Lewisian complex
The Lewisian complex or Lewisian gneiss is a suite of Precambrian metamorphic rocks that outcrop in the northwestern part of Scotland, forming part of the Hebridean Terrane and the North Atlantic Craton. These rocks are of Archaean and Paleoproterozoic age, ranging from 3.0–1.7 billion years (Ga). They form the basement on which the Stoer Group, Wester Ross Supergroup and probably the Loch Ness Supergroup sediments were deposited. The Lewisian consists mainly of granitic gneisses with a minor amount of supracrustal rocks. Rocks of the Lewisian complex were caught up in the Caledonian orogeny, appearing in the hanging walls of many of the thrust faults formed during the late stages of this tectonic event.
Distribution
The main outcrops of the Lewisian complex are on the islands of the
History of study
The first comprehensive account of the Lewisian complex was published in 1907 as part of the
Lewisian of the Scottish mainland
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
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 pegmatites 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.[1] 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.[1] Major Inverian shear zones have been identified in the Central and Southern Regions, including the Canisp Shear Zone.
Scourie dykes
This basic
The Loch Maree Group
Supracrustal rocks of the Loch Maree Group form two large areas of outcrop near
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 greenschist facies, part of which may be Grenvillian in age (about 1.1Ga).[1] 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 pegmatites, 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.[1]
South Harris igneous complex
The South Harris igneous complex consists mainly of anorthosite 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.[1]
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.[1]
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
Lewisian inliers within the Morar Group and Loch Ness Supergroup
Despite the multiple reworking that has affected Lewisian-like gneisses within the metasediments of these two sequences, they show evidence of a common history, although with some important differences. The largest, the
See also
- Tonalite–trondhjemite–granodiorite – Intrusive rocks with typical granitic composition
References
- ^ ISBN 978-1-86239-126-0. Retrieved May 23, 2010.
- ^ C.Michael Hogan. 2011. Sea of the Hebrides. Eds. P.Saundry & C.J.Cleveland. Encyclopedia of Earth. National Council for Science and the Environment. Washington DC
- ISBN 978-0-632-01605-1.
- S2CID 14471764. Retrieved 15 September 2012.
- ^ Peach, B.N.; Horne, J.; Gunn, W.; Clough C.T.; Hinxman L.W.; Teall J.J.H. (1907). The Geological Structure of the NW Highlands of Scotland. Memoirs. Geological Survey of Great Britain. p. 668.
- S2CID 129572998. Retrieved 23 May 2010.
- ^ S2CID 129375227.
- S2CID 129960850.