Journal of Petrology | Volume 39 | Number 11-12 | Pages 1875-1884 | 1998
© Oxford University Press 1998
Pb, Sr and Nd Isotopic Systematics of the Carbonatites of Sung Valley, Meghalaya, Northeast India: Implications for Contemporary Plume-Related Mantle Source Characteristics
Atomic Minerals Division, Department of Atomic Energy Hyderabad —500 016, India
Received September 30, 1997; Revised typescript accepted June 8, 1998
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ABSTRACT |
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 TOP ABSTRACT IntroductionGeological and Tectonic SettingSample Description and...Results and DiscussionConclusionsReferences |
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A 206Pb/204Pb–207Pb/204Pb age of 134 ± 20 Ma with a model µ1 of 8.19 ± 0.02 has been obtained for the carbonatites from Sung Valley, Meghalaya, placing their time of emplacement very close to the time assigned for the break-up of Gondwana. Initial
Sr (+5.3 to +7.8), Nd (+1.7 to +2.3), 206Pb/204Pb (19.02), 207Pb/204Pb (15.67) and 208Pb/204Pb (39.0) indicate a mantle source region with a somewhat higher Rb/Sr ratio than Bulk Earth, minor light rare earth element (LREE) depletion, and a time-integrated enhancement of U/Pb. The isotopic characteristics of the Sung Valley carbonatites, however, differ significantly from those of basalts from Rajmahal and the Ninetyeast Ridge, which have a relatively more depleted upper-mantle source component, indicating involvement of different mantle sources. The mantle source region of the carbonatites is characterized by an EM2–HIMU signature, similar to that observed for Ambadongar, western India, but different from the carbonatites of East Africa, which have an EM1–HIMU signature. It is speculated that the Sung Valley carbonatites were derived from a parental magma generated by partial melting of the sub-continental lithosphere, which was previously metasomatized by fluids derived from an EM2-HIMU type mantle plume. The data suggest that the pre-130 Ma sub-continental mantle exhibited preferentially an EM2 type signature. KEY WORDS: carbonatites; isotopic systematics; mantle sources; Sung Valley India
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Introduction |
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TOPABSTRACTIntroductionGeological and Tectonic SettingSample Description and...Results and DiscussionConclusionsReferences |
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Carbonatites are carbonate-rich igneous rocks of mantle origin. They are found mainly in continental settings and their emplacement ages range from Late Archaean to Quaternary (Woolley, 1989
). Their high abundances of Sr (average 7000 ppm) and Nd (average 250 ppm) preclude significant changes in Sr and Nd isotopic ratios caused by crustal contamination and provide a unique means of characterizing the evolution of the mantle from Archaean to Recent times (Bell et al., 1982; Bell & Blenkinsop, 1987). In the Indian Peninsula, carbonatites have been reported from various geological settings with ages ranging from Early Proterozoic to Tertiary (Krishnamurthy, 1988; Natarajan et al., 1994). Petrological, mineralogical and chemical data are available for most of the Indian carbonatite complexes (Krishnamurthy, 1988). However, isotopic data remain scarce and are confined to 87Sr/86Sr analyses from Sevattur (Deans & Powell, 1968; Kumar & Gopalan, 1991), an Rb–Sr study on carbonatites of Hogenekal (Natarajan et al., 1994), an isotopic study of Sr, Nd and Pb from the Ambadongar carbonatite complex (Veena et al., 1993; Simonetti et al., 1995) and a recent Pb/Pb age determination on the Newania and Sevattur carbonatites of India (Schleicher et al., 1997). This study attempts to define the combined Sr, Nd and Pb isotopic systematics of the Sung Valley carbonatites, Meghalaya, in eastern India, as part of a more detailed programme of isotopic studies on carbonatitic and related alkaline rocks from different locations in India.
The Sung Valley carbonatites and a related suite of alkaline silicate rocks are thought to represent the earliest phase of magmatic activity that was initiated by the Kerguelen mantle plume during or slightly before the break-up of India from Australia–Antarctica (Kent et al., 1992b). A study of Pb, Sr and Nd isotopic characteristics of the mantle source regions of the carbonatites of Sung Valley, therefore, has a direct bearing on the understanding of the geochemical evolution of the sub-continental mantle that existed before the rifting of Gondwana (Storey, 1995) and the formation of large, flood basalt dominated, igneous provinces (Kent et al., 1992b). In this context, the relation between the Kerguelen–Heard plume, the Ninetyeast Ridge and the continental flood basalt provinces of eastern (Rajmahal and Bengal) and northeast (Sylhet basalts) India and the Sung Valley carbonatites assumes importance, as these constitute a chain of magmatic activity that ensued during the continental break-up and opening of the Indian Ocean (Fig. 1). Our objectives here include: (1) determination of the age of emplacement of the Sung Valley carbonatite complex, (2) isotopic characterization of the carbonatites and their mantle source regions and (3) comparison and contrasting of the mantle source regions of the Sung Valley carbonatites with the sources of plume-related basalts of the Indian Ocean islands (Kerguelen, Ninetyeast Ridge) and with the Rajmahal–Bengal–Sylhet continental flood basalts.
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, hotspots; 
, hotspot traces; CMLR, Chagos–Maldive–Laccadive Ridge; 85 ER, 85°E Ridge; 90 ER, Ninetyeast Ridge; MR, Madagascar Ridge; D, Deccan basalts; S, Sylhet basalts; R, Rajmahal basalts; SV, Sung Valley. Shading indicates continents used in reconstructions. Continuous lines indicate present spreading ridge system. [Modified after Curray & Munasinghe (1991) |
Geological and Tectonic Setting |
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TOPABSTRACTIntroductionGeological and Tectonic SettingSample Description and...Results and DiscussionConclusionsReferences |
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The Sung Valley alkaline–ultramafic carbonatite complex (25°31'–25°36'N, 92°05'–92°10'E), located in the Shillong Plateau, intrudes the Early Proterozoic Shillong Group, which consists of quartzites, phyllites and quartz–sericite schists (Yusuf & Saraswat, 1977
; Krishnamurthy, 1985). Several Late Proterozoic granitic plutons (750–600 Ma) intrude rocks of the Shillong Group. The Sung Valley complex is overlain by the flat–lying Cherra sandstones, which, based on faunal evidence, have been assigned a Cenomanian (97–90 Ma) age (Krishnan, 1968). Thus, on the basis of field evidence, the carbonatites seem to be older than Upper Cretaceous. The Sylhet basalts are found 
120 km south west along the southern border of the Meghalaya plateau, between the E–W trending Raibah and Dauki faults. They occur as a 60 km long and 6 km wide belt with a monoclinal flexure to the south (Talukdar & Murthy, 1971). Structurally, the Shillong Plateau is considered to be a horst, bordered by the Dauki fault to the south and the Brahmaputra graben to the north. There are several N–S trending lineaments in the region, including the major Um–Ngot lineament, which lies east of Shillong and west of Jowai and which contains the Sung Valley complex. Besides Sung Valley, there are other carbonatite and alkaline rock complexes in northeastern India at Swangkre and Samchampi (Krishnamurthy, 1988; Kumar et al. 1996), although no age data are available on these. The close spatial association of carbonatites and basic alkaline rocks of the complex along with field, petrographic and chemical data indicate that these rocks constitute a genetically related suite both in space and time. The main rock type of the complex is pyroxenite, which forms an oval body of 35 km2, into which peridotites, ijolites, carbonatites and syenites have been emplaced. Two types of pyroxenites are present: (1) a coarse-grained variety consisting mainly of diopsidic augite with minor phlogopite, titanite and apatite and (2) a medium-grained variety consisting of diopsidic augite and aegirine-augite with minor K-feldspar, titanite and apatite. In the northern parts of the complex, stock-size bodies of olivinite and magnetite-rich peridotite are emplaced within the pyroxenite. The third most abundant rock type in the complex is the ijolites. These intrude the pyroxenite in the form of a ring dyke and consist of mainly idiomorphic nepheline and prismatic aegirine-augite. Carbonatites are found in the southern parts of the complex (Fig. 2), occurring mainly as dykes or cone sheet-like bodies intruding the pyroxenites and ijolites. Syenites and feldspathic veins occur as minor dykes in the complex cutting the pyroxenites, ijolites and quartzites.
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Sample Description and Analytical Methods |
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TOPABSTRACTIntroductionGeological and Tectonic SettingSample Description and...Results and DiscussionConclusionsReferences |
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Eleven carbonatite samples were studied for their Pb, Sr and Nd isotopic ratios. These include sövites (GC845 and GC1166), apatite sövites (GC846 and GC847), dolomitic sövites (GC848, GC1164 and GC1165), beforsites (GC849, GC1162 and GC1163) and banded phlogopite sövite (GC850). Besides major calcite and dolomite, depending upon the type, the carbonatites also contain subordinate amounts of fluorapatite, magnetite, pyrochlore, phlogopite and serpentine (after forsteritic olivine), rare hercynite, fluorite and perovskite. Sample GC857, a carbonatite–pyroxenite contact rock, contains olivine (altering to serpentine), phlogopite, apatite and carbonates.
All samples were cleaned before crushing and powdered to –200 mesh. The carbonate fractions analysed were obtained from the whole-rock powders by dissolving 500 mg of the sample in 10 ml of 1 M HCl for 5–10 min. Whole-rock samples were first dissolved in 1 M HCl followed by decomposition in 4 ml HF and 2 ml HNO3 in Teflon digestion bombs. The elemental abundances of Rb, Sr, Sm, Nd, U and Pb were determined by isotope dilution using tracers enriched in 87Rb, 84Sr, 149Sm, 146Nd, 235U and 206Pb. Th contents were determined by instrumental neutron activation analysis (INAA).
Ion exchange techniques were used to separate the elements for isotopic analysis. Rb, Sr and REE were separated using Bio-Rad AG50 x 12 cation exchange resin. Sm and Nd were further separated from the REE group using bio-beads coated with 10% HDEHP. Separation of U and Pb was carried out using Bio-Rad AG1 x 8 anion exchange resin. All isotopic analyses were carried out on a VG-354 multicollector mass spectrometer. The Sr and Nd isotopic ratios were normalized to 86Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively. SRM987 standard gave a mean 87Sr/86Sr ratio of 0.710241 ± 23 (n = 15). JM Nd2O3 spec pure solution (NDN-1) from Cambridge gave a mean 143Nd/144Nd ratio of 0.511122 ± 10 (n = 15) and La Jolla Nd standard gave a mean 143Nd/144Nd of 0.511875 ± 10 (n = 5). USGS rock standard G-2 gave the following mean values (n = 3): Rb = 170.0 ± 0.5 ppm, Sr = 480 ± 1 ppm, Sm = 7.18 ± 0.02 ppm, Nd = 53.88 ± 0.05 ppm, 87Sr/86Sr = 0.70995 ± 8, and 143Nd/144Nd = 0.512219 ± 13. The Pb isotopic ratios were corrected for mass fractionation of 0.15% per a.m.u. based on routine measurements on SRM981 isotopic standard (Pandey et al., 1997). The 2
analytical errors based on a number of duplicate analyses are 2% in 87Rb/86Sr, 1% in 147Sm/144Nd, 0.01% in 87Sr/86Sr, 0.005% in 143Nd/144Nd and 0.2% in Pb isotopic ratios and 2% in elemental abundances. The total procedural blanks were Rb < 3 ng, Sr < 5 ng, Sm < 0.1 ng, Nd < 0.1 ng, U < 3 ng and Pb < 5 ng. Errors in age and initial ratios are reported at 2 and the value of mean square weighted deviation (MSWD) was used for testing the goodness of fit after(Wendt & Carl (1991). For the calculations of single-stage model µ1 and plotting of the geochron the following values were used: age of the Earth (T) = 4.55 Ga, and 206Pb/207Pb (a0) = 9.307, 207Pb/204Pb (b0) = 10.294 and 208Pb/204Pb (c0) = 29.476 for the primodial Pb isotopic composition (Tatsumoto et al., 1973). The decay constants used were 
Rb = 1.42 x10–11 a–1, Sm = 6.54 x 10–12 a–1,U238 = 1.55125 x 10–10 a–1, U235 = 9.8485 x10–10 a–1 and Th232 = 4.9475 x 10–11 a–1.
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Results and Discussion |
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TOPABSTRACTIntroductionGeological and Tectonic SettingSample Description and...Results and DiscussionConclusionsReferences |
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The Pb isotopic ratios of 11 carbonate fractions and six whole rocks are listed in Table 1. The Rb, Sr, Sm and Nd elemental abundances and isotopic data are listed in Table 2, along with the U, Th and Pb abundances and calculated initial 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios of two whole-rock samples, GC846 and GC848.
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Age of emplacement of the Sung Valley complex and its bearing on the rifting of Gondwana
In a 207Pb/204Pb vs 206Pb/204Pb plot regression of all 17 data points yields an errorchron age (MSWD 3.7) of 142 ± 30 Ma. The data for both the calcite and whole-rock fractions of one sample, GC849, exhibit comparatively larger deviation. If these two data points (GC849C, GC849WR) are excluded, regression of the remaining 15 samples together yields a better fit with an age of 134 ± 20 Ma with a single-stage model µ1 of 8.19 ± 0.02 and MSWD of 1.8 (Fig. 3). The value of MSWD, 1.8, obtained for the regression is within the allowed limit for 13 d.f. The age obtained here is in broad agreement with the K–Ar age of 149 ± 5 Ma obtained for a phlogopite in a sövite obtained from Sung Valley (Sarkar et al., 1996
). Coeval (132 ± 9 Ma) ultramafic lamprophyres are known from East Antarctica (Kent et al., 1992b). The Pb–Pb isochron age of 134 ± 20 Ma for the Sung Valley carbonatites, though not well constrained, places its time of emplacement at the earliest stages of the Antarctica–India–Australia rifting (120 Ma) of East Gondwana (Storey, 1995). Such precursor alkaline magmatism before the onset of major continental flood basalt activity of Sylhet (110 ± 3 to 133 ± 4 Ma; Sarkar et al., 1996), Bengal and Rajmahal (117 ± 2 Ma, Baksi, 1995) and other oceanic plateau basalts in the Indian Ocean (Kerguelen plateau, 118–96 Ma, Kent et al., 1992b) has been cited as evidence for the incubation of the Kerguelen plume (Kent et al., 1992b), as well as other plume-related large igneous provinces of the world such as Siberia, Parana, Karoo and Deccan (Kent et al., 1992b).
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Initial Sr, Nd and Pb isotopic ratios
Calculated
Sr and Nd values for the carbonatite samples show a narrow variation of +5.3 to +7.8 and +1.7 to +2.3, respectively, which indicate near uniformity of Sr and Nd isotopes in the carbonatite magma. Mean initial 87Sr/86Sr and 143Nd/144Nd ratios are 0.70476 ± 0.00075 and 0.51257 ± 0.00001, respectively. Because crustal contamination is unlikely to influence the Sr and Nd isotopic compositions of carbonatites (Bell & Blenkinsop, 1989), these ratios are considered to reflect the isotopic composition of their mantle sources.
The calculated initial 206Pb/204Pb ratios of the two samples analysed (19.02 and 19.3) differ significantly. However, these differences cannot be taken as indicative of differences in their parental magma because the good fit of the data in the 206Pb/204Pb–207Pb/204Pb plot indicates a relatively uniform initial Pb isotopic composition. Therefore, the differences in the calculated initial 206Pb/204Pb ratios most probably resulted from recent disturbances of the U–Pb system in the sample, given that U has been found to be particularly mobile under surface and subsurface conditions (Rosholt & Bartel, 1969) and recent loss of Pb from U-bearing minerals is also a common observation. The metamict nature of pyrochlore in these samples may enhance the possibility of U–Pb disturbances. We therefore take the lower values of 19.02, 15.67 and 39.0 given by GC846 as the more probable estimate of the initial 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb ratios, respectively.
Implications for mantle sources
The values of Sr (+5.3 to+7.8), Nd (+1.7 to +2.3) and initial Pb ratio (to the right of the geochron; inset, Fig. 3) indicate that the carbonatite magma originated from source regions with somewhat higher Rb/Sr ratio relative to Bulk Earth, minor LREE depletion with respect to CHUR and time-integrated enhancement of the U/Pb ratio relative to Bulk Earth. This is consistent with the source characteristics reported for a number of carbonatite complexes, particularly from the southern hemisphere (Bell & Blenkinsop, 1987, 1989; Nelson et al., 1988; Simonetti et al., 1995; Bell & Simonetti, 1996). Initial Pb, Sr and Nd isotopic ratios of the Sung Valley carbonatites are compared with isotopic data from other carbonatite complexes of the Gondwana in 207Pb/204Pb–206Pb/204Pb (Fig. 4a), 208Pb/204Pb–206Pb/204Pb (Fig. 4b), Nd–Sr (Fig. 4c) and 87Sr/86Sr–206Pb/204Pb (Fig. 4d) correlation plots. Available isotopic data for some of the basalts of the Indian Ocean islands and the Ninetyeast Ridge are also shown in the above figures along with four principal mantle end-member components [N-MORB (mid-ocean ridge basalt), EM1, EM2 and HIMU] based on the data from Hart (1988).
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In Pb–Pb and Sr–Nd correlation plots the EM2 and HIMU contributions are clearly seen in the sources of the Sung Valley carbonatites. Whereas the Sung Valley data are very close to EM2 in Pb–Pb plots (Fig. 4a and b) they lie on an EM2–HIMU mixing array in the Sm–Nd plot (Fig. 4c). A slight EM1 influence can be seen in the Sr–Pb plot (Fig. 4d) only. This kind of behaviour, in which the Pb–Pb plot shows a dominant EM2 signature and the Sr–Nd plot shows a dilution of EM2 by HIMU, has also been noticed in the carbonatites of Ambadongar (Simonetti et al., 1995
). Strong enrichment of Pb in EM2 sources compared with other sources( Hart, 1988) coupled with comparable Sr and Nd, could be the cause of this, as it would result in EM2-dominated mixing of Pb isotopes whereas Sr–Nd mixing would be influenced evenly.
Comparison of mantle source characteristics of Sung Valley carbonatites
Sung Valley vs ocean island basalts
In the Sr–Nd plot (Fig. 4c), the Sung valley data fall within the fields of basalts of Kerguelen and the Ninetyeast Ridge, which show large variation stretching from a depleted (Sr = –10, Nd = +9) to an enriched EM1 end-member. More specifically, the data are close to those of mildly alkaline basalts from the Kerguelen Archipelago (Nd = -1.1 to +0.1, Sr = +5.7 to +10). The Pb initial ratios of the Sung Valley carbonatites are more radiogenic than the Kerguelen basalts and fall within the space surrounded by Amsterdam, Crozet and St Paul. The Sr-Nd composition of these islands is, however, more depleted compared with that of the Sung Valley (Fig. 4c). The isotopic data therefore suggest that though the Sung Valley carbonatite sources show a broad similarity with the data for Indian Ocean basalts there are differences when seen in detail.
An important large-scale characteristic of the mantle sources of the southern hemisphere OIBs is the existence of the DUPAL anomaly centred around the 30°S latitude characterized in terms of 
Sr > 40, 7/4 > 4 and 8/4 > 60 (Dupré & Allègre, 1983; Hart, 1984, 1988). With a Sr value of 48 and 7/4 of 12 the Sung Valley carbonatite also exhibits a somewhat DUPAL type signature.
Sung Valley carbonatites vs Rajmahal and Sylhet basalt
The available Sr–Nd–Pb data on the Rajmahal and Sylhet basalts show considerable variation and are generally consistent with their derivation from a depleted source similar to that which generated the type Indian Ocean MORB (Mahoney et al., 1983; Kent et al., 1992a; Pantulu et al., 1992). Although the Nd-Sr values from the Sung Valley carbonatites would lie within the field (not shown) that encloses the data from the Rajmahal basalts, the least contaminated Rajmahal samples have relatively more depleted characteristics [[Nd(T) = +3.0, 87Sr/86Sr = 0.70412] than the carbonatites [Nd(T) =+2.0, 87Sr/86Sr = 0.70476]. An initial magma similarisotopically to the most depleted of the Rajmahal basalts could perhaps be contaminated by crustal rocks of the Shillong Plateau, resulting in values similar to those seen in carbonatites. However, assuming a contaminant with 87Sr/86Sr = 0.725, Sr = 300 ppm; Nd = –20 and Nd = 20 ppm, this would require assimilation of 30–40% of crustal material, which seems unlikely. Because of this, the original magmas that generated the Rajmahal basalts and Sung Valley carbonatites probably originated from different mantle sources. These isotopic differences are more clearly seen with respect to the Pb data, where the Rajmahal basalts have less radiogenic Pb ratios (17.95, 15.54, 38.0, Kent et al., 1992a
Genesis of the Sung Valley carbonatite
Based on petrological evidence, a carbonated alkali peridotitic melt has been assigned a parental role from which the carbonatite separated because of liquid immiscibility. Phase equilibria evidence suggests that such melts could be generated at 20–30 kbar pressures consistent with lithospheric depths within the mantle (Eggler, 1989; Wyllie, 1989).
Mantle metasomatism has been recognized as an essential prerequisite to generate such carbonated nephelinitic melts (Olafsson & Eggler, 1983; Eggler, 1989) in which fluids and volatiles from upwelling mantle plume play a major role. The contribution of the Kerguelen plume to the petrogenesis of the Sung Valley carbonatites has to be explained in terms of the observed EM2–HIMU signatures.
To explain a mixed HIMU–EM1 type signature in East African carbonatites, Bell & Simonetti, (1996) suggested a two-stage model. First, fluids with a HIMU–EM1 signature, generated from an upwelling mantle plume, metasomatized EM1 type sub-continental lithosphere. This was followed by low-degree partial melting of this metasomatized sub-continental lithosphere to produce carbonated silicate (nephelinitic) melts.
A similar model can be envisaged in the case of the Sung Valley carbonatites. However, in this case the origin of HIMU and EM2 components in the carbonatites needs to be understood. The generally accepted view for the EM2 mantle component regards it as a subducted and recycled continental crustal component for which long-term storage is contra-indicative (Hart, 1988). However, short-term storage in a mesosphere boundary layer is certainly a possibility. An altered, subducted oceanic crust which might have undergone suitable modification during subduction is regarded as a possible source of HIMU. A composite HIMU–EM2 pair can be generated by subduction of a steeply dipping composite slab comprising both oceanic crust and continental sediments, in a tectonic setting similar to that of the Lesser Antilles island arc (Dickin, 1997). Thus it may be speculated that fluids from an upwelling mantle plume from the mesosphere boundary layer, with an EM2 (and perhaps HIMU) signature, metasomatized the pre-breakup Gondwana sub-continental lithosphere. Shortly after, partial melting of this metasomatized sub-continental lithosphere produced a carbonated nephelinitic melt with a HIMU–EM2 signature from which the Sung Valley carbonatites were produced by liquid immiscibility. A slight EM1-like influence could have come from the sub-continental lithosphere.
Opinions about the isotopic character of the Kerguelen plume are, however, equivocal. Saunders et al. (1991) and Frey & Weis (1995) could not detect any temporal trends in the isotopic composition along the Ninetyeast Ridge. However, Gautier et al. (1990) and Weis et al. (1993) noticed that Nd, Sr and Pb isotopic ratios in the Kerguelen Archipelago change systematically with age. They suggested that the isotopic composition (87Sr/86Sr = 0.7054–0.7058,Nd = –0.2 to –2.9, 206Pb/204Pb = 18.06–18.27, 207Pb/204Pb = 15.54–15.58, 208Pb/204Pb = 38.68–39.16) of Late Miocene alkalic lavas that erupted in an intraplate setting away from the spreading centre can be taken as representative of the Kerguelen plume. According to this, the Kerguelen plume composition lies between EM1 and EM2. Contrary to this view, Class et al. (1993) concluded that the plume source composition is reflected in lavas with lowest 87Sr/86Sr, highest 206Pb/204Pb and highest Nd erupted at a given time. They also contended that systematic variation can be discerned in the lavas of the Ninetyeast Ridge and the Kerguelen Plateau, and that these variations were caused by radiogenic ingrowth in an evolving plume source. They suggested a plume source with 206Pb/204Pb, 207Pb/204Pb, 87Sr/86Sr and 143Nd/144Nd ratios of 18.850, 15.583, 39.200, 0.7046 and 0.51278 at present. However, observed magnitude of variation in 206Pb/204Pb ratios with time requires substantially higher (238U/204Pb) values in the source than found in the basalts.
Mahoney et al. (1995) did not find evidence to support either of these models, and argued that significant continental lithospheric influence coupled with original proximity of some of the plateaux to spreading centres has introduced great complications plaguing the efforts to identify the post–88 Ma Kerguelen plume source composition.
The Sung Valley isotopic composition favours an EM2 and HIMU type of signature in the plume before the 130 Ma break-up. The presence of an EM2 type source in the pre-130 Ma sub-Gondwana mantle is also indicated by the volcanic rocks of the Ferrar Supergroup, Antarctica, which were associated with the first phase of break-up of Gondwana into East and West Gondwana at 180 Ma (Menzies & Kyle, 1990). Thus it appears that a preferentially EM2 type source existed until the early stages of break-up of the Indian plate from the Australia-Antarctic plate at 130 Ma, which gradually gave way to sources with a preferentially EM1 type signature as exemplified by the extremely enriched EM1 type Afnasy-Nikitin Rise (Mahoney et al., 1996).
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Conclusions |
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TOPABSTRACTIntroductionGeological and Tectonic SettingSample Description and...Results and DiscussionConclusionsReferences |
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The new Sr–Nd–Pb isotopic data, combined with the age of the Sung Valley carbonatite, bring out the following points:
- The Pb–Pb age (134 ± 20 Ma) of the Sung Valley carbonatites, combined with other available geochronological data, suggests that the alkaline magmatism manifested in northeast India–Antarctica represents an early, precursor phase before the onset of major flood basalt activity in the Middle Cretaceous. These data, therefore, seem to support the plume incubation model of Kent et al., (1992b) for the Kerguelen–Heard plume.
- The average isotopic characteristics of the source, Sr(0) = +6.0, Nd(0) = +2.0, 206Pb/204Pb = 19.02,207 Pb/204Pb = 15.67 and 208Pb/204Pb = 39.0, indicate a mantle source region with somewhat higher Rb/Sr values than Bulk Earth, minor LREE depletion, and a time-integrated enhancement of U/Pb relative to Bulk Earth. Similar characteristics have been noted for carbonatite complexes from Walloway, Australia, and Ambadongar, India.
- Mantle source characteristics show close similarities to EM2–HIMU type sources. The genesis of the parental magma to the Sung Valley carbonatites may involve metasomatism of the sub-continental lithosphere by EM2–HIMU type fluids from an upwelling mantle plume during its incubation at the base of the lithosphere and its subsequent partial melting
- The isotopic characteristics of the Sung Valley carbonatites along with available evidence from the Ferrar Supergroup, Antarctica (Menzies & Kyle, 1990) suggest that the pre-130 Ma Gondwana mantle had EM2 type source characteristics, which gradually changed to EM1 type after the break-up as seen in younger products of Indian Ocean plumes.
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Acknowledgements |
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The authors thank K. K. Dwivedy, Director, Atomic Minerals Division, for encouragement and permission to take up this work, and Tikam Chabria for useful discussions. Comments on the manuscript by Keith Bell, Mike Villeneuve, Clement Gariepy and an anonymous reviewer helped in improving the manuscript substantially and we are thankful to them. N. Satayanarayana is thanked for the INAA data and Y. V. S. Rao for improving the diagrams.
* Corresponding author. Present address: 1–10–153, Sardar Patel Road,Geochronology Laboratory, Atomic Minerals Division, Department of Atomic Energy, Begumpet, Hyderabad—500 016, India. Telephone: 0091-40-7767101, ext. 265. Fax: 0091-40-7762940. e-mail: amdhyd{at}ap.nic.in
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References |
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TOPABSTRACTIntroductionGeological and Tectonic SettingSample Description and...Results and DiscussionConclusionsReferences |
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