The formation of granite batholiths, commonly by incremental assembly of small magma batches, and their correlation with tectonic events, on both local and regional scales, is crucial to understanding the evolution of the Earth's continental crust. However, these correlations often rely on assumptions about the detailed relationship and timing of mapped units. Here we report how an integrated geochronological, structural and isotope geochemical study in only one key locality from the late Caledonian Leinster Batholith in SE Ireland provides the potential for unravelling essential questions on batholith petrogenesis. The Northern Unit of the Leinster Batholith intruded incrementally, as demonstrated by three crosscutting granite varieties at Glenmacnass Waterfall. The oldest, foliated granite, yielded a zircon U–Pb age of 417.4 ± 1.7 Ma. Following deformation, equigranular granite intruded at 409.8 ± 1.9 Ma and was cut by megacrystic granite at 404.9 ± 2.6 Ma. Hence, batholith formation lasted up to 16.8 myr, challenging the widely accepted 405 Ma age for the entire batholith. Multi-isotope geochemical investigations permit derivation of the three granites from the same sources, including both Mid- to Upper Ordovician peri-Gondwanan arc-related magmatic and Lower to Mid-Ordovician Ribband Group metasedimentary rocks. Zircon inheritance and Lu–Hf isotope geochemistry also reveal reprocessing of older granite wall rock in the younger magmas.

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