Rb-Sr whole rock isotopic studies of Lewisian metasediments and gneisses in the Loch Maree region, Ross-shire

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Abstract
Fifty-three whole rock Rb–Sr isotopic analyses are reported on two metasedimentary schist and four gneiss units in the Lewisian of the Loch Maree region. Isochron dates from the Gairloch metasediments–Ard gneisses (1980 ± 50 m.y.) and the Loch Maree gneisses (1965 ± 100 m.y.) date the main phase of metamorphism of the Laxfordian episode at 1975 ± 75 m.y. A 1745 ± 160m.y. whole rock isochron date on the Carnmore kyanite–biotite gneisses can be related to late tectonic pegmatite intrusions. Whole rock data points from the quartzofeldspathic gneisses from the Carnmore district do not define an isochron. A 1500 ± 95 m.y. isochron date for metasediments of the Loch Maree Group is interpreted in terms of final closure, during epeirogenic uplift, after a 200–300 m.y. period of isotopic exchange below an impervious cap of tectonically overthrusted hornblende schist and quartzofeldspathic gneiss. Initial 87 Sr/ 86 Sr considerations indicate both a crustal history for the gneisses of the Loch Maree and Carnmore districts going back to 2.7–2.8 b.y. and deposition of the Gairloch metasediments and Loch Maree Group after 2.2 b.y. ago, presumably unconformably on the older metamorphic complex. The Laxfordian orogenic cycle is defined, in the classical stratigraphic sense, to consist of a depositional episode 2.2–2.0 b.y. ago, an orogenic episode 2.0–1.7 b.y. ago and an epeirogenic episode 1.7–1.5 b.y. ago. There are marked similarities in the Laxfordian and Svecokarelian orogenic cycles, and the Lewisian chronology for the Loch Maree region shows striking parallelism with the Precambrian chronology of NW. Europe.