Introduction to the Metamorphism of Carbonate Rocks IN THIS LECTURE Calcite marbles Decarbonation Dolomitic marbles Calc-silicate rocks Fluid composition in marbles

Marbles The term marble is used for metamorphosed calcareous rocks in which carbonate minerals dominate. This represents essentially two end-member compositions Very pure calcite limestones Impure calcite or dolomitic limestones Metamorphism of these two end-member compositions produces two different rock types Pure calcite marbles which are petrologically not very interesting Dolomitic marbles which are petrologically interesting

Calcite Colour colourless Pleochroism non pelochroic Form variety of habits, but usually coinsist of scalenohedron and rhombohedron combinations. In most rocks calcite forms anhedral grains or grain aggregates Relief moderate negative to high positive, marked change with stage rotation nw = 1.658 ne = 1.486 Cleavage perfect rhombohedral cleavage, angle between cleavages 74°57‘ Birefringence 0.172, extreme, creamy high order colours Twinning lamellar twins parallel to one edge of the cleavage rhomb or along the long diagonal of the rhomb

Calcite Optic Character uniaxial Extinction extinction is inclined or symmetrical to cleavage traces Composition dominantly CaCO3, but substitution of Mg, Fe, Mn, or Zn and minor Sr and Ba Alteration altered to dolomite during diagenesis, calcite is soluble in natural waters and may be removed by solution Occurrence common and widespread as a major mineral in limestones, and an accessory in igneous, metamorphic and sedimentary rocks Distinguishing Features cleavage, variable relief, extreme interference colours

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Calcite vs Dolomite Distinguishing calcite, dolomite and other rhombohedral carbonates from each other can be very difficult without obtaining chemical analysis or using chemical stains. Often can use associated mineralogy to help decide We’ll come back to this

Calcite Marbles In general, metamorphism of a pure calcite limestone simply produces a pure calcite marble. Petrologically not very interesting since calcite is stable to very high pressures and temperatures. Relatively pure limestones that contain a small amount of quartz are more interesting as they show one of the simplest examples of the most common reaction type in carbonate rocks, decarbonation reactions. CaCO3 + SiO2 -> CaSiO3 + CO2 Calcite + quartz -> wollastonite + fluid However, at pressures of more than a couple of kilobars the temperature required to form wollastonite is beyond the range of normal regional metamorphism

Wollastonite Formula CaSiO3 Pyroxenoid group. Usually pure, but Mn and Fe2+ can substitute for Ca Crystal System Triclinic -> Biaxial Crystal Habit Columnar and fibrous elongate grains, often with twinning  Cleavage Perfect cleavage on {100}, good cleavages on {001} and {-102} Splitery cleavage fragments. Angles of cleavage: 84.5 degrees, and 70 degrees Color/Pleochroism Colorless, white, greyish, often with yellowish or brownish tint. Vitreous. No pleochroism

Wollastonite Refractive Indices a: 1.616-1.645 b: 1.628-1.652 g: 1.631-1.656 d: 0.013-0.017 Increase with Fe and Mn content. Wollastonite resembles tremolite and pectolite, but both have a higher birefringence. Extinction Parallel  Elongate crystals display parallel extinction. Distinguishing Features Colorless to grey in thin sectionwith moderate to moderatly high relief. First order interference color yellow-orange. One perfect cleavage and two good cleavages producing splintery cleavage fragments. H = 4.5-5. G = 2.86-3.09. Streak is colorless or white.  Occurrence Occurs commonly as a product of contact and/or regional metamorphism in limestone and dolomite. Associated minerals include calcite, and grossular in hornfels, tremolite, epidote group members, diopside, and other Ca-Mg silicates.

Wollastonite

P-T Stability of Calcite + Quartz

The Role of Fluid Composition How then do we explain the presence of wollastonite in marbles that have not been to such high temperatures? Reduce the pressure of the CO2 phase. At temperatures of the greenschist facies and above, H2O and CO2 supercritical fluids are completely miscible Hence the partial pressure of CO2 in a mixed H2O-CO2 fluid may be much less than the total fluid pressure.

P-T Stability of Calcite + Quartz

Phase Rule Constraints The observed effect of adding H2O to the calcite + quartz + wollastonite + CO2 equilibria accords with the phase rule. Recalling that F = C – P + 2 In the H2O-absent system there are four phases and three components (CaO, SiO2 and CO2) giving one degree of freedom. This means that the full assemblage can only exist along a univariant curve. If H2O is added to the system then the number of components is increased by one but the number of phases stays the same since H2O is miscible with CO2. Hence there are two degrees of freedom Therefore fluid composition is a variable in addition to T and P and by specifying one of these three variables the equilibrium conditions can be represented by a univariant curve on a plot with the other two variables as axes.

Effect of Fluid Composition

T-XCO2 diagrams These types of plots are known as isobaric T-XCO2 diagrams On these types of plots divariant equilibria plot as a line known as an isobaric univariant curve. Therefore if the P is specified there is still one degree of freedom within the system.

Dolomitic Marbles Limestones that contain dolomite provide much more useful indicators of metamorphic grade because of a range of Ca-Mg silicates can form in the more usual P-T conditions of metamorphism, such as talc, tremolite and diopside. With prograde metamorphism there is a zonal sequence of mineral-appearance isograds similar to what we saw with pelites. This zonal sequence in regionally metamorphosed dolomitic limestones appears to be Talc (not always present) Tremolite Diopside or forsterite Diopside + forsterite This zonal scheme was first identified by Eskola, one of the fathers of metamorphic petrology in 1922.

Dolomitic Marbles and the Phase Rule The zonal scheme identified by Eskola, although applying generally, is actually much more complex in natural systems. Why is this? Again look at the phase rule. Dolomitic marbles can be described by five components CaO, MgO, SiO2, H2O and CO2 No assemblages have more than five phases, normally four minerals and a mixed fluid phase. Therefore according to the phase rule, there should be two degrees of freedom in most systems and thus most mineral assemblages will occur over a wide range of pressures and temperatures depending on what the composition of the fluid phase is.

Calc-Silicates Calc-silicates are rocks rich in Ca-Mg silicate minerals but with only minor amounts of carbonate present. Like dolomitic marbles, calc-silicates are useful indicators of metamorphic grade. They can be correlated with the pelite zones in the following manner Pelite zone Calc-silicate zone Garnet Zoisite-calcite-biotite Zoisite-hornblende Staurolite Anorthite-hornblende Kyanite Sillimanite Anorthite-pyroxene

Calc-Silicates Calc-silicates contain significant amounts of other chemical components especially Al, K and Fe. Therefore their mineralogy is more complex than that of dolomite marbles and additional phases include Zoisite Garnet Hornblende Ca-pyroxene like diopside Calcic-plagioclase K-feldspar Phlogopite and vesuvianite In general zoisite and grossular garnet are only stable if the fluid phase is rich in water, while calcic-plagioclase is favoured by CO2 dominated fluids.

Diopside Formula CaMgSi2O6 Crystal System Monoclinic -> Biaxial Crystal Habit Short, stubby, prismatic crystals with square, rectangular, or eight sided cross section Granular, lamellar, or columnar masses Anhedral grains Cleavage Fair to good cleavage on (110), Partings on (100) and (001) Imperfect cleavage intersecting at 87º and 93º ie typical pyroxene cleavage Color/Pleochroism No pleochroism Colorless to pale green in thin section

Diopside Refractive Indices a = 1.664-1.745 b = 1.672-1.753 g = 1.694-1.771 d = 0.018-0.034 Extinction inclined in (010) sections Distinguishing Features light green color, cleavage Occurrence Commonly found in metamorphosed carbonate rocks like skarns and marbles. Found with: tremolite, actinolite, grossular garnet, epidote, wollastonite, forsterite, calcite and dolomite

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Epidote Formula Ca2(Al,Fe)Al2O(SiO4)(Si2O7)(OH) Complete solid solution from clinozoisite (Al: Fe 3+ = 3:0) to epidote (Al:Fe 3+ = 2:1) Crystal System Monoclinic -> Biaxial (ep –ve, czo ve) Crystal Habit coarse to fine granular ; also fibrous Cleavage {001} perfect, {100} imperfect perfect cleavage in one direction Color/Pleochroism clinozoisite: pale green to gray epidote: pistachio-green to yellowish-green to black

Epidote Clinozoisite epidote a = 1.670-1.715 1.715-1.751 b =  1.674-1.725 1.725-1.784 g = 1.690-1.734 1.734-1.797 Max Birefringence 0.004 - 0.049 Refractive indices and birefringence increase with iron content Extinction Parallel to length of elongate crystals and to the trace of cleavage. Distinguishing Features Epidote is characterized by its green color and one perfect cleavage. H= 6-7. G = 3.25 to 4.45. Streak is white to gray. Clinozoisite and epidote are distinguised from eachother by optic sign, birefringence, and color Occurrence Occurs in areas of regional metamorphism; forms during retrograde metamorphism and forms as a reaction product of plagioclase, pyroxene, and amphibole. Common in metamorphosed limestones with calcium rich garnets, diopside, vesuvianite, and calcite.

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Actinolite-Tremolite Formula Ca2(Mg,Fe2+)5Si8O22(OH)2 Crystal System Monoclinic -> Biaxial Crystal Habit occurs as columnar, bladed or acicular grains, elongated parallel to c axis, may be fibrous, basal sections are diamond shaped, with typical amphibole cleavage Cleavage two amphibole cleavages on {110}, intersect at 56 and 124° Colour/Pleochroism colourless to pale green to dark green, darker colours and stronger pleochroism associated with high Fe contents

Actinolite-Tremolite Refractive Indices a = 1.599-1.688 b = 1.612-1.697 g = 1.622-1.705 Birefringence 0.017-0.027 maximum interference colours are upper 1st to mid 2nd order Extinction Inclined extinction greater for tremolite than actinolite Distinguishing Features Can exhibit simple and lamellar twins Alters to talc, chlorite and carbonates Resembles hornblende but often has lower extinction angle Occurrence common occurrence is in contact and regional metamorphosed limestone and dolomite. Also found in metamoprhosed mafic and ultramafic rocks. It is the common fine-grained alteration product of pyroxenes.

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