1. N.Petford, A.R.Cruden, K.J.W.McCaffrey & J.-L.Vigneresse
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 to melt 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 continental crust.
- Self-propagating dykes along faults

7. 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

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
Process
Tectonic setting
Mechanism
Timescale (years)
Partial Melting
Fluid present
Fluid absent
Transpressional/ Transtensional orogens
Magmatic arcs
Crustal thickening& decompression/ astenosphere upwelling
Magmatic under/intraplating
>105
102 – 105
Segregation
Extensional/compressional
Gravity driven compaction
Deformation-enhanced flow/fracturing
105 – 109
106 – 103
Ascent
Dyke/conduit flow
Pervasive flow
Diapiric rise
Mainly extensional
Transpressional
Buoyancy or deformation-assisted flow
Viscous flow in hot permeable crust
Buoyancy
10-1 – 102
≤ 106
Emplacement
Entrainment along structural/ rheological traps, buoyancy

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
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 In: Ailleres, L. and Rawling, T