Younger Caledonian igneous rocks of the Northern Highlands of Scotland
|Johnstone, G S and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
The Caledonian igneous rocks so far described have all been involved in one or more episodes of regional folding, and some have also been modified by the effects of regional metamorphism. The Younger Caledonian igneous rocks on the other hand are all later than the main regional folding, though some have been emplaced during late thrust movements both within the Moine Thrust Zone and within the orogenic belt.
None of the major intrusions show significant recrystallisation attributable directly to regional metamorphism, but it is clear that many were intruded when the regional temperature was still relatively high. The schists surrounding the intrusions show little sign of contact metamorphism, though in several cases they show plastic deformation in response to the pressures of forceful intrusion. Moreover, many masses are cut by minor intrusions of the Microdiorite Suite, members of which show dynamothermal recrystallisation.
The available isotopic ages suggest that the major intrusions belong to the group of Late-Caledonian igneous rocks intruded in Ordovician and Silurian times. None of the ‘permitted’ Devonian intrusions (Read, 1961) found in the Grampian Highlands have been recognised north of the Great Glen Fault (however, see Chapter 11). The anomalous position of the little-deformed Strath Halladale Granite has been referred to earlier (Chapter 7).
The Younger Caledonian igneous rocks comprise:
These groups do not represent a sequence of intrusions; in some places they are contemporaneous and in others they overlap; this makes logical description rather a problem.