Rare-earth elements in the rock-forming minerals of melilitic rocks in alkaline-ultrabasic complexes
The rare-earth distributions in melilites and other coexisting minerals of melilite-containing rocks from five alkaline-ultrabasic complexes were analysed by partition paper chromatography and neutron activation. Relatively late differentiates are characterized by elevated relative alkalinity and reduced absolute basicity. The characteristics of melilitic sövites markedly differ from established trends. The minerals studied are richer in LREE than chondrites and REE concentrations in these minerals increase in the late differentiates of the melilitic series. A proportionality between logarithms of La and Sm concentrations in pyroxene and melilite imply that melilite-containing rocks (except melilitic sövite) belong to a similar comagmatic series. The degree of LREE-enrichment in rock-forming minerals directly depends on the alkalinity of the rock. The distribution of REE between coexisting melilite and pyroxene may be used as an additional genetic criterion: p.e. magmatic La Pyx/La Mel ratio is 0.91, while in metasomatic rocks this ratio is between 1.80 and 1.97.