Emplacement age and isotope geochemistry of Sung Valley alkaline–carbonatite complex, Shillong Plateau, northeastern India: implications for primary carbonate melt and genesis of

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Lithos 81 (200

Emplacement age and isotope geochemistry of Sung Valley

alkaline–carbonatite complex, Shillong Plateau, northeastern India:

Implications for primary carbonate melt and genesis of the

associated silicate rocks

Rajesh K. Srivastavaa,*, Larry M. Heamanb, Anup K. Sinhaa, Sun Shihuac

aIgneous Petrology Laboratory, Department of Geology, Banaras Hindu University, Varanasi 221 005, IndiabDepartment of Earth and Atmospheric Sciences, University of Alberta, Alberta, Canada T6G 2E3

cRCMRE, Chinese Academy of Sciences, Datum Road, Beijing 100101, PR China

Received 16 February 2004; accepted 29 September 2004

Available online 23 November 2004

Abstract

The early Cretaceous (Albian–Aptian) Sung Valley ultramafic–alkaline–carbonatite complex is one of several alkaline

intrusions that occur in the Shillong Plateau, India. This complex comprises calcite carbonatite and closely associated ultramafic

(serpentinized peridotite, pyroxenite and melilitolite) and alkaline rocks (ijolite and nepheline syenite). Field relationship and

geochemical characteristics of these rocks do not support a genetic link between carbonatite and associated silicate rocks. There

is geochemical evidence that pyroxenite, melilitolite and ijolite of the complex are genetically related. Stable (C and O) and

radiogenic (Nd and Sr) isotope data clearly indicate a mantle origin for the carbonatite samples. The carbonatite ENd (+0.7 to

+1.8) and ESr (+4.7 to +7.0) compositions overlap the field for Kerguelen ocean island basalts. One sample of ijolite has Nd and

Sr isotopic compositions that also plot within the field for Kerguelen ocean island basalts, whereas the other silicate–carbonatite

samples indicate involvement with an enriched component. These geochemical and isotopic data indicate that the rocks of the

Sung Valley complex were derived from and interacted with an isotopically heterogeneous subcontinental mantle and is

consistent with interaction of a mantle plume (e.g. Kerguelen plume) with lithosphere. A U–Pb perovskite age of 115.1F5.1 Ma

obtained for a sample of Sung Valley ijolite also supports a temporal link to the Kerguelen plume. The observed geochemical

characteristics of the carbonatite rocks indicate derivation by low-degree partial melting (~0.1%) of carbonated mantle

peridotite. This melt, containing a substantial amount of alkali elements, interacted with peridotite to form metasomatic

clinopyroxene and olivine. This process could progressively metasomatize lherzolite to form alkaline wehrlite.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Carbonatite; Silicate rocks; Sung Valley; India; U–Pb perovskite Age; Sr and Nd isotopes; Primary carbonate melt; Kerguelen plume

0024-4937/$ - s

doi:10.1016/j.lit

* Correspondi

E-mail addr

5) 33–54

ee front matter D 2004 Elsevier B.V. All rights reserved.

hos.2004.09.017

ng author. Tel.: +91 542 2570925; fax: +91 542 2368174.

ess: [email protected] (R.K. Srivastava).

R.K. Srivastava et al. / Lithos 81 (2005) 33–5434

1. Introduction

In recent years, an interesting debate has evolved

concerning the genesis of carbonate magma and its

relationship to associated silicate alkaline magmas.

Considering its importance, the Journal of Petrology

published several articles, including a special issue,

which, among other things, attempted to explain the

relationship between silicate and carbonatite magmas.

In this special issue, Bell et al. (1998) formulated some

key questions that they think outline the current

outstanding problems in research on carbonatite and

associated rocks. These are: (1) Are the parental melts

to carbonatites derived from the lithosphere or astheno-

sphere, or mixtures of both?, (2) Are carbonatites

generated as partial melts derived directly from the

mantle, or are they the products of magmatic differ-

entiation of carbonated silicate melts?, (3) How easily

can such parental melts migrate through the mantle and

what is their role in mantle metasomatism?, (4) What is

the relationship among the silicate rocks, such as

melilitite, nephelinite, ijolite, syenite and phonolite,

and spatially associated carbonatites?, and (5) How do

carbonatite magmas fractionate and evolve? To answer

these questions it is essential to have full set of

petrological, geochemical and particularly radiogenic

isotopic information. It is also important to mention

here that such data are required for both carbonatites as

well as associated silicate rocks.

Several recent researchers have tried to address

these questions using geochemical and radiogenic

isotopic studies from carbonatite and associated silicate

rocks (Bell and Dawson, 1995; Bell, 1998; Harmer et

al., 1998; Le Roex and Lanyon, 1998; Harmer, 1999;

Dunworth and Bell, 2001). A small amount of isotopic

data is also available for the Sung Valley complex

(Veena et al., 1998; Ray et al., 2000). Veena et al.

(1998) have reported Pb, Sr and Nd isotopic data for

carbonatites and Ray et al. (2000) have presented Sr

isotopic systematics for the Sung Valley carbonatites

and a pyroxenite. Except for the Sr isotope data

obtained for one pyroxenite sample, there are no

isotopic data available for the associated silicate rocks.

The study of carbonatite magmatism is important

for understanding nature of the subcontinental mantle.

The low-viscosity carbonatite magma may penetrate

the mantle and crust and is capable of separating from

its mantle matrix (MacKenzie, 1985, 1989; Watson

and Brenan, 1987; Harmer, 1999). Carbonatite and

associated alkaline magmatism may also be correlated

with mantle plume activity (Gerlach et al., 1988; Bell

and Simonetti, 1996; Simonetti et al., 1998; Ray et al.,

1999; Bell and Tilton, 2001). The spatial and temporal

association of many carbonatites and alkaline rocks

with continental flood basalts (CFBs) is further

evidence for a plume-related origin (Bell, 2001,

2002). The present paper reports a new U–Pb

perovskite age together with Nd, Sr and C–O isotopic

ratios and major and trace element data from

carbonatite and associated silicate rocks of the Sung

Valley complex. This research is an attempt to better

understand the genetic relationship between carbona-

tite and associated silicate rocks and to address many

of the questions posed by Bell et al. (1998).

2. Geological setting

The Sung Valley complex was emplaced within the

Proterozoic Shillong Group of rocks, located within

the Shillong Plateau (Fig. 1). The Shillong plateau is

an uplifted horst-like feature, bounded on all sides by

major structural discontinuities (see Fig. 1a for detail).

On the basis of Landsat images and aerial photo-

graphs, Gupta and Sen (1988) have observed that a

N–S trending lineament, known as Um Ngot linea-

ment, cuts across the general NE–SW trend of the

Shillong Plateau. This lineament developed during the

late Jurassic–early Cretaceous times. Gupta and Sen

(1988) further stated that the Um Ngot lineament

contains several alkaline intrusive bodies, including

the Sung Valley complex, and is genetically related to

the Ninety-East Ridge in the Indian Ocean. The

plateau consists mostly of Archaean gneisses and the

Proterozoic Shillong Group of rocks, comprising

orthoquartzite and phyllite. Several granite plutons

(700–450 Ma) also intrude the gneissic basement as

well as the Shillong Group cover sequence. Small

bodies of metamorphosed mafic igneous rocks are

also reported (Mazumdar, 1976; Ghosh et al., 1994).

The Sylhet Traps, a part of the Rajmahal–Sylhet flood

basalt province, are well exposed in the southern part

of the plateau and are considered to be associated with

the Kerguelen mantle plume (Storey et al., 1992; Kent

et al., 1997, 2002). The ultramafic–alkaline–carbona-

tite complexes of the Shillong Plateau are also

Fig. 1. (a) Regional geological and tectonic framework of the Shillong Plateau (compiled by Srivastava and Sinha, 2004). Number in circle

indicates locations of ultramafic–alkaline–carbonatite complexes: (1) Sung Valley, (2) Jasra, (3) Samchampi, (4) Swangkre. (b) Geological map

of the Sung Valley ultramafic–alkaline–carbonatite complex (after Krishnamurthy, 1985; Viladkar et al., 1994; Srivastava and Sinha, 2004).

Nepheline syenite and melilitolites dykes exposed around the villages Sung and Maskut are very small, hence not mentioned on the map.

R.K. Srivastava et al. / Lithos 81 (2005) 33–54 35


inferred to be associated with the Kerguelen Plume

(Veena et al., 1998; Ray et al., 1999, 2000).

Several researchers (Krishnamurthy, 1985; Viladkar

et al., 1994; Srivastava and Sinha, 2004) have

described the geology of the Sung Valley complex,

which consists of pyroxenite, serpentinized peridotite,

melilitolite, ijolite, nepheline syenite and carbonatite

(Fig. 1b). Petrographic details of these rock types are

summarised in Table 1. Pyroxenite encloses serpenti-

nized peridotite and represents the oldest rocks of the

complex. Ijolite forms a ring structure. These three rock

types form a major portion of the complex, other rock

units constitute less than 5%. Melilitolite occurs as

small dykes, intruded into the peridotite and pyrox-

enite. Nepheline syenite occurs in the form of dykes

and veins, whichmostly intrude ultramafic units as well

as ijolites (cf. Krishnamurthy, 1985). Carbonatite is the

youngest member of the complex as it intrudes all the

units and occurs as small dykes, veins and oval shaped

bodies. Here, it is important to mention that peridotites

are severely affected by extensive alteration due to

post-magmatic hydrothermal activity so fresh expo-

sures of peridotite and relict primary mineralogy is

rarely preserved. Hence, peridotite samples are not

included in the present study.

Petrographically, most pyroxenite samples show a

hypidiomorphic texture and contain clinopyroxene

(cpx)—mainly diopside. Appreciable amount of

augite/aegirine–augite is also present in all the pyrox-

enite samples. The aegirine–augite may be distin-

guished from the other clinopyroxenes by lower

Table 1

Petrographic details of the studied rock units of the Sung Valley complex

Status Rock types Mode of occurrence Pe

Te

Youngest unit Carbonatite Dyke, vein, oval

shaped bodies

So

Nepheline syenite Dyke and veins H

all

Melilitolite Small dyke H

Ijolite Ring structure H

Oldest unit Pyroxenite

Cover major portion

H

Serpentinizedgperidotite

of the complex N

(a

to

hy

extinction angles. A few grains of calcic plagioclase

are also visible in these rocks. Other constituents are

clino-enstatite, olivine, biotite, apatite, sphene, epi-

dote-clinozoisite, ilmenite and rutile. Ijolites are

typically medium-grained hypidiomorphic and mainly

consist of nepheline, aegirine–augite and aegirine,

whereas diopside, perovskite, apatite, sphene, epi-

dote-clinozoisite, zircon, melanite, calcite and opaques

are present as minor constituents. Nepheline syenites

are mainly composed of coarse-grained crystals of

orthoclase, albite, perthite, nepheline and aegirine,

whereas sphene, zircon, calcite and opaques form

miner constituents. Melilitolites are mainly composed

of melilite and diopside with minor amounts of

melanite and opaques. Carbonatites are mainly sovitic

(coarse grained, hypidiomorphic texture) consisting

mainly of calcite grains. Dolomite, siderite, apatite,

phlogopite, pyrochlore, zircon and opaques are com-

mon accessories.

3. Analytical techniques

Whole rock major and trace element analyses were

conducted at Activation Laboratories, Ancaster,

Ontario, Canada. ICP-OES (Model: Thermo-Jarre-

tAsh ENVIRO II) was used to analyse major

elements, whereas ICP-MS (Model: Perkin Elmer

Sciex ELAN 6000) was used to determine trace

element concentrations. The precision is b5% for the

major and trace elements when reported at 100�

trography

xture Main mineral composition

vite (hypidiomorphic) Calcite, dolomite, apatite,

phlogopite and pyrochlore

ypidiomorphic and

otriomorphic

Orthoclase, albite, perthite,

nepheline and aegirine

ypidiomorphic Melilite and diopside

ypidiomorphic Nepheline and aegirine–augite/aegirine

ypidiomorphic Diopside, augite/aegirine–augite

and minor calcic plagioclase

ot presented

ll serpentinized due

post-magmatic

drothermal activity)


detection limit. Several standards, such as MRG1,

W2, DNC1, STM1 and SY3, were analysed to

monitor accuracy and precision. High-precision U–

Pb perovskite age determination was conducted at the

Department of Earth and Atmospheric Sciences,

University of Alberta, Canada on the VG354 thermal

ionization mass spectrometer (for analytical proce-

dures, refer to Heaman and Kjarsgaard, 2000).

Radiogenic and stable isotopic analyses were done

at the Research Center for Mineral Exploration

(RCMRE), Beijing, PR China. A Thermo Finnigan

MAT 252 mass spectrometer was used for stable

isotope (carbon and oxygen) analyses. All the stable

isotope data are reported relative to the international

standard V-PDB; internal precision is 0.005x for

d13C and 0.01x for d18O. A Thermo Finnigan MAT

262 mass spectrometer was used for radiogenic

isotope (Sm–Nd and Rb–Sr) analyses. International

standards LA and BCR-1 were used for Nd isotope

analyses and NBS987 and NBS607 were used for Sr

isotope analyses. The internal precision for Sr and Nd

isotope ratio measurement is F0.0006 and F0.0007,

respectively, whereas external precision for both the

isotopes is F0.0009. The U–Pb perovskite results are

presented in Table 2. As Srivastava and Sinha (2004)

have already presented whole rock chemical data on

the Sung Valley complex, here we present only the

mean chemical compositions and standard deviations

for all the rocks types (see Table 3). Tables 4 and 5

report the stable and radiogenic isotope data, respec-

Table 2

U–Pb perovskite results for ijolite sample (SV/58) from the Sung Valley

Description Weight

(Ag)U

(ppm)

Th

(ppm)

Pb

(ppm)

Th/U

1. Tiny lt brn frags

15NM (57)

10 136 126 16 0.93

2. Lt grey to tan frags

15NM (48)

110 95 162 20 1.70

3. Dk brn blocky frags

15NM (35)

167 165 45 25 0.27

4. Grey blocky frags

15NM (50)

137 145 284 18 1.96

frags: fragments, irreg: irregular, Lt: light, Dk: dark, brn: brown.

Number in parentheses corresponds to the total number of grains analysed

15NM refers to a non-magnetic fraction from a Frantz Isodynamic Separa

Thorium concentrations estimated from amount of 208Pb in analyses. TCP

All errors reported at 1j and reflect the uncertainty in the last decimal po

T 0.01747F52 corresponds to 0.01747F0.00052.

tively. Locations of samples selected for isotopic

study are marked on the geological map (Fig. 1b).

4. Age of the Sung Valley complex

There are four ultramafic–alkaline–carbonatite

complexes known from the Shillong Plateau. All

have been dated by different methods using different

material (see Table 6). With the exception of the Sung

Valley intrusion, all of the other complexes show

identical emplacement ages (105–107 Ma). Unlike the

consistent age results obtained for the other intrusions,

the Sung Valley complex shows a wide range of ages,

between 90 and 150 Ma. The 106F11 Ma Rb–Sr and

107.2F0.8 Ma 40Ar/39Ar dates reported by Ray et al.

(1999, 2000) are the most precise and show excellent

agreement with the emplacement ages obtained for the

other alkaline complexes and Rajmahal–Sylhet flood

basalts (see Table 6). Considering the range in

radiometric age determinations reported for this

intrusion, based exclusively on carbonatite samples,

it is possible that the Sung Valley intrusion could

reflect multiple intrusion events. For this reason, we

attempted to constrain the emplacement age of a

silicate-rich phase of the complex (ijolite) using a

robust isotopic method (U–Pb perovskite).

In order to determine a precise U–Pb age for the

Sung Valley ijolite, we selected a sample (SV/58),

which contains abundant, grey, perovskite fragments.

complex

TCPb

(pg)

206Pb/204/Pb 238U/204Pb 206Pb/238U* 206Pb/238U

age (Ma)

121 30.99F0.12 712.8F6.2 0.01747F52 111.6F3.3

1962 24.19F0.05 333.5F1.0 0.01692F111 108.2F7.1

3321 27.60F0.09 520.1F5.9 0.01741F74 111.3F4.7

1916 29.10F0.03 649.5F1.1 0.01625F57 103.9F3.6

.

tor at full field strength (1.8 A) and 158 side tilt/108 forward tilt.

b refers to the total amount of common Pb present in picograms.

sition.

Table 3

Mean (M) and standard deviation (S.D.) of analysed samples from the Sung Valley ultramafic–alkaline–carbonatite complex, Shillong Plateau,

northeastern India

Pyroxenites Ijolites Nepheline syenites Carbonatites Melilitolites

M N S.D. M N S.D. M N S.D. M N S.D. SV/8 SV/33

SiO2 51.87 6 2.04 42.06 7 2.21 55.26 10 3.61 0.46 7 0.10 41.26 44.19

TiO2 0.57 6 0.10 2.21 7 1.76 0.48 10 0.28 0.13 7 0.21 1.89 0.84

Al2O3 3.27 6 1.55 12.66 7 4.67 18.87 10 1.83 0.19 7 0.04 8.57 4.37

Fe2O3 9.18 6 1.59 10.44 7 2.58 5.10 10 1.70 2.22 7 1.70 15.06 8.34

MnO 0.15 6 0.03 0.22 7 0.06 0.10 10 0.03 0.18 7 0.04 0.22 0.06

MgO 11.28 6 1.03 5.62 7 2.53 0.54 10 0.23 2.64 7 0.34 8.92 11.91

CaO 19.48 6 0.83 13.20 7 5.58 2.48 10 1.61 50.85 7 0.87 23.01 28.00

Na2O 2.51 6 0.67 5.08 7 2.10 8.75 10 2.12 0.04 7 0.01 0.40 1.40

K2O 0.56 6 0.31 1.52 7 0.50 5.70 10 1.48 0.12 7 0.06 0.03 0.11

P2O5 0.09 6 0.04 1.18 7 1.48 0.15 10 0.19 2.17 7 1.86 0.54 0.01

LOI 1.21 6 1.03 2.90 7 2.16 1.98 10 1.08 39.63 7 2.41 0.59 0.53

Mg# 75.26 6 4.75 55.41 7 13.27 23.72 10 6.85 74.80 7 13.72 57.11 76.25

Cr 145.67 3 50.93 194.00 3 180.26 110 387

Ni 126.00 4 19.25 73.40 5 33.03 33.50 2 7.78 107 109

Sc 36.50 6 8.73 10.43 7 14.06 15.70 7 2.56 24 39

V 125.67 6 45.25 355.43 7 158.29 130.20 10 102.62 53.43 7 56.79 223 50

Rb 19.25 4 13.07 33.00 4 17.15 116.00 7 45.81 2.67 3 0.58

Ba 119.33 6 63.10 136.71 7 211.01 919.50 10 574.93 197.71 7 70.79 38 13

Sr 280.17 6 68.35 669.71 7 385.46 469.30 10 183.14 4373.86 7 503.19 365 665

Ga 5.25 4 0.50 15.40 5 4.22 21.57 7 7.96 2.67 3 0.58 17 10

Ta 1.08 4 0.93 9.18 4 12.82 2.19 7 1.66 7.50 3 11.06 2.70

Nb 7.75 4 0.96 210.40 5 410.21 54.86 7 48.72 114.33 3 175.52 34.0 1.00

Hf 2.23 4 0.39 7.84 5 13.41 4.53 7 2.73 0.33 3 0.15 3.00 1.10

Zr 74.83 6 21.61 473.14 7 413.72 236.10 10 148.27 29.14 7 28.38 108 35

Y 7.67 6 1.21 61.43 7 50.26 9.10 10 4.84 54.00 7 7.68 20

Th 2.68 4 0.98 7.93 4 7.34 3.01 7 2.27 33.97 3 55.02 4.50

U 0.65 4 0.29 2.23 4 3.00 0.50 7 0.33 0.60 3 0.36 0.70

La 7.45 4 1.48 65.94 5 41.29 12.67 7 7.43 161.00 3 15.62 37.50 5.40

Ce 15.50 4 2.75 150.58 5 108.75 22.60 7 10.84 350.67 3 36.69 68.30 11.70

Pr 1.98 4 0.26 20.53 5 15.25 3.56 7 1.36 44.10 3 4.97 8.61 1.55

Nd 7.75 4 1.12 81.24 5 61.71 13.01 7 5.03 172.00 3 21.00 31.90 6.40

Sm 1.70 4 0.27 16.00 5 12.31 2.34 7 0.90 28.77 3 3.52 5.40 1.20

Eu 0.45 4 0.08 5.18 5 4.05 0.69 7 0.29 9.03 3 1.02 1.99 0.40

Gd 1.68 4 0.26 14.76 5 11.73 2.01 7 0.74 22.39 3 2.53 5.40 1.00

Tb 0.28 4 0.05 2.32 5 1.88 0.33 7 0.15 3.00 3 0.26 0.80 0.10

Dy 1.65 4 0.17 12.17 5 10.07 1.89 7 0.87 13.30 3 0.89 4.40 0.60

Ho 0.33 4 0.05 2.20 5 1.86 0.36 7 0.18 2.10 3 0.10 0.80 0.10

Er 0.98 4 0.05 5.78 5 4.94 1.10 7 0.64 5.17 3 0.32 2.10 0.30

Tm 0.18 4 0.02 0.75 5 0.65 0.27 7 0.22 0.52 3 0.05 0.26

Yb 1.25 4 0.17 4.70 5 3.88 1.34 7 0.72 3.37 3 0.25 1.60 0.20

Lu 0.22 4 0.05 0.67 5 0.51 0.24 7 0.12 0.41 3 0.04 0.23

N=number of analysed samples; SV/8 and SV/33=chemical data of melilitolite samples.


The U–Pb results for four multi-grain perovskite

fractions (consisting of between 35 and 57 fragments)

are presented in Table 2 and on an isochron diagram

in Fig. 2. Perovskite in this sample has moderate

uranium contents (95–165 ppm), low thorium con-

tents (45–284 ppm) and quite variable Th/U (0.27–

1.96). Perovskite from the Ice River ijolite in Canada

(Heaman, unpublished data) contains similar uranium

concentration but significantly higher thorium con-

tents (1700–2400 ppm) with corresponding high Th/U

(N12). This high Th/U is a geochemical characteristic

of most matrix perovskite in kimberlites (e.g. Heaman

Table 4

Stable carbon and oxygen isotopic compositions of carbonatite

samples from the Sung Valley UACC

Sample no. A y13C (x vs. V-PDB) y18O (x vs. SMOW)

SV/12 �3.26 7.81

SV/49 �3.27 7.57

SV/50 �3.19 7.72

SV/54 �3.29 7.77

SV/73 �3.24 7.35

SV/77 �3.12 7.93


and Kjarsgaard, 2000). As can be seen in Fig. 2, the

four perovskite analyses have range in 238U/204Pb

(334–1021) with similar 206Pb/238U dates of between

Table 5

Sm–Nd and Rb–Sr isotopic compositions in silicate and carbonatite samp

Sm–Nd compositions

S. no. A Rock type Sm (ppm) Nd (ppm) 147Sm/

SV/14 Ijolite 23.420 112.20 0.1263

SV/24A N-syenite 3.068 16.70 0.1111

SV/25 N-syenite 3.041 15.84 0.1161

SV/46 Ijolite 7.011 43.09 0.0984

SV/49 Carbonatite 37.570 220.80 0.1029

SV/54 Carbonatite 30.090 178.90 0.1018

GC1162* Carbonatite 41.500 242.00 0.1035

GC1163* Carbonatite 70.400 410.00 0.0994

GC1164* Carbonatite 14.600 85.00 0.0994

GC1165* Carbonatite 34.800 260.00 0.0815

GC1166* Carbonatite 32.700 199.00 0.0999

LA

BCR-1

Rb–Sr compositions

S. no. A Rock type Rb (ppm) Sr (ppm) 87Rb/8

SV/14 Ijolite 19.390 497.0 0.1129

SV/25 N-syenite 101.300 529.3 0.5542

SV/46 Ijolite 40.300 1262.0 0.0924

SV/49 Carbonatite 3.853 4661.0 0.0023

SV/54 Carbonatite 2.184 4988.0 0.0012

GC1162* Carbonatite 2.240 3524.0 0.0000

GC1163* Carbonatite 0.280 9525.0 0.0000

GC1164* Carbonatite 0.350 2656.0 0.0000

GC1165* Carbonatite 0.680 3667.0 0.0005

GC1166* Carbonatite 0.690 3954.0 0.0004

NBS987

NBS607

Initial Nd and Sr isotope ratios along with the epsilon values were calcul

calculated using present-day ratios of 87Sr/86Sr=0.7045 and 87Rb/86Sr=0.0147Sm/144Nd=0.1967 for CHondritic Uniform Reservoir (CHUR).

T Data taken from Veena et al. (1998) but recalculated assuming an em

103.9 and 111.6 Ma (Table 2). Three of these

perovskite analyses (#1–3) have quite similar206Pb/238U dates of between 108.2 and 111.6 Ma

with a weighted average age of 111.1F4.9 Ma

(MSWD=0.10). The preferred U–Pb perovskite age

is 115.1F5.1 Ma (MSWD=0.14) is based on a linear

regression treatment of fractions #1–3 (i.e. age

determination is not dependent on an assumed initial

common Pb composition) and is considered the

current best estimate for the emplacement age of the

Sung Valley ijolite. This age is slightly older than

the 107.2F0.8 Ma 40Ar/39Ar phlogopite age

obtained for the Sung Valley carbonatite (Ray et

al., 2000), hinting that the Sung Valley ijolite may

les from the Sung Valley UACC

144Nd 143Nd/144Nd (2j) 143Nd/144Ndinitial ENd

0.512535F15 0.512448 �1.07

0.5120140F7 0.511938 �11.02

0.5119250F9 0.511845 �12.84

0.5123710F5 0.512303 �3.90

0.5126110F7 0.512538 0.68

0.512611F11 0.512541 0.74

0.512660 0.512589 1.68

0.512640 0.512572 1.35

0.512650 0.512582 1.54

0.512650 0.512594 1.78

0.512640 0.512571 1.32

0.511854F7

0.512613F8

6Sr 87Sr/86Sr (2j) 87Sr/86Srinitial ESr

00 0.705850F11 0.705681 18.47

00 0.711198F12 0.710371 85.05

30 0.709032F11 0.708894 64.08

92 0.704834F20 0.704830 6.39

67 0.7048740F6 0.704872 6.98

20 0.704760 0.704760 5.39

90 0.704710 0.704710 4.68

40 0.704760 0.704760 5.39

40 0.704830 0.704830 6.39

90 0.704740 0.704740 5.11

0.710239F10

1.20054F12

ated assuming an emplacement age of 105 Ma. Epsilon values are

827 for Bulk Silicate Earth (BSE) and 143Nd/144Nd=0.512638 and

placement age of 105 Ma.

Table 6

Age data on basalts from the Kerguelen Plateau, Broken Ridge, Naturaliste Plateau, Bunbury and Rajmahal–Sylhet igneous province and

associated ultramafic–alkaline–carbonatite complexes of Shillong Plateau

Method Material Age (in Ma) References

Kerguelen Plateau

Ar–Ar Basalt from ODP site 1136 118–119 Coffin et al. (2002); Duncan (2002)

Basalt from ODP site 1137 107–108

Basalt from ODP sites 749 and 750 110–112

Basalt from ODP site 1138 ~100

Broken Ridge

Ar–Ar Basalt from ODP sites 1141 and 1142 ~100 Coffin et al. (2002); Duncan (2002)

Naturaliste Plateau

Ar–Ar Basalt 100.6F1.2 Pyle et al. (1995)

Bunbury, Western Australia

Ar–Ar Basalt lava 123–130 Frey et al. (1996); Coffin et al. (2002)

Rajmahal–Sylhet flood basalts province

Ar–Ar Basalts 105–118 Baksi et al. (1987); Baksi (1995);

Kent et al. (1997, 2002); Coffin et al. (2002)

Ultramafic–alkaline–carbonatite complexes (Shillong Plateau)

1. Sung Valley

Fission track Apatite 90F10 Chattopadhyay and Hashimi (1984)

K–Ar Phlogopite from carbonatite 149F5 Sarkar et al. (1996)

Pb–Pb Carbonatite (WR) 134F20 Veena et al. (1998)

Ar–Ar Pyroxenite (WR) and phlogopite from carbonatite 107.2F0.8 Ray et al. (1999)

Rb–Sr Carbonatite (WR), pyroxenite (WR) and phlogopite

from carbonatite

106F11 Ray et al. (2000)

U–Pb Perovskite from ijolite 115.1F5.1 This study

2. Jasra

U–Pb Zircon and baddeleyite from differentiated gabbro 105.2F0.5 Heaman et al. (2002)

3. Samchampi

Fission track Apatite ~105 Acharya et al. (1986)

4. Swangkre

K–Ar Lamprophyre 107F3 Sarkar et al. (1996)


have a slightly older emplacement age than the

carbonatite, consistent with field relationships. One

perovskite fraction (#5) plots below this isochron

shown in Fig. 2 with a lower 206Pb/238U date of

103.9F7.2 Ma (2j) and indicates that perovskite in

this sample may not consist of a single age population.

5. Geochemistry

5.1. Whole rock chemistry

Major and trace element whole rock compositions

were determined for 32 samples (6 pyroxenite, 7

ijolite, 10 nepheline syenite, 2 melilitolite and 7

carbonatite). A selection of the most diagnostic major

and trace elements and element ratios for these

samples are plotted against Mg number in Fig. 3.

Mg Numbers (Mg#=100*Mg2+/Mg2++Fe2+) were

automatically computed using the SINCLAS Com-

puter Programme (Verma et al., 2002). Mg# is highest

in the carbonatites, whereas the lowest values occur in

the nepheline syenite samples. Most carbonatite

samples have negligible amounts of SiO2, TiO2 and

Zr. All the main rock types of the Sung Valley

complex are easily distinguishable on a Mg#–SiO2

plot (Fig. 3a). With increasing Mg#, TiO2 in most of

the silicate rocks (except nepheline syenites) shows a

Fig. 2. U–Pb isochron plot showing the results for four perovskite analyses from ijolite sample SV/58.


decreasing trend; nepheline syenite samples plot

separately (Fig. 3b). Similar geochemical correlations

for the silicate rocks can be seen for Y and Zr (Fig.

3e,f). On most geochemical plots, the nepheline

syenite samples plot separately from the other silicate

samples. This is most noticeable when comparing

TiO2, Y, Zr, Nd contents and Ba/La among all

samples. Similarly, the carbonatite samples do not

form a geochemical continuum with any of the silicate

rocks and plot in a separate field, especially on SiO2–

Mg#, CaO–Mg# and Sr–Mg# plots (Fig. 3a,c,d).

From these bivariate geochemical variation diagrams,

it is difficult to establish any coherent chemical

relationship between carbonatite, nepheline syenite

and other silicate rocks of the Sung Valley complex.

This indicates the involvement of different magma

processes and/or origin for the carbonatites, nepheline

syenites and other silicate rocks.

Primordial-mantle normalized multi-element pat-

terns for silicate and carbonate rocks of the Sung

Valley complex are presented in Fig. 4. All samples

show enrichment in the plotted elements in compar-

ison to the undifferentiated primordial mantle (McDo-

nough et al., 1992). Different multi-element patterns

are observed for these rocks. The ijolite samples (Fig.

4a) show a wide range of chemical compositions,

specially Ba, Ta, Nb and Ti. Some of the samples

show a prominent negative Ba and Sr anomaly.

Pyroxenite samples (Fig. 4b) have different multi-

element patterns. Negative anomalies are observed for

Ba, Nd and K only, whereas positive anomalies are

noticed in Th, Sr and Zr. Ta displays a variable

behaviour with both positive and negative anomalies.

The composition of average melilitolite (Fig. 4b) is

similar to that observed for pyroxenite, except with

slightly elevated concentrations of most elements.

From the multi-element patterns of nepheline syenite

samples (Fig. 4c), two different trends are observed

(cf. Srivastava and Sinha, 2004). One group shows

negative anomalies in Ta, Nd and Ti and positive

anomaly in Zr. Nb shows either positive or no

anomaly. On the other hand, the second group

displays negative anomalies in Th, Ta, La, Ce and

Nd and positive anomalies in Nb and Zr. These groups

are defined here as group 1 syenites and group 2

syenites, respectively. It is likely that second group of

syenites might be produced by a process of ultra-

fenitization but additional geochemical and isotopic

data are required.

Carbonatite samples (Fig. 4d) also show different

multi-element patterns. Except Zr and Ti, all other

elements are enriched in comparison to primitive

Fig. 3. Geochemical variation diagram, plotted against Mg#, for the Sung Valley complex.


mantle. They show negative anomalies in Rb, K, Zr

and Ti. One sample has higher concentrations of Th,

Ta and Nb than the other two samples, probably due

to pyrochlore crystallization in this sample. Again,

from these multi-element patterns, it is difficult to

establish any genetic relationship between the silicate

members and the carbonatites of the Sung Valley

complex.

Fig. 5 presents chondrite-normalized rare-earth

element (REE) patterns for samples of the Sung Valley

complex. Samples of all rock types show enriched

REE concentrations compared to the chondritic values

(Evensen et al., 1978). The REE patterns for the ijolite

samples (Fig. 5a) are strongly fractionated, (La/Lu)N

vary from 8.34 to 18.73. REE patterns are fairly

inclined. A similar pattern is observed for average

Fig. 4. Primordial mantle normalized multi-elements spidergrams. Normalized values are after McDonough et al. (1992).


melilitolite (Fig. 5b). Pyroxenite samples (Fig. 5b)

have dUT-shaped REE patterns (downward inclination

from LREE to MREE followed by an enriched pattern

in HREE). Pyroxenite samples also display a small

negative Eu anomaly. The nepheline syenite samples

show two different REE patterns (Fig. 5c). Both

groups show dUT-shaped REE patterns but group 2

syenites also have depleted La and Ce concentrations

in comparison to group 1 syenites. The LREE to

MREE patterns for the nepheline syenites are quite

similar but there is a distinct fractionation in the

Fig. 5. Chondrite normalized rare-earth patterns. Ch

HREE. Carbonatite samples (Fig. 5d) show strongly

fractionated REE patterns compared to all other

samples from the complex, (La/Lu)N vary from

35.30 to 46.46. This reflects the control minerals

present in the carbonatites, such as apatite, perovskite,

pyrochlore, etc. have on the total REE budget as they

have very highP

REE and show high LREE/HREE

values (Hornig-Kjarsgaard, 1998). Although the Sung

Valley rocks show different REE patterns, possibly

indicating different origins or magma evolution

processes, to a first approximation that the REE

ondrite values are after Evensen et al. (1978).

Fig. 6. (a) d18O and d13C plot. Taylor’s PIC box after Taylor et al.

(1967), modified PIC box after Deines (1989) and Keller and Hoefs

(1995), and normal mantle values after Kyser (1990) and Keller and

Hoefs (1995). (b) d18O and 87Sr/86Srinitial plot. This diagram also

shows primary mantle values and hypothetical mixing model (after

James, 1981).


patterns of pyroxenites, melilitolites and ijolites could

be explained by differentiation of a single magma.

5.2. C and O isotopes

Stable isotopic, particularly C and O, studies have

been successfully applied to deciphering the origin of

carbonatites. Taylor et al. (1967) were first to

recognize the range of d13C and d18O values in

unaltered and un-weathered primary magmatic carbo-

natites. According to them and some other later

researchers, primary magmatic carbonatites exhibit a

relatively narrow range of �4x to �8x for d13C and

+6x to +10x for d18O (Taylor et al., 1967; Deines

and Gold, 1973; Sheppard and Dawon, 1973). Deines

(1989) and Keller and Hoefs (1995) have reviewed

available carbon and oxygen isotopic data for

carbonatites and suggested that most magmatic

carbonatites, termed primary igneous carbonatites

(PIC), have d13C values between �1x to �9x and

d18O values between +5x and +12x. Carbon and

oxygen isotopic compositions of the Sung Valley

carbonatite samples have been plotted on Fig. 6a. This

diagram also includes the range for un-degassed and

un-contaminated bnormalQ mantle values (this

includes mantle xenoliths, MORBs, OIBs and other

continental basalts); d18O ranges between +5.5x and

+8x and d13C ranges between �5x and �7x (after

Kyser, 1990; Keller and Hoefs, 1995). The carbonatite

samples of the Sung Valley complex analysed by Ray

et al. (1999) are also included in this diagram (open

triangles). All samples fall well within the modified

primary magmatic carbonatite field. An important

conclusion that can be gleaned from this diagram is

that d18O compositions of these carbonatite samples

plot within the field of primary magmatic carbonatites

defined by Taylor et al. (1967) and the normal mantle

range (Kyser, 1990). This implies that the carbonatite

samples were not affected either by any crustal

assimilation or loss of fluids during emplacement.

The mantle nature of these carbonatite samples is also

confirmed on the 87Sr/86Srinitial versus d18O diagram

(Fig. 6b) as they fall well within the primary mantle

(PM) field. This diagram also includes a hypothetical

mixing field that illustrates the effect of mantle source

contamination and crustal contamination on the Sr and

O isotopic compositions of derived mantle melt

(James, 1981). The Sung Valley carbonatite samples

do not show any such contamination trends and fall

well within the mantle range.

5.3. Sr, Nd and Pb isotopes

87Sr/86Srinitial and 143Nd/144Ndinitial ratios in the

Sung Valley samples show a wide range and vary

between 0.704830–0.710371 and 0.511845–0.512541,

respectively. All samples have positive ESr values

(+4.7 to +85.1). Slightly positive ENd values (+0.7 to

+1.8) are observed for carbonatite samples but all


silicate samples show negative ENd values (�1.1 to

12.8). The Sr and Nd isotopic composition of two

carbonatite samples in this study are similar to the

results reported by Veena et al. (1998) for five

carbonatite samples (listed in Table 4). The Nd initial

isotopic composition of the samples analysed in this

study are slightly lower (0.512538–0.512541) com-

pared to the Veena et al. (1998) samples (0.512571–

0.512594). A considerable difference is observed

between the isotopic composition of the carbonatite

and the silicate samples. The carbonatite and silicate

samples plot separately in the 87Sr/86Srinitial versus143Nd/144Ndinitial diagram (Fig. 7a); the carbonatite

Fig. 7. (a) 143Nd/144Nd and 87Sr/86Sr isotope correlation plot,

showing the main oceanic mantle reservoirs of Zindler and Hart

(1986). The mantle array is defined by many oceanic basalts and a

bulk Earth values of 87Sr/86Sr can be observed from this trend. (b)

qNd and qSr plot. Field of young carbonatites (b200 Ma) is taken

from Bell and Blenkinsop (1989) and field of Kerguelen OIB is

taken from Veena et al. (1998) and references therein.

samples and one sample of ijolite (SV/14) fall

within the primitive mantle field (denoted dmantle

arrayT in Fig. 7a), whereas the other two samples, an

ijolite and nepheline syenite, plot near the enriched

mantle II (EM-II) field. Carbonatite samples show

the lowest 87Sr/86Srinitial ratios and the highest143Nd/144Ndinitial ratios compared to all other sam-

ples and plot close to the Bulk Silicate Earth field in

Fig. 7a. Enriched mantle type-I (EM-I) has low87Sr/86Sr values, whereas EM-II has relatively high87Sr/86Sr values (Zindler and Hart, 1986). The high87Sr/86Sr values observed in some silicate rocks are

probably the result of metasomatism by fluids/melts

with a EM-II signature.

Different mantle reservoirs can be defined on the

basis of their isotopic taxonomy. On this basis, at

least four mantle components (DMM, HIMU, EM-I

and EM-II), are required to explain the isotopic

diversity observed for most oceanic basalts (Zindler

and Hart, 1986). In addition to these four reservoirs,

Hart et al. (1992) have proposed another mantle

component that they define as FOZO and consider

to represent focal zone basalts. Hart et al. (1992)

suggested two models to account for this FOZO

component and in both the FOZO signature is

derived by mixing of plumes containing EM and

HIMU components from the core-mantle boundary

or lower mantle. The difference between these two

is that in one model no component is derived from

the boundary layer at 670 km, whereas in the other

model it is derived from the boundary layer at 670

km. Bell and Tilton (2001) have suggested that

mantle with either HIMU or EM signature or a

mixture of both can also produce melts that can lead

to carbonatite. Although it is difficult to estimate the

exact proportion of the different mantle reservoirs

involved to generate the range in isotopic compo-

sitions preserved in the Sung Valley complex, it is

possible that mixing of HIMU and EM-II reservoirs,

similar to the FOZO model, could explain the range

in Sr and Nd isotopic composition, especially for the

silicate end members. Interaction with old continen-

tal crust could also account for the high ESr and low

eNd isotopic nature of some silicate samples;

however, the high concentration of Sr and Nd in

the Sung Valley rocks make it unlikely that

assimilation of average crust could account for these

extreme isotopic compositions.


The strontium isotopic composition for one

pyroxenite sample from the Sung Valley complex

was analysed by Ray et al. (2000). The 87Sr/86Srinitialratio obtained for this sample is 0.7051 and is

intermediate in composition between the carbonatite

samples (0.704710–0.704872) and one sample of

ijolite (0.705681; SV/14), also plotting within the

dprimitiveT mantle field. Another diagram, using ENd

and ESr values, is also prepared for the studied samples

(Fig. 7b). This diagram also shows the field for most of

the oceanic island basalts (OIB) from the Kerguelen

Island (Dosso and Murthy, 1980; Storey et al., 1992;

Weis et al., 1993; Mahoney et al., 1995) and the young

African carbonatites (b200 Ma; Nelson et al., 1988;

Bell and Blenkinsop, 1989; Bell and Tilton, 2001).

Carbonatite samples from the Sung Valley complex

analysed by Veena et al. (1998) are also included in

this plot but recalculated assuming an emplacement

age of 105 Ma. All the carbonatite samples analysed in

this study as well as those reported by Veena et al.

(1998) plot in the upper right quadrant of the diagram,

whereas silicate samples plot in the lower right

quadrant. Most of the young African carbonatites

(b200 Ma) plot in the upper left quadrant (Nelson et

al., 1988; Bell and Blenkinsop, 1989; Bell and Tilton,

2001 and references therein). All the Sung Valley

carbonatite samples and one ijolite sample fall in the

Kerguelen OIB field. It is noted from this diagram that

the carbonate and silicate components of the studied

samples define a trend away from the characteristic

depleted values of worldwide carbonatites into the

enriched quadrant. This indicates the involvement of

an enriched reservoir for the Sung Valley carbonatites

that is somewhat different from most other carbona-

tites. The cause of this enrichment may either be by the

subduction of crustal material into the mantle or by

mantle metasomatism. Whatever the cause, the iso-

topic data reported here for the Sung Valley samples

indicate that the silicate rocks clearly either contain a

greater proportion of this enriched component com-

pared to the carbonatites or have involved an isotopi-

cally distinct enriched mantle source region such as

EM-II.

The 206Pb/204Pb initial ratio (18.2) obtained for the

perovskite (extracted from ijolite) from the isochron

diagram (Fig. 2) is lower than what is reported for

most young carbonatites. Veena et al. (1998) also

reported a lower 206Pb/204Pb initial ratio (19.02–

19.30) for two carbonatite samples. These low values

indicate that either a low U/Pb source (significantly

less radiogenic than HIMU) is required for ijolite

magma formation or there is interaction with a quite

low U/Pb mantle reservoir during their generation. It

cannot be a crustal phenomenon since average crust

has quite high U/Pb.

6. Discussion

Before integrating the petrological and geochem-

ical data, it is important to review the salient field

relationships between the different rock units of the

Sung Valley complex. Some important field obser-

vations are:

1. Ijolite clearly shows a discordant relationship with

pyroxenite.

2. Melilitolite intrudes peridotite as well as pyroxenite

but not the other rock units of the complex.

3. Dykes/veins of nepheline syenite and carbonatite

intrude pyroxenite as well as ijolite and show sharp

contacts.

These field observations clearly indicate that

different magmas with distinct histories may be

involved in the generation of the Sung Valley

complex. It seems likely that silicate and carbonatite

members of the complex are derived from discrete

magma batches. In addition, the age determinations

for the Sung Valley complex are consistent with the

possibility as much as several million years has

elapsed between the intrusion of ijolite (115.1 Ma;

this study) and the carbonatite (107.2 Ma; Ray et al.,

2000). This age difference is consistent with the field

relationships that indicate that carbonatite is youngest

unit and intrudes all other rock types.

It is also difficult to establish any genetic relation-

ship between the carbonatite and silicate rocks based

on the chemical and isotopic composition of the

various Sung Valley lithologies. Nepheline syenite

and carbonatite have entirely different chemical

characteristics than the pyroxenites, melilitolites and

ijolites. The latter group of silicate rocks shows some

genetic relationship with each other, as they appear to

form a geochemical continuum for some elements.

However, even for these rocks, there are some


geochemical variations that are difficult to explain if

they were all derived from the same magma chamber.

For example, no two multi-element spidergrams are

identical. From the rare-earth element patterns, it can

be concluded that pyroxenite, melilitolite, ijolite and

nepheline syenite rocks are derived from different

melts but these melts have a common source.

Carbonatite samples show the most distinct chemical

composition with quite different REE patterns. The

carbonatites are, therefore, likely derived from an

exclusively different magma. Another important

point, reflected in the multi-element and rare-earth

element patterns of the nepheline syenite samples, is

that a group of samples show different pattern than

other, particularly one group shows remarkable differ-

ence in La concentration. The samples collected from

the Valley portion of the complex have low La

compositions. Lower La concentration may be

observed in pyroxenes (Henderson, 1984). Samples

with low La and Ce are probably due to some

metasomatic process during which other elements

have also changed their concentration, such as

depletion Th and Y. Lower LREE concentrations,

particularly La and Ce, are also observed in several

upper mantle rocks (Frey, 1984). If any melt

interacted with such upper mantle, then such melts

may show the observed REE patterns.

The isotopic compositions, both stable and radio-

genic, also indicate that the silicate and carbonatite

magmas are derived from different source. The

isotopic compositions of carbonatite samples clearly

indicate their mantle origin with little or no enriched

mantle or crustal influence. The d18O composition

of the carbonatites (7.35–7.91x; Table 4) is

identical to PM values but the d13C compositions

(�3.1x to 3.3x) is slightly higher than typical PM

(�5x to �7x), although these values are similar to

most of the magmatic igneous carbonatites. The

modelling done by the Zheng (1990) suggests that

strongly depleted values of d13C would be necessary

to propose Rayleigh fractionation of a fluid with

XCO2of ~0.4. The dominant species in the fluid

would have to be H2CO3. Carbonatite samples less

depleted in d13C would necessitate lower XCO2-

values in the degassing fluids. He further stated that

carbonates with d13C near zero would have to be

deposited from fluids very poor in CO2. Deines

(1989) has established that a Rayleigh fractionation

model successfully explains the d13C enrichment in

carbonatites. But the fractionation processes should

also be observed in d18O values, which are not

observed in the studied carbonatite samples. Another

process that may affect the d13C composition is the

assimilation of pre-existing limestone, but the gen-

eral absence of limestone in the Assam–Meghalaya

plateau precludes this possibility. These observations

suggest that the observed d13C and d18O values are

primary and do not show any isotopic fractionation.

If correct, then the carbonatites probably crystallized

under plutonic conditions and their isotopic signa-

ture reflects a mantle (source) composition.

The Sr and Nd isotopic compositions of the Sung

Valley carbonatites also suggest a relatively primitive

mantle origin. On the other hand, the Sr and Nd

isotopic compositions of the silicate samples plot

between HIMU and EM fields. The substantial

isotopic and geochemical differences between carbo-

natite and the silicate rocks in the Sung Valley

complex suggest that both are derived from two

different sources; carbonatites are derived from the

mantle (broadly similar to Bulk Silicate Earth; i.e.

equivalent to Homogeneous Primitive Mantle),

whereas associated silicate rocks are derived from

an enriched source. The enrichment of mantle may be

possible either by the subduction of crustal material

into the mantle or by mantle metasomatism.

Bell et al. (1998) have nicely outlined the details of

this interaction. They pointed out that metasomatism

is restricted only to the lithosphere, which holds melt

incursions generated from the convecting astheno-

sphere. Recently, Bell (2001, 2002) explained the

association of carbonatitic, alkalic and kimberlitic

magmatism to the plume activity. A link between

mantle plume activity and the genesis of such alkaline

rocks are supported by the presence of a HIMU-like

(mantle with high U/Pb ratio) isotopic signature. Hart

(1988) was the first to recognize the involvement of a

HIMU mantle composition in the genesis of OIBs.

The HIMU signature is also noted in some mantle

xenoliths from East Africa (Rudnick et al., 1993). The

spatial and temporal association of carbonatites and

the associated silicate rocks with many CFBs also

supports a plume-related origin. It has been noted that

many mafic–alkaline–carbonatite complexes were

emplaced at the latest stages of CFB magmatism

(Bell, 2001; Heaman et al., 2002).

Fig. 8. 87Sr/86Srinitial and qNd plot for the studied samples and the

Kerguelen Plateau basalts. Kerguelen data are taken from Michard

et al. (1986), Dosso et al. (1988), Storey et al. (1988, 1992) and

Mahoney et al. (1983, 1995).


A recent noble gas study (using Xe and Ne

isotopes) in carbonatite also corroborates a carbona-

tite–plume connection (Sasada et al., 1997). These

ratios support an origin for carbonatites from a

primitive source. These studies also suggest that

carbonatites of different ages have similar 129Xe/130Xe

and 40Ar/36Ar ratios, but quite different Sr and Nd

isotopic signatures, reflecting both enriched and

depleted sources (Sasada et al., 1997). Recently, Bell

(2002) advocated a plume model for the genesis of

alkaline–carbonatites rocks on the basis of several

criteria such as; (i) similarity in isotopic compositions

between OIBs and many carbonatites, (ii) the prim-

itive nature of the noble gas data from some

carbonatites, and (iii) the temporal and spatial

relationships of carbonatites and associated silicate

rocks with CFBs.

Many carbonatites worldwide contain mantle Nd

and Sr isotopic compositions that are similar to those

obtained for the Sung Valley carbonatites in this study.

Bell (1998) has reviewed the isotopic compositions of

the associated silicate rocks and suggested that

probably more than one mantle source is responsible

for their genesis. Tilton and Bell (1994) have noticed

that b200 Ma carbonatite complexes contain isotopic

ratios that lie between HIMU and EM-I. On the basis

of Nd and Sr isotopic signatures, similar results are

also obtained for the Kola Peninsula (Kramm and

Kogarko, 1994), East Africa (Bell and Blenkinsop,

1989; Bell and Simonetti, 1996; Kalt et al., 1997) and

the Deccan Alkaline Province (Simonetti et al., 1995,

1998) alkaline–carbonatite complexes. These studies

also emphasize the isotopically heterogeneous nature

of subcontinental mantle and that some of the isotopic

diversity can be explained by plume–lithosphere

interaction. A two-stage model is proposed to explain

the isotopic variations in many carbonatite complexes,

particularly East African (Bell and Simonetti, 1996).

This model suggests that metasomatizing agents with

HIMU-like signatures may metasomatize the subcon-

tinental lithosphere to variable degrees creating a

heterogeneous metasomatized mantle. Simonetti et al.

(1998) proposed that the large variations in isotopic

compositions in the carbonatite complexes might

result from the interaction of a mantle plume with

continental lithosphere. Recently, Bell and Tilton

(2001) suggested that HIMU and EM-I mantle

components are also present beneath the African

continent and most East African Rift carbonatites

originate predominantly from mixtures between these

two mantle end-members.

The isotopic data of the present study clearly

suggest a mantle signature for the carbonatites

whereas most of the silicate rocks require the involve-

ment of multiple reservoirs, such as mixing of HIMU

and enriched mantle (e.g. EM-II) components; close

to FOZO model. For comparison, the Sr and Nd

isotopic data obtained in this study for the Sung

Valley samples are plotted together with Kerguelen

Island OIBs and Rajmahal tholeiites (Michard et al.,

1986; Dosso et al., 1988; Storey et al., 1988, 1992;

Mahoney et al., 1983, 1995; see Fig. 8). A good

match of the isotopic compositions between these

rocks is observed suggesting a similar source for the

Kerguelen OIBs and the Sung Valley carbonatites

(also see Fig. 7b). A Kerguelen plume origin has been

proposed for these basalts (Kent et al., 1997, 2002;

Mahoney et al., 1992; Kent, 1995; Coffin et al., 2002;

Duncan, 2002) and it is possible that the same plume

is responsible for the Rajmahal–Sylhet continental

flood basalts and alkaline magmatism in the Shillong

Plateau of India (cf. Veena et al., 1998; Ray et al.,

1999). The isotopic evidence presented above com-

bined with the spatial and temporal distribution of this

magmatism (see Table 6) suggests a genetic link

between the ca. 107–115 Ma alkaline–carbonatite

complexes in the NE India with the Kerguelen Plume.


Based on the similarity in emplacement ages for

basalts from the Kerguelen Plateau, Broken Ridge,

Naturaliste Plateau, Bunbury and Rajmahal–Sylhet

Igneous Province and plate reconstructions Kent et

al. (2002) have concluded that eruption of Rajmahal

basalts and Elan’s Bank separation from eastern

India can be explained by interaction between the

Kerguelen hotspot and a spreading ridge located

close to the eastern India margin at ~120 Ma.

Spatial and temporal relationships between Rajmahal

basaltic eruptions and alkaline magmatism of the

Shillong Plateau (Table 6) suggest common link to

the Kerguelen Plume activity.

6.1. Genesis

There are three popular models for the genesis of

carbonatite magmas and their association with alka-

line silicate rocks (Le Bas, 1981; Gittins, 1989;

Bailey, 1993). Carbonatite magma is the product of

(i) fractional crystallization of primary silicate mag-

mas, normally carbonatite nephelinite (M-1), (ii) an

immiscible liquid that separates from a fractionated

silicate magma of nephelinitic/phonolitic composition

(M-2) and (iii) derived directly from low-degree

melting of a carbonated mantle peridotite (M-3).

There is a basic difference between M-1/M-2 and

M-3. Genetic relationship between carbonatites and

associated silicate rocks can be established in M-1

and M-2, whereas M-3 does not show any relation-

ship between these rocks.

Srivastava and Sinha (2004) have discussed these

models in the context of the Sung Valley carbonatites

and proposed that M-3 model is most consistent with

the origin of these rocks. The geochemical and

isotopic data presented in this study also supports

this hypothesis. On the basis of experimental work,

Hamilton et al. (1989) have provided partition

coefficient data for trace elements, particularly Ba

and La, between carbonatite and silicate melts during

the immiscibility process. They have shown that the

Ba/La ratio should be higher in an immiscible

carbonate liquid relative to associated silicate liquid

under almost any temperature and pressure, irrespec-

tive of whether the parental silicate was nephelinite or

phonolite. From the Mg# versus Ba/La plot (Fig. 3h),

it is observed that the majority of the silicate rocks

have higher Ba/La ratios than the carbonate samples.

Therefore, a liquid immiscibility origin for the

carbonatite magma is untenable.

On the other hand, experimental studies on

peridotite–CO2 clearly demonstrate that primary

carbonatite magmas can be generated at depths greater

than ~70 km (~25 kbar) (Wyllie and Huang, 1976;

Wallace and Green, 1988; Eggler, 1978, 1989; Wyllie,

1989; Ryabachikove et al., 1989; Thibault et al., 1992;

Dalton and Wood, 1993; Sweeney, 1994; Dalton and

Presnall, 1998; Lee and Wyllie, 1998; Wyllie and Lee,

1998). These experiments suggest that on rising,

primary carbonate magma, retaining equilibrium with

mantle lherzolite, will react, crystallize and release

CO2 vapour at depths of ~70 km, increasing

clinopyroxene/orthopyroxene in the rock. Given

sufficient magma, lherzolite can be converted into

wehrlite by this decarbonation reaction (Wyllie and

Lee, 1998). Wyllie and Lee (1998) further state that at

shallower depths, wehrlite can coexist with carbonate

magma relatively enriched in Ca/Mg. This melt might

dissolve an adequate amount of olivine and pyroxene

to provide Al, Fe and Si necessary for crystallization

of silicate minerals. These experiments (Wallace and

Green, 1988; Sweeney, 1994) explain that this melt

contains appreciable amount of alkalies (5–7%). Due

to low viscosity, this melt moves upward and interacts

with the peridotite to form metasomatic clinopyroxene

and olivine, which progressively metasomatizes the

lherzolite to alkaline wehrlite and release CO2 fluids.

Continued metasomatic process may form ultrabasic

alkaline silicate magma of melilitic to nephelinitic

composition.

Meen et al. (1989) have presented a model on the

basis of experiments of mantle metasomatism by

carbonated alkaline melts. Experimental studies indi-

cate that carbonated alkaline melts may metasomatize

depleted upper mantle to produce vein networks

enriched in low-temperature melting components.

These scholars have stated that when rising low-

temperature carbonated alkaline melts originating at

depths N70 km cross the peridotite–H2O–CO2 solidus,

either at 1000–1050 8C and 22 kbar or at 1050–1100

8C and 17 kbar that under these conditions melt and

wall-rock will react. At high temperature, formation of

clinopyroxene at the expense of olivine and orthopyr-

oxene will occur, whereas at lower temperature the

formation of carbonates occurs. Another important

observation from this experiment is that metasomat-

Fig. 9. Melting relationship in carbonated peridotitic mantle based

on experiments of Wallace and Green (1988) and Lee and Wyllie

(1997), compiled by Harmer (1999). Arrows symbolize carbonatitic

near-solidus melts ascending through the mantle. At ~20 kbar, the

melts react concerting mantle lherzolite to werhlite. Black solid dots

represent positions of experimental P–T conditions under which

carbonate melt exists in equilibrium with peridotite (data from

Thibault et al., 1992; Dalton and Wood, 1993; Sweeney, 1994)

Solid squires represent invariant points marked by the intersection

of the relevant solidus curve with the carbonation–decarbonation

reaction Mg Carb+Opx=Cpx+Ol+CO2.


ized veins formed at temperatures above the solidus

have relatively low Rb/Sr and Sm/Nd ratios; hence,

they develop low 87Sr/86Sr and 143Nd/144Nd isotopic

ratios and fall to the left of the mantle fields. On the

other hand, metasomatic regions produced by

complete loss of melt possess somewhat higher

Rb/Sr and significantly lower Sm/Nd ratios and they

lie in the enriched quadrant of the 87Sr/86Sr and143Nd/144Nd diagram. It is possible that the latter

conditions are responsible for the isotopic enrich-

ments in the silicate rocks of the Sung Valley

intrusion (see Fig. 8).

Similar field relationships, petrological, geochem-

ical and isotopic compositions are also observed for

the Spitskop alkaline–carbonatite complex of South

Africa (Harmer, 1999) and Dorowa and Shawa

complexes of SE Zimbabwe (Harmer et al., 1998).

These authors also advocated an origin for these

complexes from a primitive carbonate liquid pro-

duced directly by low-degree melting of carbonated

mantle peridotite. A model presented by Harmer

(1999), on the basis of field, petrological and

geochemical data for the Spitskop complex and

experimental results of Wallace and Green (1988),

Meen et al. (1989), Sweeney (1994), Lee and Wyllie

(1997) and Wyllie and Lee (1998), excellently

demonstrate the melting relationships in carbonated

peridotitic mantle (see Fig. 9). This model also

looks fit to explain the origin of carbonatite in the

Sung Valley complex. Experimental studies by

Wallace and Green (1988) and Sweeney (1994)

demonstrated that carbonate melts may be generated

by direct melting of carbonated peridotite at depths

equivalent to ~20–35 kbar. These melts are in

equilibrium with phlogopite lherzolite and rich in

magnesium and contain significant amount of

alkalies (5–7%). Sweeney (1994) clearly stated that

the K/Na ratio of the mantle component would

control this ratio in the resulting carbonatite.

Depending on the fertility of the peridotite and the

Na/K content, the carbonatite magma will be

equilibrium with phlogopiteFparagasite�richterite

amphibole (Wallace and Green, 1988; Sweeney,

1994).

Although important points related to this model

are already discussed in detail by Harmer (1999), it

is important to note that (i) due to low viscosity and

interconnectivity of these melts, they will be able to

.

percolate upward before the degree of melting

reaches 0.5–1%; (ii) metasomatic clinopyroxene

and olivine will be formed by consuming orthopyr-

oxene (Lherzolite) by the melt. This reaction will

occur under equilibrium conditions and at ~20 kbar.

During this process, free fluid (CO2) will also form.

This continuous process progressively metasomatizes

the lherzolite to an alkaline wehrlite, (iii) trace

element constituents carried by the carbonatite melt

are distributed between the metasomatic clinopyrox-

ene, amphiboleFphlogopite. This zone is recognized

as bmetasomeQ by Haggerty (1989); and (iv) with

continued metasomatic enrichment, the alkali wehr-

lite melts to form an ultrabasic alkaline silicate

magma of melilitic to nephelinitic composition.

The contrasting isotopic characteristics of the

silicate and carbonate components are the result of

carbonatitic mantle melts interacting with isotopically

anomalous lithosphere generated by trace element

enrichments trapped at the peridotite–CO2 dledgeTduring an earlier episode of alkaline magmatism. The

first silicate melts to successfully pass through the

solidus barrier and reach the surface would contain


more of the enriched component than the subsequent

melts. Thus, it may be concluded that alkaline

silicate magmatism occurs first and emplaced before

the carbonatites. This is consistent with the age

relationships obtained for the Sung Valley complex

where the ijolite emplacement date of 115.1 Ma

obtained in this study is slightly older than the most

precise 40Ar/39Ar date of 107.2 Ma obtained for the

carbonatite by Ray et al. (2000). Carbonatites are

derived from the melts originating at deeper depths

than the dmetasomeT from which silicate components

are derived. Due to this evolution, carbonatite

components have less enriched ESr–ENd signatures

than the silicate components.

7. Conclusion

Based on the geochemical and isotopic data

presented in this study for samples from the Sung

Valley alkaline complex in NE India, we propose a

model that invokes the direct melting of a carbo-

nated mantle for the origin of the carbonatite

magma. This melt might dissolve an adequate

amount of olivine and pyroxene to provide Al, Fe

and Si necessary for crystallization of silicate

minerals. Previous experimental studies indicate that

this carbonatite melt can contain appreciable

amounts of alkalies. Due to the low viscosity of

this magma, this melt moves upward and interacts

with the peridotite to form metasomatic clinopyrox-

ene and olivine, which progressively metasomatized

the lherzolite to alkaline wehrlite with the concom-

itant release of CO2 fluids. Continued metasomatism

may form an ultrabasic alkaline silicate magma of

melilitic to nephelinitic composition responsible for

the crystallization of silicate rocks. The contrasting

isotopic characteristics of the silicate and carbonate

components are the result of carbonatitic mantle

melts interacting with isotopically anomalous litho-

sphere generated by trace element enrichments

trapped at the peridotite–CO2 dledgeT during an

earlier episode of alkaline magmatism. This model

also successfully satisfies the field-relationships,

petrological, geochemical and isotopic characteristics

observed for the Sung Valley complex. Thus, it may

be concluded that alkaline silicate magmatism often

is generated first (as supported by the age dating of

the Sung Valley complex) and is emplaced before

the carbonatites. Carbonatites are derived from a

melt originating at deeper depths than the

dmetasomeT from which silicate components are

derived. It is this difference in the nature of the

mantle source regions that is responsible for

carbonatite magmas having a less enriched ESr–

ENd signature than the silicate components.

Acknowledgements

The authors are thankful to CSIR, New Delhi for

providing financial assistance for this work (Scheme

No. 24(0251)/01/EMR-II). LMH acknowledges the

financial support of NSERC for the U–Pb portion of

this study. In addition, the authors express their

sincere thanks to Prof. Yu Jie (RCMRE, Beijing,

China) for her kind help in analysing the stable and

radiogenic isotopic compositions of the Sung Valley

samples in this study. We are extremely grateful to

Keith Bell for his constructive and valuable comments

and suggestions on the earlier version of the MS,

which helped to improve this MS significantly.

Comments by an anonymous reviewer were also

helpful.

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