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Journal of Petrology Volume 41 Number 7 Pages 1155-1176 2000
© Oxford University Press 2000
Major Element Records of Variable Plume Involvement in the North Atlantic Province Tertiary Flood Basalts
1DEPARTMENT OF MINERALOGY AND PETROLOGY, CAMPUS FUENTENUEVA, UNIVERSITY OF GRANADA, 18002 GRANADA, SPAIN
2SCHOOL OF GEOSCIENCES, THE QUEEN’S UNIVERSITY OF BELFAST, BELFAST BT7 1NN, UK
3DEPARTMENT OF GEOLOGY, UNIVERSITY OF LEICESTER, LEICESTER LE1 7RH, UK
Received September 10, 1999; Revised typescript accepted February 7, 2000
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ABSTRACT |
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TOP
ABSTRACT
INTRODUCTIONMajor element variations in North Atlantic Tertiary Province primitive, early erupted, alkaline-transitional-tholeiite basalts, recalculated to a restricted value of MgO, give insights into the process of plume-related magmatism. Basalts primitive enough to be crystallizing only olivine were recalculated to a proposed primary magma composition of 15 wt % MgO. The recalculated dataset shows clear inter-element correlations including a strong, significant, negative correlation between Fe and Si indicating polybaric melt segregation. Overlap between basalt compositions and experimental melts from a fertile, Fe-rich, low mg-number (85.5) peridotite suggests that, relative to normal peridotite with mg-number > 89, the North Atlantic basalt source was Fe rich. Linear regression of the experimental data gives apparent pressures of magma segregation of 17·5–37 kbar, with intra-region variability in the depth derivation from the melt column for each sample, thus suggesting that lithospheric thickness ‘lid-effect’ control on magma generation may have been overemphasized in recent studies. Comparable source composition, magma segregation depth and calculated mantle potential temperature (1440–1460°C) throughout the Province supports the previously suggested plume impact model, arriving below East Greenland, derived from a variably enriched and depleted lower-mantle source. Given the good agreement between conclusions drawn from major element data and previously published results we suggest that restricted-MgO recalculated datasets may be usefully applied to study other large igneous provinces.
KEY WORDS: basalt; Fe-rich mantle; large igneous provinces; North Atlantic Tertiary Province; restricted-MgO major element datasets
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INTRODUCTION |
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In recent years numerous workers have used major element variations in continental flood basalt datasets recalculated to restricted values or ranges of MgO to consider the importance of plume involvement in their generation (Melluso et al., 1995
; Scarrow & Cox, 1995; Flower et al., 1996; Lightfoot et al., 1997). These studies indicate that, if rigorously applied after the effects of post magma generation processes are accounted for, huge potential exists for using major element variations in such datasets to study large igneous provinces. Information can be obtained about tectonically important factors such as depth of magma segregation, mantle potential temperatures, proximity of magma generation to the plume focus, and plume source composition.
The studies referred to above have not, however, considered the implications of spatial and temporal major element variability potentially present within a flood basalt province. The North Atlantic Tertiary Province (Fig. 1) provides an excellent opportunity to consider such compositional variations and so gain insights into the process of plume-related magmatism. The Province has long been the focus of study so a large amount of geological information is available for the area, as indicated by, and summarized in, numerous review papers (e.g. Dickin, 1988; White & McKenzie, 1989; Saunders et al., 1997). Magmatism in the region is believed to be linked to decompressive melting associated with Early Tertiary arrival of the mantle plume now located beneath Iceland (Brooks, 1973; White, 1988; White & McKenzie, 1989; Campbell & Griffiths, 1990). The magmatism has been divided into two main phases by Saunders et al. (1997) based on the work of Sinton and Duncan (1998) and others (Fig. 2), with the proviso that further dating work may reveal the two phases to be continuous. Phase 1, 63–58 Ma, terrestrial continent-based magmatism was associated with arrival of transitory, hot, low-viscosity plume material. In contrast, the more voluminous Phase 2, 56 Ma–present day, was associated with plate separation and tapping of somewhat cooler mantle.
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, palaeontological age; •, radiometric date with 2
; continuous line, possible age range; ?, uncertain date or range.
In this study we concentrate on major element variations in the primitive, Early Paleocene, Phase 1 (Saunders et al., 1997), continental flood basalts that crop out in the Tertiary Igneous Province of the British Isles and Greenland. Initially, the British Province basalts are considered in detail then a comparison is made between them and the Greenland basalts. Major element variations are used to map out the thermal and compositional influences of the ancestral Iceland plume in the Province.
The wealth of previous work in the North Atlantic Province provides us with a base against which we can compare our conclusions.
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GEOLOGICAL BACKGROUND |
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A review of basaltic magmatism in the North Atlantic Province has recently been published by Saunders et al. (1997)
and the reader is referred to that work for a more detailed description of the area. Throughout the Province early lavas were erupted, predominantly subaerially, onto a Precambrian to Tertiary age surface. The extrusive magmatism shows a temporal progression from the basal alkaline-transitional-tholeiite basalts to mid-ocean ridge basalt (MORB)-like tholeiites in the British Province, alkali basalts in West Greenland, and Fe–Ti-enriched tholeiites interbedded with MORB-like tholeiites in central East Greenland (Thompson et al., 1972; Esson et al., 1975; Hald & Pedersen, 1975; Wilson & Manning, 1978; Larsen et al., 1989a; Kerr, 1994). Ages for lavas throughout the North Atlantic Province were summarized in reviews by Dickin (1988) and Mussett et al. (1988); published dates are presented in Fig. 2.
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BASALT COMPOSITIONS |
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To compare like with like, on a province-wide basis, it is not enough to use MgO as the only criterion for sample selection. Consideration must also be given to the possibility, as in the North Atlantic Province described above, that different, albeit primitive, magma series are present within the same province. In the present work only the primitive, early erupted, alkaline-transitional-tholeiite basalts not the younger alkaline or tholeiitic basalts were selected for study.
Detailed descriptions of the geochemistry, petrography and mineral chemistry of the basalts considered in the present study have been given by the following researchers: Skye—Scarrow (1992); Mull—Kerr (1993); Small Isles—Ridley (1971) and Emeleus (1997); Antrim—Wallace (1995); West Greenland—Lightfoot et al. (1997); East Greenland—Fram & Lesher (1997). In brief, the primitive, early erupted, transitional basalts are porphyritic predominantly containing magnesian olivine phenocrysts, with or without Cr-spinel inclusions, and minor amounts of plagioclase and clinopyroxene phenocrysts.
Representative major and trace element data presented in Table 1 and Fig. 3 are consistent with olivine, with or without minor Cr-spinel, fractionation from variable composition parental magmas. In particular, the increase in CaO and Al2O3, the constant FeOT and CaO/Al2O3, and the decrease in Ni as MgO decreases preclude significant fractionation of clinopyroxene or plagioclase. It is worth noting that the MgO content below which phases other than olivine began to fractionate is different throughout the North Atlantic Province, e.g. 7 wt % MgO in Skye (Thompson et al., 1972; Scarrow, 1992) and Mull (Kerr, 1995), 9 wt % MgO in the Small Isles, and 11 wt % MgO in East Greenland (Fram & Lesher, 1997). In more evolved North Atlantic rocks major element trends change to decreasing CaO and CaO/Al2O3 and near constant Al2O3, indicating involvement of clinopyroxene or plagioclase, or both, in the fractionating assemblage (Thompson et al., 1972; Scarrow, 1992; Kerr, 1995; Fram & Lesher, 1997). Only the basalts primitive enough to be crystallizing solely olivine are considered in the present study. The broad compositional range of some elements in these rocks, e.g. FeOT and SiO2, at constant MgO (Table 1, Fig. 3) cannot be accounted for solely by olivine fractionation unless the suite had compositionally varied parental magmas; it is these major element variations that are considered below.
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NORMALIZATION OF PRIMITIVE BASALTS TO 15 WT % MgO |
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Compositional variations within a suite of rocks may usefully be considered within restricted MgO ‘windows’ either in real rock datasets (e.g. Cox, 1983
) or in datasets recalculated to a particular range, or value, of MgO either by regression of the liquid line of descent of the suite (e.g. Langmuir et al., 1992) or by addition of equilibrium olivine to primitive rocks under olivine control (e.g. Scarrow & Cox, 1995). Such datasets are assumed to represent a fixed point in the evolution of the suite and to highlight elemental variations not attributable to fractional crystallization or crustal contamination. Evidently, this approach has its limitations because it cannot be used if the basalts have been affected by low-pressure magma chamber processes such as magma mixing or RTF (replenishment–tapping–fractionation). However, by considering primitive samples that have fractionated, or accumulated, only olivine, the primary basalt compositions are unlikely to have been modified by magma chamber processes (see Cox, 1988). Therefore, the well-constrained Fe–Mg distribution between olivine and liquid (Roeder & Emslie, 1970) permits normalization to a given value of MgO by incremental fractional addition of equilibrium olivine to each analysis using the equations of Pearce (1978). The basalts selected for recalculation were generally screened for H2O+ < 2·5 wt % to minimize possible compositional effects of post-crystallization alteration.
Following the rationale of Scarrow & Cox (1995), who considered compositional variation in the Skye basalts not attributable to fractional crystallization by fractionally adding olivine to samples to create an artificial dataset at 15 wt % MgO, we recalculated all selected North Atlantic Province basalts to 15 wt % MgO. This value was selected as suitable for the parental magmas because it is slightly more magnesian than the majority of the most MgO-rich basalts (Fig. 3). Furthermore, olivines of Fo88, commonly some of the most magnesian in the North Atlantic basalts (e.g. Kerr, 1993; Scarrow & Cox, 1995; Fram & Lesher, 1997; Lightfoot et al., 1997), would have been in equilibrium with magmas with 15 wt % MgO. Accordingly, Lightfoot et al. (1997) proposed a range of 14·5–16·5 wt % MgO for parental magmas of West Greenland basalts, so revising an earlier estimate of 19 wt % by Pedersen (1985); Fram & Lesher (1997) suggested 17 wt % MgO for magmas parental to the central East Greenland basalts, and Thy et al. (1998) 15–18 wt % MgO for primary magmas of Ocean Drilling Program (ODP) leg 152 basalts off southeast Greenland. In addition, both anhydrous and hydrous experimental peridotite melts fall in the range 12–17 wt % MgO for 20–40% partial melting, with MgO being relatively insensitive to pressure (Hirose & Kushiro, 1993; Hirose & Kawamoto, 1995). In fact, changes in other major element concentrations over a range of MgO generated by olivine fractionation are minimal, e.g. a 3 wt % change in MgO corresponds to <0·5 wt % variation in SiO2 and <0·2 wt % variation in FeO or Al2O3. For these reasons, we suggest that major element compositional variations in the 15 wt % MgO datasets can be considered to be representative of those of primary magmas.
The 15 wt % MgO dataset of recalculated analyses is presented in Table 1. Elements from the recalculated dataset are denoted by a suffix, e.g. Si15. The principal inter-element relationships of the recalculated dataset, which it should be noted are also present in MgO-windows (e.g. 8–10 wt % MgO in the real rock analyses), are shown in Fig. 4. The first point to make regarding the recalculated dataset is that we see the same inter-element correlations for basalts from throughout the whole North Atlantic Tertiary Province as those that were observed in the Skye basalts (Scarrow & Cox, 1995). In summary, the most notable feature, discussed in detail below, is the strong negative correlation between Si15 and Fe15 (Fig. 4a). Although it is well established that many of the Tertiary basalts have been affected by crustal contamination (e.g. Dickin, 1988) we conclude, for several reasons stated below, that it is unlikely that in the Si15–Fe15 relationship the Fe-poor end member is restricted to 
8% wt FeO (at 0 wt % TiO2), which equates to 53 wt % SiO2 (Fig. 4). Furthermore, in a detailed study of the Skye and Mull basalts, comparable correlations were observed in restricted real rock and MgO recalculated datasets containing crustally contaminated and uncontaminated samples, indicating that incorporation of crustal material is not the main control on major element correlations in the primitive basalts (Scarrow, 1992; Kerr, 1995; J. H. Scarrow & A. C. Kerr unpublished data, 1999). This is in agreement with the conclusions of Thompson et al. (1982), who noted that, with the exception of K2O, major element compositions were comparable in contaminated and uncontaminated British Province basalts. Other inter-element relationships of the recalculated dataset, not considered in detail here, include positive correlations between Fe15 and high field strength elements Ti15 and P15, a weak negative correlation between Al15 and Fe15, and very weak or absent correlations between Ca15, K15 and Na15, and are comparable with those of the primitive Skye basalts discussed in detail by Scarrow & Cox (1995).
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The negative correlation between Si15 and Fe15 such as is seen in the recalculated North Atlantic data is analogous to that seen in ocean islands, for example, Hawaii and the Canary Islands (e.g. Langmuir et al., 1992), and in slow-spreading mid-ocean ridge segments (Klein & Langmuir, 1987, 1989; Langmuir et al., 1992); more recently it has also been identified in some other flood basalt provinces (Melluso et al., 1995; Flower et al., 1996; Campbell, 1997). The correlation has been attributed to compositional variations generated by melting over a range of depths. Other inter-element relationships in the recalculated North Atlantic dataset, for example, the positive correlation between Fe15 and Ti15, indicate that basalts from the Province have more compositional affinities with asthenosphere-derived oceanic rocks (see Flower et al., 1978; Rhodes et al., 1978; Langmuir et al., 1992), but notably higher FeO and TiO2, than with continental flood basalts (Fig. 5) such as those associated with Gondwana break-up. In the latter, mantle or crustal lithosphere, or both, is an important component and negative, or no, correlations exist between FeO and TiO2 (see Ellam & Cox, 1989).
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COMPARISON OF BASALTS AND EXPERIMENTAL COMPOSITIONS |
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The spectrum of basalt compositions that may be obtained from melting of specific mantle compositions has been well defined by experimental work (e.g. Jaques & Green, 1980
; Takahashi, 1986; Falloon & Green, 1987; Hirose & Kushiro, 1993; Baker & Stolper, 1994). From such studies it is well recognized that the dominant effect on FeO and SiO2 contents of moderate degree partial melts from the same source is pressure: the former increasing at greater depths and the latter decreasing. Therefore the negative Fe15–Si15 correlation seen in the North Atlantic, oceanic and other continental flood basalt data is what would be expected if magma segregation took place over a range of depths.
Before continuing, to make a comparison of the North Atlantic and experimental data, the validity of comparing basalt compositions with results of equilibrium batch melting experiments (e.g. Hirose & Kushiro, 1993; Baker & Stolper, 1994) must be considered. Mantle-derived magmas are probably produced by either percolation with matrix compaction (McKenzie, 1984) or fractional melting (Kinzler & Grove, 1992a, 1992b; Langmuir et al., 1992), or a combination of both (Asimow, 1999). In the first case, pooling of extracted melts generated over a range of depths is probably the dominant process, and segregation pressures estimated from compositionally comparable experimental melts probably represent the pooled average. In the case of fractional mantle melting, compositional differences from equilibrium melts may be subdued by subsequent transport of liquids by equilibrium porous flow (Asimow, 1999). Alternatively, as noted by Herzberg & O’Hara (1998), such compositional differences may be lessened when fractional melt increments are not perfectly extracted, and further reduced when progressive fractional melts of the same source are integrated. In these ways, melts may be produced by fractional processes that approximate rather closely to average equilibrium melts (Herzberg & O’Hara, 1998; Asimow, 1999), particularly with respect to components such as major elements, which have distribution coefficients close to unity (O’Hara, 1985; Plank & Langmuir, 1992). Again, segregation pressures estimated from such melts by comparison with compositionally similar experimental melts would represent averages.
In any case, whatever the melt generation process involved, with the exception of East Greenland, recalculated North Atlantic Province basalts appear to be compositionally analogous to melts of the anhydrous, low mg-number, HK-66 of Hirose & Kushiro (1993) (Fig. 6). Consistent with this is the absence of any early crystallized hydrous phases in the basalts. Compositions of other, higher mg-number (KLB-1, Hirose & Kushiro, 1993) and hydrous, experimental melts shown for comparison in Fig. 6 are not comparable with the North Atlantic data. The significance and possible implications of the Fe-rich nature of the source, and the difference of the East Greenland compositions, are discussed in more detail below. In the next section we consider the use of experimental results to determine magma segregation depths.
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APPARENT DEPTH OF MAGMA SEGREGATION |
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Determination of magma segregation pressures
Determining the pressure of melt segregation can be approached in three main ways: comparison of natural melt compositions with melts produced experimentally from a suitable source under known, realistic, pressure and temperature conditions (e.g. Albarède, 1992
); mathematical inversion of erupted melt compositions assuming fractional melting in a polybaric melt column using an assumed mantle source and an experimentally determined pressure- and temperature-dependent sub-solidus assemblage (McKenzie & O’Nions, 1991); and comparison of natural melt compositions and forward melt modelling of idealized mantle sources (e.g. Fram & Lesher, 1993; Thirlwall et al., 1994).
Comparison of results obtained from these techniques shows good agreement for the Skye basalts, with 60–120 km segregation depths from regression of experimental results of Hirose & Kushiro (1993) and 75–140 km from inversion modelling (Scarrow, 1992). Furthermore, substantially similar results were obtained for Deccan Trap basalts from regression of the Hirose & Kushiro (1993) data, 49–96 km, and the algorithm of Albarède (1992), 56–92 km (Melluso et al., 1995). Given these similarities and correspondence of the North Atlantic data and Hirose & Kushiro HK-66 experimental data, noted above (Fig. 6), we decided to determine apparent pressures of magma segregation from linear regression of the experimental data to give P (kbar) = 213·6 – (4·05 SiO2), which may be converted to depth (km) = 3·02 P(kbar) + 5 (Scarrow & Cox, 1995).
Figure 7 presents North Atlantic average magma segregation pressure histograms, each magma batch probably being a pooled mixture of magmas equilibrated over a range of pressures, calculated from the Hirose & Kushiro (1993) data. The largest spread of segregation depths in the British Province was obtained from Skye (17·5–35 kbar, 57–112 km), with similar although marginally more restricted ranges for the other main lava fields of Antrim (20–32·5 kbar, 65–103 km) and Mull (22–32·5 kbar, 74–103·5 km). When considered as a group the Small Isles magmas also separated at a similar depth range: however, when considered individually, although the datasets are limited, each island represents a limited, discrete, range (Fig. 7c): Canna (21 kbar, 69 km), Eigg (20–27 kbar, 65–85 km), Muck (27–29·5 kbar, 86–93·7 km), Rum (31·5–37 kbar, 99·5–117 km). Accordingly, the main phase of magmatism in West Greenland, contemporaneous with the British Province (Saunders et al., 1997), segregated at 21·5–30 kbar, i.e. 69–95 km (Fig. 7d). Because the East Greenland data do not coincide compositionally with the experimental data it is not possible to use the algorithm obtained by linear regression of those data to obtain pressures and depths of magma segregation. What can be noted, however, is that the higher Si15 at comparable Fe15 (Table 1, Fig. 6) indicates segregation from shallower rather than deeper in the melt column.
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To consider further the validity of the major element approach for determining pressures of magma segregation we can compare our results with published pressure estimates from the North Atlantic Province. Forward modelling of rare earth element (REE) abundances from throughout the Province has shown that although garnet must have been a component in the basalt source, mantle melting did not occur entirely within the garnet field but instead in, and somewhat above and below, the garnet–spinel transition zone (e.g. Scarrow, 1992; Fram & Lesher, 1993; Kerr, 1995; Fram et al. 1998; Tegner et al., 1998). Robinson & Wood (1998) placed the garnet–spinel transition zone at 85 km, i.e. 26 kbar. Further evidence for melting, at least initially, in the garnet zone has been suggested by Saunders et al. (1997) from Sc and Sc/Zr values of Tertiary Mull and Central Greenland basalts. Results of detailed major and trace element mantle melt modelling by Fram & Lesher (1993) extend the published melting range to 15–25 kbar, at least for East Greenland and the British Province. Evidently the possibility of polybaric asthenospheric melting is well established for the North Atlantic Province. Pressure estimates based on major element data, 17·5–35 kbar, are in good agreement with published pressure estimates, 15–30 kbar, based on inversion and forward REE modelling, and specific comparisons, for example, West Greenland from this study 21·5–30 kbar, West Greenland from Gill et al. (1995) 20–30 kbar. This clearly indicates the usefulness of comparing carefully selected natural basalt compositions with melts produced experimentally from a suitable source under known, realistic, pressure and temperature conditions.
Controls on magma segregation pressures
Placing limits on the depth of magma segregation allows the controls affecting this to be considered. Lithospheric thickness placed two important constraints on magmatism in the North Atlantic Province. First, it restricted the sites of initiation of regional volcanism to zones of already thinned crust, that is, the British Province associated with Mesozoic rift basins, and West Greenland adjacent to opening of the North Labrador Sea (Chalmers, 1991; Thompson & Gibson, 1991). Second, the lithosphere acted as a cap that halted upward progression of the melt column and truncated the low-pressure melting region (McKenzie & Bickle, 1988; Ellam, 1992).
Some workers (Fram & Lesher, 1993, 1997; Kerr, 1994) have suggested that lithospheric thickness, the ‘lid-effect’, controlled magma segregation pressures throughout the North Atlantic Province. Considering the range of apparent magma segregation pressures (Fig. 7) from spatially and temporally restricted basalt outcrops in the Province (Figs 1 and 2) to be more precise, we suggest that lithospheric thickness controlled the upper limit of the melting region below which melts segregated, and erupted, from a range of depths within the melt column. For example, rather than invoking an undulating base for the Tertiary lithosphere under the Small Isles we would suggest that each island represents a discrete magma batch from the plume. Clearly, this has important implications, discussed below, regarding conditions of magma generation; for example, the percentage of mantle melt is not necessarily, as has previously been suggested, a function of melt column length with the latter being dependent on overlying lithosphere thickness (Fram & Lesher, 1993). In agreement with our suggestion, some recent studies from East Greenland (Thy et al., 1998) and Mull (Kerr et al., 1999) have indicated that temporal variations in basalt compositions are not consistent with a simple ‘lid-effect’ controlling extent and depth of melting. In detail, to explain compositional variation in the Mull basalts by a ‘lid-effect’ 15 km of lithosphere would need to be removed in 1–2 my. Thermodynamic modelling (Fowler, 1990) indicates, however, that 1–2 km is a more realistic estimate (Kerr et al., 1999). Furthermore, no evidence exists for the substantial surface uplift that would be expected from thinning the lithosphere by 15 km. Similarly, Fram et al. (1998) concluded that the amount of lithospheric thinning, 15 km, required to generate the compositional variation in ODP leg 152 basalts by a ‘lid-effect’ was not physically possible.
However, what controls the depth at which magmas segregate from the melt column is an open question. Light may be shed upon this by detailed studies, in both the North Atlantic and other large igneous provinces, of the relative timing and volumes of magmas erupted from different depths within the melt column. Used with discretion major element data may be a simple, widely available, tool to track changes in magma segregation depths.
Geochronological considerations
Early Tertiary magmatism in the North Atlantic Province initiated at approximately the same time, over a distance of 2000 km, in West Greenland, East Greenland and the British Isles (Fig. 2). Recent dating of the Small Isles, Muck lavas has led to the conclusion that these are the earliest, 63 Ma, manifestations of British, and indeed North Atlantic, Tertiary activity (Pearson et al., 1996). We suggest from their apparent segregation depth, 86–93 km, that they came from near the middle of the melt column (Fig. 7). It might be expected that magmas segregated from shallower in the melt column would erupt before deeper segregated magmas (see Thy et al., 1998), which is in accord with the suggestion that regional lithospheric thin spots control the location of initiation of regional volcanism (Thompson & Gibson, 1991). In agreement with this, the Rum lavas that apparently segregated from deepest in the melt column, 95–120 km (Fig. 7), are some of the youngest in the British Province (Fig. 2). Given the recent, systematic detailed sampling of some British Province lava fields (e.g. Scarrow, 1992; Kerr, 1993) the relative age of the basalts related to their depth of segregation is a testable hypothesis and may indicate suitable targets for dating work to identify the oldest, and youngest, lavas in the Province.
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MANTLE POTENTIAL TEMPERATURES |
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Assuming anhydrous melting, the rationale for which is discussed above, a mantle potential temperature of
1440°C can be equated to the maximum, major element, calculated pressure of 35 kbar (McKenzie & Bickle, 1988), that is, an increase of 100–160°C above the normal mantle temperature of 1280–1300°C (McKenzie & Bickle, 1988). This estimate is in good agreement with the presence of widespread Phase 1 picritic basalts associated with elevated mantle temperatures, and with published mantle potential temperature estimates from throughout the North Atlantic Province.
In the British Province, Kent (1995) suggested a mantle potential temperature of 1350–1460°C from olivine compositions in magnesian basalt dykes from the Outer Hebrides and Skye; Kerr (1995) proposed 1420–1460°C for Mull basalts using the equations of McKenzie & Bickle (1988); and McClurg (1982) proposed 1420–1460°C for the Rum basalts source from olivines in a peridotite dyke. The McKenzie & Bickle (1988) based mantle potential temperature estimate suggested for West Greenland is somewhat higher, 1540–1600°C (Gill et al., 1992); in fact, as noted by Saunders et al. (1997), surprisingly high given the distance of the basalts from the plume axis (Fig. 1). However, the temperature estimate of Gill et al. (1992) was based on parental magmas with 19 wt % MgO, a figure that has recently been revised to 14·5–16·5 wt % MgO by Lightfoot et al. (1997) and that, again using the McKenzie & Bickle (1988) equations, gave a lower temperature similar to that estimated for the Skye basalts (Lightfoot et al., 1997) and the geographically closely associated Baffin Bay basalts, 1300–1500°C (Francis, 1985). The East Greenland basalts, by contrast, being closer to the plume axis, would be expected to have been derived from mantle with a higher potential temperature: 1500°C has been suggested for central East Greenland (Fram & Lesher, 1993; Tegner et al., 1998), and 1500–1600°C for southeast Greenland (Fitton et al., 1995). These last figures, all of which were calculated from the same equations of McKenzie & Bickle (1988), are consistent with an estimated +200°C elevated mantle temperature for the modern Iceland plume (White, 1992) but are higher than the +100°C estimate of Ribe et al. (1995) the latter being consistent with the suggestion that the plume has cooled over time (Nadin et al., 1997; Tegner et al., 1998).
The similarity in the temperature anomaly of early erupted basalts distal to the plume axis has implications for the structure and development of the ancestral Iceland plume; this is discussed in the next section.
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THE ANCESTRAL ICELAND PLUME |
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Mantle source characteristics
It is widely held that the generation of the North Atlantic Tertiary Province flood basalts was linked to the mantle plume that is now beneath Iceland (e.g. Brooks, 1973
; White & McKenzie, 1989; Campbell & Griffiths, 1990; Fitton et al., 1998). However, despite the plume being the focus of numerous studies, Tegner et al. (1998) noted that its Tertiary thermal structure, composition and position are still not well defined.
Plumes probably originate at a boundary layer deep within the Earth; this has been suggested to be the 670 km discontinuity, the 2900 km D'' core–mantle boundary layer (Allègre & Turcotte, 1985; Hofmann, 1997), or perhaps a variable-thickness, compositionally distinct, dense lower-mantle layer below 1600 km (Albarède & van der Hilst, 1999; Kellogg et al., 1999). The derivation may be different for different plumes, but it is generally agreed that most long-lived (>50 my) plume conduits ascend from the lower mantle, either from the core–mantle boundary D'' layer (Loper & Stacey, 1983; Kerr, 1991; Hauri et al., 1994) or the dense layer overlying it (Albarède & van der Hilst, 1999; Kellogg et al., 1999). Whether or not the boundary layer from which plumes are derived is compositionally distinct from its surrounding mantle should be reflected in plume-derived basalt compositions.
In fact, although as noted by Hards et al. (1995) all plumes should be compositionally heterogeneous with the proviso that this will be evident only in those where mantle or crustal lithosphere is negligible and the degree of melting is small enough not to dilute more enriched components, in general plume magmas are consistently more Fe rich than mid-ocean ridge magmas (Francis, 1995; Turner & Hawkesworth, 1995; Francis et al., 1999). For example, the Hawaiian tholeiites source has long been suggested to be Fe rich relative to the MORB source (Wilkinson, 1991). This may be a function of the two environments having different magma generation and segregation processes but, in recent years, several researchers have preferred to attribute the difference to source composition. Herzberg & Zhang (1996) proposed that partial melts solidified at depth could be an important process in creating source regions that give Fe-rich magmas at a later stage of melting. In contrast, Cordery et al. (1997) suggested that the Fe-rich nature of continental flood basalts and ocean island basalts (OIB) relative to MORB, at an equivalent stage of fractionation, as noted by Campbell (1997), resulted from a significant component of eclogite, derived from ancient subducted oceanic crust, present in the plume source at the crust–mantle boundary. More recently, Francis et al. (1999) concluded that plumes tap an Fe-rich sink residual to more Fe-rich Archean plume sources formed either in the lower mantle or at the D'' core–mantle boundary.
In addition to the effect of source composition it is well established, discussed above, that as depth of melting increases so does primary magma’s Fe content (e.g. Jaques & Green, 1980; Takahashi, 1986; Falloon & Green, 1987; Hirose & Kushiro, 1993; Baker & Stolper, 1994).
From the above we may conclude that, whatever the process involved in its generation, the low mg-number peridotite HK-66 (Hirose & Kushiro, 1993) may represent, at least with respect to major elements, plume mantle composition. The difference between this Fe-rich peridotite and most mantle nodules may reflect the paucity of analysed samples of mantle residual to plume magmatism. In particular, many high mg-number peridotite analyses are either of ophiolitic material or xenoliths that are probably representative of upper-mantle material residual to MORB formation. The Fe-rich peridotites may be inferred to be more fertile than the high mg-number peridotites because melt extraction raises residual mantle mg-number. Accordingly, plume-related basalts that are apparently derived from trace element depleted mantle sources (e.g. Gorgona) have lower Fe than typical OIBs and flood basalts. Campbell (1997) noted a general positive correlation in Fe and incompatible trace element enrichment in plume picrites; he proposed that this was proportional to the amount of eclogite in the plume source.
As noted by Saunders et al. (1997), the North Atlantic Province is a suitable place to consider plume processes given the excellent preservation of Tertiary to present plume head to axis derived basalts.
In agreement with the above suggestions of plume source location, Kerr et al. (1995) suggested an origin from the D'' boundary layer or entrained lower mantle for the Iceland plume depleted isotopic component, and Larsen & Saunders (1998) concluded that Phase 1 Tertiary North Atlantic magmatism was rapidly emplaced and resulted from melting of anomalous mantle from either the 670 km discontinuity or the D'' core–mantle boundary.
Compositional heterogeneity is observed in the North Atlantic basalts with, in addition to the Fe-rich nature of many, evidence for incompatible element and isotopic enrichment and depletion in some cases similar to that seen in modern Iceland basalts; for example, in the British Province (e.g. Carter et al., 1979; Morrison et al., 1980), West Greenland (Holm et al. 1993; Lightfoot et al., 1997), East Greenland (Tegner et al., 1998), southeast Greenland (Fram et al., 1998) and Baffin Island (Robillard et al., 1992). In terms of their major element compositions early erupted primitive basalts from the British Province and West Greenland are comparable, whereas those of East Greenland are somewhat different (Figs 3 and 4); possible reasons for this are discussed in the next section. Other datasets from West and central East Greenland (Larsen et al., 1989b; Holm et al., 1993) were recalculated to 15 wt % MgO and found to plot in the same fields in Fig. 4 as the datasets from Table 1. As noted above, the British Province and West Greenland data overlap with the compositional field of melts of low mg-number (85·5) peridotite HK-66 (residua of melting of which would have mg-number 86·5–87·5) indicating that they were derived from a more Fe-rich source than most mantle nodules, which generally have mg-number >89 (e.g. Herzberg, 1993). This observation is in agreement with the findings of Fram & Lesher (1993, 1997) and Larsen et al. (1998), who commented that the Tertiary North Atlantic basalts are more Fe and Ti rich than most of the modern basalts from Iceland. Notably, they attributed this to mantle melt systematics rather than post-magma extraction processes. Some modern Iceland basalts, however, for example from Krafla, are compositionally comparable with the British and West Greenland data of this study (e.g. Nicholson & Latin, 1992).
The relationship between depth of melting and Fe content of the North Atlantic basalts has been considered by numerous workers (Gill et al., 1992; Fram & Lesher, 1993, 1997; Larsen et al., 1998). Gill et al. (1992) suggested that the high Fe content of West Greenland basalts was related to deep melting and commented that they are compositionally similar to the highest Fe values from present-day Iceland. The same conclusion was drawn by Fram & Lesher (1993, 1997) for the central East Greenland basalts, and by Larsen et al. (1998) in a review of North Atlantic Tertiary basalts. In detail, Fram & Lesher (1993, 1997) concluded that temporal and spatial changes in primary magma compositions were most strongly controlled by lithospheric thickness and to a lesser extent by variation in mantle potential temperature, with modest melting in a restricted melt column below thick lithosphere, not melting of an enriched source responsible for Early Tertiary incompatible element enrichments. Clearly, the lithosphere can act as a lid trapping and truncating asthenospheric melting events (McKenzie & Bickle, 1988; Ellam, 1992), the ‘lid-effect’.
The possibility of the ‘lid-effect’ being superimposed on source-derived Fe-rich compositions has not, on the other hand, received much consideration for the North Atlantic rocks. We suggest that the more fundamental control on primary magma compositions, rather than the ‘lid-effect’, is the Fe-enriched nature of the plume source lower-mantle peridotite of the D'' layer or postulated overlying dense layer. Rising magma from this may be variably diluted by plume head entrainment of lower- and upper-mantle material, and later depth of melting compositional variations would be superimposed on the source-derived character.
The difference in interpretation of the main compositional control by source composition as opposed to lithospheric thickness ‘lid-effect’ is evident in consideration of the North Atlantic data. For example, in a detailed study of the lower Tertiary central East Greenland basalts, Fram & Lesher (1997) noted that FeO contents of primary melts of modern Reykjanes Ridge and Tertiary East Greenland basalts overlap. However, the REE data indicate that the East Greenland basalts were generated by melting in the garnet field and Reykjanes Ridge basalts were generated at a shallower depth in the spinel field. As Fram & Lesher (1997) concluded that ‘lid-effect’-controlled depth of melting was the dominant constraint on, and proportional to, Fe content, they suggested that the Tertiary mantle source must have had a higher mg-number for the East Greenland basalts to have similar FeO content to modern basalts that were produced by melting at shallower depth. It should be noted, however, that despite their different primary magma Fe contents Fram & Lesher (1997) grouped the modern Knipovich, Mohns, Kolbeinsey and Reykjanes Ridge basalts, assumed that they were derived from a MORB source, and, consequently assigned them a source mg-number of 0·89 [after Langmuir et al. (1992)]. This may not, of course, be the case; in particular, the high-Fe basalts may have been derived from a source with a lower mg-number.
As stated above, our conclusions regarding depth of melt segregation do not support the suggestion of Fram & Lesher (1993, 1997) that the percentage of mantle melt is necessarily a function of melt column length with the latter being dependent on the thickness of the overlying lithosphere. More probably, variable-percentage melts segregate from a range of depths in the column (Fig. 7). Therefore we suggest that the similar FeO content between the Tertiary and modern basalts is controlled by the source composition not the depth of melting. Even if the Tertiary ancestral Iceland plume mantle source did have a higher mg-number than the modern source, given the uncertainty and probable variability in the modern source noted above, it would not, de facto, preclude the Tertiary mantle plume source from being relatively Fe rich. The Fe-rich Reykjanes Ridge basalts could, consistent with their plume-dominated isotopic character (Taylor et al., 1997), have been derived from a source with lower mg-number than 0·89 typical for the MORB source. This still leaves potential for the Tertiary basalts to be derived from a source intermediate in mg-number between them and typical peridotite. An alternative explanation is that the same mantle reservoir was tapped by the plume to produce the Tertiary East Greenland and modern Reykjanes Ridge basalts but with the deeper generated, higher Tertiary Fe content diluted by entrainment of upper-mantle material in the plume head en route to the surface.
Larsen & Saunders (1998) envisaged the ancestral Iceland plume as an initial large flux caused by a large volume of thermally buoyant plume material entraining material en route to the surface, the head, that created a path for the following plume stem that had significant lower mantle flux. This model is considered below.
Proximity of magma generation to plume focus
Studies of Hawaiian basalts have shown that their compositions provide information about the extent of melting and depth of melt segregation, and that these factors vary systematically temporally and spatially in relation to the plume axis; for example, axial magmas represent higher degrees of melting at shallower depths than magmas peripheral to the axis, where the mantle is cooler (e.g. Frey & Rhodes, 1993; Yang et al., 1996). The North Atlantic is a suitable area for similar studies because of the excellent preservation of Tertiary to present plume head to axis derived basalts. Conclusions drawn from this region may be applied to studies of other large igneous provinces.
Two main locations have been suggested for the axis of the ancestral Iceland plume at 63 Ma: close to the centre of Greenland (Lawver & Müller, 1994) and near the east coast of Greenland (Brooks, 1973; White & McKenzie, 1989). Figure 1a, a tectonic reconstruction of the North Atlantic during the Early Tertiary, shows the extent of the igneous activity at that time, the two proposed plume axis positions, and the extent of associated plume head thermal anomalies of 1000 km radius suggested by White & McKenzie (1989). We favour the more easterly plume axis position for several reasons: (1) as noted by Saunders et al. (1997), there is no evidence for a hot-spot track between central and eastern Greenland; (2) in the more easterly configuration the British Province basalts fall within the, albeit assumed circular, postulated plume head area whereas in the westerly configuration they do not; (3) from geochronological considerations, magmatism started somewhat (1 my) earlier in the British Province than in West Greenland, and magmatism in East Greenland initiated 10 my earlier than predicted by the hot-spot track of Lawver & Müller (1994); (4) the suggestion of several recent studies that the character of the East Greenland basalts is consistent with close proximity to the plume axis (e.g. Storey et al., 1998; Hansen & Nielsen, 1999) also favours the eastern plume position. For these reasons, we selected samples from the British Province and West Greenland apparently located equidistant (800 km) from the plume axis, and samples from East Greenland near the plume axis. From consideration of major element data, the first two are compositionally comparable whereas East Greenland basalts show distinct compositional difference, for example, they are Si and Fe rich (Table 1), which we suggest reflects a difference in their generation related to their proximity to the plume axis.
Our conclusions regarding the major elements of the North Atlantic basalts are therefore not consistent with the proposal of Gill et al. (1995) that another plume, additional to the ancestral Iceland one, was situated under West Greenland in the Early Tertiary. Nor is the proposal of a smaller transient plume within the British Province (Nadin et al., 1997), proposed to contribute to uplift, consistent with the major element data, which indicate that the British Province and West Greenland basalts were related to the same large plume.
Modern Icelandic volcanism taps a geochemically heterogeneous mantle including depleted and relatively enriched portions; for example, in the Neovolcanic zone, basalts vary in composition from strongly light REE (LREE) depleted to moderately LREE enriched and form an array in isotope space (Hémond et al., 1993) typical of OIB (see Zindler & Hart, 1986). Isotope studies show that the Iceland plume contains a significant depleted component that is distinct from, in being slightly less depleted than, MORB (Thirlwall et al., 1994; Hards et al., 1995). Analogies can be made between the Tertiary and modern situation from similarities noted above in major element compositions and in Tertiary and modern isotopic compositions, for example, between West Greenland and modern Iceland (Lightfoot et al., 1997). Kerr et al. (1995) suggested that the source of the depleted plume component, which they proposed was an intrinsic component of plumes originating in the deep mantle (e.g. Gorgona, Hawaii, Iceland), was either from the 670 km discontinuity or, more likely, the D'' core–mantle boundary or the postulated dense layer overlying it. Accordingly, from geophysical data Wolfe et al. (1997) inferred a deep-seated source for the Iceland thermal anomaly. Both Hards et al. (1995) and Kerr et al. (1995) envisaged that the Iceland plume was composed of enriched streaks of plume source within a matrix of less enriched material derived either from the plume source or entrained during ascent, so resulting in preferential extraction of enriched material near the plume axis. In this way, more enriched, less depleted material may be expected nearer the plume axis (i.e. East Greenland) rather than at distal locations representing more entrainment-diluted plume head material (i.e. the British Province or West Greenland). This effect may be the cause of the differences we see in the major elements between the different regions. If, as discussed above, the plume source is Fe rich relative to the upper mantle, compositional zonation may also be reflected in erupted products, with Fe content being higher in plume axis rather than in entrainment-diluted plume head magmas. This compositional variation is not seen strongly in Phase 1 basalts, but as these are all interpreted to be derived from the plume head perhaps this is not surprising. In contrast, highly Fe- and Ti-rich basalts at Scoresby Sund are associated with initiation of Phase 2 magmatism associated with continental rupture; their composition has been attributed to fractionation of plagioclase and mafic phases (Larsen et al., 1989b) but, as suggested by Kerr et al. (1999) for the Mull basalts, the fractionation effect may be only enhancing a fundamental feature of the basalts, which may be attributed to them being early magmas generated in the Fe-rich plume axis. This is in agreement with the suggestion of Larsen & Saunders (1998), who envisaged an initial large flux caused by a large volume of thermally buoyant plume material, the head, that created a path for the following plume stem that had significant lower mantle flux.
So, the suggestion of Saunders et al. (1997) that radial chemical gradients may exist in the plume source from the axial region, where the hottest mantle decompresses most, outwards is supported by the major element compositions of the primitive early-erupted basalts of the North Atlantic Tertiary Province. These results could stand further examination by consideration of basalts within and outside the geographic scope considered in the present study. Suitable targets for further investigation include: onshore, Hold with Hope (NE Greenland), the Faeroe Islands and the Isle of Lundy; offshore, seaward-dipping reflector sequences sampled in ODP leg 81 (Rockall Plateau–Hatton Bank), Deep Sea Drilling Project (DSDP) leg 38, ODP leg 104 (Voring Margin), and ODP legs 152 and 163 (SE Greenland margin); and, in addition, further detailed comparison with Iceland basalts.
Plume impact or incubation
Whether large igneous provinces are the result of plume impact (Griffiths & Campbell, 1990) or plume incubation (White & McKenzie, 1989) has been the focus of much discussion in recent years (e.g. Richards et al., 1989; Kent et al., 1992). Whereas plume incubation, requiring that rifting precedes volcanism, may well be a suitable model for many large igneous provinces (Kent et al., 1992) it is not, apparently, appropriate for the North Atlantic Province, the early Phase 1 of which was extensive, widespread, pre-rift magmatism (Saunders et al., 1997). Nor is the suggestion of plate movement over a plume (Lawver & Müller, 1994) consistent with the contemporaneity and compositional similarity of Phase 1 volcanism throughout the Province. In addition, the plume incubation model of White & McKenzie (1989) suggests that plumes originate from the upper-mantle–lower-mantle boundary, whereas Griffiths & Campbell (1990), more appropriately for the long-lived Iceland plume (see Loper & Stacey, 1983; Kerr, 1991; Hauri et al., 1994), suggested that they originate from the core–mantle boundary. The latter is more in keeping with the Tertiary North Atlantic basalts’ Fe-rich and variably depleted nature. Impact, with the focus in East Greenland, has also recently been favoured for the North Atlantic Province by Fitton et al. (1998), Graham et al. (1998), Larsen & Saunders (1998) and Saunders et al. (1998). In addition, from a summary of radiometric ages from the North Atlantic Tertiary Province (Fig. 2), Saunders et al. (1997) concluded that Paleocene early magmatic activity in the British Province and SE Greenland were contemporaneous with the main phase of magmatism in Western Greenland, and Storey et al. (1998) concluded that their age data were consistent with arrival (i.e. impact) of a plume. If the plume did impact, a fall in temperature after impact may be the reason why magmatism waned after Phase 1 in regions distal to the plume axis but continued, to produce Phase 2, near the plume axis (Nadin et al., 1997; Tegner et al., 1998).
Apparent similarities in mantle source composition, and magma segregation depth, over a distance of 2000 km in early erupted basalts of the North Atlantic Province noted in the present work support a plume impact model, with magmas derived from the same plume head during its arrival (see Richards et al., 1989; Griffiths & Campbell, 1990), rather than a plume incubation model for the region.
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COMPARISON WITH OTHER CONTINENTAL FLOOD BASALT PROVINCES |
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The idea of using corrected major element compositions in continental flood basalts to look at similarities or differences relative to asthenospheric magmas has been the subject of numerous recent studies, for example, by Hergt et al. (1991)
and Turner & Hawkesworth (1995). Those workers found that at a similar stage of evolution basalts with a significant lithospheric component from the sub-continental mantle had higher SiO2, and lower Na2O, FeO and TiO2 than oceanic asthenosphere-derived basalts. Apparently, from the present work, in addition to absolute values, inter-element correlations in restricted-MgO real rock or recalculated datasets can be used to make a distinction in continental large igneous provinces between basalts that have affinities with asthenosphere-derived oceanic rocks, for example, the North Atlantic and western USA, and basalts in which lithospheric mantle is clearly an important component, for example, the Deccan, Paraná and Karoo (Fig. 5). This distinction may, in turn, reflect differences in, and be used to investigate, the relative timing of magma generation and lithospheric thinning between provinces. As noted by Saunders et al. (1997), hot mantle impinging under lithosphere thicker than 150 km will curtail asthenospheric melting at depth and cause melting of overlying lithosphere whereas hot mantle impinging under thinner lithosphere will be more asthenosphere dominated. Therefore, inter-element correlations may be useful to determine into which category flood basalts fall, that is, thick lithosphere, dominated or thin lithosphere, asthenosphere dominated. This distinction may affect whether magmas are generated by plume incubation in the former case or plume impact in the latter.
Kerr et al. (1995) showed that some continental flood basalts show evidence of being derived from an at least partially depleted source region; for example, early picrites from the North Atlantic, Siberian traps, Deccan traps and Karoo, which represent plume-derived melts not trapped and contaminated with lithosphere in a magma chamber. The isotopic depletion was suggested to come from subduction, and incorporation into the plume source, of lower oceanic lithosphere cumulates and melt residues, with the enriched plume character instead derived from subducted ocean crust and associated sediments (see Hofmann & White, 1982). Expression of depletion in plumes may be dependent on the degree of melting: large-degree melts (e.g. in the plume head) will incorporate more depleted material; lower-degree melts (e.g. in the plume axis) will be more enriched. Where the Fe-rich character of plume-derived basalts is derived from requires further study. In particular, the different end members need to be characterized to consider if, as suggested by Campbell (1997), Fe and incompatible trace element enrichments are proportional to each other and related to the amount of eclogite, or some other component, in the plume source.
Evidently, the importance of contributions from lithosphere and asthenosphere to plume-derived basalts in large igneous provinces varies from province to province; we suggest that, if rigorously applied and in particular when considered in conjunction with trace element and isotopic data, considerable potential exists for using restricted-MgO real rock or recalculated datasets (see Hergt et al., 1991; Turner & Hawkesworth, 1995) to study this question.
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SUMMARY |
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- Major element variations in restricted-MgO real rock or recalculated datasets can, potentially, give information about depth of magma segregation, mantle potential temperatures, proximity of magma generation to the plume focus, and plume source composition in large igneous provinces.
- Phase 1, 63–59 Ma, of magmatic activity in the North Atlantic Tertiary Province was dominated by polybaric melting of an anhydrous, Fe-rich, plume source mantle.
- Although there was evidently a change to shallower and greater degree of melting with time through the North Atlantic Province, recent studies may have overemphasized the importance of the ‘lid-effect’ as the main control on basalt compositions.
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ACKNOWLEDGEMENTS |
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We acknowledge K. G. Cox for initiating the current work, and whom the first author missed being around to discuss many of the questions that came up when writing this paper. D. W. Peate, C. Tegner and T. F. D. Nielsen are thanked for extremely constructive reviews that allowed us to considerably improve the manuscript. We are grateful to C. H. Emeleus for providing access to unpublished data for the Small Isles, and M. E. Dinn for help in preparation of the manuscript.
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FOOTNOTES |
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*Corresponding author. Telephone: +34-958-246176. Fax: +34-958-243368. e-mail: jscarrow{at}ugr.es
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