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Geology and Origin of Rajmahal Traps
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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
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