CALCAREOUS AND ULTRAMAFIC MET..ppt

8/11/2019 CALCAREOUS AND ULTRAMAFIC MET..ppt

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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

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CALCAREOUS AND ULTRAMAFIC MET..ppt