Metamorphism and Deformation in Early Proterozoic Basic Dykes Near Scourie, Northwest Scotland (Amphibolite)
Early Proterozoic amphibolite dykes of the Scourie Dyke swarm, exposed in the transitional Archean to Proterozoic Lewisian basement near Scourie, Northwest Scotland, exhibit complex morphologies, mineralogies, and microfabrics. Dyke morphology is often highly variable, and was controlled to a large extent by pre-existing structures within the gneissic basement. The mineralogy and microfabric of major dykes record a common sequence of metamorphism and deformation. Localization of deformation and fluid within narrow, generally anastomosing zones was responsible for the contemporaneous development of two distinct dyke fabrics. Complexity and intensity of fabric development does not correlate with complexity of dyke morphology or with structures in the basement rock. The non-schistose dyke rock records a series of processes involving unmixing of the original ferro-magnesian minerals and growth of continuously zoned amphibole rims. Microstructures record contemporaneous ductile deformation, and recovery and recrystallization processes. Schistose rock assemblages are equivalent to the rims and recrystallized grains of the non-schistose rock. These fabrics reflect local differences in amount of deformation, grain growth, recovery, and recrystallization within individual dykes. Amphibolitization was primarily confined within dyke margins. There is no correlation between the dyke characteristics and mineralogical or structural changes within the country rock. The mineralogical and deformational character of the dykes probably formed in association with, or closely following intrusion. The amphibolite facies mineralogy and two distinct dyke fabrics are preserved, with no superposition of later fabrics or metamorphic mineralogy. This observation precludes the occurrence of younger regional metamorphism or deformation in the area traversed by these dykes. The microstructural and mineralogical characteristics of these transition zone dykes are unique in comparison with Scourie Dykes of the Archean basement block (Loch Assynt area). As these characteristics were likely to have formed during or closely following intrusion, the basement must have already been divided into distinct terranes. This transitional terrane was probably reasonably mobile and at elevated temperature during dyke intrusion.
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