Which metamorphic facies is diagnosed by the presence of jadeite and lawsonite with quartz, among other minerals?

Which metamorphic facies is diagnosed by the presence of jadeite and lawsonite with quartz, among other minerals?

• A. Greenschist
• B. Amphibolite
• C. Eclogite
• D. Blueschist

Answer: (D)

Blueschist facies form in subduction zones where rocks are carried to high pressures but remain relatively cool. The minerals jadeite and lawsonite are key indicators of this environment because they stabilize only at high pressure and low temperature. Quartz is a common accompanying mineral, but the combination of jadeite and lawsonite is especially diagnostic of blueschist. This assemblage contrasts with higher-temperature metamorphism (like eclogite, which features garnet and omphacite) or with lower-grade rocks (greenschist) or mid-grade rocks (amphibolite), where you wouldn’t expect both jadeite and lawsonite to persist together. So, the presence of jadeite and lawsonite with quartz points to the blueschist facies, the hallmark of subduction-zone metamorphism at high pressure and low temperature.

Blueschist facies form in subduction zones where rocks are carried to high pressures but remain relatively cool. The minerals jadeite and lawsonite are key indicators of this environment because they stabilize only at high pressure and low temperature. Quartz is a common accompanying mineral, but the combination of jadeite and lawsonite is especially diagnostic of blueschist. This assemblage contrasts with higher-temperature metamorphism (like eclogite, which features garnet and omphacite) or with lower-grade rocks (greenschist) or mid-grade rocks (amphibolite), where you wouldn’t expect both jadeite and lawsonite to persist together. So, the presence of jadeite and lawsonite with quartz points to the blueschist facies, the hallmark of subduction-zone metamorphism at high pressure and low temperature.