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a)
b)
9000
9000
Olivine
Olivine
To garnet
8000
8000
Pyroxene
Pyroxene
An
An
Amphibole
Amphibole
7000
7000
Plagioclase
feldspar
Plagioclase
feldspar
Ab
Ab
Quartz
6000
6000
Biotite
5000
5000
4000
4000
2.5
3.0
3.5
2.5
3.0
3.5
Density (g/cm
3
)
Density (g/cm
3
)
Figure 6.33
Effects of metamorphic grade on the seismic
velocity and density of mafic rocks. (a) Low- to medium-
grade metamorphism. Based on data in Fox et al.(
1973
).
(b) Medium- to high-grade metamorphism. Based on data
actinolite, An
-
anorthite, Chl
-
chlorite, Hbl
-
hornblende.
The red dashed line is the average crustal density of
2.67 g/cm
3
. The broken lines are contours of acoustic
impedance with their separation representing the contrast
required to produce a re
Fresh
Slightly weathered gabbro
Slightly metamorph' gabbro
Chl metagabbro
Act-Chl metagabbro
Act metagabbro
Hbl-Act-Chl metagabbro
Hbl metagabbro
Rock-forming minerals
Unmetamorphosed
Lower amphibolite facies
Upper amphibolite facies
Eclogite facies (Gp I)
Eclogite facies (Gp II)
Rock-forming minerals
Zeolite?
facies
Greenschist
facies
Amphibolite
facies
Increasing metamorphic grade
ection coef
cient of 0.05.
6.6.3
Effects of metamorphism, alteration
and deformation
amphibole, biotite, garnet and pyroxene. There is an asso-
ciated decrease in velocity and a slight increase in density,
the latter in contrast with the data from the oceanic
samples. Velocity decreases owing to the change in plagio-
clase composition (note the lower density and velocity of
albite) and the addition of biotite. The transformation to
upper amphibolite facies produces a mineral assemblage
comprising plagioclase, garnet, pyroxene, plus or minus
amphibole. The disappearance of biotite and the increasing
volume of garnet cause both velocity and density to
increase.
Two groups of rocks are recognised at eclogite facies
(
Fig. 6.33b
) and are thought to be the result of different
protolith compositions. In Group I, the change to eclogite
facies creates a mineral assemblage comprising plagioclase,
garnet, amphibole and pyroxene, with the latter three
minerals occurring as a greater proportion of the rock.
The large range in physical properties reflects the variable
plagioclase content. Two samples have similar properties
to amphibolite facies rocks, but one shows a substantial
increase in velocity, density and acoustic impedance.
Group II is more ma
The relationship between metamorphic grade and seismic
properties is not well documented and is illustrated here
using a ma
c protolith. The low porosities of crystalline
rocks cause mineralogy to be the dominant control. The
effects of initial alteration and low-grade metamorphism
are demonstrated with samples of gabbroic rocks from the
Atlantic Ocean (Fox et al.,
1973
), and shown in
Fig. 6.33a
.
Metamorphic grade ranges from zero to amphibolite facies.
Development of metamorphic minerals such as albite,
actinolite and chlorite at the expense of the original assem-
blage (plagioclase, pyroxene, olivine) leads to a markedly
reduced velocity and, to a lesser extent, density.
The effects of higher grades of metamorphism on mafic
rocks are illustrated in
Fig. 6.33b
which is based on petro-
physical measurements made on compositionally similar
gabbros and metagabbros from the Grenville Province in
eastern Québec, Canada (Hurich et al.,
2001
). Meta-
morphic grade ranges from sub-greenschist to eclogite
facies, but granulite facies data are not available. As with
the oceanic samples, the igneous assemblage consists of
plagioclase, olivine and pyroxene. The lower amphibolite
facies assemblage comprises plagioclase (more sodic),
c. It does not contain plagioclase
but shows increases in the amounts of garnet, pyroxene
and amphibole, which produce signi
cant increases in
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