Magmatic complexes of the Urals as suspect parts of Large Igneous Provinces

Abstract

Petrogenetic, geochemical studies and isotope age determinations of flood basalts, dolerites, trachybasalts, picrite-basalts, rapakivi granites, layered mafic-ultramafic intrusions and also alkaline and carbonatite magmatic complexes of the Urals permit to put forward a preliminary list of objects - “candidates” at being attributed to Large Igneous Provinces (LIPs) - manifestations of superplume activity. Their petro-geochemical properties distinguish them from spreading and subduction types, and are closer to epicontinental rift zones. They are characterized by wide areas of development and very short periods of activity. In the Southern Urals near the base of the Lower Riphean (Uppermost Paleoproterozoic and Lower Mesoproterozoic) there are volcanic deposits of the Navysh Subformation, represented by trachybasalts. The age of the unit was determined as 1752 ± llMa. Volcanic rocks of the age level of 1750-1780 Ma are developed not only in some other places of Baltica, but also in the Northern Africa, Siberia, Laurentia (parts of Nuna supercontinent). Therefore, they may belong to a LIP. Higher up the section of the Riphean, at the base of the Middle Riphean (Mid-Mesoproterozoic), rhyolites of the basalt-rhyolite Mashak Formation were dated as 1380-1385 Ma. The same ages have also rapakivi granites, layered gabbro, carbonatites and dolerite dykes developed in the Southern Urals and encountered in boreholes of the East European platform; magmatic rocks of the same age are traced to Laurentia and Siberian cratons and date the beginning of Nuna supercontinent break-up. Less confidently we may speak of the younger Neoproterozoic magmatic complexes of the Southern Urals as LIPs, dated as ca. 720 Ma and 680 Ma (Arshinian and Kiryabinka complexes); they need a further study. The next in the succession of magmatic episodes, represented by subalkaline volcanics, is connected with a rift process that started at ca. 490 Ma, that led to oceanic spreading and formation of the Paleouralian ocean. The second Paleozoic episode was marked by an eruption of trachytes and carbonatites and is dated between 435 and 455 Ma. The younger complex is Devonian in age and is traced along the western slope of the Urals to Pay-Khoy and Novaya Zemlya. They belong to the LIP called Kola-Dnieper. The last but not the least are the Lower Triassic flood basalts and dykes traced from the easternmost parts of the Southern and Middle Urals to the western margin of the Polar Urals.