|
Derivatives of
melanephelinite magmas in the Kola alkaline Province: estimation of
metallogenic potential
Arzamastseva L.*,
Arzamastsev A.*
Institute
of Precambrian Geology of the Russian Academy of Sciences,
Saint-Petersburg, Russia
arz1998@yahoo.com
The main geochemical
feature of alkaline ultrabasic rocks is enrichment of Sr, Nb, Ta, Ba,
REE, F, S, P, but the ores are genetically related to carbonatites,
which intruded during the final stage of magmatic activity. The trace
element mineralization of economical interest have not been found in the
silicate alkaline rocks yet. However, recent investigations showed
relatively high ore potential of melilite-nephelinite primary magmas.
The object of our
investigations were alkaline rocks of the three different facies:
plutonic, dike and volcanic series. Geochemical data were obtained and
analysis of trace elements distribution was performed in the Khibina,
Lovozero, Kovdor, Kontozero, Khabozero and Ivanovka rocks. 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 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). |