IMPORTANT CONCEPT:

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IMPORTANT CONCEPT Metamorphic assemblages are a
function of P-T and protolith chemistry ?
Different protoliths will yield different mineral
assemblages at P-T conditions
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Metamorphism of Mafic Rocks

• Protoliths are generally mafic and intermediate
  volcanics. These represent lavas interlayered
  with other sediments. Rift basalts, island arc
  volcanics, etc.
• Prograde sequence includes zeolite,
  phrenite-pumpellyite, chlorite, actinolite,
  epidote, hornblende, pyroxenes (Cpx and Opx) in
  increasing grade order
• The mafic minerals that form are commonly
  diagnostic of the grade and facies.

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Metamorphosed Mafic Rocks (Metabasites)
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• characteristic mineral assemblage chlorite
  phrenite/pumpellyite/epidote actinolite
  albite quartz

• characteristic mineral assemblage chlorite
  zeolites calcite albite quartz

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Greenschist Facies (350 - 500ºC)

• characteristic mineral assemblage chlorite
  epidote actinolite albite quartz

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

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Greenschist ? Amphibolite Facies

• Greenschist to amphibolite facies transition
  involves two major mineralogical changes
• Plagioclase becomes more Ca-rich (albite ?
  oligoclase)
• 2. Amphiboles become more Al-, Na-, K- rich
  (actinolite ? hornblende)

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Amphibolite Facies

• classic amphibolite assemblage Hbl Plag
• Garnet in more Al-Fe-rich and Ca-poor mafic rocks
• Clinopyroxene in Al-poor-Ca-rich rocks

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Amphibolite ? Granulite Facies

• Hornblende decomposes and orthopyroxene
  clinopyroxene appear

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Granulite Facies

• characterized by anhydrous mineral assemblage

• Opx Cpx plag qtz Grt

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Blueschist and Eclogite Facies
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Med P/T
Low P/T
High P/T
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Blueschist 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.
• 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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Blueschist Facies

• classic assemblage lawsonite glaucophane
  quartz ? albite ? jadeite (high P)
• Garnet in more Al-Fe-rich and Ca-poor mafic rocks
• Aragonite in Al-poor-Ca-rich rocks

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Eclogite Facies

• Classic assemblage omphacitic pyroxene
  pyrope-grossular garnet quartz kyanite

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Metamorphism of Calcareous Rocks

• Calcareous rocks are predominantly carbonate
  rocks, usually limestone or dolostone and
  mixtures of calcerous and clastic sediments.
• Typically form in continental shelf (passive
  margin) or forearc/trench (active margins)
  environments
• They may be pure carbonate, or they may contain
  variable amounts of other precipitates (such as
  chert or hematite) or detrital material (sand,
  clays, etc.)
• Become metamorphosed when depositional basins
  becomes part of an orogenic belt

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Metamorphism of Calcareous Rocks

• Metacarbonates are metamorphosed calcareous rocks
  in which the carbonate component is predominant.
  Protoliths include limestones and dolostones.
• Calc-silicate rocks carbonate is subordinate and
  may be composed of Ca-Mg-Fe-Al silicate minerals,
  such as diopside, grossular, Ca-amphiboles,
  vesuvianite, epidote, wollastonite, etc.
  Protoliths include mixtures of calcareous and
  clastic sediments, like calcareous shale.

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Metamorphism of Calcareous Rocks
limestones
dolostones
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metacarbonate isograds
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Metamorphism of Calcareous Rocks
metacarbonate zones
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greenschist
talc
upper amphibolite
lower amphibolite
tremolite
forsterite
diopside
granulite
wollastonite
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Decarbonation Reaction

• CaCO3 SiO2 CaSiO3 CO2
• Cal Qtz Wo
• maximum thermal stability of the carbonate
  mineral assemblage occurs at pure XCO2

Reaction occurs at lower T, if you add H2O (e.g.
XCO2 lt1 )
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Dehydration Reaction

• KAl2Si3AlO10(OH)2 SiO2 KAlSi3O8 Al2SiO5
  H2O
• Ms Qtz Kfs Sill
  W

Reaction occurs at lower T, if you add CO2 (e.g.
XH2O lt1 )
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Mixed Volatile Reactions

• 5 types of devolatilization reactions, each with
  a unique general shape on a T-X diagram
• Type 3 Tmax at XCO2 determined by the
  stoichiometric ratio of CO2/H2O produced

Ca2Mg5Si8O22(OH)2 3 CaCO3 2 SiO2
Tr Cal Qtz 5
CaMgSi2O6 3 CO2 H2O Di
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IMPORTANT CONCEPT ? The composition of
the fluids involved must be constrained in order
to interpret the P-T history of the rocks
The temperature of an isograd based on a
devolatilization reaction is sensitive to the
composition of the volatile species involved
(e.g. XH2O).
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Calc-Silicate Rocks

• Protoliths are mixtures of detrital clastic
  material (qtz, feldspars, clays) and carbonate
  sediments ? warm, shallow shelf or forearc/trench
  environments.
• Bulk composition is high in Fe, Al, K, Ca, and
  Mg.
• Prograde mineral assemblages are dominated by
  silicates (Chl, Mus, Bt, Hbl, Ca-plag, Grt,
  epidote, actinolite).
• Significant mineral assemblage overlap with
  metamafic rocks especially at upper
  greenschist-amphibolite facies.

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Calc-Silicate Rocks

• Prograde Sequence characterized by
• Progressive reaction of carbonates
  (decarbonation)
• Sheet silicates epidote at low grades
• Ca-amphiboles at medium grades
• Ca-pyroxene and garnet at high grades

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Metamorphism of Calcareous Rocks
ZONE COMPARISON
Pelites
Calc-Silicates
Ankerite
Chlorite Biotite Garnet Staurolite Kyanite Sillima
nite
Biotite
Amphibole
Ziosite
Diopside
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Metamorphism of Calcareous Rocks
Figure 29-7b. Isograds mapped in the field. Note
that isograd (5) crosses the others in a manner
similar to that in part (a). This behavior is
attributed to infiltration of H2O from the
syn-metamorphic pluton in the area, creating a
gradient in XH2O across the area at a high angle
to the regional temperature gradient, equivalent
to the T-X diagram. After Carmichael (1970) J.
Petrol., 11, 147-181.
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Metamorphism of Calcareous Rocks
Figure 29-7a. T-XH2O diagram illustrating the
shapes and relative locations of the reactions
for the isograds mapped in the Whetstone Lake
area. Reactions 1, 2, and 4 are dehydration
reactions and reaction 3 is the Ky Sil
transition, all in metapelites. Reaction 5 is a
dehydration-decarbonation in calcic rocks with a
temperature maximum at XH2O 0.25. b. Isograds
mapped in the field. Note that isograd (5)
crosses the others in a manner similar to that in
part (a). This behavior is attributed to
infiltration of H2O from the syn-metamorphic
pluton in the area, creating a gradient in XH2O
across the area at a high angle to the regional
temperature gradient, equivalent to the T-X
diagram. After Carmichael (1970) J. Petrol., 11,
147-181.