Moine Supergroup

Deformation and dating of the Moine Supergroup

Introduction

The Precambrian consists of 4000 million years starting from 4600Ma to the start of the Cambrian at 544Ma. It can be subdivided into the Archaean that comprises entirely metamorphic rocks that are older than 2500 Ma and the younger division is known as the Proterozoic which includes sedimentary rocks, fossils that incorporate marine plants and soft bodied animals.

Map of the Precambrian formationsThe oldest rocks that are related to the Precambrian are located in Scotland; they compose of being ancient gneisses and belong to the Lewisian Complex. Oldest formation is the Lewisian, this formation is separated into three zones: the North that consists of amphibolites, the south that also consists of amphibolites whereas the central zone contains granulites.Next in the sequence is the Torridonian formation, a 7km thick clastic sequence split into two groups; the Stoer and the Torridon/Sleat groups, these two groups are separated by an angular unconformity that is the first and oldest unconformity in Britain. The Moine formation includes rocks of psammite, pelitic and psammopelitic metasediments. The Dalradian is the last and the youngest formation within the Lewisian complex; it's approximately 24km thick originally varied sedimentary rocks that are now metamorphosed. England and Wales contain small areas of Precambrian rocks compared to Scotland.

Moine supergroup is a 10km thick rock sequence of Proterozoic metasediments within the Caledonian Orogenic belt between the Moine Thrust and the Great Glen fault. It consists of sandstones and shales dating back to the Precambrian that have been strongly deformed and metamorphosed to schists and other rocks including psammites, semi-pelites and pelites.

The Moine supergroup was first deposited as fined grained sands, silts and muds where later the sands were metamorphosed to psammites while the silts and muds turned to pelites. During deposition of the sediments, the breakup of Rodinia, a supercontinent in 750Ma was proceeding. The Moine supergroup has been characterized into three lithostratigraphical groups; the Morar, Glenfinnan and Loch Eil groups. They show various proportions of psammites and pelites and striped units, which have been subjected to rapid alteration (Woodcock & Strachan, 2000). The Glenfinnan group is composed of psammites and pelites whereas the Loch Eil division is composed of mainly psammites, which are a result from metamorphosed sandstones. The Glenfinnan and Loch Eil have emplaced on the psammites and pelites that are part of the Morar group by a Caledonian ductile thrust (Tanner et al, 1970; Rathbone et al., 1983). Underlying the Loch Eil division is the Glenfinnan division; this is marked by a shift to psammites and pelites. It is thought that the Glenfinnan division has undergone intense Caledonian reworking (Roberts et al, 1984).

Stratigraphy

The Moine rocks have been subjected to many different interpretations, dating back to 1939 (Powell, 1964). The lower part of the Moine sequence has been agreed however, interpretation of the upper stratigraphy of the Moine succession and structure still remains indefinite.

Many studies and investigations have been carried out since the early work of Richey & Kenney 1939. The structural complexity and repetitive psammites, semi-pelites, pelites and local sequences were recognized by using way up features to establish contact with inliers of Lewisianoid basements gneisses and other lithologies including amphibolites, calc-silicate lenses and heavy mineral bands (Soper, Harris & Strachan, 1998). The Moine succession is considered to overlie granulites and gneisses that are part of the Archaean Basement of the Lewisian complex (Peach et al., 1907; Barber and May, 1976; Harris, 1983). It is believed that the Lewisian basement extends beneath the Moine across the Great Glen fault; this was demonstrated by geophysical evidence (Bramford, 1979).

Morar Group

The Morar sequence is composed of two main psammites, the upper and lower Morar Psammite formations is separated by striped and politic schists which is known as the Morar pelite formation (Soper, Harris & Strachan 1998). The lower part of the Morar is approximately 1km thick, coarse grained, feldspathic and mainly gneissose that becomes finer and richer in feldspars as it progresses upwards. In contrast the upper part of the Morar is in the region of 5km thick however, thins eastwards, slightly deformed and sedimentary structures indicating shallow marine environments are preserved. The Glenfinnan and Loch Eil group structurally overlie the Morar group where they are separated by the ductile Sgurr Beag Thrust which was formed during the Caledonian orogeny (Zeh & Millar, 1999). The Glenfinnan group and the Loch Eil form the Sgurr Beag nappe.

Glenfinnan

The Glenfinnan group is composed of interbedded quartzite, psammites, semipelites and pelites that have been irregularly migmatized in the mid-upper amphibolites facies. The Lewisian basement gneisses are found near the base and thought to represent a disrupted unconformity (Soper, Harris & Strachan 1998). The thickness of this division is unknown but is thought not to be greater than 2km thick. The Glenfinnan group is composed of mixed and muddy deposits which have been interpreted partly as corresponding to the Morar group (Soper, Harris and Strachan 1998).

Loch Eil

The loch Eil group overlies the pelites and psammites of the Glenfinnan group. The group is dominated by shallow marine arkosic sandstones and siliceous psammites that were deposited in a second (Soper, Harris and Strachan 1998). Loch Eil division is the upper part of the Moine that was formed in a basin floored by the Lewisian basement (Strachan, 1985).

The Moine rocks that are Southeast of the Great Glen fault can be distinguished into two units, the Central Highland division characterized by a gneissose assemblage and the second unit is known as the Grampian division, a younger and less deformed metasediments that is separated from the gneisses by a tectonic break, the Grampian Slide (Piasecki, 1980).

Environment

In the Moine supergroup, sedimentological and basin analysis studies have been limited due to tectonic overprinting (Gibbons, 1994).

Sedimentary deposits were recognised from the Loch Eil division where three sedimentary facies were acknowledged, these include siltstones, interbedded siltstones/sandstones and sandstones (Strachan, 1985). The environment of deposition suggests either shallow marine or fluvial/deltaic or a combination of both. Evidence to support the environment of deposition includes bipolar cross stratification, wave ripples and lenticular bedding (lense-shaped deposits of sand that have interbeddedwith mud). The bipolar cross bedding suggests tidal activity and the facies is interpreted to have built up in an offshore shallow marine environment whereas the interbedded siltstone and sandstone facies is understood to also be offshore but is characterized by high rates of silt and low level of sand deposition (Strachan, 1985). The sedimentary structures are displayed and preserved within the thick psammites of the Morar and Loch Eil groups. Cross beddings trend towards the north and northeast indicated by palaeocurrents. The upper Morar group illustrates a westward thickening of the psammites with deposition in a half graben. Soft deformation structures are visible in the west yet absent in the east. It has been suggested that the Moine sediments were most likely deposited in half grabens that trend NNE-SSW that were bounded to the west by east dipping normal faults (Woodcock & Strachan, 2000). The sands, silts and muds had been laid down in marine rift basins between 1000Ma and 873 became the Moine Supergroup as a result of Rodinia.

Problems faced in interpreting the sequences were partly due to lack of fauna and different run off conditions during the Precambrian. Another problem was distinguishing between the two depositional settings that showed little evidence favouring either environment (Long, 1978).

Tectonics

The tectonic history of the Moine and associated gneisses are still much at dispute, whether they are essentially part of the Laurentia in the early Neoproterozic or whether they formed an exotic terrane that was derived from another continental block and soon after accreted to the Laurentian margin still remains unidentified (Woodcock & Strachan, 2000).

Deformation

The Moine Supergroup in the western highlands has been entirely affected by three main phases of deformation, the fourth deformation event formed large folds (Zeh & Millar, 1999). It is considered that the first two deformations within the Moine are Precambrian in age whereas the third deformation is thought to be the main Caledonian reworking event (Holdsworth & Roberts, 1984).

Many journals agree that the Moine experienced a three-phase sequence of folding. Ramsay & Spring (1962), Powell (1966), Poole (1966), Brown et al. (1971), Tanner (1971) and Poole & Spring (1974) indicate a primary phase of isoclinal folding, a second synmmetamorphic phase creating tight folds characterized by their strong axial plane schistosity and soon after a phase of more open folds associated with crenulations of earlier planar fabrics (Powell, 1974).

However, a fourth phase is implied that the Moine consists of a major fold showing minor structures (Powell 1966), to support his idea, the fourth phase folds is associated crenulations (Brown, 1971).

Work recently carried out in the Skye-Morar-Knoydart-Ardgour region by Atldnson, MacQueen & Powell uncovered a group of primary folds before the formation of the SE Morar, Knoydart and Meith Bheinn Slides and at least three subsequent separate fold phases, each regionally developed (Powell, 1974).

Metamorphism

Little is known about the metamorphic history of the Moine Supergroup compared to the structural evolution (Zeh & Millar, 1999).

Metamorphic and structural research with the addition of using isotopic analyses propose that the rock of the Moine supergroup encountered polyphase deformation and regional greenschist, amphibolitic facies metamorphism both in the late Proterozoic and in the early Palaeozoic (Gibbons and Harris, 1994).

Regional metamorphism that affected the Moine rock ranges from greenschist to upper Amphibolite facies (Zeh & Millar, 1999). The regional metamorphism was at its peak during the second deformation event (Macqueen & Powell, 1977). Metamorphic grade increases from the west to east however majority of the metasediments lack good metamorphic index mineral assemblages due to unsuitable chemical compositions (Gibbons, 1994).

The use geochronology with pressure- temperature to approximate metamorphism is a successful way to understand and evaluate the tectonic history of a rock formation or sequence (Kelsey et al, 2007), particularly terranes that have undergone several episodes of metamorphic events that may have fundamentally destroyed growth histories of matrix mineral assemblages (Zeh, 2006).

The Northwestern part of the Caledonide in Scotland is composed of metamorphic rocks that belong to the greenschist, epidote-amphibolites facies and have been assigned to the Moine and Dalradian Successions (Harte, 1988). Metamorphism that took place in the Proterozoic and lower Palaeozoic has been acknowledged within the Moine whereas the Dalradian only show evidence for lower Palaeozoic metamorphism.

It is alleged that that the Moine rocks were affect by mid-Amphibolite facies metamorphism and intruded by syntectonic Pegmatites at 790-840Ma (Highton, 1999). The use of Sm-Nd garnet geochronology, garnet chemistry, calculated pressure and time pseudosections indicated that the metasediments within the Morar group endured burial pressures of 12-14kbar at around 700 °C at 800Ma. Burial depths of 35-40km signify crustal thickening within a compressional tectonic setting (Vance, 1998).

The garnet bearing schists and gneisses typically contain garnet, mica, quartz and plagioclase feldspar. These rocks can tell us about the P-T history due to their preservation of zonation in garnet, plagioclase and epidote; these minerals provide information about their deformation crystallization history (Zeh & Millar, 1999). Using K-Ar, Rb-Sr and U-Pb on minerals from schists, gneissose pelites and pegmatites have insinuated a thermal history, early metamorphism taking place at 750Ma followed by pegmatite emplacement at 442Ma and later metamorphism at 430Ma (Brewer, Brook &Powell, 1979).

Studies on the Moine rocks from Skye and Morar-Knoydart area illustrate garnet growth in rocks from Skye was simultaneous with F2 deformation as garnets contained rotational inclusion fabrics whereas W. Morar area it preceded and partly overlapped F movements. Further East in the garnet growth was largely pre-F, in age (Powell, 1974). Garnet textures between the SE Morar and Loch Eil are difficult to interpret but the majority in gneissose schistosity of the rocks is axial planar to the F2 folds (Powell, 1974).

The main Caledonian deformation and metamorphism that occurred in 460-430Ma is based on the structural studies (Roberts, 1984). To support the work of Roberts, Rb-Sr whole rock ages on the Caledonian pegmatites and Migmatites gave ages of 450-440Ma (Breemen, 1979). The Moine Thrust has been given an age of 430-425MA. There is strong evidence for the Caledonian metamorphism and deformation.

The minimum age that the Caledonian metamorphism could have taken place in the South-western Moine is 467±20Ma in comparison to the main metamorphism in the North-eastern Dalradian that occurred in 485Ma. Effects of the Caledonian metamorphism were displayed in the Glenfinnan division until 413±17Ma ago (Pankhurst, 1970).

The timing of the metamorphism event at 800Ma that took place in Scotland is comparable to other ages such as the North Atlantic illustrating a sizeable, protracted early to mid Neoproterozoic orogenic system that was created after the Greenville orogeny and assembly of Rodinia at 1000-1100Ma (Cutts, 2009).

Moine rocks that are north of the Great Glen fault show evidence of the Grenville Metamorphism 1000-1050Ma; dates were established from whole rock Rb-Sr data (Brewer, 1979). The Morarian pegmatites are thought to mark a major tectothermal event demonstrating ages of 800-750Ma (Harte, 1988). On the other hand it has been opposed that the Morar and Loch Eil division may be younger than Grenvillian in age (Breemen, 1983). It has also been proposed that the Loch Eil division was deposited on the Glenfinnan division uncomformably (Lambert et al, 1979).

The Moine rocks south to the Great Glen fault show dates of about 750Ma, data has been collected from pegmatites associated with the Grampian Slide (Harte, 1988). These ages are analogous to the Morarian pegmatites. Grampian deformation is present in these rocks however are absent in the Moine rocks north to the Great Glen Fault.

Age

Dating the Moine supergroup is very difficult as it contains no fossils therefore only a limited number of isotopes can be used to date orogenic events. The unconformable relationship between the Moine supergroup and the gneisses from the Lewisian Complex demonstrates that the Moine supergroup was deposited after the Laxfordian Event (Gibbons, 1994).

The main rock types are psammites, semi-pelites and pelites. They date between 1200 and 870Ma, sedimentation of the Moine occurs after 1000Ma, this is the youngest detrital zircon age dated so far. There have been at least four deformation stages that have formed isoclinal folds and 3 episodes of metamorphism that have occurred around 1000Ma, at 800 Ma and again later at 460 Ma (Powell, 1974).

The conglomerate that is in contact with the Lewisian rocks along the western side of Glenelg is considered to be the basal member of the Moine supergroup. Using Sm-Nd garnet whole rock, ages of 1000 Ma is gained. However, the contact between the Moine and Lewisian is argued to be extensional detachments (Temperley & Windley, 1997). On the other hand is has been proposed the relationship between them is an unconformable basement cover (Ramsay, 2001). Evidence supporting this case is proven by U-Pb ages on detrital zircons from the Morar also verify ages of 1000Ma, as they were ages of 1032-2712 whereas the Loch Eil division detrital zircons gave ages of 947-1889Ma (Friend, 2003).

Pegmatites within the Moine were analyzed in early attempts to gather ages for the deformation and metamorphism that affected the Moine supergroup (Long & Lambert, 1963). Early attempted ages using Rb-Sr dating on muscovite crystals up to 30cm in length from the Knoydart mica mine illustrated Precambrian ages of approximately 740Ma (Zeh & Millar, 1999). This is supported by the use of U-Pb dating of zircons on Pegmatites from the Loch Eil that shows ages of 740±30 this coincides with the Rb-Sr muscovite ages of 730±5 Ma (Breeman, 1978).

Nevertheless, U-Pb dating of zircons and monazite from the Sgurr Breac Pegmatites demonstrated ages of 770-815Ma, these seemed to be older. Evidence that concords with this is recent U-Pb dating of monazite that showed an age of 784±1 (Rogers et al. 1983) and Rb-Sr dating on muscovite has shown ages of 746-776Ma (Powell, 1983). In comparison to U-Pb dating of monazite from the same pegmatite gave results of older ages of 827±2Ma (Rogers et al. 1998). The samples on the pegmatites and muscovites were all deformed and are in much debate about their structural age (Zeh & Millar, 1999).

Moine outcrops in Sutherland show two groups of ages determined by studies on detrital zircons from the outcrop suggest ages of 1000-1250 and 1500-1760Ma (Plant, 1999).

Data from Rb-Sr has confirmed a Grenvillian age for early regional metamorphism of the Moine succession. A later Caledonian metamorphic event took place between 470 - 410Ma (Brewer, Brook & Powell 1979). Detrital and inherited zircons from the Moine succession rocks have shown ages ranging from 2707-947Ma for the Morar Group. Caledonian migmatites yield 207Pb/206Pb ages between 2940 and 926 Ma (Friend, 2003). Granites gneisses within the Glenfinnan and Loch Eil division showed ages ranging from 1889 to 947 Ma (Friend, 2003). The data implies that the Moine Supergroup was deposited in a Post Grenvillian basin.

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