Chapter 19: Continental Alkaline Magmatism

1
Chapter 19 Continental Alkaline Magmatism

• Alkaline rocks occur in all tectonic
  environments, including the ocean basins
• Conversely, Chapters 12, 15, 17, and 18 have
  shown us that magmatism on the continents can be
  highly varied, including tholeiitic and
  calc-alkaline varieties
• Now focus on the alkaline rocks that compose an
  extremely diverse spectrum of magmas occurring
  predominantly in the anorogenic portions of
  continental terranes

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Chapter 19 Continental Alkaline Magmatism

• 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
  (see Fig. 18-2) and may be either silica
  undersaturated or oversaturated

3

• Table 19.1. Nomenclature of some alkaline igneous
  rocks (mostly volcanic/hypabyssal)
• 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 gt 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
  (Na2OK2O)/Al2O3 slightly gt 1. May contain
  Na-pyroxene or amphibole
• Pantellerite peralkaline rhyolite with molar
  (Na2OK2O)/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. Table
  19-6
• Lamprophyre a diverse group of dark,
  porphyritic, mafic to ultramafic hypabyssal (or
  occasionally volcanic), commonly highly potassic
  (KgtAl) rocks. They are normally rich in alkalis,
  volatiles, Sr, Ba and Ti, with biotite-phlogopite
  and/or amphibole phenocrysts. They typically
  occur as shallow dikes, sills, plugs, or stocks.
  Table 19-7
• 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 (orangeite) 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. Table 19-3

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Chapter 19 Continental Alkaline Magmatism
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.
5
Chapter 19 Continental Alkaline Magmatism.The
East African Rift
Figure 19.2. Map of the East African Rift system
(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.
6
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7
Chapter 19 Continental Alkaline Magmatism.The
East African Rift
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.
8
Chapter 19 Continental Alkaline Magmatism.The
East African Rift
Figure 19.4. 208Pb/204Pb vs. 206Pb/204Pb (a) and
207Pb/204Pb vs. 206Pb/204Pb (b) diagrams for some
lavas (solid outline) and mantle xenoliths
(dashed) from the East African Rift . The two
distinct Virunga trends in (a) reflect
heterogeneity between two different samples.
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.
9
Chapter 19 Continental Alkaline Magmatism.The
East African Rift
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.
10
Chapter 19 Continental Alkaline Magmatism
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).
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Chapter 19 Continental Alkaline Magmatism
Figure 19.7. Phase diagram for the system
SiO2-NaAlSiO4-KAlSiO4-H2O at 1 atm. pressure.
Insert shows a T-X section from the
silica-undersaturated thermal minimum (Mu) to the
silica-oversaturated thermal minimum (Ms). that
crosses the lowest point (M) on the binary Ab-Or
thermal barrier that separates the undersaturated
and oversaturated zones. After Schairer and Bowen
(1935) Trans. Amer. Geophys. Union, 16th Ann.
Meeting, and Schairer (1950), J. Geol., 58,
512-517. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
12
Chapter 19 Continental Alkaline Magmatism
Figure 19.8. Part of the Ne-Ks-SiO2-H2O system at
1 atm, 0.1 GPa, and 0.2 GPa, illustrating the
reduction in the leucite field with increasing
PH2O. At 0.2 GPa the Lc-liquid field crosses the
Ab-Or join, and the system goes from peritectic
to eutectic behavior. Also shown are contours for
analyses of 122 undersaturated volcanics. After
Gittins, (1979), The feldspathoidal alkaline
rocks. In H. S. Yoder Jr. (ed.), The Evolution of
Igneous Rocks Fiftieth Anniversary Perspectives.
Princeton University Press. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
13
Figure 19.9. Hypothetical cross sections (same
vertical and horizontal scales) showing a
proposed model for the progressive development of
the East African Rift System. a. Pre-rift stage,
in which 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. b.
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. c. Afar stage, in which 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. In A. B. Kampunzu and R. T. Lubala
(eds.), Magmatism in Extensional Settings, the
Phanerozoic African Plate. Springer-Verlag,
Berlin, pp. 85-136 and P. Mohr (personal
communication). Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
14
Chapter 19 Continental Alkaline
Magmatism.Carbonatites
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Chapter 19 Continental Alkaline
Magmatism.Carbonatites
Figure 19.10. African carbonatite occurrences and
approximate ages in Ma. OL Oldoinyo Lengai
natrocarbonatite volcano. After Woolley (1989)
The spatial and temporal distribution of
carbonatites. In K. Bell (ed.), Carbonatites
Genesis and Evolution. Unwin Hyman, London, pp.
15-37. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
16
Carbonatites
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, 133-40.
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
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Chapter 19 Continental Alkaline
Magmatism.Carbonatites
18
Chapter 19 Continental Alkaline
Magmatism.Carbonatites
Figure 19.12. Initial 143Nd/144Nd vs. 87Sr/86Sr
diagrams for young carbonatites (dark shaded),
and the East African Carbonatite Line (EACL),
plus the HIMU and EMI mantle reservoirs. From
Bell and Blenkinsop (1987, Geology, 15, 99-102),
(1989, in K. Bell (ed.), Carbonatites Genesis
and Evolution. Unwin Hyman, London, pp. 278-300
). Also included are the data for Oldoinyo Lengai
natrocarbonatites and alkali silicate rocks (from
Bell and Dawson, 1995, in Bell, K. and J. Keller
(eds.), (1995). Carbonatite Volcanism Oldoinyo
Lengai and the Petrogenesis of Natrocarbonatites.
Springer-Verlag. Berlin, pp. 100-112 ). MORB
values and the Mantle Array are from Figure
10-15. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
19
Chapter 19 Continental Alkaline
Magmatism.Carbonatites
Figure 19.13. Solidus curve (purple) for
lherzolite-CO2-H2O with a defined ratio of CO2
H2O 0.8. Red curves H2O-saturated and
volatile-free peridotite solidi. Approximate
shield geotherm in dashed green. After Wyllie
(1989) Origin of carbonatites Evidence from
phase equilibrium studies. In K. Bell (ed.),
Carbonatites Genesis and Evolution. Unwin Hyman,
London. pp. 500-545. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
20
Chapter 19 Continental Alkaline Magmatism
Figure 19.14. Grid showing the melting products
as a function of pressure and partial melting
of model pyrolite mantle with 0.1 H2O. Dashed
curves are the stability limits of the minerals
indicated. After Green (1970), Phys. Earth
Planet. Inter., 3, 221-235. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
21
Chapter 19 Continental Alkaline
Magmatism.Carbonatites
Figure 19.15. Silicate-carbonate liquid
immiscibility in the system Na2O-CaO-SiO2-Al2O3-CO
2 (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.
Winter (2001) An Introduction to Igneous and
Metamorphic Petrology. Prentice Hall.
22
Chapter 19 Continental Alkaline
Magmatism.Carbonatites
Figure 19.16. Schematic cross section of an
asthenospheric mantle plume beneath a continental
rift environment, and the genesis of
nephelinite-carbonatites and kimberlite-carbonatit
es. Numbers correspond to Figure 19-13. After
Wyllie (1989, Origin of carbonatites Evidence
from phase equilibrium studies. In K. Bell (ed.),
Carbonatites Genesis and Evolution. Unwin Hyman,
London. pp. 500-545) and Wyllie et al., (1990,
Lithos, 26, 3-19). Winter (2001) An Introduction
to Igneous and Metamorphic Petrology. Prentice
Hall.
23
Chapter 19 Continental Alkaline
Magmatism.Lamproites
Figure 19.17. Chondrite-normalized rare earth
element diagram showing the range of patterns for
olivine-, phlogopite-, and madupitic-lamproites
from Mitchell and Bergman (1991) Petrology of
Lamproites. Plenum. New York. Typical MORB and
OIB from Figure 10-13 for comparison. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
24
Chapter 19 Continental Alkaline
Magmatism.Lamproites
25
Chapter 19 Continental Alkaline
Magmatism.Lamproites
Figure 19.18a. Initial 87Sr/86Sr vs. 143Nd/144Nd
for lamproites (red-brown) and kimberlites (red).
MORB and the Mantle Array are included for
reference. After Mitchell and Bergman (1991)
Petrology of Lamproites. Plenum. New York.
Typical MORB and OIB from Figure 10-13 for
comparison. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
26
Chapter 19 Continental Alkaline
Magmatism.Lamproites
Figure 19.18b. 207Pb/204Pb vs. 206Pb/204Pb for
lamproites and kimberlites. After Mitchell and
Bergman (1991). Mitchell and Bergman (1991)
Petrology of Lamproites. Plenum. New York.
Typical MORB and OIB from Figure 10-13 for
comparison. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
27
Chapter 19 Continental Alkaline
Magmatism.Lamprophyres
28
Chapter 19 Continental Alkaline
Magmatism.Kimberlites
Figure 19.19. Model of an idealized kimberlite
system, illustrating the hypabyssal dike-sill
complex leading to a diatreme and tuff ring
explosive crater. This model is not to scale, as
the diatreme portion is expanded to illustrate it
better. From Mitchell (1986) Kimberlites
Mineralogy, Geochemistry, and Petrology. Plenum.
New York. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
29
Chapter 19 Continental Alkaline
Magmatism.Kimberlites
30
Chapter 19 Continental Alkaline
Magmatism.Kimberlites
Figure 19.20a. Chondrite-normalized REE diagram
for kimberlites, unevolved orangeites, and
phlogopite lamproites (with typical OIB and
MORB). After Mitchell (1995) Kimberlites,
Orangeites, and Related Rocks. Plenum. New York.
and Mitchell and Bergman (1991) Petrology of
Lamproites. Plenum. New York. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
31
Chapter 19 Continental Alkaline
Magmatism.Kimberlites
Figure 19.20b. Chondrite-normalized spider
diagram for kimberlites, unevolved orangeites,
and phlogopite lamproites (with typical OIB and
MORB). After Mitchell (1995) Kimberlites,
Orangeites, and Related Rocks. Plenum. New York.
and Mitchell and Bergman (1991) Petrology of
Lamproites. Plenum. New York. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
32
Chapter 19 Continental Alkaline
Magmatism.Kimberlites
Figure 19.21. Hypothetical cross section of an
Archean craton with an extinct ancient mobile
belt (once associated with subduction) and a
young rift. The low cratonal geotherm causes the
graphite-diamond transition to rise in the
central portion. Lithospheric diamonds therefore
occur only in the peridotites and eclogites of
the deep cratonal root, where they are then
incorporated by rising magmas (mostly
kimberlitic- K). Lithospheric orangeites (O)
and some lamproites (L) may also scavenge
diamonds. Melilitites (M) are generated by more
extensive partial melting of the asthenosphere.
Depending on the depth of segregation they may
contain diamonds. Nephelinites (N) and
associated carbonatites develop from extensive
partial melting at shallow depths in rift areas.
After Mitchell (1995) Kimberlites, Orangeites,
and Related Rocks. Plenum. New York. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.