Geoscience Reference
In-Depth Information
Mineral
C'tal
Canadian
Upper
Basaltic
Basalt
Andesitic
Rhyolitic
Granite
LR tuff
Green-
crust
craton
mantle
magma
magma
magma
schist
SiO
2
58·0
66·1
45·16
50·3
49·1
62·5
75·1
70·9
75·6
42·1
Al
2
O
3
18·0
16·1
3·54
20·3
18·2
15·7
12·2
14·5
12·4
15·7
CaO
7·7
3·5
3·08
11·0
11·1
4·3
1·6
1·8
0·1
3·2
FeO
7·7
1·4
8·04
3·0
6·0
3·2
0·9
1·8
2·9
5·5
Fe
2
O
3
3·1
0·46
5·5
3·2
1·3
0·8
1·6
11·6
MgO
3·5
2·2
37·49
4·2
7·6
3·4
0·3
0·9
0·5
11·7
Na
2
O
3·6
3·9
0·57
3·3
2·5
4·2
4·2
3·3
0·2
0·8
K
2
O
1·5
2·9
0·12
0·4
0·9
2·7
3·2
4·0
7·9
0·1
TiO
2
0·5
0·71
1·0
1·0
0·6
0·3
0·4
0·2
2·2
Cr
2
O
3
0·43
NiO
0·2
MnO
0·1
0·14
0·2
0·1
0·1
0·1
0·2
P
2
O
5
0·2 0·06 0·1 0·2 0·1 0·1
H
2
O 0·7 0·4 1·8 1·0 0·8 6·9
Notes: C'tal crust, 'average' continental crust; LR Tuff, Lower Rhyolitic tuff, from the Snowdon caldera eruptions. Greenschist is metamorphosed
oceanic basalt (see below).
Igneous rocks
KEY PROCESSES
How do various mineral combinations come together and develop into distinct igneous lithologies? Magma derived
from partial melting of the asthenosphere or continental crust is converted from a hot melt to cold, solid lithified
rock. The rate and location of cooling determine its mineralogical evolution and eventual rock character. The initial
melt, at temperatures of 900-1,200C, does not cool uniformly and its homogeneous, minero-elemental composition
changes en route by
fractional crystallization
as solid minerals with successively lower melt temperatures form
and settle out through the rising magma. Fractionation proceeds in several ways. Melts become depleted of higher-
temperature products and are therefore enriched in lower-temperature elements. Denser minerals settle out faster
through the viscous melt, although they may still react with it chemically; and further mineral speciation occurs as
more subtle changes alter element ratios in solid solutions (
Table 12.1
).
Three classes of magma are recognized. Fractionation of asthenosphere peridotite proceeds through basaltic
q
andesitic
q
granitic stages, although andesite-granite magmas are also derived from continental crust wetmeltsin
subduction zones. Basaltic magmas crystallize first from asthenosphere peridotite, commencing with olivine, followed
by plagioclase feldspar (anorthite-albite solid solutions) and pyroxene. Their denser, dark minerals - Mg,Fe-rich and
relatively silicate-poor - form ultrabasic-basic or
ultramafic-mafic
rocks. Dry or anhydrous melts contain less than
0·2 per cent water. Andesitic magmas are intermediatein nature with more albite plagioclase, amphibole (hornblende)
and biotite and compositionally close to 'average' continental crust. They solidify at a temperature range of
900-1,000C. Low-temperature (500-600C) granitic or rhyolitic magma is dominated by the lighter, less dense
minerals orthoclase feldspar (potassium-rich), quartz and biotite-muscovite to form Fe,Si-rich, acid or
felsic
rocks.
