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Igneous Petrol. EPSC-423; Francis, 2013
Lab 6: Plutonic Rocks II: A Zoned Dyke
Until now, we have focused on magmas and their liquid line of descent during crystal fractionation as
reflected in the spectrum of erupted lava compositions. We have largely ignored the solid crystal
assemblage that must be extracted from the magma to produce the compositional variation seen in the
lavas that erupt at the surface. The crystal fractionation that produced the spectrum of lava compositions
occurred at depth in intrusive plutonic bodies within the crust and/or at the crust-mantle interface.
Plutonic (intrusive) rocks crystallize more slowly than volcanic rocks. As a result, they are typically
coarser-grained and more equigranular than volcanic rocks, with the actual grain-size reflecting an
interplay between cooling rate, and therefore the depth of crystallization, volatile content and major
element composition.
The solid crystal assemblages that crystallized from cooling magmas in plutonic bodies form rocks that
are known as cumulates. Their compositions reflect the mechanical accumulation of crystals rather than
a frozen liquid. Some plutonic rocks are almost completely cumulate in nature, while others reflect a
mixture of accumulated crystals and frozen interstitial liquid. When examining plutonic rocks, it is
important to try to distinguish between minerals that are interpreted to be cumulus versus those which are
post-cumulus, crystallizing from the interstitial liquid between the primary cumulus grains. The cumulus
minerals represent early crystals that accumulated from a magma to form the framework of the rock and
are commonly sub-equant (tabular cumulate plagioclase is an exception) and/or subeuhedral in habit.
Inter-cumulous minerals crystallize from the residual trapped liquid between the cumulus crystals. They
are typically anhedral and occur as either small crystals interstitial to the larger cumulus minerals, or as
large oikocrysts that poikilitically enclose smaller chadocrysts of earlier cumulus minerals. Some
cumulate rocks can be recognized in hand specimen by the "flash" of large oikocryst cleavage surfaces
containing numerous chadocryst inclusions
when
the sample is rotated in the light.
Olivine chadocrysts in
an orthopyroxene oikocryst.
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Igneous Petrol. EPSC-423; Francis, 2013
In this week’s lab, we will examine a relatively simple zoned dyke approximately 80 meter in width (see
figure). The Dash body is a late Archean dyke-like intrusion, cutting Archean granitoid rocks (DH-02,
DH08-1 and DH08-2). The samples in the attached table have been collected from a transect across the
dyke.
Dash Dyke
Major Elements in wt%
Sample: DH-02
Meter 252.00
DH-03
186.00
DH-06
163.00
DH-08
128.00
DH-09
110.00
DH-10
107.00
DH08-1 DH08-2
00.00
82.00
SiO2
TiO2
Al2O3
FeO
MgO
MnO
CaO
Na2O
K2O
P2O5
LOI
71.45
0.06
15.91
1.60
0.54
0.01
2.76
4.53
2.08
0.12
0.91
56.86
0.22
11.56
6.80
9.03
0.12
8.29
2.86
1.76
0.03
1.76
51.86
0.25
5.69
8.94
18.46
0.17
10.34
0.98
0.61
0.06
1.57
39.24
0.09
2.53
13.48
32.60
0.16
1.09
0.18
0.17
0.03
9.15
38.23
0.07
2.03
13.29
30.01
0.18
3.12
0.20
0.11
0.03
11.11
50.47
0.10
4.16
8.34
22.29
0.12
7.96
0.62
0.23
0.04
3.92
67.43
0.44
16.07
3.45
1.29
0.03
3.63
4.47
1.90
0.16
0.64
68.84
0.25
16.16
2.09
1.37
0.03
4.24
4.71
1.31
0.10
0.63
Total
99.98
99.35
99.19
99.15
98.75
98.97
99.51
99.75
Trace Elements in ppm
Ni
Cr
Zr
Ce
7.0
17.1
227.6
43.6
88.0
346.6
39.6
44.2
507.0
1496.0
23.4
31.2
1572.0
1648.0
11.1
6.5
Major Elements in wt%
Sample:DH08-3
Meter
88.00
DH08-8
152.00
DH08-10 DH08-11
169.00 167.00
SiO2
TiO2
Al2O3
FeO
MgO
MnO
CaO
Na2O
K2O
P2O5
LOI
56.18
0.25
5.84
8.00
15.63
0.15
8.98
1.17
1.85
0.05
0.84
42.29
0.15
2.97
11.20
28.09
0.20
5.30
0.37
0.14
0.03
7.93
42.64
0.18
3.40
12.24
25.06
0.18
7.98
0.46
0.11
0.03
5.95
52.05
0.29
5.74
7.82
16.86
0.16
12.66
1.04
0.92
0.05
1.43
Total
99.23
99.09
98.66
99.31
Trace Elements in ppm
Ni
Cr
Zr
Ce
699.0
1434.0
68.0
69.0
980.0
1993.0
12.0
19.0
1684.0
1560.0
11.9
25.0
375.0
1643.4
33.4
42.0
2
1283.0
1358.0
7.7
6.1
651.0
3642.0
14.3
8.6
8.0
27.6
214.2
151.0
17.0
30.7
96.8
76.0
Igneous Petrol. EPSC-423; Francis, 2013
Tasks:
1. Examine each of the rocks across the Dash dyke, as well as the two host rock samples. Identify
the minerals present and give each rock a name based on your visual estimate of the modal
mineralogy according to the classification scheme in the appendix.
2. For each rock, try to determine the order in which the mineral phases crystallized using the
following textural criteria:
“If grains of one mineral typically occur enclosed in another, then the former crystallized before the latter. A good
example is the occurrence of early olivine chadocrysts in later pyroxene or feldspar oikocrysts. Exceptions include
phenomena such as exsolution lamellae, cotectic intergrowths, late-stage alteration minerals, and accessory phases.
Phenocrysts have crystallized before their fine-grained matrices. In general, larger crystals have crystallized earlier
than smaller crystals. This criteria best applied to volcanic rocks, and is commonly problematic in plutonic rocks.
Free-forming early crystals tend to be more euhedral (show crystal faces) than later crystals. Again this criteria is best
applied in volcanic rocks. In plutonic rocks it is compromised by the decreasing tendency of the silicates to be
euhedral, in the order orthosilicate > chain silicate> sheet silicate> framework silicate. In plutonic rocks, late
crystallizing phases such as zircon, titanite and even apatite are often euhedral.”
Paraphrased after Williams et al., 1954
3. Calculate the normative mineralogy of each of the Dash samples, and plot the variation in the
normative abundance of olivine, orthopyroxene, clinopyroxene, and plagioclase with distance
across the dyke.
4. Construct plots of the variation of Mg, Ca, Al, Ni, Cr, Ce, and Zr in the whole compositions
with distance across the dyke.
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Igneous Petrol. EPSC-423; Francis, 2013
5. Construct variation diagrams of Mg versus Ca, Al, Ni, and Zr or both minerals and whole rock
compostions. Rationalize the variation in each element with Mg in terms of its relative
compatibility.
6. Describe the likely crystallization history of the Dash Dyke as inferred from the zonation that it
exhibits. How does the crystallization history of the dyke as a whole compare to the
crystallization sequence seen within individual samples of the dyke’s interior. How might this
discrepancy be explained?
7. Use the composition of the olivine in the most magnesian Dash cumulate rock (DH-08) to
calculate the Fe/Mg ratio of the liquid from which the rock crystallized, assuming the KFe/Mg
oliv/liq = 0.3. Use the composition of the clinopyroxene in the least magnesium cumulate rock
(DH-03) to estimate the Fe/Mg ratio of the liquid from which it crystallized, assuming the KFe/Mg
cpx/liq ~ 0.3. How do these numbers compare?
8. Obtain an estimate of the parental magma composition for the Dash Dyke by using AlphaMelts
to partially melt the most magnesian cumulate rock (DH-08) to the point at which pyroxenes
melt-out, leaving only olivine, plus or minus spinel, in the residue, assuming that fO2 is buffered
at FMQ-1.
9. Use Alphamelts to fractionally crystallize this parental melt composition at a pressure of 5 kbs
and a water content of 2%. Plot the liquid line of descent, as well as the compositions of the
cumulous phases, in your previous variation diagrams (#5) and explicitly compare the model
results to the observed results.
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Igneous Petrol. EPSC-423; Francis, 2013
Dash Minerals
Major Elements in wt%
Sample: DH-03
Min:
Plag
DH-03
Cpx
DH08-11 DH08-11 DH08-11
Plag
Cpx
Opx
SiO2
TiO2
Al2O3
Cr2O3
MgO
NiO
FeO
MnO
CaO
Na2O
K2O
62.17
0.00
23.49
0.00
0.01
0.00
0.01
0.00
5.24
8.39
0.22
53.33
0.07
0.86
0.02
14.57
0.00
8.14
0.27
22.97
0.46
0.00
58.19
0.03
26.38
0.00
0.00
0.00
0.08
0.00
8.37
6.62
0.16
52.71
0.07
1.46
0.19
16.60
0.00
4.11
0.11
23.95
0.19
0.01
54.93
0.00
0.74
0.03
29.30
0.00
14.51
0.31
0.25
0.00
0.02
54.28
0.01
2.51
0.12
30.08
0.14
11.14
0.25
0.24
0.01
0.00
39.10
0.00
0.00
0.00
44.12
0.17
16.37
0.22
0.00
0.00
0.00
52.91
0.08
1.50
0.18
16.77
0.00
3.66
0.12
24.07
0.16
0.01
Total
99.51
100.70
99.82
99.41
100.10
99.51
100.00
99.46
Major Elements in wt%
Sample: DH-09
Min:
Oliv
DH-10
Plag
DH-10
Cpx
DH08-3
Plag
DH08-3
Cpx
SiO2
TiO2
Al2O3
Cr2O3
MgO
NiO
FeO
MnO
CaO
Na2O
K2O
39.73
0.01
0.01
0.00
43.00
0.16
17.12
0.29
0.01
0.00
0.00
60.84
0.02
24.79
0.00
0.02
0.00
0.13
0.00
6.38
7.76
0.16
53.65
0.11
1.23
0.37
16.40
0.00
4.74
0.12
23.00
0.49
0.00
62.08
0.00
23.88
0.00
0.00
0.00
0.09
0.02
5.67
8.27
0.32
54.08
0.11
1.06
0.20
15.83
0.00
5.65
0.15
22.92
0.49
0.01
Total
100.31
100.11
100.11
100.34
100.51
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DH-08
Opx
DH-08
Oliv
DH-09
Cpx
Igneous Petrol. EPSC-423; Francis, 2013
Appendix: Nomenclature and Classification of Plutonic Rocks
Figure 4.1
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Igneous Petrol. EPSC-423; Francis, 2013
Ultramafic Rocks:
Ultramafic rocks contain over 85 % mafic minerals, and feldspar is either absent or so low in abundance
that it can not be seen in hand specimen. If present, feldspar is always an interstitial mineral.
Most ultramafic plutonic rocks are at least in part cumulates. They do not represent silicate melts, but
rather the accumulation of crystals that have crystallized from a silicate melt in a large magma chamber,
along the walls of a dyke, etc. Cumulus minerals represent early crystals that accumulated to form the
framework of the rock. They typically have subhedral to euhedral habits.
Inter-cumulus minerals crystallize from the residual trapped liquid between the cumulus crystals. They are
typically anhedral and occur as either small crystals interstitial to the larger cumulus minerals, or as large
oikocrysts that poikilitically enclose smaller chadocrysts of earlier cumulus minerals. Some cumulate
rocks can be recognized in hand specimen by the "flash" of large oikocryst cleavage surfaces containing
numerous chadocryst inclusions when the sample is rotated in the light. In addition, cumulate rocks are
generally characterized by their low number of prominent mineral species (1-3) and the common presence
of layering in terms of variations in modal mineralogy and sometimes grain-size.
Olivine-rich rocks in which clinopyroxene and/or orthopyroxene cannot be distinguished are best called
by the more general name peridotite. In the field, especially if the rock is altered, often all you can say is
that the rock is ultramafic.
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Igneous Petrol. EPSC-423; Francis, 2013
Gabbroic Rocks:
Mafic or ‘gabbroic’ rocks contain more than 15%, but less than 60% feldspar. If hornblende is present in
moderate amounts, then the prefix hormblende is used: hornblende gabbro, hornblende peridotite, etc.
Mafic rocks with over 60% hornblende are termed hornblendites.
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Igneous Petrol. EPSC-423; Francis, 2013
Granitoids:
Granitoids are felsic plutonic rocks that contain more than 60% felsic minerals. They are classified
according the nature of the feldspar and the presence or absence of quartz or nepheline, depending on
whether the rock is silica-saturated or silica-undersaturated. Syenites near the boundary between the
two typically contain either small quantities of quartz or feldspathoids (but never both) that commonly
cannot be detected in hand specimen.
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