The high plateau of the Cairngorms is Britain’s answer to the Arctic – wild and cold in winter. The mountains are made from granite – found one of the largest exposed plutons in Great Britain. Although the rocks are well-known, the granites still hold tectonic puzzles.
The Cairngorm pluton is one of a series of major granite bodies emplaced into the Grampians around 415-400 million years ago. They form the ‘Newer Granites’ of the Scottish Highlands that collectively brought the curtain down on the Caledonian orogeny in northern Britain. The primary source of these igneous granites was the melting of deep continental crust as a result of the orogeny, but their chemistry also indicates that a significant component came from the mantle and may have been the product of subduction of oceanic crust. However, this raises a problem regarding timing, as the subduction of the Iapetus – the final piece of ocean floor to be consumed as the ancient microcontinent Avalonia finally collided with Laurentia – had finished maybe 20 million years earlier. The origins of the Newer Granites have therefore been widely debated.
One possibility advanced for this apparent delay is that part of the subducted oceanic lithosphere remained in the transition zone between the upper and lower mantle (so-called ‘stagnant slab’) during the closing of the ocean and only later sank further into the mantle. But whatever the explanation, the Caledonian Newer Granites are not a lone example – other orogens also feature late granites, such as in the heart of the mountain chains arising from the Variscan orogeny across western Europe. It seems that the upper mantle may take a long time to settle down after continental collision.
Under most crustal conditions, granite (made up principally of feldspar and quarz) is a strong material compared with the quartz-mica rocks in the surrounding crust. So the addition of granite into developing zones of continental collision strengthens these parts of the crust making them less prone to continued folding of the sort associated with crustal shortening. Interestingly the Newer Granites were emplaced just as crustal shortening in the Caledonian mountain belt ended and the system evolved into so-called ‘wrench tectonics’, typified by strike-slip faults (in which movement takes place along the fault) such as the Great Glen Fault. The question is, was this cause or effect?
Further reading:
Harrison, T.N. 1986. The mode of emplacement of the Cairngorm Granite. Scottish Journal of Geology, 22, 303-314.
An overview of Caledonian igneous activity and its plate setting is provided by:
Stephenson, D. 2000. Chapter 1: Caledonian igneous rocks of Great Britain: an introduction. In: Stephenson, D., Bevins, R.E., Millward, D., Stone, P., Parsons, I., Highton, A.J. & Wadsworth, W.J., (eds), Caledonian Igneous Rocks of Great Britain, Geological Conservation Review Series, No. 17, Joint Nature Conservation Committee, Peterborough, pp. 648, ISBN 1 86107 471 9
Twinned with: New Hampshire Granites, United States
The granite formations of New Hampshire were so historically important to the economy of the state and well known in the region that they gave rise to the state’s nickname ‘The Granite State’. The moniker is attributed to the significant granite number of granite mines ...continue reading