The chemistry of accessory spinels from the ultramafic massifs of the Southern Urals as the reflect of their tectonic environment

In the present-day geological framework of the Southern Urals ultramafic rocks is formed by a number of parallel belts. The changes in the composition of accessory spinels show certain regularity and follow to West - East trend. Spinels from the western part of the fold belt have the lowest Cr/Al ratio, the highest value is in ultramafic rocks on the edge of Magnitogorsk megazone. The values for East-Urals megazone lie between the two extremes. The first group belongs to the upper mantle of the continental margin rift (Kraka, Nurali), which transforms into the mid oceanic ridge (Chabarny). The second group is similar to the ultramafic rocks of the island arc basement (Kalkan, Ishkininsky, Kulikovsky). The third group of ultramafic rocks was formed in the upper mantle of the back-arc basin (Varshavsky, Verbljugegorsky, Tatischevsky).