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The novel technique of U-Pb dating of garnet allows us to date rocks that were previously impossible to put into a temporal context, either because of the absence of traditional geochronometers such as zircon, monazite, and rutile or because of isotopic disequilibrium, e.g. Lu-Hf or Sm-Nd. The possibility to exclusively date garnet crystals can give us important information to understand the geological evolution of a particular region.In this contribution, we present data from a garnet-quartz vein from the Cabo Ortegal complex, NW Iberian Massif. In the vein, andraditic garnet is associated with chlorite and serpentine in fissures, which cut the foliation of the surrounding lithologies. Field evidence thus indicates that vein formation postdates the formation of the regional foliation (ca. 390 Ma). However, until now, it is unknown, if vein formation is related to thrusting and fluid circulation during the uplift of the area (350-330 Ma) or as the consequence of igneous intrusions (ca. 300 Ma). Vein garnet was analysed in two analytical sessions. The analyses yielded unanchored lower intercept ages of 253.5 ± 7.6 Ma (MSWD = 1.1, n = 62/64) and 252.2 ± 9.5 Ma (MSWD = 1.33, n = 40/40). Those results are much younger than the basement rocks and structures in the Cabo Ortegal complex. The obtained ages coincide with the Permo-Triassic rifting widely documented from other geological domains of the Iberian Peninsula, but has never before been recognised in the Allochthonous Complexes of NW Iberian massif. The study of these types of samples may therefore provide clues to the geological history of the Cabo Ortegal Complex, which may not be available from other techniques and lithologies.

Recent advances in analytical techniques and instrumentation allow for the analysis of increasingly smaller sample volumes and lower concentrations. This development significantly expands the possibilities of in-situ geochronology, e.g., LA-MC-ICPMS. Minerals with low U (and Pb) contents such as garnet become the target of in-situ U-Pb geochronology since ages can potentially be obtained from single (sub-)mm-sized garnet grains in thin sections. In this contribution, we explore the current limits of in-situ U-Pb geochronology: what are the minimum concentrations from which an accurate and precise U-Pb age can be obtained?For that purpose, we have analysed garnets from three different localities that were unsuccessfully analysed in the past using a single-collector sector-field Element XR instrument at FIERCE. These garnets have been re-analysed at FIERCE using a Neptune Plus MC-ICPMS coupled to a RESOLution-LR ArF Excimer laser. The analyses were performed in static mode measuring the masses 206Pb and 207Pb with Secondary Electron Multiplier (SEM) and 202Hg, 204Pb and 238U with the Multiple Ion Counters (MIC). With a spot diameter of 193 μm (round) and a fluence of 2 J/cm2 at 15 Hz, ca. 18 µm pit depth was ablated in 18s analysis time, resulting in a total of 2 µg of ablated material. This is more than 2,000 times less material compared to conventional isotope dilution analyses and 3,000 times less U than for a typical LA-ICPMS zircon analysis (20 µm spot). Although the analysed garnets typically have U contents below 10 ng/g, about 15–30 spots are commonly sufficient to define a regression line in the Tera-Wasserburg diagram, yielding a precision of typically