A new paradigm for granite generation

Abstract

Ideas about granite generation have evolved considerably during the past two decades. The present paper lists the ideas which were accepted and later modified concerning the processes acting during the four stages of granite generation: melting, melt segregation and ascent, and emplacement. The active role of the mantle constitutes a fifth stage.Fluid-assisted melting, deduced from metamorphic observations, was used to explain granite and granulite formation. Water seepage into meta-sedimentary rocks can produce granitic melt by decreasing melting temperature. CO2 released by the mantle helps to transform rocks into granulites. However, dehydration melting is now considered to be the origin of most granitic melts, as confirmed by experimental melting. Hydrous minerals are involved, beginning with muscovites, followed by biotite at higher temperatures. At even deeper conditions, hornblende dehydration melting leads to calc-alkaline magmas.Melt segregation was first attributed to compaction and gravity forces caused by the density contrast between melt and its matrix. This was found insufficient for magma segregation in the continental crust because magmas were transposed from mantle conditions (decompression melting) to crustal conditions (dehydration melting). Rheology of two-phase materials requires that melt segregation is discontinuous in time, occurring in successive bursts. Analogue and numerical models confirm the discontinuous melt segregation. Compaction and shear localisation interact non-linearly, so that melt segregates into tiny conduits. Melt segregation occurs at a low degree of melting.Global diapiric ascent and fractional crystallisation in large convective batholiths have also been shown to be inadequate and at least partly erroneous. Diapiric ascent cannot overcome the crustal brittle-ductile transition. Fracture-induced ascent influences the neutral buoyancy level at which ascent should stop but does not. Non-random orientation of magma feeders within the ambient stress field indicates that deformation controls magma ascent.Detailed gravity and structural analyses indicate that granite plutons are built from several magma injections, each of small size and with evolving chemical composition. Detailed mapping of the contact between successive magma batches documents either continuous feeding, leading to normal petrographic zoning, or over periods separated in time, commonly leading to reverse zoning. The local deformation field controls magma emplacement and influences the shape of plutons.A typical source for granite magmas involves three components from the mantle, lower and intermediate crusts. The role of the mantle in driving and controlling essential crustal processes appears necessary in providing stress and heat, as well as specific episodes of time for granite generation. These mechanisms constitute a new paradigm for granite generation.

Comparative petrogenesis and tectonics of Paleoproterozoic Malanjkhand and Dongargarh granitoids, Central India

“…Continental granite magmatism involves four stages: melting, segregation, ascent, and emplacement (Petford et al, 2000;Vigneresse, 2004). Experimental studies indicate that most granitic magma formed by dehydration melting processes (Naney, 1983;Rushmer, 1991;Rapp and Watson, 1995).…”

Petrology and geochemistry of the granitoid complex of Boroujerd, Sanandaj-Sirjan Zone, Western Iran

“…Bullen and Clynne, 1990;Roberts and Clemens, 1993;Tepper et al, 1993;Guffanti et al, 1996). The mantle is also the heat source that controls crustal melting (Vigneresse, 2004). Other mechanisms for crustal melting require either advective or diffusive heat.…”

“…Other mechanisms for crustal melting require either advective or diffusive heat. Heat transfer by the former process brings hot material from the mantle and rapidly heats up the crust (Vigneresse, 2004).…”

An introduction to magma dynamics

“…The Socorro magma body, which can be seen on seismic profiles, is sill-like (Rinehart et al 1979;Brocher 1981). Further, most igneous bodies may have been emplaced by the incremental assembly of discrete pulses Vigneresse 2004;de SaintBlanquat et al 2006). Geochronological data indicate that the assembly of some plutons and batholiths lasted several millions years.…”