Title

K.S. IVANOV, YU.V. EROKHIN, V.V. KHILLER (IGG UB RAS), N.V. RODIONOV (VSEGEI), P.A. SEROV (GI KSC RAS)

Concerning the problem of Precambrian of the Urals eastern slope (about age and nature of the Ilmenogorsk zone of the Selyankinskaya series)

Among gneiss­-magmatic complexes of the Urals eastern slope the type one is the Ilmenogorsky complex, at the basement of which one can distinguish the gneiss Selyankinskaya series of amphibolite facies. Its age has been considered as Early Proterozoic. Mineralogy and geochronology of the series key section in the N­W part of the Ilmen state reservation have been studied. New data shows that these gneisses have been formed as metamorphic rocks not in Precambrian but in Paleozoic. On the example of zircons it was shown that ancient material (~2 Ga) in these rocks composition has been noted merely in the shape of rounded grains (possibly, these are the products of washout and removal from the Russian platform). The monazite Th-­U-­Pb dating has been fulfilled, two isochrones (323±28 and 275±22 Ma), determining the gneiss metamorphism age, have been obtained. Sm­Nd isochrones on gneiss and minerals from it also display metamorphism stages (468 and 276 Ma) of ancient detrital material. Bearing in mind that the Ilmenogorsk zone metamorphites are situated directly east of the Main Uralian fault zone (in its hanging wing) being in Middle Paleozoic the subduction zone, these metamorphic complexes, possibly, present themselves deep parts of an island arc. In the same gneisses Ar­Ar determination on biotite (242 Ma) dates the moving of metamorphic complexes to the level of upper crust during the post­collision sublatitudinal extension of the Urals.
Keywords: gneisses, eastern slope of the Urals, zircons, U­-Pb and Sm-­Nd dating, monazites, chemical dating, age, Paleozoic.