Early basic rocks within the Caledonides, 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.|
Early basic rocks, introduction
Apart from small masses of ultrabasic rocks, now serpentinites, which are found mainly in Sutherland (Read, 1931), the majority of the early basic rocks of the Northern Highlands were probably intruded into the Moine as sheets and dykes. These rocks, here referred to as metabasites, comprise metagabbros and metadolerites, epidiorites, hornblende schists and amphibolites which have been classed together by D. I. Smith (1979) as a pre- to syntectonic Amphibolite Suite.
The metabasites were first metamorphosed either prior to, or during, the early phases of Caledonian folding. They are cut by pegmatites dated at 450 Ma, which sets a younger age limit for their metamorphism. Their date of intrusion is not known. Although on the whole the rocks of the suite are concordant to the main regional foliation, transgressive relationships occur locally, showing that a few of the sheets cut the earliest foliation (probably parallel to bedding). Indeed some examples are thought to post-date both the first and the second phase of deformation of the country rocks. Smith points out that members of the suite cut the Carn Chuinneag Granite (c.560 Ma), but Moorhouse and Moorhouse (1979) consider that, in the northern part of the Northern Highlands, there are two suites of very similar rocks. One of these comprises hornblende schists, which appear to have been emplaced during the Caledonian Orogeny. Peacock (1977) argues that in Glen Moriston there is a suite of hornblende schists which is earlier than the period of emplacement of the main suite of metagabbros and metadolerites. Studies are currently in progress which may throw light on the discrimination of members of the suite.
The only comprehensive description of the mineralogy and geochemistry of members of the suite is that given by Moorhouse and Moorhouse (1979). The most common types of metabasite are discussed below.
These are dark green to black rocks in which all traces of the original minerals and textures have been obliterated; they now show a strong schistosity and lineation of hornblendes. Their usual mineralogy is greenish-blue amphibole, andesine and quartz, with or without garnet, biotite and accessories. Bands of biotite-rich schist have, in places, been interpreted as being retrogressed hornblende schists.
Metagabbros and metadolerites
These form bosses and thick sills of undoubted igneous origin. They contain cores in which the original igneous textures (although not necessarily the original minerals) can be made out (Peacock, 1977; Read, 1931). They commonly have schistose margins indistinguishable from the hornblende schists (P915477).
These are sheet-like in some places, but are more often in the form of small hornblende bosses; typically, none of the original igneous minerals or textures are retained but they show no schistose foliation (although the hornblendes may show an irregular local alignment). They are usually medium- or coarse-grained rocks.
D. I. Smith (1979) has discussed the distribution of the metabasites in some detail. Metadolerites and metagabbros are found in two zones (P915477), one north and one south of the Strathconon Fault. In the western part of the southern zone, metabasites of any sort are restricted to the pelitic rocks of the Glenfinnan Division, where they occur as concordant boudins of coarse texture commonly rich in large garnets; Johnstone and others (1969) have shown that the field relations suggest that they may be of sedimentary origin. There are no metabasites within the rocks of the Morar Division in this area, but further north in Ross-shire, Winchester (1976) has found rare thin sheets of metabasite in the pelitic rocks of the Morar Division which he distinguishes as metamorphosed alkali-basalts; he differentiates them from those in the Glenfinnan Division, which he considers to be metamorphosed tholeiitic intrusions. Moorhouse and Moorhouse (1979) also consider the hornblende schists of ‘normal’ aspect of the northerly area to be tholeiitic intrusions.
To the east of the southern zone the rocks of the Loch Eil Division are cut by numerous sheets of fine-grained hornblende schist with subordinate bosses of epidiorite and metagabbro.