Why is compositional layering a common feature of mafic plutons but not of felsic plutons?

Mafic plutons are formed from relatively low-viscosity magma rich in magnesium and iron, which typically crystallize into minerals like pyroxene, olivine, and plagioclase feldspar. Felsic plutons, on the other hand, are formed from magma with a high silica content and are rich in minerals like quartz, alkali feldspar, and biotite or muscovite mica.

Compositional layering in plutons occurs when different mineral compositions precipitate out of magma, forming distinct layers. This can be a result of processes like gravity settling, where denser minerals sink to the bottom of the magma chamber or convection currents within the cooling magma.

In mafic plutons, the minerals that form are generally denser and have higher melting points than those in a felsic pluton. Since mafic magma has lower viscosity, it allows for efficient movement and mixing of minerals, such that denser minerals can easily settle to the bottom of the chamber. This leads to the formation of distinct compositional layers due to gravity settling and convection.

In contrast, felsic magma has a higher viscosity, making it more difficult for minerals to move and mix within the magma chamber. This results in a more homogeneous distribution of minerals throughout the cooling magma, rather than forming distinct layers. Additionally, since felsic minerals are generally less dense and have lower melting points, there may be less differentiation in density and melting temperature between minerals, making gravity settling less likely to produce compositional layering. In conclusion, compositional layering is more common in mafic plutons due to the low magma viscosity allowing for efficient mineral movement and mixing, as well as their dense minerals settling to the bottom of the magma chamber. In felsic plutons, the high magma viscosity and less density differentiation between minerals make compositional layering less likely to occur.