The rare earth elements (REE) have been widely used in petrogenetic studies since they are excelent indicators of geologic processes. In igneous rocks the partial melting and fractional crystalization processes lead to a relative enrichment of the light rare earth elements (LREE) in the liquid phase of the crystal-liquid silicate systems. This conduets to a LREE enrichment in crustal rocks, particularly in the more felsic ones. The minerais taking part in these processes have characteristic effects in the REE patterns of the melts allowing their identification. The sedimentary rocks show little variation in their REE patterns pointing out to the homogenization capability of the sedimentary processes. The REE content of elastie sediments reflects the composition of the source áreas subject to erosion and the variation of their REE patterns during the geological time indicates the compositional changes of the upper continental crust. The REE have been considered as immobile during metamorphism and generally they provide excellent information about the premetamorphic precursors. However, REE remobilization has been observed in several cases where there is a high fluid/rock ratio, such as during spüitization, high-temperature metasomatism, lowgrade hydrous metamorphism, weathering, etc. The REE have been also used as a monitor of the chemical changes involved in hydrothermal ore deposits
