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is the reason for their density range extending to low
values, similar to those of felsic rocks etc.
Figure 3.30
shows that metamorphosed sedimentary
rocks, e.g. marble, slate and quartzite, have much smaller
density ranges than their sedimentary precursors,
limestone, shale and sandstone respectively. Their densities
coincide with the upper limits of their precursors owing to
decreases in porosity, volume loss and the replacement of
low-density minerals, e.g. clay minerals, by higher-density
minerals such as micas. Metamorphic minerals, such as
garnet and kyanite, tend to have higher densities than most
igneous and sedimentary minerals so their appearance can
also lead to density increases.
Although hindered by the need to compare equivalent
lithotypes, a limited number of studies have shown that
there is a general correlation between metamorphic grade
and density in crystalline rocks. Bourne et al.(
1993
)
investigated the densities of mafic and ultramafic rocks
from Western Australian greenstone belts where the
rocks are at greenschist and amphibolite facies (
Fig. 3.35
).
Whole rock geochemistry confirmed the similarities of
the lithologies from the different greenstone belts. Mafic
rocks were found to increase in density by about 0.1 g/cm
3
.
Thin section analysis suggested that the density change
could be explained by the consumption of plagioclase
(2.6
3.30
3.26 g/cm
3
=3.30
-
0.00785 * (% serpentine)
Miller & Christensen (1997)
3.10
= 3.264
-
0.00748 * (% serpentine)
(K
o
mor
et al.
, 1985)
2.90
2.70
2.52 g/cm
3
2.50
0
20
40
60
80
100
Modal serpentine (vol %)
Figure 3.34
Inverse linear relationship between density and
degree of serpentinisation as reported by Komor et al.(
1985
) and,
similarly, by Miller and Christensen (
1997
). The data are from a
predominantly dunite-wehrlite suite from the Bay of Island
ophiolite in western Newfoundland, Canada. Serpentinisation in
this case primarily affects olivine. Deviations from the main trend
are caused by inter-sample variations in the amounts of unaltered
olivine, orthopyroxene, plus spinel and the magnetite and brucite
formed during serpentinisation. Redrawn with additions, with
2.8 g/cm
3
)
-
in reaction with actinolite/tremolite
3.5 g/cm
3
). Granites
from the same areas were found to have very similar dens-
ities, re
ecting their more stable felsic mineral assemblages.
from about 2.75 to 2.81 g/cm
3
, associated with the change
from amphibolite to granulite conditions in a Scandinavian
high-grade gneiss terrain. However, the scatter of densities
3.2 g/cm
3
) to hornblende (3.0
(2.9
-
-
Whole rock composition
(wt %)
25
Mafic rocks
TiO
2
+ Fe
2
O
3
20
15
Amphibolite
facies
10
Greenschist
facies
5
0
Al
2
O
3
MgO
2.5
2.7
2.9
3.1
3.3
Density (g/cm
3
)
Whole rock composition
(wt %)
25
Ultramafic rocks
TiO
2
+ Fe
2
O
3
20
Greenschist
facies
15
Amphibolite
facies
Figure 3.35
Frequency histograms of the density of ma
c
10
and ultrama
c rocks from two greenstone belts in Western
Australia showing increase in density from greenschist to
amphibolite facies. Based on diagrams in Bourne et al.
(
1993
).
5
0
Al
2
O
3
MgO
2.5
2.7
2.9
3.1
3.3
Density (g/cm
3
)
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