Abstract

Dr A. L. Harris remarked that the recognition by Drs Powell and Johnson of two orogenic periods in the western Highlands emphasized the need for strict criteria in the recognition of pre- and post tectonic gneissification. Each orogenic episode is apparently associated with the development of pelitic gneisses, referred to on the Geological Survey maps as 'migmatites'; the tracing of the Caledonian overprint on rocks metamorphosed in the Precambrian will depend partly on the recognition of deformed 'migmatites.' The quartzofeldspathic fraction of the pelitic gneisses occurring as pegmatitic veinlets or lits commonly conforms to minor fold structures, but this in itself is an insufficient criterion of deformation. Such 'folded' veinlets may be the result of post deformation segregation along already folded surfaces. The nature of the fabric within the quartzofeldspathic lits is critical. In the 'old' pegmatites, demonstrated by Dr van Breemen and his colleagues to have suffered D2 deformation, there is in addition to a mica foliation, a strong planar fabric defined by the preferred dimensional orientation of quartz and feldspar. The fact that this has survived prolonged Caledonian metamorphism in at least mid-amphibolite facies, is a strong argument in favour of the use of such fabrics as the only safe criterion for pre-tectonic gneisses. Clearly if the preferred orientation of quartz and feldspar could be broken down by prolonged static crystallization, the deformed gneisses would come closely to resemble the product of post tectonic gneissification. Their recognition as pre-tectonic gneisses would then be a matter of subjective judgement.

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