Metamorphic Facies and Metamorphosed Mafic Rocks
(Chapter 25)
last update:11/20/05
Metamorphic Facies Concept
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
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Eskola concluded that the
difference must reflect differing physical conditions between the regions |
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Concluded that Finnish rocks (with a more hydrous nature and lower volume assemblage) equilibrated at lower temperatures and higher pressures than the Norwegian ones |
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."
Dual basis for the facies concept
Descriptive: the relationship between the composition of a rock and its mineralogy
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A metamorphic facies is then a set of repeatedly associated metamorphic mineral assemblages |
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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 |
Interpretive: the range of temperature and pressure conditions represented by each facies
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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 |
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We can now assign relatively accurate temperature and pressure limits to individual facies |
Eskola (1920) proposed 5 original facies:
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Greenschist |
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Amphibolite |
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Hornfels |
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Sanidinite |
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Eclogite |
Each easily defined on the basis of mineral assemblages that develop in mafic rocks
In his final account, Eskola (1939) added:
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Granulite |
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Epidote-amphibolite |
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Glaucophane-schist (now called Blueschist) |
... and changed the name of the hornfels facies to the pyroxene hornfels facies
Since then several additional facies types have been proposed. Most notable are:
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Zeolite |
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Prehnite-pumpellyite |
...resulting from the work of Coombs in the "burial metamorphic" terranes of New Zealand
Fyfe et al. (1958) also proposed:
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Albite-epidote hornfels |
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Hornblende hornfels |
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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. |
We can 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.
Maps of metamorphic terranes often include isograds that define zones and ones that define facies boundaries
Rarely characterize a facies or zone on the basis of a single rock type, and it is most reliably done when several rocks of varying composition and mineralogy are available
Metamorphic field gradient - the array of peak metamorphic (max T) experienced in a metamorphic terrain.
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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. |
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
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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. |
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)
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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. |
Metamorphism of Mafic Rocks
Mineral changes and associations that develop with increasing metamorphic grade
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Hydrous minerals are not common in high-temperature igneous mafic protolith, so hydration is a prerequisite for the development of the metamorphic mineral assemblages that characterize most facies |
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Unless water is available, mafic igneous rocks will remain largely unaffected in metamorphic terranes, even as associated sediments are completely re-equilibrated |
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Coarse-grained intrusives are the least permeable, and thus most likely to resist metamorphic changes, while tuffs and graywackes are the most susceptible |
Plagioclase:
As temperature is lowered, the more Ca-rich plagioclases become progressively unstable
There is thus a general correlation between temperature and the maximum An-content of the stable plagioclase
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At low metamorphic grades only albite (An0-3) is stable |
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In the upper-greenschist facies oligoclase becomes stable. The An-content of plagioclase thus jumps from An1-7 to An17-20 (across the peristerite solvus) as grade increases |
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Andesine(~An40) and more calcic plagioclases are stable in the upper amphibolite and granulite facies |
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The excess Ca and Al released may released from calcite, an epidote mineral, titanite, or amphibole, etc., depending on P-T-X |
Clinopyroxene breaks down to a number of mafic minerals, depending on grade. These minerals include chlorite, actinolite, hornblende, epidote, a metamorphic pyroxene, etc., and the one(s) that form are commonly diagnostic of the grade and facies
Mafic Assemblages at Low Grades
Zeolite and prehnite-pumpellyite facies
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Do not always develop - typically require unstable protolith |
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Boles and Coombs (1975) showed that metamorphism of their tuffs in NZ was accompanied by substantial chemical changes due to circulating fluids, and that these fluids played an important role in the metamorphic minerals that were stable. Thus the classic area of burial metamorphism has a strong component of hydrothermal metamorphism as well. |
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
Metamorphism of mafic rocks is first evident in the greenschist facies, which correlates with the chlorite and biotite zones of the associated pelitic rocks
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Typical minerals include chlorite, albite, actinolite, epidote, quartz, and possibly calcite, biotite, or stilpnomelane |
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Chlorite, actinolite, and epidote impart the green color from which the mafic rocks and facies get their name |
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ACF diagram The most characteristic mineral assemblage of the greenschist facies is: chlorite + albite + epidote + actinolite ± quartz
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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 |
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ACF diagram Typically two-phase Hbl-Plag Most amphibolites are thus predominantly black rocks with up to about 30% white plagioclase Garnet occurs in the more Al-Fe-rich and Ca-poor mafic rocks and clinopyroxene in the Al-poor-Ca-rich ones, adding bits of burgundy red or green to the hand specimen |
The transition from amphibolite to granulite facies occurs in the range 650-700oC
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In the presence of an 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 |
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Not all pelites and quartzo-feldspathic rocks reach the granulite facies as a result |
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Mafic rocks generally melt at somewhat higher temperatures |
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If water is removed by the earlier melts the remaining mafic rocks may become depleted in water |
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Hornblende decomposes and orthopyroxene + clinopyroxene appear |
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This reaction occurs over a temperature interval of at least 50oC |
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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. |
The granulite facies is characterized by the presence of a largely anhydrous mineral assemblage
In metabasites the critical mineral assemblage is orthopyroxene + clinopyroxene + plagioclase + quartz
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Garnet is also common, and minor hornblende and/or biotite may be present |
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ACF diagram illustrating representative mineral assemblages for metabasites in the granulite facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. |
The origin of granulite facies rocks is complex and controversial. There is general agreement, however, on two points
1) Granulites represent unusually hot conditions
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Temperatures > 700oC, geothermometry has yielded some very high temperatures, even in excess of 1000oC |
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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 |
2) Granulites are dry
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The only reason these rocks didn’t melt was the lack of available water |
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Granulite facies terranes represent deeply buried and dehydrated roots of the continental crust |
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Touret: fluid inclusions in granulite facies rocks of S. Norway are CO2-rich, while those in the amphibolite facies rocks are more H2O-rich |
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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. |
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
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
Mafic Assemblages of the High P/T Series: Blueschist and Eclogite Facies
In contrast to low P/T metamorphism discussed in the previous section, it is the mafic rocks, and not the pelites, that 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 suggests that subducted basaltic oceanic crust becomes more dense than the surrounding mantle
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)
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The association of glaucophane + lawsonite is diagnostic. Crossite is stable to lower pressures, and may extend into transitional zones |
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Albite breaks down at high pressure by reaction to jadeitic pyroxene + quartz: |
NaAlSi3O8 = NaAlSi2O6 + SiO2 (reaction 25-3)
Ab = Jd + Qtz
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The assemblage jadeite + quartz indicates high-pressure blueschist facies |
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ACF diagram illustrating representative mineral assemblages for metabasites in the blueschist facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. |
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Eclogite facies: mafic assemblage omphacitic pyroxene + pyrope-grossular garnet 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. |
Pressure-Temperature-Time (P-T-t) Paths
There is also a temporal implication of progressive metamorphism: that rocks pass through a series of mineral assemblages as they continuously equilibrate to increasing metamorphic grade
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Consider 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 |
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Such a cycle is called a pressure-temperature-time path, or P-T-t path |
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
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
Even under the best of circumstances (1) and (2) can usually document only a small portion of the P-T-t path to which a rock was subjected
3) We thus rely on "forward" heat-flow models for various tectonic regimes to compute more complete P-T-t paths, and evaluate them by comparison with the results of the backward methods
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Chemical zoning profiles across a garnet from the Tauern Window. After Spear (1989) |
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Conventional P-T diagram (pressure increases upward) showing three modeled "clockwise" P-T-t paths computed from the profiles using the method of Selverstone et al. (1984) J. Petrol., 25, 501-531 and Spear (1989). After Spear (1989) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. |
Classic view: regional metamorphism is a result of deep burial or intrusion of hot magmas
Plate tectonics: regional metamorphism rarely produced by simple burial, but is a result of crustal thickening and heat input during orogeny at convergent plate boundaries
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 transient geotherm and must be considered in any complete model of metamorphism
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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. 25-2 and 25-3. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. |
Although the exact shape, size, and position of an orogenic P-T-t path such as path (a) may vary with the constraints of the model, 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
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Paths such as (a) are called "clockwise" P-T-t paths in the literature, and are considered to be the most common for regional metamorphism |
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Broad agreement between the forward (model) and backward (geothermobarometry) techniques regarding P-T-t paths |
We may thus assume that the general form of a path such as (a) represents a typical rock during orogeny and regional metamorphism
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
2. Pmax and Tmax do not occur at the same time
In the usual case of "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 since a mineral geobarometer should record the pressure of Tmax
