Which statement best identifies the blueschist facies in terms of mineral assemblage that includes quartz, jadeite, lawsonite, phengite, glaucophane, and chlorite?

Which statement best identifies the blueschist facies in terms of mineral assemblage that includes quartz, jadeite, lawsonite, phengite, glaucophane, and chlorite?

• A. Quartz, Jadeite, Lawsonite, Phengite, Glaucophane, and Chlorite
• B. Quartz, K-feldspar, Muscovite, Biotite
• C. Calcite, Dolomite, Gypsum, Anhydrite
• D. Olivine, Pyroxene, Calcite, Fluorite

Answer: (A)

Forming under high pressure and relatively low temperature typical of subduction zones, blueschist is defined by a distinctive suite of hydrous minerals that survive in these conditions. The minerals quartz, jadeite, lawsonite, phengite, glaucophane, and chlorite together form a classic blueschist assemblage. Glaucophane is a blue amphibole that characterizes the blueschist color and facies, while lawsonite and jadeite indicate high-pressure conditions. Phengite shows high silica in the mica, and chlorite is common in low-temperature, high-pressure environments. Quartz ties the assemblage together as a stable, widespread phase in these rocks. The other options don’t reflect blueschist metamorphism. Minerals like K-feldspar, muscovite, and biotite suggest different metamorphic grades or facies, not blueschist. Evaporite minerals such as calcite, dolomite, gypsum, and anhydrite point to sedimentary or diagenetic environments rather than high-pressure, subduction-zone metamorphism. Olivine and pyroxene with calcium-bearing minerals are typical of mantle or high-temperature contexts, not blueschist.

Forming under high pressure and relatively low temperature typical of subduction zones, blueschist is defined by a distinctive suite of hydrous minerals that survive in these conditions. The minerals quartz, jadeite, lawsonite, phengite, glaucophane, and chlorite together form a classic blueschist assemblage. Glaucophane is a blue amphibole that characterizes the blueschist color and facies, while lawsonite and jadeite indicate high-pressure conditions. Phengite shows high silica in the mica, and chlorite is common in low-temperature, high-pressure environments. Quartz ties the assemblage together as a stable, widespread phase in these rocks.

The other options don’t reflect blueschist metamorphism. Minerals like K-feldspar, muscovite, and biotite suggest different metamorphic grades or facies, not blueschist. Evaporite minerals such as calcite, dolomite, gypsum, and anhydrite point to sedimentary or diagenetic environments rather than high-pressure, subduction-zone metamorphism. Olivine and pyroxene with calcium-bearing minerals are typical of mantle or high-temperature contexts, not blueschist.