|
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).
|