U-PB GEOCHRONOLOGY OF DETRITAL ZIRCONS FROM THE INTERBASALTIC FORMATION, ANTRIM LAVA GROUP; GIANT'S CAUSEWAY, CO. ANTRIM, NE IRELAND, AND IMPLICATIONS FOR THEIR SOURCE

Detrital zircons were obtained from samples of the Interbasaltic Formation where it underlies the basal flow of the Causeway Tholeiite Member, Antrim Lava Group. Single-grain U-Pb analysis indicates that the zircons were derived from an igneous rock that crystallised at ~61 Ma. Our results agree with previously reported ⁴⁰Ar/³⁹Ar dates for the anticipated source for these zircons, the Tardree rhyolite to the south of the Causeway, but are older than other U-Pb dates. The ages are likewise old for the assumed age of the British Tertiary Province of which the Antrim Lava Group is a part.

Samples were collected from three sites within ~100 km². Twenty-six single crystals yielded ²⁰⁶ Pb*/²³⁸U ages of 60.2–62 Ma; 2s errors range from 1 to 13.5 m.y. on these numbers. Statistical analysis indicates a mean age of 60.9 ± 0.5 Ma (2s standard error). Because the zircons were collected from a geographically diverse area and are of variable morphology (i.e. acicular, tabular, barrel-shaped), we assume that the tight clustering of ²⁰⁶Pb*/²³⁸U ages is statistically relevant.

Our results have several different interpretations: 1) the zircons’ source is the Palaeocene felsic rocks of Tardree. Stratigraphic arguments suggest that mafic activity (Causeway Tholeiite) in the north of the region was contemporaneous with felsic activity (Tardree rhyolite) in the south; 2) the zircons were derived from the granophyres of Carlingford central complex in the southernmost part of the region. It is generally accepted that the granophyres, which are similar in age to zircons in the Interbasaltic Formation, reached ‘near-surface’ depths, but is not known whether they actually erupted; 3) the zircons were derived from the poorly understood Donald’s Hill ignimbrite, which is on the westernmost edge of the Antrim Lava Group. This ignimbrite has been suggested to be Palaeogene in age but some controversy exists as to its stratigraphic position and hence its importance regionally.