Solid-state redistribution of mineral particles in the upwelling mantle flow as a mechanism of chromite concentration in the ophiolite ultramafic rocks (on the example of Kraka ophiolite, the Southern Urals)

Abstract

The main regularities of the structure of chromitite-bearing zones of ultramafic rock of the ophiolitic association are considered on the example of Kraka massifs. In all studied chromitite-bearing zones, olivine demonstrates a strong preferably crystallographic orientation, indicating that plastic flow was one of the main factors of petrogenesis and ore formation. A critical review of existing ideas about the origin of ophiolitic chromitites has been carried out. It is shown that for models involving the reaction and magmatic formation of dunites and chromitites, there are a number of difficulties. In particular, the application of the magma mixing model to the mantle ultramafiс rocks for the formation of chrome ores is faced with the problem of “free space”. Free space is necessary for the deposition of large volumes of ores, which is absent in a very low-porous crystalline upper mantle. In the “melt-mantle” interaction model, it is difficult to explain the often observed abrupt contacts of dunites and harzburgites, as well as an increase in the content of orthopyroxene in the near-contact parts of harzburgites, which is very often observed in ophiolite massifs. In addition, there is no mechanism for the formation of chromitites as geological bodies in this model. We have shown that the main trend in the composition and structure of the mantle section of ophiolites is stratification, accompanied by the separation of the rheologically most “weak” aggregates of polycrystalline olivine (dunites), which are host rocks for chrome ores. The stratification of the mantle material occurred during the solid-phase redistribution of minerals in the rocks, which are a dispersion system. In this work, a thermodynamic model is substantiated, which demonstrates the possibility of the emergence of solid-state flows in the conditions of the upper mantle and which makes it possible to eliminate some of the difficulties and contradictions characteristic of the magmatic and reaction-magmatic hypotheses.