The Lewisian complex of South Harris With some observations on the metamorphosed basic intrusions of the Outer Hebrides, Scotland

Abstract

The metamorphic petrology and history of the Lewisian rocks of South Harris are described. The paragneisses in each of the two main belts are divided into three lithological and possibly stratigraphical units, the disposition of which indicates folding in tight plunging isoclines along Nw–Se (Early Laxfordian) axes. Relic textures indicating an early (pre-Lax-fordian) metamorphism and folding are recognizable in the paragneisses and also in the pyroxene-granulites which appear to intrude the paragneisses. This early metamorphism is probably Scourian in age. The pyroxene-granulites are earlier than and are not comagmatic with the igneous complex. The igneous complex, which is a metamorphosed differentiated (and possibly originally layered) anorthosite–gabbro–tonalite mass, together with the metamorphosed basic intrusions in its immediate vicinity and the widespread metamorphosed basic dykes and sills in the Outer Hebrides, constitute an essentially non-orogenic suite of post-Scourian pre-Early Laxfordian age. The geochemistry of this suite shows it to be tholeiitic in character and probably closely related to the Scourie dykes of the Scottish mainland. Early Laxfordian granulite-facies metamorphism associated with the main Nw–Se folding of the area followed the emplacement of the non-orogenic igneous suite. A south-easterly plunge and north-easterly overturning characterize the folds to the north-east of the igneous complex; to the south-west the plunge is north-west, and the overturning is to the south-west. A Late Laxfordian injection-complex situated to the north-east comprises extensive areas of migmatization and granitic intrusion and is associated with a front of hydrous retrograde metamorphism which strongly affects the Langavat paragneisses and parts of the igneouscomplex. The sinuous trend of the Langavat paragneisses is attributed to cross-folding of thisdate. All the earlier rock-types are cut by a suite of Late Laxfordian granite pegmatites containing uranium, thorium, and rare-earth minerals.