Compositional
characteristics of carbonatite magmas from the Bolshetagninskii Massif,
Eastern Sayan
Andreeva
I.A., Kovalenko V.I.
The Bol'shaya Tagna Íassif is situated in the eastern Sayan province of
alkaline ultrabasic rocks and carbonatites. This massif is a rounded and
concentrically zonal-ring complex structure (4 km in diameter) that
demonstrates the consecutive formation of ijolite-melteigite, nepheline and
subalkali syenite (microclinite), picritic porphyritic rock, and carbonatite
rocks. The abundance of syenites and microclinites and the intense development
of fluorite mineralization related to the formation of carbonatite rocks are
distinguishing features of this massif. Three types of fluorite carbonatites
are recognized: (1) fine-grained calcite carbonatites with fine- and very
fine-crystalline fluorite, hematite, and apatite; (2) massive or banded ore
consisting of fluorite, calcite, K-feldspar, and aegirine (fluorite crystals
vary from fractions of millimeter to 1 cm in size); and (3)
stringer-disseminated ore with fluorite crystals from 3-4 mm to 1 cm in size.
The studied sample is a coarse-grained fluorite
carbonatite of the second type composed of carbonate (~ 60 vol %), fluorite (~
30%), K-feldspar (~ 5%), pyrite (2-3%), and barite (up to 1-2%). Carbonate is
represented by calcite with elevated contents ofš Mnš and
FeO. The two-phase exsolution structure is often observed, Mn-calcite and
kutnahorite (carbonate of dolomite group).
The primary melt inclusions and syngenetic crystalline
inclusions were revealed in fluorite, K-feldspar, and pyrite from fluorite
carbonatite.
The crystalline inclusions represented by carbonates,
fluorite, K-feldspar, aegirine, columbite, and pyrite. Among crystalline inclusions
of carbonates were determined calcite, Na-Ca-carbonate, and kutnahorite.
Sporadic columbite ingrowths in calcite have been identified. Columbite
contains as much as 11 wt % MnO, 9.5 wt % FeO, and 4.6 wt % TiO. The
crystalline inclusions of Na-Ca-carbonate in fluorite contains up to 44.6 wt %
CaO, 18 wt % Na2O, 4.5 wt % F, 3.7 wt % MnO, and 1 wt % FeO. The
empiricalš formulaš of this mineral was calculatedš from the chemicalš analysis asššššš Na 6.05(Ca 8.27,
Mn 0.55, Fe 0.14)8.96[CO3]7.00
(F 2.49OH 1.51)4.00. The calculated H2O and CO2
contents are 1.31 and 29.66 wt %, respectively, and the total is 101.09 wt %.
Primary melt inclusions in fluorite are completely
crystallized and contain a
residual gas phase and a number of daughter minerals including carbonate, fluoride, and chloride. Carbonates are
predominant and represented by calcite, kutnahorite, Na-fluorcarbonate,
nyerereite, and burbankite. Nyerereite (Na-Ca - carbonate) and burbankite
(Ba-Sr-carbonate) are rather rare minerals and have been identified as daughter
phases in melt inclusions for the first time. Fluoride minerals are represented
by fluorite and villiaumite. Qualitative microprobe analyses have indicated the
presence of haliteš and sylviteš among the daughter minerals. In general, the
mineral assemblage of melt inclusions in fluorite from carbonatites of the
Bol'shaya Tagna carbonatite complex is consistent with mineral composition of
carbonatitic lavas of the Oldoinyo Lengai Volcano (Tanzania), where nyerereite,
gregoryite, Ba-carbonate, fluorite, and sylvite have been identified as
phenocrysts and groundmass minerals in the lavas.
The thermometric experiments with melt inclusions were
carried out on a Linkam TS 1500 microscopic heating stage with visual control.
The first indications of melting of crystalline phases were observed at 280oC.
The complete homogenization of inclusions into salt melt was attained at
520-525oC. The similar magma temperature (544oC) was
measured during the eruption of the Oldoinyo Lengai Volcano in 1988.
During the subsequent cooling, the salt melt
completely crystallized into a microgranular aggregate of quenched crystals. A
complete scanning of the chemical composition of homogenized melt inclusions in
fluorite analyzed by scanning over the whole area allowed us to estimate the
composition of the salt melt that produced the fluorite carbonatite. The melt
is characterized by extremely high contents of Na2O (up to 22 wt %)
and CaO (up to 10 wt %), F (up to 1.65), Ce2O3 (up to 1.3
wt %), Cl (up to 1.0 wt %) as well as by elevated contents of FeO (7 wt %), MnO
(4-5 wt %), K2O (3-8 wt %), SrO (0.6-0.8 wt %), and Ba (0.6 wt %),
F, and Cl. This melt are chemically similar to the compositions
of the lavas from the Oldoinyo Lengai Volcano.
A compositional difference between the rock studied
and melt inclusions, (particularly, in terms of CaO and Na2O
contents) may be accounted for by instability of sodium carbonates and
chlorides and their removal with fluid phase at the late stage of magma
crystallization. The sodic phases could also have been leached by water from
the rock, but this process is less probable because leached cavities are absent
in the rock.