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GEOL 2312
IGNEOUS AND METAMORPHIC
PETROLOGY
Lecture 18
Continental Alkaline Magmatism
March 9, 2009
ALKALINE IGNEOUS
ROCKS
Alkaline rocks generally have more alkalis than can be accommodated
by feldspars alone. The excess alkalis appear in feldspathoids, sodic
pyroxenes-amphiboles, or other alkali-rich phases
In the most restricted sense, alkaline rocks are deficient in SiO2 with
respect to Na2O, K2O, and CaO to the extent that they become “critically
undersaturated” in SiO2, and Nepheline or Acmite appears in the norm
Alternatively, some rocks may be deficient in Al2O3 (and not necessarily
SiO2) so that Al2O3 may not be able to accommodate the alkalis in
normative feldspars. Such rocks are peralkaline and may be either
silica undersaturated or oversaturated
Nepheline
Na2Al2Si2O8
Leucite
KAlSi2O6
ALKALINE ROCK SERIES
OCEANIC VS. CONTINENTAL
Winter (2001) Figure 19-1. Variations in alkali ratios (wt. %) for oceanic (a) and continental (b) alkaline series. The heavy dashed lines
distinguish the alkaline magma subdivisions from Figure 8-14 and the shaded area represents the range for the more common oceanic
intraplate series. After McBirney (1993). Igneous Petrology (2nd ed.), Jones and Bartlett. Boston. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
WHAT’S IN A NAME
1% OF IGNEOUS ROCKS ARE ALKALINE, BUT CONSTITUTE >50% OF
IGNEOUS ROCK NOMENCLATURE
Basanite
feldspathoid-bearing basalt. Usually contains nepheline, but may have leucite + olivine
Tephrite
olivine-free basanite
Leucitite
a volcanic rock that contains leucite + clinopyroxene  olivine. It typically lacks feldspar
Nephelinite a volcanic rock that contains nepheline + clinopyroxene  olivine. It typically lacks feldspar. Fig. 14-2
Urtite
plutonic nepheline-pyroxene (aegirine-augite) rock with over 70% nepheline and no feldspar
Ijolite
plutonic nepheline-pyroxene rock with 30-70% nepheline
Melilitite
a predominantly melilite - clinopyroxene volcanic (if > 10% olivine they are called olivine melilitites)
Shoshonite K-rich basalt with K-feldspar ± leucite
Phonolite felsic alkaline volcanic with alkali feldspar + nepheline. See Fig. 14-2. (plutonic = nepheline syenite)
Comendite peralkaline rhyolite with molar (Na2O+K2O)/Al2O3 slightly > 1. May contain Na-pyroxene or amphibole
Pantellerite peralkaline rhyolite with molar (Na2O+K2O)/Al2O3 = 1.6 - 1.8. Contains Na-pyroxene or amphibole
Lamproite a group of peralkaline, volatile-rich, ultrapotassic, volcanic to hypabyssal rocks. The mineralogy is
variable, but most contain phenocrysts of olivine + phlogopite ± leucite ± K-richterite ± clinopyroxene ± sanidine.
Lamprophyre a diverse group of dark, porphyritic, mafic to ultramafic hypabyssal (or occasionally volcanic),
commonly highly potassic (K>Al) rocks. They are normally rich in alkalis, volatiles, Sr, Ba and Ti, with biotitephlogopite and/or amphibole phenocrysts. They typically occur as shallow dikes, sills, plugs, or stocks.
Kimberlite a complex group of hybrid volatile-rich (dominantly CO2), potassic, ultramafic rocks with a fine-grained
matrix and macrocrysts of olivine and several of the following: ilmenite, garnet, diopside, phlogopite, enstatite,
chromite. Xenocrysts and xenoliths are also common
Group I kimberlite is typically CO2-rich and less potassic than Group 2 kimberlite
Group II kimberlite is typically H2O-rich and has a mica-rich matrix (also with calcite, diopside, apatite)
Carbonatite an igneous rock composed principally of carbonate (most commonly calcite, ankerite, and/or dolomite),
and often with any of clinopyroxene alkalic amphibole, biotite, apatite, and magnetite. The Ca-Mg-rich
carbonatites are technically not alkaline, but are commonly associated with, and thus included with, the alkaline
rocks.
ALKALINE ROCKS
ASSOCIATED WITH
CONTINENTAL RIFTS
EAST AFRICAN RIFT
Failed Arm of the Afar Triple Jct
MAGMA SERIES
Highly Alkaline
Alkaline
Tholeiitic
MAGMA SERIES OF THE EAST AFRICAN RIFT
Table 19-2. Representative Chemical Analyses of East African Rift Volcanics
Oxide
SiO2
TiO2
Series 1: Alkaline
1
2
45.6
51.7
3
46.2
Series 2: Ultra-alkaline
4
5
33.1
44.1
6
55.4
Series 3: Transitional Basalt-Rhyolite
7
8
9
10
47.6
61.8
70.3
72.5
Series 4
11
50.8
2.4
0.9
1.6
2.6
2.8
0.5
2.0
1.0
0.3
0.2
1.4
15.6
11.3
0.2
6.9
10.4
3.2
19.3
5.9
0.2
1.1
4.1
8.9
18.6
8.9
0.2
2.3
7.3
9.3
11.3
12.4
0.3
7.3
17.2
3.2
17.0
10.0
0.2
3.7
8.4
4.3
20.8
4.6
0.2
0.5
2.9
9.2
14.8
11.4
0.2
6.4
11.5
2.7
14.2
6.4
0.3
0.5
1.8
6.2
7.6
8.4
0.3
0.0
0.4
7.3
10.3
4.0
0.1
0.0
0.2
5.9
14.9
10.1
0.2
6.9
9.8
2.6
1.3
4.6
4.2
3.6
7.2
5.5
0.8
5.2
4.3
4.4
0.4
P2O5
0.6
Total
97.5
CIPW NORM
q
0.0
or
8.9
ab
31.4
an
28.3
lc
0.0
ne
0.0
di
14.2
hy
0.0
wo
0.0
ol
9.4
il
0.5
ti
4.7
ap
1.6
pf
1.0
ns
0.0
cs
0.0
0.3
97.0
0.5
99.1
1.9
92.9
1.2
98.9
0.1
99.7
0.3
97.7
0.2
97.6
0.0
98.8
97.6
0.4
97.4
0.0
29.8
28.6
0.0
0.0
28.3
6.5
0.0
3.9
0.0
0.5
0.0
0.8
1.3
0.4
0.0
0.0
27.5
8.0
0.0
0.0
39.1
13.7
0.0
5.7
0.0
0.5
0.0
1.3
2.6
1.7
0.0
0.0
0.0
0.0
7.3
20.7
18.2
15.0
0.0
0.0
10.9
0.8
0.0
5.5
4.8
0.0
16.8
0.0
31.0
0.0
6.5
13.2
22.2
16.7
0.0
0.0
1.8
0.5
0.0
3.1
4.9
0.0
0.0
0.0
34.2
30.2
0.0
0.0
27.1
2.8
0.0
4.1
0.0
0.4
0.0
0.2
0.5
0.4
0.0
2.1
5.5
26.5
30.0
0.0
0.0
20.8
8.8
0.0
0.0
0.5
5.0
0.8
0.0
0.0
0.0
9.0
33.7
48.3
0.0
0.0
0.0
2.9
0.0
0.9
0.0
0.7
1.8
0.5
0.0
2.1
0.0
41.7
28.1
16.7
0.0
0.0
0.0
0.1
0.0
0.6
0.0
0.6
0.1
0.1
0.0
12.0
0.0
35.8
27.8
30.4
0.0
0.0
0.0
0.0
0.0
0.0
0.0
0.1
0.5
0.0
0.0
5.3
0.0
9.1
2.6
25.2
31.9
0.0
0.0
12.7
13.8
0.0
0.0
0.5
3.3
1.0
0.0
0.0
0.0
Al2O3
FeO*
MnO
MgO
CaO
Na2O
K2O
1. Ave. 32 alkaline basalts, Kenya (B) 2. Ave. phonolite (B) 3. Ave. Kenyan nephelinite (B) 4. Melilitite, W. Rift (KM) 5. Leucitite, W. Rift (KM)
6. Ave. of 55 phonolites, Uganda (B) 7. Ave. of 31 transitional basalts (B) 8. Ave. 40 trachytes (B) 9. Pantellerite (KM) 10. Comendite (KM)
11. Ave. of 26 Tholeiitic basalts (KM).
KM = Kampunzu and Mohr (1991), B = Baker, 1987.
Q-F
either
/or
THERMAL DIVIDE BETWEEN
ALKALINE AND THOLEIITIC MAGMAS
1 atm Pressure
Peritectic
Eutectic
Peritectic
ISOTOPIC AND TRACE ELEMENT
GEOCHEMISTRY OF EAR VOLCANICS
Bulk Earth
Figure 19-3. 143Nd/144Nd vs. 87Sr/86Sr for East African Rift lavas
(solid outline) and xenoliths (dashed). The “cross-hair” intersects
at Bulk Earth (after Kampunzu and Mohr, 1991), Magmatic
evolution and petrogenesis in the East African Rift system. In A.
B. Kampunzu and R. T. Lubala (eds.), Magmatism in Extensional
Settings, the Phanerozoic African Plate. Springer-Verlag, Berlin,
pp. 85-136. Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
Figure 19-5. Chondrite-normalized REE variation diagram for
examples of the four magmatic series of the East African Rift
(after Kampunzu and Mohr, 1991), Magmatic evolution and
petrogenesis in the East African Rift system. In A. B.
Kampunzu and R. T. Lubala (eds.), Magmatism in Extensional
Settings, the Phanerozoic African Plate. Springer-Verlag,
Berlin, pp. 85-136. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
MAGMA SUITES
INTRA-SUITE HOMOGENEITY/INTER-SUITE HETEROGENEITY
Figure 19-6a. Ta vs. Tb for rocks of the Red Sea, Afar, and the Ethiopian Plateau. Rocks from a particular area show nearly constant
ratios of the two excluded elements, consistent with fractional crystallization of magmas with distinct Ta/Tb ratios produced either by
variable degrees of partial melting of a single source, or varied sources (after Treuil and Varet, 1973; Ferrara and Treuil, 1974).
TECTONO-MAGMATIC MODEL FOR THE
EAST AFRICAN RIFT
Pre-rift stage - an asthenospheric mantle diapir
rises (forcefully or passively) into the lithosphere.
Decompression melting (cross-hatch-green indicate
areas undergoing partial melting) produces variably
alkaline melts. Some partial melting of the
metasomatized sub-continental lithospheric mantle
(SCLM) may also occur. Reversed decollements
(D1) provide room for the diapir.
Rift stage - development of continental rifting,
eruption of alkaline magmas (red) mostly from a
deep asthenospheric source. Rise of hot
asthenosphere induces some crustal anatexis.
Rift valleys accumulate volcanics and
volcaniclastic material.
Afar stage- asthenospheric ascent reaches
crustal levels. This is transitional to the
development of oceanic crust. Successively
higher reversed decollements (D2 and D3)
accommodate space for the rising diapir.
After Kampunzu and Mohr (1991), Magmatic
evolution and petrogenesis in the East African
Rift system.
CARBONATITES ASSOCIATED WITH THE EAR
Carbonatite:
>50% carbonate minerals
Silico-carbonatite:
50-10% carbonate minerals
Table 19-3. Carbonatite Nomenclature
Name
Calcite-carbonatite
Dolomite-carbonatite
Ferrocarbonatite
Natrocarbonatite
Alternative
Coarse
Med.-Fine
sövite
alvikite
rauhaugite*
beforsite
* Rarely used, beforsite may be applied to any grain size.
FIELD CHARACTERISTICS
OF
CARBONATITES
• Commonly satellite intrusions to
alkaline intrusive centers
• Pipe-like, composite intrusions
• < 25 km across
• Ring-dike, cone sheets and plug
forms common
• Typically late in intrusive sequence
• Emplacement T – 500-1000°C
• Metasomatic halo – Fenite
carbonatized wall rock
Winter (2001) Figure 19-11. Idealized cross
section of a carbonatite-alkaline silicate complex
with early ijolite cut by more evolved urtite.
Carbonatite (most commonly calcitic) intrudes
the silicate plutons, and is itself cut by later
dikes or cone sheets of carbonatite and
ferrocarbonatite. The last events in many
complexes are late pods of Fe and REE-rich
carbonatites. A fenite aureole surrounds the
carbonatite phases and perhaps also the
alkaline silicate magmas. After Le Bas (1987)
Carbonatite magmas. Mineral. Mag., 44, 13340.
CHEMICAL ATTRIBUTES OF CARBONATITES
Table 19-5. Representative Carbonatite Compositions
%
SiO2
CalciteDolomiteFerroNatrocarbonatite carbonatite carbonatite carbonatite
2.72
3.63
4.7
0.16
TIO2
0.15
0.33
0.42
0.02
Al2O3
1.06
0.99
1.46
0.01
Fe2O3
FeO
MnO
MgO
CaO
Na2O
2.25
1.01
0.52
1.80
49.1
0.29
2.41
3.93
0.96
15.06
30.1
0.29
7.44
5.28
1.65
6.05
32.8
0.39
0.05
0.23
0.38
0.38
14.0
32.2
K2O
0.26
0.28
0.39
8.38
P2O5
2.10
1.90
1.97
0.85
H2O+
0.76
1.20
1.25
0.56
CO2
BaO
SrO
F
Cl
S
SO3
36.6
0.34
0.86
0.29
0.08
0.41
0.88
36.8
0.64
0.69
0.31
0.07
0.35
1.08
30.7
3.25
0.88
0.45
0.02
0.96
4.14
31.6
1.66
1.42
2.50
3.40
3.72
Figure 19-15. Silicate-carbonate liquid immiscibility in the system Na2OCaO-SiO2-Al2O3-CO2 (modified by Freestone and Hamilton, 1980, to
incorporate K2O, MgO, FeO, and TiO2). The system is projected from CO2
for CO2-saturated conditions. The dark shaded liquids enclose the
miscibility gap of Kjarsgaard and Hamilton (1988, 1989) at 0.5 GPa, that
extends to the alkali-free side (A-A). The lighter shaded liquids enclose the
smaller gap (B) of Lee and Wyllie (1994) at 2.5 GPa. C-C is the revised gap
of Kjarsgaard and Hamilton. Dashed tie-lines connect some of the
conjugate silicate-carbonate liquid pairs found to coexist in the system.
After Lee and Wyllie (1996) International Geology Review, 36, 797-819.
ORIGIN OF CARBONATES
IGNEOUS, METAMORPHIC, OR METSOMATIC
ULTRAPOTASSIC ROCKS
LAMPROITES AND KIMBERLITES
Table 19-8. Average Analyses and Compositional Ranges
of Kimberlites, Orangeites, and Lamproites.
SiO2
Kimberlite
33.0
27.8-37.5
Orangeite
35.0
27.6-41.9
Lamproite*
45.5
TiO2
1.3
0.4-2.8
1.1
0.4-2.5
2.3
Al2O3
FeO*
MnO
MgO
CaO
Na2O
2.0
7.6
0.14
34.0
6.7
0.12
1.0-5.1
5.9-12.2
0.1-0.17
17.0-38.6
2.1-21.3
0.03-0.48
2.9
7.1
0.19
27.
7.5
0.17
0.9-6.0
4.6-9.3
0.1-0.6
10.4-39.8
2.9-24.5
0.01-0.7
8.9
6.0
11.2
11.8
0.8
K2O
0.8
0.4-2.1
3.0
0.5-6.7
7.8
P2O5
LOI
1.3
10.9
0.5-1.9
7.4-13.9
1.0
11.7
0.1-3.3
5.2-21.5
2.1
3.5
Sc
V
Cr
Ni
Co
Cu
Zn
Ba
Sr
Zr
Hf
Nb
Ta
Th
U
La
Yb
14
100
893
965
65
93
69
885
847
263
5
171
12
20
4
150
1
20
95
1722
1227
77
28
65
3164
1263
268
7
120
9
28
5
186
1
Data from Mitchell (1995), Mitchell and Bergman (1991)
* Leucite Hills madupidic lamproite
19
66
430
152
41
9831
3860
1302
42
99
6
37
9
297
1
Lamproites – Mafic mineralogy
Kimberlites/Orangites – Ultramafic mineralogy
ULTRAPOTASSIC ROCKS
LAMPROITES
Lamproites
• K/Na > 3 (ultrapotassic)
• K/Al > 1 (perpotassic)
• (K+Na)/Al > 1 (peralkaline)
• mg# > 70
• Incompatible element-enriched
Table 19-6. Lamproite Nomenclature
Old Nomenclature
Recommended by IUGS
wyomingite
orendite
madupite
cedricite
mamilite
wolgidite
fitzroyite
verite
jumillite
fortunite
cancalite
diopside-leucite-phlogopite lamproite
diopside-sanidine-phlogopite lamproite
diopside madupidic lamproite
diopside-leucite lamproite
leucite-richterite lamproite
diopside-leucite-richterite madupidic lamproite
leucite-phlogopite lamproite
hyalo-olivine-diopside-phlogopite lamproite
olivine diopside-richterite madupidic lamproite
hyalo-enstatite-phlogopite lamproite
enstatite-sanidine-phlogopite lamproite
From Mitchell and Bergman (1991).
ULTRAPOTASSIC ROCKS
KIMBERLITES/ORANGITES