The Lewisian of Lochinver, Sutherland; the type area for the Inverian metamorphism

Abstract

The Lochinver district contains gneisses, pegmatites and dykes generated in five main episodes: (1) pre-2600 m.y, garnet pyroxene granulite facies gneisses with low angle dips, formed in their final state by the Badcallian metamorphism; (2) potash-rich pegmatites of age range 2540 to 2310 m.y.; (3) 2310–2200 m.y. amphibolite facies gneisses occurring in vertical WNW zones, produced by a broadly isochemical metamorphic event; (4) vertical NW-trending dykes emplaced at 2200 m.y. and perhaps down to 1900 m.y. and (5) epidoteamphibolite facies gneisses produced in narrow, discontinuous NW shear zones during the succeeding Laxfordian events. The petrographic and structural features of each sector of the complex are described and new whole rock Rb–Sr ages given for the pegmatite suite, suggesting a maximum age of 2540 m.y. for the suite (plus intrusion of a further suite at 2310 m.y. or recrystallisation of the first suite at 2310 m.y.). Other pegmatite ages confirm the presence of pre-2000 m.y. activity NE of Scourie and at Gruinard. Field and petrographic evidence at Lochinver demonstrates the existence of amphibolite facies metamorphism in post pegmatite pre-dyke times, which produced well-defned rock types and WNW vertical structures, and is defined as the Inverian metamorphic event.