Metamorphic grade refers to the degree of metamorphism that a rock has undergone, which is primarily determined by the temperature and pressure conditions it was subjected to during metamorphism. It reflects the extent of changes in mineral composition and texture due to increased heat and pressure relative to the original (parent) rock.
Determination of Metamorphic Grade:
-
Mineral Composition: Specific minerals are stable at certain temperature and pressure ranges. Geologists can identify these minerals in a metamorphic rock to infer the metamorphic conditions. For instance, the presence of garnet often indicates higher grades of metamorphism.
-
Textures: The texture of the rock, including grain size and foliation (the alignment of mineral grains), can also provide clues about the metamorphic grade. Higher grades typically exhibit coarser grain sizes and more pronounced foliation.
-
Index Minerals: Certain minerals, known as index minerals (like kyanite, staurolite, and sillimanite), are good indicators of particular metamorphic conditions and help delineate different metamorphic grades.
-
Pressure-Temperature (P-T) Diagrams: These diagrams depict the stability fields of different minerals and can be used to estimate the metamorphic conditions, allowing geologists to determine the grade of metamorphism.
Difference Between Grade and Facies:
-
Metamorphic Grade: As mentioned, it focuses on the temperature and pressure conditions during metamorphism and indicates how intense the metamorphism has been.
-
Metamorphic Facies: In contrast, metamorphic facies are groups of minerals that are stable together under a specific range of pressure and temperature conditions. Facies provide information about the conditions of metamorphism in a broader context than grades, often associated with a particular tectonic setting (e.g., subduction zones, regional metamorphism).
In summary, while grade addresses the intensity of metamorphism, facies relate to the specific mineral assemblages that form under those conditions.
1 answer