Metamorphic Processes
There are several processes involved in the formation of metamorphic rocks.
Recrystallization
Minerals change size and shape.Phase change
New minerals form that have the same chemical formula but have a different crystal structure. For example, Andalusite, Kyanite, and Sillmanite are all metamorphic minerals, minerals that only form as a result of metamorphism, with the chemical formula of Al2SiO5. Andalusite forms under low temperature and low pressure conditions; increasing temperature without an increase in pressure will form the mineral Kyanite. If both temperature and pressure increase, then the mineral Sillmanite will form.
Neocrystallization
New minerals form with P-T changes. The original minerals become unstable and change to new minerals. Example: clays will change into garnets and/or micas.
Pressure solution
Minerals will partially dissolve then recrystalize in response to a change in pressure and/or temperature.
Plastic deformation
As temperature and/or pressure increase, mineral grains soften and deform into a new, preferred orientation..
Degrees of Metamorphism
High-grade (obliteration of original features)
Low-grade (original features mostly retained)
As temperatures and/or pressures change, new minerals will form via the processes listed above. Some minerals are unique to a specific temperature/pressure range, and are called index minerals. These index minerals are indicators of metamorphic grade.
Folliated Metamorphic Rocks
| Protolith | Low to Medium Grade | High Grade |
|---|---|---|
| Fine-grained sedimentary rocks (eg, shale) | Slate / Phyllite / Schist | Gneiss |
| Igneous rocks (eg, basalt) | Greenschist / Blueschist | Amphibolite, Granulite, Eclogite |
Nonfolliated Metamorphic Rocks
| Protolith | Low to Medium Grade | High Grade |
|---|---|---|
| Conglomerate | Metaconglomerate | gneiss |
| Sandstone | Quartzite | Quartzite |
| Shale, mudstone or siltstone | Hornfels | Hornfels |
| Bituminous Coal | Anthracite | Anthracite |
| Igneous rocks (eg, basalt) | Hornfels | Hornfels |
| Limestone | Marble | Marble |
Retrograde - Metamorphism via an increase in temperature & pressure results in recrystallization (mineral growth) and neocrystallization (new mineral assemblages). Water is released from the minerals during this process.
Prograde - Metamorphism via a decrease in temperature & pressure. This is common in rocks that are brought from depth by erosion. The process is accompanied by addition of H2O by hydrothermal fluids.
Index Mineral Map
An index mineral map is a map showing the location of where these minerals can be found. This can be useful in interpreting past tectonic activity. For example, during a type of tectonic activity called continental-continental convergence, two tectonic plates containing continental crust collide, forming huge mountains such as the Himalayas. At the center of the collision zone, called the suture zone, temperatures and pressures are the most intense, and lessen the further away from the zone travelled. Thus, high-grade metamorphic minerals such as sillmanite can be found in the core of these mountains, and low-grade minerals such as chlorite can be found in the foothill areas.
