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VARIOUS TYPE MAGMATIC WATER-SALT FLUIDS IN PROCESSES OF THE ARY-BULAK ONGONITE MASSIF ROCKS FORMATION

Peretyazhko I.S., Tsareva E.A.

Institute of Geochemistry SB RAS, Irkutsk, Russia, pgmigor@igc.irk.ru


The Ary-Bulak dome-shaped stock (East Transbaaikalia) near 0.8 km2 by area is mainly composed by porphyritic ongonites, and marginal zone of aphyric rocks 50-100 m wide is only observed in the south-west flank. The high concentrations of CaO (5.4-22 wt %) and F (5.2-18.8 wt %) are characteristic of the most aphyric rock samples. The varieties with anomalously high CaO (up to 9.4 wt %) and F (up to 7.5 wt%) also occur far away from marginal zone, within porphyritic ongonites. The evidence of alumosilicate and fluorine-calcium melt immiscible phase joint crystallization in presence of fluid during formation of the Ca- and F-rich massif rocks have been obtained (Peretyazhko et al., 2007a).

Rock crystallization of the massif occurred at participation of low-density magmatic fluids. Essentially gaseous fluid inclusions with big bubble and thin rim of solution occur in many phenocrysts of quartz and topaz from porphyritic ongonites along with melt inclusions. In aphyric rocks, the rare phenocrysts of quartz contain primary-secondary melt inclusions along with the healed fissures, and fluid inclusions, i.e. gaseous and salt ones that are syngenetic for them. Coexistence with essentially gaseous fluid inclusions and data of thermometry allow to suggest that numerous melt inclusions formed as a result of simultaneous entrapment of melt and low-density fluid. Due to thermometric observations for melt inclusions, magmatic fluid and melt with phenocrysts of quartz, sanidine and topaz existed at 600-750C. The most possible temperature of quartz phenocrysts formation final stage in porphyritic ongonites did not exceed 700-750C. Occurrence of most melt inclusions in marginal parts or zones of phenocrysts growth suggests that quartz crystallization in melt started at higher temperatures.

The liquid phase disappears in most essentially gaseous fluid inclusions independently on its density and total salinity near critical water point at 365-390C. In some fluid inclusions isotropic crystal <1-2 m in size is contained that is not soluble up to high temperatures and rapidly decreases by volume 10-30C before its complete solution in interval 550-680C. On cooling, the crystal appears in various parts of fluid inclusions and is again solved during heating in the same temperature range. Such behaviour of crystalline phase is specific for water-salt fluids of P-Q type. In this type of fluids, surface of salt solubility crosses a critical curve in P and Q points. Salt concentration reaches minimal values in P point near critical water point, and it significantly increases above Q point at high temperatures and pressures.

Residual and remelted glasses of melt inclusions from phenocrysts of quartz and topaz in porphyritic ongonites have alumosilicate composition with wide variations in Si, Al, Na, K concentrations and contain 2-14 wt% F and 0.06-0.58 wt% Cl. By data of glass ion probe analysis (SIMS), alumosilicate melt was enriched in F and H2O on various stages of its evolution, had significant concentrations of admixture elements - Cs, B, Be, Li, Rb, Nb, Ta. Rare drops of anomalously cesium melts containing up to 17 wt% Cs existed in the process of its crystallization (Peretyazhko et al., 2007b).

Essentially gaseous fluid inclusions properties (disappearance of liquid in no dependence on its density and total salinity near critical water point, behaviour of crystalline phase during solution, capability of inclusions to resist heating to temperatures more than 1400C without opening) as well as enrichment in fluorine and peculiarities of fluid-magmatic system composition on the whole allow to suggest that the low-density NaF-containg fluids of P-Q type with slight admixture of chlorides participated in in processes of porphyritic ongonite formation.

Quartz phenocrysts contain rare primary-secondary melt and syngenetic numerous fluid inclusions, i.e. gaseous and salt ones in Ca- and F-rich aphyric rocks from the massif marginal zone. The salt inclusions are by 60-80% of volume are filled in crystalline phases. One or two isotropic phases with clear cubic forms and 2-4 anisotropic ones with different double-refraction and relief without obviously expressed crystallographic forms are more often contained in them. Anisotropic phases solve in temperature interval 150-300C, isotropic ones – to 500C (in some cases – to 650C). By EDS analyses of partially opened salt inclusions, isotropic crystals are represented by silvine (KCl) and phase of variable composition (K,Cs)Cl. Mixtures with various ratios of Na, K, Cs, Mn, Fe, Ca chlorides and K, Al, Na fluorides were analyzed in other cases. Crystalline phases and salt solution in interstitions among them were likely to fall into the field of microprobe excitation. Crystals of NaCl and NaF can also be isotropic phases. By thermometry of salt inclusions and EDS analyses, some part of anisotropic phases is crystallohydrates of chlorides and fluorides with variable amount of molecularly associated water (ex. MnCl2∙nH2O, FeCl3∙nH2O, KF∙nH2O, AlF3∙nH2O).

It is suggested that Ca- and F-rich aphyric rocks formed as a result of fluid-magmatic influence of residual alumosilicate and fluoride-calcium immiscible melts as well as P-Q type fluids, where concentrations of Cl, F, K, Cs, Na, Mn, Fe, Al increased, on the previously crystallized apical part of the massif. Accumulation of these elements resulted in transition of magmatic fluid from water-salt system of P-Q type to system of the first type that is characterized by field of liquid-steam immiscibility. Magmatic fluid changed its composition and properties, boiled and separated into immiscible phases, i.e. concentrated salt liquid and low-density water steam, when penetrating into zone of aphyric rocks. Relatively not high pressures (400-800 bar), at which massif rock formation occurred, contributed to extract of water-salt liquid with very high concentrations of Cl, F, K, Cs, Na, Mn, Fe, Al, admixture of Ca, Mg, S, As (perhaps, other elements as well).

The phenomena of alumosilicate and fluoride-calcium melt immiscibility (Peretyazhko et al., 2007a), presence of cesium melt drop phases (Peretyazhko et al., 2007b), change in composition and properties of magmatic fluids that result in boiling and extract of concentrated salt liquid testify to complicated processes of fluid-magmatic interaction and heterogeneous state of ongonite magma during the Ary-Bulak massif rocks formation.

This work is supported by Russian Foundation for Basic Research, grant 08-05-00471.


References

Peretyazhko I.S., Zagorsky V.Ye., Tsareva E.A., Sapozhnikov A.N. Immiscibility of calcium fluoride and aluminosilicate melts in ongonite from the Ary-Bulak intrusion, Eastern Transbaikal region // Doklady Akademii Nauk. 2007a. V.413. N.2. P.244-250. (Doklady Earth Sciences. 2007a. V.413. N.2. P.315–320).

Peretyazhko I.S., Tsareva E.A., Zagorsky V.Ye. A first finding of anomalously Cs-rich aluminosilicate melts in ongonite: evidence from melt inclusion study // Doklady Akademii Nauk. 2007b. V.413. N6. P.791-797. (Doklady Earth Sciences. 2007b. V.413A. N.3. P.462–468).