The evolution of the Lewisian complex in the Outer Hebrides

Abstract

Summary The oldest rocks of the Outer Hebrides are a supracrustal sequence of metasediments, metavolcanics and associated layered basics which formed at c. 2900 Ma. These rocks were buried or carried to deep levels in the crust where they were intruded by an extensive series of granodioritic and tonalitic magmas, probably during the period 2800–2900 Ma. These rocks were subjected to the polyphasal Scourian event which led to extensive migmatization, high-grade metamorphism and the development of a pervasive gneissic foliation. The culmination of this event occurred in the period 2600–2700 Ma. The pattern of gneiss types resulting from these events defined a NNE-SSW grain in the rocks which is reflected in the present distribution of metamorphic facies. This pattern, which reflects either original compositional variations or the effects of Scourian metamorphism, has controlled much of the later metamorphic and structural evolution of the Lewisian of the Outer Hebrides. The late Scourian period was marked by the emplacement of a suite of dioritic intrusions followed by numerous granitic bodies, the two events being separated by a period of deformation. This deformation died out to the extreme SE and did not affect the eastern part of Barra. These intrusive events probably marked the end of crustal formation and the accompanying deformation is the first evidence of the reworking of a crystalline crust with the development of zones or ‘augen’ of low deformation. The granitic intrusion was followed by a deformational event which, in the now more rigid crust, resulted in the development of localized steep shear zones in the period 2400–2600 Ma. The later stages of this event coincided with the intrusion of a widespread suite of metadoleritic and metanoritic rocks—the Younger Basics of ‘Scourie dyke’ age. The overlap of deformational phases from the late Scourian to the early Laxfordian highlights the difficulty of regarding the Scourian-Laxfordian boundary as one of major importance. Also there may well have been a considerable period ( c. 400 Ma) of relative crustal stability before the main Laxfordian events. There is no evidence that the late Scourian shearing events were associated with substantial uplift, although geochronological evidence from S Harris (Cliff et al. 1983) does suggest a gradual uplift and cooling, at least in that area, at about this time. Certainly the early Laxfordian events appear to have been deep-seated. The intrusion of the late Laxfordian granites at c. 1700 Ma and the associated deformation signalled the first movements which led to substantial uplift, thrusting and unroofing of the complex. The geological history draws attention to the interesting evolution of deformational style and the generation of lineaments with the changes in crustal level. Features such as the Langavat belt of S Harris may result from an early Scourian lineament that has been reactivated many times. In general the Scourian gneiss terrains are broad zones with localized sub-vertical shear zones only becoming established in the late Scourian. The Laxfordian is typified by such zones of reworking, accompanying recrystallization commonly under hydrous conditions. Ductile shear zones are found, although on a local scale the deformation pattern is typically heterogeneous. After the late Laxfordian granitic intrusion the deformation became localized in restricted mylonite zones or along brittle thrusts. The Caledonian reactivation and faulting is largely controlled by the position of this major thrust zone, with mylonization and related low-grade alteration overprinting the late Laxfordian thrust features.