Paleoproterozoic (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 southernmost Lewisian orthogneisses of Iona, SW Scotland. Syenitic orthogneisses in western Iona and granodioritic orthogneisses in the east have yielded indistinguishable secondary ion mass spectrometry 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 Neoarchean 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 and 1753 Ma (bracketed between pre-tectonic migmatization and post-tectonic granitic pegmatite intrusion) probably 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 Neoarchean orthogneisses. K-feldspar Pb isotope ratios in the orthogneisses have probably been reset during late Laxfordian metamorphism whereas 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.