Correlation of metamorphic fabrics and the age of Lewisian metasediments near Loch Maree

A. Crane
Scottish Journal of Geology, 14, 225-246, 1 September 1978, https://doi.org/10.1144/sjg14030225

Synopsis

Various views have been expressed concerning the age of the Lewisian metasediments and associated basites (Gleann Tulacha metasediments and Beinn Airigh Charr basite) near Loch Maree. Interpretation of their chronological status is complicated by:– (i) the absence of Scourie dykes intrusive in the metasediment-basite association. (ii) tectonic dislocation between the metasediment-basite outcrops and the surrounding gneissose complex.

Fabric analysis, with particular emphasis on shape-fabric anisotropy, shows that two NW–SE foliation-producing events in the gneissose complex were separated by Scourie-dyke emplacement. These foliations (coded S5 and S6) are respectively correlated with Late Scourian (Inverian) and Early Laxfordian structures elsewhere in the Lewisian.

Fabric and structural sequences in the metasediment-basite association are compared with those of the gneisses. It is concluded that Late Scourian metamorphism and deformation have affected the metasediments and associated basites. Similar findings were obtained from a narrow tract of metasediments and basites near Gruinard Bay (the Gruinard metasediment-basite association). These Gruinard rocks are probably the stratigraphical equivalents of the Loch Maree Group and contain S5 foliation cut by members of the Scourie-dyke swarm.

Absence of high grade Early Scourian (Badcallian) fabrics and structures in both the Gruinard and Loch Maree metasediment-basite associations is considered to be indicative of their post-Badcallian origin.

It is thus concluded that the metasediments near Loch Maree represent a post-Badcallian supracrustal cover which experienced penetrative metamorphism and deformation prior to Scourie-dyke emplacement.

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