Detecting Mesoproterozoic far-field Grenville tectonism in NE Laurentia using multi-method apatite thermochronology

The tectonothermal histories of basement gneiss terranes are challenging to determine, particularly in orthogneisses dominated by high-variance metamorphic assemblages that are unsuitable for thermobarometry or dating metamorphism. Apatite is a widespread accessory mineral in crystalline rocks, and apatite U–Pb and Lu–Hf closure temperatures (350–570 °C and 660–730 °C) correspond to middle-lower crustal depths. This apatite thermochronology case study investigates the thermal history of the Paleoproterozoic Rhinns Complex, a key fragment of NE Laurentian basement linking the Paleoproterozoic mobile belts of Greenland and Scandinavia. The Rhinns Complex lies immediately north of the Grenville Front in NW Ireland. Orthogneiss protoliths were emplaced at 1.78 Ga (zircon U–Pb) and underwent high-grade metamorphism at 1.73 Ga (apatite Lu–Hf). Laser–ablation U–Pb depth profiling was undertaken on primary igneous apatite with simultaneous acquisition of trace elements to identify recrystallized crystals or subdomains. Inverse modelling of U–Pb depth-profiles (constrained by zircon and epidote U–Pb, apatite Lu–Hf, and biotite Rb–Sr dates) yields a continuous deep-time thermal history, revealing rapid post-emplacement cooling and reheating to 450 °C at 1.4 Ga during the Pinwarian Orogeny, followed by exhumation. The 1.4–1.1 Ga thermal history (either isothermal holding at 375 °C or cooling and reheating) is attributed to far–field Grenville tectonism and subsequent exhumation – detected here in the Rhinns Complex for the first time. Our study highlights the power of high-resolution apatite U–Pb depth profiling to resolve deep-time crustal tectonothermal histories, especially “hidden” thermal events not captured by conventional methods.