Intraplate%20magmatism

1
Intraplate magmatism

• Hotspots
• Rift zones (often associated with hotspots)
• Intra-oceanic plate Tholeitic to alkaline
  series mostly basalts (OIB Oceanic Islands
  Basalts), some differenciated alkaline terms
• Intra-continental plate
• either large tholeitic basaltic provinces (CFB
  Continental Flood Basalts), occasionally bimodal
  (ass. with rhyolites)
• or smaller, alkaline to hyper-alkaline,
  differenciated intrusions/volcanoes
  (syenites/phonolites carbonatites kimberlites
  and more)

2
Continental alkaline series
Alkali volcanoes basaltic strombolian cone in
front, trachytic pelean dome behind in the West
European rift
3
The West-european rift sytem
4
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.
5
Continental alkaline series

• Rift (or hotspot) related
• Large diversity (possibly gt 80 of the rock
  names, for lt1 volume !)
• Strange rocks (carbonatites)

6
Common features of continental alkali series

• Alkaline (!)
• Undersaturated to just oversaturated
• Peralkaline

7
Alkaline series
Mildly alkaline
Strongly alkaline
8
Q
Quartzolite
90
90
Quartz-rich
Granitoid
60
60
Saturated
Grano-
Tonalite
Granite
Alkali Feldspar Granite
diorite
Alkali Fs.
Qtz. Diorite/
20
20
Quartz Syenite
Qtz. Gabbro
Quartz
Quartz
Quartz
Monzonite
Syenite
Monzodiorite
Alkali Fs.
5
5
Diorite/Gabbro/
Syenite
Syenite
Monzonite
Monzodiorite
Anorthosite
35
90
10
65
A
P
(Foid)-bearing
(Foid)-bearing
(Foid)-bearing
Syenite
Monzonite
Monzodiorite
(Foid)-bearing
10
10
Diorite/Gabbro
(Foid)-bearing
Alkali Fs.
Syenite
(Foid)
(Foid) Syenite
(Foid)
Monzosyenite
Monzodiorite
(Foid) Gabbro
Under-saturated
60
60
(Foid)olites
F
9
The alkali eutectic
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.
10
Figure 18-2. Alumina saturation classes based on
the molar proportions of Al2O3/(CaONa2OK2O)
(A/CNK) after Shand (1927). Common
non-quartzo-feldspathic minerals for each type
are included. After Clarke (1992). Granitoid
Rocks. Chapman Hall.
11
Trace elements enriched
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.
12
Enriched mantle source
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.
13
Generated from low to very low melt fractions
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.
14
Diversity of alkaline continental magmas some
examples

• Saturated alkaline series
• Undersaturated alkaline series
• Carbonatites
• Lamprophyres, kimberlites co.

Series with a true geological importance
Oddities and curiosities but economic
importance!
15
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.
16
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.
17
East African rift (Afar) mildly alkaline
18
Central African Rift Strongly alkaline
19
Two main series

• Basalts-Trachydandesites-Trachydacites-Rhyolites
  (stronly bimodal) (just) saturated alkali series
• A-type granites can be formed there
• Role of the preexisting crust?
• Basanite-Foidite (nephelinite)-Phonolite
  strongly undersaturated alkali series

20
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.
21
Bimodal associations(the  Daly gap )

• Mantle vs. Crustal sources?
• Remelting of underplated basalts?
• Simply an effect of the different eutectics?

22
The oddities

• Carbonatites
• Lamproites, kimberlites, etc.

23
Chapter 19 Continental Alkaline
Magmatism.Carbonatites
24
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.
25
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.
26
Chapter 19 Continental Alkaline
Magmatism.Carbonatites
Figure 19-15. 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.
27
Lamproites and kimberlites

• many, many, many rock types
• many, many different names mostly purely
  local and after the one known occurrence of that
  rock type (Vosgesite, Wyomingite, Orangeite )

28
Chapter 19 Continental Alkaline
Magmatism.Kimberlites
29
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.
30
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.
31
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.
32
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.
33
Chapter 19 Continental Alkaline
Magmatism.Kimberlites
Figure 19-20b. 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.