Which one of the following schematic P-T paths is characteristic for a rock metamorphosed in a subduction zone?
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In subduction zones, the characteristic P-T path follows a steep rise in pressure with a relatively slow increase in temperature, resulting in the formation of kyanite and sillimanite.
In metamorphic petrology, the P-T (Pressure-Temperature) path followed by a rock during metamorphism reveals critical information about the conditions under which the rock was altered. In subduction zones, the tectonic setting involves the subduction of oceanic crust into the mantle, where high-pressure, low-temperature conditions prevail. This setting results in the formation of certain minerals that are stable under these extreme conditions.
Step 1: Characteristics of Subduction Zones
Subduction zones are characterized by relatively low temperatures but extremely high pressures due to the descending oceanic plate. As the plate sinks deeper into the Earth, the pressure increases, but the temperature increases more slowly, leading to high-pressure, low-temperature metamorphism.
Step 2: Metamorphic Mineral Assemblages
In such high-pressure, low-temperature conditions, certain minerals, like kyanite and sillimanite, are stable. Kyanite is a high-pressure mineral that forms at lower temperatures, while sillimanite forms at slightly higher temperatures, but still within the high-pressure regime.
Step 3: Evaluating the Options
- Option (A): The diagram in option (A) shows a typical P-T path for a rock that is not undergoing high-pressure, low-temperature metamorphism. This would be more typical of rocks metamorphosed in a continental collision zone, not a subduction zone. Thus, option (A) is incorrect.
- Option (B): In option (B), the P-T path begins at low temperature and high pressure, which is characteristic of subduction zone metamorphism. The path rises along a relatively steep slope in the pressure range and reaches the stability field of kyanite, which then transitions to sillimanite as the temperature increases. This is the typical P-T path for rocks metamorphosed in a subduction zone. Therefore, option (B) is the correct answer.
- Option (C): The P-T path shown in option (C) does not reflect the low-temperature, high-pressure conditions found in subduction zones. It may represent a different metamorphic environment, such as regional metamorphism. Therefore, option (C) is incorrect.
- Option (D): The P-T path shown in option (D) suggests a metamorphic environment that does not involve the high-pressure conditions typical of subduction zones. It is unlikely to be associated with subduction zone metamorphism. Thus, option (D) is incorrect.
Step 4: Conclusion
The correct answer is option (B), as it represents the typical P-T path for rocks undergoing high-pressure, low-temperature metamorphism in subduction zones, where kyanite forms first and then transitions to sillimanite with increasing temperature.
Final Answer:
\[
\boxed{(B)}
\]
