SUMMARY
Large igneous provinces (LIPs) have recently been suggested to originate at the edges of low‐velocity zones on the core mantle boundary (Plume Generation Zones). If true, LIPs can potentially be used to constrain paleolongitude in plate tectonic reconstructions. To validate the hypothesis, it is essential to study LIPs of which the paleolongitude can be constrained by other methods, such as hotspot reference frames. An ideal candidate to this end is the early Cenozoic North Atlantic Igneous Province (NAIP). Despite being the largest volcanic unit of the British Tertiary Igneous Province (BTIP, part of the NAIP), the age and paleoposition of the Antrim Lava Group (ALG) in Northern Ireland, which is key to the NAIP as a whole, was hitherto poorly constrained. In this paper, we therefore present an integrated high‐resolution paleomagnetic and geochronological study.
The ALG is divided into three formations: the Lower Basalt Formation (LBF), Interbasaltic Formation (IBF) and the Upper Basalt Formation (UBF). The IBF is mostly lateritic and encloses the Tardree rhyolite. We offer new age constraints from all three formations using the 40Ar/39Ar method and propose that 62.6 ± 0.3, 61.3 ± 0.3 and 59.6 ± 0.3 Ma (1σ, internal uncertainties) are sound estimates of the age of emplacement of the LBF, Tardree rhyolite (IBF) and UBF, respectively. This constrains the nominal duration of emplacement of the ALG to 3 ± 0.6 Ma (1σ).
This reevaluation of the magnetic signature in the ALG revealed reverse polarity remanence in all three formations and an overall paleomagnetic north pole at latitude 78.9°N, longitude 167°E (A95 = 6.3; age ∼61 Ma) in the European reference system. This appears consistent with paleomagnetic poles from the rest of the NAIP; both in Europe and Greenland, as well as predictions from modern apparent polar wander paths.
The new radiometric ages span magnetochron C26r, C27n and C27r. The normal polarity chron C27n most probably occurred during the IBF hiatus, explaining why no normal polarity remanence was detected in the paleomagnetic investigation. Emplacement of the LBF falls in magnetochron C27r, making this one of the oldest lava sequence in the NAIP; older than the C27n lava pile in Western Greenland.
The 60 Ma position of the NAIP in a paleomagnetic reference frame, puts it close to the northern edge of the African large low shear wave velocity anomaly at the core–mantle boundary and therefore in the line with the Plume Generation Zone hypothesis. However, the back‐projected Icelandic hotspot, normally considered to have formed the NAIP, is located ∼1500 km north of the latitude at which the NAIP erupted. The northward motion of the north Atlantic lithosphere since the late Cretaceous challenging the existing correlation of the NAIP to the Icelandic hotspot, normally used to explain the observed pre‐ and syn‐breakup North Atlantic magmatism (63–55 Ma), and either an additional plume located further south in the North Atlantic may be invoked to create the NAIP, or the Icelandic hotspot must hav...
