multiple tectonometamorphic imprints in the lower crust: first evidence of ca. 950 ma (zircon u-pb...

Multiple tectonometamorphic imprints in the lower crust: rst evidenceof ca. 950Ma (zircon U-Pb SHRIMP) compressional reworking of UHT

aluminous granulites from the Eastern Ghats Belt, India

KAUSHIK DAS 1*, SANKAR BOSE 2, SUBRATA KARMAKAR 3,DANIEL J. DUNKLEY 4 and SOMNATH DASGUPTA5

1Department of Geology, Bengal Engineering and Science University, Howrah, India2Department of Geology, Presidency College, Kolkata, India

3Department of Geological Sciences, Jadavpur University, Kolkata, India4National Institute of Polar Research, Tokyo, Japan

5Indian Institute of Science Education and Research, Kolkata, India

Integrated structural, petrological and geochronological study on a suite of granulites from the central part of the Eastern Ghats Belt (EGB),India unveils polyphase tectonothermal evolution. We document (a) M1 ultrahigh temperature (UHT) metamorphism (10008C at6.58.5 kbar) on an anticlockwise PT trajectory simultaneously with early deformations D1D2 involving partial melting, (b) coolingdown to 8008C, 6 kbar that produced a variety of coronae/symplectites (M1R), (c) an unrelated compressional orogeny (D3) that produceddeep crustal shears and mylonitic foliation (S3m) at low angles to D1D2 structures and was associated with slight loading, and possible partialmelt extraction under granulite facies condition (M2 7 kbar, 8508C), and (d) localized retrogression (M3) in the presence of meltaccompanying D4 deformation. This is the rst record of the prograde PT path of the superimposed granulite facies metamorphism in theEGB. U-Pb SHRIMP data of zircon preserves an inherited grain domain of ca. 1700Ma (207Pb-206Pb age) that traces back the history of EGBwith a lineage of the Mesoproterozoic supercontinent, Columbia. The UHT metamorphosed (peakM1 at ca. 1000Ma) and subsequentlycooled crustal segment (M1R) was subjected to strong tectonothermal reworking (M2) along a clockwise PT path at 953 6Ma (concordiaage) that partially exhumed the rocks to mid-crustal levels. A later uid-induced retrogressive event vis-a`-vismelt crystallization occurred atca. 900Ma (207Pb-206Pb age). The post-peak evolution reveals striking similarities with those recorded in the rocks of the Rayner Complex ofeast Antarctica, thereby strengthening the notion of Indo-Antarctic correlation as part of Rodinia. Copyright# 2010 JohnWiley & Sons, Ltd.

Received 27 November 2009; accepted 29 March 2010

KEY WORDS deep crustal reworking; Eastern Ghats Belt; India; UHT aluminous granulites; zircon U-Pb SHRIMP ages

Supporting information (supplementary tables S2-S6) may be found in the online version of this paper.

1. INTRODUCTION

Many of the ultrahigh temperature (UHT, > 9008C, Harley,1998; Kelsey, 2008) metamorphosed terrains record a nearly

isobaric cooling retrograde PT trajectory (reviewed in

Harley, 1989, 1998, 2004; see also Santosh et al., 2007;

Santosh and Kusky, 2010; Liu et al., 2011). Two prominent

examples include the Archaean Napier Complex in east

Antarctica (Harley and Hensen, 1990; Harley, 2003) and the

Proterozoic Eastern Ghats Belt in India (Dasgupta and

Sengupta, 2003). Exhumation of these isobarically cooled

terranes occurred in response to unrelated later orogenic

events. In case of the Napier Complex, the Rayner orogeny

of broadly ca. 940910Ma age reworked the early

granulites, records of which are found in marginal shear

zones (Harley and Hensen, 1990; Kelly et al., 2000,

references therein). Limited radiometric dates on the Eastern

Ghats rocks show overprinting by ca. 1000950Ma orogeny

on the early UHT granulites during a second granulite

facies metamorphism characterized by near isothermal

decompression during retrogression (Grew and Manton,

1986; Shaw et al., 1997; Mezger and Cosca, 1999;

Dasgupta et al., 1994, 1995; Dasgupta and Sengupta,

2003). However, the prograde path of the second granulite

GEOLOGICAL JOURNAL

Geol. J. 46: 217239 (2011)

Published online 24 June 2010 in Wiley Online Library

(wileyonlinelibrary.com) DOI: 10.1002/gj.1246

*Correspondence to: K. Das, Department of Geology, Bengal Engineeringand Science University, Howrah 711103, India.E-mail: [email protected]

Copyright # 2010 John Wiley & Sons, Ltd.

facies metamorphism in the Eastern Ghats and the nature of

deformation associated with re-working of granulites remain

undetermined. Knowledge on these aspects is a pre-requisite

in understanding the way the isobarically cooled lower crust

reacted in response to the later orogeny.

Recent studies reveal that the Eastern Ghats Belt (EGB) is

a collage of discrete crustal provinces with characteristic

petrological, structural and isotopic histories (Rickers et al.,

2001; Dobmeier and Raith, 2003; Bose and Das, 2007;

Chetty, in press). The present study is concentrated in the

central part of the Eastern Ghats Province (Dobmeier and

Raith, 2003) or Domain 2 (Rickers et al., 2001), where

metamorphic, magmatic and isotopic signatures are well

constrained (reviewed in Dasgupta and Sengupta, 2003;

Shaw et al., 1997; Mezger and Cosca, 1999; Simmat and

Raith, 2008, Bose et al., 2008). We present structural,

petrological and geochronological data on aluminous

granulites to constrain multiple structural and metamorphic

events. Our particular emphasis here is to characterize the

structural and petrological reworking of the early UHT

granulites of this belt.

2. GEOLOGICAL BACKGROUND

The EGB represents a Proterozoic orogenic belt that extends

over 1000 km from the Brahmani river valley in Orissa to

southeastern parts of Andhra Pradesh along the eastern coast

of the Indian peninsula (Figure 1). The tectonometamorphic

events in this regional terrain are distinct from those in the

Figure 1. Geological map of the study area (Panirangini) and adjoining areas. Note the isolated occurrences of aluminous granulites within leptynite andenderbite. The inset map shows the location of the study area (Panirangini) in the broad geological framework of the Eastern Ghats Belt of India.

Copyright # 2010 John Wiley & Sons, Ltd. Geol. J. 46: 217239 (2011)

218 k. das ET AL.

surrounding cratonic zones and at the same time, there are

differences in evolutionary histories in different domains

inside the belt ranging in age from at least Meso- to

Neoproterozoic time. With all the complexity in the growth

history, this belt stands out to be crucial for reconstruction of

Proterozoic East Gondwana (Yoshida, 1995). Moreover,

recent studies have shown that the EGB preserves the

evidence for regional-scale anomalous extreme thermal

condition of metamorphism in excess of 10008C at lowercrustal pressure of 810 kbar (Dasgupta and Sengupta, 2003;

Harley, 2003). Such extreme metamorphic conditions are

inferred from reaction textures in diverse mineral assem-

blages of aluminous granulites and calc silicate granulites

exposed in different parts of this terrain (Bhowmik et al.,

1995; Bose et al., 2006, references therein).

The present study area is near Panirangini (N1881826, E8285428) in the Araku Valley in the Visakhapatnamdistrict of Andhra Pradesh, India (Figure 1). The major

lithological units in Panirangini and its adjoining

area are khondalite (garnet-sillimanite-quartz-perthite gneiss),

leptynite (plagioclase-garnet-perthite-quartz gneiss), enderbite

(orthopyroxene-plagioclase-perthite-quartz garnet gneiss),mac granulite (orthopyroxene-clinopyroxene-plagioclase-

garnet gneiss), calc-silicate granulite (scapolite-wollastonite-

garnet-plagioclase-quartz gneiss) and aluminous granulite

(sapphirine-cordierite-spinel-garnet-orthopyroxene-quartz-silli-

manite gneiss). The major part of the area is dominated by

khondalite. The unit is conformable with the associated

enderbite and leptynite. Aluminous granulite occurs as small

pods within leptynite and enderbite. Mac granulite occurs as

thick bands and/or lenses within leptynite, calc-silicate granulite

and enderbite. The gneissic foliation in leptynite is demarcated

by alternating garnet-biotite rich and quartzofeldspathic layers.

The general trend of this foliation is NNE that has been

deformed by later events (discussed later) giving rise to regional-

scale fold interference (Figure 2). The aluminous granulite near

Panirangini occurs as small podswithin sheared leptynitic gneiss

and shows two structural variations, (1)massive and the other (2)

foliated. The foliated variety shows alternating layers of quartzo-

feldspathic and mac-rich (orthopyroxene garnet) domain.This gneissic foliation in aluminous granulite and the associated

leptynitic gneiss is subvertical. Centimetre-thick quartz veins

show down-dip stretching lineation (rodding) on the foliation

plane. The massive variety is coarse-grained containing

orthopyroxene cordierite aggregate and occurs in the sameoutcrop as the foliated type. Two generations of quartzo-

feldspathic veins occur within the leptynitic gneiss and those lie

at low angle to the general foliation. Foliation-parallel elongated

Figure 2. Strike map of major foliation planes in the study area showing regional folding and other variation in different places. Note the presence of shear zoneand associated foliation near the study area.


multiple tectonometamorphic imprints in the lower crust, egb, india 219

bodies of mac granulite occur within leptynite close to the

aluminous granulite exposed at Panirangini.

3. STRUCTURAL RELATIONSHIPS

The rocks of the study area were affected by four episodes of

deformation. Superposed deformation and concomitant

metamorphic imprints have also been identied from the

nearby Borra Caves area (Bhowmik, 1997). The earliest

recognizable planar fabric in mesoscopic scale is identied

in khondalite and leptynite. This fabric is dened by layers

of ferromagnesian minerals and quartz-feldspar that could

represent modied compositional banding or a transposed

gneissic fabric on compositional banding during the

earliest D1 deformation and is termed henceforth as S1.

3.1. D2 deformation

D2 caused transposition of S1 and parallelism of S2 that is the

dominant form surface in the area. Due to intense later

deformations, folds produced on S1 are rarely preserved as

rootless, isoclinal to tight intrafolial folds (F2) (Figure 3a)

in local low-strain domains. The dominant transposed S2gneissic fabric is dened by alternating ferromagnesian

Figure 3. (a) Rootless, isoclinal, intrafolial folds (F1) on compositional banding (S1) in quartzo-feldspathic gneiss. Note the feature in the centre of the photobelow the pencil. (b) Upright, plunging fold on gneissic fabric in quartzo-feldspathic gneiss. The black arrow shows the folded limb on plan, while the whitearrow is that on the transverse surface. (c) Microscopic fractures developed during D4 that healed up with biotite. (d) Crystal plastic deformation is evident inleptynites and aluminous granulites, undulose extinction in quartz and deformation twin lamellae in alkali feldspar (Kfs) clasts. (e) Porphyroblastic garnet (Grt)containing inclusion of biotite (Bt) and quartz (Qtz) in Association A. This garnet is overgrown by a dusty intergrowth of garnet, sillimanite and quartz(Grt SilQtz). (f) Spinel (Spl) and quartz (Qtz) inclusions in coarse garnet (Grt) that is also overgrown by the intergrowth of garnet, sillimanite, quartz and/orbiotite (GrtSilQtzBt). (g) A complex intergrowth of garnet (Grt), spinel (Spl) and ilmenite (Ilm) growing at the fringe of porphyroblastic garnet (Grt)associated with matrix quartz (Qtz). Note the presence of biotite (Bt) patches along the fractures of garnet. (h) BSE image showing a spongy intergrowth of

garnet (Grt), quartz (Qtz) and sillimanite (Sil) on cordierite (Crd).


220 k. das ET AL.

(garnetiferous in favourable compositional bands) and

quartzo-feldspathic layers.

3.2. D3 deformation

D3 deformation formed small-scale shear zones accom-

panying regional-scale folding F3 on the dominant form

surface S2. F3 folds are NESW trending open, upright,

gently plunging on S2 and are pervasive to all the lithological

units of the area on regional- (Figure 2) and on mesoscopic-

scale (Figure 3b). Repetition of transposition produced a

composite S2/S3 gneissic fabric during this event. The S3 is

only identied at the low-strain zone, i.e. hinges of small-

scale parasitic folds of F3. Garnetiferous leucocratic layers,

presumably produced by partial melting (discussed later),

participated in F3 folding. D3 therefore postdates partial

melting. The geometries of F2 and F3 folds indicate that the

fold axes are at acute angles.

Localization of shear bands is conspicuous in leptynite that

overprints the dominant S2 gneissosity. Shearing produced a

CS fabric in this rock where C-surfaces (trending NESW)

are at acute angles with the pervasive S2 fabric showing a

sinistral sense on horizontal section. Apart from C-surfaces,

subsidiary shear bands develop at an angle that affects the

rock and drags the gneissic foliation resulting in small folds.

The F3 fold axes, developed on the transposed S2 with S3, are

accentuated producing sheath folds and are identied as eyed

folds on outcrop. Locally, a steeply dipping (608 westerly tosubvertical) mylonitic foliation (S3m) striking NESW is

developed in leptynite (Figure 2). The exposed foliation

planes have down-dip stretching lineation. The geometry of

folds and shear zone features illustrate bulk compression

along SENW that produced the F3 folds and small-scale

deep crustal compressional shearing. The horizontal com-

ponent of the oblique down-dip shear stress shows a sinistral

sense, i.e. the eastern part moved northeasterly and western

part moved southwesterly. Such deep crustal compressional

local shear planes characteristically have a steep dip (Ramsay,

1979).

3.3. D4 deformation

D4 deformation produced cross folding (F4) on F3 axial trace

(Figure 2). Regionally F3 folds are intersected by a NWSE

trending folds of the last deformation. The superposition

gave rise to non-plane folding and domal structure. D4 is

evident by the presence of microscopic to small-scale

fractures, healed up with biotite (Figure 3c).

4. TEXTURES AND MICROSTRUCTURES

The studied aluminous granulites have three distinct mineral

associations that developed in closely spaced domains in

outcrop scale. The mineral assemblages, key micro-structural

and textural features, mineralogical evolution with defor-

mation vis-a`-vis metamorphic events and brief mineral

chemical characteristics are summarized in Table 1.

Association A contains garnet porphyroblasts set in a ner

quartzo-feldspathic matrix. The association B is distinctly

migmatitic with alternating light and dark layers forming

composite S2/S3 gneissic fabric and a mylonitic foliation

(S3m). Evidences of crystal plastic deformation on a micro-

scale are widely preserved in this sheared variety of

aluminous granulite (Figure 3d). The association C is

developed as massive domains within the aluminous

granulite. Mineral abbreviations are after Kretz (1983).

Subscripts with minerals refer to both the mineral association

and sequence of development in each association.

In association A, garnet porphyroblasts (GrtA1) contain

inclusions of biotite (BtA1) (Figure 3e), spinel (SplA1),

quartz (Figure 3f) and ilmenite. It (GrtA2) occasionally

forms intergrowth with spinel (SplA2) and ilmenite (IlmA2)

(Figure 3g) sapphirine. Layers of GrtA1 in Association Ashow ductile deformation forming pinch-and-swell and

boudinage structure. Cordierite (Crd) is replaced by a

spongy symplectic intergrowth of garnet (GrtA3), quartz and

sillimanite (SilA3) along grain boundaries (Figure 3h). At

places, this symplectite overgrows GrtA1. This intergrown

assemblage after cordierite is possibly developed during the

high-strain event producing S3m fabric (D3).

The gneissic foliation in association B developed

during D1D2 and is characterized by quartz-perthite-

plagioclase-bearing leucosomes alternating with dark layers

comprising porphyroblastic orthopyroxene (OpxB1),

cordierite, spinel-magnetite solid solution (SplB1), quartz,

plagioclase, ilmenite and minor prismatic sillimanite (SilB1).

OpxB1, SplB1 and Crd grains are extremely stretched and

attened parallel to the foliation. Porphyroblastic OpxB1commonly shows pinch and swell and en-echelon micro-

structures. The features suggest that this assemblage

containing OpxB1 was stabilized prior to D3. The mylonitic

fabric (S3m), represented by ribbon quartz, is preserved in

high-strain areas wrapping local low-strain domains.

Dynamic recrystallization by grain boundary migration

(GBM) is shown by quartz and feldspar in quartzo-

feldspathic microdomains and also in OpxB1 and cordierite

grains (Figures 4a and b). Perthite occurs as asymmetric

grains with strings of intergrowth and quartz ribbons

sweep around these porphyroblasts resembling CS fabric

(Figure 4a) while plagioclase grains show bent twin

lamellae. Asymmetric mantle structure is shown by

recrystallized orthopyroxene grains around porphyroclasts

of the same OpxB1 (Figure 4c). OpxB1 often forms

aggregates and contains lamellae of spinel (SplB2) and

cordierite along the same crystallographic plane (Figure 4d).

OpxB1 also contains lamellae of garnet (GrtB2) along the

same crystallographic direction along with globules of



Table

1.Summaryofmineralogy,texture,mineral

chem

istry,eldandmicrostructure

andPT-tconstraintsofthethreevarieties

ofaluminousgranulites

Mineral

association

Structural/micro-structural

characters

Key

texturalfeatures

D1D2(M

1)andPost

D2-interkinem

atic

(M1R)event

D3(M

2)event

Mineral

chem

istry

AssociationA

GrtCrdIlm

Spl

Rt

QtzPl

KfsSilBt

Spr

Migmatitic

withalternate

leucosomes

(Qtz,PlandKfs)

andmelanosome(G

rtA1,Crd,

IlmA1,Rt,PlandQtz);fabric

S1/S

2;ductiledeform

ation

inGrtA1.

Bt A

1,Spl A

1,Qtz

andIlmA1

inclusionin

GrtA1,exsolved

Hem

inIlmA1;intergrowth

ofGrtA2

Spl A

2IlmA2;GrtA3SilA3Qtz

symplectiteafterCrd;Grtreplaced

byBt.

M1:peakreached

through

Bt-meltingreactionsproducing

GrtA1CrdRt

melt

(1&

2);GrtA2Spl A

2IlmA2

SprafterOpxA1.

Crd

breakdownto

GrtA3SilA3

Qtz

(3)

GrtA1/GrtA2:Alm

50Prp

46Grs2,

GrtA3:Alm

60Prp

37Grs2Sps 1;Spl A

1

(XMg0.390.42),2.83.3wt%

ZnO,Spl A

2(X

Mg0.580.60),

2.12.4wt%

ZnO;Spr

(XMg0.770.80)close

to7:9:3;Crd

(XMg0.86);Bt A

1(X

Mg0.830.88),

5.2wt%

TiO

2,XF

0.29.

AssociationB

OpxCrdSpl SSIlm

QtzPl

KfsSilGrt

Gneissic

fabric

(transposedS1/S

2)with

alternateleuco-(Q

tz

Plag

Kfs)andmelanocratic

layers

(OpxB1,Crd,Spl,Ilm,Qtz,

Pl

Sil);mylonitic

foliation

(S3m);plastic

deform

ation

inOpxB1,Spl B1,Crd

and

Qtz

grains.

OpxB2S

ilB2coronaover

Spl B1

(Ilm);exsolved

Hem

inIlm,

lamellaeofSpl B2,Crd

and

GrtB2in

OpxB1;OpxB3SilB3

Qtz

intergrowth

afterCrd.

M1:peakthroughBtmelting

reactionproducingOpxB1

Spl B1Crdmelt(5);

Spl B2CrdGrtB2intergrowth

afterOpxB1(6&

7).

M1R:OpxB2SilB2afterSpl B1

Crd/Qtz

(8, 9, 10)

Crd

breakdownto

Opx

B3SilB3

Qtz

(11).

OpxB1(X

Mg0.630.66),9.7wt%

Al 2O3(core),OpxB2(X

Mg0.65)

7.87.4%

Al2O3;OpxB3(X

Mg0.63,

6.86.3wt%

Al 2O3)Spl B1

(XMg0.370.43)2.22.8wt%

ZnO,0.63.1wt%

Cr 2O3;Spl B2

(XMg0.38)1.4wt%

ZnO;

GrtB2:Alm

58Prp

40Grs1Sps 1

AssociationC

Opx,Crd,Spr,Pl,Kfs,

Sil

Qtz

Massive,isolatedclusters

inlow

strain

domains

wrapped

byS3m.

OpxC2S

ilC2double

coronaafter

Spr

Crd/Qtz;inclusionofSplin

Spr;OpxC3SilC3Qtz

intergrowth

afterCrd;OpxC1replacedbylate

Bt.

M1:peakthroughBtmelting

reactionproducingOpxC1

Crd

C1/Spl(13, 14);stability

ofSpr(15, 17),

M1R:coronal

OpxC2SilC2

(16, 18, 19)

Crd

breakdownto

OpxC3SilC3Qtz

Included

Spl(X

Mg

0.380.42)0.71.0wt%

ZnO;OpxC1/OpxC2

(XMg0.66)8.0wt%

Al 2O3at

core

to7.1wt%

atrim;OpxC3

(XMg0.66),6.26.9wt%

Al 2O3;

Spr(X

Mg0.740.76)close

to7:9:3

Deform

ation

D1D2

Post-D

2interkinem

atic

gap

D3

D4

Metamorphicevents

Progradeto

peakM

1M

1R

M2

M3Localized

retrogressionin

presence

ofmelt

PT

conditions

Progradepathfrom

high

T/lowPconditionsto

10008C

,6.58.5kbar

atthepeak

Coolinganddecompression

to

8008C

,6kbar

Loadingto7

8kbar,8508C

,possibly

accompaniedby

partial

meltextraction

5008008C

(poor

constraint)

PT-tpath

ACW

path

Coolingwithdecompression

CW

path

Unknown

Ageconnotationand

possiblecorrelation

ca.1000Ma

ca.950MaRayner

orogeny;assembly

ofRodinia

ca.900MaD3D4

ofRSEin

Rayner

Complex

Denotesreactionnumber

asdiscussed

intext.


222 k. das ET AL.

quartz (Figure 4e). The intergrown lamellae of garnet can at

places, be traced continuously to the grain boundary.

Sillimanite (SilB3) forms a spongy (and sometimes skeletal)

intergrowth with granular orthopyroxene (OpxB3) and quartz

that replaces cordierite along the margins (Figure 4f). This

intergrowth commonly forms wispy trails and shows a

mylonitic fabric. Occasionally, cordierite grains are com-

pletely pseudomorphed by ne-grained intergrowth of

OpxB3, SilB3 and quartz (Figure 4g). This symplectic

intergrowth along with other polymineralic matrix grain

aggregates, form an oriented fabric (S3) that sweeps

asymmetrically around cordierite clasts (Figure 4f). This

indicates contemporaneous shear fabric development with

breakdown of cordierite. Extreme grain renement is

evident in quartz and feldspar grains. An oblique foliation

(as S-) is developed by the shape fabric of dynamically

recrystallized polymineralic matrix grains with attened,

recrystallized quartzose bands and ribbon quartz (C-).

OpxB1 clasts are attened with recrystallized asymmetric

trails giving s structure in unoriented section (Figure 4c).The ne-grained polymineralic matrix is the product of

dynamic recrystallization by the process of subgrain rotation

Figure 4. (a) Asymmetry in alkali feldspar clast and also bending of exsolution lamellae in alkali feldspar. (b) Orthopyroxene grains show evidences of dynamicrecrystallization. (c) Asymmetry in orthopyroxene grains is to be noted where recrystallized grain fabric wraps around it. (d) Orthopyroxene grains (Opx)containing lamellae of spinel (Spl) and cordierite along the same crystallographic plane. (e) BSE image showing orthopyroxene porphyroblast (Opx) containinglamellae of garnet (Grt) and globular quartz (Qtz) along a crystallographic plane. (f) BSE image showing symplectic intergrowths of Opx SilQtz form anoriented fabric that sweeps asymmetrically around cordierite clasts. (g) The same intergrowth of sillimanite (Sil), orthopyroxene (Opx) and quartz (Qtz)pseudomorphing cordierite (Crd) grain. (h) Bands of coarse recrystallized quartz forming ribbons engulfed within ne-grained polymineralic recrystallized

matrix with sharp boundaries.



(SR). Small grains in the matrix show development of

subgrains (recovery stage). The rock contains discontinuous

and asymmetrically tapered-head bands of coarse quartz

either engulfed within or alternating with ne-grained

polymineralic recrystallized matrix with sharp boundaries.

Grain boundary area reduction (GBAR) is evident in these

bands with curved to straight grain boundary in polygonal

aggregates that are free from any intracrystalline defor-

mation (Figure 4h). These grains are therefore statically

recrystallized. Extensive recrystallization might have been

caused by the combined effects of subgrain rotation and

grain boundary migration with increase in temperature

(Hirth and Tullis, 1992; Passchier and Trouw, 1998). These

microstructural signatures collectively imply shear defor-

mation at high-T granulite-grade metamorphic conditions.

High temperature and presence of uid along grain

boundaries generally favour recovery and recrystallization,

while high strain rate promotes distortion (Passchier and

Trouw, 1998). Secondary grain growth due to GBAR and

development of strain-free monomineralic quartz ribbons

are indicative of high temperature, above 7008008C(Passchier and Trouw, 1998). OpxB1clasts, surrounded by

mantle of recrystallized grains (Figures 4b and Figure 4c),

also suggest granulite facies metamorphic condition in the

temperature range mentioned earlier (Passchier and Trouw,

1998).

Association C develops both Qtz-rich and Qtz-poor

microdomains. Isolated clusters of porphyroblastic ortho-

pyroxene and cordierite grains in low-strain domains impart

a massive look to the rock. The quartz-absent domain is

characterized by megacrystic orthopyroxene (OpxC1),

cordierite, sapphirine, plagioclase, perthite and also

dominance of sillimanite (SilC1). Sapphirine occurs in the

interstices of porphyroblastic cordierite and is separated

from the latter by a double corona of OpxC2 and SilC2(Figure 5a). The development of these coronal phases was

possibly pre-D3. Rare inclusions of spinel are present in

sapphirine that occurs at the contact of cordierite. On the

other hand, the quartz-bearing domain also contains

porphyroblastic orthopyroxene (OpxC1), cordierite, sapphir-

ine, plagioclase, perthite and sillimanite as major minerals.

Sapphirine (SprC1) is separated from quartz by a thick

corona of sillimanite (SilC2) and orthopyroxene (OpxC2)

(Figure 5b). In both the microdomains (quartz-present

and quartz-absent) cordierite shows breakdown to

OpxC3 SilC3 quartz intergrowth during D3 event similarto that described in Association B.

5. MINERAL CHEMISTRY

Representative samples of all the types were analysed by

electron microprobe (JEOL 733) at Hokkaido University,

Figure 5. (a) Xenoblastic sapphirine (Spr) is separated from cordierite(Crd) by a double corona of sillimanite (Sil) and orthopyroxene (Opx).Note the presence of relic spinel (Spl) in sapphirine and biotite (Bt) patcheson orthopyroxene. (b) BSE image of quartz-present microdomain wherematrix sapphirine grain (Spr) is separated from quartz by a thick corona ofsillimanite (Sil) and orthopyroxene (Opx). (c) Compositional plots ofsapphirine grains to show the extent of Tschermak substitution (alongthe line joining 2:2:1 and 7:9:3 compositions). Note that all the compo-

sitions plot close to the latter point.


224 k. das ET AL.

Japan. The instrument was operated at 15 kV accelerating

voltage and 20 nA specimen current. Synthetic and natural

standards are used. Raw data were processed by ZAF

correction scheme. Individual phase chemical data are

presented in the Supporting information (Supplementary

Tables 26), while a summary of the phase chemical

characteristics are given in Table 1.

In Association A, GrtA1 and GrtA2 have similar

compositions, while GrtA3 is more iron-rich (Alm60Prp37Grs2Sps1) (Supplementary Table 2). SplA1 has variable

composition (XMg 0.390.42) and contains 2.83.3 wt%ZnO. SplA2 is more magnesian (XMg 0.580.60) withlower Zn (2.12.4wt% ZnO). Sapphirine is magnesian

(XMg 0.770.80) and is close to the 7:9:3 composition(Figure 5c). Cordierite is magnesian (XMg 0.86) andhomogeneous in composition. IlmA1 contains 1015mol%

hematite and 47mol% geikielite components. Exsolved

hematite is titaniferous containing 16mol% ilmenite and

negligible geikielite components. Included BtA1 is magne-

sian (XMg 0.830.88) and contains 5.2 wt% TiO2. Thesegrains contain 2.5wt% F (XF 0.29) and negligible Cl. Latebiotite (XMg 0.0.830.91) is compositionally similar withrespect to XMg, but contains lower Ti (TiO2 2.14.2wt%)and higher F (2.93.9wt% F; XF 0.340.44).In Association B, orthopyroxene shows compositional

variations in different textural modes. OpxB1 (XMg 0.630.66) is highly aluminous and the alumina content decreases

from intergrowth-free core regions (maximum Al2O39.7wt%) to regions having intergrowth of SplB2 (Al2O38.0wt%) as well as near the rims (Al2O3 6.8wt%).However, XMg does not change appreciably ( 0.65). OpxB2is compositionally similar in XMg (0.65) but slightly less

aluminous than the core of OpxB1. Thick coronal grains

show decrease in alumina content from the core (7.8wt%) to

rim near SplB1 (7.4wt%). OpxB3 (XMg 0.63) contains 6.36.8wt% alumina. Spinel shows compositional variations in

different modes. SplB1 grains are compositionally more or

less homogeneous (XMg 0.370.43) with ZnO 2.22.8wt% and 0.63.1wt% Cr2O3. Magnetite content in

SplB1 ranges between 34mol%. SplB2 (XMg 0.38)contains 1.4wt% ZnO and is low in Cr2O3 content. All

these phases contain insignicant amount of TiO2 (Supple-

mentary Table 3). Ilmenite contains 1013mol% hematite

and 34mol% geikielite components. Exsolved hematite

contains maximum 17mol% ilmenite component. GrtB2 is

compositionally Alm58Prp40Grs1Sps1. Cordierite is homo-

geneous in composition (XMg 0.830.84). Sillimanitecontains 0.91.1wt% Fe2O3 (total). Plagioclase is

Ab74An26. Late biotite (XMg 0.740.78) contains 4.45.3wt% TiO2 and 2.4wt% of F (XF 0.29).Spinel included within sapphirine in Association C has

XMg 0.380.42 and is low in ZnO (0.71.0wt%) andCr2O3 (0.20.3wt%). SilC1 contains 1.0wt% Fe2O3 (total).

Cordierite (XMg 0.830.85) is again homogeneous. OpxC1shows compositional homogeneity in terms of FeMg

distributions (XMg 0.66), but exhibits Al-zoning fromcore (Al2O3 8.0wt%) to rim (Al2O3 7.1wt%). A thickcorona of OpxC2 shows a comparable composition as that

of OpxC1. Symplectic OpxC3 (XMg 0.66) contains 6.26.9wt% alumina. Sapphirine is magnesian with XMg 0.740.76 and mostly 7:9:3 in composition (Figure 5c). Recalcu-

lated composition shows variable amount of ferric iron

(Fe2O3 1.94.0wt%) and appreciably high XFe3 (Supple-mentary Table 4). Late biotite (XMg 0.760.78) contains5.5 wt% TiO2, 2.3 wt% F (XF 0.27) and insignicant Clcontent.

6. EVOLUTION OF MINERAL ASSEMBLAGES

The textural and compositional features in the aluminous

granulites presented earlier attest to several stages of mineral

reconstitution.

6.1. Association A

High modal abundance of garnet, quartz and Fe-Ti oxides in

this association implies a Fe-Ti-rich silica-saturated proto-

lith. The peak metamorphic stage, culminated during the D2event, is characterized by the presence of coarse garnet

(GrtA1) in this rock. Presence of biotite, spinel and quartz

inclusions in garnet (Figures 3e and f) and presence of

cordierite in close association suggest the following

KFMASH dehydration-melting reaction during D1D2event:

BtA1 SplA1 Qtz GrtA1 Crdmelt (1)

This reaction has a steep dP/dT slope and occurs in the

temperature range 8009008C under 5 kbar pressure in thehigh fO2 KFMASH grid (Dasgupta et al., 1995). In the low

fO2 grid, however, this reaction is encountered at slightly

lower PT condition of 7408C, < 4 kbar emanating fromthe [Opx,Spr] invariant point (Scrimgeour et al., 2001).

The presence of included ilmenite in GrtA, highly

titaniferous composition of BtA1 and development of

abundant rutile would indicate a more complicated multi-

variant reaction involving TiO2, such as

BtA1 SplA1 Qtz IlmA1 GrtA1 Rt Crdmelt (2)

The mesoperthite quartz plagioclase mat surroundinggarnet could indicate the former presence of melt. Owing to

the coarse grain size of the garnet porphyroblasts and the

presence of voluminous proportion of quartzo-feldspathic



minerals, we consider that a high degree of partial melting

occurred at elevated temperatures. Both the above two

reactions may involve K-feldspar, either as reactant or

product (Harley, 2008). Experimental data on dehydration

melting of biotite indicate that such reactions occur at

T> 8128C at 10 kbar (Vielzeuf and Holloway, 1988; Patinoand Patino Douce, 1991; Gardien et al., 1995; Pickering and

Johnston, 1998), with the volume of melt increasing with

temperature. Experimental data also show that XMgBt, TiO2

Bt

and FBt increase with temperature (Munoz, 1984; LeBreton

and Thompson, 1988; Peterson et al., 1991; Patino Douce,

1993; Skjerlie and Johnston, 1993; Dooley and Patino

Douce, 1996; Stevens et al., 1997; Pickering and Johnston,

1998). The included biotite in the present study area is

magnesian, titaniferous and uorine-rich, which attests to

very high metamorphic temperatures in the study area.

Coarse garnet occurs in this rock as another textural

variety (GrtA2) where it is intergrown with sapphirine, spinel

(SplA2) and ilmenite (IlmA2) needles (Figure 3g). This

intergrowth may be a breakdown product of anomalously

high aluminous orthopyroxene formed during the pre-peak

prograde stage (Das et al., 2006a).

Fine-grained aggregate of GrtA3 SilA3Qtz thatreplaces cordierite (Figure 3h) during the high-strain

event D3 suggesting the reaction

Crd GrtA3 SilA3 Qtz (3)

This reaction has at negative dP/dT slope and operates

due to loading. This could also ensue on melt loss and

reduction of H2O activity (Harley, 2008).

Late biotite was formed during M3D4 at the fringes of

garnet grains with invasion of hydrous uid via the reaction

GrtA1 SilA3 H2O Bt Qtz (4)

6.2. Association B

Abundance of spinel solid solution, plagioclase with quartz

and rare occurrence of prismatic sillimanite and FeTi

oxides in this association suggest a Fe-rich, Ti-poor and

silica-saturated protolith. The peak assemblage in this rock

is constituted of spinel (SplB1) quartz aluminous ortho-pyroxene (OpxB1) cordierite with minor rutile, monaziteand zircon. Textural data suggests that CrdQtzOpxB1SplB1 formed a stable assemblage during the peak stage.

Stabilization of this assemblage could be achieved by biotite

dehydration melting in a semipelitic bulk composition

during the D1D2 event, viz.

Bt Pl Qtz OpxB1 SplB1 Crdmelt (5)under high Tlow P conditions (Vielzeuf and Montel, 1994).

The intergrowth of spinel, cordierite and garnet in

orthopyroxene porphyroblasts in this association can be

explained by the reaction relations involving Tschermak

molecules present in the early-stabilized anomalously high

aluminous orthopyroxene (Bose et al., 2006). The occur-

rence of spinel and cordierite in some aluminous orthopyr-

oxene porphyroblasts can be represented in MAS system via

Mg-Tschermak breakdown reaction

Mg Ts Spl Crd (6)at low Phigh T conditions.

On the other hand, participation of garnet (Prp) in the

intergrowth requires involvement of Mg-Tschermak com-

bined with enstatite (En)

Mg Ts En Prp (7)

This reaction, modelled by Gasparik (1994) occurs due to

loading or heating.

The appearance of a double corona of OpxB2 SilB2separating SplB1 against cordierite porphyroblasts in local

quartz-free microdomain indicates the operation of FMAS

divariant reaction in the pre-D3 stage

SplB1 Crd OpxB2 SilB2 (8)

A similar double corona of OpxB2 SilB2 also appears onSplB1 separating it from quartz through another FMAS

divariant reaction

SplB1 Qtz OpxB2 SilB2 (9)

These two reactions operate in closely spaced micro-

domains, possibly during the same stage of metamorphic

crystallization. These divariant reactions emanate from the

FMAS univariant reaction

SplB1 Crd Qtz OpxB2 SilB2 (10)

Reaction (8) has a at positive dP/dT slope and operates

due to cooling with or without loading.

The unique textural feature of the studied rock is

the breakdown of cordierite to ne-grained orthopyroxene

(OpxB3), sillimanite (SilB3) and quartz (Figures 4f, Figure 4g).

This suggests the reaction during high-strain event D3

Crd Opx B3 SilB3 Qtz (11)

This reaction has a at dP/dT slope and operates due to

loading. However, this reaction will occur only due to

loading in a melt-present situation (Carrington and Watt,

1995).

Late biotite formed along the margins of orthopyroxene at

the last M3D4 event due to hydration in presence of melt/

alkali feldspar via the reaction

Opx Sil H2O Bt Qtz (12)


226 k. das ET AL.

6.3. Association C

As discussed earlier, this association has two microdomains

marked by the presence or absence of quartz. In the quartz-

absent microdomain, presence of sapphirine, and abundance

of sillimanite and cordierite, coupled with the magnesian

nature of all the FeMg phases clearly indicate a silica-

undersaturated high-magnesian-aluminous domainal bulk

composition. The early-stabilized phases in this assemblage

are aluminous orthopyroxene and cordierite with occasional

relict spinel grains (now enclosed in sapphirine). Such an

assemblage could form during D1D2 by partial melting of a

biotite-sillimanite-bearing protolith through the reactions

Bt SilC1 Qtz OpxC1 Splmelt (13)

Bt Spl Qtz OpxC1 CrdC1 melt (14)

K-feldspar can participate in such reactions either as

reactant or as product depending on the K2O/H2O ratio

between biotite and melt (Carrington andWatt, 1995). These

reactions occur in the pressure range of 67 kbar at a

temperature> 8508C (Das et al., 2001). Absence of quartzin this association possibly implies complete removal of

silica during the said melting reactions.

Appearance of sapphirine of 7:9:3 composition engulng

spinel close to cordierite grains (Figure 5a) suggests the

FMAS reaction

Spl CrdC1 Sil Spr (15)

This reaction occurred during D1D2 prograde meta-

morphism as it represents a part of the metamorphic peak

assemblage. The presence of a double corona of

OpxC2 SilC2 over sapphirine separating it from cordierite(Figure 5a) suggests the reaction

Spr CrdC1 OpxC2 SilC2 (16)

This reaction has a at positive dP/dT slope and operated

due to cooling in the pre-D3 stage.

In the quartz-present microdomain of this association, the

appearance of sapphirine from spinel can be explained by

FMAS reaction during D1D2 prograde metamorphism

Spl Qtz Sil Spr (17)

Textural evidence suggests that sapphirine and quartz in

this microdomain are separated by thick coronae of

sillimanite and orthopyroxene (Figure 5b). This indicates

cooling in the pre-D3 stage across the reaction

Spr Qtz OpxC2 SilC2 (18)

Reactions (17) and (18) emanate from the FMAS

univariant reaction

Spl Spr Qtz OpxC2 SilC2 (19)

Coarse cordierite grains show breakdown to the sym-

plectic intergrowth of OpxC3 SilC3Qtz through reaction(11) during D3 as observed in Association B.

Development of late biotite during M3D4 over orthopyr-

oxene can be explained by the reaction (12) as inferred for

Association B.

7. THERMOBARIC CONDITIONS OF

METAMORPHISM

Estimation of PT conditions of metamorphism in a

polymetamorphosd UHT terrane is plagued with two

major problems. One is related to down-temperature re-

equilibration of mineral composition (Harley, 1989; Frost

and Chacko, 1989; Pattison et al., 2003). The other

constraint lies in the identication of equilibrium mineral

compositions at each metamorphic stage.

First we estimate peak metamorphic temperatures from

fossil thermometers using the reintegrated composition of

peak phases. Reintegrated composition of mesoperthitic

feldspar gives a temperature close to 10008C isotherm in thediagram of Elkins and Grove (1990). The orthopyroxene

Al-isopleth diagram of Harley (2004) gives peak meta-

morphic temperatures close to 10008C for the assemblageOpxC1-Spr-Qtz. Intergrown sapphirine and spinel in

peak garnet (GrtA2 gives temperatures around 8608708Cfollowing the formulation of Das et al. (2006b). This lower

estimate is probably due to down-temperature resetting of

mineral compositions.

Coexistence of sapphirine and quartz during the peak

metamorphism in Association C clearly indicates tempera-

tures in excess of 10008C based on both petrogenetic grids inthe system KFMASH (Dasgupta et al., 1995; Das et al.,

2003; Kelsey et al., 2004) and PT pseudosections

calculated with the internally consistent dataset of Holland

and Powell (1998) (Kelsey et al., 2004). The pressure

condition during peak metamorphism is difcult to estimate,

as there is a discrepancy of 2.53 kbars between the twoapproaches that has been ascribed to variations in aH2O(Harley, 1998; Kelsey et al., 2004; Baldwin et al., 2005).

While the experimentally constrained petrogenetic grid in

the system KFMASH (Das et al., 2003) would indicate peak

pressures of 9 kbar for the mineral assemblages (spinel-sapphirine-garnet-quartz-orthopyroxene) developed at this

stage in different associations, the computed grid and

pseudosection of Kelsey et al. (2004) would indicate 6.57 kbar for the same set of assemblages. Moreover, the



actual pressure estimate will be dependent on the Zn content

in spinel (Waters, 1991), thereby adding to further

uncertainty.

Given the above constraints, we estimate the peak

metamorphic conditions during the culmination of D1D2to be> 10008C and 79 kbars.Using the Al-isopleth diagram (Harley, 2004), we have

estimated temperatures in the range of 8008508C whereorthopyroxene equilibrated with cordierite, sillimanite and

quartz in all the three associations. Garnet (GrtB2) lamellae

in OpxB1 give a temperature 7308308C using garnet-orthopyroxene thermometer after Lee and Ganguly (1988) in

the pressure range of 79 kbar. This is obviously a reset

temperature as the same compositions give temperatures of

9309408C using the method of Pattison et al. (2003). Wehave used the composition of coexisting Spl-Crd to calculate

temperature using the formulations of Nichols et al. (1992),

which takes into consideration the gahnite component in

spinel. The estimated temperature ranges from 660 to 6908Cat assumed pressures of 78 kbars. The thermometric data

attests to signicant cooling, probably down to 8008C in thepre-D3 stage, but the pressure conditions during cooling

could not be pinpointed.

The appearance of orthopyroxene-sillimanite-quartz

intergrowth (both in Associations B and C) and of garnet-

sillimanite-quartz (in Association A) after cordierite may

imply a rise in pressure as these reactions have at dP/dT

slopes in the subsystem FMAS (Hensen, 1986). Therefore, it

is likely that the cooled granulites were subjected to loading

during D3. Using the orthopyroxene (OpxB3, C3)cordierite

barometric formulation of Bertrand et al. (1992), we obtain

pressures around 7.88.0 kbar at an assumed temperature of

8008C. The garnet (GrtA3)cordierite thermometer ofNichols et al. (1992), gives 7208C at 8 kbar. It is evidentfrom the above that minor loading of 1 kbar could havetaken place during D3. In the subsequent section, we will

explore an alternative explanation.

Garnet-biotite thermometry (after Ferry and Spear, 1978)

gives 4008C for late biotite (Bt2) patches on garnet occurringduring theM3D4 event. Since this formulation does not take

into account the Ti and F content in biotite, the estimate is

only a lower limit. An upper limit is obtained from the Ti

content of late biotite in this rock saturated with ilmenite and

quartz that plots above the 8008C isotherm (Henry et al.,2005).

8. PSEUDOSECTION AND PETROGENETIC GRID

CONSIDERATIONS

In this section we investigate further the PT conditions and

PT path of metamorphism using pseudosection modelling.

Such studies are complicated in the case of polymeta-

morphic granulites because re-metamorphism occurs in a

bulk composition that has changed substantially from that of

the protolith due to partial melting and melt loss during the

earlier event (White et al., 2001). Extraction of melt in

varying proportions is considered to be a key factor in the

preservation of peak granulite assemblages (White and

Powell, 2002; Kelsey et al., 2003). Drawing analogy, we can

argue that an indenite amount of melt was extracted from

the studied rocks as the peak UHT assemblages are

preserved. While the breakdown of cordierite to orthopyr-

oxene sillimanite quartz (Reaction 11) or to gar-net sillimanite quartz (Reaction 3) can be facilitatedby extraction of melt, preservation of all stages of textural

conversions (Figures 4f and g) show that melt loss was far

from complete. Formation of high F-bearing biotite

during D4 attests to limited melt-solid interaction signifying

presence of melt at even the last stage. At the same time,

maximum reaction progress (complete pseudomorphism)

can occur in domains of extensive melt loss. Since the

cordierite breakdown reactions occurred simultaneously

with D3 compression and shearing, melt extraction could

have been facilitated in domains by tectonic squeezing. This

would obviate the requirement of loading for cordierite to

break down. We have undertaken a PT pseudosection

approach to test whether loading is at all required to break

cordierite down.

Two general approaches have been followed to model the

protolith composition where possible melt loss occurred.

One approach is to use several hypothetical bulk compo-

sitions (White et al., 2001, 2007; Johnson and Brown, 2004),

primarily because the migmatites are themselves refractory

from which an unspecied amount of melt has been lost. A

more rigorous approach to resolve this problem is to use the

changed bulk rock composition using the theoretical

calculation matched with natural data (cf. Saha et al., 2008).

A second approach uses an effective bulk composition,

estimated through calculated mineral modes and mineral

compositions have been applied to compute PT pseudosec-

tion (Kelsey et al., 2003; Baldwin et al., 2005). In this work

we have followed the latter because of the difculty to

estimate the volume and composition of melt extracted from

the system. Even where leucosomes are present, as in the

case of Associations A and B, it is likely that part of these

could be of cumulate origin (e.g. Johnson and Brown, 2004).We have taken up two unique microdomains in the studied

rocks for phase relation modelling. These represent discrete

mineralogical domains with distinct domainal chemical

compositions. One is the intergrowth involving cordierite,

spinel, garnet and quartz in porphyroblastic orthopyroxene

(Figures 4d and e). The other one is the pseudomorphous

breakdown of cordierite porphyroblasts in the high-strain

zones to orthopyroxene, sillimanite and quartz

(Figures 4f and g). The bulk compositions for both the

microdomains have been calculated by the modal ratios of


228 k. das ET AL.

the phases present (using image analysis of microphoto-

graphs and BSE images) and their respective mineral

compositions. The recalculated bulk compositions in both

the cases can be represented in the simplied FMAS (FeO-

MgO-Al2O3-SiO2) system. Phase relations were modelled in

the FMAS system using THERMOCALC v.3.2.3 (Powell

and Holland, 1988), and the dataset of Holland and

Powell (1998, version 5.5 s, November 2003) with the

activity-composition model for sapphirine of Kelsey et al.

(2004).

The two pseudosections were constructed to characterize

the PT history of the near-peak metamorphic condition and

of the later high-strain event (D3). Porphyroblastic

aluminous orthopyroxene (zoned in terms of alumina from

core to rim) contains intergrown garnet, spinel and cordierite

along crystallographic orientations in the presence of quartz

in Association B. This assemblage is considered to have

developed at near-peak metamorphic condition. In the

earlier section, we explained the development of these

intergrowths in porphyroblastic orthopyroxene from break-

ing down of non-quadrilateral aluminous orthopyroxene. It

is, however, not possible to construct pseudosections with

ctive components, such as the Tschermak molecule in

orthopyroxene. Instead, we have modelled the reaction

orthopyroxene spinel quartz garnet cordierite (Spr,Sil) from the recalculated domainal bulk composition in the

FMAS system (compositional relationship is shown in

Figure 6a) to obtain the relative stability elds of the

different assemblages present in the intergrowths. The

pseudosection (Figure 6b) shows that the reaction has a

steep slope separating different divariant and trivariant

elds. On the low pressure side the stable trivariant

assemblage contains orthopyroxene, spinel and cordierite.

With increasing pressure the stable assemblages on the low

temperature side change to orthopyroxene-cordierite-garnet,

and subsequently to orthopyroxene-garnet-quartz through

intermediate divariant assemblages, e.g. orthopyroxene-

spinel-garnet-cordierite and orthopyroxene-cordierite-gar-

net-quartz. Although it is not possible to identify any

sequence of mineral development from Figures 4d and

Figure 4e, stabilization of quartz with garnet (Figure 4e)

may reect loading from an early-stabilized cordierite and

spinel (Figure 4d), as shown in Figure 6b. Loading near to

Figure 6. (a) Compositional plots of the phases in (AlFe3)2O3/totalFMAS vs. SiO2/total FMAS space (in atomic fraction). The tie-line ip andpiercing plane relations are elucidated in the text. Note the position ofcordierite (Crd) within the two compositional triangles, i.e. Opx-Sil-Qtz andGrt-Sil-Qtz. (b) PT pseudosection for the microdomain with recalculatedbulk composition of SiO2:Al2O3:MgO:FeO 46.55:10.25:25.28:17.91 (inmole ratio) made by THERMOCALC v3.2.3 (Powell and Holland, 1988).Internally consistent thermodynamic dataset of Holland and Powell (1998)is used. Activity-composition model of sapphirine is taken from Kelseyet al. (2004). Dashed isopleths are for y(Opx) and mixed dash-dot linerepresents isopleth for XFe of garnet, i.e. x(Grt). Note the probable PT pathshowing evolution in the near-peak metamorphic condition. (c) PT

pseudosection for the microdomain where cordierite is broken down toorthopyroxene-sillimanite-quartz assemblage in a later structuralevent, D3. Bulk composition recalculated as SiO2:Al2O3:MgO:FeO56.06:21.94:18.16:3.85 (in mole ratio). Note the disposition of differentvariance elds (v 2 and 3) and the isopleths [y(Opx) as dashed lines,x(Opx) as dotted lines and x(Crd) as thin continuous lines]. At 8508C, aslight pressure increase of 500 bars can induce this breakdown through thedivariant eld of orthopyroxene-cordierite-sillimanite-quartz at proper uidactivity. As discussed in the text, such breakdown can be aggravated further

by melt loss from the high-strained microdomains.



peak metamorphic conditions is also supported by the

garnet-spinel-sapphirine intergrowth in Association A (cf.

Das et al., 2006a). The postulated near-peak loading is

consistent with an anticlockwise PT path deduced from

adjacent areas (Sengupta et al., 1990; Bose et al., 2000).

Similar anticlockwise PT paths have also been reported

from other UHT terranes (e.g. Santosh et al., 2007). The

cooling segment of the PT path shown in Figure 6b is

construed from the Al-zoning in orthopyroxene as described

in the preceding section. Although the pseudosection is

constructed in the FMAS system, the presence of a small

amount of Fe3 in sapphirine, spinel and orthopyroxeneactually implies the relevant system be FMAS(O). At

elevated oxygen fugacity, the stability of phases like spinel

would enlarge and as a result grid topology would change

and even some of the peak assemblages would be enlarged

towards lower temperature (KFMASH system; Das et al.,

2003; Harley, 2008 and references therein).

The second pseudosection was constructed to model the

FMAS reaction orthopyroxene cordierite sillimanitesapphirine quartz (Grt, Spl) (compositional relationship isshown in Figure 6a) using the microdomainal bulk

composition in Association C (Figure 6c). Two divariant

assemblages, e.g. orthopyroxene-cordierite-sillimanite-

quartz on the lower temperature side and orthopyroxene-

cordierite-sapphirine-quartz on the high temperature side,

are associated with this univariant reaction. On the lower

temperature side, cordierite breaks down to orthopyroxene,

sillimanite and quartz with slight increase of pressure

0.5 kbar. In Association C this reaction occurredduring D3, subsequent to cooling down to 8008 C(reactions 16, 18). The above analysis suggests possible

minor loading of the cooled granulites at around 8508C(Figure 6c). As discussed earlier, breakdown of cordierite

could have been aided by extraction of melt with its

dissolved H2O along high-strain zones of the rock (tectonic

squeezing (?) during D3 event contemporaneous with the

development of S3m mylonitic foliation). Oxide total in all

analysed cordierite grains ranges between 9798wt%,

which implies the possible presence of uids. However,

in the absence of analytical data for H2O and CO2, we can

hardly quantify the nature of uid and its control over

cordierite stability in the studied rocks. It is therefore

difcult to specify whether cordierite breakdown is solely

caused by loading or melt extraction.

9. GEOCHRONOLOGY

We analysed zircon grains for U-Pb ages with an ion

microprobe to constrain major thermal imprints on the

assemblages. Additionally, we undertook EPMA chemical

dating of monazite included within orthopyroxene and

sapphirine porphyroblasts in Association C to constrain the

timing of M1 metamorphism. Monazite grains present

within cordierite grains were also analysed for

comparison.

9.1. Analytical methods

About 1 kg of the representative sample (Pan03) was

initially crushed, and zircon grains were concentrated using

water table and magnetic separation. Individual crystals

were selected and hand-picked under the optical microscope

for mounting. Analytical procedures adopted were the

same as those followed in Shiraishi et al. (2008). Spots

within the zircon crystals were selected using optical, BSE

and CL imaging (Figure 7), and were analysed on the

SHRIMP II at the National Institute of Polar Research,

Japan. Analytical data are presented in Supplementary

Table 5 and plotted in a TerraWasserburg diagram

(Figure 8a). Uncertainties quoted in the tables and in the

text for individual analyses (ratios and ages) as well as plots

are at 68% condence level (1s), whereas concordia and

mean age calculations were done at 95% condence level.

Concordia age calculation has been done using the program

SQUID (version1.02, Ludwig, 2001) which takes into

account the error in the standard. Plots are done by the

program ISOPLOT/EX (version 3.6, Ludwig, 2008). Back

Scatter Emission (BSE) and Cathodoluminescence (CL)

images of the analysed zircon grains reveal complex internal

structure with characteristic CL responses. While neoblastic

homogeneous to simple concentric zoned zircon shows

virtually no inheritance, other grains contain inherited

domains surrounded by overgrowth.

Monazite grains included within orthopyroxene or

associated with sapphirine were targeted for chemical

analysis. Those grains were analysed by the CAMECA-

SX100 electron microprobe at Indian Institute of Technol-

ogy, Kharagpur, India. The instrument was operated with

20 kV acceleration voltage and 200 nA beam current with

1mm beam diameter. Details of analytical procedure areafter Bhandari et al. (2011). Natural standards were used for

chemical analyses, while externally dated monazite standard

with an IDTIMS age of 2546Ma were used for age

calibration. During the entire analytical session, the above

standard gave an age of 2558 105Ma. Counting times forPb and Th were taken as 300 s and 200 s, respectively, while

those for other elements vary in the range 20100 s. Prior to

analysis, the target grains were mapped for U, Th, La and Ce,

so that different compositional domains could be identied.

Results of all the analyses are presented in Supplementary

Table 6. Calculation of weighted mean age is done by the

program ISOPLOT/EX (version 3.6, Ludwig, 2008).


230 k. das ET AL.

9.2. Results

Mounted zircon grains are oval to circular in shape with a

few elongated and euhedral in appearance. Grain size varies

from 100 to 200mm in length and 100150mm in width.Apart from a few neoblastic grains, most grains show core-

rim structure where the inner part is made of a thick

oscillatory-zoned domain with or without inherited cores.

Neoblastic grains are broadly homogeneous, some having

simple concentric zoning. Few apparently neoblastic grains

shows faint traces of hidden oscillatory/planar zoning at the

core region (Figure 7a). The outermost domain consists

of planar banded overgrowth with moderate CL response

(Figures 7b and c). Inherited cores in a few grains show

sector-zoning with brighter CL response compared to the

overgrowths (Figures 7d and h). The overgrowth domain is

5080mm thick and normally inclusion-free, but a fewbiotite akes were noticed in some grains. Some of these

grains show sector-zoning (Figures 7c and e), typically

found in zircon growing from anatectic melt (Vavra et al.,

1996; Schaltegger et al., 1999; Kelly et al., 2002; Kelly and

Harley, 2005). The oscillatory zoned domain is 50100mmin length often with abraded and angular outline. In a few

grains, the oscillatory zoned domain shows prominent

resorption (Figures 7f and i). In others, it is often blurred in

appearance and surrounded by a somewhat dark-CL domain

where faint traces of oscillatory zoning could be identied

(Figure 7g). Inclusions of biotite (both aky and sub-

rounded) and alkali feldspar are present within this domain.

The dark-CL domain surrounding the oscillatory zoned

domain shows evidence of recrystallization and resorption

presumably by a coupled dissolution reprecipitation process

(Geisler et al., 2007). The sharp contrast in CL responses and

Th/U ratios between the oscillatory zoned and the over-

growth domain suggest the former as inherited in origin.

Some of the oscillatory zoned zircon grains, however,

Figure 7. Cathodoluminescence (CL) images of analysed zircon grains showing internal structures and obtained ages. 207Pb-206Pb ages are shown inparentheses as quoted from Supplementary Table 5 with 1s uncertainty. Grain numbers are shown at the top left of each grain while spot numbers are labelled ineach spot. (a) Neoblastic grain, (b) Planar-zoned grain with detrital core, (c) Neoblastic grain with planar zoning, (d) Planar zoned overgrowth on multilayeredgrain with detrital core, (e) Sector zoned overgrowth on detrital core, (f) Planar zoned overgrowth on dark-CL core containing resorbed detrital core, (g)Oscillatory zoned grain showing faint traces of blurring of zoning towards the periphery, (h) and (i) Truncated bright-CL core surrounded by dark-CL

recrystallized zone.



preserve tiny relics of detrital core of rounded to irregular

outline.

Forty-four spots were analysed from 33 grains including

multiple spots from oscillatory zoned, recrystallized/

resorption and overgrowth domains. Spots from the

oscillatory zoned domain (four spots) show varying U

(2611176 ppm) and Th (133564 ppm) contents as

reected in a wide variety in the Th/U ratio (0.122.23).

Three spots give weakly discordant ages (36% discor-

dance) and show broadly similar Th/U ratio (0.120.37).207Pb-206Pb age spans ca. 16801710Ma with a weighted

mean of 1698 46Ma (95% conf., MSWD 2.7). Theother spot 33.1 shows a very strong reverse discordance,

even though the U content is below 300 ppm. An irregular

count of radiogenic Pb observed in this particular analysis

(and a few others as shown in Supplementary Table 5), could

be the major factor as noted in other studies (Compston,

1992; McFarlane et al., 2006). A large number of spots

were analysed from the dark-CL recrystallized/resorption

domain (n 20). This combined domain shows variable U(1702594 ppm), Th (43234 ppm) and Th/U ratios (0.06

0.95). Th/U ratios, however, lie in the much narrower range

of 0.060.29 except spot 1.1. Obtained ages are all

discordant (123%) in the range ca. 16001100Ma

(207Pb-206Pb age; Supplementary Table 5). Spot ages are

more concordant (14%) at the older age end and most

Figure 8. (a) TerraWasserburg diagram showing plots of data points. 24 data points from oscillatory zoned and recrystallized/resorption domains are used forcalculation of intercept ages. Three spots (out of four) from oscillatory zoned domain gives 207Pb-206Pb mean age of 1698 46Ma. A group of spots (n 7)from overgrowth and neoblastic grains denes a concordia age of 953 6Ma.Weighted average ofmean 207Pb-206Pb ages from a group of 6 data points from thesame domain denes an age of 902 17Ma. Reversely discordant data are not used for group age calculation. (b) and (c) Weighted average of mean ages of two

groups of data from overgrowth and neoblastic grain domain.


232 k. das ET AL.

discordant (923%) at the midway. This observation and

internal structure convincingly prove that the recrystalliza-

tion/resorption domain developed due to disturbance of the

oscillatory zoned domain and obtained ages imply mixing of

two thermal events. When we combine all data from this

domain with data from the oscillatory zoned domain and plot

in the TerraWasserburg diagram, the mixing line shows

upper and lower intercepts of 1689 62Ma and934 57Ma, respectively (Figure 8a) with high scatter(MSWD 6.0). The upper intercept age is in the range ofcalculated mean age and signies the crystallization age of

oscillatory zoned domain, which we equate with the timing

of magmatism in the source region from which the sediment

was derived. This age is similar to that documented recently

from central part of EGB (Bose et al., 2008). The lower

intercept age broadly matches with ages from overgrowth

and neoblastic grain domains (discussed below).

Outer overgrowth and neoblastic grains show similar CL

characters and a narrow range of U (211585 ppm) and Th

(26142 ppm) contents. Th/U ratio mostly varies in the

range 0.20.4, while a few show lower (< 0.1) and highervalues (0.60.7). Analysed spots (n 20) give concordant toweakly discordant ages. A few grains show weak to

moderate reverse discordance as well due to irregular

radiogenic Pb count (discussed before). 207Pb-206Pb ages

spread in the range ca. 1020880Ma with a distinct

cluster near ca. 950Ma (Figure 8a). A concordant age of

953 6Ma (95% conf., MSWD 1.4) is obtained from agroup of seven spots. This is broadly similar to the mean207Pb-206Pb age of 949 10Ma (95% conf., MSWD 1.6;Figure 8b). These concordant and pooled ages imply that the

major growth of zircon took place at ca. 950Ma. Spots with207Pb-206Pb ages in the range ca. 9601020Ma (n 6) areweakly discordant (16%). It is noted that this age group is

shown by spots occurring at the central part of a neoblastic

grain with hidden cores (as spot 11.1) or inner part of the

overgrowth that partly overlaps with the oscillatory zoned

domain (spot 21.1). These spot ages are thus not used for

group age calculation. A group of six spots with similar CL

character and Th/U ratios give a younger mean age of

902 17Ma (95% conf., MSWD 1.9, Figure 8c). Thismay suggest partial modication of an older zircon by an

unrelated much younger event (Pan African?) during which

no new zircon growth took place. Alternatively, this age may

signify the nal crystallization of zircon from extracted melt

and associated hydration event (M3D4). In the absence of

new zircon growth during any event younger than 900Ma,

the latter possibility seems more reasonable. The three-fold

clustering of ages is very similar to geochronologic data

from similar rocks in the adjacent localities (Bose et al.,

2008) and possibly the entire central part of the EGB.

The Th/U ratio is an important criterion to distinguish zircon

produced by magmatic (Th/U> 0.1) and metamorphic (Th/U

< 0.1) process (Rubatto, 2002). Most of the analysed spots inthe present sample show a Th/U ratio in the range 0.50.1

(Figure 9a). Notably, spots from oscillatory zoned domain

are always showing Th/U > 0.1, while a few spots fromovergrowth domain show Th/U < 0.1. No clear correlationexists between Th/U ratio and spot age (Figure 9b). A simple

correlation of Th/U ratio and zircon forming process

(magmatic or metamorphic) is problematic because enrich-

ment and/or depletion of Th over U is a complex function of

multiple factors (as discussed by Harley et al., 2007). There are

examples of zircon of undisputed metamorphic origin where

Th/U ratio exceeds 0.1 that even reached up to > 1 (Hokadaand Harley, 2004; Kelly and Harley, 2005).

The timing of M1 metamorphism is difcult to determine

from mounted zircon grains since the textural contexts are

not properly understood. Texturally constrained monazite

grains, however, proved to be extremely useful for this

purpose. Monazite included within orthopyroxene and

associated with sapphirine grains (Figure 5c) are likely to

reect the timing of near-peak condition of M1 event since

the enclosing phases are demonstrably stabilized during the

peak (Figures 10a and b). A weighted average of U-Pb spot

Figure 9. (a) Plot of Th and U contents of all the spots from oscillatory-zoned, recrystallized and overgrowth/neoblastic grain domains. (b) Plot of

Th/U ratio and 207Pb-206Pb ages from all the analysed spots.



ages gives a mean of 987 11Ma, which is the best estimatefor this event (Figures 10c and d). This age broadly coincides

with a few spot ages from mounted zircon grains,

particularly those from the central part of neoblastic zircon.

Other textural varieties of monazite grains enclosed

within cordierite grains were also analysed for comparison.

Those yield spot ages close to 900Ma (grain 4 in

Supplementary Table 5), distinctly younger than the other

population. This ca. 900Ma monazite age broadly coincides

with a zircon U-Pb SHRIMP age for the overgrowth domain.

10. DISCUSSION AND CONCLUSIONS

Integrated structural, petrological and geochronological

study on a suite of aluminous granulites and associated

gneisses from the study area in the Eastern Ghats Belt attests

to polyphase tectonothermal evolution of the rocks. The

deformation history is presented schematically in Figure 11

depicting fold interference pattern arising out of three

deformational episodes. The isoclinally folded F2 has been

superposed by a broadly NWSE compression (D3) at low

angles to that of the D2 compression. This fold interference

produced upright gently plunging folds with NESW

trending axial trace. The D3 event also witnessed develop-

ment of localized shears accompanying F3. The nal

architecture of the fold interference is manifested by cross

folding at high angles during D4 producing a non-planar fold

and domal structure.

Aluminous granulites recorded polymetamorphic signa-

tures with an early counter-clockwise metamorphic evol-

ution (M1) culminating in UHT condition (> 10008C) at6.58.5 kbar, corresponding to a crustal depth of 2030 km.

The prograde path of M1 traversed from an initial high T-low

P condition, followed by loading contemporaneous with

D1D2 deformations. Since it is not possible to denitely

assign any mineral reaction specically to either D1 or D2,

we are constrained to amalgamate these together. Never-

theless, it is evident that extensive partial melting of the

protolith occurred during prograde metamorphism, followed

by limited melt segregation/extraction. Later thermal

relaxation of the crust was achieved by cooling (M1R)

down to 8008C, during the inter-kinematic gap D2D3. M2

Figure 10. Chemical age data from included monazite (Mnz) grains (a) associated with sapphirine (Spr) porphyroblasts and (b) included within orthopyroxene(Opx) from the studied samples. Note that both the sapphirine and the monazite grains are surrounded by same layer of sillimanite (Sil) corona. Spot ages are

plotted on the respective monazite BSE images. (c) Weighted mean age and (d) probability density plot for the analysed sample.


234 k. das ET AL.

metamorphism was associated with a slight loading of the

cooled granulites, and was heralded by a compressive

event D3. D3 caused deep crustal shears accompanying

small- to regional-scale folds due to bulk SENW

compression. Crystal plastic deformation of ferromagnesian

minerals indicates shearing at elevated temperatures

under M2 granulite facies condition. It is likely that tectonic

squeezing during D3 caused partial melt extraction thereby

promoting mineral reactions. Since we identify D3 as a

distinct structural event superimposed on cooled granulites,

we propose that it is temporally unrelated to D1D2 and M1metamorphism. Regardless of the relative importance of

the two phenomena, viz. melt extraction and loading, in

promoting the mineral reactions during D3, we propose

that D3M2 is a distinctly separate tectonothermal event

superimposed on the cooled UHT granulites. Minor retro-

gression of the mineral assemblages occurred during D4 in

localized zones in the presence of melt. The tectonometa-

morphic history of the studied area is summarized in Table 1

and Figure 12.

Previous studies on the EGB granulites (summarized in

Dasgupta and Sengupta, 2003) also invoked granulite facies

re-metamorphism of isobarically cooled UHT metamor-

phosed rocks. While an anticlockwise PT trajectory has

been established for the UHT metamorphism (Sengupta

et al., 1990; Dasgupta et al., 1995), the PT path for the

second granulite metamorphism remained elusive. The

present study showed for the rst time a prograde history

Figure 12. The overall PT-t evolutionary history of the studied rockplotted on the FMAS grid to show the important stable mineral assemblages.The evolutionary path is drawn combining PT vectors from Figures 6b andc as well as geochronological data. The stability eld of SprOpxQtzand other UHT assemblages are drawn following the data of Kelsey et al.(2004). The post-M2 retrogressive segment is shown as a broken line

although there is no diagnostic textural feature present in the rock.

Figure 11. Schematic block diagram showing interference of folding developed by three episodes of deformation. Note the angular relationship between the F2and F3 fold axes and also the steeply dipping shear bands produced during the D3 event.



(loading with/without heating) of the second granulite

metamorphism (M2 in this work synchronous to a high-

strain event triggering possible melt extraction). Available

geochronological data on the EGB rocks is inconclusive

with respect to the timing of M1 metamorphism (1.5 Ga,

Shaw et al., 1997; 1.11.2 Ga, Jarick, 2000, 1.11.25 Ga,

Simmat and Raith, 2008). A recent geochronological study

from the Ongole Domain of the southern part of EGB reveals

that the UHT metamorphism occurred at ca. 1.63 Ga

(Upadhyay et al., 2009). Data presented by Upadhyay et al.

(2009) further show that the timing of metamorphism in the

type Eastern Ghats Province within the EGB (of which the

present study area belongs) is much younger (ca. 1.2

0.5Ga). The monazite chemical age data presented in our

study suggest that the M1 event was much younger (ca.

1000Ma) than envisaged by previous monazite age data

(Simmat and Raith, 2008). Weakly to strongly discordant

ages from the oscillatory zoned domains in zircon with a

mean age of ca. 1700Ma probably record a magmatic event

in the source region (see Bose et al., 2008). The granulite-

grade overprint on the UHT M1 assemblages, which is

documented as M2 in the present study is considered to be of

younger age based on earlier isotopic data (1.00.95 Ga,

Grew and Manton, 1986; Shaw et al., 1997; Mezger and

Cosca, 1999; Simmat and Raith, 2008 and references

therein). SHRIMP U-Pb data from the present study yield a

concordant age of 953 6Ma, which tightly constrainsthe timing of M2 metamorphism. This event witnessed

growth of new zircon (neoblastic grains) as well as zircon

overgrowth on inherited zircon, possibly in the presence of

melt that has subsequently been extracted during the D3mevent. This sharply dened age of M2 event matches with the

thermal and structural events in the Rayner Complex

(ca. 940910Ma, Kelly et al., 2002; Harley, 2003; Halpin

et al., 2005, 2007). The UHT M1 metamorphism in the

central EGB is temporally unrelated to the crustal

development in the Rayner Complex. However, the timing

and deduced PT path of M2 overprint in the EGB is similar

to that in the Kemp Land and Oygarden areas of the Rayner

Complex (Harley, 2003; Kelly and Harley, 2005). Ayounger

pooled age of ca. 900Ma in the studied rock possibly

signies the M3D4 event when zircon grains nally

crystallized from the extracted melt. This is again similar

to the timing of zircon growth during the D3D4 events in

rocks from Oygarden area of the Rayner Complex (Kelly

et al., 2002). Moreover, a striking similarity between the D3structural characteristics in the present study area and D3D4structural reworking in the Oygarden area (Kelly et al.,

2000) reinforces the notion that the EGB and Rayner

Complex shared a common geologic history from ca.

950Ma to ca. 900Ma as parts of Rodinia.

The present study provides record of a prolonged

geological history spanning nearly 800Ma (ca. 1700

900Ma) in the EGB, which is far longer than what has been

conceived so far. Though the retrievable tectonometa-

morphic history ranges from ca. 1000Ma to ca. 900Ma, the

earliest ca. 1700Ma age domain from zircon is intriguing, as

similar ages are reported from southern and central parts of

the EGB in recent times (Bose et al., 2008). Moreover,

tectonothermal events of this age have been reported from

different crustal blocks of India (Sarkar et al., 1989, Buick

et al., 2006) as well as crustal blocks of Antarctica,

Australia, North China and Laurentia (Goodge et al., 2001).

This timeframe witnessed the growth of the supercontinent

Columbia (Rogers and Santosh, 2002, 2009; Zhao et al.,

2004; Santosh et al., 2009). Future research in the EGB

needs to be aimed at understanding the signicance of

the older ages. The EGB rocks, however, record a more

pervasive tectonothermal imprint during the assembly of

Rodinia.

ACKNOWLEDGEMENTS

Authors thank Dr Ichiro Ohnishi for the EPMA analyses data

of biotite in certain key microdomains. T. Kuwajima, H.

Nomura andM. Ikeda of Hokkaido University helped during

analysis. They also thank Dr N.C. Pant for monazite EMP

analysis. Insightful comments by Drs. S.K. Bhowmik and T.

Tsunogae helped to improve the quality of the manuscript.

Simon Harleys comments and suggestions on an earlier

version of this manuscript were very helpful during sub-

sequent revision. Editorial handling by M. Santosh is thank-

fully acknowledged. KD and SB acknowledge DST,

Government of India for a research grant. SK has been

supported by the CAS programme of Jadavpur University.

SB acknowledges the Japan Society for the Promotion of

Science (JSPS grant no. 07043) for working in Yokohama

National University in collaboration with the National Insti-

tute of Polar Research, Japan. SB was also supported by

FIST programme of DST of Government of India at

Presidency College. SDG acknowledges nancial support

from the DST, Government of India through J.C. Bose

Fellowship.

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multiple tectonometamorphic imprints in the lower crust: first evidence of ca. 950 ma (zircon u-pb...

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