Mineralogical and geochemical aspects of rare-earth elements behavior during metamorphism (on the example of the Upper Precambrian structural-material complexes of the Bashkir megaanticlinorium, South Urals)

Abstract

The article provides new data on geochemistry and mineralogy of rare-earth elements (REE) in rocks of structural-material complexes of the Bashkir megaanticlinorium, which underwent metamorphic transformations of various nature: contact metamorphism (Suran section); syn- and postgenetic contact-dislocation metamorphism (Shatak complex) and hydrothermal metamorphism (Uluelga-Kudashmanovo zone). It has been established that when a magmatic melt is exposed to sediments, the latter are enriched with REEs with the formation of rare earth minerals (monazite, allanite, xenotime et al.). The study of the chemical composition of monazites and allanites showed that all variations of oxides in the composition of the former are due to isomorphous Ce-Ca-Th substitutions in the structure of minerals, but redistribution of these elements was an independent process characteristic of each structural-material complex. The study of allanites made it possible to establish the presence of isomorphism according to the Ca↔Ce, La, Nd principle, as well as the sharp difference between the characterized minerals in the amount of MgO, Fe* and MnO from analogues from other regions, which indicates the presence of a regional component in the chemical compositions of minerals altogether, geotectonic settings of mineralization formation. The temperature regimes of mineral-forming processes with metamorphic transformations of rocks calculated from chlorite and muscovite compositions (344-450°C – Suran section, 402-470°C – Shatak complex, 390-490°C – Uluelga-Kudashmanovo zone) indicate the possibility of stable coexistence of the association monazite-allanite. It was established that when a magmatic melt on the sedimentary substrate of the frame, the lanthanides enrich the exocontact rocks with the formation of newly formed REE-mineral associations. At the same time, the processes of formation of rare-earth mineralization are largely determined by the physicochemical parameters and thermobaric conditions of the accompanying and subsequent metamorphism.