Microstructural features of ophiolitic chromitites in the Kraka massif, Southern Urals. II. Podiform massive ores

Saveliev D. E., Shilovskykh V. V., Sergeev S. N.

Zapiski RMO (Proceedings of the Russian Mineralogical Society). 2020. V. 149. N 6. P. 52-68

https://doi.org/10.31857/S0869605520060118

Language: Russian

Abstract

We have studied microstructures and compositions of massive and nodular chromitites from the podiform ore body located in spinel peridotites of the mantle section of ophiolites andenveloped by a thin dunite rim. EBSD analysis of these chromitites allowed us to reconstruct a morphology of primary grains and to reveal an internal heterogeneity of individuals expressed in their domain structure. It is shown that chromitite heterogeneity was caused by a high temperature solid state flow of upper mantle matter during the ore body formation. It is assumed that microstructure of studied chromitites were formed while mutual influence of several processes: translation spipping, syntectonic recrystallization and growth during progressive grain aggregation. The latter is confirmed by frequently observed increase of the grain size in massive chromitites as compared with disseminated ones. Secondary processes in studied chromitites are limited by low-temperature serpentinization which takes place simultaneously with tectonic emplacement of ophiolites in the upper part of crust. This process leads to olivine conversion to mesh serpentine and is accompanied with increase in volume of rocks. Eventually, it causes a chromitite fracturing.

Keywords: podiform chromitite, ophiolite, chromian spinel, plastic deformation, recrystallization, microstructure, EBSD, ultramafic rocks, Kraka, Southern Urals