Regional metamorphic grade within the Northern Highlands Caledonides
|Johnstone, G S and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. Fourth edition. Keyworth, Nottingham: British Geological Survey.|
Regional metamorphic grade
The grade of the Lewisian metamorphism (Scourian and Laxfordian) in the Foreland rocks is discussed in Chapter 2. Post-Lewisian metamorphism has affected foreland rocks in two main areas; firstly, along the Outer Isles Thrust, where hydrous retrogression (greenschist facies) was associated with movements along the thrust planes, and secondly, immediately below the Moine Thrust, where low-grade (weak to lower greenschist facies) recrystallisation is particularly noticeable in the Torridonian of southern Skye. Lewisian basement inliers within the Caledonides generally exhibit the same metamorphic grade as the surrounding Moine. In some localities, however, relic high-grade granulite-facies assemblages can be identified, for example, in the Borgie inlier of Sutherland (Moorhouse, 1976). High-grade relics can also be identified in Lewisian basement slices in the Moine Thrust Zone, for example in the Glenelg–Attadale inlier (Barber and May, 1976; Sanders, 1979).
In the Moine rocks the widespread development of aluminosilicate polymorphs has been largely inhibited by the unsuitable chemical composition of the rocks. As a result the metamorphic grade of the rocks is generally determined by using the mineral assemblages in the calc-silicate ribs and nodules (Kennedy, 1958; Soper and Brown, 1971; Winchester, 1974; Tanner, 1976; Powell and others, 1981). The most important boundaries are the first appearance, in order of increasing grade, of oligoclase, hornblende, bytownite/ anorthite, and pyroxene. Increasing data on the distribution of index minerals in pelites show that the pelite-based isograds (garnet, kyanite, sillimanite), although broadly related to the calcsilicate-based isograds, do not parallel them in detail (Winchester, 1974; Powell and others, 1981). This necessitates certain generalisations in drawing facies boundaries.
The distribution of facies is shown in P915475, following the scheme proposed by Fettes and others (in press). This map shows the grade rising from west to east from greenschist facies to middle amphibolite facies (sillimanite + muscovite in pelites) and, locally, upper amphibolite facies (sillimanite + potash feldspar in pelites). In Inverness-shire the grade falls again eastwards to the lower amphibolite facies (kyanite + staurolite in pelites) which forms a broad belt across to the Great Glen. In general terms the high-grade areas coincide with areas of migmatisation (Winchester, 1974).
Whether this pattern reflects the effects of pre-Caledonian (M1) or of Caledonian (M2) metamorphic events is uncertain; and if both have left their mark, it is uncertain which was the more active. The problem is aggravated because there are no reliable correlations between local structural sequences, and consequently no regional framework nor widespread recognisable datum for the Caledonian event (see Powell and others, 1981).
In the extreme northern Moine of Sutherland the metamorphic grade appears to rise uniformly from west to east (Soper and Brown, 1971). The low grade of metamorphism (weakly metamorphosed to greenschist facies) in the west appears to post-date the development of the mylonites above the Moine Thrust and is therefore apparently of Caledonian age. In the east the Strath Halladale granite was intruded into high-grade (middle amphibolite-facies) rocks. This granite has given an age of 649 ± 32 Ma (Lintern and others, 1982) which would date the metamorphism as pre-Caledonian, although there is evidence of a late low-grade overprint affecting the granite. If the age for the granite is accepted, it follows that a considerable metamorphic break must exist somewhere between Strath Halladale and the Moine Thrust. It further follows that the Strath Halladale ‘block’ must have been sufficiently distant from the rocks now immediately east of the Moine Thrust to have been twice metamorphosed, the first metamorphism high grade, the second low grade, the pattern of metamorphism being distinct from that in the ‘block’ to the west. The two ‘blocks’ appear to have been juxtaposed with each other late in the Caledonian. The line of tectonic and metamorphic break could coincide with the Strath Naver slide (Moorhouse and Moorhouse, 1983) which forms the approximate western boundary of the middle amphibolite-facies rocks.
The problem of the age of the metamorphism is further complicated south of Strath Halladale where the Carn Chuinneag granite (P915475, P915478) dated at 550 ± 10 Ma (Pidgeon and Johnson, 1974) was demonstrably emplaced before the peak of metamorphism (kyanite grade) was reached (Shepherd, 1973), thus defining the dominant grade as Caledonian. If the evidence is accepted, it follows that a discontinuity must exist between Strath Halladale and Carn Chuinneag similar to that between Strath Halladale and the Moine Thrust area. This discontinuity may be postulated as an extension of the Strath Naver slide along the western limit of high-grade rocks, although no direct field evidence is available to support such an extension. In northern Ross-shire Winchester (1974) has suggested that the pre-Caledonian metamorphism increased in grade westwards from Carn Chuinneag and that the Caledonian metamorphism increased in grade eastwards.
In the south-western Moine of Inverness-shire, Powell and others (1981) Suggested that the pre-Caledonian grade, dated at 1004 ± 28 Ma (Brook and others, 1976) rose from greenschist facies (garnet grade) in Skye to middle amphibolite- facies (sillimanite grade plus migmatites) in the east. This metamorphic pattern was foreshortened by ductile sliding which took place at the beginning of the Caledonian. The sliding coincided with the Caledonian overprint, the grade rising rapidly from lower greenschists facies in western Morar to lower amphibolite facies in eastern Morar; Brewer and others (1979) give a date of 467 ± 20 Ma as the minimum age for this event. The high grade of the Caledonian event is apparently confirmed by the fact that the pyroxene isograd (in calc-silicates) cuts folds of the early Caledonian slides (Powell and others, 1981). This view is also supported by Tanner’s (1976) observation that high-grade metamorphism postdated the sliding. To the east of Morar, Roberts and others (1984) have argued on textural and structural grounds that the Glendessary syenite, dated at 457 ± 5 Ma by van Breemen and others (1979b), is earlier than the Caledonian event, and this event here gave only a low-grade retrogressive overprint. This accords with textural work in the Glen Affric-Glen Urquhart area by BGS, which suggests that the peak metamorphism in that area was early in the deformational history, and therefore of possible pre-Caledonian age.
Brewer and others (1979) and Powell and others (1981) have suggested that the Caledonian recrystallisation probably spanned the period 470–400 Ma. This accords with the views of D. I. Smith (1979), who postulated the existence of regional thermal gradients affecting the recrystallisation of microdiorite dykes in the period 450–420 Ma.
Fettes and others (in press) suggest a marked contrast between the style and time of metamorphism in the Central Highlands and those in the Northern Highlands. The Central Highlands underwent recrystallisation in the period 520–440 Ma, following initial deformation and tectonic thickening of a nappe pile. The Northern Highlands were recrystallised in the period 470–400 Ma, an event possibly associated with initial deformation.