On Granites

Abstract

Granites are important components of the Earth’s continental crust and represent the net effect of thermochemical processes that operate during partial melting, magma extraction, ascent, emplacement and crystallization. Compositional and isotopic variations in granites arise from source heterogeneities, mixing in the source, and peritectic mineral entrainment as well as crystal fractionation and assimilation. Fluid-absent hydrate-breakdown melting reactions that accompany high-temperature metamorphism are responsible for differentiating the continental crust into a granitic upper portion and a residual lower portion. In some cases, melting can proceed through the influx of a hydrous fluid, although the significance of this on the long-term compositional differentiation of the continental crust is debated. Accessory minerals in granites are important chronometers and play a primary role in transferring the radiogenic isotope signature of sources to granites. Magma ascent and emplacement are guided by deformation and pluton construction is expected to be incremental with pluton heterogeneity being related to the interaction and differentiation of individual magma batches, as well as the extent to which melt separates from solid residual source material. The nature of the source plays a crucial role in the concentration of water and volatiles that are essential for the generation of hydrothermal-magmatic mineral deposits. Future studies of granites using non-traditional stable isotope systems are expected to provide new insights into the evolution of Earth’s continental crust.