LILE, REE, and HFSE partition coefficients between crystal and melt in migmatites of granulite facies of the Nimnyr block, Aldan shield

Abstract

The behavior of Rb, Sr, Ba, Cr, V, Nb, Zr, REEs, and Y is studied in migmatites of granulite facies and the transitional zone to amphibolite facies in the western part of the Aldan granulite area. Our data allowed estimation of the partition coefficients of these elements in the crystal-melt system (Di) for orthopyroxene, garnet, clinopyroxene, and hornblende. The leucosomes of anatectic migmatites and diatectites (high-K charnockites and granites) were accepted as a melt, and the metapelites or previously granitized rocks (nebulites) and early leucosomes relative to late leucosome were the primary rocks and restite. The dependence of Di on the REE ionic radius is close to parabolic [7]. Our data allow identification of a qualitative link between the DiGrt/melt and the thermodynamic conditions of high-grade metamorphism and the accompanying partial melting, which leads to formation of a series of ultrametamorphic granitic rocks: lower temperature rocks (conditions of amphibolite facies) are characterized by higher DREE values relative to the zone of granulite facies. The late granitic rocks formed after diatectic charnockites are also characterized by higher DREE values relative to the preceding charnockites. For the conditions of granulite facies, the material belongs to rocks similar in the formation conditions; however, the series are strongly distinct in K content. The influence of the composition of the anatectic melt and acidity-alkalinity conditions on the partition coefficients of the studied elements is so strong that it eliminates the effect of temperature and pressure. The LREEs, Rb, Sr, and Ba will mostly accumulate in the garnet-orthopyroxene assemblage in the zone of melt origination, whereas HREEs will remain in restite along with Cr and V. Niobium and Zr may enrich the granitic melts. In contrast, the stability of amphibole in restite causes the depletion of melts in all REEs, Y, and Nb and enrichment in Rb, Sr, Ba, and, to a lesser extent, Zr.