1. Chapter 25. Metamorphic Facies and Metamorphosed Mafic Rocks
V.M. Goldschmidt (1911, 1912a), contact metamorphosed pelitic, calcareous, and psammitic hornfelses in the Oslo region
Relatively simple mineral assemblages of fewer than six major minerals in the inner zones of the aureoles around granitoid intrusives
Equilibrium mineral assemblage related to Xbulk
V.M. Goldschmidt (1911, 1912a) studied a series of contact metamorphosed pelitic, calcareous, and psammitic hornfelses of Paleozoic age in the Oslo region of southern Norway
Although the chemistry of the rocks varied considerably, Goldschmidt found relatively simple mineral assemblages of fewer than six major minerals in the inner zones of the aureoles around granitoid intrusives
Goldschmidt was the first to note that the equilibrium mineral assemblage of a metamorphic rock could be related to Xbulk
Aluminous pelites contained Al-rich minerals, such as cordierite, plagioclase, garnet, and/or an Al2SiO5 polymorph
Calcareous rocks contained Ca-rich and Al-poor minerals such as Di, Wo, and/or amphibole

2. Metamorphic Facies
Certain mineral pairs (e.g. anorthite + hypersthene) were consistently present in rocks of appropriate composition, whereas the compositionally equivalent pair (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
Goldschmidt thus claimed that the left side of the reaction was always stable under the metamorphic conditions existing at Oslo
If equilibrium were not attained, perhaps either pair or all four minerals may have been found

3. Metamorphic Facies
Pentii Eskola (1914, 1915) Orijärvi region of southern Finland
Rocks with K-feldspar + cordierite at Oslo contained the compositionally equivalent pair biotite + muscovite at Orijärvi
Eskola concluded that the difference must reflect differing physical conditions between the regions
Concluded that Finnish rocks (with a more hydrous nature and lower volume assemblage) equilibrated at lower temperatures and higher pressures than the Norwegian ones
Pentii Eskola (1914, 1915) worked in similar hornfelses in the Orijärvi region of southern Finland, where he confirmed the ideas of Goldschmidt by noting a consistent relationship between the mineral assemblage and the chemical composition of a wide range of rock types
Many of the assemblages were the same as at Oslo, although some rocks, chemically equivalent to some at Oslo, contained a different mineral assemblage
Using strictly qualitative thermodynamic reasoning (Chapter 5), he correctly deduced that the Finnish rocks (with a more hydrous nature and lower volume assemblage) equilibrated at lower temperatures and higher pressures than the Norwegian ones

4. Metamorphic Facies Oslo: Ksp + Cord Orijärvi: Bi + Mu Reaction:
2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH) SiO2
Bt Ms Qtz
= 3 Mg2Al4Si5O KAlSi3O8 + 8 H2O
Crd Kfs

5. Metamorphic Facies
Eskola (1915) developed the concept of metamorphic facies:
“In any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pressure conditions, 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.”
On the basis of the relationship between rock composition and mineral assemblage, and the worldwide occurrence of virtually identical mineral assemblages, Eskola (1915) developed the concept of metamorphic facies:
At the time Grubenmann’s epizone, mesozone, and catazone were very broad and poorly defined and the isograd-zones were restricted to pelitic compositions, and were too narrow for easy correlation from one locality to another
Eskola’s facies were initially based on metamorphosed mafic rocks
Basaltic rocks occur in ~ all orogenic belts, and the mineral changes in them define broader T-P ranges than those in pelites, so facies provided a convenient way to compare metamorphic areas around the world

6. Metamorphic Facies Dual basis for the facies concept
Descriptive: the relationship between the composition of a rock and its mineralogy
This descriptive aspect was a fundamental feature of Eskola’s 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
Descriptive: the relationship between the composition of a rock and its mineralogy
Eskola based his proposed facies on the mineral assemblages developed in metamorphosed mafic rocks, and this descriptive aspect was a fundamental feature of the concept

7. Metamorphic Facies
Interpretive: the range of temperature and pressure conditions represented by each facies
Eskola was aware of the temperature-pressure implications of the concept and correctly deduced the relative temperatures and pressures represented by the different facies that he proposed
We can now assign relatively accurate temperature and pressure limits to individual facies
Eskola was aware of the temperature-pressure implications of the concept, and he correctly deduced the relative temperatures and pressures represented by the different facies that he proposed
Advances in experimental techniques and the accumulation of experimental and thermodynamic data have allowed us to assign relatively accurate temperature and pressure limits to individual facies

8. Metamorphic Facies Eskola (1920) proposed 5 original facies:
Greenschist
Amphibolite
Hornfels
Sanidinite
Eclogite
Each easily defined on the basis of mineral assemblages that develop in mafic rocks
Each was easily defined on the basis of distinctive mineral assemblages that develop in mafic rocks (clearly reflected in the facies names, most of which correspond to common metamorphic mafic rock types)

9. Metamorphic Facies In his final account, Eskola (1939) added:
Granulite
Epidote-amphibolite
Glaucophane-schist (now called Blueschist)
... and changed the name of the hornfels facies to the pyroxene hornfels facies
His facies, and his estimate of their relative temperature-pressure relationships are shown in Fig. 25-1

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

11. Metamorphic Facies
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
Numerous other facies, and even sub-facies, have been proposed, but most have been dropped in favor of simplicity and ease of correlation
Now that we can assign approximate T-P limits to the facies types, we face a fundamental dilemma resulting from the dual basis behind the facies concept as we interpret facies today
Do we remain faithful to Eskola’s original method of defining facies as a set of mineral assemblages, and restrict our choice of facies types only to those defined by unique parageneses in mafic rocks?
Or do we rely on our modern ability to assign temperature-pressure limits to facies and stress the importance of these regimes?
I go for the latter
The metamorphic facies we shall use, and their generally accepted temperature and pressure limits, are shown in Fig. 25-2

12. Metamorphic Facies
Fig Temperature- pressure diagram showing the generally accepted limits of the various facies used in this text. Boundaries are approximate and gradational. The “typical” or average continental geotherm is from Brown and Mussett (1993). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The boundaries between metamorphic facies represent T-P conditions in which key minerals in mafic rocks are either added or removed, thus changing the mineral assemblages observed
They are thus separated by mineral reaction isograds
The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase
The 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient.

13. Metamorphic Facies
Table The definitive mineral assemblages that characterize each facies (for mafic rocks).
The definitive mineral assemblages that characterize each facies (for mafic rocks) are listed in Table (details and variations later)

14. 1) Facies of high pressure
It is convenient to consider metamorphic facies in 4 groups:
1) Facies of high pressure
The blueschist and eclogite facies: low molar volume phases under conditions of high pressure
The lower-temperature blueschist facies occurs in areas of low T/P gradients, characteristically developed in subduction zones
Because eclogites are stable under normal geothermal conditions, they may develop wherever mafic magmas solidify in the deep crust or mantle (crustal chambers or dikes, sub-crustal magmatic underplates, subducted crust that is redistributed into the mantle)
The name is due to the blue-gray color of most mafic blueschists, imparted by sodic amphiboles

15. Metamorphic Facies 2) Facies of medium pressure
Most metamorphic rocks now exposed at the surface of the Earth belong to the greenschist, amphibolite, or granulite facies
As you can see in Fig. 25-2, the greenschist and amphibolite facies conform to the “typical” geothermal gradient

16. Fig 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.
All three facies conform to elevated geotherms typical of most orogenic regions, such as the 30oC/km geotherm illustrated
Granulite facies rocks occur predominantly in deeply eroded continental cratons of Precambrian age

17. Metamorphic Facies 3) Facies of low pressure
The albite-epidote hornfels, hornblende hornfels, and pyroxene hornfels facies: contact metamorphic terranes and regional terranes with very high geothermal gradients
The sanidinite facies is rare and limited to xenoliths in basic magmas and the innermost portions of some contact aureoles adjacent to hot basic intrusives
Because intrusions can stall at practically any depth, and orogenic geotherms are quite variable, there is considerable overlap and gradation between the hornfels facies types and the corresponding medium-pressure facies immediately below them on Fig. 25-2

18. Fig 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.

19. Metamorphic Facies 4) Facies of low grades
Rocks often fail to recrystallize thoroughly at very low grades, and equilibrium is not always attained
The zeolite and prehnite-pumpellyite facies are thus not always represented, and the greenschist facies is the lowest grade developed in many regional terranes
The zeolite and p-p facies are thus treated separately
These facies are best developed where the protolith is very immature and susceptible to metamorphism, and where there is a high geothermal gradient and abundant hydrous fluids. As a result, they are most common in areas of burial or hydrothermal metamorphism

20. Metamorphic Facies 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 that define 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

21. Facies Series
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
Miyashiro extended the facies concept to encompass broader progressive sequences: facies series

22. Figure 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.

23. Facies Series
Miyashiro (1961) initially proposed five facies series, most of them named for a specific representative “type locality” The series were:
1. Contact Facies Series (very low-P)
2. Buchan or Abukuma Facies Series (low-P regional)
3. Barrovian Facies Series (medium-P regional)
4. Sanbagawa Facies Series (high-P, moderate-T)
5. Franciscan Facies Series (high-P, low T)
The contrast between Miyashiro’s higher-T-lower-P Ryoke-Abukuma belt (Section ) and the classical Barrovian sequence also led him to suggest that more than one type of facies series is probable
Metamorphic field gradients are highly variable (even in the same orogenic belt) and transitional series abound
Miyashiro later chose to limit the number instead to the three broad major types of facies series (also called “baric types”) illustrated in Fig. 25-3
Although the boundaries and gaps between them are somewhat artificial, the three series represent common types of tectonic environments

24. Fig Temperature- pressure diagram showing the three major types of metamorphic facies series proposed by Miyashiro (1973, 1994). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The high P/T series, for example, typically occurs in subduction zones where “normal” isotherms are depressed by the subduction of cool lithosphere faster than it can equilibrate thermally
The facies sequence here is (zeolite facies) - (prehnite-pumpellyite facies) - blueschist facies - eclogite facies.
The medium P/T series is characteristic of common orogenic belts (Barrovian type)
The sequence is (zeolite facies) - (prehnite-pumpellyite facies) - greenschist facies -amphibolite facies - (granulite facies)
Crustal melting under water-saturated conditions occurs in the upper amphibolite facies (the solidus is indicated in Fig. 25-2)
The granulite facies, therefore, occurs only in water-deficient rocks, either dehydrated lower crust, or areas with high XCO2 in the fluid
The low P/T series is characteristic of high-heat-flow orogenic belts (Buchan or Ryoke- Abukuma type), rift areas, or contact metamorphism
The sequence of facies may be a low-pressure version of the medium P/T series described above (but with cordierite and/or andalusite), or the sequence (zeolite facies) - albite-epidote hornfels facies - hornblende hornfels facies - pyroxene hornfels facies
Sanidinite facies rocks are rare, requiring the transport of great heat to shallow levels

25. Metamorphism of Mafic Rocks
Mineral changes and associations along T-P gradients characteristic of the three facies series
Hydration of original mafic minerals required for the development of the metamorphic mineral assemblages of most facies
If water is unavailable, mafic igneous rocks will remain largely unaffected in metamorphic terranes, even as associated sediments are completely re-equilibrated
Coarse-grained intrusives are the least permeable, and thus most likely to resist metamorphic changes
Tuffs and graywackes are the most susceptible
Most of the common minerals in mafic rocks exhibit extensive solid solution, so that metabasites tend to have fewer phases than pelites
This results in fewer reactions and isograds, which is one of the reasons that Eskola chose to define his facies on these rocks, since it resulted in broader subdivisions, and made it easier to categorize and correlate different areas

26. Metamorphism of Mafic Rocks
Plagioclase:
More Ca-rich plagioclases become progressively unstable as T lowered
General correlation between temperature and the maximum An-content of the stable plagioclase
At low metamorphic grades only albite (An0-3) is stable
In the upper-greenschist facies oligoclase becomes stable. The An-content of plagioclase thus jumps from An1-7 to An (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
The principal mineral changes are due to the breakdown of the two most common basaltic minerals: plagioclase and clinopyroxene

27. Metamorphism of Mafic Rocks
Clinopyroxene breaks down to a number of mafic minerals, depending on grade.
These minerals include chlorite, actinolite, hornblende, epidote, a metamorphic pyroxene, etc.
The mafic(s) that form are commonly diagnostic of the grade and facies

28. Mafic Assemblages at Low Grades
Zeolite and prehnite-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 an important role in the metamorphic minerals that were stable
The classic area of burial metamorphism thus has a strong component of hydrothermal metamorphism as well
Very low grade metabasites are usually only partly altered to very fine grained and messy-looking minerals
Coombs recognized systematic patterns in these rocks, and showed that they could be studied in the same way as other metamorphic rocks

29. Mafic Assemblages of the Medium P/T Series: Greenschist, Amphibolite, and Granulite Facies
The greenschist, amphibolite and granulite facies constitute the most common facies series of regional metamorphism
The classical Barrovian series of pelitic zones and the lower-pressure Buchan-Abukuma series are variations on this trend
We return to the Barrovian area of the Scottish Highlands and discuss the progressive mineral changes that occur in the mafic rocks across the terrane

30. Greenschist, Amphibolite, Granulite Facies
The zeolite and prehnite-pumpellyite facies are not present in the Scottish Highlands
Metamorphism of mafic rocks is first evident in the greenschist facies, which correlates with the chlorite and biotite zones of the associated pelitic rocks
Typical minerals include chlorite, albite, actinolite, epidote, quartz, and possibly calcite, biotite, or stilpnomelane
Chlorite, actinolite, and epidote impart the green color from which the mafic rocks and facies get their name
Although some schists that are not of greenschist facies may be green, it is not common, and the greenschist (and greenstone) rock names should be applied only to rocks in this facies

31. Greenschist, Amphibolite, Granulite Facies
ACF diagram
The most characteristic mineral assemblage of the greenschist facies is: chlorite + albite + epidote + actinolite  quartz
Chloritoid, you may remember, is not common in the Scottish Highlands, but is widespread in the Appalachians, where Fe-Al rich rocks are more common
Fig 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.

32. Greenschist, Amphibolite, Granulite Facies
Greenschist to amphibolite facies transition involves two major mineralogical changes
1. Transition from albite to oligoclase (increased Ca- content of stable plagioclase with temperature across the peristerite gap)
2. Transition from actinolite to hornblende (amphibole becomes able to accept increasing amounts of aluminum and alkalis at higher temperatures)
Both of these transitions occur at approximately the same grade, but have different P/T slopes
The transition from greenschist to amphibolite facies involves two major mineralogical changes
The first is the transition from albite to oligoclase (increased Ca-content of stable plagioclase with temperature across the peristerite gap)
The second is the transition from actinolite to hornblende as amphibole becomes able to accept increasing amounts of aluminum and alkalis at higher temperatures
Both of these transitions occur at approximately the same grade

33. The reactions that generate calcic plagioclase and hornblende are complex and variable, involving the breakdown of epidote and chlorite to supply the Ca and Al for the anorthite and Al-hornblende components
The transition to more calcic plagioclase involves epidote, and the temperature of the reaction(s) appear to increase with pressure more rapidly than those responsible for the actinolite-hornblende transition in Fig (Liou et al., 1974)
Thus in the higher-pressure Barrovian sequence, hornblende appears before oligoclase, prompting some workers to propose a transitional albite-epidote- amphibolite facies
In low-P contact aureoles, the plagioclase transition occurs first, resulting in a transitional zone with coexisting actinolite + calcic plagioclase (as calcic as labradorite)
The transition to amphibolite facies usually occurs in the garnet zone for associated pelitic rocks (occasionally in the upper biotite zone)
Fig 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.

34. Greenschist, Amphibolite, Granulite Facies
ACF diagram
Typically two-phase Hbl-Plag
Amphibolites are typically black rocks with up to about 30% white plagioclase
Garnet in more Al-Fe-rich and Ca-poor mafic rocks
Clinopyroxene in Al-poor-Ca- rich rocks
Metabasites in the amphibolite facies are characterized by the assemblage hornblende + plagioclase (An>17) + lesser amounts of quartz, epidote, garnet, clinopyroxene, and/or biotite
The amphibolite facies corresponds generally to the upper garnet, staurolite, kyanite, and lower sillimanite zones in associated pelitic rocks
Fig ACF diagram illustrating representative mineral assemblages for metabasites in the amphibolite facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

35. Greenschist, Amphibolite, Granulite Facies
The transition from amphibolite to granulite facies occurs in the range oC
If aqueous fluid, associated pelitic and quartzo- feldspathic rocks (including granitoids) begin to melt in this range at low to medium pressures , so that migmatites may form and the melts may become mobilized
Not all pelites and quartzo-feldspathic rocks reach the granulite facies as a result

36. Greenschist, Amphibolite, Granulite Facies
Mafic rocks generally melt at somewhat higher temperatures
If water is removed by the earlier melts the remaining mafic rocks may become depleted in water
Hornblende decomposes and orthopyroxene + clinopyroxene appear
This reaction occurs over a temperature interval of at least 50oC

37. Fig 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.

38. Greenschist, Amphibolite, Granulite Facies
The granulite facies is characterized by the presence of a largely anhydrous mineral assemblage
Metabasites critical mineral assemblage is orthopyroxene + clinopyroxene + plagioclase + quartz
Garnet, minor hornblende and/or biotite may be present
Note that a basalt that has been progressively metamorphosed from the greenschist to the granulite facies has returned to a mineralogy dominated by plagioclase and pyroxene, just like the original basalt
The texture, however, is generally gneissic, and grain shapes are decussate or polygonal, thus bearing no resemblance to the original basalt
Fig 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.

39. Greenschist, Amphibolite, Granulite Facies
The origin of granulite facies rocks is complex and controversial. There is general agreement, however, on two points
1) Granulites represent unusually hot conditions
Temperatures > 700oC (geothermometry has yielded some very high temperatures, even in excess of 1000oC)
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

40. Greenschist, Amphibolite, Granulite Facies
2) Granulites are dry
Rocks don’t 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
It is a long-held tenet that granulite facies terranes represent the deeply buried and dehydrated roots of the continental crust
Most exposed granulite facies rocks are found in the deeply eroded areas of the Precambrian continental shields
Touret showed that the fluid inclusions in granulite facies rocks of S. Norway are CO2-rich, while those in the amphibolite facies rocks are more H2O-rich
This may imply that dehydration of at least some granulite facies terranes may be due to the infiltration of CO2 replacing H2O, rather than the elimination of fluids altogether

41. Fig 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 for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

42. Mafic Assemblages of the Low P/T Series: Albite-Epidote Hornfels, Hornblende Hornfels, Pyroxene Hornfels, and Sanidinite Facies
Mineralogy of low-pressure metabasites not appreciably different from the med.-P facies series
Albite-epidote hornfels facies correlates with the greenschist facies into which it grades with increasing pressure
Similarly the hornblende hornfels facies correlates with the amphibolite facies, and the pyroxene hornfels and sanidinite facies correlate with the granulite facies
Can see this in Fig 25-2 (next frame)

43. Fig 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.

44. Mafic Assemblages of the Low P/T Series: Albite-Epidote Hornfels, Hornblende Hornfels, Pyroxene Hornfels, and Sanidinite Facies
The 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

45. Mafic Assemblages of the Low P/T Series: Albite-Epidote Hornfels, Hornblende Hornfels, Pyroxene Hornfels, and Sanidinite Facies
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 to orthopyroxene + clinopyroxene + plagioclase + quartz (without garnet), characterizing this facies
Sanidinite facies is not evident in basic rocks

46. Mafic Assemblages of the High P/T Series: Blueschist and Eclogite Facies
The mafic rocks (not the pelites) develop conspicuous and definitive mineral assemblages under high P/T conditions
High P/T geothermal gradients characterize subduction zones
Mafic blueschists are easily recognizable by their color, and are useful indicators of ancient subduction zones
The great density of eclogites: subducted basaltic oceanic crust becomes more dense than the surrounding mantle
The implications of eclogite density on subduction processes was discussed in Chapter 16, and on mantle dynamics and isotopes in Chapter 14

47. Blueschist and Eclogite Facies
Alternative paths to the blueschist facies
Fig 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.
Alternative paths to the blueschist facies depend on the local geothermal gradient, and may follow paths such as:
zeolite facies  prehnite-pumpellyite facies  blueschist facies
(zeolite facies)  (prehnite-pumpellyite facies)  greenschist facies  blueschist facies

48. Blueschist and Eclogite Facies
The blueschist facies is characterized in metabasites by the presence of a sodic blue amphibole stable only at high pressures (notably glaucophane, but some solution of crossite or riebeckite is possible)
The association of glaucophane + lawsonite is diagnostic. Crossite is stable to lower pressures, and may extend into transitional zones
Albite breaks down at high pressure by reaction to jadeitic pyroxene + quartz:
NaAlSi3O8 = NaAlSi2O6 + SiO2 (reaction 25-3)
Ab Jd Qtz
The assemblage jadeite + quartz indicates high-pressure blueschist facies
If we apply the principle we learned recently about reactions, we note that jadeite without quartz would be stable into the albite field at lower pressure
A low-pressure blueschist facies can be distinguished by the presence of jadeite without quartz (and the sodic amphibole is often more crossitic)
The Sanbagawa belt of Japan (Section ) represents the low-pressure type of blueschist facies, while the high-pressure type occurs in the Franciscan Formation of western California, New Caledonia, Sifnos (Greece) and Sesia (western Alps)

49. Blueschist and Eclogite Facies
Fig 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.

50. Blueschist and Eclogite Facies
Eclogite facies: mafic assemblage omphacitic pyroxene + pyrope-grossular garnet
Transition to the eclogite facies marked by development of mafic assemblage omphacitic pyroxene (augite-jadeite solution) + pyrope-grossular garnet, creating beautiful green and red rocks
Glaucophane + paragonite react to form this assemblage, but glaucophane alone may remain stable into the eclogite facies
Along higher geothermal gradients the amphibolite facies, or even the granulite facies, may lead to the eclogite facies as well
Much of the blueschist, and all of the eclogite facies are marked by the high- pressure instability of plagioclase, a common phase in metabasites of any other grade
Fig ACF diagram illustrating representative mineral assemblages for metabasites in the eclogite facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

51. Pressure-Temperature-Time (P-T-t) Paths
The facies series concept suggests that a traverse up grade through a metamorphic terrane should follow a metamorphic field gradient, and may cross through a sequence of facies (spatial sequences)
Progressive metamorphism: rocks pass through a series of mineral assemblages as they continuously equilibrate to increasing metamorphic grade (temporal sequences)
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.?
In other words, are the temporal and spatial mineralogical changes the same?

52. Pressure-Temperature-Time (P-T-t) Paths
The complete set of T-P conditions that a rock may experience during a metamorphic cycle from burial to metamorphism (and orogeny) to uplift and erosion is called a pressure-temperature-time path, or P-T-t path
P-T-t path, and was first introduced in Section 16.8 when we attempted to assess the conditions experienced by progressively subducted crust and convecting mantle wedge materials as they moved through the subduction zone complex

53. Pressure-Temperature-Time (P-T-t) Paths
Metamorphic P-T-t paths may be addressed by:
1) Observing partial overprints of one mineral assemblage upon another
The relict minerals may indicate a portion of either the prograde or retrograde path (or both) depending upon when they were created
We learned that thermodynamic equilibrium is generally maintained during prograde metamorphism, so that relict minerals are more common during the retrograde path, but both types have been observed

54. Pressure-Temperature-Time (P-T-t) Paths
Metamorphic P-T-t paths may be addressed by:
2) Apply geothermometers and geobarometers to the core vs. rim compositions of chemically zoned minerals to document the changing P-T conditions experienced by a rock during their growth

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

56. Fig a. 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, and Spear (1989). After Spear (1989) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1.

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

58. Pressure-Temperature-Time (P-T-t) Paths
Classic view: regional metamorphism is a result of deep burial or intrusion of hot magmas
Plate tectonics: regional metamorphism is a result of crustal thickening and heat input during orogeny at convergent plate boundaries (not simple burial)
Heat-flow models have been developed for various regimes, including burial, progressive thrust stacking, crustal doubling by continental collision, and the effects of crustal anatexis and magma migration
Higher than the normal heat flow is required for typical greenschist-amphibolite medium P/T facies series
Uplift and erosion has a fundamental effect on the geotherm and must be considered in any complete model of metamorphism
Nearly all models indicate that heat flow higher than the normal continental geotherm is required for typical greenschist-amphibolite medium P/T facies series, and that uplift and erosion has a fundamental effect on the geotherm and must be considered in any complete model of metamorphism

59. Fig illustrates some examples of modeled P-T-t paths representing common types of metamorphism
The paths illustrated are schematic, and numerous variations are possible, depending upon the style of deformation and the rates of thickening, heat transfer, magmatism, and erosion, etc.
Fig Schematic pressure-temperature-time paths based on heat-flow models. The Al2SiO5 phase diagram and two hypothetical dehydration curves are included. Facies boundaries, and facies series from Figs and Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

60. Path (a) is considered a typical P-T-t path for an orogenic belt with crustal thickening
During thickening pressure increases >> temperature, because of the time lag required for heat transfer (pressure equilibrates nearly instantaneously, but heat conducts very slowly through rocks)
Thus the thickened crustal block quickly -> Pmax while remaining relatively cool
The increased thickness of crust is rich in LIL and radioactive elements, so the heat flux increases
Subduction zone magmatism may also deliver heat the heating effect
The new geotherm is higher, but transient, and lasts only as long as the thickened crust and subduction-related heat generation lasts
Erosion soon affects the thickened crust and the pressure begins to decrease before the rocks can equilibrate with the higher orogenic geotherm
T still increasing due to the slow heat transfer (most models address heat transfer by conduction only), so that the P-T-t path has a negative slope following Pmax
Reach Tmax when cooling effect of uplift and erosion catches up to the increased geotherm, so that the thermal perturbation of crustal thickening is dampened and begins to fade
P-T-t path follows positive slope as both T and P fall while rock moves -> surface & geotherm relaxes
Fig a. Schematic pressure-temperature-time paths based on a crustal thickening heat-flow model. The Al2SiO5 phase diagram and two hypothetical dehydration curves are included. Facies boundaries, and facies series from Figs and Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

61. Pressure-Temperature-Time (P-T-t) Paths
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 “clockwise” P-T-t paths in the literature, and are considered to be the norm for regional metamorphism

62. Path (b): rocks heated and cooled at virtually constant pressure by magmatic intrusion at shallow levels
This may be an appropriate P-T-t path for contact metamorphism
Depending upon the extent of magmatic activity and its contribution to the crustal mass, any number of paths transitional between (a) and (b) can be imagined, representing a gradation from high-pressure (Barrovian) regional metamorphism to “regional contact metamorphism” with numerous plutons, to local contact metamorphism
Fig b. 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 and Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

63. Path (c): “counterclockwise” P-T-t path
Typically occurs in high-grade gneisses and granulite-facies terranes, and is believed to result from the intrusion of relatively large quantities of (usually mafic) magma into the lower and middle crust
The rapid introduction of magmatic heat and mass causes both the pressure and temperature to increase in unison below the intrusions
Followed by nearly isobaric cooling because the high density of the mafic magma does not lead to crustal buoyancy, so that uplift and erosion are limited
Fig c. Schematic pressure-temperature-time paths based on a heat-flow model for some types of granulite facies metamorphism. Facies boundaries, and facies series from Figs and Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

64. Pressure-Temperature-Time (P-T-t) Paths
Broad agreement between the forward (model) and backward (geothermobarometry) techniques regarding P- T-t paths
The general form of a path such as (a) therefore probably represents a typical rock during orogeny and regional metamorphism
If this assumption is correct, the shape of the typical P-T-t path has some instructive ramifications

65. Pressure-Temperature-Time (P-T-t) Paths
1. Contrary to the classical treatment of metamorphism, temperature and pressure do not both increase in unison as a single unified “metamorphic grade.”
Their relative magnitudes vary considerably during the process of metamorphism

66. Pressure-Temperature-Time (P-T-t) Paths
2. Pmax and Tmax do not occur at the same time
In the usual “clockwise” P-T-t paths, Pmax occurs much earlier than Tmax.
Tmax should 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 grade” should refer to the temperature and pressure at Tmax, because the grade is determined via reference to the equilibrium mineral assemblage

67. Pressure-Temperature-Time (P-T-t) Paths
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 a2 crosses the kyanite  sillimanite transition as temperature is decreasing. This may result in only minor replacement of kyanite by sillimanite during such a retrograde process

68. Fig a. Schematic pressure-temperature-time paths based on a crustal thickening heat-flow model. The Al2SiO5 phase diagram and two hypothetical dehydration curves are included. Facies boundaries, and facies series from Figs and Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

69. Pressure-Temperature-Time (P-T-t) Paths
3. Some variations on the cooling-uplift portion of the “clockwise” path (a) in Fig 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)

70. 4. Now return to the question of whether the temporal and spatial mineralogical changes associated with progressive metamorphism are the same
Fig illustrates the difference between the spatial changes along the metamorphic field gradient and the temporal changes (P-T-t paths) for several rocks along the gradient
Note that every rock follows a path involving considerably higher pressures and lower temperatures than the final locus of Tmax-(P at Tmax) points along the metamorphic field gradient suggests
Implications for blueschists: most CW paths appear to -> blueschists in early stages, but not preserved, because T gradually increases to -> med P/T at T(max) where metamorphic imprint developed
Perhaps preservation is the main reason that blueschists are relatively rare, not generation
Fig 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. Prentice Hall.

71. Figures not used
Fig ACF diagrams illustrating representative mineral assemblages for metabasites in the (a) zeolite and (b) prehnite-pumpellyite facies. Actinolite is stable only in the upper prehnite-pumpellyite facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

72. Figures not used
Fig Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the zeolite, prehnite-pumpellyite, and incipient greenschist facies. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.