Behavior of trace elements in rock-forming minerals during partial melting of granites in the migmatization process occurred in the Aidan Shield
Glebovitsky V. A., Sedova I. S.
Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2017. V. 146. N 4. P. 22-43
The evolution of rock-forming minerals (orthopyroxene, garnet, biotite, amphibole) in the series of granite and leucosome successive generations is discussed. It is shown that partition coefficients of rare elements between solid phases and melt are governed by an exponential dependence on the ionic radii (Brice, 1975), which is especially clearly manifested for REE. Influence of the composition of the anatexic melt and acidity/alkalinity of the mineral formation environment on partition coefficients is so strong that erases the effect of temperature and pressure. In the case of assemblage of garnet and clinopyroxene in the melt generation zone, they concentrate LREE, lithophile elements (Rb, Sr, Ba) while HREE, Cr, V remain in the restite. The HFS elements (Nb, Zr) can be accumulated in granitoid melts. In contrast, the stability of amphibole in the restite cause the melt to be depleted in REE, Y, Nb and enriched in Rb, Sr, Ba, Zr. Special attention is given to the behaviour of Nb, Cr, V, and Ti in biotite. Their Di is always significantly greater than 1. The migmatite melanosome is often enriched in biotite, that caused the melt to be normally depleted in these elements. The similar behave is also found for the lithofile elements (Rb, Ba) that are concentrated in the biotite restite. Zr, Y, and Sr are differently distributed and have very low DiBt/melt values. They are concentrated in the melt during migmatization and formation of crustal magma. It is of interest to note the behavior of Sr that is accumulated in the anatexic melt if garnet and biotite are stable in the restite. As for Ba, the melt enriched in this elements can be generated under conditions of garnet stability (but not stability of biotite that concentrated Ba).
Key words: migmatite, leucosome, pyroxene, garnet, amphibole, biotite, rare elements, REE, partition coefficient, Aldan Shield.