Questions: Metamorphic Grade and Pressure-Temperature Paths

Question 1

Blueschist-facies rocks containing glaucophane and lawsonite are found exhumed at the surface. Which tectonic environment most likely produced them, and what does their P-T path look like?

• A. Continental collision zone; a clockwise P-T path with initial burial followed by heating at depth
• B. Subduction zone; a high-pressure, low-temperature path where the rock was buried faster than it could heat up
• C. Mid-ocean ridge; a high-temperature, low-pressure path from proximity to an underlying magma source
• D. Continental rift; a low-pressure, high-temperature path reflecting crustal thinning and asthenosphere upwelling

Answer: (B)

Glaucophane and lawsonite are stable only at high pressure and low temperature — conditions that arise when rock is subducted rapidly into the mantle before the surrounding hot mantle can heat it. The subducting slab carries cold oceanic crust to great depth quickly, producing the characteristic high-P, low-T path of blueschist facies. In a collision zone, the slower thickening process allows more thermal equilibration, producing a clockwise path with higher temperatures at similar pressures and generating different mineral assemblages (garnet, kyanite, staurolite).

Question 2

A geologist finds garnet porphyroblasts in a schist, with inclusions of chlorite and albite trapped inside the garnet cores. What can be inferred from this?

• A. The rock simultaneously equilibrated at conditions where both chlorite-albite and garnet are stable
• B. The inclusions record earlier, lower-grade conditions; the garnet grew later as the rock reached higher grade, preserving the earlier assemblage inside it
• C. The chlorite and albite inclusions indicate the garnet is unstable and currently breaking down
• D. Nothing useful — inclusions within minerals are always contamination artifacts

Answer: (B)

When a porphyroblast like garnet grows during prograde metamorphism, it can trap mineral grains present at the time of nucleation as inclusions. These inclusions are then shielded from further reaction by the surrounding garnet and preserve a 'snapshot' of the earlier, lower-grade assemblage. The core inclusions (chlorite + albite, low grade) record conditions before garnet stability; the garnet rim may include higher-grade phases grown later. This sequential record allows geologists to reconstruct stages of the P-T path rather than just peak conditions.

Question 3

The metamorphic grade of a rock tells you both the peak conditions it reached and the full P-T path it traveled to get there.

• T. True
• F. False

Answer: False

Metamorphic grade primarily records peak (or near-peak) conditions — the most intense conditions the rock experienced, which set the dominant stable mineral assemblage. The grade does not, by itself, reveal the path: two rocks reaching identical peak conditions (same grade) could have traveled very different P-T paths to get there. Reconstructing the path requires identifying sequential assemblages — such as inclusions within porphyroblasts preserving earlier stages, or retrograde overprinting of peak minerals — rather than just identifying the dominant mineral assemblage.

Question 4

Mineral assemblages in a metamorphic rock can record conditions from multiple stages of the rock's pressure-temperature history, not just the peak metamorphic conditions.

• T. True
• F. False

Answer: True

True. Inclusions trapped within porphyroblasts during their growth can preserve earlier, lower-grade assemblages. Retrograde minerals overprinting peak-grade minerals record the cooling and decompression during exhumation. A single rock may thus contain textural evidence of prograde, peak, and retrograde stages. This multilayer record is how geologists reconstruct P-T paths rather than just identifying a single set of peak conditions.

Question 5

Why do rocks in subduction zones follow different P-T paths than rocks in continent-continent collision zones, and what mineral evidence distinguishes the two settings?

Model answer: In subduction zones, cold oceanic lithosphere descends rapidly into the hot mantle. The fast burial rate means the rock reaches great depth (high pressure) before the surrounding mantle has time to heat it — producing a high-pressure, low-temperature path. Diagnostic minerals include glaucophane and lawsonite (blueschist facies). In continent-continent collision zones, crustal thickening buries rocks more slowly, allowing more thermal equilibration. The P-T path is clockwise: burial increases pressure first, then prolonged residence at depth allows heating at roughly constant pressure, followed by exhumation. Diagnostic minerals include garnet, staurolite, and kyanite (greenschist to amphibolite facies). The two paths occupy different regions of the P-T diagram and stabilize fundamentally different mineral assemblages.

The rate of burial relative to the rate of thermal equilibration is the controlling factor. Fast subduction keeps the slab cold at depth; slow collision allows heating. These different thermal histories trace distinct trajectories on the P-T diagram, which experimental petrology has mapped to distinct mineral stability fields. Reading the minerals is reading the tectonic history.