Metamorphic Facies and Pressure-Temperature Conditions
Metamorphic facies give geologists a way to figure out the pressure and temperature conditions a rock experienced during metamorphism. Each facies is defined by a specific set of minerals that only form under certain P-T conditions. By identifying those minerals, you can work backward to determine where and how deep a rock was buried, what tectonic setting it formed in, and how intense the metamorphism was.
Significance of Metamorphic Facies
The concept of metamorphic facies exists because certain minerals are only stable within specific pressure-temperature ranges. When you find a particular mineral assemblage in a rock, it acts like a fingerprint for the conditions that rock experienced.
Facies matter for three main reasons:
- Reconstructing P-T conditions — The minerals present tell you the pressure and temperature the rock reached during metamorphism, which translates to burial depth and heat exposure.
- Identifying tectonic settings — Different tectonic environments produce different P-T paths. Shallow burial in a sedimentary basin looks very different from deep burial in a subduction zone, and the facies reflect that.
- Mapping metamorphic history — By tracking how facies change across a region, geologists can reconstruct how metamorphic conditions varied over space and time, revealing the history of mountain-building events or plate collisions.
Characteristics of Metamorphic Facies
Each facies is defined by its mineral assemblage, the specific group of minerals that are stable together under those conditions. Moving from low-grade to high-grade, the key facies are:
- Zeolite facies — Lowest grade. Forms under low pressure and low temperature. Characteristic minerals include zeolites, prehnite, and pumpellyite. These conditions occur just beyond diagenesis, where sedimentary rocks first start to become metamorphic.
- Greenschist facies — Low to medium grade. Characterized by chlorite, actinolite, epidote, and albite. The name comes from the greenish color these minerals give the rock.
- Amphibolite facies — Medium to high grade. Characterized by hornblende, plagioclase, and garnet. These minerals indicate significantly higher temperatures and pressures than greenschist conditions.
- Granulite facies — High grade. Characterized by pyroxene, plagioclase, and garnet. At these conditions, hydrous minerals (those containing water in their structure) break down, so the rock becomes largely anhydrous.
- Eclogite facies — Very high pressure with high temperature. Characterized by omphacite (a high-pressure pyroxene) and garnet. This facies is distinctive because it forms almost exclusively in subduction zones where pressures are extreme.
Interpretation of P-T Diagrams
P-T diagrams are the main tool for visualizing metamorphic facies. Here's how to read one:
- Axes — Temperature is plotted on the x-axis (increasing to the right) and pressure on the y-axis (increasing upward). Since pressure increases with depth, higher on the diagram also means deeper in the Earth.
- Facies fields — Each facies occupies a region on the diagram, representing the range of P-T conditions where its characteristic minerals are stable. The boundaries between fields mark where one mineral assemblage becomes unstable and a new one forms.
- Determining metamorphic grade — If you identify the minerals in a rock and locate the corresponding facies field on the diagram, you can estimate the P-T conditions the rock experienced. A rock plotting in the greenschist field experienced lower temperatures and pressures than one plotting in the amphibolite field.
P-T diagrams also show metamorphic field gradients, which are P-T paths that connect the conditions recorded by rocks in a particular region. These paths help distinguish between different types of metamorphism.
Concept of Metamorphic Grade
Metamorphic grade refers to the overall intensity of metamorphism, essentially how far a rock has been transformed from its original state. Grade increases with rising temperature and pressure.
- Low-grade — Corresponds to zeolite and greenschist facies. Temperatures are roughly 200–450°C. Original rock textures and some original minerals may still be recognizable.
- Medium-grade — Corresponds to amphibolite facies. Temperatures are roughly 450–700°C. Significant recrystallization occurs, and new minerals like garnet and hornblende grow.
- High-grade — Corresponds to granulite and eclogite facies. Temperatures exceed roughly 700°C (granulite) or pressures are extreme (eclogite). The rock is thoroughly recrystallized, and at the highest grades, partial melting can begin.
The transition from one grade to the next is gradual, not a sharp boundary. That's why you sometimes see rocks with minerals from two adjacent facies.
Tectonic Settings from Facies
Different tectonic environments produce characteristic P-T conditions, so the facies you find in a region point toward a specific geologic setting:
- Zeolite and greenschist facies — Found in shallow burial settings and low-grade regional metamorphism. This is typical of burial metamorphism in thick sedimentary basins where rocks are heated mainly by the geothermal gradient without major tectonic deformation.
- Amphibolite facies — Associated with regional metamorphism in orogenic (mountain-building) belts. This is the classic Barrovian metamorphism, where rocks are buried to moderate-to-deep levels during continental collision, experiencing both elevated temperature and pressure.
- Granulite facies — Found in the lower crust or the deepest parts of orogenic belts. Sometimes called Abukuma-type metamorphism when it occurs along a high-temperature, low-pressure path (as in volcanic arc settings). Granulite-facies rocks indicate deep burial with high heat flow.
- Eclogite facies — Associated with subduction zones, where oceanic crust is driven to great depths under very high pressure. Franciscan-type metamorphism (named after the Franciscan Complex in California) follows a high-pressure, low-temperature path characteristic of subduction. Finding eclogite-facies rocks at the surface means they were brought back up from extreme depths, often along fault zones.