Известия Томского политехнического университета: Инжиниринг георесурсов (Aug 2016)
Heterogeneity of plastic deformations of olivine in ultramafic rocks of the Baikal-Muya ophiolite belt (north-east of Baikal)
Abstract
Relevance of the research. Permanently identified signs of plastic flow in ultramafic rocks predetermined an approach to their study as metamorphic rocks. This approach uses non-traditional method of the petrofabric analysis. This method allows reconstructing the chronological sequence of formation and plastic deformation of ultramafic rocks in the upper mantle-crust, revealing the general trend of structural and matter evolution, and solving a number of urgent problems of the ultramafic mineral genesis. The aim of the study is to develop a scenario of the structural evolution of the Paramsky and Shamansky ultramafic massifs of Baikal-Muya ophiolite belt at the upper mantle-crust levels through the dunite and harzburgite microstructural heterogeneity, typing their microstructures, calculating olivine-chromospinelide thermal equilibrium, and olivine petrofabric analysis. Methods. The detailed petrographic characteristics of ultramafic rocks were performed by the polarizing microscope AxioScope-40 (Carl Zeiss). This study allowed carrying out microstructural typification of dunites and harzburgites. It was based on the morphological features of olivine and it was developed by many researchers. Quantification of deformation microstructures in olivine of ultramafic rocks was studied using the stereometric metallography. Microstructural analysis of olivine is an integral part of the petrofabric study of the ultramafic rocks. It allows determining the preferred orientation of minerals in the internal structure, which in their turn is a reflection of the thermodynamic conditions of their plastic deformation. The olivine and chromospinelide matter composition was determined using micro X-ray spectrum, obtained by microanalyser «Camebax» in the Institute of Petroleum Geology and Geophysics of Siberian Branch of Russian Academy of Sciences (Novosibirsk). The thermal equilibrium was calculated using olivine-chromospinelide Fabry geothermometer on the matter composition of coexisting olivine and chromospinelide. Results. The research provided the identification of the deformation microstructures of olivine for dunites and harzburgites of the Paramsky and Shamansky ultramafic massifs of the Baikal-Muya ophiolite belt. They are grouped into six types: protogranular, mesogranular porphyroclastic, porphyrolath, mosaic (mosaic-lath), mosaic-parquet-like. The spatial distribution of the allocated microstruct ures in the studied massifs from the сenter to the periphery is characterized by reducing the grain size in the rocks. This feature shows the increase in their plastic deformation degree and reflects the dynamometamorphiс zoning. The petrofabric analysis of the olivine in the identified microstructural types allowed determining thermodynamic conditions of their realization. These microstructures reflect a regressive trend of metamorphic transformations of the ultramafic rocks by high-temperature plastic deformations, syntectonic recrystallization, an d secondary annealing recrystallization in formation, transformation and consolidation in the upper mantle - earthґs crust. The identified evolution of the olivine microstructures reflects the limits of repeatedly overlaid plastic deformations, which can be decisive in l ocalization of chromite, chrysotile asbestos, jade, and jadeite within ultramafic rocks of ophiolite complexes under favorable conditions.
