Metamorphism

Geology 1001-section 4

Last updated: 02/18/98

THE CONCEPT OF METAMORPHISM (abbreviated versions of overheads)

The word metamorphism comes from the Greek words:

meta = change

morph = form

Metamorphic rocks all start out as something else (igneous or sedimentary), but the "change form" in response to changes in.

Definition of metamorphism - the mineralogic or textural change brought about in a rock in the solid state as a result of change of temperature, pressure and/or the fluid phase.

Changes occur as a result of:

recrystallization of
chemical reaction between

Metamorphic petrologist is interested in:

(1) the nature of the

(2) changes in

(3) the path that

(4) inferring the

The development of modern ideas of metamorphism

Traditional problem: Typically can't see metamorphism in action.

Dates back to James Hutton’s book Theory of the Earth (1795) in which he suggests the Highland "Primitive Strata" were originally sedimentary rocks converted by heat in the earth.

Sir James Hall became the first experimental petrologist when he placed chalk in a sealed cannon barrel and heated it to convert it to marble - appreciation of P

Our knowledge of the conditions of metamorphism are gained largely from the experimental work (e.g. N. L. Bowen)

However, thermodynamics allows us to expand this knowledge and extrapolate to new conditions. Goldschmidt (1911) was the first to do this for the wollastonite-forming reaction. Eskola (1920s) used the idea of chemical equilibrium to develop a scheme of metamorphic facies -different mineral assemblage characterize different

Barrow (1912) - Scottish Highlands - quartz-rich sandstones with some pelitic and basic igneous layers. Divided into metamorphic zones. He developed ideas of progressive metamorphism, index minerals and isograds.

Factors controlling the nature of metamorphism and metamorphic rock

(1) Composition of the protolith

Different starting compositions produce

Parent Material (Protolith)	Rock type	Important constituents
Argillaceous/clay-rich sediments (shales and mudstones)	pelites	Al, Si, K, Fe, Mg and OH
arenaceous sediments	psammites	Si, (Al), K, OH
clay-sand mixtures	semi-pelite	Si, (Al), K, Fe, Mg
quartz sandstones (arenites)	quartzite	Si
marl (lime muds)	calc-silicate	Ca, Si, (Al), Mg, (OH)
limestone or dolostone	marble	Ca, (Si), CO₂
basalt	metabasite (metamafic)	Mg, Fe, Ca, (Al), (OH)
ultramafic (altered peridotite)	meta-ultramafic	Mg, Si, Ca, (OH)
granitic rocks	metagranite	Mg, Fe, Ca, (Al), (OH)

Commonly assume isochemical. For many rocks the major change that takes place is a loss of

- However, in some instances, the bulk compositions also change i.e. they undergo

(2) Temperature and Pressure

With increase in depth the temperature increases. However, the rate of increase relative to the depth depends on:

the local

any perturbations to

P-T limits of metamorphism

- lower limit - above the PT of diagenesis (roughly

- upper limit - at the onset of

Metamorphic grade (relative intensity of metamorphic overprint)

Low grade - lower

High grade - higher

Retrograde - resetting of

T and P may occur simultaneously or independently

Increase in T produces

Increase in P produces

Mineral assemblages responses

Systematic change in

Petrogenetic grids - stability range of minerals or mineral assemblages in P-T-X(fluid) space

Differential Stress

Metamorphic textures record

Differential stress produce

Homogeneous stress produces

Textures and prograde metamorphism

Most metamorphic rocks tend to become

With differential stress foliation (planar texture of parallel oriented platy grains) and/or lineation (alignment of elongate minerals)

(a) Slaty cleavage - low grade parallel growth that produces

(b) Schistosity - medium to high grade rocks that develop

(c) Gneissosity - layering of light and dark minerals developed

(3) Presence of Fluids

Intergranular fluids play a vital role in metamorphism

Fluids carry

Fluids tend to

As P increases

porosity decreases and fluids are found as

As T-P increase (prograde metamorphism), the hydrous minerals

1. Example of dehydration in metamorphic rocks

with increasing T, hydrous minerals lose their OH and produce anhydrous minerals e.g.

KAl₃Si₃O₁₀(OH)₂ + SiO₂ = KAlSi₃O₈ + Al₂SiO₅ + H₂O

muscovite + quartz = K-feldspar + Al-silicate + H₂O

2. Example of decarbonation in metamorphic rocks

At 200°C, sandy limestone contains quartz and calcite

At 600°C quartz + calcite is no longer a stable assemblage, but they react to form wollastonite via the reaction:

SiO₂ + CaCO₃= CaSiO₃ + CO₂

The CO2 is volatile and can migrate from the rock (except for small amounts that may be trapped as fluid inclusions)

Noteworthy features

Relative amounts of Ca and Si don’t change - new minerals form with the same ratio of Ca/Si - i.e. they are isochemical with respect to these elements

CO₂ is released and can leave the rock i.e. the rock becomes decarbonated

If CO₂ leaves, the reaction can’t go backwards as the rock cools - THEREFORE, THE HIGH TEMPERATURE MINERALS CAN BE PRESERVED UPON COOLING!!!

(4) Time

Metamorphic reaction commonly takes

makes experimental duplication

allows rocks to approach

Exceptions:

- contact metamorphism at shallow depths or surface conditions

- impact metamorphism

Kinds of Metamorphism

Cataclastic-Mylonitic Metamorphism (dynamic)

found in rocks undergoing intense

At shallow crustal levels, minerals undergo

At deeper crustal levels, minerals behave

Impact Metamorphism (dynamic)

ultra-high pressure metamorphism due to

very rapid style that produces

locally important on Earth (especially for the dinosaurs)

probably the only type of metamorphism on

Subseafloor metamorphism (static)

alteration of MORB as a consequence of

results in extensive hydration and alteration of the basalts

Burial metamorphism (static)

produced as a consequence of

many relict sedimentary structures are preserved

commonly low grade (typically zeolite facies)

gradational to regional metamorphism

High pressure - Low temperature metamorphism (dynamic)

produced in a

low local

Contact metamorphism (static)

Adjacent to

involves recrystallization with

common development of metamorphic aureole whose width is a function of

igneous rock type -

size of the

the amount of

the aureole has a series of concentric zones of mineral assemblages that relate to the peak temperature attained during heating (i.e. peak metamorphic temperature)

Regional metamorphism (dynamic)

very widespread metamorphic event with distinct foliations due to differential stresses

found in the

response of the rocks to P-T changes induced by

Terminology of metamorphic rocks

Metamorphic rock names

Criteria for naming metamorphic rocks

1. Nature of the parent material

Classification by bulk composition and nature of source rocks

general (e.g. metasediment or metaigneous)

specific (e.g. metapelite) - these may be qualified by adjective that relate to mineralogy or texture (e.g. pelitic schist).

2. Metamorphic mineralogy

names of significant metamorphic minerals

(a) order of abundance (field classification) (e.g. staurolite schist)

(b) The essentially monomineralic rocks get a special name (e.g. quartzite)

3. Rock texture

determined by the shape, arrangement and orientation of constituent

Foliated rocks - classified according to the nature of their foliation, size of crystals, degree of segregation of light and dark minerals and metamorphic grade

For a shale protolith

Name	Description
Slate	Very low grade - very fine-grained (can’t see with the naked eye) with excellent planar partings (slaty cleavage)
Phyllite	Low grade - fine-grained with glossy "sheen" on the foliation plane due to orientation of micas and chlorite (can see grains only with hand lens)
Schist	Medium-to-high grade - coarser-grained rock that breaks along a foliation plane and exhibits well-developed micas and, locally, some very coarse minerals (porphyroblasts)
Gneiss	High grade - coarse-grained with alternating bands enriched light and dark minerals - rock generally does not break along a foliation plane.

For meta-mafic rock (e.g. altered basalt or gabbro)

Name	Description
Greenschist	-green, foliated metabasite - usually composed predominantly of chlorite, epidote, and actinolite
Blueschist	-dark, lilac-grey foliate metabasite, seldom truly blue in hand specimen -color due to presence of abundant sodic amphibole (glaucophane, crossite)
Amphibolite	-essentially bimineralic (hornblende and plagioclase) dark green rock-wide range of accessory minerals -can be metabasites or metasediments
Eclogite	-metabasite composed of garnet and omphacitic pyroxene with no plagioclase feldspar. Common accessories: quartz, kyanite, amphiboles, zoisite, rutile, minor sulfides
Granulite	-characterized by both a +/- equidimensional, straight-sided (polygonal) grains for all mineral species, and a very high-temperature mineralogy

Additional Textural terms

Porphyroblasts - grains significantly larger than the matrix.

Porphyroclasts - larger shear-bounded grains typically in mylonites

- Augen - eye-shaped porhyroclasts

Pseudomorph - replacement of pre-existing mineral while retaining the shape.

4. Any special name - commonly descriptive

Name	Description
Serpentinite	-green, black or reddish rock composed of predominately serpentine -formed by hydration of igneous or metamorphic peridotites
Migmatite	-a mixed rock of schistose or gneissic portion intimately mixed with veins of apparently igneous felsic melts
Skarn	-result of metasomatism where large amounts of Si, Al, Fe, and Mg introduced during intrusion. -impure marble containing crystals of calc-silicate minerals such as garnet, epidote, etc.-created by compositional changes in country rock at igneous contacts

DEFINITIONS

Metamorphic grade - a measure of the relative intensity of metamorphism that is primarily a function of T but also of P. The intensity is stated in terms of low grade, medium grade or high grade metamorphism.

Peak metamorphism - the maximum temperature conditions attained by a metamorphic rock during a distinct metamorphic event.

Index minerals - minerals that are stable over a restricted region of P-T space.

Isograds - a plane delineating the first appearance of an index mineral or assemblage of minerals in rocks of roughly similar bulk compositions. This is generally a line of equal grade.

Metamorphic zones - a mappable area in rocks of roughly similar bulk composition that are between isograds.

Metamorphic facies - contrasting, but systematic, mineral assemblages from different rock bulk compositions that equilibrated under a restricted range of P-T conditions.

P-T-t path - the path followed by a single rock in P-T space during its burial, metamorphism and uplift.

Metamorphic field gradient (or PT array or metamorphic geotherm) - a curve joining thermal maxima of rocks that experienced different P-T-t paths during a single metamorphic event.

Metamorphic facies series - a series of metamorphic field gradients that are named for the particular P-T conditions that they cover.