Geochronological investigation of the quartzofeldspathic rocks of the Lewisian of Rona, Inner Hebrides

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Abstract
Rona Island consists of Lewisian quartzofeldspathic gneisses and amphibolites that show the effects of polyphase deformation. The age of the gneisses, from a six point Rb-Sr whole rock isochron, with λ 87 Rb 1.39 × 10 −11 yr −1, is 2790 ± 210 m.y. This is not significantly different from the age of 2710 ± 20 m.y. determined from zircon fractions from a single block of the gneiss. These results are interpreted as reflecting amphibolite facies metamorphism and gneiss formation, associated with the first recognized fold phase, during the Scourian orogenic episode A Rb-Sr age for unaltered muscovite books from a pegmatite emplaced in the hinge zone of a fold of the third deformational phase is 1740 ± 10m.y. and a Rb-Sr whole rock isochron for a later granite sheet, emplaced between the fifth and sixth fold phases is 1680 ± 170 m.y. These igneous events, and the fold phases between them, belong to the later part of the Laxfordian orogenic episode. No isotopic disturbance of the Rb-Sr whole rock systems of the analysed gneisses has taken place during the Laxfordian episode and no isotopic events between the c 2700 m.y. and c 1700 m.y. episodes have been found in the quartzo, feldspathic rocks. K-Ar ages on the muscovite samples suggest minor opening of minerals to argon loss sometime after 1740 m.y.