Lewisian complex
Encyclopedia
The Lewisian complex or Lewisian Gneiss is a suite of Precambrian
metamorphic rock
s that outcrop
in the northwestern part of Scotland
, forming part of the Hebridean Terrane
. These rocks are of Archaean and Paleoproterozoic
age, ranging from 3.0–1.7 Ga. They form the basement
on which the Torridonian
and Moine Supergroup
sediments were deposited. The Lewisian consists mainly of granitic
gneiss
es with a minor amount of supracrustal rock
s. Rocks of the Lewisian complex were caught up in the Caledonian orogeny
, appearing in the hanging walls of many of the thrust fault
s formed during the late stages of this tectonic event.
, including Lewis
, from which the complex takes its name. It is also exposed on several islands of the Inner Hebrides
, small islands north of the Scottish mainland and forms a coastal strip on the mainland from near Loch Torridon
in the south to Cape Wrath
in the north. Its presence at seabed and beneath Paleozoic and Mesozoic sediments west of Shetland and in the Minches and Sea of the Hebrides
has been confirmed from the magnetic field, by shallow boreholes and hydrocarbon exploration wells. Basement rocks of similar type are found at the base of the Moine Supergroup, sometimes with well-preserved unconformable contacts, and these are generally accepted as forming part of the Lewisian, suggesting that the Lewisian complex extends at least as far southeast as the Great Glen Fault
.
memoir on the structure of the Northwest Highlands
. In 1951 John Sutton and Janet Watson built on this work by interpreting the metamorphic and structural development of the Lewisian as a series of discrete orogenic events that could be discerned in the field. They used a swarm of dolerite dykes, known as the Scourie dykes, as markers to separate the tectonic and metamorphic events into a Scourian event that occurred before the intrusion of the dykes and a later Laxfordian event that deformed and metamorphosed members of the same dyke swarm. Subsequent fieldwork, metamorphic studies and radiometric dating has refined their chronology but supported their original hypothesis.
, tonalitic
or trondhjemitic
in composition. Metasediment
ary gneisses are relatively rare. The protolith
for the Scourian gneisses are thought to be granitic, with subsidiary mafic and ultramafic plutonic rocks giving an overall bimodal character. Some variation in the age of the protoliths from different parts of the complex and their subsequent tectonic and metamorphic history suggest that there are two or possibly three distinct crustal blocks within the mainland outcrop.
The main metamorphic event in the Central Region was the 2.5 Ga granulite facies Badcallian event. The Northern Region lacks evidence of granulite facies and in the Southern Region an earlier 2.73 Ga event is recognised locally.
s dated at 2.49-2.48 Ga and predates most of the Scourie dykes, giving a possible age range of approximately 2.48 - 2.42 Ga. The deformation was accompanied by retrograde metamorphism down to amphibolite facies, similar to the later Laxfordian event. Distinguishing between these two events has proved difficult. Major Inverian shear zones have been identified in the Central and Southern Regions, including the Canisp Shear Zone.
ages come from the Central Region, giving an age for the main part of the swarm as about 2.4 Ga. Some dykes, which appear to have been intruded into cooler Scourian crust give ages of about 2.0 Ga, the same age as undated sills within the Loch Maree Group. Some of the main dyke suite show evidence of intrusion into hot country rock
. Most of the dykes are quartz-dolerite
s in terms of chemistry, with less common olivine
gabbro
, norite
and bronzite
picrite.
and Gairloch
in the Southern Region. The group consists of metasediments with intercalated amphibolite
s, interpreted to be metavolcanics with some basic sills
. They were probably deposited at about 2.0 Ga, as they contain detrital zircon
s that give a mixture of Archaean and Paleoproterozoic ages.
facies, part of which may be Grenvillian
in age (about 1.1Ga). The early event is particularly associated with shear zones in which the deformed scourie dykes form amphibolite sheets within the reworked gneisses. The original mineralogy of the dykes is also changed to an amphibolite facies assemblage, even where they remain undeformed. The early Laxfordian fabrics
are cut by a series of granites and pegmatite
s, particularly in the Northern and Southern Regions dated at 1.7 Ga.
and metagabbro, with lesser amounts of tonalitic and pyroxene-granulite gneisses. These igneous rocks are intruded into the Leverburgh and Langevat supracrustals. Radiometric dating suggests that the complex was intruded over a period from about 2.2–1.9 Ga, comparable to the age of the Loch Maree Group. The Ness Anorthosite, exposed on the northeastern tip of Lewis, is also found associated with metasediments and yields a similar Sm-Nd model age of about 2.2 Ga. It is considered possible that the South Harris and Ness bodies once formed part of a continuous body, disrupted by Laxfordian deformation.
, mylonite
and pseudotachylite
, indicating faulting at a wide range of crustal levels.
facies metamorphism within both of the tectonically juxtaposed units that make up the inlier, thought to be associated with crustal thickening during a Paleoproterozoic event at about 1.7 Ga and the Grenvillian orogenic event respectively.
Precambrian
The Precambrian is the name which describes the large span of time in Earth's history before the current Phanerozoic Eon, and is a Supereon divided into several eons of the geologic time scale...
metamorphic rock
Metamorphic rock
Metamorphic rock is the transformation of an existing rock type, the protolith, in a process called metamorphism, which means "change in form". The protolith is subjected to heat and pressure causing profound physical and/or chemical change...
s that outcrop
Outcrop
An outcrop is a visible exposure of bedrock or ancient superficial deposits on the surface of the Earth. -Features:Outcrops do not cover the majority of the Earth's land surface because in most places the bedrock or superficial deposits are covered by a mantle of soil and vegetation and cannot be...
in the northwestern part of Scotland
Scotland
Scotland is a country that is part of the United Kingdom. Occupying the northern third of the island of Great Britain, it shares a border with England to the south and is bounded by the North Sea to the east, the Atlantic Ocean to the north and west, and the North Channel and Irish Sea to the...
, forming part of the Hebridean Terrane
Hebridean Terrane
The Hebridean Terrane is one of the terranes that form part of the Caledonian orogenic belt in northwest Scotland. Its boundary with the neighbouring Northern Highland Terrane is formed by the Moine Thrust Belt...
. These rocks are of Archaean and Paleoproterozoic
Paleoproterozoic
The Paleoproterozoic is the first of the three sub-divisions of the Proterozoic occurring between . This is when the continents first stabilized...
age, ranging from 3.0–1.7 Ga. They form the basement
Basement (geology)
In geology, the terms basement and crystalline basement are used to define the rocks below a sedimentary platform or cover, or more generally any rock below sedimentary rocks or sedimentary basins that are metamorphic or igneous in origin...
on which the Torridonian
Torridonian
In geology, Torridonian describes a series of Neoproterozoic arenaceous sedimentary rocks, extensively developed in the Northwest Highlands of Scotland, and particularly in the district of upper Loch Torridon, a circumstance which suggested the name Torridon Sandstone, first applied to these rocks...
and Moine Supergroup
Moine Supergroup
The Moine Supergroup is a sequence of Neoproterozoic metamorphic rocks that form the dominant outcrop of the Scottish Highlands between the Moine Thrust Belt to the northwest and the Great Glen Fault to the southeast. The sequence is metasedimentary in nature and was metamorphosed and deformed in a...
sediments were deposited. The Lewisian consists mainly of granitic
Granite
Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite usually has a medium- to coarse-grained texture. Occasionally some individual crystals are larger than the groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic...
gneiss
Gneiss
Gneiss is a common and widely distributed type of rock formed by high-grade regional metamorphic processes from pre-existing formations that were originally either igneous or sedimentary rocks.-Etymology:...
es with a minor amount of supracrustal rock
Supracrustal rock
Supracrustal rocks are rocks that were deposited on the existing basement rocks of the crust, hence the name. They may be further metamorphosed from both sedimentary and volcanic rocks....
s. Rocks of the Lewisian complex were caught up in the Caledonian orogeny
Caledonian orogeny
The Caledonian orogeny is a mountain building era recorded in the northern parts of the British Isles, the Scandinavian Mountains, Svalbard, eastern Greenland and parts of north-central Europe. The Caledonian orogeny encompasses events that occurred from the Ordovician to Early Devonian, roughly...
, appearing in the hanging walls of many of the thrust fault
Thrust fault
A thrust fault is a type of fault, or break in the Earth's crust across which there has been relative movement, in which rocks of lower stratigraphic position are pushed up and over higher strata. They are often recognized because they place older rocks above younger...
s formed during the late stages of this tectonic event.
Distribution
The main outcrops of the Lewisian complex are on the islands of the Outer HebridesOuter Hebrides
The Outer Hebrides also known as the Western Isles and the Long Island, is an island chain off the west coast of Scotland. The islands are geographically contiguous with Comhairle nan Eilean Siar, one of the 32 unitary council areas of Scotland...
, including Lewis
Lewis
Lewis is the northern part of Lewis and Harris, the largest island of the Western Isles or Outer Hebrides of Scotland. The total area of Lewis is ....
, from which the complex takes its name. It is also exposed on several islands of the Inner Hebrides
Inner Hebrides
The Inner Hebrides is an archipelago off the west coast of Scotland, to the south east of the Outer Hebrides. Together these two island chains form the Hebrides, which enjoy a mild oceanic climate. There are 36 inhabited islands and a further 43 uninhabited Inner Hebrides with an area greater than...
, small islands north of the Scottish mainland and forms a coastal strip on the mainland from near Loch Torridon
Loch Torridon
Loch Torridon is a sea loch on the west coast of Scotland in the Northwest Highlands. The loch was created by glacial processes and is in total around 15 miles long. It has two sections: Upper Loch Torridon to landward, east of Rubha na h-Airde Ghlaise, at which point it joins Loch Sheildaig;...
in the south to Cape Wrath
Cape Wrath
Cape Wrath is a cape in Sutherland, Highland, in northern Scotland. It is the most northwesterly point on the island of Great Britain. The land between the Kyle of Durness and the lighthouse that is situated right at the tip, is known as the Parph, two hundred and seven square kilometers of...
in the north. Its presence at seabed and beneath Paleozoic and Mesozoic sediments west of Shetland and in the Minches and Sea of the Hebrides
Sea of the Hebrides
The Sea of the Hebrides is a portion of the North Atlantic Ocean, located off the coast of western Scotland, separating the mainland and the northern Inner Hebrides islands from the southern Outer Hebrides islands...
has been confirmed from the magnetic field, by shallow boreholes and hydrocarbon exploration wells. Basement rocks of similar type are found at the base of the Moine Supergroup, sometimes with well-preserved unconformable contacts, and these are generally accepted as forming part of the Lewisian, suggesting that the Lewisian complex extends at least as far southeast as the Great Glen Fault
Great Glen Fault
The Great Glen Fault is a long strike-slip fault that runs through its namesake the Great Glen in Scotland. However, the fault is actually much longer and over 400 million years old.-Location:...
.
History of study
The first comprehensive account of the Lewisian complex was published in 1907 as part of the Geological SurveyBritish Geological Survey
The British Geological Survey is a partly publicly funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. The BGS headquarters are in Keyworth, Nottinghamshire, but other centres...
memoir on the structure of the Northwest Highlands
Northwest Highlands
The Northwest Highlands are the northern third of Scotland that is separated from the Grampian Mountains by the Great Glen . The region comprises , Assynt, Caithness and Sutherland. The Caledonian Canal, which extends from Loch Linnhe in the west, via Loch Ness to the Moray Firth in the north...
. In 1951 John Sutton and Janet Watson built on this work by interpreting the metamorphic and structural development of the Lewisian as a series of discrete orogenic events that could be discerned in the field. They used a swarm of dolerite dykes, known as the Scourie dykes, as markers to separate the tectonic and metamorphic events into a Scourian event that occurred before the intrusion of the dykes and a later Laxfordian event that deformed and metamorphosed members of the same dyke swarm. Subsequent fieldwork, metamorphic studies and radiometric dating has refined their chronology but supported their original hypothesis.
Scourie complex
The oldest part of the Lewisian complex is a group of gneisses of Archaean age that formed in the interval 3.0–2.7 Ga. These gneisses are found throughout the outcrop of the Lewisian complex in the mainland. The dominant lithology of the Scourie complex is banded grey gneisses, typically granodioriticGranodiorite
Granodiorite is an intrusive igneous rock similar to granite, but containing more plagioclase than orthoclase-type feldspar. Officially, it is defined as a phaneritic igneous rock with greater than 20% quartz by volume where at least 65% of the feldspar is plagioclase. It usually contains abundant...
, tonalitic
Tonalite
Tonalite is an igneous, plutonic rock, of felsic composition, with phaneritic texture. Feldspar is present as plagioclase with 10% or less alkali feldspar. Quartz is present as more than 20% of the rock. Amphiboles and pyroxenes are common accessory minerals.In older references tonalite is...
or trondhjemitic
Trondhjemite
Trondhjemite is a leucocratic intrusive igneous rock. It is a variety of tonalite in which the plagioclase is mostly in the form of oligoclase. Trondhjemites are sometimes known as plagiogranites....
in composition. Metasediment
Metasediment
In geology, metasediment is sediment or sedimentary rock that shows evidence of having been subjected to metamorphism. The overall composition of a metasediment can be used to identify the original sedimentary rock, even where they have been subject to high-grade metamorphism and intense...
ary gneisses are relatively rare. The protolith
Protolith
Protolith refers to the precursor lithology of a metamorphic rock.For example, the protolith of a slate is a shale or mudstone. Metamorphic rocks can be derived from any other rock and thus have a wide variety of protoliths. Identifying a protolith is a major aim of metamorphic geology.Sedimentary...
for the Scourian gneisses are thought to be granitic, with subsidiary mafic and ultramafic plutonic rocks giving an overall bimodal character. Some variation in the age of the protoliths from different parts of the complex and their subsequent tectonic and metamorphic history suggest that there are two or possibly three distinct crustal blocks within the mainland outcrop.
The main metamorphic event in the Central Region was the 2.5 Ga granulite facies Badcallian event. The Northern Region lacks evidence of granulite facies and in the Southern Region an earlier 2.73 Ga event is recognised locally.
Inverian event
This tectonic and metamorphic event postdates the main granulite facies metamorphic event in the Scourian complex but mostly predates intrusion of the Scourie dykes. This event deforms a suite of post-Badcallian pegmatitePegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
s dated at 2.49-2.48 Ga and predates most of the Scourie dykes, giving a possible age range of approximately 2.48 - 2.42 Ga. The deformation was accompanied by retrograde metamorphism down to amphibolite facies, similar to the later Laxfordian event. Distinguishing between these two events has proved difficult. Major Inverian shear zones have been identified in the Central and Southern Regions, including the Canisp Shear Zone.
Scourie dykes
This basic dyke swarm cuts the banding of the Scourie complex gneisses and therefore postdates the main igneous, tectonic and metamorphic events that created them. Due to the degree of later metamorphism and deformation in other parts of the mainland outcrop, the only reliable radiometricRadiometric dating
Radiometric dating is a technique used to date materials such as rocks, usually based on a comparison between the observed abundance of a naturally occurring radioactive isotope and its decay products, using known decay rates...
ages come from the Central Region, giving an age for the main part of the swarm as about 2.4 Ga. Some dykes, which appear to have been intruded into cooler Scourian crust give ages of about 2.0 Ga, the same age as undated sills within the Loch Maree Group. Some of the main dyke suite show evidence of intrusion into hot country rock
Country rock (geology)
Country rock is a geological term meaning the rock native to an area. It is similar and in many cases interchangeable with the terms basement and wall rocks....
. Most of the dykes are quartz-dolerite
Quartz-dolerite
An intrusive rock, similar to dolerite, but with an excess of quartz. Dolerite is similar in composition to basalt, which is eruptive , and gabbro, which is plutonic. The differing crystal sizes are due to the different rate of cooling, basalt cools quickly and has a very fine structure, while...
s in terms of chemistry, with less common olivine
Olivine
The mineral olivine is a magnesium iron silicate with the formula 2SiO4. It is a common mineral in the Earth's subsurface but weathers quickly on the surface....
gabbro
Gabbro
Gabbro refers to a large group of dark, coarse-grained, intrusive mafic igneous rocks chemically equivalent to basalt. The rocks are plutonic, formed when molten magma is trapped beneath the Earth's surface and cools into a crystalline mass....
, norite
Norite
Norite is a mafic intrusive igneous rock composed largely of the calcium-rich plagioclase labradorite and hypersthene with olivine. Norite is essentially indistinguishable from gabbro without thin section study under the petrographic microscope...
and bronzite
Bronzite
Bronzite is a member of the orthopyroxene group of minerals, belonging with enstatite and hypersthene to the orthorhombic series of the group...
picrite.
The Loch Maree Group
Supracrustal rocks of the Loch Maree Group form two large areas of outcrop near Loch MareeLoch Maree
Loch Maree is a loch in Wester Ross in the Northwest Highlands of Scotland. At long and with a maximum width of , it is the fourth largest freshwater loch in Scotland; it is the largest north of Loch Ness. Its surface area is ....
and Gairloch
Gairloch
Gairloch is a village, civil parish and community on the shores of Loch Gairloch on the northwest coast of Scotland. A popular tourist destination in the summer months, Gairloch has a golf course, a small museum, several hotels, a community centre, a leisure centre with sports facilities, a local...
in the Southern Region. The group consists of metasediments with intercalated amphibolite
Amphibolite
Amphibolite is the name given to a rock consisting mainly of hornblende amphibole, the use of the term being restricted, however, to metamorphic rocks. The modern terminology for a holocrystalline plutonic igneous rocks composed primarily of hornblende amphibole is a hornblendite, which are...
s, interpreted to be metavolcanics with some basic sills
Sill (geology)
In geology, a sill is a tabular sheet intrusion that has intruded between older layers of sedimentary rock, beds of volcanic lava or tuff, or even along the direction of foliation in metamorphic rock. The term sill is synonymous with concordant intrusive sheet...
. They were probably deposited at about 2.0 Ga, as they contain detrital zircon
Zircon
Zircon is a mineral belonging to the group of nesosilicates. Its chemical name is zirconium silicate and its corresponding chemical formula is ZrSiO4. A common empirical formula showing some of the range of substitution in zircon is 1–x4x–y...
s that give a mixture of Archaean and Paleoproterozoic ages.
Laxfordian events
The Laxfordian was originally recognised from the presence of deformation and metamorphism of the Scourie dykes. The Laxfordian can be divided into an early event before 1.7 Ga, associated with retrogression of the Scourie gneisses from granulite to amphibolite facies and a later event with local further retrogression to greenschistGreenschist
Greenschist is a general field petrologic term applied to metamorphic or altered mafic volcanic rock. The term greenstone is sometimes used to refer to greenschist but can refer to other rock types too. The green is due to abundant green chlorite, actinolite and epidote minerals that dominate the...
facies, part of which may be Grenvillian
Grenville orogeny
The Grenville Orogeny was a long-lived Mesoproterozoic mountain-building event associated with the assembly of the supercontinent Rodinia. Its record is a prominent orogenic belt which spans a significant portion of the North American continent, from Labrador to Mexico, as well as to Scotland...
in age (about 1.1Ga). The early event is particularly associated with shear zones in which the deformed scourie dykes form amphibolite sheets within the reworked gneisses. The original mineralogy of the dykes is also changed to an amphibolite facies assemblage, even where they remain undeformed. The early Laxfordian fabrics
Fabric (geology)
In geology, a rock's fabric describes the spatial and geometric configuration of all the elements that make it up.-Types of fabric:* Primary fabric — a fabric created during the original formation of the rock, e.g...
are cut by a series of granites and pegmatite
Pegmatite
A pegmatite is a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm in size; such rocks are referred to as pegmatitic....
s, particularly in the Northern and Southern Regions dated at 1.7 Ga.
Lewisian of the Outer Hebrides
Much of the Lewisian outcrop of the Outer Hebrides consist of rocks of the Scourie complex cut by post-Scourian granites. Laxfordian reworking is extensive and very little unmodified Scourian crust has survived. Amphibolite sheets, interpreted to be deformed members of the Scourie Dykes, are much less common than on the mainland. More of the outcrop area consists of supracrustal rocks, about 5% of the total. The relationship between the supreacrustal rocks and the Scourian gneisses remains unclear.South Harris igneous complex
The South Harris igneous complex consists mainly of anorthositeAnorthosite
Anorthosite is a phaneritic, intrusive igneous rock characterized by a predominance of plagioclase feldspar , and a minimal mafic component...
and metagabbro, with lesser amounts of tonalitic and pyroxene-granulite gneisses. These igneous rocks are intruded into the Leverburgh and Langevat supracrustals. Radiometric dating suggests that the complex was intruded over a period from about 2.2–1.9 Ga, comparable to the age of the Loch Maree Group. The Ness Anorthosite, exposed on the northeastern tip of Lewis, is also found associated with metasediments and yields a similar Sm-Nd model age of about 2.2 Ga. It is considered possible that the South Harris and Ness bodies once formed part of a continuous body, disrupted by Laxfordian deformation.
Langevat and Leverburgh metasediments
These two belts of metasediments flank the South Harris igneous complex, and form the largest outcrop of such rocks in the Outer Hebrides. Radiometric dating has shown these metasediments to be of Paleoproterozoic age, similar to the rocks of the Loch Maree Group. The relationship between these metasediments and Scourian gneisses remains unclear.Outer Isles fault zone
This fault zone stretches the entire length of the Outer Hebrides, a distance of about 200 km, dipping 20°–30° to the ESE. The fault rock within the fault zone shows a long and complex history of movement with the development of fault brecciaFault breccia
Fault breccia, or tectonic breccia, is a breccia that was formed by tectonic forces....
, mylonite
Mylonite
Mylonite is a fine-grained, compact rock produced by dynamic recrystallization of the constituent minerals resulting in a reduction of the grain size of the rock. It is classified as a metamorphic rock...
and pseudotachylite
Pseudotachylite
Pseudotachylite is a fault rock that has the appearance of the basaltic glass, tachylyte. It is dark in color and has a glassy appearance. However, the glass has normally been completely devitrified into very fine-grained material with radial and concentric clusters of crystals...
, indicating faulting at a wide range of crustal levels.
Lewisian inliers within the Moine
Despite the multiple reworking that has affected Lewisian like gneisses within the Moine they show evidence of a common history, although with some important differences. The largest, the Glenelg-Attadale inlier, shows evidence of eclogiteEclogite
Eclogite is a mafic metamorphic rock. Eclogite is of special interest for at least two reasons. First, it forms at pressures greater than those typical of the crust of the Earth...
facies metamorphism within both of the tectonically juxtaposed units that make up the inlier, thought to be associated with crustal thickening during a Paleoproterozoic event at about 1.7 Ga and the Grenvillian orogenic event respectively.