Results of structural-petrological and mineralogical-geochemical studies of chromitite-bearing zone within spinel peridotites of the South Kraka ophiolite massif are presented. Symmetric petrographic zonation of the deposit (peridotites – dunites – chromitites – dunites – peridotites) is accompanied by mineralogical-geochemical zonation, which is manifested in a regular increase in the Cr# value of spinel and Mg# value of olivine from peridotites to dunites and chromitites. A chemical gap between the composition of spinel from peridotites and dunites/chromitites is replaced by gradual compositional transition at the contacts between host peridotite and fne dunite veins. The contact zones exhibit the features of deformation-induced processes: 1) segregation of trace elements in olivine in form of newly formed acicular chromite and 2) decomposition of large enstatite-1 crystals leading to the formation of Na-Cr-pargasite and smallest grains of chromites, forsterite, and enstatite-2 depleted in elements as a restite. It is shown that all rocks of the studied area are characterized by deformation textures and structures, which are caused by plastic fow under high-PT conditions. The host dunites, where olivine was deformed predominantly along the {0kl}[100] sliding system, represented the most mobile zone of the plastic fow. Evidences for rheomorphic diferentiation of the mantle ultramafc rocks, which results in the formation of chromite ores, are given.

Figures 13. Tables 5. References 23.

Key words: mantle ultramafc rocks, chromite ore, plastic deformation, olivine, enstatite, chromite, Kraka massif, South Urals.

D.E. Saveliev,  Institute of Geology USC RAS, Ufa, savl71@mail.ru

I.A. Blinov, Institute of Mineralogy UB RAS, Miass;

I.I. Musabirov, Institute for Metals Superplasticity Problems RAS, Ufa

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