Journal of Petrology | Volume 39 | Number 11-12 | Pages 1847-1864 | 1998
© Oxford University Press 1998
Geochemical and Nd, Pb, and Sr Isotope Data from Deccan Alkaline Complexes—Inferences for Mantle Sources and Plume–Lithosphere Interaction
1 Max-Planck-Institut Für Chemie, Abteilung Geochemie Postfach 3060, 55020 Mainz, Germany
2 Department of Earth and Planetary Sciences, McGill University 3450 University Street, Montréal, Québec, Canada, H3A 2A7
3 Department of Geology, St Xavier's College Bombay 400 001, India
Received October 1, 1997; Revised typescript accepted May 21, 1998
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ABSTRACT |
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Previous chemical and isotopic studies based on alkaline rocks and carbonatites associated with large, continental flood basaltic provinces indicate their important role in monitoring plume–lithosphere interaction. We report new major and trace element data, and Nd, Pb, and Sr isotope ratios for various alkaline silica-undersaturated rocks and carbonatites from several Decca alkaline complexes in an attempt to evaluate the relative contributions of Réunion plume and Indian sub-continental mantles in their source regions. Major and trace element abundances for the most primitive silicate samples are consistent with an origin via small-degree partial melting of metasomatized mantle. Initial 87Sr/86Sr, 143Nd/144Nd and Pb isotope ratios for the most primitive alkaline silicate samples and associated carbonatites exhibit a large variation, and are attributed to mixing of three distinct mantle components—Réunion plume, continental lithosphere and asthenosphere (Indian MORB-like). For the silicate rocks, isotope ratios correlate with major and trace element composition and support derivation from distinct mantle sources. The data obtained here are consistent with previous models invoking Réunion plume–continental lithosphere interaction to explain the origin of Deccan alkaline complexes, which suggest a more prominent role of Réunion mantle during the early stages of Deccan volcanism involving small-degree melting of plume-modified lithosphere. With time, the isotope systematics of both alkaline and tholeiitic magmatism record a larger lithospheric imprint.
KEY WORDS: carbonatite; Deccan province; lithosphere; mantle plume; Réunion
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Introduction |
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TOPABSTRACTIntroductionAnalytical MethodsResultsDiscussionConclusionsReferences |
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Although the exact origin of carbonatite magma has yet to be agreed upon, whether derived vi liquid immiscibility (e.g. Kjarsgaard & Hamilton, 1989
; Church & Jones, 1995; Jones et al., 1995; Kjarsgaard et al., 1995), magmatic separation from a carbonated parental silicate magma or primary carbonatite melt (e.g. Wallace & Green, 1988; Dalton & Wood, 1993), the similarity between the Nd, Pb, and Sr isotope data from young (<200 Ma) carbonatites world wide and ocean island basalts (OIBs) clearly points to their mantle origin (e.g. Bell & Blenkinsop, 1987a, 1987b; Nelson et al., 1988; Tilton & Bell, 1994). Recent Nd, Pb, and Sr isotope data from East African complexes, however, have led to questioning of the validity of some of these models, as these data indicate that (1) carbonatites and associated silicate rocks evolve along separate melt differentiation pathways, and (2) both represent discrete, partial melts derived from an isotopically heterogeneous mantle source consisting of HIMU and EMI components (e.g. Napak, Simonetti & Bell, 1994a; Mount Elgon, Simonetti & Bell, 1995; Oldoinyo Lengai, Bell & Dawson, 1995). In addition, Bell & Simonetti (1996) proposed that the variations in Nd, Pb, and Sr isotope ratios from East African carbonatites may be attributed to interaction between old, radiogenic continental lithosphere (EMI-like mantle component) and upwelling (‘plume’), HIMU-like mantle.
Other studies have also proposed the involvement (both direct and indirect) of mantle plumes in carbonatite genesis (e.g. for the Cape Verdes, Gerlach et al., 1988; th Canary Islands, Hoernle & Tilton, 1991; Brazil, Toyoda et al., 1994; Huang et al., 1995; Namibia, le Roex & Lanyon, 1998). The link between mantle plume activity and alkaline–carbonatite magmatism is further supported by the spatial relationship of carbonatite occurrences within flood basaltic provinces. Examples include those found in Brazil and Namibia associated with Paraná–Etendeka volcanism, those in Canada associated with Keeweenawan volcanism, and those in India associated with Deccan volcanism. In the Nd, Pb, and Sr isotope study of the Amba Dongar carbonatite complex from west–central India, Simonetti et al. (1995) proposed that the Réunion ‘hotspot’, responsible for the generation of the voluminous Deccan Traps, may have been involved in producing the parental melt to the Amba Dongar carbonatite.
The Deccan igneous province consists of a volumetrically large succession of predominantly tholeiitic lava flows (
0.5 x 106 km2), which mark the first surface expression of the Réunion hotspot on the Indian sub-continent Vandamme et al., 1991). Geochronological data define the main period of Deccan flood basaltic volcanism at 66 Ma (e.g. Courtillot et al., 1988; Duncan & Pyle, 1988; Vandamme et al., 1991; Venkatesan et al., 1993; Baksi, 1994) and suggest that volcanic activity occurred over a time interval of probably <1 my (Courtillot et al., 1986a, 1986b). Devey & Stephens (1992) proposed that Deccan alkaline magmatism post-dated tholeiitic volcanism by up to 3 my. More recent 40Ar/39Ar ages for several of the Deccan alkaline complexes examined in this study are listed in Table 1, and their locations are shown in Fig. 1. These ages clearly show that Decca alkaline magmatic activity overlapped with the main period of tholeiitic volcanism. Moreover, on the basis of 40Ar/39Ar ages, Basu et al. (1993) concluded that Deccan alkaline magmatism was slightly older (3 my older) in the northern part of the province (68.5 Ma for Barmer and Mundwara) compared with areas further south (e.g. 65 Ma for Phenai Mata). Figure 1 also shows that occurrences of alkaline magmatism are mainly restricted to the northern part of the Deccan igneous province, north of the Narmada Rift Valley. A summary of the geological settings and petrography of the samples investigated in this study is provided in Table 1.
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) examined in this study; these include Amba Dongar, Barmer, Bhuj, and Mundwara. Also shown are sample locations (
) for mildly alkaline picrites and basaltic flows (from Peng & Mahoney, 1995We report new geochemical and Nd, Pb, and Sr isotope data for various silica-undersaturated rocks and carbonatites from the Deccan alkaline complexes of Barmer, Bhuj, Mundwara and Amba Dongar. The main objectives are (1) to characterize the chemical and isotopic nature of the mantle responsible for derivation of the Deccan alkaline complexes, and (2) to evaluate the role of Réunion ‘plume’–type mantle–lithosphere interaction, and to assess their respective contributions in the generation of Deccan alkaline magmatism.
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Analytical Methods |
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Samples selected for chemical and isotopic analyses contained little or no evidence of subsolidus alteration. Before acid digestion, all sample powders (between 0.1 and 0.2 g were spiked with two mixed isotope tracer solutions, one for 87Rb–84Sr and the other for 150Nd–149Sm. For silicate rocks, the sample powder was treated with a mixture of concentrated HF–HNO3 in a Savillex vial and left at 140°C for a period of at least 48 h. Acid digestion of carbonatite samples involved mixture of 6 N HCl and concentrated HNO3 placed in a Savillex vial, also for 48 h. Procedures for chemical separation of Sr and Nd were similar to those described by White & Patchett (1984)
. Sr aliquots were loaded using a TaF solution on single W filaments whereas Nd was loaded with HCl using a double Re filament technique. Nd and Sr isotope measurements were obtained by thermal ionization mass spectrometry at the Max-Planck-Institut für Chemie (Mainz) using a MAT 261 mass spectrometer operated in the static multicollection mode. Separation of lead was done using anion-exchange chromatography (after Manhès et al., 1980), and U and Th determination followed the technique of Edwards et al. (1986), both conducted at GEOTOP, Université du Québec à Montréal. Lead, thorium and uranium concentrations were determined by isotope dilution using 206Pb, 229Th, and 233U–236U spikes. Pb isotope measurements were obtained using either a single Faraday cage detector or Daly analogue detecto (in peak switching mode), whereas U and Th were measured using an ion-counting Daly detector. |
Results |
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Mineral analyses
Representative average microprobe analyses of olivine and clinopyroxene phenocrysts from a basanite from Bhuj (95–BHUJ–016) and a melilitite from Barmer (95–BMR–037) are listed in Tables 2 and 3. In addition, representative average compositions for clinopyroxene phenocrysts from an ijolite from Mundwara (95–MUN–028) are shown in Table 3. The olivines in the samples from Bhuj and Barmer are characterized by hig Fo contents (81–90), with those for the latter being slightly less Fo rich. The compositions of the clinopyroxene phenocrysts (Table 3) correspond to those for diopside, an the range in mg-numbers (89–92) is restricted for samples from Bhuj and Barmer; in comparison, those from Mundwara suggest crystallization from a slightly more differentiated mel (mg-numbers 84–86).
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Chemical data
Table 4 lists X-ray fluorescence (XRF) whole-rock chemical analyses of samples examined in this study. With the exception of the phonolites, the remaining silicate samples contain moderately high MgO contents >6 wt %, corresponding to mg-numbers {100[atomic Mg/(Mg + Fe2+)]} ranging from 54 to 71, indicative of their relatively primitive nature. Figure 2 shows the variations in the contents of several majo element oxides vs mg-number. Also shown in Fig. 2 for comparison are data for olivine basalts (primitive shield phase) and for the differentiated series (gabbro, basalt, syenite) from the Piton des Neiges volcano, Réunion (Fisk et al., 1988
). With the exception of the data for the melilitites from Barmer and for the more differentiate samples (mg-numbers <40) from the Deccan alkaline complexes, those for the remaining samples overlap the trends defined by the samples from Réunion (Fig. 2) Of interest in Fig. 2, the melilitites from Barmer contain significantly higher CaO/Al2O3 (>1.29) ratios and TiO2 contents (>5.8 wt %) compared with the remaining Deccan alkaline samples at similar mg-numbers.
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The samples of basanite–nephelinite from Bhuj contain the highest abundances of both N (168–332 ppm) and Cr (429–995 ppm), and these are negatively correlated with mg-numbers (Fig. 3). Similar arrays are also defined by the data from Réunion (Fig. 3). For samples from Barmer and Bhuj with mg-numbers >50, the arrays shown in Fig. 3 are consistent with melt differentiation involving predominantly clinopyroxene and olivine fractional crystallization, a feature confirmed by both their phenocryst assemblage (Tables 1–3) and by results from melt experiments using lavas from Réunion (Fisk et al., 1988
). Variations in the abundances of incompatible trace elements, such as Zr and Rb, with melt differentiation (Fig. 3) are sma for the more primitive samples (mg-numbers >50) from Barmer, Bhuj and Mundwara, a feature consistent with crystal fractionation involving predominantly olivine and clinopyroxene. In contrast, the more differentiated samples from Barmer and Mundwara exhibit large variations in the abundances of Rb and Zr, a feature difficult to reconcile with closed-system crystal fractionation. It is also clear from Fig. 3 that the samples from the Deccan alkaline complexes contain higher absolute Rb contents compared with the Réunion samples for similar mg-numbers.
Primitive mantle-normalized patterns for the average contents of trace elemen differentiated samples (mg-number >50) from each of the alkaline complexes are shown in Fig. 4. Also included are normalized patterns for primitive olivine basalts from the oceanite series, Réunion (Fisk et al., 1988), and for N-MORB (Sun & McDonough, 1989). Important points to note from Fig. 4 are: (1) all the alkaline, silica-undersaturated samples show enrichment in highly incompatible trace elements, consistent with an origin via small degrees of partial melting; (2) the patterns for the most primitive samples from Bhuj, Barmer and Mundwara are fairly similar, with the level of enrichment correlating positively with the degree of alkalinity (undersaturation); (3) all patterns are anchored at similar Y-normalized values.
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Nd, Sr and Pb isotopic data
For all complexes, only the most primitive samples (mg-numbers >50) were selected for isotopic analysis, and results are listed in Tables 5 (Nd and Sr) and 6 (Pb). Initial 87Sr/86Sr, 143Nd/144Nd and Pb isotope ratios were calculated assuming an emplacement age of
65 Ma. Given the extremely high abundances of Sr (2400 to >3 x 104 ppm) for the carbonatites (Table 3), and the relatively young age of the complexes, their measured 87Sr/86Sr ratios are considered to approximate initial values.
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Compared with the initial Sr and Nd isotope ratios for all samples (Table 3), the basanite–nephelinites from Bhuj contain the lowest initial 87Sr/86Sr (0.70357–0.70396) and highest initial 143Nd/144Nd (0.51281–0.51287) ratios. In a plot of initial 87Sr/86Sr vs initial 143Nd/144Nd ratios (Fig. 5), the samples from Bhuj fall between the fields defined for present-day Indian MORB and that representing the composition of Réunion ‘plume’ mantle at
65 Ma (Dupré & Allègre, 1983; Fisk et al., 1988). The melilitites from Barmer also fall within the ‘depleted’ quadrant of Fig. 5, but contain more radiogenic Sr and lower Nd isotope values compared with the samples from Bhuj. In addition, the melilitite from Barmer contain similar initial 87Sr/86Sr ratios but slightly lower initial 143Nd/144Nd ratios compared with the composition of the Réunion plume (Fig. 5). Figure 5 also clearly shows that the initial 143Nd/144Nd ratios for the carbonatites and melilitites from Barmer do not entirely overlap, with the former having slightly lower values. This feature suggests that the carbonatite and associated silicate rocks from Barmer do not share a common melt evolutionary history, and/or derivation from an isotopically heterogeneous mantle. This result is consistent with similar findings based on isotope results from previous investigations of several East African carbonatite complexes (e.g. Napak, Uganda—Simonetti & Bell, 1994a; Chilwa Island, Malawi—Simonetti & Bell, 1994b; Mount Elgon, Uganda—Simonetti & Bell, 1995; Oldoinyo Lengai, Tanzania—Bell & Dawson, 1995; Bell & Simonetti, 1996). Also shown in Fig. 5 are the initial Sr and Nd isotope data for nephelinites and phonolites from Mundwara, and these plot at higher initial Sr but similar initial Nd isotope ratios compared with those for samples from Barmer. In addition, initial Nd and Sr isotope ratios for most of the samples from Barmer and Mundwara (Fig. 5) cluster at the ‘depleted’ end of the isotopic array defined by data for picritic and basaltic lavas from the northwestern Deccan Traps region (Peng & Mahoney, 1995; locations shown in Fig. 1). This array was interpreted as representing mixing between mantle end-members similar in composition to that of Réunion and continental lithosphere (Peng & Mahoney, 1995). In the lower right quadrant of Fig. 5, the initial Nd and Sr isotope ratios for carbonatite dyke 95-AMDO-002 plot within the previously established field for Amba Dongar (Simonetti et al. 1995). In contrast, the initial Nd and Sr isotope ratios for the basanite from the central plug at Amba Dongar plot close to the Réunion field, and are distinct from those for the surrounding carbonatite (Fig. 5). The isotope systematics for carbonatite and adjacent basanite at Amba Dongar are not uniform, a finding also observed for Barmer and other alkaline complexes world wide.
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= 1.42 x 10–11 per year) and 143Nd/144NdCHUR = 0.512638 and 147Sm/144NdCHUR = 0.1967 (Present-day Pb isotope ratios for carbonatite are considered to approximate initial ratios because of the relatively young age of the complexes (
65 Ma), and the generally low U/Pb and Th/Pb ratios characteristic of carbonatites (e.g. Grünenfelder et al., 1986; Nelson et al., 1988; Kwon et al., 1989; also for natrocarbonatites—Simonetti et al., 1997; this study, Table 6). A plot of initial 206Pb/204Pb vs initial 207Pb/204Pb (Fig. 6) shows the data listed in Table 6, in addition to the fields for Indian MORB, the Réunion plume component (Oversby, 1972), the carbonatites from Amba Dongar (Simonetti et al., 1995), and both arrays defined by picrites and basaltic lavas from the northwestern Deccan province (Peng & Mahoney, 1995). As in the case for their initial Nd and Sr isotope data (Fig. 5), the basanite–nephelinites from Bhuj contain initial Pb isotope ratios that plot between the Réunion and Indian MORB fields (Fig. 6). The initial Pb isotope ratios for samples from Bhuj and basanite from Amba Dongar are clearly less radiogenic compared with the remaining samples (Fig. 6). Data for carbonatites and melilitites from Barmer shown in Fig. 6 are similar (with the exception of sample 95-BMR-038, Table 6), and fall very close to the Réunion field. Initial Pb isotope ratios for the Mundwara nephelinites overlap those for the samples from Barmer (Fig. 6). Carbonatite sample 95-AMDO-001 contains initial Pb isotope ratios that plot within the field previously defined for Amba Dongar carbonatites (Simonetti et al., 1995). A noteworthy feature of Fig. 6 is that the initial Pb isotope ratios for most of the samples from Barmer, Mundwara, and Amba Dongar form a near-vertical array and overlap one of the trends (1) defined by the mildly alkaline picritic and basaltic Deccan lavas from Peng & Mahoney (1995).
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With the exception of the samples from Bhuj, the various silicate rock types from the Deccan alkaline complexes share several characteristics. These include having approximately the same level of incompatible trace element enrichment and similar primitive mantle-normalized patterns (Fig. 4). The large variation in the initial Nd, Pb, and Sr isotope ratios for the least differentiated rock types from the individual complexes, however, are clearly indicative of derivation from isotopically distinct mantle sources.
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Discussion |
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Mantle sources
Compared with the initial Nd and Sr isotope ratios from Amba Dongar and Mundwara, the complexes of Bhuj and Barmer are characterized by lower initial 87Sr/86Sr an higher initial 143Nd/144Nd ratios, and these correspond more closely to those for Réunion mantle (Fig. 5). Basu et al. (1993
reported initial Sr isotope and 3He/4HeRA ratios (from clinopyroxene) (where RA is the air ratio) for pyroxenite and gabbro from the complexes of Mundwara, Sarnu (Barmer) and Phenai Mata. More importantly, the range in observed 3He/4HeRA ratios for the slightly older complexes of Mundwara (8.5–13.9), and Barmer (12.4–12.8) are similar to the 3He/4HeRA value of 14 measured for the Réunion hotspot (Craig & Rison, 1982; Graham et al., 1990). The He isotope data provide, therefore, evidence for the participation of Réunion plume mantle in the derivation, at the very least, of the slightly older (northern) Deccan alkaline complexes (Basu et al., 1993).
With the exception of the carbonatites from Amba Dongar, the initial 143Nd/144Nd and 87Sr/86Sr isotope values from the remaining complexes plot either close to (Bhuj) or at slightly lower Nd isotope ratios compared with that for Réunion mantle (Fig. 5). In addition, the initial Pb isotope data for most of the samples studied here plot close to the composition of Réunion mantle (Fig. 6). The Nd, Pb and Sr isotope data for samples from Barmer, Mundwara and Amba Dongar also overlap those for mildly alkaline picrites and basaltic lavas from the northwestern Deccan flows (trend 1, Peng & Mahoney, 1995). The latter define near-vertical arrays in Pb–Pb isotope diagrams (Fig. 6) and an Nd–Sr isotope array (Fig. 5), which were both attributed to mixing between a Réunion plume mantle and continental lithosphere (Peng & Mahoney, 1995). Moreover, based on the incompatible trace element enriched nature of mildly alkaline picritic and basaltic lava flows from the northwestern Decca (Fig. 1), Peng & Mahoney (1995) proposed that melting within the upper mantle occurred at greater average depths and over a shorter depth interval for these basalts than for their counterparts located in the southern part of the province. The data shown in Figs 5 and 6 suggest, therefore, that continental lithosphere played an important role in the generation of Deccan alkaline magmatism. The isotope results shown in Figs 5–7 clearly indicate, however, that a third end-member is required to explain the variation in the isotopic data. The most likely candidate is asthenospheric, Indian MORB-type mantle because of its isotopic similarities to the basanite–nephelinites from Bhuj. Experimental results indicate that entrainment of ambient mantle during the ascent of thermal plume is possible because of coupling between heat conduction and laminar stirring driven by plume motion (Griffiths & Campbell, 1990). On the basis of results from recent numerical models investigating the role of thermal entrainment and melting in mantle plumes, Farnetani & Richards (1995) suggested that the isotopic heterogeneity is inherent to the plume itself or the result of contamination from crust and lithosphere through which the primar magmas ascend. With regard to Réunion plume–lithosphere interaction, however, the Nd, Pb and Sr isotopic compositions for the samples from Bhuj are solely consistent with contamination by lithosphere similar in composition to Indian MORB mantle.
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Correlations between isotope ratios and major and trace element data would lend support to the generation of Deccan alkaline complexes involving distinct mantle sources. First, it must be shown that initial isotopic ratios are inherited from their mantle source region, and not perturbed by secondary processes, such as crustal contamination. The data listed in Tables 4 and 5 indicate that no correlations exist between Sr and Nd contents and mg-number, and initial 87Sr/86Sr and initial 143Nd/144Nd therefore are not consistent with crustal contamination.
Figure 2 clearly shows that the melilitites from Barmer are characterized by much higher CaO/Al2O3 ratios compared with the silicate rocks from the remaining Deccan alkaline complexes. According to the experimental work of Herzberg (1995), partial melting at higher pressures (
30 kbar) stabilizes garnet relativ to olivine and pyroxene, resulting in melt products with lower Al2O3 contents and higher CaO/Al2O3 ratios. In addition, the low SiO2 contents, high incompatible element concentrations and steep REE patterns typical of melilitite indicate that they are the result of small-degree partial melting (<<5%) of an enriched mantle source (relative to MORB-source mantle) interpreted to be carbonated phlogopite-garnet lherzolite (e.g. Brey & Green, 1977; Maaløe et al., 1992; Rogers et al., 1992). The CaO-rich (up to 17 wt %) and Al2O3-poor (8–10 wt %) nature of melilitites probably reflects the role of carbonate during melting and the residual nature of garnet in the source (Wilson et al., 1995). Nb/Y ratios may serve to evaluate the role of residual garnet during melting, as Y would preferentially partition into residual garnet during melting, resulting in higher Nb/Y ratios in the liqui. Figure 8 plots Nb/Y ratios against mg-number, Nb/Zr and initial 143Nd/144Nd values for the least differentiated samples (mg-number >50) from the Deccan alkaline complexes. These indicate that: (1) Nb/Y ratios are constant for each individual complex in the most primitive samples and samples from Barmer and Mundwara contain higher values (3–4) than those for Bhuj and Amba Dongar (1–2); (2) there is a positive correlation with Nb/Zr values, the latter being a function of degree of partial melting (higher for smaller-degree melts); (3) the high Nb/Y for the samples from Barmer and Mundwara correlate with lower initial Nd isotopic composition. In general, this correlation also seems to hold true between CaO/Al2O3 ratios and initial Nd isotope compositions (Fig. 8). The data shown in Fig. 8 suggest that the basanite–nephelinites from Bhuj and Amba Dongar, and samples from Réunion were generated at similar mantle depths (and from similar mantle sources). Results from melting experiments indicate that al Piton des Neiges lavas may be derived from the parent magma composition for Réunion by fractional crystallization (augite ± olivine) at pressures of 1–5 kbar (Fisk et al., 1988). In contrast, the higher CaO/Al2O3 ratios for the melilitites from Barmer suggest an origin by small-degree partial melting at deeper levels within (plume-) metasomatized lithosphere. This interpretation is consistent with phase equilibria studies, which indicate that melilititic or nephelinitic melts are generated at pressures of 20–30 kbar (Eggler, 1974, 1978, 1989; Brey & Green, 1975, 1977; Edgar, 1987).
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plume–lithosphere interaction
In recent years, attempts at explaining the large variations in Nd and Sr isotope data typically exhibited by carbonatite complexes (of similar age) from individual alkaline volcanic provinces, such as the Kola Peninsula (Kola carbonatite mixing array, Kramm & Kogarko, 1994
) and East Africa (e.g. Bell & Blenkinsop, 1987a; Bell & Simonetti, 1996; Kalt et al., 1997) have included: (1) melt derivation from an isotopically heterogeneous sub-continental mantle; (2) mixing of HIMU and EMI mantle components; (3) plume–lithosphere interaction. In relation to the last, Bell & Simonetti (1996) proposed a two-stage model to explain the isotope variations shown by East African carbonatites which involved (1) release of metasomatizing agents with HIMU-like signatures from upwelling mantle (plume) source, which in turn metasomatize the sub-continental (EMI-like) lithosphere, and (2) variable degrees and discrete partial melting of the resulting heterogeneous, metasomatized lithosphere.
The Nd and Sr isotope data from the Deccan alkaline complexes obtained in this study are compared with the position of the East African Carbonatite Line (EACL, from Bell & Blenkinsop, 1987a), a line based on Nd and Sr isotope data from carbonatite complexes younger (0–30 Ma) than the Deccan alkaline complexes (Fig. 5). The Nd and Sr isotope data from the Deccan alkaline complexes are distinct from those of the East African carbonatites and do not plot along the EACL (Fig. 5). In addition, age correction of the EACL would drive it slightly further to the lower left in Fig. 5, and this feature furthe eliminates the need for involving HIMU and EMI mantle components (from Hart, 1988) the proposed end-members of the EACL, in the generation of the Deccan alkaline complexes. The close similarity between the isotope data from the Deccan alkaline complexes examined here and from previous studies (e.g. Basu et al., 1993) and those for present-day volcanics from Réunion supports the significant involvement of plume-type mantlein the derivation of the former. Recent isotopic investigations of the Okenyenya igneous complex (Milner & le Roex, 1996) and Damaraland lamprophyre and carbonatite occurrences (le Roex & Lanyon, 1998) both from northwestern Namibia, which are temporally and spatially associated with the Etendeka volcanic province, also argue for the involvement of the Tristan plume in their generation. Le Roex & Lanyon (1998) have proposed a similar model for the generation of lamprophyre and carbonatite magmatism at Damaraland to the one being advocated in this study. The alkaline magmatism in northwestern Namibia is thought to be the result of melting of metasomatic vein material introduced into the sub-continental lithospheric mantle by alkaline melts or fluids derived from the upwelling Tristan mantle plume at the time of continental break-up (le Roex & Lanyon, 1998).
Based on the isotope results obtained in this study, we propose that the large variations in isotope data defined by carbonatite provinces world wide may result from the interaction betwee mantle perturbations or upwellings (plume component) and continental lithosphere. The excellent correlation between the temporal distribution of carbonatites and major orogenic cycles for the past 3.0 b.y. (Woolley & Kempe, 1989; Veizer et al., 1993) lends support to this interpretation, a conclusion also stated by Bell & Simonetti (1996).
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Conclusions |
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TOPABSTRACTIntroductionAnalytical MethodsResultsDiscussionConclusionsReferences |
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The isotope results obtained from the least differentiated, silica-undersaturated samples from the complexes of Barmer, Bhuj, Mundwara and Amba Dongar show large variations attributable to the mixing of at least three distinct mantle end-members—asthenosphere (Indian MORB), (old) enriched continental lithosphere, and Réunion plume mantle. Correlations between major element chemistry and isotope ratios are also indicative of melt generation from distinct mantle sources. Furthermore, the isotope results obtained here are consistent with previous tectonic–petrogenetic models that propose an important contribution of Réunion plume-type mantle in the source of the alkaline complexes and Deccan lavas found in the north and northwestern regions of the province. This input may have been especially important during the early stages of plume interaction with the Indian sub-continental lithosphere. With time, and as the Indian sub-contine moved northwards over the Réunion plume, increased heating of continental lithosphere (an its consequent thinning) resulted in larger-degree partial melting and an increased input of lithospheric isotopic ‘signal’ in subsequent production of alkaline and tholeiitic magmatism of the Deccan igneous province.
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Acknowledgements |
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We thank Al Hofmann, director of the Max-Planck-Institut für Chemie (MPI) für Chemie (Mainz), for his support of this project and scientific input. We thank also Hans-Peter Meyer Mineralogisches Institut–Universität Heidelberg, for technical assistance in obtaining microprobe analyses. We are especially grateful to S. B. Vora, chief scientific officer, Research and Development, of the Gujarat Mineral Development Corporation (GMDC), for providing assistance in collecting samples, logistical support and accommodation during our stay in India. A. Simonetti acknowledges financial support of an NSERC postdoctoral fellowship. B. Ghaleb (GEOTOP) is thanked for help in obtaining U and Th isotope dilution analyses. Professor D. Francis and G. Keating, both from McGill University, are thanked for assistance in obtaining XRF analyses. Reviews by J. Davidson, T. Skulski and G. Wörner helped improve the quality of the manuscript. Comments on an earlier version of the manuscript by C. Gariépy and R. Stevenson (both at GEOTOP) are much appreciated. A. le Roex (University of CapeTown) is thanked for providing a copy of a manuscriptin press.
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FOOTNOTES |
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Present address: Lamont–Doherty Earth Observatory of Columbia University, Palisades, NY 10964, USA. 
* Corresponding author. Present address: Centre de recherche GEOTOP, Université du Québec à Montréal, succursale Centre-ville, CP 8888, Montréal, Québec, Canada, H3C 3P8. Telephone: (514) 987-3000, ext. 7019. Fax: (514) 987-3635. e-mail: c3204{at}er.uqam.ca
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References |
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TOPABSTRACTIntroductionAnalytical MethodsResultsDiscussionConclusionsReferences |
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