Peculiarity of ultrametamorphism in the juncture zone of the Aldan shield and Dzhugdzhur-Stanovoi folded area

Abstract

The processes of ultrametamorphism in the juncture zone between the Aldan shield and Stanovoi folded area are manifested in granitization (volume-for-volume replacement of gneisses by trondhjemite gneisses Lc1) and subsequent migmatization with formation of several leucosome generations Lc2, Lc3, Lc4, and Lc5, which is confirmed by U-Pb zircon dating. It was established that the granitization stage is marked by the input of Si, Na, and Ba and removal of practically all major (including K) and minor elements. Formation of migmatite leucosomes is accompanied by further depletion in transition (Ti, Mg, Fe, V, Cr, Ni) and light rare-earth (La, Ce, Nd, and Eu) elements, and accumulation of HFSE (Pb, U, Th, Nb, Ta, Y) as well as medium and heavy rare-earth elements (Sm, Gd, Yb, Lu). Leucosomes Lc4, in addition, are enriched in K, Rb, and especially HREE due to the appearance of garnet, while Lc5 leucosomes become higher in K, Sr, and Pb. The study of relations of trondhjemite gneisses and migmatite leucosomes with protolith, geochemical features, and opposite trends in variations of Zr/Hf, Zr/Nb, Nb/La, and Eu/Eu*, and LREE/HREE ratios in the series of granitization and migmatization indicate that the trondhjemite gneisses were formed during deep-fluid-assisted infiltration granitization under the amphibolite facies conditions, while migmatite leucosomes were generated during evolving anatexis under conditions of subsequent diatexis and continuing fluid reworking. With time, the composition of the fluid changed changed, the role of K increased, and leucosomes acquired granitic composition. Unlike common K and K-Na types of ultrametamorphism, the considered juncture zone is characterized by specific type of ultrametamorphism-Na type, with formation of granitic leucosomes in subordinate amounts at the final stages.