In carbonate rocks, the mineral assemblage wollastonite, anorthite, and diopside would most likely correspond to which metamorphic facies?

In carbonate rocks, the mineral assemblage wollastonite, anorthite, and diopside would most likely correspond to which metamorphic facies?

• A. Greenschist facies
• B. Blueschist facies
• C. Sanidinite facies
• D. Granulite facies

Answer: (C)

The main idea here is how mineral assemblages reveal the pressure–temperature conditions of metamorphism. The combination of wollastonite, diopside, and anorthite in carbonate rocks signals extremely high temperature with very low pressure, which corresponds to the sanidinite facies. Sanidinite forms during intense, shallow, high-temperature contact metamorphism, often around intrusions, and it yields calc-silicate minerals like wollastonite (CaSiO3) and calcium-rich phases such as diopside (CaMgSi2O6) and anorthite (CaAl2Si2O8). This specific assemblage is not typical of the lower-temperature, higher-pressure greenschist or blueschist facies, nor of the granulite facies, which have different mineral suites. So, the presence of wollastonite–diopside–anorthite in carbonate rocks points to sanidinite facies conditions.

The main idea here is how mineral assemblages reveal the pressure–temperature conditions of metamorphism. The combination of wollastonite, diopside, and anorthite in carbonate rocks signals extremely high temperature with very low pressure, which corresponds to the sanidinite facies. Sanidinite forms during intense, shallow, high-temperature contact metamorphism, often around intrusions, and it yields calc-silicate minerals like wollastonite (CaSiO3) and calcium-rich phases such as diopside (CaMgSi2O6) and anorthite (CaAl2Si2O8). This specific assemblage is not typical of the lower-temperature, higher-pressure greenschist or blueschist facies, nor of the granulite facies, which have different mineral suites. So, the presence of wollastonite–diopside–anorthite in carbonate rocks points to sanidinite facies conditions.