Not all metamorphism takes place under the same physical conditions. For example, rocks carried deep below a mountain range undergo more intense metamorphism than rocks closer to the surface. metamorphic grade Geologists use the term metamorphic grade in a somewhat informal way to indicate the intensity of metamorphism, that is, the amount or degree of metamorphic change.
To provide a more complete indication of the intensity of metamorphism, geologists use the concept of metamorphic facies.
The classification of metamorphic grade depends mainly on temperature, because temperature plays a dominant role in determining recrystallization and neocrystallization during metamorphism.
Metamorphic rocks that form at relatively low temperatures (between about 250°C and 400°C) are low-grade rocks, and metamorphic rocks that form at relatively high temperatures (about 600°C) are high-grade rocks..
Intermediate grade metamorphic rocks form at temperatures between these two extremes (figure 2).
Different degrees of metamorphism produce different sets of metamorphic minerals (assemblies). As grade increases, recrystallization and neocrystallization tend to produce coarser grains and new mineral assemblies that are stable at higher temperatures and pressures (figure 3).
Geologists discovered that the presence of certain minerals, known as index minerals, in a rock indicates the approximate metamorphic grade of the metamorphic rock. The line on a map along which an index mineral first appears is called the isograde.
All points along an isograd have approximately the same metamorphic grade. Metamorphic zones are regions between two isogrades; zones are named after an index mineral that was not present in the previous lower-grade zone.
Metamorphic facies
Geologists realized that metamorphic rocks, in general, do not consist of a hodgepodge of minerals formed at different times and in different places, but instead consist of a distinct set of minerals that grew in association with each other at a given pressure and temperature..
It seemed that the assemblages represent more or less a condition of chemical equilibrium.
Geologists also determined that the specific mineral assemblage in a metamorphic rock depends on pressure and temperature conditions, and on the composition of the protolith.
This discovery led geologists to propose the concept of metamorphic facies.
The metamorphic facies correspond to a set of metamorphic minerals indicative of a certain range of pressure and temperature. Each specific assemblage on a facie reflects the original composition of the protolith.
According to this definition, a given metamorphic facie includes several different types of rocks that differ from each other in terms of chemical composition and therefore mineral content, but all rocks of a given facie formed under the same conditions of temperature and pressure.
Geologists recognize several facies, the main ones being zeolite, hornfels, greenschist, amphibolite, blueschist, eclogite, and granulite. The names of the different facies are based on a distinctive or mineralogical feature found in some of the metamorphic rocks corresponding to the facie.
We can represent the approximate conditions under the metamorphic facies formed by using a temperature-pressure graph.
Each area of the graph, labeled with a facies name, represents the approximate range of temperatures and pressures in which mineral assemblages characteristic of that particular facies form. For example, a rock subjected to the pressure and temperature at point A (4.5 kbar and 400 °C) develops a mineral assemblage characteristic of greenschist facies.
Also, the graph can represent the geothermal gradients of different regions of the crust.
Beneath the ridges, for example, the geothermal gradient passes through zeolite, greenschist, amphibolite, and granulite facies.
On the contrary, in the accretionary prism that forms in a subduction zone, the temperature increases slowly as the depth increases, so blue shales assemblages can form.