December 2016 LIP of the Month

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Improving the geochronology of the North Atlantic Igneous Province

Lars E. Augland, Morgan T. Jones, Henrik H. Svensen, Sverre Planke

Centre for Earth Evolution and Dynamics (CEED), University of Oslo, PO Box 1028 Blindern, 0315 Oslo, Norway

Emails: l.e.augland@geo.uio.no; m.t.jones@geo.uio.no; hensven@geo.uio.no; planke@vbpr.no

Overview

The North Atlantic Igneous Province (NAIP) was one of the larger known large igneous provinces (LIPs) that was dominantly emplaced during the Palaeogene (Saunders et al., 2007; Ernst, 2014) (Figure 1). The total volume of magma emplaced during the Palaeocene and Eocene is estimated to be 6-10 x 106 km3. Magmatism continues to the present day in the form of the Iceland hotspot, albeit at a reduced flux rate. The initial activity was manifest as two main pulses of activity:

  • Phase 1 - A pre-break-up phase of continental flood basalt volcanism with associated subvolcanic intrusions and plutonism that started at ~63 Ma. The majority of magmas were largely erupted through and onto continental crust, with remnants of Phase 1 activity found across much of the NAIP region.
  • Phase 2 - The main, more voluminous phase of thick lava flows, large subvolcanic intrusions, and plutonic complexes that began at ~56 Ma. This larger pulse is closely associated with the break-up of the North Atlantic (Saunders et al., 2007; Storey et al., 2007b; White and McKenzie, 1995). Remnants of Phase 2 are found in East and West Greenland, on the Faroe Islands and on the Norwegian Sea shelf.


Figure 1:
A plate reconstruction at 55 Ma (Abdelmalak et al., 2015; Jones et al., 2016) around the time of North Atlantic breakup. Paleo-shorelines and marine seaways (Golonka, 2009) are indicated in blue. Present day coastlines are shown in black. The dashed red lines indicate projected plate boundaries. The translucent red overlay areas denote the known extent of the NAIP at this time, with individual centres shows are solid red areas.

Mantle Plume Source?

A current and ongoing source of contention in this scientific field is the possible role of deep-fed mantle plumes in the emplacement of LIPs (Ernst, 2014). The majority of studies argue for a plume source in the formation of continental flood basalts (White and McKenzie, 1995), oceanic plateaus (Coffin and Eldholm, 1994), and volcanic rifted margins (White et al., 1987). However, a vocal group of scientists question the prevalence of an assumed plume source in the literature. They argue that not all interpretations are fully explored, and that published interpretations of a plume source are often inconsistent with other datasets from other sources. Alternative proposals have included thermal blanketing of overlying lithosphere (Anderson et al., 1992) and enhanced mantle convection (Mutter et al., 1988). The NAIP is an ideal locality to study whether a LIP emplacement can be connected to a mantle plume, as much of the volcanic and magmatic products are still exposed across the province. This allows for an evaluation of thermal and compositional variations across the LIP. We are currently working on several sites around the NAIP, focusing on volcanic products to improve the geochronology of plate reconfigurations and the emplacement of the NAIP.

There is already fairly convincing evidence that the NAIP emplacement was due to the arrival of the proto-Iceland plume below thick continental lithosphere in Greenland (Torsvik and Cocks, 2016). The simultaneous and widespread initiation of magmatism across the province points to a major thermal event in the mantle with an abnormally high flux rate (Saunders et al., 2007). The high magnesian content of some magmas suggest anomalously hot melts (Kent, 1995). Moreover, the isotopic and compositional diversity observed in Iceland is mirrored in many of the Palaeocene sequences, even after crustal contamination and pressure of melt segregation are taken into account (Saunders et al., 2007). Finally, seaward dipping reflector series erupted during Phase 2 show evidence of subaerial or shallow marine eruptions, indicating thermal uplift during emplacement (Hinz, 1981). Modern seafloor bathymetry (Haigh, 1973), ocean crust thicknesses around Iceland (White and McKenzie, 1995), and major element composition variations along the mid-Atlantic ridge (Klein and Langmuir, 1987) all suggest that thermal buoyancy effects beneath Iceland continue to this day.

If the initial phase of the NAIP was driven by a plume arriving at the base of the lithosphere, one would expect there to be significant plate reorganisations in response. Indeed, there is an abrupt change to the deformation style within the Eurasian plate at 62-61 Ma (Nielsen et al., 2007) and in northern Canada (e.g. Harrison et al. 1999), marking the boundary between the Danian and Selandian stages. The Danian-Selandian boundary marks the end of 40 million years of carbonate deposition in the North Sea, with a shift to siliciclastic deposition sourced from the uplift of the Scotland-Shetland area (Clemmensen and Thomas, 2005). There is also evidence of widespread shear deformation along the Greenland-Eurasia margin around this time (Guarnieri, 2015). Several potential triggers have been proposed, including a temporary hiatus in convergence between Africa and Europe (Nielsen et al., 2007), the propagation of seafloor spreading in the Labrador Sea (Oakey and Chalmers, 2012), and the emplacement of Phase 1 of the NAIP (Storey et al., 2007b). However, one of the large gaps in our current understanding of the NAIP is the relative ages of events, meaning that the relative contributions of forces driving changes to plate motions remain contentious. Therefore, improving the geochronology of events would shed considerable light onto not only the NAIP itself, but also onto the potential correlation of NAIP events with observed plate reorganisations in the Palaeocene and Eocene.

Current Geochronology

At present, the available modern geochronological data are restricted to 40Ar/39Ar mineral ages and a few U-Pb ages for subvolcanic intrusions. There is only one modern (single zircons) high precision chemical abrasion isotope dilution thermal ionization (CA-ID-TIMS) study of NAIP rocks, from the Skaergaard intrusion (Wotzlaw et al., 2012). The robust (but not highly precise) ages that exist (40Ar/39Ar mineral ages [recalculated to conform to the Fish Canyon sanidine age of (Kuiper et al., 2008)], U-Pb zircon ID-TIMS ages) show a spectrum of ages from ca. 63 to 54 Ma that cluster in two groups, one at ca. 63 to 57.0 Ma and a second at ca. 57.3 to 53.7 Ma (Chambers et al., 2005; Ganerød et al., 2010; Hamilton et al., 1998; Larsen et al., 2016; Storey et al., 1998; Storey et al., 2007b; Svensen et al., 2010; Wilkinson et al., 2016). Although the radiogenic ages for Phase 1 and 2 magmatic rocks for NAIP in general overlap, there is a somewhat sharper picture in a regional view, where hiatuses can be identified between the two phases of volcanism (Larsen et al., 1999).

In East Greenland available ages of the Main Basalts (Phase 2) range from 57.3 ±1 Ma to 53.8 ±0.8 Ma, whereas the older Lower Basalts (Phase 1) range from 58.4 ±0.5 Ma to 57.5 ± 2 Ma, with some sills from southeast Greenland being as old as 62.6 Ma, possibly reflecting closely the age of the lowermost flow of the Lower Basalt series that has not been successfully dated (Storey et al., 2007b). Recent high precision U-Pb ages of the Skaergaard intrusion indicate that it was initially emplaced at ca. 56.02 Ma (Wotzlaw et al., 2012). This age overlaps the ages of basalt flows dated by (Storey et al., 2007b), but the 40Ar/39Ar data suggest that flood basalt volcanism of Phase 2 in East Greenland started up to 1.3 m.y. before emplacement of the Skaergaard intrusion. Alternatively, solidification pressures at different levels of this intrusion can be interpreted to reflect rapid burial under increasing amounts of basaltic flows extruding contemporaneously with its crystallisation (Larsen and Tegner, 2006). Combined with the high resolution crystallisation history of the Skaergaard intrusion (Wotzlaw et al., 2012), this could indicate that Phase 2 basalts on the East Greenland margin was emplaced within ca. 100 kyr.

In West Greenland, Phase 1 volcanic rocks have been dated by 40Ar/39Ar and range from younger than 62 ±1 Ma to 57.0 ±0.6 Ma and Phase 2 lavas have been dated to between 57.3 ±1 Ma to 54.0 ±0.3 Ma (Larsen et al., 2016; Storey et al., 1998). Available data from the Faroe Islands indicate that Phase 1 igneous activity started at 60.8 ±0.6 Ma or earlier and lasted until after 57.5 ±0.6 Ma, and that Phase 2 activity had started by ca. 55.7 Ma (Storey et al., 2007b). The NAIP magmatic activity on the British Isles, as recorded by 40Ar/39Ar and U-Pb ages, ranges from 62.9 to 58.9 Ma (Chambers et al., 2005; Ganerød et al., 2011; Ganerød et al., 2010; Hamilton et al., 1998; Storey et al., 2007b). A sill from the Vøring margin off the west coast of Norway was dated by zircon U-Pb to 55.6 Ma (Svensen et al., 2010). Minor volumes of post break-up igneous rocks were emplaced more or less continuously until the Late Eocene, probably related to the East Greenland margin crossing westward over the Iceland plume axis (Storey et al., 2007b).

Volcano-Climate Interactions

Phase 2 volcanic and subvolcanic activity of the NAIP has been linked to the severe climatic changes that occurred over the Palaeocene-Eocene boundary, termed the Palaeocene-Eocene Thermal Maximum (PETM; Figure 2). The PETM occurred at ca. 55.8 Ma (Charles et al., 2011) and was marked by a rapid, global >5 °C increase in global surface temperatures within 20 ka (Cui et al., 2011). It was caused by the release of vast amounts of carbon to the atmosphere and is characterized by a sharp negative δ13C excursion that gradually returned to elevated pre-PETM values (Zachos et al., 2008). Associated with the event there were prominent changes of the terrestrial and marine realms, with effects on ocean circulation (Kennett and Stott, 1991; Nunes and Norris, 2006), and ocean acidification (Zachos et al., 2005). As a consequence, considerable biotic turnover occurred. The onset of the extreme greenhouse conditions during the PETM has been suggested to be linked to igneous activity in the NAIP (Storey et al., 2007a; Wotzlaw et al., 2012), with degassing from organic-rich sedimentary rocks as one of the main mechanisms proposed (Svensen et al., 2010; Svensen et al., 2004). Published age data from the voluminous Phase 2 NAIP magmatism, however, range from ca. 57 to 54 Ma, thus potentially predating the PETM with more than 1 million years.


Figure 2:
A stacked record of 13C values in the Palaeogene (Zachos et al., 2008). The two main pulses of NAIP are shown at the bottom along with the time interval covered by the Fur Formation. The blown up δ13C curve above shows the measured excursion in the Frysjaodden Formation, Svalbard (Charles et al., 2011), spanning ~200 kyr.

Future Work

Whether the Palaeocene to Eocene igneous activity in the NAIP occurred in two discrete phases or was more or less continuous cannot be properly resolved outside uncertainties of individual ages with the presently available geochronological data. Accepting that there were two distinct phases of igneous activity separated by a hiatus (Larsen et al., 1999; Storey et al., 2007b), the time span for the two phases indicated by published data is on the order of 6 and 3-4 million years for Phase 1 and 2, respectively. In order to test if the NAIP represents punctuated, short-lived LIP magmatism where the major magmatic events correlate with important tectonic and climatic changes as has previously been suggested (Storey et al., 2007a), an extensive high precision geochronological study that covers the different regions of the NAIP is required. We are currently working in on volcanic ash layers in Svalbard, Greenland, and Denmark (Figure 3). These deposits span an age of 62-55 Ma, with provenances related to the emplacement of the NAIP and continental rifting associated with plate reconfigurations (Jones et al., 2016). In the coming years we hope to significantly improve the geochronology of the NAIP emplacement and its relation to plate reconfigurations and climate perturbations.


Figure 3:
An exposed sequence of volcanic ash layers preserved in shallow marine sediments in northern Denmark (1m tall child for scale). Over 180 ash layers over 1 cm thickness are present, with some up to 15 cm thickness, despite being more than 1800 km from the nearest known NAIP centre.

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