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REE Mineralogy and Resources Chapter 279 149 REEs, especially HREEs are not incorporated in the accessory minerals significantly, resulting in the enrichment in the fractionated melt. Because the granitoids in the Ryoke belt have been deeply eroded and only relatively deeper portions are exposed on the surface, such enrichment is not clear. However, in the Sanyo belt, where highly fractionated ilmenite-series I-type granitoids are exposed, the granitoids are highly enriched in HREEs in addi- tion to strong negative europium anomalies (Fig. 12C). Thus, oxidation state of granitoid melts affects REE concentration by the presence or absence of highly REE partitioned accessory minerals although the REE concentrations and compositions of the primitive oxidized and reduced granitoid magmas are not significantly different. 2.3.2 Continental Rift Zone The magmatism of the continental rift zones is characterized by the occur- rence of kimberlite and carbonatite in addition to alkali basalt. These three rock types commonly occur together, forming an igneous complex. Because of the incompatible characteristic of REEs, they are concentrated more in the magmas formed by low degree of partial melting such as peralkaline and carbonatite magmas. Carbonatites are defined as igneous rocks composed of more than 50 modal percent primary carbonate and containing less than 20 wt.% SiO2 (Le Maitre, 2002). Carbonatites are known to range in age from the Archean to the Recent, with an increasing abundance toward more recent time (Rukhlov and Bell, 2010). Carbonatite magmatism has been almost exclusively restricted to the continental environments characterized by thickened lithosphere and relatively stable cratons of Archean age. Especially, carbonatites cluster in large igneous provinces such as 66Ma Deccan flood basalt province, 130Ma Parana ́- Etendeka, 250Ma Siberian province, and 370Ma Kola alkaline province (Ernst and Bell, 2010). Most carbonatites are associated with prominent geolog- ical structures such as rift systems or grabens, major faults, large-scale uplifts, and cratonic domains (Rukhlov and Bell, 2010). Carbonatite magmas are able to form by direct melting of a carbonate- bearing mantle source, by immiscible separation from a carbonated silicate melt, and by crystal fractionation of a carbonated alkali silicate melt (Halama et al., 2005), among which the mantle-source carbonatites are most dominant. The models of carbonatite magma generation from mantle source are debatable; it forms by sublithospheric source (Bell and Tilton, 2002; Nelson et al., 1988) or by metasomatized lithosphere with mantle materials (Bell and Simonetti, 2010; Chakhmouradian, 2006). Whichever the origin, it is accepted that car- bonatite magmas form at pressures exceeding 1.9 GPa in the garnet- and amphibole-stable fields (Fig. 13). Primary carbonatite melts can generate near the solidus greater than 70 km from dolomitic or carbonated amphibole lherzo- lite by very low degree of partial melting. This is consistent with LREEPDF Image | HANDBOOK ON THE PHYSICS AND CHEMISTRY OF RARE EARTHS
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