chapter 25. metamorphic facies and metamorphosed mafic rocks l v.m. goldschmidt (1911, 1912a),...

Chapter 25. Metamorphic Facies and Chapter 25. Metamorphic Facies and Metamorphosed Mafic RocksMetamorphosed Mafic Rocks

V.M. Goldschmidt (1911, 1912a), contact V.M. Goldschmidt (1911, 1912a), contact metamorphosed pelitic, calcareous, and metamorphosed pelitic, calcareous, and psammitic hornfelses in the Oslo regionpsammitic hornfelses in the Oslo region

Relatively simple mineral assemblages of fewer Relatively simple mineral assemblages of fewer than six major minerals in the inner zones of the than six major minerals in the inner zones of the aureoles around granitoid intrusivesaureoles around granitoid intrusives

Equilibrium mineral assemblage related to XEquilibrium mineral assemblage related to Xbulkbulk

Certain mineral pairs (e.g. anorthite + hypersthene) Certain mineral pairs (e.g. anorthite + hypersthene) were consistently present in rocks of appropriate were consistently present in rocks of appropriate composition, whereas the composition, whereas the compositionally compositionally equivalentequivalent pair (diopside + andalusite) was not pair (diopside + andalusite) was not

If two alternative assemblages are X-equivalent, If two alternative assemblages are X-equivalent, we must be able to relate them by a reactionwe must be able to relate them by a reaction

In this case the reaction is simple:In this case the reaction is simple:

MgSiOMgSiO33 + CaAl + CaAl22SiSi22OO88 = CaMgSi = CaMgSi22OO66 + Al + Al22SiOSiO55

EnEn An An Di Di Als Als

Metamorphic FaciesMetamorphic Facies

Pentii Eskola (1914, 1915) Orijärvi region of Pentii Eskola (1914, 1915) Orijärvi region of southern Finlandsouthern Finland

Rocks with Rocks with K-feldspar + cordieriteK-feldspar + cordierite at Oslo at Oslo contained the compositionally equivalent pair contained the compositionally equivalent pair biotite + muscovitebiotite + muscovite at Orijärvi at Orijärvi

Eskola concluded that the difference must reflect Eskola concluded that the difference must reflect differing physical conditions between the regionsdiffering physical conditions between the regions

Concluded that Finnish rocks (with a more Concluded that Finnish rocks (with a more hydrous nature and lower volume assemblage) hydrous nature and lower volume assemblage) equilibrated at lower temperatures and higher equilibrated at lower temperatures and higher pressures than the Norwegian onespressures than the Norwegian ones


Oslo: Oslo: Ksp + CordKsp + Cord

Orijärvi: Orijärvi: Bi + MuBi + Mu

Reaction: Reaction: 2 KMg2 KMg33AlSiAlSi33OO1010(OH)(OH)22 + 6 KAl + 6 KAl22AlSiAlSi33OO1010(OH)(OH)22 + 15 SiO + 15 SiO22

BtBt MsMs Qtz Qtz

= 3 Mg= 3 Mg22AlAl44SiSi55OO1818 + 8 KAlSi + 8 KAlSi33OO88 + 8 H + 8 H22OO

CrdCrd Kfs Kfs


Eskola (1915) developed the concept of Eskola (1915) developed the concept of metamorphic facies:metamorphic facies:

““In any rock or metamorphic formation which has In any rock or metamorphic formation which has arrived at a chemical equilibrium through arrived at a chemical equilibrium through metamorphism at constant temperature and pressure metamorphism at constant temperature and pressure conditions, conditions, the mineral composition is controlled only the mineral composition is controlled only by the chemical compositionby the chemical composition. We are led to a general . We are led to a general conception which the writer proposes to call conception which the writer proposes to call metamorphic facies.”metamorphic facies.”


Dual basis for the facies conceptDual basis for the facies concept DescriptiveDescriptive: the relationship between the composition of : the relationship between the composition of

a rock and its mineralogya rock and its mineralogy This descriptive aspect was a fundamental feature of This descriptive aspect was a fundamental feature of

Eskola’s conceptEskola’s concept A metamorphic facies is then a set of repeatedly A metamorphic facies is then a set of repeatedly

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

group of compatible assemblages covering a range of group of compatible assemblages covering a range of compositions) in the field, then a certain facies may compositions) in the field, then a certain facies may be assigned to the areabe assigned to the area


InterpretiveInterpretive: the range of temperature and pressure : the range of temperature and pressure conditions represented by each faciesconditions represented by each facies Eskola was aware of the temperature-pressure Eskola was aware of the temperature-pressure

implications of the concept and correctly deduced the implications of the concept and correctly deduced the relativerelative temperatures and pressures represented by temperatures and pressures represented by the different facies that he proposedthe different facies that he proposed

We can now assign relatively accurate temperature We can now assign relatively accurate temperature and pressure limits to individual faciesand pressure limits to individual facies


Eskola (1920) proposed 5 original facies:Eskola (1920) proposed 5 original facies: GreenschistGreenschist AmphiboliteAmphibolite HornfelsHornfels SanidiniteSanidinite EclogiteEclogite

Each easily defined on the basis of mineral Each easily defined on the basis of mineral assemblages that develop in assemblages that develop in maficmafic rocks rocks


In his final account, Eskola (1939) added:In his final account, Eskola (1939) added: GranuliteGranulite Epidote-amphiboliteEpidote-amphibolite Glaucophane-schist Glaucophane-schist (now called (now called BlueschistBlueschist))

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

His facies, and his estimate of their relative His facies, and his estimate of their relative temperature-pressure relationships are shown in temperature-pressure relationships are shown in Fig. 25-1Fig. 25-1


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


Formation of Zeolites

Temperature

Pre

ssur

e GreenschistFacies

Epidote-Amphibolite

Facies

AmphiboliteFacies

Pyroxene-HornfelsFacies

Glaucophane-Schist Facies Eclogite

Facies

GranuliteFacies

SanadiniteFacies

Several additional facies types have been proposed. Several additional facies types have been proposed. Most notable are:Most notable are: ZeoliteZeolite Prehnite-pumpellyitePrehnite-pumpellyite...resulting from the work of Coombs in the “burial ...resulting from the work of Coombs in the “burial

metamorphic” terranes of New Zealandmetamorphic” terranes of New Zealand

Fyfe Fyfe et alet al. (1958) also proposed:. (1958) also proposed: Albite-epidote hornfelsAlbite-epidote hornfels Hornblende hornfelsHornblende hornfels



Fig. 25-2.Fig. 25-2. Temperature-Temperature-pressure diagram pressure diagram showing the generally showing the generally accepted limits of the accepted limits of the various facies used in this various facies used in this text. Boundaries are text. Boundaries are approximate and approximate and gradational. The gradational. The “typical” or average “typical” or average continental geotherm is continental geotherm is from Brown and Mussett from Brown and Mussett (1993). (1993). Winter (2001) An Winter (2001) An Introduction to Igneous Introduction to Igneous and Metamorphic and Metamorphic Petrology. Prentice Hall.Petrology. Prentice Hall.

Table 25-1. The definitive mineral assemblages Table 25-1. The definitive mineral assemblages that characterize each facies (for mafic rocks).that characterize each facies (for mafic rocks).


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.

It is convenient to consider metamorphic facies in 4 groups:It is convenient to consider metamorphic facies in 4 groups:

1) Facies of high pressure1) Facies of high pressure The The blueschistblueschist and and eeccllooggiittee facies: low molar volume facies: low molar volume

phases under conditions of high pressurephases under conditions of high pressure The lower-temperature The lower-temperature blueschistblueschist facies occurs in facies occurs in

areas of low T/P gradients, characteristically areas of low T/P gradients, characteristically developed in developed in subduction zonessubduction zones

BecauseBecause e eccllooggiitteess are stable under normal geothermal are stable under normal geothermal conditions, they may develop wherever mafic magmas conditions, they may develop wherever mafic magmas solidify in the deep crust or mantle (crustal chambers solidify in the deep crust or mantle (crustal chambers or dikes, sub-crustal magmatic underplates, subducted or dikes, sub-crustal magmatic underplates, subducted crust that is redistributed into the mantle)crust that is redistributed into the mantle)

2) Facies of medium pressure2) Facies of medium pressure Most metamorphic rocks now exposed at the surface Most metamorphic rocks now exposed at the surface

of the Earth belong to the of the Earth belong to the greenschistgreenschist, , amphiboliteamphibolite, or , or granulitegranulite facies facies

As you can see in Fig. 25-2, the As you can see in Fig. 25-2, the greenschistgreenschist and and amphiboliteamphibolite facies conform to the “typical” geothermal facies conform to the “typical” geothermal gradientgradient


Fig. 25-2.Fig. 25-2. Temperature-Temperature-pressure diagram pressure diagram showing the showing the generally generally accepted limits of accepted limits of the various facies the various facies used in this text. used in this text. Winter (2001) An Winter (2001) An Introduction to Introduction to Igneous and Igneous and Metamorphic Metamorphic Petrology. Petrology. Prentice Hall.Prentice Hall.

3) Facies of low pressure3) Facies of low pressure The The albite-epidote hornfelsalbite-epidote hornfels, , hornblende hornfelshornblende hornfels, and , and

pyroxene hornfelspyroxene hornfels facies: contact metamorphic facies: contact metamorphic terranes and regional terranes with very high terranes and regional terranes with very high geothermal gradientsgeothermal gradients

The The sanidinitesanidinite facies is rare and limited to xenoliths in facies is rare and limited to xenoliths in basic magmas and the innermost portions of some basic magmas and the innermost portions of some contact aureoles adjacent to hot basic intrusivescontact aureoles adjacent to hot basic intrusives


4) Facies of low grades4) Facies of low grades Rocks often fail to recrystallize thoroughly at very low Rocks often fail to recrystallize thoroughly at very low

grades, and equilibrium is not always attainedgrades, and equilibrium is not always attained The The zeolitezeolite and and prehnite-pumpellyiteprehnite-pumpellyite facies are thus not facies are thus not

always represented, and the always represented, and the greenschistgreenschist facies is the facies is the lowest grade developed in many regional terraneslowest grade developed in many regional terranes


Combine the concepts of isograds, zones, and faciesCombine the concepts of isograds, zones, and facies Examples: “chlorite zone of the greenschist facies,” the Examples: “chlorite zone of the greenschist facies,” the

“staurolite zone of the amphibolite facies,” or the “staurolite zone of the amphibolite facies,” or the “cordierite zone of the hornblende hornfels facies,” etc. “cordierite zone of the hornblende hornfels facies,” etc.

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

Determining a facies or zone is most reliably done Determining a facies or zone is most reliably done when several rocks of varying composition and when several rocks of varying composition and mineralogy are availablemineralogy are available


A traverse up grade through a metamorphic terrane should A traverse up grade through a metamorphic terrane should follow one of several possible metamorphic field gradients follow one of several possible metamorphic field gradients (Fig. 21-1), and, if extensive enough, cross through a (Fig. 21-1), and, if extensive enough, cross through a sequence of faciessequence of facies

Facies SeriesFacies Series

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: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.

Miyashiro (1961) initially proposed five facies series, Miyashiro (1961) initially proposed five facies series, most of them named for a specific representative “type most of them named for a specific representative “type locality” The series were:locality” The series were:

1. Contact Facies Series (very low-P)1. Contact Facies Series (very low-P)

2. Buchan or Abukuma Facies Series (low-P 2. Buchan or Abukuma Facies Series (low-P regional)regional)

3. Barrovian Facies Series (medium-P regional)3. Barrovian Facies Series (medium-P regional)

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

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

Facies SeriesFacies Series

Fig. 25-3.Fig. 25-3. Temperature-Temperature-pressure diagram pressure diagram showing the three showing the three major types of major types of metamorphic metamorphic facies series facies series proposed by proposed by Miyashiro (1973, Miyashiro (1973, 1994). 1994). Winter Winter (2001) An (2001) An Introduction to Introduction to Igneous and Igneous and Metamorphic Metamorphic Petrology. Petrology. Prentice Hall.Prentice Hall.

Mineral changes and associations along T-P gradients Mineral changes and associations along T-P gradients characteristic of the three facies seriescharacteristic of the three facies series HydrationHydration of original mafic minerals required for the development of original mafic minerals required for the development

of the metamorphic mineral assemblages of most faciesof the metamorphic mineral assemblages of most facies If water is unavailable, mafic igneous rocks will remain largely If water is unavailable, mafic igneous rocks will remain largely

unaffected in metamorphic terranes, even as associated sediments unaffected in metamorphic terranes, even as associated sediments are completely re-equilibratedare completely re-equilibrated

Coarse-grained intrusives are the least permeable, and thus most Coarse-grained intrusives are the least permeable, and thus most likely to resist metamorphic changeslikely to resist metamorphic changes

Tuffs and graywackes are the most susceptibleTuffs and graywackes are the most susceptible

Metamorphism of Mafic RocksMetamorphism of Mafic Rocks

Plagioclase:Plagioclase: More Ca-richMore Ca-rich plagioclases become progressively unstable plagioclases become progressively unstable

as T loweredas T lowered General correlation between temperature and the maximum General correlation between temperature and the maximum

An-content of the stable plagioclaseAn-content of the stable plagioclase At low metamorphic grades only At low metamorphic grades only albitealbite (An (An0-30-3) is stable) is stable In the upper-greenschist facies In the upper-greenschist facies oligoclaseoligoclase becomes stable. The becomes stable. The

An-content of plagioclase thus jumps from AnAn-content of plagioclase thus jumps from An1-71-7 to An to An17-2017-20 (across (across

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

amphibolite and granulite faciesamphibolite and granulite facies The excess Ca and Al The excess Ca and Al calcite, an epidote mineral, calcite, an epidote mineral,

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


ClinopyroxeneClinopyroxene breaks down to a number of mafic minerals, breaks down to a number of mafic minerals, depending on grade. depending on grade.

These minerals include chlorite, actinolite, hornblende, These minerals include chlorite, actinolite, hornblende, epidote, a metamorphic pyroxene, etc.epidote, a metamorphic pyroxene, etc.

The mafic(s) that form are commonly diagnostic of the The mafic(s) that form are commonly diagnostic of the grade and faciesgrade and facies


ZeoliteZeolite and and prehnite-pumpellyite faciesprehnite-pumpellyite facies Do not always occur - typically require unstable protolithDo not always occur - typically require unstable protolith Boles and Coombs (1975) showed that metamorphism of Boles and Coombs (1975) showed that metamorphism of

tuffs in NZ accompanied by substantial chemical changes tuffs in NZ accompanied by substantial chemical changes due to circulating fluids, and that these fluids played an due to circulating fluids, and that these fluids played an important role in the metamorphic minerals that were important role in the metamorphic minerals that were stablestable

The classic area of The classic area of burial metamorphismburial metamorphism thus has a strong thus has a strong component of component of hydrothermal metamorphismhydrothermal metamorphism as well as well

Mafic Assemblages at Low GradesMafic Assemblages at Low Grades

The The greenschistgreenschist, , amphiboliteamphibolite and and granulitegranulite facies constitute facies constitute the most common facies series of regional metamorphismthe most common facies series of regional metamorphism

The classical Barrovian series of pelitic zones and the The classical Barrovian series of pelitic zones and the lower-pressure Buchan-Abukuma series are variations on lower-pressure Buchan-Abukuma series are variations on this trend this trend

Mafic Assemblages of the Medium P/T Mafic Assemblages of the Medium P/T Series: Greenschist, Amphibolite, and Series: Greenschist, Amphibolite, and

Granulite FaciesGranulite Facies

The zeolite and prehnite-pumpellyite facies are not present The zeolite and prehnite-pumpellyite facies are not present in the Scottish Highlandsin the Scottish Highlands

Metamorphism of mafic rocks is first evident in the Metamorphism of mafic rocks is first evident in the greenschistgreenschist facies, which correlates with the facies, which correlates with the chlorite and chlorite and biotite zonesbiotite zones of the associated pelitic rocks of the associated pelitic rocks Typical minerals include Typical minerals include chlorite, albite, actinolite, chlorite, albite, actinolite,

epidote, quartzepidote, quartz, and possibly calcite, biotite, or , and possibly calcite, biotite, or stilpnomelanestilpnomelane

Chlorite, actinolite, and epidote impart the green color Chlorite, actinolite, and epidote impart the green color from which the mafic rocks and facies get their name from which the mafic rocks and facies get their name

Greenschist, Amphibolite, Granulite FaciesGreenschist, Amphibolite, Granulite Facies

ACF diagramACF diagram The most characteristic The most characteristic

mineral assemblage of mineral assemblage of the greenschist facies is: the greenschist facies is: chlorite + albite + chlorite + albite + epidote + actinolite epidote + actinolite quartzquartz


Fig. 25-6.Fig. 25-6. ACF diagram illustrating ACF diagram illustrating representative mineral assemblages for representative mineral assemblages for metabasites in the greenschist facies. The metabasites in the greenschist facies. The composition range of common mafic rocks is composition range of common mafic rocks is shaded. shaded. Winter (2001) An Introduction to Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Igneous and Metamorphic Petrology. Prentice Hall.Hall.

Greenschist to amphibolite facies transition Greenschist to amphibolite facies transition involves involves twotwo major mineralogical changes major mineralogical changes

1. Transition from 1. Transition from albite to oligoclasealbite to oligoclase (increased (increased Ca-content of stable plagioclase with temperature Ca-content of stable plagioclase with temperature across the peristerite gap)across the peristerite gap)

2. Transition from 2. Transition from actinolite to hornblendeactinolite to hornblende (amphibole becomes able to accept increasing (amphibole becomes able to accept increasing amounts of aluminum and alkalis at higher amounts of aluminum and alkalis at higher temperatures)temperatures)

Both of these transitions occur at approximately Both of these transitions occur at approximately the same grade, but have different P/T slopesthe same grade, but have different P/T slopes


Fig. 26-19.Fig. 26-19. Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined univariant reactions in the CaO-MgO-Alunivariant reactions in the CaO-MgO-Al

22OO33-SiO-SiO22-H-H

22O-(NaO-(Na22O) system (“C(N)MASH”). O) system (“C(N)MASH”). Winter (2001) An Winter (2001) An

Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

ACF diagramACF diagram Typically two-phase Hbl-Plag Typically two-phase Hbl-Plag Amphibolites are typically Amphibolites are typically

black rocks with up to about black rocks with up to about 30% white plagioclase30% white plagioclase

Garnet in more Al-Fe-rich and Garnet in more Al-Fe-rich and Ca-poor mafic rocksCa-poor mafic rocks

Clinopyroxene in Al-poor-Ca-Clinopyroxene in Al-poor-Ca-rich rocksrich rocks


Fig. 25-7.Fig. 25-7. ACF diagram illustrating representative ACF diagram illustrating representative mineral assemblages for metabasites in the amphibolite mineral assemblages for metabasites in the amphibolite facies. The composition range of common mafic rocks is facies. The composition range of common mafic rocks is shaded. shaded. Winter (2001) An Introduction to Igneous and Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Metamorphic Petrology. Prentice Hall.

The transition from The transition from amphibolite to granulite faciesamphibolite to granulite facies occurs in the range 650-700occurs in the range 650-700ooCC

If aqueous fluid, associated pelitic and quartzo-If aqueous fluid, associated pelitic and quartzo-feldspathic rocks (including granitoids) begin to feldspathic rocks (including granitoids) begin to melt in this range at low to medium pressures , so melt in this range at low to medium pressures , so that that migmatitesmigmatites may form and the melts may may form and the melts may become mobilizedbecome mobilized

Not all pelites and quartzo-feldspathic rocks reach Not all pelites and quartzo-feldspathic rocks reach the granulite facies as a resultthe granulite facies as a result


Mafic rocks generally melt at somewhat higher Mafic rocks generally melt at somewhat higher temperaturestemperatures

If water is removed by the earlier melts the If water is removed by the earlier melts the remaining mafic rocks may become depleted in remaining mafic rocks may become depleted in waterwater

Hornblende decomposes and Hornblende decomposes and orthopyroxene + orthopyroxene + clinopyroxeneclinopyroxene appear appear

This reaction occurs over a temperature interval of This reaction occurs over a temperature interval of at least 50at least 50ooCC


Fig. 26-19.Fig. 26-19. Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined univariant reactions in the CaO-MgO-Alunivariant reactions in the CaO-MgO-Al

22OO33-SiO-SiO22-H-H

22O-(NaO-(Na22O) system (“C(N)MASH”). O) system (“C(N)MASH”). Winter (2001) An Winter (2001) An

Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

The The granulite faciesgranulite facies is characterized by the presence of a is characterized by the presence of a largely anhydrous mineral assemblagelargely anhydrous mineral assemblage


Metabasites critical Metabasites critical mineral assemblage is mineral assemblage is orthopyroxene + orthopyroxene + clinopyroxene + clinopyroxene + plagioclase + quartzplagioclase + quartz Garnet, minor Garnet, minor

hornblende and/or hornblende and/or biotite may be biotite may be presentpresent

Fig. 25-8.Fig. 25-8. ACF diagram for the granulite facies. The ACF diagram for the granulite facies. The composition range of common mafic rocks is shaded. composition range of common mafic rocks is shaded. Winter Winter (2001) An Introduction to Igneous and Metamorphic (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Petrology. Prentice Hall.

The origin of granulite facies rocks is complex and The origin of granulite facies rocks is complex and controversial. There is general agreement, however, on two controversial. There is general agreement, however, on two pointspoints

1) Granulites represent unusually hot conditions1) Granulites represent unusually hot conditions Temperatures > 700Temperatures > 700ooC (geothermometry has yielded C (geothermometry has yielded

some very high temperatures, even in excess of 1000some very high temperatures, even in excess of 1000ooC)C) Average geotherm temperatures for granulite facies Average geotherm temperatures for granulite facies

depths should be in the vicinity of 500depths should be in the vicinity of 500ooC, suggesting C, suggesting that granulites are the products of that granulites are the products of crustal thickening and crustal thickening and excess heatingexcess heating


2) Granulites are dry2) Granulites are dry Rocks don’t melt due to lack of available waterRocks don’t melt due to lack of available water Granulite facies terranes represent deeply buried and dehydrated Granulite facies terranes represent deeply buried and dehydrated

roots of the continental crustroots of the continental crust Fluid inclusions in granulite facies rocks of S. Norway are COFluid inclusions in granulite facies rocks of S. Norway are CO22--

rich, whereas those in the amphibolite facies rocks are Hrich, whereas those in the amphibolite facies rocks are H22O-richO-rich


Fig. 25-9.Fig. 25-9. Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the 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 medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison. for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Mineralogy of low-pressure metabasites not Mineralogy of low-pressure metabasites not appreciably different from the med.-P facies series appreciably different from the med.-P facies series

Albite-epidote hornfelsAlbite-epidote hornfels facies correlates with the facies correlates with the greenschist facies into which it grades with greenschist facies into which it grades with increasing pressureincreasing pressure

Similarly the Similarly the hornblende hornfels facieshornblende hornfels facies correlates correlates with the amphibolite facies, and the with the amphibolite facies, and the pyroxene pyroxene hornfels and sanidinite facieshornfels and sanidinite facies correlate with the correlate with the granulite faciesgranulite facies

Mafic Assemblages of the Low P/T Series: Albite-Mafic Assemblages of the Low P/T Series: Albite-Epidote Hornfels, Hornblende Hornfels, Pyroxene Epidote Hornfels, Hornblende Hornfels, Pyroxene

Hornfels, and Sanidinite FaciesHornfels, and Sanidinite Facies

The facies of contact metamorphism are readily The facies of contact metamorphism are readily distinguished from those of medium-pressure distinguished from those of medium-pressure regional metamorphism on the basis of:regional metamorphism on the basis of: MetapelitesMetapelites (e.g. andalusite and cordierite) (e.g. andalusite and cordierite) Textures and field relationshipsTextures and field relationships Mineral thermobarometryMineral thermobarometry



The innermost zone of most aureoles rarely reaches the The innermost zone of most aureoles rarely reaches the pyroxene hornfels faciespyroxene hornfels facies If the intrusion is hot and dry enough, a narrow zone If the intrusion is hot and dry enough, a narrow zone

develops in which amphiboles break down to develops in which amphiboles break down to orthopyroxene + clinopyroxene + plagioclase + quartz orthopyroxene + clinopyroxene + plagioclase + quartz (without garnet), characterizing this facies(without garnet), characterizing this facies

Sanidinite facies is not evident in basic rocksSanidinite facies is not evident in basic rocks



The The maficmafic rocks (not the pelites) develop conspicuous and rocks (not the pelites) develop conspicuous and definitive mineral assemblages under high P/T conditionsdefinitive mineral assemblages under high P/T conditions

High P/T geothermal gradients characterize High P/T geothermal gradients characterize subduction zonessubduction zones Mafic Mafic blueschistsblueschists are easily recognizable by their color, and are are easily recognizable by their color, and are

useful indicators of ancient subduction zonesuseful indicators of ancient subduction zones The great density of The great density of eclogiteseclogites: subducted basaltic oceanic crust : subducted basaltic oceanic crust

becomes more dense than the surrounding mantlebecomes more dense than the surrounding mantle

Mafic Assemblages of the High P/T Series: Mafic Assemblages of the High P/T Series: Blueschist and Eclogite FaciesBlueschist and Eclogite Facies

Alternative paths to the blueschist faciesAlternative paths to the blueschist facies

Blueschist and Eclogite FaciesBlueschist and Eclogite Facies

Fig. 25-2.Fig. 25-2. Temperature-Temperature-pressure diagram showing the pressure diagram showing the generally accepted limits of generally accepted limits of the various facies used in this the various facies used in this text. text. Winter (2001) An Winter (2001) An Introduction to Igneous and Introduction to Igneous and Metamorphic Petrology. Metamorphic Petrology. Prentice Hall.Prentice Hall.

The The blueschist faciesblueschist facies is characterized in metabasites by is characterized in metabasites by the presence of a the presence of a sodic blue amphibolesodic blue amphibole stable only at high stable only at high pressures (notably glaucophane, but some solution of pressures (notably glaucophane, but some solution of crossite or riebeckite is possible)crossite or riebeckite is possible)

The association of The association of glaucophane + lawsoniteglaucophane + lawsonite is diagnostic. is diagnostic. Crossite is stable to lower pressures, and may extend into Crossite is stable to lower pressures, and may extend into transitional zonestransitional zones

Albite breaks down at high pressure by reaction to jadeitic Albite breaks down at high pressure by reaction to jadeitic pyroxene + quartz:pyroxene + quartz:

NaAlSiNaAlSi33OO88 = NaAlSi = NaAlSi22OO66 + SiO + SiO22 (reaction 25-3)(reaction 25-3)

AbAb Jd Jd Qtz Qtz



Fig. 25-10.Fig. 25-10. ACF diagram illustrating ACF diagram illustrating representative mineral assemblages for representative mineral assemblages for metabasites in the blueschist facies. The metabasites in the blueschist facies. The composition range of common mafic rocks is composition range of common mafic rocks is shaded. shaded. Winter (2001) An Introduction to Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Igneous and Metamorphic Petrology. Prentice Hall.Hall.

Eclogite facies:Eclogite facies: mafic assemblage omphacitic pyroxene mafic assemblage omphacitic pyroxene + pyrope-grossular garnet+ pyrope-grossular garnet

Fig. 25-11.Fig. 25-11. ACF diagram illustrating ACF diagram illustrating representative mineral assemblages for representative mineral assemblages for metabasites in the eclogite facies. The metabasites in the eclogite facies. The composition range of common mafic rocks is composition range of common mafic rocks is shaded. shaded. Winter (2001) An Introduction to Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Igneous and Metamorphic Petrology. Prentice Hall.Hall.


The The facies seriesfacies series concept suggests that a traverse up grade concept suggests that a traverse up grade through a metamorphic terrane should follow a through a metamorphic terrane should follow a metamorphic field gradientmetamorphic field gradient, and may cross through a , and may cross through a sequence of facies (sequence of facies (spatialspatial sequences) sequences)

Progressive metamorphism:Progressive metamorphism: rocks pass through a series of rocks pass through a series of mineral assemblages as they continuously equilibrate to mineral assemblages as they continuously equilibrate to increasing metamorphic grade (increasing metamorphic grade (temporaltemporal sequences) sequences)

But does a rock in the upper amphibolite facies, for But does a rock in the upper amphibolite facies, for example, pass through the same sequence of mineral example, pass through the same sequence of mineral assemblages that are encountered via a traverse up grade assemblages that are encountered via a traverse up grade to that rock through greenschist facies, etc.? to that rock through greenschist facies, etc.?

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

The complete set of T-P conditions that a rock may The complete set of T-P conditions that a rock may experience during a metamorphic cycle from burial to experience during a metamorphic cycle from burial to metamorphism (and orogeny) to uplift and erosion is metamorphism (and orogeny) to uplift and erosion is called a called a pressure-temperature-time path, or P-T-t pathpressure-temperature-time path, or P-T-t path


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

1) Observing partial overprints of one mineral assemblage 1) Observing partial overprints of one mineral assemblage upon anotherupon another The relict minerals may indicate a portion of either the prograde The relict minerals may indicate a portion of either the prograde

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


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

2) Apply geothermometers and geobarometers to the core 2) Apply geothermometers and geobarometers to the core vs. rim compositions of chemically zoned minerals to vs. rim compositions of chemically zoned minerals to document the changing P-T conditions experienced by a document the changing P-T conditions experienced by a rock during their growthrock during their growth


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

Fig. 25-13a.Fig. 25-13a. Conventional P-T diagram (pressure increases upward) showing three modeled “clockwise” P-T-t paths Conventional P-T diagram (pressure increases upward) showing three modeled “clockwise” P-T-t paths computed from the profiles using the method of Selverstone computed from the profiles using the method of Selverstone et alet al. (1984) . (1984) J. Petrol.J. Petrol., 25, 501-531 and Spear (1989). , 25, 501-531 and Spear (1989). After Spear (1989) After Spear (1989) Metamorphic Phase Equilibria and Pressure-Temperature-Time PathsMetamorphic Phase Equilibria and Pressure-Temperature-Time Paths . Mineral. Soc. Amer. . Mineral. Soc. Amer. Monograph 1.Monograph 1.

Metamorphic P-T-t paths may be addressed by:Metamorphic P-T-t paths may be addressed by: Even under the best of circumstances (1) overprints and Even under the best of circumstances (1) overprints and

(2) geothermobarometry can usually document only a (2) geothermobarometry can usually document only a small portion of the full P-T-t pathsmall portion of the full P-T-t path

3) We thus rely on “forward” heat-flow models for various 3) We thus rely on “forward” heat-flow models for various tectonic regimes to compute more complete P-T-t paths, tectonic regimes to compute more complete P-T-t paths, and evaluate them by comparison with the results of the and evaluate them by comparison with the results of the backward methodsbackward methods


Classic view: regional metamorphism is a result of deep burial or Classic view: regional metamorphism is a result of deep burial or intrusion of hot magmasintrusion of hot magmas

Plate tectonics: regional metamorphism is a result of crustal Plate tectonics: regional metamorphism is a result of crustal thickening and heat input during orogeny at convergent plate thickening and heat input during orogeny at convergent plate boundaries (not simple burial)boundaries (not simple burial)

Heat-flow models have been developed for various regimes, Heat-flow models have been developed for various regimes, including including burial, progressive thrust stacking, crustal doubling by burial, progressive thrust stacking, crustal doubling by continental collision, and the effects of crustal anatexis and magma continental collision, and the effects of crustal anatexis and magma migrationmigration Higher than the normal heat flow is required for typical Higher than the normal heat flow is required for typical

greenschist-amphibolite medium P/T facies seriesgreenschist-amphibolite medium P/T facies series Uplift and erosion has a fundamental effect on the geotherm and Uplift and erosion has a fundamental effect on the geotherm and

must be considered in any complete model of metamorphismmust be considered in any complete model of metamorphism


Fig. 25-12.Fig. 25-12. Schematic pressure-temperature-time paths based on heat-flow models. The AlSchematic pressure-temperature-time paths based on heat-flow models. The Al22SiOSiO

55 phase diagram and phase diagram and

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

Fig. 25-12a.Fig. 25-12a. Schematic pressure-temperature-time paths based on a Schematic pressure-temperature-time paths based on a crustal thickeningcrustal thickening heat-flow model. The Al heat-flow model. The Al22SiOSiO

55

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

Most examples of crustal thickening have the same Most examples of crustal thickening have the same general looping shape, whether the model assumes general looping shape, whether the model assumes homogeneous thickening or thrusting of large masses, homogeneous thickening or thrusting of large masses, conductive heat transfer or additional magmatic riseconductive heat transfer or additional magmatic rise

Paths such as (a) are called Paths such as (a) are called “clockwise”“clockwise” P-T-t paths in the P-T-t paths in the literature, and are considered to be the norm for regional literature, and are considered to be the norm for regional metamorphismmetamorphism


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

22SiOSiO55 phase diagram and two hypothetical dehydration curves are included. Facies boundaries, and facies series phase diagram and two hypothetical dehydration curves are included. Facies boundaries, and facies series

from Figs. 25-2 and 25-3. from Figs. 25-2 and 25-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

Fig. 25-12c.Fig. 25-12c. Schematic pressure-temperature-time paths based on a heat-flow model for some types ofSchematic pressure-temperature-time paths based on a heat-flow model for some types of granulite facies granulite facies metamorphismmetamorphism. Facies boundaries, and facies series from Figs. 25-2 and 25-3. . Facies boundaries, and facies series from Figs. 25-2 and 25-3. Winter (2001) An Introduction to Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Igneous and Metamorphic Petrology. Prentice Hall.

Broad agreement between the forward (model) and Broad agreement between the forward (model) and backward (geothermobarometry) techniques regarding P-backward (geothermobarometry) techniques regarding P-T-t pathsT-t paths

The general form of a path such as (a) therefore probably The general form of a path such as (a) therefore probably represents a typical rock during orogeny and regional represents a typical rock during orogeny and regional metamorphismmetamorphism


1. Contrary to the classical treatment of 1. Contrary to the classical treatment of metamorphism, metamorphism, temperature and pressure do not temperature and pressure do not both increase in unison as a single unified both increase in unison as a single unified “metamorphic grade.”“metamorphic grade.”

Their relative magnitudes vary considerably during Their relative magnitudes vary considerably during the process of metamorphismthe process of metamorphism


2. P2. Pmaxmax and T and Tmaxmax do not occur at the same time do not occur at the same time In the usual “clockwise” P-T-t paths, In the usual “clockwise” P-T-t paths, PPmaxmax occurs much occurs much

earlier than Tearlier than Tmaxmax. .

TTmaxmax should represent the maximum grade at which should represent the maximum grade at which

chemical equilibrium is “frozen in” and the chemical equilibrium is “frozen in” and the metamorphic mineral assemblage is developedmetamorphic mineral assemblage is developed

This occurs at a pressure well below PThis occurs at a pressure well below Pmaxmax, which is , which is

uncertain because a mineral geobarometer should uncertain because a mineral geobarometer should record the pressure of Trecord the pressure of Tmaxmax

““Metamorphic grade”Metamorphic grade” should refer to the temperature should refer to the temperature and pressure and pressure at Tat Tmaxmax, because the grade is determined , because the grade is determined

via reference to the equilibrium mineral assemblagevia reference to the equilibrium mineral assemblage


3. Some variations on the cooling-uplift portion of the 3. Some variations on the cooling-uplift portion of the “clockwise” path (a) indicate some surprising “clockwise” path (a) indicate some surprising circumstancescircumstances For example, the kyanite For example, the kyanite sillimanite transition is sillimanite transition is

generally considered a prograde transition (as in path generally considered a prograde transition (as in path aa11), but path a), but path a22 crosses the kyanite crosses the kyanite sillimanite sillimanite

transition as temperature is transition as temperature is decreasing. decreasing. This may result This may result in only minor replacement of kyanite by sillimanite in only minor replacement of kyanite by sillimanite during such a retrograde processduring such a retrograde process


Fig. 25-12a.Fig. 25-12a. Schematic pressure-temperature-time paths based on a Schematic pressure-temperature-time paths based on a crustal thickeningcrustal thickening heat-flow model. The Al heat-flow model. The Al22SiOSiO

55

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

3. Some variations on the cooling-uplift portion of the 3. Some variations on the cooling-uplift portion of the “clockwise” path (a) in Fig. 25-12 indicate some “clockwise” path (a) in Fig. 25-12 indicate some surprising circumstancessurprising circumstances If the P-T-t path is steeper than a dehydration reaction If the P-T-t path is steeper than a dehydration reaction

curve, it is also possible that curve, it is also possible that a dehydration reaction can a dehydration reaction can occur with decreasing temperatureoccur with decreasing temperature (although this is (although this is only likely at low pressures where the dehydration only likely at low pressures where the dehydration curve slope is low)curve slope is low)


Fig. 25-14.Fig. 25-14. A typical Barrovian-type metamorphic field gradient and a series of metamorphic P-T-t paths for rocks 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.found along that gradient in the field. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Prentice Hall.

Figures not usedFigures not used

Fig. 25-4.Fig. 25-4. ACF diagrams illustrating representative mineral ACF diagrams illustrating representative mineral assemblages for metabasites in the (a) zeolite and (b) assemblages for metabasites in the (a) zeolite and (b) prehnite-pumpellyite facies. Actinolite is stable only in the prehnite-pumpellyite facies. Actinolite is stable only in the upperupper prehnite-pumpellyite facies. The composition range of prehnite-pumpellyite facies. The composition range of common mafic rocks is shaded. common mafic rocks is shaded. Winter (2001) An Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Prentice Hall.

Figures Figures not usednot used

Fig. 25-5.Fig. 25-5. Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the zeolite, prehnite-pumpellyite, and incipient greenschist facies. zeolite, prehnite-pumpellyite, and incipient greenschist facies. Winter (2001) An Introduction to Igneous and Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.Metamorphic Petrology. Prentice Hall.

chapter 25. metamorphic facies and metamorphosed mafic rocks l v.m. goldschmidt (1911, 1912a),...

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