Final answer:
The compositional banding in gneiss results mainly from the segregation of minerals into different composition bands, stretching and smearing of compositional differences in the protolith, and the reorientation of minerals due to differential stress. Complete melting and recrystallization do not typically result in such banding, as they would produce an igneous rock, not a metamorphic one.
Step-by-step explanation:
Compositional Banding in Gneiss
The compositional banding in gneiss, a high-grade foliated metamorphic rock, can be attributed to several processes. One such process is the segregation of minerals into bands of different composition.
During the metamorphic process, minerals can self-segregate into discrete zones, forming the characteristic alternating bands of light and dark minerals. Another relevant process is the compositional differences in the protolith, which are stretched and smeared out due to intense heat and pressure, redistributed across different bands. Additionally, the reorientation of platy minerals like micas or elongate minerals like amphiboles can occur due to differential stress, contributing to foliation and gneissic banding. This reorientation results in a layered, foliated texture in the metamorphic rock.
Such banding is not likely due to complete melting and recrystallization since that would lead to an igneous rock rather than a metamorphic one. Rock textures produced by shearing and breaking, such as in a fault zone, are referred to as mylonitic, which is different from the foliated textures seen in gneiss. Heat and pressure induce these textural changes, where heat allows ions in minerals to migrate and pressure forces minerals to reorient. The orientation of minerals, termed foliation, increases with pressure, and gneiss represents a high-grade metamorphic rock formed from materials such as schists or granites.