age of southern granulite terrain

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Please cite this article in press as: Collins, A.S. et al., Age and sedimentary provenance of the Southern Granulites, South India:

U-Th-Pb SHRIMP secondary ion mass spectrometry, Precambrian Res. (2007), doi: 10.1016/j.precamres.2007.01.006

ARTICLE IN PRESS+Model

PRECAM-2771; No.of Pages14

Precambrian Research xxx (2007) xxxxxx

Age and sedimentary provenance of the SouthernGranulites, South India:

U-Th-Pb SHRIMP secondary ion mass spectrometry

Alan S. Collins a,, M. Santosh b, I. Braun c, C. Clarka

a Continental Evolution Research Group, Geology & Geophysics, School of Earth and Environmental Sciences,

The University of Adelaide, Adelaide, SA 5005, Australiab Faculty of Science, Kochi University, Akebono-cho 2-5-1, Kochi 780-8520, Japan

c Mineralogisch-Petrologisches Institut, Universitat Bonn, Poppelsdorfer Schlo, 53115 Bonn, Germany

Received 4 August 2006; received in revised form 17 January 2007; accepted 29 January 2007

Abstract

Southern India lies at a junction in the Gondwana-forming orogenic belts, between the East African Orogen and the Kuunga

Orogen. It contains voluminous high-grade metasedimentary gneisses that make up an important component of the record of

collision and amalgamation of Gondwana. Here we present U-Pb Secondary Ion Mass Spectrometry (SIMS) isotopic data from

detrital zircon cores from throughout southern India that demonstrate dominant Neoarchaean to Palaeoproterozoic age components

that are incompatible with the known ages of potential southern and central Indian source regions. The original sediments to the

Trivandrum Block gneisses were deposited between1900 and515 Ma, whereas a sample from the Achancovil Unit, and possible

also a sample from the Madurai Block, were deposited in Neoproterozoic times. We speculate that these rocks broadly correlate

with southern and western Malagasy metasedimentary rocks (including the Itremo and Molo Groups) and formed an extensive basin(or basins) that lay on the west side (present orientation) of the Neoproterozoic continent Azania. In addition, metamorphic zircon

from four samples yielded an age of 513 6 Ma that is interpreted as dating high-grade metamorphism throughout much of the

Southern Granulite Terrane.

2007 Elsevier B.V. All rights reserved.

Keywords: Gondwana; SIMS U-Pb geochronology; Zircon; Southern India; Provenance; Metamorphic age

1. Introduction

The Ediacaran-Cambrian assembly of Gondwana

occurred by the collision and amalgamation of numerous

continental blocks along a number of disparate orogenic

belts (see Collins and Pisarevsky, 2005). In recent Neo-

proterozoic palaeogeographic reconstructions, India did

not amalgamate with the other Gondwanan continents

Corresponding author. Fax: +61 8 8303 3174.

E-mail address: [email protected](A.S. Collins).

until latest Neoproterozoic or Cambrian times (Torsvik

et al., 2001; Powell and Pisarevsky, 2002; Meert, 2003;

Boger and Miller, 2004; Collins and Pisarevsky, 2005).

In these reconstructions, southern India and adjacent

regions of Madagascar, Sri Lanka and Antarctica are

located at the meeting point of a number of separate

orogenic belts that formed as India, Australia, Azania,

Kalahari and Antarctica terranes collided to form Gond-

wana (Fig. 1).

Despite this key location within the Gondwana coali-

tion, and the potential of the protoliths to the high-grade

metasedimentary rocks of southern India to delineate

0301-9268/$ see front matter 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.precamres.2007.01.006
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U-Th-Pb SHRIMP secondary ion mass spectrometry, Precambrian Res. (2007), doi:10.1016/j.precamres.2007.01.006



2 A.S. Collins et al. / Precambrian Research xxx (2007) xxxxxx

Fig. 1. (a)Reconstructionof part of central Gondwana at530 Ma (present continental outlines reconstructed for Gondwana usingthe reconstruction

ofReeves and de Wit, 2000). The details of southern India summarised from Geological Survey of India (1994). (b) Palaeogeographic reconstructionof the Neoproterozoic continents in the Gondwana fit of Reeves and de Wit (2000) highlighting the location of the present study (after Collins

and Pisarevsky, 2005). Achan: Achancovil shear zone; Az: Azania; Congo: Congo/Tanzania/Bangweulu Block; Ir: Irumide Belt; KKPT: Karur-

Kamban-Painavu-Trichur isotopic boundary after Ghosh et al. (2004); MB: Madurai Block; PCSS: Palghat-Cauvery shear zone system after Chetty

et al. (2003); Ruker: Ruker Terrane of the southern Prince Charles mountains, Antarctica; Tanz: Tanzania craton; TB: Trivandrum Block; Ubende:

Ubende belt; Us: Usagaran belt. Locations of samples discussed in this paper are illustrated.

the shapes of the colliding continents and constrain

the collisional history, very little is known about their

age, or provenance. Recent work on metasedimentary

rocks from southern and central Madagascar (adjacent

to southern India in GondwanaFig. 1a) has shown

how the U-Pb isotopic information locked up in the

cores of detrital zircon grains can not only constrain

the age of metasedimentary rocks, but can also help

unravel the locations of sediment source regions and

the sites of suture zones (Collins et al., 2003b; Cox

et al., 2004; Fitzsimons and Hulscher, 2005). In this

contribution we examine the U-Th-Pb isotopic record
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A.S. Collins et al. / Precambrian Research xxx (2007) xxxxxx 3

preserved in detrital zircon grains from metasedimentary

rocks of the Indian Southern Granulite Belt to constrain

the age of deposition of the protoliths and examine the

age-provenance record preserved in the zircon cores.

2. Geological framework

Southern India (Fig. 1a), south of the Archaean/

Palaeoproterozoic Dharwar craton, consists of Palaeo-

proterozoic (Peucat et al., 1993) orthogneisses, metased-

imentary rocks, and charnockites that continue as far

south as the Palghat-Cauvery shear zone system (Drury

and Holt, 1980; Drury et al., 1984; Chetty, 1996; Chetty

et al., 2003). The >100 km wide, crust-cutting (Reddy

et al., 2003a), Palghat-Cauvery system of anastomos-

ing shear zones, also known as the Cauvery shear zone

(Chetty et al., 2003; Chetty and Bhaskar Rao, 2006),cuts migmatitic mafic gneisses, and high-pressure gran-

ulites (Bhaskar Rao et al., 1996; Srikantappa et al.,

2003; Shimpo et al., 2006; Collins et al., in press).

The Madurai and Trivandrum Blocks (Harris et al.,

1994) lie south of the Palghat-Cauvery shear zone sys-

tem, separated from each other by the Achancovil shear

zone, an enigmaticstructure with a pronounced magnetic

(Rajaram et al., 2003) and seismic anomaly (Rajendra

Prasad et al., 2006). Both the Madurai and Trivandrum

Blocks were extensively deformed and metamorphosed

to granulite-facies during the Neoproterozoic (Bartlett

et al., 1998; Braun et al., 1998; Braun and Kriegsman,

2003; Santosh et al., 2003, 2005a; Braun and Brocker,

2004).

Geochronological information from the Madurai and

Trivandrum Blocks of southern India consist of sepa-

rate studies using Sm/Nd data (Harris et al., 1994, 1996;

Jayananda et al., 1995; Bhaskar Rao et al., 1996, 2003;

Meiner et al., 2002), Rb/Sr data (Bhaskar Rao et al.,

1996, 2003), electron-probe U-Th-Pb data (Braun et al.,

1998; Santosh et al., 2003, 2005a,b, 2006a,b), single zir-

con 207Pb-206Pb data (Jayananda et al., 1995; Bartlett et

al., 1998; Ghosh et al., 2004) and reconnaissance ther-mal ionisation U-Pb zircon data (Soman et al., 1995). To

date, the only ion microprobe studies have been under-

taken by Ghosh et al. (2004). These data have broadly

indicated the age of igneous protoliths and highlighted

the Neoproterozoic age of high-grade metamorphism,

but have not shed much light on the age of the protoliths

to the extensive metasedimentary rocks found through-

out the region. Here, we present new U-Th-Pb ion-probe

data, obtained using the Sensitive High Resolution Ion

MicroProbe (SHRIMP), from both the Trivandrum and

Madurai Blocks.

3. Analytical techniques

Zircon grains were separated from crushed rock sam-

ples by conventional magnetic and methylene iodide

liquid separation. Grains were handpicked and mounted

in epoxy resin discs that were coated with a thin mem-

brane of gold that produced a resistively of 1020 across the disc. The mounts were then imaged using

a CL detector fitted to a Phillips XL30 scanning elec-

tron microscope at a working distance of 15 mm and

using an accelerating voltage of 10 kV. The resulting

images (Fig. 2) highlight distortions in the crystal lat-

tice (Stevens Kalceff et al., 2000) that are related to

trace-element distribution and/or radiation damage (e.g.

Rubatto and Gebauer, 2000; Nasdala et al., 2003).

Zircon U-Th-Pb isotopic datawere collectedusing the

Sensitive High Resolution Ion Microprobe Mass Spec-

trometer (SHRIMP II)based in the John de Laeter Centreof Mass Spectrometry, Perth, Western Australia. The

sensitivity for Pb isotopes in zircon using SHRIMP II

was 18 cps/ppm/nA, the primary beam current was

2.53.0 nA and mass resolution was 5000. Correc-

tion of measured isotopic ratios for common Pb was

based on the measured 204Pb in each sample and often

represented a


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Fig.2. Cathodoluminescence images of selectedzircon grains. Ages presented as 206Pb/238U ages or, if indicatedby an asterisk, 207Pb/206Pb ages. All

errorsquoted at the1level. (A) Sample I04-01. Poorly cathodoluminescent grains with rarely preserved oscillatory-zoned detrital cores. (B) Sample

I03-18. Subhedral zircon with partial oscillatory-zoned discordant Mesoproterozoic core mantled with low Th and U brightly cathodoluminescent

Cambrian rim. (C) Sample I03-18. Three subhedral to anhedral detrital zircon grains. (D) Sample I04-04. Multiple detrital zircon grains. (E) Sample

K1/2. Detrital zircon grains with oscillatory-zoned cores partially reset during the Neoproterozoic/Cambrian. (F) Sample I03-20. Euhedral zircon

with a series of rims around a poorly cathodoluminescent Palaeoproterozoic core. The luminescent zones in the core are parallel with those in

the rims, suggesting that the rims may have formed by solid-state recrystalisation or an originally prismatic crystal. (G) Sample I03-20. Bright

cathodoluminescent Archaean oscillatory-zoned core mantled by a poorly luminescent inner rim that irregularly truncates the core. This inner rim

is in turn mantled by an oscillatory-zoned Cambrian rim. (H) Sample I03-21. Subhedral zircon with oscillatory-zoned Archaean core surrounded by

a poorly luminescent irregular metamict inner rim that is mantled with a homogenous brightly luminescent rim. (I) Sample I03-21. Late Archaean

sector zoned core mantled with a brightly luminescent homogenously luminescent rim.

preserve oscillatory zoning with bright centres and dark

margins (Fig. 2A).

4.1.2. I03-18

I03-18 is a garnet + biotite gneiss sample from a

sunken quarry directly south of the Ovari to Nan-

guneri road30 km south of Tirunelveli (N082818.6,

E774022.2) (Fig. 1a). The rock is compositional

banded and folded into open folds. Dark nebulous

orthopyroxene + quartz bearing veins and pods occur

that resemble incipient charnockite veins reported by

Santosh et al. (1990). These veins and pods commonly

follow cross-cutting felsic veins and locally inter-finger

along foliation planes. Graphite veins cut the outcrop.

Cathodoluminescent images of sectioned zircon

grains show a diverse series of morphologies and lumi-

nescence responses (Fig. 2B and C). Zircon grainsrange from subhedral prismatic morphologies with

aspect ratios of5:1 to nearly equant squat anhedral

ovoids. Many zircon grains have distinct cathodo-

luminescent cores that preserve oscillatory zoning

(Fig. 2B) and are mantled with more homogenous

rims that show up as bright (Fig. 2B) or dark

(Fig. 2C) luminescent zones. Many grains, and grain

cores, display truncated cathodoluminescent zones that,

along with the rounded nature of the grains, are

interpreted here as resulting from sedimentary abra-

sion.


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4.1.3. I04-04

This sample is of a garnet + biotite + sillimanite +

cordierite metapelite and was collected from a small hill

near the village of Thalavapuram, 1 km southwest of

Eruvadi (N082444 E773634). The rock is boudi-

naged and shows evidence of in-situ charnockitization

(Chetty and Bhaskar Rao, 2004). Braun (in Chetty andBhaskar Rao, 2004) described how this charnockitiza-

tion grades into massive garnet-bearing and garnet-poor

enderbites in adjacent outcrops. Zircon grains from this

sample are rounded and preserve complex CL patterns

(Fig.2D). Relatively bright CL cores grade outwardsinto

dark margins. Distinct rims are uncommon, but where

they exist, they are thin and luminesce brightly (Fig. 2D).

4.1.4. K1-2

K1-2 is a garnet-biotite gneiss from Mannanthala,

very close to Trivandrum. Zircon grains from this sampleare rounded and commonly preserve oscillatory-zoned

cores under CL. These zones are often truncated by the

grain margins, probably due to sedimentary abrasion

(Fig. 2E). Nebulous, homogenous, poorly luminescent

rims to some grains invade the cores forming concave

and linear salients (Fig. 2E).

4.2. Sample from the Madurai block (between the

Achancovil lineament and the Palghat-Cauvery

shear zone system)

4.2.1. I03-20

I03-20 is a quartzite sample from the hills

directly north of Ganguvarpatti village (N101030.8,

E774146.3) (Fig. 1a). The quartzite forms a promi-

nent 15m thick ridge dipping 70 towards the

south-east. The ridge can be traced at least 2 km across

the landscape. Below the quartzite band are leucocratic

gneisses with biotite pseudomorphs after garnet. Above

the quartzite are sapphirine-bearing pelites that record

peak-thermal metamorphic temperatures of >1000 C

(Mohan and Windley, 1993; Raith et al., 1997).

Zircongrains from I03-20have a variety of morpholo-gies ranging from euhedralprismatic crystals with aspect

ratios of 3:1 (Fig. 2F) to anhedral equant rounded

grains. Many crystals preserve cores, when imaged using

cathodoluminescence, with complex multiple rim tex-

tures (Fig. 2F and G). In some grains, fine luminescent

zones appear to transgress rim boundaries (e.g. ghost

zoning passing from bright inner rim to dark outer rim

in Fig. 2F). These crystals also preserve similar crystal

faces in the core as in the rim (Fig. 2F). Similar features

have been used to argue for a solid-state recrystallisa-

tion mechanism of rim formation (Hoskin and Black,

2000; Collins et al., 2004; Love et al., 2004). In many

casesthe rimspreserveoscillatory zoning thatcommonly

indicates zircon crystallisation from a melt (Corfu et al.,

2003). No granitic veins were seen in the outcrop mak-

ing it unclear whether these textures represent either: (1)

zircon crystallisation from a melt that has subsequently

migrated out of the rock; (2) zircon crystallisation from alow-volume metamorphic fluid that uncharacteristically

resulted in oscillatory zoning; or, (3) relict ghost zon-

ing of the original detrital grain that is still preserved

although isotopic and Th and U abundances have been

reset.

5. Secondary ion microscopy results (SIMS)

SIMS data are presented in Appendix B available

from the on-line version of this paper, geochronologi-

cal interpretations are presented in Figs. 35 whereas Thand U geochemical results are presented in Fig. 6.

5.1. I04-01

Fourteen zircon cores were analysed from sam-

ple I04-01. Four analyses were


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Fig. 3. U-Pb concordia plots of pre-latest Neoproterozoic analyses.


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Fig. 4. U-Pb concordia plots of Neoproterozoic-Cambrian analyses.

analysis is included, the resulting discordia curve has

an upper intercept of 529 18 Ma (2, MSWD = 1.02)

and a zero lower intercept (Fig. 4). The observation that

these analyses are predominantly low Th/U rims (Fig. 6)

lead to our interpretation that these ages date new zircon

growth during high-grade metamorphism.

The detrital cores from this sample dominantly

yield Mesoproterozoic and Palaeoproterozoic ages

(Figs. 3 and 4), but the presence of a number of10% discordant data that are interpreted as reflect-

ing Late Archaean to Palaeoproterozoic sources for the

protolith zircon grains (Figs. 3 and 5). Core analy-

ses that are


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Fig. 5. Probability density distribution plots.

three analyses yields an age of 2005 24Ma (2,

MSWD = 0.51, Fig. 3). Along with the lone analyses

of 2190 27 Ma, three Palaeoproterozoic source com-

ponents appear to be present. The >10% discordant

analyses also have a similar trimodal 207Pb/206Pb age

distribution (Fig. 5) that strengthens the suggestion that

these 1907, 2005 and 2190 Ma ages are significant

components of the source region.

A weighted mean of three concordant analyses

from sample K1-2 gives an age of 514 16Ma (2,

MSWD=2.7, Fig. 4) that is identical to the lower inter-

cept of a discordant array of detrital core analyses


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Fig. 6. Thorium and uranium geochemistry of analysed zircon grains.


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(518 32 Ma, Fig. 4). These zircons are homogenous in

CL. The coincidence of the lower intercept of the detri-

tal discordant array with new zircon growth (with Th/U

ratios 10% concordant (67%) and show that the

protolith to the sample contains zircon grains sourced

from Late Archaean and Palaeoproterozoic rocks with

major age components at 2700, 2260, 2100 and

1997 Ma (Figs. 3 and 5). One detrital core yields and

age of 695 16 Ma, which may indicate that this rock

was deposited in Neoproterozoic times. A distinct pop-

ulation of concordant zircon rims is interpreted to

date high-grade metamorphism at 508.3 9.0 Ma (2,MSWD = 1.7, Fig. 4).

6. Discussion

6.1. Th-U geochemistry of zircon grains and

processes of rim formation

The concentrations of thorium and uranium, and the

ratio of these cations, commonly vary between pro-

tolith zircon cores and zircon that formed, or was highly

altered, during a subsequent metamorphic event (Maaset al., 1992; Hoskin and Schaltegger, 2003; Collins et al.,

2004). In the samples analysed here, three relationships

between protolith zircon composition and interpreted

metamorphic zircon (either rims or homogenous zircon

interpreted as completely recrystallised pre-existing zir-

con) are seen (Fig. 6). Samples I03-18, K1-2 and I04-04

show decreased Th/U ratios from pre-metamorphic to

metamorphic zircon caused by an increase in U in the

syn-metamorphic zircon. In sample I03-20, the concen-

tration of U and Th values from core to rim does not

change dramatically, but instead covers a much more

restricted range of values. These analyses still maintaina reasonable spread of Th/U ratios (0.6-0.1).

A decrease in Th/U ratio is predicted during solid-

state recrystallisation (Hoskin and Black, 2000) of

pre-existing zircon due to greater incompatibility of the

Th ion in the zircon lattice than the smaller U ion (Maas

et al., 1992; Hoskin and Schaltegger, 2003). All sam-

ples show a broad trend of decreasing 232Th/238U with

age with distinct CL rims and unassigned analyses in all

samples except I04-01 showing lower Th232/U238 values

than core analyses (Fig. 6). The analysed samples show

that this decrease in Th232

/U238

ratio was largely due

to an absolute decrease in Th and increase in U in the

metamorphically-altered analyses (Fig. 6).

6.2. Age of metamorphism

A number of different samples have yielded data

that we interpret as providing estimates for the age ofhigh-grade metamorphismin the SouthernGranuliteTer-

rane. These data are from: (1) concordant analyses that

approximate the lower intercepts of discordant arrays

of partially reset detrital cores (I04-03 and K1-2); (2) a

discrete, concordant, population of zircon rims (sample

I03-20); and (3) the upper intercept of a slightly discor-

dant array of zircon rim analyses (I03-18). A weighted

mean of these results from individual samples yields an

age of 513 6 Ma (2, MSWD = 1.4), which is our best

estimate of the time of high-grade metamorphism. We

note that the estimates from individual samples collectedfrom throughout the Trivandrum and Madurai Blocks do

not differ in age, and therefore, there is no diachroneity

in the timing of high-grade metamorphism between the

Trivandrum and Madurai Blocks.

Ediacaran-Cambrian U-Th-Pb Electron Probe Micro-

Analysis (EPMA) monazite ages from the region have

been interpreted by a number of authors as estimates of

the timing of metamorphism. The results of the present

study are broadly consistent with the EPMA ages that

range from 610 to 470 Ma (Cenki et al., 2002; Braun

andBrocker,2004; Santosh et al., 2005a, 2006a,b)notethat the EMPA technique can not investigate the pres-

ence of common Pb or discordance in the analyses and

therefore has more inherent uncertainties than isotopic

techniques such as SIMS. The presented zircon data do

not contain evidence of an earlier Neoproterozoic high-

grade event that has been reported from EMPA monazite

studies (e.g. Braun and Appel, 2006). This may be a con-

sequence of sampling bias during preparation and in the

future could be addressed with an in-situ isotopic zircon

analytical campaign.

6.3. Age constraints of deposition

All samples contain evidence for a considerable

Palaeoproterozoic detrital input into their original sedi-

mentarymakeup(Figs.3and5), with theyoungest


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analyses interpreted as constraining the age of deposi-

tion to younger than 695 16 Ma, in the case of I03-20

from the Madurai Block, and 728 21 Ma, inthe case of

I03-18fromthe Achancovil unit ofBraun andKriegsman

(2003).

The samples analysed here were collected from a

large area of high-grade metasedimentary rocks andtherefore there is no necessity that the protoliths to

these rocks were deposited within the same sedimentary

system. Three samples from south of the Achancovil

shear zone (I04-01, I04-04 and K1-2) are conceivably

Palaeoproterozoic in age. Whereas the protoliths to I03-

18, from the Achancovil Unit close to the Achancovil

shear zone, and I03-20, from the central Madurai Block,

are likely to have been deposited in Neoproterozoic

times. The Achancovil Unit has younger Nd model ages

than the surrounding terranes (Nd TDm model ages of

1.4 1.3 Ga, Harris et al., 1994; Bartlett et al., 1998)that support the detrital zircon evidence that the pro-

toliths to these metasedimentary rocks were deposited

in Neoproterozoic times.

6.4. Provenance implications

There are possible Indian sources for some of the

detrital zircon grains. For example, rocks dated between

1880 and 1700 Ma are found in the Krishna province of

the Eastern Ghats (Dobmeier and Raith, 2003), zircon

xenocrysts date back to 2431 Ma and a detrital grain hasbeen recorded as having a 207Pb/206Pb age of 2747 Ma

(Shaw et al., 1997). However, no source in the Eastern

Ghats is known for the prominent 23001990 Ma peaks

in the age spectra from many of the samples (Fig. 5).

The Mesoarchaean to Neoarchaean Dharwar craton lies

directly north of the Southern Granulite Terrane and pro-

vided >3.0 Ga detritus into Dharwar-derived sediments

throughout the Proterozoic (Collins et al., 2003b). No

pre-3.0 Ga zircon ages were obtained from any sam-

ples analysed in this study. This lack of zircon detritus

that can be fingerprinted to southern India as well as

the missing Indian sources for the Palaeoproterozoicdetrital zircon grains suggests that the protoliths of these

Southern Granulite Terrane metasedimentary rocks may

be sourced from non-Indian terranes.

Madagascar lay directly west (present direction) of

India in Gondwana (Fig. 1) and contains similar high-

grade metasedimentary rocks in the south and west

of the island (Nicollet, 1983; Windley et al., 1994;

Collins et al., 2003a; Fernandez et al., 2003; Cox et al.,

2004; Fitzsimons and Hulscher, 2005; Collins, 2006).

These metasedimentary rocks comprise of two succes-

sions; one successionof probable Palaeoproterozoic age,

known as theItremo Group (Coxetal.,1998), which con-

tains Palaeoproterozoic and Neoarchaean detrital zircon

grains; a second succession of Neoproterozoic age (the

Molo Group) with Neoarchaean, Palaeoproterozoic and

Neoproterozoic detrital zircon grains (Cox et al., 2004;

Fitzsimons and Hulscher, 2005). These successions lay

adjacent to each other in Gondwana (Fig. 1a) and havebroadly similar detrital zircon recordsdominated by

Palaeoproterozoic sources that are not easily attributed

to India and havean absence of >3.0 Ga detritus. We here

suggest that Malagasy and southern Indian sequences

represent parts of the same sedimentary basins.

Both Cox et al. (2004) and Fitzsimons and Hulscher

(2005) suggested that the Itremo and Molo Groups were

sourced from eastern Africa. Eastern Africa has poten-

tial source rocks for all the age peaks derived from

southern India in this study. The Tanzanian craton and

Usagaran/Ubende and Irumide orogens contain numer-ous 2.7, 2.3 1.8 Ga granitoid rocks (Lenoir et al.,

1994; Borg and Krogh, 1999; Reddy et al., 2003b;

Sommer et al., 2003; Collins et al., 2004; Johnson

et al., 2005; de Waele et al., 2007). Mesoproterozoic

rocks dated at 1.4 Ga are found in the Kibaran belt

(Kokonyangi et al., 2004) and early/mid Neoproterozoic

magmatic rocks occur in central Madagascar (Handke et

al., 1999; Kroner et al., 2000) and the Mozambique belt

(Kroner et al., 2003).

In the palaeogeographicinterpretationpresentedhere,

the Palghat-Cauvery shear zone represents a Neopro-terozoic suture zone delineating the southern margin of

Neoproterozoic India and separating it from a south-

ern microcontinent represented by the Neoarchaean

metagranitoids exposed in the northern Madurai Block

(Fig. 1, Bhaskar Rao et al., 2003; Ghosh et al., 2004) that

is correlated with Azania.

7. Conclusions

Granulite-facies metasedimentary rocks from south-

ern India preserve detrital zircon cores that indicate

that these rocks were sourced from a predominantlyNeoarchaean to Palaeoproterozoic source region more

compatible with eastern Africa than with Peninsula

India. Two samples preserve near-concordant Neopro-

terozoic zircon cores that suggest that the Achancovil

unit, at least, was deposited between 728 21 Ma and

the age of metamorphismthe best estimate of which

comes from zircon rims that yield an age of 513 6Ma.

This 515 Ma age appears consistent across the South-

ern Granulite Terrane.

The metasedimentary rocks for the Southern Gran-

ulite Terrane are correlated with the metasedimentary


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rocks of southern and western Madagascar in partic-

ular, the Itremo and Molo groups and are interpreted

as a southern fragment of the Neoproterozoic continent

Azania (Collins and Pisarevsky, 2005).

Acknowledgements

The zircon analyses were carried out on the Sensitive

High-mass Resolution Ion Microprobe (SHRIMP II) at

the John de Laeter Centre for Mass Spectrometry, Perth

and operated by a consortium consisting of Curtin Uni-

versity of Technology, the Geological Survey of Western

Australia, and the University of Western Australia with

the support of the Australian Research Council. We

appreciate the assistance of Ian Fitzsimons, Peter Kinny,

Sasha Nemchin and Allen Kennedy during SHRIMP

analysis and data reduction. This manuscript contributes

to IGCP project 509 (Palaeoproterozoic SupercontinentsandGlobal Evolution).This is a contribution to Grant-in-

aid No. 17403013 to M. Santosh from the Japan Ministry

of Education, Sports, Culture, Science and Technology.

The authors thank two anonymous reviewers and the

editorial assistance of Peter Cawood for suggesting sub-

stantial improvements to the manuscript.

Appendix A. Supplementary data

Supplementary data associated with this article can be

found, in the online version, at doi:10.1016/j.precamres.2007.01.006.

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