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Ophiolite complexes represent slices of upper mantle and oceanic crust that have been thrust onto continental crust during subduction. In Scotland there are three such complexes: Ballantrae, the Highland Border Complex (e.g. at Loch Lomond) and in Shetland – all formed during the Caledonian orogeny approximately 500 million years ago. The Shetland ophiolite covers much of the islands of Unst and Fetlar and is perhaps the best of the Scottish ophiolites. This includes a classic exposure of the transition between mantle and crust.
No topmost pillow lavas or sedimentary rocks are found in Shetland, but the lower parts of a classic ophiolite sequence are well exposed. Two tectonic units have been identified: the Upper and Lower Nappes, each underlain by imbricates of metasediments and metavolcanics. The Upper Nappe is composed mostly of metaharzburgite of mantle origin, while the Lower Nappe consists of layers of much of the classic sequence: metaharzburgite, metadunite, metagabbro, wehrlite and sheeted dyke complex.
The mantle-crustal boundary is best exposed on the east side of the island. Here, the following sequence can be identified:
- Metadunite: transition zone between Geophysical Moho and Petrological Moho (base of the crust)
- These rocks are serpentinised and chromitised (the chromite is found in podiform masses which are now mostly quarried)
- Petrological Moho
- Metaharzburgite: uppermost mantle
- These rocks are serpentised with accessory chromite
- Includes pods and sheets of metadunite, showing rhythmic banding and pyroxenite veins
The exact origins of the metadunite zone and its inclusions into the metaharzburgite is as yet undetermined.
Keen of Hamar National Nature Reserve – an exposure of the Unst ophiolite. This serpentinite rock once formed the floor of an ancient ocean Scotland’s National Nature Reserves.© Scottish Natural Heritage.
Trewin, N. H. (ed.) 2002. The Geology of Scotland. The Geological Society, London.