Fractionation of the rare earths in a suite of highly evolved metaluminous granitoids and felsic dykes from the Western Cape Province, South Africa
The rare earth element (REE) distribution in a comagmatic, metaluminous, igneous suite, consisting of a monzogranite, differentiating to an even-grained granite and eventually a sequence of dyke rocks, was investigated. During the early stages of the differentiation process, the REE behaved compatibly, the LREE being preferentially removed by fractionation of early allanite and monazite. With increasing fractionation, higher silica content and increasing alkalinity in the dyke rocks, evidence for the incompatible behaviour of the REE were obtained. LREE, Eu and, to a lesser extent, Gd were fractionated from the HREE and concentrated in the dyke centres. Partitioning of F and F-REE complexes towards the highly fractionated melt phase contributed towards this process. An increase in oxidation conditions during the very late evolutionary phases of the dykes again stabilized allanite as an accessory phase. LREE were sufficiently enriched at this stage to be incorporated in the mineral, leading to a subsequent lowering of the La Sm ratio in some highly evolved dyke centres. Late crystallizing fluorite in dyke centres accommodated some of the enriched LREE by a process of coupled substitution. Evidence for these fractionation processes in evolved metaluminous felsic rocks have implications for the use of REE in modelling the generation of such rocks. © 1995.