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

该项目旨在应用40Ar/39Ar激光微束和显微取样测年技术，对与低品位岩石(叶理、应变条纹和矿脉)中的微结构有关的新生长、同变形和变形后的白色云母进行研究。目标区域是爱尔兰西南部的欧洲瓦里斯坎前陆，这是一个暴露良好、构造约束良好的褶皱带，主要由低级变质古生代沉积物组成。对白云母在上云母/下云母变质条件下变形时形成的白色云母进行测年，得出的生长年龄低于白云母中Ar扩散的闭合温度(约350℃)。这种材料随后没有长时间的冷却历史。对不同世代的低等级微结构组构的直接测年将准确地确定它们的形成日期，并确定不同变形阶段的持续时间。此外，在薄片尺度上测年将最大限度地减少试验性白色云母的贡献，这些白色云母将扭曲Ar同位素数据，并产生虚假的(较老的)年龄。为了实现对Am微观结构尺度的分辨，将使用40Ar/39Ar原位(颗粒内)紫外线激光对抛光样品进行烧蚀，并结合红外激光对从薄片和手工挑选的云母分离物中钻取的微观样品进行红外激光阶梯加热。

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.