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Schilling Et Al Journal of Geodynamics 38 (2004) 57–83 Volcanism on the Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71◦N): age, source, and relationship to the Iceland and putative Jan Mayen plumes Dieter F. Mertz a,b,c,∗, Warren D. Sharp c, Karsten M. Haase d a Johannes Gutenberg-Universität, Institut für Geowissenschaften, 55099 Mainz, Germany b Max Planck-Institut für Chemie, Postfach 3060, 55020 Mainz, Germany c Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, CA 94709, USA d Institut für Geowissenschaften der Christian-Albrechts-Universität zu Kiel, Olshausenstr. 40, 24118 Kiel, Germany Received 8 December 2003; received in revised form 1 March 2004; accepted 5 March 2004 Abstract The Eggvin Bank (Central Norwegian-Greenland Sea, latitude ∼71◦N) is a topographically anomalous shallow area with scattered volcanic peaks extending between the island of Jan Mayen and East Greenland and straddling the northern segment of the mid-Atlantic Kolbeinsey Ridge axis. Basalts dredged from the Eggvin Bank range from variably depleted, tholeiitic, near-axis lavas to enriched, transitional-to-alkaline, off-axis seamount lavas. In terms of normalised incompatible element patterns, the most depleted, near-axis tholeiite is similar to neighbouring Kolbeinsey Ridge basalts, whereas the off-axis, transitional-to-alkaline lavas are similar to other alkaline basalts occurring close to the Eggvin Bank region, e.g., those of Jan Mayen. 40Ar/39Ar step heating data indicate that the off-axis seamount lavas are coeval with other alkaline lavas erupted in the Central Norwegian-Greeland Sea at ca. 0.6–0.7 Ma. In contrast, the Eggvin near-axis tholeiites are <0.1 Ma. Volcanic peaks west and north of Jan Mayen show no indication of a systematic age progression. Therefore, the Jan Mayen hot spot hypothesis is not supported by the available radiometric age data. Sr, Nd, and Pb isotope composi- tions of near-axis and off-axis Eggvin Bank lavas are distinct, implying differences in their mantle sources. Isotope ratios of the off-axis basalts (87Sr/86Sr = 0.70344–0.70352, 143Nd/144Nd = 0.51283–0.51288, 206Pb/204Pb = 18.82–18.85) resemble those of neighbouring alkali basalt occurrences, however, isotope ratios of the near-axis tholeiites correspond to lavas erupting in the south-eastern volcanic zone of Iceland, e.g., at Vestmannaeyjar. The near-axis tholeiites are generated by an unusual source with highly radiogenic Pb (206Pb/204Pb = 18.95) to- gether with relatively radiogenic Nd (143Nd/144Nd = 0.51295) and low-radiogenic Sr (87Sr/86Sr = 0.70314), respectively, representing an unique composition in the mantle north of central Iceland. The overlap in isotope ∗ Corresponding author. Tel.: +49-6131-3922857. E-mail address: [email protected] (D.F. Mertz). 0264-3707/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.jog.2004.03.003 58 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 compositions between Eggvin Bank near-axis tholeiites and south-east Iceland alkaline lavas could be an indication that the Eggvin Bank tholeiite source was derived from the Iceland plume and that it was emplaced in the upper mantle by the original Iceland plume head during the Early Tertiary as suggested by Trønnes et al. [Trønnes, T., Planke, S., Sundvoll, B., Imsland, P., 1999. Recent volcanic rocks from Jan Mayen: low degree melt fractions of enriched north-east Atlantic mantle. J. Geophys. Res. 104, 7153–7167]. Isotope and trace element data indicate an abrupt change in source composition along the Kolbeinsey Ridge axis at latitude ca. 70.6◦N, apparently reflecting a boundary between two chemically distinct mantle domains with limited interaction. Based on Pb versus Pb isotope diagrams, no dispersion of enriched material is observed adjacent to the hypothetical Jan Mayen/Jan Mayen Platform plume, neither to the north-east along the Southern Mohns Ridge nor to the south along the Central Kolbeinsey Ridge. © 2004 Elsevier Ltd. All rights reserved. 1. Introduction In 1983, Schilling et al. (1999) published a comprehensive major and trace element data set on Mid-Atlantic ridge samples dredged from latitude 29◦Nto73◦N providing an overview on the geo- chemistry of the North Atlantic basalts from the ridge segments south of the Azores via Iceland up to the north of Jan Mayen. Recently, the high-latitude part of this data set comprising the ridge segments north of Iceland, i.e., essentially Kolbeinsey, Mohns and Knipovich Ridges (Fig. 1) has been completed by isotope and additional trace element (Hanan et al., 2000) measurements. Together with new geochem- ical and isotope data on Iceland (e.g., Hanan and Schilling, 1997; Chauvel and Hémond, 2000) and Jan Mayen (e.g., Trønnes et al., 1999) volcanism, a consistent, large-scale picture of the composition of the high-latitude North Atlantic mantle has emerged. In contrast, the nature of the mantle sources, their interactions and the causes of magma generation for Eggvin Bank–Jan Mayen intraplate volcanism located between the northern Kolbeinsey Ridge and the southern Mohns Ridge segments (Central Norwegian-Greenland Sea; Fig. 1) remains controversial. Jan Mayen volcanism has been interpreted as recent hotspot activity (e.g., Johnson and Campsie, 1976; Morgan, 1983; Schilling et al., 1983; Vink, 1984) and the topographically anomalous shallow Eggvin Bank west of Jan Mayen (Fig. 1) with discontinuous volcanic peaks is thought to represent the Jan Mayen hot spot track (Morgan, 1981). Mohns Ridge spreading axis lavas occurring north of Jan Mayen—with radiogenic 87Sr/86Sr and Pb isotope ratios, relatively unradiogenic 143Nd/144Nd compositions, and in- compatible element enrichment relative to normal-type mid-ocean ridge basalt (N-type MORB)—are regarded as the result of contamination of their asthenospheric source by material from a hot mantle plume underneath Jan Mayen or the Jan Mayen Platform (Schilling et al., 1983, 1999; Neumann and Schilling, 1984). In contrast, seismic, tectonic, petrological, and geochemical data are interpreted to indi- cate that no anomalously hot mantle underlies the Jan Mayen region (e.g., Imsland, 1980; Saemundsson, 1986; Havskov and Atakan, 1991; Haase et al., 1996). For example, Haase et al. (1996, 2003) concluded that Jan Mayen magmas are generated by melting of volatile-enhanced, passively up- welling mantle influenced by the adjacent Mohns Ridge spreading axis (Fig. 1). Moreover, they suggested that the shallow bathymetry of the Eggvin Bank is caused by an iron-depleted mantle, which is less dense than its surrounding mantle. Trønnes et al. (1999) proposed that Jan Mayen magma originates from low-degree partial melts of enriched material emplaced in the NE Atlantic mantle by the ancestral Ice- land Plume at about 60 Ma. D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 59 Fig. 1. Map of the Eggvin Bank–Jan Mayen region (Central Norwegian-Greenland Sea) based on Perry (1986) showing ma- jor tectonic and bathymetric features as well as K–Ar (in italics) and 40Ar/39Ar ages (Fitch et al., 1965; Mertz and Renne, 1995; Upton et al., 1995; this work) on volcanic rocks. The inset presents the high-latitude North Atlantic with the Eggvin Bank–Jan Mayen region. Numbers in brackets indicate sampling locations (see Table 1). Depth contours in meter, JMFZ: Jan Mayen Fracture Zone (indicated by hatching), SFZ: Spar Fracture Zone (ca. 69◦N), TFZ: Tjörnes Fracture Zone (ca. 67◦N), JMP: Jan Mayen Platform, JMR: Jan Mayen Ridge, JM: Jan Mayen Island, MR: Mohns Ridge, CKR: Central Kolbeinsey Ridge (ca. 69◦–70.6◦N, also termed Middle Kolbeinsey Ridge in other papers), NKR: North Kolbeinsey Ridge (ca. 70.6◦–72◦N, other authors apply the acronym NKR to the segment 69◦–72◦N), OSC: Overlapping Spreading Center (ca. 70.6◦N). Reykjanes Ridge extends south of Iceland approximately at latitude 64◦–55◦N. Only limited geochemical and geochronological data are available for Eggvin Bank lavas (Pedersen et al., 1976; Schilling et al., 1983, 1999; Campsie et al., 1990) and their relationship to lavas of Jan Mayen island as well as those of neighbouring Kolbeinsey and Mohns Ridge is not clear. During FS Polarstern expedition ARK VII/1 volcanic rocks from the Eggvin Bank region were dredged. Here we present new 40Ar/39Ar step heating data, geochemical compositions and Sr, Nd, Pb isotope ratios on these rocks in order to establish their ages, characterise their mantle sources, and evaluate source interactions. 2. Geological setting Fig. 1 is a simplified map of the Eggvin Bank–Jan Mayen region showing relevant bathymetric and tectonic features. Jan Mayen is located in a topographically anomalous area at the northern end of the Jan Mayen Ridge, which is at least in part a continental fragment (e.g., Grønlie et al., 1979; Myhre et al., 1984). With the northward propagation of the mid-Atlantic Kolbeinsey Ridge about 43 Ma ago, this fragment was split off from Greenland and drifted to its present position (Nunns, 1982). Vesterisseamount is an isolated volcanic edifice located about 350 km north-west of Jan Mayen that was built on Middle Eocene oceanic crust about 3000 m below sea level. To the west of Jan Mayen, the northern segment of the Kolbeinsey Ridge penetrates the Eggvin Bank, a shallow region with seamounts reaching up to few tens of meters below sea level (Fig. 1). East Greenlandic basalts at latitude 72◦–75◦N (e.g., Upton et al., 60 D.F. Mertz et al. / Journal of Geodynamics 38 (2004) 57–83 1984) occur in the western extension of the Eggvin Bank–Jan Mayen shallow region. 40Ar/39Ar dating of these basalts yields eruption ages of 57–58 Ma as well as later volcanic activity at ca. 32 Ma (Upton et al., 1995). East Greenland basalts south of latitude 72◦N yield Paleogene K–Ar total rock ages with the oldest Paleocene ages of ca.
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