The construction of the Donegal composite batholith, Irish Caledonides: temporal constraints from U-Pb dating of zircon and titanite

Magmatic and tectonic processes can transport large volumes of magma generated in the deep crust as discrete pulses to shallower crustal depths, resulting in the incremental construction of large composite batholiths over thousands to tens of millions of years. The Silurian to early Devonian Donegal composite batholith in Ireland is a classic example for which regional geological syntheses and lithogeochemical data show emplacement was syn- and post-kinematic with respect to the terminal phases (ca. 437-415 Ma) of the Caledonian orogeny. We used U-Pb dating of zircon and titanite to investigate the construction of the batholith over time. Imaging of these minerals reveals complex, zoned grains with distinct autocrystic (growth during pluton emplacement), as well as inherited domains comprising antecrysts (growth during lower crustal incubation), and xenocrysts (incorporated from wall rocks). To determine the age(s) of emplacement and of inherited domains, discrete growth zones were targeted for dating using LA44 ICP-MS (laser ablation inductively coupled plasma mass spectrometry). Taken together, the zircon and titanite U-Pb isotopic data indicate that batholith emplacement occurred over at least 30 myr, between ca. 430 Ma and 400 Ma, bracketed by emplacement of the ca. 427-423 Ma Ardara pluton and the latest phases in the Main Donegal and Trawenagh Bay plutons (ca. 400 Ma). Although apparently volumetrically minor, U-Pb data from spatially associated mafic rocks (appinite suite, lamprophyre dikes, and mafic enclaves in granitoid plutons) yield ages ranging from ca. 431-416 Ma, which indicates ongoing mafic magmatism during emplacement of much of the Donegal composite batholith.