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GEO136-C1
IGNEOUSANDMETAMORPHICPETROLOGY
Metamorphism ofCalcareous
and Ultramafic Rocks
Karra Fenine Lopez
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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.)
Become metamorphosed when the passive margin
becomes part of an orogenic belt
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METAMORPHISMOFCALCAREOUSROCKS
TYPESOFMETA-CALCAREOUSROCKS
Metacarbonatesare metamorphosed calcareous rocks in which
the carbonate component is predominant
Marblesare nearly pure carbonate
Calc-silicate rocks: carbonate is subordinate and may becomposed 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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METAMORPHISMOFCALCAREOUSROCKS
THECMS-HC CHEMOGRAPHICSYSTEM
Winter (2001) Figure 29-1.
Chemographics in the CaO-
MgO-SiO2
-CO2
-H2
O system,
projected from CO2and 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. Carbonatedultramafics occupy the right half
of the triangle.
Cal Calcite
Wo WollastoniteDi Diopside
Tr Tremolite
Tlc Talc
Fo Mg-olivine
Bru Brucite
Per Periclase
Dol - Dolomite
Impure
Limestone
Dolostone
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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) Am er. J. Sci., 276, 502-524.
Winter (2010) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
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Figure 29.3.T-XCO2phase 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 fordolomite + 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.
METAMORPHISMOFCALCAREOUSROCKSMETAMORPHISMATLOWPRESSURES
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Figure 29.4.The sequence of CaO-MgO-SiO2-H2O-CO2compatibility 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 dashedisograd requires that tremolite is more abundant than either calcite or quartz, which is rare in siliceous carbonates. After
Spear (1993) Metamorphic Phase Equil ibr ia and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1.
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METAMORPHISMOFCALCAREOUSROCKSMETAMORPHISMATHIGHERPRESSURES
Reactions at
higher T
Diopside larger
stability field
Talc stable only
at low XCO2
Figure 29.6.T-XCO2phase 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.
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METAMORPHISMOFCALCAREOUSROCKSCALC-SILICATES
Winter (2001) 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.
- Pelitic rocks
- Calc-silicate rocks
Calc-silicate rocks present
challenges in simply defining theircomplex compositions
Necessary components include
Si, Ca, Mg, Fe, Na, and K
Resultant metamorphic mineral
suites are hybrids of calcareousand pelitic systems
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Occurences of Ultramafic Rocks in
the CrustAlpine peridotites: uppermost mantle
attached to the base of oceaniclithosphere slabs (ophiolites) that becomeincorporated into the continental crust
along subduction zones; commonly gettrapped between two terranes during anaccretion event
Originally composed of olivine +orthopyroxene + clinopyroxene (6:3:1) =Lherzolite
Due to hydrothermal alteration at oceanridges, the ultramafic rock comes intoorogenic belts being strongly altered(serpentinized). Regional metamorphismthen overprints this.
METAMORPHISMOFULTRAMAFICROCKS
Chain of ultramafic bodies
in Vermont indicating a
suture zone of the
Ordovician Taconic
Orogeny. The ultramafics
mark a closed oceanicbasin 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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The suture zone is marked by the mlange and
particularly by the occurrence of ultramafic rocks
composing the mantle portion of the ocean lithosphere
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METAMORPHISMOFULTRAMAFICROCKSCOMMONMETAMORPHICMINERALS
Qtz - Quartz
Di
DiopsideTr Tremolite
Tlc Talc
Ath Anthophyllite
En Enstatite
Atg Antigorite (Serpentine)
Crs
Chrysotile (Serpentine)Fo Mg-olivine
Bru Brucite
Per PericlaseEnAth
Atg, Crs
- Typical
Mantle
Lherzolite
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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 thecomposition of a typical mantle lherzolite. Dashed reactions represent those that do not occur in typical ultramafic rocks, but rather in unusually SiO2-rich or
SiO -poor varieties After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths Mineral Soc Amer Monograph 1
METAMORPHISMOFULTRAMAFICROCKS