In adjacent rocks that show a gradual increase in metamorphic intensity, the first appearance of a particular index mineral indicates
A) the location of the minimum pressure and temperature conditions needed for the formation of that mineral.
B) the location of the maximum pressure and temperature conditions needed for the formation of that mineral.
C) the rocks formed under the same broad conditions of temperature and pressure.
D) the location on Bowen's reaction series where that mineral is in equilibrium.

Question

Quizplus · Accepted Answer

The first appearance of an index mineral in a sequence of metamorphic rocks marks the minimum pressure and temperature conditions required for the formation of that mineral. This is because index minerals are used to indicate specific conditions of metamorphism, and their initial occurrence signifies the point at which the conditions were just sufficient for the mineral to form.

In Adjacent Rocks That Show a Gradual Increase in Metamorphic