27.02.03 · earth-science / minerals-rocks

Metamorphic and sedimentary rocks: facies, diagenesis, classification

stub3 tiersLean: nonepending prereqs

Anchor (Master): Eskola, P. — On the relations between chemical and mineralogical composition (1920)

Intuition Beginner

Sedimentary rocks are Earth's history books, recording past environments in their layers. Sandstone forms when sand grains from beaches and deserts are compressed and cemented together. Limestone forms from accumulated shells and coral fragments on the ocean floor. Shale forms from mud compressed layer by layer in quiet water. Because these rocks form at the surface, they preserve evidence of ancient climates, sea levels, and ecosystems. Fossils — the preserved remains or traces of organisms — are found almost exclusively in sedimentary rocks, making them indispensable for reconstructing the history of life on Earth.

Metamorphic rocks are existing rocks transformed by heat and pressure without melting. Limestone becomes marble when heated deeply enough. Shale undergoes a progressive series of transformations: first it becomes slate, then phyllite, then schist, and finally gneiss, as temperature and pressure increase step by step. Each stage produces distinct minerals and textures. Geologists use these index minerals — chlorite, biotite, garnet, staurolite, kyanite, and sillimanite — to gauge how intensely the rock was cooked. Foliation, the layered banding visible in many metamorphic rocks, develops when minerals align under directed pressure, much like leaves stacking in a pile.

Sedimentary and metamorphic rocks are linked through the rock cycle. Sedimentary rocks buried by tectonic forces begin metamorphosing as depth increases. Sandstone becomes quartzite, a hard non-foliated rock. Shale transforms progressively from slate through schist to gneiss. If metamorphic rocks are heated enough, they melt entirely, beginning the cycle again as igneous rock. At the surface, weathering breaks all rock types back into sediment, restarting the sedimentary pathway. This continuous transformation means every rock carries a record of its history.

Visual Beginner

Protolith	Low grade	Medium grade	High grade	Rock type
Shale	Slate	Schist	Gneiss	Foliated
Limestone	Marble (fine)	Marble (coarse)	Marble (coarse)	Non-foliated
Sandstone	Quartzite (fine)	Quartzite (coarse)	Quartzite (coarse)	Non-foliated
Basalt	Greenschist	Amphibolite	Granulite	Foliated

Sediment size	Name	Rock	Common environment
Gravel (>2 mm)	Conglomerate	Conglomerate	River beds, beaches
Sand (0.0625–2 mm)	Sand	Sandstone	Beaches, deserts, river channels
Silt (0.004–0.0625 mm)	Silt	Siltstone	Floodplains, deep ocean
Clay (<0.004 mm)	Clay	Shale	Deep ocean, lake floors

Worked example Beginner

A geologist examining a cliff face in the Scottish Highlands finds a dark, shiny rock that splits easily into flat sheets. Under a hand lens, the rock shows tiny sparkling flakes of mica aligned in parallel planes. Nearby, a coarser rock displays visible bands of light and dark minerals, with garnet crystals up to one centimeter across embedded in the dark layers. What metamorphic rocks are these, and what do they tell us about conditions deep underground?

The first rock, with its flat cleavage and tiny mica flakes, is slate if the grains are too small to see individually, or phyllite if the surface has a distinctive silky sheen. Both form from shale at relatively low metamorphic grades. The aligned mica flakes indicate foliation caused by directed pressure during mountain building.

The second rock, with its banded appearance and visible garnet crystals, is schist if the minerals are large but not segregated into continuous bands, or gneiss if the light and dark minerals form distinct layers. The garnet crystals are index minerals indicating medium-to-high metamorphic grade. Garnet forms at temperatures around 500 degrees Celsius and pressures of several kilobars, conditions found at depths of 15 to 25 kilometers during continental collision.

The sequence from slate to schist to gneiss represents increasing metamorphic grade and records the progressive burial and heating of sedimentary rocks during the Caledonian orogeny, the mountain-building event that created the Scottish Highlands approximately 470 to 430 million years ago.

Check your understanding Beginner

Formal definition Intermediate+

Clastic sedimentary rocks are classified by grain size using the Wentworth scale. Gravel (particles greater than 2 mm) lithifies into conglomerate if the clasts are rounded or breccia if angular. Sand (0.0625 to 2 mm) produces sandstone, further subdivided into quartz arenite (greater than 95 percent quartz), arkose (greater than 25 percent feldspar), and lithic sandstone (greater than 25 percent rock fragments). Silt (0.004 to 0.0625 mm) forms siltstone. Clay (less than 0.004 mm) forms shale or mudstone. The Wentworth scale is logarithmic: each boundary differs by a factor of two from the next, with the phi scale defined as $ϕ = - lo g_{2} (d)$ where $d$ is the diameter in millimeters.

Chemical and biochemical sedimentary rocks form by precipitation from solution. Limestone, composed of calcite (CaCO $_{3}$ ), is the most abundant chemical sedimentary rock and forms through both inorganic precipitation and biochemical processes involving marine organisms. Dolostone, composed of dolomite (CaMg(CO $_{3}$ ) $_{2}$ ), forms by the chemical alteration of limestone through the addition of magnesium. Chert, composed of microcrystalline quartz (SiO $_{2}$ ), forms from the accumulation of silica-shelled organisms (radiolaria, diatoms) or by direct precipitation. Evaporites — including halite (NaCl), gypsum (CaSO $_{4} \cdot$ 2H $_{2}$ O), and anhydrite (CaSO $_{4}$ ) — form by evaporation of saline water in restricted basins.

Diagenesis encompasses all physical, chemical, and biological changes that affect sediment after deposition and during lithification, at temperatures below approximately 200 degrees Celsius and pressures below approximately 2 kilobars. Compaction reduces pore space as overlying sediment weight expels water. Cementation precipitates minerals (silica, calcite, iron oxides) from groundwater into pore spaces, binding grains. Recrystallization changes the crystal form or size of existing minerals without altering composition, as when fine aragonite shells recrystallize to coarser calcite. Dissolution removes soluble minerals, creating secondary porosity. Replacement substitutes one mineral for another, as when pyrite replaces calcite fossils.

Sedimentary structures record depositional processes. Bedding (stratification) represents successive depositional episodes and is the most fundamental sedimentary structure. Cross-bedding forms when sediment is deposited at an angle to the main bedding plane by migrating ripples or dunes, recording current direction. Graded bedding displays a vertical decrease in grain size within a single bed, produced by deposition from a decelerating current, often a turbidity current. Ripple marks form by the interaction of currents or waves with a sediment surface: asymmetric ripples indicate unidirectional current flow, while symmetric ripples indicate oscillatory wave motion. Mud cracks form by desiccation of wet mud, indicating subaerial exposure.

Sedimentary environments are settings where sediment accumulates, each producing distinctive rock assemblages. Fluvial environments (rivers) produce channel sandstones, floodplain shales, and conglomeratic lag deposits. Deltaic environments produce coarsening-upward sequences of shale, siltstone, and sandstone as the delta progrades into the basin. Shallow marine environments produce well-sorted sandstones, limestones, and shales with abundant fossils. Deep marine environments produce thin-bedded turbidites interbedded with pelagic shales. Aeolian environments produce well-sorted, well-rounded sandstones with large-scale cross-bedding.

Foliated metamorphic rocks display planar mineral alignment produced by directed stress. Slate, derived from shale, has fine grain and excellent rock cleavage along foliation planes. Phyllite has a glossy sheen from aligned muscovite and chlorite grains just visible to the eye. Schist contains medium-to-coarse grains of platy minerals (muscovite, biotite, chlorite) aligned in parallel, giving a scaly appearance. Gneiss displays compositional banding of light (felsic) and dark (mafic) mineral layers, reflecting high-grade segregation of minerals under extreme conditions.

Non-foliated metamorphic rocks lack planar fabric because they formed under uniform stress or are composed of minerals that do not develop preferred orientation. Marble, from limestone or dolostone, consists of interlocking calcite or dolomite grains. Quartzite, from sandstone, consists of interlocking quartz grains with the original quartz cement fused to the clastic grains. Hornfels, produced by contact metamorphism, is a fine-grained, hard rock with a granular texture and no preferred mineral orientation.

Metamorphic facies, introduced by Pentti Eskola in 1920, are groups of mineral assemblages that repeatedly occur together in rocks of diverse chemical composition and that indicate specific ranges of pressure and temperature. The principal facies, in order of increasing metamorphic grade, are:

Zeolite facies (very low grade): characterized by zeolite minerals, produced by burial metamorphism at low temperature and pressure.
Greenschist facies (low grade): chlorite, muscovite, albite, and epidote. The namesake green color comes from chlorite and epidote.
Amphibolite facies (medium grade): hornblende and plagioclase, with garnet, staurolite, and kyanite as index minerals in aluminous rocks.
Granulite facies (high grade): pyroxene and plagioclase, with garnet and sillimanite. Represents the highest grade of regional metamorphism at crustal depths.
Blueschist facies (high pressure, low temperature): glaucophane and lawsonite, diagnostic of subduction zone metamorphism where cold crust is rapidly buried.
Eclogite facies (very high pressure): omphacite (sodic clinopyroxene) and pyrope-rich garnet, formed at mantle depths in subduction zones.

Contact metamorphism occurs in a narrow aureole around an igneous intrusion, driven primarily by elevated temperature at relatively low pressure. The aureole width depends on the size and temperature of the intrusion: a large pluton may produce an aureole several kilometers wide. Rocks in contact aureoles are typically non-foliated (hornfels, marble, quartzite) because directed stress is absent. Mineral zones within the aureole reflect decreasing temperature with distance from the intrusion.

Regional metamorphism affects large volumes of rock during mountain building, driven by both elevated temperature and directed pressure associated with tectonic deformation. It produces foliated rocks on a regional scale and is responsible for the metamorphic cores of mountain belts such as the Appalachians, Alps, and Himalaya. Regional metamorphism displays systematic zonation of index minerals that George Barrow mapped in the Scottish Highlands in 1893, establishing the chlorite, biotite, garnet, staurolite, kyanite, and sillimanite zones of increasing metamorphic grade.

Metamorphic grade describes the intensity of metamorphism, ranging from low grade (slight recrystallization at low temperature and pressure) through medium grade to high grade (extensive recrystallization at high temperature and pressure). Grade correlates broadly with temperature: low grade (200 to 400 degrees Celsius), medium grade (400 to 600 degrees Celsius), and high grade (600 to 800 degrees Celsius and above). The grade of a rock is inferred from its mineral assemblage, which serves as a paleothermometer and paleobarometer.

Pressure-temperature-time (P-T-t) paths describe the trajectory a rock follows through pressure-temperature space during the entire metamorphic cycle, from burial and heating through peak conditions to exhumation and cooling. The shape of the P-T-t path reveals the tectonic setting and rate of geologic processes. Clockwise P-T-t paths, where peak pressure precedes peak temperature, are typical of regional metamorphism during continental collision: rocks are first buried rapidly (pressure increases faster than temperature), then heat up during residence at depth (temperature increases while pressure remains high), and finally are exhumed (pressure and temperature decrease). Counter-clockwise paths occur in some contact and extensional metamorphic settings.

Key result: metamorphic facies and the equilibrium mineral assemblage Intermediate+

Eskola's facies concept rests on the principle of equilibrium thermodynamics applied to mineral systems. If a rock of given bulk composition reaches chemical equilibrium at a particular pressure and temperature, the mineral assemblage is determined entirely by $P$ , $T$ , and composition $X$ . Different bulk compositions metamorphosed under identical $P$ - $T$ conditions will produce different mineral assemblages, but all belong to the same metamorphic facies because they record the same $P$ - $T$ conditions.

This principle can be expressed through the Gibbs phase rule for a system at equilibrium:

F = C - P + 2

where $F$ is the number of degrees of freedom (variance), $C$ is the number of independent chemical components, and $P$ is the number of phases (minerals plus any melt or fluid). For a typical pelitic rock with approximately six components (SiO $_{2}$ , Al $_{2}$ O $_{3}$ , FeO, MgO, K $_{2}$ O, H $_{2}$ O), a mineral assemblage of five or fewer phases is divariant ( $F \geq 2$ ), meaning it is stable over a range of pressures and temperatures. A divariant assemblage defines a field on a $P$ - $T$ diagram. A univariant assemblage ( $F = 1$ ) plots as a curve, representing a metamorphic reaction boundary.

The Gibbs free energy of a mineral at pressure $P$ and temperature $T$ relative to standard-state conditions ( $P_{0}$ , $T_{0}$ ) can be expressed as:

Δ G = Δ H - T Δ S + \int_{P_{0}}^{P} V d P

where $Δ H$ is enthalpy, $Δ S$ is entropy, and $V$ is molar volume. At equilibrium, the total Gibbs free energy of the mineral assemblage is minimized. A metamorphic reaction occurs when the Gibbs free energies of the reactant and product assemblages are equal. The slope of a reaction boundary on a $P$ - $T$ diagram is given by the Clausius-Clapeyron relation:

\frac{d P}{d T} = \frac{Δ S}{Δ V}

where $Δ S$ and $Δ V$ are the entropy and volume changes of the reaction. Reactions that consume volume and produce entropy (such as devolatilization reactions that release H $_{2}$ O) have shallow slopes on $P$ - $T$ diagrams. Reactions involving a large volume decrease (such as the transformation of plagioclase + olivine to garnet + pyroxene in the granulite-to-eclogite transition) have steep slopes and are sensitive pressure indicators.

Index minerals and Barrovian zones

The Barrovian zonal scheme, established by George Barrow in the Scottish Highlands, maps the sequential appearance of index minerals in metamorphosed pelitic rocks with increasing grade: chlorite, biotite, garnet, staurolite, kyanite, sillimanite. Each index mineral appears at a specific metamorphic reaction. Garnet appears when chlorite + muscovite react to produce garnet + biotite + water. Staurolite appears when garnet + chlorite + muscovite react to produce staurolite + biotite + water. Kyanite appears at the staurolite breakdown reaction. Sillimanite replaces kyanite at the aluminosilicate polymorphic transition near 500 to 600 degrees Celsius at middle crustal pressures.

The three Al $_{2}$ SiO $_{5}$ polymorphs — andalusite, kyanite, and sillimanite — are particularly useful because their stability fields are determined by pressure and temperature alone, independent of bulk composition. The Al $_{2}$ SiO $_{5}$ phase diagram has a triple point at approximately 500 degrees Celsius and 4 kilobars. Andalusite is stable at low pressure, kyanite at high pressure, and sillimanite at high temperature. The presence of any one polymorph constrains the $P$ - $T$ conditions; finding two together pins conditions to a reaction boundary; finding all three would indicate the triple point.

Diagenetic regimes and the diagenesis-catagenesis-metamorphism boundary

Diagenesis grades into metamorphism through a continuum of increasing temperature and pressure. Early diagenesis (near-surface, less than 50 degrees Celsius) involves microbial activity, bioturbation, and shallow compaction. Mesodiagenesis (50 to 150 degrees Celsius) encompasses quartz cementation, clay mineral transformations (smectite to illite), and hydrocarbon generation from organic matter. Metagenesis or catagenesis (150 to 200 degrees Celsius) involves extensive mineral dissolution and reprecipitation. The conventional boundary between diagenesis and metamorphism is placed at approximately 200 degrees Celsius, marked by the onset of greenschist-facies mineral assemblages.

The smectite-to-illite transition is one of the most widely studied diagenetic reactions. Smectite, an expandable clay, transforms to illite, a non-expandable mica-like clay, through a series of mixed-layer intermediate stages as temperature increases. The reaction releases water, silica, sodium, calcium, and iron, which affect pore water chemistry, cementation, and porosity. The transition serves as a geothermometer in sedimentary basins and is critical for predicting reservoir quality in petroleum geology.

Exercises Intermediate+

Exercise 3 (medium, short answer).

Explain why contact metamorphism produces non-foliated rocks while regional metamorphism produces foliated rocks.

Hint

Foliation requires directed stress (differential pressure), not just elevated temperature. Consider the stress regime around a pluton versus within a collisional orogen.

Answer

Contact metamorphism is driven primarily by elevated temperature from a nearby igneous intrusion, with stress being approximately equal in all directions (lithostatic). Without differential stress, platy minerals have no preferred orientation, producing non-foliated rocks like hornfels. Regional metamorphism occurs during orogenesis, where tectonic forces impose directed stress (differential pressure) that aligns platy minerals perpendicular to the maximum compression direction, producing foliation. The difference is the presence or absence of non-hydrostatic stress, not temperature alone.

Exercise 4 (medium, short answer).

A sandstone contains 80 percent quartz, 10 percent feldspar, and 10 percent rock fragments. Classify this sandstone using the standard QFL (quartz-feldspar-lithic) ternary diagram and explain what the composition reveals about its provenance.

Hint

On the QFL diagram, quartz-rich sandstones (greater than 95 percent quartz) are quartz arenites. Between 75 and 95 percent quartz with more feldspar than lithics, the rock is subarkose.

Answer

This sandstone is a subarkose. With 80 percent quartz and feldspar content exceeding lithic fragment content, it falls in the subarkose field of the QFL diagram (75 to 95 percent quartz, feldspar greater than lithic fragments). The high quartz content indicates a mature sediment that has undergone extensive chemical weathering and transport, likely from a cratonic interior setting. The presence of 10 percent feldspar suggests the source area included granitic or gneissic basement rocks, and the moderate feldspar content indicates either relatively short transport distance or an arid climate where chemical weathering was less effective at destroying feldspar grains.

Exercise 6 (medium, short answer).

Describe the differences between fluvial and aeolian sandstones in terms of sorting, rounding, grain size, and sedimentary structures.

Hint

Consider the transport medium (water versus wind) and how it affects grain selection, abrasion, and bedform development.

Answer

Fluvial sandstones are moderately to well-sorted, sub-rounded to rounded, and contain a range of sand sizes including some silt and clay. They display trough cross-bedding, planar cross-bedding, and ripple marks, and often show fining-upward sequences from channel to floodplain deposits. Aeolian sandstones are extremely well-sorted and well-rounded because wind selectively transports a narrow size range and prolonged saltation abrades grains effectively. They display large-scale cross-bedding with set heights of meters to tens of meters, far larger than fluvial cross-beds. Aeolian deposits lack clay and contain almost no mica, which winnowed away. Grain frosting (matte surface texture) is characteristic of aeolian quartz grains due to grain-to-grain impacts during saltation.

Exercise 7 (hard, short answer).

A metamorphic rock contains the assemblage garnet + clinopyroxene + kyanite + quartz. Using the Gibbs phase rule, determine the variance of this assemblage in a system with six independent components (SiO $_{2}$ , Al $_{2}$ O $_{3}$ , FeO, MgO, CaO, H $_{2}$ O) and explain whether this assemblage could be used for geothermobarometry.

Hint

Apply $F = C - P + 2$ . If the variance is zero or one, the assemblage constrains $P$ - $T$ conditions tightly. Consider whether H $_{2}$ O is present as a phase.

Answer

With six components ( $C = 6$ , assuming H $_{2}$ O is a component but not a separate fluid phase if the system is dry) and four mineral phases ( $P = 4$ ), the Gibbs phase rule gives $F = 6 - 4 + 2 = 4$ . If H $_{2}$ O is present as a fluid phase ( $P = 5$ ), then $F = 6 - 5 + 2 = 3$ . In either case the assemblage is divariant or higher, meaning it is stable over a range of pressures and temperatures and cannot by itself uniquely determine $P$ and $T$ . However, the garnet-clinopyroxene Fe-Mg exchange reaction is temperature-sensitive (geothermometer), and the garnet-clinopyroxene-kyanite-quartz assemblage constrains pressure through a net-transfer reaction (geobarometer). By analyzing the compositions of coexisting garnet and clinopyroxene using the garnet-clinopyroxene Mg-Fe exchange thermometer combined with a garnet activity model, both $P$ and $T$ can be estimated simultaneously, reducing the effective variance to zero at the point where both reactions are satisfied.

Exercise 8 (hard, short answer).

Explain why the transformation of blueschist to eclogite in a subducting slab has major implications for the buoyancy of the slab and the driving force of subduction.

Hint

Consider the density change associated with the blueschist-to-eclogite transition and how this affects the slab's buoyancy relative to the surrounding mantle.

Answer

The blueschist-to-eclogite transition involves the breakdown of glaucophane + lawsonite (relatively low-density minerals) to omphacite + garnet (high-density minerals), increasing the rock's density by approximately 10 to 15 percent (from roughly 3.0 to 3.5 g/cm $^{3}$ ). This transformation occurs at depths of 40 to 60 kilometers in cold subduction zones. The density increase makes the subducting slab negatively buoyant relative to the surrounding mantle, enhancing the slab pull force that drives subduction. This positive feedback — metamorphism increases slab density, which increases slab pull, which drives faster subduction — may explain why subduction, once initiated, becomes a self-sustaining process. The transition also releases fluid from the breakdown of hydrous minerals (lawsonite, glaucophane contain structurally bound water), which fluxes the overlying mantle wedge and triggers arc volcanism.

Advanced results Master

Phase petrology of metamorphic systems

Quantitative metamorphic petrology uses phase equilibria to predict stable mineral assemblages as functions of pressure, temperature, and bulk composition. The AFM (Al $_{2}$ O $_{3}$ -FeO-MgO) projection, developed by Thompson (1957), reduces the multicomponent pelitic system to three coordinates by projecting from muscovite (or K-feldspar at high grade) and quartz onto the Al-Fe-Mg plane. AFM diagrams display the topology of mineral assemblages: each triangle on the diagram represents a three-phase assemblage, and points within triangles represent rock compositions that produce those assemblages. As metamorphic grade changes, tie-lines flip as reactions break old assemblages and produce new ones, providing a graphical representation of metamorphic reactions.

Petrogenetic grids are $P$ - $T$ diagrams showing the reaction curves relevant to a particular chemical system. For pelitic rocks, the KFMASH system (K $_{2}$ O-FeO-MgO-Al $_{2}$ O $_{3}$ -SiO $_{2}$ -H $_{2}$ O) captures the essential reactions. Each reaction curve represents a univariant equilibrium where $F = 1$ , and the intersections of reaction curves are invariant points where specific assemblages coexist. Petrogenetic grids provide a theoretical framework for interpreting mineral assemblages, but their construction requires accurate thermodynamic data for all participating minerals.

Equilibrium thermodynamics and the Gibbs method

The Gibbs method, formalized by Spear and others in the 1980s, applies differential thermodynamics to metamorphic systems. Rather than computing full phase equilibria, the Gibbs method linearizes the system around a known reference state and solves for changes in mineral compositions, modes, and fluid composition as functions of changes in $P$ and $T$ . The method expresses mass balance, thermodynamic equilibrium, and phase constraints as a system of linear equations:

A \cdot x = b

where $A$ is a matrix of stoichiometric and thermodynamic coefficients, $x$ is a vector of changes in mineral compositions and abundances, and $b$ is a vector of changes in $P$ and $T$ . The solution provides the partial derivatives of mineral compositions with respect to $P$ and $T$ , enabling construction of isopleths (lines of constant mineral composition) on $P$ - $T$ diagrams. Isopleth intersections provide precise geothermobarometric estimates.

Geothermobarometry

Quantitative geothermobarometry exploits the temperature and pressure dependence of element partitioning between coexisting minerals. Exchange thermometers use the distribution of two elements (typically Fe and Mg) between a pair of minerals. The garnet-biotite thermometer, calibrated by Ferry and Spear (1978), uses the equilibrium:

Fe-garnet + Mg-biotite ⇌ Mg-garnet + Fe-biotite

The distribution coefficient $K_{D} = (Fe/Mg)^{gar} / (Fe/Mg)^{bt}$ is primarily a function of temperature, with a weak pressure dependence. The garnet-clinopyroxene thermometer applies the same principle to mafic and eclogitic rocks, using Fe-Mg exchange between garnet and clinopyroxene.

Net-transfer barometers use reactions that change the number of moles of phases, making them sensitive to pressure. The garnet-plagioclase-aluminosilicate-quartz (GPAQ) barometer uses the equilibrium:

grossular garnet + sillimanite + quartz ⇌ anorthite

The large volume change of this reaction makes the equilibrium constant strongly pressure-dependent. Combining a thermometer and a barometer on the same rock sample yields an intersection that determines both $P$ and $T$ .

Garnet is especially valuable because it grows zonally, with each growth zone recording the $P$ - $T$ conditions at the time of its formation. Zoned garnet crystals thus preserve a segment of the P-T-t path, allowing reconstruction of the metamorphic history from a single grain.

Pseudosection modelling

Pseudosections (or equilibrium phase diagrams) show the stable mineral assemblage for a specific bulk composition across a range of $P$ - $T$ conditions. Unlike petrogenetic grids, which show reaction curves for the entire chemical system, pseudosections display only the assemblages relevant to one rock composition. They are computed using internally consistent thermodynamic datasets (such as the Holland and Powell dataset) and solution models for minerals with variable composition.

Software packages for pseudosection calculation include THERMOCALC (Holland and Powell, maintained at the University of Cambridge) and Perple_X (Connolly, maintained at ETH Zurich). The user inputs a bulk composition, and the software calculates the stable assemblage, mineral proportions, and mineral compositions across a specified $P$ - $T$ window. Contours of mineral composition (isopleths) can be overlaid on the pseudosection, allowing direct comparison with measured compositions from a real rock to determine the $P$ - $T$ conditions of equilibration.

Pseudosection modelling has become the standard approach for quantitative metamorphic petrology because it accounts for the full multicomponent, multiphase nature of real rocks, including the effects of solid solution in minerals. The main limitations are the quality of thermodynamic data and solution models for certain mineral groups, and the assumption that the rock achieved equilibrium.

Reaction textures and overprinting

Metamorphic rocks rarely preserve a single equilibrium state. Reaction textures — features such as coronas (rims of one mineral surrounding another), symplectites (intergrowths of two or more minerals replacing a single phase), and inclusion trails in porphyroblasts — record changes in $P$ - $T$ conditions that outpace the kinetics of equilibration. A garnet surrounded by a corona of plagioclase + pyroxene may record the decompression reaction that produced the corona after peak pressure conditions.

Porphyroblasts (large crystals that grow during metamorphism in a finer-grained matrix) often contain inclusions of earlier minerals arranged in trails (S-shaped, spiral, or straight) that record the deformation history. The relationship between inclusion trail orientation and external foliation reveals whether the porphyroblast grew before, during, or after the deformation that produced the external fabric. Textural analysis of porphyroblast-inclusion relationships provides constraints on the relative timing of metamorphic reactions and deformation events, essential for constructing P-T-t-D (pressure-temperature-time-deformation) paths.

Ultra-high-pressure metamorphism

The discovery of coesite (a high-pressure polymorph of SiO $_{2}$ stable above approximately 28 kbar, equivalent to depths greater than 80 kilometers) in crustal rocks of the Western Alps (Chopin 1984) and the Dora Maira massif revolutionized understanding of subduction and exhumation dynamics. Subsequent discoveries of microdiamond inclusions in garnet and zircon from crustal rocks in the Kokchetav massif (Kazakhstan), the Western Gneiss Region (Norway), and the Sulu terrane (China) demonstrated that continental crust can be subducted to depths exceeding 120 kilometers and returned to the surface.

The exhumation of UHP rocks presents a mechanical problem: how does buoyant continental crust reach mantle depths, and what mechanism brings it back? Current models invoke slab extraction, where the dense oceanic portion of the subducting slab detaches and sinks, allowing the buoyant continental portion to rise. Channel flow models propose that the subducted crust flows upward within a low-viscosity channel between the subducting slab and the overriding plate. The preservation of UHP minerals requires rapid exhumation to prevent retrograde reaction to lower-pressure mineral assemblages, with estimated exhumation rates of centimeters per year, comparable to plate velocities.

Subduction zone metamorphism and paired metamorphic belts

Subduction zone metamorphism follows a characteristic P-T-t path determined by the thermal structure of the slab. Cold oceanic lithosphere subducting at rates of several centimeters per year experiences rapid pressure increase with minimal heating, following a low-geothermal-gradient trajectory through blueschist and then eclogite facies. The metamorphic evolution is recorded by the sequential appearance of hydrous minerals: prehnite-pumpellyite, lawsonite, glaucophane, epidote, and finally the anhydrous eclogite-facies assemblage.

Akiho Miyashiro (1961) recognized that many circum-Pacific orogens contain paired metamorphic belts: a high-pressure, low-temperature (HPLT) belt on the oceanward side, recording subduction zone metamorphism, and a low-pressure, high-temperature (LPHT) belt on the continentward side, recording magmatic arc metamorphism. The paired belts are separated by a major fault or tectonic boundary. The HPLT belt contains blueschist and eclogite, while the LPHT belt contains andalusite- and sillimanite-bearing schists and gneisses formed at high temperatures but shallow crustal levels. The pairing reflects the dual thermal structure of convergent margins: cold subduction on one side, magmatic heating on the other.

The concept of paired metamorphic belts has been applied to ancient orogens to identify former subduction zones. The Franciscan Complex (blueschist) and Sierra Nevada batholith (contact metamorphism) in California represent a Cenozoic pair. The Sanbagawa (blueschist) and Ryoke (andalusite-sillimanite) belts in Japan represent a Cretaceous pair. However, the concept requires modification for continental collisional orogens, where the two belts may be telescoped together or overprinted by subsequent metamorphism.

Orogenic metamorphism and Barrovian zones

The classic Barrovian metamorphic pattern, documented by George Barrow in the southeastern Highlands of Scotland and elaborated by C.E. Tilley, represents medium-pressure regional metamorphism during continental collision. The progressive zonation from chlorite zone through sillimanite zone records increasing temperature from approximately 300 to 700 degrees Celsius at pressures of 4 to 8 kilobars (depths of 15 to 30 kilometers). The Barrovian pattern is reproduced in orogens worldwide and reflects the typical thermal gradient during crustal thickening.

The Buchan metamorphic pattern, documented in northeastern Scotland, represents a low-pressure, high-temperature metamorphic series characterized by the sequence chlorite, andalusite, sillimanite (with biotite and cordierite). The absence of kyanite and the presence of andalusite indicate low pressures (less than 4 kilobars), suggesting thin crust or elevated heat flow, possibly due to magmatic underplating. The contrast between Barrovian and Buchan metamorphism in the same orogen illustrates how tectonic setting controls the metamorphic P-T trajectory.

Sedimentary provenance analysis

Sedimentary provenance analysis reconstructs the source area of sedimentary rocks by examining their mineralogical, geochemical, and geochronological signatures. Detrital zircon U-Pb geochronology has become the most widely used provenance tool. Zircon is physically and chemically durable, survives multiple sedimentary cycles, and can be precisely dated. The age distribution of detrital zircons in a sandstone provides a fingerprint of the source terrane: a population of 1.0 to 1.2 Ga zircons indicates derivation from the Grenville orogen, while 2.6 to 2.8 Ga zircons indicate Archean crust.

Heavy mineral analysis supplements zircon geochronology. The assemblage of dense accessory minerals (zircon, tourmaline, rutile, garnet, apatite, epidote, staurolite, kyanite, sillimanite) reflects the lithology and metamorphic grade of the source area. Chemical weathering selectively destroys less stable minerals: olivine and pyroxene are lost rapidly, while zircon, tourmaline, and rutile survive extensive recycling. The ZTR index (zircon + tourmaline + rutile as a percentage of the total heavy mineral assemblage) measures mineralogical maturity.

Basin analysis and sequence stratigraphy

Sedimentary basins subside through three principal mechanisms: tectonic subsidence (extensional, flexural, or thermal), sediment loading (the weight of accumulated sediment causes further subsidence), and compaction (volume reduction as pore fluids are expelled). Backstripping analysis removes the effects of sediment loading and compaction to isolate the tectonic subsidence component, producing a subsidence curve that reveals the thermal and mechanical evolution of the basin.

Sequence stratigraphy, developed by Peter Vail and colleagues at Exxon in the 1970s, organizes sedimentary strata into unconformity-bounded units called sequences. Each sequence records a cycle of relative sea-level change: a sea-level fall produces an unconformity (sequence boundary), a subsequent rise produces transgressive deposits that onlap the margin, and the highstand deposits prograde basinward. Sequences are subdivided into systems tracts (lowstand, transgressive, highstand) defined by their geometric relationships to the sequence boundary and maximum flooding surface.

The origin of sequence boundaries — eustatic (global sea-level change) versus tectonic (regional subsidence or uplift) — has been debated since the framework was proposed. Eustatic controls produce synchronous sequence boundaries worldwide, while tectonic controls produce diachronous boundaries. Distinguishing the two requires correlation of sequence boundaries across different basins on different tectonic plates, a nontrivial exercise.

Milankovitch cycles in sedimentary records

Milankovitch theory posits that variations in Earth's orbital parameters — eccentricity (100 kyr and 400 kyr cycles), obliquity (41 kyr cycle), and precession (19 kyr and 23 kyr cycles) — modulate the distribution and intensity of solar radiation, driving climate cycles that are recorded in sedimentary strata. These orbital cycles produce rhythmic bedding patterns (cyclothems) in which lithological variations (limestone-shale, sandstone-shale, coal-clastic alternations) repeat at orbital frequencies.

The identification of Milankovitch cycles in sedimentary records provides a high-resolution geochronometer. Cyclostratigraphy matches the pattern of bed thickness and lithology to the predicted orbital signal, allowing correlation and dating at resolutions of tens of thousands of years — far more precise than biostratigraphy or radiometric dating alone. The Triassic-Jurassic lacustrine succession of the Newark Basin (eastern North America) and the Cretaceous pelagic limestone of the Piobbico core (Italy) are classic examples of astronomically tuned stratigraphic records.

Connections Master

Connections to plate tectonics

Metamorphic facies patterns directly record plate tectonic processes. Blueschist and eclogite facies form exclusively in subduction zones, where high pressure and low temperature result from rapid burial of cold oceanic lithosphere. Regional metamorphic belts record continental collision, with Barrovian zonation mapping the thermal structure of thickened crust. Paired metamorphic belts, first described by Miyashiro, reflect the thermal duality of convergent margins: cold subduction on one side, magmatic arc heating on the other. The discovery of ultra-high-pressure metamorphic rocks in continental collision zones demonstrates that continental crust can be subducted to mantle depths and exhumed, a process that requires plate-scale tectonic forces.

Sedimentary basins are also products of plate tectonics. Rift basins form at divergent boundaries, foreland basins form by flexural loading during continental collision, and forearc and backarc basins form at convergent boundaries. The composition and geometry of sedimentary fill record the tectonic history of the basin.

Connections to the rock cycle

This unit completes the rock cycle triad begun in Unit 27.02.01. Igneous rocks weather into sediment, which is lithified into sedimentary rocks through diagenesis. Sedimentary rocks are metamorphosed by heat and pressure into metamorphic rocks. Metamorphic rocks heated sufficiently melt to form magma, which cools into igneous rocks. Each transformation involves specific processes: weathering and erosion break rocks down, transport and deposition sort the fragments, diagenesis lithifies them, and metamorphism transforms them through solid-state reactions. The rates and paths of these transformations are controlled by plate tectonics.

Connections to Earth history

Sedimentary rocks are the primary archive of Earth history. Stratigraphy provides the temporal framework for interpreting the geologic past. Sequence stratigraphy relates sedimentary packages to changes in relative sea level, which are driven by tectonics and climate. Fossils preserved in sedimentary rocks document the evolution of life, while sedimentary structures record ancient environments. Metamorphic rocks in orogenic belts record the timing and conditions of mountain-building events. Detrital zircon geochronology in sedimentary rocks reveals the ages of eroded source terranes, reconstructing ancient continental configurations that preceded current ones.

Connections to economic geology

Many economically important resources are hosted in sedimentary rocks. Petroleum systems require a source rock (organic-rich shale), reservoir rock (porous sandstone or carbonate), seal (impermeable shale or evaporite), trap (structural or stratigraphic), and appropriate thermal history for hydrocarbon generation. Understanding diagenesis is essential for predicting reservoir quality because cementation and compaction reduce porosity. Uranium roll-front deposits form in fluvial sandstones where oxidizing groundwater encounters reducing conditions. Banded iron formations, the world's primary source of iron ore, are chemical sedimentary rocks deposited during the Great Oxygenation Event.

Metamorphic rocks host important ore deposits. Skarn deposits form by metasomatism at the contact between igneous intrusions and carbonate rocks, producing garnet-pyroxene-wollastonite assemblages with concentrated copper, zinc, tungsten, and gold. Ophiolite-hosted chromite deposits are associated with metamorphosed oceanic crust.

Connections to climate science

Sedimentary records preserve paleoclimate information at multiple timescales. Lithological indicators (coal, evaporites, glacial diamictites, reefal limestones) reveal past climate zones. Geochemical proxies — oxygen isotopes in carbonate fossils, carbon isotopes in organic matter, magnesium-calcium ratios in foraminifera — provide quantitative estimates of past temperature and ice volume. Milankovitch cycles in sedimentary records demonstrate orbital forcing of climate and provide the astronomical timescale that calibrates the geologic time scale. Sequence stratigraphic records of sea-level change constrain the amplitude and timing of eustatic fluctuations driven by glacial-interglacial cycles and tectonics.

Connections to geotechnical engineering

The engineering properties of rocks depend on their mineralogy, texture, and degree of weathering — all of which are determined by sedimentary and metamorphic processes. Shale, the most common sedimentary rock, exhibits low shear strength and is prone to slope failure when wetted. Slate, the metamorphosed equivalent, has much higher strength along cleavage planes but is anisotropic. Schist and gneiss are competent when intact but may fail along foliation planes. Understanding the relationship between metamorphic grade, mineralogy, and mechanical properties is essential for tunnel, foundation, and slope design.

Historical and philosophical context Master

Eskola and the facies concept

Pentti Eskola (1883-1964), a Finnish geologist, introduced the metamorphic facies concept in a series of papers beginning in 1914 and culminating in his 1920 paper "On the relations between chemical and mineralogical composition in metamorphic rocks" published in Norsk Geologisk Tidsskrift. Eskola recognized that rocks of different chemical composition metamorphosed under the same physical conditions develop different mineral assemblages, but these assemblages are systematically related and can be grouped into facies that reflect specific pressure-temperature conditions.

Eskola initially defined five facies. His original classification was expanded by subsequent workers: Coombs, Ellis, and others added the zeolite facies in the 1950s; Turner and Verhoogen systematized the classification in their influential 1960 textbook; and Miyashiro added refinements for subduction zone metamorphism. The facies concept remains the foundational framework for metamorphic petrology, though modern quantitative approaches using pseudosections and thermodynamic modelling have largely superseded the qualitative facies classification for detailed work.

Barrow and the mapping of metamorphic zones

George Barrow (1853-1932), a survey geologist with the Geological Survey of Scotland, mapped the metamorphic zones of the southeastern Highlands between 1893 and 1912. Working in difficult terrain with limited analytical equipment, Barrow recognized that certain minerals appeared in a consistent sequence with increasing metamorphic grade: chlorite, biotite, garnet, staurolite, kyanite, sillimanite. He drew zone boundaries (isograds) on his geological maps, creating the first systematic map of metamorphic intensity.

Barrow's work was initially met with skepticism. His colleague C.E. Tilley at Cambridge confirmed and extended the zonal scheme in the 1920s, demonstrating that the same sequence of index minerals appeared in orogens worldwide. The Barrovian zonal scheme became one of the most powerful tools in field metamorphic petrology and remains in use today, although modern geothermobarometry has provided quantitative P-T estimates that refine and sometimes revise the original zonal boundaries.

Miyashiro and paired metamorphic belts

Akiho Miyashiro (1920-2008), a Japanese petrologist, proposed the concept of paired metamorphic belts in 1961 based on his studies of circum-Pacific orogens. Miyashiro recognized that many Mesozoic and Cenozoic orogens around the Pacific rim contain two parallel metamorphic belts: a high-pressure, low-temperature belt (blueschist to eclogite facies) on the oceanward side and a low-pressure, high-temperature belt (andesite-sillimanite facies) on the continentward side. He interpreted this pairing as a direct consequence of subduction: the cold subducting slab produces HPLT metamorphism, while the magmatic arc above produces LPHT metamorphism.

The paired belt concept was later applied to ancient orogens and became a key line of evidence for the operation of plate tectonics in the geologic past. However, subsequent work has shown that not all orogens display simple paired belts: continental collision orogens may telescope the two belts together, and some subduction zones lack one or the other component. The concept nonetheless remains a powerful framework for interpreting the metamorphic record of plate tectonics.

The development of sedimentology as a discipline

Sedimentology emerged as a distinct discipline in the mid-20th century, building on earlier work in stratigraphy and sedimentary petrology. Henry Clifton Sorby (1826-1908), a British microscopist, pioneered the use of thin sections to study sedimentary rocks and recognized that grain shape and sorting reflect transport processes. His work anticipated the quantitative approach to sedimentology by nearly a century.

The modern quantitative era began with the work of Francis Shepard on marine sediments in the 1930s and 1940s, and was advanced by the development of the Wentworth grain-size scale, the application of statistical methods to sedimentary data, and the recognition that sedimentary structures record specific depositional processes. The publication of "Primary Sedimentary Structures and Their Hydrodynamic Interpretation" by the Society of Economic Paleontologists and Mineralogists (SEPM) in 1965 established the process-based approach that defines modern sedimentology.

Sequence stratigraphy, developed by Peter Vail and colleagues at Exxon Production Research Company in the 1970s, revolutionized stratigraphic analysis by relating sedimentary packages to changes in relative sea level. The method was initially controversial because it proposed that sequence boundaries were globally synchronous, implying eustatic control. Subsequent work has shown that both eustatic and tectonic controls operate, and sequence stratigraphy is now a standard tool in both academic and industrial stratigraphy.

The thermodynamics revolution in petrology

The application of equilibrium thermodynamics to metamorphic petrology, pioneered by John B. Thompson Jr. at Harvard in the 1950s and 1960s, transformed the field from a descriptive to a quantitative science. Thompson introduced graphical methods (AFM diagrams, petrogenetic grids) for analyzing mineral assemblages and demonstrated that metamorphic reactions could be predicted from thermodynamic principles. His 1957 paper "The graphical analysis of mineral assemblages in pelitic schists" is one of the most influential papers in petrology.

The development of internally consistent thermodynamic datasets, beginning with the work of Holland and Powell in the 1980s, enabled quantitative calculation of phase equilibria. Software packages such as THERMOCALC and Perple_X made pseudosection modelling accessible to practicing geologists. The result has been a revolution in the precision and rigor of metamorphic petrology: modern studies routinely estimate P-T conditions to within 25 degrees Celsius and 0.5 kilobars, a level of precision that was unattainable before the thermodynamic approach.

Bibliography Master

Tarbuck, E.J. and Lutgens, F.K. (2018). Earth Science (15th ed.). Pearson. Chapters 6 (Sedimentary rocks) and 7 (Metamorphic rocks).
Best, M.G. (2003). Igneous and Metamorphic Petrology (2nd ed.). Blackwell Publishing. Chapters 8-12 on metamorphic facies, textures, and thermobarometry.
Eskola, P. (1920). "On the relations between chemical and mineralogical composition in metamorphic rocks." Norsk Geologisk Tidsskrift, 6, 143-194. The foundational paper introducing the metamorphic facies concept.
Boggs, S. (2014). Principles of Sedimentology and Stratigraphy (5th ed.). Pearson. Chapters 1-4 on sedimentary processes, transport, and deposition.
Miyashiro, A. (1961). "Evolution of metamorphic belts." Journal of Petrology, 2(3), 277-311. The paper introducing paired metamorphic belts.
Barrow, G. (1893). "On an intrusion of muscovite-biotite gneiss in the south-eastern Highlands of Scotland." Quarterly Journal of the Geological Society, 49, 330-358. The first description of Barrovian metamorphic zones.
Thompson, J.B. Jr. (1957). "The graphical analysis of mineral assemblages in pelitic schists." American Mineralogist, 42, 842-858. Introduction of AFM projections.
Chopin, C. (1984). "Coesite and pure pyrope in high-grade blueschists of the Western Alps: a first record and some consequences." Contributions to Mineralogy and Petrology, 86, 107-118. Discovery of ultra-high-pressure metamorphism in continental crust.
Holland, T.J.B. and Powell, R. (1998). "An internally consistent thermodynamic data set for phases of petrological interest." Journal of Metamorphic Geology, 16, 309-343. The thermodynamic dataset underlying THERMOCALC.
Connolly, J.A.D. (2005). "Computation of phase equilibria by linear programming: a tool for geodynamic modelling." Earth and Planetary Science Letters, 236, 524-541. The Perple_X pseudosection modelling software.
Spear, F.S. (1993). Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineralogical Society of America Monograph. Comprehensive treatment of thermobarometry and P-T-t paths.
Ferry, J.M. and Spear, F.S. (1978). "Experimental calibration of the partitioning of Fe and Mg between biotite and garnet." Contributions to Mineralogy and Petrology, 66, 113-117. The garnet-biotite thermometer calibration.
Vail, P.R., Mitchum, R.M. Jr., and Thompson, S. III (1977). "Seismic stratigraphy and global changes of sea level, Part 4: Global cycles of relative changes of sea level." AAPG Memoir, 26, 83-97. Foundational work on sequence stratigraphy.
Wentworth, C.K. (1922). "A scale of grade and class terms for clastic sediments." Journal of Geology, 30, 377-392. The grain-size classification still in use today.

Prerequisites

27.02.01 pending

Tier anchors

beginner: Tarbuck and Lutgens, Earth Science (15e), Ch. 3, 6-7
intermediate: Best, M. G. — Igneous and Metamorphic Petrology, 2nd ed. (2003)
master: Eskola, P. — On the relations between chemical and mineralogical composition (1920)

References

Tarbuck, F. K. & Lutgens, E. J. — Earth Science, 15th ed. (Pearson, 2018) · Ch. 6 Sedimentary rocks, Ch. 7 Metamorphic rocks
Best, M. G. — Igneous and Metamorphic Petrology, 2nd ed. (Blackwell, 2003) · Ch. 8-12 Metamorphic facies and textures
Eskola, P. — On the relations between chemical and mineralogical composition in metamorphic rocks · Norsk Geol. Tidsskr. 6 (1920) 143-194; metamorphic facies concept
Boggs, S. — Principles of Sedimentology and Stratigraphy, 5th ed. (2014) · Ch. 1-4 Sedimentary processes

Estimated time

beginner: 15m
intermediate: 30m
master: 45m