Литосфера (Jun 2017)
Sandyites and rocks of monzonite composition of the Ilmenogorsk miaskit massive (the Southern Urals)
Abstract
The results of study of rocks of the Ilmenogorsk miaskite block - mylonitized miaskites, “sandyites” and monzonitic rocks, which were found for the first time, are presented in the paper. We studied textural-structural features of rock and their mineral and chemical composition. The composition of petrogenic elements in rocks were determined by atomic absobtion method, rare earth, rare and trace elements - ICP-MS method. Microprobe analyses compositions of minerals were made on the scanning electron microscope REMMА-202 M with energy-dispersive console Link systems LZ with Si-Li detector. Correlation of data was performed using the program “Magellanes”. These data allowed us to establish the degree of transformation of rocks and multistage metasomatic processes. The mineral assemblages indicate amphibolite facies of metamorphism, which was accompanied by participation of the F-bearing fluid and formation of specific accessory minerals - highly aluminous titanite and allanite. The highly variable Al contents of these minerals is evidence of high alkalinity at the presence of the fluorine in the fluid. The minerals of the banalsite-stronalsite group in sandyites points to a wide range of temperatures during metasomatic processes. Alteration of rocks results of the change of their chemical composition: the ТiO2, MgО, СаО, total Fe, and LREE contents increased and SiO2, Al2O3, and К2O contents decreased. The rocks are characterized by high REE contents in contrast to the host mylonitized miaskites and the high contents of lithophile elements (Rb, Ba, Sr, Th) and low contents of Co, Cu, W, Ni, Cr, and Pb. Our data indicate the metasomatic origin of the studied rocks. The increase in the contents of trace and rare earth elements from milonitized miaskites to sandyites and monzonitic rocks reflect their mobility and significant role of assimilation of the continental crust. The mobility of these elements and their redistribution in the rocks increase during active influence of fluid and growth in it the contents of alkalis and fluorine. Thus, sandyites of the Ilmenogorsky miaskite block are metasomatic rocks which produced after mylonitized miaskites in the linear tectonic zones during late postcollisional shear stage. The monzonitic rocks are ortho-rocks and were probably formed after country diorites. They kept relics of primary structures, but were transformed simultaneously with sandyites under influence of multistage tectonic-metamorphic processes.
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