The Scottish Geology Trust is keen to offer people involved with Scottish geology the chance to share information about Scotland’s geology with a wider audience.

The Hebridean Terrane – a simplified history of Scotland’s oldest rocks, by Alex G. Neches

Preamble

The concept of millions or billions of years is hard to conceive and even harder to comprehend. Given our very limited existence on this planet, such an extended period of time is almost unimaginable and perhaps frightening. Yet over its long course, evolution yielded an exceptionally complex and diverse world. Modern geology, in its attempt to organise and explain such a wealth of phenomena, abounds with names of rocks, minerals, fossils, processes and events. Like all sciences, it seems to have developed a language of its own, which may sometimes be disheartening and confusing. Novices will be excused for assuming that geologists are rather unsocial individuals who examine with forensic precision extinct continents and oceans and are not very good at sharing their work outside specialised circles. In fact, geoscientists have become more and more aware that geological knowledge remains little understood and appreciated and make sustained efforts to communicate it and ensure it becomes accessible to everyone. The Scottish Geology Festival represents one such example of collective effort to provide a glimpse into the geological history of a very special and geodiverse part of the world, in hope that it will instil a sense of wonder and value for its heritage.

Indeed, few countries in the world have inherited such old and varied landscapes as Scotland, whose geological record spans 3 billion years. This equals 2/3 of the age of planet Earth itself and 1/5 of that of the Universe! It should not be surprising that throughout its existence, Scotland has not always been as we know it and as we see it on maps, but rather ‘slices’ of it evolved independently of one another, sometimes hundreds of kilometres apart, and were gradually assembled and shaped into their current configuration.

Scotland consists of five major ‘slices’ called terranes or crustal blocks. They represent buoyant fragments of Earth’s primeval crust, or lithosphere, that over the course of hundreds of millions of years broke off one continent and often travelled a long way to join another. Each of them formed in unique conditions, preserves a succession of rocks with distinct physical characteristics and holds a key to unravelling the complex and fascinating history of Scotland.

The Hebridean Terrane

Image of Scotland

Map of Scotland (not including Shetland) illustrating the location of major crustal blocks and faults: 1. Hebridean Terrane; 2. Northern Highlands Terrane; 3. Grampian Terrane; 4. Midland Valley Terrane; 5. Southern Uplands Terrane; A. Moine Thrust; B. Great Glen Fault; C.Highland Boundary Fault; D. Southern Uplands Fault; E. Iapetus Suture Basemap source: Pope, A. (2017). SRTM-DEM (90m). EDINA ShareGeo. Dataset derived from NASA/JPL/NGA/DLR/ASI

The first and oldest of Scotland’s crustal blocks does not represent a single, compact mass, as was believed for many years. Instead, it was formed by gradual assemblage of smaller blocks, most of which formed in the Archaean after Earth’s surface had cooled enough following its first five hundred million years of existence. The rest formed in Palaeoproterozoic as continental volcanic arcs, whose development preceded and accompanied the formation of a supercontinent, Columbia.

The Hebridean Terrane is famous for containing the oldest rock formations in Scotland and some of the oldest in the world. It is a playground for scientists attempting to solve an ancient puzzle: reconstructing the exact sequence of events in the distant past – an immense period still shrouded in mystery, the Precambrian.

These famous rocks, known as the Lewisian (Gneiss) Complex, represent a reworking of previous rocks, some of which date back as far as 3.2 billion years. They occur in all of the Outer Hebrides, including part of the submerged continental shelf, the northern Inner Hebrides plus the islands of Coll and Tiree, and a narrow belt of mainland between Cape Wrath and Loch Torridon. The Complex was later eroded and overlain in part by younger sediments.

The original rocks, also called parent rocks or protoliths, were long thought to be sedimentary rocks and lavas. It is now known that at various intervals during Archaean, magma with different chemical compositions rose from inside the Earth, intruded a number of existing crustal fragments, cooled down and crystallized into solid bodies called plutons. These were later buried, squeezed and deformed by metamorphic events separated by episodes of magmatism.

The first event, the Badcallian, was rather intense. Parent rocks were exposed to high pressures and heat, but not necessarily great depths. The second event, the Inverian, was less intense; conditions of medium pressure and heat prevailed. This is how gneisses originated. These events may not have affected all of the Lewisian Complex and some crustal fragments may not have been affected at all. The Lewisian Complex is, after all, a general name for rocks that formed in different locations, at different times and in different settings.

Lewisian Gneiss

Lewisian Gneiss (grey-pink colour) intruded by Scourie dykes (black colour), Isle of Lewis, Outer Hebrides. Gneiss is an easily recognizable rock, with alternating bands of light and dark minerals, as seen in the section between the dykes. When heated and folded, the bands exhibit a wave pattern, as seen in the lower left foreground. Image source: Plate 5. Johnstone, G. S. and Mykura, W. 1989. British regional geology: Northern Highlands of Scotland. 4th edition. Keyworth, Nottingham: British Geological Survey Copyright of UKRI-NERC-BGS (P008263)

The Badcallian and Inverian events were followed by rifting and magmatic infusions. The continental crust was being pulled apart while magma rose once again from inside the Earth. This magma was rich in magnesium, iron and calcium, was very hot and fluid and filled pre-existing fissures and cracks in the recently-formed gneisses, after which it cooled and solidified into a suite of rocks that geologists named the ‘Scourie dykes’. Recent research disproved long-held assumptions that these represent a single group, but four individual groups that formed in at least a couple of distinct episodes, all throughout early Palaeoproterozoic.

In mid-Palaeoproterozoic times, formation of a new supercontinent – Columbia – began. A chain of volcanoes developed along areas of subsidence. These are areas where the denser oceanic plates, rich in magnesium and iron, were pushed underneath the lighter continental plates, composed of silica and quartz. The chain of volcanoes, also referred to as continental arcs, eventually collided with margins of neighbouring continents, which resulted in additional deformation events.

These events are known as the Laxfordian and Somerledian and were both characterized by medium-grade metamorphism. Part of the gneisses and dykes were exposed to medium pressures and heat, were strained and folded. The Laxfordian has only been recorded in crustal fragments of mainland Scotland. The Somerledian occurred in both mainland Scotland and the Outer Hebrides. These events marked the final assemblage and stabilization of the Lewisian Complex, which at the time was part of the supercontinent Columbia.

Precambrian Timeline illustrating major events in the formation of the Lewisian Gneiss Complex (dates are approximate): P. formation of parent rocks; B. Badcallian event; I. Inverian event; S. Scourie dykes; M+C.a. Magmatism and formation of continental arcs; C. gradual assemblage of Columbia; L.+S. Laxfordian and Somerledian events. Timeline abbreviations: Cr. Cryogenian; Ed. Ediacaran

Geological knowledge, much like the shape of continents and oceans, is very malleable; it can change in rapid and unpredictable ways and it sometimes happens that well-established facts are undermined by new evidence.

Many years ago, when sophisticated instruments were not available, it was assumed, based on observed rock characteristics, that the Lewisian Complex was homogenous across all of the Hebridean Terrane. Reconstructed chronology using modern instruments and refined techniques is far more accurate and detailed, but local ages and events cannot be generalized. The Outer Hebrides, for example, and most of the Isle of Lewis, are very different from mainland Scotland. This may seem counter-intuitive, since the whole Complex owes its name to this very island. It will take years and the cumulated effort of many scientists to finally assemble a complete picture of the geological history of the Lewisian Complex, which in a perhaps ironic coincidence, proves to be far more complex than originally thought.

Alex G. Neches

Based on information from the following sources:

Chew, D.M. and Strachan, R.A., 2014. The Laurentian Caledonides of Scotland and Ireland. Geological Society, London, Special Publications, 390(1), pp.45-91

Friend, C. and Kinny, P., 2001. A reappraisal of the Lewisian Gneiss Complex: geochronological evidence for its tectonic assembly from disparate terranes in the Proterozoic. Contributions to Mineralogy and Petrology, 142(2), pp.198-218

Goodenough, K.M., Krabbendam, M., Shaw, R.A. and Crowley, Q.G., 2013. Making and breaking Columbia (Nuna): formation of a critical metal province? A North Atlantic view. [Lecture] In: Building Strong Continents, Portsmouth, UK, 2-4 Sept 2013

Goodenough, K., Krabbendam, M. and Crowley, Q., 2013. Overview of the Proterozoic evolution of the Lewisian Gneiss Complex, Scotland-constraints from the SE corner of Laurentia. Geophysical Research Abstracts, 15, EGU2013-8058, EGU General Assembly 2013, 7-12 April, Vienna, Austria

Goodenough, K.M., Crowley, Q.G., Krabbendam, M. and Parry, S.F., 2013. New U-Pb age constraints for the Laxford Shear Zone, NW Scotland: Evidence for tectono-magmatic processes associated with the formation of a Paleoproterozoic supercontinent. Precambrian Research, 233, pp.1-19.

Goodenough, K.M., Macdonald, J.M., Johnson, T.E., Hughes, H., Shaw, R.A. and Millar, I., 2014. Tectonic history and mineralisation in the North Atlantic Craton: a view from Scotland. [Lecture] In: North Atlantic Craton Conference, St Andrews, UK, 19-21 March 2014

Hughes, H.S., McDonald, I., Faithfull, J.W., Upton, B.G. and Downes, H., 2015. Trace-element abundances in the shallow lithospheric mantle of the North Atlantic Craton margin: Implications for melting and metasomatism beneath Northern Scotland. Mineralogical Magazine, 79(4), pp.877-907

Imber, J. and Strachan, R. A. and Holdsworth, R. E. and Butler, C. A. (2002) ‘The initiation and early tectonic significance of the Outer Hebrides Fault Zone, Scotland.’, Geological magazine., 139 (6). pp. 609-619

Kinny, P.D., Friend, C.R.L. and Love, G.J., 2005. Proposal for a terrane-based nomenclature for the Lewisian Gneiss Complex of NW Scotland. Journal of the Geological Society, 162(1), pp.175-186

Love, G.J., Friend, C.R.L. and Kinny, P.D., 2010. Palaeoproterozoic terrane assembly in the Lewisian Gneiss Complex on the Scottish mainland, south of Gruinard Bay: SHRIMP U–Pb zircon evidence. Precambrian Research, 183(1), pp.89-111

Park, R.G., Stewart, A.D., Wright, D.T. and Trewin, N.H., 2002. The Hebridean terrane. In Trewin, N.H. (Ed.) The Geology of Scotland, 4th Edition, Geological Society, London, 45, pp.45-80

Park, R.G., 2005. The Lewisian terrane model: a review. Scottish Journal of Geology, 41(2), pp.105-118

Spencer, C.J., Cawood, P.A., Hawkesworth, C.J., Prave, A.R., Roberts, N.M., Horstwood, M.S. and Whitehouse, M.J., 2015. Generation and preservation of continental crust in the Grenville Orogeny. Geoscience Frontiers, 6(3), pp.357-372

Trewin, N.H. and Rollin, K.E., 2002. Geological history and structure of Scotland. In Trewin, N.H. (Ed.) The Geology of Scotland, 4th Edition, Geological Society, London, 45, pp.1-25