Lecture ELEVEN Lecture ELEVEN Metamorphism of Metamorphism of carbonate carbonate

rocksrocks (Metcarbonates)

What is the Metacarbonates?MetacarbonatesMetacarbonates, are metamorphosed calcareous (limestone are metamorphosed calcareous (limestone and dolomite) rocks in which the carbonate component is and dolomite) rocks in which the carbonate component is predominant, with predominant, with granoblasticgranoblastic polygonalpolygonal texture texture

MetacarbonatesMetacarbonates include: include: i) i) MarblesMarbles are nearly pure carbonate (carbonate >50%)are nearly pure carbonate (carbonate >50%) ii) ii) Calc-silicate rocksCalc-silicate rocks:: carbonate is subordinate (carbonate carbonate is subordinate (carbonate <50%) and may be composed of Ca-Mg-Fe-Al silicate minerals, <50%) and may be composed of Ca-Mg-Fe-Al silicate minerals, such as such as diopside, grossular, Ca-amphiboles, vesuvianite, diopside, grossular, Ca-amphiboles, vesuvianite, epidote, wollastonite, plagioclase, talc, anthophyllite, etcepidote, wollastonite, plagioclase, talc, anthophyllite, etc. . iii) iii) SkarnSkarn:: calc-silicate rock formed by metasomatism between calc-silicate rock formed by metasomatism between carbonates and silicate-rich rocks or fluidscarbonates and silicate-rich rocks or fluids

Carbonate rocks Carbonate rocks are predominantly carbonate minerals, are predominantly carbonate minerals, usually limestone or dolostone. They may be pure carbonate, or usually limestone or dolostone. They may be pure carbonate, or they may contain variable amounts of other precipitates (such they may contain variable amounts of other precipitates (such as chert or hematite) or detrital material (sand, clays, etc.)as chert or hematite) or detrital material (sand, clays, etc.)

ChemicallyChemically, the carbonate rocks are rich in CaO, CO, the carbonate rocks are rich in CaO, CO22, MgO, , MgO, and mad may SiOand mad may SiO22, Al, Al22OO33, FeO, and other subordinate oxides if , FeO, and other subordinate oxides if the carbonate are impure.the carbonate are impure.

Mineralogy of MetacarbonatesMetacarbonate contain the Metacarbonate contain the following mineral following mineral assemblageassemblage::

Carbonate minerals Carbonate minerals (Calcite (Calcite and dolomite),and dolomite),

AmphiboleAmphibole (anthophyllite (anthophyllite Enstatite, Tremolite) Enstatite, Tremolite)

PyroxenePyroxene (Diopside) (Diopside)

OlivineOlivine

talc, talc,

wollastonitewollastonite

quartzquartz

The metacrbonates will discussed for The metacrbonates will discussed for metamorphism in the following conditions:metamorphism in the following conditions:

Pure limestone and dolomite

Impure limestone and dolomite

-1-Pure Metacarbonates

Calcite and dolomite marbles

1- Pure Carbonates (Limestone and dolomite) Metamorphism of pure carbonate rocks yielded Metamorphism of pure carbonate rocks yielded calcitecalcite

and/or and/or dolomitedolomite marblesmarbles. Many marbles are composed . Many marbles are composed only of calcite and/or dolomite with minor quartz and only of calcite and/or dolomite with minor quartz and

phyllosilicates, originally of detrital originphyllosilicates, originally of detrital origin. .

The grade of metamorphism is function in grain size, The grade of metamorphism is function in grain size, where grain size increases with grade increase.where grain size increases with grade increase.

At very At very HPHP, the polymorph , the polymorph aragonite becomes stable and becomes stable and aragonite marble is known from high pressure terrains. is known from high pressure terrains.

At At HT/LPHT/LP (>600°C) calcite and quartz react to produce (>600°C) calcite and quartz react to produce wollasonite and CO and CO22. The reaction occurs only at high . The reaction occurs only at high

temperature thermal aureole, and is inhibited by high temperature thermal aureole, and is inhibited by high fluid pressures of COfluid pressures of CO22..

CaCOCaCO33 +SiO +SiO2 2 CaSiO CaSiO33 + CO + CO22

A- Calcite marble

1- Pure Carbonates (Limestone and dolomite)

At At HT/LPHT/LP, dolomite marble loses CO, dolomite marble loses CO22 to form to form periclasepericlase

(MgO) in condition <900 °C, and consequently reacts with (MgO) in condition <900 °C, and consequently reacts with

water to form water to form brucitebrucite (MgO(OH) (MgO(OH)22). Therefore, the common ). Therefore, the common

result of decarbonation of dolomite or dolomitic marble is a result of decarbonation of dolomite or dolomitic marble is a

mixture of mixture of brucite and calcitebrucite and calcite..

Quartz bearing dolomitic marbles (Quartz bearing dolomitic marbles (calcite + dolomite + calcite + dolomite +

quartzquartz) develop a characteristic sequence of Ca- and/or Mg-) develop a characteristic sequence of Ca- and/or Mg-

silicate as follows:silicate as follows:

(i) talc(i) talc

dolomite + qurtz + Hdolomite + qurtz + H22O = talc + calcite + COO = talc + calcite + CO22

(ii) tremolite(ii) tremolite in the greenschist facies, in the greenschist facies,

talc + calcite + quartz = tremolite + Htalc + calcite + quartz = tremolite + H22O + COO + CO2 2 (quartz rich)(quartz rich)

talc+calcite = tremolite + dolomite + COtalc+calcite = tremolite + dolomite + CO22 + H + H22O (quartz O (quartz poor)poor)

A- Dolomite

marble

1- Pure Carbonates (Limestone and dolomite)

(iii) diopside and/or forsterite(iii) diopside and/or forsterite in the amphibolite facies in the amphibolite facies

tremolite+calcite+quartz = diopside+Htremolite+calcite+quartz = diopside+H22O +COO +CO22

tremolite + dolomite = forsterite + calcite + Htremolite + dolomite = forsterite + calcite + H22O + COO + CO22

And, And,

(iv) diopside + forsterite(iv) diopside + forsterite at higher grade. at higher grade.

tremolite + calcite = diopside + forsterite + Htremolite + calcite = diopside + forsterite + H22O+COO+CO22

Sheet-silicate impurity in calcite and dolomite marble Sheet-silicate impurity in calcite and dolomite marble

adds variety by the following Al-bearing minerals to adds variety by the following Al-bearing minerals to

feature in the assemblage: typically they include feature in the assemblage: typically they include zoisite, zoisite,

epidote and Ca-rich garnetepidote and Ca-rich garnet in the greenschist facies and in the greenschist facies and

anorthiteanorthite in the amphibolite facies. in the amphibolite facies.

A- Dolomite marble, cont.

Metamorphic zones developed in regionally metamorphosed dolomitic rocks of the Lepontine

Alps

-2-Metamorphism of impure

carbonates and marls (Calc-silicates)

2 -Calc-silicates Calc-silicates are rocks rich in Ca-Mg-silicate are rocks rich in Ca-Mg-silicate

minerals but poor in carbonate,minerals but poor in carbonate,

They form via the metamorphism of very impure They form via the metamorphism of very impure

calcite or dolomite limestones, or from limy calcite or dolomite limestones, or from limy

mudstones (marls). mudstones (marls).

Since calc–silicates contain significant amounts Since calc–silicates contain significant amounts

of other chemical components, such as Al, K and of other chemical components, such as Al, K and

Fe, minerals such as Fe, minerals such as zoisite (epidote group), zoisite (epidote group),

garnet, Ca-plagioclase, K-feldspar, hornblende garnet, Ca-plagioclase, K-feldspar, hornblende

and diopsideand diopside could formed. A generalized zonal could formed. A generalized zonal

sequence can be summarized as follows:sequence can be summarized as follows:

I- Ankerite zone

-The lowest grade rocks

- It characterized by the assemblage ankerite

Ca(Mg,Fe)(CO3)2) + quartz + albite + muscovite ±

chlorite II- Biotite zone

This zone is characterized by the coexistence of

biotite and chlorite without amphibole, via a reaction

such as:

Ms +Qtz + ankerite + H2O Cal + Chl + Bt + CO2

The upper part of this zone also characterize by the

replacement of albite by a more Ca-rich plagioclase

and a reduction in the amount of muscovite present:

Chl + Cal + Ms + Qtz + Ab Bt + Pl + H2O + CO2

III- Amphibole zone

The appearance of Ca-amphibole is accompanied by a

further increase in the Ca content of the plagioclase:

Chl + Cal + Qtz + Pl Ca-amph + Ca-Pl + H2O + CO2

IV- Zoisite zone

Zoisite (Ca2(Al,Fe)3[SiO4](OH)) often first appears

rimming plagioclase at contacts with calcite grains,

suggesting growth is due to the reaction:

Ca-plagioclase + calcite + H2O zoisite + CO2

V- Diopside zone

At the highest grades diopside appears due to the

breakdown of amphibole:

Ca-amphibole + calcite + quartz diopside + H2O +

CO2