GRANITE: FROM GENESIS TO EMPLACEMENT

To produce a significant volume of anatectic melt at low temperatures (680–750ºC) requires an influx of aqueous fluid along structurally-controlled pathways or recycling of fluid via migration of melt and exsolution during crystallization. At higher temperatures melting occurs by hydrate-breakdown reactions involving micas and amphiboles, which produces significant melt volumes at temperatures >800°C for micaeous protoliths and >900ºC for amphibolites. Calculated phase diagrams for average pelite illustrate the types of mineralogical controls on melt production and composition. Modal mineralogy, accessory phase accessibility and disequibrium melting control the chemistry and isotope signature of crustal melts; peritectic assemblage entrainment is important to the final composition of some granites. Preservation of peritectic minerals in residual migmatites requires that most of the melt produced was extracted, which implies a flux of melt through the suprasolidus crust. Suprasolidus crust becomes porous at a low melt volume, as evidenced by microstructures in residual migmatites, whereas leucosome networks record evidence of melt accumulation at outcrop scale. Melting leads to weakening of the crust, so that feedback between deformation and melting creates a dynamic rheological environment leading to melt segregation as melt volume reaches the melt connectivity transition. With increasing melt volume and decreasing effective pressure the behavior of anatectic crust changes from distributed shear-enhanced compaction to localized dilatant shear failure. The resulting deformation bands form a high-permeability network that allows for accumulation of melt and fast transport at the initiation of a melt extraction event. Melt escapes from the source in multiple melt-loss events with increasing temperature during an orogenic event. Buoyancy-driven melt ascent is via discordant dikes in ductile-to-brittle fractures that propagate from dilation or shear bands or via concordant high-permeability zones controlled by tectonic fabrics. Emplacement of horizontal tabular and wedge-shaped plutons occurs around the ductile-to-brittle transition zone, whereas vertical lozenge-shaped and blobby plutons represent crystallization of melt in the ascent conduit.