by isothermal decompression is

inferred (the hollow arrow with dashed outline in Fig. 5a).

6.2 P-T pseudosection for jadeite-lawsonite-bearing garnet- schist (sample H71-4)

Fig. 5c and d presents P-T pseudosections, calculated for sample H71-4 in the

MnNCKFMASH system with quartz/coesite, phengite and H2O in excess, contoured

with Prp and Grs isopleths and garnet’s modal proportion.

Prp isopleths are positively correlated with T, comparable to those from eclogite

H608-14; Grs isopleths, however, show a much stronger T-dependence. GIA zoning

profile from sample H71-4 (points 1-10 in Fig. 5c; numbers refer to those in Fig. 3c;

points α and β are core compositions of two other garnet grains; see Table 2)

corresponds to a two-stage P-T vector similar to sample H608-14, however, with a

steeper slope from UHP conditions (core: ~31 kbar at 510 °C) to HP conditions (rim:

~22 kbar at 580 °C). The outermost rim (point 10) is plotted in a supplementary P-T

pseudosection (not shown), which bases on an effective bulk composition obtained by

17

subtraction of garnet and glaucophane fractionation from the original one (Fig. 5c). In

addition, GIB profile corresponds to P-T conditions (Fig. 5d) equivalent to those of

GIA rim compositions. Along the P-T vector, the mineral assemblage evolves from

garnet + glaucophane + jadeite + lawsonite + carpholite via garnet + glaucophane +

jadeite + lawsonite to garnet + glaucophane + jadeite + lawsonite + paragonite with

excess quartz/coesite and phengite, in well agreement with the occurrences of

lawsonite in zircon, coesite and lawsonite pseudomorphs and jadeite in garnet.

On account of the matrix mineral assemblage garnet + glaucophane + paragonite +

quartz + phengite stable in a penta-variant field below ~20 kbar at 530-620 °C,

isothermal decompression after garnet growth is inferred (hollow arrow with dashed

margin in Fig. 5c).

Jadeite and omphacite, both being present in the sample, are predicted to coexist

within a narrow strip at 19-20 kbar and 560-620 °C, reflecting the presence of both

minerals and the miscibility gap reported by Green et al. (2007).

According to the pseudosections described above, glaucophane eclogite H608-14 and

its host (jadeite-lawsonite-bearing garnet-glaucophane-phengite schist H71-4) have

evolved along similar clockwise P-T paths with early decompression and apparent

heating and late isothermal decompression. Both share the same UHP peak

metamorphic conditions of ~490-510°C and 31-32 kbar.

18

6.3 P-T pseudosection for a representative MORB composition

A P-T pseudosection (Fig. 6a) for a representative MORB composition (sample

104-16; Table 3) taken from Sun and McDonough (1989) was calculated aiming to

explore compositional characteristics of UHP garnet for general basaltic bulk

compositions. In the pseudosection, glaucophane and lawsonite show reduced and

expanded stabilities with respect to sample H608-14, respectively. Despite of these

differences, Grs and Prp isopleths strongly depend on pressure and temperature,

respectively, especially in lawsonite-glaucophane-bearing eclogite facies

assemblages.

7 Discussion

7.1 The mineral assemblage jadeite-lawsonite-glaucophane

Lawsonite-jadeite-glaucophane-bearing metapelites are rarely exposed in several

LT-HP metamorphic terranes, e.g. the Tavşanlı zone, Turkey (Okay, 2002), the Rio

San Juan Complex, the Dominican Republic (Schertl et al., 2012) and south of the

Motagua Fault, Guatemala (Harlow et al., 2011). In the Chinese southwestern

Tianshan HP-UHP metamorphic belt, though not observed, the peak-P

jadeite-lawsonite-glaucophane-bearing assemblages were previously predicted for

(U)HP pelitic schists (Wei et al., 2009; Du et al., 2011). Our discovery of

well-preserved lawsonite included in zircon and lawsonite pseudomorphs and jadeite

in garnet from the pelitic sample H71-4 further supports this prediction. In the P-T

19

pseudosection calculated for sample H71-4 (Fig. 5c), the mineral assemblage

jadeite-lawsonite-glaucophane is stable above 21 kbar at 420-590°C. Given that cores

of garnet from sample H71-4 imply UHP conditions (~31 kbar, 510°C), it is so far the

only reported instance of an UHP jadeite-lawsonite-glaucophane-bearing mineral

assemblage worldwide. Likely, pelitic-felsic schists having such mineral assemblages

occur in other LT-UHP terranes but have hitherto escaped notice due to difficulty in

preserving it during uplift.

Rare omphacite occurs, comparable to lawsonite discovered in lawsonite-bearing

chloritoid-glaucophane schist (Du et al., 2011), as inclusions in quartz, on its part

included in GIB from jadeite-lawsonite-bearing garnet-glaucophane schist H71-4, yet

never together with jadeite. The occurrence of two clinopyroxenes might be explained

by: 1) The effective bulk composition in local domains differs from the one of the

whole rock and leads to omphacite growth instead of jadeite at the HP stage or to

transition from jadeite into omphacite, for instance by:

(Wei and Powell, 2006). The lack of an equilibrium texture among them supports this.

2) The coeval omphacite and jadeite appearance reflects a miscibility gap, i.e. they

were in equilibrium with each other during the HP stage, which is supported by the

pseudosection predicting coexistence within a narrow strip at 19-20 kbar and

560-620 °C. Actually, a miscibility gap between omphacite and jadeite is inferred

from their compositions (Fig. 4e).

20

7.2 Metamorphic evolution of UHP rocks from Chinese southwestern Tianshan and

comparison with previous studies

7.2.1 Peak P-T conditions and comparison with previous studies

Published estimates of peak metamorphic P and T of UHP rocks from Chinese

southwestern Tianshan (Fig. 1) are summarized in Table 4 and vary from 24 to 51

kbar and from 470 to 630°C. Conventional methods, such as the garnet-clinopyroxene

thermometer and the garnet-clinopyroxene-phengite barometer, have commonly been

applied in early studies (e.g., Lü et al., 2008; Zhang et al., 2002a, 2003b). However,

the uncertainty of omphacite’s Fe2+/Mg-ratio, arising from the difficulty to estimate

its actual Fe3+-content, critically affects the accuracy of the garnet-clinopyroxene

thermometer. Moreover, if P-T conditions at the peak-T stage (in this study: garnet

rim) are mistaken for those at the peak-P stage (in this study: garnet core), the

calculated pressure would be underestimated due to decoupling of P and T peaks (e.g.,

Zhang et al., 2002a). In recent years, phase equilibria modeling not affected by these

problems has been performed for several UHP eclogites (Lü et al., 2009; Tian and

Wei, 2013) and metapelites (Lü et al., 2012b; Wei et al., 2009; Yang et al., 2013), but

ambiguity on the peak metamorphic conditions still exists (Table 4).

For two new samples, a glaucophane eclogite and a jadeite-lawsonite-bearing

garnet-glaucophane schist, we derived similar P-T vectors from 29-32 kbar, ~490 °C

to ~580 °C, 27 kbar and ~32 kbar, 510°C to ~22 kbar, 580°C, respectively. Peak-P

estimates are little higher than the one obtained for coesite-bearing eclogite H607-5

21

(24-27 kbar) by Lü et al. (2009); in their isopleth diagrams, however, some garnet

compositions adjacent to coesite or its pseudomorphs plot below the coesite-quartz

transition, in disagreement with the petrological observation, which is why a P-T

pseudosection (Fig. 6b) has been recalculated in the model system MnNCFMASH

using the same bulk composition (Table 3) as Lü et al. (2009). Compared with

glaucophane eclogite H608-14 (Fig. 5a and b), this pseudosection shows a much

larger stability field of quartz/coesite, which is reflected by the rarity of quartz in

H608-14 and the abundance of quartz/coesite in H607-5. In the stability field of

garnet + glaucophane + lawsonite + omphacite + jadeite + coesite, the measured

compositions of garnet having inclusions of coesite and its pseudomorphs (data from

Lü et al., 2009) plot above the coesite-quartz transition and correspond to a P-T vector

ranging from ~33kbar at 520ºC to ~29kbar at 550ºC, quite different from the results

of Lü et al. (2009). This discrepancy is potentially a consequence of the different

programs, databases and/or solution models applied. P-T pseudosections with Prp and

Grs isopleths for sample H608-14 calculated using different programs but the same

database and solid solution models are nearly identical (Figs. 5a and B.1-2),

suggesting the modeling results do not dependent on the modeling algorithms. Given

that the modeling results obtained in this study with the database of Holland and

Powell (1998) agree better with the observation, because all garnet compositions plot

in the coesite stability field, this database and the new estimates are more reliable.

The new P-T estimates for H607-5 and peak-P of 32-33 kbar at ~490-520 ºC of our

samples H608-14 and H71-4 reliably determine the peak P-T conditions of the UHP

22

metamorphic rocks from Habutengsu. They agree well with P-T-values up to 32 kbar

and 550-570ºC derived for coesite-pseudomorph-bearing garnet-glaucophane-mica

schist T311 (Wei et al., 2009) from the same valley, yet differ little from estimates for

UHP eclogites from Kebuerte (Tian and Wei, 2013). All correspond to a geothermal

gradient of 4-5 ºC/km and indicate that the UHP belt of Chinese southwestern

Tianshan hosts the most deeply buried slice of subducted oceanic crust (Agard et al.,

2009), which returned back to Earth’s surface.

7.2.2 Heating-decompression P-T paths

In consistency with Lü et al. (2009) and Tian and Wei (2013) we state that UHP

eclogites experienced peak metamorphism in the glaucophane-lawsonite-bearing

eclogite facies along similar heating–decompression P-T paths. The predicted mineral

assemblages garnet + glaucophane + lawsonite + omphacite ± coesite ± chlorite are

consistent with the occurrence of coesite and lawsonite and their pseudomorphs.

Zoning of garnet from jadeite-lawsonite-bearing garnet-glaucophane-phengite schist

H71-4 corresponds to a heating–decompression P-T path, whose UHP portion (the

core of GIA; points α, β, 1-7 in Fig. 5c) is subparallel to isopleths of garnet’s modal

proportion (Fig. 5d), i.e. at this stage the total amount of garnet in the rock is

approximately constant. Therefore, either GIA grew during this stage on consumption

of, if present, early-formed (i.e., pre-UHP) garnet, which would give a possible

explanation for the absence of pre-UHP garnet or this part of the zoning of GIA was

distinctly modified by diffusion. On the contrary, the portion of the P-T path derived

23

from GIA rim and GIB compositions (points 8-10 in Fig. 5c and d) progressively cuts

across the isopleths of garnet’s modal proportion, indicating GIA rims and GIB have

crystallized at the expense of matrix phases.

Previous and own phase equilibria modeling applied to both UHP eclogites and UHP

metapelites from Chinese southwestern Tianshan predict initial garnet growth (=

garnet cores) only at UHP conditions, yet no or little garnet prior to it (Lü et al., 2009,

2012b; Tian and Wei, 2013; Wei et al., 2009) in discrepancy to other (U)HP terranes

(Agard et al., 2009). This might be traced back to the very low geothermal gradient

(4-6 °C/km) during subduction of the rocks (Fig. 5b and d). Calculated isopleths of

garnet’s modal proportion (MnO-bearing system) and the garnet-in isograde

(MnO-free system) are much more sensitive to T than P (Fig. 5b and d), which means,

the onset of garnet growth is mainly controlled by T. As a consequence, whether

garnet appears (long) before the rock reached peak-P or only at peak-P itself, is

mainly controlled by the geothermal gradient. At moderately low geothermal

gradients (Fig. 5b), garnet can record the pre-peak-P evolution at UHP (e.g., Tian and

Wei, 2013; Wei et al., 2009) or HP conditions (e.g. Yang et al., 2013; and eclogites

from Zermatt–Saas Zone (Groppo et al., 2009)), whereas at very low geothermal

gradients, as is the case for Chinese southwestern Tianshan, garnet may not record it

(e.g., Lü et al., 2009, 2012b; this study).

Ca-rich garnet inclusions (GII in H608-14 and H71-4) within porphyroblastic garnet

from Chinese southwestern Tianshan were previously interpreted as lawsonite

pseudomorphs or relict of an earlier mineral assemblage (Lü et al., 2009).

24

Alternatively, they may represent a pre-UHP garnet generation. For those occurring in

both eclogite H608-14 and metapelite H71-4, we prefer descent from lawsonite on the

basis of their box-shaped occurrence together with paragonite or quartz (Figs. 2d, e

and 3f), which is comparable to that of clinozoisite + paragonite, and their Ca-rich and

Mn-poor chemical characteristics (Tables 1 and 2). Grs and Prp isopleths of Ca-rich

garnet do not intersect in P-T pseudosections, but are sub-parallel in small areas at the

lawsonite-out lines (bright ellipses in Fig. 5a and c). As shown in Fig. 7, the following

reactions may model the two types of lawsonite pseudomorphs:

7.2.3 Tectonic implications

The glaucophane eclogite H608-14 and its host metapelite H71-4 experienced similar

peak UHP metamorphism at 32-33 kbar and ~490-520 ºC, which was followed by

similar heating-decompression P-T paths (Figs. 5 and 11). These P-T paths resemble

those of eclogites from Sanbagawa and New Caledonia, which were proposed to

result from exhumation caused by continental subduction and subsequent collision

(Agard et al., 2009; Ernst, 1988). This type of exhumation, however, is ruled out for

Chinese southwestern Tianshan eclogites, giving lacking evidence for continental

subduction contemporaneous with the exhumation of subducted oceanic crust. Here,

we emphasize the important role that metapelites played during the exhumation of

eclogites. In the HP-UHP metamorphic belt of Chinese southwestern Tianshan,

25

metapelites with minor serpentinites (Fig. 1) wrap eclogite layers, lenticular bodies,

massive blocks or pillows (Zhang et al., 2001, 2002a, 2005a). Metapelites and even

eclogites may have much lower densities than the surrounding mantle at depths of

<110-120 km (Chen et al., 2013; Wei et al., 2009) and thus the exhumation of UHP

eclogites could be caused by their buoyancy with respect to the mantle and may be

facilitated by the much higher buoyancy of metapelites.

7.3 Identification of UHP metamorphism from garnet composition

Index minerals such as coesite and diamond prove whether a rock underwent UHP

metamorphism. However, if they are absent, because they have been decomposed

during uplift or they did not form, e.g., due to Si-undersaturation, sluggish reaction

kinetics, lack of C or unfavorable f(O2), it is still possible to identify UHP. For

instance, the peak P of the glaucophane-bearing eclogite H608-14 is higher than the

one required to transform quartz into coesite, the mineral assemblage predicted at

these conditions, however, lacks coesite (Fig. 5a) due to SiO2-undersaturation.

According to phase equilibria modeling for UHP eclogites from Chinese southwestern

Tianshan (Figs. 5a and 6b) and for MORB (Fig. 6a), Grs and Prp isopleths strongly

depend on pressure and temperature, respectively, especially in eclogites having

glaucophane- and lawsonite-bearing assemblages, and determine therefore P-T

conditions. Based on the hypothesis that garnet composition is similar among rocks, if

the rocks have similar bulk compositions and undergo similar metamorphic evolutions,

26

we infer that a specific garnet composition is another indicator for UHP

metamorphism.

A series of P-X pseudosections (Fig. 8) calculated for glaucophane eclogite H608-14

at a fixed T of 530 ºC in the P range of 15-35 kbar shows that for eclogites the

stabilities of mineral assemblages are strongly affected by the bulk-rock CaO/(CaO +

MnO + MgO + FeOtotal) (Fig. 8a), CaO/(CaO + Na2O) (Fig. 8b), CO2/(CO2 + H2O)

(Fig. 8c), Al2O3/(Al2O3 + MgO + FeOtotal) (Fig. 8d) and Fe3+/FeOtotal ratios (Fig. 8e).

Contrastingly, Grs isopleths in mineral assemblages involving garnet, omphacite,

glaucophane and lawsonite are only subtly, i.e. < 2 kbar, influenced by variations in

all of these ratios except for Fe3+/FeOtotal, on which Grs isopleths negatively depend.

P-X pseudosections calculated for a MORB composition by Wei and Clarke (2011)

lead to the similar conclusion that Grs isopleths are subtly affected by the bulk-rock

CaO⁄(MgO + FeOtotal + MnO + Na2O) ratio and H2O content in the

glaucophane-lawsonite-bearing eclogite facies. In general, variations in the bulk

composition involve variations in several to all of the ratios named above, and,

therefore, the effect of a single ratio is weakened. For instance, although Fe3+/FeOtotal

varies from 0.042 in glaucophane eclogite H608-14 to 0.18 in 104-16 (MORB

composition), Grs isopleths show only a variation of ~ 2 kbar as illustrated by the

P-X(H608-14 to MORB) pseudosection (Fig. 8f). This indicates that Grs largely

depends on P rather than the bulk composition in glaucophane- and lawsonite-bearing

assemblages in eclogites.

27

Comparatively, for garnet-glaucophane-phengite schist H71-4 in eclogite facies,

glaucophane-lawsonite-bearing assemblages, the effect of bulk-rock attributes on Grs

isopleths depend on the stable mineral assemblage, which is highlighted by the

P-X[CaO/(CaO + MnO + MgO + FeOtotal)] pseudosection (Fig. 9).

This phenomenon can be described and interpreted in the NCMASH system. The

invariant point (Fig. 7) involving grossular, pyrope, jadeite, diopside, lawsonite,

glaucophane, quartz/coesite and H2O:

can be regarded to govern the grossular and pyrope content of garnet coexisting with

omphacite/jadeite, lawsonite, glaucophane and quartz/coesite. It is revealed that

activities of grossular and pyrope are high at low P and high T, respectively, in

consistency with the pseudosection modeling, but opposite to the following reaction:

suggesting P-T estimates of lawsonite eclogites based on

garnet-clinopyroxene-phengite geobarometer (e.g., Ravna and Terry, 2004) only

obtain lower pressure limits for garnet composition and therefore probably

28

underestimate metamorphic P-T conditions. The pyrope- and grossular-absent

reactions (5) and (9) display shallow and steep dP/dT slopes, respectively, and

independently show the strong control of P and T on garnet’s grossular and pyrope

contents in glaucophane- and lawsonite-bearing eclogites; the glaucophane-, the

diopside- and the lawsonite-absent reactions (6), (7) and (8) show medium positive

and negative dP/dT slopes, in good agreement with Grs isopleths in the corresponding

assemblages (e.g., Figs. 5 and 6).

In conclusion, garnet’s grossular content, fairly pressure sensitive but subtly

influenced by T and the bulk composition in glaucophane-lawsonite-bearing eclogites,

may serve as a new indicator for UHP metamorphism in combination with its pyrope

content.

7.3.1 Application of new UHP indicator to eclogites from SW Tianshan

Calculated garnet compositions (Prp and Grs) corresponding to the quartz-coesite

transition lie close to each other in Prp-Grs diagrams (green, blue and red lines, Fig.

10) and divide them into a UHP region with lower and an HP region with higher Grs.

Previously reported compositions of garnet from eclogites, mafic blueschists and

vein-bearing rocks from Chinese southwestern Tianshan were recalculated and plotted

in the Prp-Grs diagrams too. The uncertainties of Prp and Grs propagated from the

oxides through cation calculation are 0.4-1.0 and 0.5-1.2 (2σ), respectively. This

deviation will lead to a deviation of the localities of symbols within their sizes.

29

Generally, the garnet cores are poorer in both Prp and Grs than the rims. A simple

explanation is presented below.

UHP eclogites

All garnet rims from reported UHP eclogites from Chinese southwestern Tianshan

plot in the HP region, while most of the cores plot in the UHP region, in general

agreement with our conclusion that the garnet cores grew under UHP, but the rims

under HP conditions (Fig. 10a). However, there are still some compositions of garnet

from UHP eclogites that are located in the HP rather than the UHP region (e.g., those

reported by Lü et al. (2012a) and Zhang et al. (2002a)). Several factors may be

responsible for this phenomenon: 1) not every grain and every part of garnet record

the UHP stage due to multi-stage growth and, therefore, the textural occurrence and

the zoning of garnet have to be carefully examined; 2) the influence of diffusion,

especially in fine-grained garnet, must not be ignored; 3) this Prp-Grs diagram is not

applicable to extremely Ca-rich bulk-rock compositions (Fig. 8a). As most possible

factor among them, however, garnet compositions were not fully reported: although

the garnet compositions reported in the data tables of both studies plot in the HP

region, more compositions shown in the compositional diagrams (Fig. 7a of Lü et al.

2012a and Fig. 5 of Zhang et al. 2002a) display a Grs-poor characteristic (< 20

mole-%), consistent with our prediction of Ca-poor UHP garnet.

HP eclogites

Most of the reported garnet compositions in HP eclogites from Chinese southwestern

Tianshan plot in the HP region, but a few of them in the UHP region (Fig. 10b; Table

30

5). We infer that the latter actually have experienced UHP metamorphism, as well as

few in the HP region (e.g. sample H504-8 (Lü et al., 2007), due to the low grossular

content of the garnet core as reported in Fig. 3c of their paper), though neither coesite

nor its pseudomorphs were reported from them. These newly identified UHP eclogites

crop out well inside the UHP sub-belt defined by Lü et al. (2012a) (for detailed

information, see Fig. 1 and Tables 5 and A.1). Peak P of 12-25 kbar have been

reported based on jadeite content in omphacite (Gao et al., 1999), the existence of

paragonite instead of kyanite and lawsonite (Gao and Klemd, 2000; Klemd, 2003;

Zhang et al., 2001), the garnet-clinopyroxene-phengite geothermobarometer (Lin and

Enami, 2006; Lü et al., 2007) or pseudosection modeling (Wei et al., 2003; Lü et al.,

2007). However, according to our modeling (Figs. 5 and 6a), lawsonite instead of

paragonite is a stable phase during peak (U)HP conditions in eclogites and

metapelites from Chinese southwestern Tianshan, which is supported by the

well-preserved lawsonite inclusions in glaucophane (Du et al., 2011) and zircon (Fig.

3g and h) and the numerous lawsonite pseudomorphs of czo + pg in garnet in these

rocks. This indicates that previous studies of Gao and Klemd (2000), Klemd (2003)

and Zhang et al. (2001) underestimated peak P-T conditions, as did the pseudosection

approach of Lü et al. (2007) and Wei et al. (2003), due to not fully considering garnet

composition.

In order to cross-check the qualitative approach described afore, Grs and Prp isopleth

intersections were calculated for newly identified UHP garnet from the literature cited

above (white circles with dashed outline in Fig. 11), if the bulk composition is

31

available. Furthermore, all were plotted into the P-T pseudosection for sample

H608-14 (white circles with solid outline in Fig. 11). The newly identified UHP

garnet has a Grs range of 9.3-20.0 and a Prp range of 4.8-18.1 corresponding to a P

range of 26-41 kbar at 430-580 °C (Table 5; Fig. 11). These UHP conditions display

heating with slight decompression, which is in line with the P-T vector derived from

sample H608-14. More accurate P-T estimates for these eclogites would be available

if pseudosection modeling for all samples could be performed. However, the

semi-quantitative approach here demonstrates that earlier studies underestimated

pressures distinctly. For instance, conventional grt-cpx-ph geothermobarometry

(Ravna and Terry, 2004) yields 20-25 kbar for sample H504-8 (Lü et al., 2007), but

when plotted into the pseudosection for sample H608-14, the garnet compositions

correspond to P>26 kbar (Fig. 11).

Mafic blueschists and vein-bearing rocks

Nearly all of the reported compositions of garnet from mafic blueschists plot in the

HP region, in consistency with the rocks’ nature. However, some compositions of

garnet from vein-bearing rocks are located in the UHP region. Yet, except those

reported by Lü et al. (2012a), no vein-bearing UHP eclogites have been identified so

far. For instance, garnet from eclogite sample D12-2, which contains

quartz-garnet-omphacite veins (Xiong et al., 2006), has Grs as low as 14-20, which

plots within the UHP field, similar to that of vein garnet (Grs 15-20); however, it

shows a gradual contact with the surrounding mafic blueschist, suggesting P-T

conditions at the boundary between mafic blueschist and eclogite facies. This can be

32

explained by the low CaO activity in fluids in the open system (Xiong et al., 2006). In

comparison, garnets from vein-bearing rocks vary in a wide Grs range from 14-41,

probably indicating a significant variation of the CaO content in the coexisting fluid.

Metapelites

In addition, a Prp-Grs diagram for metapelites with Prp and Grs at the quartz-coesite

transition of garnet-glaucophane-phengite schist H71-4 is presented in Fig. 12a and

reported compositions of garnet from pelitic schists from Chinese southwestern

Tianshan are plotted. It demonstrates that our method of using garnet composition to

identify HP and UHP metamorphism is consistent with results from the literature (Lü

et al., 2008, 2012b; Wei and Powell, 2004; Wei et al., 2009; Yang et al., 2013). As an

exception, garnet compositions from magnesite-bearing schists (Zhang et al., 2003a)

are located in the HP rather than the UHP region, which might be caused by the

intensive retrograde metamorphism the sample experienced, i.e. garnet reequilibrated

during uplift, or the distinct difference in the bulk CaO content.

7.3.2 Application of new UHP indicator to typical lawsonite-bearing

eclogites worldwide

Besides Chinese southwestern Tianshan, eighteen localities of lawsonite- or

lawsonite-pseudomorph-bearing eclogites have been reported worldwide (Groppo and

Castelli, 2010; Tsujimori et al., 2006; Wei and Clarke, 2011; Zucali and Spalla, 2011).

Selected garnet compositions of these lawsonite eclogites are plotted in the Prp-Grs

diagram (Fig. 12b). Among them, several eclogites, such as those from the Garnet

Ridge, Colorado Plateau (Usui et al., 2006; label 15 in Fig. 12b), Sulu (Mattinson et

33

al., 2004; label 16 in Fig. 12b), Dabieshan (Wei et al., 2010; label 17 in Fig. 12b) and

Verpeneset, Western Gneiss Region (Krogh, 1982; Root et al., 2004; label 18 in Fig.

12b), were previously identified as UHP eclogites (Type-U according to Wei and

Clarke (2011)) due to presence of coesite or coesite pseudomorphs. Garnets in these

UHP eclogites have low Grs (8-29, mostly below 23) and low to extremely high Prp

(8-57, mostly above 0.25) far beyond the modeled Prp range in this study, suggesting

not only UHP but also HT conditions. Correspondingly, these UHP eclogites have

peak mineral assemblages involving garnet + omphacite + coesite + rutile ± kyanite ±

glaucophane ± talc; lawsonite is rarely preserved and commonly replaced by

pseudomorphs of kyanite + clinozoisite/epidote ± paragonite. Garnet in the lawsonite

eclogite from the Sesia-Lanzo Zone, Italian western Alps (Zucali and Spalla, 2011;

label 14 in Fig. 12b) shows similar Prp (31-44) but higher Grs (20-24) than the other

UHP garnets named above, suggesting similar T but possibly HP rather than UHP

conditions. Other lawsonite-bearing eclogites, either Type-L or Type-E, mostly have

peak mineral assemblages involving garnet + omphacite + glaucophane + lawsonite +

quartz/coesite with other minor phases (Tsujimori et al., 2006; Wei and Clarke, 2011;

and references therein). In general, garnet from these two types of lawsonite-bearing

eclogites shows a higher Grs (11-41) and a much lower Prp (0-29) with respect to

those from the Type-U eclogites, which suggests lower P-T conditions (see Figs. 5, 6

and 8). Only some compositions of garnet from lawsonite-bearing eclogite from the

Monviso meta-ophiolite, western Alps (Groppo and Castelli, 2010; label 13 in Fig.

12b) are located in the UHP region suggesting possible UHP metamorphism which is

34

documented by coesite pseudomorphs in eclogites from the same area (Angiboust et

al., 2012), while most lie in the HP region. This indicates either that Type-L and

Type-E lawsonite-bearing eclogites rarely experienced UHP metamorphism or that

UHP lawsonite-bearing eclogites are rarely exhumed.

Comparing the three types of lawsonite-bearing eclogite, lawsonite is very rarely

preserved in the type-U eclogites, even in the rigid garnet, due to higher temperature

metamorphism they experienced than the other ones, suggesting lawsonite will be less

likely preserved when ultra-deep subduction continues. Therefore, the Chinese

southwestern Tianshan metamorphic terrane and Garnet Ridge, Colorado Plateau,

probably as well as the Monviso Meta-ophiolite, western Alps, provide rare examples

of exposed LT-UHP lawsonite-bearing eclogites (Tsujimori et al., 2006; Wei and

Clarke, 2011; and references therein).

7.3.3 Caveats

If the garnet composition is used to identify UHP metamorphism as suggested in this

study, several caveats have to be kept in mind.

The first one concerns the scope of the application of the new method. According to

the modeling above, Grs strongly depends on P but only subtly on T and bulk

composition in mineral assemblages involving garnet + glaucophane + omphacite +

lawsonite, regardless of the modal proportion of each phase and whether other phases,

such as chlorite, talc, quartz/coesite, epidote, jadeite and carbonates, coexist with

them. In a wider context, Grs is controlled by both P, T and bulk composition in other

35

types of (U)HP blueschists and eclogites (see Figs. 5, 6 and 8). The composition of

garnet from omphacite-free blueschists and glaucophane-epidote eclogites exhibits

much higher Grs than that calculated for the quartz-coesite transition (green, blue and

red lines in Fig. 10c), which matches well with the fact that they commonly did not

experience UHP metamorphism. However, Grs in garnet from glaucophane-absent

lawsonite eclogites, kyanite eclogites and dry eclogites, strongly depends on T and the

bulk composition (Figs 5a, 6 and 8; Wei and Clarke, 2011; Wei et al., 2013), and it is

therefore not a suitable P indicator for these eclogites. Summarized, the new method

for identifying UHP metamorphism using the garnet composition is applicable only

for lawsonite-omphacite-bearing blueschists and glaucophane-lawsonite-bearing

eclogites as well as omphacite-free blueschists and glaucophane-epidote eclogites, all

being low-T (U)HP metamorphics (mostly < 600 °C).

For metapelites, Grs isopleths in the assemblages garnet + lawsonite + glaucophane +

omphacite ± jadeite with excess phengite and quartz/coesite trend comparably with

those in glaucophane-lawsonite-bearing eclogite, and in this case, for the

identification of UHP metamorphism in omphacite-bearing metapelites (e.g., Lü et al.,

2008) the Prp-Grs diagram for eclogites could be used. In the assemblages garnet +

lawsonite + glaucophane + carpholite/chloritoid ± jadeite, Grs subtly dependents on

the bulk composition (Fig. 9), and for these rocks (e.g., those reported by Lü et al.

(2012b), Wei et al. (2009), Yang et al. (2013)), a Prp-Grs diagram like Fig. 12a could

be applied as an indicator of UHP metamorphism. In other low-T (U)HP assemblages,

however, Grs content is strongly controlled by the bulk composition, and thus, for

36

these rocks, the simple method introduced here is less suitable for identifying UHP

metamorphism and pseudosections has to be calculated using their bulk compositions.

Secondly, the modeling performed in this study is restricted to closed systems,

because the garnet composition in open systems has not been explored, yet a natural

rock system, at least when considering fluid liberation and supply, is never completely

closed. Garnet, especially in veins or vein-bearing rocks (Fig. 10d), might therefore

have grown in equilibrium with a bulk composition deviating from the current one,

which is why the method should be applied to these rocks with caution.

Thirdly, in many UHP metamorphic rocks, only a part of the garnet grain grew under

UHP conditions, thus, unless the entire garnet profile is examined, UHP

metamorphism cannot be excluded for a rock from a single garnet composition

plotting in the HP field. To improve the method, EMP traverses on garnets separated

from the sample, mounted in epoxy and polished to expose their cores should be

recorded.

Fourth, yet most important, the Prp-Grs diagrams introduced here are a useful and

qualitative tool to identify UHP metamorphism. Their strength is a preliminary and

quick examination of large datasets comprising samples with similar bulk

composition and peak metamorphic mineral assemblages and they are particularly

useful to constrain the extend of a UHP-slice. To obtain exact P-T conditions of a

rock, however, pseudosection modeling is indispensable and has to be performed for

every sample.

37

8 Conclusions

The existence of jadeite-lawsonite-glaucophane-bearing assemblages in metapelites

from Chinese southwestern Tianshan was confirmed the first time by the discovery of

inclusions of lawsonite in zircon and jadeite and lawsonite pseudomorphs in garnet.

Phase equilibria modeling for a metapelite, a glaucophane eclogite enclosed in it and a

coesite-bearing eclogite earlier described by Lü et al. (2009) reveals that zoning

profiles of garnet from these rocks preserve the record of similar clockwise P-T paths

characterized by similar UHP lawsonite eclogite facies P-T conditions at 490-520°C

and 31-33 kbar.

In P-T pseudosections of eclogites, contours of Prp and Grs are sub-parallel to the P

axis and T axis, respectively, and are only subtly influenced by the bulk rock

composition in the stability fields of glaucophane-lawsonite-bearing eclogite facies

mineral assemblages. Thus, the composition of garnet from low-T eclogites is a

potential qualitative indicator for UHP metamorphism in, at least,

glaucophane-lawsonite-bearing eclogites. In practice, the measured composition of

garnet from eclogites and mafic blueschists can be plotted in a Prp-Grs diagram with

calculated garnet compositions on the quartz-coesite equilibrium. Using this method,

several eclogite occurrences from Chinese southwestern Tianshan and

lawsonite-bearing eclogite from the Monviso meta-ophiolite, western Alps, are

identified to have probably experienced UHP metamorphism. On the contrary, garnet

from most of the other lawsonite-bearing eclogites, either Type-L or Type-E, records

38

only HP rather than UHP metamorphism. However, for more accurate P-T estimates,

pseudosection modeling has to be performed for each sample.

ACKNOWLEDGEMENTS

This study was financially supported by National Science Foundation of China

(Grants 41121062, 41090371 and 41272069) and Major State Basic Research

Development Program (Grant 2009CB825007). Mark Scheltens is thanked for

constructive comments, suggestions and English polishing of an earlier version. We

thank J.G. Liou for editorial handling and acknowledge gratefully constructive

reviews of Donna Whitney, Sean R. Mulcahy, Nadia Malaspina and an anonymous

reviewer, which led to significant improvement of the manuscript.

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54

Vitae

Jinxue Du is a Ph.D. candidate supervised by Prof. Lifei zhang at School of Earth and

Space Sciences in Peking University. He is now studying on petrology and phase

equilibria modelling of HP-UHP eclogites and metapelites from Southwestern

Tianshan, China. His research interests also include geochronology study of both

accessory and rock-forming minerals in these rocks.

Lifei Zhang received his B.S. and Ph.D. degrees from Jilin University and Peking

University, respectively. He is now a professor and a doctoral supervisor in Peking

University. He is mainly engaged in teaching and researches in ultrahigh pressure

metamorphism. His research interests include petrology, geochronology and

geochemistry of HP-UHP eclogites and fluid activities in these rocks.

Thomas Bader

Thomas Bader is currently a post-doctoral researcher at Peking University. He

obtained his diploma degree from Bergakademie Freiberg in 2005 and his PhD degree

from Basel University in 2011 under supervision of Leander Franz and Christian de

Capitani. His research focuses on modeling of metamorphic phase equilibria,

pressure-temperature-time-paths, and the orogenic evolution of the Qinling and

Tianshan belts.

55

Figure Captions

Fig. 1. Geological map of the LT-HP/UHP belt in Chinese southwestern Tianshan,

(modified after Lü et al. (2012a)). The inset map shows the location of the South

Tianshan orogenic belt in central-east Asia. Green outlined, solid red and blue stars

show localities of samples studied in this paper, previously reported UHP occurrences

and newly identified UHP eclogites respectively. For details, see Tables 4 and 5. GPS

coordinates of some of the UHP rocks are provided in Table A.1.

Fig. 2. Field photograph (a), backscattered electron (BSE) images (b, d-f) and thin

section photomicrograph in plane-polarized light (c) of glaucophane eclogite H608-14

from Chinese southwestern Tianshan. (a) Eclogite occurs as massive lens in phengite

schist. (b) A subhedral garnet crystal (GI) with a distinct core-rim zoning shows

inclusions of omphacite, rutile, quartz with cracks surrounding it, Ca-rich garnet (GII),

aggregates of carbonates + paragonite (enlarged in Fig. 2d) and barroisite + quartz.

The black line shows the location of the EMP profile (Fig. 4b). (c) Fine-grained

omphacite aggregate, paragonite and medium-grained glaucophane mantled by

barroisite are major matrix constituents. (e-f) Box-shaped aggregates of Ca-rich

GII/clinozoisite + quartz/paragonite in GI.

Fig. 3. BSE images (a, c-f), photomicrographs in plane-polarized light (b, g), and a

Raman spectrum of lawsonite (h) of jadeite-lawsonite-bearing

garnet-glaucophane-phengite schist H71-4. (a) A fine-grained garnet crystal (GIA)

with rare inclusions of jadeite rimed by chlorite, rutile and quartz surrounded by

cracks. (b) A coarse-grained diablastic garnet crystal (GIB) with numerous inclusions

56

of quartz, jadeite, glaucophane, dolomite, and clinozoisite + paragonite (enlarged in (d)

and (e)). (c) Rutile and omphacite inclusions in quartz, on its part included in GIB. (f)

Box-shaped aggregates of Ca-rich GII/clinozoisite + paragonite included in GIB. (g)

An anhedral zircon with inclusions of lawsonite and rutile. (h) Raman spectrum of the

lawsonite in Fig. 3g. The peaks at 567.9, 696.4 and 939.3 cm-1 are diagnostic of

lawsonite, and the ones without labels stem from zircon and epoxy resin.

Fig. 4. Mineral chemistry diagrams showing compositional variation of garnet,

clinopyroxene and amphibole from glaucophane eclogite H608-14 and

garnet-glaucophane-phengite schist H71-4. (a) Garnet plotted in a part of the

Alm+Sps (almandine + spessartine) – Grs (grossular) – Prp (pyrope) diagram;

core-rim variations are indicated by the hollow arrows. (b) Compositional profile of

GI from sample H608-14 along the line in Fig. 2d. (c) and (d) Compositional profiles

of GIA and GIB from sample H71-4. (e) Clinopyroxene from H608-14 and H71-4

plotted in the WEF (wollastonite + enstatite + ferrosilite) – Jd (jadeite) – Aeg

(aegirine) diagram. Phase relations proposed by Green et al. (2007) are indicated by

the dashed lines. (f) Si-NaM4 p.f.u. diagram showing compositions of earlier

glaucophane and late-stage barroisite/katophorite from H608-14 and H71-4

(nomenclature follows Leake et al. (1997)). The arrows show their compositional

evolution.

Fig. 5. P-T pseudosections for glaucophane eclogite H608-14 (a and b) and

jadeite-lawsonite-bearing garnet-glaucophane-phengite schist H71-4 (c and d)

calculated in the systems MnNCFMASHO and MnNCKFMASH with H2O in excess,

57

respectively, with the bulk rock compositions listed in Table 3. The pseudosections

are contoured with Prp and Grs isopleths, garnet’s modal proportion (based on

one-oxide), lawsonite-out isograds calculated after subtracting fractionated garnet

from the bulk rock compositions and garnet-in isograds calculated in MnO-free

subsystems. The white-colored quadrangle and circles represent intersections of Grs

and Prp isopleths: points 1-12 in (a) correspond to EMP spots of GI in H608-14

labeled the same as in Fig. 4b; points α, β and 1-10 in (c) correspond to GIA in H71-4

labeled as in Table 2 and Fig. 4c; Points Gc and Gr in (d) correspond to GIB labeled

the in Fig. 4d. Among them, circles with dashed outlines represent outermost garnet

rims plotted taken chemical fractionation into account. The shaded ellipses

correspond to GII compositions. Hollow arrows represent the derived P-T paths, solid

and dashed outline correspond to garnet growth and inference from matrix

assemblages, respectively. For comparison, reported P-T paths derived from garnet

profiles of UHP eclogites (T12E: Tian and Wei, 2013) and metapelites (W09M: Wei

et al., 2009, L12M: Lü et al., 2012b, Y13M: Yang et al., 2013) from Chinese

southwestern Tianshan are shown in (b).

Fig. 6. P-T pseudosections calculated for the coesite-bearing eclogite H607-5 of Lü et

al. (2009) from Chinese southwestern Tianshan (a) and a typical MORB composition

(sample 104-16, b) taken from Sun and McDonough (1989) in the MnNCFMASH(O)

systems with H2O in excess. The intersections of Grs and Prp isopleths of garnet

adjacent to coesite Lü et al. (2009) are represented by white-colored circles in Fig. 6a.

Other explanations are the same as for Fig. 5.

58

Fig. 7. P-X pseudosections calculated at 530 °C for glaucophane eclogite H608-14

showing the effect of bulk-rock variations on Grs isopleths. In (a, b, d & e), the actual

X-ratios of H608-14 are shown as the vertical dashed lines joining the two stars. In (a,

b & d), the limits of X-ratios of natural lawsonite eclogites (Wei and Clarke, 2011) are

shown as thick dashed lines. Other explanations are the same as for Fig. 5.

Fig. 8. P-X[CaO/(CaO + MnO + MgO + FeOtotal] pseudosection calculated at 530 °C

for jadeite-lawsonite-bearing garnet-glaucophane-phengite schist H71-4. Explanations

are the same as for Fig. 7.

Fig. 9. (a) Simplified P-T grid for end-member reactions (black) involving typical

minerals of the glaucophane-lawsonite eclogite facies calculated in the NCMASH

system. Dashed lines represent metastable parts of these reactions. Furthermore

shown are the reaction (grey) which defines maximum

stability of lawsonite, and reactions (2) and (3) calculated using compositions of GII

and omphacite from glaucophane eclogite H608-14 which may be response for the

box-shaped GII/clinozoisite + quartz/paragonite inclusions. For simplification, only

one polymorph of SiO2, i.e. quartz, is considered. (b) P-T pseudosection for

glaucophane eclogite H608-14 (the same as Fig. 5a) with intersections of

representative Grs and Prp isopleths (white circles) of garnet from newly identified

UHP eclogites from Chinese southwestern Tianshan (Table 5). For comparison, P-T

conditions of several eclogites (white circles with dashed outlines) estimated by P-T

pseudosections recalculated with bulk compositions available in the corresponding

papers (see Table 5) and the P-T path derived from H608-14 are shown.

59

Fig. 10. Prp-Grs diagrams for garnet from eclogites and mafic blueschists from

Chinese southwestern Tianshan. (a) Previously identified UHP eclogites. (b)

Previously reported HP eclogites. (c) Mafic blueschists. (d) Vein-bearing mafic

blueschists and eclogites. These diagrams are contoured with Grs-Prp-values

calculated at the quartz-coesite transition for MORB (Sun and McDonough, 1989),

coesite-bearing eclogite H607-5 (Lü et al., 2009) and glaucophane eclogite H608-14

(red) in the same model systems as Figs. 5a and 6. The garnet compositions plotted

into these diagrams are taken from: A Gao et al. (1995), B Gao et al. (1999), C Gao

and Klemd (2001), D Zhang et al. (2001), E Klemd et al. (2002), F Zhang et al.

(2002a), G Zhang et al. (2002b), H Gao and Klemd (2003), I Klemd (2003), J Li et al.

(2003), K Wei et al. (2003), L: Zhang et al. (2003b), M Zhang et al. (2005), N Lin and

Enami (2006), O Su et al. (2006), P Xiong et al. (2006), Q Gao et al. (2007), R Lü et

al. (2007), S John et al. (2008), T van der Straaten et al. (2008), U Zhang et al.

(2008a), V Lü et al. (2009), W Beinlich et al. (2010), X Lü et al. (2012a) and Y Tian

and Wei (2013).

Fig. 11. (a) Prp-Grs diagram for metapelites from Chinese souwestern Tianshan. The

plotted garnet compositions are taken from: I Zhang et al. (2003a), II Wei and Powell

(2004), III Lü et al. (2008), IV Wei et al. (2009), V Lü et al. (2012b) and VI Yang et

al. (2013) are shown. (b) Prp-Grs diagram for representative lawsonite eclogites

worldwide. 1 Schistes Lustrés, Corsica (Brovarone et al., 2011), 2 Ward Creek,

Franciscan Complex (Shibakusa and Maekawa, 1997), 3 Port Macquarie, Australia

(Och et al. 2003), 4 Pinchi Lake, British Columbia (Ghent et al., 2009), 5 Guatemala

60

(Endo et al. 2012), 6 Central Pontides, Turkey (Altherr et al., 2004), 7 Sivrihisar,

Turkey (Davis and Whitney, 2006; Çetinkaplan et al., 2008; Davis and Whitney,

2008), 8 Samana Peninsula, Dominica (Zack et al., 2004), 9 Pam Peninsula,

Caledonia (Clarke et al., 1997), 10 North Qilian, China (Song et al., 2007; Zhang et

al., 2007b), 11 South Sulawesi, Indonesia (Miyazaki et al., 1996), 12 Motalafjella,

Spitsbergen (Hirajima et al., 1988), 13 Monviso meta-ophiolite (Groppo and Castelli,

2010), 14 Sesia-Lanzo Zone, western Alps (Zucali and Spalla, 2011), 15 Garnet Ridge,

Colorado Plateau (Usui et al., 2006), 16 Sulu, China (Mattinson et al., 2004), 17

Dabieshan, China (Wei et al., 2010), 18 Verpeneset, Western Gneiss Region, Norway

(Krogh, 1982). Other explanations are the same as for Fig. 10.

61

Table 1 Representative microprobe analyses of minerals from glaucophane eclogite

H608-14.

Mineral GI-c GI-r GII gln brs omp-m omp-in pg ep ab

SiO2 37.47 38.75 38.27 58.85 48.78 55.26 55.07 46.89 38.01 68.52

TiO2 0.16 0.01 0.12 0.04 0.22 0.03 0.02 0.04 0.07 0.00

Al2O3 20.73 22.29 21.05 11.91 11.50 9.70 9.84 38.78 26.00 19.30

Cr2O3 0.00 0.05 0.39 0.01 0.05 0.05 0.02 0.06 0.08 0.00

Fe2O3 0.61 0.00 1.03 0.02 1.29 5.91 3.10 0.00 6.79 0.30

FeO 30.96 25.48 23.27 5.87 10.75 0.28 4.02 0.73 1.90 0.00

MnO 2.39 0.33 0.61 0.01 0.18 0.09 0.07 0.00 0.15 0.01

MgO 1.90 5.84 2.17 12.28 11.81 8.11 7.28 0.21 0.01 0.00

CaO 6.31 7.67 13.99 0.97 8.51 12.85 12.73 0.15 23.61 0.23

Na2O 0.02 0.03 0.04 7.23 3.54 7.67 7.00 7.56 0.10 11.54

K2O 0.00 0.00 0.00 0.01 0.25 0.00 0.00 0.86 0.02 0.02

Totals 100.55 100.45 100.94 97.20 96.88 99.95 99.15 95.28 96.74 99.92

O 12.00 12.00 12.00 23.00 23.00 6.00 6.00 11.00 12.50 8.00

Si 3.00 2.99 2.99 7.96 7.05 1.97 1.99 3.01 3.03 3.00

Ti 0.01 0.00 0.01 0.00 0.02 0.00 0.00 0.00 0.00 0.00

Al 1.95 2.03 1.94 1.90 1.96 0.41 0.42 2.93 2.44 1.00

Cr 0.00 0.00 0.02 0.00 0.01 0.00 0.00 0.00 0.01 0.00

Fe3+ 0.04 0.00 0.06 0.00 0.14 0.16 0.08 0.00 0.41 0.01

Fe2+ 2.07 1.64 1.52 0.66 1.30 0.01 0.12 0.04 0.13 0.00

Mn 0.16 0.02 0.04 0.00 0.02 0.00 0.00 0.00 0.01 0.00

Mg 0.23 0.67 0.25 2.47 2.54 0.43 0.39 0.02 0.00 0.00

Ca 0.54 0.63 1.17 0.14 1.32 0.49 0.49 0.01 2.01 0.01

Na 0.00 0.00 0.01 1.90 0.99 0.53 0.49 0.94 0.02 0.98

K 0.00 0.00 0.00 0.00 0.05 0.00 0.00 0.07 0.00 0.00

Cations 8.00 8.00 8.00 15.04 15.44 4.01 4.00 7.03 8.05 4.99

Alm 69.0 55.3 51.0 Jd 39.5 41.7

Sps 5.4 0.7 1.3 Aeg 15.4 8.2

Prp 7.5 22.6 8.5 WEF 45.1 50.1

Grs 18.0 21.3 39.2

-c, core; -r, rim; -in, inclusions in garnet; -m matrix. The other abbreviations are the

same as in the text.

62

Table 2 Representative microprobe analyses of minerals from

jadeite-lawsonite-bearing garnet-glaucophane-phengite schist H71-4.

Mineral GIA-c GIA-α GIA-β GIA-r GIB-c GIB-r GII brs gln omp jd ph pg ab

SiO2 37.34 37.68 37.24 38.38 37.76 37.54 37.78 49.88 58.81 56.37 58.61 49.93 46.89 68.2

TiO2 0.06 0.07 0.07 0.05 0.04 0.08 0.05 0.12 0.02 0.08 0.00 0.31 0.08 0.0

Al2O3 20.99 21.14 21.41 22.05 21.20 21.16 21.87 10.98 12.52 11.43 21.69 27.63 39.59 19.9

Cr2O3 0.04 0.04 0.00 0.07 0.00 0.02 0.02 0.04 0.03 0.04 0.00 0.02 0.00 0.0

Fe2O3 0.35 0.23 0.42 0.00 0.62 0.54 0.00 1.95 0.26 3.49 4.11 0.00 0.00 0.3

FeO 34.02 33.87 33.27 27.38 31.80 27.18 25.95 11.70 9.24 2.72 0.19 2.14 0.40 0.0

MnO 2.27 2.55 3.14 0.16 1.06 0.18 0.98 0.07 0.01 0.03 0.04 0.03 0.00 0.0

MgO 2.80 3.03 2.78 3.58 2.85 3.49 1.78 11.39 10.18 6.69 0.28 3.16 0.21 0.0

CaO 2.32 2.19 2.23 9.11 5.50 8.66 11.58 7.48 0.37 12.01 0.53 0.00 0.15 0.6

Na2O 0.09 0.10 0.07 0.02 0.04 0.06 0.00 4.20 7.30 8.06 15.61 0.84 7.34 10.

K2O 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.25 0.01 0.00 0.02 9.86 0.89 0.0

Totals 100.29 100.89 100.63 100.80 100.87 98.91 100.01 98.06 98.75 100.92 101.08 93.92 95.55 99.

O 12.00 12.00 12.00 12.00 12.00 12.00 12.00 23.00 23.00 6.00 6.00 11.00 11.00 8.0

Si 3.00 3.00 2.98 2.99 2.99 2.99 2.98 7.14 7.93 1.99 1.99 3.37 2.99 2.9

Ti 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.00 0.00 0.00 0.02 0.00 0.0

Al 1.99 1.99 2.02 2.03 1.98 1.99 2.04 1.85 1.99 0.48 0.87 2.20 2.98 1.0

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.0

Fe3+ 0.02 0.01 0.03 0.00 0.04 0.03 0.00 0.21 0.03 0.09 0.11 0.00 0.00 0.0

Fe2+ 2.29 2.26 2.23 1.78 2.11 1.81 1.71 1.40 1.04 0.08 0.01 0.12 0.02 0.0

Mn 0.15 0.17 0.21 0.01 0.07 0.01 0.07 0.01 0.00 0.00 0.00 0.00 0.00 0.0

Mg 0.34 0.36 0.33 0.42 0.34 0.41 0.21 2.43 2.05 0.35 0.01 0.32 0.02 0.0

Ca 0.20 0.19 0.19 0.76 0.47 0.74 0.98 1.15 0.05 0.45 0.02 0.00 0.01 0.0

Na 0.01 0.02 0.01 0.00 0.01 0.01 0.00 1.17 1.91 0.55 1.03 0.11 0.91 0.8

K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.05 0.00 0.00 0.00 0.85 0.07 0.0

Cations 8.00 8.00 8.00 8.00 8.00 8.00 7.99 15.49 15.02 4.00 4.04 6.99 7.01 4.9

Alm 76.8 75.8 75.2 60.1 70.7 60.9 57.7 Jd 47.1 87.5

Spss 5.2 5.8 7.2 0.4 2.4 0.4 2.2 Ae 9.0 10.6

Py 11.3 12.1 11.2 14.0 11.3 13.9 7.1 WEF 43.9 2.0

Gr 6.7 6.3 6.5 25.6 15.7 24.8 33.0

GIA-α and GIA-β refer to additional core compositions of GIA plotted in Fig. 5c (points

α and β). The other abbreviations are the same as in Table 1.

63

Table 3 Bulk rock compositions of samples from Chinese southwestern Tianshan and

a representative mid-ocean-ridge basalt (MORB) used in phase equilibria modeling

(normalized to 100 mole-%).

Sample SiO2 Al2O3 CaO MgO FeO* Na2O K2O MnO O** References

H607-5 MnNCFMASH 56.76 10.88 7.73 7.36 12.75 4.29 0.23 Lü et al. (2009)

H608-14 MnNCFMASHO 52.92 9.91 9.02 9.63 12.63 5.03 0.34 0.53 This study

H71-4 MnNCKFMASH 64.21 11.73 4.15 4.98 11.37 2.13 1.30 0.12 This study

MORB

(104-16) MnNCFMASHO 53.14 8.65 12.45 13.05 8.37 2.69 0.16 1.50***

Sun and

McDonough

(1989)

*, total FeO; **, Fe2O3; ***, according to the discussion in Rebay et al. (2010).

Table 4 Summary of previously reported UHP eclogites and metapelites from

Chinese southwestern Tianshan.

Sam Type Mineral assemblage Pr G P/kba Estimation Refe La

64

ple p rs r-T/º

C in

refer

ence

method renc

e

bel

s

in

Fig

s

10,

11

963

0-4

Quartz-exsolutio

n-lamellae-bearin

g eclogite

Grt+omp+ep+qz+cal+pg 8.

4

2

3.

2

25.7-

26.7,

496-5

98

Ca-Eskola

content in

omphacite

and

grt-cpx-ph

geotherm

obaromet

er

Zhan

g et

al.

(200

2a)

F

in

Fig

.

10

965

7

coesite-pseudom

orph-bearing

eclogite

Grt+omp+zo+gln+pg+qz 6.

2

2

1.

0

519-

1

Magnesite-bearin

g eclogite

Cal/dol/mgs+grt+omp+gl

n+zo+pg

1

2.

3

2

1.

5

27-28

,

525-6

07

Magnesite

-bearing

equilibriu

m

calculated

by

Thermocal

c and

grt-cpx

geotherm

ometer

Zhan

g et

al.

(200

2b)

G

in

Fig

.

10

519-

2

1

0.

0

2

2.

4

508 1

1.

8

2

0.

0

302 Magnesite-bearin

g eclogite

Grt+omp+ph+mgs+dol 1

0.

0

2

3.

0

24.6-

28.9,

525-6

07

Grt-cpx-ph

geobarom

eter

Zhan

g et

al.

(200

3b)

L

in

Fig

.

10

303 Grt+omp+ph+mgs+dol 9.

8

2

2.

9

305 Grt+omp+ph+mgs+dol 1 2

65

0.

5

1.

9

H61

2

Magnesite-bearin

g metapelite

Grt+cld+gln+pg+cal+qz+r

t+ph+mgs

4.

8

2

3.

4

49.5-

50.7,

560-6

00

Cal-dol

geotherm

ometer

and the

reaction

dol=mgs+a

rg

calculated

using

Thermocal

c

Zhan

g et

al.

(200

3a)

I in

Fig

.

11

105-

34

coesite-exsolutio

n-lamellae-bearin

g eclogite

Grt+omp+gln+zo+cal+pg 9.

2

2

2.

9

50,

600

After

Zhang et

al. (2003a)

Zhan

g et

al.

(200

5)

M

in

Fig

.

10

H60

1-9

coesite- and

omphacite-bearin

g schist

Grt+omp+coe+ph/pg+gl

n/brs+ab

9.

8

8.

1

27-33

,

570-6

30

Grt-cpx

geotherm

ometer

and

occurrenc

e of

coesite

Lü et

al.

(200

8)

III

in

Fig

.

11

H60

7-5

coesite-bearing

eclogite

Grt+omp+coe+gln/brs+p

g+rt

1

0.

6

1

3.

2

24-27

,

470-5

10

Phase

equilibria

modeling

calculated

by Domino

Lü et

al.

(200

9)

V

in

Fig

.

10

T31

1

glaucophane-phe

ngite schist

Grt+gln+hbl+ph+pg+ep+

ab+qz

2

3.

0

7.

1

32,

550-5

70

Phase

equilibria

modeling

calculated

by

Thermocal

c

Wei

et al.

(200

9)

IV

in

Fig

.

11

104-

1

Albite schist Ab+grt+brs+coe/qz+ph/

pg+cb+gln+rt/spn+ep+ch

l

- - - - Lü

and

Zhan

g

(201

2)

-

101- Albite schist Ab+grt+brs+coe/qz+ph/ - -

66

1 pg+gln+rt/spn+ep+chl+cl

d

AT1

03-6

Albite schist Ab+brs+grt+qz+gln+ph/p

g+rt/spn

- -

AT1

03-2

Vein-bearing

eclogite

Omp+grt+cb+rt+coe/qz+

ep+brs+ph/pg+rt+ab

6.

6

2

8.

9

>27,

490

Grt-cpx

geotherm

ometer

and

occurrenc

e of

coesite

Lü et

al.

(201

2a)

X

in

Fig

.

10

K10

28

Vein-bearing

eclogite

Omp+grt+pg/ph+brs/gln

+czo+rt

1

1.

0

1

4.

4

H60

1-18

Vein-bearing

eclogite

2

4.

4

2

2.

3

H60

7-1

Vein-bearing

eclogite

1

6.

3

2

1.

0

H50

4-4

Albite-poor schist Qz+ab+grt+ph/pg+chl+rt

/spn±gln±czo±cb

8.

6

8.

1

25-31

,

430-5

10

Phase

equilibria

modeling

calculated

by domino

Lü et

al.

(201

2b)

V

in

Fig

.

10

H71

3-71

Metapelite 1

2.

1

9.

7

H50

3-1-

4

Albite-rich schist 6.

8

5.

4

H60

6-19

Metapelite 1

0.

2

6.

8

H60

1-13

Albite-poor schist 1

2.

4

7.

8

H71

-2

Metapelite 1

1.

3

9.

8

K94

9

Eclogite Grt+omp+brs+ep+ph+pg

+qz+rt/spn

1

3.

8

1

7.

2

29-30

,

526-5

40

Phase

equilibria

modeling

calculated

Tian

and

Wei

(201

Y

in

Fig

.

67

by

Thermocal

c

3) 10

K95

0

Eclogite Grt+omp+gln/brs+ep+ph

+pg+qz+rt/spn

1

3.

3

1

6.

2

T84

8

Eclogite Grt+omp+gln/brs+ep+ph

+qz+rt/spn

1

0.

0

1

5.

6

28.2,

518

K93

1

Coesite-bearing

schist

Grt+ph/pg+qz/coe+chl+

pg+ab+zo+cld+rt/spn

1

7.

0

6.

2

29,

565

Phase

equilibria

modeling

calculated

by

Thermocal

c

Yang

et al.

(201

3)

VI

in

Fig

.

11

-, not available.

Table 5 Summary UHP eclogites from Chinese southwestern Tianshan newly

identified on the basis of garnet composition.

Sa

mpl

Type Mineral

assemblage

Pr

p

G

rs

P/kba

r-T/º

Estimation

method

Refe

renc

P-T

by

La

be

68

e C in

the

refer

ence

e Grs

and

Prp

isop

leth

s

ls

in

Fig

.

10

952

5-1

1

Clinozoisite

eclogite

Grt+omp+czo+h

bl+ph+rt+qz+cal

8.

5

9.

3

>12-1

4,

520

Grt-cpx

geothermometer

and jadeite

content in

omphacite

Gao

et al.

(199

9)

40.5

,

431

B

95a

5

Eclogite Grt+omp+rt+qz+

czo+gln/brs+ph/

pg

5.

0

1

8.

2

14-21

,

500-6

00

29.0

,

463

965

11

Massive

eclogite

Grt+omp+zo+gln

+pg

1

0.

4

1

8.

4

16-19

, 530

The reaction:

gln+czo=pg+prp+o

mp+qz and

grt-cpx

geothermometer

Zhan

g et

al.

(200

1)

28.7

,

503

D

965

33

Gneissic

eclogite

Grt+omp+ph+qz

+cal

1

0.

2

1

7.

5

29.6

,

498

965

14

Pillow

eclogite

Grt+omp+cal+gln

+zo+pg

9.

3

1

6.

6

30.6

,

488

R3-

1

magnesite-

bearing

eclogite

Grt+omp+dol+gl

n+czo+pg+mgs+p

h

1

1.

3

2

0.

0

20-25

,

470-5

30

Magnesite-parago

nite-quartz-bearin

g equilibrium

calculated by

Thermocalc

Kle

md

(200

3)

27.0

,

521

I

AK1

7

Glaucophan

e eclogite

Grt+omp+gln+pg

+qz

1

3.

9

1

7.

3

15-19

, 580

Phase equilibria

modeling using

Thermocalc

Wei

et al.

(200

3)

29.1

,

552

28.6

,

560*

K

AK1

0

Hornblende

eclogite

Grt+omp+pg+hbl

+ep+qz

1

3.

2

1

7.

1

17-18

,

610-6

30

29.5

,

547

29.2

,

554

69

*

AK3

0

1

4.

0

1

7.

8

28.6

,

555

29.9

0,

548*

CG4

-4

Omphacite-

glaucophan

e epidosite

Grt+zo+gln+ph+o

mp

7.

3

1

6.

9

20,

520

Grt-cpx

geothermometer

and the reaction:

ky+jd+H2O=pg

Gao

and

Kle

md

(200

3)

30.2

,

477

H

ws2

4-7

Eclogite Grt+omp+rt+zo+

ph+pg+qz+cal+br

s

4.

8

1

4.

4

14-21

, 520

After Klemd et al.

(2002) and Wei et

al. (2003)

Su

et al.

(200

6)

32.9

,

442

O

H50

4-8

Hornblende

eclogite

Grt+omp+brs+ca

l+ms+ep+rt+spn

1

8.

1

1

9.

9

20-25

,

570±

30

Grt-cpx-ph

geothermobarom

eter

Lü et

al.

(200

7)

25.9

,

581

R

H50

4-1

0

Paragonite

eclogite

Grt+omp+ms+br

s+qz

1

3.

6

1

6.

5

30.0

,

547

31.0

,

543*

*, estimated by P-T pseudosections recalculated with bulk compositions available in

the corresponding papers; other results are derived from the P-T pseudosection of

H608-14.

Highlights

70

� The first report of UHP jd-lws-gln-bearing metapelites from SW Tianshan.

� UHP metapelites and eclogites experienced similar metamorphic evolution.

� Grs content in garnet from LT eclogites can be an indicator of UHP metamorphism.

aC a1

S a3S q3

2aγ42dγ4

bC1a木扎

尔

特

水

克木

扎

尔

特

东

都

不

东德沟里

萨依

阿

克

牙孜古

尔布拉

克

阿克 牙 孜

卡朴

亭苏

萨

依

科

布

尔特

萨

依

2dγ4

eΣ 24eΣ 24

Thrust Fault,uncategorized

Strike-slipFault

Precambrian amphibolitefacies Rocks

UltramaficRocks

Granitoid

Late PaleozoicVolcanics

RhyolitePorphyry

PaleozoicMarble

PaleozoicSedimentary Rocks

LT-HP/UHPComplex

Precambrian BasementPaleozoic Granite

LP Granulite

10 km0

Eclogite

UHP sub-belt

HP sub-belt

HP-UHPBoundary

Newly identifiedUHP localities

Cenozoic cover

Study area

N

42°20′

42°40′

80°30′ 81°00′ 81°30′

ChinaJunggar Plate

UrumqiYining

Tarim Plate

5

Haerkeshan Fault

South Tianshan Fault

Habutengsu

Keb

uert

e

Akeyazi

Muzhaerte R

iver

Don

gdel

igou

Riv

er

CarboniferousVolcanics

River Previously reportedUHP localities

Yili CentralTianshan Plate

North Central Tianshan Fault

South Central Tianshan Fault

H612

K1028K949K950K931

AT103-2AT103-6

AK17

101-1

104-1 WS24-7

AK30

9630-4, 9657519-1, 519-2T848, 508, 302303, 305, 105-34

9525-1195a5

9651196514

T311

96533

H504-4H504-8H504-10

H601-9, H601-13H601-18, H607-1H607-5, H71-2H608-14, H71-4

H713-71H503-1-4

H606-19

New samples

42°

44°

42°

44°

82° 84° 86°

82° 84° 86°

100 km

Fig. 1

500um

omp

gln

brs

pg f

phengite Schist

Eclogite

a b

300um

GII ompbrs

qz

qz

rt

GI

ec

d

dolcal

rt

GIpg

c

50um

qz

GII

GI

omp

czo

czo

GII

pgchl

50um

qz

50um

GI

Fig. 2

900 800 700 600 500-1Raman Shift (cm )

500

1000

1500

Inte

nsi

ty

93

9.3

69

6.4

56

7.9

20um

rt

lws

zrn

90um

gln

pl qz

jd

chlrt

AGI

30um

BGI

omp

qz

rt

500um

phba

c

BGI

qz

de

30um

pg

BGI

GII

czo

rtf

gln

jd

pgchl

100um

d

BGI

eczo

pgph

100um

hg

BGI

Fig. 3

a b

dc

e f

rim core rim0

20

40

60

80

AlmSpsPrpGrs

Jd Aeg

WEF

Omphacite

Jadeite Aegirine

Aegirine-Augite

Quad

In the matrixIn GIIn H71-4

H608-14

6 7 80

1

2

Si

(Na)

M4 gln

wnc/rct

acthbl/ed

brs/ktp

ts/prg

H608-14H71-4

core

rim

rim core rim0

20

40

60

80

AlmSpsPrpGrs

rim core rim

AlmSpsPrpGrs

0

20

40

60

80

10987654321

11

1234567

89

Alm+Sps

Grs Prp

100

90

80

70

60

50

90

80

70

60

50

H608-14GI GII

12

10

core

rim

Gr

Gc

H71-4

AGI

BGI GII

Fig. 4

15

20

25

30

35

coeqz

P/k

bar

gln lwsomp chl

gln lws om

p

gln lws om

p qz

gln omp qz

omp coe

omptlc coe

gln lwsomp pg

gln omp pg qz

gln lw

s

omp c

hl pg

glnomp chlpg ep

gln pg

omp chl

gln omp pg

gln hblomp pg qz

p5

p2

p10

p25

p24

p20

p15g15

g20

g25

g30

g35

g40

core

rim

1-7

8

912

10

11

g30p10

GrsPrp

coeqz

(c) H71-4 MnNCKFMASH(+grt+qz/coe+ph+H O) (d)2

gln jd lws

jd lws

jd lw

s ky

jd ky

gln jd

lws car

gln lws car

gln lwscar chl

gln lws

car cld

gln jdlws cld

gln

lws cld

gln lws

car chl cld

gln

lws

chl c

ld

jd

omp gln pggln pg

gln chl pg

gln lwschl cld

gln jdlws ky

gln

lws p

g

ompgln pg pl

p2

p5

p10 p15

p20

p21

p21

g5

g3

g10

g15

g20

g25g30

g35

5

43

12

core

rim

g30p10

GrsPrp

(a) H608-14 MnNCFMASHO(+grt+H O) (b)2

98

76

αβ

lws-out

lws-out400 450 500 550 600 65015

20

25

30

35

coeqz

T/°C

P/k

bar

M10 Mgrt

grt-in

M1

M5

M8

M10

M16

M14M

12

M17.5

450 500 550 600 650

coe

T/°C

qz

M8

M12

M10

M14

M16

M18

M19

M5

M1

400 grt-in

gln lws chl pg

jd glnomp pg

gln jd pg

jdky gln

glnjd lws pg

M10 Mgrt

4 °C

/km

5 °C/k

m

6 °C/k

m

7 °C/km

8 °C/km

4 °C

/km

5 °C/k

m

6 °C/k

m

7 °C/km

8 °C/km

W09M

Y13M

L12M

T12E

10 Gr

Gc

Fig. 5

coeqz

gln lwsomp tlc

gln lws om

p

gln lwsomp jd

lwsomp jd

omp jd

gln omp jd

gln lwsomp act

gln lws act

gln lws

act tlcgln

lws act chl

gln lws

act chl tlc

p10p15

p20

p5

g15

g18

g20

g25

g20p10

GrsPrp

24

26

28

30

32

34

450 500 550 600 650

H607-5 MnNCFMASH(+grt+qz/coe+H O) (b)2

P/kbar

T/°C

20

22

24

26

450 500 550 600

coeqz

MORB(104-16) MnNCFMASHO(+grt+qz/coe+H O) (a)2

400

30

T/°C

28

omp lws tlc

gln omp lws tlc

gln o

mp l

ws t

lc ep

gln o

mp t

lc epgln omp lws tlc chl

gln omp lws chl

gln omp lw

s tlc-qz

gln omp lw

s tlc chl-qz

g20

22g

52g

g18

g16

g30

21p

61p

02p

42p

g20p10

GrsPrp

Fig. 6

jd di prp qz H

O2

gln grs

jd di lws

gln grs qz H O2

jd di prp lwsgln grs H O2

jd di prp qz HO2

gln lws

400 500 600 700 800 90010

15

20

25

30

35

40

lws

czo

ky qz

HO

2

di lw

sgr

s prp

qz

HO

2

gln

lws

jd g

rs p

rp q

z H

O2

T/°C

P/kbar

jd di prp lws qzgln grs

jd lw

s49

grs pg

qz H

O

35

2

jd lw

s49

czo pg

qz H

O2

Fig. 7

X[Al O /(Al O + MgO + FeO )] × 1002 3 2 3 total

gln lws omp

gln lwsomp tlc coegln omp tlc

gln omp

gln om

p chl

gln lwsomp chl

gln lwsomp jd

gln lws chl jd

gln lws chl jd pg

gln lwsomp chl jd

gln lws omp chl pg

gln omp chl pg epgln lws omp chl pg ep

coeqz

g25

g20

g15

g30

g35

g38

(d): T=530 ˚C+grt+HO2

g30 Grs

gln lws jd

5030 4020X[CO/(CO + HO)] × 1002 2 2

(c): T=530 ˚C+grt+fluid

g30 Grs

gln lws om

p

gln lwsomp chl

gln lws omp dol

gln lwsomp dol tlc coe

gln lws ompdol qz/coe

gln lws ompdol jd qz/coe

gln lwsdol jd qz/coe

grt glnomp chl pg

grt gln lwsomp dol pg

grt glnomp dol pg grt gln lws omp dol pg qz

grt gln omp chl pg ep

gln ompdol pg ep

gln ompdol pg ep qz

gln omp dol pg qz

gln lwsomp tlc coe

g30

g35

g38

g15

g20

g25

15

20

25

30

35

P/kbar

0 5 10 15 20

coeqz

gln lwsomp chl

gln lws omp

gln lwsomp qz/coe

gln omp

gln omp chl pg

gln ompchl pg ep

gln ompchl pg lws

gln ompep qz

gln ompep lws qz

gln lwsomp tlc qz/coe

lws omptlc qz/coe

lws

omp

tlc d

i qz

/coe

gln lwsomp tlc act qz

gln lwsomp act qz

gln om

p

ep act q

z

gln

lws

act d

i qz

gln epact qz

g15

g20

g25

g30

g35

g38

60X[CaO/(CaO + NaO)] × 1002

coeqz

g30 Grs

70 80 90

gln

omp

chl

gln lwsomp act ep qz

(b): T=530 ˚C+grt+H O2

15

20

25

30

35

P/kbar

g15

g20

g25

g30

g35

g38

gln lws omplw

s om

p

lws ompcoe tlc

lws

omp tlc

gln lws omp tlc

gln lws omp tlc coe

gln lws omp coe

gln lws omp chl

gln lws omp chl pg

gln lws omp ep pg

gln ompep pg

gln ompep pg chl

gln ompchl pg

coeqz

g30 Grs

20 5030 40X[CaO/(CaO + MnO + MgO + FeO )] × 100total

(a): T=530 ˚C+grt+H O2

Fig. 8 to be continued

gln lwsomp qz

gln lws omp

gln lwsomp chl

gln lws omp tlc qz/coe

lws omp tlc qz/coegln lws om

p tlc

gln epomp chl

gln ep ompgln epomp qz gln ep omp chl qz

gln lws omp chl qz

g15

g20

g25

g30

g35

g39

H608-14 MORB

coeqz

(f): T=530 ˚C+grt+HO2

g30 Grs

gln lwsomp di qz gln lws

di qz

gln lws omp chl pg

gln lws omp

gln lws omp tlc

gln lws omp tlc coe

gln lwsomp tlc hem

lws omp tlc

gln lwsomp hem

gln lwsomp hem chl

gln lws omphem ep chl

gln lws omp chl ep

gln omphem ep pg

gln ompchl ep pg

gln omp

chl ep pg hemgln ompchl pg

gln lws omp chl

g15

g20

g25

g30

g35

g38

3+X[Fe /FeO ] × 100total

15

20

25

30

35

P/kbar

0

(e): T=530 ˚C+grt+HO2

g30 Grs

20 40 60

coeqz

X(H608-14 to MORB) × 10020 806040

Fig. 8 continuing

15

20

25

30

35

P/kbar

10 20X[CaO/(CaO + MnO + MgO + FeO )] × 100total

gln lws pg

gln

lws

jd

gln lwsgln lws omp

gln lws omp pg

gln lws jd omp

gln lws pg cld

gln lws jd cld

gln lws jd car

lws om

p

lws om

p jd

gln cld

gln pg cld

gln jd cld

gln

jd c

ar

lws jd car

gln omp pg czo

gln pg czo

gln pg

gln

pg ch

l

g30 Grs

T=530 ̊ C+grt+ph+qz/coe+HO2

30 40 50

coeqz

g15

g6

g10

g20

g25

g30

g35

g38

Fig. 9

0

15

30

45

HP Region

UHP Region

a

Grs

UHP eclogites

HP Region

UHP Region

b

HP eclogites

0 10 20 300

15

30

45

HP Region

UHP Region

c

Prp

Grs

Blueschists

HP Region

UHP Region

d

MORBH607-5H608-14

0 10 20 30Prp

Vein-bearing rocks

Core

Rim

A B C D E F G H I J K L M N O P Q R S T U V W X Y

Qz

Coe

Qz

Coe

Qz

Coe

Qz

Coe

Fig. 10

400 450 500 550 600 65015

20

25

30

35

coeqz

T/°C

p5p2

p1

p25

p24p2

p15

g15

g2

g25

g3

g35

g4g30p10

GrsPrp

95a596511

96533

96514

R3-1

AK17

AK10AK30

ws24-7

H504-8

H504-10CG4-4

Fig. 11

1 2 3 5 6 7 8 9 104 1112 1314 1516 17 18

0 10 20 30 40 50 60Prp

MORBH607-5H608-14

UHP Region

HP Region

b

Lawsonite eclogites worldwide

Rim

Core

Type-LType-EType-U

Type-L Type-E Type-U

Metapelites

Rim

H71-4

HP Region

UHP Regiona

Prp

0

15

30

45

Grs

Core

0 10 20 30

I II III IV V VI

Qz

Coe

Qz

Coe

Fig. 12