Many granites result from anatexis of common crustal rock types and the segregation, aggregation, ascent and emplacement of the resultant magma. What then is the connection between migmatites, rocks which preserve evidence at outcrop-scale for the presence of former melt now frozen as granite, whether in situ or locally displaced with respect to the site of melting, and map-scale bodies of crustally-derived granite, clearly removed from the site of melting? Both water-rich volatile phase-present melting and volatile phase-absent dehydration melting can occur in the middle and lower crust, but dehydration melting that involves the decomposition of mica and amphibole likely is the more important process in the generation of plutonic volumes of magma with sufficient mobility to reach the upper crust. Both volatile phase-present and dehydration melting can occur in each of the two main types of orogenic belt, those that result from thickening before maximum temperatures are achieved (clockwise in P-T space) and those that result from heating prior to or concomitant with thickening (anticlockwise in P-T space). Depending upon the particular tectonic circumstances, the thermal perturbation to provide the heat necessary for crustal anatexis may be caused by internal radiogenic heat production in overthickened crust, intraplating/underplating of mantle-derived magma, an enhanced flux from the mantle, or some combination of these mechanisms. The tectonic environment to a large extent also controls the segregation, ascent and emplacement of granite magma. For example, at the present time a majority of convergent plate margins exhibit an oblique net displacement vector, and it is likely, therefore, that oblique convergence was important in the past. Retreating subduction boundaries will result in regional deformation of the overriding plate by horizontal extension or transtension in contrast to advancing subduction boundaries that will result in regional deformation of the overriding plate by horizontal shortening or transpression. Transpression can be considered as a zone of transcurrent shear accompanied by horizontal shortening across and vertical lengthening along the shear plane. It plays a vital role in overthickening of the crust, in structurally shuffling and thickening sedimentary basins, in assisting with the segregation of crustally-derived melts, and ultimately allowing for the ascent and emplacement of granite magma in extensional segments of the associated strike-slip system. Segregation of granite melt depends upon a number of factors that include how the liquid is distributed within the matrix of a partially melted rock and the viscosity of that liquid. The geometrical structure of partially molten rock is crucial to understanding its behavior. Migration of melt is a complex process that is achieved by compaction and buoyancy segregation, and flow into extensional and shear fractures and other dilatant sites. The driving force for segregation may be chemical or physical or a combination of both. In this review I stress the important role of deformation in enhancing segregation of melt. Magma buoyancy is a primary driving force for ascent, but diapirism no longer appears to be a viable mechanism. Rather, fracture-controlled mechanisms and deformation-enhanced ascent are considered to be of prime importance. Magma ascends in dykes to feed tabular batholiths that are constructed from hundreds of individual magma pulses due to magma ponding at roughly horizontal discontinuities in the upper crust. I emphasize the role of ductile shear zones and fault systems in the ascent and emplacement of magma. Many granites appear to have been constructed from sheets emplaced in transient dilational sites along transpressional strike-slip fault systems undergoing net contructional deformation. Emplacement is synkinematic, a void or cavity will not exist and filling at a suitable site occurs simultaneously with dilation. Rates of ascent are fast, consistent with a pulsed magma supply. Some examples cited in the literature of diapiric emplacement may be interpreted better as local ballooning by magmatic flow of granite after upward transport along shear zones. Thus, granite contact relationships likely reflect local emplacement mechanisms rather than regional, crustal-scale ascent mechanisms. A general model for granite magma genesis, ascent and emplacement that may apply to other orogenic belts is developed from relationships interpreted from different crustal levels exposed within the late-Precambrian Cadomian orogenic belt of northwest France. Here, thickening of a volcano-sedimentary basin during transpression led to upper amphibolite facies water-rich volatile phase-present anatexis and development of migmatites (St. Malo migmatite belt) and granite melt production (Mancellian granites), with some evidence of dehydration melting in granites emplaced at the highest structural level. Transcurrent shear was regionally focussed within this zone of softened crust. Granite magma was transported to higher crustal levels in megadike bodies (c. 0.5-1 km width), themselves constructed from multiple sheets, located within major ductile shear zones. Magma entered the shear zones at points of local extension and was expelled upwards in zones of compression, a mechanism referred to as strike-slip dilatancy pumping. The shear zones are inferred to have been linked to major brittle fault zones in the upper crust, and extensional jogs within such systems have provided sites for assembly of plutons (tens of km across) from magmas arriving from below. Granite is generated also by decompression melting during uplift of orogenic belts and commonly has an important role to play in exhumation of the high-grade cores of thickened orogens. The Variscan metamorphic belt of western France provides an example of these interactions between tectonic processes and granite in a thickened orogen. Here, the Eo-Variscan to Variscan P-T-t-d evolution required fast uplift and exhumation of the metamorphic belt at c. 330-300 Ma as a coherent block without internal penetrative strain. Coeval granite was produced by decompression melting during the fast uplift. The granite facilitated exhumation by accommodating strain along a major intracontinental transcurrent shear zone and along thrusts, reactivated to allow tectonic unroofing by ductile normal faulting.
