Chapter 29: Metamorphism of Calcareous and Ultramafic Rocks

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Chapter 29 Metamorphism of Calcareous and
Ultramafic Rocks

• Calcareous rocks are predominantly carbonate
  rocks, usually limestone or dolostone
• Typically form in a stable continental shelf
  environment along a passive margin
• 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.)
• The spectrum from pure carbonate to purely
  clastic rocks is essentially complete
• Become metamorphosed when the passive margin
  becomes part of an orogenic belt

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Chapter 29 Metamorphism of Calcareous and
Ultramafic Rocks

• Metacarbonates are metamorphosed calcareous rocks
  in which the carbonate component is predominant
• Marbles are nearly pure carbonate
• 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.
• Skarn calc-silicate rock formed by metasomatism
  between carbonates and silicate-rich rocks or
  fluids
• Contact between sedimentary layers
• Contact between carbonate country rocks and a
  hot, hydrous, silicate intrusion, such as a
  granite

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Metamorphism of Calcareous Rocks
Figure 29-1. Chemographics in the
CaO-MgO-SiO2 -CO2 -H2O system, projected from CO2
and H2O. The green shaded areas represent the
common composition range of limestones and
dolostones. Due to the solvus between calcite and
dolomite, both minerals can coexist in carbonate
rocks. The dark red left half of the triangle is
the area of interest for metacarbonates.
Carbonated ultramafics occupy the right half of
the triangle. Winter (2001) An Introduction to
Igneous and Metamorphic Petrology. Prentice Hall.
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Figure 29-2. A portion of the Alta aureole in
Little Cottonwood Canyon, SE of Salt Lake City,
UT, where talc, tremolite, forsterite, and
periclase isograds were mapped in metacarbonates
by Moore and Kerrick (1976) Amer. J. Sci., 276,
502-524. Winter (2001) An Introduction to Igneous
and Metamorphic Petrology. Prentice Hall.
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Figure 29-3. T-XCO2 phase diagram for siliceous
carbonates at P 0.1 GPa. Calculated using the
program TWQ of Berman (1988, 1990, 1991). The
green area is the field in which tremolite is
stable, the reddish area is the field in which
dolomite diopside is stable, and the blue area
is for dolomite talc. Compatibility diagrams,
similar to those in Figure 29-4, show the mineral
assemblages in each field. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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Figure 29-4. The sequence of CaO-MgO-SiO2-H2O-CO2
compatibility diagrams for metamorphosed
siliceous carbonates (shaded half) along an
open-system (vertical) path up metamorphic grade
for XCO2 lt 0.63 in Figure 29-3. The dashed
isograd requires that tremolite is more abundant
than either calcite or quartz, which is rare in
siliceous carbonates. After Spear (1993)
Metamorphic Phase Equilibria and
Pressure-Temperature-Time Paths. Mineral. Soc.
Amer. Monograph 1.
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Metamorphism of Calcareous Rocks
Figure 29-5. Metamorphic zones developed in
regionally metamorphosed dolomitic rocks of the
Lepontine Alps, along the Swiss-Italian border.
After Trommsdorff (1966) Schweiz. Mineral.
Petrogr. Mitt., 46, 431-460 and (1972) Schweiz.
Mineral. Petrogr. Mitt., 52, 567-571. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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Figure 29-6. T-XCO2 phase diagram for siliceous
carbonates at P 0.5 GPa, calculated using the
program TWQ of Berman (1988, 1990, 1991). The
light-shaded area is the field in which tremolite
is stable, the darker shaded areas are the fields
in which talc or diopside are stable. Winter
(2001) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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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. After
Carmichael (1970) J. Petrol., 11, 147-181,
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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.
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Figure 29-8. Schematic T-XCO2 diagram
illustrating the characteristic shape of typical
dehydration reactions, such as those that
generate orthopyroxene from hornblende or
biotite. Notice that the amphibolite facies to
granulite facies can be accomplished by either an
increase in temperature or infiltration of CO2 at
a constant temperature. Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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Metamorphism of Calcareous Rocks
Figure 29-9. Map of isograds in the pelitic
Waterville and calcareous Vassalboro formations
of south-central Maine. After Ferry (1983) J.
Petrol., 24, 343-376.
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Metamorphism of Ultramafic Rocks
Figure 29-10. Petrogenetic grid for
water-saturated ultramafic rocks in the system
CaO-MgO-SiO2-H2O produced using the TWQ software
of Berman (1988, 1990, 1991). The green arrow
represents a typical medium P/T metamorphic field
gradient. The dark blue area represents the
stability range of anthophyllite in normal
ultramafic compositions. The lighter blue area
represents the overall stability range of
anthophyllite, including more siliceous
ultramafic rocks. After Spear (1993) Metamorphic
Phase Equilibria and Pressure-Temperature-Time
Paths. Mineral. Soc. Amer. Monograph 1.
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Metamorphism of Ultramafic Rocks

• Alpine peridotites uppermost mantle base of
  slivers of oceanic lithosphere that become
  incorporated into the continental crust along
  subduction zones
• Dismembered portions of ophiolites pieces of
  oceanic crust and mantle that either separate
  from the subducting slab and become incorporated
  into the accretionary wedge of the subduction
  zone, or (more commonly) get trapped between two
  terranes during an accretion event
• Strings of ultramafic bodies in orogens follow
  major fault zones separating contrasting rock
  bodies. Interpreted as remnants of oceanic crust
  mantle that once separated collisional
  terranes, and thus mark the suture zone
• Association of blueschist facies rocks with the
  ultramafics further supports a subduction-related
  origin

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Metamorphism of Ultramafic Rocks
Chain of ultramafic bodies in Vermont indicating
a suture zone of the Ordovician Taconic Orogeny.
The ultramafics mark a closed oceanic basin
between North American rocks and an accreted
island arc terrane. From Chidester, (1968) in Zen
et al., Studies in Appalachian Geology, Northern
and Maritime. Wiley Interscience.
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Metamorphism of Ultramafic Rocks
Figure 29-11. Chemographics of ultramafic rocks
in the CMS-H system (projected from H2O) showing
the stable mineral assemblages (in the presence
of excess H2O) and changes in topology due to
reactions along the medium P/T metamorphic field
gradient illustrated in Figure 29-10. The star
represents the composition of a typical mantle
lherzolite. Dashed reactions represent those that
do not occur in typical ultramafic rocks, but
rather in unusually SiO2-rich or SiO2-poor
varieties. After Spear (1993) Metamorphic Phase
Equilibria and Pressure-Temperature-Time Paths.
Mineral. Soc. Amer. Monograph 1.
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Metamorphism of Ultramafic Rocks
Figure 29-12. Simplified T-XCO2 phase diagram for
the system CaO-MgO-SiO2-H2O-CO2 at 0.5 GPa,
calculated using the program TWQ of Berman (1988,
1990, 1991). The diagram focuses on
ultramafic-carbonate rocks and omits reactions
involving quartz. The shaded fields represent the
stability ranges of serpentine-antigorite
(purple), anthophyllite in typical low-SiO2
ultramafics (blue), and tremolite in low-SiO2
ultramafics (green). Winter (2001) An
Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.