SEISMOGENIC FRICTIONAL MELTING AND ITS ROLE IN THE REDISTRIBUTION OF MATTER IN THE METAMORPHIC COMPLEXES OF ZAVALLIA

Abstract

Using the example of the Zavallivske graphite deposit (Ukrainian Shield), the role of palaeoseismic activity in the formation of hydrothermal systems and ore zones is examined. A marker of ancient earthquakes is pseudotachylites – black vitrified rocks, represented by slip films and thin veins. Pseudotachylites are products of frictional melting in fault zones. Their formation is accompanied by the localisation of thermal energy, rapid cooling and the formation of an amorphous glass-like substance, as confirmed by X-ray diffraction studies. Microprobe analysis of the pseudotachylites revealed a high SiO₂ content (up to 73.84 wt. %) and significant variations in FeO (from 5.56 to 34.79 wt. %) and MgO (3.62–11.61 wt. %). Calculations of the oxides indicated the presence of water (up to 36 wt. %), which acted as a kind of flux and a melting point depression factor. The source of water was likely the dehydration of minerals under conditions of rapid heating during seismic events. This is evidence of a close link between friction melting processes and the mobilisation of fluids in the fault zone. As a result, a local environment enriched with volatile components was formed. About 10 % of the volume of the studied rocks consists of fragments of primary rocks and minerals that either did not melt completely or were entrapped by the melt during formation. Pseudotachylites formed at the transition boundary from plastic to brittle deformation, at depths of 10–12 km, corresponding to the conditions of the amphibolite facies of metamorphism. Mylonites formed below this boundary, and cataclastic rocks ‒ above it. Friction melting processes were accompanied by the dehydration of the surrounding primary rocks, the formation of fluids and fracture systems. The fluids, in turn, facilitated the migration and redistribution of chemical elements. And the extensive fracture systems facilitated the circulation of hydrothermal solutions and the development of vein-type mineralisation. Pseudotachylites are an important indicator of palaeoseismic activity and a trigger for hydrothermal processes. Hydrothermal fluids played a significant role in the formation of ore mineralisation at the Zavallivske deposit, particularly in the redistribution of material and the formation of graphite deposits.