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Granite magma formation, transport and emplacement in the Earth’s crust

Granite magma formation, transport and emplacement in the Earth’s crust. N.Petford, A.R.Cruden, K.J.W.McCaffrey & J.-L.Vigneresse. Main topics. Partial melting of continental crust Melt transport Emplacement Three-dimensional shapes of granitic intrusions Mechanisms of pluton growth

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Granite magma formation, transport and emplacement in the Earth’s crust

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  1. Granite magma formation, transport and emplacement in the Earth’s crust N.Petford, A.R.Cruden, K.J.W.McCaffrey & J.-L.Vigneresse

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

  3. Partial melting of continental crust • Temperatures in crust are generally not high enough tomelt crustal rocks • Only <25% 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

  4. 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

  5. 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

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

  7. EmplacementDefinition: 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

  8. 3D-shapes of granitic intrusions • Flat-lying to open • Funnel-shaped • Central/marginal feeding zones

  9. Mechanism of pluton growth • Laterally spreading • Upward thickening • Evolves according to a power-law: L=kTa width thickness • Therefore, first horizontal traveling of magma, then vertical thickening

  10. Timescales of pluton growth

  11. 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)

  12. References • Granite magma formation, transport and emplacement in the Earth’s 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.

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