lecture 9 - intro. to metam. rocks

7/29/2019 Lecture 9 - Intro. to Metam. Rocks

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FUNDAMENTAL CONCEPTS IN METAMORPHIC PETROLOGY

Study of metamorphism deals with the physico-chemical conditions of recrystallization. The study of

metamorphic rocks gives us very important information about 1) the pressure and temperature

conditions of tectonic processes and 2) the nature of fluid flow in the deep crust. Changes in P, T, and

fluid compositions result in reactions among minerals to produce new minerals. Therefore, by studyingmineral assemblages in metamorphic rocks we can deduce the conditions of metamorphism.

I. Types of Metamorphism

1. Contact metamorphism is the result of intrusion of magmas into colder, upper crustal rocks. The

zone of metamorphism is called the contact aureole. The change in metamorphic grade, as expressed in

change in the mineral assemblage, is generally concentric to the intrusion. Contact metamorphism can

be thought of being isobaric at a given crustal level. It involves fairly rapid changes temperature.

(Time interval is 200,000 years.)

Factors influencing nature of a contact aureole:

Size of igneous body. The larger the body, the larger the aureole.

Temperature difference between magma and wall-rocks. The larger the difference, the


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larger the apparent effect of metamorphism.

2. Regional metamorphism

A. Orogenic (most important)

related to formation of orogenic belts

hundreds to thousands km2

T up to 800 C observed

geothermal gradient > normal (30-50C/km)

metamorphism isdynamothermal, meaning it is associated with deformation (get foliated

rocks)

Burial regional metamorphism

result of simple burial

T is normal geothermal gradient (max ~400C)

Ocean floor metamorphism

involves the alteration of the ocean crust by seawater while the crust is still hot

typical minerals include serpentine, chlorite, epidote, zeolites, etc.

3. Hydrothermal metamorphism

mineral reactions are controlled by fluid flow and compositions of fluids

is usually associated with all other types of metamorphism

metasomatism involves the movement of chemicals, such as Si, Cl, Na, metals, by

hydrothermal fluids

4. Fault-zone (cataclastic) metamorphism

involves crushing and grinding by friction during movement along faultsmetamorphism is local in extent

low temperature; high pressure

causes formation offault breccia, or fine-grained, foliated rock calledmylonite

5. Impact (shock) metamorphism

occurs during impact of meteorites and other extraterrestrial objects


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involves very high pressures and temperatures generated by energy of the impact.

involves the formation of high-pressure polymorphs of minerals (e.g. coesite and

stishovite) or loss of internal structure of minerals (e.g. feldspars)


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II. Pressure and Temperature Regimes in the Crust

1. Pressure

Many metamorphic reactions are dehydration or decarbonation reactions. An often made assumption in

metamorphic petrology is that the fluid pressure equals the total pressure on the rock system. However,in the upper few kilometers in the crust where pores in rocks are filled with fluids and the pore spaces

are connected, the fluid pressure in pore-spaces is that exerted by overlying column of the fluid. Thus

thefluid pressure (Pfluid) is hydrostatic. It is given by the weight of the overlying column of the water.

Pfluid = fluidgh

where fluid is the density of the fluid, g is gravitational constant, and h is depth. Because the fluids in

metamorphic rocks are typically not pure H2O, to total

Pfluid =pH2O + PCO2 + ... .

However, at depths >~6 km, fluid pressure becomes equal to the lithostatic pressure:

Pfluid= Plith,

because at the high pressures the fluid cannot keep pore-spaces open anymore as the minerals push

against each other. Lithostatic pressure is given by the weight of the overlying rocks:

Plith = rockgh .


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2. Metamorphic field gradients (metamorphic series)

In a given regional metamorphic terrane, a given prograde metamorphic sequence usually indicates an

increase in both temperature and/or pressure. However, the prograde sequence may not necessarily

represent an originally vertical section through the crust at the time of metamorphism. Therefore, the

apparent P-Tgradient is not indicative of the geothermal gradient that existed in the crust during

metamorphism. However, because the gradient is observed in the field, it is called themetamorphic

field gradient.

A) Hornfels series isobaric contact metamorphism. However, it can occur at higher pressures than

indicated.

B) Buchan series low-P regional metamorphism, usually cause by intrusion of large volume of

magmas into the upper crust.

C) Barrovian series occurs in orogenic terranes by dynamothermal metamorphism.

D) Blueschist (Franciscan) series high-P/low-T series occurs in subduction zone environments.


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P

T

a

b

c

max in P

max in T

III. Metamorphic Pressure-Temperature-Time Paths

Assume that a several km thick section of a sedimentary pile, represented by samples a-c, gets buried

during crustal convergence in an orogen and then gets exposed at the surface:

a

b

c

mantle

The pressure on the rocks increases faster than they get heated up. Therefore, the three samples undergoclockwise P-T-t paths:

Metamorphic Field

Gradient

Thus, metamorphism of rocks needs to be viewed as a dynamic process in which pressure and

temperature conditions change. Note that the maximum P on a given path does not correspond to the


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maximum T. If the three samples were collected in a metamorphic terrane, mineral assemblages

corresponding to the maximum Treached by each sample would define a metamorphic field gradient.

(Typically, near-maximum T conditions are preserved in mineral assemblages of metamorphic rocks

because retrograde reactions are energetically difficult.)


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IV. Nomenclature of Metamorphic Rocks

The best classification of metamorphic rocks is the faciesclassification, in which mineral assemblages

are used to define ranges of P-T conditions. We will discuss metamorphic facies later. However, names

are also given to metamorphic rocks that may be indicative of their morphology, mineralogy, or bulk

chemical composition.

1. Structural (morphological) classification

Hornfels - non-schistose, fine-grained rock with 'granoblastic' fabric (mosaic of small mineral grains).

May have porphyroblasts. Hornfelsic texture is usually produced by contact metamorphism where there

is no deviatoric stress.

Slate - Fine-grained rock with perfect foliation usually defined by sericite and chlorite.

Phyllite - Fine-grained foliated rock but coarser-grained than slate. Muscovite imparts sheen to it.

Schist Medium to coarse grained foliated and commonly lineated rock in which most individual

mineral grains can be recognized without a hand-lens.

(Slate, phyllite, schist most commonly develop in pelitic rocks and tend to indicate a progression in

grade.)

Gneiss - Medium to coarse-grained rock that is discontinuously banded, with banding generallyseparating feldspar/quartz from biotite/hbld/px. The production of some gneisses may also involve

partial melting.

2. Bulk chemical composition (indicates the protolith of a metamorphic rock)

Pelitic - derived from aluminous sediments (shales). Micas are abundant as are other aluminous

minerals, including the aluminosilicates, pyralspite garnets, staurolite, and cordierite.

Quartzo-feldspathic - derived from sandstones, tuffs, granites. The principal minerals are quartz andfeldspars.

Calcareous - derived from calcareous rocks. The principal minerals are either calcite or dolomite.

Calc-silicate- derived from mixed calcareous and pelitic protoliths. Calcite, dolomite, muscovite,

chlorite, and biotite are common at low grades, whereas diopside, tremolite, grossular garnet,

wollastonite, and vesuvianite are common at high grades.


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Basic - Derived from mafic igneous rocks. Typical minerals are chlorite, hornblende, plagioclase,

epidote, pyroxene.

Magnesian - derived from ultramafic rocks (peridotites). Serpentine, talc, magnesite, and brucite

common.

The metamorphism of the last two groups is essentially retrograde.


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3. Traditional names indicating the dominant mineralogy

Marble - non-foliated or weakly foliated rock composed dominantly of calcite or less commonly

dolomite.

(Meta-)Quartzite - recrystallized sedimentary quartzite (sandstone).

Amphibolite - composed essentially of hornblende and plagioclase. It is often foliated and lineated.

Eclogite - Dominantly omphacite clinopyroxene and garnet (kyanite).

omphacite - (Ca,Na)(Mg,Fe2+,Fe3+,Al)Si2O6

garnet - pyrope-almandine solid solution.

4. Modifiers and prefixes

Modifiers may be placed in front of the above names, e.g. :

tremolite marble

hornblende gneiss

garnet-biotite schist

staurolite-andalusite schist

Common prefixes:

Meta - Used to describe an originally igneous or sedimentary rock that was metamorphosed to indicate

the protolith. E.g. meta-basalt, meta-graywacke, meta-gabbro, meta-arkose

Ortho - Indicates that the metamorphic rock was originally igneous. E.g. orthogneiss, orthoamphibolite

Para - Indicates that the metamorphic rocks was originally sedimentary, e.g. paragneiss,

paraamphibolite.


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V. Concept of Metamorphic Facies

Terms such as low grade, medium grade, or high grade are only useful for comparing the metamorphic

grade in a given area. These terms are worthless when comparing different terranes that may follow

different metamorphic field gradients. For example, a blueschist series rock may be metamorphosed at

high pressures but fairly low temperature, whereas a contact-metamorphic rock may be metamorphosed

at high temperature but low pressure. Much more useful is the concept ofmetamorphic facies, where

each facies defines a certain range of P-Tconditions. The facies concept was proposed by Turner in

1968:

"A metamorphic facies is a set of metamorphic mineral assemblages, ... , such that there is a constant

and therefore predictable relationship between mineral composition and chemical composition.

Eclogite

Blueschist

PrehnitePumpelyite

Zeolite

Greenschist

Amphibolite

Granulite

Albite-epidotehornfels

Hornblendehornfels

Pyroxenehornfels Sanidine

10

12

8

6

4

2

0100 200 300 400 500 600 700 800 900

temperature(C)

Explanation of the definition:

Each facies defines a restricted set of P-T conditions.

To belong to the same facies, rocks of the same chemical composition must have the same

mineral assemblage. However, rocks of different composition may have a different assemblage

at the same facies. For example, in the greenschist facies:


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basic composition: plagioclase, chlorite, actinolite, epidote

pelitic composition: quartz, chlorite, muscovite, garnet, biotite.


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Facies boundaries may not be shown by all compositions:

epidote + tremolite

pyrophyllite +

muscovite +coriderite+ qtz

andalusite +

muscovite +cordierite+ qtz

albite-epidote

hornfles

hornblende

hornfles

pyroxene

hornfles

pelitic

basic

calcareous

garnet + tremolite +

plagioclase

garnet + diopside +plagioclase

calcite calcite calcite

sillimanite +

cordierite + Kspar

lecture 9 - intro. to metam. rocks

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