Slide 1Metamorphism of Calcareous Rocks Types of Meta-Calcareous RocksMetamorphism of Calcareous Rocks Limestone and DolostoneMetamorphism of Calcareous Rocks The CMS-HC Chemographic SystemSlide 5Slide 6Slide 7Slide 8Slide 9Slide 10Slide 11Slide 12Slide 13Slide 14Slide 15GEOL 2312 IGNEOUS AND METAMORPHIC PETROLOGYLecture 28Metamorphism of Calcareous and Ultramafic RocksApril 13, 2009METAMORPHISM OF CALCAREOUS ROCKSTYPES OF META-CALCAREOUS ROCKSMetacarbonates are metamorphosed calcareous rocks in which the carbonate component is predominantMarbles are nearly pure carbonateCalc-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 graniteMETAMORPHISM OF CALCAREOUS ROCKSLIMESTONE AND DOLOSTONEBecause 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 metacarbonatesMETAMORPHISM OF CALCAREOUS ROCKSTHE CMS-HC CHEMOGRAPHIC SYSTEMWinter (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. Cal – CalciteWo – WollastoniteDi – DiopsideTr – TremoliteTlc – TalcFo – Mg-olivineBru – BrucitePer – PericlaseDol - DolomiteImpure LimestoneDolostoneMETAMORPHISM OF CALCAREOUS ROCKSMAJOR LOW PRESSURE ISOGRADSWinter (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-inTr-inFo-inPer-inDi-inT-XCO2 phase diagram for siliceous carbonates at P = 0.1 GPaMETAMORPHISM OF CALCAREOUS ROCKS *XCO2 of fluid is externally buffered at some fixed value; here = 0.41)LOW PRESSURE ~ 0.1 GPaOPEN FLUID CONDITIONS* XCO2< 0.671) Tlc + Cal + CO2Dol + Qtz + H2O 2) Tr + H2O + CO2Tlc + Qtz + CalRemember, T cannot increase at a reaction until one of the phases is consumed3) Tr + Dol + H2O + CO2Tlc + CalOnly if Qtz remains after reaction #1(unlikely in calc rx)4) Di + H2O + CO2Tr + Cal + Qtz5) Tr + Fo + H2O + CO2Tlc + DolOnly if Qtz remains after reaction #2(only in calc-sil rx)6) Tr + Fo + H2O + CO2Tr + DolOnly if Tlc remains after reaction #3(rare in calc rx)METAMORPHISM OF CALCAREOUS ROCKS LOW PRESSURE ~ 0.1 GPaOPEN FLUID CONDITIONS* XCO2< 0.677) Wo + CO2Cal + Qtz8) Per + Cal + CO2DolIf Qtz remains after prev. reactions(e.g. calc-sil rxor Si-metasomatized calcareous rx)6x) Tr + Cal = Di + Fo (Tr-out)6x) Tr + Cal = Di + Fo (Tr-out)METAMORPHISM OF CALCAREOUS ROCKS LOW PRESSURE ~ 0.1 GPaOPEN FLUID CONDITIONS* XCO2< 0.67METAMORPHISM OF CALCAREOUS ROCKSPROGRESSIVE METAMORPHISM UNDER CLOSED CONDITIONSRED LINE - FLUID COMPOSITION CONTROLLED BY REACTIONSFive-phaseInvariant PointsA. Dol + Qtz + Tlc + Cal + TrB. Dol + Qtz + Tr + Di + CalC. Tr + Cal + Di + Fo + DolPhase in Red are may be consumed at invariant points, which then allows reactions to proceed along univariant curvesReaction produces 1:1 moles of H2O and CO2 , therefore fluid composition is fixed at XCO2=0.5 until a phase is lostMETAMORPHISM OF CALCAREOUS ROCKSMETAMORPHISM AT HIGHER PRESSURES• Reactions at higher T• Diopside larger stability field•Talc stable only at low XCO2METAMORPHISM OF CALCAREOUS ROCKSCALC-SILICATESWinter (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 rocksCalc-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 systemsOccurences of Ultramafic Rocks in the CrustAlpine 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 eventOriginally 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. METAMORPHISM OF ULTRAMAFIC ROCKSChain 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.METAMORPHISM OF ULTRAMAFIC ROCKSCOMMON METAMORPHIC MINERALSQtz - QuartzDi – DiopsideTr – TremoliteTlc – TalcAth – AnthophylliteEn – EnstatiteAtg – Antigorite (Serpentine)Crs – Chrysotile (Serpentine)Fo – Mg-olivineBru – BrucitePer – PericlaseEnAthAtg, Crs- Typical Mantle LherzoliteFigure 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.METAMORPHISM OF ULTRAMAFIC ROCKSMETAMORPHISM OF ULTRAMAFIC ROCKSWinter (2001) Figure 29-10.