Sokli Carbonatite

The Kola Alkaline Province (KAP) covers an area of approximately 100.000 km2, and is one of a few regions in the world where the Paleozoic ultramafic-alkaline magmatism is well developed. There are more than 20 massifs of the Paleozoic ultramafic and alkaline rocks in the KAP (Fig.1), and sixteen of them contain carbonatites.
A characteristic feature of the Kola carbonatites in many complexes is their close relation with phoscorites, rocks consisting essentially of apatite, magnetite and forsterite or diopside (± phlogopite). Phoscorites and the paired carbonatites generally show an extremely intricate association (at the meter scale) and share the same mineral assemblages which evolve from stage to stage.

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Fig.1: Map of NW Europe showing the location of the Sokli carbonatite intrusion (red dot) and the other carbonatites of the KAP(Kola Alkaline Province) . Modified after Vartiainen, H., (1976).



Sokli complex
The Sokli complex is located in the eastern Finish Lapland and intruded, into Archean rocks, at about 360 Ma. It was discovered in 1967 by airborne geophysical survey and its geological structure and petrology were established by extensive campaigns of drilling and trenching with detailed geophysical investigations. Satellite pictures of central Lapland indicate that the Sokli intrusion lies on a major fault which also intersects the Kovdor carbonatite in the Kola Peninsula.

The dominant rocks of the area are gneissose granites and associated pegmatites, but there are also extensive areas of amphibolite and hornblende schist occurring in a broad zone extending north eastwards across the eastern part of the complex and ultramafic rocks and syenite. The contact relationships between these ultramafic masses and the surrounding rocks are everywhere obscured by glacial deposits.
The plug occupies an oval area covering some 20 km2 (Fig.2), and thus is one of the largest carbonatites known. The overall outline of the complex is a vertical pipe with about 6.4 km in diameter on the surface level and walls steeply dipping inward (Fig.3).
Overlying part of the western and eastern side of the carbonatite and the innermost fenites is a phosphate-rich regolith which is probably a product of complex weathering processes of the carbonatite. Carbonatite also forms dykes, veins and veinlets which penetrate the fenites up to 1.3 km from the contact. These carbonatite dykes probably form tangential sheets, probably similar to the system of cone-sheets (deseribed also in the Alnö complex).
The carbonatite intrusion comprises essentially two groups of rock types: sövites and phoscorites. The sövites consist of Calcite, mica (mainly phlogopite), amphibole (variously tremolite or a blue alkaline type), serpentine, apatite and opaque minerals. Texturally are generally fine to medium grained, massive rocks. Dyke rocks tend to be fine to medium grained and relatively massive. The phoscorites occur as fragmental rocks within sövites. They are characteristically coarse grained massive rocks of variable composition. The principal minerals are magnetite, green and red brown phlogopite, apatite, calcite and dolomite. Where the sulphides exceed 10%, the rocks are distinguished as sulphide phoscorites.

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Fig.2: Sokli complex.



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Fig.3: The structure of Sokli carbonatite massif (surface and cross section) and rock types.





Sample doned by Anton Chakhmouradian (University of Manitoba)

Bibliography



• Vartiainen, H., & Woolley, A. R. (1976). The Petrography, Mineralogy, and Chemistry of the Fenites of the Sokli Carbonatite Intrusion, Finland. Geologinen tutkimuslaitos.
• Lee, M. J., Lee, J. I., Garcia, D., Moutte, J., Williams, C. T., Wall, F., & Kim, Y. (2006). Pyrochlore chemistry from the Sokli phoscorite-carbonatite complex, Finland: implications for the genesis of phoscorite and carbonatite association. Geochemical Journal, 40(1), 1-13.

Photo
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Phlogopite (brown), calcite, apatite (high relief, colorless) and magnetite (black) crystals in a carbonatite from Sokli carbonatite complex. PPL image, 1x (Field of view = 9mm)
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Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 1x (Field of view = 9mm)
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Phlogopite (brown), calcite, apatite (high relief, colorless) and magnetite (black) crystals in a carbonatite from Sokli carbonatite complex. PPL image, 1x (Field of view = 9mm)
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Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 1x (Field of view = 9mm)
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Phlogopite (brown), calcite, apatite (high relief, colorless) and magnetite (black) crystals in a carbonatite from Sokli carbonatite complex. PPL image, 1x (Field of view = 9mm)
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Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 1x (Field of view = 9mm)
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Phlogopite (brown), calcite, apatite (high relief, colorless) and magnetite (black) crystals in a carbonatite from Sokli carbonatite complex. PPL image, 1x (Field of view = 9mm)
sokli(21).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 1x (Field of view = 9mm)
sokli(22).jpg

Phlogopite (brown), calcite, apatite (high relief, colorless) and magnetite (black) crystals in a carbonatite from Sokli carbonatite complex. PPL image, 1x (Field of view = 9mm)
sokli(23).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 1x (Field of view = 9mm)
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Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(30).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 2x (Field of view = 7mm)
sokli(31).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(32).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 2x (Field of view = 7mm)
sokli(33).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(34).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 2x (Field of view = 7mm)
sokli(35).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 2x (Field of view = 7mm)
sokli(36).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 2x (Field of view = 7mm)
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Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(25).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 2x (Field of view = 7mm)
sokli(26).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
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Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(5).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(6).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 2x (Field of view = 7mm)
sokli(7).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(8).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
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Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(10).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
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Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 10x (Field of view = 1mm)
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Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 10x (Field of view = 1mm)
sokli(27).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 2x (Field of view = 7mm)
sokli(28).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(37).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(38).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. XPL image, 2x (Field of view = 7mm)
sokli(39).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) and magnetite crystals in a carbonatite from Sokli carbonatite complex. PPL image, 2x (Field of view = 7mm)
sokli(13).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) crystals in a carbonatite from Sokli carbonatite complex. XPL image, 10x (Field of view = 2mm)
sokli(44).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) crystals in a carbonatite from Sokli carbonatite complex. XPL image, 10x (Field of view = 2mm)
sokli(45).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) crystals in a carbonatite from Sokli carbonatite complex. XPL image, 10x (Field of view = 2mm)
sokli(46).jpg

Phlogopite (high birefringence), calcite, apatite (I order gray) crystals in a carbonatite from Sokli carbonatite complex. XPL image, 10x (Field of view = 2mm)