The Lewisian

The Lewisian complex shows a very long history. We can reconstruct this on the basis of field relationships. A last great event in the history of the Lewisian was an episode of intense deformation and accompanying metamorphism - called the Laxfordian. However, this episode was predated by a long history of older metamorphism and deformation. Rocks which show little evidence for Laxfordian deformation lie within the Scourie Block. Early structures can be recognised, cross-cut by a suite of basic intrusions - the Scourie dykes.

These intrusives are deformed and metamorphosed (to amphibolite facies) by the Laxfordian deformation. Approaching the Laxford Front from the Scourie Block the dykes become increasingly deformed. Some sectors show discordance between dyke and country rock gneisses and the development of small shear zones inside . But inside the belt of rocks fully deformed by the Laxfordian orogenic episode the Scourie dykes are sheared and boudinaged.

So a key to establishing the relative sequence of geological events in high-grade metamorphic terrains like the Lewisian is to use intrusions to provide crucial time markers. Radiometric dating can then be used to find the absolute ages.

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