Moine Metasediments Research Articles

Fenites, breccia dykes, albitites, and carbonatitic veins near the Great Glen Fault, Inverness, Scotland

In the Abriachan-Dochfour-Moniack, Rosemarkie and Foyers areas near Inverness, Moine metasediments and gneisses, amphibolites, Caledonian granitic rocks, and meta-limestones of uncertain age are veined or replaced by metasomatic assemblages of blue fibrous magnesio-riebeckite (crocidolite), green aegirine-rich pyroxene, albite, calcite, hematite, and anatase. The field characteristics, textures and parageneses of these assemblages duplicate those of classical fenites around igneous carbonatite complexes. In the Abriachan granite, the chemistry of metasomatism also recalls classical fenitization in correlating removal of SiO 2 , K 2 O and Rb with addition of Na 2 O, Fe, MgO, CaO, F, Zn, Zr, Ba, Ce and CO 2 , while sums of cations such as [Si + Na] and fSi + A1 + Fe + Mg + Ca] remain virtually constant. At Moniack and near Rosemarkie, the fenites border calcitic veins, whose high contents—especially of Sr, Nb, Ba, REE, and Mn—ally them with late-stage carbonatites rather than with limestones or hydrothermal carbonates. Associated dykes of ffuidized, often carbonate-rich breccias, and bodies of brick red, sometimes radioactive albitites, also have close analogues in carbonatite complexes. The fenites are distinguished by petrology, field relationships and age from other Scottish alkali metasomatized rocks. All these rocks are believed to have formed broadly coevally, during the emplacment of at least one carbonatite mass deep into the Great Glen Fault zone, most probably during early Devonian (c. 390 Ma) fault movements.

Structural dating of a Precambrian pegmatite in Moine rocks of northern Scotland and its bearing on the status of the 'Morarian Orogeny'

A cross-cutting pegmatite emplaced into low grade, non-migmatitic, Proterozoic, Moine metasediments on the western sea-board of Inverness-shire, Scotland, gives Rb-Sr muscovite ages of 746–776 Ma which are interpreted as minimum estimates for the age of formation of the pegmatite. Intrusion followed at least one phase of folding and schistosity production in the country rocks and an early phase of syn-kinematic garnet growth, correlated with the regional D 2 tectono-metamorphic event. The pegmatite was folded and suffered considerable grain size reduction under metamorphic conditions during the regional D 3 deformation episode. The dilational character and low grade context of the pegmatite show that it was not a product of melting in place during metamorphism of the adjacent Moine Schists. Its lack of initial, syn-kinematic crystallization textures and fabrics, together with the absence of evidence that might relate its emplacement to any regional phase of compressive ductile deformation, would be compatible with a considerable time lapse between its intrusion and the earlier deformation and metamorphism of the host rocks. We have, however, no direct evidence for the extent of this interval. The relationship of the pegmatite to the earlier deformation and metamorphism of the host rocks provides clear evidence for substantial Precambrian orogenic activity but the age of this activity, whether 'Morarian' and, or, 'Grenvillian', remains a matter of interpretation.

Probable Old Red Sandstone conglomerates around Tongue and adjacent areas, north Sutherland

Synopsis Small outliers of conglomerate and sandstone occur on the N Sutherland coast at Kirtomy, Tongue and on the Roan Islands. At Kirtomy sediment has infilled three N–S trending valleys cut into the Moine metasediments. The following three-stage depositional history is recorded in outliers near Tongue: (1) dissection of Moine and Lewisian terrain forming N–S valleys; (2) quiescence, in situ weathering of metamorphics, probable meandering rivers on alluvial plain; and (3) tectonic rejuvenation, thick conglomerate sequences laid down in braided rivers flowing north. On the Roan Islands 350 m or more of coarse conglomerate was deposited by a braided river system flowing from the NW probably from uplands bordering the Orcadian Basin. A Lower or Middle Devonian age for all the outliers is preferred.

Moine and Lewisian near the Great Glen Fault in Easter Ross

Synopsis Two inliers of metamorphic rock—the Cromarty and Rosemarkie inliers—occur adjacent to the Great Glen Fault and within the Devonian sediments of Easter Ross. The Cromarty Inlier consists of striped psammitic Moine rocks, probably Glenfinnan Division, which have suffered high grade regional metamorphism and which have probably been locally transformed into mobilised granite. In the Rosemarkie Inlier, which corresponds to a well marked positive anomaly on the aeromagnetic map, Moine metasediments are accompanied by Lewisian hornblendic gneisses. The earliest of at least four episodes of deformation (D1–D4) is inferred to have interleaved the Moine and Lewisian while the Moine/Lewisian boundary was folded during three subsequent episodes. Granitic veins, including pegmatite, were emplaced in the Rosemarkie Inlier during or before the second deformation episode and carry intense D2 LS fabrics involving strains of 18:2.5:1 which have locally been modified during the third episode. Pegmatites and microgranites which occur in the Cromarty Inlier have not suffered comparable ductile deformation and postdate garnetiferous metabasic intrusions which are a feature of the Cromarty Inlier. Details of lithology and igneous history serve so to distinguish the two inliers that it is possible that, pre-D3, they were much further apart than at present.

Structural evolution in the northern part of the Moine thrust belt, NW Scotland

ABSTRACTFour balanced cross sections, supported by longitudinal sections, structure contour maps, stratigraphic separation diagrams and hangingwall sequence diagrams are keys to this interpretation of the Moine thrust, which forms the western margin of the Caledonides in NW Scotland. New basement and cover correlations between foreland and thrust belt give new slip estimates for the Moine thrust (∼ 77 km), the Loch More klippe (≥ 43 km), Glencoul sheet (20–25 km), Ben More sheet (∼28 km), Achall and Dundonnell ‘sheet II’ (∼28 km).Like other major thrusts the Moine thrust moved in a smooth or rough fashion at different places and times, and many structures are a footwall response to its passage. Widely developed duplexes vary in thickness so that the roof thrust is folded and occasionally faulted; many late Caledonian folds in the Moine metasediments are of this origin. The presence of igneous bodies with contact aureoles increased the propensity to rough slip and this, by causing thickening in the footwall to the Moine thrust, is partly responsible for the Assynt culmination.The previously accepted sequence of thrusting from foreland to hinterland, which has been deduced from the concept of ‘overstep’ of the Moine thrust across lower thrusts, is considered to be a misconception of thrust geometry. Instead, a ‘piggy-back’ sequence of thrusts, from higher to lower, is proposed.

A Lewisian basement sheet within the Moine at Ribigill, north Sutherland

Synopsis A Lewisian basement sheet, consisting of hornblende and hornblende-biotite gneiss and amphibolite with a central zone of feldspathic migmatite, tectonically emplaced within the Moine is described using 98 major and trace element analyses of Lewisian and Moine rocks. The Lewisian rocks are quite distinct from the unmigmatized local Moine metasediments and can be distinguished from a Moine pre-metamorphic basic intrusion (the ‘Ben Hope Sill’) which forms part of the boundary of the Lewisian sheet. It is inferred, by comparison with published data, that the Ribigill Lewisian originally had a ‘Scourian-type’ geochemistry depleted in K, Rb, Pb, etc., during an early Lewisian high grade metamorphism. The rocks were retrogressed and migmatized during a later Lewisian event, with the introduction of Ce, Th, K, Rb, and possibly Na. The Ribigill migmatites differ from published accounts of Laxfordian migmatites in several respects most notably in having high K/Rb ratios.

Stratigraphy and structure of the western Moine and the problem of Moine orogenesis

The stratigraphy and major structure of some 650 km 2 of Precambrian, Moine metasediments in western Scotland are discussed and it is shown that at least four major deformation phases involving primary isoclinal folding with fold amplitudes of at least 20 km, and two major metamorphic episodes affect both the Lewisian basement and the Moine cover. It is argued that there is sufficient evidence to regard the first two deformation phases together with the earlier metamorphism as Precambrian in age and the subsequent deformations and metamorphism as Caledonian.

Discussion

D r A. L. H arris asked if it were possible to distinguish between progressive and retrogressive mineral assemblages in the calc-silicates. Where studied by him at Loch Quoich and in River Loyne, the Quoich granitic gneiss and the adjacent Moine metasediments clearly share a common history of penetrative deformation followed by thorough annealing recrystallization. The Quoich granitic gneiss must originate either by migmatization involving potash enrichment in a zone of high strain (cf. Johnstone et al. 1969) or by the introduction of slices of granitic basement into the Moine cover, again presumably along zones of high strain. Discordance of these zones to the metamorphic isograds traced by Dr Winchester is an important demonstration that the pattern of metamorphic grade in this district was established later than the main penetrative espisodes to affect the rocks. If it can be demonstrated conclusively by textural evidence that the calc-silicate assemblages, on which the isograds are based, are retrogressive assemblages then the theory of the origin of the gneisses by migmatization may still be valid. On the other hand, if the assemblages are progressive, it should be recognized that the migmatization, which is unlikely to have taken place during progressive metamorphism at garnet grade, cannot be solely responsible for the presence and nature of the gneiss. In this event, the granitic gneiss must in all probability consist of basement slices, but of a completely different lithological type from those recognized along the Sgurr Beag Slide and analogous structures elsewhere in the Northern Highlands. T he A uthor

Major element composition of the Loch Coire migmatite complex, Sutherland, Scotland

Analyses of Moine metasediments and migmatites in the Loch Coire area of northern Scotland are used to delineate the fields of composition of the major rock groups. An assessment is made of the relative merits of the hypotheses of isochemical metamorphism and metasomatism in the genesis of the migmatites. Within the migmatites the Fe″/Fe‴ ratio is constant, a feature not shown by the metasediments. This is interpreted as evidence of permeation of the migmatites by metasomatising fluids. Granitic sheets which are common in the migmatites have a very variable and predominantly sodium-rich composition more easily reconciled with metasomatism than a simple magmatic or anatectic origin. Attention is also drawn to the sodic nature of the large mass of Loch Coire granite which occurs in the centre of the migmatite area and the possibility is considered that the uprise of sodic magma provided sodium-rich fluids for permeation of the rest of the complex.

The relationship of the Moine Schists and Lewisian Gneisses near Mallaigmore, Inverness-shire

A new map of the Mallaigmore area defines the area of the Lewisian gneiss more accurately than maps hitherto available, and shows the extent of transitional rock types produced by mechanical and metasomatic processes, occurring between the Lewisian and the Moine metasediments. Analyses of rocks from both major divisions show systematic differences in major elements and their ratios, but do not reveal any striking anomalies in minor element distribution. The criteria for recognising the Lewisian and the transitional rocks are given, and a major recumbent fold is postulated to account for the presence of the Lewisian.

The structure of the Rogart igneous complex, Sutherland, Scotland

The Rogart igneous complex, Sutherland, is of Newer Granite age and consists of a zoned tonalite-granodiorite-granite body (the central granodiorite complex) emplaced in Moine metasediments which to the east and north are involved in a peripheral zone of migmatization (the migmatite complex). Although the migmatite complex is genetically related to the central complex, it is not a static contact-effect. Evidence is presented which suggests the following sequence of events. An ascending body of granodiorite magma was preceded by a 'front' of migmatization. When it reached a higher crustal level, migmatization ceased and continued forceful intrusion of the magma deformed the migmatites into a plastic envelope around the expanding magma body, which consolidated as tonalite and granodiorite. Lateral distension exceeded upward movement and a partly funnel-shaped structure resulted. The granite cross-cuts structures in the tonalite and granodiorite and was emplaced later under more brittle conditions. An analysis of the deformation of 'regional' linear structures around the complex confirms the forcefully intrusive nature of the igneous mass. The geometry of the deformation of the migmatites during the emplacement of the central complex is related to the stress field obtaining at this period as deduced from planar and linear structures in the tonalite.

Zircons from the granitic gneiss of western Ardgour, Argyll; their bearing on its origin

Zircons from the granitic gneiss of western Ardgour, Argyll, have been studied in order to obtain supplementary evidence regarding its origin. They are compared with zircons from the surrounding Moine metasediments, the Strontian “granite” and the orthogneisses of Carn Chuinneag and Inchbae. It is concluded that extensive zircon recrystallisation has not occurred in the granitic gneiss, which is therefore unlikely to have passed through a magmatic stage, and is probably migmatitic in origin as the structural evidence indicates.

A Study of Two Directions of Folding: The Structural Geology of the Monadhliath and Mid-Strathspey

A review of the different ways in which more than one direction of folding has been thought to occur in one area and a classification of the different varieties of cross folding described in the literature are given. An area of complex folding in Moine metasediments is described. Consideration of the style of folding and the orientation of axial planes supports the view that an initial phase of homoaxial folding developed about axes plunging southeastward. This folding was accompanied by slide tectonics repeating the succession in large tectonic inclusions. The deformation was monoclinic and apparently in response to a regional sinistral movement (looking northward along the fold axes). A second phase of folding turned the folds of half the area through about 48°, producing a dominant southwesterly trend and a sharp junction line with crossing (mainly east-west) structures at the boundary between the originally disposed and the rotated block. Dislocation of the line of junction proves the magnitude of the...