Facies and Mafic

8/10/2019 Facies and Mafic

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Metamorphic Facies

contact metamorphosed pelitic, calcareous, and

psammitic hornfelses in the Oslo region

Relatively simple mineral assemblages (< 6major minerals) in the inner zones of the

aureoles around granitoid intrusives

Equilibrium mineral assemblage related to Xbulk


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Certain mineral pairs (e.g. anorthite + hypersthene)were consistently present in rocks of appropriate

composition, whereas the compositionally

equivalentpair (diopside + andalusite) was not

If two alternative assemblages are X-equivalent,

we must be able to relate them by a reaction

In this case the reaction is simple:

MgSiO3+ CaAl2Si2O8 = CaMgSi2O6+ Al2SiO5

En An Di Als

Metamorphic Facies


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Pentii Eskola (1914, 1915) Orijrvi, S. Finland

Rocks with K-feldspar + cordieriteat Oslocontained the compositionally equivalent pair

biotite + muscoviteat Orijrvi

Eskola: difference must reflect differing physicalconditions

Finnish rocks (more hydrous and lower volume

assemblage) equilibrated at lower temperaturesand higher pressures than the Norwegian ones

Metamorphic Facies


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Oslo: Ksp + Cord

Orijrvi: Bi + Mu

Reaction:

2 KMg3AlSi3O10(OH)2+ 6 KAl2AlSi3O10(OH)2+ 15 SiO2

Bt Ms Qtz

= 3 Mg2Al4Si5O18+ 8 KAlSi3O8+ 8 H2O

Crd Kfs

Metamorphic Facies


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Eskola (1915) developed the concept ofmetamorphic facies:

In any rock or metamorphic formation which has

arrived at a chemical equilibrium through

metamorphism at constant temperature and pressureconditions, the mineral composition is controlled only

by the chemical composition. We are led to a general

conception which the writer proposes to call

metamorphic facies.

Metamorphic Facies


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Dual basis for the facies concept

Descriptive: relationship between the Xbulk& mineralogy

A fundamental feature of Eskolas concept

A metamorphic facies is then a set of repeatedly

associated metamorphic mineral assemblages If we find a specified assemblage (or better yet, a

group of compatible assemblages covering a range of

compositions) in the field, then a certain facies may

be assigned to the area

Metamorphic Facies


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Interpretive: the range of temperature and pressure

conditions represented by each facies Eskola aware of the P-T implications and correctly

deduced the relativetemperatures and pressures of

facies he proposed

Can now assign relatively accurate temperature and

pressure limits to individual facies

Metamorphic Facies


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Eskola (1920) proposed 5 original facies:

Greenschist

Amphibolite

Hornfels

Sanidinite

Eclogite

Easily defined on the basis of mineral

assemblages that develop in maficrocks

Metamorphic Facies


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In his final account, Eskola (1939) added:

Granulite

Epidote-amphibolite

Glaucophane-schist (now called Blueschist)

... and changed the name of the hornfels facies to

thepyroxene hornfelsfacies

Metamorphic Facies


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Fig. 25-1The metamorphic facies proposed by Eskola and their relative temperature-pressurerelationships. After Eskola (1939) Die Entstehung der Gesteine. Julius Springer. Berlin.

Metamorphic Facies

Formation of Zeolites

Temperature

Pressure Greenschist

Facies

Epidote-Amphibolite

Facies

AmphiboliteFacies

Pyroxene-HornfelsFacies

Glaucophane-Schist Facies Eclogite

Facies

GranuliteFacies

SanadiniteFacies


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Several additional facies types have been proposed.

Most notable are:

Zeolite

Prehnite-pumpellyite

...resulting from the work of Coombs in the burial

metamorphic terranes of New Zealand

Fyfe et al. (1958) also proposed:

Albite-epidote hornfels

Hornblende hornfels

Metamorphic Facies


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Metamorphic Facies

Fig. 25-2.Temperature-

pressure diagram

showing the generally

accepted limits of the

various facies used in this

text. Boundaries are

approximate andgradational. The

typical or average

continental geotherm is

from Brown and Mussett

(1993). Winter (2001) An

Introduction to Igneous

and Metamorphic

Petrology. Prentice Hall.


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Table 25-1. The definitive mineral assemblages

that characterize each facies (for mafic rocks).

Metamorphic Facies

Facies Definitive Mineral Assemblage in Mafic Rocks

Zeolite zeolites: especially laumontite, wairakite, analcime Prehnite-Pumpellyite prehnite + pumpellyite (+ chlorite + albite)

Greenschist chlorite + albite + epidote (or zoisite) + quartz actinolite

Amphibolite hornblende + plagioclase (oligoclase-andesine) garnet

Granulite orthopyroxene (+ clinopyrixene + plagioclase garnet

hornblende)

Blueschist glaucophane + lawsonite or epidote (+albite chlorite)

Eclogite pyrope garnet + omphacitic pyroxene ( kyanite)

Contact Facies

After Spear (1993)

Table 25-1. Definitive Mineral Assemblages of Metamorphic Facies

Mineral assemblages in mafic rocks of the facies of contact meta-

morphism do not differ substantially from that of the corresponding

regional facies at higher pressure.


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It is convenient to consider metamorphic facies in 4 groups:

1) Facies of high pressure

Theblueschistand eclogitefacies: low molar volumephases under conditions of high pressure

Blueschistfacies occurs in areas of low T/P gradients,

characteristically developed in subduction zones Eclogitesare stable under normal geothermal

conditions

May develop wherever mafic magmas solidify in the deep

crust or mantle: crustal chambers or dikes, sub-crustalmagmatic underplates, subducted crust that is redistributed

into the mantle


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2) Facies of medium pressure

Most metamorphic rocks now exposed belong to the

greenschist, amphibolite, or granulitefacies

The greenschistand amphibolitefacies conform to the

typical geothermal

gradient

Metamorphic Facies


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Metamorphic Facies

Zeoliteandprehnite-

pumpellyitefacies arethus not always

represented, and the

greenschistfacies is

the lowest gradedeveloped in many

regional terranes

4) Facies of low grades

Rocks often fail to recrystallize thoroughly at very low

grades, and equilibrium is not always attained


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Combine the concepts of isograds, zones, and facies

Examples: chlorite zone of the greenschist facies, the

staurolite zone of the amphibolite facies, or the

cordierite zone of the hornblende hornfels facies, etc.

Metamorphic maps typically include isograds thatdefine zones and ones that define facies boundaries

Determining a facies or zone is most reliably done

when several rocks of varying composition and

mineralogy are available

Metamorphic Facies


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Fig. 25-9.Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the

medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included forcomparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


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A traverse up grade through a metamorphic terrane should

follow one of several possible metamorphic field gradients(Fig. 21-1), and, if extensive enough, cross through a

sequence of facies

Facies Series


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Figure 21-1.Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for

several metamorphic areas. After Turner (1981). Metamorphic Petrology: M ineralogical, F ield, and Tectonic Aspects. McGraw-

Hill.


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Miyashiro (1961) proposed five facies series, most of

them named for a specific representative type localityThe series were:

1. Contact Facies Series (very low-P)

2. Buchan or Abukuma Facies Series (low-Pregional)

3. Barrovian Facies Series (medium-P regional)

4. Sanbagawa Facies Series (high-P, moderate-T)

5. Franciscan Facies Series (high-P, low T)

Facies Series


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Fig. 25-3.

Temperature-

pressure diagram

showing the three

major types of

metamorphic

facies series

proposed byMiyashiro (1973,

1994). Winter

(2001) An

Introduction to

Igneous and

Metamorphic

Petrology.

Prentice Hall.


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Mineral changes and associations along T-P gradients

characteristic of the three facies series Hydrationof original mafic minerals generally required

If water unavailable, mafic igneous rocks will remain largely

unaffected, even as associated sediments are completely re-

equilibrated

Coarse-grained intrusives are the least permeable and likely to

resist metamorphic changes

Tuffs and graywackes are the most susceptible

Metamorphism of Mafic Rocks


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Plagioclase:

More Ca-richplagioclases become progressively unstable

as T lowered

General correlation between temperature and maximum

An-content of the stable plagioclase At low metamorphic grades only albite(An0-3) is stable

In the upper-greenschist facies oligoclasebecomes stable. The

An-content of plagioclase thus jumps from An1-7to An17-20

(across the peristerite solvus) as grade increases Andesine and more calcic plagioclases are stable in the upper

amphibolite and granulite facies

The excess Ca and Al calcite, an epidote mineral,

sphene, or amphibole, etc., depending on P-T-X

Metamorphism of Mafic Rocks


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Zeoliteandprehnite-pumpellyite facies

Do not always occur - typically require unstable protolith

Boles and Coombs (1975) showed that metamorphism of

tuffs in NZ accompanied by substantial chemical changes

due to circulating fluids, and that these fluids played animportant role in the metamorphic minerals that were

stable

The classic area ofburial metamorphismthus has a strong

component of hydrothermal metamorphismas well

Mafic Assemblages at Low Grades


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The greenschist, amphiboliteand granulitefacies constitute

the most common facies series of regional metamorphism

The classical Barrovian series of pelitic zones and thelower-pressure Buchan-Abukuma series are variations on

this trend

Mafic Assemblages of the Medium P/T

Series: Greenschist, Amphibolite, and

Granulite Facies


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ACF diagram

The most characteristicmineral assemblage of

the greenschist facies is:

chlorite + albite +

epidote + actinolite

quartz

Greenschist, Amphibolite, Granulite Facies

Fig. 25-6.ACF diagram illustrating

representative mineral assemblages for

metabasites in the greenschist facies. The

composition range of common mafic rocks is

shaded. Winter (2001) An Introduction to

Igneous and Metamorphic Petrology. Prentice

Hall.


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Fig. 26-19.Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined

univariant reactions in the CaO-MgO-Al2O3-SiO2-H2O-(Na2O) system (C(N)MASH). Winter (2001) AnIntroduction to Igneous and Metamorphic Petrology. Prentice Hall.


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Amphibolitegranulite facies~ 650-700oC

If aqueous fluid, associated pelitic and quartzo-

feldspathic rocks (including granitoids) begin to

melt in this range at low to medium pressures

migmatitesand melts may become mobilized

As a result not all pelites and quartzo-feldspathic

rocks reach the granulite facies



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Mafic rocks generally melt at higher temperatures

If water is removed by the earlier melts the

remaining mafic rocks may become depleted in

water

Hornblende decomposes and orthopyroxene +clinopyroxeneappear

This reaction occurs over a T interval > 50oC



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Fig. 26-19.Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined

univariant reactions in the CaO-MgO-Al2O3-SiO2-H2O-(Na2O) system (C(N)MASH). Winter (2001) An

Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


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Granulite faciescharacterized by a largely

anhydrousmineral assemblage


Critical meta-basite

mineral assemblage is

orthopyroxene +clinopyroxene +

plagioclase + quartz

Garnet, minor

hornblende and/orbiotite may be

presentFig. 25-8.ACF diagram for the granulite facies. The

composition range of common mafic rocks is shaded. Winter

(2001) An Introduction to Igneous and Metamorphic

Petrology. Prentice Hall.


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Origin of granulite facies rocks is complex and controversial.

There is general agreement, however, on two points1) Granulites represent unusually hot conditions

Temperatures > 700oC (geothermometry has yielded

some very high temperatures, even in excess of 1000o

C) Average geotherm temperatures for granulite facies

depths should be in the vicinity of 500oC, suggesting

that granulites are the products of crustal thickening and

excess heating



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2) Granulites are dry

Rocks dont melt due to lack of available water

Granulite facies terranes represent deeply buried and dehydrated

roots of the continental crust

Fluid inclusions in granulite facies rocks of S. Norway are CO2-

rich, whereas those in the amphibolite facies rocks are H2O-rich



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Fig. 25-9.Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the

medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included forcomparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Mafic Assemblages of the Low P/T Series: Albite


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Mineralogy of low-pressure metabasites not

appreciably different from the med.-P facies series

Albite-epidote hornfelsfacies correlates with the

greenschist facies into which it grades with

increasing pressure

Hornblende hornfels faciescorrelates with the

amphibolite facies, and thepyroxene hornfels and

sanidinite faciescorrelate with the granulite facies

Mafic Assemblages of the Low P/T Series: Albite-

Epidote Hornfels, Hornblende Hornfels, Pyroxene

Hornfels, and Sanidinite Facies


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Fig. 25-2.

Temperature-

pressure diagram

showing the

generally accepted

limits of the

various facies

used in this text.

Winter (2001) An

Introduction to

Igneous and

Metamorphic

Petrology.

Prentice Hall.



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Facies of contact metamorphism are readily

distinguished from those of medium-pressure

regional metamorphism on the basis of:

Metapelites(e.g. andalusite and cordierite)

Textures and field relationships

Mineral thermobarometry






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The innermost zone of most aureoles rarely reaches the

pyroxene hornfels facies

If the intrusion is hot and dry enough, a narrow zone

develops in which amphiboles break down toorthopyroxene + clinopyroxene + plagioclase + quartz

(without garnet), characterizing this facies

Sanidinite facies is not evident in basic rocks





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Blueschist and Eclogite Facies


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Alternative paths to the blueschist facies


Fig. 25-2.Temperature-

pressure diagram showing the

generally accepted limits ofthe various facies used in this

text. Winter (2001) An

Introduction to Igneous and

Metamorphic Petrology.

Prentice Hall.

Bl hi t d E l it F i


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Theblueschist faciesis characterized in metabasites by

the presence of a sodic blue amphibolestable only at highpressures (notably glaucophane, but some solution of

crossite or riebeckite is possible)

The association of glaucophane + lawsoniteis diagnostic.

Crossite is stable to lower pressures, and may extend intotransitional zones

Albite breaks down at high pressure by reaction to jadeitic

pyroxene + quartz:

NaAlSi3O8 = NaAlSi2O6+ SiO2 (reaction 25-3)

Ab Jd Qtz




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Fig. 25-10.ACF diagram illustrating

representative mineral assemblages for

metabasites in the blueschist facies. The

composition range of common mafic rocks is

shaded. Winter (2001) An Introduction to

Igneous and Metamorphic Petrology. Prentice

Hall.


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P Ch i


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Pyroxene Chemistry

Non-quad pyroxenesJadeite

NaAlSi2O6

Ca(Mg,Fe)Si2O6

Aegirine

NaFe3+Si2O6

Diopside-Hedenbergite

Ca-Tschermacks

molecule CaAl2SiO6

Ca / (Ca + Na)

0.2

0.8

Omphaciteaegirine-

augite

Augite

Pressure Temperature Time (P T t) Paths


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The facies seriesconcept suggests that a traverse up grade

through a metamorphic terrane should follow ametamorphic field gradient, and may cross through a

sequence of facies (spatialsequences)

Progressive metamorphism:rocks pass through a series of

mineral assemblages as they continuously equilibrate toincreasing metamorphic grade (temporalsequences)

But does a rock in the upper amphibolite facies, for

example, pass through the same sequence of mineral

assemblages that are encountered via a traverse up grade

to that rock through greenschist facies, etc.?

Pressure-Temperature-Time (P-T-t) Paths



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The complete set of T-P conditions that a rock may

experience during a metamorphic cycle from burial tometamorphism (and orogeny) to uplift and erosion is

called apressure-temperature-time path, or P-T-t path




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Metamorphic P-T-t paths may be addressed by:

1) Observing partial overprints of one mineral assemblageupon another

The relict minerals may indicate a portion of either the prograde

or retrograde path (or both) depending upon when they were

created



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Fig. 25-13a.Chemical zoning profiles across a garnet from the Tauern Window. After Spear (1989)


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Fig. 25-13a.Conventional P-T diagram (pressure increases upward) showing three modeled clockwise P-T-t paths

computed from the profiles using the method of Selverstone et al. (1984) J. Petrol ., 25, 501-531 and Spear (1989). After

Spear (1989) Metamorphi c Phase Equi l ibri a and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1.



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Metamorphic P-T-t paths may be addressed by:

Even under the best of circumstances (1) overprints and(2) geothermobarometry can usually document only a

small portion of the full P-T-t path

3) We thus rely on forward heat-flow models for various

tectonic regimes to compute more complete P-T-t paths,and evaluate them by comparison with the results of the

backward methods



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Fig. 25-12.Schematic pressure-temperature-time paths based on heat-flow models. The Al2SiO5phase diagram and

two hypothetical dehydration curves are included. Facies boundaries, and facies series from Figs. 25-2 and 25-3.Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


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Most examples of crustal thickening have the same

general looping shape, whether the model assumes

homogeneous thickening or thrusting of large masses,

conductive heat transfer or additional magmatic rise

Paths such as (a) are called clockwiseP-T-t paths in the

literature, and are considered to be the norm for regionalmetamorphism



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Fig. 25-12b.Schematic pressure-temperature-time paths based on a shallow magmatism heat-flow model. The Al2SiO5

phase diagram and two hypothetical dehydration curves are included. Facies boundaries, and facies series from Figs.25-2 and 25-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.


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1. Contrary to the classical treatment of

metamorphism, temperature and pressure do notboth increase in unison as a single unified

metamorphic grade.

Their relative magnitudes vary considerably during

the process of metamorphism




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2. Pmaxand Tmaxdo not occur at the same time

In the usual clockwise P-T-t paths, Pmaxoccursmuch earlier than Tmax.

Tmaxshould represent the maximum grade at which

chemical equilibrium is frozen in and the

metamorphic mineral assemblage is developed

This occurs at a pressure well below Pmax, which is

uncertain because a mineral geobarometer should

record the pressure of Tmax Metamorphic gradeshould refer to the temperature

and pressure at Tmax, because the grade is determined

via reference to the equilibrium mineral assemblage




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3. Some variations on the cooling-uplift portion of the

clockwise path (a) indicate some surprising

circumstances

For example, the kyanite sillimanite transition is

generally considered a prograde transition (as in path

a1), but path a2crosses the kyanite sillimanitetransition as temperature is decreasing. This may result

in only minor replacement of kyanite by sillimanite

during such a retrograde process



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Fig. 25-12a.Schematic pressure-temperature-time paths based on a crustal thickeningheat-flow model. The Al2SiO5

phase diagram and two hypothetical dehydration curves are included. Facies boundaries, and facies series from Figs.25-2 and 25-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.



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3. Some variations on the cooling-uplift portion of the

clockwise path (a) in Fig. 25-12 indicate some

surprising circumstances

If the P-T-t path is steeper than a dehydration reaction

curve, it is also possible that a dehydration reaction can

occur with decreasing temperature(although this is

only likely at low pressures where the dehydration

curve slope is low)



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Fig. 25-14.A typical Barrovian-type metamorphic field gradient and a series of metamorphic P-T-t paths for rocks

found along that gradient in the field. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. PrenticeHall.

Figures not used


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Figures not used

Fig. 25-4.ACF diagrams illustrating representative mineral

assemblages for metabasites in the (a) zeolite and (b)

prehnite-pumpellyite facies. Actinolite is stable only in the

upperprehnite-pumpellyite facies. The composition range of

common mafic rocks is shaded. Winter (2001) An

Introduction to Igneous and Metamorphic Petrology.Prentice Hall.


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Figures

not used

Fi 25 5 T i l i l h th t t k l i t b i k d i i t hi i th lit

Documents

Facies and Mafic