Marine Geology 261 (2009) Contents lists available at ScienceDirect. Marine Geology. journal homepage:

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1 Marine Geology 261 (2009) 3 16 Contents lists available at ScienceDirect Marine Geology journal homepage: Interaction of mantle derived melts with crust during the emplacement of the Vøring Plateau, N.E. Atlantic Romain Meyer a,, Jan Hertogen a, Rolf B. Pedersen b, Lothar Viereck-Götte c, Michael Abratis c a Geo-Instituut, Katholieke Universiteit Leuven, Celestijnenlaan 200E, B-3001 Leuven-Heverlee, Belgium b Institutt for geovitskap, Universitetet i Bergen, Allegaten 41, N-5007 Bergen, Norway c Institut für Geowissenschaften, Friedrich-Schiller-Universität Jena,Geochemie, Burgweg 11, D Jena, Germany article info abstract Article history: Received 15 November 2007 Received in revised form 4 February 2009 Accepted 23 February 2009 Keywords: volcanic margins continent breakup petrogenesis North Atlantic Igneous Province-NAIP trace elements SrNdPb isotopes Vøring Plateau SE Greenland Trace element and isotopic signatures of magmatic rock samples from ODP Hole 642E at the Vøring Plateau provide insight into the interaction processes of mantle melt with crust during the initial magma extrusion phases at the onset of the continental breakup. The intermediate (basaltic andesitic) to felsic (dacitic and rhyolitic) Lower Series magmas at ODP Hole 642E appear to be produced by large amounts of melting of upper crustal material. This study not only makes use of the traditional geochemical tools to investigate crust mantle interaction, but also explores the value of Cs geochemistry as an additional tool. The element Cs forms the largest lithophile cation, and shows the largest contrast in concentration between (depleted) mantle and continental crust. As such it is a very sensitive indicator of involvement of crustal material. The Cs data reinforce the conclusion drawn from isotopic signatures that the felsic magmas are largely anatectic crustal melts. The down-hole geochemical variation within ODP Hole 642E defines a decreasing continental crustal influence from the Lower Series into the Upper Series. This is essential information to distinguish intrinsic geochemical properties of the mantle melts from signatures imposed by crustal contamination. A comparison with data from the SE Greenland margin highlights the compositional asymmetry of the crust mantle interactions at both sides of the paleo-iapetus suture. While Lower Series and Middle Series rocks from the SE Greenland margin have isotopic signatures reflecting interactions with lower and middle crust, such signatures have not been observed at the mid- Norwegian margin. The geochemical data either point to a dissimilar Caledonian crustal composition and/or to different geodynamic pre-breakup rifting history at the two NE Atlantic margin segments. Published by Elsevier B.V. 1. Introduction The opening of the Atlantic Ocean gave rise to the formation of three Large Igneous Provinces (LIP): the Central Atlantic Magmatic Province (CAMP), the Paraná-Etendeka province, and North Atlantic Igneous Province (NAIP) which is the subject of the present paper (Fig. 1). Continental breakup between Eurasia and Greenland at the Palaeocene Eocene transition marked the culmination of the predominantly extensional setting in the northern North Atlantic area since the end of the Caledonian orogeny. Magmatic activity was limited in the early stages of the rift history, but toward breakup the NE Atlantic conjugate margins were characterized by extensive magmatic events. In the following seafloor spreading setting, the Icelandic mantle anomaly has left its imprint on the oceanic lithosphere. These Corresponding author. Present Address: Dept. Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Massachusetts Ave. 77, Cambridge, MA, USA. address: mail@romain-meyer.eu (R. Meyer). magmatic events prior to and during continental separation, and the ongoing activity of Iceland, comprise an exceptional LIP: the NAIP. The NAIP record includes in addition to Iceland, central volcanoes, and extensive volcanic margin Seaward Dipping Reflector Sequences (SDRS). Moreover, the SDRS are underlain by magmatic rock successions that penetrated the crust just prior to the transition from rift-todrift geodynamics (Eldholm et al., 1989). Magmatic records in the NAIP have played a central role in the development of igneous petrology and some of the initial petrogenesis concepts for LIP are derived from NAIP rock samples. Bailey et al. (1924) described on Mull (Scotland) for the first time the existence of the two predominant types of basalt in LIP, which were later classified as alkalic and tholeiitic basalts. In these early years of modern petrology it was a debate on principles whether the observed rock suites from tholeiitic basalts to andesites represented a differentiation trend and/or contamination of primary mafic mantle melts with SiO 2 - rich material derived from continental crustal. The contribution of the continental crust to either the production of melt or the contamination of mantle melts during the magmatic episodes accompanying continental breakup is still a matter of debate. Divergent opinions /$ see front matter. Published by Elsevier B.V. doi: /j.margeo

2 4 R. Meyer et al. / Marine Geology 261 (2009) 3 16 Fig. 1. The North Atlantic Igneous Province. Ocean Drilling Program (ODP) Legs that drilled into a Lower Series on the NE Atlantic VRMs (Leg 104: Eldholm et al., 1987; Leg 152: Larsen et al., 1994; Leg 163: Duncan et al., 1996) are indicated, as well as onshore and offshore magmatic formations. originate mainly from the difficulties to distinguish trace element and isotopic characteristics of crustal contamination from those acquired by the involvement either as melt source or as contaminant of the non-convective lithospheric mantle section. However, the discovery of crustal anatectic volcanics dating back to the rift-to-drift geodynamic transition of the NE Atlantic was undoubtedly a major achievement of Ocean Drilling Program (ODP) Leg 104. After drilling on the mid- Norway at the Vøring Plateau (ODP Leg 104) recovered such volcanic rock successions, drilling along the 63 N transect off SE Greenland (ODP Legs: 152 and 163) sampled magmas that erupted just prior to the opening of the NE Atlantic Ocean and that carry a clear imprint of mantle continental crust interaction. These magmas underwent different magnitudes and styles of crustal mantle interactions. In addition to these multifaceted contamination histories during the injection of hot mantle melts into the crust, the mantle material traversed continental crust of variable composition. The large spectrum of trace elements and radiogenic isotopes in crustal rocks and the limited available information on the composition of the crust underlying these extrusive igneous units complicates the identification of involved end-member and quantification of crustal contamination. Only careful geochemical investigations including the newest developments in petrogenetic concepts and analytical facilities have a potential to define an original geochemical signature of the magma with some degree of confidence. As a result a better reconstruction of the magmatic processes just prior to the continental breakup is indispensable knowledge to recognize intrinsic geochemical properties of the involved asthenospheric mantle domain. Such a characterisaton is essential, as plate tectonic and delamination processes have been recently linked to heterogeneities in the peridotitic mantle, and as a result proposed as sources for LIP magmatism (e.g., nearly pure pyroxenitic sources for the Siberia and Karoo provinces (Sobolev et al., 2007)). Furthermore, a geochemical description of the implicated continental crustal material sheds light on the structure and compositional features of the underlying paleo- Iapetus suture at the time of the continental breakup. This paper is focussed on the interactions of ascending mantle melts with continental crust during the initial stages of the NE Atlantic opening. Such interactions are highly probable when magma has to pass through thick lithosphere and may be common when melt is stored in crustal magma reservoir systems. Three concurrent processes had to be considered for the genesis of this silicic LIP volcanism: (1) differentiation of primary mantle melts; (2) mixing of crustal anatectic melts with uncontaminated primitive tholeiitic magmas; and (3) pure anatexis of continental crustal melts, due to the thermal perturbation of high density mafic sills emplaced in the crust or underplated at the base of crust. 2. Geological setting The available geological, geochemical and geophysical datasets relevant to the NAIP formation have been recently summarized, and the currently proposed hypotheses of its genesis the mantle plume and alternative models have been critically evaluated (Meyer et al., 2007). One of the major conclusions of this paper is that the existing datasets and geodynamic concepts are incomplete, which hinders a

3 R. Meyer et al. / Marine Geology 261 (2009) more conclusive statement on whether or not the mantle plume or alternative models can be accepted or rejected. As a result, only ongoing improvements in geodynamic models and new geochemical data will help us to understand which of the geochemical variations observed in basalts reflect intrinsic variations of the postulated deep-mantle sources. The Vøring rifted passive margin investigated in this study is located at the central segment of the Norwegian Margin and forms part of the NAIP. Drilling at the Vøring margin recovered volcanic rock successions that erupted during the initial stages of breakup of Greenland from Fennoscandia. Hence, the mid-norwegian margin now represents the transition region between an old Proterozoic cratonic lithosphere and a geologically young Cenozoic oceanic lithosphere (e.g., Scheck-Wenderoth et al., 2007). The ODP Leg 104 Hole 642E core of the Vøring Plateau consists of a ca. 320 m thick sequence of marine sedimentary rocks, a 770 m thick magmatic Upper Series (US) of transitional-type, enriched mid-ocean ridge tholeiitic basalts (E-MORB), and an underlying Lower Series (LS) of a rhyolitic ignimbrite, tholeiitic basaltic dykes, basaltic andesites and dacites (Eldholm et al., 1987, 1989 and references therein). The Lower Series assemblage is clearly indicative of interaction of mantle melts with crustal material and/or of significant crustal melting by underplating. Unfortunately drilling stopped after 170 m penetration into the Lower Series. Mantle crustal interactions are more probable during the generation of volcanic rifted margins erupting initially through continental lithosphere, compared to MORB or OIB magmas passing through thin or thick oceanic lithosphere, respectively. This makes the Vøring Plateau ideally suited to investigate mantle melt crustal rock relationships, due to the specific NAIP geodynamic setting including volcanic rifted margins, mid-ocean ridges and an ongoing excess magmatic production below Iceland. 3. Sampling and analytical techniques In the framework of a EUROMARGINS sub-project (01-LEC-13F) the ODP Leg 104 core was macroscopically reinvestigated (Meyer et al., 2009) and resampled at the ODP East Coast Core Repository at the Lamont Doherty Earth Observatory. The resampling strategy aimed to attain a much improved sampling density of the heterogenous Lower Series and the transition to the much more homogeneous Upper Series lava flow sequence (SDRS). Representative samples from the Upper Series were taken for an accurate determination of the involved mantle end-member. Thin sections were prepared from representative samples and petrographically studied. The crystal chemistry and glass chemistry of selected samples has been investigated by electron microprobe analysis (Abratis et al., in preparation). The core samples were prepared by cutting inclusion-free specimens, with weathered surfaces removed. These small blocks were crushed with a pestle and only the fresh fragments were subsequently powdered in an agate ball mill. Table 1 Major element (wt.%) concentrations of (a) Upper Series magmas and (b) Lower Series melts. Subdivision SiO 2 TiO 2 Al 2 O 3 Fe 2 O 3 (T) MnO MgO CaO Na 2 O K 2 O P 2 O 5 Leg R1/140 US Leg R2/092 US Leg R2/070 US Leg R1/000 US n.a Leg R1/000b US Leg R3/128 US Leg R1/128 US Leg R2/040 US Leg R2/075 US Leg R1/025 US Leg R3/056 US Leg R2/005 US Leg R3/092 US Leg R2/078 US Leg R1/069 US Leg R3/084 US Leg R1/050 US Leg R2/062 US Leg R4/126 S Leg R2/057 B Leg R1/075 B Leg R1/031 B Leg R1/098 D Leg R1/003 B Leg R3/116 D Leg R3/023 D Leg R1/097 D Leg R2/130 D Leg R3/073 D Leg R4/046 D Leg R4/093 S Leg R1/090 B Leg R1/036 B Leg R2/104 D Leg R2/081 D Leg R4/063 S Leg R1/129 S Leg R1/075 S Leg R2/041 S Leg R2/051 A Leg R1/023 A Leg R1/024 A Leg R1/050 A Leg R2/066 A

4 6 R. Meyer et al. / Marine Geology 261 (2009) 3 16 Whole rock major element data were determined by Inductively Coupled Plasma-Optical Emission Spectrometry (ICP-OES) with a Perkin Elmer OPTIMA 3300DV at the K.U. Leuven. powders weighing 200 mg were fused with 1 g of LiBO2. The fusion bead was dissolved in 100 ml of 0.4 M HNO 3. A 5 ml aliquot of the solution was diluted to 50 ml with 0.4 M HNO 3 prior to ICP-OES analysis. ICO-OES data for Fe, Na and K data of crucial samples have been verified with flame atomic absorption spectrometry (FAAS) measurements with a Thermo Electron Corporation machine (SOLAAR S4). A 10 ml aliquot of the fusion solution was used for a determination of the lanthanides, Zr, Nb, Hf, Ta and Th by Quadrupole ICP-MS (Agilent HP4500 apparatus). To reduce the Li and borate content of the measurement solution, the trace elements of interest were isolated by a Fe-hydroxide scavenge, and finally taken up in 50 ml of 0.4 M HNO 3. Blank values of the LiBO2 were significant only for La. Additional low-blank ICP-MS trace element analyses have been made for selected trace elements after a three acid (ultrapure, subboiled HF HNO 3 HClO 4 ) digestion of 100 mg of powder in a Milestone Ethos 900 microwave digestion and evaporation unit using the procedure of Mareels (2004). In this procedure the decomposition of the samples and the evaporation of the siliciumfluoride and acids takes place in closed PTFE vessels. International reference rocks and secondary in-house rock samples were used for calibration and quality control in both the ICP-OES and ICP-MS procedures. Isotope analyses were made at the Department of Earth Science in Bergen. Sr and Nd radiogenic isotope ratios were performed on a multicollector Finnigan MAT 262 RPQ and Pb isotope data on a multicollector ICP MS (Finnigan Neptune). The precision of the 143 Nd/ 144 Nd measurements is illustrated with multiple La Jolla standard runs of ± The strontium isotope 87 Sr/ 86 Sr ratio was monitored with the international standard SRM987 and the 87 Sr/ 86 Sr precision of repeated SRM987 runs was ± Repeated analysis of standard NBS981 lead isotopic ratios yielded the following values: 206 Pb/ 204 Pb ±0.0002; 207 Pb/ 206 Pb ± ; 208 Pb/ 204 Pb ± ; 207 Pb/ 204 Pb ± All errors are 2σ values. 4. Results The restudy of the ODP Leg 104 Site 642E cores has led to a slight change of the petrological of the Lower Series. The initial Lower Series into A1, A2 and B (Parson et al., 1989) has been extended to four groups. A1, A2, B1 and B2. The classification criteria and Fig. 2. Total alkali-silica (TAS) diagram used to illustrate the compositional variability of the Vøring magmas. observations of the lower series groups are given in Meyer et al. (2009). The stratigraphic succession of the drill core can be used to reconstruct magmatic process changes during the rift-to-drift transition. Major element concentrations for the different groups are reported in Table 1. Since the mineral mode could not be determined in many samples (e.g. in the glassy dacites) we followed the recommended IUGS total alkali versus silica (TAS) chemical classification of rocks (Le Bas et al., 1986) (Fig. 2). A potentially weak point of this classification is that results of weathered, altered and metamorphosed rocks should be used with caution (Le Maitre et al., 2005). However the application of the TAS classification to the comparable volcanics from the British Isles, has been evaluated to be satisfactorily correct by Sabine et al. (1985). In addition, Viereck et al. (1989) believed that most Vøring igneous rocks have suffered only limited alteration, and so the TAS diagram should be an appropriate classification tool. The new sample set better defines the different processes involved in the formation of the Lower Series magmas: e.g., the major element geochemistry of the Lower Series confirms the known trend from the glassy dacite flows to the ignimbritic rhyolite, and also indicates a differentiation trend of basalts to basaltic trachy-andesites (Fig. 2). The Upper Series is mainly defined by the homogeneity of typical tholeiitic MORB composition (Fig. 2). Trace element data obtained by ICP-MS are presented in Tables 2 3. Lithological unit assignments were defined by the initial shipboard party and by the onshore core reinvestigation (Meyer et al., 2009). Isotopic data are listed in Tables Discussion 5.1. REE patterns The Upper Series shows a range of REE contents, but is homogeneous with respect to the shape of the chondrite normalized patterns, which are generally parallel to each other. The REE patterns of the Lower Series are much more diverse with LaN data between 50 and 300 (Figs. 3 and 4). Even if none of the Vøring tholeiites can be regarded as a primary mantle melt (Viereck et al., 1988, 1989) the mafic Lower Series dykes can be regarded in a first approximation as a potential mantle end-member. The REE have the highest potential for the investigation of initial melt whose signatures could be obscured by hydrothermal alteration, weathering or low grade metamorphism. The Upper Series lava flows are characterized by chondrite-normalized REE patterns that are slightly LREE-enriched compared to typical N-MORB patterns (Fig. 3). Similar enriched MORB patterns are reported from different NAIP sub-areas (Meyer et al., 2007; and references therein). The trace elements of Lower Series dykes samples D5 and D6 illustrate that no equivalent magmas have been erupted in the Upper Series. Hence, the Lower Series dykes are not simply feeder dykes of the Upper Series sequence. The REE patterns of dykes D5 and D6 are quite similar, but rather uncommon among MORB. However, such REE patterns have been reported for dykes from the Central Atlantic Magmatic Province (CAMP) (Salters et al., 2003, and references therein). The dykes differ from the Upper Series lavas in two respects. Firstly, they have higher heavy REE abundances than most of the Upper Series lavas, which implies that they have been formed by smaller degrees of melting. Secondly, they show a pronounced depletion of the Pr to Gd light to middle REE relative to heavy REE, coupled to an enrichment of the lightest REE La and Ce. Since there is no known mechanism for selective depletion of Pr to Gd, the La and Ce enhancement indicates that the REE patterns have been further complicated by contamination with crustal material. These feature of the dykes were already documented in two samples by Viereck et al. (1989). The additional analyses confirm that it is a consistent characteristic of the dykes. It is unlikely that it is the result of some fluke alteration effect, and rather implies that the source of

5 R. Meyer et al. / Marine Geology 261 (2009) Table 2 ICP-MS trace element (ppm) concentrations of the Upper Series. 007R2/ R1/ R3/ R1/ R2/ R2/ R1/ R2/ R2/ R4/ R2/ R2/003 US US US US US US US US US US US US Sc n.a n.a n.a Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W 0.52 n.a n.a n.a Pb n.a n.a n.a n.a Th U R2/ R2/ R3/ R1/ R1/ R3/ R1/ R1/ R2/ R3/ R1/ R2/040 US US US US US US US US US US US US Sc Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W n.a n.a. n.a. Pb n.a n.a Th U R2/ R3/ R3/ R2/ R1/ R2/ R4/ R2/ R4/ R2/ R2/ R3/063 US US US US US US US US US US US US Sc n.a n.a n.a Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er (continued on next page)

6 8 R. Meyer et al. / Marine Geology 261 (2009) 3 16 Table 2 (continued) 059R2/ R3/ R3/ R2/ R1/ R2/ R4/ R2/ R4/ R2/ R2/ R3/063 US US US US US US US US US US US US Tm Yb Lu Hf Ta W n.a n.a n.a. n.a. n.a. n.a. n.a. n.a. n.a. Pb n.a Th U R1/ R3/ R2/ R1/ R1/ R1/ R3/ R2/ R3/ R2/ R2/ R1/046 US US US US US US US US US US US US Sc n.a. n.a n.a Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W n.a. n.a n.a n.a. n.a n.a n.a Pb Th n.a. n.a n.a U R2/ R1/ R1/ R2/ R3/ R3/ R1/ R2/ R2/ R1/ R3/ R3/082 US US US US US US US US US US US US Sc n.a n.a. n.a n.a n.a. Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W n.a. n.a. n.a n.a. n.a. n.a. n.a. n.a. n.a. Pb Th n.a. n.a n.a. U R3/ R4/ R1/ R2/ R2/ R2/ R1/ R1/ R1/ R3/ R3/ R3/023 US US US US B2 B2 B2 B2 D4 D5 D5 D5 Sc n.a n.a n.a Y Zr Nb Ba La

7 R. Meyer et al. / Marine Geology 261 (2009) Table 2 (continued) 093R3/ R4/ R1/ R2/ R2/ R2/ R1/ R1/ R1/ R3/ R3/ R3/023 US US US US B2 B2 B2 B2 D4 D5 D5 D5 Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W 0.65 n.a. n.a. n.a. n.a n.a n.a. n.a. Pb Th 0.21 n.a U R1/ R2/ R2/ R2/ R3/ R3/ R4/ R4/ R1/ R2/ R2/ R3/142 D5 D5 D5 D5 D5 D5 D5 S46 B1 D6 D6 S48 Sc n.a. n.a Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W n.a. n.a n.a. n.a Pb Th U R4/ R1/ R1/ R1/ R2/ R2/ R1/ R1/ R2/ R2/ R2/ R1/028 S48 S48 S48 S48 S48 A2 A2 A2 A1 A1 A1 D7 Sc n.a Y Zr Nb Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta W n.a n.a Pb Th U

8 10 R. Meyer et al. / Marine Geology 261 (2009) 3 16 Table 3 Low blank ICP-MS trace element measurements of selected samples (ppm). 45R3/128 55R1/128 59R2/040 73R2/075 80R1/025 85R3/056 94R1/050 94R2/042 98R2/057 99R1/ R3/ R3/023 US US US US US US US US B2 B2 D5 D5 Sc V Co n.a Ni n.a. n.a Cu Zn Ga Ge Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb n.a. n.a. n.a Bi n.a n.a. n.a Th n.a U n.a R1/ R4/ R4/ R2/ R4/ R1/ R1/ R2/ R1/ R2/ R2/092 D5 D5 S46 D6 S48 S48 S48 S48 A2 A1 A1 Sc V Co n.a. n.a n.a Ni n.a n.a n.a. n.a Cu Zn Ga Ge Sr Y Zr Nb Cs Ba La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Hf Ta Pb Bi n.a Th U

9 R. Meyer et al. / Marine Geology 261 (2009) Table 4 Sr, Nd and Pb isotopic variations in the Upper Series rocks. 87 Sr/ 86 Src 143 Nd/ 144 Nd 145 Nd/ 144 Nd εnd(0) 206 Pb/ 204 Pb 207 Pb/ 206 Pb 208 Pb/ 204 Pb 207 Pb/ 204 Pb Leg R1/ Leg R2/ Leg R1/ Leg R3/ Leg R1/ Leg R2/ Leg R3/ Leg R2/ Leg R1/ Leg R3/ Leg R3/ Leg R2/ Leg R1/ Leg R3/ Leg R1/ Leg R2/ Leg R2/ Leg R1/ Leg R3/ Leg R4/ Leg R1/ Leg R2/ Table 5 Sr, Nd isotopic data of the Lower Series. 87 Sr/ 86 Src 143 Nd/ 144 Nd 145 Nd/ 144 Nd εnd(0) Leg R2/ Leg R1/ Leg R2/ Leg R1/ Leg R2/ Leg R1/ Leg R2/ Leg R1/ Leg R2/ Leg R2/ Leg R3/ Leg R3/ Leg R1/ Leg R1/ Leg R3/ Leg R1/ Leg R2/ Leg R3/ Leg R3/ Leg R4/ Leg R4/ Leg R5/ Leg R1/ Leg R1/ Leg R1/ Leg R2/ Leg R2/ Leg R2/ Leg R3/ Leg R3/ Leg R4/ Leg R4/ Leg R5/ Leg R1/ Leg R1/ Leg R1/ Leg R1/ Leg R2/ Leg R2/ Leg R1/ Leg R1/ Leg R2/ Leg R2/ Leg R1/ Leg R1/ dykes was more depleted than the Upper Series lava source. This depleted source character leaves room for two different interpretations. Either, the source of the dykes showed a long term LREE depletion, or the dykes are derived from a source which lost the first fraction of melts formed only shortly before the main melting event. The other Lower Series dykes D4 and D7 have different REE patterns that are much more genetically linked to the Lower Series silicic magmas (Fig. 4). Negative europium anomalies are more pronounced, and point either to contamination of the mafic magmas by crustal melts from feldspar bearing source rocks, or to feldspar fractionation during magma ascent to the surface. In terms of REE geochemistry dyke D4 is not distinguishable from the Lower Series subgroup B2 silicic magmas. This supports the new core reinterpretation of Abratis et al. (in preparation), that D4 is the aphanitic lower section of the thick cordierite dacite flow F113. The REE geochemistry of D7 shows also a strong link between this dyke and the silicic volcanics Isotopes The isotopic variation of the Voring Plateau samples is summarized in the Nd Sr isotopic variation plot shown in Fig. 5. Four main groups can be distinguished. The Upper Series lavas form a tight cluster with isotopic ratios that are more radiogenic than for a typical MORB mantle source, but rather usual for the basalts from the NAIP (Figs. 5 and 6). Most of the Lower Series lavas cluster together around 87 Sr/ 86 Sr=0.71 and 143 Nd/ 144 Nd= This is outside the accepted Table 6 Pb isotopic compositions of the Lower Series. Subdivision 206 Pb/ 204 Pb 207 Pb/ 206 Pb 208 Pb/ 204 Pb 207 Pb/ 204 Pb Leg R3/116 D Leg R3/023 D Leg R1/097 D Leg R3/073 D Leg R1/133 B Leg R2/004 B Leg R1/018 D Leg R2/082 D Leg R1/017 A Leg R2/090 A Leg R1/044 D

10 12 R. Meyer et al. / Marine Geology 261 (2009) 3 16 Fig. 3. Chondrite-normalized diagram comparing REE compositions of the Upper Series MORBs (orange field) and the mafic Lower Seriesdykes (similar dykes D5 and D6 green field; the light green pattern is D7, and the bluish-green represents D4). Different partial melting degrees have been modelled for an 55 wt.% Ol, 25 wt.% Opx, 18 wt.% Cpx, 2 wt.% Sp depleted peridotite mantle source (dashed lines) (K d values from the Eartref.org database; Koppers, 2008). Normalizing values and N-MORB from Sun and McDonough (1989). Fig Nd/ 144 Nd vs. 87 Sr/ 86 Sr isotopic variation of the Vøring margin magmas suggesting assimilation with progressive fractionation (AFC) and a binary mixing curve from the Upper Series MORBs over the mafic dykes D5 and D6 to the silicic Lower Series flows toward the sediments with the highest 87 Sr/ 86 Sr values for an ignimbrite sample. range of pristine mantle ratios, and is a strong indication of the involvement of radiogenic crustal material. The Lower Series dykes occupy an intermediate position between the Upper Series tholeiites and the Lower Series. In principle this could reflect a long-term different mantle source or contamination with crustal material. The second explanation is supported by the enrichment of La and Ce in the dykes discussed above. A fourth group of samples shows a larger variation of Sr isotopic ratios, but limited variation of Nd-isotopic composition. Since most of these rocks are volcaniclastic sediments, it could be argued that this mainly reflects mechanical mixing of volcanic pyroclasts with supracrustal pelitic sediments. Such a view is supported by the presence of light mica crystals. However, the fourth group also includes more massive glassy dacites. This in turn can be seen as evidence that supracrustal material was not only a mechanical mixing component, but also a component in magmatic interaction processes Caesium as an indicator of crustal contribution In order to better understand mixing processes we have analyzed a number selected samples for the element caesium with a low-blank ICP- MS procedure (see above). The justification of the choice of caesium is Fig. 4. Rare earth elements patterns of the various silicic magmas in the Lower Series, showing parallel REE patterns throughout the whole series. Normalizing values from Sun and McDonough (1989). Fig Nd/ 144 Nd vs. 87 Sr/ 86 Sr for igneous NAIP products. A) Primary mafic mantle magma compositions. B) Observed isotopic ratio variability in NAIP magmas. A) The isotopic signature of most NAIP mafic magmas suggest influence by the Iceland plume, whereas the signature of the Kolbeinsey Ridge likes to be the depleted N Atlantic N- MORB source. Mixing by contamination may also contribute to the isotope characteristics as illustrated in B) by the SE Greenland Lower Series and MS reflecting continental contamination by granulitic- and amphibolitic-facies rocks (data from: this work and Mertz and Haase, 1997; Fitton et al., 2000; Kokfelt et al., 2006 and references therein).

11 R. Meyer et al. / Marine Geology 261 (2009) illustrated in Fig. 7. This trace element shows the greatest contrast in abundance between pristine mantle material, mantle melts, lower crust and especially upper crust. The range in concentrations spans 2.5 orders of magnitude. The strong enrichment in the crust and as a corollary, the strong depletion in depleted mantle is largely due to the large size of the Cs + ion. Cs + with its [Xe] electron configuration has a strong preference for ionic bonds. The Cs partition coefficients are exceedingly small for most rock forming minerals (bb1), except for biotite (N1). This high K d for biotite is due to similarity of ionic radii between potassium and Cs (Cs + is about 15% larger than K + ), although substitution for K+ in potash feldspars is much more limited. Caesium is the most lithophile of all elements and shows the greatest relative increase during magmatic processes (protracted crystallisation or decreasing degree of melting). The Cs concentrations in the C1 Chondrites (0.188 ppm), the Primordial Mantle (0.018 ppm), and the Lower Continental Crust (LCC) (0.3 ppm; Rudnick and Gao, 2003) are very low, while the Middle Continental Crust (MCC) and Upper Continental Crust (UCC) contains much larger values, typically 2.2 and 4.9 ppm Cs respectively (Rudnick and Gao, 2003). In our modelling we used only the two extreme end-members UCC and the LCC (Rudnick and Gao, 2003) as crustal components, and for the mafic mantle end-member we used an average model E-MORB (Sun and McDonough, 1989). Most of the silicic magmas can be described as partial melts from a geochemical source similar to the lower crust, sometimes mixed with small melt fractions of upper crustal material. It is not feasible to unambiguously define the nature of the local crust, because the crustal structure could have been quite complex due to the Caledonian orogeny, and could consist of juxtaposed fragments of material that previously made up lower and upper crust. During the orogeny and its subsequent collapse the crust was certainly more heterogeneous than a standard model continental crustal segment. Because of the uncertainties involved in estimating the endmember compositions of the mantle source and the continental crust, the differences between magma groups are better constrained than the absolute melt fraction and mixing percentages. Fig. 8A and B show the variation of Cs content with Sr and Nd isotopic composition. The general correlation trends are clear in spite of some scatter, which may be partly due to analytical error and partly due to secondary alteration processes. Many of the Upper Series lavas have very low Cs abundances, down to 5 ppb, which is even below primordial mantle. The Lower Series dykes D5 and D6 plot well on the trend expected from a mixing process, between crustal and mantle end-members. As these dykes are not greatly affected by alteration at least not more than the Upper Series Fig. 8. A) Correlation between the trace element Cs and the Sr isotope data. B) Cs correlates with the Sm Nd isotopic system being more resistant to alteration as methods based on concentrations of elements like Rb and Sr. lavas it is unlikely that the enhanced Cs content of the dykes primarily reflects redistribution of Cs by secondary seawater alteration processes. Fig. 9 presents results of modelling of the Cs and La/Sm variation in melts from LCC and UCC. The Lower Series samples (except the dykes D5 and D6) have Cs contents expected for 10 to 50% partial melts of Lower Crust material. But the calculations allow other combinations, such as assimilation of crustal material by mantle melts. It is also obvious that Cs contents of Upper Series lavas are extremely sensitive to even very minor contamination with crust-derived material Geodynamic implications of the new Vøring escarpment data Fig. 7. Trace element characteristics of the continental crust: UCC (upper line), MCC (middle line), and LCC (lowermost line) illustrating the largest variability between the continental reservoirs for Cs. Data from Rudnick and Gao (2003) and normalizing values from Sun and McDonough (1989). The inaccessibility of pristine mantle material presents a problem for geochemical modelling, which limits the direct isotopic characterization of the original source in the breakup processes. The designation of pristine or uncontaminated primary mantle melts, reflecting the signature of the North Atlantic mantle is not a simple question. For example the results obtained for the Vøring margin show that the Upper Series SDRS tholeiitic MORB and the mafic Lower

12 14 R. Meyer et al. / Marine Geology 261 (2009) 3 16 Fig. 9. Crustal anatexis modelling for the Upper Continental Crust (UCC) and the Lower Continental Crust (LCC). Trace element compositions for the postulated end-members UCC and LCC are taken from Rudnick and Gao (2003) and for the E-MORB source from Sun and McDonough (1989). The UCC batch melting trend is marked with black dots, the LCC partial melt fractions with red. Realistic melt fractions are indicated with this model. For the Lower Series magmas a mainly LCC component is suggested. (K d values from the Eartref.org database; Koppers, 2008). Series dykes do not derive from the same source, which points to spatial or temporal heterogeneities in the mantle. In addition, ongoing mafic magmatism in the NAIP is often influenced by the Iceland mantle anomaly, and the potential effects of crust magma interactions are often underestimated or discounted for NAIP melts rising through thick oceanic lithosphere (Iceland), or a micro continent (Jan Mayen). Radiogenic isotope investigations of NAIP basalts have played a key role to support Morgan's mantle plume hypothesis (Morgan, 1971) during the last decades. An example is the work of Kempton et al. (2000), who postulated a zoned mantle plume, consisting of a heterogeneous core encircled by a thick sheath of depleted mantle underlying the Iceland melt anomaly. In contrast, Foulger et al. (2005) propose extensive remelting of subducted Iapetus crust relicts as the source for the geochemistry of Iceland magmas. This illustrates that the chemical and isotopic heterogeneity in the mantle is continuously debated, and a particular point of disagreement is the origin of the heterogeneities: (1) being a random distribution (Albarède, 2005) or (2) being related to large scale mantle plumes. Volcanic Rifted Margins (VRM) like the Vøring Plateau which are built up of mafic SDRS underlain by silicic volcanic formations ( Lower Series ) are key areas for a better understanding of geodynamic processes leading to continental breakup. They provide an opportunity to investigate in space and time geochemical variations before and during excess magmatism. The Vøring Plateau is petrogenetically important because it provides an extensive series of heterogeneous rocks from the breakup history. In comparison with other North Atlantic volcanic rifted margin sampled by ODP (SE Greenland, Fig. 1), both Upper Series are isotopically rather similar. The use of isotope ratios for estimates on crustal mantle interactions in the Upper Series from both margins is compromised by the isotopic variability of subducted materials involved in the recycling process in the mantle and the large range of the continental crustal reservoirs. In contrast, the Lower Series from both areas are fundamentally different (Fig. 6) from each other in many aspects. The Lower Series of the VRM on SE Greenland has 87 Sr/ 86 Srb0.702 while the Lower Series from the Vøring Plateau has 87 Sr/ 86 SrN The 143 Nd/ 144 Nd data for SE Greenland vary from to (Fitton et al., 2000) while at the Vøring this ratio lies between and This marked asymmetry of the geochemical signature of the SE Greenland and Vøring Plateau samples points to a substantial difference in either the pre-breakup crustal composition at the two localities (Fig. 10), or to different styles of mantle crust interactions. One hypothetical scenario could be, that the different mantle crustal relationships are the result of an asymmetric rift system (Mosar et al., 2002) just prior Fig. 10. Schematic sketches of the geological situation during the closure of the Iapetus (A) and the pre-breakup crustal structure (B). Both illustrate the different crustal segments on the Norwegian and Greenland side.

13 R. Meyer et al. / Marine Geology 261 (2009) Fig. 11. Conceptual petrogenetic model for the formation of the different rock series of the Vøring margin. Long term active continental rift systems allowed asthenospheric mantle to rise up (B). Partial melting started, due to the decompression of this hotter mantle. These melts intruded the lower continental crust (C). The heat transport from these mafic mantles melts into the crust generated anatexis of wet crustal segments (D). The anatectic processes weakened the crust so that that the final continental breakup resulted in the emplacement of the MORB-like SDRS (E). to the opening of the NE Atlantic. Testing of this hypothesis requires systematic proximal sampling and analysis of SDRS on both sides of the Jan Mayen Fracture Zone. More geochemical data are needed to identify the relationship between crustal contamination processes and the rift-to-drift geodynamics. Clearly, the generalizations that may be made from any volcanic margin geochemistry without conjugate margin chemical data are limited. Each region and borehole may include only a restricted record of the magmatism. Interpretations of crustal mantle interactions depend critically on the definition and characterization of the involved end-members. The Cs signature of the Lower Series rocks ( ppm) confirms the crustal anatectic contribution of these rocks, and the data of the Upper Series ( ppm) points out that some crustal material has also been involved during the SDRS formation. A systematic investigation of the Cs in the Hole 642E core defined a decreasing influence trend of continental crust during the magma formation. The data presented in this paper are consistent with a conceptual geodynamic model for the rift-to-drift transition at the Vøring margin (Fig. 11), where the lithosphere has been substantially thinned during rifting periods prior to the breakup. Wangen and Faleide (2008) define four rift episodes from the end of the Caledonian orogeny until today at the Vøring margin. area The Eocene magmatic breakup along the mid-norway rifted margin was preceded by extreme Jurassic Cretaceous crustal thinning (Wangen et al., 2008). Due to this severe lithospheric thinning, hot asthenospheric material started to rise into the lithosphere. Under lithospheric pressure conditions the hot asthenospheric material starts to melt. The produced melts are intruding and underplating the crystalline crust. The intruded crust reacts with the melts by assimilation processes and/or anatexis of intruded wet crustal segments. The chemistry and isotope variability of these felsic melts is strongly dependent on the crustal composition. The crustal melting processes are weakening the crust in such a way that further rifting results in the final breakup. This rift-to-drift stage is defined by the eruption of massive asthenospheric MORB-like lava flows. Acknowledgments Project supported by EUROMARGINS CRP-01-LEC-EMA13F. R.M. acknowledges funding by FWO-Vlaanderen (project G ), by the Ministère de la Culture, de l'enseignement supérieur et de la Recherche Luxembourg (BFR05/133), and by the Norwegian Science foundation. Ms. Helena De Cock is thanked for her assistance with the chemical analyses. We are grateful to E. Smolders of the Laboratory of Soil and Water Management of the K.U. Leuven for the use of the ICP-OES equipment. We thank the initial scientific community of Leg 104 and especially O. Eldholm, J. Thiede, and E. Taylor. The authors are grateful for the help during resampling of the core at the ODP East Coast Repository. The paper benefited from the constructive reviews by Gillian R. Foulger and an anonymous referee. References Abratis, M., Viereck-Götte, L., Hertogen, J., Pedersen, J.B. and Meyer, R., in preparation. Melting of continental crust during the formation of volcanic rifted margins insights from the Vøring Plateau Lower Series, ODP Leg 104, Site 642.