7.4 Metamorphic facies and pressure-temperature conditions

Metamorphic facies are key to understanding the pressure and temperature conditions rocks experienced during transformation. They reveal the environments where rocks formed, from shallow burial to deep subduction zones, helping geologists reconstruct a region's metamorphic history.

Each facies has unique mineral assemblages that form under specific conditions. By analyzing these minerals and using P-T diagrams, geologists can determine metamorphic grade and intensity. This information provides crucial insights into tectonic settings and burial depths of metamorphic rocks.

Metamorphic Facies and Pressure-Temperature Conditions

Significance of metamorphic facies

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• Provide insight into pressure and temperature conditions during metamorphism allows geologists to understand the environment in which the rocks formed
• Help understand tectonic settings and burial depths of metamorphic rocks such as shallow burial in low-grade regional metamorphism or deep burial in subduction zones
• Allow reconstruction of metamorphic history of a region by analyzing the spatial distribution and changes in facies over time

Characteristics of metamorphic facies

• Zeolite facies forms under low pressure and temperature conditions characterized by zeolites, prehnite, and pumpellyite minerals
• Greenschist facies forms under low to medium pressure and temperature conditions characterized by chlorite, actinolite, epidote, and albite minerals
• Amphibolite facies forms under medium to high pressure and temperature conditions characterized by hornblende, plagioclase, and garnet minerals
• Granulite facies forms under high pressure and temperature conditions characterized by pyroxene, plagioclase, and garnet minerals
• Eclogite facies forms under very high pressure and high temperature conditions characterized by omphacite (high-pressure pyroxene) and garnet minerals

Interpretation of P-T diagrams

• Display stability fields of metamorphic minerals and facies with temperature on x-axis and pressure on y-axis
• Metamorphic facies plotted as regions on the diagram show the pressure and temperature conditions under which they form
• Can be used to determine the metamorphic grade and intensity of metamorphism a rock has undergone

Concept of metamorphic grade

• Refers to intensity or degree of metamorphism with low-grade at lower temperatures and pressures and high-grade at higher temperatures and pressures
• Low-grade metamorphism corresponds to zeolite and greenschist facies indicating shallow burial or low-grade regional metamorphism
• Medium-grade metamorphism corresponds to amphibolite facies indicating moderate to deep burial in orogenic belts
• High-grade metamorphism corresponds to granulite and eclogite facies indicating deep burial in the lower crust or subduction zones

Tectonic settings from facies

• Zeolite and greenschist facies indicate low pressure and temperature conditions found in shallow burial settings or low-grade regional metamorphism (burial metamorphism)
• Amphibolite facies indicate medium to high pressure and temperature conditions associated with regional metamorphism in orogenic belts suggesting moderate to deep burial depths (Barrovian metamorphism)
• Granulite facies indicate high pressure and temperature conditions found in the lower crust or deep portions of orogenic belts suggesting deep burial depths and high-grade regional metamorphism (Abukuma metamorphism)
• Eclogite facies indicate very high pressure and high temperature conditions associated with subduction zones or ultra-high pressure metamorphism suggesting very deep burial depths, typically in subduction settings (Franciscan metamorphism)