Vendian tectono-magmatic events in mantle ophiolitic complexes of the polar Urals: U-Pb dating of zircon from chromitite

Geological and isotopic evidence of Late Vendian magmatic events in restitic ultramafic mantle rocks of the Voikar-Syn’ya ophiolitic massif are considered and correlated with events at the eastern margin of the East European Platform. The geological and isotopic data show that the ophiolitic complexes of the Polar Urals were formed during several stages. The percolation of melts through peridotites was recorded in the newly formed mineral assemblages, for example, olivine + chromite ± zircon. Zircon crystallized from the residual fraction of the evolved basic melt that impregnated peridotite. The active interaction of hot restitic harzburgite with the migrating melt resumed repeatedly and could have led to the formation of several generations of chromite-bearing dunite. An important geological inference can be made from this suggestion: There is a high probability that isotopic markers of different age have been retained in restitic mantle complexes of ophiolites. The U-Pb dating of zircons with a SHRIMP-2 ion microprobe has shown that the isotopic age of seven grains is 585.3 ± 6 Ma (MSWD is 0.036 and the probability of concordance is 0.85). The obtained age of zircon from chromitite marks a Vendian tectonomagmatic event that occurred in the upper mantle of the transitional zone between the East European Plate and the oceanic basin. The island-arc complexes of the Polar Urals developed on the tectonically juxtaposed fragments of the Early Paleozoic and pre-Paleozoic oceanic crust. These crustal rocks were reworked during younger magmatic events related to the origin of the Middle Paleozoic island arcs. As a result, the rocks that formed in different geological epochs were locally retained in the restitic mantle complexes of a spatially indivisible ophiolitic association.