Alnö Complex
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The Alnö Complex or Alnö Alkaline Complex is a group of
carbonatite Carbonatite () is a type of intrusive or extrusive igneous rock defined by mineralogic composition consisting of greater than 50% carbonate minerals. Carbonatites may be confused with marble and may require geochemical verification. Carbonati ...
and alkaline
igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma o ...
s in Alnö in the eastern coast of central Sweden that intruded the basement in Late Ediacaran times. The Alnö Complex is made up by a series of concentric dykes within a radius of 25 km of a main "central complex" of intrusions. In addition the Alnö Complex proper is surrounded by a 500 to 600 m broad zone of
metasomatic Metasomatism (from the Greek μετά ''metá'' "change" and σῶμα ''sôma'' "body") is the chemical alteration of a rock by hydrothermal and other fluids. It is the replacement of one rock by another of different mineralogical and chemical com ...
rock that was formed by metasomatic alteration of the existing Precambrian
migmatite Migmatite is a composite rock found in medium and high-grade metamorphic environments, commonly within Precambrian cratonic blocks. It consists of two or more constituents often layered repetitively: one layer is an older metamorphic rock th ...
gneiss Gneiss ( ) is a common and widely distributed type of metamorphic rock. It is formed by high-temperature and high-pressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. Gneiss forms at higher temperatures a ...
basement. The specific type of metasomatic rock is referred by some authors as "
fenite Fenite is a metasomatic alteration associated particularly with carbonatite intrusions and created, very rarely, by advanced carbon dioxide alteration (carbonation) of felsic and mafic rocks. It is characterised by the presence of alkali feldspar, ...
". The dykes of the complex consist of carbonatite and alkaline rocks such
melilite Melilite refers to a mineral of the melilite group. Minerals of the group are solid solutions of several endmembers, the most important of which are gehlenite and åkermanite. A generalized formula for common melilite is ( Ca, Na)2( Al, Mg, ...
and sövite. It has been proposed that both the
Fen Complex The Fen Complex ( no, Fensfeltet) in Nome, Telemark, Norway is a region noted for an unusual suite of igneous rocks. Several varieties of carbonatite are present in the area as well as lamprophyre, ijolite and other highly alkalic rocks. It is the ...
in Southern Norway and the Alnö Complex formed as consequence to mild extensional tectonics in the ancient continent of
Baltica Baltica is a paleocontinent that formed in the Paleoproterozoic and now constitutes northwestern Eurasia, or Europe north of the Trans-European Suture Zone and west of the Ural Mountains. The thick core of Baltica, the East European Craton, ...
following the opening of the Iapetus Ocean. Harry von Eckermann published a landmark study on the Alnö Complex in 1948 correctly claiming a
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
tic origin of carbonatite, albeit his finds were only widely accepted after the
Ol Doinyo Lengai Ol Doinyo Lengai (Oldoinyo Lengai), "Mountain of God" in the Maasai language, is an active volcano located in the Gregory Rift, south of Lake Natron within the Arusha Region of Tanzania, Africa. Part of the volcanic system of the East African Ri ...
eruption of carbonatite lava in the 1960s showed contemporary evidence on the existence of such magmas. As result of decreased foreign trade during
World War II World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposing ...
apatite was mined from the carbonates of Alnö Complex from 1943 to 1945. Aparite was separated by flotation but results were meager. While the separation process had improved in 1945 the end of the war the same year meant that apatite mining became unprofitable.


References

{{Geology of Fennoscandia Cambrian Europe Cambrian geology Carbonatite occurrences Geography of Västernorrland County Geology of Sweden