CH8 Rajmahal

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Geology and Origin of Rajmahal Traps

Text of 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