Remnants of an Early Metasedimentary Assemblage in the Lewisian Complex of the Outer Hebrides

Abstract

Abstract: Remnants of a varied association of rocks, including psammitic, pelitic, semi-pelitic and calcareous metasediments, have been mapped in all the larger islands of the Outer Hebrides. Most of the remnants are concentrated in relatively narrow zones in which they are associated with, and pass into, quartzo-feldspathic gneisses. These zones are flanked by areas made predominantly of quartzo-feldspathic gneisses which appear to lack metasedimentary remnants. The metasediments were metamorphosed, migmatised and partially transformed into gneisses prior to the emplacement of the Scourie Dyke suite. Their least-modified portions are in many places intimately associated with bodies of basic rock which are thought to have shielded them from the effects of successive phases of metamorphism. Their present mineral assemblages and fabrics date from the Laxfordian episodes of regeneration. Relict mineral assemblages suggest that the earliest recognised Laxfordian metamorphism was of amphibolite facies over much of the Hebrides but was locally of granulite facies. The metasedimentary rocks are regarded as derivatives of a supracrustal group deposited prior to the main Scourian period of gneiss-formation and much of the gneiss is thought to be derived from such a group. A still older basement could, however, be represented by some of the areas of quartzo-feldspathic gneisses which intervene between the metasedimentary zones, though the existence of such a unit remains open to discussion.