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

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

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

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

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

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

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

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

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

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 102 - 104

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)

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.