Ch' 10 Igneous Rocks of the Continental Lithosphere

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Ch. 10Igneous Rocks of the Continental
Lithosphere
Kilimanjaro
Carbonatites in the caldera of Ol Doinyo Lengai
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Continental Igneous Rocks

• Do not involve plate interactions
• Continental shield areas or rift zones long after
  orogenesis
• Diverse tholeiite-, alkali-basalts,
  carbonatite, nephelenite, phonolite, rhyolite,
  anorthosite, anorogenic granites, lamproites and
  kimberlites
• Volcanic Eg Columbia Rv basalts Pacific
  Northwest, Karoo basalts of S. Africa, Deccan
  traps of India
• Intrusive Eg Bushveld, Stillwater and Skaergard
  complexes

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Continental Basalt Provinces
Mambai, India
Antarctica
Deccan Traps - India
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Continental Basalt Provinces

• Continental Flood Basalts CFBs
• Eg Colombia Rv basalts - Enormous outpourings of
  tholeiitic basalt gt100,000 km3 in little over 3
  Ma
• Few if any phenocrysts primary magma?
• MgFe ratio too low for typical mantle melts an
  atypical mantle source with 2 possible
  complications 1) east dipping subduction under
  the Cascades must have passed under the Columbia
  Rv Plateau and could have contributed subduction
  zone magmas 2) The linear belt of volcanics in
  the Snake River province has been ascribed to the
  passage of a hot spot beneath North America
  Yellowstone Hot Spot
• Currently thought that the subcontinental mantle,
  including lithosphere and asthenosphere, is
  heterogeneous, with depleted and enriched
  portions. Unlike oceanic lithosphere, continental
  lithosphere is not readily recycled into the
  mantle and instead is welded or attached to the
  overlying continental crust quite early. CFBs
  originate from melting of enriched
  sublithospheric (asthenospheric) mantle of the
  type that also produces E-MORBs and OIBs.

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Columbia Rv Plateau
Snake Rv Plain
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Continental Basalt Provinces

• Layered Mafic Intrusions
• Increasing tectonic evidence suggesting they are
  related to periods of major crustal extension and
  rifting in continental lithosphere and may be the
  intrusive deep crustal feeders for the CFBs also
  associated with rifting
• Bushveld, Stillwater (Montana), Skaergard
  (Iceland), Dufek (Antarctica)
• Overall basaltic in composition and subdivided
  into thick units ranging from ultramafic at the
  bottom to felsic at the top.

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Skaergard intrusion of east Greenland
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Continental Basalt Provinces

• Komatiites
• Rare rocks believed to represent ultramafic lava
  flows
• Occur in layered stratigraphic patterns with
  pillow morphologies at the top
• Odd rocks because ultramafics have liquidus T
  between 1400-1600? C at low P and are thus
  unlikely to occur as liquids even in the Earths
  interior
• Notable for their spinefex texture, after the
  large elongated bladelike phenocrysts of olivine
  in a groundmass of cpx
• To generate ultramafic melts in the Archean the
  mantle must have been much hotter at shallow
  depths as it would require 80 partial melting
• Profound implications for plate tectonics
  sea-floor spreading and subduction may have
  operated more effectively and rapidly in the
  Archean than today, allowing for efficient
  recycling of the early products of crust building
  which may be why the rocks from this earliest
  part of the Earths history are so rare

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Continental Rifts

• Large scale continental extensional environments
  probably represent the initial rifting stages of
  single continental plates as they break up and
  drift apart. Include failed rifts
• Eg East African Rift system, which is
  volcanically active due to large scale
  extensional stress in the crust combined with
  normal faulting which provides pathways for magma
  to reach the surface.
• Continuous volcanism for the past 30 Ma.
  Dominantly alkalic. Nephelinites and alkali
  basalts occur early in the sequence and are
  followed by phonolites, trachytes and rhyolites.
  This increase in silica along with increasing
  FeMg ratios in phenocrystic ferromagnesian
  minerals are key indicators of fractionation
  processes operating in deep magma chambers
• Late magmas are characterised by highly alkalic
  and silica-deficient lavas and even by
  carbonatites.

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East African Rift-Dead Sea Graben- Red Sea Graben
extensional system
Volcanism of East African Rift
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Chemistry, Petrography and Petrogenesis of
Continental Rift Magmas

• The large diversity of igneous rocks at rifts
  raises many petrological questions. How are these
  magmas related to each other? How many are mantle
  derived and how many are the product of
  fractionation in mantle or deep crustal magma
  chambers? Has crustal melting or contamination
  played a role in magma evolution?
• The chemistry of the silica-undersaturated alkali
  basalts, basanites and nephelinites is similar to
  that of OIBs ie a chemical signature of deep
  melting or enriched mantle, possibly
  asthenosphere.
• The interesting rocks of continental rifts are
  the more evolved ones eg phonolite, trachyte,
  rhyolite. The majority of these are peralkaline.
  As reflected by the presence of sodic px and
  amphibole eg aegerine, aegerine-augite,
  riebeckite and high FeMg ratios in
  ferromagnesian minerals and the presence of
  fluorite.
• The highly alkaline nature and high conc of
  incompatible elements suggests small degrees of
  partial melting at great depth (60-100 km). The
  mantle beneath East Africa is not normal
  subcontinental lithosphere mantle but may be
  upward bulging asthenosphere. The continenal
  crust is thinned to 20km

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Carbonatites

• Rare igneous rocks containing gt50 carbonate
  minerals (calcite, dolomite, magnesite and sodium
  carbonate). Rich in Na-rich pyroxenes and
  amphiboles apatite, phlogopite, magnetite,
  fluorite, perovskite, monazite, pyrochlore and
  barite
• Usually accompanied by nepheline syenite
• Abundant fracturing by alkali-rich fluids
  resulting in metasomotism referred to as
  fenetization
• Originate by small degrees of fractional melting
  of mantle peridotite which contains carbonate
  minerals or CO2-rich fluid phase and is important
  in the role of melting

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Anorogenic Granites

• Granite related plutonic rocks with no obvious
  relationship to subduction or plate convergence
• A-type granites. However substantial overlap with
  I-type
• Usually either metaluminous or peralkaline, high
  FeMg ratio in ferromagnesian minerals
• Characteristic occurrence on the liquidus of only
  a single alkali feldspar, rather than the
  separate crystallisation of a K-rich alkali
  feldspar and a Na-rich feldspar. Upon cooling,
  this single feldspar decomposes (exsolves) into
  the lamellar intergrowth called perthite with
  K-rich and Na-rich lamellae referred to as
  hypersolvus crystallisation and is an important
  clue as to low P and shallow depth
• Simultaneous crystallization of 2 feldspars is
  called subsolvus texture

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Anorthosites

• Plutonic rocks containing gt90 plagioclase
• Lunar anorthosites
• Archean Megacrystic Anorthosites plagioclase up
  to 1m across!
• Massif Anorthosites includes the charnockite
  series (opx granite fayalite, hedenburgite)

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Kimberlites and Lamproites

• Volumetrically miniscule but economically
  important diamonds
• Kimberlites are potassic ultramafic rocks that
  occur as small plugs or pipes
• Lamproites are ultramfic end members of a family
  of highly porphyritic ultrapotassic rocks
• Contain abundant xenoliths

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Kimberlites and Lamproites
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Study Exercises