The petrography and geological significance of the post-Cambrian minor intrusions of Assynt and the adjoining districts of North-West Scotland 1

Abstract

<h3>Summary</h3> The post-Cambrian sills and dykes described occur mainly in the Assynt district of the North-West Highlands of Scotland. The principal types are grorudites, Canisp porphyry (biotite, albite-oligoclase, aegirine-augite porphyrite), hornblende-porphyrites, nordmarkitic rocks, vogesites and ledmorites and related rocks. They are emplaced in Lewisian Gneiss, Torridon Sandstone and Cambrian sediments, and some also occur in the Moine schists. The intrusions are affected by the Caledonian thrust movements. The tectonic distribution of the grorudites leads to the recognition of the Glencoul and Ben More thrust-masses as parts of a single tectonic unit. South of Conival the Benmore thrust-plane is of greater importance than to the north, and is renamed the Assynt thrust-plane. The sills in the Moine Series are metamorphosed and are thought to have been emplaced after the inception, but before the close, of the Caledonian thrust movements; some part at least of the Mo^ne metamorphism is thus of post-Cambrian age. Assimilation of limestone has probably no relation to the production of lime-magnesia pyroxene in the vogesites, nor to the production of highly alkaline rock types. A mechanism of differentiation of the post-Cambrian suite is suggested. Both major and minor intrusions have been emplaced by injection; in places the thickness of the sills amounted to several hundred feet, and had considerable effect in up-doming the strata over which the Moine schists were thrust.