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Unst
Ophiolite complexes represent slices of upper mantle and oceanic crust that
have been thrust onto continental curst 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 rythmic banding and pyroxenite
veins
The exact origins of the metadunite zone and its inclusions into the metaharzburgite
is as yet undetermined.
An exposure of the Unst ophiolite in the Keen of Hamar
National Nature Reserve. This serpentinite rock once formed the floor of
an ancient ocean Scotland's
National Nature Reserves.
Image provided by SNH
Further reading:
Craig, G.Y. 1991. Geology of Scotland. 3rd edn. The Geological Society, London.
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