Introduction to the Metamorphism of Carbonate Rocks

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

2. 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

3. 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 rotationnw = 1.658ne = 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

4. 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

5. Calcite ppl xpl

6. 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

7. 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

8. 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

9. Wollastonite • Refractive Indicesa: 1.616-1.645b: 1.628-1.652g: 1.631-1.656d: 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.

10. Wollastonite

11. P-T Stability of Calcite + Quartz

12. 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.

13. P-T Stability of Calcite + Quartz

14. 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.

15. Effect of Fluid Composition

16. 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.

17. 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.

18. 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.

19. 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

20. 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.

21. Diopside • FormulaCaMgSi2O6 • Crystal SystemMonoclinic -> Biaxial • Crystal HabitShort, stubby, prismatic crystals with square, rectangular, or eight sided cross sectionGranular, lamellar, or columnar masses Anhedral grains • CleavageFair to good cleavage on (110), Partings on (100) and (001)Imperfect cleavage intersecting at 87º and 93º ie typical pyroxene cleavage • Color/PleochroismNo pleochroism Colorless to pale green in thin section

22. Diopside • Refractive Indicesa = 1.664-1.745b = 1.672-1.753g = 1.694-1.771d = 0.018-0.034 • Extinctioninclined in (010) sections • Distinguishing Featureslight green color, cleavage • OccurrenceCommonly found in metamorphosed carbonate rocks like skarns and marbles. Found with: tremolite, actinolite, grossular garnet, epidote, wollastonite, forsterite, calcite and dolomite

23. Diopside ppl xpl

24. 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

25. Epidote • Clinozoisite epidotea = 1.670-1.715 1.715-1.751b =  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.

26. Epidote ppl xpl

27. 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

28. Actinolite-Tremolite • Refractive Indices a = 1.599-1.688b = 1.612-1.697g = 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.

29. Actinolite-Tremolite ppl xpl