TOWARD A NEW PARADIGM FOR GRANITE GENERATION

A new paradigm for granite generation lists what was accepted, and later modified during the melting, melt segregation and ascent, and emplacement. Fluid (water or CO2) assisted melting, deduced from metamorphic situations explained joint formation of granite and granulite generation. Dehydration melting of hydrous minerals is now considered as the origin of most granitic melts, as observed from experimental melting. Minerals are first muscovite, then biotite at higher temperature and hornblende in deeper conditions. Melt segregation was first attributed to compaction and gravity forces owing to density contrast between melt and its matrix. The mode revealed unlikely since it ported studies in the mantle conditions (decompression melting) to crustal conditions (dehydration melting). Rheology of two-phase materials documents that melt segregation is irregular in time, with successive bursts as confirmed by Analogue and Lagrangian numerical models. Compaction and shear localisation non-linearly interact, so that melt segregates into tiny conduits. Melt segregation occurs whatever the degree of melting. Global diapiric ascent as large batholiths also revealed inadequate and partly erroneous. Diapiric ascent cannot overcome the crustal brittle-ductile transition. Fracture-induced ascent faces the neutral buoyancy level at which depth the ascent should stop, though it doesn't. Non-random orientation of magma feeders within the ambient stress pattern indicates that deformation controls magma ascent. Melts concentrate into smaller conduits under the ambient rheology, as shown by independent numerical and analogue models. Detailed gravity and structural analysis indicate that granites are built from several episodes of magma, each of small size and evolving chemical composition. Contacts between successive magma batches document either continuous feeding, leading to normal petrographic zoning, or by successive periods of time, leading to inverse zoning. Magma emplacement is controlled by the local deformation field, imposing the shape of plutons. A typical source for granite magmas involves three components from the mantle, lower and intermediate crust. The role of the mantle appears necessary in providing stress, heat and specific episodes of granite generation.