Chapter 6 Laxfordian Structure and Metamorphism

Abstract

The key to the Lewisian structural chronology in Gairloch, as elsewhere in the Lewisian complex, is the Scourie dyke swarm, which, following Peach et al. (1907) and Sutton & Watson (1951), has been used to divide the complex into pre-dyke and post-dyke elements; the post-dyke structures and events are referred to the Laxfordian. However, as explained in Chapter 3, none of the dykes attributed to the Scourie dyke swarm in the Gairloch area, or in the Southern Region generally, have been dated (except for K-Ar dates interpreted as metamorphic cooling ages). Therefore they may correlate with either the 2.4 Ga or 2.0 Ga dykes of the Central Region (see Section 1.2.6), or could represent a third swarm of some other age. It is also possible that dykes of two or more different ages are represented. While the later Laxfordian structural and metamorphic elements can be correlated throughout the Lewisian complex, some of the earlier post-dyke structures assigned here to the Laxfordian may not correspond to similar structures elsewhere in the complex if the dykes themselves are not of the same age (see discussion on chronology in Section 1.3). Structures are also assigned to the Laxfordian if they affect the c. 2.0 Ga rocks of the Loch Maree Group (LMG). Although the Archaean gneisses share in the Laxfordian deformation, the following structural analysis depends critically on the evidence from the dykes and the LMG. Laxfordian deformation in the Southern Region of the mainland Lewisian is distributed through a wide belt over