SOME PETROPHYSICAL PROPERTIES OF THE MAIN ROCKS FROM THE ARCHAEAN SECTION OF THE KOLA SUPERDEEP BOREHOLE SG-3

The values of density and velocity in the samples of the main rocks from the Archaean section of the Kola superdeep borehole (SG-3)  drilled in the northern frame of the Pechenga palaeorift structure  were studied. The predominant part of rocks of the SG-3 Archaean  section is represented by gneisses, schists and amphibolites. Their  main rock forming minerals are plagioclase, hornblende, mica and  quartz. The structure of the rocks is mainly medium-grained, the  texture is nematogranoblastic and lepidogranoblastic. The elastic  anisotropy and orientation of the rock texture were estimated by the acoustopolariscopy method, the density and velocity of compression and shear waves were determined in the laboratory and the  mentioned characteristics were calculated by the mineral  composition. The measured sample velocities are unusually low, which can be explained by the decompaction effect of deep rocks.  The values of the velocity characteristics that are close to those for  the conditions of the rock deep occurrence are obtained by  calculation with regard to their specific mineral composition. The  average values of the velocity of compression and shear waves for gneisses calculated by the mineral composition are 6.38 ± 0.16  km/s and 3.52 ± 0.14 km/s, respectively. The average of the  compression wave velocity for schists is 6.40 ± 0.13 km/s, of the  shear wave velocity – 3.46 ± 0.09 km/s, and for amphibolites 6.84±0.13 km/s and 3.82±0.08 km/s, respectively. The ratios of the values of compression and shear moduli in different rocks exhibit the same trends as the average velocities. However, their values are  lower in schists, medium in gneisses and grater in amphibolites.

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