1. Geological map of the massifs of Lake Chudzjavr
Density and velocity model of metamorphic rock properties in the upper, middle and lower crust in the geospace of the Kola superdeep borehole (SG-3)
Geological map of the massifs of Lake Chudzjavr
Twenty samples of gabbro, tonalite, enderbite and garnet-biotite gneiss selected at the outcrops of Lake Chudzjavr have been studied.
Dependences of temperature and stresses
along the Kola superdeep borehole section
Geological map of the north-western Kandalaksha coast of the Kola Peninsula
The distribution of the values of density  (a), shear VS (b) and compression VP (c) wave velocities along the Kola SG-3 section (experimental data) and within the crystalline crust (calculation)
(I) The distribution of the density values  along the SG-3 section. 1 - unit values measured on individual core samples; 2 - mean interval values.
(II) - The values of the compression and shear wave velocities in the SG-3 section. 1 – method of vertical seismic profiling (VSP); 2 - method of acoustic logging (AL); 3 - calculation method (CM) applied to rocks of gneiss-granite composition; 4 - calculation method for amphibole-containing rocks; 5 - calculation method, confidence limits of the averages over the intervals.
(III) - Variations in the density, compression and shear wave velocities for the 10 ‑ 25 km interval: 1 ‑ calculation of the , Vp, VS values from the depth for sample 31115; 2 ‑ ditto for sample 35400; 3 ‑ ditto for 36058; 4‑ 38098S; 5 ‑ 43560; 6‑ to (IV) The data for different depth intervals 1 ‑ km (SG-3), 2 ‑ km (Lake Chudzjavr), 3 – 21 ‑ 37 km (Elovy Island).
In the north-western part of the Kola Peninsula, the lower boundary of the upper crust is drawn at depths of km. Below down to depths of km and km the middle crust of supposedly diorite composition and the basic lower crust, respectively, have been distinguished.
According to the geological evaluation rocks from the Lake Chudzjavr area formed under the PT conditions (P = 3-5 kbar, T = °C) adequate to the conditions of the middle crust, but as a result of the geological process they were evacuated to the earth's surface.
Changes in the density of SG-3 core samples versus the applied confining pressure (a) at a constant temperature of 20°C and temperature increase (b) at the confining pressure of 600 MPa [Керн et al., 2001]
1 Palaeozoic igneous rocks: a ‑ dykes of ultrabasic lamprophyres, carbonatites, melilitites; b – volcanic pipes; c ‑ the intrusion of alkaline-ultrabasic composition; 2 ‑ sandstones of Tersky and Turinsky Formations (Riphean); 3-6 ‑ Palaeoproterozoic rocks: 3 ‑ enderbite granitic rocks of the Umba complex; 4 ‑ gabbro-anorthosites, gabbros, diorites; 5 ‑ basic (а) and acidic (b) granulites; 6 – andesite basalts, amphibolites; 7 ‑ gneisses, tonalites, amphibolites of the base complex (Late Archaean); 8 – the area of investigations.
The trends in petrophysical parameters reveal a possibility of estimating the content of various associations of rock-forming minerals using the deep exploration methods.
Horizontal tectonic stresses vary from 7 MPa to 125 MPa along the Kola superdeep borehole section. The average ratio of the working horizontal stresses (including the value of the lateral pressure that emerged under the action of the vertical component, and the tectonic component) to the vertical stresses is
The average gradient of the temperature rise at depths over 5 km is °C/100 m.
Legend: 1- dykes of diabases, 2 monzonites, monzodiorites, 3 tonalite gneisses, 4 aluminous gneisses.
Acoustopolarigrams of xenolith samples with predominant plagioclase content
Acoustopolarigrams of the samples from the Lake Chudzjavr massif
Conclusions The gradient of the increase in seismic velocities in the crystalline crust is caused not by the increase in PT conditions with depth, but by the increase in the proportion of high-velocity minerals. It is known that the change in the mineral composition of rocks is going on for a long time, as a result of changes in the PT conditions in depth and metamorphic transformation of some minerals into others, for example, plagioclase and amphibole into pyroxene and garnet.
The obtained quantitative characteristics of the elastic anisotropy of garnet granulites showed that within the lower crust there are rocks with less elastic anisotropy than within the upper and middle crust.
Changes in the compression VP and shear wave VS velocities in sample versus the confining pressure(а, c) and temperature (b, d) [Керн et al., 2001]
k2-09-1
k2-09-2
k B
k B
Characteristics of the lower crust rocks (according to xenoliths of Elovy Island)
Variations in the xenolith density values are g/cm3 (mean ρC = 3.1 g/cm3). The velocity values of VPC and VSC for 15 xenolith samples are in the range of 6.51‑7.73 km/s and 3.61‑4.57 km/s, respectively. The value of VPC is quite clearly related to the amount of garnet and to a lesser extent ‑ to clinopyroxene.
The determinations of anisotropy on xenolith samples allow one to conclude that the differences between the vertical and horizontal components of the stress field at great depths (~30 km) are close to quasi-hydrostatic ones.
Acoustopolarigrams of xenolith samples with predominant garnet content
Photos of thin sections of of xenolith samples of Elovy Island (nikols crossed)
а) k2-09-1
b) k B
c) k2-09-2
d) k A
e) k B
f) k B
g) k B
h) k A
Rocks at the depth of occurrence (in situ) corresponding to the depth of the mineral parageneses formation have a density of ρ = g/cm3. The change range of the compression wave velocities for rocks under РТ-conditions of the middle crust is limited to km/s. The shear wave velocity remains practically constant with depth. Variations of its changes are km/s.
Photos of thin sections of gneisses and granitoids of Lake Chudzjavr (nikols crossed) 1 - Metagabbro (Chu-10-01a); 2 - Enderbite (Chu-10-02); 3 - Ophitic gabbro (Chu ); 4 - Enderbite (Chu ); 5 - Tonalite (Chu-10-09б); 6 - Garnet-biotite gneiss (Chu ); 7 - Leucoenderbite (Chu-10-17); 8 - Tonalite (Chu ).
The distribution of the density values r (a), shear VS (b) and compression VP (c) wave velocities along the Kola SG-3 section (experimental data). 1 - method of vertical seismic profiling (VSP); 2 - method of acoustic logging (AL); 3 - calculation method (CM).
Feliks F. Gorbatsevich, Mikhail V. Kovalevskiy, Olga M. Trishina. Geological Institute, Kola Science Centre RAS, Russia.
k B
k A
k A