Литосфера (Oct 2017)
Petrology, u-pb isotope age and metamorphism of nyashevo polygenic mafit-ultramafite massive (the southern urals)
Abstract
In the article is presented data of mineralogy, petrography, geochemistry and isotope-geochronology study Nyashevo mafic-ultramafic massif in Ilmeny-Vishnevogorsk polymethamorphic shear zone. The ultramafic rocks consist of serpentine and chlorite (up to 90 %), accessory and rare minerals - talc, carbonate, tremolite, cromium-magnetite, amphibole, garnet, biotite, muscovite, kyanite, scapolite, potassium-feldspar, zircon and quartz. The microprobe analysis of the composition of minerals was performed with a REMMA-202M scanning microscope with a microanalyzer. The contents of petrogenic, rare and rare-earth elements are determined by the atomic absorption method and by the ICP-MS mass spectrometer. U-Pb age in zircon was obtained on microprobe SHRIMP II. The Nyashevo ultramafic rocks have variations of the content of SiO2 from 38 to 42 wt % and of #Mg from 0.89 to 0.92, minor concentration of Al2O3 from 0.6 to 2.0 wt % and CaO from 0.3 to 2.8 wt %, increase contents Cr (550-2400) and Ni (800-2000 ppm), low - Ti (to 75), HSFE (6.0-117.4) и REE (∑ = 1.3-4.3ppm). Relatively high content Rb = 0.10-0.57, Sr = 3-42, Ba = 3-95 ppm and relation Ba/Sr = 0.4-7.7. The 1892 ± 23 Ma of zircons dating from ultramafic rocks of massif are the minimum age of the mantle substrate, probably is his restite. The appearance of pyrope-almandine ( Py18-26 Grs3-4) and almandine-grossular ( Py6-8 Grs54-57) garnets, high- aluminous and magnesian alumoenstatite (#Mg = 0.77) and spinel (#Mg = 0.72-0.75) in ultramafic rocks suggests formation-transformation occurred at Т = 900-1050°С и Р ≥ 9 kbar. The formations of Nyashevo massif is the age of the Late Ordovician (443 ± 12 Ma) and almost coincides with the maximum marks of age Buldym ultramafic alkaline-carbonatite massif. The Permian (275.8 ± 2.1 Ma) dating reflects the processes of serpentinization and scapolitization in ultramafic rocks and it correlated with the age parameters of zircons from late miaskites, carbonatites and pegmatites. Nyashevo mafic-ultramafic massif was formed in the deep zone of the Proterozoic continental rift, it undergone a multistage metamorphic transformation during decompression ascent in the lower crust and subsequent disintegration in regional post-collision shear zone.
