Palaeoproterozoic (late “Laxfordian”) reworking of juvenile Neoarchean Lewisian orthogneisses, Iona, Inner Hebrides, Scotland.

Abstract

A geochronological and isotopic study grounded by field observations is presented on the southern-most Lewisian orthogneisses of Iona, southwest Scotland. Syenitic orthogneisses in western Iona and granodioritic orthogneisses in the east have yielded indistinguishable SIMS U-Pb zircon c . 2710 Ma protolith ages, among the youngest recorded from the Lewisian. Whole-rock Sm-Nd and zircon Lu-Hf data indicate largely juvenile Neoarchaean crustal additions. Based on this evidence, a north-south trending mylonite zone separating eastern and western Iona is unlikely to be a terrane boundary. Extensive reworking during the “late Laxfordian” deformational event between 1779-1753 Ma (bracketed between pre-tectonic migmatisation and post-tectonic granitic pegmatite intrusion) likely corresponds to accretion of the Rhinns Complex to the Nuna/ Columbia supercontinent. Zircon Lu-Hf data indicate that late Laxfordian melts were largely derived from the Neoarchaean orthogneisses. K-feldspar Pb isotope ratios in the orthogneisses have likely been reset during late Laxfordian metamorphism while those in a post-tectonic syenitic pegmatite, with a U-Pb zircon age of 1688 ± 8 Ma are considered to be original and consistent with an exotic source. Correlation of the Iona Lewisian with Coll and Tiree is possible but the younger age of the Iona orthogneisses does not support correlation with the Rona terrane.