Incremental growth of granitoid plutons and highly crystalline magmatic differentiation

Understanding the genesis of granites is fundamental to understanding the formation and differentiation of continental crust. Geological, geophysical, geochronological and field studies, combined with modeling of thermal evolution of plutons, indicate that many granitic bodies emplaced in the upper crust result from the amalgamation of several, discrete magma pulses over several million years or even a longer timescale. Hypothesized batholith-scale magma chambers may not exist in the crust, and magma bodies with the capacity for flow of melts are generally small (< 1000m). A magma body 1000m wide would cool down to solidus on a timescale of thousands of years. The formation of composite intrusions generally has three stages: source magma upwelling along dikes, transformation from dykes to sill-like intrusions at the brittle-ductile transition of the crust, and growth of the magma body by the vertical stacking of numerous sill-like magma bodies. Magma chambers in the crust, especially those successively-intruded magma bodies, are mainly composed of crystal mush. The crystal mush is adverse to convection, differentiation or mixing owning to the high crystal content, high viscosity and weak activity. However, the viscous mushy magma can be heated, becoming more highly melted and less viscous when the mantle-derived mafic magmas intrude into the crust. This leads to differentiation inside one magma body and mixing between magmas with distinct compositions. Finally, when the buoyancy of the bottom highly molten magma is high enough, or with an injection of volatiles, it will rise rapidly, penetrate the upper mushy magma and trigger large-scale volcanic eruptions. The activity of crustal magmatism is enhancing when there is an increase in the flux of mantle-derived magma. Thus, large-scale felsic magma may form a super volcano. It is proposed in this paper that understanding relationships between plutonism-volcanism and felsicmafic rocks is fundamental for a better understanding of the genesis of granites. Moreover, we must pay close attention to multiple factors, such as time- and spatial-scale of the intrusions, evolution of the magma fluxes, differentiation mechanism of highly crystalline rocks, contribution of mantle heat and materials, and the role of the volatiles during magma differentiation and volcanic eruption. These factors should be combined with a comprehensive study of field observation, petrology, geochemistry, isotopic chronology and magma dynamics to achieve a more complete understanding of the formation and evolution of continental crust.