~Classification of Metamorphic Rocks~
The classification of metamorphic rocks is complex due to the fact that metamorphism takes place through heat, pressure, and chemically active fluids, but since there are many combinations of heat, pressure, and chemically active fluids, there are many different responses. Additionally, any rock - igneous, sedimentary, or metamorphic - can be a parent rock, and so the same heat/pressure/fluid combination can produce markedly different results depending on the type of parent. There is no direct relationship between the processes of metamorphism and the rocks that result- each situation is different. Finally, different parent rocks undergoing the same kind of metamorphism can end up looking virtually the same. For example, Greenschist (Shale parent) and greenstone (mafic igneous parent).
The texture terminology use for metamorphic rocks can be misleading. For example, schist is a type of layering or foliation found of metamorphic rocks where minerals large enough to be seen by eye tend to line up all in the same direction. But schistose rocks include Greenschist (Chlorite dominated), Blueschist (glaucophane dominated, plus many other minerals), and just plain Schist (Biotite or Muscovite, Quartz, and feldspar dominated), and each of these form under different tectonic conditions. To add confusion some rocks with a schist-like texture do not include the term in the name, like Amphibolite.
Systematic classification is the only way to accurately identify metamorphic rocks;
Eclogite, . . . produces the rock Eclogite.
Contact, . . . produces the rocks Hornfels and marble.
Blueschist, . . . produces the rock Blueschist.
Hydrothermal, . . . produces rocks such as Pegmatite, Serpentinite, and Soapstone.
As mentioned above, the same texture terms are used to describe rocks that come from totally different metamorphic processes, so we must avoid the mistake of assuming that a texture term refers to a particular type of metamorphism. A texture term only applies to a description of texture.
Metamorphic Rocks are divided into two basic divisions, summarized in the table below:
|Complex composition, many different kinds of minerals.
||Simple composition, only a few minerals such as Calcite or Quartz.
|Many new minerals produced with a change in T and/or P.
||No new minerals form with a change in T and/or P.
|Texture is layered, foliated, lineated, banded. Minerals have a preferred orientation.
||Texture is granular and equi- dimensional. No preferred orientation.
|Many rocks with diverse compositions. See Barrovian metamorphism.
||Few rocks with simple compositions.
Qtz SS - Quartzite
Limestone - Marble
Dolostone - Dolomitic Marble
Shale - Hornfels
Contributed by Lynn Fichter