Tertiary granites and associated rocks of the Marsco area, Isle of Skye

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Abstract
The early Tertiary intrusive igneous rocks of the Marsco area, Skye, form part of the Western Redhills Complex. They are predominantly granitic in composition, but also include small amounts of gabbro and ferrodiorite. The intrusions are arcuate, steep-sided, dyke-like bodies. Portions of the roofs of several successive plutons are preserved on Marsco. The margins, against granite, of the gabbro on top of the mountain give evidence of the chilling of basic against acid magma. Seven high-level granites are described. They show a considerable variety of textures and mineralogy; usually containing feldspar phenocrysts, and ground-mass fayalite, ferrohedenbergite, calciferous or alkali amphibole, biotite and numerous accessory minerals. Mineralogical data and twelve new whole-rock chemical analyses are presented. It is shown that the ferrodiorite is probably contaminated by acid magma. Two granites south of Marsco are truncated by the Western Redhills intrusions, and appear to belong to a separate, possibly earlier, intrusive centre. The six major granites in the Marsco area are classified structurally into three pairs. The second members of each pair are very alike mineralogically and chemically. It is postulated that the acid magmas were produced by partial melting of Lewisian gneisses and Torridonian sediments in the thermal aureole of a large basic intrusion. The phase-chemical relationships of the granites to their supposed source rocks are discussed. A petrogenetic scheme is given, which attempts to explain the compositions and forms of the granitic dykes, and their emplacement in pairs. It is thought that intrusion took place during the tilting of a foundering crustal block.