What is the correct name for a black and white banded rock with coarse crystals of biotite, quartz, and garnet, where foliation is associated with the folds?
The described rock is a metamorphic rock called gneiss, identified by its characteristic banding, coarse crystals of biotite, quartz, and garnet, and foliation associated with folds.
Gneiss: A Metamorphic Rock
Gneiss is a metamorphic rock that exhibits distinct banded textures, often composed of alternating layers of light and dark minerals. The rock you are describing, with its black and white banding, coarse crystals of biotite, quartz, and garnet, and foliation associated with folds, fits the characteristics of gneiss perfectly.
Formation Process:
Gneiss forms from the metamorphism of pre-existing rocks, usually sedimentary or igneous in origin, under high temperature and pressure conditions deep within the Earth's crust. During this process, the minerals in the original rock are recrystallized and reoriented, resulting in the characteristic banding and foliation seen in gneiss.
Characteristics:
The coarse crystals of biotite, quartz, and garnet in the rock indicate the presence of these minerals in the protolith (original rock) that underwent metamorphism to become gneiss. The distinctive banding is a result of mineral segregation during metamorphism, with darker minerals like biotite and garnet forming one band and lighter minerals like quartz forming another.
Importance of Foliation:
The foliation seen in gneiss is a result of the alignment of minerals within the rock due to directed pressure during metamorphism. This alignment gives gneiss its layered appearance and allows geologists to study the history of deformation and tectonic events that affected the rock.
In conclusion, the rock described in the question fits the characteristics of gneiss, a metamorphic rock known for its banded texture, coarse crystals, and foliation. Understanding the formation process and characteristics of gneiss can provide valuable insights into Earth's geological history and processes.