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154 Handbook on the Physics and Chemistry of Rare Earths 2.4.1 Accumulation Concentration of REEs in the cumulates is found in large peralkaline igneous complexes such as the Khibina and Lovozero complexes in Kola Peninsula of Russia, and the Il ́ımaussaq complex in southern Greenland. Kola Peninsula includes more than 20 main alkaline rock complexes, ages of which cluster between 360 and 390 Ma (Downes et al., 2005). The Khibina complex is a cylindrical shape body about 30 km in diameter, and it has a con- centric zonal structure formed by successive phases of intrusion including nepheline syenite, ijolite, and carbonatite. The ijolite unit contains an apatite-rich layer that is composed of magnetite, titanite, and apatite in the hanging wall of an intrusion. This layer partly attains more that 80% apatite in volume, forming high-grade apatite ores in the complex. Because the apa- tite typically contains up to 5 wt.% REEs as oxides (Belousova et al., 2002; Sha and Chappell, 1999), the apatite ores are regarded as a potential REE resource (Zaitsev and Kogarko, 2012). This apatite concentration is inter- preted as that apatite crystallized from silicate magma and accumulated in the upper part of the magma chamber due to crystal sorting in the convecting melt (Kogarko and Khapaev, 1987). Lovozero is another large cylindrical complex located east of the Khibina complex. This complex is about 26 km in diameter and is divided into three units: phase I nepheline syenite, phase II layered nepheline syenite, and phase III eudialyte-bearing nepheline syenite. The phase II unit is composed of repeating layers, each of which has a basal apatite–loparite-enriched part and an upper alkali feldspar and nepheline rich part (Femenias et al., 2005). This segregation is ascribed to the different specific gravity among the constituent minerals: loparite (4.6–4.9), apatite (3.2), and nepheline (2.7), forming a cumu- lus layer of heavy minerals at the base of the magma chamber (Kogarko et al., 2002; Fig. 15). Because loparite at Lovozero contains 28–35 wt.% REEs as oxides (Kogarko et al., 2002), REEs are produced as by-product of niobium in loparite mined from the loparite-enriched layers of the complex (Hedrick et al., 1997) (Fig. 15). The Il ́ımaussaq complex in Greenland is northwesterly elongated peral- kaline body, 17km7km in size, which was associated with continental rifting between 1350 and 1140 Ma (Upton et al., 2003) and is one of the highly evolved and silica undersaturated peralkaline intrusions. The body is divided into a lower part of kakortokites and lujavrites and upper naujaite part (Pfaff et al., 2008). The lowermost kakortokites, 200 m thick, include 29 units (8m thick on average) composed of a set of black-colored arfvedsonite-rich layer, red-colored eudialyite-rich layer, and white-colored K-feldspar and nepheline rich layer. Pfaff et al. (2008) interpreted the layers formed by different mineral abundance in each unit is as a result of their different specific gravities. Among these minerals, eudialyte contains 2.5–6.8 wt.% REO, and thus eudialyte-rich layers and/or units are concen- trated in REEs.PDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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