Igneous

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Igneous Mineral Associations
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• How we know the mineralogy of the mantle
• Seismic velocities indicate rocks are either
  peridotite or eclogite
• Natural samples of the mantle xenoliths in
  lava, rocks from tectonically disrupted
  lithosphere (ophiolites) are mostly peridotite
• Mantle rocks must melt to form basaltic magma

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3 types of primary basaltic magma

• At divergent plate margins (mid ocean ridges)
  magma rises from asthenosphere - decompression
  melting at low pressure - tholeiitic basalt
• At hot spot (intra-plate volcanoes) magma rises
  from deep mantle - decompression melting at high
  pressure - alkali basalt
• At convergent plate margins (volcanic arcs)
  water added to the mantle from the subducted
  lithosphere causes melting - flux melting -
  calc-alkaline or high alumina basalt
• Special magmas from the mantle kimberlite,
  carbonatite

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Intrusive and extrusive igneous rocks

• Intrusive (plutonic) igneous rocks form from
  magma that cools beneath the earths surface
• Extrusive (volcanic) igneous rocks form from
  magma that cools at the earths surface
• Lava
• Pyroclastics

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Coarse grained Plutonic or intrusive rocks
TEXTURE COMPOSITION
Fine grained Volcanic or extrusive
rocks
Mafic rocks Basalt and gabbro contain mafic
minerals
Felsic rocks Rhyolite and granite contain
feldspar and quartz
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The common igneous rocks are classified on the
basis of SiO2 and the percentage of mafic
minerals, in addition to texture.
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Why are there different types of magma ?
Experiments on molten rocks showed that there was
a sequence in which minerals crystallized from
basaltic magma as the magma cooled
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Magmatic differentiation by fractional
crystallization

• Liquid part of magma changes composition because
  early formed minerals settle out and are
  subtracted from it

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Other magmatic differentiation processes-assimil
ation of crustal rocks-magma mixing
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Magmatic differentiation

• There are some special mineral associations
  related to differentiation
• 1) Layered Mafic Intrusions
• 2) Pegmatites

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