Journal of Petrology Volume 41 Number 7 Pages 951-966 2000
© Oxford University Press 2000
Palaeogene Continental to Oceanic Magmatism on the SE Greenland Continental Margin at 63°N: a Review of the Results of Ocean Drilling Program Legs 152 and 163
1DEPARTMENT OF GEOLOGY AND GEOPHYSICS, UNIVERSITY OF EDINBURGH, WEST MAINS ROAD, EDINBURGH EH9 3JW, UK
2GEOLOGICAL SURVEY OF DENMARK AND GREENLAND, THORAVEJ 8, DK-2400 COPENHAGEN NV, DENMARK
3DEPARTMENT OF GEOLOGY, UNIVERSITY OF LEICESTER, LEICESTER LE1 7RH, UK
4SCOTTISH UNIVERSITIES ENVIRONMENTAL RESEARCH CENTRE, EAST KILBRIDE G75 0QF, UK
5NERC ISOTOPE GEOSCIENCES LABORATORY, KEYWORTH, NOTTINGHAM NG12 5GG, UK
Received September 27, 1999; Revised typescript accepted February 18, 2000
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONTHE ODP TRANSECT AT...MAGMATIC DEVELOPMENT OF THE...CONCLUSIONSREFERENCES |
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Drilling along a 63°N transect off SE Greenland during Ocean Drilling Program (ODP) Legs 152 and 163 recovered a succession of volcanic rocks representing all stages in the break-up of the volcanic rifted margin. The rocks range from pre-break-up continental tholeiitic flood basalt, through syn-break-up picrite, to truly oceanic basalt forming the main part of the seaward-dipping reflector sequence (SDRS). All the lava flows recovered from the transect were erupted in a subaerial environment. 40Ar–39Ar dating shows that the earliest magmas were erupted at
61 Ma and has confirmed that the main part of the SDRS was erupted during C24r (56–53 Ma) following continental break-up. Magma represented by the pre-break-up lava flows was stored in crustal reservoirs where it evolved by fractional crystallization and assimilation of continental crust. Trace element and radiogenic isotope data show that the contaminant changed, through time, from lower-crustal granulite to a mixture of granulite and amphibolite, suggesting storage of magma at progressively shallower levels in the crust. The degree of contamination declined rapidly as break-up proceeded, and the youngest rocks sampled in the transect are uncontaminated by continental basement. Variation of, for example, Sc/Zr and Sm/Lu through the succession suggests a shallowing of the top of the mantle melting zone, accompanied by an increase in the average degree of melting with time from 4% to 12%. These modest degrees of melting imply mantle temperatures only 100°C hotter than normal upper mantle. Upwelling mantle must therefore have been fed dynamically to the melt zone to generate the igneous crust of 18 km thickness deduced from seismic and gravity studies. N-MORB-like magmas dominated the earliest part of the succession although a few flows of ‘Icelandic’ basalt were erupted in the pre-break-up phase. In contrast, the post-break-up magmas had an Icelandic mantle source. This suggests that the developing head of the ancestral Iceland plume was compositionally zoned, with a core of Icelandic mantle surrounded by a thick outer zone of hot, depleted upper mantle. KEY WORDS: flood basalt; geochemistry; Greenland; Palaeogene; Sr–Nd–Pb isotopes
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONTHE ODP TRANSECT AT...MAGMATIC DEVELOPMENT OF THE...CONCLUSIONSREFERENCES |
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Rifting along the North Atlantic continental margin during the Early Tertiary was preceded and accompanied by an intense phase of basaltic magmatism, now represented by onshore tholeiitic flood basalt outcrops and offshore seaward-dipping reflector sequences (SDRSs). The latter mark the transition from continental volcanism to normal ocean-floor volcanism. SDRS on the SE Greenland margin were mostly erupted during magnetic chron C24r (56–53 Ma; Berggren et al., 1995
) and are >6 km thick (Larsen & Jakobsdóttir, 1988). Seismic and gravity data from 63°N on the SE Greenland margin (H. C. Larsen et al., 1998) define a continent-to-ocean transition of 40 km width in which the continental crust thins from 28 km to almost zero. Thinning of the continental crust was accompanied by the addition of 18 km of new igneous crust composed of SDRS lava flows, feeder dykes and a lower plutonic layer 8 km thick (H. C. Larsen et al., 1998). These enormous volumes of magma require the presence of anomalously hot mantle at the time of rifting and continental break-up (White & McKenzie, 1989). In the case of the North Atlantic, there is a clear genetic link between the excess magmatism represented by the SRDS and the presence of a mantle plume now situated beneath Iceland.
The SE Greenland margin provides an excellent and arguably unique area in which to investigate the link between continental rifting, large igneous provinces and mantle plumes. The Greenland continental margin south of Kangerlussuaq (Fig. 1) is tectonically uncomplicated and is linked across the North Atlantic to its conjugate margin by the aseismic Greenland–Iceland–Faeroes ridge. Thus all magmatic stages in the development of the province, from the earliest manifestations of volcanism on the Greenland margin to present-day magmatism in Iceland, are available for study. One of the objectives of Ocean Drilling Program (ODP) Legs 152 and 163 was to sample a transect across the SE Greenland SDRS at 63°N and to investigate the timing and character of the transition from continental to oceanic magmatism. Two previous Deep Sea Drilling Program (DSDP) and ODP Legs have sampled other portions of the North Atlantic SDRS. Leg 81 sampled the southern tip of the SDRS on Hatton Bank (Roberts et al., 1984) and Leg 104 sampled the Vøring Plateau on the Norwegian continental margin (Eldholm et al., 1987; Viereck et al., 1988, 1989). The location of all the drill sites is shown in Fig. 1.
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A full description of the core recovered during Legs 152 and 163 has been published in the ODP Initial Reports (Larsen et al., 1994; Duncan et al., 1996). Detailed reports of individual scientific studies resulting from the two Legs can be found in the respective Scientific Results volumes (Saunders et al., 1998b; H. C. Larsen et al., 1999). The purpose of the present paper is to provide an overview of the magmatic development of the SE Greenland continental margin, based on the results of Legs 152 and 163. Chemical and isotopic data discussed in this paper are available on the Journal of Petrology web site, at http://www.petrology.oupjournals.org.
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THE ODP TRANSECT AT 63°N |
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Figure 2 is an interpreted seismic section across the SE Greenland margin at 63°N showing the location of holes drilled during Legs 152 and 163. Two further sites are not included on this section. Site 919 lies farther to the east and comprises three shallow holes drilled to collect sediment samples only. Drilling at Site 988, at 66°N, penetrated a thick unit of fresh basalt overlying the thoroughly altered rubbly top of an underlying flow. Elemental and isotopic data for the basalt recovered at Site 988 have been given by Saunders et al. (1999)
and will not be discussed further here because the site lies 300 km from the main transect (Fig. 1). Five holes along the 63°N transect reached igneous basement, and data from these will form the basis of this paper.
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Drilling at Site 917, 50 km from the Greenland coast, was by far the most successful part of the transect. Hole 917A penetrated the feather-edge of the SDRS all the way to a metasedimentary basement, and a 779 m volcanic sequence comprising at least 91 flow units (Fig. 3) was recovered although some of the lowest part of the succession was missing because of normal faulting (Fig. 2). Lavas from Site 917 can be divided into three series on the basis of petrography. The Lower Series lavas vary from picrite and olivine basalt to more evolved quartz tholeiite. A pronounced shift to more evolved compositions marks the beginning of the overlying Middle Series, which is composed of dacite lava flows, welded and unwelded tuffs, and evolved quartz tholeiite flows. The dacite units consist of mixed rocks containing abundant partly digested xenoliths of basalt, some with lobate margins suggesting mixing between basaltic and acid magma. A 67 cm sedimentary unit composed of volcaniclastic conglomerate, sandstone and siltstone marks the top of the Middle Series. The time interval represented by this unit is indeterminate but it coincides with a shift to much more basic lava flows (Fig. 3). The Upper Series consists of thin, dominantly pahoehoe flows of olivine basalt and picrite.
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Site 915, 8 km farther offshore, yielded a single basalt lava flow from a stratigraphic level only 100–200 m above the top of the Upper Series at Site 917. This single flow was much less magnesian than any of the Upper Series flows and was very similar in composition to flows of oceanic basalt sampled at Site 918, 72 km farther offshore in the centre of the SDRS. Drilling the unsampled interval between Sites 917 and 915 was a major objective of Leg 163, and recovery at Site 990 (close to Site 915) was more successful. Thirteen basaltic lava flows with a total thickness of 130·8 m, and with compositions similar to the flow at Site 915, were sampled at this site. Unfortunately, drilling was terminated before lithological contiguity could be established and this critical part of the transect remains unsampled.
At Site 918, a 120·7 m section of truly oceanic basalt (18 flow units) was recovered. The flows were overlain by shallow-marine sediments apparently intruded by a sheet of fresh olivine basalt. The sheet is probably a sill but, as the actual contacts were not recovered, this cannot be established with certainty.
Site 989, located 43 km off the Greenland coast on the extreme western edge of the SDRS, was drilled to determine the nature of the pre-eruptive unconformity and the underlying basement rocks. From the seismic profiles, it was predicted that the basalt at this site would be older than that recovered at Site 917, but subsequent 40Ar–39Ar studies showed that it was about 4 my younger than the Lower and Middle Series (Tegner & Duncan, 1999). It is comparable in composition and age with the basalt at Site 990. Two basaltic flow units were recovered at Site 989. The upper unit, composed of vesicular aphyric basalt and at least 69 m thick, was interpreted as a compound lava flow consisting of several lobes 0·1–10 m thick (Duncan et al., 1996). Lesher et al. (1999) have used experimental studies and textural evidence to suggest that the flow was emplaced rapidly and close to its eruptive vent. The lower unit is at least 11 m thick and composed of slightly to moderately plagioclase–augite–olivine-phyric basalt.
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MAGMATIC DEVELOPMENT OF THE CONTINENTAL MARGIN |
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Age and character of volcanism
Determining the age of the SDRS, and hence the rate of magma production, was a major objective of Legs 152 and 163, but the state of alteration of most of the samples recovered has made this difficult. Figure 4 summarizes the SDRS stratigraphy, the magnetic polarity (Duncan et al., 1996
; Vandamme & Ali, 1998) and 40Ar–39Ar ages (Sinton & Duncan, 1998; Tegner & Duncan, 1999) obtained from the drill cores. Several important conclusions can be drawn from the results of these studies. First, volcanism on the margin pre-dated sea-floor spreading by at least 5 my. Second, the recognition of normally magnetized lava flows at Sites 990 (overlying flows with reversed polarity) and 989, coupled with reliable 40Ar–39Ar dates at both sites, suggests that the oldest flows with the composition of those in the main part of the SDRS were erupted during C25 (57–56 Ma). And, third, the dating of the sill at Site 918 (52 Ma) effectively constrains the age of the main part of the SDRS to C24r (56–53 Ma). The relatively young age of the lava flows recovered at Site 989, on the feather-edge of the SDRS, was surprising. The emplacement of these flows close to their eruption site indicates that the eruption of lava flows of the main phase of SDRS magmatism extended onto the continental shelf.
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All the volcanic rocks recovered from the transect appear to have been erupted subaerially. Both aa and pahoehoe lava flows, often with reddened and oxidized tops, can be recognized in the drill cores. Red soil horizons were found at Site 917 (Fig. 3). No pillow-like structures were found, but brecciated and quenched bases to some of the flows, especially in the lower parts of the succession, suggest emplacement into shallow water or onto a wet land surface (Saunders et al., 1998).
The stratigraphic succession that has been established from the drill cores (Fig. 4) can be used to assess changes in the character of magmatism as break-up proceeded to sea-floor spreading. Figure 5 shows the variation of several compositional parameters, with the data stacked to reflect their relative stratigraphic position. The vertical separation of data points on these plots reflects the actual stratigraphic thickness at individual sites but the lateral distance between sites varies enormously. Sites 915/990 and 918, for example, are separated by 72 km. The two dykes sampled in the drill cores at Sites 917 and 990 are placed arbitrarily below the Site 918 data.
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Compositional variation through the SDRS, coupled with 40Ar–39Ar dating, has been used to divide the lava flows into pre-, syn- and post-break-up successions (Fig. 4). The earliest lava flows [Site 917 Lower (LS) and Middle Series (MS)] comprise a pre-break-up succession varying in composition from picrite to dacite. Peaks and troughs in mg-number, superimposed on a general trend towards more evolved compositions (Fig. 5), suggest evolution of the magma in periodically recharged magma reservoirs with a dwindling supply of parental magma (Fitton et al., 1995). At least two magma sources are represented in the Lower Series with respectively low and high Nb/Zr. The six high-Nb/Zr flows are concentrated towards the top, intercalated with the more common low-Nb/Zr flows. The two groups both have basic and evolved members, implying that at least two independent magma reservoirs were feeding the SDRS during this early phase.
A sandstone unit of 67 cm thickness (Fig. 4) marks a dramatic change from the thick silicic flows of the Middle Series to a sequence of 34 thin flow units of olivine basalt and picrite forming the Upper Series (Fig. 5). The time interval represented by this sandstone is unknown because it has not been possible to date any of the Upper Series samples. Some of the more magnesian flows are strongly olivine-phyric and have clearly accumulated olivine. However, the high magnesium contents of the aphyric flows (up to 18 wt % MgO with mg-number = 77), the high forsterite contents (up to Fo92; Demant, 1998) of the olivine phenocrysts, the skeletal morphology of some of the phenocrysts, and the composition of Cr-spinel grains (Allan et al., 1999) suggest that many of the units represent high-magnesium liquids close to primary magma in composition (Thy et al., 1998). The magma could not have been stored in large reservoirs for long periods, as was inferred for the Lower and Middle Series, but was probably erupted rapidly through a system of fissures that penetrated the entire lithosphere. This change in the style of magmatism may mark the final stage of break-up of the Greenland margin (Larsen et al., 1994; Fitton et al., 1995).
Compositional variation within the flows of the Upper Series is dominated by separation and accumulation of olivine, although there is some slight variation in incompatible-element ratios (Fig. 5). The flows become significantly more depleted (e.g. lower Nb/Zr) up the succession and a group of five flows near the base of the Upper Series have distinctly higher contents of incompatible elements in relation to their MgO contents than do the other flows in the series (L. M. Larsen et al., 1998). Flows near the top of the succession are more depleted in light rare earth elements (LREE) than are flows from the base of the succession (Fitton et al., 1998b). This variation may represent an increase, with time, in degree of melting, or the effects of progressive depletion of the mantle source supplying the volcanic system, or both.
Between the top of the Upper Series at Site 917 and the base of the drill hole at Site 990, the basalt composition changes dramatically once again (Fig. 5). The basalts at Site 915/990, 989 and 918 (apart from the sill) are remarkably uniform in composition, with mg-number between 50 and 61 and MgO between 6·8 and 8·4 wt % (calculated on a volatile-free basis and omitting the most altered samples). Nb/Zr is low and constant, but not quite as low as in the later flows of the underlying Upper Series at Site 917. The relatively evolved and uniform character of these basalts suggests that their composition has been buffered. The narrow range of MgO content is similar to the range observed in mid-ocean ridge basalt (MORB) from the North Atlantic Ocean (Schilling et al., 1983). By the time the basalts sampled at Sites 915/990, 989 and 918 were erupted, the rift system seems to have evolved towards the re-establishment of permanent magma reservoirs, this time in denser basaltic crust (Fitton et al., 1995). These basalts form the main part of the SDRS and are thought to have been erupted after break-up of the margin. Larsen & Saunders (1998) proposed a division of the SDRS lava flows into continental and oceanic successions, respectively below and above the sandstone layer between the Middle and Upper Series at Site 917 (Fig. 4).
The sill at Site 918 differs from the other post-break-up basalts in being rather more enriched in incompatible elements (Fig. 5). The sill is certainly younger than the main part of the SDRS and may be related to a nearby, post-SDRS volcano seen in seismic profiles across the area (Larsen et al., 1994).
The transition from continental to oceanic volcanism recorded during the drilling of the transect is the first to be described from a continental margin. However, Francis et al. (1983) described a virtually identical sequence from the Proterozoic Cape Smith fold belt of northern Quebec. Here a succession of continental flood basalts (the Povungnituk basalts) are related to continental rifting and show evidence for storage, evolution and contamination in crustal magma reservoirs. These give way to a series of picritic basalts (the Lower Chukotat Group) erupted rapidly through a conduit system dominated by fractures rather than magma reservoirs. The picrites are overlain by a sequence of basalts whose ‘compositional homogeneity ... implies the presence of large magma reservoirs, within or along the base of an oceanic crust, which buffered the compositions of the magmas rising from the underlying mantle’. The whole sequence is time-transgressive into the Cape Smith basin and remarkably reminiscent of the SE Greenland transect. Francis et al. (1983) speculated that their volcanic sequence was erupted on a developing continental margin.
Crustal contamination
Chemical and isotopic data (Fitton et al., 1998a, 1988b; Saunders et al., 1999) provide convincing evidence for crustal contamination in the magmas represented by the continental succession (Lower and Middle Series at Site 917). This can be seen in Fig. 5, which shows high and variable Ba/Zr coupled with low 143Nd/144Nd and 206Pb/204Pb in these rocks. The degree of crustal contamination drops off rapidly in the syn-break-up and early post-break-up basalts, and the basalts from Site 918, erupted in a truly oceanic environment, are uncontaminated by continental basement.
Sr- and Nd-isotopic data (Fig. 6) show that at least two different contaminants must have been involved in the evolution of the SE Greenland SDRS magmas. The first of these, which strongly affected the samples from the lower part of the Lower Series at Site 917 (below Unit 73A; Figs 3 and 5), has low 87Sr/86Sr and low 143Nd/144Nd. These six samples define a distinct field in Fig. 6. Unit 73A shows no evidence for contamination in its Sr- and Nd-isotope ratios, and heralds a significant change in the nature of the contaminant. Units higher in the Lower Series represent magmas contaminated by crust with distinctly more radiogenic Sr (Fig. 6). The transition from a low-87Sr/86Sr to a high-87Sr/86Sr contaminant occurred progressively from the flows of the Lower Series above Unit 73A through the Middle Series. A large increase in 87Sr/86Sr through the Middle Series is shown by the three analysed samples, and is indicated with arrows in Fig. 6. Some of the olivine basalt and picrite flows of the Upper Series at Site 917 (notably Unit 17) were also affected by a high-87Sr/86Sr, low-143Nd/144Nd contaminant, but to a lesser extent. The post-break-up magmas represented by the lava flows sampled at Site 915/990 appear to have been affected by both high- and low-87Sr/86Sr contaminants (Fig. 6).
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Evidence for a change in the nature of the contaminant with time can also be seen in the Pb-isotopic composition of the volcanic rocks. All of the contaminated rocks fall on a single array on a 207Pb/204Pb vs 206Pb/204Pb diagram (Fitton et al., 1998a; Saunders et al., 1999), but show more scatter in 208Pb/204Pb (Fig. 7). Some rocks, notably the Middle Series at Site 917 and sample 17 from the Upper Series, show higher 208Pb/204Pb, relative to 206Pb/204Pb, than do those from the lower part of the Lower Series (Fig. 7). This implies that the later magmas at Site 917 were contaminated with crust with higher time-integrated Th/U than were the earlier magmas, consistent with the observation (Fitton et al., 1998b) of an abrupt decline in La/Th above Unit 73A in the Lower Series at Site 917.
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The style of crustal contamination recorded in the SE Greenland SDRS closely resembles that which affected the contemporaneous magmas of the Hebridean igneous province of NW Scotland, on the conjugate continental margin. Isotopic variation in the Hebridean rocks can be modelled by variable contamination with granulite- and amphibolite-facies Lewisian gneiss (Carter et al., 1978; Dickin, 1981) representing, respectively, the lower and upper continental crust in this area. Dickin’s (1981) average granulite- and amphibolite-facies Lewisian gneiss compositions provide plausible analogues for the contaminants in the SE Greenland magmas (Figs 6 and 7). The Lewisian crust of NW Scotland is very similar in its history and isotopic composition to that of southern Greenland (e.g. Kalsbeek et al., 1993).
By analogy with Hebridean magmatism, the contaminant affecting the SE Greenland magmas during continental extension and break-up seems to have changed through time. The oldest rocks in the SDRS show only the effects of contamination with granulite but the later magmas were contaminated with both amphibolite and granulite. It is tempting, therefore, to conclude that the magma reservoirs moved to shallower depths in the crust over this period. A similar shallowing of magma reservoirs has been proposed by Fram & Lesher (1997) for the East Greenland Tertiary lava successions on the basis of basalt phase relations. The nature of the crustal contaminant does not necessarily relate to depth of magma storage, as both granulite- and amphibolite-facies gneisses are exposed at the surface today. However, the observed pattern of contamination could only be produced in magma stored at one level in the crust if amphibolite-facies gneiss were more refractory than granulite-facies gneiss. Even if this were so, it is unlikely that such a clear temporal pattern of contamination (Figs 6 and 7) would result.
Some of the basaltic flows from the Lower Series at Site 917 have Pb-isotope ratios comparable with those in Lewisian granulite (Fig. 7) implying that the original mantle isotopic signatures have been completely overprinted. The problem of apparently large proportions of crustal Pb in relatively primitive basalt was also encountered by Dickin (1981) in his study of the Tertiary volcanic rocks of Skye. Dickin (1981) noted that some rocks showing no other evidence for contamination still contained a significant proportion of crustal Pb. He proposed that magmas can be selectively contaminated with Pb as a result of the greater mobility of this element. An alternative explanation (Fitton et al., 1998a) might be that the uncontaminated primitive magmas in both provinces resembled N-MORB in their very low concentrations of Pb and other incompatible elements (Wood, 1980; Thompson et al., 1982; Fitton et al., 1997, 1998b) and that both were therefore extremely susceptible to the effects of contamination with high-Pb crust. Fitton et al. (1998a) used AFC calculations based on the most primitive magma compositions and plausible crustal end members to show that the range of 143Nd/144Nd in the volcanic rocks from the SDRS can easily be modelled by assimilation coupled with fractional crystallization. The lowest 143Nd/144Nd values found in the Lower Series can be produced by simple bulk assimilation of 8–15% of crust, depending on the crustal component chosen. These values are comparable with those deduced by Thompson et al. (1982) for the most contaminated basalts from Skye and Mull.
Magma chamber processes
The presence of long-lived magma reservoirs is implied by the differentiation shown by the Lower and Middle Series lavas at Site 917, and by the constant and low MgO content of the oceanic part of the SDRS. Variation of the crustal contaminant noted in the previous section is consistent with magma storage initially at lower-crustal levels (granulite) but shallowing with time as the continental crust was stretched and replaced by igneous crust. Seismic and gravity studies (H. C. Larsen et al., 1998) along the 63°N transect across the SDRS suggest that the igneous crust is composed of a 10 km basaltic layer (unit 3; flows and feeder dykes) overlying 4 km of gabbro (unit 5) and 4 km of ultramafic cumulates (unit 6). This observation fits well with Cox’s (1980) model for the evolution of flood basalt magmas in lower-crustal, sill-like bodies. In this model, primary magma is trapped because of its density at the base of the continental crust and evolves through crystallization of olivine, which accumulates as a lower, ultramafic layer. The evolved magma composition is buffered on an olivine–plagioclase–augite cotectic, and this explains the constancy of composition of flood tholeiite. Lower-crustal igneous cumulate bodies resulting from this process would consist of gabbro overlying more olivine-rich layers. The same model can be used to explain the constant composition of the oceanic SDRS lavas.
If this model is valid, then the estimated thickness of the underplated cumulate layers should match those expected from the differentiation of primary basaltic magma. This test has been applied by L. M. Larsen et al. (1999), who used the COMAGMAT program (Ariskin et al., 1993) to predict the crystallization sequence of a primary SDRS magma with 18% MgO at the fayalite–magnetite–quartz (FMQ) oxygen buffer and at pressures equivalent to the depth of the cumulate sequence. The primary magma composition was based on the composition of the most magnesian aphyric Upper Series basalt (Fitton et al., 1995) and on melting experiments carried out by Thy et al. (1998). For this primary magma to evolve to a liquid with 7·3% MgO (the average composition of the oceanic SDRS basalts) requires the crystallization of 25 wt % olivine followed by 13 wt % of a gabbro assemblage (olivine–plagioclase–augite). Thus the primary magma supplying the SDRS should differentiate into 38% of cumulates, of which the lower two-thirds are composed of olivine, and 62% of evolved basaltic magma (dykes and lava flows). Melt retained in the cumulates will increase their volume and reduce that of the basalt layer. The predicted distribution of cumulates and evolved basalt is comparable with the geophysical estimates (H. C. Larsen et al., 1998) and demonstrates the applicability of Cox’s (1980) model to SDRS magmatism. The only discrepancy lies in the proportion of gabbro and olivine cumulates, but H. C. Larsen et al. (1998) noted that the boundary between their units 5 and 6 is poorly constrained seismically.
Depth and degree of melting
Figure 5 shows a large increase in Sc/Zr upwards through the SDRS, with a corresponding decrease in Sm/Lu. Much of the change in these two ratios takes place through the syn-break-up Upper Series at Site 917. Sc and Lu are much more compatible in garnet than are Zr and Sm and so variation in Sc/Zr and Sm/Lu probably reflects variation in the proportion of garnet in the mantle source. As none of the four elements is very incompatible in mantle phases, these ratios are likely to be insensitive to the effects of source depletion. For example, Sm/Lu in N-MORB, and hence in the depleted upper mantle, is only 5% lower than in primitive mantle (Sun & McDonough, 1989). The most likely explanation for the variation is that the top of the melting zone was rising rapidly through the garnet–spinel transition during the syn-break-up phase. Once sea-floor spreading was established in the post-break-up phase, these ratios settled down to values reflecting a mainly spinel lherzolite source. Thinning of the lithospheric lid during break-up would also have resulted in more decompression and therefore larger degrees of melting.
Fram et al. (1998) have attempted to quantify the syn-break-up thinning of the lithosphere and the resulting increase in degree of mantle melting. They calculated the pooled composition of individual melt increments during progressive decompression and compared the results with their data from Leg 152 samples. Using Lu/Hf, La/Sm and La/Th they were able to show that the average melt fraction increased from 4% to 12% as the lithosphere lid thinned from about 60 km to <25 km over the time interval represented by the Upper Series at Site 917.
Mantle temperature
The addition of 18 km thickness of igneous crust during continental break-up requires anomalously hot mantle beneath the region (e.g. White & McKenzie, 1989). The occurrence of aphyric basalt with up to 18% MgO and olivine phenocrysts with high forsterite contents (up to Fo92; Demant, 1998) seems to support this. Furthermore, the buoyancy of anomalously hot mantle would counteract the subsidence expected during rifting and break-up and explain why the SDRS were erupted subaerially. An important question is just how hot the mantle was at the time of continental break-up and whether it was hotter then than it is now beneath Iceland. The SE Greenland SDRS were formed soon after the initiation of the Iceland plume at 62 Ma (e.g. Saunders et al., 1997) and may therefore reflect melting in the transient head of the plume, whereas Iceland may represent the steady-state plume stem.
The production of 18 km of igneous crust through melting caused by passive upwelling beneath a 25 km lithospheric lid requires a mantle potential temperature 200°C hotter than ambient upper mantle (McKenzie & Bickle, 1988). Fram et al. (1998) have discussed the implications of this observation and pointed out that the large melt fractions that would result from decompression of mantle with such a high temperature are difficult to reconcile with the 4–12% implied by the composition of the Upper Series picrites. These modest melt fractions are comparable with those expected beneath normal segments of mid-ocean ridge where only 7 km of igneous crust are produced (McKenzie & Bickle, 1988). Fram et al. (1998) concluded that the most likely explanation for this apparent contradiction is that upwelling beneath the developing continental margin was not a simple passive process but that the melt zone was fed dynamically by the plume at a rate faster than that resulting solely from plate separation. A similar conclusion was reached for Iceland by Ribe et al. (1995), who showed that dynamic upwelling of mantle with a temperature anomaly of only 90°C can produce the observed crustal thickness. This is less than half the temperature anomaly required by passive upwelling (White et al., 1995). Independent support for cooler mantle, and therefore dynamic upwelling, is provided by subsidence analysis of the North Atlantic SDRS by Clift (1997), who suggested a temperature anomaly of 50–100°C.
There is thus no evidence from the composition of the SDRS that requires the temperature of their mantle source to have been significantly different from that beneath Iceland today. The eruption of picrite magma at the time of break-up does not necessarily imply very high temperatures in the mantle. Fram et al. (1998) have argued that the high MgO contents of these magmas resulted from their generation at moderately high average pressure as a result of the thick lithospheric lid present at the onset of break-up. These magmas were erupted in a near-primary state because rapid stretching and break-up of the lithosphere at this time gave them unrestrained access to the surface (Fitton et al., 1995, 1998b). We cannot, however, exclude the possibility that the picrites represent a transient pulse of hotter mantle at the time of plume initiation (Saunders et al., 1997). It has not been possible to date the Upper Series at Site 917, nor can we be certain about the thickness of the lithosphere at the time of their eruption.
Mantle sources
The bathymetry of the Mid-Atlantic Ridge to the south of Iceland suggests that the thermal anomaly surrounding Iceland extends 1700 km from the centre of the plume. Major-element analyses of dredged basalt samples from this anomalous region have higher Fe and lower Na contents compared with N-MORB, consistent with hotter mantle (Klein & Langmuir, 1987). Anomalously high 3He/4He is broadly coincident with the thermal anomaly suggesting some input of lower-mantle He (Taylor et al., 1997). However, other chemical and isotopic plume signatures occur over a much more restricted area closer to Iceland. Anomalous values of 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb and La/Sm extend southwards only to 60°N on the Reykjanes Ridge, 750 km from the plume centre (Taylor et al., 1997). The Iceland plume today appears to be zoned, therefore, with an axial zone composed of compositionally distinct plume mantle and a peripheral zone composed of anomalously hot upper mantle. One of the objectives of the SE Greenland transect at 63°N was to assess the extent to which the ancestral plume head was zoned. It was already known that the southern edge of the SDRS on Hatton Bank (Fig. 1) is composed of basalt indistinguishable from N-MORB (Joron et al., 1984; Merriman et al., 1988), and DSDP Leg 49 (Sites 407–409), on the southern flank of the Greenland–Iceland Ridge (Fig. 1), had traced the influence of the plume back to 35 Ma (Wood et al., 1979). The drilling transect on the Greenland margin was therefore located at 63°N, on a mantle flow line from the modern limit of plume influence at 60°N on the Reykjanes Ridge.
Identifying plume and non-plume components in Palaeogene basalts erupted through continental crust presents problems. Modern Icelandic basalt overlaps with N-MORB in most elemental and isotopic parameters, although Thirlwall (1995) has shown that the two form parallel and separate arrays on a plot of 208Pb/204Pb vs 207Pb/204Pb. However, Pb-isotope ratios are very susceptible to disturbance through crustal contamination (Fig. 7) and cannot be used to discriminate between mantle sources for the SDRS magmas. Fitton et al. (1997) showed that Icelandic basalt and N-MORB define separate and parallel arrays on a logarithmic plot of Nb/Y vs Zr/Y, and this provides a useful discriminant. Nb, Zr and Y are relatively immobile during alteration of basalt and are incompatible in the phases (principally olivine and plagioclase) crystallizing from tholeiitic magma at low pressure. Icelandic (plume) basalt and N-MORB may be discriminated through 
Nb, which is the deviation, in log units, of Nb from the boundary between the arrays. Nb has been given by Fitton et al. (1997):
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Nb > 0 and N-MORB has Nb < 0. Continental crust generally has slightly negative Nb and so enrichment of N-MORB through crustal contamination produces a mixture that can readily be distinguished from Icelandic basalt. Contamination of Icelandic basalt with crust could produce a mixture with negative Nb, but this would not resemble N-MORB in any other respect. Because Icelandic basalt and N-MORB form distinct parallel arrays on a plot of log(Nb/Y) vs log(Zr/Y), Nb is insensitive to variation in degree of melting and is, therefore, a mantle source characteristic.
Figure 5 shows the stratigraphic variation in Nb through the SDRS. Most of the earliest, pre-break-up flows have negative Nb, suggesting a source in the depleted upper mantle. It should be noted that contamination with continental crust has little effect on Nb, even in the evolved rocks of the Middle Series. A few basalt flows, mostly the high-Nb/Zr group, have positive Nb, suggesting that plume mantle was also available at 63°N at this time. This is an important observation because it shows that the SDRS at this latitude were erupted close to the interface between plume mantle to the north (closer to the plume track; Fig. 1) and depleted upper mantle to the south (the source of the SDRS forming Hatton Bank). The same interface occurs today at the conjugate point (at 60°N) on the Reykjanes Ridge.
The syn-break-up Upper Series basalts and picrites have negative Nb, suggesting that they, too, had a source in the depleted upper mantle. Fram et al. (1998) carried out a detailed geochemical study of the Upper Series lava flows and divided these into two groups. Group 1, comprising most of the Upper Series flows, is strongly depleted in incompatible elements, whereas Group 2, from the upper half of the series (Units 1, 2, 16, 17 and 18 in Fig. 3), is enriched in Ba, Th, Pb and LREE. We have measured Sr-, Nd- and Pb-isotope ratios in one of these (Unit 17; Fitton et al., 1998a) and this sample shows clear isotopic evidence for crustal contamination (Figs 6 and 7). Fram et al. (1998) discussed the possibility that the enrichments seen in Group 2 lavas resulted from crustal contamination, but concluded that they reflect the incoming of an Icelandic mantle source. We disagree with this conclusion because our isotopic data (Fitton et al., 1998a) strongly favour crustal contamination as an explanation.
As the SDRS entered the post-break-up phase (Site 990 and younger parts of the succession), they began to tap Icelandic mantle (positive Nb), and this source persisted through to the end of the succession studied in the transect. The small dyke sampled at Site 990 is the only recorded occurrence of N-MORB in the post-break-up phase. A transition from negative to positive Nb occurred in the basalt lava succession of Mull, NW Scotland, at about this time (Chambers & Fitton, 2000), suggesting that this may be a general feature of the ancestral Iceland plume. The overall trend in basalt composition through time, from N-MORB-like to Icelandic (Fig. 5), implies that the developing head of the plume was zoned, with an outer sheath of hot, depleted upper mantle surrounding a core of relatively enriched (Icelandic) mantle.
The outer zone of hot mantle with negative Nb appears to have spread rapidly outwards at 62 Ma, followed by mantle with positive Nb from the core of the plume (Saunders et al., 1997). The outer sheath is too thick to have formed by conductive heating of ambient upper mantle around the plume head, and this observation led Fitton et al. (1997) to suggest that this large volume of depleted mantle originated in the thermal boundary layer above the 670 km discontinuity. High 3He/4He in the outer zone today (Taylor et al., 1997) and at 60 Ma (Stuart et al., 2000) supports such an origin. Kempton et al. (2000) have shown that basaltic magmas generated in the depleted outer sheath of the Iceland plume have higher 176Hf/177Hf than that found in N-MORB emplaced in regions remote from mantle plumes. These workers have proposed that convective instability in the boundary layer at the base of the upper mantle is triggered by plume material that rises through the lower mantle, possibly from the core–mantle boundary, and then stalls and spreads out at 670 km. Numerical modelling of instability under such conditions (Yuen et al., 1998) shows that material from the lower layer is likely to be drawn up to form the core of the plume, whereas the boundary layer contributes a thick outer plume sheath. This process readily explains the thermal and chemical zonation observed in the Iceland plume.
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CONCLUSIONS |
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TOPABSTRACTINTRODUCTIONTHE ODP TRANSECT AT...MAGMATIC DEVELOPMENT OF THE...CONCLUSIONSREFERENCES |
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ODP Legs 152 and 163 successfully sampled a transect across the SE Greenland SDRS, and documented the development of a volcanic rifted margin from a pre-break-up continental flood tholeiite phase all the way through break-up to the establishment of sea-floor spreading. This sequence is summarized schematically in Fig. 8. 40Ar–39Ar dating of rock samples from the transect confirms that most of the SDRS were erupted during C24r (56–53 Ma) although the continental phase was active at 61 Ma. The onset of volcanism at
61 Ma was probably triggered by the arrival of the head of the ancestral Iceland plume beneath the area (Saunders et al., 1997).
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The pre-break-up continental succession is composed of lava flows and pyroclastic rocks that evolved in crustal magma reservoirs with a dwindling magma supply. Following a volcanic hiatus, a resurgence of mantle melting accompanying break-up generated primary magmas that were able to reach the surface without significant periods of storage. After some indeterminate time interval, magma reservoirs were once again established, this time in a sea-floor spreading situation. Drilling the interval between the syn- and post-break-up phases was a priority objective of Leg 163 but, regrettably, this Leg had to be abandoned through bad weather before this was completed. Differentiation of primary magmas to a point where their density was low enough for them to erupt through the oceanic part of the SDRS produced a thick layer of underplated igneous cumulates at the base of the crust. These would comprise an olivine-rich layer overlain by gabbroic cumulates, and would have thicknesses comparable with those inferred from seismic and gravity studies.
The continental succession shows clear evidence for contamination with crustal rocks. Magmas represented by the earliest part of the continental succession appear to have interacted only with basic granulite (low 87Sr/86Sr and 143Nd/144Nd) inferred to form the lower continental crust. Later continental magmas interacted with a contaminant with high 87Sr/86Sr and low 143Nd/144Nd, probably amphibolite-facies upper crust. This suggests that the magma reservoirs rose to shallower levels in the crust as stretching proceeded towards break-up. Once sea-floor spreading was established, the degree of crustal contamination declined rapidly, and the succession drilled at Site 918 is uncontaminated by continental basement.
Stratigraphic variation in, for example, Sm/Lu, Lu/Hf and Sc/Zr suggests that the top of the mantle melting zone rose rapidly out of the garnet stability field as the lithosphere thinned from 60 to 25 km during the break-up phase. A rapid phase of sea-floor spreading immediately following break-up generated 18 km of igneous crust through only modest degrees of mantle melting. This observation implies that the mantle was probably only 100°C hotter than normal upper mantle, and that upwelling mantle was fed dynamically to the melt zone.
The transect is located on the SE Greenland SDRS at the end of a mantle flow line from a point at 60°N on the Reykjanes Ridge at which the chemical and isotopic influence of the modern Iceland plume declines almost to background N-MORB levels (Taylor et al., 1997). Significantly, the SDRS magmas show input from both Icelandic and N-MORB-like mantle sources, implying that the ancestral Iceland plume was zoned in much the same way as it is today. The continental to syn-break-up succession is dominated by N-MORB-like basalt but with a few intercalated ‘Icelandic’ flows. The post-break-up SDRS basalts had an Icelandic mantle source. A hot outer plume sheath composed of depleted upper mantle from the thermal boundary layer above the 670 km discontinuity can account for both the volume and the N-MORB-like character of the pre- and syn-break-up magmas. The post-break-up ‘Icelandic’ basalts represent magma generated from the core of the plume. This material must have originated in the lower mantle, possibly at the core–mantle boundary, and its emplacement beneath the 670 km discontinuity may have triggered the boundary-layer instability that initiated the Iceland plume. Understanding the nature and origin of mantle plumes is a major goal in the earth sciences, and the results of ODP Legs 152 and 163 have added significantly to this understanding.
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ACKNOWLEDGEMENTS |
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Keith Cox was one of the original proponents for drilling the North Atlantic margins, and his ideas were incorporated in the Detailed Planning Group report that led ultimately to ODP Legs 152 and 163. Hans Christian Larsen chaired this Group and was the principal author of the SE Greenland drilling proposals. We thank the Captain and crew of the JOIDES Resolution for the successful completion of the drilling, and Stefan Bernstein, Peter Clift and Don Francis for their thoughtful and constructive reviews of this paper. The analytical work was supported by research grants from NERC (GST/02/673 and GST/02/1227) to J.G.F., A.D.S. and P.D.K. L.M.L. publishes with the permission of the Geological Survey of Denmark and Greenland.
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FOOTNOTES |
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*Corresponding author. Telephone: +44-131-650-8529. Fax: +44-131-668-3184. e-mail: Godfrey.Fitton{at}ed.ac.uk
Extended data set can be found at: http://www.petrology.oupjournals.org
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 J. H. Natland {Delta}Nb and the role of magma mixing at the East Pacific Rise and Iceland Geological Society of America Special Papers, January 1, 2007; 430(0): 413 - 449. [Abstract] [Full Text] [PDF]
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R. Meyer, J. van Wijk, and L. Gernigon The North Atlantic Igneous Province: A review of models for its formation Geological Society of America Special Papers, January 1, 2007; 430(0): 525 - 552. [Abstract] [Full Text] [PDF]
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 C. J. Parkin, Z. C. Lunnon, R. S. White, P. A.F. Christie, and Integrated Seismic Imaging & Modelling of Margins Imaging the pulsing Iceland mantle plume through the Eocene Geology, January 1, 2007; 35(1): 93 - 96. [Abstract] [Full Text] [PDF]
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 A. H. F. Robertson Overview of tectonic settings related to the rifting and opening of Mesozoic ocean basins in the Eastern Tethys: Oman, Himalayas and Eastern Mediterranean regions Geological Society, London, Special Publications, January 1, 2007; 282(1): 325 - 388. [Abstract] [Full Text] [PDF]
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 N. G. Direen, N. G. Direen, and A. J. Crawford Fossil seaward-dipping reflector sequences preserved in southeastern Australia: a 600 Ma volcanic passive margin in eastern Gondwanaland Journal of the Geological Society, December 1, 2003; 160(6): 985 - 990. [Abstract] [Full Text] [PDF]
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 F. A. FREY, D. WEIS, A. YU. BORISOVA, and G. XU Involvement of Continental Crust in the Formation of the Cretaceous Kerguelen Plateau: New Perspectives from ODP Leg 120 Sites J. Petrology, July 1, 2002; 43(7): 1207 - 1239. [Abstract] [Full Text] [PDF]
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