Enriched Subcontinental Upper Mantle beneath Southern India: Evidence from Pb, Nd, Sr, and CO...

JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 PAGES 1765–1785 1998

Enriched Subcontinental Upper Mantlebeneath Southern India: Evidence from Pb,Nd, Sr, and C–O Isotopic Studies on TamilNadu Carbonatites

HELMUT SCHLEICHER1∗, ULRICH KRAMM2, ERNST PERNICKA3,MANFRED SCHIDLOWSKI4, FATEMEH SCHMIDT5,V. SUBRAMANIAN6, WOLFGANG TODT4 ANDSHRIVINAS G. VILADKAR7

1MINERALOGISCH–PETROGRAPHISCHES INSTITUT UNIVERSITAT HAMBURG, GRINDELALLEE 48, D-20146 HAMBURG,

GERMANY2INSTITUT FUR MINERALOGIE UND LAGERSTATTENLEHRE, RHEINISCH–WESTFALISCHE TECHNISCHE HOCHSCHULE

AACHEN, AACHEN, GERMANY3MAX-PLANCK-INSTITUT FUR KERNPHYSIK, POSTFACH 10 39 80, D-69029 HEIDELBERG, GERMANY4MAX PLANCK-INSTITUT FUR CHEMIE, POSTFACH 3060, D-55020 MAINZ, GERMANY5MINERALOGISCH–PETROGRAPHISCHES INSTITUT UNIVERSITAT HEIDELBERG, IM NEUENHEIMER FELD 236,

D-69120 HEIDELBERG, GERMANY6STATE GEOLOGICAL SURVEY, MADRAS, TAMIL NADU, INDIA7GEOLOGY DEPARTMENT, ST XAVIER’S COLLEGE, 400 001 BOMBAY, INDIA

RECEIVED OCTOBER 1, 1997; REVISED TYPESCRIPT ACCEPTED APRIL 16, 1998

The major, trace and rare earth element distributions as well as the contrast to the lead data, the Nd–Sr isotope data show a clearNd, Sr, Pb, C and O isotope geochemistry of four carbonatite signature of a mantle source excluding crustal contamination. Theoccurrences (Sevattur, Jogipatti, Samalpatti, Pakkanadu) from Tamil data are compatible with either an EM I-like mantle componentNadu, southern India, have been investigated. The majority of these 800 my ago or a mixing process between two mantle reservoirs.carbonatites are highly enriched in light rare earth elements (LREE)

and exhibit stable isotope patterns similar to those reported for

primary mantle-derived carbonatitic melts. In Sevattur, a well-

defined lead/lead isochron yields an age of 801 ± 11 Ma for

ankeritic carbonatites. Most of the samples from the other Tamil KEY WORDS: carbonatites; geochemistry; India; isotope geochemistry; REE

Nadu carbonatites define an array in the lead system which is

interpreted as a mixing line between two reservoirs, namely a depleted

mantle and a U-enriched component (either crustal lead or anINTRODUCTIONenriched mantle reservoir). The carbonatites are characterized byOn account of some very peculiar features, such as (1)very low present-day eNd values between –8·8 and –20·1 and highhigh to extremely high Sr and Nd contents which in87Sr/86Sr ratios (0·7045–0·7054). At 800 Ma, the isotopic

signature was similar to EM I, but somewhat more enriched. In most of the cases buffer their primary isotopic signatures

∗Corresponding author. e-mail: [email protected] Oxford University Press 1998

JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 OCTOBER 1998

against crustal contamination (Bell & Blenkinsop, 1987; GEOLOGY AND PETROGRAPHYSchleicher et al., 1990; Simonetti et al., 1995), (2) a deep- In southern India, in North Arcot and Dharampuriseated origin within the mantle, and (3) widespread districts of Tamil Nadu a large number of Proterozoicoccurrence in continental areas, carbonatitic rocks are carbonatite complexes (Fig. 1) are emplaced within theparticularly suitable to characterize the nature and the Precambrian granulite terrains (Southern Ghats Terrain,isotopic evolution of the subcontinental upper mantle SGT) along NE–SW trending fault systems (e.g. Su-(e.g. Bell & Blenkinsop, 1989). So far, only few carbonatite bramanian et al., 1978; Subramanian, 1983; Viladkar &complexes are known for which evidence for crustal Subramanian, 1995). These complexes belong to Pre-contamination not only in the Pb isotopic system, but cambrian alkaline magmatism (~1600–600 Ma) withinalso in the Nd and Sr isotopic systems was reported (e.g. the Eastern Ghats Mobile Belt (EGMB) of eastern andGardar Province, Andersen, 1997; Pearce & Leng, 1996; southern India. Ratnakar & Leelanandam (1989) listedFen, Andersen, 1987). For most of the carbonatite oc- more than 40 alkaline plutons in this region. As a segmentcurrences of the world, a time-integrated Rb/Sr- and of the Gondwana granulite belt the EGMB representsNd/Sm-depleted mantle source can be assumed (e.g. Bell an isostatically uplifted boundary zone of the Dharwar& Blenkinsop, 1987, 1989; Nelson et al., 1988; Schleicher province and is considered to represent a fault-boundedet al., 1990). However, Nd and Sr isotope measurements ensialic rift zone (Leelanandam, 1993). Most of theseof carbonatites from continental rift environments have alkaline complexes are located near the western marginclearly demonstrated a heterogeneity of the mantle and of the EGMB and seem to be confined to the junctionthe mixing of two or even more mantle components. between the cratonic (non-charnockitic) and the mobileThis is particularly true for the young East African belt (charnockitic) regions of southern India.carbonatite complexes (Bell & Blenkinsop, 1987; Bell & According to Ratnakar & Leelanandam (1989), theSimonetti, 1996), where the nature of the mantle res- alkaline magmatic activity of the EGMB is generally laterervoirs most probably can be attributed to HIMU and than the Eastern Ghat orogeny. They considered theEM I. Also in the case of Amba Dongar, India (Simonetti alkaline and carbonatitic rocks to represent the latestet al., 1995) and, to a lesser degree, for the Kaiserstuhl intrusive stage within this mobile belt. However, somecarbonatites (Schleicher et al., 1990, 1991) mantle mixing alkaline complexes are distinctly older and were involvedprocesses were shown to be involved. Carbonatites typ- in the orogeny (e.g. Elchuru; Czygan & Goldenberg,ically are associated with alkaline rocks, and both pref- 1989). In the southern part, the carbonatitic complexeserentially occur within regions of continental rifting. were mainly emplaced within extensive shear zones (seeThus, although they originate from the upper mantle Fig. 1, inset), which are interpreted as the boundarytheir emplacement is mainly governed by the tectonic between crustal fragments of provinces of different ageresponse of the continental crust to mantle processes. (e.g. the Palghat–Cauvery shear zone, Unnikrishnan-

So far only few isotopic data from Indian carbonatites Warrier et al., 1995). The investigated carbonatites arehave been published. Deans & Powell (1968) were the first located within a dominant shear zone between the Madrasto present Sr isotope ratios for some Indian carbonatite Block and the Northern Block of the SGT; they arecomplexes. Later, Krishna et al. (1991) reported Nd, Pb associated mostly with pyroxenites and nepheline-freeand Sr ratios for Sung Valley carbonatites (Shillong), and syenites (usually with small amounts of modal quartz),Anil Kumar & Gopalan (1991) reported initial Sr isotope and with dunites which form small plugs in some areas.compositions for Sevattur carbonatites and pyroxenites Within the Sevattur carbonatite complex (SCC) car-by Rb/Sr mineral isochron dating. Simonetti et al. (1995) bonatites show a wide variation in their mineralogicalpublished Pb, Sr, Nd and C–O isotopic data for Amba composition. The main mass, consisting entirely of coarse-Dongar carbonatites (Gujarat), and Viladkar & Schid- grained dolomitic carbonatite, forms a crescent-shapedlowski (in preparation) following with a comprehensive intrusive body which is in contact with pyroxenites insurvey of the Amba Dongar carbon–oxygen geo- the west and northwest, with Peninsular gneiss in thechemistry. More recently, Schleicher et al. (1997) pre- southwest, and with trachytic syenite in the east (Fig. 2).sented lead/lead dating on the Newania and Sevattur The carbonatite incorporates a number of xenoliths ofcarbonatite complexes. The aim of the present study basement gneisses, syenite and pyroxenite (Borodin et al.,is (1) to provide new insight into the nature of the 1971; Krishnamurthy, 1977; Viladkar & Subramanian,subcontinental mantle below the southernmost part of 1995). Both sovite and ankeritic carbonatite form isolatedPeninsular India (Tamil Nadu), and (2) to contribute to outcrops as thin dykes. Besides the carbonates—calcite,the problem of crust–mantle interaction during car- dolomite and ankerite—phlogopite, magnetite and ap-bonatite emplacement, on the basis of high-precision Pb, atite are very common, with minor occurrences of mon-Nd, Sr, C and O isotope ratios, and trace element azite, zircon, amphiboles, baddeleyite, urano-pyrochlor

and allanite in various proportions. Bands of apatite,concentrations, particularly those of REE.

1766

SCHLEICHER et al. TAMIL NADU CARBONATITES

Salem

Tirupattur

N

Kolar Gold Fields

Syenite

Dunite

Carbonatite

F

F

EG

MB

SGT

Alkalinecomplexes

Sevattur

Jogipatti

Samalpatti

Pakkanadu

Fig. 1. Geological sketch map showing the location of Tamil Nadu carbonatite occurrences. Inset: dots, carbonatite and alkaline complexes;grey bands, major shear zones; SGT, Southern Granulite Terrain; EGMB, Eastern Ghats Mobile Belt; F–F, Fermor Line, which separates thecharnockitic from the non-charnockitic (cratonic) region (Fermor, 1936).

magnetite and ferromagnesian minerals are interpreted of 700 ± 30 Ma. The individual carbonatites occurmostly in the form of elongated bodies and dykes inas the result of igneous cumulate crystallization (Krishna-

murthy, 1977; Viladkar & Subramanian, 1995). Ac- pyroxenites and, to a lesser extent, in syenites ( Jogipatti)along a discontinuous ring around a central syenitecording to Udas & Krishnamurthy (1970), sovite was

emplaced first, and later veined by dolomitic carbonatite. mass. The main types are calcite–dolomite carbonatite,dolomite carbonatite, ankeritic carbonatite and peg-In contrast, Borodin et al. (1971) considered the dolomitic

carbonatite was later calcitized by hydrothermal al- matitic carbonatite. Some varieties are characterizedby the presence of minerals such as mica, monazite,teration. The ankeritic carbonatites are clearly the young-

est group of the sequence and intrude into the other riebeckite, ilmenite or benstonite; other reported mineralsare bastnaesite, pyrochlore, thorite, aegirine and barite.carbonatites, producing thereby prominent reaction

zones with replacement of dolomite by ankerite (Viladkar Xenoliths of gneisses, pyroxenites and syenites, alongwith compositional banding and foliation, are typical& Subramanian, 1995).

The Samalpatti carbonatite–alkaline complex, located features of the carbonatites (Subramanian et al., 1978).The carbonatite occurrence of Pakkanadu, Salem dis-in the Dharmapuri district ~30 km southwest of Ti-

rupattur town (Fig. 1), has a size of >125 km2 (Su- trict, situated 155 km SW of Tirupattur town (Fig. 1)and intruded into charnockites, gneisses and granulites,bramanian et al., 1978; Viladkar & Subramanian, 1995).

The complex comprises serpentinized dunite, pyroxenite, exhibits some peculiar features. Syenites, pyroxenites,albitites and orthoclasites are common associates of thesyenite and carbonatite (Fig. 2). It intruded into horn-

blende–epidote gneisses; a K/Ar mineral age (Moralev carbonatites. Some silico-carbonatites contain ap-preciable quantities of mica (greenish biotite and phlo-et al., 1975) on phlogopite from Jogipatti yielded an age

1767


Fig. 2. Geological sketch maps of the Samalpatti and Sevattur carbonatite complexes [according to Subramanian et al. (1978) and Viladkar &Subramanian (1995), respectively], with location of the analysed samples.

gopite; e.g. sample Pak 203) and show metamorphic for biotite from pyroxenite adjacent to carbonatite atfolding. In addition to calcite and mica, other important Sevattur (Deans & Powell, 1968). Further investigationsminerals include monazite (up to 10 cm in diameter), used phlogopite (K/Ar), pyrochlore (Pb/Pb), zircon (U/allanite, barite, feldspar and quartz. According to Pb, fission track) and apatite (fission track) from Sevattur,Krishnamurthy (1988), some minerals, especially allanite Jogipatti and Pakkanadu, respectively (Moralev et al.,and monazite, belong to a pegmatitic stage, whereas 1975; Nagpaul & Metha, 1975; Parthsarathy & SankarBorodin et al. (1971) considered the rocks as ‘late phase Das, 1976). The results of these studies suggested ageshydrothermal carbonatites’. between 600 and 845 Ma; the fission track dating of

Within the Samalpatti complex some carbonate bodies zircon from Sevattur yielded an age of 1330 ± 40 Maat Garikalpalli have been described by Borodin et al. (Nagpaul & Metha, 1975). All these are mineral dates(1971). Subramanian et al. (1978) suggested that these and, therefore, strictly speaking only cooling ages. Anilrocks were carbonatitic, because of the presence of xeno- Kumar & Gopalan (1991) have published rather preciseliths and minerals such as ilmenorutile, monazite, apatite Rb/Sr mineral isochron ages for carbonatite and fenitizedand phlogopite. Though their carbonatitic nature is ques-

pyroxenite from the Sevattur carbonatite complex. Intionable, we collected some carbonate rock samples fromboth cases we are dealing with internal isochrons whereboth the Sevattur (Sev 179, Sev 180) and the Samalpattithe whole-rock samples represent the initial Sr isotopiccomplex (samples Kud 186, Kud 187, Kudamandapatticomposition and the slopes are defined in the case of thelocation) which consist of mineral assemblages typicalcarbonatite by biotites and phlogopite, and in the caseof high-grade silicate marbles. These comprise calcite,of the pyroxenite by three biotite fractions. The isochronsdiopside, garnet, epidote, vesuvianite, microcline andshow concordant ages of 771 ± 18 Ma for carbonatitequartz; the garnet commonly shows poikiloblastic fea-and 773 ± 13 Ma for pyroxenite; the initial 87Sr/86Srtures. At Kudamandapatti, wollastonite occurs togetherratios are 0·70521± 4 and 0·70536± 13, respectively.with calcite, poikiloblastic garnet, diopside, olivine, epi-Because the ages are only defined by micas, these datesdote, feldpar and quartz in rhythmically banded rocks.again represent cooling ages, which date only the latestThe first radiometric dates of Tamil Nadu carbonatites

using the K/Ar method yielded an age of 720± 30 Ma cooling event below a temperature of ~500°C.

1768


Mineral dating usually does not tell us whether these analysed as a metal on a MAT 261 mass spectrometerin static multicollector mode. The mean 143Nd/144Nddates relate to magmatic cooling or to a subsequentratio in the La Jolla standard was 0·511844 ± 12 (1r)metamorphic event. The carbonatite occurrence of Pak-during the course of this study. This value is fractionationkanadu shows a dominant metamorphic foliation withincorrected for a 146Nd/144Nd value of 0·7219. Fractionationthe silico-carbonatites. Because of their position withineffects during the Sr isotope composition (IC) runs wereprominent shear zones, clear indications of tectonic stresseliminated by normalizing to a 86Sr/88Sr value of 0·1194.are found within some other Tamil Nadu carbonatiteA mean of 0·710257± 13 (1r) was obtained on 19 runsoccurrences, e.g. strongly deformed twin lamellae inof the NBS 987 standard.calcite. It would thus seem that parts of the Tamil

For the carbon and oxygen isotope work, the finelyNadu carbonatites suffered a metamorphic overprinting.powdered (200 mesh size) carbonatite samples wereRecently, however, Schleicher et al. (1997) reported atreated with anhydrous (100%) phosphoric acid at 25°Cwhole-rock lead/lead isochron age for for Sevattur whichaccording to the procedure described by McCrea (1950)yields an age of 801 ± 11 Ma and can be interpretedand Craig (1953). This treatment was employed for 72 has intrusion age of the carbonatites.to ensure quantitative reaction of dolomite and ankeritecomponents. Carbon and oxygen isotope ratios weredetermined for the CO2 obtained from these procedures.ANALYTICAL METHODSMeasurements were performed on a VG PRISM massWe analysed 22 samples from the Sevattur and Samalpattispectrometer equipped with a three-collector systemcomplexes and from Pakkanadu with respect to major andallowing the simultaneous collection of the masses 44,trace element composition, rare earth elements (REE), as45 and 46. Results were corrected for 17O after Craigwell as d18O and d13C, and Pb-, Nd- and Sr-isotopic(1957) and are reported as d13C and d18O values (incomposition.permil) relative to the Peedee Belemnite (PDB) andDetails of the lead chemistry and lead mass spec-Standard Mean Ocean Water (SMOW) standards. Stand-trometry have been given by Arndt & Todt (1994). Toard deviations of individual measurements were usuallyminimize contamination during crushing we used whole-smaller than ±0·1‰.rock chips that were crushed down to grains of 1 mm in

Major element data were acquired by routine X-raysize, sieved and washed. For most samples we processedfluorescence (XRF) analysis; the CO2 contents weretwo or three aliquots of 100–500 mg each includingmeasured volumetrically with a so-called ‘carbonate-separate chemical preparation (indicated in Table 3,bomb’ designed by Muller & Gastner (1971). The con-below, with sub-numbers /1, /2, etc.). All data werecentrations of 22 elements (Na, K, Sc, Cr, Fe, Co, Zn,obtained using a MAT 261 mass spectrometer with aRb, Zr, Ba, La, Ce, Nd, Sm, Eu, Tb, Yb, Lu, Hf,nine-cup multicollector which allowed all Pb and inter-Ta, Th, U) were determined by instrumental neutronfering isotopes (masses 201 and 203 for monitoring) toactivation analysis (INAA). For this, powdered samplesbe measured simultaneously. Static measurements of allof ~100 mg were packed in small polyethylene containersisotopes yield an internal precision of <±50 ppm (2rand irradiated together with an in-house clay standardmean). Pb standard 982 was measured before and after(TONY) and the USGS standard reference materialeach group of samples and the mean of both meas-SDO-1 (shale) in the reactor of the Deutsches Krebsfor-urements was used to correct for fractionation, whichschungszentrum, Heidelberg, for 4 h at a flux of 4× 1913

was in the range of 1·45± 0·05‰ per atomic mass unitneutrons/cm2 per s. Gamma-ray spectra of both samples(DMU) (Todt et al., 1996). For the regression lines Yorkand standards were measured several times at differentII and ISOPLOT 2/92 were used and the error onintervals after the irradiation with hyperpure germaniumage was multiplied by the square-root of mean square detectors. The elemental concentrations were calculated

weighted deviation (MSWD). External errors of these using modified versions of a computer program especiallycalculations were [0·5‰ per DMU assuming that the adjusted for the laboratory (Schubiger et al., 1975). Cor-fractionation in sample runs is not as consistent as in rections were applied to the concentrations of Zr, La,standard runs, which are routinely measured with an and Ce for (n,f ) reactions on uranium. Interference toexternal error of 2× 10–4. the major gamma line of 131Ba is caused by 103Ru with

Nd and Sr isotopic compositions and concentrations a half-life of 39·35 days, which is also produced by thiswere determined on totally spiked samples. For sample reaction and was taken into account.digestion and chemical separation of Sm, Nd, Rb andSr, standard prodecures were applied. For the Sm–Nd

RESULTSmeasurements, a 150Nd/149Sm tracer solution was used;Geochemistryseparation of Nd and Sm from the REE group was

performed on Teflon powder coated with 2 ml hydrogen- Concentrations of major and trace elements of selectedsamples are summarized in Table 1 and plotted in Figsdiethyl-hexyl-phosphate (HDEHP). Nd and Sm were

1769


Tab

le1:

Maj

or(w

t%

)an

dtr

ace

elem

ent

data

(ppm

)fo

rse

lect

edsa

mpl

esof

carb

onat

ites

and

asso

ciat

edro

cks

from

Sev

attu

r,Sam

alpa

tti,

Jog

ipat

ti,

Pak

kana

duan

dK

udam

anda

patti

occu

rren

ces

Sam

ple

:S

ev17

4S

ev18

0bS

ev-1

81S

ev-1

82S

ev-Z

R.K

Jog

gi-

191

Rie

b-J

og

gJo

gg

i-A

pS

am19

9S

am20

0P

ak-2

03R

ock

typ

e:C

arb

on

atit

eC

arb

on

atit

eC

arb

on

atit

eC

arb

on

atit

eC

arb

on

atit

eC

arb

on

atit

eC

arb

on

atit

eC

arb

on

atit

eC

arb

on

atit

eC

arb

on

atit

eC

arb

on

atit

eLo

calit

y:S

evat

tur

Sev

attu

rS

evat

tur

Sev

attu

rS

evat

tur

Jog

gip

atti

Jog

gip

atti

Jog

gip

atti

Sam

alp

atti

Sam

alp

atti

Pak

kan

adu

SiO

20·

012·

133·

71·

963·

560·

073·

860·

450·

024·

0923

·5Ti

O2

0·03

0·03

0·09

0·53

00·

020

0·05

0·04

0·43

Al 2

O3

0·03

0·13

0·1

0·07

0·15

0·02

0·06

0·03

00·

576·

94Fe

2O3t

0·99

6·14

9·28

6·71

11·1

85·

661·

140·

272·

371·

224·

22M

nO

0·37

0·77

1·18

0·06

0·88

0·72

0·12

0·05

1·05

0·53

0·67

Mg

O4·

0316

·74

16·8

213

·26

16·3

414

·51·

820·

492·

022·

99·

07C

aO49

·34

31·6

631

·29

25·9

529

·21

36·3

745

·845

·11

46·1

22·1

411

·01

Na 2

O0·

050·

190·

080·

130·

140·

130·

370·

250·

090·

40·

31K

2O0·

140·

080·

070·

160·

010·

050

0·02

0·04

3·23

P2O

50·

223·

541·

83·

223·

499·

5115

·35

21·5

50·

2218

·11

0·94

LOI

CO

242

·95

40·4

38·8

938

·09

36·8

834

·91

23·5

17·1

641

·93

11·7

410

·19

Su

m97

·99

101·

8710

3·25

89·6

110

2·52

101·

992

·09

85·3

693

·87

61·7

870

·51

Rb

0·09

0·91

0·87

2·11

0·11

0·2

0·06

0·24

0·99

62S

r81

1076

6011

310

1028

081

7088

4043

2035

2093

0012

640

2977

0B

a19

0083

021

0019

2025

9015

6016

2062

034

030

4150

2200

00Y

9639

3652

5497

112

130

147

972

291

Nb

171

1718

838

440

21

514

234

Zr

1656

464

432

1440

357

154

116

320

Hf

0·52

0·89

11·

411·

071

13·4

1·17

V10

1513

1222

827

2838

585

Cr

3020

1821

2832

2431

1011

620

Co

0·85

1260

2627

89

1·93

62·

664·

8Z

n37

4940

2110

3146

3T

h2·

4620

0·21

23

744

941

1640

700

U1·

3912

34·

771

·517

317

·12·

734·

54·

5S

c23

·527

24·6

2331

920

·21·

6613

0·06

265

Ta59

·23·

0142

·912

67·

50·

130·

450·

610

·8La

430

226

169

274

308

506

414

470

115

3240

011

500

Ce

790

494

410

610

717

1115

950

1090

302

2180

017

200

Nd

224

217

207

304

367

587

390

628

196

2470

030

00S

m45

·737

·233

·356

·373

·595

·883

115·

349

·027

0048

0E

u14

·29

8·15

12·2

13·6

23·8

22·2

28·9

15·4

403

86T

b5

2·62

2·37

3·57

47·

16·

89

5·97

7117

Yb

6·6

2·44

2·47

1·79

3·39

5·6

6·4

9·61

216

16·8

Lu0·

840·

140·

280·

20·

430·

570·

981·

285·

12·

66

1770


Sam

ple

:P

ak-2

04P

ak-2

05S

ev17

9S

ev18

0S

ev18

4K

ud

a18

6K

ud

a18

7S

am19

3S

am19

7-II

Sam

197-

IIIS

am-2

01R

ock

typ

e:C

arb

on

atit

eFe

nit

eC

alcs

ilica

tem

.C

alcs

ilica

tem

.S

yen

ite

Cal

csili

cate

m.

Cal

csili

cate

m.

Du

nit

e(f

en.)

Pyr

oxe

nit

eP

yro

xen

ite

‘Pyr

oxe

nit

e’Lo

calit

y:P

akka

nak

uP

akka

nak

uS

evat

tur

Sev

attu

rS

evat

tur

Ku

dam

and

ap.

Ku

dam

and

ap.

Sam

alp

atti

Sam

alp

atti

Sam

alp

atti

Sam

alp

atti

SiO

28·

0337

·51

24·1

98·

852

·08

19·0

526

·37

56·3

340

·04

46·2

52·8

4Ti

O2

0·22

0·53

0·33

0·13

1·01

0·22

0·38

0·24

1·29

0·96

0·57

Al 2

O3

3·01

147·

372·

1916

·93

5·22

9·42

18·2

73·

886·

3911

·07

Fe2O

3t2·

626·

32·

581·

0410

·74

1·83

2·85

1·36

12·7

99·

9810

·55

Mn

O0·

180·

130·

060·

020·

190·

050·

080·

020·

190·

130·

15M

gO

0·94

1·47

1·76

2·08

3·78

2·88

1·63

0·64

9·05

12·9

112

·53

CaO

43·6

915

·58

39·0

448

·37

7·65

40·5

340

·06

11·1

614

·43

20·2

36·

97N

a 2O

0·68

4·29

0·24

0·12

4·24

0·05

0·11

0·23

3·08

0·77

2·19

K2O

13·

161·

50·

843·

411·

560·

111

·49

2·11

1·12

5·47

P2O

51·

160·

170·

210·

040·

990·

030·

070·

030·

440·

040·

66LO

I0·

320·

214·

031·

220·

17C

O2

32·4

813

·93

20·1

134

·26

26·1

211

·81

3·84

Su

m94

·01

97·0

797

·39

97·8

910

1·34

97·5

492

·88

99·9

891

·33

99·9

510

3·17

Rb

27·9

598

2934

·96

80·7

240

3·06

410·

669

4319

7·7

Sr

1591

067

3028

035

212

1232

961

·374

010

776

0B

a53

0096

0046

029

333

0046

117

012

7010

4010

1019

90Y

3322

1310

2517

218

913

Nb

2375

144

3511

2Z

r72

934

820

123

102

452

6723

38

Hf

1·47

1·91

1·74

12·

052

2·72

0·52

3·4

2V

3611

124

918

221

1639

212

117

6C

r27

846

2780

2962

4958

576

586

5C

o8·

719

·47·

85

25·1

158·

864

5658

Zn

3073

3913

118

6472

129

9882

Th

17·3

5710

23·

45

7·5

73·

57·

67

U9·

231

1·15

1·04

50·

472·

221·

9S

c0·

531·

636·

92

22·3

58·

612

30·9

43·2

25Ta

1·24

4·4

0·45

0·56

0·4

0·69

0·53

0·3

La87

024

015

·620

·359

·224

·513

·618

61·4

16·6

27·5

Ce

1320

380

3334

·411

048

·331

513

445

53·3

Nd

390

110

1713

·954

23·1

175

9332

24·7

Sm

43·1

17·7

3·08

2·47

8·7

4·30

3·35

17·3

7·3

4·83

Eu

10·2

4·41

0·62

0·49

2·85

0·93

0·71

4·44

1·41

1·4

Tb

2·42

1·03

0·39

0·26

0·91

0·51

0·45

1·59

0·84

0·48

Yb

1·94

1·93

1·41

0·63

2·24

1·17

1·8

1·75

1·42

1·08

Lu0·

250·

660·

250·

090·

290·

170·

320·

270·

270·

2

Maj

or

elem

ents

by

XR

F;tr

ace

elem

ents

and

RE

Eb

yIN

AA

,is

oto

pe

dilu

tio

nan

dX

RF.

LOI,

loss

on

ign

itio

n.

1771


10000

1000

100

10

1

0.1

100000

Yb

CarbonatitesSevatturPakkanadu

Pyroxenites

strongly fenitizedweakly fenitized

CalcsilicateMarbles

Carbonatites

JogipattiSamalpatti

Sam

ple

/ Prim

itive

Man

tle

RbBa

ThU K

Nb LaCe

Sr NdHf

ZrSm

EuTi

TbY LuP

(a) (b)

(d)(c)

RbBa

ThU K

Nb LaCe

Sr NdHf

ZrSm

EuTi

TbY LuP

Yb RbBa

ThU K

Nb LaCe

Sr NdHf

ZrSm

EuTi

TbY LuP

Yb

RbBa

ThU K

Nb LaCe

Sr NdHf

ZrSm

EuTi

TbY LuP

Yb

10000

1000

100

10

1

0.1

100000

Sam

ple

/ Prim

itive

Man

tle

10000

1000

100

10

1

0.1

100000

Sam

ple

/ Prim

itive

Man

tle

10000

1000

100

10

1

0.1

100000

Sam

ple

/ Prim

itive

Man

tle

Fig. 3. Primitive mantle normalized trace element diagrams for carbonatites (Sevattur, Pakkanadu, Samalpatti and Jogipatti), fenitized pyroxenitesand sedimentary derived calcsilicate rocks. For normalization the data of Sun & McDonough (1989) were used.

3a–d and 4a–d, respectively. The carbonatite rocks of In Figs. 3a and b, and 4a and b two samples withextremely high concentrations of incompatible elementsSevattur (Fig. 3a) and Jogipatti (Fig. 3b) show similar

enrichment patterns, with enrichment factors up to 1000 are shown. Their compositions are not typical of car-bonatitic melts but seem to be characterized by unusuallyfor Sr and light REE (LREE). This corresponds to Sr

contents of 3000–11 000 ppm and La and Ce con- high modes of certain minerals. Sample Pak 203 (Pak-kanadu) with 22% Ba, 2·9% Sr, 1·1% La and 1·7% Cecentrations of 170–500 and 400–1100 ppm, respectively.

All carbonatites show strong depletions of Rb, K and Ti. is dominated by barite and monazite, whereas sampleSam 200 (Samalpatti) with 1·2% Sr, 3·2% La, 2·2% Ce,Such geochemical characteristics are typical for car-

bonatites in general. 2·5% Nd and 0·16% Th is extraordinarily rich in apatiteand allanite.In contrast to the Jogipatti and the Sevattur rocks,

which show similar enrichment patterns for trace ele- The samples that have been classified as calcsilicatemarbles on the basis of their mineralogical compositionsments and REE, at Samalpatti and Pakkanadu re-

markably large variations are observed for some elements also show trace element patterns significantly differentfrom the true carbonatites (Figs 3d and 4d). They re-(Th, U, Nb, Zr), indicating variable mineral composition

of the accessory phases. Some characteristic differences semble those of the calcsilicate rocks of Borra (AndhraPradesh), which were interpreted by Bhowmik et al. (1995)between Jogipatti and Sevattur are of special interest.

Jogipatti rocks are significantly enriched in Th and P as metasedimentary rocks from a granulite facies (Fig. 5aand b).combined with low Nb concentrations, whereas the

ankerites of Sevattur are highly enriched in U and in The geochemical patterns of the pyroxenite samplesand one serpentinite–dunite sample of the Samalpattisome samples also in Zr, corresponding to significantly

higher contents of apatite in the samples from Jogipatti complex (Figs 3c and 4c) very clearly indicate a fen-itization process. This is in agreement with the min-than in the investigated Sevattur rocks.

1772


Fig. 4. Chondrite-normalized REE patterns for selected carbonatites, pyroxenites and calcsilicate rocks from Tamil Nadu carbonatite complexes.

eralogical and petrographical observations. With in- the range for pyroxenites of the carbonatite complex ofcreasing fenitization the contents of potassium (up to Sung Valley (Shillong; Viladkar et al., 1994) whose min-11·5%) and rubidium (up to 410 ppm) as well as Ba (up eral assemblage is a relic one with cumulate textureto 9600 ppm), Sr (up to 6700 ppm) and P (up to 0·66%) and without the formation of new minerals during theincrease, whereas Nb and Zr decrease significantly. fenitization process. Interestingly, their trace element

In Fig. 6b the REE patterns of two representative patterns are very similar to those of the Tamil Nadupyroxenite samples from Zabargad (St John’s Island, pyroxenites.Red Sea, Kurat et al., 1993) are plotted together withpyroxenites from Tamil Nadu to compare the latter withthe typical REE geochemistry of undisturbed pyroxenitesderived from a depleted upper-mantle source. This plot

Stable isotope resultsclearly demonstrates the unusual pattern of the TamilThe results of the carbon and oxygen isotope meas-Nadu pyroxenites. Also plotted in Fig. 6b, as well as in

the primordial mantle normalized diagram of Fig. 6a, is urements are listed in Table 2 and plotted in Fig. 7. The

1773


1000

100

10

1

Sam

ple/

Cho

ndrit

e

Tamil Nadu calcsilicate rocks

Borra calcsilicates (granulites)

La Ce Nd EuSm Tb LuYb

10000

1000

100

10

1

0.1Rb

BaTh

U KNb La

CeSr Nd

HfZr

SmEu

TiTb

YYb

LuP

Tamil Nadu calcsilicate rocks

Borra calcsilicates (granulites)

(b)

(a)

Sam

ple/

Prim

itive

Man

tle

Fig. 5. Range of trace element (a) and REE (b) patterns normalized to primitive mantle and chondrite, respectively (Sun & McDonough, 1989)for calcsilicate rocks from Tamil Nadu carbonatite complexes, compared with granulite facies calcsilicate rocks from Borra (Andhra Pradesh).

d13C and d18O values are normalized relative to PDB Lead, neodymium and strontium isotopicand SMOW, respectively. results

No significant differences could be detected between The lead isotope results are listed in Table 3 and thethe different occurrences, as these carbonatites have neodymium and strontium results in Table 2. Our leadoverlapping ranges of isotopic composition. With one isotope work was concentrated on the Sevattur car-single exception, all carbonatite samples analysed plot bonatite complex, where a lead/lead secondary isochronwithin the box for ‘primary igneous carbonatites’ of age of 801 ± 11 Ma was obtained by Schleicher et al.Keller & Hoefs (1995). Specifically, the d18O range was (1997) for ankeritic and dolomitic carbonatites. Becausefound to be very small, with values confined between 7·5 the ankeritic carbonatites at Sevattur represent the young-and 9·9‰. According to Keller & Hoefs (1995), however, est stage of carbonatite formation, this age relates to thethese values are already slightly too high compared end of the carbonatitic magmatism in this complex. Thewith primary mantle compositions as defined by oceanic lowermost part of this Sevattur isochron is shown inbasalts. For comparison, different types of carbonatites Fig. 8a. Most samples which define the isochron arefrom Amba Dongar published by Simonetti et al. (1995) excluded from the figure because of their extremely highand Viladkar & Schidlowski (1998) are also plotted. present-day isotope ratios (206Pb/204Pb up to 270). TheSample Sam 200, which shows an increased d18O value, isochron plots somewhat above the Stacey & Kramersbelongs to a rock extremely rich in apatite. In contrast (1975) growth curve (SK in Fig. 8a); the initial leadto the carbonatites, all calcsilicate marbles investigated isotope composition at 800 Ma can be described by aplot outside the igneous carbonatite box, with a large simple first-stage model l value of 10·11, starting fromscatter towards higher d18O and d13C values. the Stacey & Kramers growth curve at 3·7 Ga.

1774


10000

1000

100

10

1

0.1Rb

BaTh

U KNb La

CeSr Nd

HfZr

SmEu

TiTb

YYb

LuP

Tamil Nadu

Sung Valley

Pyroxenites

1000

100

10

1

La Ce Nd EuSm Tb LuYb

Sam

ple

/ Cho

ndrit

e

Tamil Nadu

Sung Valley

Zabargad

Pyroxenites

Sam

ple/

Prim

itive

Man

tle

(a)

(b)

Fig. 6. Range of trace element (a) and REE (b) patterns for pyroxenites from Tamil Nadu carbonatite complexes, compared with pyroxenitesfrom Sung Valley (Shillong; Viladkar et al., 1994) and two unfenitized pyroxenites from Zabargad, Red Sea (Kurat et al., 1993).

radiogenic 206Pb/204Pb ratios around 16·5–17·5, whenIn Fig. 8a the present-day lead isotope ratios of somecompared with other (but mostly younger) carbonatiteother Tamil Nadu carbonatite occurrences as well asoccurrences world wide. Only the Jacupiranga complexsome pyroxenites and some calcsilicate marbles are plot-in Brazil (though also younger, i.e. ~130 Ma) showsted in a 207Pb/204Pb vs 206Pb/204Pb diagram. Because thecomparable lead isotope composition. In contrast toU and Pb concentrations are not determined, an agethe carbonatites, the calcsilicate rocks show very highcorrection to obtain initial ratios is not possible. However,radiogenic lead isotope ratios.an 800 Ma event seems also to be exhibited by the other

In the 208Pb/204Pb vs 206Pb/204Pb diagram (Fig. 9) mostsamples, as may be indicated from the samples Sam 199/of the Tamil Nadu samples plot close together somewhat1 and Sam 199/2 (Samalpatti), which represent two chipsabove the Stacey & Kramers evolution curve (at ~1of different mineralogy (and hence different l) from oneGa). The samples are significantly less radiogenic insample. Perhaps this age value can even be detectedcomposition than other (mostly younger) carbonatitewithin the calcsilicate marbles. The majority of the car-complexes, and again they resemble the Jacupirangabonatite samples ( Jogipatti, Samalpatti, Pakkanadu andcomplex. They plot on a broad array for initial Pbthe Sevattur calcite carbonatites Sev 174) plot on anratios of Indian mantle-derived rocks like the Ramagiriarray roughly between the Doe & Zartman (1979) leadmetabasalts (2·7 Ga; Zachariah et al., 1995), Indianevolution curves for depleted Earth mantle (M) and forMORB [compiled by Barling et al. (1994)], some Deccana continental upper crust (UC). In accordance with their

Proterozoic age, the Tamil Nadu samples exhibit low basalts (Peng & Mahoney, 1995) and Amba Dongar

1775


Tab

le2:

C,

O,

Sr

and

Nd

isot

ope

resu

lts

for

carb

onat

ites

,py

roxe

nite

s,fe

nite

s,ca

lcsi

lica

tem

arbl

esan

don

esy

enite

Sam

ple

Loca

lity

d13C

d18O

87S

r/86

Sr

87R

b/86

Sr

87S

r/86

Sr

e Sr(

i)14

3 Nd

/144 N

d14

7 Sm

/144 N

d14

3 Nd

/144 N

de N

d(i

)

(PD

B)

(SM

OW

)(8

00M

a)(8

00M

a)

Car

bona

tites

Sev

174

Sev

attu

r−

5·63

7·51

0·70

529

0·00

003

0·70

529

24·6

50·

5119

120·

1231

0·51

1266

−6·

65

Sev

180b

Sev

attu

r−

5·03

9·32

0·70

511

0·00

035

0·70

510

21·9

70·

5118

000·

1039

0·51

1255

−6·

86

Sev

-181

Sev

attu

r0·

7050

00·

0002

20·

7050

020

·53

0·51

1857

0·10

100·

5113

27−

5·45

Sev

-182

Sev

attu

r0·

7052

10·

0005

90·

7052

123

·45

0·51

1801

0·11

700·

5111

87−

8·19

Sev

-ZR

·KS

evat

tur

−4·

859·

190·

7050

90·

0035

0·70

505

21·1

90·

5118

400·

1260

0·51

1179

−8·

35

Jog

gi-

191

Jog

gip

atti

−4·

878·

870·

7051

40·

0000

40·

7051

422

·53

0·51

1727

0·09

860·

5112

10−

7·75

Rie

b-J

og

gJo

gg

ipat

ti−

5·77

9·20

0·70

514

0·00

014

0·70

514

22·4

8

Jog

gi-

Ap

Jog

gip

atti

−5·

819·

290·

7051

30·

0000

50·

7051

322

·42

0·51

1928

0·11

100·

5113

45−

5·10

Sam

199

Sam

alp

atti

−6·

479·

420·

7045

50·

0000

80·

7045

414

·06

0·51

2186

0·15

090·

5113

94−

4·15

Sam

200

Sam

alp

atti

−4·

5913

·52

0·70

537

0·00

023

0·70

537

25·7

9

Pak

-203

Pak

kan

adu

−5·

649·

880·

00

Pak

-204

Pak

kan

aku

−7·

068·

670·

7050

80·

0050

80·

7050

320

·90

0·51

1608

0·06

690·

5112

57−

6·83

Pyro

xeni

tes

and

feni

tes

Sam

197-

IIS

amal

pat

ti0·

7081

50·

270

0·70

507

21·5

10·

5118

340·

1170

0·51

1220

−7·

54

Sam

197-

IIIS

amal

pat

ti0·

7209

71·

157

0·70

775

59·6

20·

5117

480·

1440

0·51

0993

−11

·99

Sam

-201

Sam

alp

atti

0·71

457

0·75

00·

7059

934

·68

0·51

1412

0·11

790·

5107

93−

15·8

9

Sam

193

Sam

alp

atti

0·93

358

19·8

00·

7073

453

·79

Pak

-205

Pak

kan

aku

−6·

698·

690·

0000

00·

00

Cal

csili

cate

mar

bles

Sev

-178

Sev

attu

r0·

7073

80·

0000

00·

00

Sev

179

Sev

attu

r0·

7135

00·

0000

00·

000·

5118

050·

1140

0·51

1207

−7·

80

Sev

180

Sev

attu

r2·

0818

·54

0·71

288

0·28

70·

7096

085

·89

0·51

1691

0·10

720·

5111

29−

9·33

Ku

da

186

Ku

dam

and

ap.

0·09

23·8

60·

7148

80·

384

0·71

050

98·6

50·

5116

580·

1127

0·51

1067

−10

·54

Ku

da

187

Ku

dam

and

ap.−

2·55

11·1

80·

7093

40·

0269

0·70

903

77·7

80·

5118

120·

1393

0·51

1081

−10

·27

Sye

nite

Sev

184

Sev

attu

r0·

7059

80·

193

0·70

378

3·20

All

dat

aar

eco

rrec

ted

for

frac

tio

nat

ion

.

1776


SevatturJoggipattiSamelpattiPakkanadu

Tamil NaduCarbonatites

0

– 2

– 4

– 6

– 8

2

5 10 15 20 25

"primary igneous carbonatites"

13

C

18 O [ ‰ , ]SMOW

Amba Dongar:calcite carbonatite

Amba Dongar:dolomite fromferrocarbonatite

Amba Dongar:calcite fromferrocarbonatiteBorra

[ ‰ ,

]P

DB

Tamil NaduBorra

Calcsilicate marbles

Fig. 7. d13C and d18O stable isotope diagram for carbonatites and calcsilicate marbles from Tamil Nadu carbonatite complexes. For comparison,one calcsilicate rock from Borra (Andhra Pradesh) was also analysed. Also shown are fields for Amba Dongar ferrocarbonatites (Simonetti et al.,1995) and Amba Dongar calcite carbonatites (Simonetti et al., 1995; Viladkar & Schidlowski, 1998). Box for ‘primary igneous carbonatites’according to Keller & Hoefs (1995).

carbonatite complex (Simonetti et al., 1995; H. Schleicher concentrations) suggest a rather fractionated stage foret al., unpublished data, 1997). Some of these Indian the Tamil Nadu carbonatites, their d18O and d13C isotopemantle-derived rocks also plot close to EM I and EM II data (Fig. 7) indicate that they originated from mantle-(Hart, 1988). For comparison, we also plotted the fields derived melts. With a single exception (sample Sam 200),for some Early Proterozoic (~2·5 Ga) gneisses from all samples plot very close to primary mantle compositionssouthern India in the immediate surroundings of the as defined by oceanic basalts and lie within the ‘primaryKolar Gold Field (Krogstad et al., 1995), just north of igneous carbonatite box’ according to Keller & Hoefsthe Tamil Nadu carbonatite complexes. A significantly (1995). In contrast, all rocks classified on the basis oflarger scatter and significantly more radiogenic com- petrographical criteria as calcsilicate marbles differ sig-positions are evident on this plot. It is worth noting that nificantly in terms of their d18O and d13C isotope com-the calcsilicate rocks associated with the carbonatites of positions. Although comparable stable isotopeTamil Nadu have strongly different lead isotope com- compositions are also known for late differentiates ofpositions which mainly reflect high j values. carbonatitic rocks (e.g. the Amba Dongar ferro-

Similar to the lead system, the present-day eNd–eSr carbonatites, Fig. 7; Simonetti et al., 1995; compare alsovalues of the Tamil Nadu carbonatites plot far away Deines, 1989), these isotope ranges are more commonlyfrom all other carbonatite isotopic compositions so far seen in carbonate rocks of crustal derivation.known (compare Fig. 11). These rocks have present-day

A metasedimentary character of the calcsilicate marbleseNd(0) values between –14 and –20 (with the exception ofis also indicated by our geochemical data (Figs 3c, 4csample Sam 199 with a value of –9) and eSr(0) valuesand 6); the mineralogical composition of these rocksaround+5 (Sam 199 has a value+1). In their present-suggests formation at high-grade granulite facies con-day Sr and Nd isotope compositions, the Tamil Naduditions, comparable with those noticed in the calcareouscarbonatites resemble the alkaline rocks from Leucitegranulites at Borra, Andhra Pradesh (Bhowmik et al.,Hills, Wyoming (Vollmer et al., 1984), though they are1995). This would be in agreement with the location ofof very different age.these occurrences within the charnockite belt of theEGMB of southern India. However, the appearance of

DISCUSSION calcsilicate rocks in close connection to true car-bonatites—some of them were even described as specialAlthough the geochemical features (e.g. high to very high

Sr–Ba–REE contents, variability of Th, U, Nb and Zr kinds of carbonatites, e.g. ‘wollastonite carbonatites’ of

1777


Table 3: Lead isotope results for carbonatites, pyroxenites and calcsilicate rocks

Sample Rock type 208Pb/204Pb 207Pb/204Pb 206Pb/204Pb

Sev 174/1 Carbonatite 37·44 ± 0·03 15·57 ± 0·01 17·21 ± 0·01

Sev 174/2 37·39 ± 0·01 15·55 ± 0·002 17·20 ± 0·002

Sev 174/3 37·60 ± 0·005 15·59 ± 0·003 17·59 ± 0·001

Sev 181/1 Carbonatite 37·32 ± 0·03 15·68 ± 0·01 18·62 ± 0·01

Sev 181/2 38·15 ± 0·01 32·11 ± 0·01 268·00 ± 0·09

Sev 181/3 37·72 ± 0·02 19·86 ± 0·01 82·13 ± 0·02

Sev 182 Carbonatite 38·20 ± 0·06 16·86 ± 0·02 35·30 ± 0·03

Sev ZrK Carbonatite 38·53 ± 0·07 18·85 ± 0·03 67·14 ± 0·12

Joggi 191/1 Carbonatite 37·88 ± 0·02 15·36 ± 0·01 16·66 ± 0·005

Joggi 191/2 37·94 ± 0·01 15·36 ± 0·003 16·63 ± 0·002

Joggi 191/3 38·55 ± 0·01 15·53 ± 0·01 16·84 ± 0·01

Joggi-Ap Carbonatite 37·75 ± 0·06 15·57 ± 0·03 17·06 ± 0·03

Sam 199/1 Carbonatite 36·61 ± 0·01 15·66 ± 0·003 21·26 ± 0·003

Sam 199/2 36·67 ± 0·01 15·39 ± 0·003 17·30 ± 0·002

Sam 199/3 37·14 ± 0·01 15·57 ± 0·01 17·20 ± 0·005

Pak 204/1 Carbonatite 37·96 ± 0·004 15·52 ± 0·002 17·45 ± 0·001

Pak 204/2 92·73 ± 0·03 16·30 ± 0·02 24·80 ± 0·002

Sam 201/1 Pyroxenite (fenitized) 37·57 ± 0·14 15·45 ± 0·05 17·07 ± 0·04

Sam 201/2 37·41 ± 0·01 15·42 ± 0·004 16·97 ± 0·004

Sam 201/3 37·46 ± 0·01 15·45 ± 0·01 16·94 ± 0·01

Sev 179/1 Calcsilicate marble 42·67 ± 0·004 15·80 ± 0·001 19·82 ± 0·001

Sev 179/2 41·81 ± 0·01 15·79 ± 0·002 19·62 ± 0·002

Sev 179/3 41·54 ± 0·01 15·80 ± 0·003 19·92 ± 0·004

Sev 179/4 44·63 ± 0·01 15·87 ± 0·002 21·04 ± 0·002

Sev 180 Calcsilicate marble 45·66 ± 0·05 15·81 ± 0·01 19·05 ± 0·01

Errors are 2r in run errors. All data are corrected for fractionation.

the Samalpatti complex (Subramanian et al., 1978)—is For the broad array of the data points, we have toconsider the fact that the data are not age corrected andvery strange, and the field and structural relations are

not completely clear. In any case, these rocks may indicate the 800 Ma event may also be exhibited by other samples.The data array obviously must be interpreted as a mixinga deep lower-crustal intrusion level for the carbonatite

itself. array between two lead reservoirs, one of them being amantle reservoir which can be described not by theIn Fig. 8a the Tamil Nadu carbonatites form a relatively

broad array between the Doe & Zartman (1979) mantle Stacey & Kramers (compare Fig. 8a) but by the Doe &Zartman mantle curve, and another with much highercurve and a lead evolution curve corresponding to a

much higher l value. Also, the Sevattur 800 Ma sec- 207Pb/204Pb ratios corresponding to a much higher lvalue. We expect an even more linear arrangement ifondary isochron lies above the Stacey & Kramers growth

curve and marks an enriched initial lead isotopic com- the data were age corrected; however, the actual shifttowards higher 206Pb/204Pb ratios cannot be very extensiveposition. As has already been pointed out, this initial

lead (at 800 Ma) corresponds to a simple first-stage model because otherwise the age correction would result inunrealistically low isotope ratios, assuming the mixingl value of 10·11, starting from a Stacey & Kramers

evolution at 3·7 Ga, i.e. this model l value already points took place during or around carbonatite formation.As is evident from Fig. 9, the majority of those Tamiltowards the existence of an enriched source reservoir.

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Fig. 8. (a) 207Pb/204Pb vs 206Pb/204Pb diagram for lead isotopic compositions of Tamil Nadu carbonatites and calcsilicate rocks compared withother carbonatite occurrences [compiled by Nelson et al. (1988) and Schleicher et al. (1991)] and with present-day compositions for EM I, EMII and HIMU (Hart, 1988). Amba Dongar data are from Simonetti et al. (1995). Lead growth lines for mantle (M) and upper crust (UC)according to the model of Doe & Zartman (1979); SK refers to the Stacey & Kramers (1975) lead evolution curve. Also shown is the lower partof the Sevattur ankerite lead/lead isochron (800 Ma) of Schleicher et al. (1997). (b) 207Pb/204Pb vs 206Pb/204Pb diagram for lead isotopic compositionsof Tamil Nadu carbonatites and calcsilicate rocks compared with crustal rocks from southern India (Krogstad et al., 1995) and Sri Lanka (Liewet al., 1991), some Deccan basalts (Peng & Mahoney, 1995), and Early Proterozoic metabasalts from Ramagiri (southern India; Zachariah et al.,1995).

Nadu carbonatites that are not characterized by ex- 208Pb/204Pb–206Pb/204Pb array of Indian mantle-derivedrocks. This ‘mantle array’ is fairly well defined whentraordinarily high l or j values plot within the broad

1779


Fig. 9. 208Pb/204Pb vs 206Pb/204Pb diagram for Tamil Nadu carbonatites and calcsilicate rocks, compared with some other carbonatites (referencesas in Fig. 8a) and other mantle-derived rocks from India (Barling et al., 1994; Peng & Mahoney, 1995; Zachariah et al., 1995). Also shown arepresent-day compositions for EM I, EM II and HIMU (Hart, 1988) and fields for some crustal rocks from southern India (Krogstad et al., 1995).SK, lead growth curve according to Stacey & Kramers (1975).

compared with the scatter of crustal rocks from southern on a crustal contaminant was convincingly demonstratedby Andersen (1987) and Andersen & Taylor (1988) forIndia, which show significantly more radiogenic com-

positions. The latter is also true for the 207Pb/204Pb–206Pb/ the Fen (Norway) carbonatite complex, and by Andersen(1997) for the Qassiarsuk (Gardar rift, Greenland) car-204Pb system (Fig. 8b) and seems to be a general char-

acteristic of the continental crust of southern India as bonatite complex.However, a U-enriched mantle reservoir may also bewell as of Sri Lanka. In Fig. 8b, in addition to the Dod

and Dosa gneisses and Kambha gneisses, which are a possible end-member of the observed mixing array.For an example, if enriched mantle component EM IIalready shown in Fig. 9, we plot some granite–gneisses

from Patna (Krogstad et al., 1995) and the field for (Hart, 1988) was involved into the mixing, it would needa model l value of 10 to generate the appropriate Pbgranulite rocks from Sri Lanka according to Liew et al.

(1991) to underline this feature. isotope compositions, when calculated back to 800 Ma(Fig. 10). Because the l value of EM II is actually notWe can use l values of South Indian Kambha gneisses

published by Krogstad et al. (1995) for an age correction known, this is only a more general statement concerningthe possible existence of an enriched mantle reservoir. Ifto the time of the carbonatite intrusion, i.e. 800 Ma

(Sevattur data). This is shown in Fig. 10. It is evident the observed array indicates mixing between two distinctmantle components, then either this mixing process mustthat the higher radiogenic end-member of the mixing

array may be lead from the continental crust, perhaps have happened before the carbonatite formation, orboth components were equally involved in the meltingeven from the wall rocks of the carbonatite intrusions.

Because of the usually very low contents of lead in processes responsible for the formation of the primarymantle melts from which the carbonatites result.carbonatites the lead system is very sensitive to con-

tamination. Such a dependence of the carbonatite lead On the other hand, in the case of simple contamination

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Fig. 10. 207Pb/204Pb(t) vs 206Pb/204Pb(t) diagram for age-corrected (800 Ma) data of Kambha gneisses (southern India), Deccan basalts and EM II(for references see Figs 8 and 9). Age corrections for Deccan and EM II were made by assuming a l value of 10, and for Kambha gneisses bythe use of individual l values given by Krogstad et al. (1995). The data for Tamil Nadu carbonatites and calcsilicate rocks are not age correctedpresent-day values.

by crustal lead, we are dealing with secondary over- which are very close to our measured carbonatite values,e.g. 7–13‰ for charnockites and enderbites of Sri Lankaprinting of the existing carbonatites. This could be a

contamination of the melts themselves at a deeper crustal (Hoernes et al., 1991). Because of this feature, even amore pronounced lower-crustal contamination would notlevel, or a younger contamination or overprinting by

meteoric fluids. Also, metamorphic fluids could be pos- be detected by the C–O system.In Fig. 11 age-corrected (800 Ma) eNd and eSr data ofsible contaminants; this may be supported by the fact

that some of the Tamil Nadu carbonatites show evidence the investigated Tamil Nadu rocks and some selectedIndian rocks (South Indian gneisses, granite–gneisses andfor a metamorphic overprinting (compare the section on

geology and petrography). charnockites, Ramagiri metabasalts; Krogstad et al., 1995;Unnikrishnan-Warrier et al., 1995; Zachariah et al., 1995)In conclusion, the lead isotope data show the mixing

between two distinct arrays but do not provide un- are plotted, together with a possible 800 Ma position forEM I (calculated with 147Sm/144Nd = 0·18 and 87Rb/equivocal evidence for the nature of the enriched end-

member component. However, because of the well- 86Sr = 0·10) and present-day fields of Indian MORB,Indian OIB (Barling et al., 1994), Mahale–Deccan basaltsknown sensitivity of the carbonatite Pb system to crustal

contamination, the participation of crustal fluids seems (Peng & Mahoney, 1995), HIMU and EM II (Hart,1988). For comparison, also shown are initial fields forto be most likely. The fact that the d18O and d13C isotopic

compositions of the Tamil Nadu carbonatites lie very the Amba Dongar carbonatites, some selected othercarbonatites (Nelson et al., 1988; Schleicher et al., 1990),close to primary mantle compositions (Fig. 7) does not

contradict this interpretation. The granulite facies the Kimberlite Group I and Group II (Smith, 1983),and the East African Carbonatite Line (EACL; Bell(charnockitic) lower-crustal rocks from Southern India

and Sri Lanka are well known for their very high CO2 & Blenkinsop, 1989), which has been interpreted as aplume–enriched mantle mixing (Bell & Simonetti, 1996).abundances, which suggest a pervasive infiltration of

mantle-derived CO2 into the lower crust (e.g. Hoernes The Tamil Nadu rocks plot very close to the EM Iposition. The pyroxenites show a somewhat larger scatter,et al., 1991; Wickham, 1992). Equilibration of this mantle

CO2 with high-grade crustal rocks results in d18O values but mostly this is only due to insufficient element con-

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Sr (t)200100– 40

10

- 30

Kambha gneissesand charnockitesDod and

Dosa gneisses

Agali-Coimbatore dolerites

Patna granite

carbonatites

calcsilicate marbles

Tamil Nadu

pyroxenites

A D Amba DongarJ JacupirangaKST KaiserstuhlMC Magnet CoveW Walloway

Carbonatites :

EACL

HIMU

EM I 800 Ma

EM II

Indian MORB

- 20

Deccan

A D

Reunion

Nd(t)

LeuciteHills

JW

MC

KST

Tamil Nadu800 Ma

Kimberlite Group II

Kimberlite Group I

Indian OIB

Fig. 11. eNd(t)–eSr(t) diagram for Tamil Nadu carbonatites, pyroxenites and calcsilicate rocks, corrected for an age of 800 Ma. The correction oftwo of the three pyroxenites is only based on XRF data. Also age corrected to 800 Ma are the data for South Indian granites, gneisses andcharnockites, for the Agali–Coimbatore dolerites (Radhakrishna et al., 1995) and for the EM I field (see text for details). The data for IndianMORB, Indian OIB, Reunion (Peng & Mahoney, 1995), HIMU and EM II are present-day compositions; Amba Dongar, Deccan and selectedother carbonatites (compilation by Nelson et al., 1988; Schleicher et al., 1990) are initial values. EACL, East African Carbonatite Line (Bell &Blenkinsop, 1989); data for Leucite Hills are from Vollmer et al. (1984).

centration data. Significantly more radiogenic are the contamination allowed in the Tamil Nadu carbonatites,some simple mass balance calculations were performedSr and Nd isotopic compositions of the investigated

calcsilicate marbles, which are clearly of crustal character, including lower-crustal gneisses from southern India[Kambha gneisses; Krogstadt et al. (1995), with 87Sr/although they differ from the gneissic and charnockitic

rocks shown in Fig. 11. The Ramagiri metabasalts situ- 86Sr(t=800Ma) = 0·72; 143Nd/144Nd(t) = 0·511; Sr = 550ppm, Nd = 16.6 ppm] as a contaminant. Starting fromated in the Coimbatore region of southern India strongly

indicate mixing between a mantle and a highly enriched a slightly depleted primary mantle composition 800 myago (87Sr/86Sr= 0·703, 143Nd/144Nd= 0·512), ~70% ofcrustal reservoir.

According to their Nd and Sr isotope compositions the Sr and 95% of the Nd must be of crustal origin toadjust the observed Sr(i) and Nd(i) ratios of an averageshown in Fig. 11, the Tamil Nadu carbonatites may

either directly characterize an enriched upper mantle of Tamil Nadu carbonatite. This would lower the Sr con-tents to 3500 ppm (from 10 400) and the Nd contents to800 my ago which was roughly similar to present-day

EM I compositions, or they represent mixing of a depleted 35 ppm (from 350). However, if we accept that the highREE, Sr (and Ba) concentrations of most of the Tamiland an enriched reservoir, analogous to the lead isotope

compositions. In contrast to the lead isotope system, the Nadu carbonatites are the result of fractionation processessubsequent to the contamination, we have to test theNd and the Sr isotopic information in carbonatites seems

only hardly to be changed by crustal contaminants, conditions for the most primitive carbonatite samples(compare Fig. 4b, Samalpatti). But even in this case (Sr=because of the extremely high Sr and Nd contents in

carbonatites (e.g. Bell & Blenkinsop, 1987; Schleicher et 3500 ppm, Nd = 196 ppm), ~43% of the Sr and 94%of the Nd must be of crustal origin, with a reduction inal., 1990; Simonetti et al., 1995). The Sr contents in the

Tamil Nadu carbonatites range between 3500 and 30 000 the overall Sr content to 2200 ppm and the Nd contentto 30 ppm. Such high amounts of contamination wouldppm, and the Nd contents between 200 and 25 000

ppm (Table 1). To define the limits to the amount of dramatically change the character of the resulting melt,

1782


and the undoubtedly carbonatitic composition of the the upwelling carbonatite magma with crustal or meteoricfluids.Tamil Nadu rocks thus strongly argues against a severe

contamination in the Sr and especially in the Nd isotopic (2) The initial lead isotope ratios of the Tamil Nadurocks indicate mixing of two lead reservoirs. One of themsystem. Thus, if the Tamil Nadu eNd(t) and eSr(t) com-

positions (Fig. 11) do not relate to a single and distinct can be characterized as a mantle component with a lowerl value, and the other one as a high-l reservoir leadingmantle component but to a mixture, the mixing event

must have occurred within the mantle either before the to 207Pb/204Pb ratios of at least 15·6 at 800 Ma. On thebasis of the lead isotopes alone, the problem of the naturecarbonatite formation or during the carbonatite forming

processes by the interaction of different mantle com- of this U-enriched reservoir (enriched mantle or crust)cannot be solved, but because of the sensitivity of theponents with variable isotopic ratios in an isotopically

heterogeneous mantle. carbonatitic Pb system a crustal contribution can beassumed.For the Tertiary Amba Dongar carbonatite complex

of northwestern India in Gujarat State, Simonetti et al. (3) The Tamil Nadu carbonatites are characterizedby very low 143Nd/144Nd and corresponding eNd(0) ratios(1995) reported EM II-like lead isotopic compositions

(compare Fig. 8a) and Nd and Sr initial isotope ratios (0·5116–0·5122; –9 to –20). The Sr isotopic ratios arerather high for carbonatite rocks (0·7045–0·7054). Be-which point towards an interaction between a Reunion

hotspot-related component and an enriched sub- cause of the high to extremely high Sr and Nd con-centrations in the carbonatites, in these systems a crustalcontinental lithosphere (see Fig. 11). In contrast to this

finding, geologically young (<200 Ma) carbonatites gen- contamination can probably be excluded, at least forthe Nd system. To obtain these isotopic signatures theerally have lead isotopic signatures that lie between

HIMU and EM I mantle components (compare Fig. 11), existence of an enriched mantle reservoir at ~800 Ma isrequired. This enriched mantle component is either dir-which is well expressed by the East African Carbonatite

Line (EACL; Bell & Blenkinsop, 1989).The special feature ectly defined by the initial Sr and Nd ratios, which weremoderately more enriched even than present-day EM I,of the Tamil Nadu carbonatites is the evidence that they

most probably provide for analogous conditions already or more probably point towards an interaction of twomantle components within an isotopically heterogeneousexisting 800 my ago, i.e. in a totally different geotectonicmantle, one of them being even more enriched (sub-environment (the interior part of the later Gondwana).continental lithosphere).Whereas the lead isotope results only indicate the inter-

(4) The fenitization of pyroxenites by carbonatite canaction of two different reservoirs, the initial Sr and Ndproduce garnet, Ba-feldspar and biotite-bearing diopsideisotopic ratios can be interpretedrocks which show high LREE and highly incompatible(a) to characterize a single and distinct mantle com-trace elements.ponent which was possibly moderately more enriched

(5) The presence of non-carbonatitic, sedimentary-than the present-day EM I component of Hart (1988),derived calcsilicate rocks within the carbonatite–syenite(b) to be the result of mixing of a depleted (HIMU?)complexes has been shown on the basis of petrographic,mantle source and a subcontinental lithosphere, orgeochemical and isotopic (C, O, Pb, Sr, Nd) data.(c) to indicate the interaction between an EM I type

mantle component and a more enriched subcontinentallithospheric mantle.

Regardless of which of these interpretations is chosen,ACKNOWLEDGEMENTSthe Tamil Nadu carbonatites prove the existence of aThis work was partially supported by a grant from theProterozoic enriched mantle component.German Research Foundation (DFG). The assistanceand help of H. Schlegel and J. Otto(Mineralogic–Petrographical Institute of the Universityof Freiburg) in performing the XRF analyses is greatlyCONCLUSIONSappreciated. We are especially grateful to Tom Andersen,Our investigations show the following:Keith Bell and an anonymous reviewer for very con-(1) The d18O and d13C values of the Tamil Nadustructive reviews.carbonatites are indicative of a primary mantle origin,

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Enriched Subcontinental Upper Mantle beneath Southern India: Evidence from Pb, Nd, Sr, and CO...

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