In situ, high spatial resolution 40Ar/39Ar laser probe dating of microstructures in very low-grade metasediments from the Variscan foreland, southern Ireland

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This project aims to apply 40Ar/39Ar laser microbeam and microsamling dating techiques on newly grown, syn- and post-deformational white micas associated with microstructures in low-grade rocks (foliations, strain-fringes and veins). The target area is the European Variscan foreland in south-west Ireland, an excellently exposed and structurally wellconstrained fold belt predominantly composed of low-grade metamorphic Palaeozoic sediments. Dating of white micas that formed during deformation under upper anchizone/lower epizone metamorphic conditions will yield growth ages as they formed below closure temperature of argon diffusion in white mica (c. 350°C). this material did not subsequently a protracted cooling history. Direct dating of different generations of low-grade microstructural fabrics will precisely and accurately date their formation, and resolve and bracket the duration of discrete stages of deformation. In addition, dating on thin section scale will minimise contributions of detrial white micas that will skew the argon isotope data and yield spurious (older) ages. In order to achieve resolution on am microstructural scale, the dating techniques used will be 40 Ar/39 Ar in situ (intra grain) ultra-Violet laser ablation on polished samples, in combination with infra-red laser step-heating on micro-samples drilled from thin sections and hand-picked mica separates.