Тезисы международной конференции

Рудный потенциал щелочного, кимберлитового

 и карбонатитового магматизма

Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

Melilite-group minerals in rocks from the Tazheran alkaline massif (Western Baikal aria)

Starikova A.E.

 V.S. Sobolev Institute of Geology and Mineralogy SB RAS, Novosibirsk, Russia

a_sklr@mail.ru

The Tazheran alkaline massif is the part of the Olkhon collision system. It is consist of assemblage of alkaline and nepheline syenites and mafic rocks.  In addition to the igneous rocks within the massif there are a lot of marble bodies of different size, which have various irregular shapes. The strip of dolomite-bearing calcitic marbles is mapped in the central part of the massif. Zone of titanfassaitic and nepheline-fassaitic rocks (pyroxenites in (Konev, Samoylov, 1974)) is exposed on the northern boundary of that marble strip. It is 10-100 m. of thickness. Near the contact marbles are saturated with xenoliths of pyroxenites ranging in size from a few centimeters up to tens of meters. One of the best explanations for the genesis of pyroxenites (Skyarov et al., in press) suggests the crystallization (with automagmatic metasomatic change) of trachydolerite magma that assimilated some carbonate material.

The main minerals of pyroxenites are titanfassait (Al₂O₃ = 5-12 wt.%, TiO₂ = 0,6-3 wt.%) and nepheline. Grandite garnet, pargasite, phlogopite and spinel are often present. Pyroxenite xenoliths in the marbles are most enriched in CaO and depleted in SiO₂. Melilite-group minerals were found in these xenoliths. Melilite forms compositional range between the end members åkermanite (Ca₂MgSi₂O₇ - 30-36 mole%) and Na-melilite (CaNaAlSi₂O₇ - 50-59 mole%) with small amount of gehlenite component (Ca₂Al₂SiO₇ up to 11 mole %) (Table, Fig.). Such high content of Na₂O (6.9 wt.%) is close to the maximum value, known for natural (Wiedenmann, 2010) and for synthetic melilites (Yoder, 1973). Melilite with a similar composition has been described as a new mineral alyumoakermanita in the rocks of carbonatite volcano Oldonyo Lengai (Wiedenmann, 2009).

It was showed in the experimental studies (Mysen et al., 1976) that the crystallization of melilite depends on the fugacity of CO₂. The addition of carbonate material to the mafic magmas will cause the formation of minerals with radical (Si₂O₇)^6- instead of minerals with radical SiO₄^4- (in this case, nepheline). Thus with increasing of contamination degree in pyroxenites melilite begins the crystallization instead of nepheline. Na and Al in this case are admitted in the structure of melilite isomorphically replacing Ca and Mg. Some grains of alumoakermanit are partly or completely replaced by association of nepheline and calcite.

Melilite is also one of the main minerals composing the rocks, which are on the extension of pyroxenite zone. There it is associated with garnet (grossular-andradite series), wollastonite, monticellite, and calcite. These melilites have a lower content of Na₂O (3.5-5.7 wt.%), higher concentrations of FeO_tot (1.0-2.5 wt.%) and amount of gehlenite component is negligible (<2 mole%) (Table). The formation of similar rocks most often is associated with high-temperature contact metamorphism (or metasomatism) of limestones (Valley, Essene, 1980; Pascal et al., 2001; et alias). In the Tazheran massif the occurrence of such rocks on the continuation of the pyroxenite zone can be explained by the influence of transmagmatic fluids enriched in alkalis on carbonate host rocks.

The work was supported by the Russian Foundation of Basic Research (projects nos. 12-05-00229) and special grant of OPTEC Company.

Figure. Composition diagrams of melilite From the Tazheran massif.

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