ロシア北西部,コラ半島のアルカリ深成岩瀬と希土類鉱物資源

抄録

Devonian peralkaline igneous rocks of the Kola Alkaline Carbonatite Province (KACP), occur mostly in the Kola Peninsula (Russia) and parts of northeastern Finland of the northern Baltic Shield. KACP consists mainly of nepheline syenites, evolved alkaline syenites and ijolite accompanied with ultramafic to mafic cumulates, and small amount of carbonatites dyke and neck intruded at a late stage. Lithological zonation by multiphase intrusions and fractional crystallization are observed in the representiative phases of the Khibina and Lovozero plutons. Rare earth minerals of apatite, loparite and eudialyte are cumulate phases of the fractionated peralkaline syenitic magmas. Based on whole-rock Sr and Nd isotopic compositions of plutonic bodies, the peralkaline magmas of KACP are considered generated from isotopically heterogeneous mantle. Mineral compositions of apatite and loparite and homogeneous isotopic compositions in each plutonic body suggest that fractional crystallization is main process of magmatic evolution and lithological layering in each plutonic body.
The peralkaline syenites of KACP contain abundant rare earth minerals, and loparite, eudialyte and apatite would be economically important. The Khibina body is well known for the large apatite deposits and loparite from the Lovozero body is mined for rare earth resources in Russia. Heavy rare earth contents of eudialyte (Y=400-900ppm) from the Lovozero body are often higher than those contents of apatite (Y=140-700ppm) and loparite (Y<150ppm). Rare earth minerals from peralkaline syenites of KACP would have a high potentiality for rare earth resources.