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348 CHAPTER 8 Kerguelen plume volcanism in Eastern India and geochemistry of lost Indian Lithospheric roots Abstract The Archean East Indian craton was affected by the Kerguelen plume at ~117 Ma causing flood basalt eruptions at the cratonic margin giving rise to the Rajmahal- Bengal-Sylhet Traps. Until recently, there was considerable disagreement among workers concerning the Kerguelen plume being the source for the Rajmahal traps lavas in eastern India. It is now recognized that Rajmahal-age volcanic rocks are widely spread in and around the Bengal Basin, from the intrusive lamproites and lamprophyres in the west and Sikkim in the north, to the Sylhet basalts of the Shillong plateau and the Mikir hills of Assam in the east. These volcanic rocks occur as groups of alkalic-ultrabasic rocks and carbonatites along with basalts, exposed over an area of ~ 1.5 million km 2 , including the Rajmahal hills of Bihar, and beneath the Tertiary sediments of the Bengal basin in West Bengal and Bangladesh. The central hypothesis of this study is that all these diverse volcanic rocks, including the flood basalts, are caused by the Kerguelen plume activity that also caused the erosion of the Indian lithospheric roots. We provide an isotope tracer study of the Rajmahal Traps and associated alkalic complexes, and relate them to the Sylhet Traps, Kerguelen Plateau basalts and associated volcanics in the Southern Indian

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CHAPTER 8

Kerguelen plume volcanism in Eastern India and geochemistry of lost Indian

Lithospheric roots

Abstract

The Archean East Indian craton was affected by the Kerguelen plume at ~117

Ma causing flood basalt eruptions at the cratonic margin giving rise to the Rajmahal-

Bengal-Sylhet Traps. Until recently, there was considerable disagreement among

workers concerning the Kerguelen plume being the source for the Rajmahal traps

lavas in eastern India. It is now recognized that Rajmahal-age volcanic rocks are

widely spread in and around the Bengal Basin, from the intrusive lamproites and

lamprophyres in the west and Sikkim in the north, to the Sylhet basalts of the Shillong

plateau and the Mikir hills of Assam in the east. These volcanic rocks occur as groups

of alkalic-ultrabasic rocks and carbonatites along with basalts, exposed over an area

of ~ 1.5 million km2, including the Rajmahal hills of Bihar, and beneath the Tertiary

sediments of the Bengal basin in West Bengal and Bangladesh.

The central hypothesis of this study is that all these diverse volcanic rocks,

including the flood basalts, are caused by the Kerguelen plume activity that also

caused the erosion of the Indian lithospheric roots. We provide an isotope tracer study

of the Rajmahal Traps and associated alkalic complexes, and relate them to the Sylhet

Traps, Kerguelen Plateau basalts and associated volcanics in the Southern Indian

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Ocean. We report Nd-Sr-Pb isotopic and multiple trace element data of 21 discrete

lava flows from four sections of the Rajmahal Traps, 56 mafic, alkalic, ultrabasic, and

carbonatitic rocks from four alkalic complexes associated with the Rajmahal-Bengal-

Sylhet Traps, and four dikes from the Bokaro coal fields southwest of the Rajmahal

Traps.

In Nd-Sr-Pb isotopes, the Rajmahal Traps lavas of this study show remarkable

similarity with the Rajmahal Groups I and II basalts, Sylhet Traps, Bunbury basalts

and lavas from the Kerguelen Plateau. The combined geochemical data and their

correlation with the Rajmahal, Bunbury basalts, and some of the Kerguelen Plateau

lavas in the Indian Ocean, imply a relatively primitive Kerguelen plume source for

some of the Rajmahal lavas similar to the Rajmahal Group I basalts. We propose the

average composition of this plume source to be: εNd(I) = +2, 87Sr/86Sr(I) = 0.7045, with

relatively flat REE patterns. Rajmahal lavas similar to the Group II Rajmahal basalts

have slightly enriched LREE patterns with εNd(I) = -5, 87Sr/86Sr(I) = 0.7069. We

suggest these lavas to be slightly contaminated by the Indian lithospheric granulites of

the Eastern Ghats Belt. We suggest the incorporation of the lithospheric contaminant

in the Kerguelen plume by thermal-chemical erosion resulted in reducing the

thickness of the Indian subcontinental lithosphere. The combined Nd-Sr-Pb isotopic

evidence also reflects absence of MORB and upper continental crustal components in

these lavas.

Rocks from the four alkalic complexes, Sung, Samchampi, Barpung, and

Sikkim, have been divided into two groups: the mafic rock group consisting of

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pyroxenites, nephelinites, lamproites, soviets, melteigite, uncompahgrites and

carbonatites, and the second group consisting of syenites and ijolites. All the mafic

rocks of this study have extremely enriched LREEs with Nd-Sr ratios consistent with

the Rajmahal lavas of this study as well as previous studies and thus are concluded to

be derived from the Kerguelen plume. The syenites and ijolites have a much wider

range of Nd-Sr compositions relative to the mafic rocks, and are interpreted to be

contaminated by the mid-continental crust after emplacement by magma chamber

processes.

Collectively these data imply a zone of influence of the plate-motion-

reconstructed Kerguelen plume for ~500 km in an east-west and north-south

direction, linking this plume head to its vestiges of the Rajmahal-Bengal-Sylhet Traps

in northeastern India and the Ninetyeast ridge in the Bay of Bengal. The present day

location of the Kerguelen Plume is beneath the Kerguelen Plateau in the southern

Indian Ocean.

8.1. Introduction

Large volume basaltic volcanism that erupted in the Early Cretaceous on the

eastern Indian continental margin (Rajmahal-Bengal-Sylhet Traps), southwestern

Australia (Bunbury-Naturaliste Plateau), and Antarctica are considered to have

caused the opening of the Indian Ocean (Fig. 8.1). This large and widespread

volcanism is attributed to the melting of a major plume head, the remnant of which is

now present as a hot spot beneath the Kerguelen Plateau in the Indian Ocean

(Mahoney et al., 1983; Storey et al., 1989; Weis et al., 1989; Kent et al., 1997; Frey

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et al., 2000). The episode of volcanism is also believed to have formed a flood basalt

province in eastern India comprising the Rajmahal, Sylhet and Bengal Traps of 117 +

2 Ma age (Baksi, 1995; Kent et al., 2002).

Volcanic rocks recovered by recent drilling from the Kerguelen plateau

demonstrate isotopic and geochemical similarity with the continental Rajmahal flood

basalts of eastern India as well as Bunbury basalts of southwestern Australia,

suggesting possible role of the Kerguelen plume in the fragmentation of part of the

Gondwana supercontinent (Frey et al., 2000). Kumar et al. (2007) claimed to have

determined the thickness of the Indian lithosphere with unprecedented accuracy to be

100 km, almost half to one-third as thick as those of South Africa, Australia and

Antarctica. These authors concluded that the “…plume that partitioned

Gondwanaland may have also melted the lower half of the Indian lithosphere…”,

leaving the Indian fragment of Gondwanaland with the thinnest lithosphere. From

Rayleigh wave phase velocity measurements (Mitra et al., 2006), the thickness of the

lithosphere under the Indian shield was estimated to be ~155 km, in agreement with

Ritzwoller and Levshin (1998) and with the multimode Rayleigh wave tomographic

model of Priestley and Mckenzie, (2006). These results clearly suggest a somewhat

thinner Indian cratonic root than that found for many other cratons in different parts

of the world. The most important implication of these seismic studies is the strong

correlation between India’s lost lithospheric roots and its very rapid northward

movement from about 130 Ma until its collision with Tibet around 50Ma.

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Figure 8.1. Map of the Indian Ocean and surrounding continents with physiographic

features, after Frey et al. (2002), showing locations of the Sylhet and Rajmahal Traps

in northeastern India. Also shown in gray is the extended Eastern Ghats – Shillong

orogenic belt (Yin et al., 2010) in the east coast of India. Basalt provinces attributed

to the Kerguelen Plume (Frey et al., 2002) include Kerguelen Plateau, Broken Ridge,

Ninety East Ridge, Bunbury basalts and Rajmahal Traps. Abbreviations used: BB –

Bunbury Basalt drill core sites; NKP – North Kerguelen Plateau; CKP – Central

Kerguelen Plateau; SKP – South Kerguelen Plateau; CG – Chilka Granulites

(Chakrabarti et al., 2010). Black crosses are ODP sites. Sites 253, 254, 756, 757, 214,

216, and 758 are from the Ninety East Ridge and are grouped as NER in subsequent

Nd-Sr-Pb isotopic plots.

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This correlation suggests degeneration of the Indian lithosphere and its subsequent

passage over the Kerguelen and the Reunion hotspot that resulted in flood basalt

eruptions of the Rajmahal and Deccan traps, respectively, during the breakup of

Gondwana.

The 117 Ma Rajmahal-Bengal-Sylhet Traps occur in eastern India (Fig. 8.2)

and occupy an area of 2 X 105 km2 (Baksi, 1995; Ray and Pande, 1999; Kent et al.,

2002). There has been disagreement, however, among several workers concerning the

Kerguelen plume head being the feeder in supplying the Rajmahal lavas. Curray and

Munasinghe (1991) suggested that the Rajmahal volcanism in north-eastern India

(Fig. 8.2b) was related to the Crozet hotspot via the Eighty-five East Ridge rather

than the Kerguelen plume; the relationship between the Rajmahal Traps, Ninetyeast

Ridge and the Kerguelen plume was also questioned (Mahoney et al., 1983).

However, based on revised model calculations for plate motions, Muller et al., (1993)

considered these above contentions to be unrealistic. Contrary to plume links,

Anderson et al., (1992) proposed that these Cretaceous lavas were the surface

manifestation of decompressional melting above a ‘hot cell’. The early geochemical

studies of Sr, Nd and Pb isotopes in the Rajmahal traps and their comparison with

Kerguelen plateau basalts did not allow a suggested link between the Kerguelen

plume basalts and the volcanism in eastern India (Mahoney et al., 1983; Baksi et al.,

1987; Storey et al., 1992). However, the Kerguelen plume was considered in some of

these studies to have provided the heat source for mantle melting to produce the

basaltic traps from a compositionally ‘normal’ asthenosphere”. These authors divided

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the Rajmahal basalts into Group I lavas having variable amounts of MORB

contaminant and being the “least contaminated source”, and Group II lavas with

predominant crustal contamination. Kumar et al., (2003) analyzed a kimberlite

intrusion and identified a pristine Kerguelen plume source for these basalts, similar to

Group II kimberlites. In contrast, recent studies of Cretaceous basalts recovered by

Ocean Drilling Program from the Kerguelen plateau (Fig. 8.1) show strong

geochemical similarities to the Rajmahal Traps and southwestern Australian Bunbury

basalts; both the Nd, Sr and Pb isotopic compositions and plate tectonic

reconstructions constrain the origin of this volcanism, currently separated by

thousands of kilometers in the Indian Ocean, from the Kerguelen hotspot (Ingle et al.,

2002b).

It has also been recently recognized that Rajmahal-age volcanic rocks are

widely spread in and around the Bengal Basin in eastern India. These volcanic rocks

are manifested as diverse groups of alkalic, ultrabasic, and carbonatitic rocks along

with basalt, widely exposed over an area of ~ 1.5 million km2 (Fig. 8.2a). From the

Rajmahal hills of Bihar in the west, these volcanic suites are contiguous to the east

and covered beneath the Tertiary sediments in the Bengal basin in West Bengal, and

alkalic rocks are distributed widely in and around the Bengal basin that include

lamproite dikes in the Gondwana sediments to the west, carbonatite-alkalic

complexes in the Shillong plateau (Meghalaya), Mikir hills (Assam) in the northeast,

and lamproite dikes of the Sikkim Himalayas to the north (Fig. 8.2a). These alkaline

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intrusives are similar to those found in the Deccan and Siberian continental flood

basalt provinces (Basu et al., 1993; Basu et al., 1995; Basu et al., 1998).

An association between alkalic magmatism and mantle plumes has frequently

been proposed (Basu et al., 1993; Basu et al., 1995; Basu et al., 1998; Franz et al.,

1999; Basu et al., 2001; Bell, 2002). Alkaline magma may be generated by low-

degree melting of an enriched mantle source or by pronounced differentiation of a

mafic magma, or even by crustal contamination of a mantle derived magma; volatile

contents and mantle mineralogy also affect magma compositions (Mahoney et al.,

1985; Wilson, 1989; Winter, 2001). It is also observed that the last phases of

continental flood basalts are spatially and temporally associated with alkalic and

carbonatitic magmatism (e.g. Bell, 2001; 2002; Heaman et al., 2002).

In this study we report the trace element and Nd-Sr-Pb isotopic data of 21

basalts and andesites from four different locations of the Rajmahal Traps, 56 alkalic,

ultrabasic, and carbonatitic rocks from four alkalic complexes associated with the

Rajmahal-Bengal-Sylhet Traps, and four dikes from the Bokaro coal fields southwest

of the Rajmahal Traps (Figs. 8.2, 8.3). Nd-Sr-Pb data have not been reported from

Harinsingha or Taljhari area (Fig. 8.2, 8.3) of the Rajmahal Traps or from the Sikkim,

Samchampi and Barpung alkalic complexes presented in this study. Although several

workers have presented geochemical data for the Sung Valley complex (Ray and

Pande, 2001; Ray et al., 2005; Srivastava et al., 2005), our data show a much wider

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Figure 8.2. (a) Geological map showing structural features and locations of the Rajmahal

and Sylhet Traps in and around the Bengal Basin including bore hole sites where basalts

have been encountered (Sengupta, 1966; Baksi et al., 1987; Ray et al., 2005). Alkalic and

ultra-basic rocks also occur in Samchampi, Sung, Sikkim, and the Bokaro dykes related

to the Rajmahal-Sylhet Traps. Associated volcanic rocks are also reported from the

Bangladesh part of the basin (personal communications with Bangladesh geologists).

(b) Map showing the distribution of volcanic and sedimentary rocks in the Rajmahal Hills

and surrounding areas (Kent et al., 1997), and locations of sample sites.

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range of geochemical data than those previously reported. We have compared the 81 rock

samples mentioned here with similar available data in the published literature on the

ocean drilling and dredge samples of the Kerguelen plateau crust, Bunbury and Ninety

East Ridge basalts and with the published and available geochemical data of the Archean

Indian lithosphere to evaluate the possible connection between these alkalic complexes

and the Rajmahal basalts and ultimately with the Kerguelen plume and the lost eastern

Indian lithospheric root. We use this data to establish a more extensive and more

complete aerial extent of this widespread volcanism in eastern India, caused by the

Kerguelen plume. It appears that this aerial extent may exceed 1.5 million km2 (Fig. 8.1).

The above mentioned geochemical results and their interpretation are important in

assessing how ancient cratonic roots survive and persist through geological time in the

presence of mantle plumes and which parts of cratons are susceptible for melting during

plume impingement.

8.2. Geological History

8.2.1 Rajmahal Traps

The Rajmahal basalts are exposed in the Rajmahal hills (Fig. 8.2b, 8.3a-d) in

eastern Bihar and parts of West Bengal, India. They cover an area of ~4100km2, with

exposed thickness of up to 230m. Toward the west, the basalts unconformably overly

Gondwana Supergroup sediments. To the east, down faulting of the basement causes the

sequence to be lost beneath the massive amounts of sediment of the Bengal basin

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Figure 8.3a. Geological map of Sahibganj with sample locations. Six basalt samples

from the six flows shown here have been analyzed in this study.

Figure 8.3b. Geological map of Taljhari with sample locations. Four basalts and one

andesite samples from the three flows have been analyzed for this study.

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Figure 8.3c. Geological map of Tinpahar with sample locations. Four basalts and two

andesite dikes have been sampled from the three flows for this study. The dikes intersect

all the lava flows.

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Figure 8.3d. Geological map of Harinsingha with sample locations. Three basalts have

been sampled from the two flows for this study. One Dubrajpur sandstone has also been

analyzed in this study.

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basin shed from the Himalayas. However, exploration drilling in the area indicates that

these basalts are also present to the east underlying much of the Bengal Basin where the

thickness of the lava is as much as 332m, continuing farther south of Calcutta (Sengupta,

1966).

40Ar/39Ar data in basalts of the Rajmahal traps recovered by drilling from the

Bengal Basin suggest a formation age of ~117 Ma (Baksi, 1995; Ray et al., 2005). More

recent 40Ar/39Ar results (Kent et al., 2002) from the Rajmahal hills are consistent with an

~118 Ma age for the magmatic activity, contemporaneous with the final stage of

volcanism in ODP program site 1136 in the southern Kerguelen Plateau (119-118 Ma)

(Duncan, 1978).

In this study we present data from four different regions of the Rajmahal hills, sparsely

sampled and analyzed in previous studies. There are six individual lava flows in the Sahibganj

area shown in Fig. 8.3a. The location of the basaltic samples as collected from three different

areas in Ambadihi, Rangamatia and Adro Bedo traverses. The Taljhari location (Figs. 8.3b) has

three recognizable basaltic-andesitic and andesitic-dacitic flows. Five samples were collected

representing these three flows. The Tinpahar location (Figs. 8.3c) made of three hillocks, show

four basaltic lava flows each of which was sampled for this study. In addition, an andesitic dike is

seen to cut all four flows here. Two samples from this dike are also included in our study. The

Harinsingha location (Fig. 8.3d) is the southernmost location of the Rajmahal hills sampled for

this study. Two tholeiitic flows in this location are separated by an inter-trappean bed and the

bottom flow is in contact with the Gondwana Dubrajpur sandstone. Four samples were collected,

two of which are in contact with the sediments.

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8.2.2. Bokaro Dikes

The Gondwana coalfields of Bihar-West Bengal are known to host a variety of

occurrences of intrusive ultrabasic bodies (Sarkar et al., 1980; Rock et al., 1992; Kent et

al., 1998). They have been examined for their economic potential as diamond-bearing

rocks, much like the kimberlites of South Africa. Sarkar et al. (1980) determined the K-

Ar age of 113-105 Ma for biotites from ultramafic alkaline rocks in this region. We have

determined a 40Ar/39Ar laser step heating spectra of a phlogopite from a lamproite dike in

the eastern Bokaro coal field (Fig. 8.2), courtesy of P. R. Renne, Berkley Geochronology

Center, that gave an integrated age of 114.4 + 0.1 Ma. Thus these dike emplacements

were slightly younger than the 118 Ma-old tholeiitic flood volcanism in Rajmahal Hills

(Baksi, 1995; Kent et al., 2002).

The mineralogical composition of the alkalic dikes intruding the Gondwana

sediments are commonly characterized by olivine, phlogopite, apatite, aegirine,

amphibole (usually K-richterite), carbonate, spinel, and perovskite. This observation

allows these dikes to be classified as minettes, lamproites, or olivine-lamproites. Many of

the sills show varying mineralogy due to differentiation, and may contain both minette

and lamproite in gradational contact with the sill rock (Rock et al., 1992).

In addition to the lamprophyric intrusions, the Gondwana sediments also host

many diabase dikes and sills. Four samples of dikes from the Bokaro field (Fig. 8.2) have

been collected for this study. Two of the samples are diabase dikes, one is a lamprophyre,

and one is an unusual carbonatite sample with predominant carbonate and some altered

silicates. This carbonatite sample was found in contact with a diabase dike.

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8.2.3 Sikkim

In the foothills of the Himalayas, near Darjeeling, India (Fig. 8.3e) is the Rangit

structural window. This tectonic feature exposes sub-Himalayan thrust sheet lithologies.

These thrust sheets have been piling up for the last 43 Ma since India collided with Asia

(Gansser, 1964). The thrusts are northerly dipping and create nappes that place older

rocks on younger material, and result in the obduction of the Indian craton and overlying

sediments.

The Rangit tectonic window is located within the Main Central Thrust sheets of

the Himalayas, which are made up of high-grade crystalline rocks such as the Lingtse

gneiss, which constitute most of the Higher Himalayas. Exposed within the window are

rocks carried by the Main Boundary Thrust sheets, which comprise most of the Lesser

Himalayan region. These include the Gondwana, Daling and Siwalik metasediments. The

Gondwana sediments consist mainly of feldspathic sandstones and carbonaceous shales

with frequent seams of semi-anthracitic coal. At the southern end of the Rangit window is

a linear east-west exposure of the Gondwanas, with lamprophyre and lamproite dikes

cutting across the coal and adjacent sediments. The Daling sediments have a consistent

lithology of greenish fissile slates to green quartzitic schists (Gansser, 1964). These

sediments are exposed in the central portion of the window, hosting a series of syenitic

dikes. Recent kinematic modeling has shown that the Rangit window has experienced

shortening of ~125 km since the collision of the Indian with the Asian plate 65 million

years ago (Mitra et al., 2010).

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Figure 8.3e. Geological map of Sikkim with sample locations (Weaver, 2000). Six ultra-

potassic syenites, six lamproites, and one nephelinite have been analyzed for this study.

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We have selected 12 samples from the Sikkim-Darjeeling area. These samples are

ultra-potassic syenites, peridotites, minettes, nephelinites and lamproites that comprise

the most unusual alkalic rock associations of any flood basalt province, perhaps akin to

those of the Parana, Deccan and Siberian flood volcanic provinces. The 6 ultra-potassic

syenites are rocks occurring as intrusives into the Daling group of metasediments which

occur as a nappe overlying the Gondwana rocks in a zone of thrust wedges. The

remaining rocks occur as intrusives into the Gondwana sediments occurring below the

Daling Nappe. Samples are collected from three different locations along an east-west

belt parallel to the trend of the mountain belt. From east to west the locations are

Tindharia, Lish Nasi, and Manzing Khola.

8.2.4 The Sung Valley Complex

The Sung Valley Complex is an intrusive alkaline igneous complex located within

the Meghalaya craton and exposed on the Shillong Plateau in northeast India (Fig 8.3f). It

is ~26 km2 in size and intruded into the Proterozoic quartzites and phyllites of the

Shillong group. The Shillong plateau is a block of uplifted Precambrian basement,

bordered by the Dauki fault to the South and the Brahmaputra graben to the north. A N-S

trending lineament (Um-Ngot lineament) cuts across the Shillong plateau and contains

several alkalic intrusive bodies including the Sung Complex (Kumar et al., 1996). The

Sung complex has been dated at 134 + 20 Ma using Pb isotopes (Veena et al., 1998).

However, 40Ar/39Ar analysis of carbonatites from this complex yield an age of 108 + 2

Ma (Ray and Pande, 2001).

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Figure 8.3f. Geological map of Sung Valley modified from (Veena et al., 1998).

Seventeen samples from this location have been analyzed for this study including,

syenites, carbonatites, pyroxenites, and uncompahgrites.

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The petrology of the Sung Valley complex has been described by several

researchers (e.g. Krishnamurthy, 1985; Srivastava and Sinha, 2004; Srivastava et al.,

2005). An oval pyroxenite body forms the main rock type of the complex, intruded by

peridotites, ijolites, carbonatites and syenites (Veena et al., 1998). Two varieties of

pyroxenites are present in the complex: (1) a coarse grained variety composed of

diopsidic augite with minor amounts of phlogopite, titanite and apatite and (2) a medium

grained variety composed of diopside and aegirine augite with minor K-feldspar, titanite

and apatite. Stock-sized bodies of olivine and peridotite are emplaced in the northern part

of the complex. Ijolites are the third most abundant rock-type in the complex and they

intrude the pyroxenites in the form of a ring dike. Carbonatites are found in the southern

part of the complex and occur mainly as dikes and cone sheets. Minor syenitic and

felspathic veins and dikes cut pyroxenites, ijolites and quartzites. We have collected 19

samples of various lithologies from this complex.

8.2.5 The Samchampi Alkalic Province

Located in the Mikir Hills, the northeast extension of the Shillong Plateau, the

Samchampi complex (Kumar et al., 1989) (Fig. 8.3g) shows many similarities to the

Sung Valley complex. The age of the Samchampi complex overlaps the age of the Sung

Valley complex (108 + 2 Ma) (Ray and Pande, 2001) indicating that these alkaline

complexes are coeval and together they post date the Rajmahal and Sylhet Traps by ~10

My.

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Figure 8.3g. Geological map of the Samchampi Complex after Hoda et al., (1997).

Fifteen samples from this location analyzed for this study include syenites, pyroxenites,

melteigites, sovites, and ijolites.

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The Samchampi complex is a relatively circular stock-like body, and is cored

by a titano-magnetitie-perovskite rock. Syenites of variable mineralogic compositions

occur as discontinuous lenses and ovoid bodies of primarily alkali syenites and

nepheline syenites. Pyroxenites occur only as small patches, and are composed

mainly of aegirine-augite with interstitial potassium feldspars (Hoda et al., 1997). An

ijolite-melteigite suite is confined to the northern part of the complex and has an

outcrop pattern suggestive of a ring dike or a cone-shaped body, much like the Sung

Valley complex. The ijolite-melteigites are composed primarily of aegirine-augite,

nepheline, biotite and carbonate minerals. Apatite and sphene occur as accessory

phases and may control the trace element concentrations of these rocks. Carbonatites

occur as discontinuous dike-like bodies, and as small veins and lenses, consisting of

carbonate minerals with accessory ilmenite and magnetite. Biotite, phlogopite, apatite

and olivine are also present in different parts in each of the carbonatite dikes. The

carbonatites represent the last stage of emplacement of the Samchampi alkaline

complex (Kumar et al., 1989).

For our study, 15 samples have been collected from the Samchampi Alkalic

complex. These samples include alkali pyroxenites, syenites, ijolites, and sovites.

There is a remarkable parallelism in the lithology of the complex with the ijolites and

jacupirangites associated with the Parana flood basalt province (e.g. Piccirillo et al.,

1989).

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8.2.6. Barpung Alkalic Complex

The Barpung alkalic complex is very similar to Samchampi and is located to

the southeast of Samchampi (Fig. 8.2). These rocks are intruded into amphibolite

facies metamorphosed host rock (Kumar et al., 1989). This complex is composed

primarily of pyroxenites and aegirine-bearing syenites. Ten samples of pyroxenite

and syenite were collected for this study. The Barpung alkalic complex contains the

only monomineralic potassium-feldspar syenites of all the rocks of this study

(Weaver, 2000). Its age has not yet been determined.

8.3. Analytical Results

In this section we present the geochemical results of the basaltic and andesitic

lavas from the Rajmahal Traps, dikes from Bokaro, and the alkaline rocks from Sung,

Samchampi, Barpung and Sikkim alkaline complexes. These data comprise multiple

trace element concentrations including the rare earths, and the isotopic compositions

of Nd, Sr, and Pb. The data are presented in tables 8.1-8.2 and figures 8.4-8.8. Basalts

and andesites from the four locations of the Rajmahal Traps are compared with

similar data obtained from previous work on volcanic rocks related to the Kerguelen

plume activity, including the Rajmahal-Bengal-Sylhet Traps, Broken Ridge,

Ninetyeast Ridge, Bunbury basalts and basalts from the northern, central and southern

parts of the Kerguelen Plateau. The alkalic rocks from the Sung, Samchampi, Sikkim,

and Barpung Complexes are divided into two groups: one group consisting of the

mafic rock types (pyroxenites, nephelinites, lamproites, soviets, melteigite,

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uncompahgrites and carbonatites), and the other group consisting of syenites and

ijolites. These two rock groups from the alkaline complexes are compared to the

Kerguelen plume related volcanics, granulites from th eEastern Ghats Belt in India as

well as Lamproites from India, Gaussberg (Antarctica), and Australia. Analytical

methods are described in Appendix-2.

8.4.1 Trace Element Geochemistry

Trace element data for the all the basaltic and alkalic rocks of this study are

presented in Table 8.1. The Chondrite normalized (Evensen et al., 1978) rare earth

element (REE) patterns for the basaltic and andesitic laavs from the four Rajmahal

Traps sections are shown in figure 8.4a. The five basaltic flows from Sahibganj are

slightly enriched light rare earth (LREE) with average LaN/SmN = 1.8 and LaN/YbN =

3.5 (Fig. 8.4a). In the Taljhari section (Fig. 8.4a) the four basalt flows have flat to

slightly enriched rare earth element (REE) patterns (LaN/SmN = 1.1 to 2.0; LaN/YbN =

1.9 to 3.9) while the andesite flow has relatively higher LREE (LaN/SmN = 3.8) and a

much steeper overall REE slope (LaN/YbN = 28). Two of the Tinpahar basaltic flows

have flat REE patterns (LaN/SmN = 1.0; LaN/YbN = 1.7) (Fig. 8.4a) while the

remaining two basaltic flows and two andesite dikes from this section have slightly

enriched REEs (LaN/SmN = 1.8 to 2.0; LaN/YbN = 3.6 to 4.1) similar to the Sahibganj

and Taljhari sections. The three basaltic flows of the Harinsingha section, including

one flow in contact with sandstone, are nearly flat in their REEs (LaN/SmN = 1.1;

LaN/YbN = 1.7) (Fig. 8.4a) and are similar to the other flat basaltic flows from

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Tinpahar and Taljhari. One sandstone sample from Harinsingha in contact with the

basaltic flows has extremely depleted REE except La, Ce, Pr and Nd. The overall

chondrite normalized REE concentration of the lavas from the Rajmahal Traps have

either nearly flat or slightly enriched LREE (La ~20-50 times chondrite), and flat

heavy rare earth elements (HREE) (~10 times chondrite), with one andesite flow from

Taljhari showing a striking different REE pattern.

Four dikes from Bokaro have been analyzed for this study (Table 8.2, Fig.

8.4b). The two diabase and one lamprophyre dikes are strikingly similar to the nearly

flat basalts from the Rajmahal Traps (LaN/SmN = 1.2; LaN/YbN = 2.7) with LREEs

~20 times more enriched than chondrite. The carbonatite dike is similar to the mafic

rocks from the alkaline complexes (LaN/SmN = 6.9; LaN/YbN = 100; DyN/YbN = 2.5)

with extreme LREE enrichment, steep HREE slopes, and no Eu-anomaly.

Our analytical results show that the pyroxenites, lamproites, carbonatites,

meltiegites, and soviets from the four alkaline complexes have high concentrations of

Ba (average ~900 ppm), Sr (average ~1900 ppm), Zr (average ~400 ppm) and La

(average ~125 ppm), conforming to the chemical characteristics of global lamproites

(Mitchell and Bergman, 1991; Woolley et al., 1996). The concentrations of these

elements are lower in the corresponding syenites and ijolites from these complexes,

except for Sikkim where the ultra potassic syenites are very similar in their trace

element concentrations and patters to the lamproites and nephelinites (Fig. 8.4c).

Trace element data for the rocks from the alkalic complexes are reported in Table 8.1.

All the rocks from the four alkaline complexes have high concentrations of REEs

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Figure 8.4a. Chondrite normalized REE patterns of basalts from the Rajmahal Traps.

Basalts with a relatively flat REE pattern in the CH section are similar to Group I and

those with LREE enrichments are comparable to Group II Rajmahal basalts (Kent et al.,

1997).

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Figure 8.4b. Chondrite normalized REE patterns of the four Bokaro dikes. The

lamprophyre and diabase dikes are similar to the Rajmahal Traps data (Fig. 8.4a),

whereas the carbonatite dike is similar to the alkalic complex data (Fig. 8.4c).

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Figure 8.4c. Chondrite normalized REE patterns of the alkalic-carbonatitic rocks (left

column) and the syenites and ijolites (right column) from the four alkalic complexes of

this study. Notice extreme LREE enrichments in all these rocks.

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with extreme LREE enrichment (LaN/SmN = 3.3 to 5.5), ~200-1000 time chondrite, and

relatively less steep HREE (DyN/YbN = 1.7 to 3.0). The overall LaN/YbN range of the

mafic rocks for the alkaline complexes are more restricted (30 to 55) compared to the

syenites and ijolites (13 to 103). In general, all the mafic rocks and some of the syenites

of the four alkaline complexes are similar to most global lamproites and group II

kimberlites (Cullers et al., 1985; Foley et al., 1987; Mitchell and Bergman, 1991;

Chakrabarti et al., 2007). All the mafic rocks are characterized by the notable absence of

Eu-anomaly.

Twenty-two compatible and incompatible trace element concentration patterns for

the Rajmahal Traps lavas of this study are shown, normalized to primitive mantle (Sun

and McDonough, 1989) in figure 8.5a, with progressively less incompatible elements to

the right. Primitive mantle-normalized basalts of this study show low Rb, U, and high Ba,

Sr, Pb concentrations. In general, all basalts that show nearly flat REE patterns from

Harinsingha, Taljhari and Tinpahar (Fig. 8.4a), are also similar to and ~10-20 times more

enriched than primitive mantle except in the elements mentioned above. The remaining

basalts and andesite dikes from Tinpahar and Sahibganj that show slightly enriched

LREEs in figure 8.4a, have negative Nb-Ta anomalies and a relatively steeper HREE

slope when normalized over primitive mantle; these lavas are ~20-50 times more

enriched than the primitive mantle. One andesite flow from Taljhari and one sandstone

from Harinsingha are different from the rest of the Rajmahal lavas and have more

enriched LREEs, negative Nb-Ta anomaly, positive Zr-Hf anomaly, and a much steeper

HREE slope in figure 8.5a.

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Figure 8.5a. Multiple trace element concentrations normalized to Primitive Mantle for

the Rajmahal Traps lavas of this study. The lava with flat REE patterns (Fig. 8.4a)

display small variations in their elemental concentration patterns. In contrast, the lavas

with LREE enrichment (Fig. 8.4a) display enrichment in incompatible elements relative

to primitive mantle, characteristically in Rb, Ba, Sr, and Pb, while being depleted in

general in Nb-Ta.

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Figure 8.5b. Multiple trace element concentrations normalized to Primitive Mantle

for the four Bokaro dikes. While the diabase dikes are similar to Rajmahal data with

flat REE and the lamprophyre dike is similar to LREE enriched Rajmahal data, the

carbonatite dike is strikingly different.

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Figure 8.5c. Multiple trace element concentrations normalized to Primitive Mantle

for all the alkalic rocks of this study.

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Tab

le 8

.1. T

race

ele

men

t con

cent

ratio

ns a

nd se

lect

ed e

lem

ent r

atio

s of a

ll th

e ro

cks o

f thi

s stu

dy

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384

Tab

le 8

.1 c

ontin

ued.

Page 38: CH8 Rajmahal

385

Tab

le 8

.1 c

ontin

ued.

Page 39: CH8 Rajmahal

386

Tab

le 8

.1 c

ontin

ued.

Page 40: CH8 Rajmahal

387

Tab

le 8

.1 c

ontin

ued.

Page 41: CH8 Rajmahal

388

Tab

le 8

.1 c

ontin

ued.

Page 42: CH8 Rajmahal

389

The two diabase dikes from Bokaro are extremely flat with only slight Ba

enrichment (Fig. 8.5b) and are ~10 times more enriched than the primitive mantle.

The lamprophyre is enriched in Ba, La and Ce, depleted in Ta, Sr, and Sm, with no

Nb-Ta anomaly and a steep HREE slope. The carbonatites is in general flat and ~10

time primitive mantle with depletions in Rb, Ba, Pb, and Sr, as well as negative Nb-

Ta anomaly.

The rocks from the four alkaline complexes, i.e. Sung, Samchampi, Barpung,

and Sikkim, have wide ranges in the primitive mantle normalized concentrations of in

the most incompatible elements (Rb, Ba, Th, U) (Fig 8.5c). In general, these rocks

have Pb-depletion, both positive and negative Sr peaks, and negative Zr-Hf

anomalies, with steep HREE slopes. Most of the mafic rocks have no Nb-Ta anomaly

where as the syenites and ijolites have both positive and negative Nb-Ta anomalies.

8.3.2 Nd-Sr-Pb isotopic geochemistry

Rb-Sr and Sm-Nd isotope systematics data for all the rocks of this study are

reported in Table 8.2. In the Nd-Sr plot (Fig. 8.6a) the basalts and andesites of this

study are compared with the primitive Kerguelen plume component (shaded gray

region) (Ghatak and Basu, 2010) which includes Sylhet Traps basalts, Bunbury

Casuarina lavas (Frey et al., 1996), and drill core sites from the Kerguelen plateau

(Weis and Frey, 1991; Frey et al., 1996; Frey et al., 2000; Frey et al., 2002; Neal et

al., 2002a; Weis et al., 2002) as well as the Rajmahal group I and II basalts (Kent et

al., 1997), Indian MORB (Mahoney et al., 1992; Mahoney et al., 2002), Niinetyeast

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Ridge (NER)(Weis and Frey, 1991), and ancient continental fragments from drill hole

1137 (Ingle et al., 2002a). The basalts and andesites from the Rajmahal Traps have

have been calculated at 117 Ma which is the 40Ar-39Ar age for these Sylhet Trap

tholeiites (Ray et al., 2005). The initial εNd values for these rocks range from +2.8 to

-7.7 and the 87Sr/86Sr(I) data range from 0.70347 to 0.70804.

The alkalic complex rocks of this study have been compared to primitive

Kerguelen plume component (shaded gray region)(Ghatak and Basu, 2010), Rajmahal

group I and II basalts (Kent et al., 1997), Indian MORB (Mahoney et al., 1992;

Mahoney et al., 2002), Niinetyeast Ridge (NER) (Weis and Frey, 1991), ancient

continental fragments from drill hole 1137 (Ingle et al., 2002a), basalts from bore

hole 738, Groups I and II kimberlites (Chakrabarti et al., 2010), lamproites from India

(Chakrabarti et al., 2007), Australia (Mitchell and Bergman, 1991), and Gaussberg

(Murphy et al., 2002), as well as the Eastern Ghats Belt of India (Rickers et al., 2001;

Chakrabarti et al., 2010) in figure 8.6b.

The pyroxenites, lamproites, sovites, carbonatites and melteigite comprising

the mafic rocks from the alkalic complexes have initial εNd values of +1.2 to -6.9 and

87Sr/86Sr(I) of 0.70471 to 0.70879. Syenites and ijolites from these alkalic complexes

have a much larger range in Nd-Sr isotopes with εNd(I) = -1.5 to -17.8 and 87Sr/86Sr(I)

= 0.70138 to 0.72734. It is interesting to note that the mafic rocks from the alkalic

complexes have a similar range of data to the Rajmahal Traps data of this study as

well as previous studies (Mahoney et al., 1983; Baksi et al., 1987; Storey et al., 1992;

Kent et al., 1997; Kumar et al., 2003; Ghatak and Basu, 2010) while the majority of

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the syenites and ijolites have negative initial εNd and radiogenic Sr-isotopic

composition and lie in the time-integrated Rb/Sr and Sm/Nd enriched quadrant of the

Nd-Sr isotopic correlation diagram (Fig. 8.6b). Also notice that the four carbonatite

samples of this study from Sung overlap with Rajmahal Group I basalts (Kent et al.,

1997) as well as the least contaminated Kerguelen plume component in the upper left

quadrant in figure 8.6b.

Pb-Pb isotopic ratios of all the basaltic and andesitic lavas and the alkalic

rocks of this study are plotted in figure 8.7a-b along with the groups I and II

Rajmahal Traps (Kent et al., 1997), primitive Kerguelen plume component (Ghatak

and Basu, 2010), NER (Weis and Frey, 1991), Bunbury basalts (Frey et al., 1996),

various Kerguelen Plateau drill core basalts (Weis et al., 1993; Weis et al., 1998; Frey

et al., 2000; Frey et al., 2002; Neal et al., 2002a; Weis et al., 2002), Indian MORB

(Mahoney et al., 1992; Mahoney et al., 2002), Chilka granulites (CG) of the Eastern

Ghats Belt (Chakrabarti et al., 2010), ancient non-volcanic continental fragments

from drill core 1137 (Ingle et al., 2002a), and lamproites from Gaussberg (Murphy et

al., 2002). Various continental crustal and mantle reservoirs and the Northern

Hemisphere Reference Line (NHRL) are plotted in the Pb-Pb plots for reference

(Zartman and Doe, 1981; Hart and Zindler, 1989).

Initial 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb of the Rajmahal Trap basalts

and andesites at 117 Ma have ranges of 17.36-18.33, 15.48-15.71, and 37.12-40.40

respectively as reported in Table 8.2. Most of the Rajmahal lavas are clustered around

the field of the primitive Kerguelen plume component (gray shaded area) and have a

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Tab

le 8

.2. P

rese

nt d

ay (0

) and

initi

al (I

) at 1

17 M

a fo

r Nd,

Sr a

nd P

b is

otop

e da

ta fo

r all

the

rock

s of t

his s

tudy

.

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Tab

le 8

.2 c

ontin

ued.

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394

Tab

le 8

.2 c

ontin

ued.

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395

Tab

le 8

.2 c

ontin

ued.

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Tab

le 8

.2 c

ontin

ued.

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397

Tab

le 8

.2 c

ontin

ued.

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Tab

le 8

.2 c

ontin

ued.

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399

Tab

le 8

.2 c

ontin

ued.

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nearly vertical trend along the field of the bulk silicate Earth (BSE) (Fig. 8.7a). The

pyroxenites, lamproites, sovites, melteigite, carbonatites, syenites and ijolites from

the alkalic complexes on the other hand have a much larger range of Pb-isotopic

compositions (17.60-19.54, 15.46-15.76, 35.78-41.37) and show the mixing trend

between the least contaminated Kerguelen plume component, ancient continental

fragments from 1137, and the average upper continental crust (Fig. 8.7b). There is no

distinction in the Pb-isotopic data between the mafic rocks and the syenites and

ijolites from the alkalic complexes. Correlation between 207Pb/206Pb(I) and

208Pb/206Pb(I) at 117 Ma for all the Rajmahal Traps lavas as well as the alkalic rocks of

this study is shown in figure 8.8. All the data presented in this study are compared in

this plot with the Rajmahal Traps (Groups I and II) (Kent et al., 1997), Ninetyeast

Ridge (NER) (Weis and Frey, 1991), Bunbury basalts (BB) (Frey et al., 1996), some

Kerguelen Plateau drill core samples (e.g. Frey et al., 2002; Ingle et al., 2002b; Neal

et al., 2002a), and mantle reservoirs DMM, EMI, EMII, and HIMU (Saal et al.,

1998). In addition to these fields the alkalic rocks complex are also compared to a

common component ‘C’ as derived from the Pacific, Atlantic, and Indian MORB data

by Hannan and Graham (Hanan and Graham, 1996), Groups I and II Kimberlites

(Collerson et al., 2010), Gaussberg lamproites (Murphy and Collerson, 2002), and

Oldoinyo Lengai (Bell and Simonetti, 1996; Bell and Tilton, 2001; Bizimis et al.,

2003).

All the Rajmahal Traps basalts and andeites of this study and the lamprophyre

and diabase dikes from Bokaro broadly overlap with the least contaminated

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Kerguelen plume component, and Rajmahal Groups I and II, Bunbury basalts, and

Kerguelen Plateau drill core basalts (Fig. 8.8a). In general a linear trend is visible for

these lavas, beginning near the fields of RJI, BB and drill core 749 basalts and ending

near the EM-1 field for all these lavas. On the other hand, the mafic rocks and the

syenites from the alkalic complexes show a much larger range in composition and fall

along a linear trend from the field of ‘C’ to EM-I.

8.4. Discussion

In this section we discuss the trace element and Nd-Sr-Pb isotopic results

presented in section 8.3. We document from our geochemical results of the Rajmahal

volcanics that they may have 80-100% of the primitive Kerguelen plume component

with up to 20% contamination from the lower crustal and continental mantle

lithospheric source that can be identified to be granulites of the Eastern Ghats in India

(Fig. 8.1). We also discuss the geochemical data of the alkalic rocks from four

different complexes that post date the Rajmahal-Bengal-Sylhet Traps and suggest a

model for the evolution of these varies rock types in association with the Rajmahal

flood basalt province.

8.4.1 Discussion of the geochemistry of Rajmahal Trap Lavas and Bokaro dikes

The lavas flows from Harinsingha, two basalts from Tinpahar, one from

Taljhari, and the two diabase dikes from Bokaro show remarkably flat REE patterns

(Fig. 8.4a, b). These lavas are strikingly similar to some of the Sylhet Traps 500km to

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the east of the Rajmahal Traps (Ghatak and Basu, 2010), and to the Rajmahal Group I

basalts, defined on the basis of only three analyses by Kent et al. (1997). The

relatively flat REE patterns of the group I Rajmahal Traps rocks has been attributed to

have formed by decompressional melting of the asthenosphere and passive upwelling

of these basalts through the rifted margin of eastern India (e.g. Baksi et al., 1987;

Kent et al., 1997). Another probable scenario is that the relatively primitive nature of

these lavas with flat REE patterns is better explained by the large degree melting of a

less contaminated and relatively primitive component of the Kerguelen plume.

Primitive mantle-normalized trace element data for basalts and dikes with flat

REE patterns are similar to the least contaminated Rajmahal group I basalts (Kent et

al., 1997) as well as the Sylhet Traps lavas (Ghatak and Basu, 2010), with relatively

low Rb, U and Pb and higher Ba, and Sr with respect to the primitive mantle (Fig.

8.5a, b). Overall, these rocks are distinctly different from both NMORB and upper

continental crust in trace element patterns and ratios in this multi-element plot (Fig.

8.5a, b). This pattern could not have resulted from crustal contamination of a magma

derived from the depleted mantle as has been proposed by several authors for the

Rajmahal Traps (e.g. Baksi et al., 1987; Kent et al., 1997).

The REE patterns of the remaining basalts and three andesites of this study

from the Rajmahal Traps and the laprophyre dike from Bokaro are more LREE

enriched (Fig. 8.4a, b). The LREE-enrichment seen in these basalts are similar to the

Group II Rajmahal basalts as defined by Kent et al., (1997) on the basis of five basalt

analyses. This LREE enrichment may be due to contamination from upper or lower

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Figure 8.6a. Initial εNd vs. 87Sr/86Sr at 117 Ma for the Rajmahal Traps of this study

compared with the Rajmahal Traps, Bunbury basalts, Kerguelen Plateau basalts, and

Eastern Ghat Belt (EGB) Granulites (Rickers et al., 2001). The field of Indian MORB

also includes the southeast Indian Ridge (Mahoney et al., 2002). Partial mixing lines

resulting from the modeling of the Nd-Sr data with a modeled plume component and

two EGB granulites as end members are from (Ghatak and Basu, 2010).

Abbreviations used: NER – Ninety East Ridge; BB-C – Bunbury, Casuarina; RJ I –

Rajmahal Traps Group I; RJ II – Rajmahal Traps Group II; Data sources: Indian

MORB (Mahoney et al., 1992); NER (Weis et al., 1991); Bunbury (Frey et al., 1996);

Rajmahal basalts (Kent et al., 1997);. South Kerguelen Plateau sites 738, 749, 750,

747 (Frey et al., 2002); Elan Bank 1137, and ancient continental fragment from 1137

(Ingle et al., 2002a).

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Figure 8.6b. Initial εNd vs. 87Sr/86Sr at 117 Ma for the alkalic rocks of this study

compared with the Rajmahal Traps, primitive Kerguelen plume component, Indian

MORB, NER, Groups I and II kimberlites, Krishna lamproites, drill core 738 basalts,

Gaussberg lamproites and Eastern Ghat Belt (EGB) Granulites (Rickers et al., 2001).

Data sources: Group I Kimberlites (Kumar et al., 2003); Krishna lamproites

(Chakrabarti et al., 2007); Gaussberg and Group II kimberlites (Mitchell and

Bergman, 1991); Australian Lamproites (McCulloch et al., 1983). Other data sources

are as in figure 8.6a.

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continental crust or from the mantle lithosphere. In contrast to the basalts with flat

REE, the remaining basalts and andesites from the Rajmahal Traps show very

different primitive mantle normalized trace element patterns (Fig. 8.5a, b). These

patterns are similar to Rajmahal group II basalts (Kent et al., 1997), some Sylhet

Traps lavas (Ghatak and Basu, 2010), and also to the granitoid rocks recovered from

drill core site 1137 (Ingle et al., 2002a) on the Kerguelen Plateau (Fig. 8.1). In

addition, these rocks show negative Nb-Ta anomalies that are characteristic

signatures of continentally derived crustal rocks.

The absence of a negative Eu anomaly in all the Rajmahal Traps lavas in their

REE plots (Fig. 8.4a) indicates the absence of an upper continental crustal component

in these rocks. Thus we can eliminate the possibility of an ancient subducted

continental crustal component in the sources of these lavas. Therefore, the continental

component present in LREE enriched basalts with Nb-Ta anomalies (Fig. 8.4a) must

be sourced from the lower continental crust, possibly from the eastern Indian

continental margin granulites (Ghatak and Basu, 2010).

In the Nd-Sr isotopic correlation (Fig. 8.6a) all the basalts and andesites as

well as the Bokaro diabase and lamprophyre dikes show affinity with the Rajmahal

Groups I and IIb lavas, Kerguelen Plateau basalts, as well as the field of the primitive

Kerguelen plume component as defined by Ghatak and Basu (2010). Based on these

correlations, a Rajmahal–Sylhet–Ninetyeast Ridge–Kerguelen Plateau connection is

strongly indicated. It is interesting to note that all the basalts described as having

nearly flat REE patterns correspond with Rajmahal Group I basalts in the quadrant

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with unradiogenic Sr and positive εNd values in the Nd-Sr plot, where as the LREE

enriched basalts and andesites lie near fields of RJ IIb, drill core 738, and ancient

continental fragments from 1137 which are considered to be contaminated by the

lower continental crust (Fig. 8.6a) (Frey et al., 2002; Ingle et al., 2002a; Ghatak and

Basu, 2010).

The two mixing lines shown in this figure (Fig. 8.6a) are from Ghatak and

Basu (2010) and they represent mixing between the relatively primitive Kerguelen

plume component (Ghatak and Basu, 2010) and granulites from the Eastern Ghats

Belt commonly believed to constitute the lower continental crust (Rickers et al.,

2001; Chakrabarti et al., 2010) in the eastern Indian continental margin and widely

exposed along the Eastern Ghats Belt (Fig. 8.1) (Yin et al., 2010). This modeling

indicates less than 20% of the granulite contamination for the Rajmahal Traps lavas

of this study as well as previous studies (Mahoney et al., 1983; Storey et al., 1989;

Baksi, 1995; Kent et al., 1997; Ghatak and Basu, 2010).

The proximity of the Rajmahal Traps lavas and the diabase and lamprophyre

dikes of this study with the field of Rajmahal Groups I, IIa, and IIb lavas near the

4.45 Ga Geochron (at μ = 8.3-8.5) in the 207Pb/204Pb vs. 206Pb/204Pb plot (Fig. 8.7a) is

noteworthy. This correspondence of the Rajmahal basalts and andesites and Bokaro

dikes in Pb-isotopes with Rajmahal I, IIa, and IIb basalts is also consistent with their

trace element patterns as discussed previously in this section. A few lavas fall close to

the field of average lower crust in figure 8.7a, close to the least contaminated CH

section basalts while others show higher values of both 207Pb/204Pb(I) (Fig. 8.7a) and

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208Pb/204Pb(I) (Fig. 8.7a) suggesting a greater degree of crustal/lithospheric

contamination for these rocks because these rocks fall closer to the fields of Chilka

granulites (CG) as well as the most contaminated Kerguelen plateau basalts from drill

cores 738 (Fig. 8.2a).

Although a few samples fall close to the field of Indian MORB (Fig. 8.7a)

these samples also overlap with Kerguelen Plateau basalts which are commonly

accepted to be derived from the Kerguelen plume (Mahoney et al., 1992; Frey et al.,

1996; Frey et al., 2000; Ingle et al., 2002b; Neal et al., 2002a). There is no

correspondence of any of the Rajmahal Trap lavas of this study with the Ninetyeast

Ridge (NER) basalts in figure 8.7. We also note from figure 8.7 that the Rajmahal

basalts and andesites plot far removed from the average upper continental crust in

their Pb-isotopic compositions. An important conclusion that can be drawn from these

observations in the Pb-variation diagram (Fig. 8.7) is that the lithospheric

contaminants in the Rajmahal-Bengal-Sylhet basalts as well as the Kerguelen Plateau

basalts are clearly of lower crustal-lithospheric affinity and have no upper crustal

component. This inference of a lower crustal contaminant is also supported from the

trace element evidence as discussed before.

Pb-isotopic ratios plotted as 208Pb/206Pb(I) and 207Pb/206Pb(I) in figure 8.8 are

important in distinguishing the mantle components EM-I, EM-II, depleted mantle

(DMM), and HIMU (e.g. Saal et al., 1998). The Rajmahal lavas trend from Bunbury

and Rajmahal group I basalts towards contaminated basalts from drill core 738 and

EM-I, this mixing trend is similar to the Rajmahal Group II basalts. A likely

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408

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Figure 8.7a. (a) 208Pb/204Pb(I) vs. 206Pb/204Pb(I) and (b) 207Pb/204Pb(I) vs. 206Pb/204Pb(I)

plots for the Rajmahal Traps lavas compared with Kerguelen Plateau basalts,

Bunbury Basalts, Rajmahal Traps, primitive Kerguelen plume component and Chilka

Granulites at the 117 Ma age of eruption of the Sylhet Traps. µ values of 8.3, 8.4, and

8.5 are also shown in (b) where µ = 238U/204Pb. The field of ancient continental

fragments in drill core 1137 (Ingle et al., 2002a) are present day values. Data sources

and symbols are as in figure 8.6a.

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Figure 8.7b. (a) 208Pb/204Pb(I) vs. 206Pb/204Pb(I) and (b) 207Pb/204Pb(I) vs. 206Pb/204Pb(I)

plots for the alkalic rocks of this study compared with Kerguelen Plateau basalts,

Bunbury Basalts, Rajmahal Traps, primitive Kerguelen plume component and Chilka

Granulites at the 117 Ma age of eruption of the Sylhet Traps. µ values of 8.3, 8.4, and

8.5 are also shown in (b) where µ = 238U/204Pb. The field of ancient continental

fragments in drill core 1137 (Ingle et al., 2002a) are present day values. Data sources

and symbols are as in figure 8.6a and b.

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component for the lavas of this study as well as some of the Kerguelen Plateau

basalts, including basalts of ODP site 738 is an EM-I like end member (Fig. 8.8a). It

is interesting to note from figure 8.8a that both EM-II, considered as upper

continental crustal, and depleted mantle (DMM, NER) components are unlikely

sources in the Rajmahal samples of this study as well as in Rajmahal-Sylhet-

Bunbury-Kerguelen Plateau basalts.

The presence of lower crustal and lithospheric geochemical signatures in the

Rajmahal-Sylhet Traps has important implications for understanding the

subcontinental lithospheric mantle which can be isolated from the convecting

asthenosphere for billions of years and can thus evolve to isotopic compositions that

deviate significantly from crustal or asthenospheric ratios. This subcontinental

reservoir cannot yield large magmas but can locally contaminate continental flood

basalts (e.g. Widom et al., 1999; Neal et al., 2002a) as seen in the case of the

Rajmahal-Sylhet basalts of this study and previous studies (Mahoney et al., 1983;

Storey et al., 1989; Baksi, 1995; Kent et al., 1997; Ghatak and Basu, 2010). Hence

these locally contaminated basalts are an important source for understanding these

geochemically isolated subcontinental lithospheric roots.

The Rajmahal Traps and Bokaro dikes described in this study can be divided

into those with flat REE patterns and mantle-like Nd-Sr-Pb isotopic ratios, and those

with LREE enrichments that show a clear mixing between a plume end member and a

lower crustal-lithospheric end member that is similar to the Eastern Ghat granulites of

India. These traps reside on a late Archean eastern Indian craton which once had deep

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lithospheric roots as evidenced by the presence of diamond bearing Proterozoic

kimberlites from the Indian craton (Basu and Tatsumoto, 1979; Rao et al., 2004). The

thickness of the present day Indian lithosphere has recently been estimated to be 100

km (Kumar et al., 2007) which is half to one-third the thickness of the Gondwana

Supercontinent lithosphere, comprising South Africa, Australia and Antarctica. Thus

the source of contamination in the LREE lavas of this study as well as other similarly

contaminated rocks from the Kerguelen Plateau (e.g. drill hole 738 and 1137) may

have been derived by the erosion of this “lost” Indian lithosphere, referred to by

Kumar et al., (2007), by the impact of the Kerguelen plume head prior to the

Rajmahal-Bengal-Sylhet flood basalt eruption. This scenario has recently been

suggested for similar LREE enriched lavas from the Sylhet Traps ~500km east of the

Rajmahal Traps (Ghatak and Basu, 2010).

We suggest a relatively primitive Kerguelen plume source for the lavas with

nearly flat REEs, that was also responsible for the Sylhet, Bunbury-Casuarina,

Rajmahal Group I and the least contaminated Kerguelen Plateau basalts, in contrast

with the previous proposal by several workers (Weis et al., 1993; Weis et al., 1998;

Kumar et al., 2003) for a generally enriched end-member for the Kerguelen plume

without the signature of continental crust. We suggest this relatively primitive plume

source to have the same geochemical composition as the uncontaminated Sylhet

Traps (Ghatak and Basu, 2010) and drill core 1138 (Neal et al., 2002b).

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8.4.2 Discussion of the rocks from the alkalic complexes Sung, Samchampi, Barpung,

and Sikkim

The extreme enrichment of LREEs in the mafic rocks of this study from the

four alkalic complexes, with as high as 1000 times chondritic abundances (Fig. 8.4c)

precludes significant contamination by continental crust (Collerson and McCulloch,

1983; McCulloch et al., 1983; Fraser et al., 1985). The comparison of chondrite

normalized REE plots of these mafic rocks and average continental crust (Taylor and

McClennan, 1985) shows that although the continental crust is enriched in LREE, it is

not nearly to the extent of the mafic rocks of this study. If crustal contamination had

occurred it would have resulted in general reduction of the LREE concentrations from

their original composition. These mafic rocks are also depleted in HREE relative to

both continental crust and MORB, hence contamination would cause the slope of the

REE patterns to flatten. This flattening effect is seen in the silicate rocks (syenites and

ijolites) having lower Sr and Rb concentrations that are more easily contaminated

(Fig. 8.4c). The absence of an upper crustal component for the mafic rocks is further

evidenced by the absence of a negative Eu anomaly (Fig. 8.4c). However, a few of

the syenites do exhibit a small negative Eu anomaly and may have been crustally

contaminated. Hence, it is likely that the mafic rocks reflect source chemistry and

source mineralogy with no significant contamination from the continental crust or

MORB. The syenites, on the other hand, may reflect crustal or MORB contamination,

especially in the Sung and Barpung complexes.

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The extreme LREE enrichment of the alkalic-carbonatitic rocks of this study

is also seen in the primitive mantle normalized patterns in figure 8.5c. This

enrichment may be due to metasomatism of an anomalous mantle source prior to

melting as has been suggested for lamproites, kimberlites, and carbonatites world-

wide (e.g. Mitchell and Bergman, 1991; Ringwood et al., 1992). The absence of any

Nb-Ta anomaly in the alkalic-carbonatitic further confirms the absence of an upper

crustal contaminant for these carbonatites, pyroxenites etc. The syenites have both

positive and negative Nb-Ta anomalies indicating some crustal contamination in these

rocks. The extremely negative Zr-Hf anomalies in some of the mafic rocks may be

due to removal of a heavy mineral phase. A prominent negative Pb spike and positive

Sr spike for the mafic rocks (Fig. 8.5c) could not have originated during melting

because Pb and Sr have similar incompatibility during dry melting (Hofmann, 1988).

Since high concentrations of Sr in these rocks preclude the possibility of

contamination, low Pb concentrations may be a result of post emplacement processes

or due to metasomatism of these rocks by a fluid with low Pb and high Sr

concentrations. Both Pb and Sr are variable in the syenites and have both positive and

negative peaks (Fig. 8.5c). The ultra potassic syenites (with a major alkali feldspar

component in their mineralogy) have high Pb and low Sr peaks have clearly

undergone either metasomatism or contamination or both.

In the Nd-Sr isotopic variation diagram (Fig. 8.6b) the mafic rocks have an

overall negative correlation with most of them falling in the domain of negative εNd

and radiogenic Sr indicative of time-integrated high Rb/Sr and Nd/Sm ratios in their

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source. This correlation is also observed for global lamproites (McCulloch et al.,

1983; Murphy et al., 2002; Chakrabarti et al., 2007). Although the Sr-Nd ratios of the

pyroxenites, lamprophyres, melteigites, and uncompahgrite are similar to the

Rajmahal group II type lavas derived from the Kerguelen plume, they have

significantly different trace element patterns (Figs. 8.4c, 8.5c). A model for the

petrogenesis of these mafic rocks and the associated carbonaties, syenites and ijolites

is discussed in section 8.4.3 below.

Carbonatites from Sung fall in a tight cluster and overlap with group I

kimberlites, Rajmahal Group I basalts, and most primitive Kerguelen plume

component in the upper left quardrant of the Nd-Sr plot (Fig. 8.6b) similar to most of

the young African carbonatites (Bell and Blenkinsop, 1987; Nelson et al., 1988; Bell

and Tilton, 2001). However, all the carbonatite samples of the Sung Valley Complex

previously analyzed by Srivastava et al., (2005) as well as those reported by Veena et

al., (1998) plot in the upper right quadrant of the Nd-Sr diagram. These authors

suggested cause of Sr enrichment in the Sung carbonatites away from global

carbonatites and into the enriched quadrant to be due to an enriched mantle source

involvement (Veena et al., 1998; Srivastava et al., 2005), specifically, an EM-2

source (Veena et al., 1998). Interaction with old continental crust could account for

the high 87Sr/86Sr and low εNd 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 Sung carbonatites must therefore be representative of the original composition

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and therefore must have been derived from a relatively uncontaminated Kerguelen

plume.

In contrast to the carbonatites and other mafic rocks, the syenites of this study

have extreme Nd-Sr isotopic ratios typical of old continental crust (Fig. 8.6b).

However, the multi-element spider diagrams of the mafic rocks and the corresponding

syenites from the four alkalic provinces are nearly identical, especially for the Sikkim

alkalic complex (Fig. 8.5b). The substantial isotopic and geochemical differences

between carbonatities and the syenites suggest that both are derived from two

different sources. However, the similarity in the trace element patterns of these two

rock groups (Fig. 8.4c, 8.5c) indicates that these melts have a common source.

The alkalic complex rocks of this study have a wide range in their Pb-isotopic

compositions (Fig. 8.7b, 8.8b). The Sikkim lamproites have high initial 207Pb/204Pb

and low initial 206Pb/204Pb values (Fig. 8.7b) and have close affinity to the Gaussberg

lamproites (Fig. 8.8b) (Murphy and Collerson, 2002), indicating an Archean crustal

component in these rocks. Alumina depleted komatiites with low silica and depleted

HREE patterns (Nesbitt et al., 1979; Blichert-Toft and Arndt, 1999; Polat et al., 1999)

have been suggested to be present in the sources of other global lamproites such as

the Krishna lamproites in India (Chakrabarti et al., 2007). We propose that a

subducted Archean Al-depleted komatiite was present in the source of the Sikkim

lamproites in the peridotite mantle.

The carbonatites and mafic rocks of this study from the alkalic complexes,

except Sikkim lamproites, have a strong Pb-depletion when normalized to the

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Figure 8.8a. Initial 207Pb/206Pb and 208Pb/206Pb ratios of the Rajmahal Traps at 117

Ma compared with Rajmahal Group I and II basalts (RJI and RJII respectively),

Bunbury basalts (BB), South Kerguelen Plateau lavas (sites 738, 747, 749, 750),

primitive Kerguelen plume component, and Ninety-East Ridge basalts. Note mantle

reservoirs EM-I, EM-II, DMM, and HIMU (Saal et al., 1998) and the correspondence

of the lavas of this study with the Rajmahal and Kerguelen plateau lavas in the close

proximity of the EM-I field. Ninety-East Ridge lavas plot closer to the DMM field.

Data sources and symbols as in figure 8.6a.

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Figure 8.8b. Initial 207Pb/206Pb and 208Pb/206Pb ratios of the alkalic rocks of this study

compared with Rajmahal Group I and II basalts (RJI and RJII respectively), Bunbury

basalts (BB), South Kerguelen Plateau lavas (sites 738, 747, 749, 750), primitive

Kerguelen plume component, and Ninety-East Ridge basalts. Note mantle reservoirs

EM-I, EM-II, DMM, and HIMU (Saal et al., 1998) and the correspondence of the

lavas of this study with the Rajmahal and Kerguelen plateau lavas in the close

proximity of the EM-I field. Ninety-East Ridge lavas plot closer to the DMM field.

Data sources as in figure 8.6a. Symbols as in figure 8.6b.

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primitive mantle (Fig. 8.5c) possibly due to post emplacement processes or due to

metasomatism as discussed previously. These rocks as well as the syenites show

evidence of both upper and lower mantle contamination in their Pb-isotopic data (Fig.

8.7b, 8.8b). Notice that the Sung carbonatites fall in close correspondence to the ‘C’

component, interpreted to be a common mantle source region for ocean island basalts

sampled by mantle plumes (Hanan and Graham, 1996), in figure 8.8b.

8.4.3 Models for the genesis of the alkalic rocks

There are three general models for the genesis of carbonatite magmas and

their associated silicate rocks (Le Bas, 1981; Gittins, 1989; Bailey, 1993): (i)

fractional crystallization of primary silicate magmas, normally carbonatite

nephelinite; (ii) an immiscible liquid that separates from a fractionated silicate

magma of nephelinitic/phonolitic composition; (iii) derived directly from low-degree

melting of a carbonated mantle peridotite. A fourth model for the generation of

alkaline magmas is in association with other alkalic rocks at continental rift-like

extensional tectonic settings (Wilson, 1989; Verma, 2006).

The isotopic similarity of the ultramafic rocks of the alkalic provinces, their

contemporaneous ages with the Rajmahal-Bengal-Sylhet Traps and lack of any

geological or geophysical feature suggesting rift-tectonics point towards a plume

model for the genesis of these alkalic provinces, although an extensional setting is

conceivable prior to plume impingement beneath this region.

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On the basis of experimental work Hamilton et al., (1989) showed that Ba/La

ratios should be higher in an immiscible carbonate liquid relative to associated silicate

liquid under any temperature or pressure. In our study most of the syenites and ijolites

have significantly higher Ba/La than the alkalic-carbonatitic rocks (Table 8.1).

Therefore the liquid immiscibility origin for the carbonatite magma is untenable.

On the other hand, experimental studies on peridotite-CO2 demonstrate that

primary carbonatite magmas can be generated at depths greater than ~70 km

(~25kbar) (Wyllie and Huang, 1976; Eggler, 1978; 1989; Thibault et al., 1992;

Dalton and Wood, 1993; Sweeney, 1994; Lee and Wyllie, 1997; Wallace and Green,

1998; Wyllie and Lee, 1998; Harmer, 1999; Verma, 2006). These models suggest that

primary carbonate magmas will release CO2 vapor on rising to depths of ~70 km,

increasing pyroxenes in the rock, and assist in thinning of the lithosphere due to high

pore pressure of CO2 rich fluids. This decarbonation reaction can convert lherzolite

into wherlite, which can coexist with carbonated magmas and dissolve an adequate

amount of olivine and pyroxene to provide Al, Fe, and Si necessary for the

crystallization of ultrabasic alkaline silicate magmas of nephelinitic composition

(Sweeney, 1994; Lee and Wyllie, 1997; Wallace and Green, 1998; Wyllie and Lee,

1998). Upton (1967) suggested that pyroxenites in alkalic environments may result

from reaction between silicate rock and carbonatite magma. Dasgupta et al., (2004)

have experimentally shown that carbonated eclogites rather than carbonated

peridotites are the potential source of continental carbonatites.

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Based on the geochemical data discussed in this study as well as experimental

results discussed above we suggest a model that invokes the direct melting of a

carbonated eclogite. This has also been suggested by Srivas tava et al., (2005) for the

Sung Valley carbonatites. This melt might dissolve enough olivine and pyroxene

required for the crystallization of nepheline syenites, syenites and ijolites. Pyroxenites

and lamproites result from the reaction of the silicates with and carbonated melts.

Carbonatites are derived from greater depths than any other alkalic or tholeiitic rocks

in this region.

8.5. Conclusions

When combined with previous studies of lavas associated with the Kerguelen

plume, this study of Rajmahal flood basalts and associated rocks from alkalic

complexes in the Shillong Plateau and Sikkim leads to the following conclusions:

1. The data presented in this study for the Rajmahal Traps can be correlated to the

Sylhet Traps ~550km to the east as well as to basalts recovered from the Kerguelen

Plateau. These data fall into two groups, one group comprising the least contaminated

volcanics with flat REE patterns, which are similar to the Rajmahal Group I

geochemical data as well as the least contaminated Kerulen plume-derived basalts

(Neal et al., 2002b; Ghatak and Basu, 2010). The other group is similar to the

Rajmahal Group II basalts; we have shown that these Group II basalts are

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contaminated by the lower crustal Eastern Ghat granulites. Similar contaminated

lavas have also been reported from the Sylhet Traps (Ghatak and Basu, 2010).

2. We conclude that the Rajmahal Traps lavas of this study are results of partial

melting of a relatively primitive Kerguelen plume source, that were contaminated by

partially melted components derived from the lower continental granulitic crust as

represented by the Eastern Ghats Belt in India. We also suggest that this lower crustal

component in these basalts resulted from the incorporation by melting-erosion of the

eastern Indian continental lithosphere by the Kerguelen plume, reducing the thickness

of the Indian lithospheric plate. We infer that the plume head eroded parts of the

subcontinental mantle lithosphere and the lower continental crust prior to eruption of

the Rajmahal-Bengal-Sylhet Traps.

3. The two diabase dikes from the Bokaro coal field are similar to the uncontaminated

Group I type Rajmahal-Sylhet Trap lavas while the lamprophyre dike is similar to the

Group II type Rajmahal lavas and is likely contaminated by the lower continental

granulitic crust of the Eastern Ghats Belt in India.

4. There is no evidence of any contamination of the Rajmahal Traps lavas and the

Bokaro dikes by the upper continental crust or by any MORB component.

Contamination in this flood basalt province varies from relatively uncontaminated

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plume-derived basalts to assimilation of variable amounts of lithospheric components

by hot mafic magmas with little fractionation.

5. The Sung valley carbonatites are likely derived from a carbonated eclogite, from

greater depths in the mantle than any of the other rocks of this study. The Nd-Sr

isotopic composition (Fig. 8.6b) and 206Pb-207Pb-208Pb isotopic ratios (Fig. 8.8b) of

these carbonatites indicate a mantle origin for these rocks with no enriched mantle or

crustal influence. The 206Pb/204Pb ratios of these rocks (Fig. 8.7b) may have been by

post-emplacement or metasomatic processes.

6. The remaining mafic rocks and syenites are suggested here to be products of

metasomatization of the carbonated eclogite as discussed in section 8.4.3. These melts

have likely been contaminated by the upper and lower continental crustal components

during emplacement or during their crystallization-differentiation after emplacement.

Although the Sikkim lamproites and syenites collectively show close affinity

Rajmahal-Sylhet Traps basalts in their Nd-Sr-Pb-isotopic ratios, their extreme

enrichment in LREEs indicates these rocks to be of similar origin as the remaining

mafic and syenitic rocks of this study. Lamproites and nepheline bearing syenites are

ultrabasic rocks that cannot be generated by melting of the lower crust. These rocks

likely interacted with a subducted Archean Al-depleted komatiite.

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7. The isotopic evidence presented in this study combined with the spatial and

temporal distribution of a wide variety of mafic-alkaline-carbonatitic rocks of the

Rajmahal-Sylhet volcanic province in northeast India ranging in age from 117-

105Ma, and derived from the Kerguelen plume. This observation along with studies

of ~150 km shortening of the Rangit window (Mitra et al., 2010) that contains the

Sikkim alkalic rocks suggests that the Kerguelen plume-generated flood basalt that

erupted in eastern India at 117 Ma, gave rise to a large igneous province that includes

the Rajmahal-Bengal-Sylhet Traps and the alkalic rocks in an around an area of

~1000 km in diameter. We suggest this volcanic province in northeastern India

constituted the major remnants of the Kerguelen plume head.

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