burenjargal_et_al-2014-journal_of_metamorphic_geology

Thermal evolution of the Tseel terrane, SW Mongolia and itsrelation to granitoid intrusions in the Central Asian OrogenicBelt

U. BURENJARGAL,1 A. OKAMOTO,1 T. KUWATANI,1 S. SAKATA,2 T. HIRATA2 AND N. TSUCHIYA1

1Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan([email protected])2Department of Geology and Mineralogy, Graduate School of Science, Kyoto University, Kyoto, Japan

ABSTRACT The timing and thermal effects of granitoid intrusions into accreted sedimentary rocks are importantfor understanding the growth process of continental crust. In this study, the petrology and geochro-nology of pelitic gneisses in the Tseel area of the Tseel terrane, SW Mongolia, are examined to under-stand the relationship between igneous activity and metamorphism during crustal evolution in theCentral Asian Orogenic Belt (CAOB). Four mineral zones are recognized on the basis of progressivechanges in the mineral assemblages in the pelitic gneisses, namely: the garnet, staurolite, sillimaniteand cordierite zones. The gneisses with high metamorphic grades (i.e. sillimanite and cordierite zones)occur in the central part of the Tseel area, where granitoids are abundant. To the north and south ofthese granitoids, the metamorphic grade shows a gradual decrease. The composition of garnet in thepelitic gneisses varies systematically across the mineral zones, from grossular-rich garnet in the garnetzone to zoned garnet with grossular-rich cores and pyrope-rich rims in the staurolite zone, andpyrope-rich garnet in the sillimanite and cordierite zones. Thermobarometric analyses of individualgarnet crystals reveal two main stages of metamorphism: (i) a high-P and low-T stage (as recordedby garnet in the garnet zone and garnet cores in the staurolite zone) at 520–580 °C and 4.5–7 kbar inthe kyanite stability field and (ii) a low-P and high-T stage (garnet rims in the staurolite zone andgarnet in the sillimanite and cordierite zones) at 570–680 °C and 3.0–6.0 kbar in the sillimanite stabil-ity field. The earlier high-P metamorphism resulted in the growth of kyanite in quartz veins withinthe staurolite and sillimanite zones. The U–Pb zircon ages of pelitic gneisses and granitoids revealthat (i) the protolith (igneous) age of the pelitic gneisses is c. 510 Ma; (ii) the low-P and high-T meta-morphism occurred at 377 � 30 Ma; and (iii) this metamorphic stage was coeval with granitoid intru-sion at 385 � 7 Ma. The age of the earlier low-T and high-P metamorphism is not clearly recordedin the zircon, but probably corresponds to small age peaks at 450–400 Ma. The low-P and high-Tmetamorphism continued for c. 100 Ma, which is longer than the active period of a single granitoidbody. These findings indicate that an elevation of geotherm and a transition from high-P and low-Tto low-P and high-T metamorphism occurred, associated with continuous emplacement of severalgranitoids, during the crustal evolution in the Devonian CAOB.

Key words: garnet; granitoid intrusion; metamorphic history; Tseel terrane; U–Pb zircon age.

INTRODUCTION

Low-P and high-T metamorphism, which occurs atmiddle to shallow crustal depths with high geothermalgradient (>35 °C km�1), is one of the fundamentalprocesses during the evolution of continental crust.Several models have been proposed to explain anelevation of the geotherm of shallow crust, includingemplacement of basaltic magma or granitoids fromdepths (De Yoreo et al., 1989; Collins & Vernon,1991; Sandiford et al., 1991; Bodorkos et al., 2002;Okudaira & Suda, 2011), shallowing of a thermalboundary layer in an extensional setting (Wickham &Oxburgh, 1985; Craven et al., 2012) and radioactive

heat production (Sandiford et al., 1998). There havebeen many studies on low-P and high-T metamorphiccomplexes, including the Wongwibinda and Coomacomplexes in southeastern Australia (Vernon, 1982;Danis et al., 2010; Craven et al., 2012), metamorphicterranes from the Proterozoic northern Arunta inlier,central Australia (Collins & Vernon, 1991), the Aca-dian metamorphic terrane in the northern Appala-chians (Holdaway et al., 1982), Ryoke metamorphicbelt in Japan (Okudaira, 1996; Ikeda, 2004; Miyazaki,2004; Okudaira & Suda, 2011). One of the commonfeatures of the low-P and high-T metamorphic com-plexes is intrusion of granitoids into sedimentaryrocks, and thus it is important to evaluate (i) the

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extent of thermal perturbation induced by granitoidemplacement and its relation to regional thermalstructure and tectonic setting and (ii) temporal evolu-tion of metamorphic conditions of sedimentary rocksfrom burial or accretion to granitoid intrusion stages.However, it is generally difficult to reconstruct themetamorphic P–T history prior to the granitoid intru-sions, as the microstructural record, including garnetzoning, is modified or even obliterated by the heating.In addition, granitoids may be intruded in severalstages, resulting in a complicated thermal history.

The Tseel terrane of SW Mongolia is a high-T andlow-P crustal segment of an early Palaeozoic arc sys-tem within the Central Asian Orogenic Belt (CAOB;Kozakov et al., 2002; Burenjargal et al., 2012; Jianget al., 2012; Fig. 1). The terrane is composed mainlyof greenschist to amphibolite facies metasedimentaryand mafic rocks that are intruded by granitoids(e.g. Kozakov et al., 2002; Sukhorukov, 2007; Buren-jargal et al., 2012). Previous studies have proposedthat ridge subduction may have been the heat sourceof the high-T metamorphism of the Tseel terrane(Kozakov et al., 2002; Windley et al., 2007). How-ever, notwithstanding previous geochemical and chro-nological investigations (Bibikova et al., 1992;Kozakov et al., 2002; Helo et al., 2006; Demouxet al., 2009a,b; Jiang et al., 2012), the thermal historyof the terrane remains poorly understood because ofa lack of detailed petrological studies.

In this study, we examined the petrology of peliticgneisses in the Tseel terrane and undertook U–Pbzircon dating. In the Tseel area, in the eastern block ofthe Tseel terrane, pelitic gneisses are continuouslyexposed across regions of low to high metamorphicgrade, are associated with voluminous granitoids andsome contain garnet with clear growth zoning (Buren-jargal et al., 2012). Here, spatial variations in garnetcompositions are examined throughout the differentmineral zones in the Tseel area and the P–T conditionsare estimated for garnet growth. The U–Pb zircon agesare also obtained for representative pelitic gneissesand granitoids. Combining the data on thermobaricstructures and the ages of granitoid intrusions andmetamorphic events, the metamorphic evolution ofthe Tseel area is reconstructed and the relationbetween metamorphism and granitoids in the crust isdiscussed.

GEOLOGICAL SETTING

The CAOB is a long-lived orogenic belt (c. 1000–250 Ma) that extends from the Urals in the westthrough Kazakhstan, Mongolia, southern Siberia,northern China and the Okhotsk Sea in the east(Fig. 1a). It is bordered by the Siberian Craton to thenorth and by the Tarim and Sino–Korean cratons tothe south (Seng€or et al., 1993; Jahn et al., 2000; Wind-ley et al., 2007). The collision of these cratons led tothe formation of the CAOB through the accretion of

island arcs, ophiolites, oceanic islands, seamounts,accretionary wedges and microcontinents at a conver-gent margin (e.g. Khain et al., 2002; Windley et al.,2007).Mongolia is located in the central part of the

CAOB (Fig. 1a) and its basement structure is subdi-vided into southern and northern domains, separatedby the Main Mongolian Lineament (MML; Fig. 1b).The two domains have different structural styles andyield contrasting ages. The Caledonian orogenic beltof the northern domain is composed of Precambrianblocks and Neoproterozoic to early Palaeozoic arc-related terranes and ophiolitic belts (Zonenshain &Kuzmin, 1978; Kuzmichev et al., 2001; Badarchet al., 2002; Kozakov et al., 2007; Demoux et al.,2009b; Kr€oner et al., 2010; Lehmann et al., 2010).The Hercynian orogenic belt of the southern domainconsists of middle to late Palaeozoic arc-relatedassemblages (Zonenshain et al., 1975; Badarch et al.,2002) and fragments of ophiolite and serpentinitem�elange (Rippington et al., 2008).The Tseel terrane is located south of the MML

(Fig. 1b) and extends for more than 600 km fromeast to west (Tomurtogoo, 1997; Badarch et al.,2002). Although it comprises eastern and westernblocks, it is considered a single terrane because oflithological and structural similarities between theblocks (Badarch et al., 2002). The Chinese Altai,forming the westernmost extent of the Tseel terrane(Fig. 1b), may also have belonged to the same arcsystem, based on its comparable protolith ages, andthe timing and conditions of metamorphism (Jianget al., 2012).The Tseel terrane records widespread greenschist

to amphibolite facies metamorphism (Sukhorukov,2007; Burenjargal et al., 2012), with granulite faciesassemblages reported in migmatitic mafic gneiss fromthe Tsogt area (~50 km east of the Tseel area; Koza-kov, 1986). The Tseel area, in the eastern block ofthe Tseel terrane (Fig. 1b), is composed mainly ofpelitic gneisses and amphibolites intruded by numer-ous granitoids (Fig. 1c). The dominant foliationstrikes E–W and is steep, although its orientation islocally influenced by granitoid emplacement (Fig. 1c).The granitoids occur as large kilometre-scale bodies(Fig. 2a) or as layers up to several metres thick,interlayered with the pelitic gneisses (Fig. 2b). In thecentral Tseel area, aluminosilicate-bearing quartzveins occur, in which the three aluminosilicatesformed in the order of kyanite, sillimanite and finallyandalusite (Fig. 1c; Burenjargal et al., 2012).Pelitic gneiss located near the granitoid bodies con-

tains garnet porphyroblasts up to 10 mm across orlarger (Fig. 2c). Garnet in the gneisses becomes lessabundant and finer grained with increasing distancefrom the granitoids (Fig. 2d). Amphibolites occur aslayers oriented subparallel to the foliation in thepelitic gneiss (Fig. 2e), although some appear as largemassive bodies.

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766 U. BUREN JARGAL ET AL .

PETROGRAPHY AND MINERAL ZONES

Eighty-two samples of pelitic gneiss were examinedfrom the Tseel area, from which four mineral zones

were identified: garnet, staurolite, sillimanite andcordierite (Fig. 1c). The characteristic mineral assem-blage of each zone is summarized in Fig. 3. The dis-tribution of mineral zones is symmetrical about an

1

5

2

34

Tsogt

300 km

Altay City Ulaanbaatar

ChineseAltai

Tseel MML

Tibet plateau

Mongolia

60o

50o

40o

60o

N

40o

50o

130oE110oE90oE

44o

N

47o 93o99o 105o 111o

47o

44o

(a)

(b)

85

56

70

20

63

85

10 km

68

Sil

Mineral assemblage of pelitic gneisses (Pl, +Qtz)

Legend

N

Grt

Grt

BtGrt+Bt

St

Crd

Grt+Sil+St+BtGrt+Sil+BtGrt+Sil+Crd+Bt

(c)

SilGrt

St42

68

Aluminosilicate-bearing veins

Aluminosilicate-bearing quartz veins

70

45 68

68

85

65

70

63

20

Sedimentary cover

Granitoids

Metapelites

AmphibolitesFaults

Foliation

8470

4530

B

A

58

30

G2505

M0901

G0903

M2507

M2706

M2602

M3001

Sino-Koreancraton

Tarim

Siberian craton

Ural Mt.

160oE120oE80oE

West Siberian basin

Central AsianOrogenic Belt

Fig. 1. (a) Major tectonic components of the Central Asian Orogenic Belt (CAOB). Dark grey areas indicate Archean toMesoproterozoic cratons (modified after Jahn et al., 2000). (b) Tectonostratigraphic terrane map of southern Mongolia: 1. Tseelterrane, 2. Gobi–Altai terrane, 3. Mandalovoo terrane, 4. Gurvansaikhan terrane, 5. Edren terrane (Badarch et al., 2002). MML,Main Mongolian Lineament. (c) Geological map of the Tseel area showing the four mineral zones: garnet (Grt), staurolite (St),sillimanite (Sil) and cordierite (Crd). Open diamonds indicate the localities of aluminosilicate-bearing quartz veins.


P–T EVOLUT ION OF THE TSEEL TERRANE 767

E–W trending axis, with the high-grade sillimaniteassemblages occurring along a central strip, and thegrade decreasing to a sillimanite-absent biotite � gar-net assemblage to the north and south (Fig. 1c).Granitoids are common in the sillimanite and cordie-rite zones, but are rare in the garnet zone (Fig. 1c).

Garnet zone

The garnet zone, more than 5 km wide, is situated inthe northernmost and southernmost parts of theTseel area (Fig. 1c). The pelitic rocks are composedmainly of biotite, chlorite, muscovite, quartz and

Fig. 2. Photographs of rocks in the Tseel area. (a) Massivegranitoid body at Tseel town. (b) Deformed granitoid layers andveins, and pelitic gneisses. (c) Coarse-grained garnet (>10 mm)in biotite-rich gneiss from the cordierite zone. (d) Handspecimen of pelitic gneiss containing fine-grained garnet (garnetzone). (e) Amphibolite interlayered with pelitic gneiss.



plagioclase, with minor garnet, ilmenite, rutile, mona-zite, calcite and apatite (Figs 3 & 4a,b). In this zone,garnet is rare in pelitic rocks (Fig. 1c), and it appearsas small poikiloblasts (commonly <1 mm in size;Figs 2d & 4b) with a dusty appearance under themicroscope, due to fine inclusions of quartz, calciteand plagioclase (Fig. 4b). Plagioclase in the matrixoccurs as anhedral to subhedral grains of 0.05–0.50 mm in size (Fig. 4a,b). The foliation in theserocks is defined by the preferred orientation of biotiteand muscovite (Fig. 4a,b). In this zone, alumino-silicate minerals are absent.

Staurolite zone

The staurolite zone, which occurs between the garnetand sillimanite zones, is defined by the first appear-ance of sillimanite and staurolite in pelitic gneisses(Fig 1c). In this zone, the pelitic gneisses are com-posed of garnet, biotite, plagioclase, quartz, stauroliteand sillimanite with minor ilmenite, calcite, epidote,apatite, rutile, monazite, zircon and graphite (Fig. 3).Garnet occurs as euhedral to subhedral porphyro-blasts of 0.1–6.0 mm in size (Fig. 4c). Garnet corescontain fine inclusions of quartz, plagioclase, epidote,ilmenite, calcite and zircon, whereas the rims containonly inclusions of quartz (Fig. 4c).

Staurolite is pale yellow and occurs as porphyro-blasts of generally 50–300 lm in size (Fig. 4d).Biotite grains are >4.0 mm long and have a shape-preferred orientation that defines the foliation(Fig. 4c), along with fibrolitic sillimanite (Fig. 4d).Plagioclase is subhedral and 0.5–5.0 mm in size.

Sillimanite zone

The sillimanite zone occupies an E–W trending stripthrough the centre of the Tseel area (Fig. 1c). Thepelitic gneisses in this zone are composed of garnet,biotite, plagioclase, quartz and sillimanite with minormuscovite, chlorite, ilmenite, zircon and graphite(Fig. 3). Garnet occurs as euhedral to subhedralporphyroblasts of 1.0–5.0 mm in size (Fig. 4e) andcontains inclusions of quartz, ilmenite and calcite.Biotite of >0.4 mm in size occurs with fibrolitic silli-manite, and their shape-preferred orientation definesthe foliation (Fig. 4e). Plagioclase is euhedral to sub-hedral, and larger than 0.5 mm.

Cordierite zone

The cordierite zone, defined by the appearance of cor-dierite (Fig. 4f), occupies a small area of <10 km2 inthe eastern part of the sillimanite zone (Fig. 1c). Inaddition to cordierite, the pelitic gneisses in this zonecontain garnet, biotite, plagioclase, quartz, sillimaniteand Mg–Fe amphibole, with minor muscovite, chlo-rite, ilmenite, calcite, apatite, monazite, zircon andgraphite (Fig. 3). Garnet porphyroblasts are up to10 mm across or larger (Fig. 2c) and contain inclu-sions of quartz, biotite, cordierite, ilmenite and zircon(Fig. 4f). Amphibole occurs as elongate blades longerthan 0.3 mm. Cordierite is subhedral and 0.1–0.5 mmin size (Fig. 4f). Biotite is present in the matrix and is0.4 mm in size. Plagioclase is euhedral to subhedral,and >0.4 mm in size.

ALUMINOSILICATE-BEARING QUARTZ VEINS

Aluminosilicate-bearing quartz veins, which containall three aluminosilicate polymorphs, occur in thecentral parts of the Tseel area and have beendescribed previously by Sukhorukov (2007) andBurenjargal et al. (2012). These previous studiesreported the veins to be restricted in occurrence;however, we found them to be widely distributed inthe sillimanite and staurolite zones in the Tseel area(Fig. 1c). The occurrence, microstructures and P–Tconditions of the aluminosilicate-bearing quartz veinswere described in detail by Burenjargal et al. (2012),and thus are only briefly outlined here.The aluminosilicate-bearing quartz veins are 0.1–

1.0 m wide and 2–10 m long, and are oriented sub-parallel to the gneissosity in the host pelitic rocks(Fig. 5a). The veins consist mainly of coarse-grainedquartz (0.1–1.0 cm), aluminosilicates (kyanite–sillimanite–andalusite) and muscovite along withminor staurolite, chlorite, ilmenite, calcite, apatiteand rutile. Visible columnar kyanite crystals occurnear the vein walls (Fig. 5b), whereas quartz grainsoccur in the central parts of the veins.Kyanite is the largest aluminosilicate mineral in the

veins, and occurs as euhedral to subhedral columnar

GrtBtPlQtzSil

Grt zone St zone Sil zone

StCrdMsChl

Ep

RtlIlmCal

Crd zone

Grph

Fe-Mg amp

Fig. 3. Mineral assemblages of pelitic gneisses andmetamorphic zones in the Tseel area.



Fig. 4. Photomicrographs of pelitic gneisses from the various mineral zones in the Tseel area. (a) Biotite gneiss of the garnet zone,in which oriented biotite and muscovite define the foliation. (b) Garnet with an irregular shape and dusty appearance due to fineinclusions, in pelitic gneiss from the garnet zone. (c) Garnet porphyroblast with a dusty core and clear rim in pelitic gneiss fromthe staurolite zone. (d) Subhedral staurolite grains with fibrolitic sillimanite in pelitic gneiss from the staurolite zone. (e) Garnetporphyroblast wrapped by a fibrolitic sillimanite (Sil) and biotite (Bt), in pelitic gneiss from the sillimanite zone. (f) Subhedralcordierite grains and garnet porphyroblast in pelitic gneiss from the cordierite zone. All images were taken under plane polarizedlight.



grains of 0.5–6.0 mm in size. Kyanite crystals com-monly contain fractures along the {100} cleavageplane, and the fractures are filled with muscoviteand fibrous sillimanite (Fig. 5c). Kyanite crystalsare partially surrounded by anhedral andalusite. Silli-manite occurs as fibrous, columnar to prismatic euhe-dral crystals of varying size, up to 1 mm in length(Fig. 5c,d). Sillimanite also occurs as inclusions inandalusite grains (Fig. 5d). These observations indi-cate that aluminosilicate polymorphs in the veinsformed in the order of kyanite followed by sillimaniteand finally andalusite (Burenjargal et al., 2012).

MINERAL COMPOSITIONS

Thin sections of the samples were cut normal to thefoliation and parallel to the lineation. The composi-tions of minerals were determined by wavelength dis-persive X-ray spectrometry using an electron probe

microanalyser (JEOL 8200) at the University ofTokyo, Japan. An acceleration voltage of 15 kV andbeam current of 12 nA were used for quantitativeanalyses, and a beam current of 120 nA was used formap analyses.Garnet in the pelitic gneisses is predominantly

almandine-rich, with XAlm (= Fe2+/(Fe2++Mg+Ca+Mn)) values of 0.38–0.72 (Fig. 6a; Table 1), andit shows a wide compositional variation. Three typesof garnet were identified: grossular-rich, zoned andpyrope-rich (Figs 6 & 7). The grossular-rich andpyrope-rich types tend to be compositionally homo-geneous (Figs 6a & 7a,c,d), whereas zoned garnet hasdistinct grossular-rich cores and pyrope-rich rims(Figs 6a & 7b). The garnet type varies systematicallyacross the mineral zones (Fig. 6b): the grossular-richgarnet occurs in the garnet zone, the zoned garnet inthe staurolite zone, and the pyrope-rich garnet in thesillimanite and cordierite zones. X-ray colour maps

Fig. 5. Occurrences and textures of aluminosilicate-bearing quartz veins. (a) Outcrop photograph of aluminosilicate-bearing quartzveins hosted by pelitic gneisses of the sillimanite zone. (b) Hand specimen of an aluminosilicate-bearing quartz vein containingcentimetre-scale kyanite crystals. (c) Photomicrograph of kyanite crystals cut by columnar sillimanite crystals in a vein (cross polarizedlight). (d) Photomicrograph of sillimanite inclusions in large andalusite crystals in a vein (cross polarized light). Qtz = quartz;Ky = kyanite; Sil = sillimanite; Ms = muscovite.



of Ca, Mg, Fe and Mn contents in garnet from eachof the mineral zones, and electron microprobe lineprofiles, are shown in Figs 6 and 7 respectively. Rep-resentative compositions of garnet are listed inTable 1.

The compositions of biotite and plagioclase, sum-marized in Tables 2 and 3, respectively, do notchange systematically among the mineral zones.

Biotite shows little compositional zoning in a singlesample, but varies among samples. The compositionof biotite is Fe2+ (0.95–1.12), Mg (1.29–1.48), AlVI

(0.36–0.64) and Ti (0.05–0.10) (values are based on11 oxygen per formula unit; Table 2). One represen-tative sample was selected from each mineral zonefor detailed petrological analyses, as described below.

Sample M2706 (garnet zone)

The grossular-rich garnet in sample M2706 is rela-tively homogeneous in composition. The garnet isirregular in shape, contains many inclusions ofquartz, calcite and plagioclase, and does not showchemical zoning (Figs 7a & 8a; Table 1). The alman-dine content, XAlm, ranges from 0.38 to 0.43 and islower than in the other mineral zones (Fig. 8;Table 1). The pyrope content is low, with XPrp

(= Mg/(Fe2++Mg+Ca+Mn)) values of 0.03–0.06. Thegrossular content, XGrs (= Ca/(Fe2++Mg+Ca+Mn)),is 0.29–0.39, which is higher than in the other mineralzones. The spessartine content is also high in thissample, with XSps (= Mn/(Fe2++Mg+Ca+Mn)) valuesof 0.20–0.28. In the garnet zone, grossular content ishigher than spessartine content in some samples, butit is lower in the other samples. The garnet composi-tion is similar among samples in the northern andsouthern parts of the garnet zone.Biotite in sample M2706 is relatively composition-

ally homogeneous, with XMg,Bt (= Mg/(Mg+Fe2+))ranging from 0.59 to 0.60. Ti content in the biotiteranges from 0.05 to 0.07 per formula unit (p.f.u.)(Table 2). Plagioclase occurs as anhedral grains inthe matrix and shows marked compositional zoningin XAn,Pl (= Ca/(Ca+Na+K)) from 0.82 in the core to0.34 in the rim (Table 3). Plagioclase also occurs asinclusions in garnet, with XAn,Pl value of 0.63–0.90.

Table 1. Representative compositions of garnet in peliticgneisses of the Tseel area.

Zone Grt zone St zone Sil zone Crd zone

Sample M2706 M2507 M2602 M0901

Core Rim Core Rim Core Rim Core Rim

SiO2 37.25 37.19 36.23 36.80 37.91 37.39 38.07 37.85

TiO2 0.04 0.16 0.11 0.01 0.01 0.06 0.00 0.00

Al2O3 21.13 20.98 20.32 20.84 21.45 21.53 21.42 21.26

FeO 18.33 18.00 28.80 31.69 31.67 32.44 31.95 31.51

MnO 11.69 9.26 5.14 3.90 0.84 0.91 0.60 0.55

MgO 1.10 1.11 2.31 4.00 6.49 5.48 5.92 5.46

CaO 10.95 14.21 5.55 1.64 1.65 1.63 2.14 2.06

Na2O 0.03 0.02 0.00 0.08 0.00 0.00 0.01 0.01

K2O 0.00 0.01 0.00 0.00 0.00 0.00 0.02 0.00

Cr2O3 0.01 0.01 0.08 0.00 0.01 0.01 0.00 0.00

Total 100.53 100.94 98.53 98.95 100.03 99.43 100.14 98.70

Normalized to 8 cations and 12 oxygen

Si 2.97 2.95 2.97 2.98 3.00 2.98 3.00 3.02

Al 1.98 1.96 1.96 1.99 1.99 2.02 1.99 2.00

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Ti 0.00 0.01 0.01 0.00 0.00 0.00 0.00 0.00

Fe3+ 1.22 0.03 0.00 0.00 0.00 0.00 0.01 0.02

Fe2+ 0.01 1.19 1.97 2.15 2.10 2.16 2.10 2.10

Mn 0.79 0.13 0.36 0.27 0.04 0.06 0.04 0.04

Mg 0.13 1.21 0.28 0.48 0.70 0.65 0.70 0.65

Ca 0.94 0.62 0.49 0.14 0.18 0.14 0.18 0.18

Na 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00

K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

XAlm 0.40 0.38 0.64 0.71 0.69 0.72 0.70 0.71

XPrp 0.04 0.04 0.09 0.16 0.25 0.22 0.23 0.22

XGrs 0.30 0.38 0.16 0.05 0.05 0.05 0.06 0.06

XSps 0.26 0.20 0.12 0.09 0.02 0.02 0.01 0.01

(a) Ca

Mg Fe0.

4

0.6

0.8

1.0

0.0

0.2

0.4

0.6

Ca

Mg Fe

Zoned(M2507)

Prp-rich(M2602)

Prp-rich(M0901)

Grs-rich(M2706)

10 km

(b)

56

N

70

G2505

M2507

M2602

M3001

G0903

Grt

St

Sil

Sil

Garnet types

Pyrope-rich garnetZoned grt (Grs-rich core and poor rim)Grossular-rich garnet

Garnet-absent pelitic gneisses

M2706

Crd

M0901

St

Grt

Grt

Fig. 6. (a) Ternary Ca–Mg–Fe diagram showing compositional zoning trends in garnet from each metamorphic zone of the Tseel area.The data are from Grs-rich garnet (sample M2706), zoned garnet (M2507) and Prp-rich garnet (M2602, M0901). (b) Map of the Tseelarea showing the spatial distribution of garnet types across the metamorphic zones: grossular-rich (red), zoned (yellow) and pyrope-rich(blue). Grey squares indicate outcrops of garnet-absent pelitic gneisses.



(a) (b)

(c) (d)

Fig. 7. Colour X-ray element maps of Ca, Mg, Fe and Mn in garnet. (a) Grossular-rich garnet from the garnet zone (sampleM2706). (b) Zoned garnet from the staurolite zone (M2507). (c) Pyrope-rich garnet from the sillimanite zone (M2602). (d) Pyrope-rich garnet from the cordierite zone (M0901). The compositional profiles along the lines marked A–B are shown in Fig. 8. Scalebars are 500 lm.



Sample M2706 consists of garnet (10 vol.%), biotite(7 vol.%), plagioclase (23 vol.%), quartz (49 vol.%)and epidote (4 vol.%), with minor muscovite, chlo-rite, calcite and rutile (Table 5).

Sample M2507 (staurolite zone)

Garnet in sample M2507 contains core, mantle andrim zones defined by grossular and spessartine(XSps = Mn/(Fe2++Mg+Ca+Mn)) contents (Figs 7b &8b; Table 1). The composition of garnet cores ischaracterized by high grossular contents (XGrs = 0.13–0.16) and spessartine contents (XSps = 0.04–0.13). Thespessartine content decreases from core to mantle,where the lowest XSps value of 0.01 is recorded, andthen increases again in the rim (XSps = 0.09). The gros-sular content is almost constant in the core and man-tle, and decreases to ~0.05 in the rims. The almandinecontent, XAlm, is 0.64–0.70 in the core, 0.75 in the man-tle and 0.71 in the rim. The pyrope content is 0.09 inthe core and shows a steady increase to 0.16 in the rim.The grossular content of garnet cores in sample M2507is lower than that of garnet in the garnet zone(Fig. 8a), but much higher than that of garnet in thesillimanite and cordierite zones (Fig. 8c,d). The garnetcores in sample M2507 contain numerous fine inclu-sions of plagioclase, quartz, calcite and zircon, whereasthe rims contain no inclusions except for quartz(Fig. 7b). The zoning pattern and occurrence of inclu-sions in garnet from sample M2507 are similar to thezoned garnet reported in our previous study (sampleM3001 in Burenjargal et al., 2012). However, it shouldbe noted that in the mantle of garnet in sample M2507,the contour in the Ca map is different from that in theMn map (Fig. 7b); in particular, the Ca zoning showsirregular lobes that extend along a single axis from

Table 2. Representative compositions ofbiotite in pelitic gneisses of the Tseel area.


Sample M2706 M2507 M2602 M0901

High

XMg,Bt

Low XMg,Bt High

XMg,Bt

Low

XMg,Bt

High

XMg,Bt

Low

XMg,Bt

High

XMg,Bt

Low

XMg,Bt

SiO2 37.09 36.87 35.74 35.01 37.56 35.99 36.29 36.48

TiO2 0.97 1.26 1.53 1.47 1.58 1.71 1.17 1.06

Al2O3 18.54 17.93 18.44 18.44 20.42 19.20 17.55 17.59

FeO 15.17 16.22 17.50 17.45 13.62 15.08 16.67 16.31

MnO 0.36 0.39 0.05 0.07 0.01 0.11 0.00 0.00

MgO 13.02 13.17 11.39 11.58 12.15 12.23 12.68 13.12

CaO 0.00 0.29 0.02 0.00 0.01 0.08 0.24 0.16

Na2O 0.08 0.13 0.42 0.36 0.30 0.32 0.64 0.52

K2O 10.75 8.77 9.46 9.54 8.59 9.28 8.20 8.44

Cr2O3 0.08 0.05 0.06 0.07 0.09 0.11 0.01 0.02

Total 96.07 95.08 94.59 93.98 94.31 94.12 93.45 93.70

Biotite (O = 11)

Si 2.77 2.77 2.69 2.72 2.77 2.72 2.77 2.77

Ti 0.05 0.07 0.09 0.09 0.09 0.10 0.06 0.07

Aliv 1.23 1.23 1.23 1.19 1.14 1.19 1.17 1.17

Alvi 0.40 0.36 0.44 0.46 0.64 0.52 0.40 0.41

Fe3+ 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Fe2+ 0.95 1.02 1.12 1.11 0.84 0.95 1.04 1.06

Mn 0.02 0.02 0.00 0.00 0.00 0.01 0.00 0.00

Mg 1.45 1.47 1.33 1.29 1.34 1.38 1.48 1.44

Ca 0.00 0.02 0.00 0.00 0.00 0.01 0.01 0.02

Na 0.01 0.02 0.05 0.06 0.04 0.05 0.08 0.09

K 1.02 0.84 0.93 0.92 0.81 0.89 0.82 0.80

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 7.90 7.83 7.89 7.86 7.68 7.81 7.83 7.82

XMg,Bta 0.60 0.59 0.54 0.54 0.61 0.59 0.59 0.58

aXMg,Bt = Mg/(Mg+Fe2+).

Table 3. Representative compositions of plagioclase in peliticgneisses of the Tseel area.


Sample M2706 M2507 M2602 M0901

Core Rim Inc in Grt Matrix Core Rim Core Rim

SiO2 46.25 60.50 44.07 59.24 59.01 58.77 59.23 61.08

TiO2 0.02 0.04 0.00 0.01 0.00 0.01 0.02 0.01

Al2O3 34.00 25.15 35.76 25.51 25.22 25.77 24.88 24.77

FeO 0.04 0.06 0.64 0.09 0.04 0.51 0.28 0.06

MnO 0.03 0.01 0.05 0.02 0.02 0.05 0.05 0.05

MgO 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.00

CaO 17.59 6.85 19.54 7.15 7.36 8.09 6.99 6.47

Na2O 2.12 7.34 0.58 7.70 7.84 7.55 7.19 7.90

K2O 0.03 0.10 0.03 0.08 0.10 0.03 0.03 0.04

Cr2O3 0.00 0.02 0.01 0.03 0.00 0.03 – –Total 100.09 100.07 100.68 99.82 99.58 100.80 98.67 100.36

Plagioclase (O = 8)

Si 2.13 2.69 2.03 2.65 2.65 2.62 2.67 2.70

Al 1.85 1.32 1.94 1.34 1.33 1.35 1.32 1.29

Cr 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Ti 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Fe3+ 0.00 0.00 0.02 0.00 0.00 0.02 0.01 0.00

Mn 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Mg 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Ca 0.87 0.33 0.97 0.34 0.35 0.39 0.34 0.31

Na 0.19 0.63 0.05 0.67 0.68 0.65 0.63 0.68

K 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.00

XAna 0.82 0.34 0.95 0.34 0.34 0.37 0.35 0.31

aXAn = Ca/(Ca+Na+K).



core to rim. This irregular zoning in the mantleindicates that the zoning was not produced by simplegrowth zoning, but was probably modified by meta-somatic reactions. Therefore, only the compositions ofthe core and rim of garnet in sample M2507 (and notthe mantle) are used for the P–T estimates below.

Biotite in sample M2507 has a relatively homo-geneous composition, with XMg values of 0.53–0.54(Table 2). In contrast, plagioclase compositions arehighly variable (Table 3): plagioclase inclusions ingrossular-rich garnet cores have high XAn values of0.90–0.95, whereas plagioclase in the matrix has rela-tively low XAn,Pl values of 0.34–0.40. Similarly highXAn,Pl values have been reported in a zoned garnetfrom the same mineral zone (sample M3001 inBurenjargal et al., 2012), and the values are similarto those in sample M2706 from the garnet zone. Theanorthite content of plagioclase inclusions shows asystematic decrease from garnet cores to rims; conse-quently, it is thought that the plagioclase inclusionswere trapped when in near-equilibrium with the gar-net. Staurolite, which is present only in the staurolitezone, has XMg (= Mg/(Fe2++Mg)) values of 0.20–0.23 (Table 4). Sample M2507 contains garnet (28vol.%), biotite (22 vol.%), plagioclase (10 vol.%),quartz (33 vol.%), staurolite (5 vol.%) with minorsillimanite (<1 vol.%) (Table 5).

Sample M2602 (sillimanite zone)

Pyrope-rich garnet in sample M2602 is composi-tionally homogeneous, with XAlm = 0.69–0.72,XGrs = 0.05–0.06, XSps = 0.02 and XPrp = 0.22–0.25(Figs 6, 7c & 8c; Table 1). A thin retrograde rimshows a slight increase in XAlm, and decreases in XGrs

and XPrp. The core composition of garnet in sampleM2602 (Fig. 8c) is similar to the rim composition of

zoned garnet in sample M2507 (Fig. 8b), althoughthe pyrope content is slightly higher in the formerthan in the latter. Mineral inclusions in garnet fromsample M2602 are mainly quartz. This sample showsminor compositional variations in biotite (XMg,Bt

= 0.59–0.61) and plagioclase (XAn,Pl = 0.34–0.37),and contains garnet (13 vol.%), biotite (25 vol.%),

Core RimRim

1440 µm

BA

Prp-rich garnet

Alm Prp Grs Sps

Core RimRim

1320 µm

BA

Prp-rich garnetZoned garnet

A BCoreRim Rim

5100 µm

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0Core RimRim

1058 µm

BA

Sil zone M2602 Crd zone M0901St zone M2507 Grt zone M2706(a) (b) (c) (d)

Grs-rich garnet

Core

Man

tleR

im Man

tleR

im

Mol

e fra

ctio

n

Fig. 8. Compositional profiles of garnet along the lines A–B in Fig. 7. (a) Garnet zone (sample M2706). (b) Staurolite zone(M2507). (c) Sillimanite zone (M2602). (d) Cordierite zone (M0901). Black arrows indicate the compositions used for P–Testimates.

Table 4. Representative compositions of staurolite, cordieriteand amphibole in pelitic gneisses of the Tseel area.

Mineral Staurolite Cordierite Fe-Mg amphibole

Zone St zone Crd zone Crd zone

Sample M2507 M0901 M0901

Core Rim

SiO2 26.47 48.79 45.73 52.137

TiO2 0.62 0.06 0.18 0.116

Al2O3 51.04 33.39 12.19 4.398

FeO 13.89 5.16 21.18 21.54

MnO 0.22 0.00 0.18 0.095

MgO 2.32 10.78 15.91 18.556

CaO 0.01 0.02 0.42 0.289

Na2O 0.01 0.20 1.08 0.26

K2O 0.01 0.01 0.00 0

Cr2O3 0.01 – – –Total 94.60 98.41 96.87 97.39

Staurolite (O = 48), cordierite (O = 18), amphibole (O = 23)

Si 7.97 4.95 Si 6.71 7.56

Ti 0.14 0.00 Ti 0.02 0.01

Al 18.11 4.00 Aliv 1.29 0.44

Alvi 0.81 0.31

Fe3+ 0.13 0.03

Fe2+ 3.50 0.44 Fe2+ 2.47 2.58

Mn 0.06 0.00 Mn 0.02 0.01

Mg 1.04 1.63 Mg 3.48 4.01

Ca 0.00 0.00 Ca 0.07 0.04

Na 0.00 0.04 Na 0.31 0.07

K 0.00 0.00 K 0.00 0.00

Cr 0.01 – Cr – –Total 30.84 11.07 Total 15.31 15.07

XMga 0.23 0.79 0.61 0.59

aXMg = Mg/(Mg+Fe2+).



plagioclase (10 vol.%), quartz (46 vol.%) and sillima-nite (6 vol.%) (Table 5).

Sample M0901 (cordierite zone)

Pyrope-rich garnet in sample M0901 is composition-ally homogeneous with a thin retrograde rim(Figs 6, 7d & 8d), similar to pyrope-rich garnet inthe sillimanite zone (sample M2602; Fig. 8c); how-ever, the former is richer in pyrope content(XPrp = 0.22–0.23) and poorer in Mn (XSps = 0.01)(Fig. 8c,d). Garnet porphyroblast contains inclusionsof quartz, calcite, biotite, cordierite and fine-grainedzircon.

The compositions of biotite (XMg,Bt = 0.58–0.59)and plagioclase (XAn,Pl = 0.31–0.35) are homogeneous(Tables 2 & 3). Cordierite occurs in the matrix and asinclusions in garnet (Fig. 7d), and has XMg (=Mg/(Mg+Fe2+)) values of 0.76–0.79 (Table 4). This sam-ple contains Fe–Mg amphibole with zoning related toTschermak substitution, with AlVI decreasing from1.29 (gedrite) in the core to 0.44 (anthophyllite) in therim (Table 4). The XMg value of amphibole showslittle variation within the sample, ranging from 0.58to 0.61. The sample contains garnet (29 vol.%), biotite(21 vol.%), plagioclase (5 vol.%), quartz (36 vol.%)and cordierite (5 vol.%) (Table 5), with minor sillima-nite and Fe–Mg amphiboles (<1 vol.%).

P–T ESTIMATES FROM GARNET IN PELITICGNEISS

Method

The P–T conditions recorded by pelitic gneiss fromeach mineral zone were estimated using garnet-isopleth (GIP) thermobarometry based on pseudo-sections. The P–T pseudosections were calculated

in the system MnO–CaO–Na2O–K2O–FeO–MgO–Al2O3–SiO2–H2O (MnCNKFMASH) using the soft-ware Perple_X 07 (Connolly, 1990) and the internallyconsistent data set of Holland & Powell (1998). TheP–T range of the pseudosections is 1–10 kbar and450–700 °C. The Fe3+ component in the system wasnot considered, because the presence of graphite inpelitic gneisses (Fig. 3) indicates low oxygen fugacity.The solution models for garnet, chlorite and stauro-lite were taken from Holland & Powell (1998), andthose for biotite and plagioclase from Powell & Hol-land (1999) and Newton et al. (1980). The solutionmodel for muscovite was taken from Coggon & Hol-land (2002). For Mg-Fe amphibole, a simple Mg-Femixing model of anthophyllite was used (data fromPerple_X 07). In the calculation, melt was not consid-ered. Isopleths calculated for the grossular, alman-dine and spessartine components were plotted on thepseudosections, and the P–T conditions for eachsample were obtained from points at which theseisopleths intersected the appropriate composition(e.g. Spear, 1988; Vance & Holland, 1993; Vance &Mahar, 1998).Bulk-rock compositions used in the calculations

(Table 6) were obtained from the modal abundancesof minerals observed in thin sections (Table 5) andfrom the representative compositions of minerals(Tables 1–4). The modal abundances of mineralswere determined by point counting under an opticalmicroscope, except for the fined-grained sample fromthe garnet zone (sample M2706), for which themineral mode was determined by image analysis ofan electron probe microanalyser map with an areaof 3 9 3 mm. The garnet in the staurolite zone (sam-ple M2507) is strongly zoned; consequently, separateP–T calculations were undertaken for the core andrim (Fig. 8b). To calculate the effective bulk-rockcomposition at the stage of garnet core growth, the

Table 5. Modal abundances (vol.%) of minerals measured in pelitic gneisses and as predicted by P–T pseudosection modelling.

Observed mineral modea Normalized mineral modeb Calculated mineral modec

Sample M2706 M2507 M2602 M0901 M2706 M2507 M2602 M0901 M2706 M2507 M2602 M0901

Mineral zone Grt St Sil Crd Grt St Sil Crd Grt St Sil Crd

Garnet 10 28 13 29 22 43 24 48 7 33 20 46

Biotite 7 22 25 21 16 34 46 35 17 34 47 34

Plagioclase 23 10 10 5 50 16 18 8 70 20 18 8

Quartz 49 33 46 36 – – – – – – – –Chlorite 1 <1 <1 1 1 <1 <1 1 – 3 – –Muscovite <1 1 <1 – 1 1 <1 – 6 5 – –Staurolite – 5 – – – 7 – – – 4 – –Sillimanite – <1 6 <1 – <1 12 <1 – 1 15 –Cordierite – – – 5 – – – 8 – – <1 5

Calcite 1 <1 <1 – – – – – – – – –Rutile 1 <1 <1 1 – – – – – – – –Epidote 4 – – – 9 – – – –Fe-Mg amp – – – <1 – – – <1 – – – 7

Total 100 100 100 100 100 100 100 100 100 100 100 100

aModal abundances of minerals measured in thin section.bNormalized modal abundances of minerals used for pseudosection modelling (MnNCKFMASH with excess SiO2).cModal abundances of minerals calculated by pseudosection modelling.



average garnet composition of a zoned garnet wasused. The garnet core and mantle were neglectedwhen calculating the effective bulk-rock compositionat the stage of garnet rim growth. The bulk-rockcompositions of the analysed pelitic gneisses varyamong the mineral zones (e.g. the Al2O3 contentranges from 13.40 to 19.13 wt%; Table 6). However,this variation does not represent a systematic differ-ence among mineral zones; instead, it probably repre-sents local variations in the bulk-rock compositionsof the pelitic gneisses (i.e. compositional banding,coarse-grained garnet), as the Al2O3 content wasmeasured from thin sections.

To assess the validity of our P–T estimates, themodal abundances of minerals calculated by pseudo-section modelling were compared with the abun-dances measured in thin section (Table 5). The P–Tconditions were also evaluated by the garnet–biotite(GB) and garnet–biotite–plagioclase–quartz (GBPQ)thermobarometers (Holdaway, 2000; Wu et al.,2004), hereafter referred to as GBPQ. TheseGBPQ geothermobarometers are applicable in thepresent case, as the assemblage of Grt+Bt+Pl+Qtz isobserved in all samples, and the P–T pseudosectionmodelling indicates the stability of this assemblagethroughout the entire period of garnet growth, asshown below. For garnet cores in each garnet (i.e.black arrows in Fig. 8), the possible P–T rangeswere determined by making four separate sets of cal-culations accounting for the highest and lowest val-ues of XMg,Bt for biotite (Table 2) and XAn,Pl forplagioclase (Table 3), following Burenjargal et al.(2012).

Results

Garnet zone (sample M2706)

On the pseudosection, garnet is stable across theentire P–T space (Fig. 9a). The mineral assemblagein the calculated P–T space is garnet � biotite � pla-gioclase � chlorite � muscovite � zoisite � K-feld-

spar. Muscovite and chlorite are stable in lowertemperature and higher pressure regions, whereasbiotite is stable at higher temperatures. Muscovitedisappears at the relatively lower temperatures inM2706 (e.g. 480 °C at 2 kbar) than in typical peliticgneisses (cf. ~600 °C at 2 kbar, fig. 13 of Wei et al.,2007). This is probably due to the lower K2O contentin this sample (0.93 wt%) than those of the typicalpelitic rocks (3.38–3.53 wt%, Mahar et al., 1997).The grossular content increases with decreasing tem-perature and increasing pressure. In the low-T region(<~550 °C), the almandine isopleths show a steep dP/dT slope, with XAlm increasing with increasingtemperature, whereas in the high-T region (>~550 °C)the dP/dT slope is gentle and the XAlm valueincreases with decreasing pressure. The spessartineisopleths also have a concave upward pattern, withXSps increasing with decreasing pressure. The P–Tconditions estimated from the garnet isopleths are540–570 °C and 5.7–6.5 kbar, located in the fieldcontaining the assemblage garnet+biotite+plagio-clase+muscovite (Fig. 9a). The calculated modes ofbiotite (17 vol.%) and plagioclase (70 vol.%) are sim-ilar to those observed (Bt 16, Pl 50 vol.%; Table 5),whereas the calculated mode of garnet (7 vol.%) isslightly lower than that observed (22 vol.%). Epidoteis absent in the calculated results (Table 5), probablydue to the uncertainties in the estimates of the CaOcontent in the bulk-rock composition, because plagio-clase compositions are highly variable in this sample(Table 3).The P–T conditions estimated by GBPQ geo-

thermobarometry for the garnet zone are 520–540 °Cand 6.5–8.0 kbar (Fig. 9a). These values are 10–20 °Clower and ~1.5 kbar higher than those obtained usingGIP thermobarometry respectively (Fig. 9a).

Staurolite zone (sample M2507)

Separate P–T pseudosections were obtained for thegarnet core (Fig. 9b) and rim (Fig. 9c) in sampleM2507. For both core and rim, garnet is stable in theentire calculated P–T space. The calculated mineralassemblage for the core stage is garnet � biotite �plagioclase � chlorite � muscovite � aluminosilicates� zoisite � cordierite (Fig. 9b). Staurolite does notappear in the P–T pseudosection for the garnet core.The P–T conditions estimated for the garnet coreusing garnet isopleths are 520–540 °C and 4.5–5.8 kbar, for which the predicted assemblage is gar-net+biotite+plagioclase+chlorite+muscovite. Althoughit is impossible to know the modal abundances ofminerals during growth of the garnet core from thethin section, the inclusion minerals of the core insample M2507 (i.e. plagioclase and muscovite) appearto be consistent with those predicted. The P–T condi-tions estimated by GBPQ geothermobarometry forthe garnet core of the staurolite zone are 550–560 °Cand 5.2 kbar (Fig. 9b). This pressure estimate is

Table 6. Bulk-rock compositions (wt%) of the pelitic gneissesused for P–T pseudosection modelling.


Sample M2706 M2507 M2602 M0901

Core Rim

SiO2 71.68 54.97 60.69 61.15 57.86

Al2O3 14.30 17.75 16.63 19.13 13.40

TiO2 1.13 0.40 0.53 0.49 1.75

FeO 2.69 15.93 10.96 7.42 16.54

MgO 1.26 3.70 4.19 5.32 5.90

MnO 0.57 0.97 0.61 0.07 0.17

CaO 6.25 2.30 1.11 0.88 1.20

Na2O 0.64 0.67 0.90 0.74 0.28

K2O 0.93 2.31 3.06 3.20 2.07

H2O 0.55 1.00 1.32 1.61 0.82

Total 100 100 100 100 100



Pre

ssur

e (k

bar)

Temperature (°C) 700

1.0500 650450

10

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

St zone: M2507 core 0.16 Grs; 0.64 Alm; 0.12 Sps

1. Grt Pl Chl Ms Zo 2. Grt Bt Chl Ms

3. Grt Bt Pl Chl Ms Zo 4. Grt Bt Pl Chl Ms St

5. Grt Bt Pl St Ms

7. Grt Bt Pl St Crd

6. Grt Bt Pl St Crd Sil

0.60

0.62

0.64

0.66

0.68

0.68

0.10

0.120.1

40.1

60.1

8

0.20

0.06

0.08

0.10

0.12

0.14

Grt Bt Pl Ms

Grt

Bt M

s

Grt Bt Pl Ms Sil

Grt Bt Pl Sil

Grt Bt Pl Crd Sil

Grt Bt Pl Crd

Grt Bt Pl Chl Ms Zo

Grt Bt Pl Ms

Grt Chl MsZo Pl

Grt Chl Ms Pl

Grt

Bt P

l Chl

Ms

Grt Bt Chl Ms

Grt Bt Pl Chl Ms Pl

1

2

3

4

5

6

7

GIP GBPQ

550 600

(b)P

ress

ure

(kba

r)

Temperature (°C) 700

1.0500 650450

10

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

St zone: M2507 rim 0.05 Grs; 0.71 Alm; 0.09 Sps

0.61

0.63

0.65

0.67

0.69

0.71

0.050.0

70.0

90.1

10.1

3

0.05

0.07

0.090.11

0.13

Grt Bt Pl Crd

Grt Bt Pl Chl Ms

Grt Bt Pl Sil

Grt Bt Pl Ms

Grt Bt

Pl M

s St Grt Bt Pl

Ms Sil

Grt

Bt M

s

Grt Bt Chl Ms

Grt Bt Pl M

s Ky

Grt Bt Pl Crd Sil

Grt

Bt P

l C

hl M

s Zo

Grt Bt Pl Chl Ms

Grt Bt

Pl C

hl Crd

Grt Bt PlChl Ms Zo

34

Grt Bt Pl St Sil

56

7

2

1

1. Grt Pl Chl Ms 2. Grt Bt Pl Chl Ms

3. Grt Bt Pl Chl St Ms 4. Grt Bt Pl Chl St Sil

5. Grt Bt Pl Chl St 6. Grt Bt Pl St Crd

7. Grt Bt Pl Crd Kfs

GIP

GBPQ

550 600

(c)

Grt Bt Pl Ms

GIP

GBPQ

Pre

ssur

e (k

bar)

7001.0

500 650450

10

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

Grt zone: M2706

Temperature (°C)

0.31 Grs; 0.40 Alm; Sps

1. Grt Bt Chl Ms Zo 2. Grt Bt Ms Zo

3. Grt Bt Pl Ms Kfs 4. Grt Bt Pl Kfs

0.36 0.

38 0.40

0.42

0.42

0.440.20

0.22

0.240.26

0.28

0.25

0.29

0.310.3

3

Grt Bt Pl

Grt Bt Pl Chl

Grt Bt Pl Ms Zo

Grt Bt P

l

Chl Ms

Grt

Bt P

l Chl

Ms

Zo

Grt

Pl C

hl M

s Zo

Grt Pl Chl Ms Zo

1

2

3

4

550 600

(a) 0.26

Pre

ssur

e (k

bar)

Temperature (°C)

GIP

GBPQ

7001.0

500 650450

10

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

Sil zone: M2602 0.05 Grs; 0.69 Alm; 0.02 Sps

0.65

0.67

0.690.71

0.05

0.07

0.09

0.11

0.02

0.040.06

0.02

Chl Ms Zo Pa

Pl Chl M

s Zo Pa

Pl Chl M

s Pa

Pl Chl Ms

Bt Pl Crd Kfs

Bt Pl Crd Sil

Bt Pl Ms Crd

Grt Bt Pl Crd Sil

Grt Bt Pl Sil

Grt Bt Pl Ky

12

3

4

56

7

8

9 1011

1213

14

15 1617 18 19

20

21

22

2324

25

26

1. Grt Chl Ms Zo Pa 2. Grt Chl Ms Pa 3. Grt Pl Chl Ms Pa 4. Grt Pl Chl Ms St Pa5. Grt Bt Pl Ms Ky Pa 6. Grt Bt Pl Ms Ky 7. Grt Pl Chl Ms Ky 8. Grt Pl Chl Ms Pa Ma

9. Pl Chl Ms Pa Ma 10. Pl Chl Ms Ma 11. Grt Bt Pl Chl Ms St 12. Grt Bt Pl Chl St Ky 13. Grt Bt Pl St Ky 14. Bt Pl Chl Ms St 15. Bt Pl Chl St Ky 16. Bt Pl Chl St17. Grt Bt Pl Chl St 18. Grt Bt Pl St 19. Grt Bt Pl St Sil 20. Bt Pl Chl St Sil21. Grt Bt Pl Chl St Sil 22. Grt Bt Pl Crd St Sil 23. Bt Pl Chl Ms Sil 24. Bt Pl Chl Sil25. Bt Pl Ms Crd Sil 26. Bt Pl Sil Crd Kfs

550 600

(d)

Fig. 9. Calculated P–T pseudosections with compositional garnet isopleths for (a) the garnet zone (sample M2706), (b, c) thestaurolite zone (M2507), (d) the sillimanite zone (M2602) and (e) the cordierite zone (M0901). Pseudosections for garnet in thestaurolite zone were made separately for cores (b) and rims (c), by considering the effect of garnet zoning on the effective bulk-rock compositions (see text for details). The P–T pseudosections were calculated in the system MnNCKFMASH, with excessquartz and H2O, using the software PerpleX 07 (Connolly, 1990). Grt = garnet; Bt = biotite; Pl = plagioclase; Chl = chlorite;Ms = muscovite; Pa = paragonite; St = staurolite; Crd = cordierite; Zo = zoisite; Ky = kyanite; And = andalusite; Sil = sillimanite;oAmph = orthoamphibole; Kfs = K-feldspar. Dark and light shaded areas labelled with GIP and GBPQ indicate the P–Tconditions estimated by GIP geothermobarometry and by garnet–biotite–plagioclase–quartz geothermobarometry respectively.



similar to that obtained using GIP thermobarometry,although the temperature is ~30 °C higher.

The calculated mineral assemblage for the rim stage(Fig. 9c) is garnet � biotite � plagioclase � chlorite �muscovite � aluminosilicates � zoisite � cordierite �staurolite � K-feldspar. Staurolite appears undermedium-T and medium-P conditions (550–630 °C and3–7 kbar). The P–T conditions estimated for the gar-net rim using garnet isopleths are 570–600 °C and4–5 kbar, for which the predicted assemblage isgarnet+biotite+plagioclase+staurolite+sillimanite. Thecalculated modal abundances of minerals (Grt 33, Bt34, Pl 20, St 4, Sil 1, Chl 3, Ms 5 vol.%) are similar tothose observed (Grt 43, Bt 34, Pl 16, St 7, Sil 7,Chl <1, Ms 1 vol.%; Table 5). The P–T conditionsestimated by GBPQ geothermobarometry for the gar-net rim of the staurolite zone are 620–625 °C and 4.3–4.4 kbar (Fig. 9c). This pressure estimate is similar tothat obtained using GIP thermobarometry, althoughthe temperature is ~30 °C higher.

From core to rim in the garnet, the temperatureshows an increase of ~50 °C and the pressuredecreases slightly by ~1 kbar. Such a decompressionand heating P–T path obtained from sampleM2507 is similar to that reported previously froma different sample in the staurolite zone of theTseel area (sample M3001 in Burenjargal et al.,2012).

Sillimanite zone (sample M2602)

The calculated mineral assemblage for pelitic gneissfrom the sillimanite zone (Fig. 9d) is combinations of

biotite � plagioclase � garnet � chlorite � muscovite� zoisite � aluminosilicates � staurolite � cordierite� paragonite � margarite in the calculated P–T space.In this P–T pseudosection, garnet appears athigher temperatures (>500 °C) and higher pressures(>4 kbar). The grossular content increases withdecreasing temperature and increasing pressure, andthe spessartine content decreases with increasing tem-perature. The almandine isopleth shows a complexshape, and XAlm decreases with increasing pressure.The P–T region of 550–650 °C and 5–8 kbar isdivided into several small regions (nos 11–21 inFig. 9d). The mineral assemblage of these smallregions changes readily with a small change in thebulk-rock composition; in contrast, the mineral assem-blage in the other larger P–T regions is robust tochanges of 10% in the bulk-rock composition used inthe calculation.The P–T conditions estimated for garnet of sample

M2602 are 670–690 °C and 5.8–6.0 kbar. The min-eral assemblage predicted at this P–T is garnet+biotite+plagioclase+sillimanite+cordierite. The calcu-lated modal abundances of minerals (Grt 20, Bt 47,Pl 18, Sil 15, Crd <1 vol.%) are similar to those pre-dicted (Grt 24, Bt 46, Pl 18, Sil 12, Chl <1, Ms <1vol.%) (Table 5). The P–T condition estimated byGBPQ geothermobarometry is 630–660 °C and 5.2–5.9 kbar. This pressure estimate is similar to thatobtained using GIP thermobarometry, although thetemperature is ~30 °C lower.

Cordierite zone (sample M0901)

The calculated mineral assemblages for pelitic gneissfrom the cordierite zone (Fig. 9e) are combinations ofbiotite � plagioclase � garnet � chlorite � muscovite� zoisite � staurolite � cordierite � orthoamphibolein the calculated P–T space. The garnet-in reactionline for sample M0901 from the cordierite zone plotsat ~530 °C on the pseudosection at pressures of2–6 kbar (Fig. 9e). Garnet is absent in the low-Pregion (<1.5 kbar). Cordierite, which occurs both inthe matrix and as inclusions in garnet (Figs 4f & 7d),and Fe-Mg amphibole (orthoamphibole) appear in thehigher temperature (>500 °C) and lower pressureregion (<7 kbar). The P–T conditions estimated fromgarnet isopleths for this sample are 665 °C and5.2 kbar, which is close to those of the sillimanite zonesample (Fig. 9d). At this P–T, the predicted mineralassemblage is garnet+biotite+plagioclase+cordierite+orthoamphibole (oAmph) (Fig. 9e). The predictedmodal abundances of minerals (Grt 46, Bt 34, Pl 8,Crd 5, oAmph 7 vol.%) are similar to those observed(Grt 48, Bt 35, Pl 8, Crd 8, Chl 1, Sil <1, oAmph <1vol.%).The P–T conditions calculated by GBPQ geo-

thermobarometry are 650–670 °C and 3–3.5 kbar(Fig. 9e). This temperature is similar to that esti-

Pre

ssur

e (k

bar)

Temperature (°C) 550 600 700

1.0500 650450

10

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

Crd zone: M0901 0.06 Grs; 0.70 Alm; 0.01 Sps

1. Grt Bt Chl Ms Zo 2. Grt Bt Pl Ms3. Bt Pl Chl Crd 4. Grt Bt Pl Chl Crd

0.68

0.700.72

0.74

0.740.720.700.680.66

0.06

0.08

0.10

0.1241.0

0.01

0.030.05

0.070.09

Grt Bt Pl

Grt Bt Pl oAmp Crd

Grt Bt Pl oAmp

Bt Pl oAmp Crd

Bt Pl Chl Ms

Bt Pl C

hl

Grt Bt P

l Chl

Bt Pl Chl Ms Zo

Bt Pl Chl Ms

Zo

Grt Bt Pl Chl Ms Zo

Grt Bt

Pl

Chl M

s Zo

Grt Bt

Pl C

hl M

s

Grt Bt Chl Ms

1 2

3

4

GBPQ

GIP

(e)

Fig. 9. (continued)



mated by GIP geothermobarometry, but the pressureis 2–3 kbar lower.

Uncertainties in P–T estimates

The P–T conditions estimated by GIP geothermoba-rometry deviate from those estimated by conventionalGBPQ geothermobarometry; the maximum deviationsin temperature and pressure between the two methodsare ~30 °C and ~2 kbar respectively (Fig. 9). There aretwo possible factors that cause errors in GIPgeothermobarometry: the activity models used forminerals, and uncertainty in the bulk-rock composi-tions. The activity models for minerals used in GBPQgeothermobarometry (Wu et al., 2004) [i.e. the modelfor garnet is an average from the models of Berman &Aranovich (1996), Ganguly et al. (1996) and Mukho-padhyay et al. (1997); for plagioclase is the model ofFuhrman & Lindsley (1988); and for biotite is themodel of Holdaway (2000)] are different from thoseused in the pseudosection calculation by Perple_X 07.For example, the P–T conditions of sample M2602were re-estimated by GBPQ geothermobarometryusing the compositions of garnet, plagioclase andbiotite obtained by pseudosection calculations. Thiscalculation yields lower temperature and pressure(641 °C and 4.3 MPa respectively) than obtained byGIP geothermobarometry (670–690 °C and 5.8–6.0 kbar). These results suggest that the choice ofactivity model for minerals could introduce systematicerrors into P–T estimates. In contrast, the P–Tconditions estimated by GIP geothermobarometry donot change significantly even if the amount of eachcomponent of the original bulk-rock composition isincreased by 10% (such changes result in changes intemperature and pressure of <15 °C and <0.5 kbarrespectively).

In spite of the slight difference in P–T conditionsbetween GIP and GBPQ geothermobarometry, thesystematic relationship among the different mineralzones in P–T space does not change significantly withthe choice of geothermobarometry. Accordingly, inthe discussion below, the thermal evolution of theTseel terrane is discussed based on the P–T valuesobtained using GIP geothermobarometry.

ZIRCON U–Pb DATING OF PELITIC GNEISSESAND GRANITOIDS

Samples and analytical methods

Five samples were selected for U–Pb zircon age dat-ing: two granitoids (G0903, G2505) and three peliticgneisses (M3001, M2507, M0901). The samplinglocalities are shown in Fig. 1c. Sample G0903 is acoarse-grained granitoid taken from the large bodyin the central part of the Tseel area, and is com-posed of quartz+plagioclase+K-feldspar+biotite+mag-netite+zircon+apatite. Sample G2505 is a gneissose

granitoid from the western part of the Tseel area,and is composed of quartz+plagioclase+amphibolewith lesser amounts of biotite+muscovite+zircon.Samples M3001 and M2507 were collected from thestaurolite zone and contain zoned garnet. The P–Tconditions inferred from sample M3001 have beenreported previously (Burenjargal et al., 2012) andthose inferred for sample M2507 are shown inFig. 9b. Sample M0901 was collected from the cor-dierite zone and contains garnet that is composition-ally homogeneous (Figs 7d, 8d & 9e). Before zirconseparation, garnet grains were separated from thepelitic gneiss samples, and U–Pb dating was per-formed separately for zircon inclusions in garnet andfor zircon in the matrix.Zircon grains were separated from rock samples

through standard crushing, grinding, sieving,magnetic and heavy-liquid separation techniques, fol-lowed by hand picking under a binocular microscope.The grains were mounted in Teflon and polished toapproximately half the mean grain thickness. Theinternal structures of the zircon were assessed usingtransmitted and reflected light microscopy, as well ascathodoluminescence (CL) imaging to reveal zoningand to select optimum sites in the cores and rims forin situ U–Pb dating.In situ zircon U–Pb dating was performed using a

Nu AtttoM single-collector inductively coupled massspectrometry (ICP-MS; Nu Instruments, Wrexham,UK) coupled to a NWR-193 laser ablation system(ESI, Portland, OR, USA) using a 193 nm ArFexcimer laser, all housed at the Department of Geol-ogy and Mineralogy, Kyoto University, Japan. A fulllist of instrumental parameters is included in TableS1. The laser was operated with an output energy of~3.2 mJ per pulse, a repetition rate of 8 Hz and alaser spot size of 5 or 15 lm in diameter, providingan estimated power density to the sample of<2.5 J cm�2. This focused laser spot enabled the rela-tively fine-grained zircon grains (<50 lm) to be anal-ysed. The pulse count was ~70 shots. Ablationoccurred in helium gas within a micro-cell of <1 ml,and the ablated sample aerosol and helium gas weremixed with argon gas downstream of the cell. Thehelium minimizes re-deposition of ejecta or conden-sates, while argon provides efficient sample transportto the ICP–MS (Eggins et al., 1998; Gunther & Hein-rich, 1999; Jackson et al., 2004). A signal-smoothingdevice was applied to minimize the introduction oflarge aerosols into the ICP, thereby reducing signalspikes (Tunheng & Hirata, 2004).The ICP–MS was optimized by continuous abla-

tion of a 91500 zircon standard (Wiedenbeck et al.,1995, 2004) and NIST SRM 610 to providemaximum sensitivity while maintaining low oxide for-mation (UO+/U+<1%). Data for U-Pb age determi-nation were acquired on six isotopes, 202Hg, 204Pb,206Pb, 207Pb, 232Th and 238U using a low-resolutiondeflector jump mode. Mass peak of 232Th were not



measured for all spots, measured only for typicalgrains to obain the ratio of Th and U.

Background and ablation data for each analysiswere collected over ~90 and ~9 s respectively. Back-grounds were measured without laser ablation, butwith the same settings and gas flows as those usedduring ablation. Data were acquired in multiplegroups of 10 sample unknowns bracketed by quartetsof the 91500 zircon standard (Wiedenbeck et al.,1995, 2004) following a single background analysis.

202Hg was monitored to correct for isobaric inter-ference of 204Hg on 204Pb. To reduce the isobaricinterference, an Hg-trap device with an activatedcharcoal filter was applied to the Ar make-up gasbefore mixing with the He carrier gas (Hirata et al.,2005). Prior to each individual analysis, regions ofinterest were pre-ablated using a few pulses of thelaser with laser spot size of 10 or 20 lm in diameterto remove potential surface contamination, therebyreducing contamination by common Pb (Iizuka & Hi-rata, 2004).

All data reduction was conducted off-line using anin-house Excel spreadsheet. Background intensitieswere interpolated using the average of two back-ground values acquired before and after eachunknown sample group. The mean of the measuredratios was calculated for the eight analyses of the91500 zircon standard bracketing each group ofunknown samples, and this mean value was used forage estimates. Measurement uncertainties were calcu-lated based on counting statistics of the signal intensi-ties and the eight separate measurements of the 91500zircon. All uncertainties are quoted at the 2r level.235U was calculated from 238U using a 238U/235U ratioof 137.88 (Jaffey et al., 1971). The Ple�sovice zircon,which has the age of 337 � 0.37 Ma as determinedby isotope dilution, thermal ionization mass spec-trometry (Sl�ama et al., 2008), was measured; the aver-age ages and 2r of the 15 measurements are338 � 5 Ma for spot size of 15 lm, and 337 � 8 Mafor spot size of 5 lm respectively.

Results

Representative CL images of zircon in individualsamples are shown in Fig. 10. The concordia dia-grams and histograms of ages for individual samplesare shown in Figs 11 and 12a–d respectively. Therelative probability curves in Fig. 12a–d wereobtained by using ISOPLOT (version 3.7; Ludwig,2012). The age data, which are <10% discordant, arelisted in Table S2. The Th/U ratio was measured forselected zircon grains from individual samples (TableS2; Fig. 12e).

In sample M3001 (pelitic gneiss) from the stauro-lite zone, the matrix contains many euhedral zircongrains of >100 lm in size (Fig. 10a). These grainshave bright or grey cores in CL images, and showoscillatory zoning. Zircon inclusions in garnet are

rare and are smaller (<100 lm) than in the matrix.Both the matrix and inclusion zircon crystals show alargest cluster of similar Cambrian ages, 560–480 and550–460 Ma respectively (Fig. 11a,b). Combining thematrix and inclusion age data, an age of510 � 24 Ma is obtained for sample M3001(Fig. 12b). The cores of the matrix zircon are com-monly 5–20 Ma older than the rims (Fig. 10a). Afew zircon grains yielded old ages of 1800–1000 Main the core and much younger ages (550–450 Ma) inthe rim (Figs 11a & 12a). These older zircon coresare interpreted to be detrital in origin, derived froman old craton (possibly the Tuva–Mongolian micro-continent; Jiang et al., 2012). The rims of somegrains yield ages of c. 400, 300 and 260 Ma(Fig. 12b).Sample M2507 is also a pelitic gneiss sample from

the staurolite zone, but in contrast to sampleM3001, zircon grains in this sample are rare andfine-grained (mostly <60 lm) in both the matrix andas inclusions in garnet. In CL images, zircon grainsin the matrix have a bright core, a dark mantle anda bright rim, and the core is subhedral to anhedral(Fig. 10b). The cores (and mantles) of zircon grainscommonly yield Cambrian ages (550–460 Ma),whereas the rims yield younger ages (c. 410, 310 and260 Ma; Figs 10b, 11c & 12b). Six zircon inclusionswere analysed in garnet that shows no zoning in CLimages. Similar to the matrix zircon, three of the zir-con inclusions yield Cambrian ages (560–500 Ma)and the other one a Devonian age (421 Ma)(Fig. 11d). Two zircon inclusions yield a much olderage of 1900–1800 Ma (Fig. 12a). The combinedinclusion and matrix zircon age data show a peakage of 516 � 27 Ma (Fig. 12b), with some minorpeaks of younger ages.The age distributions of two staurolite zone sam-

ples (M2507 & M3001) are similar to each other(Fig. 12b). The combined data of the two samplesyield a main peak at 511 � 24 Ma (Cambrian) andthree minor clusters at 406 � 11 Ma (Devonian),309 � 9 Ma (Late Carboniferous to Early Permian)and 256 � 7 Ma (Late Permian to Early Triassic).The Th/U ratio of the staurolite zone samples varieswith age, with the ratio being 0.5–1.5 for ages of550–500 Ma, but commonly <0.2 for younger ages(Fig. 12e; Table S2).Sample M0901 (pelitic gneiss) of the cordierite

zone contains subhedral zircon grains with sizes of<150 lm, both in the matrix and as inclusions in gar-net. In CL images, the zircon grains commonly havebright cores and dark rims (Fig. 10c). The zirconcores show anhedral outlines, and commonly haveages of c. 500 Ma (Fig. 10c). The peak age of thematrix zircon (430–360 Ma; Fig. 11e) is similar tothat of the inclusions (410–350 Ma; Fig. 11f). Thecombined matrix and inclusion age data (grey arrowin Fig. 12c) show a broad age peak at 377 � 30 Ma(Devonian), with a small shoulder at c. 420 Ma



(Fig. 12c). In addition, minor peaks occur at491 � 15 Ma (Cambrian) and 288 � 11 Ma (Perm-ian). The Th/U ratio of the zircon grains in sampleM0901 varies from <0.01 to 1.8, and small values(<0.1) are found for ages of 400–350 Ma (Fig. 12e).

Sample G2505 is a granitoid that contains coarse-grained (>100 lm) euhedral zircon grains. In CLimages, the grains show clear zoning with brightcores, dark mantles and grey rims (Fig. 10d). Thecombined age data show a main peak at 385 � 7 Ma

(Figs 11g & 12d). The Th/U ratios of zircon in sam-ple G2505 range from 0.8 to 3.58, which is higherthan in the other samples (Fig. 12e; Table S2).The granitoid sample G0903 contains coarse-

grained zircon crystals of >100 lm in size. The zircongrains are euhedral and show clear oscillatory zoning(Fig. 10e). The main peak age is 297 � 11 Ma(Figs 11h & 12d). The Th/U ratios of zircon in thissample range from 0.4 to 0.8, except for one value of0.18 (Fig. 12e; Table S2).

Fig. 10. Representative cathodoluminescence images of zircon in matrix with analysed ages from (a) sample M3001 in the St zonepelitic gneisses, (b) M2507 in the St zone pelitic gneisses, (c) M0901 in the Crd zone pelitic gneisses and (d) G2505 (granitoid) and(e) G0903 (granitoid). In (b), the same zircon grains are shown in the backscattered electron image (BSE). Scale bar indicates50 lm.



0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.4

M3001 inclusion

0.3

0.13

1.0

0.10

0.20

0.30

0.40

0.003.0 5.0

18001600

14001200

1000

0.04

0.05

0.06

0.08

0.09

0.25 0.35 0.40 0.45 0.55 0.60 0.70

M0901 inclusion

0.30 0.50 0.65

0.07

400

350

300

500

450

550

400

350

0.04

0.05

0.06

0.08

0.09

0.25 0.35 0.40 0.45 0.55 0.60 0.70

M0901 matrix

0.30 0.50 0.65

0.07

300

450

500

0.0

0.02

0.04

0.12

0.14

0.0 0.2 0.3 0.7 1.0

M2507 matrix

0.1 0.5 0.9

0.08

0.8

0.06

0.10

200

300

400

600

0.04

0.05

0.06

0.09

0.25 0.35 0.45

G0903 granite

0.55 0.75

0.08

0.65

0.07

350

450

(c)

(e)

(h)

0.04

0.045

0.05

0.07

0.3 0.35 0.4

G2505 granite

0.45 0.55

0.065

0.5

0.055

0.60

(g)

600

0.13

0.06

0.07

0.08

0.09

0.10

0.11

0.12

0.4 0.5 0.6 0.7 0.8 0.90.5 0.6 0.7 0.8 0.90.3

1.3 1.8 2.3 2.8 3.3

0.16

0.20

0.24

0.28

0.12

15001400

13001200

1100

900

400

450

650

M3001 matrix(a)

206 P

b/23

8 U20

6 Pb/

238 U

206 P

b/23

8 U20

6 Pb/

238 U

206 P

b/23

8 U20

6 Pb/

238 U

206 P

b/23

8 U20

6 Pb/

238 U

207Pb/235U

207Pb/235U 207Pb/235U

207Pb/235U 207Pb/235U

207Pb/235U 207Pb/235U

0.0

0.02

0.04

0.12

0.14

0.0 0.2 0.30.4 0.6 0.4 0.6 0.7 1.0

M2507 inclusion

0.1 0.5 0.9

0.08

0.8

0.06

0.10

200

300

600

(d)

400

(b)

(f)

300

350

550

500

500

500

400

600

400

450

650

550

500

Fig. 11. Concordia diagrams of U–Pb zircon ages from pelitic gneisses (a–f) and granitoids (g, h). Zircon grains occurring in thematrix and as inclusions in garnet porphyroblasts are shown separately. (a) Matrix and (b) inclusion zircon crystals from sampleM3001 in the staurolite zone; (c) matrix and (d) inclusion zircon crystals from sample M2507 in the staurolite zone; (e) matrix and(f) inclusion zircon crystals from sample M0901 in the cordierite zone; and zircon in granitoid samples (g) G2505 and (h) G0903.



DISCUSSION

Two-stage metamorphism recorded in garnet andaluminosilicates

Figure 13a summarizes the P–T conditions of eachmineral zone (samples M2706, M2507, M2602,M0901) in the Tseel area, as obtained by the GIPgeothermobarometry (Fig. 9), along with additionaldata (samples M3001, M0701c & M0702b) fromour previous study (Burenjargal et al., 2012). TheP–T conditions estimated from the pelitic gneisses inthe Tseel area are 520–680 °C and 3–7 kbar(Fig. 13). The estimated P–T conditions are roughlydivided into two groups: high-P and low-T condition,and low-P and high-T condition respectively(Fig. 13a). The high-P and low-T conditions, which

are located in the kyanite stability field (520–570 °Cand 4.5–7 kbar), were obtained from the grossular-rich garnet in the garnet zone (M2706) and the gros-sular-rich cores of zoned garnet in the staurolite zone(samples M2507 & M3001). The low-P and high-Tconditions, which correspond to the sillimanitestability field (570–680 °C, 3–6 kbar), were obtainedfrom the pyrope-rich rims of zoned garnet in thestaurolite zone (M2507 & M3001), the pyrope-richgarnet in the sillimanite zone (samples M2602,M0701c & M0702b) and in the cordierite zone(sample M0901). The temperature of the cordieritezone (650–670 °C) is >50 °C higher than thatobtained from the rim of the staurolite zone(570–600 °C), whereas the pressure is similar to eachother (Fig. 13a). The temperatures estimated for thesillimanite zone samples cover a range from the

n = 11309±9

M0901 n = 104

25

20

15

10

5

0

25

20

15

10

5

0

20

15

10

5

0

M2507 n = 27M3001 n = 105

G0903; n = 65G2505; n = 38

(b)

(c) (d)

Num

ber

Num

ber

Num

ber n = 91

510±24(M3001)

n = 33 385±7

n = 53 297±11

n = 19 516±27(M2507)

80

0

60

40

20

500 1000 1500 20000

M2507M3001M0901

Age (Ma)

(a)

Num

ber

n = 5256±7

n = 110 511±24

n = 5406±11

n = 12491±15

St zone

Crd zone

n = 86 377±30

n = 4288±11

Pelitic gneisses

Granite

Relative probability




0

1

2

3

4

Th/U

ratio

Age (Ma)

M3001 (St zone)M2507 (St zone)M0901 (Crd zone)

G0903 (granite)G2505 (granite)

200Age (Ma)

200Age (Ma)

200

200 250 300 350 400 450 500 550 600

250 300 350 400 450 500 550 600

250 300 350 400 450 500 550 600 250 300 350 400 450 500 550 600

(e)

Age (Ma)

Fig. 12. U–Pb age histograms with relative probability curves forpelitic gneisses and granitoids in the Tseel area. (a) Agehistogram in the range of 2000–0 Ma for the pelitic gneisssamples. (b–d) Age histograms in the range of 600–200 Ma,showing (b) pelitic gneiss samples in the staurolite (St) zone(M3001, M2507), (c) pelitic gneiss in the cordierite (Crd) zone(M0901) and (d) granitoid samples (G2505, G0903). The agehistograms for the individual samples have several peaks. Themean and standard deviation (1r) for each age cluster (indicatedby double-headed arrows) are labelled at each peak. In (c), thelarge age cluster at 377 � 30 Ma has a shoulder at c. 430 Ma(grey arrow). (e) Th/U ratio of zircon against age (Ma) forindividual samples.



staurolite to cordierite zones. The zoning in garnetfrom the staurolite zone (Figs 7b & 8b) reveals thatthe high-P and low-T metamorphic event occurredprior to the low-P and high-T event (Figs 9b,c &13a; Burenjargal et al., 2012).

Garnet in the sillimanite and cordierite zones iscompositionally homogeneous (Figs 6a, 7c,d & 8c,d)and contains no record of the earlier high-P and low-Tevent (which is recorded by grossular-rich cores in thegarnet and staurolite zones). The absence of grossular-rich cores in the sillimanite and cordierite zones mayindicate that these zones did not experience the high-Pand low-T event or that grossular-rich cores did formbut were modified by diffusion during the low-P andhigh-T metamorphism. We consider that the lattercase is more likely, because aluminosilicate-bearingquartz veins occur not only in the staurolite zone butalso in the sillimanite zones (Fig. 1c). In these veins,aluminosilicates formed in the order of kyanite ? silli-

manite ? andalusite (Fig. 5; Burenjargal et al., 2012),which suggests a transition from the high-P and low-Tto low-P and high-T conditions.Garnet is rare and fine-grained in the pelitic

gneisses of the garnet zone (Figs 2d & 4a,b), whereasalmost all of the pelitic gneisses in the sillimanite andcordierite zones contain many large garnet porphyro-blasts (Figs 2c & 4e,f). These observations suggestthat garnet grew mainly under the low-P and high-Tconditions. For example, in sample M0901 from thecordierite zone, cordierite, which is only stable underhigh-T and low-P conditions (Fig. 9e), occurs asinclusions in garnet porphyroblasts as well as in thematrix (Figs 4f & 7d), indicating that coarse-grainedgarnet grew rapidly at high temperatures (>600 °C).In contrast to the higher grade zones, the grossular-

rich garnet in the garnet zone shows no evidence of thelow-P and high-T metamorphism (Figs 9a & 13a).Given that garnet overgrowths generally form withincreasing temperature (Burenjargal et al., 2012), thelack of overgrowths in the garnet zone suggests theexhumation of this zone without heating at the laterstage.

Protolith ages and timing of metamorphic events andgranitoid intrusions

Most of the zircon grains in the pelitic gneisses yieldages of 600–200 Ma, although some yield older agesof 2000–600 Ma (Fig. 12a). In the interval of 600–200 Ma, the pelitic gneisses in the Tseel area yieldtwo major peaks in U–Pb ages: Cambrian (570–440 Ma) and Devonian (450–300 Ma) (Fig. 12b,c).The cluster of Cambrian ages is found in all the

pelitic gneiss samples (Fig. 12b,c), although the peakis especially prominent in the staurolite zone samples(511 � 24 Ma in samples M3001 and M2507;Fig. 12b). This Cambrian age in the pelitic gneisses isinterpreted as the protolith (igneous) age (Fig. 13b),based on the facts that (i) the zircon grains of Cam-brian age commonly show clear oscillatory zoningwith a euhedral outline (Fig. 10a) and (ii) thesegrains commonly have high Th/U ratios (>0.4)(Fig. 12e).The large cluster of Devonian ages is found in the

cordierite zone sample (377 � 30 Ma for sampleM0901; Fig. 12c). In the staurolite zone samples(M2507 & M3001), there are two minor peaks in theinterval of 450–300 Ma (i.e. 406 � 11 and309 � 9 Ma; Fig. 12b). The zircon grains with Devo-nian ages are characterized by dissolution and over-growths (Fig. 10b,c), and the low Th/U ratio (<0.1;Fig. 12e) indicates that the rims of these zircon grainsare metamorphic in origin. Because the Devonian ageis recorded mainly in the cordierite zone samples, itis reasonable to infer that the low-P and high-Tmetamorphism occurred during the Devonian.Although the number of samples was limited in

this study, the U–Pb zircon age data clearly show

High-P–low-T

Temperature (°C)

Pre

ssur

e (k

bar)

550 600 7001.0

500 650450

10

2.0

3.0

4.0

5.0

6.0

7.0

8.0

9.0

Sil

And

M2602(Sil)

M2507 core (St)

M3001 rim (St)

Ky

M3001 core (St)

M2706(Grt)

M0701c (Sil)M0702b (Sil)

Ky

Sil

M0901 (Crd)

M2507 rim (St)

And

Low-P–high-T

(a)

(b)

Protolith depositionGranitoidintrusion 1

Granitoidintrusion 2

Metamorphicevents High-P

Low-T Low-PHigh-T

Retrograde ?

Igneousevents

500 Ma400300200

Fig. 13. (a) Summary of the P–T conditions obtained fromzoned garnet for each of the mineral zones (see Fig. 9) and fromprevious analyses (M3001; M0701c; M0702b) by Burenjargalet al. (2012). Two metamorphic events are recognized: a high-Pand low-T event (kyanite stability field) followed by a low-P andhigh-T event (sillimanite stability field). Grt = garnet zone;St = staurolite zone; Sil = sillimanite zone; Crd = cordieritezone. (b) Schematic illustration showing the age relationshipbetween the metamorphic events and granitoid intrusions.



that the Tseel area has experienced at least two stagesof granitoid intrusions: in the Devonian (385 �7 Ma; G2505) and Permian (297 � 11 Ma; G0903)(Fig. 12d). The ages of granitoids reported in theTseel terrane range from 580 to 270 Ma (Bibikovaet al., 1992; Kozakov et al., 2002; Kr€oner et al.,2007; Demoux et al., 2009a; Jiang et al., 2012), butmost yield Middle Devonian ages (400–380 Ma;e.g. Bibikova et al., 1992; Kozakov et al., 2002; De-moux et al., 2009a). A deformed granitoid in theeastern block of the Tseel terrane yields a zircon U–Pb age of 385 � 5 Ma (Bibikova et al., 1992), whichis consistent with the Devonian age of granitoid sam-ple G2505 (385 � 7 Ma; Figs 11g & 12d). It isimportant to note that the timing of this earlier phaseof granitoid intrusion (385 � 7 Ma; Figs 11g & 12d)is consistent with the timing of the low-P and high-Tmetamorphism (377 � 30 Ma; Fig. 13b), suggestingthat the granitoids were the heat source of the high-Tmetamorphism at middle crustal depths.

The exact timing of high-P and low-T metamor-phism is unclear, although it pre-dated the low-P andhigh-T metamorphism (Fig. 13a). The lack of signifi-cant zircon growth during the high-P and low-Tmetamorphism in the staurolite zone samples is prob-ably due to the low-T and/or low-fluid activity. Nev-ertheless, a small age peak at 450–400 Ma in thestaurolite zone samples (Fig. 12b) and in the cordie-rite zone sample (Fig. 12c) is thought to correspondto the high-P and low-T metamorphism (Fig. 13b).

The staurolite zone samples also have a small agepeak in the Carboniferous (309 � 9 Ma; Fig. 12b).This peak overlaps with the large age cluster of thecordierite zone (450–300 Ma) and with the laterphase of granitoid intrusion (297 � 11 Ma, sampleG0903; Fig. 12d). These findings indicate that igne-ous activity and low-P and high-T metamorphismcontinued for c. 100 Ma, reflecting the multi-stagenature of granitoid intrusions in the Tseel area; how-ever, the main heating events are likely to haveoccurred at c. 385 Ma.

Demoux et al. (2009a) found that some granitoids(e.g. feldspar porphyry) intruded the Early Devonianlow-grade volcano-sedimentary rocks during theearly Permian (c. 280 Ma). These younger granitoidsare interpreted to have been emplaced after the mainmetamorphic events, as their intrusion occurred afterthe Devonian amphibolite facies metamorphism (De-moux et al., 2009a). This age of 280 Ma proposed byDemoux et al. (2009a) is slightly younger than thelater phase of granitoid intrusion identified in thisstudy (297 � 11 Ma, G0903; Figs 11h & 12d). Asmall age peak at c. 260 Ma is found in the stauro-lite zone samples (Fig. 12b) and in the cordieritezone sample (Fig. 12c). These younger ages of thepelitic gneisses after the later phase of granitoidintrusion could be associated with retrograde meta-morphism after the peak metamorphic event(Fig. 13b).

Thermal effects of granitoid intrusions on apparentmineral zones

Figure 14a shows the peak temperatures recordedacross the metamorphic zones along a SSE–NNWtransect (line A–B in Fig. 1). It should be noted thatthe peak pressure of a particular sample doesnot always correspond to the peak temperature(Fig. 9b,c). In the Tseel area, the peak temperatureincreases when passing from the garnet to the cordie-rite zone. Peak temperatures are highest (600–660 °C)in the centre of the Tseel area, within the sillimaniteand cordierite zones, and decrease in a symmetricalfashion towards the garnet zones in the north andsouth (Fig. 14a). The peak temperature of the garnetzone is ~150 °C lower than those of the sillimaniteand cordierite zones. There are no systematic differ-ences in the mineral zones that are exposed in thenorthern and southern parts of the Tseel area.The occurrence and distribution of granitoid

intrusions in the Tseel area are spatially variable:some intrusions occur as layers interlayered with pe-litic gneiss (Fig. 2b) and oriented subparallel to thefoliation, whereas others occur as massive kilometre-scale bodies (Figs 1c & 2a). From the map in

BA

60

500

600

700

Tem

pera

ture

(°C

)

Distance (km)

0

100

80

60

40

20Gra

nito

ids

(are

a %

)

Grt Grt

0 20 40 80 100

(a)

St Sil Crd St

Garnet isoplethGBPQ

(b)

Fig. 14. Spatial variations in (a) peak temperature and (b) theareal proportion (per cent) of granitoid rocks along line A–Bin Fig. 1c. The peak temperatures and pressures wereestimated from garnet isopleths (Fig. 9) and are not alwayssynchronous. The areal proportion of granitoid rock wasestimated from a geological map (as described in the text).Grt = garnet zone; St = staurolite zone; Sil = sillimanite zone;Crd = cordierite zone.



Fig. 1c, the area percentage occupied by granitoidswas estimated using a 4 9 4 km square along theline marked A–B. The results, shown in Fig. 14b,indicate a correlation between granitoid abundanceand metamorphic grade (Fig. 14b). In the sillimaniteand cordierite zones, which are ~30 km wide(Fig. 1c), there occur many granitoid intrusions. Asdiscussed in the previous section, some granitoidintrusions are associated with the low-P and high-Tmetamorphism at c. 385 Ma, whereas others (c.297 Ma) are younger than the metamorphic events.Although not all of the granitoids associated withthe low-P and high-T metamorphism in the Tseelarea have been identified, the spatial correlationbetween granitoids and the highest peak tempera-tures (Fig. 14) strongly supports the scenario thatthe granitoid intrusions caused a regional contactmetamorphism at middle to upper crustal levels dur-ing the Middle Devonian.

Tectonic implications

Previous studies have examined the geochemistry andgeochronology of the Chinese Altai, which is the east-ward continuation of the Tseel terrane (e.g. Salnikovaet al., 2001; Dijkstra et al., 2006; Long et al., 2007;Sun et al., 2008; Cai et al., 2011; Jiang et al., 2012;Fig. 1b), as well as the petrology of the region (Weiet al., 2007). Wei et al. (2007) carried out a detailedpetrological analysis of the Xinjiang area in the Chi-nese Altai, and found a transition in metamorphicseries from medium-P kyanite type to low-P andalu-site–sillimanite type. This finding is consistent withthe present result that the earlier high-P and low-Tevent was followed by the low-P and high-T meta-morphism (Fig. 13a). Jiang et al. (2012) carried outthe U–Pb zircon age dating of the pelitic gneisses inthe Tseel terrane, and based on the CL-textures andTh/U ratio of zircon grains, they showed that theTseel terrane and Chinese Altai have similar protolithages of c. 500 and c. 470 Ma, respectively, with meta-morphism dated at c. 390 and c. 385 Ma respectively.Based on these age similarities, Jiang et al. (2012) sug-gested that these terranes belonged to the same crustalsegment of an early Palaeozoic arc system, whichextended from western Mongolia to the Chinese Altai.Our petrological and geochronological data indicate aprotolith age of c. 510 Ma (580–450 Ma) and a low-Pand high-T metamorphism age of 377 � 30 Ma(Fig. 12c) associated with granitoid intrusions (c.385 Ma), which also supports similarities between theTseel terrane and the Chinese Altai.

Combining the data of this study with existing datafrom the Tseel terrane and the Chinese Altai, we pro-pose the following scenario on the evolution of ther-mobaric structure of the Tseel area. During theCambrian (c. 510 Ma), terrigenous sedimentary rockswere accreted at an active continental margin (Longet al., 2007; Sun et al., 2008). The accreted sedimen-

tary rocks underwent high-P and low-T meta-morphism in deeper levels of the accretionary prism(15–20 km depth) during the Silurian (450–400 Ma).During this stage, the grossular-rich garnet grew inthe pelitic gneisses, but dissolution and overgrowthof zircon did not occur significantly due to low-Tand low-fluid activity. The accreted sedimentaryrocks were moved farther from the trench as a resultof ongoing accretion. After the high-P and low-Tmetamorphism, low-P and high-T metamorphismoccurred in association with granitoid intrusions inthe Middle Devonian (c. 385 Ma). This metamor-phism involved higher temperatures and slightlylower pressures than the earlier event, resulting in thegrowth of coarse-grained pyrope-rich garnet in thepelitic gneisses. The concentration of granitoid intru-sions in the central part of the Tseel area could haveproduced the symmetrical east–west trending mineralzones. Near the granitoids, the temperature increasedto ~650 °C, leading to the growth of garnet, sillima-nite and cordierite. The cordierite zone lies especiallyclose to the granitoids and was heated at the middlecrustal depths (~15 km; Fig. 13a). At this stage, zir-con grains were dissolved, and overgrew to form themetamorphic rims. In areas farther from the grani-toids, the effects of the low-P and high-T metamor-phism are minor, so that the effects of the earlierhigh-P and low-T metamorphism are well preserved.

Possible heat source of the low-P and high-Tmetamorphism

A remaining problem is the heat source of the Devo-nian low-P and high-T metamorphism and granitoidformation within the crust. Wang et al. (2006) pro-posed a model that the Devonian magmatism (c. 460,408 & 375 Ma) of the Chinese Altai occurred in anextensional setting associated with an opening of theback-arc basin. Lower Devonian S-type granites havebeen reported from the Chinese Altai (Cai et al.,2011). In contrast, based on the geochemical andgeochronological studies including Hf isotopic com-positions of zircon in the gneissic granitoids, Sunet al. (2008) discussed that the plutonic activity atthese Devonian ages is characterized by I-type gran-ites formed in a subduction environment. In such acase, the heat source would have been subduction ofa hot young slab (i.e. ridge subduction). It is cur-rently difficult to determine which setting is morelikely, and additional systematic geochemical studieson the granitoids and other intrusions of various agesin the Tseel area are required to improve our under-standing of the changes in tectonic setting thataccompanied the evolution of the crust in the CAOB.The age histogram of the cordierite zone sample

(M0901) has a broad peak at 450–300 Ma (377 �30 Ma; Fig. 12c). Because each of the age data isplotted on the concordia curve (Fig. 11; Table S2),these broad age ranges do not always result from the



error in the U–Pb zircon age dating, but indicatesthat the low-P and high-T metamorphism continuedfor c. 100 Ma (400–300 Ma; Figs 12c & 13b). Thisduration of c. 100 Ma was clearly longer than thatrecorded in a single granitoid body at the same age(<50 Ma for G2505; Fig. 12d), suggesting that an ele-vation of geotherm during the Devonian was causedby continuous intrusion of several granitoid bodiesand/or radioactive heat production from the grani-toids (Sandiford et al., 1998).

CONCLUSIONS

1 The Tseel area contains four E–W trending min-eral zones defined by index minerals in metapel-ites: the garnet, staurolite, sillimanite andcordierite zones. The highest peak metamorphictemperatures are recorded by pelitic gneisses fromthe sillimanite and cordierite zones, located in thecentre of the Tseel area; from this central area,the metamorphic grade decreases to the north andto the south. The sillimanite and cordierite zoneswere intruded by voluminous granitoids.

2 The composition of garnet in the pelitic gneissesshows a systematic change across the mineralzones from grossular-rich in the garnet zone topyrope-rich in the sillimanite and cordierite zones.The staurolite zone contains zoned garnet withgrossular-rich cores and pyrope-rich rims.

3 Analysis of the pelitic gneisses, based on GIPthermobarometry, P–T pseudosections and con-ventional garnet–biotite–plagioclase–quartz ther-mobarometry, reveals two metamorphic events: anearlier high-P and low-T metamorphism (kyanitestability field) and a later low-P and high-T meta-morphism (sillimanite stability field).

4 U–Pb dating of zircon from granitoids in theTseel area reveals two stages of granitoid intru-sions at 385 � 7 and 297 � 11 Ma. U–Pb datingof zircon from pelitic gneisses indicates that theprotolith (igneous) age of the sedimentary rocks isc. 510 Ma. The low-P and high-T metamorphismoccurred during the Devonian (377 � 30 Ma),which was coeval with the first period of granitoidintrusions. The high-P and low-T metamorphismprobably corresponds to small age peaks duringthe Silurian (450–400 Ma).

5 Our petrological and chronological analyses ofthe Tseel area suggest that sedimentary rocksaccreted at a continental margin and experiencedhigh-P and low-T (kyanite-type) metamorphism atmid-crustal depths, and subsequent heating. Intru-sion of granitoids during the Devonian resulted ina regional low-P and high-T metamorphism (silli-manite-type), producing the apparent mineralzones of the Tseel area. The low-P and high-Tmetamorphism continued for c. 100 Ma, which isclearly longer than the active period of a singlegranitoid body, suggesting the elevation of geo-

therm during the Devonian was caused by contin-uous intrusion of several granitoid bodies and/orradioactive heat production subsequent to thegranitoid intrusions.

ACKNOWLEDGEMENTS

We thank O. Gerel, J. Lkhamsuren, B. Munkhtsengel,B. Batkhishig and A. Chimedtseren (Mongolian Uni-versity of Science and Technology) for their help andadvice during the course of this study. D. Tsendbazaris thanked for providing assistance in the field. We arealso grateful to M. Toriumi and M. Uno for valuablediscussions on various aspects of this work. We thankS. Yamasaki and R. Yamada for help with the XRFanalyses and Y. Kouketsu for assistance with thePerpleX software. We thank T. Okudaira and ananonymous reviewer for their constructive comments.This work was financially supported by a Grant-in-Aid for Scientific Research from the Japan Societyfor the Promotion of Science (nos. 22246115 and25000009 awarded to N. Tsuchiya) and by a Grant-in-Aid for Challenging Exploratory Research (no.23654180 awarded to A. Okamoto).

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SUPPORTING INFORMATION

Additional Supporting Information may be found inthe online version of this article at the publisher’sweb site:Table S1. Instrumental parameters used for U–Pb

zircon dating by LA–ICP–MS.Table S2. LA-ICPMS analytical data for zircon

from pelitic gneisses and granitoids in the Tseel ter-rane.

Received 24 September 2013; revision accepted 17 April 2014.



burenjargal_et_al-2014-journal_of_metamorphic_geology

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