Unusual rocks

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Unusual rocks
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• Alkaline rocks
• Excess alkalis in feldspathoids, sodic px/amph
• SiO2 deficient so no quartz
• lt1 exposed igneous rocks are alkaline

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Table 19-1. Nomenclature of some alkaline igneous
rocks (mostly volcanic/hypabyssal) Basanite felds
pathoid-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. Xenocryst s 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)
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Common occurrence

• Continental rifts
• Intraplate settings with no clear tectonic
  control
• End of volcanic activity (Hawaii)

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Rift associated

• Rhine graben, Baikal Rift, Oslo Rift, East
  African Rift
• Uplift, extension, 3 km deep grabens
• Likely plume causing uplift and magma source
• Also in rift carbonatites

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carbonatites

• gt50 carbonate minerals
• Igneous carbonate rocks
• 1/2 in Africa, also in Arkansas Ontario
• Need stable continental craton

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• Crazy mineralogies!

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• Crazy chemistries
• Look at
• SiO2 content

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• Crazy chemistries
• Look at
• SiO2 content
• REE content

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Carbonatite source

• Mantle source (isotopes)
• Direct melt of hydrous carbonated mantle
• How does mantle get CO2 in it?
• Deep primordial mantle
• Subducted limestones/altered ocean crust

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natrocarbonatite

• Tanzania Ol Doinyo Lengai volcano
• On east African rift
• Sodium carbonate lava
• Very low viscosity
• Rich in CO2

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natrocarbonatite

• Tanzania Ol Doinyo Lengai volcano
• Sodium carbonate lava
• Very low viscosity
• Rich in CO2
• Only volcano like in on Earth
• Similar to flows on Venus?

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• Black flows lt24 hours
• Then turn gray, then powdery white

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• Long exposure photography shows red flows
• Eruption T 500C
• Basalt 1000C

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• Active spatter cone
• CO2 makes lava foam like sodas

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Other alkaline rocks on cratons

• Lamprophyres porphyritic dikes, basaltic but
  crazy chemistry/mineralogy. Commonly with plag
  and nepheline
• Lamporites similar, but no plag and nepheline.
  Found in Leucite Hills of Wyoming. Ultrapotassic
• Kimberlites

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Kimberlites

• Emplaced as explosive breccia from deep in mantle
• Hard to tell true magma composition
• So much contamination

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Kimberlites

• Famous for diamonds only on Archean crust
• Most occur in South Africa
• Diamonds are older than host rock (900-3300 Ma
  vs. 90-1600 Ma)

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• Diamonds form and preserved at base of craton
  (120-200 km thick)
• Picked up by younger kimberlite and lamproite
  magmas
• Inclusions in diamond found in harzburgite found
  in kimberlites hosted in Archean country rock

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Anorthosites

• Plutonic rocks gt90 plag
• No known volcanic equivalent
• Light colored highlands of moon
• Archean and Proterozoic

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Fiskenaesset anorthosites
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• This is where all the Eu ends up!

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• How do we concentrate so much plag from a mantle
  melt?

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Model for origin

• a. Mantle-derived magma underplates the crust as
  it becomes density equilibrated.

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• b. Crystallization of mafic phases (which sink),
  and partial melting of the crust above the ponded
  magma. The melt becomes enriched in Al and Fe/Mg

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• c. Plagioclase forms when the melt is
  sufficiently enriched. Plagioclase rises to the
  top of the chamber whereas mafics sink.

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• d. Plagioclase accumulations become less dense
  than the crust above and rise as crystal mush
  plutons.

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• e. Plagioclase plutons coalesce to form massif
  anorthosite, whereas granitoid crustal melts rise
  to shallow levels as well. Mafic cumulates remain
  at depth or detach and sink into the mantle.

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Anorthosite on the moon

• Highlands anorthosite of 4.4 Ga
• Maria basalt and younger
• Several km thick layer of magma formed at surface
• Magma ocean crystallizes, plag floats
• Occurs just after formation based on age