2012

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Abstracts of International conference

Ore potential of alkaline, kimberlite

and carbonatite magmatism

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).