Geol 2312 Igneous and Metamorphic Petrology

1. Geol 2312 Igneous and Metamorphic Petrology Lecture 28 Metamorphism of Calcareous and Ultramafic Rocks April 13, 2009

2. Metamorphism of Calcareous RocksTypes of Meta-Calcareous Rocks • 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 • Contact between sedimentary layers • Contact between carbonate country rocks and a hot, hydrous, silicate intrusion, such as a granite

3. Metamorphism of Calcareous RocksLimestone and Dolostone 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. Metamorphism of Calcareous RocksThe CMS-HC Chemographic System Cal – Calcite Wo – Wollastonite Di – Diopside Tr – Tremolite Tlc – Talc Fo – Mg-olivine Bru – Brucite Per – Periclase Dol - Dolomite Impure Limestone Winter (2001) Figure 29-1. Chemographics in the CaO-MgO-SiO2 -CO2 -H2O system, projected from CO2 and 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. Carbonated ultramafics occupy the right half of the triangle. Dolostone

5. Metamorphism of Calcareous RocksMajor Low Pressure Isograds T-XCO2 phase diagram for siliceous carbonates at P = 0.1 GPa Per-in Di-in Fo-in Tr-in 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

6. Metamorphism of Calcareous Rocks Low Pressure ~ 0.1 GPa Open Fluid Conditions* Xco2< 0.67 3) Tr + Dol + H2O + CO2 Tlc + Cal *Xco2 of fluid is externally buffered at some fixed value; here = 0.41) 2) Tr+ H2O + CO2 Tlc + Qtz + Cal Only if Qtz remains after reaction #1 (unlikely in calc rx) 1) Tlc + Cal + CO2 Dol + Qtz + H2O Remember, T cannot increase at a reaction until one of the phases is consumed

7. Metamorphism of Calcareous Rocks Low Pressure ~ 0.1 GPa Open Fluid Conditions* Xco2< 0.67 6) Tr + Fo + H2O + CO2 Tr + Dol 5) Tr + Fo+ H2O + CO2 Tlc + Dol Only if Tlc remains after reaction #3 (rare in calc rx) 4) Di+ H2O + CO2 Tr + Cal + Qtz Only if Qtz remains after reaction #2 (only in calc-sil rx)

8. Metamorphism of Calcareous Rocks Low Pressure ~ 0.1 GPa Open Fluid Conditions* Xco2< 0.67 8) Per + Cal+ CO2 Dol 7) Wo + CO2 Cal + Qtz If Qtz remains after prev. reactions (e.g. calc-sil rx or Si-metasomatized calcareous rx) 6x) Tr + Cal = Di + Fo (Tr-out)

9. Metamorphism of Calcareous RocksProgressive Metamorphism under Closed Conditions Red Line - Fluid composition controlled by reactions • Five-phase • Invariant Points • Dol + Qtz + Tlc + Cal + Tr • Dol + Qtz + Tr + Di + Cal • Tr + Cal + Di + Fo + Dol Reaction produces 1:1 moles of H2O and CO2 , therefore fluid composition is fixed at XCO2=0.5 until a phase is lost Phase in Red are may be consumed at invariant points, which then allows reactions to proceed along univariant curves

10. Metamorphism of Calcareous Rocks Metamorphism at Higher Pressures • Reactions at • higher T • Diopside larger • stability field • Talc stable only • at low XCO2

11. Metamorphism of Calcareous Rocks Calc-Silicates 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 • Pelitic rocks • Calc-silicate rocks 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.

12. Metamorphism of Ultramafic Rocks Occurences 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. 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. Metamorphism of Ultramafic Rocks Common Metamorphic Minerals 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 • Typical • Mantle • Lherzolite Ath En Atg, Crs

14. Metamorphism of Ultramafic Rocks 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 the composition of a typical mantle lherzolite. Dashed reactions represent those that do not occur in typical ultramafic rocks, but rather in unusually SiO2-rich or SiO2-poor varieties. After Spear (1993) Metamorphic Phase Equilibria and Pressure-Temperature-Time Paths. Mineral. Soc. Amer. Monograph 1.

15. Metamorphism of Ultramafic Rocks Winter (2001) Figure 29-10. Petrogenetic grid for water-saturated ultramafic rocks in the system CaO-MgO-SiO2-H2O 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.