Insights into the P–T evolution path of Tso Morari

Insights into the P–T evolution path of Tso Morarieclogites of the north-western Himalayas: Constraints

on the geodynamic evolution of the region

Preeti Singh, Ashima Saikia∗, Naresh Chandra Pant and Pramod Kumar Verma

∗∗

Department of Geology, University of Delhi, Delhi 110 007, India.∗Corresponding author. e-mail: [email protected]

The present study is on the Ultra High Pressure Metamorphic rocks of the Tso Morari Crystalline Com-plex of the northwestern Himalayas. Five different mineral associations representative of five stages ofP–T (pressure–temperature) evolution of these rocks have been established based on metamorphic tex-tures and mineral chemistry. The pre-UHP metamorphic association 1 of Na-Ca-amphibole + epidote ±paragonite ± rutile ± magnetite with T–P of ∼ 500◦C and 10 kbar. This is followed by UHP metamor-phic regime marked by association 2 and association 3. Association 2 (Fe>Mg>Ca-garnet + omphacite +coesite + phengite + rutile ± ilmenite) marks the peak metamorphic conditions of atleast 33 kbar and∼ 750◦C. Association 3 (Fe>Mg>Ca-garnet + Na-Ca amphibole + phengite ± paragonite ± calcite ±ilmenite ± titanite) yields a P–T condition of ∼28 kbar and 700◦C. The post-UHP metamorphic regimeis defined by associations 4 and 5. Association 4 (Fe>Ca>Mg-garnet + Ca-amphibole + plagioclase(An05) + biotite + epidote ± phengite yields a P–T estimate of ∼14 kbar and 800◦C) and association5 (Chlorite + plagioclase (An05) + quartz + phengite + Ca-amphibole ± epidote ± biotite ± rutile ±titanite ± ilmenite) yields a P–T value of ∼7 kbar and 350◦C.

1. Introduction

Reported occurrence of coesite, the high pressurepolymorph of quartz as inclusions in the garnetsof eclogitic rocks from Norway and the Alps regionin the early 80’s (Chopin 1984; Smith 1984) wasinstrumental in renewing the interest in the studyof eclogite-bearing terrains. The presence of coesiteestablished beyond doubt that the source of originof eclogite was quite deep, at least 100 km beneaththe earth’s surface. Thereafter, numerous studieswere carried out to understand the mechanism ofplate subduction leading to formation of eclogiteas a result of ultra high pressure metamorphism

(UHPM) and their subsequent exhumation andpreservation at surface conditions (e.g., UHPMrocks from the Kokchetav massif, Kazakhstan;Dabie-Shan, China and western Gneiss Region,Norway, Dora Maria Massif, W. Alps; FranciscanComplex, California). Ultra high pressure meta-morphic eclogite rocks in the Himalayan terrainhas been reported from the Tso Morari CrystallineComplex (TMC) in India, the Kaghan and theNeelum valleys in Pakistan and the Ama Drimeregion and the Arun river valley in Nepal (Guillotet al. 1997; Sachan et al. 1999; Pognante andSpencer 1991; O’Brien et al. 2001; Groppo et al.2007).

Keywords. UHPM (Ultra high pressure metamorphism); eclogites; Himalayas; Tso Morari Crystalline Complex (TMC).

∗∗ Since deceased.Supplementary data pertaining to this article are available on the Journal of Earth System Science Website at http://www.

ias.ac.in/jess/jun2013/supp/Ashima.pdf

J. Earth Syst. Sci. 122, No. 3, June 2013, pp. 677–698c© Indian Academy of Sciences 677

http://www.ias.ac.in/jess/jun2013/supp/Ashima.pdf

http://www.ias.ac.in/jess/jun2013/supp/Ashima.pdf

678 Preeti Singh et al.

The Tso Morari Crystalline Complex was the firstarea from where the occurrence of eclogite rocksin the Himalayan terrain was reported in mid 20thcentury (Berthelsen 1953). This was corroboratednearly half a century later by reporting of coesiteinclusions in garnets (Mukherjee and Sachan2001). The peak UHP assemblage and its subse-quent exhumation through amphibolite facies togreenschist facies in the TMC is generally agreedupon by most workers (de Sigoyer et al. 2004;Guillot et al. 1997) but textural and mineralogicalconstraints for the prograde path for attainmentof UHP are lacking. Recently, Guillot et al. (2008)suggested a refined P–T–t path for the rocks wherea pre-UHP path through blueschist facies regimewas envisaged but not documented in terms ofmineral chemistry or textural evidence.

The present work defines a new P–T evolutionpath for the UHP rocks of the Tso Morari Crys-talline Complex. It works out a pre-UHPM blue-schist facies mineral association (association 1)based on textural and mineral chemical data. Anew transitional mineral association (association 3)gradational between eclogite and amphibolitesfacies is being reported for the first time for theserocks. We show that the rocks of TMC were notexhumed in a single step. Our work refines the P–Tevolution path for the TMC eclogites which is usedto comment upon the geodynamic evolution of thisregion in terms of slab kinematics and availablegeochronological constraints.

2. Geological setting

The Himalayan terrain is the most tectonicallyactive zone of the Indian plate. It is characterizedby four major tectonic elements that can be seenthroughout from the Nanga Parbat in the west toNamcha Barwa in the east. The Himalayan rocksbetween these tectonic elements exhibit distinctgeological characteristics. Based on it the Hima-layas are classified into four divisions (Gansser1964) from south to north: the Sub Himalayas, theLesser Himalayas, the Higher Himalayas and theTethyan or the Tibetan Himalayas (figure 1a).

2.1 The Tso Morari dome

The Tso Morari area is a NW–SE trending beltlying between the Tethyan Himalayas in the southand the Zildat ophiolite melange of the IndusSuture zone to the north. Lithostratigraphically,the area consists of three formations, viz., the PugaFormation, the Taglang La Formation and theRupshu and the Polokong La granites.

The Puga Formation comprises of schists andgneisses and hosts the boudins of eclogites studied

in this work. The Taglang La Formation comprisesof metamorphosed calcareous, marly and argilla-ceous sediments with concordant bands of amphi-bolites. The Rupshu and Polokong La granites,the third lithostartigraphic unit of the Tso Morariarea are intrusive in the Puga Formation and theTaglang La Formation (figure 1b).

Structurally, the area forms a doubly plunginganticline or a dome. The Puga Formation formsthe core of this dome followed by the Taglang LaFormation on the outer periphery. The Rupshuand Polokong La granites crosscut these forma-tions in the dome. Three generations of structuresare revealed in the field (Guillot et al. 1997; Jainet al. 2003). The first set of structures is verti-cal rootless folds and the associated lineation andcleavage. The second generation of structures arepreserved as comparatively large recumbent foldswith near horizontal axial plane. The third gen-eration of deformation formed normal faults andthe Tso Morari dome that delineates eclogite faciesrocks from the surrounding lower grade rocks.

2.2 Current status of metamorphic studies

Focus so far in the metamorphic studies of the TsoMorari eclogites has been on establishing the UHPmineral assemblage. Based on metamorphic stud-ies the eclogite rocks of the Tso Morari area areinferred to exhibit three mineral assemblages, viz.,eclogite facies assemblage, blueschist-amphibolitefacies assemblage and greenschist facies assemblage(de Sigoyer et al. 1997; Guillot et al. 1997; Sachanet al. 1999). For the eclogitic assemblage a pressureestimate varying from 14 to 16 kbar and temper-ature estimate of 550 ± 50◦C to 580 ± 60◦C hasbeen documented; for the retrograde assemblage ofamphibolite facies a pressure estimate of ∼8 ± 3to 10 ± 3 kbar and temperature of ∼570 ± 70◦Cto 630 ± 30◦C and for the third retrograde assem-blage of grenschist facies the P–T estimates of ∼5to 8 ± 3 kbar and ∼500◦C to 590 ± 90◦C. As isevident the variability of the reported data lies welloutside the error ranges of the data.

However, with the report of coesite inclusionin garnet of the eclogites from Tso Morari area(Mukherjee and Sachan 2001; Sachan et al. 2004)the minimum pressure attained by these rockswere redefined to be atleast 28 kbar at 650◦C.Following this report a lot of revisions went intoP–T estimates for rocks of this area. For the eclog-ite facies assemblage, the temperature estimatesvary between ∼600 and 800◦C (Jain et al. 2003;de Sigoyer et al. 2004). A carbonate bearing assem-blage has also been reported from the TMC indi-cating P–T as ∼39 kbar and ∼750◦C (Mukherjeeet al. 2003). Geodynamic evolution models havebeen proposed by de Sigoyer et al. (2000, 2004),

P–T evolution path of Tso Morari eclogites of NW Himalayas 679

70o 80o 90o

35o

25o

0 100 300300 500

kms

KAGHAN VALLEYKarakoram

Indus Suture

Trans Himalayas

TsangpoSutlu

j

Ind

us

DELHI

Ganges

Brahmaputra

TSO MORARI CRYSTALLINE COMPLEX

AMA DRIME

Tibetan or Tethyan Himalayas Lesser Himalayas

Krol Belt

Sub Himalayas

Higher Himalayas

Main Boundary Thrust Fault

Orogenic sediments or ophiolites Main Central Thrust fault

(a)

(b)

++

+

++

++

077 50’

33°2

5’

078 40’ 33°25’

078 40’

32°4

0’

077 50’

32°40’

+

+

++

+

+

+

Kiagar Tso

POLOKONG LA

PUGA FORMATION

Tso Morari

TAGLANG LA FORMATION

TETHYAN HIMALAYAS

INDUS SUTUREZONE

LADAKH BATHOLITH

RUPSHU

Zilda t Detachment Fault

NyimalingIndus

Tso Kar

Karzok

Mata

Scale: 10km

Mahe

Chumantang

Sumdo

P hi rse fu

6150

6100

6540

6270

4550

Figure 1. (a) Geological map showing the sub-divisions of the Himalayas (modified after Gansser 1964). The three differentlocations of Ultra High Pressure (UHP) metamorphic rocks reported so far from the Himalayan terrain have been shownmarked as stars in the map. (b) Geological map of the Tso Morari Crystalline Complex, modified after Berthelsen (1953) andThakur (1983). Three different litho units comprising the study area has been shown with the sample collection locationsmarked as circles. Filled black circles and open circles mark two different field seasons.


Guillot et al. (2007, 2008) and Epard and Steck(2008) and mathematical estimations have beencarried out to arrive at the rate and angle of sub-duction of the lithospheric plate (Kaneko et al.2003; Leech et al. 2005).

It is quite clear that the peak UHP eclogite faciesassemblage was followed by the development of am-phibolite facies assemblage with a slight increasein temperature and a sharp decrease in pressure.Finally, the rocks attained greenschist facies assem-blage with decrease in pressure and tempera-ture. The prograde path leading to eclogite facies,however, remains unreported.

Figure 2. Field occurrence of eclogite boudins of the TsoMorari region hosted in Puga Gneisses. Boudins have beenhighlighted using white coloured boundaries. Sample loca-tion is on the road section from Sumdo towards PolokongLa, GPS reading: 33 degree 183 minutes N; 78 degree 358minutes E; Altitude: 15265 ft. The largest boudin in thisphotograph is about a meter in width. However, at differ-ent locations boudin sizes vary. We observed a maximum of5–6 m length and 2–3 m width boudins.

2.3 The sample localities and field characteristicsof the sampled rocks

The eclogite samples for the present work were col-lected from the Puga Formation and their locationsare marked in figure 1(b). Eclogite occur as boudinswithin the metapelites of the Puga Formation. Inthe large boudins, unfoliated cores were rimmed byfoliated margins (figure 2).

The UHP rocks which occur as boudins in thePuga Formations of the Tso Morari dome are min-eralogically zoned on both megascopic as well as thinsection scale. The core portions of these boudinsconsist predominantly of garnet with pyroxenes oc-curring as inclusions within the garnet. Amphiboleand phengite occur with garnet as the other min-eral constituents of this core portion. The rimportion of the boudin comprises Ca-amphibole/chlorite, biotite (± quartz, K-feldspar and epidote).

3. Petrography and mineral chemistry

3.1 Methods and analytical conditions

The present study was carried out with focus on theeclogites of the Tso Morari area. Textural featuresof these rocks specially the inclusion minerals/assemblages, zoning, garnet in garnet growth andsymplectitic intergrowth of minerals have pre-served records of prograde and retrograde path.

The textural signatures associated with eachmetamorphic episode of the eclogites were observedin thin sections. We established five mineral asso-ciations (table 1).

Mineral chemistry were determined using aCAMECA SX51 electron microprobe with acceler-ation voltage of 15 kV and beam current of 12 nA

Table 1. Mineral associations of the eclogites of the Tso Morari Crystalline Complex, NW Himalayas with associatedcharacteristic textures as observed in rock thin sections.

Association 1 Poikilitic garnet with inclusion assemblage of epidote/ Na-Ca-amphibole + epidote ±clinozoisite, pargonite and Na-Ca amphibole; Zoned paragonite ± rutile ± magnetite.

amphibole with Na rich core and Na-Ca rich rim.

Association 2 Poikilitic garnet with inclusion assemblage of omphacite, Fe>Mg>Ca-garnet + omphacite +

phengite, rutile; coesite pseudomorph in garnet; two coesite/usually quartz + phengite +

phases of growth in garnet; prograde zoning in garnet. rutile ± ilmenite.

Association 3 Conversion of omphacite into Na-Ca amph; Na-Ca rich Fe>Mg>Ca-garnet + Na-Ca amphibole +

rim of the zoned amphibole. phengite ± paragonite ± calcite ±ilmenite ± titanite.

Association 4 Atoll garnet; symplectite of Ca-amphibole and plagioclase Fe>Ca>Mg-garnet + Ca-amphibole +

after garnet and omph; zoned amph with Na-Ca rich core plagioclase (An05) + biotite +

and Ca rich rim. epidote ± phengite.

Association 5 Fractured and skeletal garnets with fractures filled Chlorite + plagioclase (An05) + quartz +

by chlorite and biotite; pseudomorph of secondary phengite + Ca-amphibole ± epidote ±minerals after garnet. biotite ± rutile ± titanite ± ilmenite.


Table

2.

Rep

rese

nta

tive

min

eralanaly

ses

ofdiff

eren

tm

iner

alass

ocia

tions

pre

serv

edin

the

eclo

gite

sofT

soM

ora

riC

ryst

allin

eC

om

ple

x.T

he

oxy

gen

num

bers

use

dfo

rm

iner

al

form

ula

calc

ula

tion

have

been

mark

edalo

ngs

ide

the

min

eralnam

e.End

mem

ber

calc

ula

tions

are

also

shown

for

impo

rtantm

iner

alco

nst

ituen

ts.

Ass

ocia

tion

1=

Na-C

aam

ph

+Epi+

Parg

+R

t+

Mag

Na-C

aam

phib

ole

(23O

)Epid

ote

(12O

)Para

gonit

e(2

2O

)

Min

eral

ingt(

8)

ingt(

5)

ingt(

2)

inm

tx(C

)in

gt(

1)

ingt(

3)

ingt(

5)

ingt(

3)

ingt(

1)

ingt(

1)

Sam

ple

no.

40/47

65/107

67/119

40/47

67/119

37/122

40/126

67/119

40/47

40/131

SiO

245.7

642.1

540.6

554.7

337.0

638.0

22

38.7

51

45.5

246.5

446.5

7

TiO

20.5

0.7

90.5

30.1

10.2

10.1

78

0.0

85

0.0

80.1

60.1

9

Al 2

O3

14.9

516.2

616.3

612.2

826.5

826.0

51

30.0

27

38.9

638.1

638.3

9

Cr 2

O3

0.0

90

00.0

10

00

0.0

80.1

10.2

4

FeO

14.8

716.4

517.1

29.8

48.8

10.0

81

5.0

37

10.8

81.3

1

MnO

0.1

80.0

40.3

30

0.0

90.0

41

0.0

06

0.0

40

0.0

4

MgO

9.3

27.8

19.1

511.5

30.1

40

0.1

87

0.1

20.3

20.4

1

CaO

7.4

97.5

58.6

33.0

523.2

222.8

92

23.2

92

0.4

70.2

0.2

6

Na2O

4.1

34.5

43.2

86.3

40

00

7.7

96.9

76.9

8

K2O

0.3

90.2

30

0.1

30

00

0.0

31.3

70.9

P2O

50

00

00

00

00

0.0

3

Sum

97.6

895.8

296.2

598.0

296.1

297.2

65

97.3

85

94.0

994.7

195.3

3

Am

ph

Tota

lN

a(B

)0.8

42747

0.7

9375

0.6

37078

1.5

51026

Tota

lN

a(A

)0.3

11989

0.5

18851

0.3

00312

0.1

37847

Tota

lC

a1.1

57253

1.2

0625

1.3

62922

0.4

48974


Table

2.

(Continued

)

Ass

ocia

tion

2=

Garnet

(Mg-C

a)

+P

yx

+C

oe/Q

tz+

Phe

+R

t

Poin

tof

Garnet

(12O

)C

linopyroxene(6

O)

Phengite

(22O

)R

utile

analy

ses

Gt(

C)

(5)

Gt(

Int)

(2)

Gt(

C)(

5)

Gt(

R)(

1)

Gt(

R)(

6)

inn

Gt(

R)(

5)

inn

Gt(

C)(

5)

ingt(

6)

ingt(

2)

ingt(

1)

ingt(

2)

ingt

ingt(

3)

Ingt(

2)

Sam

ple

no.

75/145

37/122

40/47

40/131

40/04

63/101

63/101

40/47

37/122

65/107

75/145

40/47

40/47

40/47/5

SiO

239.1

32

38.0

539.1

139.0

437.9

42

39.0

139.2

657.0

455.4

555.9

83

56.0

43

51.1

849.6

0.0

79

TiO

20

00.0

30.2

60.0

35

0.0

40.1

10.0

80.0

80

0.0

53

0.3

40.5

5100.4

61

Al 2

O3

21.9

46

21.8

21.8

521.8

621.8

822.4

422.6

913

10.3

110.0

18.5

93

25.4

128.5

30.0

51

Cr 2

O3

0.0

23

0.1

60

00

0.0

60.0

70.1

0.0

10.0

75

0.1

08

0.0

20.1

20

FeO

26.8

76

28.2

23.5

721.9

227.8

18

25.6

924.3

53.7

57.6

16.6

19

7.9

11

2.0

61.9

40.9

7

MnO

0.2

57

0.2

40.2

70.2

10.5

91

0.5

0.3

10.0

30

00.2

22

00

0.0

13

MgO

6.8

26.2

18.4

27.2

45.8

02

7.7

28.5

77.2

66.9

77.4

23

7.8

82

4.1

72.8

80.0

28

CaO

5.6

93

5.8

46.3

68.5

36.0

65

5.2

86.0

911.6

511.3

812.2

06

13.8

88

00.0

20

Na2O

00.0

20.1

0.0

70

00.0

87.7

97.7

7.7

66.3

58

0.4

61.0

50

K2O

00

00.0

20

00.0

20

0.0

10.0

07

09.5

88.5

10

P2O

50

00.0

40.0

20

00

00

0.0

71

00

00

Sum

100.7

47

100.5

399.7

599.1

6100.1

33

100.7

4101.7

9100.7

99.6

2100.1

54

101.0

58

93.2

393.1

9101.6

02

Gt

Om

ph

Alm

57.6

869

58.6

1357

49.6

4618

47.6

8388

58.7

902

54.9

4584229

49.8

1765661

XJd

36.3

0159

39.1

1293

36.3

1064

28.2

5783

Pyr

26.0

9869

24.3

7715

32.2

5701

28.0

7963

22.7

7972

29.4

8026538

32.7

7297399

Wo

30.0

0045

31.9

4372

31.5

6161

34.1

092

Grs

15.6

557

16.4

7408

17.5

092

23.7

7381

17.1

1191

14.4

8922152

16.7

3591129

Ens

26.0

1605

27.2

257

26.7

0965

26.9

3837

Fes

7.5

37274

1.5

59996

5.4

18096

10.6

0321


Table

2.

(Continued

)

Associa

tio

n3

=G

t(M

g-C

a)

+N

a-C

aam

ph

+Phe

+C

al+

Parg

Garnet

(12O

)N

a-C

a-a

mphib

ole

(23O

)P

hengit

e(22O

)Paragonit

e(22O

)

Sam

ple

Gt(

R)(

2)

Gt

inam

ph(4

)G

t(R

)(2)

Gt(

R)(

2)

Gt(

R)(

2)

adjG

t(3)

adjG

t(6)

adjG

t(11)

adjG

t(3)

adjG

t(1)

inm

tx(r

im)

adjG

t(7)

adjG

t(1)

adjG

t(1)

mtx

(1)

mtx

(2)

mtx

mtx

(1)

no.

40/47

67/119

40/126

63/104

37/122

40/48

40/47

67/119

71/134

37/122

40/47

40/126

40/131

40/47

37/122

40/126

40/131

40/127

SiO

238.7

837.3

78

39.5

79

38.7

438.0

547.1

24

46.6

751.1

154.1

39.5

250.8

37.8

65

49.2

351.8

950.9

247.9

647.2

747.7

3

TiO

20.0

60

00.0

09

00.2

65

0.1

0.2

10.0

40.0

80.2

50.9

24

0.2

0.2

30.2

50.0

50.1

20.0

4

Al 2

O3

21.8

321.3

88

22.1

46

22.1

33

21.8

12.3

42

11.6

911.1

65.5

818.4

712.6

19.1

32

25.3

826.1

525.0

639.1

338.9

340.5

13

Cr 2

O3

00.0

09

0.0

88

0.0

28

0.1

60

0.1

20

00.0

30.1

30.0

16

3.2

90

00.0

60.0

80.0

26

FeO

26.5

330.9

69

23.8

86

26.6

328.2

13.6

73

16.5

68.9

66.2

418.4

710.2

720.0

54

1.4

22.3

1.8

10.3

40.2

70.1

9

MnO

0.1

70.4

79

0.2

17

0.3

29

0.2

40.2

18

0.1

70.2

30.0

118.3

10

0.0

99

1.4

20.0

50.1

0.1

30.0

70.0

2

MgO

6.7

94.3

61

7.1

94

8.0

91

6.2

112.3

01

9.0

513.9

18.0

90.2

411.5

35.9

13

1.4

24.2

34.3

20.3

40.2

80.2

04

CaO

5.4

5.5

17

7.4

39

4.9

23

5.8

49.6

89

7.6

66.5

99.7

95.8

55.1

69.2

09

1.4

20

00.2

90.3

90.2

48

Na2O

0.0

20.0

16

00.0

29

0.0

22.1

16

4.0

73.5

22.4

63.9

85.4

84.0

59

1.4

20.5

70.3

36.8

97.1

36.9

47

K2O

00.0

01

00.0

01

00.2

47

0.3

50.1

90.1

70.8

80.1

70.2

35

1.4

210.4

610.3

90.8

50.8

60.6

5

P2O

50

00

00

00.0

10

00

00

1.4

20

00

0.1

0

Sum

99.6

3100.1

18

100.5

49

100.9

13

100.5

397.9

75

96.4

595.8

796.4

896.5

696.3

997.5

06

1.4

295.8

893.1

896.0

395.5

96.5

68

Gt

Am

ph

Alm

58.0

1896

65.6

5464

51.4

0898

54.4

0872

58.6

1357

Tota

lN

a(B

)0.5

26277

0.7

87294

0.9

55

0.5

31071

0.6

1141

1.2

15716

0.8

07315

Pyr

26.4

7428

17.4

2092

27.6

05

31.2

1954

24.3

7715

Tota

lN

a(A

)0.0

56147

0.3

78731

00.1

36873

0.5

10499

20.3

22914

Grs

15.1

3022

15.8

3744

20.5

1299

13.6

5058

16.4

7408

Tota

lC

a1.4

73723

1.2

12706

0.9

89

1.4

68929

0.9

11263

0.7

84284

1.1

92685

Associa

tio

n4

=G

t(C

a-M

g)

+C

a-a

mph

+P

lag

+B

t+

Epi+

Phe

Garnet

(12O

)C

a-a

mphib

ole

(23O

)P

lagio

cla

se

(8O

)B

iotit

e(22O

)Epid

ote

(12)

Sam

ple

Gt(

C)

Gt

Gt

adjG

tm

txsy

mp

mtx

sym

psy

mp

sym

pm

txm

txm

txm

tx(7

)m

tx(2

)

no.

71/134(5

)40/48(3

)76/149(5

)40/48(2

)76/149(8

)67/119(2

)71/134(2

)40/126(4

)40/126(5

)67/119(2

)40/48(3

)63/101(2

)37/122(2

)71/134

76/149

SiO

237.6

838.3

02

37.8

549.6

34

55.1

637.4

33

39.4

148.9

88

66.4

67.6

14

36.4

739.2

837.3

239.3

12

39.3

4

TiO

20.0

80.2

27

0.1

80.3

0.0

20

0.1

90.2

22

00.0

43

1.1

60.9

10.3

35

0.0

72

0.2

Al 2

O3

21.4

421.0

52

21.4

89.8

65

3.9

820.6

82

19.4

11.3

95

19.5

119.6

25

17.3

216.2

718.8

47

32.5

91

29.7

3

Cr 2

O3

0.0

30.0

50

0.0

77

00

0.0

20

00

0.2

10.0

70

00.0

2

FeO

26.2

424.9

05

34.0

313.5

82

9.0

418.7

09

15.1

812.0

43

0.3

20.3

86

16.5

711.7

910.1

72

2.3

39

5.1

9

MnO

0.3

21.9

08

0.2

40.4

69

0.0

30.2

43

0.0

60.0

34

0.0

20

0.1

50.0

70.0

05

0.0

23

0.0

6

MgO

2.5

84.8

05

1.9

912.7

62

17.2

46.5

94

7.7

611.7

28

0.8

60.0

36

12.1

515.4

115.8

73

0.0

20.0

9

CaO

11

9.8

91

6.5

510.1

94

11.5

59.5

27

9.6

7.7

95

1.3

30.7

49

0.1

30.0

70

24.8

61

24.7

9

Na2O

00

01.6

42

1.2

13.3

53.7

53.7

06

10.4

811.1

26

0.1

80.4

30.2

55

00

K2O

00

0.0

50.2

14

0.0

70.0

39

0.4

90.2

67

0.0

60.0

23

8.3

98.1

210.0

23

00.0

2

P2O

50

00

00

0.0

87

00

00

00

00

Sum

99.3

8101.1

4102.3

798.7

39

98.3

96.6

64

96.1

796.1

78

98.9

899.6

02

92.7

292.4

192.8

399.2

18

99.4

4

Gt

Am

ph

Fsp

Epi

Alm

57.8

6478

49.6

502

73.0

9239

Tota

lN

a(B

)0.4

58902

0.2

72383

0.4

77874

0.4

5666

0.4

59726

XN

a0.9

31

0.9

6287

XPs

0.0

48455

0.1

10168

Pyr

10.1

9155

18.6

1473

7.8

36086

Tota

lN

a(A

)0

0.0

55136

0.4

90684

0.6

34298

0.0

99031

XC

a0.0

65

0.0

3582

Grs

31.2

2557

27.5

3598

18.5

3465

Tota

lC

a1.5

41098

1.7

27617

1.5

22126

1.5

4334

1.5

40274

XK

0.0

03

0.0

0131


Table

2.

(Continued

)

Ass

ocia

tion

5=

Chl+

Phe

+Q

tz+

Pla

g+

Ca-a

mph

±Epi

±B

t±

Rt

±Sph

±Ilm

Chlo

rite

(28O

)P

hengit

e(2

2O

)Feld

spar

(6O

)C

a-A

mphib

ole

(23O

)B

ioti

te(2

2O

)

Sam

ple

adjG

t(2)

mtx

(2)

mtx

(6)

mtx

(3)

mtx

(2)

mtx

(1)

mtx

(2)

mtx

(1)

mtx

(4)

mtx

(9)

mtx

(1)

mtx

(2)

mtx

(2)

no.

40/48

23/30

40/127

38/124

38/124

75/146

75/146

38/124

23/30

23/30

38/124

75/146

23/30

SiO

226.2

11

26.5

31

25.0

28

26.3

22

37.1

29

51.3

666.9

666.6

67

64.2

13

49.7

19

45.8

51

37.1

737.4

44

TiO

20.0

22

0.0

08

0.1

15

0.0

72

1.6

23

0.3

90

00

0.1

67

0.4

24

0.9

61.5

46

Al 2

O3

21.2

22

21.2

69

21.6

28

21.2

35

15.6

88

26.6

820.3

420.5

61

23.0

45

7.0

83

13.9

35

16.5

916.8

54

Cr 2

O3

00

00.0

92

0.2

22

00.0

90

00.0

83

0.0

75

00

FeO

20.5

84

20.7

84

23.9

81

19.6

32

17.2

21

3.1

80.0

70

0.2

411.4

74

14.1

15

17.0

515.4

97

MnO

0.1

58

0.2

16

00.1

83

0.1

65

0.0

90

00

0.1

36

0.0

19

0.2

30.1

76

MgO

18.3

47

18.3

81

14.5

76

18.6

22

12.8

93

3.2

80

00.0

46

13.8

95

10.7

58

11.8

212.2

8

CaO

00

0.0

28

0.1

18

0.0

27

01.0

11.5

39

3.8

612.5

68.6

64

00.1

73

Na2O

00

0.0

57

0.0

19

0.0

70.4

211.7

310.5

33

9.8

25

0.5

81

2.7

04

0.1

20.1

08

K2O

0.0

52

0.0

27

0.0

43

09.4

05

9.2

30.1

30.0

51

0.0

35

0.1

12

0.4

23

8.6

49.0

17

P2O

50

00

0.0

44

0.0

66

00

00

00.0

53

00

Sum

98.1

23

98.8

28

96.6

14

86.3

39

94.5

09

95.0

2100.3

299.3

51

101.2

64

95.8

197.0

21

92.5

793.0

95

Fsp

Am

ph

XN

a0.9

4862

0.9

22571

0.8

19704

Tota

lN

a(B

)0.0

27364

0.6

634245

XC

a0.0

44719

0.0

7449

0.1

78325

Tota

lN

a(A

)0.1

37764

0.0

914386

XK

0.0

0666

0.0

02939

0.0

0197

Tota

lC

a1.9

72636

1.3

365755

Abbre

via

tions:

ingt

=in

clusi

on

inth

egarn

et,in

mtx

(c)

=co

nst

ituen

tofth

em

atr

ixm

iner

als

,G

t(C

)=

Garn

etco

re,G

t(R

)=

Garn

etri

m,G

t(5)

=G

arn

etdata

ofav

erage

5analy

ses

for

that

poin

t,sy

mp

=data

poin

tfr

om

sym

ple

ctit

icgro

wth

ofass

oci

ati

on

4.


at the Electron Microprobe Analyzer Laboratory,Geological Survey of India, Faridabad. The analy-ses were carried out with the effective beam diam-eter ∼1 μ. Standards used for calibrations includesynthetic MnTiO3 for Mn and Ti, albite for Si andNa, corundum for Al, magnetite for Fe, andraditefor Ca, orthoclase for K, apatite for P, chromitefor Cr and olivine for Mg. The raw data was cor-rected using inbuilt PAP (Pouchou and Pichoir1987) correction.

Table 2 lists the mineral data for the five mineralassociations containing all the phases represen-tative of that metamorphic stage in the P–T path.The amphiboles in eclogites are classified after Leakeet al. (1997) and Schumacher (2007) (figure 3a andb); Pyroxene classification is after Morimoto (1988)(figure 4a and b); Mica classification is after Deeret al. (1992) and Tischendorf (1997); Tischendorfet al. (2004) (figure 5). Additional data for eachassociation is supplied in supplementary data –table 1.

0.00

0.20

0.40

0.60

0.80

1.00

5 5.5 6 6.5 7 7.5 8

amph of association 1



Si

XN

a

0

0.4

0.8

1.2

1.6

2

0.4 0.9 1.4 1.9

Alv

i

Aliv



(a)

(b)

Figure 3. Plots showing variation in amphibole chemistryfor different mineral associations of eclogites as reported

in this study. (a) Si versus XNa; (b) Aliv versus Alvi

(amphibole has been abbreviated as amph).

Q%

Jd%

Jadeite Aegirine

Augite+AegirineOmphacite

Aeg%

Jd

Aeg Aug

Cpx as inclusion in Gt

Cpx as relict within amph

(a)

(b)

Figure 4. Plots showing chemistry of clinopyroxenes fromthe eclogites of the Tso Morari Crystalline Complex.(a) Classification of pyroxenes after Morimoto (1988). Allthe pyroxenes of the eclogites are omphacite. (b) A trian-gular plot showing variation in pyroxene chemistry whenpresent as inclusion in garnet and while occurring in matrixas relict in amphibole (Q% = Ca + Mg+Fe2+, Aeg% =aegerine%, Jd% = Jadeite%, Cpx = clinopyroxene, Aug =Augite).

3.2 Petrographic, mineral chemical attributes andassociated metamorphic reactions

Garnet, clinopyroxene, amphibole (Na-, Na-Caand Ca-rich), phengite, quartz/coesite, paragonite,biotite, epidote, clinozoisite, rutile, ilmenite, cal-cite, plagioclase, chlorite, titanite, apatite and


Figure 5. Composition of different types of mica miner-als present in the eclogites of the Tso Morari CrystallineComplex (after Tischendorf et al. 2004).

magnetite are the minerals identified in the fiveassociations present in eclogites from the TsoMorari area.

Garnet is the most dominant mineral in theeclogites. It contains a variety of inclusions in thecore and in the rim which are representative of dif-ferent stages of metamorphism. Generally, garnetis a solid solution of almandine, pyrope, grossularand spessartine with XFe = 0.36–0.75, XMg = 0.04–0.46, XCa = 0.13–0.38 and XMn = 0.00–0.13. Fol-lowing garnet composition, these eclogites belongto group ‘C’ (Coleman et al. 1965) (figure 6a). Allthe garnets are almandine rich but the ratio of theMg and Ca content of garnet grains varies accord-ing to different textural settings (figure 6b). Sowe refer Fe>Mg>Ca to relatively Mg-rich garnetand Fe>Ca>Mg refers to Ca-rich garnets. Otherdominant minerals are the amphiboles with vary-ing proportions of Na and Ca content and the Napyroxenes.

The five different associations identified can beclassified into three different metamorphic regimesnamely pre-UHP, UHP and post-UHP stages. Asso-ciation 1 has been established to be of pre-UHPregime, associations 2 and 3 lie in UHP regime andassociations 4 and 5 are representative of post-UHPregime.

3.2.1 Pre-UHP metamorphic regime: (Association1 – Na-Ca-amphibole + epidote ± paragonite ±

rutile ± magnetite)

The inclusion minerals namely sodic-calcic amphi-bole, paragonite, epidote, rutile and magnetite inthe garnet (figure 7a and b) constitute the firstpreserved prograde association of minerals in theeclogites.

The Na-Ca amphibole (mainly barroisite) of theassemblage contains total Na(+K) = 0.94–1.71

(a)

Pyr 30 mol%

Group A

Group B

Group C

Pyr 55 mol%

MgCa

Fe+Mn

(b)

Fe+Mn

Ca Mg

Gt with Cpx(UHP stage)Gt with Ca-amph(post-UHP stage)

Figure 6. Triangular plots showing composition of garnetfrom eclogites of the Tso Morari Crystalline Complex.(a) The eclogites belong to Group C eclogites based on gar-net chemistry (Coleman et al. 1965). (b) A triangular plotshowing variation in composition of garnets found in asso-ciation with pyroxenes (association 2) and Ca-amphiboles(association 4) (Gt: garnet, Pyr: pyrope, Cpx: clinopyroxene,Ca-amph: calcic amphibole).

with Na(B) = 0.64–1.55; Na(A) = 0.14–0.52 andtotal Ca = 0.45–1.36; Alvi = 0.83–1.51 and Aliv =0.48–2.01 (figure 3). The epidote contains Xps =0.11–0.22 where, Xps = Fe3+/(Fe3+ + Al2+) (Deeret al. 1992) and the Na content of paragonite grainsvary from 1.74 to 1.96 a.p.f.u. on the basis of22 oxygens (O) (table 2). Na-amphibole (glauco-phane) has also been observed from core portions ofzoned amphiboles which are present in the matrix.Comparison of the Aliv and Alvi content of thesetwo types of amphiboles (Na-amphibole and Na-Caamphibole) show that they represent a similar


amph

PargNa-Ca

CzoGt

Na-Caamph

Rt

Gt

EpiParg

Mag

(a)

(b)

Figure 7. (a) A back scattered electron (BSE) image of a garnet grain from the eclogite (sample no. TD 38/125) with aninclusion assemblage of Na-Ca-amphibole, clinozoisite and paragonite representative of the association 1. (b) A BSE imageof a garnet grain in an eclogite (sample no. TD 67/119) containing inclusions of paragonite, epidote, Na-Ca-amphibole,rutile and magnetite characteristic of association 1 (Gt: garnet, Czo: clinozoiste, Parg: paragonite, Na-Ca-amph: sodic calcicamphibole, Mag: magnesite, Rt: rutile, Epi: epidote).

pressure stability field and can be considered tobelong to the same metamorphic stage.

In the investigated rocks association 1 hasresulted from metamorphism of a mafic protolith.The preserved association is indicative of blueschistfacies.

3.2.2 UHP metamorphic regime: (Association 2 –Fe>Mg>Ca-garnet + omphacite + coesite/usually

quartz + phengite + rutile ± ilmenite)

The textural feature leading to the establishmentof association 2 is the garnet in garnet growth with

omphacite as inclusion within garnets. Here theinner garnet contains inclusion of omphacite alongwith amphibole and rutile. We have also observedquartz pseudomorphs after coesite associated withradiating fractures in the host garnet in differentsections of the same samples. Our identification istexture based and no Raman spectroscopy datais available with us. However, presence of coesitefrom these rocks is well established (Mukherjee andSachan 2001) and hence considering coesite in ourassemblage is justified (figure 8).

The inner garnets associated with omphacitesare Mg rich with XPy = 0.24–0.35 and XGr =


Coe psuedomorph

Gt

Omph

(a)

Omph

Na-Ca-amph

Phe

Rt

Gt

(c)

Inner GtOmph

Omph

Outer Gt

Outer Gt

Inner Gt

(b)

.

0.14–0.21 (figure 6b). There is a well defined pro-grade zoning in the inner garnet where XMg andXCa increase and XFe decreases from core to rim.The omphacite is Na rich with XJd of omphacite =

38.16–44.21 (figure 4b). The Si content of phengitevaries from 6.70 to 6.94 with Al content rangingfrom 4.1 to 4.5 a.p.f.u. (22O) (see table 2).

Association 2, which is the ultra high pressureassemblage, has resulted from association 1. Thefollowing reactions were possibly responsible forthe formation of association 2.

Epidote + Glaucophane = Garnet + Omphacite± Paragonite + Quartz + H2O

Quartz ↔ CoesiteTitanite + Epidote = Garnet + Rutile + Coesite

+ H2ONa-Ca amphibole = Garnet + Omphacite

From the textural evidence of two stages of garnetgrowth it is likely that fluids present in the systemreacted with the core garnet to form amphibole andphengite first. As the temperature rose garnet re-crystallization took place (second phase of garnet)from amphibole and phengite.

Fluid source in such a setting can be many.Engvik et al. (2000, 2001) pointed out that it couldbe the water coming off from the dehydrating sub-ducting slab which migrate up the subduction zoneand hydrates the system all over again. In manyterrains progressive dehydration and decarbona-tion reactions during prograde HP (high pressure)and UHP metamorphism leads to transformationof hydrous protolith to an anhydrous assemblage,but we do find hydrous minerals like phengite,amphibole, zoisite, etc., in HP and UHP rocks(Mottana et al. 1990; Peacock 1993; Poli andSchmidt 1995, 1997). Such minerals serve as impor-tant phase to deliver H2O in the subduction zone(Schmidt and Poli 1998; Fumagalli and Poli 2005).Conversion of glaucophane + clinozoisite = garnet +omphacite + H2O has been considered to demar-cate the transition of blueschist to eclogite faciesin many UHP metamorphic terrains (e.g., Ridley1984; Evans 1990). It has been experimentallyshown that a large amount of water can be releasedby this transition which can get stored as grainboundary fluid or as fluid inclusion (Liu et al.1996). The fluid inclusion data in Tso Morari

Figure 8. (a) A BSE image of the garnet grain with inclu-sions of omphacite and quartz pseudomorph after coesite.Radial cracks developed around the coesite inclusions as canbe seen in the image. (b) A BSE image of garnet in garnetgrowth texture in eclogite with two distinct rings of gar-nets. The core garnet with inclusion of omphacite is partof the association 2 (sample no. TD 63/101). (c) A BSEimage of the eclogite (sample no. TD 40/47) showing garnetgrain with inclusions of rutile, omphacite, phengite andNa-Ca-amphibole representative of association 2 (Gt: garnet,coe: coesite, oomph: omphacite, Phe: phengite, Rt: rutile,Na-Ca-amph: sodic calcic amphibole).

�


eclogite supports this idea (Mukherjee and Sachan2009).

Experimental studies show that amphibole canbe stable up to 22 to 30 kbar pressure depending onthe bulk chemistry of the rock (Schmidt and Poli1998). Infrared spectroscopy (IR) and transmis-sion electron microscopic (TEM) study has shownthat garnet and clino-pyroxene can host a lot ofwater as molecular water under these conditions(Su et al. 2002; Katayama and Nakashima 2003).Thus, anhydrous minerals too can be a transporterof water to the earth’s deeper level.

Another important factor regarding the source offluid in UHP environment is that the top and baseof the subducting slab may follow different thermalpath during subduction, which leads to redistribu-tion of fluid rather than its loss (Liu et al. 1996).The base of the crust being cooler than the topof the crust will not undergo much devolatilizationreactions.

3.2.3 Association 3 – Fe>Mg>Ca-garnet +Na-Ca amphibole + phengite ± paragonite ±

calcite ± ilmenite ± titanite

The outer garnet is separated from the inner gar-net by a rim of Na-Ca amphibole. At places theamphiboles are zoned with a Na-rich core and Na-Ca rich rim. In the matrix also an associationof Na-Ca amphiboles along with garnet, phengite,paragonite, calcite and rutile is present.

The outer garnet contains relatively less Mg andmore Ca than the inner garnet (XMg = 0.17–0.31and XCa = 0.16–0.25). The associated Na-Ca richamphiboles with total Na = 0.63–1.58; Na(B) =0.53–1.45; Na + K(A) = 0.01–0.51; Ca = 0.55–1.48;Aliv = 0.42–2.25 and Alvi = 0.5–1.42 mol fractiona.p.f.u (23O) and phengites have an Al content of4.1–4.5 a.p.f.u (22O) (table 2).

The most conspicuous changes in assemblage ofassociation 3 are related to the type of amphibolepresent (see figure 3). The Na-Ca amphibole whichis a part of this association is present in the matrixof eclogites (figure 9). These amphibole grains haveformed from clinopyroxene as is evidenced by the

Figure 9. (a) A BSE image of a zoned grain of amphi-bole with Na-rich core and Na-Ca rich rim (sample no. TD40/47). The Na-rich amphibole present in the core is anamphibole of earlier generation. (b) A BSE image of aneclogite sample showing relict Ca-pyroxene within a grainof Na-Ca amphibole in the matrix (sample no. TD 71/134).(c) A BSE image of a garnet grain in an eclogite withinclusion of relict clinopyroxene rimmed by Na-Ca amphi-bole (sample no. TD 40/47) (Na-Ca amph: sodic calcicamphibole, Cpx: clinopyroxene, Gt: garnet).

�

replacement texture. The compositional differencebetween the pyroxene away from amphibole andthose which are replaced by Na-Ca amphibole isdistinct. Probably initially all the pyroxenes wereomphacite as indicated by the preserved inclusions

(a)

Na-rich core

Na-Ca rich rim

(b)

Relict Cpx

Na-Ca amph

Na-Ca amph

Relict Cpx

(c)

Relict Cpx

Na-Ca amph

Host Gt

.


in the garnet (i.e., pyroxene of association 2). Whilethe pyroxene inclusions in the garnet retained theoriginal composition, those in the matrix got con-verted to Na-Ca amphibole. In the later case, mostof the Na composition in the pyroxene migrated toform amphibole (figure 4b).

This is well illustrated in sample no. TD40/47 where pyroxenes are preserved as bothinclusion and as relict in amphibole grain. Theinclusion pyroxene has composition (Na0.58Ca0.43)(Mg0.35Fe0.15Al0.50)(Si1.99)O6 representative of theoriginal pyroxene while the matrix pyroxenewhich is present as relict within the amphi-bole grain has lower Na and Fe[(Na0.53Ca0.44)(Mg0.38Fe0.11Al0.53)(Si1.99)O6]. The surroundingamphibole has a composition Na0.51(Na0.62Ca1.38)(Mg1.82Fe2+

1.73Fe3+0.36Al1.08) (Si6.55Al1.45)O22(OH) show-

ing transfer of the Na and Ca component (seetable 2 and supplementary data table 1).

3.2.4 Post-UHP metamorphic regime (Association4 –Fe>Ca>Mg-garnet+ Ca-amphibole +

plagioclase (An05) + biotite + epidote ± phengite)

A characteristic texture observed in the eclogitesis the atoll garnets (figure 10a). Here the coreis entirely occupied by Ca-amphibole, phengite,± epidote, ± biotite, ± albite (An5) while the rim ismade up of garnet. These minerals are also presentin symplectitic association around garnet grains.Assemblage 4 is essentially a replacement or sym-plectitic breakdown texture of UHP stage. Thereare zoned matrix amphiboles where the core partis Na-Ca amphibole and the rim is Ca-amphibole.The symplectitic growth of Ca-amphibole and pla-gioclase adjacent to garnet is also documented. Atplaces Ca-amphibole and plagioclase symplectitereplaces pyroxene grain. Biotite growth along thecleavage plane of phengite is seen in many samplesas well (figure 10b and c).

Amphiboles of this association has a Ca contentvarying between 1.52 and 1.73 and Na contentof 0.34–1.1 (see figure 3); garnet is also Ca-richwith XGr = 0.16–0.31 (see figure 6b); plagioclaseis albitic in composition with XAb = 0.90–0.98;

Figure 10. (a) A BSE image of a garnet grain, showingatoll texture characteristic of the association 4 in eclogite(sample no. TD 40/47). (b) A BSE image showing clinopy-roxene breakdown to Ca-amphibole and plagioclase whichoccurs as symplectitic intergrowth in eclogite (sample no.TD 40/126). (c) A BSE image showing symplectitic inter-growth of Ca-amphibole and plagioclase occurring adjacentto garnet in eclogite (sample no. TD 67/119) (Cpx: clinopy-roxene, Phe: phengite, Na-Ca amph: sodic-calcic amphibole,Plag: plagioclase, Gt: garnet).

�

biotite is Mg rich (XMg = 0.51–0.74, XFe = 0.30–0.50)and epidote has XPs = 0.05–0.18 (table 2). Thefollowing reactions have resulted in association 4

Cpx

Plag

Ca amph

Atoll Gt ring

Cpx

Na-Ca amph

Phe

(a)

(b)

Gt

Ca-amph(light colored)

Plag(dark colored)

(c)

.


(a)

(b)

(c)

Pseudomorph of Phe, Chl, Bt and Qtz after Gt

1000µm BSE 15.kV

Gt

Phe

Chl

Gt

1000µm BSE 15.kV

Gt Chl

Phe

Ca-amph

Chl

1000µm BSE 15.kV

196

172

147

122

98.0

73.5

49.0

24.5

.00

.from association 3 which is also a part of the highpressure assemblage.

�

Jadeite + Quartz = AlbiteGarnet + Omphacite = Ca-amphibole

+ Plagioclase

Phengite (high Si) = Phengite (low Si) + Feldspar+ Phlogopite/biotite+ Quartz

3.2.5 Association 5 –Chlorite + plagioclase(An05) + quartz + phengite + Ca-amphibole ±epidote ± biotite ± rutile ± titanite ± ilmenite

The last stage of metamorphism preserved in theeclogites is documented by the presence of tex-tures like skeletal garnets where garnet as replacedby chlorite, phengite and Ca-amphiboles; manyof the samples show chlorite pseudomorphs aftergarnet with association of biotite, quartz andplagioclase. Locally highly fractured garnets withfracture filling of chlorite and biotite are alsopresent (figure 11).

Chlorites of this association are Mg rich (Mg =4.67–5.85 and Fe = 3.25–4.31). Phengite containsSi = 5.65–6.86 and K = 1.55–1.82 p.f.u. (22O);Feldspar is albitic in composition with XNa = 0.92–0.98; biotite is Mg rich with (XMg = 0.50–0.60 andXFe = 0.41–0.47) and amphibole present in somesamples are Ca rich with total Ca of 1.34–1.73 p.f.u.(23O) (table 2). Breakdown of minerals in associa-tion 4 through following reaction is inferred for theassociation 5.

Plagioclase + Ca-amphibole= Epidote + Chlorite + Quartz

4. P–T estimates for the eclogites

For temperature estimation, Fe2+–Mg2+ exchangegeothermometry between coexisting garnet-clinopyroxene, garnet-phengite and garnet-biotitemineral pairs has been employed. Apart fromthis, a number of experimental studies such asQuartz ↔ Coesite transition reaction, Si contentin phengite, Albite = Jadeite + Quartz; Titanite +Epidote = Garnet + Rutile + Clinopyroxene +Coesite/Quartz + H2O reaction and conversion ofclinopyroxene to Na-Ca amphibole have been usedto deduce the pressure and temperature limits ofthe associated minerals. Besides the conventional

Figure 11. (a) A BSE image of pseudomorph of a phengite,chlorite, biotite and quartz after garnet in eclogite (sampleno. TD 38/124) representing Association 5. Pseudomorphoutline has been highlighted in the image using a boundaryline. (b) A BSE image showing relict garnet replaced bychlorite, phengite and quartz in eclogite (sample no. PF-36/04). (c) A BSE image of embayed and fractured garnetgrains in the matrix of chlorite, Ca-amphibole, phengite andquartz in eclogites (sample no. TD 76/149) (Phe: phengite,Chl: chlorite, Bt: biotite, Qtz: quartz, Gt: garnet, Ca: amph:calcic amphibole).


thermo-barometers, THERMOCALC has alsobeen used to determine the P–T conditions forvarious mineral associations established by pet-rographic and mineral chemistry data (table 3).Henceforth, mineral name and abbreviationswill be used in tables and figures: Albite (Ab),Aegerine (Aeg), Augite (Aug), Almandine (Alm),Amphibole (Amph), Biotite (Bt), Calcite (Cal),Chlorite (Chl), Clinopyroxene (Cpx), Clinozoisite(Cz), Coesite (Coe), Epidote (Epi), Garnet (Gt),Glaucophane (Glau), Grossular (Gr), Ilmenite(Ilm), Jadeite (Jd), Magnetite (Mag), Omphacite(Omph), Paragonite (Parg), Phengite (Phe), Pla-gioclase (Plag), Pyrope (Py), Pyroxene (Pyx),Quartz (Qtz), Rutile (Ru), Titanite (Ttn).

For the association 1 of Na-Ca amphibole +epidote/clinozoisite ± paragonite pressure of 11 kbarand temperature of 500◦C has been establishedbased on the stability limits of the minerals (Evans1990). Using the garnet-clinopyroxene core assem-blage in the garnet in association 2, the averagetemperature has been estimated to be ∼580◦C at27 kbar (Ellis and Green 1979). Garnet-phengitethermometry has also been used for the temper-ature estimate of 637◦C at 27 kbar (Green andHellman 1982). A minimum pressure limit of

27 kbar for this association is indicated from thequartz to coesite transition reaction (Bohlen andBoettcher 1982).

To estimate the peak metamorphic conditionsattained by association 2 we have used the reac-tion Titanite + Epidote ↔ Garnet + Rutile +Coesite + H2O (Manning and Bohlen 1991) for theend-member reaction in CASTH system. Applica-tion of the garnet-clinopyroxene thermometer ofEllis and Green (1979) and the reaction aboveyielded a well constrained estimate of 834 ± 12◦Cand ∼33 kbar.

However, the Manning and Bohlen (1991) curveis for end member compositions and it cannotbe used without correction for solid solutions. Thelocus of this reaction in rocks is not univariantbut depends critically on grossular content in gar-net and clinozoisite content in epidote. Using theterm epidote rather than clinozoisite implies Fe3+,and the reaction if balanced with epidote wouldthen depend on fO2, the Fe3+# of epidote and theFe2+# of garnet (Donohue and Essene 2000). Pageet al. (2003, 2007) applied the quartz equivalent ofthe reaction for eclogites from North Carolina andCalifornia. Their calculations for solid solutionsin titanite, epidote and garnet with Holland and

Table 3. Thermobarometric data for the eclogites of the Tso Morari Crystalline Complex.

Metamorphic Minerals usedThermocalc

stages for calculations T (◦C) P (kbar) T (◦C) P (kbar)

Pre-UHP Association 1

Glu ± Epi/czo ± pg 515 8.5

UHP Association 2

Gt-pyx (in core of Gt) 555(±45)(EG) 27(BB) 760 39.5

20±3(WM)

33(MB)

Gt-Pyx (in rim of Gt) 837(±11)(EG)

27(BB)

Gt-Phe (in core of Gt) 637(±24)(GH)

Association 3

Gt-Phe 689(±17)(GH) 28(FH) 810 25

(Gt rim and adj Phe) 700(FH) 28(FH)

Post-UHP Association 4

Gt-Bt (Gt rim and adj Bt) 796(±50)(FS) 14(H) 840 14.4

Association 5

Gt-Phe (gt-rim and adj Phe) 473(±13)(GH) 8(MS) 425 6.5

Gt-Bt (Gt rim and adj Bt) 370(±25)(FS) 8(MS)

T (◦C) – temperature in degree celcius; P (kbar) – pressure in kilobars. Formulations used for calculations are: EG =Ellis and Green (1979); GH = Green and Hellman (1982); FR = Forneris and Holloway (2004); FS = Ferry and Spear(1978); BB = Bohlen and Boettcher (1982); MB = Manning and Bohlen (1991); H = Holland (1980); MS = Massonneand Schreyer (1987); WM = Waters and Martin (1993). Mineral abbreviations used are: Glu: glaucophane, Epi: epidote,Czo: clinozoisite, Parg: paragonite, Gt: garnet, Pyx: pyroxene, Coe: coesite, Qtz: quartz, Phe: phengite, Cal: calcite,Na-Ca amph: sodic calcic amphibole, Rt: rutile, Bt: biotite. Thermocalc window version 3.21, based on internally consis-tent thermodynamic dataset of Holland and Powell (1998) was used. Mineral activities were calculated by A-X Program ofHolland and Powell (1998). All the possible reactions between given end member phases with P–T values were calculatedfor the Na2O-CaO-K2O-FeO-MgO-Al2O3-SiO2-H2O system.


Powell (1998) data set show a strongly back-bending reaction that does not cross into thecoesite field for those compositions.

Manon et al. (2008) recalculated this reactionand placed it above 45 kbar for temperatures esti-mated by us for association 2. They measuredCp (heat capacity) of titanite and found that theV(P,T) data for titanite in Holland and Powell(1998) fit experimental data much better than thedata of Berman (1988). Manon et al. (2008) recom-mended using Holland and Powell (1998) datasetfor titanite-bearing equilibria and showed that thelocus of the reaction is shifted upward by 1 kbarrelative to Page et al. (2003, 2007). Tropper andManning (2008), pointed out that Al can also sub-stitute Ti in titanite which needs to be consideredfor geobarometric calculations involving reactiondiscussed above. In our study, in the presence ofrutile and epidote, we can at least use end-memberclinozoisite and titanite to get a lower P limit forour association 2.

Pressure calculation using Thermocalc based onHolland and Powell (1998), data set for the associa-tion 2 (Garnet + omphacite + phengite + rutile +coesite/quartz) has yielded a pressure of 39.5 kbar.Thus, conservatively we estimate pressures exceed-ing 33 kbar for association 2.

For the association 3 an average P–T ∼ 28 kbarand 740◦C has been obtained on the basis of con-version of omphacite to Na-Ca amphibole based onthe experimental study of Forneris and Holloway(2004). The average temperature estimation forassociation 4 is calculated on the basis of garnet-biotite KD thermometry as ∼800◦C (Ferry andSpear 1978). Average pressure calculation is doneby using Jd (jadeite) barometry given by Holland(1980) as ∼14 kbar. The P–T estimate for the asso-ciation 5 have been determined using garnet-biotiteand garnet-phengite thermometry and phengitebarometry as ∼370◦C and ∼7 kbar (Ferry andSpear 1978 and Green and Hellman 1982).

5. Evolution of the P–T path

In discussing the P–T path for the Tso Morarieclogites the time markers are based on the work ofde Sigoyer et al. (2000, 2004), Kaneko et al. (2003),Leech et al. (2003, 2005) and Schlup et al. (2003).The reconstruction of P–T path implies explainingthe development of above-described five mineralassociations in the Tso Morari eclogites.

The mineral association 1 has a P–T estimateof about 11 kbar and 500◦C. In the context ofthe tectonic setting of the Tso Morari area we believethat initiation of metamorphism of this UHP ter-rain is coincident or began soon after the collisionof the Indian plate with the Asian plate at ∼57 Ma.

This event is well constrained by geologic, pale-ontological and paleomagnetic records (Klootjwiket al. 1992; Garzanti et al. 1996; Rowley 1996;Leech et al. 2005). With the initiation of subduc-tion of the Indian plate, the mafic protolith under-went a burial to a depth of about 30 km. Thisresulted in sudden rise in pressure with relativelyless rise in temperature and the protolith experi-enced a blueschist facies regime of metamorphismresulting in mineral association 1 (figure 12).

Further burial to a depth of up to ∼130 kmraised the temperature and pressure regime toabout 750◦C and 33 kbar leading to the transfor-mation of blueschist facies assemblage to eclogiticassemblage of association 2. This ultra high pres-sure rocks possibly formed at about 53 Ma on thebasis of U–Pb SHRIMP dating of zircon (Leechet al. 2005), and 53±0.2 Ma 39Ar/40Ar age ofphengite (Schlup et al. 2003).

The mineral association 3 of Fe>Mg>Ca–garnet-Na-Ca amphibole – phengite ± paragonite± carbonate ± ilmenite is reported for the firsttime in the P–T evolution path of the Tso Morarieclogites. The P–T range for this transition is∼ 28 kbar and 780◦C. Hence, association 3 isalso in the UHP regime that represents decreasingpressure suggesting a phase of uplift.

There is evidence of fluid activity in the eclog-ites from this region as is reflected in the con-tinuous presence of phengite, amphiboles andcalcite in equilibrium with association 3. Associ-ation 3 was followed by association 4 consistingof Fe>Ca>Mg-garnet–Ca-amphibole – plagioclase(Ab95) – biotite – clinozoisite/epidote ± phengite.The average P–T of formation for this particularmineral association is estimated to be 14 kbar and800◦C at a depth of about 45 km. This is the timewhen slab moved up at T ∼ 47 Ma, marking theinitiation of retrograde phase of metamorphism.During this exhumation, these rocks underwentslight increase in temperature. This could be aresult of slab break-off or dumping of graniticmaterial with radioactive elements on top of therock-pile (formation of the Himalayan nappes) assuggested by some researchers (de Sigoyer et al.2004; Leech et al. 2005). The later decompressionepisode as recorded by the mineral association 5has pressure–temperature regime of ∼7 kbar and400◦C corresponding to an uplift to a depth of30 km at about 29 Ma.

The initial burial rate along the P–T slopechanged the confining P from 11 to 33 kbar (ca. 35to 130 km, transition from association 1 to 2)in 57 to 53 = 4 Ma (rate = 100 km/4 Ma =2.4 cm yr−1). Further, after the formation of assem-blage 3, the decompression of the Tso Morari rockstook place from 33 to 14 kbar from a depth of 130to 45 km (for association 3 to 4 transition) took


53–47 = 6 Ma (rate of 85 km/6 Ma = 1.4 cm yr−1).The exhumation of the eclogitic unit from 45 to28 km occurred over a time interval of 47–29 =18 Ma, suggesting a linear rate of 17 km/18 Ma ∼1 cm/yr−1.

The significance of the present study is that aprograde sequence is indicated through the epi-dote blueschist facies regime that sheds light on thenature of the burial. Our findings show a highervalue of pressure being attained by rocks of theTso Morari area. The retrograde path is also shownto have attained a higher temperature and is wellconstrained with respect to mineral assemblages,reactions and the pressure temperature data.

The coesite bearing assemblage from the Kaghaneclogite yielded an age of 46.4 ± 0.1 Ma (Parrishet al. 2006). Zircons from the host rock of the samearea also yielded almost identical ages of 46.2 ±

0.7 Ma (Kaneko et al. 2003; Parrish et al. 2006).There is a difference of about 8 to 9 Ma in theage of UHP metamorphic assemblage from the TsoMorari and that of Kaghan Valley samples.

5.1 Implications on the geodynamics of the region

There have been propositions that the protolithfor the metamafic rocks of the Tso Morari ter-rain is the Zildat Ophiolitic Melange as both haveconsanguineous trace elements and Nd isotopicdata (Ahmad et al. 1996, 2006 and UnpublishedPhD thesis, Preeti Singh 2008). Although the exacttiming of the initiation of subduction leading tothe formation of this melange suit is lacking, paleo-magnetic, geochronological and stratigraphic datapoint to 65–57 Ma as its beginning (Garzanti et al.1987; Klootwijk et al. 1992; Rowley 1996). This

Temp( C)°

P(k

bar)

Dep

th(k

m)

350 550 750

29 0.4Ma±

57 1Ma±

53 0.7Ma±

47 Ma±6

Gt+Rt+Coe+H O2

Gt+Rt+Coe+H O2

Ttn+EpiTtn+Epi

CoeCoe

QtzQtz

AlbAlb

Pause ?

Jd+QtzJd+Qtz

7

12

17

22

27

32

950

Gt+Pyx

Gt+Pyx

Gt+Om

ph

Gt+Om

ph

Glau+E

pi

Glau+E

pi

35

75

125

2.5c

m/y

r

Ca-amph + Plag

Ca-amph + Plag

1

2

3

4

5

1.4cm/yr

Gui

llot e

t al.,

(200

8)

Si

)g

004oye

al (2r et

Figure 12. The proposed P–T path for the eclogites of the Tso Morari Crystalline Complex. Rectangles with numbers markeach mineral association. Errors in estimation of pressure and temperature conditions for each association have been markedalong with the rectangular boxes. The P–T path has been marked in dark coloured solid line passing through each rectangle.The subduction rate calculated for the proposed P–T path has been shown in dotted lines capped with an arrow markalong with the P–T path drawn. P–T paths proposed by previous researchers have been shown alongside for comparison.The age quoted for each association is, viz., for Association 1: Rowley (1996), Garzanti et al. (1996), Klootwijk et al.(1992); Association 2: Leech et al. (2005), Schlup et al. (2003); Association 4: Schlup et al. (2003); Association 5: de Sigoyeret al. (2004). Crucial reactions used for establishing the P–T evolution path of the Tso Morari Crystalline Complex havealso been marked alongside [Glaucophane(Glau) + Clinozoisite(Czo) ↔ Garnet(Gt) + Omphacite(Omph) (Evans 1990),Quatrz(Qtz) ↔ Coesite(Coe) (Bohlen and Boetcher 1982), Titanite(Ttn) + Epidote(Epi) ↔ Garnet(Gt) + Rutile(Rt) +Coesite(coe)/Quartz(Qtz) + H2O (Manning and Bohlen 1991), Jadeite(Jd) + Quartz(Qtz) ↔ Albite(Ab), (Newton andSmith 1966; Holland 1980), Garnet + Clinopyroxene ↔ Amphibole + Albite(Plg) (Holland and Powell 1990)]. Reactantmineral phases are being marked on one side of the reaction curve and the other side shows product phases.


suggests that the subduction of Indian plate andthe obduction of the melange may have initiatedclose in time. It is possible that some of these maficrocks, now the eclogites were emplaced within thesubducting continental Indian plate at the time offormation of the melange.

The depth to which a subducting slab wouldtravel has bearing on the pressure–temperature–time evolution path of the rocks associated with theslab and is invariably a function of the subduc-tion angle. Many studies on calculation of sub-duction angle for the Indian plates exist and arebased on trigonometric calculations taking intoaccount convergence rate and geochronology data.Our findings have established a prograde path andhence a precise subduction angle can be calculated.If we consider the rate of change of convergencefrom 18 to 10 cm−1yr at the time of initiation ofthe subduction (Klootwijk et al. 1992; Epard andSteck 2008), the subduction angle would be ∼10◦

[subduction angle=(tan−1(depth/distance)]; wheretotal depth is assumed to be the distance fromabout 35 km to the depth of maximum UHP, i.e.,130 km.

It is quite likely that the subducting Indianplate along with the overlying sediments experi-enced rigidity up to a depth of about 20 km duringsubduction (e.g., Bilek and Lay 1999). Experimen-tal structural modeling work by Chemenda et al.(2000), has shown that it generates numerous steepdipping faults in such kind of subduction setting.The Indian plate on its downward journey mighthave developed a number of steep brittle faults atdifferent levels. As a result, the downthrown blocksmust have sunk deeper than rest of the downgoinglithospheric slab. As and when the fault-boundedblocks reached the deeper higher temperature duc-tile environment, the density difference betweenthe dense slab and that of the mantle would havedecreased progressively resulting in slowing downof the downward travel of the slab.

During the course of downward movement, min-eralogical constituents of the slab also underwentphase changes from lighter to denser structuresfacilitating the break off of the sinking slab (Vander Voo et al. 1999). This could have initiated ahalt in subduction after an initial phase of slow-ing down the downward movement. While the con-vergence of the Indian and Asian plates was stillcontinuing, parts of the subducting plate may havebeen squeezed up. Contemporaneously the upperparts of the subducting plate may have failed andwere squeezed back up the tapered angle of theinterface. The subduction channel now became eas-ier pathway to exhume the buoyant rocks underthe action of fluids and associated serpentinisation.The exhumation would continue and the depth atwhich this would stop depends on the strength of

the lithosphere as well as the depth to which itsank.

The process of subduction and exhumation ofthe Tso Morari region till the slab came to a haltmay have taken ∼10 Ma. However, the durationof the pause in the exhumation path of the slabis not certain. It could be as much as 10 Mabecause the earliest medium grade metamorphismas documented by presence of kyanite-staurolite inthe Central Crystallines of the Himalayas is dur-ing the Oligocene, i.e., ∼37–29 Ma (Vance andHarris 1999). We suggest that the exhumation ofthe Central Crystallines and the final exhumationof the Tso Morari Complex (including eclogitesof all levels) took place simultaneously, i.e., postMid-Miocene (Liou et al. 2004).

Exhumation of plates may be caused by ero-sion and isostatic adjustments, extensional collapse(DeCelles et al. 2002), or underplating accompa-nying corner flow (de Sigoyer et al. 2004). How-ever, the erosional rates calculated by previousresearchers for the Tso Morari region are too highto be accounted for by the isostatic adjustment.The case of underplating would require entireIndian Plate to go down to great depths with con-sequent widespread UHP metamorphism. We pro-pose that the downgoing segments used the alreadyavailable shear zones to flow back up to ca 45 kmat current plate tectonic (seafloor spreading) veloc-ities presumably as part of one continuous episodeduring the subduction of the Indian Plate underthe Asian Plate (Thompson et al. 1997). The pro-posed mechanism thus accounts for the short dura-tion of the ultra high pressure metamorphism aswell as its restrictive outcrop nature and the fieldsettings.

New findings presented in this paper have beenable to sharpen our concepts regarding the evo-lution of UHP rocks. The basic process of gen-erating them through subduction and subsequentexhumation was one single process. In this lightit is easier to explain why only restricted areaswithin the Tso Morari Complex have UHP assem-blages. Incorporation of mafic protoliths fromthe ophiolites occurred at different levels; onlythose parts which crossed through a depth equiv-alent to the 27 kbar pressure attained garnet-clinopyroxene-phengite-rutile assemblage and restshow only up to Ca-amphibole-plagioclase-biotite-epidote assemblage attained when these rocks werecoupled with rest of the Himalayan crust.

Acknowledgements

Preeti Singh acknowledges the JRF she obtainedfrom UGC, India and Mr A Kundu and Mrs S Joshifor probe work at GSI Faridabad. Prof. T Ahmad


is acknowledged for his help during field session in2004 and suggestions at various levels.

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MS received 26 October 2012; revised 26 December 2012; accepted 27 December 2012

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Insights into the P–T evolution path of Tso Morari