March 2008 LIP of the Month

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Corresponds to event #5 in LIP record database.

Tracking the evolution of magmatic plumbing systems from temporal variations in crustal assimilation in the East Greenland flood basalt province.

David W. Peate, Department of Geoscience, University of Iowa, 121 Trowbridge Hall, Iowa City,  IA 52242, USA. (david-peate@uiowa.edu)

Abigail K. Barker, School of Earth and Ocean Sciences, University of Victoria,
PO Box 3055 STN CSC, Victoria, British Columbia, Canada.

Morten S. Riishuus, Geological and Environmental Sciences, 450 Serra Mall, Brown Hall, Building 320, Stanford University, Stanford,  CA 94305, USA.

Rasmus Andreasen, Department of Earth Sciences, Dartmouth College, 6105 Sherman Fairchild Hall, Hanover,  NH 03755, USA.

For the expanded version of this note and additional references, see Peate et al. (2008) (http://dx.doi.org/10.1016/j.lithos.2007.08.009)

INTRODUCTION

The dissected glaciated terrain in the Blosseville Kyst region of central East Greenland (Fig. 1) provides a comprehensive exposed record of Palaeogene magmatism related to the initial stages of the Iceland hot spot activity and the subsequent continental rifting that lead to the opening of the North Atlantic Ocean. This magmatism is part of the larger North Atlantic Igneous Province that covers a wide area from Baffin Island and central West Greenland to the continental margins of East Greenland and north-west Europe (e.g. Saunders et al., 1997). Magmatism throughout the North Atlantic Igneous Province, including central East Greenland, occurred principally during two episodes (c. 62-57 and 56-54 Ma: e.g. Saunders et al., 1997; Tegner et al., 1998a; Storey et al., 2007) that have been related to the initial arrival and rapid dispersal of plume head material over a broad area in the shallow upper mantle, and to the initial plate break-up and opening of the North Atlantic Ocean, respectively.

Magmatism in central East Greenland spanned a period mainly between ~61 Ma and 45 Ma, and comprised a diverse range of lithological types from early shield volcanoes, extensive tholeiitic flood basalt lavas, minor late-stage alkaline lavas, coastal dyke swarms, and numerous mafic and syenitic intrusive complexes. Magmatism initiated with eruption of ~1.2 km of interbedded basaltic lavas, hyaloclastites and pyroclastic deposits (‘Lower Basalts’: Nielsen et al., 1981; Hansen et al., 2002; Ukstins Peate et al., 2003). These were followed by the regional eruption of 2-6 km of voluminous sheet-like tholeiitic flood basalt lavas (‘Plateau Basalts’) that have been divided into four regionally mappable formations (from bottom to top: Milne Land, Geikie Plateau, Rømer Fjord, and Skrænterne Formations: Larsen et al., 1989; Pedersen et al., 1997). Coast-parallel dyke swarms related to both magmatic phases are found along the Blosseville Kyst and in the basement terrains south of Kangerlussuaq (Nielsen, 1978; Klausen & Larsen, 2002; Hanghøj et al., 2003). In a pre-drift reconstruction, the Faroe Islands would have fitted close to the southern part of the Blosseville Kyst, less than 100 km south-east of Kangerlussuaq (Fig. 1). Larsen et al. (1999) have determined stratigraphic correlations between the lava successions of the Faroes and central East Greenland. Volumetrically-minor post-break-up transitional to alkaline magmatism continued sporadically throughout the central East Greenland region until c. 13 Ma.

We review radiogenic isotope data (>350 samples) on suites of magmatic rocks within the central East Greenland flood basalt province to evaluate the types of crustal assimilants and the extent of crustal assimilation involved in each suite. We use these observations to build a regional picture of how magmatic plumbing systems changed with time and location during the sequential development of the province, as magmatism responded to the development of a volcanic rifted margin and eventual plate separation. The central East Greenland region is an ideal place for this type of study because:
1. there is an excellent temporal record of magmatism preserved within a geographically small area that spans the full range of tectonic environments from pre-break-up, break-up, to post-break-up magmatism.
2. there is such a marked contrast in isotopic composition (εNd and 206Pb/204Pb) between mantle sources and the local crust and also because there are significant variations in isotopic composition (87Sr/86Sr and 208Pb/204Pb) of different crustal lithologies.


Figure 1: Map of the East Greenland region, showing the location of the Palaeogene magmatism (modified from Bernstein et al., 2001 & Larsen et al., 1999).

ASSIMILATION AND THE EAST GREENLAND CRUST

The flood basalts in the Blosseville Kyst region straddle the Caledonian Front (Fig. 1), which marks the western extent of deformation associated with the Silurian-Devonian Caledonian orogeny (Escher & Watt, 1976). West of the Caledonian Front, the basement rocks comprise a sequence of high-grade Archaean gneisses (c. 2.6-3.0 Ga) reworked during the Proterozoic (Leeman et al., 1976; Taylor et al., 1992; Kalsbeek et al., 1993). They all have low εNd (–15 to –41), but with variable 87Sr/86Sr (0.703-0.755) and 208Pb/204Pb (33-40, with some samples up to 60) depending on the lithology (Fig. 2). The most striking feature is their unradiogenic 206Pb/204Pb composition, a result of U-depletion during Archaean granulite-facies metamorphism: >80% of analyses have 206Pb/204Pb of 13.4-15.9. Although the basement rocks are lithologically diverse and cover a wide continuum in isotopic composition (Fig. 2), many studies have simplified this complexity in terms of two end-members: “amphibolitic” gneisses with 87Sr/86Sr of 0.738-0.755 and relatively high 208Pb/204Pb, “granulitic” gneisses with 87Sr/86Sr of 0.703-0.718 and relatively low 208Pb/204Pb (e.g. Carter et al., 1978; Hansen & Nielsen, 1999). East of the Caledonian Front, the crust consists of Palaeozoic-Mesozoic sedimentary basins and Pre-Cambrian basement material that was reworked to differing extents during the Caledonian orogeny. This Caledonian basement comprises Late Proterozoic – Early Palaeozoic supracrustal sequences overlying an Early Proterozoic mobile belt that contains some reworked Archaean gneisses, particularly in the western parts and southern parts near Scoresby Sund (Henriksen & Higgins. 1976; Tucker et al., 1993). This crust has high 206Pb/204Pb (> 18.5) and does not appear to have been involved as an assimilant in the Blosseville Kyst lavas, although it did influence the compositions of Palaeogene basalts further north at Hold With Hope and Shannon Ø (Fig. 1; Thirlwall et al., 1994).

The central East Greenland magmas have radiogenic isotope compositions that plot between the data fields for local basement rocks and local asthenosphere-derived melts (as represented by recent Icelandic basalts and North Atlantic MORB), indicating that these magmas interacted to variable extents with different crustal lithologies within the local basement during their ascent through the crust (Fig. 2). The εNd vs. 206Pb/204Pb diagram is useful because mantle heterogeneity and crustal assimilation produce data trends with markedly different slopes. Crustal assimilation will displace magma samples towards the low εNd and 206Pb/204Pb values shown by all samples of the local crustal basement. In contrast, mantle heterogeneity, as shown by Icelandic lavas and North Atlantic MORB, has a negative slope where the samples with the highest 206Pb/204Pb have the lowest εNd values. The 208Pb/204Pb vs. 206Pb/204Pb diagram is useful to discriminate between different crustal lithologies.


Figure 2: Nd-Pb isotopic composition of Palaeogene magmatism in central East Greenland and the Faroes (red crosses) compared with local basement crust (green field) and asthenosphere-derived melts (recent Icelandic basalts & North Atlantic MORB: purple field). Data sources can be found in Peate et al. (2008).

PHASE ONE (c. 62-57 Ma): PRE-BREAK-UP MAGMATISM

Virtually all samples from the pre-break-up phase have isotopic compositions that do not overlap with the Iceland or North Atlantic MORB fields. Many samples are displaced to extreme isotopic compositions with 206Pb/204Pb of 15.0-18.1 and εNd as low as –5, which indicates extensive crustal contamination (Fram & Lesher, 1997; Hansen & Nielsen, 1999). The scatter of analyses on the 206Pb/204Pb vs. 208Pb/204Pb diagram requires that several crustal assimilants of different compositions were involved. Hansen & Nielsen (1999) showed that the contaminant type changed with time, with the earlier lavas contaminated by amphibolite-type material (higher 208Pb/204Pb & 87Sr/86Sr), while the upper lavas were contaminated by granulite-type material (lower 208Pb/204Pb & 87Sr/86Sr). Hansen & Nielsen (1999) argued that in times of low magma supply, conduit systems would be more ephemeral in nature and magmas would stall at various levels within the crust and preferentially assimilate crustal material with the lower solidus temperatures (i.e. hydrous amphibolite-facies lithologies). During times of higher magma supply rates, conduit systems would be more robust, and the higher supply of heat to the crust from the magmas would allow melting and assimilation of the more refractory granulite-facies rocks. In addition, during the earliest stage of magmatism, there would have been minimal perturbation of the crustal geotherm allowing only the most fusible crustal lithologies to be assimilated, while the progressive build up of heat in the crust due to continued intrusion of mafic magmas would allow a greater contribution from the more refractory lithologies.


Figure 3: Nd-Pb isotope variations in phase 1 pre-break-up samples and phase 2 break-up samples. Data sources can be found in Peate et al. (2008).

PHASE TWO (c. 56-54 Ma): BREAK-UP MAGMATISM

The Plateau Basalts along the Blosseville Kyst make up the dominant preserved volume of break-up magmatism in central East Greenland (Pedersen et al., 1997). Most isotopic data are from the Sortebræ composite profile through the Plateau Basalts (Tegner et al., 1998b), but they include stratigraphically-equivalent lavas and dykes from the Faroes. The break-up magmatism saw a general reduction in overall extent of crustal assimilation compared to the pre-break-up magmatism, with many flows having isotopic compositions that overlap with those found in recent Icelandic basalts. This transition is marked by a change in eruptive style and eruption rates from the small-volume shield volcanoes of the Lower Basalts to the voluminous sheet flood basalt flows of the Plateau Basalts (e.g. Ukstins Peate et al., 2003). The feeder systems to both suites were in the same general geographical area (Pedersen et al., 1997), and so it is likely that the most easily fusible parts of the crust were assimilated during eruption of the Lower Basalts. Eruption of the Plateau Basalts saw the establishment of major new plumbing systems, and their continued use would have reduced the availability of easily assimilated crustal material to later magmas, leading to overall lower extents of crustal contamination.

There are systematic variations in the contamination process within the Plateau Basalts (Fig. 4). The average extent of assimilation decreased with time within the lower two-thirds of the Plateau Basalts from the Milne Land to the Rømer Fjord Formation lavas, and then became more significant again in the uppermost Rømer Fjord Formation and the Skrænterne Formation. The earliest stages contain some highly contaminated flows as new robust conduit systems were being established but the extent of assimilation soon diminished due to a decrease in the availability of suitable crustal material for assimilation in the mature plumbing systems. Despite the low extents of contamination in the Geikie Plateau Formation, through a detailed flow-by-flow study (Andreasen et al., 2004) it has been possible to resolve packets of lavas that stalled and fractionated at different crustal levels where the wall rocks had different compositions. There is a return to more contaminated lavas in the later stages (Skrænterne Formation) and coupled with the range in inferred assimilant compositions and significant inter-flow variations in incompatible element ratios, this suggests a shift from a few robust conduit systems to a greater number of smaller new conduits established as rifting proceeded. It also requires that continental crust was still present beneath the developing rift, and that rifting had not yet focused into a single true oceanic rift.


Figure 4: Stratigraphic variations in 206Pb/204Pb within the Sortebræ profile through the Plateau Basalts. Samples that do not overlap with the field of local asthenospheric melts (recent Icelandic basalts and North Atlantic MORB) in Sr-Nd-Pb isotope space are considered to be ‘contaminated’ (Peate & Stecher, 2003; Andreasen et al., 2004; Barker et al., 2006).

PHASE THREE (c. 54-13 Ma): POST-BREAK-UP MAGMATISM

The post-break-up magmatism is dominated by magmas with clear evidence for high levels of crustal assimilation, greater than in the main break-up magmatic phase. This reflects a change in the style of magmatism, which involves the eruption of small-volume alkalic lava flows from newly established conduits through the thicker inland crust (e.g. Peate et al., 2003; Storey et al., 2004) and larger mafic and silicic alkalic intrusions being emplaced at shallow levels into the Archaean basement terrains now exposed along the coastal regions and undergoing local assimilation of basement rocks (e.g. Bernstein et al., 1998; Hanghøj et al., 2003; Riishuus et al., 2005, 2006, 2008).


Figure 5: Nd-Pb isotope variations in phase 3 post-break-up volcanic and intrusive rocks. Data sources can be found in Peate et al. (2008).

ACKNOWLEDGEMENTS:   This article is based on work carried out while the authors were staff and students at the now defunct Danish Lithosphere Centre, which was funded by the Danish National Research Foundation. Additional support was provided by NSF grant EAR-0439888 and the University of Iowa. We thank many colleagues associated with the former Danish Lithosphere Centre for numerous stimulating discussions about the magmatism of East Greenland, including Joel Baker, Stefan Bernstein, Kent Brooks, Karen Hanghøj, Henriette Hansen, Paul Martin Holm, Adam Kent, Lotte Melchior Larsen, Hans Christian Larsen, Chip Lesher, Troels Nielsen, Ingrid Ukstins Peate, Asger Ken Pedersen, Ole Stecher, Michael Storey, Christian Tegner and Tod Waight.

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