Which metamorphic facies is formed at low pressure and very high temperature?

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

Answer: (D)

Metamorphic facies map P–T conditions to specific mineral assemblages. The one formed at low pressure and very high temperature occurs when rocks are heated intensely by nearby magma but aren’t buried under thick overburden. This ultra-hot, low-pressure environment produces a sanidinite assemblage, with minerals like sanidine, cristobalite, tridymite, and quartz dominating. It signals extreme heating from intrusion or near-vent settings rather than burial metamorphism.

In contrast, greenschist forms at relatively low temperatures and pressures, blueschist forms under high pressure and relatively low temperature, and amphibolite forms at higher temperatures and pressures than greenschist but not as extreme as sanidinite. Hence, the low-pressure, very high-temperature condition points to the sanidinite facies.