Reconstruction of the deep process based on the analysis of the magmatic rocks composition (on the example of the Osnytsk and Buky complexes of the Volyn megablock of the Ukrainian Shield)

The article summarizes the data on the composition of the Osnitsk and Buky magmatic complexes within the Osnytsko-Mikashevytsky volcano-plutonic belt and Volyn megablock, Ukrainian Shield. The РT-conditions for the melts formation, from which, the rocks of the specified complexes crystallized were determined. This makes it possible to determine the depth of the thermal asthenosphere from where the melts came from the composition of the fluids accompanying the differentiation and to outline the probable tectonic setting under which they were formed. Analysis of both complex compositions makes it possible to assume basalts and magnesianandesites may appear in the melting layer at Р~1.2÷1.5 GPa, and Т>1400 °С. Here were mixed ultrabasic melts formed with the participation of carbonate fluids from the deep source, and andesite-basalts, formed with the presence of water fluids with chlorine. These could have arisen due to the melting of the crust, which subducted into the layer of partial melting at the depth of about 50 km. Granitoids are developed within the Osnytsko-Mikashevytsky volcano-plutonic belt, indicating a sell of melting in the crust, which is at least 40 km thick. Layering in the massifs located near the surface was not a result of crystallization differentiation in the magmatic cell, but a result of the multiple income of melt from melting layers during periodic tectonic movements. The composition of magmatic complexes formed 2.0―1.97 billion years ago does not contradict the existing scheme of the Osnytsko-Mikashevytsky volcano-plutonic belt formation as a result of Fennoscandia subduction under Sarmatia, but makes it mandatory to include advective link ― the plume process in the adjacent territory.