Ion microprobe U-Pb zircon geochronology and correlation of Archaean gneisses from the Lewisian Complex of Gruinard Bay, northwestern Scotland

摘要: We present ion microprobe UPb zircon geochronology for three samples from the late Archaean Lewisian gneiss complex at Gruinard Bay, northwestern Scotland, in an attempt to resolve discrepancy between SmNd and PbPb isochron ages trondhjemite hornblendite components. Whole-rock imply ca. 2.8 Ga. both components (Whitehouse et al., 1996), but separated minerals have yielded 2.4–2.5 Ga 3.3 (Burton et. 1994). Zircons representative lithologies Bay (trondhjemite, hornblendite, banded tonalite) yield concordant or near data with 207Pb206Pb mostly 2.85 2.75 Ga, a few outliers 2.65 Backscattered electron images of zircons allow differentiation cores overgrowths; latter are interpreted as igneous on basis field relationships other samples. Morphology is more consistent crystallisation under metamorphic rather than conditions although previous arguments Whitehouse al. (1996) protolith age not greatly excess this remain valid. The new suggest that magmatism occurred over relatively short time interval, accord observations whole-rock isotopic data. There no evidence analysed magmatic respectively. mineral probably represent resetting related regional 2.5 high-grade event recorded central region. For confirmed spurious, resulting post-crystallisation open-system isotope behaviour scale. Regional comparison within Complex suggests ≤5 TTG gneisses might been amalgamated ≥ 2.96 region before during 2.76 subsequently experienced similar geological evolution.