The Geology of Ireland

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The Geology of Ireland
Part Two
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Main Tectonic Zones Granites
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Tectonic Environments of Granites

• Ocean Ridge Granites (ORG)
• Volcanic-Arc Granites
• Collisional Granites (COLG)
• Syn-tectonic granites associated with
  continent-continent collision
• Post-tectonic granites associated with
  continent-continent collision
• Syn-tectonic granites associated with
  continent-arc collision
• Within-Plate Granites (WPG)

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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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The Caledonian orogeny
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Crustal Melting Granite Magmas

• Partial Melting
• Segregation
• Aggregation
• Ascent
• Emplacement

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Protoliths Partial Melting

• Typical geothermal gradients of 20C/km do not
  generate temps gt800C at 35 km depth required to
  melt most crustal rocks (Thompson 1999).
• Three main factors in inducing partial melting
• Increase in temperature
• Decrease in pressure (adiabatic decompression)
• Introduce H2O-rich volatiles
• One or more of these may be met by the influence
  of proximal mantle-derived basaltic magmas

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Partial Melting Melt Segregation

• Melt segregation is the separation of the melt
  fraction from its restite and source during
  partial melting
• Melt segregation depends on the permeability of
  the source.
• Melts first forms at grain boundaries between
  mineral phases

Image source www.indiana.edu
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Protoliths Melt Composition

• Small degrees of partial melting of an
  amphibolite will produce a Si-rich melt.
• Granitic melts can be produced from a mafic
  protolith

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Granite Mineralogy
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Granite Mineralogy
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Mafic Enclaves
Close-up view of the Drogheda Granite with mafic
enclaves (www.gsi.ie)
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Mafic Enclaves
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Magma Mixing Mingling
Galway Granite (www.gsi.ie)
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Mafic Felsic Magma Interaction
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Magma Mixing and Mingling
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Porphyry Deposits

• Mineralisation associated with porphyritic
  intrusive rocks
• Ore occurs as disseminations along hairline
  fractures as well as within larger veins, which
  often form a stockwork
• The orebodies typically contain between 0.4 and
  1  copper with smaller amounts of other metals
  such as molybdenum, silver and gold
• They are formed when large quantities of
  hydrothermal fluids carrying small quantities of
  metals pass through fractured rock within and
  around the intrusive and deposit the metals

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Mineralisation
Molybdenite mineralisation
Fluorite mineralisation