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Granite Magma Formation and Emplacement in Earth's Crust: Mechanisms and Timescales

This article explores the complex processes involved in the formation, transport, and emplacement of granite magma within the Earth's crust. Topics covered include partial melting of continental crust, melt transport mechanisms, three-dimensional shapes of granitic intrusions, and the timescales of pluton growth. The text delves into the various factors influencing granitic melt formation, volume changes during melting, and the role of gravity in magma transport. Emplacement processes, 3D shapes of intrusions, mechanisms of pluton growth, and the timescales of granite formation are also discussed. A comprehensive overview is provided with references for further exploration.

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Granite Magma Formation and Emplacement in Earth's Crust: Mechanisms and Timescales

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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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