1. G EOL 2312 I GNEOUS AND M ETAMORPHIC P ETROLOGY Lecture 22 Metamorphism of Calcareous and Ultramafic Rocks April 13, 2016

2. M ETAMORPHISM OF C ALCAREOUS R OCKS T YPES OF M ETA -C ALCAREOUS R OCKS Metacarbonates are metamorphosed calcareous rocks in which the carbonate component is predominant Marbles are nearly pure carbonate Calc-silicate rocks: carbonate is subordinate and may be composed 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 F Contact between sedimentary layers F Contact between carbonate country rocks and a hot, hydrous, silicate intrusion, such as a granite

3. M ETAMORPHISM OF C ALCAREOUS R OCKS L IMESTONE AND D OLOSTONE Because calcite has a broad P-T stability range, the metamorphism of pure limestone to marble results in coarsening, but little change in mineralogy. Metamorphism of impure limestones and dolostones (containing minor amounts of other minerals like quartz, feldspar and heavy minerals) create a far more diverse assemblage of Ca-Mg- Si-bearing minerals contained in metacarbonates

4. M ETAMORPHISM OF C ALCAREOUS R OCKS T HE CMS-HC C HEMOGRAPHIC S YSTEM Winter (2001) Figure 29-1. Chemographics in the CaO- MgO-SiO 2 -CO 2 -H 2 O system, projected from CO 2 and H 2 O. 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. Carbonated ultramafics occupy the right half of the triangle. Cal – Calcite Wo – Wollastonite Di – Diopside Tr – Tremolite Tlc – Talc Fo – Mg-olivine Bru – Brucite Per – Periclase Dol - Dolomite Impure Limestone Dolostone

5. M ETAMORPHISM OF C ALCAREOUS R OCKS M AJOR L OW P RESSURE I SOGRADS Winter (2001) 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) Amer. J. Sci., 276, 502-524. Tlc-in Tr-in Fo-in Per-in Di-in T-X CO2 phase diagram for siliceous carbonates at P = 0.1 GPa

6. M ETAMORPHISM OF C ALCAREOUS R OCKS *X CO 2 of fluid is externally buffered at some fixed value; here = 0.41) L OW P RESSURE ~ 0.1 GPa O PEN F LUID C ONDITIONS * X CO 2 < 0.67 1) Tlc + Cal + CO 2 Dol + Qtz + H 2 O 2) Tr + H 2 O + CO 2 Tlc + Qtz + Cal Remember, T cannot increase at a reaction until one of the phases is consumed 3) Tr + Dol + H 2 O + CO 2 Tlc + Cal Only if Qtz remains after reaction #1 (unlikely in calc rx)

7. 4) Di + H 2 O + CO 2 Tr + Cal + Qtz 5) Tr + Fo + H 2 O + CO 2 Tlc + Dol Only if Qtz remains after reaction #2 (only in calc-sil rx) 6) Tr + Fo + H 2 O + CO 2 Tr + Dol Only if Tlc remains after reaction #3 (rare in calc rx) M ETAMORPHISM OF C ALCAREOUS R OCKS L OW P RESSURE ~ 0.1 GPa O PEN F LUID C ONDITIONS * X CO 2 < 0.67

8. 7) Wo + CO 2 Cal + Qtz 8) Per + Cal + CO 2 Dol If Qtz remains after prev. reactions (e.g. calc-sil rx or Si-metasomatized calcareous rx) 6x) Tr + Cal = Di + Fo (Tr-out) M ETAMORPHISM OF C ALCAREOUS R OCKS L OW P RESSURE ~ 0.1 GPa O PEN F LUID C ONDITIONS * X CO 2 < 0.67

9. M ETAMORPHISM OF C ALCAREOUS R OCKS P ROGRESSIVE M ETAMORPHISM UNDER C LOSED C ONDITIONS R ED L INE - F LUID COMPOSITION CONTROLLED BY REACTIONS Five-phase Invariant Points A.Dol + Qtz + Tlc + Cal + Tr B.Dol + Qtz + Tr + Di + Cal C.Tr + Cal + Di + Fo + Dol Phase in Red are may be consumed at invariant points, which then allows reactions to proceed along univariant curves Reaction produces 1:1 moles of H 2 O and CO 2, therefore fluid composition is fixed at X CO2 =0.5 until a phase is lost

10. M ETAMORPHISM OF C ALCAREOUS R OCKS M ETAMORPHISM AT H IGHER P RESSURES Reactions at higher T Diopside larger stability field Talc stable only at low X CO2

11. M ETAMORPHISM OF C ALCAREOUS R OCKS C ALC -S ILICATES 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 their complex compositions Necessary components include Si, Ca, Mg, Fe, Na, and K Resultant metamorphic mineral suites are hybrids of calcareous and pelitic systems

12. Occurrences of Ultramafic Rocks in the Crust Alpine peridotites: uppermost mantle attached to the base of oceanic lithosphere slabs (ophiolites) that become incorporated into the continental crust along subduction zones; commonly get trapped between two terranes during an accretion event Originally composed of olivine + orthopyroxene + clinopyroxene (6:3:1) = Lherzolite Due to hydrothermal alteration at ocean ridges, the ultramafic rock comes into orogenic belts being strongly altered (serpentinized). Regional metamorphism then overprints this. M ETAMORPHISM OF U LTRAMAFIC R OCKS Chain of ultramafic bodies in Vermont indicating a suture zone of the Ordovician Taconic Orogeny. The ultramafics mark a closed oceanic basin 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.

13. M ETAMORPHISM OF U LTRAMAFIC R OCKS C OMMON M ETAMORPHIC M INERALS Qtz - Quartz Di – Diopside Tr – Tremolite Tlc – Talc Ath – Anthophyllite En – Enstatite Atg – Antigorite (Serpentine) Crs – Chrysotile (Serpentine) Fo – Mg-olivine Bru – Brucite Per – Periclase En Ath Atg, Crs - Typical Mantle Lherzolite

14. Figure 29-11. Chemographics of ultramafic rocks in the CMS-H system (projected from H 2 O) showing the stable mineral assemblages (in the presence of excess H 2 O) and changes in topology due to reactions along the medium P/T metamorphic field gradient illustrated in Figure 29-10. The star represents the composition of a typical mantle lherzolite. Dashed reactions represent those that do not occur in typical ultramafic rocks, but rather in unusually SiO 2 -rich or SiO 2 -poor varieties. After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1. M ETAMORPHISM OF U LTRAMAFIC R OCKS

15. Winter (2001) Figure 29-10. Petrogenetic grid for water-saturated ultramafic rocks in the system CaO-MgO-SiO 2 -H 2 O produced using the TWQ software of Berman (1988, 1990, 1991). The green arrow represents a typical medium P/T metamorphic field gradient. The dark blue area represents the stability range of anthophyllite in “normal” ultramafic compositions. The lighter blue area represents the overall stability range of anthophyllite, including more siliceous ultramafic rocks. After Spear (1993) Metamorphic Phase Equilibria and Pressure- Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1.