Analysis of a mica subfabric

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Abstract
Synopsis: The mica (muscovite, biotite, and/or phlogopite) in the Lewisian rocks near Furnace, Loch Maree, exhibits two habits, phylloblastic and decussate nemato-blastic crystals, which are deduced to have grown during one period of deformation and metamorphism called main phase. All the micas, along with the associated mesoscopic fabric elements, are deformed by two subsequent periods of deformation, mid phase and late phase, during which no recrystallisation of mica occurred. These observations are supplemented by a petrofabric analysis, the interpretation of which involves the interplay of various factors: Schnitteffekt, the amount of minerals other than mica in the rock, the influence of active and passive s-planes versus the crystal boundary effect and interfacial tension, the degree of mid phase deformation, and the composition of the mica; all of these combine to produce the mica subfabric. The effect of late phase deformation on this subfabric is negligible within the area. On this evidence the age dates obtained by Giletti, Moorbath, and Lambert (1961) on two muscovite samples from this area—1510±40 m.y. and 1540±45 m.y.—represent the latter part of the main phase metamorphism, thus placing it in the Laxfordian orogeny.