Geochronology of the Tardree Rhyolite Complex, Northern Ireland: Implications for zircon fission track studies, the North Atlantic Igneous Province and the age of the Fish Canyon sanidine standard

摘要

The British-Irish Palaeogene Igneous Province (BIPIP) is part of the larger North Atlantic Igneous Province and includes the lava fields of Antrim, Mull, and Skye. The Tardree Rhyolite Complex (TRC) in Northern Ireland forms an important stratigraphic unit between the Lower and Upper Basalt Formations of the Antrim Lava Group (ALG). Previous zircon age determinations obtained from the TRC have been used as a standard in zircon fission track studies, but contradict several ~(40)Ar/~(39)Ar sanidine and U-Pb zircon results. We provide new ~(40)Ar/~(39)Ar sanidine and U-Pb CA-TIMS zircon ages which resolve this discrepancy. Two sanidine samples from the Sandy Braes vent and the columnar-jointed dome-forming rhyolites of Tardree Forest yield a weighted mean ~(40)Ar/~(39)Ar age of 61.13±0.42Ma (2α, internal error). Ten U-Pb CA-TIMS zircon analyses were undertaken, eight of which employed the CA-TIMS approach on both multi-grain fractions and single grains. Six of the CA-TIMS data yield a disequilibrium-corrected weighted mean 206Pb-238U age of 61.32±0.09Ma (2σ). The consistency of the ~(40)Ar/~(39)Ar ages with the CA-TIMS U-Pb zircon age, points to a closed system of both K and Ar since eruption. We propose that the crystallization age of the TRC be taken as 61.32±0.09Ma and that the currently used age of the zircon fission track standard (58.4±0.7Ma) be changed accordingly. This also places the eruption of the TRC in magnetochron C26r, which is consistent with the reversed polarity magnetic remanence observed in the ALG, and supports the conclusion of Ganer?d et al. (2010) that the Lower Basalt Formation is older than the Vaigat Formation in Western Greenland. No resolvable zircon inheritance has been detected by the TIMS analyses, consistent with the fact that the temporal and geographic extent of rhyolitic magmatism within this sector of the BIPIP was very limited, and hence was unlikely to provide inherited magmatic zircons from slightly older magmas (antecrysts). Potentially older zircon xenocrysts would be derived from the underlying Caledonian basement (>400Ma) or yet older rocks. These should be easily detectable if the Tardree zircon was to be employed as a U-Pb zircon standard. The paired ~(40)Ar/~(39)Ar and 206Pb-238U results from this study indicate an age of 28.393±0.194Ma for the widely used Fish Canyon sanidine standard and gives further support to the recent calibrations of Kuiper et al. (2008) and Renne et al. (2010).