2011

Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

ON THE RATE OF RELATIONS OF KIMBERLITES, MELILITITES AND CARBONATITES

E.A. Chernysheva

P.P. Shirshov Institute of Oceanology RAS, Atlantic Branch, Kaliningrad, Russia; elcher37@mail.ru

Ore-bearing intrusive carbonatites have got the rare elements from the primary melilitites via long and complex magmatic and metasomatic transformations, when the rocks have lost the signs of their relations. Geochemical comparison of the carbonatites and their “ancestor” should account  the history therefore.

Experiments proved many times, that the melts of kimberlites, melilitites and carbonatites could be generated at 3-8 GPa, 1390-1800⁰ C by partial melting of the mantle peridotites in the presence of CO₂ [Gudfinnson, Presnall, 2005]. The natural kimberlites and melilitites have signs of deep generated melts enriched in the incompatible trace elements much more than oceanic basalts. The both types of rock have some similarity in composition, areas of their occurring sometimes are disposed very near, or overlapped. So melilitites are accepted sometimes as a low-pressure facies of kimberlites. Geochemical features reflect very low degree of the mantle substance melting and big depth of the melt generation, with a differences between kimberlites and melilitites. Indicator ratios Zr/Nb and La/Yb (as a measure of degree of melting) vary from 1.55-2.4 and usually > 170 in kimberlites [Becker, Le Roex, 2006], when in melilitites these ratios are < 4 and only > 50 (higher degree of melting). In the same way, the values of the ratios Ce/Y and Gd/Yb consist 17.5- 27.5 and 12.6-9.0 in kimberlites and sufficiently less in melilitites 5.7-7.7 and 5-7 (shallower depth of melting).

The primary carbonatite melts, generated simultaneously with silicate melts at the high pressure, can not remain long in the equilibrium with carbonated peridotite because of their high reactivity at any changing of  parameters [Harmer, Gittins, 1998]. In metasomatic reactions carbonate liquids is eliminated and let CO₂  loose. Any new step of the similar reactions is followed by changes of the carbonate and of the coexisting silicates composition, and by new trace elements behaviour. The main “mission” of the primary deep generated carbonatite melts consists in saturation of kimberlitic and melilititic melts by CO₂ , what helps to their transporting to the high levels of the Earth crust [Girnis, Ryabchikov, 2005].

The last reviews on carbonatites [Mitchell, 2005; Woolley, Kjarsgaard, 2008] and our observations [Chernysheva, 2006] show, that from all diversity of endogenous carbonate rocks only two kinds of them are related to deep generated melilitite melts: the  extrusive carbonatites from some young volcanoes in Africa [Bailey et al., 2005], and the intrusive carbonatites in the  ultrabasic alkaline rock complexes (UAC). It is few known yet about “ore” extrusive carbonatites. But intrusive carbonatites in the most remarkable large complexes of Siberia, Kola peninsula, Scandinavia, Africa et al., are well-known and recognizable on their typomorphous paragenesis of ore (oxides of Nb, Ta, Ti, Zr) and other minerals, and represent value source of rare-metal raw material. The carbonatites in the complexes UAC are connected with repeated associations of plutonic alkaline silicate rocks (differentiated intrusions, cumulates, metasomatic rocks), the main of them are alkaline pyroxenites, melilitolites, ijolites, nepheline syenites and so on. They were formed in the middle or shallow depths, they have no xenoliths of mantle rock and no signs of primitive melts because of many transformations, but they are enriched in incompatible trace elements (Ti, Nb, Zr, Sr, Ba, et al., along with P and REE).

 Series of the dykes and diatremes occur in many UAC, with primitive deep generated alkaline picrites and melilitites, what represent straight evidence of their relationship. The volume of these rocks is not large, they mark the late impulses of alkaline magmatism, but it shows continuous tectono-magmatic connection of UAC with parent source (and, may be, with mantle diapir). Constant influx of volatiles and alkalies to the intrusive chamber, provided metasomatic transformations of the rock in UAC, evidences about the same.

The detail researching of the UAC of Siberia have shown, that carbonatitic “plug” or “neck”, usually mapping in the frame of silicate rocks on these massifs, are in reality the filds, composed by large veins or lenses of carbonatites with relict blocks of silicate rocks between. Vein body have a zonality, the character of ores is different in carbonatites of different stages. According to thermometric data, intrusive carbonatites could be crystallized from the melt. Carbonate melt transformed to the fluid and hydrothermal solution on the rate of its cooling; the composition of carbonates and new forming ore minerals were transformed subsequently [Pozharitskaya, Samoilov, 1972].

On researching of the composition and relations of the different rocks in Siberian UAC we have come to conclusion, that the appearance of the early carbonatitic melts here is preferably connected with a widespread process of metasomatic ijolitization of the intrusive melilitolites and early alkaline pyroxenites [Chernysheva et al., 1994; Chernysheva, 2006]. Veins and layer bodies of carbonate-bearing ijolites contain relic blocks of melilitolites and abundant veinlet zone and pools of carbonate, easily migrated in tectonic fissures. The early segregations of carbonatitic melt, coexisting with ijolitic melt,  had a temperature near 650-680⁰ C, but on the rate of its accumulation and cooling below 500⁰ C, the mass crystallization of the carbonatitic ores with pyrochlore occured in the tectonically favourable zones – as a result of destruction of the former existed complexes of Nb [Pozharitskaya, Samoilov, 1972].

So, the most full extraction of the rare elements, at first contained in the primary melilititic melt, is possible only in very long and many-steps metasomatic transformation, when the signs of the rocks relationship are fully lost.

References

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 Pozharitskaya L.K., Samoilov V.S. Petrology, mineralogy and geochemistry of the carbonatites of the Eastern Siberia.  Moskow: Nauka. 1972. 265 p. (in Russian).

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