Granite magma formation, transport and emplacement in the Earths crust

1
Granite magma formation, transport and
emplacement in the Earths crust
N.Petford, A.R.Cruden, K.J.W.McCaffrey
J.-L.Vigneresse
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Main topics

• Partial melting of continental crust
• Melt transport
• Emplacement
• Three-dimensional shapes of granitic intrusions
• Mechanisms of pluton growth
• Timescales of pluton growth

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Partial melting of continental crust

• Temperatures in crust are generally not high
  enough to melt crustal rocks
• Only lt25 partial melt obtained by fluid-present
  melting
• More efficient way heat obtained from mantle by
  basaltic underplating
• This type of partial melting is more rapid
• Amphibole and mica breakdown is NB for formation
  of granitic melts (fluid absent conditions)
• Compositions of granitic melt differ with higher
  temperatures

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• Positive volume changes in fluid-absent melting
• Volume changes, deviatoric stress gradients and
  regional tectonic strain lead to higher fracture
  permeability, aiding in melt segregation

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Melt transport

• Transport process uses 2 length scales
• (i) SEGREGATION-small (dm or cm)
• (ii) ASCENT large (km)
• SEGREGATION
• - the physical properties of a granitic melt
  determines its ability to segregate
  mechanically from its matrix
• - viscosity and density

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• ASCENT
• - Gravity is the most obvious driving force for
  large vertical magma transport in continental
  crust.
• - Self-propagating dykes along faults

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Emplacement Definition Switch from upward to
horizontal flow1

• Final stage of granite formation in continental
  crust
• Mechanical interactions and density effects
  control the emplacement of granites
• Episodic processes
• Space for incoming magma needs to be created

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3D-shapes of granitic intrusions

• Flat-lying to open
• Funnel-shaped
• Central/marginal feeding zones

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Mechanism of pluton growth

• Laterally spreading
• Upward thickening
• Evolves according to a power-law
• LkTa
• width thickness
• Therefore, first horizontal traveling of magma,
  then vertical thickening

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Timescales of pluton growth
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Conclusion

• Formation of granite intrusions in middle-upper
  crust is goverened by 4 processes each with their
  own timescale and environment (as seen in
  foregoing table)

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References

• Granite magma formation, transport and
  emplacement in the Earths crust - N.Petford,
  A.R.Cruden, K.J.W.McCaffrey J.-L.Vigneresse
• http//terragalleria.com/pictures-subjects/granite
  /granite.9.html
• The rapid formation of granitic rocks more
  evidence - John Woodmorappe
• Analogue modelling of segregation and ascent of
  magma - Bons, P. D., Elburg, M. A. and
  Dougherty-Page, J. 2001. In Ailleres, L. and
  Rawling, T. 2001.