Carbonatites at the Alnö Complex, Sweden and along the East African Rift : a literature review

University essay from Lunds universitet/Geologiska institutionen

Abstract: Carbonatite is an unusual type of rock that, for a long time, were not accepted for what they really are. This group of rocks is now considered to be of igneous origin and is classified after its content of carbonate minerals. If it exceed 50% the rock will be named a carbonatite. This group of rocks can be found all over the world and two of these places, which this review will focus on, are the Alnö island of Sweden and the East African Rift. The alkaline and carbonatite complex of Alnö lays on the north part of the island and shows several intrusions in the surrounding migmatitic gneiss and granite. The complex holds many of the features that are expected to be found around carbonatites, for example signs of fenitization processes. The carbonatites appear mostly as dykes and are named sövite, alvikite and beforsite. The complex has been dated to be around 560-580 million years old and there are theories that the complex is formed in a similar way of a caldera. The East African Rift is an active place where the continental plate is being split apart. Volcanic activity shapes the landscape along with faults and graben structures. As with Alnö, the rift contains carbonatites, but one thing that is different, is that the rift holds a very unique version of carbonatites which are known as natrocarbonatites. These rocks are produced by the volcano of Oldoinyo Lengai in Tanzania and have not been found anywhere else in the world. A common feature for both these places is that, although much is known about carbonatites, no one knows for sure where they come from and how they are being formed.

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