#8 Britain’s Only Carbonatite

(A brief introfuction to the Loch Borralan Carbonatite)

Between the NE shore of Loch Urigill and Loch Borralan lies Britain’s only carbonatite pluton. The presence of a carbonatite in the Assynt area was first reported to the geological community by Young, Parsons and Threadgould in 1994 . The pluton was actually discovered by an undergraduate from the University of Aberdeen in the late 1980s who was mapping in the area. He puzzled over the rocks and eventually was bold enough to confront his supervisors about it. Sure enough, his speculation was soon corroborated and a fuller investigation was launched.

So what is known about the carbonatite? Well there has been relatively little work conducted on it since it was reported in the Journal of the Geological Society of London in 1994. It was reported that there are 4 varieties that occur:

1. porphyritic white sövite
2. phlogopite sövite
3. sövite breccia
4. foliated silicocarbonatite

The latter of three of the four types above were only observed in situ after part of the pluton was excavated. Since then mineralogical and whole rock analyses were conducted on the four different lithologies. Those chemical analyses revealed that the Loch Borralan carbonatite is chemically and isotopically (carbon) distinct from the surrounding Durness Dolomites in which the pluton is enveloped.

There are many unanswered questions about the pluton. Amazingly, there has been little interest in the body since the mid 1990s. I visited the carbonatite earlier this summer, and collected a sample of the porphyritic sövite. A thin section of this has revealed some interesting minerals which I am in the process of having analysed. Hopefully a report into further findings in the mineralogy of the body will follow some time next year.

A sample of Sövite from the Loch Borralan Carbonatite, showing well equilibrated calcites amongst some other as yet unidentified minerals.

#7 The Kerlingarfjöll Mountains of Central Iceland

The geothermally active Kerlingarfjöll mountains in Central West Iceland.

The Kerlingarfjöll mountains are located in the Central Highlands of Iceland. The Kerlingarfjöll massif was formed asa tuya volcano erupted under more than 550 m of ice (see research by Dr. John Stevenson), and is mostly composed mostly of intermediate to evolved  volcanic rocks such as andesite and rhyolite. The ground surface is mostly gravel, with very little exposed bedrock. This is because of prolific weathering via freeze-thaw which has acted on the mountains since their exposure after the retreat of the glacier which once covered the area. Evidence for these glaciers is provided in the form of glacial clays found in valleys between dolerite ridges, more so in the north of the plateau. There is also evidence that there was once an ice-dammed lake that stood for long enough for a delta to develop inside, a cross section of which can be seen by the riverside at the Kerlingarfjöll camping site.

Seen in the picture above, hot spring valleys cut the mountain range from west to east and give entire mountain sides a rich palette of autumn oranges and browns set agains the white and blue of the snow, ice and hot spring water. For more information about the geology of this wonderful area of Iceland please consult the following links:

Report – A contribution to the Geology of Kerlingarfjöll (1946)

Paper – Stevenson et. al., 2009

Paper – Stevenson et. al., 2011