Precise U-Pb zircon dating of the Donegal composite batholith, Irish Caledonides

Composite intrusions are incrementally constructed over thousands to tens of millions of years. Emplacement of the Donegal composite batholith in NW Ireland was syn- and post-kinematic with respect to the terminal phases of the Caledonian orogeny (ca. 437–415 Ma) in a slab-failure tectonic setting. Published LA-ICP-MS (laser ablation inductively coupled plasma mass spectrometry) U-Pb data from zircon and titanite indicate emplacement occurred between ca. 430 Ma and 400 Ma. Here, we use CA-ID-TIMS (chemical abrasion isotope dilution thermal ionization mass spectrometry) to precisely date zircon from three samples; weighted mean 206 Pb/ 238 U dates are 426.77 ± 0.11 (Ardara pluton), 414.71 ± 0.11 (Main Donegal pluton), and 414.25 ± 0.11 Ma (Thorr pluton). Collectively, U-Pb dates from the Donegal composite batholith support interpretations indicating a prolonged and dynamic magma emplacement spanning at least 12 Ma. The new CA-ID-TIMS dates are equivalent to published LA-ICP-MS dates from the same samples. Taken together, these data indicate pulsed batholith emplacement commenced by ca. 437 Ma with the emplacement of the appinite suite and associated lamprophyres. By ca. 400 Ma, magmatism terminated with the emplacement of the latest phases of the Main Donegal pluton and the Trawenagh Bay pluton.