Thermodynamic model of sulphide formation in the carbonatites of Turiy Alkaline Complex, Kola Peninsula, Russia
✍ Scribed by P.I. Karchevsky
- Publisher
- Elsevier Science
- Year
- 2001
- Tongue
- English
- Weight
- 244 KB
- Volume
- 32
- Category
- Article
- ISSN
- 1464-343X
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✦ Synopsis
Most of the silicate rocks are primitive silica poor (35-48%) and TiO,-rich (2.2-5.2%) and vary from the least silica undersaturated OV to the strongest undersaturation in YV. OV are hy-normative basalts, picro basalts, alkali basalts to tephrites with le+ ne + cs up to 7. The YV has a few primitive tephrites but are mostly nephelinites with le + ne + cs from 12 to 55. Most dykes are tephrites, fractionated to varying degrees and some nephelinites.
A few phonolites were sampled among YV and D. The RV fall among the most undersaturated of YV. The increasing silica undersaturation with time is described by the index Na* [Na,O/(SiO,-3311, which correlates positively with CaO, P,O, and Nb, Ba, Sr and REE among primitive (6% MgO and 10% CaO) rocks.
The carbonatites are bimodally distributed with magnesio-carbonatites, MgO 1 O-20% and Ca* ICaO/ (CaO + MgO + Fe0 + MnO)] 5.4-6.7, calcio carbonatites with Ca* 8.4-9.8, plus two ferro carbonatites. The carbonatites have low Na,O + K,O (< 1 %I, SiO, (O-14%) and there is a positive relation between TiO, and A&O, which suggest some contamination from silicate magma or rocks. Compared to the silicate rocks, the carbonatites are enriched in P,O,, Sr, Ba, Pb, Y and rare earth elements (REE), and are poorer in Zr and mostly also in Nb.
The genesis of the carbonatites is unlikely to be a result of liquid immiscibility because of their low contents of alkalis and the absence of fenitisation on the island. Fractional crystallisation of silicate magma is an unlikely process to create these carbonatites because of the lower concentrations of Zr and Nb, which are incompatible during most crystal fractionation. Preliminary isotopic data on Pb show significantly higher *0sPb/204Pb and lower U/Pb ratios in the carbonatites than in the silicate rocks, which excludes the possibility that the carbonatites are the product of hydrothermal activity. The carbonatites are thought to be primary melts modified by crystal fractionation to various degrees.
The stratigraphical variation of silica saturation and contents of trace elements in the silicate rocks indicate that a decreasing fraction of melt was produced at increasing mantle depth. The stratigraphical position of the carbonatites documents that they are younger than the OV and dykes. This indicates that the source region of the carbonatites were spatially separate from the early silicate magmatism and suggest that the carbonatites were residing in a source region that were heated conductively by the early silicate magmatism. Extrusion of carbonatite would metasomatically line the path, and the evolution of silicate magmas would subsequently be increasingly contaminated by carbonatites that supplies CaO, P,O,, Sr, Ba and REE.