Behavior of Trace Elements in Rock-Forming Minerals during Partial Melting and Migmatization of Granites in the Aldan Shield

Аннотация

The evolution of rock-forming minerals (orthopyroxene, garnet, biotite, and amphibole) is studied in successive generations of granites and leucosomes. It is shown that the trace element partition coefficients between solid phases and melt are governed by an exponential dependence on the ionic radii (Brice, 1975), which is manifested especially clearly for REE. The influence of the composition of an anatectic melt and acidity/alkalinity of the mineral environment on the partition coefficients is high enough to erase the effect of temperature and pressure. The garnet–orthopyroxene assemblage in the melt generation zone results in accumulation of LREE and lithophile elements (Rb, Sr, and Ba), while HREE, Cr, and V are retained in the residue together with Cr and V. HFS elements (Nb, Zr) can accumulate in granitoid melts. In contrast, an amphibole-bearing residue causes the melt to be depleted in REE, Y, Nb, and enriched in Rb, Sr, Ba, and Zr. Special attention is given to the behavior of Nb, Cr, V, and Ti in biotite. Their D_i is always significantly greater than unity. The migmatite leucosome is often enriched in biotite, which causes the melt to be depleted in these elements. A similar behavior is also found for lithophile elements (Rb, Ba), which are concentrated in biotite residue. Zr, Y, and Sr show different distributions with very low D_i^Bt/melt values. Therefore, they are concentrated in the melt during migmatization and formation of crustal magma. It is interesting to note the behavior of Sr, which accumulates in the anatectic melt in equilibrium with garnet–biotite residue. Ba-rich melts can be generated only in the garnet stability field, because biotite-bearing residue concentrates Ba.