Structural setting and geochronological significance of the West Highland Granitic Gneiss, a deformed early granite within Proterozoic, Moine rocks of NW Scotland

Abstract

A re-investigation of the six major outcrops of granitic gneiss within the Glenfinnan and Loch Eil divisions of the Moine has shown that they are a suite of deformed and metamorphosed granite intrusions. They record all the tectonic events recognized in their host metasediments, while the absence of thermal aureoles suggests that their emplacement was synmetamorphic, i.e. syn-D 1 rather than pre-D 1 . Their dominant gneissic fabric is of S 2 age, but in low-strain augen an earlier S 1 fabric is preserved which is cut by MP 1 leucocratic segregations contemporaneous with, but compositionally distinct from, those in the pelitic regional migmatites of the Moine. The deformed granites underwent local MP 2 remelting and have been widely intruded by later, Caledonian pegmatites. The gneiss has a uniformly granitic (sensu stricto) bulk composition, regardless of country-rock lithology or location within the Glenfinnan and Loch Eil divisions. It has strongly S-type chemical affinities, being reduced and peraluminous with a high initial 87 Sr/ 86 Sr ratio. It was probably produced at least in part by partial melting of Moine-like rocks at depth. A published Rb/Sr whole-rock isochron of c. 1028 Ma probably dates intrusion of the Ardgour body and confirms the reality of a 'Grenvillian' event in the Glenfinnan and Loch Eil divisions of the Moine.