Transcript Ch 19 Continental Alk mod 8

Chapter 19: Continental Alkaline
Magmatism
Ol Doinyo Lengai volcano
Alkaline rocks generally have more alkalis than can be
accommodated by feldspars alone. The excess alkalis appear in
feldspathoids, sodic pyroxenes and amphiboles, or other alkalirich 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
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 > 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. Table 19-6
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 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
For more details, see Sørensen (1974), Streckeisen (1978), and Woolley et al. (1996)
Mt Erebus a basanite in Antarctica
Basanite a feldspathoid-bearing basalt
Example of Alkali Magma: Nepheline Syenite
Mostly Orthoclase, no quartz, excess alkali to Nepheline
Continental alkaline series are much more varied than OIAs
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.
Alkaline
Magmatism 1 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.
Alkaline Magmatism.
The East African Rift REEs
These authors
compared isotope ratios
(not shown) and
incompatible
frequencies and, again,
found no correlation.
This was taken to mean
that enrichment in
incompatibles occurs
just before magma
generation.
High LREEs
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.
Alkaline Magmatism:
The East African Rift
East Africa Rift lavas are
enriched in Rubidium and
Nd incompatibles, as
expected in alkaline
rocks*. These, over time,
should decay, resulting in
high 87Sr/86Sr and
143Nd/144Nd ratios.
This is not true, suggesting
that the incompatibles
enrichment occurs just
before magma generation,
and the Rb in the magma
just got there in these
young lavas.
Rb and Sr are relatively mobile alkaline elements
and as such are relatively easily moved around
by the hot, often carbonated hydrothermal fluids
present during metamorphism or magmatism.
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. SpringerVerlag, Berlin, pp. 85-136.
•*For example, 87Rb is a LIL, so is
expected in late fractionation/ lower
temperature solids. 87Sr is its daughter
http://en.wikipedia.org/wiki/Rubidium-strontium_dating
Alkaline Magmatism in
The East African Rift
Pb data are
similar to OIBs
OIBs are thought
to be plume
generated
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. SpringerVerlag, Berlin, pp. 85-136. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
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
Figure 19-6a. Ta vs. Tb for rocks of the Red Sea, Afar, and the Ethiopian Plateau. (after Treuil and Varet, 1973;
Ferrara and Treuil, 1974).
You either get Tridymite or
Nepheline, not both.
. 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.
Figure 19-7. Phase diagram for the system SiO2-NaAlSiO4-KAlSiO4-H2O at 1 atm. pressure 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.
Figure 19-9.. a. Pre-rift stage 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.
2 - Carbonatites
Rare, mantle-derived igneous rock
dominated by Calcite and Dolomite
with associated silicates
Ol Doinyo Lengai volcano
Continental Alkaline
Magmatism:Carbonatites
Table 19-4. Some Minerals in Carbonatites.
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.
Carbonates
Calcite
Dolomite
Ankerite
Siderite
Strontanite
Bastnäsite (Ce,La)FCO3)
* Nyerereite ((Na,K) 2Ca(CO3)2)
* Gregoryite ((Na,K) 2CO3)
Silicates
Pyroxene
Aegirine-augite
Diopside
Augite
Olivine
Monticellite
Alkali amphibole
Allanite
Andradite
Phlogopite
Zircon
Source: Heinrich (1966), Hogarth (1989)
Sulfides
Pyrrhotite
Pyrite
Galena
Sphalerite
Oxides-Hydroxides
Magnetite
Pyrochlore
Perovskite
Hematite
Ilmenite
Rutile
Baddeleyite
Pyrolusite
Halides
Fluorite
Phosphates
Apatite
Monazite
* only in natrocarbonatite
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. 1537. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology.
Prentice Hall.
Ijolite
plutonic nepheline-pyroxene rock with 30-70% nepheline
Urtite
plutonic nepheline-pyroxene (aegirine-augite) rock with over 70%
nepheline and no feldspar
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 Sovite) 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. Winter (2001) An
Introduction to Igneous and
Metamorphic Petrology. Prentice
Hall.
Carbonatite
Origins
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.
Ratios plot along lines from
HIMU to EMI
Isotopic signatures of
carbonatites and associated
silicates indicates they are
genetically related
Carbonatites
as primary magmas
At about 70 km depth, the
presence of CO2 begins to
convert silicates to carbonates:
CaMgSi2O6 +2 Mg2SiO4 + 2 CO2
CPx
Ol
= CaMg(CO3)2 + 4 MgSiO3
dolomite +
Opx
Making a Hbl + Dol region
V= vapor M= melt
As much as 45 wt. % CO2 is
dissolved in the melt
The presence of H2O brings the
melting pt. of Calcite down to
600 C
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.
If sufficient CO2 and H2O in
rising aesthenosphere
plume, melting will occur as
rising Lherzolite passes 2.
Rise to solidus at 3, then
solidfy
3 - Lamproites
Ultramafic rock with
uniquely high alkali
(especially K)
content that exceeds
the alumina on a
molar basis, so they
are called peralkaline.
3- Lamproites
Peralkaline, volatile rich,
ultra-potassic rocks
Ti and K-rich amphibole, Olivine
Diopside, leucite and sanadine.
No plagioclase, nepheline, or
Sodalite
Little differentiation, strictly in
thick continental settings, on
craton margins over extinct
subduction zones.
Figure 19-17. Chondrite-normalized
rare earth element diagram showing
the range of patterns for olivine-,
phlogopite-, and madupiticlamproites 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.
Lamproites
Enriched wrt bulk
earth low Nd/Nd
and high Sr/Sr
Lamproites are
thought nevertheless
to be from a Mantle
source, the Sub
Continental
Lithospheric Mantle
SCLM, not crust
contamination
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.
Lamprophyres
Porphyritic dike
rocks with large
phenocrysts of
mafic minerals
Many types with
different origins
Lamprophyres
Table 19-7. Lamprophyre Nomenclature
Light-colored
constituents
Predominant mafic minerals
biotite,
hornblende,
Na- Ti- amphib., melilite, biotite,
feldspar
foid
diopsidic augite, diopsidic augite,
Ti-augite,
± Ti-augite
(± olivine)
(± olivine)
olivine, biotite ± olivine ± calcite
or > pl
-minette
vogesite
pl > or
-kersantite
spessartite
or > pl
feld > foid
sannaite
pl > or
feld > foid
camptonite
-glass or foid
monchiquite
polzenite
--alnöite
Lamprophyre branch:
Calc-alkaline
Alkaline
Melilitic
After Le Maitre (1989), Table B.3, p. 11.
Only common feature is hydrated mineralogy amphiboles and micas
Polygenetic
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.
Kimberlites
Kimberlite
Pipe
Kimberlite Sample rich in Olivine
Kimberlites
Differentiation of HREE
suggests a deep Garnet
Lherzolite, and the
greatest known LREE
enrichment suggest
enrichment from a
subduction zone during
ascent
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.
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.