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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: hschl@mineralogie.uni-hamburg.de 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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 OCTOBER 1998
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
SCHLEICHER et al. TAMIL NADU CARBONATITES
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 OCTOBER 1998
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
SCHLEICHER et al. TAMIL NADU CARBONATITES
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 OCTOBER 1998
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
SCHLEICHER et al. TAMIL NADU CARBONATITES
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 OCTOBER 1998
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
SCHLEICHER et al. TAMIL NADU CARBONATITES
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 OCTOBER 1998
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
SCHLEICHER et al. TAMIL NADU CARBONATITES
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
JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 OCTOBER 1998
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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SCHLEICHER et al. TAMIL NADU CARBONATITES
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
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JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 OCTOBER 1998
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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SCHLEICHER et al. TAMIL NADU CARBONATITES
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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JOURNAL OF PETROLOGY VOLUME 39 NUMBER 10 OCTOBER 1998
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
SCHLEICHER et al. TAMIL NADU CARBONATITES
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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