The Age of the Antrim Lava Group, Northern Ireland, and its correlation to the North Atlantic Igneous Province
Abstract
Modern radiometric dating techniques enable reliable dating of flood basalt provinces and in doing so, lead to a deeper understanding of eruption dynamics. Reliable ages have been published for much of the pre-drift lava succession of the North Atlantic Igneous Province. However, chronology of emplacement of the extensive Antrim Lava Group (ALG) in Northern Ireland was until now, poorly constrained. The ALG consists in stratigraphic order of the lower basalt Fm (LBF), interbasaltic Fm (IB) with the Tardree rhyolite, and upper basalt Formation (UBF). A study by Thompson (1985), using the 40Ar/39Ar method, left the age of the LBF inconclusive, but a reliable date of 61 ± 0.6 Ma was obtained for the Tardree rhyolite (IB), and 58.3 +/- 1.1 Ma for the UBF. The date of 61 +/- 0.6 Ma for the Tardree rhyolite corresponds well with a recent U-Pb date of 60.9 +/- 0.5 Ma (Gould, 2004) of detrital zircons from a laterite layer underlying the Causeway Tholeiites (IB) in the north, zircons that may have Tardree as a source. However, Gamble et al. (1999) reported a younger date of 58.4 +/- 0.7 Ma using the U-Pb SHRIMP method for the Tardree rhyolite, so a discrepancy exists between different investigations to obtain the age of emplacement of these rhyolites. The magnetic polarity of the ALG has been reinvestigated by us and shown to be of reverse polarity. Based on the reverse magnetic polarity and the close spatial relationship with other trap sub-provinces in the British Tertiary Igneous Province, the literature has placed ALG to C26r on the geomagnetic polarity timescale. We have dated the principal lava formation in the ALG using the 40Ar/39Ar method. We obtained a weighted mean age (3 samples) of 61.4 +/- 0.5 for the LBF and 60.5 +/- 0.5 (1s, including the error on J) for the Tardree rhyolite (IB). These dates overlap at the 95% confidence level using a conservative 1% error on the J-value, but are separate using a less conservative approach. We obtained a date of 57.7 +/- 1.5 on the UBF. These new radiometric dates are consistent with the stratigraphic position of the units and are consistent with the previous work of Thompson (1985) and Gould (2004). The reliability of the older age of LBF is enhanced taking into account that Tardree rhyolite is overlying 19 meters of red basaltic bole and lithomarge of LBF, representing a considerable time of volcanic dormancy between the LBF and IB. Therefore, the reverse magnetic polarity of the LBF, combined with the new ages, places the eruption of the lower basalt formation to C27r on the geomagnetic polarity time scale, not C26r as previously thought, thus a normal polarity epoch must be hidden in the interbasaltic hiatus. This being so, we can now show that the ALG is older than the Vaigat Fm. in West Greenland (C27n) and that the major trap formations in the British Tertiary Igneous Province show a time progressive pattern from south-west to north-east.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFM.V53A2124G
- Keywords:
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- 8137 Hotspots;
- large igneous provinces;
- and flood basalt volcanism