Synopsis
The Lewisian rocks between Loch Shieldaig and Loch Bràigh Horrisdale consist mainly of acid gneisses, hornblende schists, metadolerites and pegmatites together with quartzo-felspathic and hornblendic segregation veins, lenses and pods. The structural deformation is shown to have occurred in three phases. The dominant foliation and lineation are the result of deformation during the main phase, the N.W.-S.E. trend of the lithological banding and the present lineation pattern are the result of mid phase deformation, while the open folds, which affect both banding and foliation, are the products of late phase deformation.
The acid gneiss/hornblende schist banded sequence is considered to have been derived from a concordant sequence of sediments and basic masses during main phase metamorphism and felspathisation. Emplacement of basic dykes took place after the mid phase deformation, and the pegmatites were developed by an extension of the felspathisation processes.
During the Laxfordian orogenic period dolerite intrusions crosscut and were chilled against vertical bands of hornblende schist that trend N.W.-S.E. Subsequent metamorphism affected both the regional hornblende schists and the dolerites, and narrow bands of hornblende schist were formed in places at the expense of doleritic chilled margins. Previous interpretations of Lewisian geochronology, tectonics and metamorphic history made on the basis of a unique pre-Laxfordian basic dyke phase, cannot be applied to the rocks south of Gairloch.
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