Moine Assemblage Research Articles

Neoproterozoic extensional basic magmatism associated with the West Highland granite gneiss in the Moine Supergroup of NW Scotland

Pre-tectonic metagabbroic rocks emplaced into the Glenfinnan and Loch Eil groups of the Moine Supergroup give a U–-Pb zircon age of 873 ± 6 Ma. This new age for the metagabbros confirms the absence of Grenvillian ( c. 1.0 Ga) tectonic events in the Moine assemblage. The metagabbros are spatially associated with the Glen Doe body of the West Highland granite gneiss, and were emplaced soon after the granite gneiss protolith. The metagabbros have chemical characteristics indicating contamination with local country rocks. A regionally developed suite of tholeiitic metadolerite dykes post-dates the metagabbros. These dykes are geochemically similar to modern mid-ocean ridge basalt, albeit modified by interaction with metamorphic fluids. The presence of abundant MORB-like basaltic dykes, coupled with the lack of major compressional structures associated with the intrusive events, suggests that the c. 873 Ma event may have been dominated by extensional tectonics.

The pre‐Caledonian Inishkea Division of northwest Co. Mayo, Ireland: Its geochemistry and probable stratigraphic position

AbstractThe dominantly metasedimentary schists and gneisses of the Inishkea Division of the Erris Complex form a distinct rock group, structurally overlying the Grenvillian Annagh Division orthogneisses and underlying Dalradian metasediments, in the northwest County Mayo metamorphic inlier. A regression line for the Inishkea Division schists, defined by Rb–Sr whole rock analysis, suggests a metamorphic age of about 800 Ma, with a provenance age of about 1000–1300 Ma. The major and trace element chemistry is distinct from both the Annagh Division and Dalradian rocks, although sometimes similar to that of the Erris Group, and it suggests that the Inishkea Division originated as greywackes with associated intrusives. Local variations of the chemistry exist, but no systematic subdivision of the Division has been made. Amphibolite pods within the schists are metamorphosed and deformed tholeiite dykes, which are similar in chemistry to the younger metadolerites seen in the Annagh Division. While the geochemistry suggests that the Inishkea Division rocks are similar to both the Moine Assemblage and Grampian Group rocks of Scotland, the isotopic data suggest that they may have been deposited and then initially metamorphosed during the interval between the Grenville metamorphism of the Moine Assemblage and the onset of Dalradian and Erris Group deposition.

Intrusive metabasite belts within the Moine assemblage, west of Loch Ness, Scotland: evidence for metabasite modification by country rock interactions

Metabasites representing an extensive differentiated sill-complex (with minor dykes) now form two c . 10 km-wide NE-SW ‘belts’ in western Inverness-shire: a Western Belt, overlapping the ‘Quoich Line’, and an Eastern Belt, beside the Great Glen Fault. Regional evidence and, where interpretable. Rb-Sr isotope data, imply Grenvillian emplacement. More than one intrusive episode may be represented by contrasting adjacent discordant/unfoliated versus concordant/foliated bodies. although some such contrasts reflect later tectonism. Despite variations in form, size, internal homogeneity, paragenesis, grain-size and fabric, 120 newly analysed metabasites have much more uniformly tholeiitic chemistry than metabasites in the Moine of Ross-shire or Sutherland. Discriminant analysis nevertheless reveals that the Eastern Belt is compositionally invariable from NE to SW. whereas the Western differs between three sub-areas studied in detail, and shows more numerous evolved compositions. A single andesitic rock ( c . 57% SiO 2 ) is the most evolved metabasite so far reported from the Moine. Metabasite–country rock interactions during igneous emplacement and/or regional metamorphism caused major chemical transfers. For example, host psammites and granite-gneisses gained Ca. causing hornblende to crystallize; metabasites in limestones gained Ca, Mg, Sr, and lost Fe, Ti, Rb, Rb, Y and Zr, totally suppressing garnet; metabasites in pelites lost Na, enhancing garnet relative to plagioclase; metabasites within psammites gained K and Rb, enhancing biotite; and metabasites within granite-gneisses gained Na, K, Mn, Rb and Sr, suppressing epidote. Overall chemical differences between metabasites with or without biotite, epidote and garnet may reflect the same interactions, rather than original magmatic variations. Even the ‘immobile’ element chemistry of high-grade metabasites may be an unreliable tracer of magmatic chemistry, unless country rock interactions are allowed for.

The geochemistry of the Strathconon amphibolites, Northern Scotland

Synopsis The Strathconon ortho-amphibolites were intruded into Glenfinnan and Loch Eil Division metasediments of the Moine Assemblage before the area was deformed in late Precambrian time. A chemical study has indicated that they consist of a single suite of variously fractionated tholeiitic sills and dykes which can be distinguished chemically both from the Y-poor amphibolites of the Morar Nappe and from more Y-rich amphibolites within the Central Ross Nappe spatially associated with the Quoich Line in Inverness-shire. The chemical distinction between amphibolites from the two nappes requires that major transport of the overlying Central Ross Nappe occurred after the intrusion of the basic sills.

Field and isotopic evidence for a c. 750 Ma tectonothermal event in Moine rocks in the Central Highland region of the Scottish Caledonides

ABSTRACTEvidence is presented for a late Proterozoic, tectonothermal event which affected the rocks of the Moine assemblage in the Central Highland region of the Scottish Caledonides c. 750 Ma ago. This is about 200 Ma before the early Palaeozoic Grampian orogeny, whose effects are superimposed on the Moine rocks as well as dominating the tectonism in the Dalradian Supergroup.Field and isotopic studies are integrated in zones of ductile thrusting (sliding) which are typified by belts of tectonic schists with related swarms of quartz and muscovite-bearing pegmatite veins. Of particular significance is a ductile thrust (the Grampian slide) which separates deeper crustal rocks (the Central Highland division), interpreted as showing the imprint of the Grenville orogeny, from shallower rocks (the Grampian division) representing a supracrustal assemblage formed between the Grenville and the c. 750 Ma events.The Grampian slide is the structurally highest member of a system of related, previously unrecorded slides affecting the Central Highland division. New structural, petrographic and Rb-Sr isotopic data, obtained largely from a recently recognised inlier of the Central Highland division at Laggan, bear out that the quartz and pegmatite veins are segregations formed during ductile shearing under amphibolite facies conditions. Muscovites from these veins yield ages between 780 and 730 Ma, and a regression analysis of tectonic schists and the muscovites gives an age of 740 ± 40 Ma. These data substantiate our previous hypothesis that the deeper-seated Moine rocks were affected by a distinct orogenic event at 750 ± 30 Ma.In the Northern Highlands, similar vein swarms are related to the Sgurr Beag slide and to belts of previously unrecorded tectonic schists in the Glenfinnan division of the Moine assemblage. A new, 755 ± 8 Ma age obtained from such a tectonic schist at Kinloch Hourn, combined with previous, similar age data from lensoid pegmatites, imply that the c. 750 Ma event may have also affected the Morar and Glenfinnan divisions.

Microdiorite sheet intrusions as incompetent time- and strain-markers in the Moine assemblage NW of the Great Glen fault, Scotland

ABSTRACTRadiometrie evidence suggests a complex evolution for some parts of the Moine assemblage NW of the Great Glen fault in Scotland. Four generations of microdiorite sheet intrusions are distinguished and used as time- and strain-markers in this region. Two are found to record different tectonic episodes in the Morar and Glenfinnan divisions and their Loch Eil division cover during the c 750 Ma Knoydartian orogeny. These demonstrate that F1 and F2 (of the Grenvillian orogeny) and F3 folds (of the Knoydartian orogeny) in the Morar and Glenfinnan divisions are absent from the Loch Eil division. Two later generations of microdiorites record successive episodes (F4 and F5) of the c. 455 Ma Caledonian orogeny.The tectonic history of the region is considered after a discussion of the use of microdiorite sheet intrusions as strain-markers. This is because, rather than fold or boudinage, they deformed as incompetent single layers and foliated, mullioned or thinned uniformly instead. The attitudes of different elements of the microdiorites and their internal foliation are used to construct strain ellipsoids for four tectonic episodes on scales of 4–400 m and >4000 m. Three of these episodes are found empirically to have involved irrotational plane strain and all are close to coaxial even though the principal axes of strain appear to have swapped one for another in space as well as time.

Precambrian and Palaeozoic rocks of the Inner Hebrides

SynopsisThe Lewisian complex, which forms the continental basement to north-west Scotland, crops out on the Inner Hebridean islands of Rona, Raasay, Skye, Coll, Tiree, Iona and Islay. Upon this basement, four major rock successions were deposited before the Caledonian orogeny. The upper Precambrian Moine assemblage forms only a small area of metamorphosed rocks on Mull but the c. 790 m.y. (million year) old Torridonian sediments are found on Raasay, Scalpay. the Sleat of Skye, Soay and Rhum. The upper Precambrian to Cambrian Dal radian Supergroup dominates Islay, Jura, Gigha and the islands of the Firth of Lome whilst also forming a partial rim around the Tertiary Northern granite in Arran. Other Precambrian rocks of uncertain affinity are found on Islay, Oronsay, Colonsay and Iona. Cambro-Ordovician sediments are found on Skye where they have been partially metamorphosed by Tertiary intrusions.South-east of the Moine thrust zone, the Precambrian and lower Palaeozoic rocks were deformed and metamorphosed during the late Cambrian to early Devonian Caledonian orogeny which resulted in the development of the Caledonian mountain chain. Subsequently, Scotland north-west of the Highland Boundary fault has tended to persist as a land area undergoing erosion with sedimentation restricted to peripheral areas now preserved around the present coast. Upper Palaeozoic rocks are therefore only well represented on Arran although Old Red Sandstone (Devonian) sediments are found in the Firth of Lome and a very small area of possible Permian rocks occurs on Islay.

A multicomponent, dual-polarity palaeomagnetic regional overprint from the Moine of northwest Scotland

Palaeomagnetic remanence may be reset by orogeny in a manner analogous to isotopic systems. The heavy mineral seams and associated psammite in the deformed and metamorphosed Lower Morar Division of the Moine Assemblage of northwest Scotland have a palaeomagnetic signature that is characteristic over a wide area. The various components of the magnetization are resolved by detailed thermal demagnetization. These components are post-tectonic, as they are not affected by any structural fabric. They may be classified according to blocking temperature and, when statistically distinguishable, show a systematic change of direction from high to lower temperature possibly associated with apparent polar wander. Comparison to the European palaeomagnetic record suggests a late Silurian to early Devonian age of magnetization for the localities south of the Strath Conon Fault. The distribution of remanance directions and blocking temperatures is not consistent with the pattern of potassium-argon ages in this part of the British Caledonides. If the magnetization is purely of thermal origin, the palaeomagnetic record indicates higher temperatures at later time than does the potassium-argon record. This remagnetization event is related to the closing stages of the Caledonian orogeny.

Caledonian thrusting in NW Scotland

Summary Several major thrusting events, ranging in age from Precambrian to Middle Devonian, have affected the Moine assemblage and adjacent foreland. An early thrusting event is primarily responsible for the emplacement of lenticular Lewisian Gneiss inliers within the Moines. The main Caledonian ( sensu stricto ) event (F 2 ) at about 480 Ma was complex in nature with widespread marginal and internal ductile thrusting, related recumbent folding, metamorphism and locally migmatisation. Later open to tight folding (F 3 , F 4 ) and minor ductile thrusting (F 3 ) modified this pattern. Subsequently in Silurian to Lower Devonian times marginal major brittle thrusting, and internal faulting, minor thrusting and uplift, occurred.

Structural age of the Vagastie Bridge granite, Sutherland

Synopsis The Vagastie Bridge granite is observed to cut folds associated with the main metamorphism of the surrounding psammites of the Moine assemblage, confirming recent radiometric evidence that the granite belongs to the suite of Caledonian Newer Granites in the Northern Highlands, and is not equivalent to the Carn Chuinneag granite.