Geochemistry and geochronology of migmatites of the Kurul’ta-Nyukzha segment and the problem of correlation between metamorphic events in the Dzhugdzhur-Stanovoi folded area, Eastern Siberia

Abstract

Crystalline schists of the El’gakan unit (Nyukzha River) were affected by penetrative (volume) replacement by plagiogneisses and granite-gneisses (Lc1) and were then transformed into a polymigmatite complex with successively developing leucosomes Lc2, Lc3, and Lc4. After a thrust-nappe structure was formed in response to collision processes, a new generation of granite veins was produced (Lc5), and then tonalite gneisses Lc6avt and branching migmatites with leucosomes Lc6all were formed along strike-slip fault zones. Zircons from granite-gneisses Lc1 were classified into four types (populations) based on SHRIMP II data. Type I (rhythmically zonal cores) were dated at 2960 and 3010 Ma, which is correlated with the age of the magmatic (predominantly volcanic) protolith. Types II and III were dated at 2703 Ma, which corresponds to granitization under amphibolite-facies conditions and the origin of the Stanoi granite-gneiss. This event is correlated with granulite metamorphism and ultrametamorphism over the whole territory of the Dzhugdzhur-Stanovoi folded area. The most widely spread type IV of the zircons has an age of 1915 Ma, which corresponds to the metamorphism coeval with overthrusting and, hence, with the collision of the Stanovoi plate and a margin of the Siberian Platform. Concentrations of REE, U, and Th and the Th/U ratio were determined to systematically decrease from type I to IV of the zircons (except their type III, whose Th/U ratio increases to >1). Zircons from Lc5 have a concordant age of 139 Ma, which is comparable with the age of the Late Stanovoi granites. The compositional changes from the older cores to younger rims of zircons from Lc5 are analogous to those mentioned above for zircon from Lc1. The concordant age of zircons from Lc6avt is 127–130 Ma. Their Th/U ratio increases from cores ( 1), which suggests that melt may have appeared when Lc6avt was formed. ICP-MS analyses of 53 rock samples reveal differences in the character of the trend (increase/decrease) and magnitude of the changes in the concentrations of trace elements in the distinguished granitization and migmatization series; correlations were revealed between the concentrations of elements and composition of the rock groups. For example, the development of Lc1 was associated with enrichment in Rb, Sr, Ba, LREE, Th, Zr, and Hf at depletion in Nb, Ta, U, and HREE relative to the original rocks. The leucosomes of the Lc2, Lc3, and Lc4 migmatites are depleted in all of these elements except LILE, which is thought to be explained by infiltration-controlled granitization with volume replacement and partial melting at the development of vein leucosome and the subsequent mobilization of the melts together with residues. The different signs of the changes in the LREE and LILE concentrations is unusual for anatectic processes and can be modeled by equilibrium or disequilibrium melting.