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Geochemical indicators
of the evolution of the ultrabasic alkaline series of the Kola Province
Arzamastsev A.*,
Arzamastseva L.*
Institute
of Precambrian Geology of the Russian Academy of Sciences,
Saint-Petersburg, Russia
arz1998@yahoo.com
A comparative study of
foidite series from alkaline carbonatite intrusions, dyke series and
volcanic rocks was conducted. Maximum content of phosphorus in the
plutonic rock sequence of has been found in highly melanocratic
varieties of ultramafic foidites. The distribution patterns for
picrite-alnoite-olivine melanephelinite-nephelinite-phonolite reveal a
constant enrichment in P2O5 of the initial members
of dyke series.
The Sr distribution
patterns differ in the Khibina foidite and ultramafic alkaline series of
Kovdor, Turiy Mys, Salmagora. The maximum Sr content in the Khibina
foiditic series has been found in melanocratic foidolites. By contrast,
in the Kovdor type series the maximal concentration of Sr is typical
for melilitolites, whereas the residual foidolites are depleted in these
elements as well as the accessory apatite. The above differences between
the Khibina and Kovdor type ultramafic alkaline series exhibit two
particular evolutionary trends of fractional crystallization process,
which was responsible for the entire ultrabasic-foiditic-phonolitic
series. Parental olivine nephelinite melts are shown to be able to
produce phonolite or melilitite derivatives, depending on fluid
pressure. Hydrous phases, appearing on the liquidus instead of
clinopyroxene and olivine, result in melt compositions evolving toward
the melilitite melt field, as is observed in the Kovdor and Turiy Mys
massifs. Crystallization of melilite, which has a relatively high Sr
distribution coefficient causes depletion in Sr of the complementary
members (foidolites and nepheline syenites). In the Khibina foidite
series no important role of amphibole and phlogopite in the liquid
evolution following the "phonolitic" trend should be expected.
Melilitolite members of the series played no significant role and were
reduced. This resulted in the enrichment of the Khibina melteigites and
ijolites in Sr. Thus, the chemical composition of apatite and partly
titanite in the Kola foidite series reflects the type of crystallization
trends and depends on the presence or absence of melilitolite members.
Financial support:
Russian foundation for Basic Research (Grant
12-05-00244). |