Download Chapter 29: Calcareous and Ultramafic Rocks

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
Metamorphism of Calcareous 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
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 (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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 (2010) An
Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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 (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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 < 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.
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 (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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 (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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.
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.
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 synmetamorphic 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.
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 (2010) An Introduction to Igneous and Metamorphic
Petrology. Prentice Hall.
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.
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 PressureTemperature-Time Paths. Mineral. Soc. Amer. Monograph 1.
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
The suture zone is marked by the mélange and
particularly by the occurrence of ultramafic rocks
composing the mantle portion of the ocean lithosphere
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.
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.Star represents
composition of a typical mantle lherzolite. Dashed reactions represent those that do not occur in typical ultramafic rocks, but rather in unusually SiO2rich or SiO2-poor varieties. After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monogr. 1.
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 lowSiO2 ultramafics (green). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.