Mineral association of alkaline metasomatic rocks and
carbonatites of Gremyakha Vyrmes massif,
the Kola Peninsula.
Shpachenko A.K.*, Sorokhtina N.V.**, Zakharov D.O.***
* GI KSC RAS, Apatity, Russia; ** Vernadsky
Institute, Moscow, Russia; *** MGRI-RSGPU,
Proterozoic alkaline ultramafic massif Gremyakha-Vyrmes
consists of different
rock complexes: 1)
ultramafic-mafic rocks 2)
foidolites 3) alkali granites and syenogranites [Polkanov
et al., 1967]. Interpretation
of sequence is complicated because of late tectonic and metamorphic
processes that are occurred here intensive.
Alkaline complex rocks and carbonatites replaced protolite whose
interpretation is incomplete still. In this case it
is important to explore their mineral composition and sequence of
crystallization of phases in appropriate degree. Particularly it is
valuable due to
rare-metal mineralization occurs in the complex [Sorokhtina et al,
2010]. The rare-metal metasomatic rocks occur as submeridionally
orientated zones extending up to 6-8 km and have several hundred meters
thickness. They exist in form of plate and lenticular bodies in varying
degrees of altered ore (ilmenite) pyroxenites and foidolite,
Basically, alkaline metasomatic rocks are albitites,
aegirinite. Their composition varies a lot depending on the protolith
composition range. We believe that due to hard tectonics especially
tectonic blocks shifting different complexes of rocks were involved in
metasomatic processes. The rocks that were altered are: 1. ore
pyroxenites ultrabasite-mafic complex, 2. microcline syenites, alkali
granites complex syenogranites 3. urtites, foyaites and
nepheline-aegirine pegmatite foidolite complex.
Despite large range of minerals in rocks the main
constituents are albite, aegirine, phlogopite-annite series, microcline,
accessories are calcite, sulfides, graphite, pyrochlore group, zircon.
Rarely in albitized microcline syenites quartz and fluorite could be
found. Sometimes in the aegirine-albite metasomatic rocks arfvedsonite
and biotite replace aegirine and titanite replaces ilmenite.
In thin sections calcite occurs as rare grains and
aggregates, small veins. In this case we suppose
that the protolith was a basic rock (Fig. 1a). Also there are
nepheline relicts replaced by cancrinite, natrolite,
prehnite and secondary mica aggregate.
Calcite carbonatites occur as concordant
and semi-concordant viens in the metasomatic rocks. The veins are few
tens of centimeters thick. Rock has a fine-grained and medium grained
texture. Uneven distribution of carbonatite veins was discovered during
the drilling. Under microscope carbonatites have a hypidiomorphic
texture and contains 80-90%vol. of calcite (Fig.
1b). The texture of calcite aggregate is homoblastic. Signs of strain
effects on the whole rock have not been identified.
Apatite, aegirine, phlogopite-annite minerals, albite,
microcline are also found here. Titanite, ilmenite, prehnite, graphite,
pyrite, ferrialanite-(Ce), serpentine occur here in less quantity.
Calcite forms polyhedral well-shaped crystals,
without traces of corrosion, the faces of adjacent grains are often
found at an angle of 120 º. Twins are very common in the calcite and
shown in form of flat parallel bands or rhomb system. Sometimes curved
cleavage cracks and fine-grained aggregates of calcite are found here.
Apparently this could be a result of later deformations. Graphite forms
microspherulites aggregates in carbonate or tabular crystals in albitite
matrix Mineral is also found as inclusions or
intergrowth with pyrochlore, zircon, biotite.
Aegirine occurs in single elongated prismatic crystals (1-2 mm on
average) in calcite matrix; sometimes twins are shown here.
Small cracks inside the crystals contain calcite, sulfides and fibrous
silicate mineral which is less common. The common mica constituent is a
phlogopite-annite series. Mineral forms a large
(up to 1.8 mm) tabular crystals that are intergrown with albite and are
often bent and has split edge areas that are replaced by chlorite group
minerals. Albite and calcite occur along
cleavage fractures in phlogopite. Albite forms
large relic poikiloblasts up to 3 mm, with a characteristic albite
twinning. The twins are dislocated, separate blocks of crystals are
shifted here. Aegirine,
mica, calcite, sulfides, and graphite are found here as the inclusions
in poikiloblast. Orthoclase usually forms xenomorphic relict grains. The
size varies up to 1.5mm. Distribution of potash feldpar in calcite
matrix is extremely uneven. The inclusions of calcite, albite, and
zeolites are identified in relict crystals of potassium feldspar.
Fluorapatite occurs in the calcite matrix in the form of a large (up to
1.5 mm) elongate-prismatic crystals with smooth outlines and strong
fracturing, inclusion of calcite also develops in cracks. In thin
sections fluorapatite crystals are oriented in one direction. Titanite
forms large metacrystals several centimeters long as well as in
albitites. Almost all minerals of carbonatite are established as
inclusions in titanite crystals. Sulfides such as pyrrhotite and
chalcopyrite form small flattened grains and aggregates which develop in
the interstices of calcite.
Proceedings of the petrographic study of thin sections allow
us to conclude that formed metasomatites were involved in
carbonatization process intensively and microcline, aegirine,
phlogopite-annite, ilmenite, titanite were saved here as a relicts.
Fluorapatite and graphite are syngenetic to calcite, other accesory
minerals formed on final stages of crystallization.
The relationship of
minerals in the
altered ultramafic rocks (a)
and carbonate (b).
a) The aegirinite
reaction in contact
the process of replacement of ilmenite
(Ilm) by titanite
(Aeg) by alkaline
Transmitted polarized light,
with relict of albite
Transmitted light, crossed
alkaline metasomatic rocks
could be classified as fenites-nepheline
type of metasomatism [Zharikov
et al, 1998]. The
process described as apomagmatic
without regard to the
granite or gneisses and
the composition of paragenetic association
varies depending on what kind of
the block was involved in
metasomatism. The process appeared on
the final stage of foidolite intrusion and was formed by residual
Rocks of foidolite group and associated
with them syenogranites, hyperbasic rocks were involved in process of
metasomatic alteration. That caused forming of different and complicated
mineral associations and several generations of forming minerals such as
microcline, albite, aegirine, micas.
Marakushev А.А., Zaraisky G.P.,
Metasomatism and metasomatic rocks / М.:
Nauchnii mir, 1998. 492 p.
Polkanov A.A., Eliseev N.A., Eliseev N.E., Kavardin
G.I. Massif Gremyakha-Virmes, Kola peninsula. М.,
Nauka, 1967. p.236.
Sorokhtina N.V., Kogarko L.N., Shpachenko A.K. New
geochemical and mineralogical data about rare-element occurrence on
Gremyakha-Virmes massif // Docladi RAN. 2010. Т. 434. № 2.