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Abstracts of International conference
A Method for Petrochemical Differentiation of Kimberlites from Other Igneous Carbonatite Associations
V.B. Vasilenko and L.G. Kuznetsova
Sobolev Institute of Geology
and Mineralogy, Siberian Branch of the Russian Academy of Sciences,
The tenuous view of carbonatites as rocks whose rock-forming minerals include nepheline is common even in experts. An example is the attribution of the Seligdar apatite deposit to the gneiss–marble association (Mineralogical Map of the USSR, 1985). Our studies have proven that this deposit belongs to carbonatites (Vasilenko et al., 1982, 1994). The conclusion of the carbonatite nature of Seligdar apatites agrees with the data reported by Kukharenko (1966), who has shown that carbonatites are typical of platforms and regions of completed orogeny joined to platforms. There are two subfacies of platform carbonatites in central-type complexes: kimberlite (presumably deeper) and alkaline–ultrabasic (less deep). The carbonatites of completed orogeny regions are associated with rocks of the alkaline–gabbro association. The depth of associated rocks is of great importance for carbonatite identification. We emphasize this statement because alkaline–ultrabasic magmas formed at depths of tens of kilometers, and kimberlites, at hundreds of kilometers.
An important feature of the rocks of these associations is that carbonate and silicate rocks are complementary. An excess of alumina and alkalis in rocks of alkaline–ultrabasic associations is compensated by the deficiency of calcium in silicate rocks. The excess of calcium in kimberlites is compensated by its deficiency in silicate rocks. It is pertinent to consider the classification of peridotites and ultrabasic foidolites (Bogatikov et al., 1981). The former are classified mainly according to CaO content, and the latter, according to Al2O3. In both cases, the high chemical affinity of calcium to carbonate is reflected. Uniform calcium leaching reactions occurred in peridotites and alkaline basaltoids saturated with aqueous–carbonate fluids of mantle plumes. At moderate pressures, the immiscibility of silicate and carbonate melts was apparent, but at high pressures, this phenomenon was absent. Nevertheless, all rock types contained silicate rocks, rocks transitional to carbonate, and carbonate rocks.
Table 1. Mean rock compositions.
* Here and in the following tables: n, number of analyses; U/b, ultrabasic; 1 authors' collection; 2 V. S. Samoilov (1977).
The approach to the petrographic identification of rocks from different depths reduces itself to the skill of discrimination of kimberlite rocks from other rock types. Mean compositions of various carbonatites and kimberlites are shown in Table 1. At the first glance, carbonatites are characterized by elevated phosphorus contents, but it is not true in some cases. Comparison of correlations of CaO with Al2O3 and MgO (Table 2) is less ambiguous. Carbonatites generally demonstrate a negative correlation between CaO and Al2O3, which illustrates the degradations of feldspars of alkaline gabbroids. Kimberlites and other alkaline picrite types are universally characterized by a negative correlation between CaO and MgO, resulting from clinopyroxene cotectics melting. Our recommendations can be tried out. If we find a negative CaO–MgO correlation in a new pipe with the composition close to the compositions of other pipes, it will bring us to the conclusion that these rocks belong to the alkaline picrite family, which, in addition to kimberlites, includes other types of alkaline picrites. By comparison of TiO2 contents in rocks under study with the limits of TiO2 contents in picrites of other types, we determine whether the rocks belong to kimberlites.
Let us apply this approach to the hypothesis that diamondiferous lamproites belong to kimberlites. This hypothesis must be rejected for both leucite and olivine lamproites, because they are characterized by a highly negative correlation between Al2O3 and MgO. Such correlations are most typical of oceanic submelapicritoids (Vasilenko et al., 1994). It is reasonable to suggest that lamproites arose in magma formation zones containing oceanic crust fragments enriched in high-potassium sedimentary matter.
Table 2. Correlation profiles of major oxides. Correlation coefficients (Ð = 0.99) between the contents of CaO and other rock-forming oxides
Table 3. Examples of petrochemical reconstructions
3 Jaques A.L. et al. (1986).
Vasilenko V.B., Kuznetsova L.G., and Kholodova L.D. Apatitic rocks of Seligdar. Novosibirsk: Nauka. 1982 [in Russian].
Vasilenko V.B., Zinchuk N.N., Kuznetsova L.G., and Serenko V.P. Petrochemistry of subalkaline carbonatite-containing associations in Siberia. Novosibirsk: Nauka. 1997 [in Russian].
Jaques A. L., Lewis J. D. and Smith C. B. The kimberlites and lamproites of Western Australia: Geological Survey of Western Australia Bulletin. 1986.
Bogatikov O.A., Gon'shakova V.I., Efremova S.V., et al. Classification and nomenclature of igneous rocks. Moscow: Nedra. 1981 [in Russian].
Kukharenko A.A. On the nature of carbonatite origin, in: Proceedings of the 2nd conference on wall-rock metasomatism. Leningrad: Leningrad State University. 1966, pp. 34–47 [in Russian].
Note on the Mineragenic map of the USSR. Phosphate materials. Leningrad: VSEGEI. 1985 [in Russian].
Samoilov V.S. Carbonatites. Moscow: Nauka. 1977 [in Russian].