2011

Òåçèñû ìåæäóíàðîäíîé êîíôåðåíöèè

Ðóäíûé ïîòåíöèàë ùåëî÷íîãî, êèìáåðëèòîâîãî

 è êàðáîíàòèòîâîãî ìàãìàòèçìà

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Petrology of the Elet’ozersky Alkaline—Mafic Layered Massif, Karelia: New Data

Chistyakov A.V.

Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of

Sciences, Staromonetnyi per. 35, Moscow, 119017 Russia

e-mail:chist@igem.ru

Syenite—gabbro layered massifs were formed at the territory of the Eastern Baltic Shield in the Early Paleoproteorzoic (around 1.9 Ga). The largest of them are the Elet’ozersky and Gremyakha Vyrmes Massifs at the Karelian and Kola cratons, respectively (Fig. 1, inset).

The massifs of this type are promising for ilmenite—Ti-magnetite mineralization. This work represents the results of studying the layered series of the Elet’ozersky Massif, which is of great interest in terms of petrology and ore potential, but due to its weak exposure remains poor studied.

The Elet’ozersky Massif has a concentrically—zoned structure (Fig. 1). The peripheral part is mainly made up of the compositionally diverse gabbroids, the fine-grained varieties of which in the near-contact zones are subsequently replaced inward by the medium- and coarse-grained rocks (Fig. 1). The inner part of the massif comprises the rocks of differentiated series including olivine gabbros, peridotites, pyroxenites, and gabbroids (from leucogabbro to anorthosites), as well as ore-bearing varieties of these rocks. The core of the massif is represented by nepheline syenites.

The study of the rock-forming minerals (olivine, clinopyroxene, and plagioclases) revealed their steady composition. Plagioclases frequently demonstrate the presence of normal zoning, which is expressed in the core-to-rim decreasing anorthite content by 3-4%, whereas clinopyroxene has practically invariable composition. In composition, Cpx corresponds to diopside, while plagioclases are plotted in bytownite field (Fig. 2). The composition of olivine corresponds to Fo@65-70.  

All the rocks of the differentiated (layered) series are variably enriched in Ti-magnetite and (or) ilmenite, content of which is around 10 wt % and reaches up to 30-40 wt % only in the ore varieties. Sometimes, ore minerals form complex intergrowths. Ilmenite contains around 50 wt % TiO₂ and 5.7 wt % V₂O₃.

Mafic minerals are often surrounded by reaction rims mainly secondary amphibole in composition.

In the AFM diagram (Fig.3), all studied rocks of the Elet’ozersky Massif form a compact cluster in the tholeiite field, practically separately from the rocks of the layered series of the Gremyakha-Vyrmes Massif.

The trace-element distribution patterns of the Elet’ozersky Massif revealed their significant enrichment relative to primitive mantle and their similarity to OIB compositions (Fig. 4), except for depletion in Ni and Cu. The rocks of the Gremyakha-Vyrmes massif have practically similar patterns.

Thus, the study of the rocks of the differentiated series of the Elet’ozersky Massif showed that they have similar composition of major mineral phases, and in terms of geochemistry, most approximate OIB composition. Practically identical geochemical characteristics for the rocks of the layered series of the Elet’ozersky and Gremyakha-Vyermes massifs suggest their formation from similar parental melts.  

REFERENCES:

Arzamastsev A.A., Bea F., Arzamastseva L.V., Montero P. Proterozoic Gremyakha-Vyrmes Polyphase Massif, Kola Peninsula: An Example of Mixing Basic and Alkaline Mantle Melts // Petrology. 2006. V. 14. ¹4. P.412-441.

Kukharenko A.A., Orlova M.P., Bagdasarov E.A., Alkaline gabbroids of Karelia. LGU, 1969.184 p.

Hofmann A.W. Chemical differentiation of the Earth: the relationship between mantle, continental crust, and oceanic crust // Earth and Planetary Science Letters. 1988. V.90. P.297-314.