CH8 Rajmahal

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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: Nd(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 Nd(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 Indias 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