Ch 25 Metamorphic Facies. V.M. Goldschmidt (1911, 1912a) contact metamorphosed pelitic, calcareous, and psammitic hornfelses Oslo s. Norway fewer than six major minerals in the aureoles around granitoid intrusives
V.M. Goldschmidt (1911, 1912a) contact metamorphosed pelitic, calcareous, and psammitichornfelses Oslo s. Norway
fewer than six major minerals in the aureoles around granitoid intrusives
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
Certain mineral pairs (e.g. anorthite + enstatite) 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
PentiiEskola (1914, 1915) Orijärvi, S. Finland
Rocks with K-feldspar + cordierite at Oslo contained the compositionally equivalent pair biotite + muscovite at Orijärvi
Eskola: difference must reflect differing physical conditions
Finnish rocks (more hydrous and lower volume assemblage) equilibrated at lower temperatures and higher pressures than the Norwegian ones
Oslo: Kfs + Mg-Crd
Orijärvi: Phlo + Ms + Qtz
2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH)2 + 15 SiO2
Phlo Ms Qtz
= 3 Mg2Al4Si5O18 + 8 KAlSi3O8 + 8 H2O
CrdKfswater (missing at Oslo)
PentiiEskola (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.”
Eskola ‘s dual basis facies : Xbulk & mineralogy
A metamorphic facies is a set of repeatedly associated metamorphic mineral assemblages
Eskola aware of the P-T implications and correctly deduced the relative temperatures and pressures of facies he proposed
Modern lab results: can now assign relatively accurate temperature and pressure limits to individual facies
Eskola (1920) proposed 5 original facies:
Easily defined on the basis of mineral assemblages that develop in mafic rocks
In his final account, Eskola (1939) added:
Glaucophane-schist (now called Blueschist)
... and changed the name of the hornfels facies to the pyroxene hornfels facies
Fig. 25-1The metamorphic facies proposed by Eskola and their relative temperature-pressure relationships. After Eskola (1939) Die Entstehung der Gesteine. Julius Springer. Berlin.
Several additional facies types have been proposed. Most notable are:
...resulting from the work of Coombs in the “burial metamorphic” terranes of New Zealand
Fyfe et al. (1958) also proposed:
Fig. 25-2.Temperature-pressure diagram showing the generally accepted limits of the various facies used in this text.
Table 25-1. The definitive mineral assemblages that characterize each facies (for mafic rocks).
Fig. 25-9.Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Miyashiro extended the facies concept to encompass broader progressive sequences: facies series
A traverse up grade through a metamorphic terrane should follow one of several possible metamorphic field gradients (Fig. 21-1, next slide), and, if extensive enough, cross through a sequence of facies
Mineral changes and associations along T-P gradients characteristic of the three facies series
Hydration of original mafic minerals generally required
If water unavailable, mafic igneous rocks will remain largely unaffected, even as associated sediments are completely re-equilibrated
Coarse-grained intrusives are the least permeable and likely to resist metamorphic changes
Tuffs and graywackes are the most permeable and so subject to metamorphic changes.
The principal mineral changes are due to the breakdown of the two most common basaltic minerals: plagioclase and clinopyroxene
More Ca-rich plagioclases become progressively unstable as T lowered
General correlation between temperature and maximum An-content of the stable plagioclase
At low metamorphic grades only albite (An0-3) is stable
The An-content of plagioclase jumps as grade increases
More Anorthite content (= more calcic) plagioclases are stable in the upper amphibolite and granulitefacies
Clinopyroxene breaks down to a number of mafic minerals: (less hot) chlorite, actinolite, hornblende (hotter), etc. depending on T & P
The greenschist, amphibolite and granulitefacies constitute the most common facies series of regional metamorphism on Mafic rocks
The most characteristic mineral assemblage of the greenschistfacies is: chlorite + albite + epidote + actinolite quartz
Fig. 25-6.ACF diagram illustrating representative mineral assemblages for metabasites in the greenschist facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Greenschist®amphibolitefacies transition involves two major mineralogical changes
1. Albite ® oligoclase (increased Ca-content with temperature across the peristerite gap)
2. Actinolite® hornblende (amphibole accepts increasing aluminum and alkalis at higher Ts)
Both transitions occur at approximately the same grade, but have different P/T slopes
Exist exsolution lamellae on cooling in the peristerite miscibility gap, ~An5-An18
Typically two-phase: Hbl-Plag
Amphibolites are typically black rocks with up to about 30% white plagioclase
Like diorites, but differ texturally
Garnet in more Al-Fe-rich and Ca-poor mafic rocks
Clinopyroxene in Al-poor-Ca-rich rocks
Fig. 25-7.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.
Mafic rocks generally melt at higher temperatures
If water is removed by the earlier melts the remaining mafic rocks may become depleted in water
Hornblende decomposes and orthopyroxene + clinopyroxene appear
Fig. 26-19.Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined univariant reactions in the CaO-MgO-Al2O3-SiO2-H2O-(Na2O) system (“C(N)MASH”). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Granulitefacies characterized by a largely anhydrousmineral assemblage
Origin of granulitefacies: general agreement 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 granulitefacies depths should be in the vicinity of 500oC, suggesting that granulites are the products of crustal thickening and excess heating
2) Granulites are dry
Rocks don’t melt due to lack of available water
Granulitefaciesterranes represent deeply buried and dehydrated roots of the continental crust
Fluid inclusions in granulitefacies rocks of S. Norway are CO2-rich, whereas those in the amphibolitefacies rocks are H2O-rich
Fig. 25-2.Temperature-pressure diagram showing the generally accepted limits of the various facies used in this text. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Mafic rocks (not pelites) develop 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. Formation requires abundant water.
The great density of eclogites: subducted basaltic oceanic crust becomes more dense than the surrounding mantle
Begins to sink , extended necks thin and break off pieces
The blueschistfacies is characterized in metabasites by the presence of a sodic blue amphibole stable only at high pressures (notably glaucophane)
The association of glaucophane + lawsonite(CaAl2Si2O7(OH)2·H2O) is diagnostic of the high pressure.
Albite breaks down at high pressure by reaction to jadeite (a pyroxene) + quartz:
NaAlSi3O8 = NaAlSi2O6 + SiO2 (reaction 25-3)
Jadeite without quartz would be stable into the albite field at lower pressure =>low pressure blueschist
Specimen: Jadeite + Glaucophane w/o quartz
Fig. 25-10.ACF diagram illustrating representative mineral assemblages for metabasites in the blueschist facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Glaucophane + Paragonite = Pyrope + Jadeite + Qtz +H2O
Paragonite is NaAl2(AlSi3O10)(OH)2
Pyrope is Mg3Al2(SiO4)3
Jadeite is NaAlSi2O6
Eclogite facies: mafic assemblage omphacitic pyroxene + pyrope-grossular garnet
Fig. 25-11.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.
Na / (Na + Ca)
Augite (Ca, Na) (Fe, Al) Si2O6
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)
Are the temporal and spatial mineralogical changes the same?
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
Muscovite + chlorite + quartz + staurolite = andalusite / sillimanite + biotite + H20
Staurolite (St) as relict within poikiloblastic andalusite (And).
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
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)
Isochemical phase transformations (the polymorphs of SiO2 or Al2SiO5 or graphite-diamond or calcite-aragonite
The transformations depend on temperature and pressure only
Aragonite is the stable CaCO3 polymorph commonly found in blueschist facies terranes
Independent of other minerals present, fluids, etc.
Andalusite -> Sill as T and P increase
regardless of other phases Stau, Mus, Qtz
Small DS for most polymorphic transformations
small DG between two alternative polymorphs, even several tens of degrees from the equilibrium boundary
little driving force for the reaction to proceed ® common metastable relics in the stability field of other
Coexisting polymorphs may therefore represent non-equilibrium states (overstepped equilibrium curves)
Classic example K-spar and Ab form a solid solution at 1000C but separate into Microcline + Albite -> Perthitic Texture
Figure 6-16. T-X phase diagram of the system albite-orthoclase at 0.2 GPa H2O pressure. After Bowen and Tuttle (1950). J. Geology, 58, 489-511. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Differ from polymorphic transformations: involve solids of differing composition
Material must diffuse from one site to another for the reaction to proceed
NaAlSi2O6 + SiO2 = NaAlSi3O8
MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5
4 (Mg,Fe)SiO3 + CaAl2Si2O8 =
Reaction curves typically pretty straight
DS and DV change little
If minerals contain volatiles, but the volatiles conserved in the reaction so that no fluid phase is generated or consumed
For example, the reaction:
Mg3Si4O10(OH)2 + 4 MgSiO3 = Mg7Si8O22(OH)2
Talc Enstatite Anthophyllite
involves hydrous phases, but conserves H2O
It may therefore be treated as a solid-solid net-transfer reaction
For example the location of the reaction line on a
P-T phase diagram of the dehydration reaction:
KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O
MuscQtzKfld Sill Water
depends upon the partial pressure of H2O (pH2O)
Here the equilibrium curve represents equilibrium between the reactants and products under water-saturated conditions (pH2O = PLithostatic)
P-T phase diagram for the reaction Ms + Qtz = Kfs + Al2SiO5 + H2O showing the shift in equilibrium conditions as pH2O varies (assuming ideal H2O-CO2 mixing). Calculated using the program TWQ by Berman (1988, 1990, 1991). After Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Suppose H2O is withdrawn from the system at some point on the water-saturated equilibrium curve: pH2O < Plithostatic
According to Le Châtelier’s Principle, removing water at equilibrium will be compensated by the reaction running to the right, thereby producing more water
This has the effect of stabilizing the right side of the reaction at the expense of the left side
So as water is withdrawn the Kfs + Sill + H2O field expands slightly at the expense of the Mu + Qtz field, and the reaction curve shifts toward lower temperature
Figure 26-5. T-XCO2 phase diagram for the reaction Cal + Qtz = Wo + CO2 at 0.5 GPa assuming ideal H2O-CO2 mixing, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Figure 26-1. A portion of the equilibrium boundary for the calcite-aragonite phase transformation in the CaCO3 system. After Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, 28-38. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
Occur when F 1, and the reactants and products coexist over a temperature (or grade) interval
Fig. 26-9.Schematic isobaric T-XMg diagram representing the simplified metamorphic reaction Chl + Qtz Grt + H2O. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
where DG0 is the Gibbs Free Energy at the pressure and temperature of interest
are the corrections as T,P change
Mg3Al2Si3O12 + KFe3Si3AlO10(OH)2
The Garnet - Biotite geothermometer
lnK = - DH/RT +DS/R - (DV/RT) dP
This is a line!
From (27-26) we can extract
DH from the slope and DS from
Figure 27-5. Graph of lnK vs. 1/T (in Kelvins) for the Ferry and Spear (1978) garnet-biotite exchange equilibrium at 0.2 GPa from Table 27-2. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
The GASP geobarometer
Figure 27-8. P-T phase diagram showing the experimental results of Koziol and Newton (1988), and the equilibrium curve for reaction (27-37). Open triangles indicate runs in which An grew, closed triangles indicate runs in which Grs + Ky + Qtz grew, and half-filled triangles indicate no significant reaction. The univariant equilibrium curve is a best-fit regression of the data brackets. The line at 650oC is Koziol and Newton’s estimate of the reaction location based on reactions involving zoisite. The shaded area is the uncertainty envelope. After Koziol and Newton (1988) Amer. Mineral., 73, 216-233