Discussion on a terrane-based nomenclature for the Lewisian Gneiss Complex of NW Scotland Journal , Vol. 162, 2005, pp. 175–186

Abstract

Graham Park writes: The terrane model of Kinny et al. (2005; see also Friend & Kinny 2001) presents a radically different picture of the Lewisian complex from the ‘traditional’ view of a single piece of Archaean crust that has been subsequently modified and augmented in various Proterozoic events. The authors are to be congratulated on their attempt to rationalize a very confused terminology that has arisen over many decades, and for forcing us to rethink our views. I have reservations about their re-definition of ‘Laxfordian’, but my comments mainly concern the validity of the terrane model as applied to the Lewisian and suggestions to refine it (see Park 2005). I agree that uncritical use of terms such as ‘Scourian’, ‘Laxfordian’ and ‘Scourie dyke suite’ can obscure genuine differences of rock, process or event (e.g. see Park et al . 2002). Most of the authors’ suggested revisions seem sensible, with the exception of their proposed restriction of ‘Laxfordian’ to a c. 1.74 Ga metamorphic event at Loch Laxford. I believe that this term is more useful in its current sense as a general name for the series of events etc. that affected the entire Lewisian complex after the incorporation of the early Palaeoproterozoic material (e.g. the Loch Maree Group and the ‘Scourie’ dykes). I have retained the traditional usage below for convenience. According to this concept, distinction is made between ‘suspect terranes’ and ‘displaced terranes’ i.e. those that are exotic or allochthonous. In order to qualify as a displaced terrane, a piece of crust should display evidence of significant displacement relative to its neighbour. Differences in geological history between two adjacent pieces of crust may not in themselves be sufficient evidence of large relative displacements between them. Transverse or oblique convergence between two continental plates inevitably involves the subduction …