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Archaean granulite facies metamorphism at the Singhbhum Craton–Eastern
Ghats Mobile Belt interface: implication for the Ur supercontinent assembly
S. N. MAHAPATRO
1
, N. C. PANT
2
*, S. K. BHOWMIK
3
, A. K. TRIPATHY
4
and J. K. NANDA
5
1
Geological Survey of India, Training Institute, Hyderabad, India
2
Department of Geology, University of Delhi, Delhi, India
3
Department of Geology and Geophysics, IIT, Kharagpur, India
4
Geological Survey of India, Bhubaneswar, Orissa, India
5
S‐4/113, Niladri Vihar, Bhubaneswar, Orissa, India
In this study, we investigate the geological significance and the antiquity of lower crustal granulite facies metamorphism from the Rengali
Domain, which lies in between the Singhbhum Craton in the north and the Eastern Ghats Mobile Belt in the south. Petrographic, mineral
compositional, metamorphic reaction history and geothermobarometric studies of two representative metapelite granulite samples reveal
widespread biotite melting at peak granulite facies metamorphic conditions of 7.8 ± 0.13 kbar, 849 ± 31 °C and subsequent melt extraction,
producing a mixture of residual granulites and melts in the Rengali Province. Depending on local bulk rock compositional variations,
biotite melting produced peak metamorphic assemblages of garnet + cordierite in the more aluminous compositional domain, while gar-
net + orthopyroxene+cordierite resulted in domains of intermediate alumina. During post‐peak cooling, there were localized developments of
biotite + sillimanite+quartz symplectites replacing garnet and cordierite and biotite+quartz intergrowths after orthopyroxene. Application of
garnet‐orthopyroxene and garnet‐biotite Fe–Mg exchange thermometers to co‐existing garnet rim and symplectitic biotite show the extent
of cooling to 610–660°C. Electron microprobe geochronology of texturally well constrained monazites indicates the timing of peak granulite
metamorphism at 3057 ± 17 Ma and its metamorphic reheating at 2781 ± 16 Ma. The present findings when collated with available geological
and geophysical data appear to indicate that the studied granulites and the associated granite gneisses, charnockite and enderbite suite of
rocks of the Rengali Domain are part of the exhumed lower continental crust of the Singhbhum Craton. The significance of this Neoarchaean
orogenesis in the ‘Ur’supercontinent assembly is discussed. Copyright © 2011 John Wiley & Sons, Ltd.
Received 29 November 2010; accepted 5 May 2011
KEY WORDS Singhbhum Craton; Eastern Ghats Mobile Belt; Archaean granulites; monazite geochronology
Supporting supplementary information may be found in the online version of this article.
1. INTRODUCTION
Although the craton–mobile belt interface is one of the criti-
cal areas in an orogen to study geodynamic processes of
terrane amalgamation, the study is beset with difficulties,
particularly in high‐grade orogens (Basu Sarbadhikari and
Bhowmik, 2008). Because of localization of penetrative,
moderate to high‐T ductile shear zone deformations at craton
margins, rocks from these domains undergo widespread fab-
ric transpositions and recrystallization, neo‐mineralization
and also compositional resetting of primary mineralogy,
leading to obliteration of signatures of early metamorphic
parageneses (Bhowmik et al., 1999). This makes it extremely
difficult to distinguish litho‐tectonic components of the cra-
ton from those of the mobile belt. The problem becomes
even more complex, if there is similarity of metamorphic
grades, and where the lack of suitable age constraints does
not allow discrimination of metamorphic P–T–t paths of
cratons from those of younger mobile belts. In a seminal
contribution on the classification of the Mesoproterozoic
Grenville orogen, Rivers et al. (1989) highlighted this issue
by recognizing a thrust‐bounded litho‐tectonic unit, named
the ‘allochthonous polycyclic domain’. This unit, which
occurs in between the mildly reworked cratonic domain
(parautochthonous domain of the authors) and the mobile
belt domain, is generally polymetamorphic, and may bear
*Correspondence to: N. C. Pant, Department of Geology, University of
Delhi, Delhi, India. E‐mail: pantnc@hotmail.com
Copyright © 2011 John Wiley & Sons, Ltd.
GEOLOGICAL JOURNAL
Geol. J. (2011)
Published online in Wiley Online Library
(wileyonlinelibrary.com). DOI: 10.1002/gj.1311
no relation either with the cratons or with the mobile belts.
In contrast, rocks of the mobile belt record a polycyclic meta-
morphic history, commonly with clockwise metamorphic
P–T path. Several recent studies suggest that such polymeta-
morphic domains at craton margins are not uncommon,
and may be general features of orogens (Basu Sarbadhikari
and Bhowmik, 2008; Bhowmik et al., 2010).
Central to identify the ancestry of litho‐tectonic com-
ponents from craton‐mobile belt contacts is, therefore,
to differentiate pre‐orogenic evolution from tectonic evo-
lution during an overprinting orogenic event. In this in-
vestigation, we address this issue using metasedimentary
granulites from a narrow, high‐grade granulite‐granite
gneiss domain around Bhuban‐Murdanga areas within
the Rengali Province (Figure 1a,b). The granulites are lo-
cated in between the Eastern Ghats Mobile Belt (EGMB)
and the Singhbhum Craton (SC) (Figure 1c). Previous stud-
ies by Moitra (1996), Misra et al. (2000) and Bhattacharya
et al. (2004) established an Archaean lineage of some of
the granites, and the associated granulites were considered
to be part of the EGMB. However, there were no detailed
metamorphic and geochronological investigations of the
granulites to link these with those of the EGMB. In this
work, we have carried out petrography, mineral chemistry,
metamorphic reaction history, geothermobarometry and
monazite chemical geochronology to deduce a medium‐
pressure, high‐temperature granulite facies metamorphic event
of Archaean age for the studied granulites. The significance of
the exhumed lower crust of the Archaean Singhbhum Craton
in supercontinent assembly is discussed.
2. GEOLOGICAL SETTING
The Indian Peninsular Shield is a composite collage of
several Precambrian cratonic blocks with a Mid‐Archaean
to Late‐Archaean nucleus, bounded by major fold belts
(mobile belts), along with cover rocks of Proterozoic and
Phanerozoic age, that were accumulated in intracratonic
sags and rifts (Radhakrishna and Naqvi, 1986). These cratonic
Dominant foliation in SC
S - S foliation in EGMB
Mylonitic foliation
Quaternary/Recent sediments Phanerozoic cover Eastern Ghats Province
Krishna Province Jeypur Province Rengali Province
Palaeo-/Meso-Proterozoic Basins
Archaean metamorphic rocks
Pyroxenite/gabbro/dolerite dykes
+ + + Microgranodiorite - granophyre
Peridotite/websterite/gabbro
Tremolite-chlorite schist
Kyanite-muscovite quartzite/BMQ
Migmatitic / banded granite gneiss
Charno-enderbite
Mafic (Opx+Cpx Hbl) granulite+
Sillimanite ( garnet) quartzite+
Cpx bearing gneissic granite
Pyroxenite/gabbro/dolerite dykes
Khondalite
Charnockite-enderbite with cm scale
interbands of megacrystic granite gneiss
Megacrystic granite gneiss
Mafic (Opx+Cpx Hbl Grt) granulite++
......
......
Grt+Pl+Sil+Crd+Opx+Spl gneiss
......
......
Eastern Ghats
Mobile Belt
Rengali Domain Singhbhum Craton
Tectonic Contact
I N D E X
Tectonic Contact
Shear Zone
(1) Rengali, (2) Angul, (3) Tikarpara, (4) Khariar, (5) Rampur, (6) Phulbani, (7) Chilka ake, (8) Vishakhapatnam, (9) Jeypur, (10) Ongole, (11) Vinjamuru and
(12) Udayagiri Domains The star indicates the study area.
L
after Dobmeier and Raith (2003).
Chromite pits
EK-7
CHR-4
?
??
?
?
?
?
?
?
?
?
EGMB
1a
1b
1c
Diatexitic granite
1 0 1 2 km
Figure 1. (a) Inset map of India showing location of Eastern Ghats Mobile Belt, (b) Map of the Eastern Ghats Belt showing different Provinces and
Domains (after Dobmeier and Raith, 2003) and location of study area at the eastern margin of the Rengali Province, (c) Geological map of the study (Bhuban–
Hatibari–Nihalprasad) area in the contact zone between EGMB and SC. This figure is available in colour online at wileyonlinelibrary.com/journal/gj
s. n. mahapatro ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
blocks (viz: the Dharwar or Karnataka Craton, Bastar or
Bhandara Craton, Singhbhum Craton, Bundelkhand Massif
and Aravalli Craton) are flanked by fold belts, with or with-
out discernible suture or shear zones (e.g. Santosh et al.,
2009; Dharma Rao et al., 2011; Kaur et al., 2011; Mohanty,
2011). These may represent the ancient microplates which
moved against each other and collided to generate these fold
belts (Naqvi, 2005) or alternatively, these are the result of
fragmentation of a large craton that constituted the Indian
shield (Sharma, 2009). The fold belts (or mobile belts)
may have evolved through multiple episodes involving sev-
eral tectonic/orogenic processes.
The focus of attention in this contribution is on a narrow
linear WNW–ESE trending zone at the interface between
the southern fringe of the SC and the northern margin of
the EGMB around Bhuban. This intervening area constitutes
the northern extension of the ‘Western Charnockite Zone’
(WCZ) (Ramakrishnan et al., 1998). It has also been de-
scribed variously as the ‘Rengali Assemblage’(Mahalik,
1994), the ‘Rengali Domain’(Nash et al., 1996) or the
‘Rengali Province’(Crowe et al., 2003; Dobmeier and
Raith, 2003). In the present study, the terminology of
Rengali Domain is used, as in this classification different
provinces are demarcated by major lineaments (Nash et al.,
1996). The status of the Rengali Domain (in the north) and
the Jeypur Province towards the west (Figure 1b) separating
the EGMB (Eastern Ghats Province of Dobmeier and Raith,
2003) from the Archaean cratonic blocks has remained unre-
solved, though Archaean ages have been documented from
rocks of these domains (Sarkar et al., 2000; Kovach et al.,
2001; Rickers et al., 2001; Simmat and Raith, 2008). The
Archaean charnockite–enderbite massifs of Riamal–Rengali
(Sarkar et al., 2000) and Bhuban‐Jenapur (Bhattacharya
et al., 2001) areas occurring to the north of the Eastern Ghats
Province are considered as an eastern extension of the WCZ
(Nanda and Pati, 1998; Ramakrishnan et al., 1998).
While the ‘Rengali Province’is considered correlatable
with the Bhandara Craton (Crowe et al., 2003; Dobmeier
and Raith, 2003), the WCZ is considered to be the exhumed
root zone of the cratonic blocks adjoining the EGMB
(Ramakrishnan et al., 1998, Nanda, 2008). The amphibolite
facies lithological assemblage of Rengali Province and the
associated charnockite massifs are thus tacitly inferred to repre-
sent the moderate and deeper tectonic levels of the Bhandara
(Nash et al., 1996; Crowe et al., 2003; Dobmeier and Raith,
2003) and Singhbhum (Nanda, 2008; Ramakrishnan et al.,
1998) cratonic blocks which have been exhumed and subse-
quently emplaced in juxtaposition with the lithological
assemblage of lower metamorphic grade developed at
shallower crustal levels possibly as a result of tectonic
interdigitation and strike slip faults.
The present work on a suite of metasedimentary
supracrustal rocks from this zone aims to provide a
better insight on metamorphic and geochronologic evo-
lution of this segment.
The Singhbhum Craton (or East Indian Craton) and the
Eastern Ghats Mobile Belt bordering to its south have con-
trasting lithologies. The former consists of five principal
litho‐tectonic components viz. (i) supracrustals of the Older
Metamorphic Group (OMG) and the Older Metamorphic
Tonalite Gneiss (OMTG), (ii) massifs of Singhbhum Granite
(phases I, II and III), Bonai Granite, Kaptipada Granite and
Pal Lahara Gneiss, (iii) Iron Ore Group (IOG) dominantly
comprising Banded Iron Formation (BIF) at the margins of
the Singhbhum Granite, (iv) greenstone belts dominantly
comprising volcanics of Simlipal, Dhanjori, Dalma and
several others, and (v) mafic dyke swarm known as the
Newer Dolerite (Bose, 1986; Naqvi and Rogers, 1987;
Saha et al., 1988; Saha, 1994; Goswami et al., 1995; Misra
et al., 1999, 2000; Mukhopadhyay, 2001; Mukhopadhyay
et al., 2008; Sharma, 2009; Ghosh et al., 2010; Tait et al.,
2010). The Precambrian crust of eastern India grew by accre-
tion of younger terranes around this nucleus (Mukhopadhyay,
2001). The sediment deposition is at least 3.5 Ga old in SC
and a metamorphic imprint of ~3.2 Ga is also indicated
(
207
Pb/
206
Pb ion microprobe ages of zircon; Goswami
et al., 1995).
In contrast, the EGMB, extending for over 1000 km in
the southeastern part of the Indian Peninsula (Figure 1a),
is a multiply‐deformed and polymetamorphosed Proterozoic
granulite facies terrain exposed along the east coast of penin-
sular India (Sengupta et al., 1990, 1999; Dasgupta et al.,
1995; Dasgupta, 1998; Ramakrishnan et al., 1998; Dasgupta
and Sengupta, 2003; Gupta, 2004; Das et al., 2008; Vijaya
Kumar and Leelanandam, 2008; Karmakar et al., 2009;
Mukhopadhyay and Basak, 2009 and references cited
therein). The EGMB broadly comprises granulite facies lith-
ological association derived from both sedimentary and
igneous protoliths. These include garnet + sillimanite +
K‐feldspar ± quartz ± graphite ± green spinel‐bearing pelitic
granulite, sapphirine/spinel‐bearing high Mg–Al granulite,
garnet–sillimanite quartzite, calc‐silicate granulite, dolomitic
marble, charnockite suite of rocks comprising orthopyroxene‐
bearing felsic and mafic granulites and gneisses, garnetiferous
gneissic granite and leptynite, massif‐type anorthosite com-
plexes, miaskitic nepheline syenite and ultrapotassic rocks
(Bhowmik et al., 1995; Bhowmik, 1997, 2000; Ramakrish-
nan et al., 1998; Joshi et al., 2004). The EGMB has been de-
scribed as a heterogeneous collage of a number of
longitudinal litho‐tectonic domains (Narayanaswami, 1975;
Ramakrishnan et al., 1998), terranes (Chetty, 1999, 2001,
2010), provinces or domains (Rickers et al., 2001; Dobmeier
and Raith, 2003) having distinct structural and isotopic
characteristics. These domains are said to be separated by a
network of major shear zones and lineaments. The western
contact of EGMB with the Bastar Craton is envisaged as a
ARCHAEAN GRANULITE FACIES METAMORPHISM
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
thrust contact (Nanda, 1995; Biswal et al., 2000; Gupta
et al., 2000; Das et al., 2008). The east–west trending north-
ern contact with SC has also been inferred to be tectonic with
a geomorphic expression (Brahmani River), vaguely known
as the Brahmani lineament and is variously designated as
the Sukinda Thrust (Prasad Rao et al., 1964; Banerji et al.,
1987; Mahalik, 1994; Saha, 1994) and Gohira–Sukinda
shear/thrust belt (Sarkar and Nanda, 1998).
Metamorphic studies over the last two decades have
revealed that the EGMB is one of the few metamorphic
belts, which experienced ultra‐high temperature (UHT)
granulite facies metamorphism at middle to lower crustal
conditions (see Dasgupta and Sengupta, 2003 for a review).
Such extreme metamorphism was largely deduced from high
Mg‐Al granulites (Sengupta et al., 1990, 1999; Dasgupta
et al., 1995; Bose et al., 2006) and rare calc‐silicate granu-
lites (Bhowmik et al., 1995; Bhowmik, 1997). The UHT
metamorphism has a counterclockwise metamorphic P–T
path, involving a prograde, peak and post‐peak nearly
isobarically cooled metamorphic segments (Sengupta et al.,
1990, 1999; Dasgupta et al., 1995; Bose et al., 2006; Das
et al., 2006). The isobarically cooled UHT granulites were
subsequently reworked by a second granulite facies meta-
morphic event. Das et al. (2011) recently constrained a
clockwise metamorphic P–T path for the second granulite
event. These workers additionally established a terminal
shear zone controlled fluid‐infiltration event.
There is controversy on the timing ofthe UHTmetamorphic
event in the EGMB. Available geochronological data show
a spread of dates 1.5 Ga (Shaw et al., 1997) to 1.1–1.2 Ga
(Jarick, 2000) through 1.1–1.25 Ga (Simmat and Raith,
2008). From the Ongole Domain lying to the south of the
Godavari Graben, Upadhyay et al. (2009) recently estab-
lished the timing of the UHT at 1.63 Ga. In a recent
SHRIMP U–Pb zircon and monazite chemical geochronol-
ogy investigations, Das et al. (2011) and Bose et al. (2011)
have provided much younger time constraints of the UHT
metamorphism (~1.0 Ga) from the type Eastern Ghats
Province located north of the Godvari Graben. Such dis-
tinct differences in the age of the granulite facies meta-
morphism across the Godavari Graben were previously
inferred by Mezger and Cosca (1999). Das et al. (2011)
additionally established the timings of the second granulite
facies metamorphic event and its terminal shear zone
reworkings at 0.95 Ga and 0.90 Ga respectively. The impor-
tant outcomes of the study of Das et al. (2011) are: the type
Eastern Ghats Orogeny is of broadly Grenvillian age, and it
involved several short‐lived high‐T granulite facies meta-
morphic events. The Grenvillian age of the granulite
metamorphism, at least from the Eastern Ghats Province
was also predicted by previous studies (Grew and Manton,
1986; Shaw et al., 1997; Mezger and Cosca, 1999; Simmat
and Raith, 2008 and references therein). Recent studies also
predict that several sectors of the EGMB have recorded sig-
natures of pre‐Grenvillian magmatic events and granulite
facies metamorphism in the Early Mesoproterozoic to Late
Palaeoproterozoic time periods (Bose et al., 2011).
A variety of granitoids and gneisses is exposed in the
southern margin of the SC. Granite gneisses occurring in
the south‐central and south‐western parts of SC, in the
Deogarh–Pal Lahara–Kamakhyanagar Belt are variously
designated as Pal Lahara Gneiss (Sarkar et al., 1990),
Palkam Gneiss (Mahalik, 1994). This mesocratic medium‐
grained grey granite gneiss with a subordinate coarse‐
grained leucophase also occurs further to the east in the
Bhuban –Hatibari area, located to the north of the
Brahmani River. These gneisses have several features,
which distinguish them from the Singhbhum Granite
Complex and its temporal equivalents (e.g. Bonai Granite
and Nilgiri Granite). These include (i) syeno‐monzonitic
affinity, (ii) presence of amphibole (hornblende, riebeckite)
as the main mafic mineral with/without biotite, (iii) pres-
ence of accessory magnetite, allanite, zircon and primary
sphene (Mohanty et al., 2008 and Saha, 1994). Besides
these deformed gneisses, a suite of undeformed volcanic
and subvolcanic felsic rocks ranging in composition from
microgranite–granodiorite to rhyolite and rhyodacite occurs
in the southern fringe region of the SC. The granite gneiss
at Bhuban gives minimum ages of their formation as
~2.8 Ga (
207
Pb–
206
Pb of Zircon; Misra et al., 2000). These
workers estimated the age of the cataclastic deformation at
~2.48 Ga by dating the overgrowth over zircons.
3. GEOLOGY OF THE STUDY AREA
Situated at the conjunction of the Eastern Ghats Belt in the
south and the Singhbhum Craton in the north (Figure 1b),
the study area exposes a variety of rock types in an ex-
tremely complex geological setting.
The Balibo–Hatibari region in the northern fringe of
the study area (Figure 1c) exposes amphibole and/or biotite‐
bearing granite gneisses and low‐grade metasupracrustals
(banded chert, banded magnetite chert, kyanite–muscovite
quartzite, tremolite–chlorite schist) that are typical of SC.
Besides, undeformed microgranodiorites and felsic sub-
volcanics are also present. A linear ridge of fine‐to very
fine‐grained, undeformed, mesocratic microgranodiorite‐
granophyre is exposed ~4km north of Hatibari (Figure 1c).
This is intrusive into both the supracrustal sequences and
mafic–ultramafic rocks (harzburgite–websterite–gabbro).
Geological mapping has led to the identification of two
distinct granulite facies lithological associations, exposed
to the south of SC (Figure 1c). The northern part (Bhuban–
Chandar–Ektali area), immediately south of SC, exposes
granulite–granite assemblage dominated by expansive
s. n. mahapatro ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
medium‐grained mesocratic gneissic charnockite, enderbite
and mafic granulite. The supracrustal association, subordinate
in proportion, composed of aluminous metapelite granulite
and quartzite often with sillimanite and garnet, is reported for
the first time from the Chandar and Ektali areas. The granulites
are associated with granite diatexites and clinopyroxene/
amphibole/biotite‐bearing gneissic granite. These metapelites
are interlayered and co‐deformed with the Bhuban charnockite
massif and associated gneissic granites. Locally these contain
veinlets of pseudotachylite, possibly related to subsequent
reactivation. Occurrences of patchy charnockite in outcrops
and gradational relationship between granite gneiss with
charnockite–enderbite at places are observed, indicating sta-
bilization of orthopyroxene in the S‐type, peraluminous gar-
netiferous granites at an intermediate tectonic level where
the dry granulite (charnockite) and amphibole‐biotite bear-
ing gneisses coexist. Boudinaged meta‐mafic dykes of
two‐pyroxene granulite traverse these charnockite‐enderbite
and gneissic granites. In addition, mafic–ultramafic dykes
and sills of websterite, gabbro and dolerite frequently traverse
the granulite–granite lithological association. Small bodies of
layered chromiferous ultramafic–anorthosite–leucogabbro
and pods of chromiferous serpentinized ultramaficrocks
comprising massive chromitite, olivine chromitite, serpenti-
nized dunite, altered orthopyroxenite, peridotite and leuco-
gabbro occur in this domain.
In the area to the south of this charnockite–enderbite–
granite dominant zone, a lithological assemblage of khondalite–
charnockite–leptynite–megacrystic granite gneiss of EGMB
is exposed in the Nua Kashipur, Pingua and Nihalprasad
areas (Figure 1c). Khondalite with garnet + sillimanite +
biotite + quartz + K feldspars ± plagioclase assemblage hav-
ing thin interbands of quartzite (±garnet ± sillimanite) occurs
co‐deformed with charnockite–enderbite. The associated
granitoids of the area include (i) medium‐grained granite
gneiss, (ii) leptynite, and (iii) very coarse‐grained blastopor-
phyritic to porphyroblastic granites with abundant K‐feldspar
megacrysts. The granitoids exhibit metatexite–diatexite
relationship with the khondalite and a mutual structural
conformity. The charnockite and enderbite are commonly
traversed by thin to thick interbands of very coarse‐
grained (biotite ± garnet) megacrystic granite gneiss con-
formable with the gneissosity in the host. This segment is
devoid of websterite, gabbro and dolerite dykes unlike in
the charnockite–enderbite–granite dominant zone, but is
traversed by narrow linear bodies of mafic two‐pyroxene
granulite.
The regional strike of the dominant foliation in all lith-
ological units and the litho‐contacts in this sector is
WNW–ESE with subvertical dip which is in contrast to
the NNE–SSW to NE–SW trend in the southern and cen-
tral segments of the EGMB. The charnockite–enderbite are
characterized by the presence of both mafic schlierens,
as well as a set of penetrative streaky gneissic foliation
superimposed by a mylonitic fabric. Khondalite shows a
compositional banding defined by metamorphic segregation
of alternating quartz–feldspar‐rich and garnet + sillimanite‐
rich layers, besides a pervasive penetrative gneissosity.
Though the regional strike of the dominant foliation in
all the three domains is WNW–ESE, there are differences
in deformation history of each domain. In the northern
part of the EGMB, the earliest identifiable coaxial defor-
mation (D
1
) is manifested by rootless intrafolial tight iso-
clinal reclined folds (F
1
) and a pervasive axial planar
gneissic foliation (S
1
). The S
1
foliation is folded into sub‐
vertical folds (F
2
), some times very steeply plunging either
to east or west, during the second deformation (D
2
) with
ESE–WNW to nearly E–W striking axial planar S
2
foliation.
In this domain, the D
2
deformation is the dominant deforma-
tion episode that controls the regional disposition of the
interbanded charnockite–khondalite litho‐sequence. The
steep southerly and occasional northerly dipping nature
of the lithological contacts (S
0
) and S
1
fabric may be attrib-
uted to the geometry of the F
2
folds. The N–S trending up-
right nature of the third deformation (D
3
) is inferred from
occasional plunge reversal of the F
2
folds. In the granulite–
granite domain between the EGMB and the SC, the earliest
identifiable isoclinal reclined folds (F
1
) are recorded in the
Mg–Al metapelite with the development of a pervasive
gneissic foliation observed both in ortho‐and para‐gneisses.
The second deformation (D
2
) has folded the S
1
fabric into
WNW–ESE trending asymmetric low plunging folds. The
regional disposition of different litho‐units of this domain
is also controlled by D
2
deformation with a distinctly differ-
ent geometry. The third deformation is manifested as N–S
trending upright folds and warps. Dome and basin type inter-
ference pattern is observed due to superposition of D
3
defor-
mation over D
2
deformation. In the craton margin, the third
domain, the disposition of the chert bands and the bedding
(S
0
)–cleavage (S
1
) relation indicate northeasterly plunging
open reclined fold pattern.
The three domains are separated from each other by tec-
tonic contact zones marked by several anastomosing mi-
cro‐domains of dislocation and ductile shearing associated
with rotation of structural trends. In addition, the feature of
grain flattening is common in the entire zone. ENE–WSW,
E–W and WNW–ESE trending anastomosing to sub‐parallel
discrete shear zones characterized by mylonite, ultramylo-
nite and local pseudotachylite veins are delineated. The
contact between khondalite–charnockite dominated EGMB
domain and Bhuban–Chandar–Ektali granite–granulite do-
main is marked by a major brittle–ductile shear zone
with extensive pseudotachylite veins (Patro et al., 2011).
Similarly, the contact region between the SC and granite–
granulite dominant Bhuban–Chandar–Ektali Domain is also
marked by WNW–ESE and E–W trending anastomosing
ARCHAEAN GRANULITE FACIES METAMORPHISM
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
to sub‐parallel discrete shear zones affecting migmatitic
granite gneiss, banded chert occurring in the southern mar-
gin of the SC and the charnockite, sillimanite quartzite of
the Bhuban Domain.
4. PETROGRAPHY AND MINERAL CHEMISTRY
Two metasedimentary granulite samples from Ektali (Sample
EK‐7) and Chandar (Sample CHR‐4) localities are selected
for metamorphic and geochronological studies. In both these
locations, the samples are interlayered with charnockite–
enderbite and granite gneiss (Figure 1c). Sample EK‐7 has
a mineral association of garnet + plagioclase + biotite +
cordierite + quartz + orthopyroxene + ilmenite + rutile +
sillimanite. Chandar sample (CHR‐4) lacks orthopyroxene.
The distribution of the minerals in EK‐7 is shown with
an Al X‐ray element map (Figure 2a) in which the black
colour represents quartz, blue is orthopyroxene, light green
is garnet, dark green is biotite, yellow represents plagioclase
while cordierite is represented in orange colour. This map
shows that EK‐7 is dominated by garnet (~40–50% by mode),
followed by plagioclase (~20–30%), biotite (~10–15%), quartz
(~5–8%), cordierite (~2–5%) with minor orthopyroxene, il-
menite, rutile and sillimanite. Garnet is generally coarse‐
grained, occurring as clusters, particularly in the lower half of
thethinsection(Figure2a). It occurs in two textural modes:
(i) Garnet
1
, coexisting with anhedral cordierite (cordierite
1
),
is porphyroblastic with corroded grain margins and is
replaced and rimmed by biotite (Figure 2b,c,g); (ii) Garnet
2
occurs as idioblastic grains with rare inclusions of tiny silli-
manite and has fine intergrowth of plagioclase and quartz
around it (Figure 2c,e). Porphyroblasts of garnet
1
are locally
surrounded by delicate intergrowth of plagioclase + biotite+
quartz (Figure 2d). The grain boundary contact between gar-
net
2
and biotite is generally straight and smooth (Figure 2e).
Orthopyroxene occurs as scattered porphyroblasts often
in close spatial association with garnet
1
and cordierite
(Figure 2f). Orthopyroxene is replaced by delicate biotite +
quartz symplectites (Figure 2f,g). Cordierite occurs in two
textural modes: (i) cordierite
1
as equant, anhedral grains in
association with garnet
1
(Figure 2a,c) and, (ii) cordierite
2
as criss‐crossly orientated elongated, subhedral to euhedral
grains occurring in close spatial association with garnet
2
and plagioclase + quartz assemblage (Figure 2a,f,g). Ru-
tile in association with ilmenite occurs as inclusions within
garnet
1
(Figure 2g).
In sample CHR‐4, garnet, cordierite and plagioclase por-
phyroblasts are the dominant constituents along with biotite,
quartz and sillimanite (Figure 3a; Back Scattered Electron
(BSE) image of the thin section). Orthopyroxene is charac-
teristically absent in this rock. Garnet (~40%), appearing
brighter in the BSE images, occurs in two modes: (i)
garnet
1
—mainly as anhedral porphyroblasts defining a pla-
nar fabric (Figure 3a,b) and, (ii) garnet
2
—as euhedral grains
in the matrix in contact with plagioclase representing crys-
tallization in the presence of melt (Figure 3c). Subidioblastic
cordierite porphyroblasts (dark in BSE images) are also com-
mon (Figs. 3a,b). Biotite (~15%), having intermediate grey
tone in the BSE image (Figure 3a), is distributed within the
sample. Melt rich domains marked by intergrown plagioclase
and quartz is locally present (Figure 3c). Besides the absence
of orthopyroxene, the presence of prismatic sillimanite in the
matrix in CHR‐4 (Figure 3d) is notable in contrast to EK‐7
where it is present only as rare and tiny inclusions within
garnet
2
. Garnet
1
is replaced by biotite, biotite + quartz,
biotite + plagioclase + quartz with or without sillimanite
(Figure 3d). Rutile occurs as a matrix mineral. Monazite is
present as a significant accessory phase in both the samples,
but its concentration is higher in CHR‐4 where it occurs as
inclusions in garnet
1
as well as in the matrix (Figure 3e).
Representative mineral chemical analysis of EK‐7 and
CHR‐4 are given in Tables 1 and 2 respectively. Analyses
were carried out on CAMECA SX‐100 Electron Probe
Microanalyzer at Geological Survey of India, Kolkata and
Faridabad and Department of Geology and Geophysics,
Indian Institute of Technology, Kharagpur. The accelerat-
ing voltage was kept at 15 kV and the beam current be-
tween 10 and 15 nA. The beam size was kept at ~1 µm for
all analyses. Natural mineral standards were used for most
elements (Orthoclase‐Si and K; Jadeite‐Na; Corundum‐Al;
Wollastonite‐Ca; hematite‐Fe; Rhodonite‐Mn; Apatite‐P;
Chromite‐Cr and synthetic MnTiO
3
for Ti). Representative
mineral chemical data for sample EK‐7 and CHR‐4 are
given in Tables 1 and 2 respectively. In EK‐7, some biotite
inclusions in garnet
1
in proximity to orthopyroxene are sig-
nificantly more magnesian (X
Mg
= 0.89) compared to matrix
biotite (X
Mg
= 0.74) and contain very low titanium (Ti =
0.035 per atomic formula unit (a.f.u.) on 11(O) basis) than
the matrix biotite (Ti = 0.225 a.f.u.) (Table 1). The Mg con-
tent of garnet
2
(X
Prp
~ 0.38) is marginally higher than that of
the garnet
1
(X
Prp
~ 0.35). No significant zoning is noticed in
garnets (Table 1). Orthopyroxene has a uniform composition
(X
Mg
= 0.64–0.67) though the Al‐content shows some vari-
ability (Al = 0.197–0.213 a.f.u. on 6(O) basis). Both textural
varieties of cordierite (X
Mg
= 0.89) have nearly uniform
composition range. Ilmenite is slightly magnesian (MgO =
1.80%). Garnets in CHR‐4 are slightly more magnesian (X
Prp
of garnet
1
~ 0.37 and X
Prp
of garnet
2
~ 0.41–0.42) than in
EK‐7 and both textural variants do not show zoning (Table 2).
However, similar to EK‐7 garnet
2
(~Prp
41
Alm
55
Grs
03
Sps
01
)
is more magnesian than garnet
1
(~Prp
37
Alm
59
Grs
03
Sps
01
).
Matrix biotite (X
Mg
~ 0.75) and plagioclase (~An
30
) in both
the samples are nearly of similar composition while cordier-
ite in CHR‐4(X
Mg
~ 0.86) is less magnesian than in EK‐7
(X
Mg
~ 89) (Tables 1 and 2).
s. n. mahapatro ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
EK-7
Grt1
Bt
Pl
b
Grt1
a
Grt1
Pl+Bt+Qtz
Pl
d
Rt
Ilm
Bt
Opx
Bt+Qtz
g
Grt1
Crd1
Pl+Qtz
Bt+Qtz
Grt1
Grt2
c
Grt2
Bt
Bt
Pl
Qtz
Grt2
e
Bt
Crd1
Pl
Opx
Crd2
Qtz
Pl
Spl
Bt
f
Grt2
Sil
Bt+Qtz
Grt1
Grt2
Figure 2. (a)AlKαX‐ray element map of Sample EK‐7 (see text for description), (b) Back Scattered Electron (BSE) image showing anhedral garnet
1
porphyr-
oblasts (Grt
1
) rimmed by biotite, Note irregular and jagged grain boundary contacts between garnet and biotite (c) BSE image depicting garnet
1
–cordierite
1
(Crd
1
) association. Note delicate plagioclase + quartz (Pl + Qtz) and biotite + quartz (Bt + Qtz) around garnet
1
.(d) Plagioclase + biotite + quartz ((Pl + Bt + Qtz)
intergrowth around garnet
1
. Note large matrix plagioclase (Pl) representing melt. (e) Euhedral garnet
2
(Grt
2
) coexisting with plagioclase (Pl) and quartz
(Qtz). Biotite has smooth grain boundary contacts with this garnet. (f) Orthopyroxene with biotite + quartz (Bt+ Qtz) symplectite around it. Garnet
1
(Grt
1
) which
occurs in association with anhedral cordierite
1
(Crd
1
)is‘floating’within the melt assemblage of plagioclase (Pl) and quartz (Qtz). Note coexisting euhedral
cordierite
2
(Crd
2
) and garnet
2
(Grt
2
) in the lower right half. (g) BSE image showing biotite (Bt) and biotite + quartz (Bt + Qtz) coronas around garnet
1
and ortho-
pyroxene. (Mineral abbreviations—Bt, biotite; Crd, cordierite; Grt, garnet; Ilm, ilmenite; Opx, orthopyroxene; Pl, plagioclase; Qtz, quartz; Rt, rutile; Sil, silli-
manite; Spl, spinel). This figure is available in colour online at wileyonlinelibrary.com/journal/gj
ARCHAEAN GRANULITE FACIES METAMORPHISM
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
CHR-4
a
Pl
Sil
Crd
Bt
Bt Grt1
Bt
b
Grt2
Grt1
Bt
Pl
Qtz
Pl
c
Grt1
Grain 2
Grain 3
Grain 5
Grain 6
Grain 4
e
Sil
Sil
Sil
Sil
Pl
Grt1
Bt+Qtz
d
Figure 3. (a) BSE map of sample CHR‐4, bright anhedral porphyroblast are garnet (see text for further description) (b) BSE image showing porphyroblasts of
garnet
1
(Grt
1
) and cordierite containing inclusions of biotite, sillimanite and quartz, (c) Garnet
1
(Grt
1
) as anhedral porphyrobasts with irregular grain margins
and rimmed by biotite and euhedral garnet
2
(Grt
2
) in juxtaposition to the melt phase represented by plagioclase (Pl) and quartz (Qtz). (d) Extensive biotite +
quartz (Bt +Qtz) symplectites around garnet
1
(Grt
1
). Note presence of prismatic sillimanite in the matrix as well as in inclusion. (e) BSE image showing bright
grains of monazite occurring as inclusions in garnet as well as in the matrix. This figure is available in colour online at wileyonlinelibrary.com/journal/gj
s. n. mahapatro ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
5. METAMORPHIC REACTION HISTORY
5.1. Biotite melting and the stability of peak
metamorphic assemblage
Textural and mineral compositional features attest to a
highly residual character of the studied metapelite granulite
samples with the possibility of significant volumes of melt
loss. The textural features of (a) close network of coarse pla-
gioclase + quartz assemblage with porphyroblastic phases of
garnet
1
, cordierite and orthopyroxene in EK‐7 and with gar-
net and cordierite in CHR‐4, (b) the commonly intergrowth
nature of plagioclase and quartz and (c) the occurrence of
the retrograde biotite + quartz, biotite + plagioclase + quartz
and biotite + sillimanite + quartz intergrowths around the
porphyroblastic garnet and orthopyroxene in contact with
the plagioclase + quartz assemblage collectively indicate
that plagioclase + quartz ± K‐feldspar assemblage represents
crystallized partial melt that were left over after a phase of
melt extraction. Garnet
1
+ aluminous orthopyroxene + cordi-
erite in sample EK‐7 and garnet
1
+ cordierite in CHR‐4
represent peak metamorphic assemblages. The rare appearance
of garnet
1
+ orthopyroxene + cordierite by biotite melting
reactions have been recently investigated by P–T pseudosec-
tion approach (Johnson et al., 2008) and in natural rocks
(Bhandari et al., 2011). Using pseudosection modelling in
the system NCKFMASHTO, and on different greywacke
bulk rock compositions, Johnson et al. (2008) demonstrated
that garnet + orthopyroxene + cordierite assemblage with
biotite + K‐feldspar + plagioclase + quartz + ilmenite + rutile
has a limited stability in P–T space. Bhandari et al. (2011)
noted domainal occurrences of garnet, garnet + cordierite
and garnet + orthopyroxene assemblages in ultrahigh‐
temperature metamorphosed (T ~ 900 °C at P ~ 8 kbar) mag-
nesian metagreywacke protolith from the southern margin
of the Central Indian Tectonic Zone. The authors suggested
Table 1. Representative mineral chemical data from sample no. EK‐7
123456789101112
Mineral Grt
1
Grt
1
Grt
2
Grt
2
Bt Bt Opx Opx Crd Crd Pl Pl
Texture Incl. Matrix
SiO
2
39.31 38.78 39.43 39.56 38.82 37.89 51.61 52.76 49.96 50.01 61.72 61.71
TiO
2
0.00 0.00 0.00 0.01 0.65 4.12 0.04 0.07 0.00 0.00 0.06 0.00
Al
2
O
3
21.80 21.59 22.06 22.27 18.47 16.13 4.56 5.02 33.00 33.35 24.69 24.06
Cr
2
O
3
0.20 0.12 0.01 0.08 0.31 0.54 0.28 0.25 0.11 0.00 0.07 0.04
FeO 28.43 27.91 27.88 26.68 5.64 10.39 20.79 20.32 3.27 3.38 0.00 0.00
MnO 0.86 0.52 0.65 0.55 0.02 0.00 0.09 0.21 0.03 0.05 0.00 0.03
MgO 9.19 9.06 9.77 9.92 21.34 16.94 22.93 23.22 11.84 11.90 0.01 0.02
CaO 1.08 1.10 0.95 1.06 0.00 0.01 0.07 0.03 0.03 0.01 6.26 6.04
Na
2
O0.00 0.04 0.01 0.00 0.00 0.00 0.01 0.01 0.07 0.09 7.96 8.04
K
2
O0.02 0.07 0.00 0.01 9.94 9.63 0.03 0.01 0.00 0.02 0.28 0.17
Total 100.89 99.19 100.76 100.14 95.19 95.65 100.41 101.90 98.31 98.81 101.05 100.11
Oxygen 12 12 12 12 11 11 6 6 18 18 8 8
Si 3.006 2.990 3.007 3.017 2.757 2.756 1.894 1.900 5.019 5.001 2.714 2.736
Ti 0.000 0.000 0.000 0.001 0.035 0.225 0.001 0.002 0.000 0.000 0.002 0.000
Al 1.965 1.963 1.983 2.003 1.547 1.383 0.197 0.213 3.908 3.932 1.280 1.258
Cr 0.012 0.007 0.000 0.005 0.018 0.031 0.008 0.007 0.008 0.000 0.003 0.001
Fe
3+
0.013 0.061 0.004 0.000 0.050 0.000 0.007 0.000 0.055 0.057 0.000 0.000
Fe
2+
1.811 1.800 1.774 1.702 0.285 0.632 0.632 0.612 0.220 0.226 0.000 0.000
Mn 0.056 0.034 0.042 0.036 0.001 0.000 0.003 0.006 0.003 0.004 0.000 0.001
Mg 1.048 1.042 1.110 1.128 2.259 1.836 1.254 1.246 1.773 1.774 0.001 0.001
Ca 0.088 0.091 0.077 0.087 0.000 0.001 0.003 0.001 0.003 0.001 0.295 0.287
Na 0.000 0.005 0.001 0.000 0.000 0.000 0.000 0.001 0.014 0.017 0.678 0.691
K0.001 0.006 0.000 0.001 0.900 0.894 0.001 0.001 0.000 0.002 0.016 0.010
Sum 8.000 8.000 8.000 7.979 7.852 7.759 4.000 3.989 11.002 11.014 4.989 4.985
X
Mg
0.89 0.74 0.66 0.67 0.89 0.89
X
Sps
0.02 0.01 0.01 0.01
X
Prp
0.35 0.35 0.37 0.38
X
Alm
0.60 0.61 0.59 0.58
X
Grs
0.03 0.03 0.03 0.03
X
Or
0.01 0.01
X
An
0.30 0.29
X
Ab
0.69 0.71
ARCHAEAN GRANULITE FACIES METAMORPHISM
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
Table 2. Representative mineral chemical data from sample no. CHR‐4
1 2 3 4 5 6 7 8 9 10 11 12 13
Mineral Grt
1
Grt
1
Grt
2
Grt
2
Grt
1
Grt
1
Grt
2
Grt
2
Bt Bt Crd Crd Pl
Texture RCRCRCRCC
SiO
2
38.77 38.72 39.09 39.17 38.77 38.72 39.09 39.17 36.29 37.07 49.83 49.69 59.82
TiO
2
0.02 0.00 0.06 0.26 0.02 0.00 0.06 0.26 5.21 4.44 0.00 0.01 0.00
Al
2
O
3
21.90 22.09 21.85 21.51 21.90 22.09 21.85 21.51 15.99 16.74 33.67 33.63 24.81
Cr
2
O
3
0.16 0.29 0.05 0.26 0.16 0.29 0.05 0.26 0.52 0.25 0.00 0.00 0.00
FeO 27.30 27.65 25.37 25.90 27.30 27.65 25.37 25.90 9.71 9.90 3.48 3.19 0.10
MnO 0.45 0.56 0.52 0.62 0.45 0.56 0.52 0.62 0.00 0.00 0.00 0.02 0.00
MgO 9.60 9.62 10.95 10.76 9.60 9.62 10.95 10.76 15.61 16.26 11.64 11.53 0.03
CaO 0.99 1.00 1.11 1.23 0.99 1.00 1.11 1.23 0.00 0.00 0.01 0.02 6.97
Na
2
O0.00 0.00 0.02 0.06 0.00 0.00 0.02 0.06 0.16 0.05 0.09 0.05 8.01
K
2
O0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.00 9.98 10.37 0.00 0.01 0.22
P
2
O
5
0.08 0.08 0.07 0.02 0.08 0.08 0.07 0.02 0.00 0.02 0.07 0.00 0.14
Total 99.28 100.01 99.09 99.79 99.28 100.01 99.09 99.79 93.47 95.10 98.79 98.15 100.10
Oxygen 12 12 12 12 12 12 12 12 11 11 18 18 8
Si 3.000 2.981 3.004 3.000 3.000 2.979 3.004 3.000 2.712 2.720 4.996 5.002 2.673
Ti 0.001 0.000 0.003 0.015 0.001 0.000 0.003 0.015 0.293 0.245 0.000 0.001 0.000
Al 1.997 2.005 1.979 1.942 1.997 2.003 1.979 1.942 1.408 1.448 3.978 3.990 1.307
Cr 0.010 0.018 0.003 0.016 0.010 0.018 0.003 0.016 0.031 0.015 0.000 0.000 0.000
Fe
3+
0.000 0.000 0.000 0.000 0.000 0.022 0.000 0.000 0.000 0.000 0.000 0.000 0.000
Fe
2+
1.767 1.781 1.631 1.659 1.767 1.756 1.631 1.659 0.607 0.608 0.292 0.269 0.004
Mn 0.029 0.037 0.034 0.040 0.029 0.036 0.034 0.040 0.000 0.000 0.000 0.002 0.000
Mg 1.108 1.104 1.255 1.229 1.108 1.103 1.255 1.229 1.739 1.779 1.740 1.730 0.002
Ca 0.082 0.083 0.091 0.101 0.082 0.082 0.091 0.101 0.000 0.000 0.001 0.002 0.334
Na 0.000 0.000 0.003 0.009 0.000 0.000 0.003 0.009 0.023 0.007 0.017 0.010 0.694
K0.001 0.000 0.000 0.000 0.001 0.000 0.000 0.000 0.951 0.971 0.000 0.001 0.013
Sum 7.995 8.007 8.003 8.011 7.995 8.000 8.003 8.011 7.763 7.792 11.024 11.007 5.026
X
Mg
0.74 0.75 0.86 0.87
X
Sps
0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
X
Prp
0.37 0.37 0.42 0.41 0.37 0.37 0.42 0.41
X
Alm
0.59 0.59 0.54 0.55 0.59 0.59 0.54 0.55
X
Grs
0.03 0.03 0.03 0.03 0.03 0.03 0.03 0.03
X
Or
0.01
X
An
0.32
X
Ab
0.67
s. n. mahapatro ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
that biotite melting producing the diverse mineral assem-
blages in the scale of a thin section was dictated by local
bulk rock composition. Garnet grew in exclusion of cordi-
erite and orthopyroxene in relatively ferroan bulk rock
composition, while garnet and cordierite appeared in more
magnesian and aluminous compositional domain. The
authors speculated an intermediate to ferroan bulk rock
composition for the growth of the garnet + orthopyroxene
assemblage. The biotite melting reactions suggested by
the authors for NCKFMASH system are listed below:
For garnet alone,
biotite þplagioclase þquartz→garnet
þK−feldspar þmelt (R1)
For garnet + cordierite,
biotite þsillimanite þplagioclase
þquartz→garnet þcordierite
þK−feldspar þmelt
(R2a)
biotite þplagioclase þquartz→garnet
þcordierite þK−feldspar þmelt
(R2b)
For garnet + orthopyroxene,
biotite þplagioclase þquartz→garnet
þorthopyroxene þK−feldspar þmelt
(R3)
In the granulites of the present study, a similar control
of protolith composition on biotite melting and stability
of the peak metamorphic assemblage can be inferred.
Though the ferromagnesian minerals such as garnet, cor-
dierite and biotite in both the samples have the similar
range of X
Mg
, sample CHR‐4 is distinctly aluminous by
having prismatic sillimanites in the matrix. This implies that
the variations in the peak metamorphic assemblages in the
two samples can be attributed to differences in alumina con-
tents in the two protoliths, having broadly the same range of
Mg#. Orthopyroxene‐bearing garnet + cordierite assem-
blage in EK‐7 appeared in relatively alumina‐poor composi-
tional domains, while orthopyroxene‐free garnet + cordierite
assemblage could develop in a more aluminous bulk, fol-
lowing a model reaction of the type (R2a). Reaction (R2a)
operates both in pelitic and greywacke bulk rock composi-
tions and generally marks intermediate‐pressure granulite
facies metamorphism. For sample EK‐7, the combinations
of reactions (R2b) and (R3) in separate microdomains may
explain the stability of the peak metamorphic assemblage
of garnet + orthopyroxene + cordierite. The occurrence of il-
menite + rutile association as inclusion within granet
1
in
EK‐7 implies operation of more complex reactions in the
system NCKFMASHTO. In a recent study on metamorphic
phase relations in metagreywacke bulk rock compositions in
the system NCKFMASHTO predict that reaction (R3) could
progress at temperatures in excess of 820–840 °C in the
pressure range of 5.4–9.2 kbar (Johnson et al., 2008).
Several lines of evidence suggest that the granulite meta-
morphism stabilising peak metamorphic assemblages in the
studied samples via reactions (R1) to (R3) occurred at high
metamorphic temperatures. These are: (a) the restricted oc-
currence of titaniferous biotite as inclusions within garnet
and the near complete elimination of prograde/peak biotite,
the latter occurring as retrograde minerals in the matrix.
This, coupled with the stability of rutile, implies that biotite
completely reacted out, which experimental and pseudosec-
tion modelling studies in appropriate bulk rock compositions
predict temperatures in excess of 900 °C (Stevens et al.,
1997; Nair and Chacko, 2002; Johnson et al., 2008);
(b) The stability of aluminous orthopyroxene and garnet from
biotite melting reaction (R3) (Nair and Chacko, 2002); and
(c) The lack of K‐feldspar in the peak metamorphic assem-
blages also infers peak metamorphic temperatures in excess
of K‐feldspar‐out curve, which is generally placed at tem-
peratures, upgrade to the biotite‐out curve. Calculated P–T
pseudosections in different metagreywacke and metapelites
bulk rock compositions predict K‐feldspar‐out to occur at
temperatures in excess of 850–950 °C for intermediate to
lower crustal granulite metamorphism (White et al., 2001;
Johnson et al., 2008) and it is also supported by experimen-
tal work (Stevens et al., 1997). There are, however, alternate
interpretations, which attribute the lack of K‐feldspar in the
studied rocks to a function of H
2
O/K
2
O ratio in the melt
vis‐à‐vis biotite (Carrington and Watt, 1995). For melts
having relatively lower H
2
O/K
2
O ratio than that in biotite,
K‐feldspar will be a reactant in biotite melting reactions
and will be dissolved in the melt with low a
H2O
.
The texturally distinct garnet
2
in both the samples has
composition quite similar to that of the garnet
1
with slight
Mg enrichment in the former. The euhedral nature of garnet
2
could be on account of growth in the presence of melt during
peak metamorphism. The presence of euhedral cordierite
2
in
association with garnet
2
in EK‐7 suggests two granulite
events.
5.2. Retrograde Metamorphism
In both the samples, there is replacement of the porphyro-
blastic phases by biotite‐bearing assemblages. In sample
EK‐7, the occurrence of biotite + plagioclase + quartz inter-
growths around garnet
1
, and biotite + quartz symplectites
around orthopyroxene, and the development of biotite + silli-
manite + plagioclase + quartz intergrowths around garnet and
cordierite in CHR‐4, collectively, indicate the operations of
the reactions (R1) to (R3) in opposite sense and mark
melt‐crystal interactions during cooling.
ARCHAEAN GRANULITE FACIES METAMORPHISM
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
Summarizing, the peak mineral assemblages in EK‐7 and
CHR‐4 bear evidence for high‐T granulite metamorphism at
intermediate to deep crustal levels. The preservation of a
high‐T mineral assemblage of garnet + cordierite + orthopyr-
oxene + rutile and garnet + cordierite, except localized re-
placement to secondary biotite‐bearing assemblages is at-
tributed to substantial volumes of melt extraction during
peak or subsequent metamorphism.
6. P–T CONDITIONS OF METAMORPHISM
The peak metamorphic condition is further quantified by
applying geothermobarometry to the assemblages, garnet–
orthopyroxene–cordierite–plagioclase–quartz in EK‐7 and
garnet–cordierite–sillimanite‐plagioclase–quartz in CHR‐4.
Mineral chemistry and metamorphic reaction history specify
that the combination of cores of garnet
1
, aluminous orthopyr-
oxene, plagioclase and cordierite with quartz in EK‐7 and
garnet
1
, cordierite, plagioclase with sillimanite and quartz
in CHR‐4 may represent equilibrium assemblages during
peak metamorphism. For quantification of retrograde meta-
morphism, the combination of garnet rim and co‐existing
biotite symplectite is chosen. Temperatures were estimated
using garnet–biotite (Ferry and Spear, 1978; Dasgupta
et al., 1991; Bhattacharya et al., 1992), garnet–cordierite
(Nichols et al., 1992) and garnet–orthopyroxene Fe–Mg ex-
change thermometry (Harley, 1984; Sen and Bhattacharya,
1984; Lee and Ganguly, 1988; Bhattacharya et al., 1991)
and orthopyroxene Al thermometry. The latter is based on
the solubility of Al
2
O
3
in orthopyroxene, co‐existing with
garnet. The temperature is estimated at a reference pressure
in the calibrations of Aranovich and Berman (1997) and
Harley and Green (1982) while for the calibration of Pattison
et al. (2003), which is a Fe–Mg–Al thermobarometer, a con-
vergence technique is used that accounts for late Fe–Mg ex-
change. For the estimations of pressures in EK‐7, the Fe‐
and Mg‐end members (Bhattacharya et al., 1991; Pattison
et al., 2003) and Newton and Perkins (1982) garnet–
orthopyroxene–plagioclase–quartz (GOPS) barometric
formulations are used. For CHR‐4, the pressure is esti-
mated using garnet–aluminosilicate–silica–plagioclase
(GASP) barometer, for which the calibration of Newton
and Haselton (1981) is employed. The results of these cal-
culations are presented in Tables 3A and 3B.
Irrespective of calibrations and mineral assemblages used,
the table shows that the Fe–Mg exchange thermometers record
consistently lower than expected temperatures for peak meta-
morphism in the range of 558–646 °C (garnet–cordierite),
566–651 °C (garnet–biotite) and 606–751 °C (garnet–
orthopyroxene) and the garnet–cordierite–sillimanite–quartz
thermometer also records similar temperatures (~700 °C).
The low temperature estimates are due to Fe–Mg exchange
during cooling and indicate the limitations of conventional
Fe–Mg exchange thermometry to obtain peak metamorphic
temperatures in the granulites (Frost and Chacko, 1989;
Harley, 1989, 1998; Fitzsimons and Harley, 1994; Pattison
and Begin, 1994; Pattison et al., 2003). The calibration of
Pattison et al. (2003), and assuming model 2 for estimating
mole fraction of Al (VI) in orthopyroxene (X
Al
(Opx)¼(Al/2)/2) yields P–T estimate of 7.6–7.9 kbar and
804–845 °C (Table 3). Similar temperatures of 835–897 °C
(Table 3) have been obtained for the formulations of
Harley and Green (1982) and Aranovich and Berman
(1997). Similar to the variation in the T estimates between
the exchange thermometers and the Al‐solubility in ortho-
pyroxene, the GOPS (5.2–6.6 kbars with P
Fe
~6.6 kbar)
and the GASP (6.6 kbar) geobarometers record lower pres-
sures than the calibration of Pattison et al. (2003) which esti-
mates peak pressure in the range of 7.6–7.9 kbars
(Table 3A).
Summarising, we consider P ~ 7.8 ± 0.13 kbar and
T ~ 849 ± 31 °C (the errors being 2σvalues) as peak meta-
morphic conditions for the studied granulites. Thus, the
Ektali and Chandar granulites record near Ultra High Tem-
perature (UHT) metamorphic conditions.
7. DATING OF METAMORPHISM
Chemical geochronology of 6 grains of monazite from meta-
pelite samples CHR‐4 and EK‐7 was carried out using a
4‐spectrometer SX‐100 CAMECA Electron Microprobe
(EPMA) at the Department of Geology and Geophysics,
Table 3A. Results of geothermobarometry in EK‐7
Fe–Mg exchange (Grt–Opx) Fe–Mg exchange (Grt–Crd) Al solubility GOPS Barometry
T
LG
T
H
T
SB
T
BKRS
T
N
P
Ref.
T
P
T
HG
T
AB
P
P
P
NP
P
B(Mg)
P
B(Fe)
T
Ref
692 616 639 648 558 8 845 835 873 7.9 5.2 5.2 6.4 850
681 606 625 638 567 8 826 849 899 7.8 5.8 5.8 6.6 850
738 662 700 682 807 835 871 7.8
751 674 717 693 804 849 897 7.6
s. n. mahapatro ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
IIT Kharagpur and 2 grains of monazite using SX‐100
CAMECA EPMA from the Geological Survey of India
(GSI), Faridabad. The line measured included PbMα,
UMβ, ThMα,YLα, LaLα, CeLα, PrLβ, SmLα,HoLβ,
DyLα, GdLβ, SiKα, Al,Kα, CaKαand PKα. Pb was mea-
sured on LPET crystal for better counting statistics. Inter-
ference of YLλon PbMαand ThMζon PbMαwas cor-
rected by measuring the interfering lines and applying
the overlap correction. The other experimental conditions
are detailed in Pant et al. (2009), Bhowmik et al. (2010)
and Bhandari et al. (2011). Besides checking the calibrations
by analyzing the standards as unknown, the veracity of the
chemical geochronology was verified using an IDTIMS
dated monazite standard (Crowley et al. 2005). Against the
given age of 2645 Ma, the chemical geochronology during
the session of analysis gave an age of 2579 ± 62 Ma. At
GSI, Faridabad an IDTIMS dated monazite (Ravikant et al.
2007) of 639 ± 9 Ma was simultaneously analyzed and an
age of 623 ± 39 Ma was obtained. Six grains from CHR‐4
(Figure 4a) and two grains from EK‐7 (Figure 4b) have been
investigated in detail. Grain 2 from CHR‐4 is from a domi-
nantly restitic domain (Figure 3e). Because garnets are ex-
tensively fractured, it is difficult to find completely
shielded inclusion of monazites in garnet. These six mona-
zite grains annotated with ages are shown in Figure 4a.
Yttrium Lαmap of the grain 2 of CHR‐4 shows an early
stability of Y‐rich monazite, which now occurs as variously
resorbed relics, being surrounded by Y‐poor monazite.
Relics of Y‐rich monazite within Y‐poor monazite composi-
tional domain are also noted in monazite grain 6 of CHR‐4.
Monazite spot ages in both the Y‐rich and Y‐poor composi-
tional domains from grain 2 vary in the range 2.9 to 3.1 Ga.
In grain 6, the spot ages range from 2.6 to 3.3 Ga with the
dominant cluster is at 2.9 Ga. In the Y‐poor compositional
domain of the same grain, the spot ages show a range from
2.6 to 3.1 Ga. Grain 3, which has an idioblastic habit and
shows weak zoning in Th shows monazite spot ages in the
range of 2.8 to 3.2 Ga, with a cluster at 3.1–3.2 Ga. Monazite
grains 4 and 5 are ovoid in shape. Monazite spot ages in both
these grains show a spread from 3.1–3.2 Ga to 2.8–2.9 Ga.
Grain 7 is elongate, has an idioblastic habit and is fractured.
Monazite spot ages in this grain are slightly younger and
show a spread from 3.0 to 2.5 Ga. Two monazite grains
of EK‐7 record near identical spot ages in the range of
2.7–2.8 Ga, with complete absence of 3.0–3.2 Ga age
domain as recorded in CHR‐4.
The age data for these two samples are further processed
and displayed in Figure 5. The Chandar sample (CHR‐4)
when unmixed (Ludwig, 2001) is resolved into two popula-
tion i.e. 3058 ± 17 and 2798 ± 23 Ma (Figure 5a). In EK‐7a
dominant 2776 ± 21 Ma population (representing 90% frac-
tion) and a subordinate 2517 ± 99 Ma age population are
observed (Figure 5b). In Figure 5c, the spot age data of the
two samples are combined and processed for unmixing.
Two age populations of 3057 ± 17 Ma and 2781 ± 16 Ma of
nearly equal proportion are distinguishable.
While the analysis of the monazite spot ages reveal an
unequivocal Archaean ancestry to the investigated metape-
lite granulites, it becomes important to correlate the two
age domains to specific metamorphic events. Monazite
textures give some indication in this regard. Textural features
in monazite grains 2 and 6 attest to fluid or melt‐assisted
partial dissolution of a Y‐rich monazite and subsequent
re‐precipitation of Y‐poor monazite. Since the rock recorded
a high‐T (T ~ 850 °C) biotite melting, which also produced
incongruent garnet solid, the latter having high partition co‐
efficient of Y (with respect to melt), the Y‐poor monazite
can be considered to be in equilibrium with garnet. This
implies that the partial dissolution of an early monazite
andgrowthofneo‐monazite could be explained within
the realm of granulite facies partial melting and crystalli-
zation of anatectic melt. If true, this would suggest that
the growth of Y‐poor monazite was linked to the peak
granulite facies metamorphism. The near identical ages
of both older (Y‐rich) and younger (Y‐poor) monazite
domains also suggest complete compositional resetting
Table 3B. Results of geothermobarometry in CHR‐4
Fe–Mg exchange (Grt–Bio) Fe–Mg exchange (Grt–Crd) Grt–crd–sil–qtz thermometer Grt–aluminosilcate–plag–qtz barometry
T
B
T
D
T
FS
T
N
P
Ref.
T
C
P
NH
570 614 644 646 8 698 6.6
572 611 639 645 8 700 6.6
566 610 631
Abbreviations used for T (°C) and P (kbar) estimates: P
Ref
/T
Ref
: Reference P and T; T
LG
,T
H
,T
SB
,T
BKRS
,T
N
,T
HG
,T
AB
; T calculated using
the calibrations of Lee and Ganguly (1988), Harley (1984), Sen and Bhattacharya (1984), Bhattacharya et al. (1991), Nichols et al. (1992);
Harley and Green (1982); Aranovich and Berman (1997); P
NP
,P
B(Mg/Fe)
:P
NP
,P
B
; P calculated using the calibrations of Newton and Perkins
(1982), Bhattacharya et al. (1991); T
p
and P
p
: temperature and pressure estimate using the calibration of Pattison et al. (2003) with XAlOpx
(1‐site Opx) = (Al–(2‐Si))/2; T
B
,T
D
,T
FS
,T
C
,P
NH
: T calculated using Bhattacharya et al. (1992), Dasgupta et al. (1991), Ferry and Spear
(1978), Currie (1971); P calculated using Newton and Haselton (1981).
ARCHAEAN GRANULITE FACIES METAMORPHISM
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
CHR-4
Grain 5
3167
3134
2828
3144
2947
2992
2880
3110
Grain 3
3165
3128
3012
3188
3131
2824
Grain 4
2800
2960
2862
2911 2804
2775
3152
3022
3079
2946
3194
3020
3061
3070
3076
3021
3008
2937
3024
2947
2904
Grain 2
2624
2860
2982
2885 2963
2745
2876
2867
3284
3060
2580 2909
2895 2711
2989
2897
2957
2544
2689
2987
2819
Grain 6 Grain 7
s. n. mahapatro ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
of Th, U and Pb at 3.06 Ga, which is also consistent
with high metamorphic temperatures. Based on these
arguments, we consider 3.06 Ga as the age of peak gran-
ulite facies metamorphism. 2.78 Ga age domain may re-
flect a second high‐grade metamorphic event, the support
of which comes from the presence of younger 2.7 to
2.8 Ga monazite spot ages in a milieu of 3.1 to 3.2 Ga
age domains in sample CHR‐4. This can be attributed
to partial Pb‐loss due to metamorphic re‐heating of the
older monazites.
8. DISCUSSION
The petrographical, mineral chemical, metamorphic and
geothermobarometric studies of the metapelite interbands
within charnockites at Ektali and Chandar reveal a medium‐
pressure, high‐T granulite metamorphism at the contact
zone between the EGMB and the SC. The peak granulite
metamorphism caused extensive biotite melting in the meta-
sedimentary protoliths, producing porphyroblastic garnet +
orthopyroxene + cordierite incongruent solids in relatively
alumina‐deficient bulk rock composition and porphyroblas-
tic garnet + cordierite‐bearing ones in more aluminous com-
positions. Subsequent melt extractions produced a residual
character of the studied metapelite granulites. A phase of
post‐peak cooling is indicated by the developments of bio-
tite + quartz symplectites replacing orthopyroxene and bio-
tite + sillimanite + quartz symplectites replacing garnet and
cordierite. Geothermobarometric study constrains the ex-
tent of cooling to ~600 °C. The appearance of garnet
2
asso-
ciated possibly with cordierite
2
indicates a second garnet
forming stage. This garnet might be a product of the same
metamorphic event or a product of an unrelated later meta-
morphic event. Available geochronological data (this study,
see below) appear to support the second possibility.
Based on electron microprobe geochronology of monazite
grains, we constrain the timing of the peak granulite meta-
morphism at 3.06 Ga. This puts the granulites of the investi-
gated area as part of select Archaean granulite terrains of the
Peninsular India. This age is close to the 3.09 Ga age
reported for the last major plutonic activity in the SC (Misra
et al., 1999). Although meagre, the available monazite age
data also raise the possibility that the 3.06 Ga granulites
were re‐heated by a second metamorphic event at 2.78 Ga.
Evidence of a 2.8 Ga event in the same belt was previously
documented by Misra and his co‐workers in the form of
magmatic emplacement of granites in Bhuban–Rengali sec-
tor (Bhattacharya, 1997; Misra et al., 2000). However, de-
tailed U–Pb zircon studies are required to further confirm
the polymetamorphic history of the studied granulites.
High‐grade metamorphic rocks along ancient sutures are
thought to represent upthrusted exposures of lower continent
crust (Fountain and Salisbury, 1981), although some of the
recent studies relate them to a wedge extrusion model asso-
ciated with subduction–collision process (Maruyama et al.,
2010; Santosh et al., 2010). The interface between the
two contrasting terrains can be evaluated through: (a) juxta-
position of dissimilar terrains across major structures, (b)
contrasting ages, (c) different heat flows between the juxta-
posed terrains, (d) paired gravity anomaly, (e) dipping sub‐
Grain 1
2814
2815
Grain 2
2766
28322775
2702
2772
2517
2746
2741
EK-7
Figure 4. Chemical age data from monazite grains (a) six grains from
CHR‐4 with spot ages plotted on the respective monazite BSE images.
For grains 2,3,4 and 5, the bar scale represents 50 µm while for grains 6
and 7 it represents 100 µm. For grains 2,6 and 7 the spot chemical ages
are plotted on yttrium LαX‐ray map and for grains 3,4 and 5 on BSE
images. (b) two grains from EK‐7 with spot ages plotted on the respective
monazite BSE images. Bar scale is 20 µm for grain 1 and 50 µm for grain
2. This figure is available in colour online at wileyonlinelibrary.com/
journal/gj
ARCHAEAN GRANULITE FACIES METAMORPHISM
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
0
2
4
6
8
10
12
14
16
18
1600 2000 2400 2800 3200 3600 4000 4400
Age fraction
2798 23 0.41 0.18
3057.9 17 0.59 ---
relative misfit = 0.717
0
1
2
3
4
5
6
7
2200 2400 2600 2800 3000 3200
Age fraction
2517 99 0.10 0.20
2776 21 0.90 ---
relative misfit = 0.848
CHR-4 (6 Grains)
EK-7 (2 Grains)
CHR-4 & EK-7 (8 Grains)
0
2
4
6
8
10
12
14
1600 2000 2400 2800 3200 3600 4000 4400
Age fraction
2780.8 16 0.50 0.18
3057.3 17 0.50 ---
relative misfit = 0.635
a)
b)
c)
±2σ
±2σ
±2σ
±2σ
±2σ
±2σ
Age in million years
Age in million years
Age in million years
Frequency
Frequency
Frequency
Figure 5. Probability density plot for chemical age data and weighted mean ages of analysed monazite grains for (a) sample CHR‐4 (six grains) (b) sample
EK‐7 (two grains) and (c) all eight grains of samples CHR‐4 and EK‐7. This figure is available in colour online at wileyonlinelibrary.com/journal/gj
s. n. mahapatro ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
surface structures, and (f) contrasting metamorphic grades.
The Bhuban–Hatibari–Nihalprasad area is located at the in-
terface of the Archaean Singhbhum Craton to the north and
the Proterozoic EGMB to the south. The heat flow in
EGMB is higher than in the SC across this interface (Senthil
Kumar et al., 2007). A paired gravity anomaly is present in
the area north and south of Bhuban–Hatibari–Nihalprasad
area (Kumar et al., 2004) with EGMB having steep‐dipping
(−50 to +30 mGal) higher gravity signatures and nearly uni-
form low ~ −60 mGal gravity values in the adjacent cratonic
areas (Ramesh et al., 2010). Using P‐and S‐receiver func-
tion data, preservation of a pre‐Grenvillian, westerly‐dipping
palaeosuture has recently been advocated extending up to
160–200 km depths (Ramesh et al., 2010). In the tectonic
model envisaged by these authors, formation of the Western
Charnockite Zone (WCZ of Ramakrishnan et al., 1998) is
considered as a distinctly earlier event than the main phase
of EGMB and involves a pre‐Grenville age continent–
continent collision.
The collision of two crustal blocks of different mechan-
ical boundary layer thickness, differing gravity, as well as
heat flow attributes, is likely to produce a complex lithol-
ogy at the suture zones, wherein some of the lighter
cratonic lithosphere may be delaminated and upthrusted,
thereby exposing the lower continental crust. The granu-
lites of the lower continental crust of the cratonic block
will, thus, be juxtaposed against the high‐grade metamor-
phic rocks of the mobile belt. Since, the grade of meta-
morphism is likely to be similar, the most distinguishing
criteria for cratonic and mobile belt signature in such
zones can be the different age signature of the two ter-
ranes. The enclave association in the present area with
respective terrane components appears to have been essen-
tially derived on the basis of the grade of metamorphism,
with the granulitic enclaves being assigned to EGMB and
the low‐to medium‐grade enclaves to the SC (Mahalik,
1994; Moitra, 1996).
The Archaean ages reported from SC and EGMB are
summarized in Tables 4 and 5 respectively. Archaean ages
in EGMB have been recorded only in the marginal areas
(Rengali Province in the northern part and Jeypore Province
in the western part) (Table 5) where formation of the igneous
protoliths before 3.0 Ga (Kovach et al., 2001; Rickers et al.,
2001) and granulite facies metamorphism at ~2.5 Ga
(Kovach et al., 2001; Simmat and Raith, 2008) were sug-
gested. No Archaean age granulites have been reported from
Table 4. Some relevant geochronologic data from the Singhbhum Craton
Age Method Reference
Older metamorphic tonalitic gneisses (OMTG)
3.55 Ga U–Pb zircon Goswami et al. (1995)
3.38 Ga Whole rock Rb–Sr Moorbath et al. (1986)
3.0–3.2 Ga Rb–Sr, K–Ar and Ar–Ar Saha (1994)
Singhbhum granitic complex
3.3 Ga (Phases I and II) Moorbath and Taylor (1988),
Ghosh et al. (1996), Saha
(1994),
Misra et al. (1999)
3.1 Ga (Phase III) Saha et al. (1988), Saha (1994)
Bonai granitic complex
3.37–3.38 Ga Pb–Pb and U–Pb (zircons) Sengupta et al. (1991, 1996)
For high Al
2
O
3
trondjhemite xenoliths
3.16 Ga Pb–Pb for host trondjhemite
Mayurbhanj granite
3092 ± 5 Ma and 3080 ± 8 Ma
207
Pb/
206
Pb zircon ages Misra et al. (1999)
Granitoids of Deogarh–
Pallahara–Bhuban Belt
2.8 Ga
207
Pb/
206
Pb (zircon) Misra et al. (2000)
3.5 Ga Xenocrystic zircons from granitic gneisses zircon
overgrowth
Misra et al. (2000)
ca. 2.48 Ga
Dacitic lava from southern
IOG greenstone succession,
Tamka–Daitari Range
3.51 Ga U–Pb SHRIMP zircon age Mukhopadhyay et al. (2008)
Tamparkola granite‐acid volcanics
2809 ± 8 Ma (Granite) and 2836 ± 67
(Rhyolite)
In situ Pb–Pb (zircon) dating Bandyopadhyay et al. (2001)
Baula–Nausahi Complex
3122 ± 5 Ma Zircon (SHRIMP) age of gabbroic rocks Auge et al. (2003)
ARCHAEAN GRANULITE FACIES METAMORPHISM
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
interiors of EGMB. Thus, the Rengali Province has an
Archaean age affinity, which is distinctly different from the
rest of the EGMB. The present area corresponds to the
southeastern part of the Rengali Province. Though the
Kerajang Fault in this province has been proposed to be
the boundary between SC and EGMB (Nash et al.,
1996), the presence of amphibolite facies assemblages to
the south and granulite grade assemblage to the north of this
fault (Crowe et al., 2003) does not indicate it to be a bound-
ary fault. The available geochronological data in this prov-
ince indicate dominantly Archaean signatures (Table 5).
Rb–Sr whole‐rock isochron ages for charnockite in the west-
ern extension of this segment in the Rengali–Riamal sector
are 2743 ± 103 and 2735 ± 44 Ma respectively (Sarkar
et al., 2000). The granulite facies metamorphism and gener-
ation of charno‐enderbites in Jenapore have been dated at
3030 ± 200 Ma (Sm–Nd and Rb–Sr whole‐rock) and
2930 ± 42 Ma (
207
Pb/
206
Pb zircon; Bhattacharya et al.,
2001). Misra et al. (2000) infer a minimum age of 2.8 Ga
(
207
Pb/
206
Pb of zircon) for gneisses of the Bhuban area at
the SC–EGMB interface.
Our present work unequivocally shows a high tempera-
ture granulite facies metamorphic event of ~3.06 Ga age in
the Bhuban–Hatibari–Nihalprasad area, which does not ap-
pear to belong to the tectonometamorphic history of the
EGMB. In conjunction with the geological, geophysical
and geochronological data it is inferred that the supracrustal
rocks at Chandar and Ektali (CHR‐4 and EK‐7) represent the
upthrusted slices of lower continental crust of the Archaean
Singhbhum Craton.
9. IMPLICATIONS FOR UR ASSEMBLY
The major cratons that became stable in the late Archaean
(~3000 Ma) include Kaapvaal, Madagascar, Aravalli,
Dharwar, Bundelkhand, Singhbhum, Napier, western Dron-
ning Maud Land, Vestfold, Pilbara, Kimberley, Gawler and
Yilgarn and since these belonged to the same region of
Pangaea, they were considered to have together formed one
large Archaean age continent named as Ur (Rogers, 1993,
1996; Rogers and Santosh, 2003; Eriksson et al., 2009).
Typically, the stabilization of a continent is through several
processes which include juvenile crustal growth, deposition
of supracrustal rocks and orogenic processes normally at the
continent margins, anorogenic plutonism and deposition of
platform sediments. Evidence of these processes, thus, will
have significant bearing on the nature of the supercontinent.
The largest preserved component of the Ur is present in the
Indian shield in the form of Aravalli, Dharwar, Bundelkhand
and Singhbhum nuclei. The reworking of the early
formed rocks in these nuclei should provide the strongest
evidence of the existence of Ur. In this context, there is
Table 5. Archaean dates from Eastern Ghats Belt
Rock Locality Method Age (Ma) References
Rengali Province
Charnockite Riamal Rb/Sr whole rock isochron 2743 ± 103 Sarkar et al. (2000)
Charnockite Rengali Rb/Sr whole rock isochron 2735 ± 44 Sarkar et al. (2000)
Charnockite and mafic granulite Jenapore Sm–Nd whole rock isochron 3030 ± 200 Bhattacharya et al. (2001)
207
Pb–
206
Pb Zircon 2930 ± 42
87
Sr /
86
Sr 3236 ± 206
Granite gneiss Bhuban
207
Pb–
206
Pb Zircon 2811 ± 3 Misra et al. (2000)
87
Rb /
86
Sr 2924 ± 35
Granite gneiss North of the Kerajang Fault
207
Pb–
206
Pb ages of zircon minimum
age for the granulite facies metamorphism
~2.8 Ga Crowe et al. (2003)
Misra et al. (2000)
Jeypore Province
Charnockite WCZ TIMS U–Pb of Zircon 2703 ± 7 to 3433 ± 27 Kovach et al. (2001)
Enderbites Nd model ages (TDM) 3.0–3.9 Ga Kovach et al. (2001)
Rickers et al. (2001)
Granitoid emplacement and
granulite facies metamorphism
Jeypore Province U–Pb of zoned zircons ~2.7 Ga Kovach et al. (2001)
~2.5 Ga Simmat and Raith (2008)
s. n. mahapatro ET AL.
Copyright © 2011 John Wiley & Sons, Ltd. Geol. J. (2011)
DOI: 10.1002/gj
increasing new data from the Indian shield, as well as
from other Archaean nuclei, which sheds light on the
neo‐Archaean tectonic processes and indicates similarities
with the modern plate tectonic processes (Rapp et al., 2003;
Moyen et al., 2006; Menneken et al., 2007; Saha et al.,
2011). Though in the initial proposal the Bundelkhnad–
Aravalli nucleus was not considered to be part of Ur, the
geochronological data indicates it to be part of this assembly
(Mondal, 2009), with the Central Indian Tectonic Zone
(CITZ) being the present‐day suture between Bundelkhand–
Aravalli and the rest of the Archaean nuclei, and the age of
the orogenesis of the suture being ~1.6 Ga (Bhandari et al.,
2011). The oldest ages in the Singhbhum nucleus are
~3.55 Ga (Goswami et al., 1995). The Chandar and Ektali
supracrustals (metapelites) which have been inferred to be
the components of the Singhbhum Craton, indicate that prior
to ~3.07 Ga, sedimentary processes were operative. Orogen-
esis is also indicated by recent data in the Dharwar nucleus
(Jayananda et al., 2012; High T–low P metamorphism at
~2625 Ma) and in Bundlekhand nucleus (Saha et al., 2011;
High P metamorphism at ~2.78 Ga), thereby indicating that
the stabilization of Ur possibly continued up to ~2.6–2.8 Ga.
ACKNOWLEDGEMENTS
We thank M. Santosh and Somnath Dasgupta for inviting us
to present our work in the Special Issue of the Geological
Journal on ‘Indian Precambrian: correlations and con-
nections’. SNM and AKT acknowledge the Director
General, Geological Survey of India for permitting the pub-
lication of the work. The electron microprobe work was
carried out at DST‐IIT Kharagpur National facility at
Department of Geology and Geophysics, IIT Kharagpur
and Geological Survey of India, Faridabad while the SEM‐
EDS were done at Department of Geology, University of
Delhi. A. Kundu, Sonalika Joshi and Aloka Dey provided
support in the analysis. The paper was greatly improved by
critical reviews by Rosaria Palmeri and an anonymous
reviewer.
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