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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)
Dependences of temperature and stresses
along the Kola superdeep borehole section
In the north-western part of the Kola Peninsula,
the lower boundary of the upper crust is drawn
at depths of 12-15 km. Below down to depths
of 20-21 km and 37-38 km the middle crust of
supposedly diorite composition and the basic
lower crust, respectively, have been
distinguished.
Twenty samples of gabbro,
tonalite, enderbite and garnetbiotite gneiss selected at the
outcrops of Lake Chudzjavr
have been studied.
Geological map of the massifs
of Lake Chudzjavr
According to the geological
evaluation rocks from the Lake
Chudzjavr area formed under
the PT conditions (P = 3-5 kbar, T
= 710-790°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]
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 0.7-0.8.
The average gradient of the temperature rise at
depths over 5 km is 2.0-2.3°C/100 m.
Legend: 1- dykes of diabases, 2
monzonites, monzodiorites, 3 tonalite
gneisses, 4 aluminous gneisses.
The trends in petrophysical
parameters reveal a possibility of
estimating the content of various
associations of rock-forming
minerals using the deep
exploration methods.
(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.
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.
(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 43726.
(IV) The data for different depth intervals 1 - 10-25 km (SG-3),
2 - 12-21 km (Lake Chudzjavr), 3 – 21 - 37 km (Elovy Island).
Acoustopolarigrams of
xenolith samples with
predominant plagioclase
content
Acoustopolarigrams of the
samples from the Lake Chudzjavr
massif
Changes in the compression VP and shear wave
VS velocities in sample 31115 versus the
confining pressure(а, c) and temperature (b, d)
[Керн et al., 2001]
a)
b)
c)
d)
k2-09-1
k2-09-2
k2-09-02-5B
k2-09-02-6B
Acoustopolarigrams of
xenolith samples with
predominant garnet content
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).
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)
Geological map of the north-western Kandalaksha
coast of the Kola Peninsula
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-10-07-1); 4
- Enderbite (Chu 10-08-1); 5 Tonalite (Chu-10-09б); 6 Garnet-biotite gneiss (Chu-1012-1); 7 - Leucoenderbite (Chu10-17); 8 - Tonalite (Chu-10-201).
Rocks at the depth of occurrence (in
situ) corresponding to the depth of the
mineral parageneses formation have a
density of ρ = 2.65-3.0 g/cm3. The
change range of the compression wave
velocities for rocks under РТ-conditions
of the middle crust is limited to 6.0-6.6
km/s. The shear wave velocity remains
practically constant with depth.
Variations of its changes are 3.5-3.7
km/s.
Photos of thin sections of of
xenolith samples of Elovy
Island (nikols crossed)
а) k2-09-1
b) k2-09-02-1B
c) k2-09-2
d) k2-09-02-2A
e) k2-09-02-5B
f) k2-09-02-2B
g) k2-09-02-6B
h) k2-09-02-3A
a)
b)
c)
d)
k2-09-02-1B
k2-09-02-1B
k2-09-02-2A
k2-09-02-3A
Characteristics of the lower crust
rocks (according to xenoliths of
Elovy Island)
Variations in the xenolith density values
are 2.74-3.55 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.
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.
Feliks F. Gorbatsevich, Mikhail V. Kovalevskiy,
Olga M. Trishina. Geological Institute, Kola
Science Centre RAS, Russia. E-mail:
[email protected]