Long-lived postbreakup magmatism along the East Greenland margin: Evidence for shallow-mantle metasomatism by the Iceland plume M. Storey Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark A.K. Pedersen Geological Museum, Øster Voldgade 5–7, DK-1350 Copenhagen, Denmark O. Stecher S. Bernstein Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark H.C. Larsen L.M. Larsen Geological Survey of Denmark and Greenland, Øster Voldgade 10, DK-1350 Copenhagen, Denmark J.A. Baker Danish Lithosphere Center, Øster Voldgade 10, DK-1350 Copenhagen, Denmark R.A. Duncan College of Oceanography, Oregon State University, Corvallis, Oregon 97331, USA ABSTRACT 40Ar/39Ar dating has identified a succession of middle Miocene (14–13 Ma) basaltic lavas in East Greenland that overlie Eocene flood basalts that were erupted during continental breakup ca. 56– 55 Ma. The long postbreakup magmatic history (;40 m.y.) of the East Greenland margin precludes a simple relationship between this later igneous activity and the track of the Iceland hotspot. Chemical and isotopic data suggest that the postbreakup magmas were produced from mantle that had been metasomatized by light rare earth element–enriched, H2O- and CO2-bearing melts originating from the Iceland plume. Episodic melting of recently metasomatized shallow mantle beneath Greenland and the North Atlantic can explain both the composition and the long-lived nature of postbreakup magmatism along the East Greenland margin, as well as lavas on Jan Mayen Island that have enriched, Icelandictype isotopic signatures.
emental and isotopic signatures, as the result of pervasive metasomatism of ambient upper mantle by volatile-rich melts that originated by incipient melting of the Iceland plume. SAMPLES AND RESULTS The Miocene lavas (Vindtop Formation) crop out on the top of the early Eocene flood basalt plateau on a number of nunataks within an 18 km2 area, ;200 km northeast of the assumed Iceland plume track (Fig. 1). We sampled 19 lavas from the Vindtop Formation along with 3 associated dikes. Two flows contain plagioclase megacrysts, which were separated for 40Ar/39Ar dating at Oregon State University (OSU). Whole-rock major and trace element analyses were determined by X-ray fluorescence and inductively coupled plasma–mass spectrometry (ICP-MS) at the Geological Survey of Denmark and Greenland and OSU. The Pb, Sr, and Nd isotope compositions were measured by using
Keywords: Greenland, North Atlantic, Iceland, Jan Mayen, Miocene, continental margin, mantle plumes, flood basalts, metasomatism. INTRODUCTION The East Greenland volcanic rifted margin is characterized by voluminous flood basalts, erupted ca. 56–55 Ma, synchronous with the formation of a spreading center and emplacement of seaward-dipping reflector sequences (Fig. 1). This magmatism has been explained by continental breakup in the vicinity of the ancestral Iceland hotspot (e.g., Saunders et al., 1997). In addition to the 56–55 Ma magmatism, however, studies have demonstrated the presence of widespread postbreakup magmatism (e.g., Tegner et al., 1998). A significant part of this magmatism took place within the first 15 m.y. after breakup, hypothesized to have been caused by the drift of the Greenland margin over the Iceland plume (Lawver and Mu¨ller, 1994; Tegner et al., 1998; Bernstein et al., 1998). However, a review of available age data shows that postbreakup magmatism along the East Greenland margin has occurred sporadically over a time period much longer than can satisfactorily be explained by the hotspot track hypothesis alone (Fig. 1). Here we report the discovery of a sequence of middle Miocene basaltic lavas, thus demonstrating that the postbreakup magmatic history of the East Greenland margin spans at least 40 m.y. Younger volcanism at Vesteris Seamount and Jan Mayen Island also represents igneous activity in the North Atlantic realm unrelated to the oceanic spreading center and not easily explained by the presence of the Iceland mantle plume. We examine the origin of the extended postbreakup magmatic history of the East Greenland margin and explain the apparent longterm regional presence of enriched-mantle peridotite, with distinct el-
Figure 1. East Greenland margin (COB 5 continent-ocean boundary) with location (star) and stratigraphy of Miocene lavas (Vindtop Formation). 1Si—two high Si-lavas divide Vindtop Formation into lower and upper parts; Skrænterne—55 Ma flood-basalt formation (Pedersen et al., 1997); small circles—sampled lava flows; Lilloise and Borgtinderne—early Tertiary intrusions. Column at right shows age distribution of igneous activity (green) along East Greenland margin; age data are from this paper, Tegner et al. (1998), Hansen et al. (2002), Gleadow and Brooks (1979). Inset: Suggested age progression of Iceland hotspot track (Lawver and Mu¨ller, 1994) and location of main map.
q 2004 Geological Society of America. For permission to copy, contact Copyright Permissions, GSA, or
[email protected]. Geology; February 2004; v. 32; no. 2; p. 173–176; DOI 10.1130/G19889.1; 3 figures; Data Repository item 2004021.
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Figure 2. Trace element variations for Vindtop Formation (Fm.) lavas. Inset: lower and upper Vindtop Formation compositions normalized to primitive mantle (PM; Sun and McDonough, 1989). A: Chondritenormalized (N) Yb/Dy vs. Nd/Dy showing Vindtop Formation and other postbreakup East Greenland magmas to have excess light rare earth elements compared to synbreakup flood basalts, Iceland (on axis), and calculated liquid compositions aggregated from spinel (sp) and garnet (gt) peridotite melting (two trajectories: melting parameters in Bernstein et al., 1998); circled C symbol—highSi, strongly crustally contaminated lavas. B: Lavas from lower part of Vindtop Formation have high Zr/Hf (>50) and low Yb/Dy (N). Zr/Hf ratios decrease in upper part of Vindtop Formation, possibly reflecting exhaustion of amphibole in source. Data sources for both A and B: East Greenland (EG) flood basalts—Danish Lithosphere Center database. EG postbreakup transitional (trans.) dikes—Hanghøj et al. (2003). EG postbreakup gabbros—Bernstein et al. (1998). EG-POW (Prinsen of Wales Bjerge [mountains])—Peate et al. (2003). EG melilitites—Bernstein et al. (2001). Iceland—Furman et al. (1991), Chauvel and He´mond (2000), Kempton et al. (2000), Breddam (2002). Jan Mayen—Haase et al. (1996), Trønnes et al. (1999). Vesteris Seamount—Haase and Devey (1994). Mid-oceanic-ridge basalt (MORB) and oceanic-island basalt (OIB) fields—Dupuy et al. (1992).
the Danish Lithosphere Center multicollector ICP-MS facility. Six other East Greenland postbreakup lavas and intrusions were also analyzed; two alkaline lavas that erupted very shortly (,1 m.y.) after breakup, and two samples each from the basic-ultrabasic 50 Ma Lilloise intrusion and the 47 Ma syenitic Borgtinderne intrusion (Fig. 1). The complete data set on these rocks, along with analytical details, is available.1 40Ar/39Ar
GEOCHRONOLOGY The 40Ar/39Ar results are given in Table DR1 (see footnote 1). Plagioclase from sample 436144 gave a robust plateau age of 13.4 6 0.2 Ma for the first three steps, in which nearly 80% of the total amount of 39Ar was released. The 1250 8C and 1400 8C steps gave older ages of 18.0 Ma and 25.0 Ma, respectively, indicating a high-temperature site of excess 40Ar. The isochron (first three heating steps) gave an age of 13.6 6 0.2 Ma with an atmospheric initial 40Ar/36Ar ratio of 294.4 6 8.9. Sample 436153 produced a saddle-shaped age vs. temperature profile, indicative of excess 40Ar. Steps 2 and 3 were concordant and gave a maximum age estimate of 14.2 6 0.4 Ma. A 40Ar/36Ar vs. 39Ar/ 36Ar regression sample defines a nonatmospheric intercept (40Ar/36Ar 5 425.7 6 56.8). However, the slope corresponds to an age of 13.6 6 0.5 Ma, consistent with the age given by sample 436144. CHEMISTRY The Vindtop Formation comprises transitional to mildly alkaline basalts with 5–9.6 wt% MgO and 47–49 wt% SiO2 (Table DR2; see footnote 1). Two adjacent flows, about halfway up the Vindtop succession, have higher SiO2 (51–53 wt%). Notable features of the Vindtop Formation are the high Al2O3 contents and low CaO/Al2O3 ratios compared to most mid-oceanic-ridge basalt (MORB) and oceanicisland basalt (OIB) (Table DR2; see footnote 1). Compared to breakup-related East Greenland flood basalts, the Vindtop Formation is enriched in the light rare earth elements (LREEs), as are postbreakup coastal dikes (Hanghøj et al., 2003) and intrusions (Bernstein et al., 1998) (Fig. 2A). This feature is also shown by basaltic 1GSA Data Repository item 2004021, Tables DR1–DR3, is available online at www.geosociety.org/pubs/ft2004.htm, or on request from editing@ geosociety.org or Documents Secretary, GSA, P.O. Box 9140, Boulder, CO 80301-9140, USA.
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lavas from Jan Mayen, Vesteris Seamount, and off-axis Icelandic volcanoes (Fig. 2A). The Vindtop Formation has high contents of the incompatible elements Ba, Nb, and La, but exhibits relative depletion of Rb, K, Th, Zr, and Hf (Fig. 2 inset). The lower Vindtop Formation has lower Yb/Dy, higher Nd/Dy, and suprachondritic Zr/Hf ratios (.45) compared to upper Vindtop Formation lavas (Fig. 2). Postbreakup coastal dikes in Greenland and rocks from Jan Mayen and Vesteris Seamount also trend toward high Zr/Hf values (Fig. 2B). Pb isotopic data for the Vindtop Formation and other postbreakup magmas (Table DR3; see footnote 1) produce a strongly correlated trend in a plot of 207Pb/204Pb vs. 206Pb/204Pb (Fig. 3A), that has a slope corresponding to an age of 2846 6 76 Ma (mean square of weighted deviates 5 6.7). The regression line intercepts the Stacey and Kramers (1975) evolution curve at 3014 Ma, which is similar to Sm-Nd model ages (ca. 3.0–2.8 Ga) for basement gneisses and granulites (Taylor et al., 1992), which also plot along this trend (Fig. 3A inset). The simplest explanation is that the regression line is a mixing line and that the postbreakup magmas have been subjected to variable contamination by Archean crustal rocks. Similarly, the hyperbolic arrays in Sr vs. Pb and Nd vs. Pb isotopic space (Fig. 3B, 3C) are consistent with the mantle source of the Vindtop Formation sharing the same isotopic character as Oræfajo¨kull (Iceland) and Jan Mayen, the difference being that the Vindtop Formation magmas have undergone variable contamination by ca. 3.0 Ga basement rocks. METASOMATIZED MANTLE SOURCE FOR EAST GREENLAND POSTBREAKUP MAGMATISM The high Al2O3 contents and low CaO/Al2O3 ratios of the Vindtop Formation cannot be explained by either clinopyroxene fractionation or plagioclase accumulation. The lack of correlation between CaO/ Al2O3 and either Cr or Sc rules out clinopyroxene fractionation, and Sr/Nd shows a weak positive correlation with CaO/Al2O3, opposite to what would be expected for plagioclase accumulation. High Al2O3, however, is consistent with an amphibole-bearing mantle source (Foley et al., 1999), as has been suggested for the Vesteris Seamount (Haase and Devey, 1994). Melts produced immediately above the solidus of an amphibole-bearing peridotite will not necessarily have low CaO/ Al2O3 ratios, but as the degree of melting increases, CaO/Al2O3 deGEOLOGY, February 2004
Figure 3. Pb-Sr-Nd isotopic variations for Vindtop Formation and other postbreakup rocks from East Greenland. Data are age corrected to time of extrusion. Other data: Oræfajo¨kull, Iceland (Prestvik et al., 2001), Jan Mayen (JM) (Trønnes et al., 1999), normal midoceanic-ridge basalt (N-MORB) from Mohns Ridge north of Jan Mayen (Mertz and Haase, 1997; Schilling et al., 1999), and Greenland basement gneisses from Kræmer Ø (island) (Taylor et al., 1992). Iceland compositional field (ICE) is from Stecher et al. (1998). A: Best-fit line for postbreakup data has slope that corresponds to age of 2846 6 76 Ma (MSWD [mean square of weighted deviates] 5 6.7) and that intersects Stacey and Kramers (1975) growth curve at 3014 Ma (see inset). Note that two high-Si lavas from middle part of Vindtop Formation have lowest 206Pb/204Pb ratios. B and C: Postbreakup rocks define hyperbolic mixing arrays in Sr vs. Pb and Nd vs. Pb space between local Archean gneisses (silicic and mafic granulites) and end member of Iceland compositional field having high 87Sr/86Sr and low 143Nd/144Nd ratios. Calculated mixing curves are based on following end-member compositions: enriched Iceland—Sr 5 900 ppm, 87Sr/86Sr 5 0.7035–0.7037, Nd 5 50, 143Nd/ 144 Nd 5 0.51285–0.51291, Pb 5 5 ppm, 206Pb/204Pb 5 18.70, 207Pb/ 204 Pb 5 15.50, 208Pb/204Pb 5 38.30; silicic granulite—Sr 5 120 ppm, 87 Sr/86Sr 5 0.7600, Nd 5 20, 143Nd/144Nd 5 0.5108, Pb 5 14 ppm, 206 Pb/204Pb 5 14.00, 207Pb/204Pb 5 14.500, 208Pb/204Pb 5 33.50; mafic granulite—Sr 5 400 ppm, 87Sr/86Sr 5 0.7055–0.7060, Nd 5 5, 143Nd/ 144 Nd 5 0.5114, Pb 5 10 ppm, 206Pb/204Pb 5 14.00, 207Pb/204Pb 5 14.500, 208Pb/204Pb 5 33.50.
creases from ;1.1 to values as low as ;0.4 owing to the incongruent melting of amphibole, which by reaction produces clinopyroxene (in addition to olivine) with much higher CaO/Al2O3 (Foley et al., 1999). This transition from melts with CaO/Al2O3 of ;1.1–0.4 happens within a temperature increase of only 25 8C, which corresponds to the exhaustion of amphibole (Foley et al., 1999). Hence we interpret the low CaO/Al2O3 in the Vindtop Formation as a result of melting of amphibole-bearing peridotite, a view supported by the lavas’ depletion in K relative to Th (Fig. 2 inset) (LaTourette et al., 1995). The LREE-enriched character of the Vindtop Formation (Fig. 2A) GEOLOGY, February 2004
is not inherited from contamination by local Archean gneisses, from the fact that 206Pb/204Pb correlates positively with Nd/Dy (not shown). Bernstein et al. (1998) suggested that the mantle source of the East Greenland postbreakup magmatism was enriched in LREEs by some recent process, as required by the positive «Nd values. The addition of LREEs to the Vindtop Formation’s mantle source happened without a concomitant contribution of the high field strength elements Zr, Hf, and Ti (Fig. 2 inset). The suprachondritic Zr/Hf ratios in the lower Vindtop Formation, and to a lesser extent in the postbreakup coastal dikes and rocks from both Jan Mayen and Vesteris, are features consistent with mantle that has been metasomatized by CO2-bearing melts. In general, this type of metasomatism will lead to high LREE/(Zr 1 Hf) and high Zr/Hf ratios (Dupuy et al., 1992; Ionov et al., 1993). Melting and crystal-fractionation processes are unlikely explanations for the high Zr/Hf ratios because of the near-identical bulk distribution coefficients of Zr and Hf in basaltic melts (Halliday et al., 1995). Crustal contamination can also be ruled out as an explanation because the most crustally contaminated Vindtop Formation lava (highest Si, lowest 206Pb/204Pb) has a normal (chondritic) Zr/Hf ratio (Table DR2; see footnote 1). The high Ba/Rb and Ba/K ratios of the Vindtop Formation (implied in Fig. 2 inset) support the conclusion that the metasomatizing agent included CO2-rich fluids or melts. Ionov and Hofmann (1995) reported high Ba/Rb, Ba/Th, and Ba/K ratios for separated amphibole from mantle xenoliths, and high Ba/Rb ratios are typical for most carbonatites (Hoernle et al., 2002). Isotopic variations for the Vindtop Formation and other postbreakup igneous rocks are consistent with mixing between ca. 3.0 Ga Archean high-grade basement rocks and a component represented by the end member of the Iceland compositional field having high 87Sr/86Sr and low 143Nd/144Nd ratios (Fig. 3). The chemical and isotopic data for the postbreakup magmas are best explained by production from mantle that had previously been metasomatized by LREE-enriched, H2O- and CO2-bearing melts originating from the Iceland plume. Tapping of a metasomatized mantle source can also explain the origin of recent volcanic rocks from Jan Mayen volcano, which is located on a transform fault next to a microcontinent that rifted from East Greenland during the early Eocene to Miocene, without the need to propose a separate Jan Mayen plume (Schilling et al., 1999). Episodic melting of North Atlantic metasomatized mantle, with frozen-in Icelandic isotopic compositions, may have been triggered by a variety of mechanisms including small thermal perturbations or mantle decompression due to lithospheric tectonism such as leaky transforms, crustal extension, or exhumation by uplift and erosion. Fissiontrack data for the Greenland margin show that the eruption of the Vindtop Formation lavas coincided with a period of crustal exhumation (Hansen and Brooks, 2002). Small amounts of volatiles will enable incipient melting (K1%) of plume mantle, with dehydration and decarbonation, at depths of ;170 km, ;50 km below the dry solidus for mantle with a potential temperature of ;1480 8C (Hirschmann et al., 1999). The low-viscosity, volatile- and incompatible element–rich melts formed by such a low degree of partial melting would separate and rapidly ascend along a channelized network, pervasively metasomatizing the shallow mantle, as proposed by several authors for the lithospheric mantle (Hauri and Hart, 1994; Halliday et al., 1995; Baker et al., 1998) and for ambient asthenosphere (Halliday et al., 1995). This metasomatized mantle may later be the source of enriched intraplate volcanism (Baker et al., 1998) or enriched OIBs and MORBs (Halliday et al., 1995). Widespread metasomatism of the shallow mantle may have occurred ca. 61 Ma, when the Iceland plume is thought to have impacted and rapidly spread out beneath the Greenland lithosphere (e.g., Storey et al., 1998). Further large-scale metasomatism of the North Atlantic mantle probably occurred during the subsequent passage of the Iceland 175
plume to its present-day position. We conclude that metasomatism of Earth’s upper mantle by high-pressure volatile-rich melts produced by incipient melting of plumes, like those beneath Iceland and Hawaii, may be a significant process for the redistribution of incompatible elements between different mantle reservoirs. ACKNOWLEDGMENTS The Vindtop Formation was discovered during the 1995 Danish Lithosphere Center field expedition to East Greenland. We thank the participants, particularly Troels Nielsen for organizing logistics. We are grateful to Jørgen Kystol, Tod Waight, Christian Tegner, and Lew Hogan for analytical help. We thank Karsten Haase, Tanya Furman, and Godfrey Fitton for reviewing the manuscript. The Danish National Research Foundation supported this work. REFERENCES CITED Baker, J., Chazot, G., Menzies, M., and Thirlwall, M., 1998, Metasomatism of the shallow mantle beneath Yemen by the Afar plume—Implications for mantle plumes, flood volcanism, and intraplate volcanism: Geology, v. 26, p. 431–434. Bernstein, S., Kelemen, K.B., Tegner, C., Kurz, M.D., Blusztajn, J., and Brooks, K.C., 1998, Post break-up basaltic magmatism along the East Greenland Tertiary rifted margin: Earth and Planetary Science Letters, v. 160, p. 845–862. Bernstein, S., Brooks, C.K., and Stecher, O., 2001, The ‘‘enriched’’ component of the proto-Icelandic mantle plume revealed in alkaline Tertiary lavas from East Greenland: Geology, v. 29, p. 859–862. Breddam, K., 2002, Kistufell: Primitive melt from the Iceland mantle plume: Journal of Petrology, v. 43, p. 345–373. Chauvel, C., and He´mond, C., 2000, Melting of a complete section of recycled oceanic crust: Trace element and Pb isotopic evidence from Iceland: Geochemistry, Geophysics, Geosystems 1, paper 1999GC000002. Dupuy, C., Liotard, J.M., and Dostal, J., 1992, Zr/Hf fractionation in intraplate basaltic rocks: Carbonate metasomatism in the mantle source: Geochimica et Cosmochimica Acta, v. 56, p. 2417–2423. Foley, S., Musselwhite, D.S., and van der Laan, S.R., 1999, Melt compositions from ultramafic vein assemblages in the lithospheric mantle: A comparison of cratonic and noncratonic settings, in Gurney, J.J., et al., eds., Proceeding of the 7th International Kimberlite Conference, Dawson Volume: Cape Town, Red Roof Design, p. 238–246. Furman, T., Frey, F.A., and Park, K.-H., 1991, Chemical constraints on the petrogenesis of mildly alkaline lavas from Vestmannaeyjar, Iceland: The Eldfeld (1973) and Surtsey (1963–1967) eruptions: Contributions to Mineralogy and Petrology, v. 109, p. 19–37. Gleadow, A.J.W., and Brooks, C.K., 1979, Fission track dating, thermal histories and tectonics of igneous intrusions in East Greenland: Contributions to Mineralogy and Petrology, v. 71, p. 45–60. Haase, K.M., and Devey, C.W., 1994, The petrology and geochemistry of Vesteris Seamount, Greenland Basin—An intraplate alkaline volcano of nonplume origin: Journal of Petrology, v. 35, p. 295–328. Haase, K.M., Devey, C.W., Mertz, D.F., Stoffers, P., and Garbe-Scho¨nberg, D., 1996, Geochemistry of lavas from Mohns Ridge, Norwegian-Greenland Sea: Implications for melting conditions and magma sources near Jan Mayen: Contributions to Mineralogy and Petrology, v. 123, p. 223–237. Halliday, A.N., Lee, D.C., Tommasini, S., Davies, G.R., Paslick, C.R., Fitton, J.G., and James, D.E., 1995, Incompatible trace-elements in OIB and MORB and source enrichment in the suboceanic mantle: Earth and Planetary Science Letters, v. 133, p. 379–395. Hanghøj, K., Storey, M., and Stecher, O., 2003, An isotope and trace element study of the East Greenland Tertiary dyke swarm: Constraints on temporal and spatial evolution during continental rifting: Journal of Petrology, v. 44, p. 2081–2112. Hansen, K., and Brooks, C.K., 2002, The evolution of the East Greenland margin as revealed from fission track studies: Tectonophysics, v. 349, p. 93–111. Hansen, H., Pedersen, A.K., Duncan, R.A., Bird, D.K., Brooks, C.K., Fawcett, J.J., Gittins, J., Gorton, M., and O’Day, P., 2002, Volcanic stratigraphy of the southern Prinsen af Wales Bjerge region, East Greenland, in Jolley, D.W., and Bell, B.R., eds., The North Atlantic igneous province: Stratigraphy, tectonics, volcanic and magmatic processes: Geological Society [London] Special Publication 197, p. 183–218. Hauri, E.H., and Hart, S.R., 1994, Constraints on melt migration from mantle plumes: A trace element study of peridotite xenoliths from Savaii, western Samoa: Journal of Geophysical Research, v. 99, p. 24,301–24,321. Hirschmann, M.M., Asimow, P.D., Ghiorso, M.S., and Stolper, E.M., 1999, Calculation of peridotite partial melting from thermodynamic models of min-
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