Cnoc an Droighinn

[NC 263 226]

Description

This is a classic region of structural complexity in Assynt, overlooking Inchnadamph (Figure 7.2), in which the Glencoul Thrust brings Cambrian quartzites over thick duplexes (imbricated lenses) of Durness Group dolomitic limestones. The reader is referred to figure 15 of Elliott and Johnson (1980), and the accompanying text, for a modern structural interpretation. Numerous 'hornblende porphyrite' sills are structurally repeated in the duplexes beneath the Glencoul Thrust, but sills were also demonstrably emplaced repeatedly at several levels in the quartzites above the thrust. They may be seen in various stages of cataclastic deformation ranging from highly sheared to unsheared. Sabine's map (1953, figure 5) records the relative amount of deformation he observed. 'Grorudite' dykes and sills also occur in this site, but only above the Glencoul thrust plane.

Interpretation and conclusions

Cnoc an Droighinn is a structurally complex GCR site close to Inchnadamph, providing a major concentration of 'hornblende porphyrite' (and 'grorudite) sills cutting various Cambro-Ordovician rock types. Some sills are repeated structurally, others are emplaced at different levels in the succession. The 'hornblende porphyrites' are clearly pre-deformational and, despite structural repetition on the lower part of Cnoc an Droighinn, they were emplaced repeatedly at various levels in the quartzites above the Glencoul Thrust. The area is an object lesson in the difficulties of achieving correlations of igneous bodies in a region which, while largely non-metamorphic, is nonetheless structurally complex. The distribution of the 'hornblende porphyrites' provides a very useful marker of relative structural displacements in the thrust belt.

References