Types of Metamorphic Facies
Metamorphic facies are distinct mineral assemblages that form under specific pressure-temperature (P-T) conditions. They give geochemists a way to interpret the thermal and tectonic history of a rock, essentially telling you how deep and how hot a rock got during metamorphism. Each facies corresponds to a particular P-T window, and recognizing the right one is key to reconstructing a rock's metamorphic journey.
Zeolite Facies
The zeolite facies is the lowest-grade metamorphic facies, forming at temperatures below ~300°C and low pressures. Diagnostic minerals include laumontite and heulandite. You'll find this facies most often in burial metamorphism of sedimentary sequences in deep basins, where volcanic glass alters to zeolites and clay minerals without reaching the temperatures needed for higher-grade reactions.
Prehnite-Pumpellyite Facies
This facies sits between the zeolite and greenschist facies, forming at roughly 200–350°C and low to moderate pressures. The diagnostic minerals are prehnite, pumpellyite, and albite. It's commonly associated with low-grade metamorphism of mafic volcanic rocks in oceanic crust, making it a useful indicator of early-stage ocean-floor metamorphism.
Greenschist Facies
Greenschist facies forms at moderate temperatures (300–500°C) and low to moderate pressures. The name comes from the characteristic green minerals: chlorite, actinolite, and epidote. This facies develops widely during regional metamorphism of mafic rocks and produces foliated rocks like phyllites and schists. It's one of the most commonly encountered facies in orogenic belts.
Amphibolite Facies
At higher grades (500–700°C, moderate pressures), the amphibolite facies is defined by the assemblage of hornblende and plagioclase feldspar. Rocks at this grade are strongly foliated and often show visible mineral banding. Regional metamorphism of continental crust commonly produces amphibolite-facies rocks, and this is where you start to see significant recrystallization and grain coarsening.
Granulite Facies
The granulite facies represents the highest-grade conditions in the normal continental geotherm, forming above ~700°C at moderate to high pressures. The key feature is the dominance of anhydrous minerals like pyroxenes and garnets, since most hydrous phases have broken down by this point. These rocks typically form in the lower continental crust or deep roots of mountain belts and display coarse-grained, granoblastic (non-foliated) textures.
Blueschist Facies
Blueschist facies forms under high pressure (>6 kbar) but relatively low temperature (200–500°C), a combination unique to subduction zone environments. Diagnostic minerals include glaucophane (a sodic amphibole that gives the rock its blue color), lawsonite, and jadeite. Finding blueschist-facies rocks at the surface is significant because it means material was buried to great depth and then exhumed rapidly enough to preserve these high-pressure minerals.
Eclogite Facies
The eclogite facies represents the highest-pressure conditions (>12 kbar, >400°C) and forms in deeply subducted oceanic crust or during continental collision. The diagnostic assemblage is omphacite (a sodic clinopyroxene) + pyrope-rich garnet, producing dense, coarse-grained rocks with a striking red-and-green appearance. Eclogites are denser than surrounding mantle peridotite at equivalent depths, which has important implications for slab dynamics.
Pressure-Temperature Conditions
P-T conditions are the fundamental variables that determine which metamorphic facies and mineral assemblages develop. By reconstructing the P-T history of a rock, you can trace its path through the crust and infer the tectonic processes responsible.
P-T Diagrams
P-T diagrams are the primary tool for visualizing metamorphic facies. Pressure is plotted on the y-axis (increasing downward corresponds to increasing depth) and temperature on the x-axis. Each facies occupies a distinct stability field on the diagram, bounded by reaction lines where one mineral assemblage transforms into another. Isograds (lines marking the first appearance of an index mineral) also appear on these diagrams. Reading a P-T diagram lets you determine what facies a rock belongs to and what path it followed to get there.
Facies Series
A facies series is a sequence of metamorphic facies that develops along a particular geothermal gradient. Different tectonic settings produce different gradients, so the facies series tells you about the tectonic environment:
- High P / low T series (blueschist → eclogite): characteristic of subduction zones
- Intermediate P series (greenschist → amphibolite → granulite): typical of continental collision and regional metamorphism
- Low P / high T series (hornfels → granulite): associated with contact metamorphism or regions of elevated heat flow
Recognizing which series is present in a metamorphic terrane is one of the fastest ways to identify the tectonic setting.
Barrovian Metamorphism
Barrovian metamorphism, first described by George Barrow in the Scottish Highlands, represents the classic intermediate-P/T regional metamorphic sequence. It's characterized by a progression of index minerals that appear with increasing grade:
chlorite → biotite → garnet → staurolite → kyanite → sillimanite
This sequence reflects increasing temperature and pressure with depth in thickened continental crust. In the field, these index minerals define concentric metamorphic zones across a mountain belt, with the highest-grade rocks in the core.
Buchan Metamorphism
Buchan (also called Abukuma-type) metamorphism follows a low-P, high-T path. Instead of kyanite, you get andalusite and cordierite as diagnostic minerals. This type of metamorphism is typically associated with contact aureoles around igneous intrusions or with extensional tectonic settings where heat flow is elevated. The rocks produced tend to be hornfels and other non-foliated types, since directed stress is minimal.
Mineral Assemblages
A mineral assemblage is the specific combination of minerals that coexist in equilibrium under particular P-T conditions. Identifying the assemblage in a rock is how you assign it to a metamorphic facies and determine its grade.
Index Minerals
Index minerals are key species that first appear at specific metamorphic conditions. In a Barrovian sequence, for example, the appearance of garnet signals a higher grade than a rock containing only chlorite and biotite. These minerals appear in a predictable order with increasing grade, which makes them powerful tools for mapping metamorphic terranes. By tracing where each index mineral first appears across a region, you can map out the spatial variation in metamorphic intensity.
Diagnostic Mineral Associations
While a single mineral can be suggestive, it's the combination of minerals that defines a facies. Some key associations to know:
- Greenschist facies: chlorite + actinolite + epidote + albite
- Amphibolite facies: hornblende + plagioclase (andesine or higher)
- Blueschist facies: glaucophane + lawsonite (± jadeite)
- Eclogite facies: omphacite + pyrope-rich garnet
These associations allow for quick facies identification in hand specimen or thin section. The absence of certain minerals matters too: for instance, the lack of plagioclase in eclogite (replaced by omphacite + garnet) distinguishes it from amphibolite.
Reaction Isograds
An isograd is a line on a metamorphic map marking the first appearance of a particular index mineral. The "garnet-in" isograd, for example, traces the boundary where garnet first becomes stable. Isograds represent specific P-T conditions where metamorphic reactions occur, and they separate different metamorphic zones. Mapping isograds across a region reveals the spatial distribution of metamorphic grades and helps reconstruct the thermal structure of the crust at the time of metamorphism.
Metamorphic Grade
Metamorphic grade refers to the overall intensity of metamorphism a rock has experienced, primarily reflecting the peak temperature (and to a lesser extent, pressure) conditions.
Low-Grade vs. High-Grade Metamorphism
Low-grade metamorphism occurs at relatively low temperatures and pressures, producing fine-grained minerals like chlorite, muscovite, and albite. The resulting rocks (slates, phyllites) have weak to moderate foliation. High-grade metamorphism involves significantly higher temperatures and pressures, forming minerals like garnet, sillimanite, and pyroxenes. The rocks produced (gneisses, granulites) are coarse-grained with strong foliation or granoblastic textures. The transition between these extremes is gradational, not a sharp boundary.
Prograde vs. Retrograde Metamorphism
Prograde metamorphism describes the path of increasing temperature and pressure. As conditions intensify, new minerals replace earlier ones, creating the characteristic zonal patterns seen in metamorphic terranes. Most of the mineral assemblages you observe in metamorphic rocks record peak or near-peak prograde conditions.
Retrograde metamorphism occurs during cooling and decompression (the return path). High-grade minerals begin breaking down into lower-grade assemblages. Retrograde reactions often require fluid infiltration to proceed, since many involve hydration (adding water back into the mineral structure). Retrograde overprinting can partially or completely obscure the prograde history, which is why careful petrographic work is needed to distinguish primary from secondary assemblages.
Geotectonic Settings
Different tectonic environments impose different P-T paths on rocks, producing characteristic metamorphic facies. Recognizing these associations connects metamorphic petrology to plate tectonics.
Subduction Zones
Subduction zones produce the distinctive high-P, low-T metamorphism that generates blueschist and eclogite facies rocks. The rapid burial of cold oceanic crust into the mantle creates conditions where pressure increases much faster than temperature. Diagnostic minerals like glaucophane, lawsonite, and omphacite form under these conditions. P-T paths in subduction zones are often complex, showing rapid burial followed by exhumation, sometimes recorded as "hairpin" paths on P-T diagrams.
Collision Zones
Continental collision generates regional metamorphism over large areas, typically producing Barrovian-type sequences. The deeper parts of collision zones reach amphibolite and granulite facies, forming gneisses and migmatites (partially melted rocks). Thrust faulting can create inverted metamorphic sequences, where higher-grade rocks are stacked on top of lower-grade rocks. At the deepest levels, temperatures may be high enough for anatexis (partial melting), generating granitic magmas.
Contact Metamorphism
Contact metamorphism occurs in aureoles surrounding igneous intrusions. Heat from the magma drives metamorphic reactions in the country rock, producing concentric zones of increasing grade toward the intrusion. Because pressure is low (shallow crustal levels), the mineral assemblages are low-P/high-T, with porphyroblasts of andalusite or cordierite growing in hornfels. The width of the aureole depends on the size and temperature of the intrusion.
Regional Metamorphism
Regional metamorphism affects large volumes of continental crust, typically in orogenic belts. It produces a wide range of grades and facies, from greenschist through granulite, and creates the distinctive metamorphic zoning patterns mapped using index minerals and isograds. Because regional metamorphism is accompanied by deformation, the rocks are typically foliated. This is the most volumetrically significant type of metamorphism on Earth.
Facies Transitions
Transitions between facies occur as P-T conditions change, driving metamorphic reactions that transform one mineral assemblage into another. Understanding these transitions is central to interpreting a rock's metamorphic history.
Metamorphic Reactions
Metamorphic reactions fall into several categories:
- Dehydration reactions: Release during prograde metamorphism (e.g., breakdown of muscovite or chlorite at higher temperatures)
- Decarbonation reactions: Release , particularly important in carbonate-bearing rocks (e.g., calcite + quartz → wollastonite + )
- Solid-solid reactions: Involve changes in mineral assemblage without fluid involvement (e.g., the kyanite → sillimanite transition)
Each reaction occurs at specific P-T conditions, and the products tell you which side of the reaction boundary the rock ended up on.
Phase Equilibria
Phase equilibria use thermodynamic principles to predict which mineral assemblages are stable under given P-T conditions. These relationships are displayed graphically in petrogenetic grids and P-T diagrams, where reaction curves divide the diagram into stability fields. By comparing the observed mineral assemblage in a rock to a petrogenetic grid, you can estimate the equilibrium P-T conditions. This is the theoretical foundation for geothermobarometry.
Gibbs Phase Rule
The Gibbs phase rule is a fundamental constraint on metamorphic systems:
where is the degrees of freedom (the number of intensive variables like P and T that can change independently), is the number of components, and is the number of phases. A system at a univariant reaction line () means that if you fix pressure, temperature is determined. An invariant point () fixes both P and T. The phase rule helps you understand why certain mineral assemblages are stable over a range of conditions (divariant fields) while others exist only along narrow reaction boundaries.
Geochemical Changes
Metamorphism doesn't just rearrange minerals; it can also change the bulk chemical composition of rocks through fluid-mediated processes.
Element Mobility
During metamorphism, some elements are mobile and others are not. Mobile elements like K, Na, Ca, and Si can dissolve in metamorphic fluids and migrate through the rock or into adjacent lithologies. Immobile elements like Al, Ti, Zr, and the high field strength elements (HFSE) tend to stay put, which is why they're used as monitors of protolith composition even in heavily altered rocks. The degree of element mobility depends on temperature, pressure, fluid composition, and the duration of fluid-rock interaction.
Fluid-Rock Interactions
Metamorphic fluids (dominantly and , with dissolved solutes) facilitate reactions by transporting reactants and products. These fluids can infiltrate along grain boundaries, fractures, and shear zones, driving mineral reactions that wouldn't proceed in a dry system. Fluid-rock interaction produces veins (quartz veins are the most common), alteration halos, and in extreme cases, wholesale compositional changes. The fluid/rock ratio is a key variable: high ratios lead to more extensive chemical modification.
Metasomatism
Metasomatism is the process of changing a rock's bulk composition through fluid-rock interaction. Unlike isochemical metamorphism (where the bulk composition stays constant), metasomatism adds or removes elements. Examples include:
- Serpentinization: hydration of ultramafic rocks (olivine + water → serpentine)
- Granitization: introduction of alkalis into country rock
- Skarn formation: metasomatism of carbonate rocks at intrusive contacts, often producing economically important ore deposits (W, Sn, Cu, Fe)
Metasomatism can operate on scales from millimeter-wide reaction zones to kilometer-scale alteration systems.
Metamorphic Textures
The textures of metamorphic rocks record both the conditions of metamorphism and the deformation history. They're essential for interpreting the sequence and timing of metamorphic events.
Foliation vs. Lineation
Foliation is a planar fabric produced by the alignment of platy or tabular minerals (micas, chlorite) perpendicular to the maximum compressive stress. It develops progressively with increasing grade:
- Slaty cleavage (very fine-grained alignment in slates)
- Schistosity (visible alignment of mica and other sheet silicates in schists)
- Gneissic banding (compositional layering of light and dark minerals in gneisses)
Lineation is a linear fabric element created by the alignment of elongate minerals (amphiboles, sillimanite) or stretched mineral aggregates. It records the direction of tectonic transport or mineral growth and provides three-dimensional information about the strain field.
Porphyroblasts
Porphyroblasts are large crystals (garnet, staurolite, andalusite, etc.) that grow within a finer-grained matrix during metamorphism. They're valuable because they often overgrow and preserve earlier fabrics as inclusion trails. By comparing the geometry of inclusion trails inside the porphyroblast to the external foliation, you can determine whether the porphyroblast grew before, during, or after deformation. This makes porphyroblasts powerful tools for establishing the relative timing of metamorphic and deformational events.
Reaction Rims
Reaction rims (or coronas) are zones of new minerals that form around pre-existing grains when those grains become unstable under changing P-T conditions. A classic example is a corona of orthopyroxene + plagioclase forming around olivine in a granulite-facies rock. Reaction rims indicate that the rock did not fully equilibrate; they preserve evidence of the reaction in progress. This makes them useful for reconstructing the sequence of P-T changes and identifying disequilibrium textures.
Analytical Techniques
Several laboratory methods are essential for characterizing metamorphic rocks and extracting quantitative P-T information.
Petrographic Microscopy
Thin-section petrography under polarized light remains the foundational technique. It allows you to identify minerals by their optical properties (birefringence, extinction angle, pleochroism), observe textures and microstructures, and determine mineral relationships (what's growing at the expense of what). Petrographic work is always the first step before moving to more expensive analytical methods, because it tells you which minerals and textures to target.
X-Ray Diffraction (XRD)
XRD identifies crystalline phases based on the characteristic diffraction pattern produced when X-rays interact with a crystal lattice. It's particularly useful for identifying fine-grained minerals that are difficult to distinguish optically (e.g., different clay minerals or zeolites in low-grade rocks) and for quantifying the proportions of minerals in a bulk sample. XRD complements petrography by confirming mineral identifications and detecting minor phases.
Electron Microprobe Analysis (EMPA)
The electron microprobe uses a focused electron beam to excite characteristic X-ray emission from a polished sample, providing quantitative chemical analysis of individual mineral grains at the micron scale. This is critical for geothermobarometry, since thermometers and barometers require precise mineral compositions (e.g., the Fe/Mg ratio in garnet and biotite for garnet-biotite thermometry). EMPA also reveals chemical zoning within crystals, which records the changing P-T conditions during crystal growth.
Applications in Geochemistry
Metamorphic facies concepts connect directly to broader questions in geochemistry and tectonics.
Geothermobarometry
Geothermobarometry estimates the P-T conditions of metamorphism from the compositions of coexisting minerals that were in thermodynamic equilibrium. Thermometers use temperature-sensitive exchange reactions (e.g., Fe-Mg exchange between garnet and biotite). Barometers use pressure-sensitive net-transfer reactions (e.g., the garnet-aluminosilicate-quartz-plagioclase, or GASP, barometer). Combining thermometry and barometry for the same sample gives you a point on a P-T diagram, and analyzing multiple zones within zoned minerals can yield a full P-T path.
Petrogenetic Grids
Petrogenetic grids compile experimentally and thermodynamically determined reaction curves onto a single P-T diagram for a given bulk composition. They show which mineral assemblages are stable in each region of P-T space and predict the sequence of reactions a rock will undergo along a given P-T path. Modern computational tools like Thermocalc and Perple_X generate pseudosections (petrogenetic grids calculated for a specific bulk composition), which are now standard in metamorphic petrology.
Tectonic Reconstructions
P-T paths derived from metamorphic rocks are among the most powerful tools for reconstructing past tectonic settings. A clockwise P-T path (burial followed by heating, then exhumation) is typical of continental collision zones. A counterclockwise path can indicate contact metamorphism or magmatic underplating. Hairpin P-T paths with very high pressures at low temperatures point to subduction and rapid exhumation. By combining P-T data with geochronology (dating the timing of metamorphism), you can reconstruct the full tectonic evolution of a metamorphic terrane.