Abstract
Synopsis Mafic dykes within the Lewisian Complex on Tiree and Coll were emplaced after a granulite-facies metamorphic event in the host gneisses. They were subsequently deformed and metamorphosed under amphibolite-facies conditions and cut by quartzofeldspathic pegmatites. The dykes are quartz-tholeiites and have a typical ‘continental’ trace element signature; showing enrichment in large ion lithophile and light rare earth elements and depletion in the high field strength elements. The intrusions are indistinguishable from Proterozoic (2.4–2.0 Ga) quartz-dolerite Scourie dykes of mainland Scotland, in terms of their field relationships, petrography, geochemistry and Sm-Nd isotope systematics. The occurrence of Scourie dyke-like bodies enables pre-and post-dyke events within the Lewisian on Tiree and Coll to be tentatively assigned to Scourian (2.9–2.5 Ga) and Laxfordian (1.9–1.5 Ga) events respectively. Sm-Nd whole-rock-mineral ages of c. 1.75 Ga are interpreted as dating garnet growth during the Laxfordian event and provide a minimum emplacement age for the dykes.
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