In subduction zones, the accretionary prism is characterized by the accumulation of sediments and other materials that are scraped off the subducting oceanic plate as it descends into the mantle. The mineral assemblage characteristics in these prisms are primarily influenced by the temperature, pressure, and depth conditions present during metamorphism.
Two key facies that are likely to form in the accretionary prism are the subduction zone metamorphic facies and certain blueschist facies. Here's a breakdown of how these form and why they are associated with specific conditions:
Blueschist Facies
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Temperature and Pressure:
- The blueschist facies forms under high-pressure and relatively low-temperature conditions. Specifically, it typically forms at pressures greater than 1 GPa (approximately 30 km depth) and lower thermal gradients, usually between 200°C and 500°C.
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Depth:
- The conditions favoring blueschist facies are found at significant depths in subduction zones. As the oceanic plate descends, it experiences increased pressure but not an equivalent increase in temperature due to the cold nature of the subducting slab.
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Mineral Assemblage:
- Characteristic minerals that form include glaucophane, lawsonite, and other hydrous minerals. These indicate the presence of water-rich conditions during metamorphism which plays a crucial role in facilitating the formation of blueschist.
Other Facies
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Eclogite Facies:
- Deeper within the subduction zone, as the oceanic crust continues to descend beyond the blueschist conditions (approximately above 70 km or pressures exceeding 1.5 GPa), the transition may lead to the formation of eclogite facies. This occurs at higher temperatures (600°C–1,200°C) and pressures.
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Mineral Assemblage:
- Eclogite is characterized by the presence of garnet and omphacite and indicates a significant change in the metamorphic conditions caused by the higher temperatures and pressures.
Summary
In summary, the accretionary prism at subduction zones predominantly develops blueschist facies at relatively shallow depths (important for tectonic movements) due to high pressure and low temperature conditions. As the crustal materials descend further into the subduction zone, they can eventually transition to eclogite facies under higher temperature and pressure conditions. The complex interplay of these variations in pressure, temperature, and depth is critical to understanding the metamorphic processes occurring in subduction zones.
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