Chapter 7 Tectonic Interpretation and Regional Significance

Abstract

The fragmentary nature of the outcrop of least modified Scourian rocks limits any attempt at interpretation of their tectonic history. However, the three main areas where such rocks can be studied (Loch Braigh Horrisdale, Creag Mhor Thollaidh and Ard Ialltaig) all show certain common features. The rocks are all orthogneisses, dominantly granodioritic in the SW (Braigh Horrisdale), but quartz-dioritic to dioritic with abundant mafic inclusions at Creag Mhor Thollaidh and Ard Ialltaig. Structurally, there is considerable uniformity; the early gneissose banding/foliation is generally steeply dipping with a NEerly to northerly trend. This structure (S2) has been produced by the D2 deformation of the igneous protoliths of the gneisses, which also locally contain a previous foliation, Sj. A similar structural pattern is found to the south of the Gairloch area, at Ruadh Mheallan in the Torridon district, and also to the north, at Gruinard Bay. Thus, all the areas throughout the Southern Region of the Lewisian complex where the Scourian structure can be recognized in a relatively unmodified state, involving an outcrop width of around 35 km, appear to present a common structural style. This has obvious implications for any collisional model for the Palaeoproterozoic, suggesting that the Archaean blocks on either side of the Loch Maree Group outcrop may have had a common origin, or may originally have been contiguous (see Section 7.3). The intense deformation, gently-dipping foliation and high pressure granulite-facies metamorphism that characterize the Scourian of the type area in Scourie and Assynt are not matched in the