Antrim lava field : flow patterns and provenance of interbasaltic zircons

Abstract

A multidisciplinary investigation was conducted on the Antrim Lava Group (ALG). The first aspect of the research involved dating key stratigraphical horizons within the ALG, using single crystal U-Pb thermal ionisation mass spectrometry (TIMS) analysis of zircons. One such stratigraphical horizon was a bole that is situated between the first flow of the Causeway Tholeiite Member (CTM) and the Port na Spaniagh Member (PSM) of the Interbasaltic Formation (IBF). This unit contained detrital zircons that have a mean age 60.9 ± 0.5 Ma (2o standard error) therefore suggesting that the CTM has a maximum age of 60.9 ± 0.5 Ma and the PSM is older than 60.9 ± 0.5 Ma. To determine the provenance of the detrital zircons from the Interbasaltic Formation, crystals from both this formation and the potential sources were characterised using their petrographical features and mineral geochemistry. Petrographically, the 'interbasaltic' zircons are very similar to those obtained from the Tardree rhyolite. Cathodoluminescence microscopy and back-scattered electron imaging show dark cores in both the detrital and Tardree zircons, indicative of radiation damage produced by a rich core. Electron microprobe and synchrotron-XRF analysis determined the major and trace element composition of the zircons; though the results proved not to be definitive, there was a broad correlation between the Tardree rhyolite and the 'interbasaltic' zircons.