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The oldest rocks in the British Isles are to be found in NW Scotland, forming the ancient basement to the sedimentary rocks - Torridonian, Cambrian and Moine. These old rocks are a complex arrangement of metamorphic and igneous rocks collectively termed the Lewisian. They tell a story not only of how continental crust is formed but also give insight on the processes that happen 30 km down in the continental crust.
The oldest part of the Lewisian complex is the Scourie Block. Current understanding suggests this became a broadly stable part of the continental crust by about 2600 million years ago, at the end of the Archaean era. Most of the rocks are banded grey gneisses, generally thought to be the deformed and metamorphosed equivalent of extensive plutons. There are thin strips of material that are thought to be of metasedimentary origin. Some of these may represent the material into which the early plutons were intruded. However intense deformation has obliterated all the evidence of this. But what is clear is that the early history of the Lewisian is one of growing continental crust - the growth caused by the repeated injection of magma. The most likely place for this to have happened is above a subduction zone, either as part of a volcanic island arc (e.g. like modern-day Japan) or in an Andean setting. These sites are the factories of continental crust in the modern world. The chemistry of the Lewisian gneisses is in broad agreement with this tectonic model for their origin. The oldest material probably dates from about 3 billion years ago with extensive deformation happening at about 2750 million years ago (an episode called the "Badcallian"). The associated metamorphism was under so-called granulite facies conditions (11-15 kbar, 950-1000 C) . This equates to depths of 35-50 km - (over the normal thickness of continental crust). Granulites are characterised by being anhydrous - all the water has been driven off, commonly along with small volumes of melt. So the Lewisian tells a story of ancient crustal growth and thickening.
crust can also grow through the addition of magma during rifting. In these situations the magma generally has
a basic composition (i.e. it is basaltic) and very commonly comes in batches intruded along cracks, preserved
geologically as dykes. This type of geology is found within the Lewisian - which is intruded by widespread NW-SE
trending basic sheets - the so-called Scourie dykes. In the Scourie block these dykes preserve igneous textures
- although commonly they contain granulite-facies metamorphic minerals. Elsewhere the dykes have been metamorphosed
into amphibolites. The age of these intrusions has been controversial but it now seems clear that they were intruded
over perhaps a 400 million year period, ending about 2000 million years ago.
The history of the Lewisian might seem complex but it didn't end there. Large tracts of gneisses with Scourie dykes have been strongly deformed and metamorphosed and amalgamated with other crustal units. In general the metamorphic grade is lower than within the Scourie block, characterised by amphibolite facies. In places the gneisses are invaded by granitic intrusions including coarse-grained pegmatites. It is worth noting however that most of these appear to have been produced by melting pre-existing continental crust. So these late intrusions represent a remobilisation of existing material rather than true growth of new crust. All this activity of deformation, metamorphism and magmatism is generally termed the Laxfordian. The best estimates at present suggest that this happened around 1600 million years ago. It certainly was all over well before the first Torridonian sediments were deposited on Lewisian. Before this could happen the Lewisian must have stopped deforming, been uplifted and eroded.
Find out more about the formation of continental crust.
The Lewisian opens a window on how the continental crust deforms at depth. We can understand how the crust deforms near the surface today by studying earthquakes and measuring movements directly. But in areas of active deformation within the continents earthquakes are restricted to the top 10 to 12 km. What does this tell us?
Earthquakes tell us that rocks are breaking - the formation and movement on faults. But just because there are few deep earthquakes in the continents does not mean that the deep levels are not deforming. Rather than breaking the rocks are flowing - albeit very slowly. The Lewisian shows us what happens.
The northern edge of the Scourie Block is a structural zone called the Laxford Front. This is the transition into widespread Laxfordian deformation. From the Laxford front north to the coast at Durness all the Scourie dykes have been extensively sheared. So too are the gneisses. In general everything is smeared out to become parallel. Then this parallel fabric is folded into broad structures. So from this we can deduce two things. First, the banding in the gneisses is primarily due to deformation. It does not represent primary layering. Second, the deformation in the Lewisian crust during the Laxfordian was very widely distributed. It looks like all the deep crust was flowing - rather like toffee.
It used to be thought that the tract of strongly Laxfordianised gneisses north of the Laxford Front were merely the deformed equivalent of the Scourian gneisses found to the south. However, modern geochemical studies suggest that the two sets of gneisses were derived from continental crust of distinctly different age. Furthermore the northern units contain not a trace of the granulite facies metamorphism that characterises the Scourie block. So perhaps the intense shearing relates to displacements that have brought together distinct crustal units. It is interesting to note that almost all the shearing has been taken up by the northern gneisses with little happening to the Scourie unit. But this might be due to their geology. Dry granulites are amongst the strongest rocks and so tend to resist deformation. In contrast "wetter" amphibolites are much weaker. So it is no surprise that the intense shearing is concentrated in the weaker material.
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