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 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.

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 Pressure-Temperature-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 SiO2-rich 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 low-

SiO2 ultramafics (green). Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.