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Journal of Petrology | Volume 43 | Number 7 | Pages 1241-1257 | 2002
© Oxford University Press 2002
Indian Continental Crust Recovered from Elan Bank, Kerguelen Plateau (ODP Leg 183, Site 1137)
1DÉPARTEMENT DES SCIENCES DE LA TERRE ET DE L’ENVIRONNEMENT, UNIVERSITÉ LIBRE DE BRUXELLES, FACULTÉ DES SCIENCES, CP 160/02, AVE. F. D. ROOSEVELT, 50, BRUSSELS, B-1050, BELGIUM
2DEPARTMENT OF EARTH, ATMOSPHERIC AND PLANETARY SCIENCES, MASSACHUSETTS INSTITUTE OF TECHNOLOGY, BUILDING 54-1226, CAMBRIDGE, MA 02139, USA
Received June 13, 2001; Revised typescript accepted January 14, 2002
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ABSTRACT |
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At Site 1137 on Elan Bank of the Kerguelen Plateau, a large igneous province in the southern Indian Ocean, a fluvial, volcaniclastic, polymict conglomerate and a fluvial sandstone are intercalated with subaerially erupted tholeiitic basalt flows. Clasts recovered from the conglomerate have highly variable lithologies, including alkali basalt, rhyolite, trachyte, granitoid and gneiss. Major and trace element abundances and whole-rock isotopic data for the sandstones, the conglomerate matrix and representative clasts from the conglomerate are used to infer the origin of these diverse rock types. The gneiss clasts show an affinity to crustal rocks from India, particularly those of the Eastern Ghats Belt and its possible East Antarctic corollary, the Rayner Complex. The felsic volcanic clasts are not genetically related to the intercalated basalt flows, despite being erupted contemporaneously with these basaltic magmas. These felsic volcanic clasts probably formed from partial melting of evolved upper continental crust. The granitoid also probably formed by partial melting of continental crust and could be an intrusive equivalent of the felsic volcanic rocks. In contrast, the alkali basalt clasts have isotopic compositions that are more similar to those of the tholeiitic basalt flows recovered at Site 1137; however, these clasts are highly alkalic (tephrite to phonotephrite) and have a distinct petrogenesis from the tholeiitic basalt flow units.
KEY WORDS: geochemistry; Indian Ocean; Kerguelen Plateau; large igneous provinces; Ocean Drilling Program
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Elan Bank is a western salient of the Kerguelen Plateau, extending from between the Southern and Central Kerguelen Plateau (Fig. 1a). Drilling at Site 1137 on Elan Bank penetrated basement to a depth of 152 m and recovered 10 basement units (Fig. 1b). Site 1137 contains anomalous, non-volcanic units including a fluvially deposited volcaniclastic polymict conglomerate (Unit 6) and an overlying, finer-grained, volcaniclastic sandstone (Unit 5; Shipboard Scientific Party, 2000a
). A crystal-vitric tuff was also recovered (Unit 9), indicating that evolved and explosive volcanism occurred contemporaneously with mafic volcanism (Pringle et al., 2000; Duncan, 2002). All other basement units at Site 1137 are basalts, either emplaced into shallow sediment or subaerially erupted. These basalt flows have been shown to be contaminated by continental crust (Weis et al., 2001; Ingle et al., 2002). Although these studies of the Site 1137 basalt flows provide information about the extent and type of crustal contamination, they do not provide constraints regarding the origin of the continental crustal component. This paper focuses on the fluvial units at Site 1137 and the information that they reveal regarding their provenance and origin.
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, dredge locations. Elan Bank on the Kerguelen Plateau (location of Site 1137) is delineated by a bold rectangle. The four physiographic provinces on the Kerguelen Plateau are also labeled: SKP, Southern Kerguelen Plateau; CKP, Central Kerguelen Plateau, upon which Heard and McDonald Islands are located; Elan Bank; NKP, Northern Kerguelen Plateau, upon which the Kerguelen Archipelago is located. (b) Site 1137 downcore hardrock log (mbsf, meters below sea floor; Shipboard Scientific Party, 2000a
Clasts within the conglomerate range from pebble to cobble sized (averaging in the pebble-size range, 1·5 cm x 2 cm to 3 cm x 5 cm; Fig. 2) and have variable lithologies including (in order of decreasing abundance) alkali basalt, rhyolite, trachyte, granitoid, garnet-bearing gneiss and other metamorphic rocks (Table 1). The gneisses, sandstone and conglomerate matrix contain detrital zircons and monazites with Neoproterozoic ages and one zircon with a late Archean age (Nicolaysen et al., 2001). Therefore, it seems incontrovertible that continental crust was proximal during the formation of the fluvial conglomerate at Site 1137. Basaltic volcanism on Elan Bank occurred around 109 Ma (Pringle & Duncan, 2000). At this time, the nearest continents include India, Antarctica and Australia, and derivation of the continental material from one of these is a possibility. Another possibility is that the continental material was derived from Elan Bank itself. Recent geophysical studies of the crustal structure of Elan Bank have concluded that it may be constructed mostly of continental crust, and that basalts form only a thin veneer on top of this crust (Charvis et al., 1997; Borissova et al., 2000). Coffin et al. (2000) and Frey et al. (2000) considered Elan Bank to be a ‘crustal fragment’ potentially rifted from Antarctica, Australia or India during the breakup of Eastern Gondwana. The diversity of rock types and the large clast sizes suggest a proximal source where uplifted crust was present, such as a volcanic arc, orogenic belt or rifted margin. The conglomerate and clasts may hold important clues regarding the origin of Elan Bank.
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The subaerially erupted basalt flows at Site 1137 are divisible into a lower and upper group, separated by these anomalous fluvial units; the lower group is more extensively contaminated by continental crust (Weis et al., 2001; Ingle et al., 2002). The hiatus between the lower and upper basalt groups is not resolvable within error of the 40Ar/39Ar dating method (109·1 ± 0·3 Ma for the upper group and 109·3 ± 0·2 Ma for the lower group; Pringle & Duncan, 2000). No evidence for development of a soil horizon between the sandstone and the overlying basalt flow unit was found during shipboard observations, but rare burrows are present and imply that eruption of the overlying basalt flow did not immediately follow deposition of the sandstone (Shipboard Scientific Party, 2000a). Sanidines within one rhyolite clast give an 40Ar/39Ar age of 109·2 ± 0·2 Ma, contemporaneous with basalt eruption (Pringle et al., 2000). The alkali basalt clasts, however, have not been dated.
The occurrence of a fluvial conglomerate and sandstone that formed during a period of quiescence within a large igneous province setting is unusual and interesting. We use major and trace element abundances and radiogenic (Sr, Nd and Pb) isotopic compositions of the sandstone, conglomerate matrix and representative clasts within the conglomerate to constrain the origin and provenance of these anomalous units within the Kerguelen Plateau basement. We examine the relative contributions from mantle sources and continental crust in the petrogenesis of the mafic and felsic volcanic clasts to decipher their relationship to the contemporaneous tholeiitic basaltic volcanism. Finally, using the whole-rock isotope data for the gneiss clasts, we build upon the study by Nicolaysen et al. (2001) and place important constraints on their provenance.
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ANALYTICAL TECHNIQUES |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Fourteen samples including clasts, a fine-grained conglomerate matrix and sandstones were selected for major and trace element chemistry and Sr, Nd and Pb isotope analysis on the basis of (1) a complete representation of the rock types present, (2) sample diversity, (3) a lack of obvious alteration and/or weathering and (4) sample homogeneity (particularly important for the sandstones and conglomerate matrix). Rock powders were analyzed by X-ray fluorescence (XRF) methods (University of Massachusetts) for major and some trace elements, and loss on ignition (LOI); other trace elements and the rare earth elements (REE) were analyzed by ICP-MS at Acme Analytical Laboratories in Vancouver, Canada (Table 2). All XRF data reported for major elements are averages of duplicate analyses. Details of accuracy and precision for XRF methods have been reported by Rhodes (1996)
. A duplicate analysis of a trachyte clast indicates reproducibility (in relative percent) of ICP-MS measurements as follows: Cu, Cs, Ce, Yb, Lu, Hf, Ta and Th 
±2; La, Pr, Nd, Gd, Er and Tm ±3; Sm, Eu and U ±4; Co ±6.
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All samples were also analyzed for Sr, Nd and Pb isotopic compositions (Table 3). Both leached and unleached mafic volcanic clasts were analyzed using 250–300 mg of rock powder dissolved in a Teflon vial [leaching methods follow those of Mahoney (1987) with some modifications by Weis & Frey (1991)]. Because of the possibility of refractory minerals such as garnet, sphene and zircon in all non-mafic rocks, these samples were dissolved in Teflon-lined steel bombs for 4 days at 180°C. Chemical procedures for dissolution follow those of Weis et al. (1987) and Weis & Frey (1991). All radiogenic isotopic compositions were determined with a VG Elemental Sector 54 multicollector thermal ionization mass spectrometer at the Université Libre de Bruxelles. Sr and Nd isotopic compositions were measured on single Ta and triple Re–Ta filaments, respectively, in dynamic mode. Sr isotopic ratios were normalized to 86Sr/88Sr = 0·1194 and Nd isotopic data were normalized using 146Nd/144Nd = 0·7219. The average 87Sr/86Sr value for the NBS 987 Sr standard was 0·710279 ± 7 on the basis of 12 analyses. Nd standard Rennes values for 143Nd/144Nd were 0·511967 ± 9 for 12 runs. Pb isotopic ratios were measured on single Re filaments using the H3PO4–silica gel technique. All Pb isotopic ratios were corrected by 1·2
per a.m.u. to account for mass fractionation on the filament, based on 14 analyses of NBS 981 Pb standard run at a temperature between 1090°C and 1200°C. Samples were run between 1050°C and 1200°C.
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RESULTS |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Major element chemistry
The two mafic volcanic clasts have extremely high total alkali (Na2O + K2O) contents of 7·1–9·3 wt % and are classified as a tephrite and a phonotephrite (Fig. 3). The felsic volcanic clasts plot as either trachytes or rhyolites and the data tightly cluster for each rock type. The sandstones are rich in silica, and fall into the same two groups represented by the felsic clasts. The conglomerate matrix falls between these two groups. Alteration has not strongly affected the felsic volcanic clasts; all have LOI
1·11 wt %. The granitoid and the gneiss clasts are very rich in silica (>70 wt %).
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Trace element chemistry
Primitive mantle-normalized incompatible trace element variation diagrams show similar levels of enrichment for all the volcanic clast samples (Fig. 4). Negative Sr anomalies are present in all volcanic clasts and negative Ti anomalies are present in the felsic volcanic clasts. These anomalies are the most pronounced in the rhyolites, which is consistent with their highly fractionated compositions. A negative Ti anomaly is typical of continental crust generated in a subduction zone environment at any time in the past and is inherited during any subsequent modification of the crust (e.g. Green & Pearson, 1986). A negative Sr anomaly may form in one of two ways: (1) by anatectic partial melting of the lower continental crust, leaving plagioclase as a residual phase; (2) by extensive fractionation of plagioclase during crystallization.
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The alkali basalt clasts also show high levels of enrichment in the incompatible trace elements, similar to that of the felsic volcanic clasts (Fig. 4). Compared with the felsic volcanic clasts, the alkali basalts are somewhat less enriched in Rb and Th and have slightly positive Ti anomalies. In comparison with the basalt flows that make up the majority of the units at Site 1137, the alkali basalt clasts are much more incompatible element enriched overall, and have negative anomalies in Sr and Ti.
The non-volcanic rocks also have high abundances of incompatible trace elements but their mantle-normalized patterns show some notable differences when compared with the volcanic clasts (Fig. 4). Both gneiss clasts have a pronounced Nb–Ta trough and a minor negative Zr anomaly. The gneisses, granitoid, sandstones and conglomerate matrix all have negative Sr anomalies, similar to those observed in the rhyolites and trachytes; this negative Sr anomaly is most pronounced in the granitoid. The gneisses, granitoid, sandstones and conglomerate matrix are highly enriched in Rb and Th (up to 400 times primitive mantle).
The primitive mantle-normalized trace element patterns of the felsic volcanic rocks and granitoid are broadly similar to those of the garnet–biotite gneiss, except that the latter has a much more pronounced negative Nb–Ta trough (Fig. 4). The continental crust is also strongly depleted in Nb and Ta (Rudnick & Fountain, 1995; Plank & Langmuir, 1998; Barth et al., 2000) and the pattern of the gneiss clasts more closely matches that of average upper continental crust than lower continental crust.
The volcanic clasts have steep light rare earth element (LREE) chondrite-normalized patterns with somewhat more shallow heavy REE (HREE) patterns (Fig. 5). The rhyolites are the most enriched in the LREE, with a pronounced negative Eu anomaly. The trachytes are somewhat less enriched than the rhyolites in the LREE but have a slight positive Eu anomaly and are more enriched in the HREE. The alkali basalt clasts share the same level of enrichment in the REE as the trachytes, but have a very minor negative Eu anomaly.
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The non-volcanic gneiss and granitoid clasts, sandstones and conglomerate matrix have REE patterns similar to those of the volcanic clasts (Fig. 5). All non-volcanic samples have a negative Eu anomaly and this anomaly is the most pronounced in the granitoid. With the exception of Eu, the granitoid is the most enriched in total REE and the gneisses the least rich. In comparison, the sandstones and conglomerate have intermediate levels of total REE enrichment.
Isotopic results
Strontium isotope data for the felsic volcanic clasts form a well-defined linear array that can be interpreted as an isochron with an age of 112·8 ± 3 Ma (2
, MSWD = 2·3) and an initial 87Sr/86Sr value of 0·71009 ± 14 (Fig. 6). This age agrees well with a 40Ar/39Ar age on sanidine phenocrysts from a rhyolite clast of 109·2 ± 0·2 Ma (Pringle et al., 2000). One rhyolite sample was excluded from the regression (and its distance from the isochron is outside analytical error); its inclusion lowers the age to 108·7 ± 3 Ma, which is within error of the Pringle et al. (2000) age. However, this sample was analyzed for Rb and Sr on the shipboard XRF system and the high Rb/Sr ratio for the rhyolite clasts makes the data quality very important (minor errors in the measurements will strongly affect the age-corrected isotopic values); data determined by the shipboard XRF system may be less accurate.
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Strontium and Nd isotopic compositions are age-corrected to 109 Ma [the age of felsic and basaltic volcanism at Site 1137; Pringle & Duncan (2000); Pringle et al. (2000)] for the clasts, sandstones and conglomerate matrix so that they can be compared with data for the rest of the Kerguelen Plateau. A diagram of 
Nd(T) vs (87Sr/86Sr)T shows that the felsic volcanic clasts are distinct compared with the rest of the Kerguelen system (Fig. 7a). The felsic volcanic clasts extend to more radiogenic (87Sr/86Sr)T for a comparable Nd(T) than even the most contaminated basalts on the Kerguelen Plateau, Site 738 (Mahoney et al., 1995). Interestingly, the alkali basalt clasts fall near the more contaminated Site 1137 lower basalt flows and the Rajmahal Traps Group 2 lavas, a potential on-land expression of Kerguelen plume activity (Storey et al., 1992; Mahoney et al., 1995; Ingle et al., 2002).
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) from a felsic volcanic unit also at Site 1137 serve as a good estimate of the maximum possible correction as a result of in situ decay for the felsic volcanic clasts (S. Ingle, unpublished data, 2001). The leached sanidine Pb isotope composition is inferred to represent common Pb in the magma source at the time of crystallization, as the leaching procedure removes Pb not incorporated in the sanidine crystal lattice (Weis & Deutsch, 1984
Measured Pb isotopic ratios on the sandstones, conglomerate matrix, granitoid and felsic volcanic clasts are very homogeneous for the 12 samples analyzed, 206Pb/204Pb = 18·572 (SD = ±0·027), 207Pb/204Pb = 15·826 (SD = ±0·009, within analytical error), and 208Pb/204Pb = 39·882 (SD = ±0·077). These values are significantly higher than those found in the basalt flows at Site 1137 (Weis et al., 2001; Ingle et al., 2002) and are much more radiogenic, in terms of 207Pb/204Pb and 208Pb/204Pb, than any other rocks studied in the Kerguelen system (Fig. 7b). However, because of the fractionated nature of these rocks and their high concentrations of U and Th in comparison with basaltic rocks in the Kerguelen system, it is possible that in situ decay has elevated their Pb isotopic ratios. Leached sanidine phenocrysts from a felsic tuff recovered deeper in the Site 1137 core may provide a good indication of the maximum age-decay correction for the felsic volcanic clasts, as this tuff has trace element and isotopic compositions similar to those of the felsic volcanic clasts (S. Ingle, unpublished data, 2001). The difference between the Pb isotopic compositions of the felsic volcanic clasts and those of the leached sanidines from the tuff is not statistically significant (Fig. 7b), and suggests that the felsic volcanism at Site 1137 may not be related to the basaltic volcanism.
Leached and unleached aliquots of the alkali basalt clasts were analyzed for Sr, Nd and Pb isotopic compositions (Table 3). For each isotope ratio determined, the value for the leached sample is lower than that of the unleached sample, indicating that the leaching procedure is highly effective. In the Pb isotopic results, however, the difference is substantial. The results for the 36R-1-UL and 36R-2-UL samples are within error and the same holds true for the leached results. The leached samples, 36R-1-L and 36R-2-L, plot near the Site 1137 basalt flows and well within the field defined by other Kerguelen Plateau basalts. The unleached samples have Pb isotopic compositions intermediate between the basalt flows and the felsic volcanic clasts, suggesting the fluids that altered the alkali basalts may have been, in part, derived from the felsic magmas.
The gneiss clasts are isotopically distinct from all other Site 1137 samples. This is not surprising considering that reported ages on biotites from the gneiss clasts record a peak metamorphic event at 
550 Ma (Nicolaysen et al., 2001). Therefore, they have had a vastly different history than the 109 Ma felsic clasts and basalt flows (Pringle & Duncan, 2000; Pringle et al., 2000).
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Origin of the felsic volcanic clasts
The felsic volcanic clasts clearly have a common provenance, as demonstrated by their comparable trace element abundances and Sr, Nd and Pb isotopic ratios. The relationship between the felsic volcanic clasts and the tholeiitic basalt flows at Site 1137 is less clear. There are two processes that could result in the formation of these felsic volcanic rocks. One possibility is by extensive fractional crystallization of mafic magmas coupled with significant assimilation of continental crust (AFC processes). However, this seems an unlikely case for several reasons. First, the common initial 87Sr/86Sr and the formation of a well-defined isochron would require derivation of the felsic volcanic rocks from a homogeneous magma; that is, the assimilate would have to be extremely well mixed into the magma. Second, these initial 87Sr/86Sr isotopic ratios for the felsic volcanic clasts are dissimilar to, and much higher than, those of the tholeiitic basalt flows (and alkali basalt clasts). Third, assimilation of continental crust by basaltic magmas is unlikely to affect strongly the initial Sr isotopic ratio of basaltic magmas because Sr concentrations are normally much lower in the continental crust than in mantle-derived magmas (Taylor, 1980
). In fact, these felsic volcanic clasts have significantly lower Sr concentrations than the basalt lavas (200 ppm in the trachytes, 40 ppm in the rhyolites and 400–560 ppm in the basalts; Ingle et al., 2002).
An origin by partial melting of thinned continental crust is an alternative possibility to extreme AFC processes. If all the felsic volcanic clasts were generated by partial melting of an isotopically enriched source, such as upper continental crust, initial isotopic ratios would be homogeneous for magmas derived from the melts of the same crust. The trace element compositions for the granitoid are most comparable with those compositions typical of within-plate granites, which generally result from partial melting of attenuated continental crust that is hot and undergoing stretching (Pearce et al., 1984). The Elan Bank salient has geophysical characteristics that suggest it is dominantly continental crust with only thin veneers of basalt flows on top (Charvis et al., 1997; Borissova et al., 2000; Gladczenko & Coffin, 2000). The presence of the gneiss clasts with their indisputable continental origin confirms the local existence of continental crust (Coffin et al., 2000; Frey et al., 2000; Nicolaysen et al., 2001). Derivation of the felsic volcanic clasts by partial melting of continental crust is supported by both the circumstantial evidence (the presence of continental crust and heat available for melting this crust in the form of the Kerguelen plume) and the data stipulations (an isotopically homogeneous source that is very different from that recognized in all basaltic volcanism at Site 1137).
Origin of the alkali basalt clasts
The origin of the alkali basalt clasts remains somewhat enigmatic. The Site 1137 basalt flows cannot be parental to the alkali basalt clasts, as the clasts are significantly more alkalic than the basalt flows. In addition, the alkali basalt clasts are silica undersaturated and cannot have been the parental magmas for the evolved, silica-saturated felsic clasts that also occur within the conglomerate. In terms of trace elements and REE, the alkali basalt clasts are extremely enriched in the large ion lithophile elements and LREE, a similar level of enrichment to that seen in the felsic volcanic clasts. However, the Sr, Nd and Pb isotope compositions of the alkalic basalts are similar to those of the Site 1137 basalt flows. If the parental magmas for these clasts were generated beneath thick continental lithosphere as the initial Kerguelen plume impinged at the base of the Indian subcontinent, then alkali basalts, strongly enriched in incompatible trace elements and exhibiting plume-like isotopic characteristics, might be expected. Storey et al. (1992) suggested such an origin for alkalic volcanic rocks on the Indian margin and attributed the isotopic signature to that of the subcontinental mantle lithosphere. In fact, alkalic volcanism is often the precursor to tholeiitic continental flood basalt volcanism (Wooden et al., 1993; Baksi, 1995). It remains debatable whether the isotopic signature in ‘precursor’ alkalic volcanism results from melting the underlying subcontinental mantle lithosphere (e.g. Hawkesworth et al., 1990; Abraham et al., 2001) or from melts derived directly from the plume (e.g. Arndt & Christensen, 1992). However, these clasts have isotopic compositions well within the range of tholeiitic basalts considered to be derived from the Kerguelen plume (e.g. Ingle et al., 2002; Neal et al., 2002).
Provenance of the gneiss clasts and sedimentary rocks
The provenance of the gneiss clasts has been discussed by Nicolaysen et al. (2001). They reported Neoproterozoic U–Pb zircon ages for detrital zircons and monazites and one late Archean U–Pb zircon age from the gneiss clasts, sandstone and conglomerate matrix, and suggested a local source (an uplifted part of Elan Bank). Elan Bank’s continental crust-like basement originally must have evolved alongside one of the three juxtaposed continents (Australia, Antarctica and India; Fig. 8a) that separated shortly before the first expressions of Kerguelen plume activity occurred at 119 Ma (Duncan, 2002), isolating Elan Bank as a microcontinent. The Elan Bank microcontinent may then be treated as an exotic terrane, within the Kerguelen Plateau environment, and investigated by combining the detrital zircon study of Nicolaysen et al. (2001) with our whole-rock isotope data.
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] and 550 Ma [the age of thermal metamorphic overprint; Nicolaysen et al. (2001)
]. Data for the four domains of the Eastern Ghats Belt are from Rickers et al. (2001)
A Paleoproterozoic protolith, or a mixture of older, Archean crust with more juvenile, mantle-derived material at some later time is suggested by the Depleted Mantle model ages (TDM) of the gneiss clasts, 2100 Ma. The Australian Yilgarn Craton is generally characterized by TDM >3000 Ma and little late Archean or Paleoproterozoic material would have been available; this margin shows no evidence for significant addition of juvenile material during the Proterozoic (Bosch et al., 1996; Yeats et al., 1999; Mueller & McNaughton, 2000). However, the conjugate margins of East Antarctica and eastern India have coalesced several times and undergone extensive reworking and/or additions of juvenile material during the Proterozoic (e.g. Brandon & Meen, 1995; Unrug, 1997; Young et al., 1997; Zhao et al., 1997; Fitzsimons, 2000; Piper, 2000; Krause et al., 2001). The Eastern Ghats Belt of eastern India and the Rayner Complex of East Antarctica have similar isotopic signatures and may be equivalent terranes (Rickers et al., 2001). Rickers et al. (2001) recently determined the whole-rock Sr and Nd isotope and Pb isotope compositions of leached feldspars of rocks from the Eastern Ghats Belt in eastern India. They divided the belt into four domains on the basis of their isotopic characteristics and lithology. In an Nd vs 87Sr/86Sr diagram, the gneiss clasts fall in the field defined by Domain 2 of the Eastern Ghats Belt of eastern India (Fig. 8b). The gneiss clasts also plot in the Eastern Ghats Domain 2 field in a crustal evolution diagram of DePaolo (1981; Fig. 8c). The gneiss clasts and the leached sanidines (from the Unit 9 tuff) in a diagram of 207Pb/204Pb vs 206Pb/204Pb (Fig. 8d), again fall in the field defined by Domain 2. The position of the gneiss clasts in this diagram is well above the upper continental crust evolution curve as defined by Doe & Zartman (1979), and a source that had a long-term residence in the continental crust and a complex history of Th and U enrichment is required. Thus, the Elan Bank microcontinent could have originated along with the Eastern Ghats and Rayner Complex as a crustal fragment that records the coupled interactions between these two terranes. The sandstones have younger TDM values (1300 Ma) and this probably reflects their mixture of older continental material, like the gneiss, with younger, mantle-derived material, such as the tholeiitic basalt flows.
Tectonic implications for the Kerguelen plume
It is now clear that some continental material was incorporated at a shallow level in the early construction of the Kerguelen Plateau. When and how was this continental material isolated from the conjugate margins of eastern India and/or Antarctica? Coffin et al. (2002) have suggested that the Elan Bank microcontinent evolved alongside the eastern Indian margin until just after 119 Ma, the age of earliest Kerguelen Plateau construction (Duncan, 2002), but that Elan Bank was separated from India by the time basaltic volcanism was active at Site 1137. However, Kent et al. (2002) have pointed out that plate tectonic reconstructions for ages greater than 93 Ma in the Indian Ocean are subject to large uncertainties and the timing of the separation of Elan Bank cannot be well constrained.
The Kerguelen plume may or may not have played a causal role in the breakup of Antarctica, Australia and India, but it probably fragmented the eastern Indian margin and isolated some continental crust. Müller et al. (2001) proposed that a thermal anomaly, created when a mantle plume is emplaced beneath a young, continental rifted margin, could cause a ridge-jump; this process might serve as a mechanism in the isolation of microcontinents. Those workers noted that isolated microcontinents may be accreted later to continental margins during subduction. Our study of rocks of continental origin from a fluvial conglomerate at Elan Bank strengthens geophysical arguments that a microcontinent has been incorporated into the structure of an oceanic plateau. It has been proposed that oceanic plateaux may make up a significant contribution to the total crustal growth budget (e.g. Ben-Avraham et al., 1981; Stein & Hofmann, 1994; Abbott & Mooney, 1995; Saunders et al., 1996). However, if some plateaux are not entirely juvenile in nature, this could significantly complicate crustal growth models.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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Drilling on Elan Bank of the Kerguelen Plateau recovered rocks of unequivocal continental origin, particularly garnet–biotite gneiss. The gneiss clasts were derived from a Paleoproterozoic terrane that probably originated from east India. The most feasible candidate for this terrane is the Eastern Ghats Belt, which has a long-term shared history with the Rayner Complex of East Antarctica. Our study also provides evidence that felsic volcanism, contemporaneous with basaltic volcanism, resulted from partial melting of upper, evolved continental crust. Alkali basalts clasts also recovered from the conglomerate have an uncertain origin, but are certainly distinct petrogenetically from the tholeiitic basalt flows at Site 1137 on the Elan Bank.
Previously, most geochemical variations in Kerguelen Plateau basalts were largely attributed to mantle processes. Our current study provides evidence that rocks of continental origin are present at shallow levels in the Cretaceous Kerguelen Plateau, as evidenced by pebbles within a fluvial conglomerate. These rocks have disparate isotopic characteristics. This continental material may have been isolated as fragments of crust of variable origin during interaction of the Kerguelen plume with the continental margins that bordered the embryonic Indian Ocean. The Kerguelen plume may therefore have been responsible for the distribution of isotopically heterogeneous continental material into the young Indian Ocean lithosphere.
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ACKNOWLEDGEMENTS |
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We thank J. S. Scoates and M. F. Coffin for discussions that improved some of the ideas presented here. K. Mezger, R. Pankhurst and R. W. Kent are greatly thanked for their insightful reviews. M. Rhodes and the University of Massachusetts are thanked for X-ray fluorescence analyses. The first author is supported by, and the work presented here is funded by an ARC grant (98/03-233) from the Communauté Française de Belgique. The Fonds National de la Recherche Scientifique funds Belgian membership in the Ocean Drilling Program (ODP) and supported the second author’s participation in ODP Leg 183 (FNRS grant 2.4579.99). This research used samples and data provided by the ODP. The ODP is sponsored by the US National Science Foundation and participating countries under management of Joint Oceanographic Institutions, Inc. Additional funding for this research was provided by the US Science Support Program.
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FOOTNOTES |
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*Corresponding author. Telephone: 32-2-650.22.40. Fax: 32-2-650.37.48. E-mail: single{at}ulb.ac.be
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REFERENCES |
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TOPABSTRACTINTRODUCTIONANALYTICAL TECHNIQUESRESULTSDISCUSSIONCONCLUSIONSREFERENCES |
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