Metamorphic Rocks

CONTENTS Introductions Metamorphism and Type of metamorphic agents. Metamorphic processes. Types of Metamorphism Classification of metamorphic rocks Textures of metamorphic rocks Mineral assemblages and Metamorphic grade and facies of

metamorphic rocks. Graphic representation of metamorphic mineral

parageneses.

Metamorphic Rocks

• Recrystallization in the solid state• Caused by changes in T, P or fluids• New environment = new minerals• Growing minerals create a new texture

Meta"= Change (Grk) "Morph"= form (Grk)

a rock that has been changed from its original form (parent ) by Heat, pressure , and fluid activity into a new rock (daughter ).

Sources of metamorphic change

METAMORPHISM

Metamorphism is the process of mineralogical and structural changes of rocks in their solid state in response to physical and chemical conditions which differ from the conditions prevailing during the formation of the rocks, however, the changes occurring within the domains of weathering and diagenesis are commonly excluded.

METAMORPHISM

• Features of Metamorphism – It principally formed in solid state and before melting,– Metamorphism can be considered to be isochemical, except

perhaps for removal or addition of volatiles (H2O, CO2),– The process of extensive chemical changes during

transformation is known as metasomatism.• Metamorphism characterized by

– Growth of new minerals from pre-existing minerals through recrystallization

– Deformation of existing minerals• Change in shape• Change in orientation

• Growth of new minerals

Mica garnet schist

Garnet crystal

• Deformation of rocks and minerals

Layers are folded and broken

• Recrystallization– T, P or change in pore fluids initiate

change in the mineral assemblage– Reaction occur entirely in the solid

state

Shale Schist

• Textural changes occur during metamorphism–New minerals grow during

metamorphism–Precursor rock textures are

modified

Protolith

The parent rock subjected to Metamorphism

-can be any rock type: igneous, sedimentary, or metamorphic

Shale Schist

Protolith

Limits of metamorphism• The lower temperature limit, at metamorphism takes place, marks the

boundary between metamorphism and diagenesis.• The temperature lower limit of metamorphism is considered to be between

100 - 200 ゜ C for the more unstable types of protolith, to exclude diagenetic changes, due to compaction, which result in sedimentary rocks. There is no agreement as for a pressure lower limit. Some workers argue that changes in atmospheric pressures are not metamorphic, but some types of metamorphism can occur at extremely low pressures.

• The upper boundary of metamorphic conditions is related to the onset of melting processes in the rock. The maximum temperature for metamorphism is typically between 700 - 900 ゜ C, depending on the pressure and on the composition of the rock. Migmatites are rocks formed at this upper limit, which contain pods and veins of material that has started to melt but has not fully segregated from the refractory residue. Since the 1980s, it has been recognized that rarely, rocks are dry enough, and of a refractory enough composition, to record without melting "ultra-high" metamorphic temperatures of 900 - 1100°C. Once melting begins, the process changes to an igneous precess rather than a metamorphic process.

• Migmatites (“mixed rocks”) are gradational

• Migmatites: metamorphic or igneous rocks?

Origin of Metamorphic Rocks

• Metamorphism Temperature exceeds 200OC

Pressure exceeds 300 M Pa (approx. 10,000 ft deep)

• Metamorphism ends when melting begins Melting begins at ~700oC

Stability ranges for Al2 SiO5 minerals

TYPES OF METAMORPHIC AGENTS AND LIMITS OF METAMORPHISM

TYPES OF METAMORPHIC AGENTS :

Temperature Pressure Chemical active fluids

1- Temperatures: Typically the most important factor in metamorphism (leads to Recrystallization results in new, stable minerals, increase

in grain size, increases the rate of reactions) - Limits of temperatures lower limit (150±50 °C)

higher limit (beginning of melting, 650-1100 °C)- Low limit depend on the original protolith

lower T (shale, organic matters) higher T (Igneous rocks and carbonates)

- Beginning of melting depend on: protolith composition

the presence of aqueous fluids

Example: - At 5 kbar and presence of aqueous fluid - granites begin to melt at ~ 660 °C

- basalts begin to melt at ~800 °C - At 5 kbar and dry conditions - granites begin to melt at ~ 1000 °C

- basalts begin to melt at ~1120 °C

Source of Temperature for metamorphism:- heat flowing into the base of the crust from the mantle- heat brought into the crust by rising magma bodies - heat generated from radioactive decay- the effect of rapid uplift and erosion - heat related to burial effect and geothermal gradient

Geothermal gradient: (rate of increasing temperature with depth, mean = 25 °C/km) - Subduction zone (10 °C/km) - Precambrian Shields (12-20 °C/km) - Collisionl orogens (25-30 °C/km) - Active arc-margin (30-35 °C/km) - Extensional orogens (40-50 °C/km) - Mid-ocean ridges (~ 60 °C/km)

Heat as a Metamorphic AgentGeothermal Gradient

• Source of heatProximity to igneous intrusions

• Contact metamorphism• Different metamorphic grade ring

the intrusionDepth of burial

• 15-30oC increase per km• Geothermal Gradient

Magma Body-source of heat

Zone of ContactMetamorphism

2- Pressures: (leads to reducing grain size and deformation) -

--- Pressure is define as force/unit area

- Unit of pressure (bar, kbar), 1 bar = 0.987 atmosphere = 14.5 pound/inch2

- pressures types confining pressure or lithostatic pressure (Plith)

directive or deviatoric pressure

fluid pressure (Pfluid)

effective pressure (Pe)

Pe = Plith – Pfluid

Pressures:

- Limits of pressure

lower limit (a few of bars, at Earth’s surface)

Higher limits (30-40 in the collisional orogen or up to 100 kbar

in the ultrahigh pressure metamorphism)

- Source of pressure

burial influence of an overlying rock column

Plate tectonic and movement of plate segments

- Geobaric gradient (change of pressure with depth )

average = 0.285 kbar/km or ~1kbar/3km

Pressure and Stress

Lithostatic Pressure- pressure increases with depth due to the weight of the overlying rocks.

-equal in all directions: uniform stress

-a cubic foot of granite weighs 167.9 pounds.

Lithostatic Pressure - equal from all directions

Tectonic Pressure/Stress-directional/non-uniform -referred to as deviatoric stress

Folded Metamorphic Rocks-subjected to non-uniform deviatoric stress

Pressure and fabric changes

►Lithostatic pressure = uniform stress (hydrostatic)

► Deviatoric stress = unequal pressure in different directions.

Deviatoric stress can be resolved into three mutually

perpendicular stress () components:

i) 1 is the maximum principal stress

ii) 2 is an intermediate principal stress

iii) 3 is the minimum principal stress

In hydrostatic situations all three are equal

Pressure and fabric changes, Cont.

► Stress is an applied force acting on a rock (over a particular

cross-sectional area)

► Strain is the response of the rock to an applied stress (=

yielding or deformation)

► Deviatoric stress affects the textures and structures, but not

the equilibrium mineral assemblage

Deviatoric stresses come in three principal types: – Tension– Compression– Shear

Tension: 3 is negative, and the resulting strain is extension, or

pulling apart. Tension fractures may open normal to the extension

direction and become filled with mineral precipitates.

original shape strain ellipsoid

s1

s3

Compression: 1 is dominant; therefore, folding or more

homogenous flattening are caused.

s1

s3

Shear motion occurs along planes at an angle to 1 and causing

slip along parallel planes and rotation.

s1

Foliation is a common result, which allows us to estimate the orientation of 1

- 1 > 2 = 3 foliation and no lineation

- 1 = 2 > s3 lineation and no foliation

- 1 > 2 > 3 both foliation and lineation

s1

3- Metamorphic fluids (leads to chemical changes)

mostley are H2O and CO2 types

- include Ascending fluids from Magma chamber Descending fluids of the meteoric water

- Proofs of importance of fluids in metamorphism

most metamorphic minerals are hydrous, so water should

be present

most of metamorphic reactions involves dehydration or

decarbonation

ms + chl bt + grt + qtz + H2O

CaCO3 + SiO2 CaSiO3 + CO2 Fluids could preserved as inclusion

in neoblasts in metamorphic rocks.

• T & P determine degree of metamorphism

Low-grade metamorphism- 200 to 350 OC Intermediate-grade metamorphism-350

to 550 OC High-grade metamorphism - very high

temperatures, above 550OC

Metamorphic Processes• Recrystallization: The most common process in the

formation of metamorphic rocks is recrystallization. In this process the grains making up the protolith change shape and size. The identity of the mineral does not change during this process, only the texture. Recrystallization occurs due to heating of the protolith. The temperature at which this occurs can vary depending on the minerals present. For example, the sedimentary rock, limestone. Limestone is composed of microscopic crystals of calcite that once formed the shells of marine creatures. When metamorphosed, those tiny crystals are begin to form larger, more visible crystals of calcite found in the metamorphic rock marble.

• Phase Change: Phase change metamorphism is the creating of new minerals with the same chemical formula as the protolith. This involves a rearrangement of the atoms in the crystals.

• Neocrystallization: Neocrystallization involves the creation of new mineral crystals different from the protolith. Chemical reactions digest the minerals of the protolith which yields new minerals. This is a very slow process as it can also involve the diffusion of atoms through solid crystals. This is most often observed with the sedimentary rock shale, which is composed of fine grained quartz, feldspars, and other clay minerals. When metamorphosed, these minerals grains will change to muscovite mica and garnet.

• Pressure Solution: Pressure solution is a metamorphic process that requires a rock to be under strong pressure from one direction and in the presence of hot water. During this process minerals of the protolith partially dissolve, diffuse through the water and precipitate elsewhere.

• Plastic Deformation: In plastic deformation pressure is applied to the protolith, which causes it to shear or bend, but not break. In order for this to happen temperatures must be high enough that brittle fractures do not occur, but not so high that diffusion of crystals takes place.

• Metasomatism: The process results in the loss or

addition of chemicals. This can best be seen in the production of anthracite coal, which is almost pure carbon. The parent rock bituminous coal is produced from the aggregation of dead plant material, and anthracite is produced by the loss of the more volatile materials such as nitrogen, oxygen, and methane.

The Types of MetamorphismDifferent approaches to classification

1. Based on principal process or agent• Dynamic Metamorphism• Thermal Metamorphism• Dynamo-thermal Metamorphism

2. Based on setting• Contact (Thermal) Metamorphism

Pyrometamorphism• Regional Metamorphism

Orogenic Metamorphism Burial Metamorphism Ocean Floor Metamorphism

• Hydrothermal Metamorphism• Fault-Zone Metamorphism • Impact or Shock Metamorphism

We’ll look at the classifivation based on setting

Contact metamorphic rocks form under conditions of high temperature and low pressure. They are arrayed in aureoles, or metamorphosed zones, around plutons and other intrusive igneous bodies.

Contact (thermal) Metamorphism

Contact metamorphism occurs typically around intrusive igneous rocks as a result of the temperature increase caused by the intrusion of magma into cooler country rock. The area surrounding the intrusion where the contact metamorphism effects are present is called the metamorphic aureole. Contact metamorphic rocks are usually known as hornfels. Rocks formed by contact metamorphism may not present signs of strong deformation and are often fine-grained.

The size and shape of an aureole is controlled by:

1- The nature of the pluton• Size• Shape• Orientation• Temperature• Composition

2- The nature of the country rocks• Composition• Depth and metamorphic grade prior to intrusion• Permeability

Dikes generally have small aureoles with minimal metamorphism whereas large ultramafic intrusions can have significantly thick and well-developed contact metamorphism.

Contact metamorphism produces non-foliated (rocks without any cleavage) rocks such as marble, quartzite, and hornfels.

The metamorphic grade of an aureole is measured by the peak metamorphic mineral which forms in the aureole. This is usually related to the metamorphic temperatures of pelitic or alumonisilicate rocks and the minerals they form.The metamorphic grades of aureoles are andalusite hornfels, sillimanite hornfels, pyroxene hornfels.

Magmatic fluids coming from the intrusive rock may also take part in the metamorphic reactions. Extensive addition of magmatic fluids can significantly modify the chemistry of the affected rocks. In this case the metamorphism grades into metasomatism. If the intruded rock is rich in carbonate the result is a skarn. Fluorine-rich magmatic waters which leave a cooling granite may often form greisens within and adjacent to the contact of the granite. Metasomatic altered aureoles can localize the deposition of metallic ore minerals and thus are of economic interest.

Contact Metamorphism• Adjacent to igneous intrusions

• Thermal (± metasomatic) effects of hot magma intruding cooler shallow rocks

• Occurs over a wide range of pressures, including very low

• Contact aureole

Metamorphic EnvironmentsContact Metamorphism

Regional Metamorphism

Regional Metamorphism : metamorphism that affects a large body of rock, and thus covers a great lateral extent

Three principal types: Orogenic metamorphism Burial metamorphism Ocean-floor metamorphism

• Orogenic metamorphism: is the type of metamorphism associated with convergent plate margins

Regional or Barrovian metamorphism covers large areas of continental crust typically associated with mountain ranges, particularly subduction zones or the roots of previously eroded mountains. Conditions producing widespread regionally metamorphosed rocks occur during an orogenic event. The collision of two continental plates or island arcs with continental plates produce the extreme compressional forces required for the metamorphic changes typical of regional metamorphism.

These orogenic mountains are later eroded, exposing the intensely deformed rocks typical of their cores. The conditions within the subducting slab as it plunges toward the mantle in a subduction zone also produce regional metamorphic effects. The techniques of structural geology are used to unravel the collisional history and determine the forces involved. Regional metamorphism can be described and classified into metamorphic facies or metamorphic zones of temperature/pressure conditions throughout the orogenic terrane. This metamorphism usually produces rocks such as gneiss and schist.

Regional Metamorphism

Burial metamorphism : for low-grade metamorphism in sedimentary basins due to burial. A term coined by Coombs (1961) for low-grade metamorphism that occurs in sedimentary basins due to burial by successive layers.

• Mild deformation, no igneous intrusions discovered

• Metamorphic effects attributed to increased temperature and pressure due to burial

• The main minerals produced are often Zeolites. Burial metamorphism overlaps, to some extent, with diagenesis, and grades into regional metamorphism as temperature and pressure increase.

• The Gulf of Mexico may represent a modern example• Areas of burial metamorphism may thus become areas of

orogenic metamorphism

Ocean-Floor Metamorphism: affects the oceanic crust at ocean ridge spreading centers. Variety of metamorphic minerals in ocean-floor rocks, representing a wide range of temperatures at relatively low pressure. Alteration concentrated along vein systems, presumably associated with hydrothermal activity. Seawater penetrates down ubiquitous fracture systems, where it becomes heated, and leaches metals and silica from the hot basalts. Considerable metasomatic alteration, notably loss of Ca and Si and gain of Mg and Na.

• Highly altered chlorite-quartz rocks- distinctive high-Mg, low-Ca composition

• Exchange between basalt and hot seawater

• Another example of hydrothermal metamorphism

•Hydrothermal metamorphismHydrothermal metamorphism is the result of the

interaction of a rock with a high-temperature fluid of variable composition. The difference in composition between existing rock and the invading fluid triggers a set of metamorphic and metasomatic reactions. The hydrothermal fluid may be magmatic (originate in an intruding magma), circulating groundwater, or ocean water. Convective circulation of hydrothermal fluids in the ocean floor basalts produces extensive hydrothermal metamorphism adjacent to spreading centers and other submarine volcanic areas. The fluids eventually escape through vents in the ocean floor known as black smokers. The patterns of this hydrothermal alteration is used as a guide in the search for deposits of valuable metal ores.

Metamorphic EnvironmentsHydrothermal Metamorphism

•Shock metamorphism

This kind of metamorphism occurs when either an extraterrestrial object (a meteorite for instance) collides with the Earth's surface or during an extremely violent volcanic eruption. Impact metamorphism is, therefore, characterized by ultrahigh pressure conditions and low temperature. The resulting minerals (such as SiO2 polymorphs coesite and stishovite) and textures are characteristic of these conditions.

Metamorphic EnvironmentsImpact Metamorphism

•Dynamic metamorphismDynamic metamorphism is associated with zones of high to moderate

strain such as fault zones. Cataclasis, crushing and grinding of rocks into angular fragments, occurs in dynamic metamorphic zones, giving cataclastic texture.

The textures of dynamic metamorphic zones are dependent on the depth at which they were formed, as the temperature and confining pressure determine the deformation mechanisms which predominate. Within depths less than 5 km, dynamic metamorphism is not often produced because the confining pressure is too low to produce frictional heat. Instead, a zone of breccia or cataclasite is formed, with the rock milled and broken into random fragments. This generally forms a mélange. At depth, the angular breccias transit into a ductile shear texture and into mylonite zones.

Dynamic metamorphism:Rock deformation associated with fault zones

Mylonite from shear zone

Within the depth range of 5–10 km pseudotachylite is formed, as the confining pressure is enough to prevent brecciation and milling and thus energy is focused into discrete fault planes. Frictional heating in this case may melt the rock to form pseudotachylite glass.

Within the depth range of 10–20 km, deformation is governed by ductile deformation conditions and hence frictional heating is dispersed throughout shear zones, resulting in a weaker thermal imprint and distributed deformation. Here, deformation forms mylonite, with dynamothermal metamorphism observed rarely as the growth of porphyroblasts in mylonite zones.

Overthrusting may juxtapose hot lower crustal rocks against cooler mid and upper crust blocks, resulting in conductive heat transfer and localised contact metamorphism of the cooler blocks adjacent to the hotter blocks, and often retrograde metamorphism in the hotter blocks. The metamorphic assemblages in this case are diagnostic of the depth and temperature and the throw of the fault and can also be dated to give an age of the thrusting.

CLASSIFICATION OF METAMORPHIC ROCKS

Classification of metamorphic rocks is based on mineral assemblage, texture, protolith, and bulk chemical composition of the rock.

•TextureIn metamorphic rocks individual minerals may or

may not be bounded by crystal faces. Those that are bounded by their own crystal faces are termed idioblastic. Those that show none of their own crystal faces are termed xenoblastic. From examination of metamorphic rocks, it has been found that metamorphic minerals can be listed in a generalized sequence in order of their tendency to be idioblastic known as the crystalloblastic series.

• Crystalloblastic series In the series, each mineral tends to develop

idioblastic surfaces against any mineral that occurs lower in the series. This series is listed below:

– rutile, sphene, magnetite – tourmaline, kyanite, staurolite, garnet, andalusite – epidote, zoisite, lawsonite, forsterite – pyroxenes, amphiboles, wollastonite – micas, chlorites, talc, stilpnomelane, prehnite – dolomite, calcite – scapolite, cordierite, feldspars – Quartz

This series can, in a rather general way, enable us to determine the origin of a given rock.

For example a rock that shows euhedral plagioclase crystals in contact with anhedral amphibole, likely had an igneous protolith, since a metamorphic rock with the same minerals would be expected to show euhedral amphibole in contact with anhedral plagioclase.

Another aspect of the crystalloblastic series is that minerals high on the list tend to form porphyroblasts (the metamorphic equivalent of phenocrysts), although K-feldspar (a mineral that occurs lower in the list) may also form porphyroblasts. Porphyroblasts are often riddled with inclusions of other minerals that were enveloped during growth of the porphyroblast. These are said to have a poikioblastic texture.

Most metamorphic textures involve foliation. Foliation is generally caused by a preferred orientation of sheet silicates. If a rock has a slatey cleavage as its foliation, it is termed a slate, if it has a phyllitic foliation, it is termed a phyllite, if it has a schistose foliation, it is termed a schist. A rock that shows a banded texture without a distinct foliation is termed a gneiss. All of these could be porphyroblastic (i.e. could contain porhyroblasts).

A rock that shows no foliation is called a hornfels if the grain size is small, and a granulite, if the grain size is large and individual minerals can be easily distinguished with a hand lens.

• ProtolithProtolith refers to the original rock, prior to metamorphism. In low grade metamorphic rocks, original textures are often preserved allowing one to determine the likely protolith.  As the grade of metamorphism increases, original textures are replaced with metamorphic textures and other clues, such as bulk chemical composition of the rock, are used to determine the protolith.

• Bulk Chemical CompositionThe mineral assemblage that develops in a metamorphic rock is dependent on – The pressure and temperature reached during metamorphism– The composition of any fluid phase present during metamorphism,

and– The bulk chemical composition of the rock.

Just like in igneous rocks, minerals can only form if the necessary chemical constituents are present in the rock (i.e. the concept of silica saturation and alumina saturation applies to metamorphic rocks as well).  Based on the mineral assemblage present in the rock one can often estimate the approximate bulk chemical composition of the rock.  Some terms that describe this general bulk chemical composition are as follows:Pelitic.  These rocks are derivatives of aluminous sedimentary rocks like shales and mudrocks.  Because of their high concentrations of alumina they are recognized by an abundance of aluminous minerals, like clay minerals, micas, kyanite, sillimanite, andalusite, and garnet.Quartzo-Feldspathic.  Rocks that originally contained mostly quartz and feldspar like granitic rocks and arkosic sandstones will also contain an abundance of quartz and feldspar as metamorphic rocks, since these minerals are stable over a wide range of temperature and pressure.  Those that exhibit mostly quartz and feldspar with only minor amounts of aluminous minerals are termed quartzo-feldspathic.

• Calcareous.  Calcareous rocks are calcium rich.   They are usually derivatives of carbonate rocks.  At low grades of metamorphism calcareous rocks are recognized by their abundance of carbonate minerals like calcite and dolomite.   With increasing grade of metamorphism these are replaced by minerals like brucite, phlogopite (Mg-rich biotite), chlorite, and tremolite.  At even higher grades anhydrous minerals like diopside, forsterite, wollastonite, grossularite, and calcic plagioclase.

• Basic.  The general term basic refers to low silica content.  Basic metamorphic rocks are generally derivatives of basic igneous rocks like basalts and gabbros.  They have an abundance of Fe-Mg minerals like biotite, chlorite, and hornblende, as well as calcic minerals like plagioclase and epidote.

• Magnesian. Rocks that are rich in Mg with relatively less Fe, are termed magnesian.  Such rocks would contain Mg-rich minerals like serpentine, brucite, talc, dolomite, and tremolite.  In general, such rocks usually have an ultrabasic protolith, like peridotite, dunite, or pyroxenite.

• Ferriginous. Rocks that are rich in Fe with little Mg are termed ferriginous.  Such rocks could be derivatives of Fe-rich cherts or ironstones. They are characterized by an abundance of Fe-rich minerals like greenalite (Fe-rich serpentine), minnesotaite (Fe-rich talc), ferroactinolite, ferrocummingtonite, hematite, and magnetite at low grades, and ferrosilite, fayalite, ferrohedenbergite, and almandine garnet at higher grades.

• Manganiferrous. Rocks that are characterized by the presence of Mn-rich minerals are termed manganiferrous.  They are characterized by such minerals as Stilpnomelane and spessartine.

• ClassificationThree kinds of criteria are normally employed. These are:

1. Mineralogical - The most distinguishing minerals are used as a prefix to a textural term. Thus, a schist containing biotite, garnet, quartz, and feldspar, would be called a biotite-garnet schist.

2. Chemical - If the general chemical composition can be determined from the mineral assemblage, then a chemical name can be employed. For example a schist with a lot of quartz and feldspar and some garnet and muscovite would be called a garnet-muscovite quartzo-feldspathic schist.

3. Protolithic -  If a rock has undergone only slight metamorphism such that its original texture can still be observed then the rock is given a name based on its original name, with the prefix meta- applied. For example: metabasalt, metagraywacke, meta-andesite.

In addition to these conventions, certain non-foliated rocks with specific chemical compositions and/or mineral assemblages are given specific names. These are as follows: • Amphibolites: These are medium to coarse grained, dark colored rocks whose principal minerals are hornblende and plagioclase. They result from metamorphism of basic igneous rocks.  Foliation is highly variable, but when present the term schist can be appended to the name (i.e. amphibolite schist).• Marbles: These are rocks composed mostly of calcite, and less commonly of dolomite. They result from metamorphism of limestones and dolostones.  Some foliation may be present if the marble contains micas.• Eclogites: These are medium to coarse grained consisting mostly of garnet and green clinopyroxene called omphacite, that result from high grade metamorphism of basic igneous rocks. Eclogites usually do not show foliation.

• Quartzites: Quartz arenites and chert both are composed mostly of SiO2.  Since quartz is stable over a wide range of pressures and temperatures, metamorphism of quartz arenites and cherts will result only in the recrystallization of quartz forming a hard rock with interlocking crystals of quartz.   Such a rock is called a quartzite.

• Serpentinites:  Serpentinites are rocks that consist mostly of serpentine.  These form by hydrothermal metamorphism of ultrabasic igneous rocks.

• Soapstones: Soapstones are rocks that contain an abundance of talc, which gives the rock a greasy feel, similar to that of soap.   Talc is an Mg-rich mineral, and thus soapstones from ultrabasic igneous protoliths, like peridotites, dunites, and pyroxenites, usually by hydrothermal alteration.

• Skarns: Skarns are rocks that originate from contact metamorphism of limestones or dolostones, and show evidence of having exchanged constituents with the intruding magma.  Thus, skarns are generally composed of minerals like calcite and dolomite, from the original carbonate rock, but contain abundant calcium and magnesium silicate minerals like andradite, grossularite, epidote, vesuvianite, diopside, and wollastonite that form by reaction of the original carbonate minerals with silica from the magma.  The chemical exchange is that takes place   is called metasomatism.

• Mylonites: Mylonites are cataclastic metamorphic rocks that are produced along shear zones deep in the crust.  They are usually fine-grained, sometimes glassy, that are streaky or layered, with the layers and streaks having been drawn out by ductile shear.

Metamorphic Rocks

• Classified by texture and composition–Texture is divided into foliated &

nonfoliated rocks–Mineral composition qualifies

textural name

Foliated Nonfoliated textures

Nonfoliated Rocks• Marble

– Interlocking, coarse grained calcite

–Recrystallization of limestone or dolostone

– Sedimentary features – destroyed

• Quartzite–Metamorphism of quartz sandstone–Entire rock is recrystallized

Metamorphic Rock Textures Metamorphic rocks exhibit a variety of textures.  These

can range from textures similar to the original protolith at low grades of metamorphism, to textures that are purely produced during metamorphism and leave the rock with little resemblance to the original protolith. 

FoliationFoliation is defined as a pervasive planar structure that results from the nearly parallel alignment of sheet silicate minerals and/or compositional and mineralogical layering in the rock. Most foliation is caused by the preferred orientation of phylosilicates, like clay minerals, micas, and chlorite.  Preferred orientation develops as a result of non-hydrostatic or differential stress acting on the rock (also called deviatoric stress). 

Foliation:-preferred orientation or location of minerals

Stress and Preferred OrientationPressure is defined as a force acting equally from

all directions.  It is a type of stress, called hydrostatic stress or uniform stress.  If the stress is not equal from all directions, then the stress is called a differential stress.  Normally geologists talk about stress as compressional stress.  Thus, if a differential stress is acting on the rock, the direction along which the maximum principal stress acts is called σ1, the intermediate principal stress direction is called σ2 and

the minimum principal stress is called σ3.   Note that extensional stress would act along the direction of minimum principal stress. 

• If differential stress is present during metamorphism, it can

have a profound effect on the texture of the rock. 

Minerals that crystallize or grow in the differential stress field may develop a preferred orientation. Sheet silicates and minerals that have an elongated habit will grow with their sheets or direction of elongation orientated perpendicular to the direction of maximum stress.  

Rounded grains can become flattened in the direction of maximum compressional stress.

• Example - metamorphism of a shale (made up initially of clay minerals and quartz)

• Shales have fissility that is caused by the preferred orientation of clay minerals with their {001} planes orientated parallel to bedding. Metamorphic petrologists and structural geologists refer to the original bedding surface as S0.

• Slate  Slates form at low metamorphic grade by the growth of fine grained chlorite and clay minerals. The preferred orientation of these sheet silicates causes the rock to easily break planes parallel to the sheet silicates, causing a

slatey cleavage.

The foliation or surface

produced by this deformation

is referred to S1.

• Phyllite– Similar to slate– More Recrystallization

• Crystals are larger– Common Protolith: – shales

Slate

• Schist - The size of the mineral grains tends to enlarge with increasing grade of metamorphism.  Eventually the rock develops a near planar foliation caused by the preferred orientation of sheet silicates (mainly biotite and muscovite).  Quartz and feldspar grains, however show no preferred orientation.  The irregular planar foliation at this stage is called schistosity

• Schist–Strongly foliated rock–Medium to coarse

grained-dominated by platy minerals

–Protoliths: multiple??

[Garnet] Schist w/porphyroblasts

• Gneiss  As metamorphic grade increases, the sheet silicates become unstable and dark colored minerals like hornblende and pyroxene start to grow. These dark colored minerals tend to become segregated into distinct bands through the rock (this process is called metamorphic differentiation), giving the rock a gneissic banding.  Because the dark colored minerals tend to form elongated crystals,  rather than sheet- like crystals, they still have a preferred orientation with their long directions perpendicular to the maximum differential stress.

• Gneiss–Coarse grained granular rock–Foliation: alternating bands of

light & dark minerals• Quartz, feldspar, amphibole, • biotite

–Protoliths: ??

• Granulite - At the highest grades of metamorphism most of the hydrous minerals and sheet silicates become unstable and thus there are few minerals present that would show a preferred orientation.  The resulting rock will have a granulitic texture that is similar to a phaneritic texture in igneous rocks.

Metamorphism and Deformation

The result of compressional stress acting on rocks that behave in a ductile manner (ductile behavior is favored by higher temperature, higher confining stress [pressure] and low strain rates) is the folding of rocks.  Original bedding is folded into a series of anticlines and synclines with fold axes perpendicular to the direction of maximum compressional stress. Note that since the axial planes are oriented perpendicular to the maximum compressional stress direction, slatey cleavage or foliation should also develop along these directions.   Thus, slatey cleavage or foliation is often seen to be parallel to the axial planes of folds, and is sometimes referred to axial plane cleavage or foliation.

Metamorphic Differentiation

• Metamorphic differentiation is alternating somewhat discontinuous bands or layers of dark colored ferromagnesian minerals and lighter colored quartzo-feldspathic layers as in gneisses, and to some extent schists. 

Throughout the history of metamorphic petrology, several mechanisms have been proposed to explain metamorphic differentiation.1- Preservation of Original Compositional Layering.  In some rocks the compositional layering may not represent metamorphic differentiation at all, but instead could simply be the result of original bedding.  For example, during the early stages of metamorphism and deformation of interbedded sandstones and shales the compositional layering could be preserved even if the maximum compressional stress direction were at an angle to the original bedding.

• In such a case, a foliation might develop in the shale layers due to the recrystallization of clay minerals or the crystallization of other sheet silicates with a preferred orientation controlled by the maximum stress direction.

Here, it would be easy to determine that the compositional layers represented original bedding because the foliation would cut across the compositional layering.

• In highly deformed rocks that have undergone both folding and shearing, it may be more difficult to determine that the compositional layering represents original bedding. As shearing stretches the bedding, individual folded beds may be stretched out and broken to that the original folds are not easily seen.

2- Transposition of Original Bedding.  Original compositional layering a rock could also become transposed to a new orientation during metamorphism.  The diagram below shows how this could occur.  In the initial stages a new foliation begins to develop in the rock as a result of compressional stress at some angle to the original bedding.  As the minerals that form this foliation grow, they begin to break up the original beds into small pods.  As the pods are compressed and extended, partly by recrystallization, they could eventually intersect again to form new compositional bands parallel to the new foliation.

3- Solution and Re-precipitation. In fine grained metamorphic rocks small scale folds, called kink bands, often develop in the rock as the result of application of compressional stress. A new foliation begins to develop along the axial planes of the folds.  Quartz and feldspar may dissolve as a result of pressure solution and be reprecipitated at the hinges of the folds where the pressure is lower.  As the new foliation begins to align itself perpendicular to s1, the end result would be alternating bands of micas or sheet silicates and quartz or feldspar, with layering parallel to the new foliation.

4- Preferential Nucleation.  Fluids present during metamorphism have the ability to dissolve minerals and transport ions from one place in the rock to another.

Thus felsic minerals could be dissolved from one part of the rock and preferentially nucleate and grow in another part of the rock to produce discontinuous layers of

alternating mafic and felsic compositions. 

5- Migmatization.  As discussed previously, migmatites are small pods and lenses that occur in high grade metamorphic terranes that may represent melts of the surrounding metamorphic rocks.  Injection of the these melts into pods and layers in the rock could also produce the discontinuous banding often seen in high grade metamorphic rocks.  The process would be similar to that described in 4, above, except that it would involve partially melting the original rock to produce a felsic melt, which would then migrate and crystallize in pods and layers in the metamorphic rock.  Further deformation of the rock could then stretch and fold such layers so that they may no longer by recognizable as migmatites. 

• Amphibolites & Eclogites– Metamorphism of mafic rocks– Mineral assemblage gives green color

Eclogite from Norway

Amphibolite [pillow basalt]

Progressive metamorphism of shale

Fig. 6.5. Stability ranges for Al2O5 minerals

Index Minerals

METAMORPHIC GRADE• One of the primary goals of metamorphic petrology is to

interpret P-T conditions under which a rock (or set of rocks) formed

• Metamorphic grade– Relative temperature and pressure conditions under which metamorphic

rocks form • Low-grade metamorphism

– T ~200 to 320ºC, relatively low pressure

– abundant hydrous minerals • High-grade metamorphism

– T >320ºC and relatively high pressure

– Dehydration; less hydrous minerals common

• Prograde metamorphism – T and/or P, grade of

metamorphism increases • Retrograde metamorphism

– T and/or P, grade of metamorphism decreases 

Metamorphic Grade

Migmatite

Metamorphic Grade

The sequence of zones (Barrovian Zones) now recognized, and the typical metamorphic mineral assemblage in each, are:

• Chlorite zone. Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albite

• Biotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite

• Garnet zone. Schists with conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase

• Staurolite zone. Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persist

• Kyanite zone. Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and staurolite

• Sillimanite zone. Schists and gneisses with sillimanite, biotite, muscovite, quartz, plagioclase, garnet, and perhaps staurolite. Some kyanite may also be present (although kyanite and sillimanite are both polymorphs of Al2SiO5)

Regional Metamorphic Zones

• Index minerals–A mineral that forms within a

specific,often narrow range of conditions

– Identifies a specific grade of metamorphism

–Allows further subdivision of rock types

Metamorphic Index Minerals

• Protolith controls final mineral assemblage– Different mineral assemblages form

at a given P & T for various parent rocks

– Metamorphic facies describe a group of minerals formed under similar conditions• May be found in different rocks

MINERAL ASSEMBLAGES AND METAMORPHIC FACIES

The mineral assemblages that occur in metamorphic rocks depend on four factors: 1. The bulk chemical composition of the original rock.2. The pressure reached during metamorphism.3. The temperature reached during metamorphism.4. The composition of any fluid phase that was present during metamorphism.

If a rock is taken to some higher pressure and temperature then the mineral assemblage that develops should represent stable chemical equilibrium if the conditions are held for a long enough period of time that equilibrium can be achieved.  Since metamorphism usually involves long periods of geologic time, most metamorphic rocks represent an equilibrium mineral assemblage.

METAMORPHIC FACIESIn general, metamorphic rocks do not drastically

change chemical composition during metamorphism, except in the special case where metasomatism is involved. The changes in mineral assemblages are due to changes in the temperature and pressure conditions of metamorphism.  Thus, the mineral assemblages that are observed must be an indication of the temperature and pressure environment that the rock was subjected to.  This pressure and temperature environment is referred to as Metamorphic Facies (The metamorphic facies are groups of mineral compositions in metamorphic rock that are typical for a certain field in pressure –temperature). (This is similar to the concept of  sedimentary facies).  The sequence of metamorphic facies observed in any metamorphic terrain, depends on the geothermal gradient that was present during metamorphism.

A high geothermal gradient such as the one labeled "A" , might be present around an igneous intrusion, and would result in metamorphic rocks belonging to the hornfels facies.  Under a normal to high geothermal gradient, such as "B", rocks would progress from zeolite facies to greenschist, amphibolite, and eclogite facies as the grade of metamorphism (or depth of burial) increased. 

Thus, if we know the facies of metamorphic rocks in the region, we can determine what the geothermal gradient must have been like at the time the metamorphism occurred.  

If a low geothermal gradient was present, such the one labeled "C" in the diagram, then rocks would progress from zeolite facies to blueschist facies to eclogite facies. 

Metamorphic Facies and Mineral Assemblages

Contact and Regional Metamorphism

Metamorphic Facies

• = set of rocks characterized by equilibrium mineral assemblage that reflect specific range of metamorphic (T-P) conditions

• Mineral assemblage present depends on protolith composition and P-T conditions– Ex: marble, metabasalt and schist all in amphibolite facies

Metamorphic Series

• Progression or sequence of facies across a metamorphic terrain

• Identified by field and experimental work

1. Contact Series – Hi T, low P = contact metamorphism

2. Dynamic Regional Series – Mod T & P = Barrovian

3. Static Regional Series – Lo T, hi P = Blueschist

1.

2.

3.

RegionalMetamorphicGradients

Metamorphic facies

Stable Mineral Assemblages in Metamorphic Rocks

• Equilibrium Mineral Assemblages• At equilibrium, the mineralogy (and the

composition of each mineral) is determined by T, P, and X

• “Mineral paragenesis” refers to such an equilibrium mineral assemblage

• Relict minerals or later alteration products are excluded unless specifically stated

THE PHASE RULE FOR METAMORPHISM

Phase rule, as applied to systems at equilibrium:

F = C + 2 – P where

F = the variance of the system or number of degrees of freedom (the number of independently variable intensive parameters of state (such as temperature, pressure, the composition of each phase, etc.)

C = the number of components in the system (the minimum number of chemical constituents required to specify every phase in the system) P = the number of phases present in the system.

• So, for F=2, C=P, the number of phases present in a rock for the more common divariant assemblage will be equal to the number of components.  If P is greater than C, then one of three possibilities exist for the mineral assemblage:

1. The assemblage represents a non-equilibrium assemblage (perhaps due to incomplete chemical reactions or due to the presence of retrograde minerals that developed during cooling, uplift, or unroofing of the metamorphic rock).

2. The assemblage represents univariant (F=1) or invariant (F=0) equilibrium.

3. The number of components have not been chosen correctly.

• The number of components, as stated in the phase rule, must be chosen so as to represent the minimum number necessary to form all phases possible in the rock. Recall that the number of components is not strictly the number of oxide components or the number of elements as reported in a chemical analysis of the rock.  If we just consider the major phases that make up metamorphic rocks and consider that some ions freely substitute for one another in solid solutions, then the number of components can often be reduced to 7 or 8.  For example:

1. K2O.Al2O3 (based on the ratio of K to Al in the alkali feldspars)

2. (Ca,Na2)O.Al2O3 (based on the ratio of Ca and Na to Al in the plagioclase)

3. (Si,Ti)O2 -  based on the common substitution of Ti into tetrahedral sites in most silicates)

4. (Fe,Mn)O - based on the common substitution of Mn for Fe in minerals.

5. MgO - usually needed because Fe-Mg solid solution compositions are both temperature and pressure dependent.  (although sometimes these two are combined, which would reduce the total number of components by 1).

6. (Al,Fe+3) 2O3 - based on the commonly observed substitution of Fe+3 for Al+3Al in minerals.

7. H2O - usually present in a fluid phase, but also an important component of hydrous minerals.

8. CO2 - also usually present in a fluid phase, but also an important component in carbonate minerals.

• If  H2O and CO2 are assumed to be always present and available to form hydrous and carbonate minerals, then the number of components can be reduced to 5 or 6.  Thus for a divariant assemblage (F=2) we would expect to find 5 or 6 different mineral phases present in a metamorphic rock, or up to 8 phases if the assemblage is invariant. 

• This is the basis for the construction of the AKF and ACF diagrams, where the number of components have been reduced to 4

TRIANGULAR PLOTS IN METAMORPHIC PETROLOGY

• Recall, that the number of components in any given system is the minimum number required to define the composition of all phases in the system.  Thus, since constituents like Na2O and K2O are not usually found as separate mineral phases, we can combine these with other constituents, like Al2O3 and SiO2 in the feldspars, and thus reduce the number of components required to define our system.

• the best way to graphically display chemical rock and mineral data is to attempt to reduce the number of components to 3, so that we can plot the compositions of rocks and minerals on a triangular composition diagram. 

General Three Component Compositional Diagrams

• Specific minerals present within a facies (P-T conditions) depends largely on protolith composition

• Equilibrium assemblages can be shown on ternary phase diagrams (with some assumptions and simplifications)

• Hypothetical A-B-C– 7 possible minerals can occur at

this temp & pressure– Mineral phases that coexist with

each other at this temperature and pressure are connected by lines, called tie lines.  

– Smaller triangles show regions where sets of 3 minerals are stable together

• Changing the P-T conditions changes– Location of the tie lines– Regions of coexisting stable mineral assemblages

• Due to chemical reactions between minerals

Change in conditions

AB + A2C  => 2A + ABC

• Many minerals that occur in nature are solid solutions, and thus they can have a variable composition. Solid solution minerals, because of their possible range in chemical composition, do not plot at single point on the composition diagrams, but instead plot along a line or within a field that represents the possible range in chemical compositions. 

• When solid solutions are present, the tie lines become spread out over a range of compositions as is seen in the diagram shown here for the hypothetical system X, Y, Z.

• In this diagram the mineral X(Y,Z) shows limited solid solution with variable amounts of Z substituting for Y.  This is shown by a solid line extending from pure XY into the ternary diagram.

Similarly, mineral X2(Z,Y) shows limited solid solution of Y substituting for Z.  The minerals XYZss and Zss show a range of possible compositions that are represented by a shaded field on the diagram.  

Common Triangular Plots Used in Metamorphic Rocks

• Most common natural rocks contain the major elements: SiO2, Al2O3, K2O, CaO, Na2O, FeO, MgO, MnO , H2O and CO2 such that C = 10

• Three components is the maximum number that we can easily deal with in two dimensions

• What is the “right” choice of components? • Some simplifying methods:

1) Simply “ignore” components• Trace elements• Elements that enter only a single phase (we

can drop both the component and the phase without violating the phase rule)

• Perfectly mobile components

2) Combine componentsComponents that substitute for one

another in a solid solution: (Fe + Mg)3) Limit the types of rocks to be shown

Only deal with a sub-set of rock types for which a simplified system works

4) Use projectionsThat is assume that a constituent will always be

present and project compositions from that constituent in a four or five component system to the 3 component system. 

The ACF Diagram• Illustrate metamorphic mineral assemblages in mafic

rocks on a simplified 3-C triangular diagram• Concentrate only on the minerals that appeared or

disappeared during metamorphism, thus acting as indicators of metamorphic grade

• Useful for plotting most common metamorphic rocks

• However, the A value we want is the value of excess Al2O3 left after allotting Na2O and K2O to form alkali feldspar. The CaO value we want is the excess CaO after allotting P2O5 to form apatite, assuming that any P2O5 in the rock will suck up CaO to form apatite.  We will assume then that all mineral assemblages plotted may also contain alkali feldspar and quartz (and apatite). 

To plot a rock on the ACF diagram, the chemical analysis of the rock is first recalculated to molecular proportions

A = Al2O3

C = CaO

F = FeO + MgO

a = [Al2O3 + Fe2O3] - [Na2O + K2O] c = [CaO] - 3.33[P2O5] f = [FeO + MgO + MnO] Since we are only plotting these 3 components, they have to be normalized so that they add up to 1 (or 100 if we are plotting %).

The ACF Diagram

Example 1:

Anorthite CaAl2Si2O8

A = 1 + 0 - 0 - 0 = 1, C = 1 - 0 = 1, and F = 0

A+C+F= 2, so we can normalize to 100

A = 100* 1/2 = 50

C = 100* 1/2 = 50

F = 0

Plotting minerals on the ACF diagram is somewhat easier if you know the chemical formula of the mineral, since mineral formulae are already in the form of molecular proportions.

Where does Ab plot? Plagioclase?

Figure 24.4. After Ehlers and Blatt (1982). Petrology. Freeman. And Miyashiro (1994) Metamorphic Petrology. Oxford.

Example 2:

hypersthene, (Mg,Fe)SiO3 , we have 1 molecule of  (FeO + MgO) for every 1 molecule of SiO2. Thus:

a = 0c = 0f = 1t =1

• so, the plotting parameters become• A = 100 * 0/1 = 0• C = 100 * 0/1 = 0• F = 100 * 1/1 = 100%• and we see that hypersthene would plot at the F corner

of the ACF diagram.

The AKF DiagramBecause pelitic sediments are high in Al2O3 and K2O, and low

in CaO, Eskola proposed a different diagram that included K2O to depict the mineral assemblages that develop in them.

In AKF diagrams we assume that both alkali feldspar and plagioclase feldspar can be present, thus the amount of Al2O3 that we use is the excess Al2O3 left after allotting it to all of the feldspars. To obtain the plotting parameters for AKF diagrams, calculate the following:

a = [Al2O3 + Fe2O3] - [Na2O + K2O + CaO]k = [K2O]f = [FeO + MgO + MnO]

Let t = a + k + f, then the plotting parameters in % are:A = 100 * a/t K = 100 * k/t F = 100 * f/t

Figure 24.6. After Ehlers and Blatt (1982). Petrology. Freeman.

Minerals are plotted in the same way as was done for the ACF diagrams, and an example AKF diagram showing the potting positions of common metamorphic minerals is shown below.

• Example 1: chemical formula of K-feldspar KAlSi3O8

• Rewrite it in oxide form as 1/2K2O 1/2Al2O3 3SiO2. Then:

a = ½ - ½ = 0k = ½f = 0t = ½

• So, A = 100 * 0/½  = 0%K = 100 * ½ /½ = 100%F = 100 * 0/½ = 0%

• The K-feldspar plots in the lower right hand corner

• Example 2: Muscovite KAl3Si3O10 (OH)2 or • 1/2K2O 3/2Al2O3 3SiO2 H2O.  For muscovite:

a = 3/2 - 1/2 = 1k = 1/2f = 0t = 1½ = 1.5

• So,A = 100 * 1/1.5 = 66.7%K = 100 * 0.5/1.5 = 33.33%F = 100 * 0 = 0%

• Note that AKF diagrams are used for CaO-poor, K2O-rich rocks, whereas ACF diagrams should be used for Al2O3  and CaO - rich rocks.

Three of the most common minerals in metapelites: andalusite, muscovite, and microcline, all plot as distinct points in the AKF diagram

• And & Ms plot as the same point in the ACF diagram, and Micr doesn’t plot at all, so the ACF diagram is much less useful for pelitic rocks (rich in K and Al)

AKFM Projection onto AFM• One of the problems associated with ACF and AKF diagrams is

that Fe and Mg are assumed to substitute for one another and act as a single component.

• J.B. Thompson developed a projected diagram that takes into account possible variation in the Mg/(Mg+Fe) ratios in ferromagnesium minerals, and has proven very useful in understanding metamorphosed pelitic sediments.

• Thompson starts with the 5 component system SiO2 - Al2O3 - K2O - FeO - MgO  and ignores minor components in pelitic rocks like CaO and Na2O.  Because quartz is a ubiquitous phase in metamorphosed pelitic rocks, the five component system is projected into the four component system Al2O3 - K2O - FeO - MgO as shown below.  

• Next, because muscovite is also a common mineral in these rocks, all compositions are projected from muscovite onto the front face of the diagram. (Al2O3 - FeO - MgO). The front face

of the diagram becomes the AFM diagram. • Minerals that contain no K2O like

andalusite, kyanite and sillimanite plot at the A corner of the diagram, and minerals like staurolite, chloritoid (Ctd), chlorite, and garnet plot on the front face of the diagram.

Biotite, however, does contain K2Oand has varying amounts of Al2O3 and thus is a solid solution that lies in the four component system. Because muscovite is relatively K - poor, this results in biotite being projected to negative values of Al2O3.

• A = [Al2O3 - 3 K2O]• F = [FeO]• M = [MgO]• Using these parameters, one

can grid off the AFM diagram with the vertical scale represented by the normalized values for the A parameter -

• [Al2O3 - 3 K2O]/[Al2O3 - 3 K2O + FeO + MgO]

• and the horizontal position based on the ratio of MgO/(FeO + MgO).

• Of course these values are obtained after converting the chemical analysis of the rock to molecular proportions.

• To calculate the plotting parameters for the AFM diagram the following formulae are used:

• The projection from muscovite works well for metamorphic rocks that contain muscovite.  But, at higher grades of metamorphism, in the upper amphibolite facies and the granulite facies, muscovite becomes unstable and is replaced by K-feldspar + quartz + an Al2SiO5 mineral.  In order to show rocks and mineral assemblages at these higher grades of metamorphism, a new projection is made from K-spar, as shown below.• For this diagram the plotting parameters

are much more straight forward, with - A = [Al2O3]F= [FeO]M =[ MgO]

• All on a molecular basis and then renormalized to sum to 100%.

• Note the absence of all hydrous phases (staurolite, chloritoid, muscovite) except biotite in this projection.

Resolving Problems• a common medium grade assemblage in a pelitic rock is

staurolite, garnet, muscovite, biotite, quartz, and plagioclase.  Plotting these minerals on ACF, AKF, and AMF diagrams, as shown below creates a problem.  For divariant equilibrium we expect the number of components to equal the number of phases (c=3, so p=3) at least for the ternary part of our system. 

• Thus, in the ACF diagram, a rock like composition x would have plagioclase, garnet, staurolite (+quartz + muscovite), but biotite cannot be resolved from garnet because they plot near the same point(s).  Still, in the ACF diagram, x plots within a 3 phase triangle. 

• In the AFM diagram the same rock of composition x is seen to have garnet, staurolite, and biotite (+quartz + muscovite). Plagioclase is ignored by the diagram (because CaO is not plotted), but we can resolve biotite and garnet because they clearly have different compositions in the AFM plot.

• The problem in the AKF diagram implies one of the following:1. The assemblage represents one where equilibrium has not

been achieved in the rock.2. The assemblage is really a univariant assemblage, rather

than a divariant assemblage.3. The AKF diagram is not truly behaving as a three

component system because of information lost to create the projection.

• In the AKF plot there is an ambiguity.  Composition x plots in the correct 4 phase field of muscovite, garnet, staurolite, and Biotite, but divariant equilibrium requires that it plot in a 3 phase triangle.  

149

Example of Progressive Metamorphism

• Minerals present reflect equilibrium P-T conditions for different protoliths

• As P-T conditions increase (prograde metamorphism) minerals react with each other and with fluids– Some minerals no longer stable; new minerals appear

What happened in jumping from the pressure/ temperature conditions of the first diagram to those

of the second diagram. Phases  Disappeared New Phases

andalusite sillimanite

epidote/zoisite plagioclase

chlorite grossularite

talc cordierite

actinolite biotite

albite anthophyllite

hornblende

The chemical reactions that must have occurred between the two sets of pressure/temperature conditions that would explain the new mineral assemblages are as the following:

Al2SiO5 => Al2SiO5

Andalusite       Sillimanite

4Ca2Al3Si3O12(OH) + SiO2 => 5CaAl2Si2O8 + Ca3Al2Si3O12 + 2H2O       Epidote/Zoisite     Qtz      Anorthite (plag)   grossularite     fluid

(Mg,Fe)5Al2Si3O10(OH)2 + KAl3Si3O10(OH)2 + 2SiO2 =>             Chlorite                     Muscovite               Qtz                                        K(Fe,Mg)3AlSi3O10(OH)2 + (Mg,Fe)2Al4Si5O18 + H2O                                                Biotite                        Cordierite                fluidthe reactions result in the evolution of H2O in a fluid phase are called dehydration reactions.

Retrograde Metamorphism

If retrograde metamorphism were a common process then upon uplift and unroofing metamorphic rocks would progressively return to mineral assemblages stable at lower pressures and temperatures

Three factors inhibit retrograde metamorphism, two of which involve the fluid phase. 1. Chemical reactions run faster at higher temperatures. Thus, during prograde metamorphism reaction rates are faster.  As temperature is lowered on a rock, the reaction rates are much slower.

2. During prograde metamorphism a fluid phase is driven off as a result of the devolatilization reactions.  As pressure increases, porosity of rocks also decreases, and thus this fluid phase will likely be driven out of the rock body.  In the absence of the fluid phase it is impossible to form hydrous minerals and carbonates, since H2O and CO2, two of the key components needed in such reactions, may not be present.

3. The fluid phase also helps to catalyze chemical reactions. If the fluid phase is driven off during prograde metamorphism, then it will not be available to catalyze the reactions to produce the retrograde mineral assemblage as pressure and temperature are lowered.

TYPES OF METAMORPHIC REACTIONS

A metamorphic reaction is an expression of how the minerals got to their final state, but a reaction does not necessarily tell us the path that was actually taken to arrive at this state. Sometimes it is possible to deduce the path by means of a reaction mechanism. 

Types of metamorphic reactions:

1. Univariant Reactions

2. Divariant Reactions 

Univariant Reactions• A univariant reaction is one that plots as a line or

curve on a pressure-temperature diagram.  If all phases in the reaction are present in the rock, then we know that the rock must have been metamorphosed at some pressure and temperature along the reaction boundary. Consider for example the simple Al2SiO5 system with excess SiO2 and H2O.  In low grade metamorphic in this system, the reaction:

Al2Si4O10(OH) 2 <=> Al2SiO5 + 3SiO2 + H2O

  Pyrophyllite       Ky or Andal    Qtz      fluid

Metamorphic Reactions and Critical Minerals

• plot a reaction boundary on a P-T diagram.  This boundary can be determined experimentally or can be calculated using thermodynamic properties of the phases involved. 

• For example, if the mineral is andalusite, then we know the rock was metamorphosed at a pressure less than about 2.5

kilobars.  If the mineral is kyanite, then we know that the pressure was greater than about 2.5 kilobars.

• we can use the diagram to illustrate another point.  Imagine that a group of rocks are buried along the geothermal gradient shown in the diagram. From this diagram we can deduce that:

1. Rocks buried to a pressure less than about 4 kb and a temperature less than about 420oC should have pyrophyllite

2. Rocks buried to pressures between about 4 and 5 kb and temperatures between 420 and about 600oC should have kyanite + quartz

3. Rocks buried to pressures along the geothermal gradient greater than about 5 kb and temperatures greater than about 600oC should have Sillimanite + Quartz.

Metamorphic Isograds• Reactions and appearance/disappearance of critical

minerals can be used to determine P-T conditions• Isograd = “line” to indicate same grade of metamorphism

– First appearance of mineral of increasing metamorphic grade

Divariant Reactions

• The univariant reactions can be considered discontinuous reactions because they result in a sudden change in mineral assemblage which occur along specific pressure temperature curves.

• Divariant reactions result in a gradual change in composition of the solid solutions minerals, which occur over a wide range of pressure and temperature conditions.  

Consider the hypothetical case of rocks that contain minerals like chlorite and garnet, which are both Mg-Fe solid solutions.

The reaction that occurs with increasing temperature (at constant pressure) is:

Chlorite + Qtz  => Garnet + Mg-richer Chlorite + H2O

We say that this reaction is a continuous reaction because there is no change in mineral assemblage between T1 and T3, but there is a reaction occurring and its effect is to change the compositions of the solid solution minerals. 

Assemblages, Facies, and Series• For a given parent rock composition, mineral assemblage

reflects P-T conditions • A variety of

different rocks metamorphosed under the same conditions indicate facies

• Regional changes between facies indicate series (and tectonic setting)

Metamorphic Reaction Mechanisms

Contact Metamorphism

• Contact aureole = surrounding rocks metamorphosed by heat of intrusion

• Size/shape of aureole depends on:– Size, temp, cooling history of the igneous intrusion– Properties of the country rock (conductivity, presence of water)

• Result of high geothermal gradient produced locally around intruding magma

• Restricted to relatively shallow depths (low pressure)

• Rocks generally not foliated• Produce fine grained rocks

called hornfels

Contact Metamorphism: Hornfels

• Mineral assemblage characterized by hornfels facies series – Albite-epidote

hfls– Hornblende hfls– Pyroxene hfls– Sanidinite

1.

Pyroxene hornfels

163

Contact Metamorphism: Skarn• Contact metamorphism of carbonate or siliceous carbonate• Usually involves significant metasomatism (chemical

exchange between magmatic fluids and rock)• Can be hosts of Au, Cu, Fe, Mo, Sn, W, Zn-Pb

Wollastonite-garnet skarn

Regional Metamorphism

• Affects large areas of earth’s crust

• Produced during orogenic (mountain-building) event

1.

• Foliated rocks developed under med-high T and P:– 1. Buchan 2. Barrovian

• Rocks developed under low-med T and high P:– 3. Sanbagawa 4. Franciscan

3.2.

4.

Regional Metamorphism: Barrovian• “Normal” metamorphic series recognized in mountain belts

world-wide• Medium to high T; low to

high P• Moderate to high

geothermal gradient• Critical mineral sequence:

– Kaolinite => pyrophyllite => kyanite => sillimanite

• Textural sequence (in pelitic rocks):– Slate => phyllite => schist => gneiss

• Facies sequence:– Zeolite => prehnite-pumpellyite => greenschist => amphibolite =>

granulite

Regional Metamorphism: Franciscan

• Low T; high P• Low geothermal gradient• Little directed stress

(little deformation)• Distinguished by

presence of blue, sodic amphiboles

• Facies sequence:– Zeolite => prehnite-

pumpellyite => blueschist => eclogite

Regional Metamorphism and Tectonics

• Barrovian (mod-high T and P):– Develop in orogenic belts at convergent margins– Regional heat flow increased by upward movement of magma

and migration of fluids– Thrust faulting thickens crust, increasing pressure and directed

stress– Geothermal gradient of ~20-40ºC/km

• “Blueschist” (Franciscan) (low T and high P):– Occur world-wide in orogenic belts of Phanerozoic age– Associated with subduction zones

• rapid subduction of oceanic crust depresses isotherms– Geothermal gradient of ~10-20ºC/km

Regional Metamorphism and Tectonics

• Paired metamorphic belts – specific to subduction zones

• Barrovian– Formed in rocks

beneath and around the arc

– Heat due to magmatic activity

• Blueschist– formed in the

mélange of the trench

Identification of Metamorphic Rocks

Foliation Grain Size Hardness Usual Color Other Rock Type

foliated fine very soft light greasy feel Soapstone

foliated fine soft dark "tink" when struck Slate

foliated fine soft dark shiny; crinkly foliation Phyllite

foliated coarse hard mixed dark and light crushed and stretched fabric; deformed large crystals Mylonite

foliated coarse hard mixed dark and light wrinkled foliation; often has large crystals Schist

foliated coarse hard mixed banded Gneiss

foliated coarse hard mixed distorted "melted" layers Migmatite

foliated coarse hard dark mostly hornblende Amphibolite

nonfoliated fine soft greenish shiny, mottled surface Serpentinite

nonfoliated fine or coarse hard dark dull and opaque colors, found near intrusions Hornfels

nonfoliated coarse hard red and green dense; garnet and pyroxene Eclogite

nonfoliated coarse soft light calcite or dolomite by the acid test Marble

nonfoliated coarse hard light quartz (no fizzing with acid) Quartzite

References• Vernon, Ronald Holden, 2008, Principles of Metamorphic Petrology,

Cambridge University Press, p.1• Bucher, Kurt, 2002, Petrogenesis of Metamorphic Rock, Springer, p. 4• Marshank, Stephen, 2009, Essentials of Geology, Third Edition, Norton, p.

187.• Marshank, Stephen, 2009, Essentials of Geology, Third Edition, Norton, p.

190.• Gillen, Cornerlius, 1982, Metamorphic geology : an introduction to tectonic

and metamorphic processes, London; Boston: G. Allen & Unwin, p. 31.• Winkler, H.G.F., 1979, Petrogenesis of Metamorphic Rock, Springer-Verlag

New York Inc. • Ron H. Vernon, Geoffrey Clarke: Principles of Metamorphic Petrology.• Kurt Bacher, Martin Frey: Petrogenesis of Metamorphic Rocks. 8th Edition,

Springer.• Norman Frey: The Field Description of Metamorphic rocks.

Metamorphic Rocks & Tectonics

• Most metamorphic rocks develop due to plate collisions– Processes are not observable in the

field– Conditions at various plate

boundaries define possible P & T range

– Parent rocks present control mineral assemblages