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

Abstract

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