Successfully reported this slideshow.
Your SlideShare is downloading. ×

Ch 21 Intro to Met.ppt

Ad
Ad
Ad
Ad
Ad
Ad
Ad
Ad
Ad
Ad
Ad
Upcoming SlideShare
Volcanoes.ppt
Volcanoes.ppt
Loading in …3
×

Check these out next

1 of 72 Ad

More Related Content

More from KimberlyAnnePagdanga1 (20)

Recently uploaded (20)

Advertisement

Ch 21 Intro to Met.ppt

  1. 1. Chapter 21: Metamorphism Fresh basalt and weathered basalt
  2. 2. The IUGS-SCMR proposed this definition: “Metamorphism is a subsolidus process leading to changes in mineralogy and/or texture (for example grain size) and often in chemical composition in a rock. These changes are due to physical and/or chemical conditions that differ from those normally occurring at the surface of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting.” Chapter 21: Metamorphism
  3. 3. The Limits of Metamorphism Low-temperature limit grades into diagenesis • Processes are indistinguishable • Metamorphism begins in the range of 100-150oC for the more unstable types of protolith • Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary
  4. 4. The Limits of Metamorphism • High-temperature limit grades into melting • Over the melting range solids and liquids coexist • Xenoliths, restites, and other enclaves? • Migmatites (“mixed rocks”) are gradational
  5. 5. Metamorphic Agents and Changes • Temperature: typically the most important factor in metamorphism Figure 1.9. Estimated ranges of oceanic and continental steady-state geotherms to a depth of 100 km using upper and lower limits based on heat flows measured near the surface. After Sclater et al. (1980), Earth. Rev. Geophys. Space Sci., 18, 269-311.
  6. 6. Metamorphic Agents and Changes Increasing temperature has several effects 1) Promotes recrystallization  increased grain size 2) Drive reactions (endothermic) 3) Overcomes kinetic barriers
  7. 7. Metamorphic Agents and Changes Pressure • “Normal” gradients perturbed in several ways, most commonly:  High T/P geotherms in areas of plutonic activity or rifting  Low T/P geotherms in subduction zones
  8. 8. Figure 21.1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw- Hill.
  9. 9. Metamorphic Agents and Changes • Metamorphic grade: a general increase in degree of metamorphism without specifying the exact relationship between temperature and pressure
  10. 10. Metamorphic Agents and Changes • Lithostatic pressure - uniform stress (hydrostatic) • Deviatoric stress = pressure unequal in different directions • Resolved into three mutually perpendicular stress (s) components: s1 is the maximum principal stress s2 is an intermediate principal stress s3 is the minimum principal stress • In hydrostatic situations all three are equal
  11. 11. Metamorphic Agents and Changes • Stress • Strain  deformation • Deviatoric stress affects the textures and structures, but not the equilibrium mineral assemblage • Strain energy may overcome kinetic barriers to reactions
  12. 12. • Foliation is a common result, which allows us to estimate the orientation of s1  s1 > s2 = s3  foliation and no lineation  s1 = s2 > s3  lineation and no foliation  s1 > s2 > s3  both foliation and lineation Figure 21.3. Flattening of a ductile homogeneous sphere (a) containing randomly oriented flat disks or flakes. In (b), the matrix flows with progressive flattening, and the flakes are rotated toward parallelism normal to the predominant stress. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. s1 Strain ellipsoid
  13. 13. Metamorphic Agents and Changes Shear motion occurs along planes at an angle to s1 Figure 21.2. The three main types of deviatoric stress with an example of possible resulting structures. b. Shear, causing slip along parallel planes and rotation. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. s1
  14. 14. Metamorphic Agents and Changes Fluids Evidence for the existence of a metamorphic fluid: • Fluid inclusions • Fluids are required for hydrous or carbonate phases • Volatile-involving reactions occur at temperatures and pressures that require finite fluid pressures
  15. 15. Metamorphic Agents and Changes • Pfluid = S partial pressures of each component (Pfluid = pH2O + pCO2 + …) • Mole fractions of components must sum to 1.0 (XH2O + XCO2 + … = 1.0) • pH2O = XH2O x Pfluid • Gradients in T, P, Xfluid •  Zonation in mineral assemblages
  16. 16. The Types of Metamorphism Different approaches to classification 1. Based on principal process or agent • Dynamic Metamorphism • Thermal Metamorphism • Dynamo-thermal Metamorphism
  17. 17. The Types of Metamorphism Different approaches to classification 2. Based on setting • Contact Metamorphism  Pyrometamorphism • Regional Metamorphism  Orogenic Metamorphism  Burial Metamorphism  Ocean Floor Metamorphism • Hydrothermal Metamorphism • Fault-Zone Metamorphism • Impact or Shock Metamorphism
  18. 18. The Types of Metamorphism Contact Metamorphism The size and shape of an aureole is controlled by: • The nature of the pluton • The nature of the country rocks  Size  Shape  Orientation  Temperature  Composition  Composition  Depth and metamorphic grade prior to intrusion  Permeability
  19. 19. Contact Metamorphism • Adjacent to igneous intrusions • Thermal (± metasomatic) effects of hot magma intruding cooler shallow rocks • Occurs over a wide range of pressures, including very low • Contact aureole
  20. 20. The Types of Metamorphism Contact Metamorphism Most easily recognized where a pluton is introduced into shallow rocks in a static environment  Hornfelses (granofelses) commonly with relict textures and structures
  21. 21. The Types of Metamorphism Contact Metamorphism Polymetamorphic rocks are common, usually representing an orogenic event followed by a contact one • Spotted phyllite (or slate) • Overprint may be due to:  Lag time for magma migration  A separate phase of post-orogenic collapse magmatism (Chapter 18)
  22. 22. The Types of Metamorphism Pyrometamorphism Very high temperatures at low pressures, generated by a volcanic or sub-volcanic body Also developed in xenoliths
  23. 23. The Types of Metamorphism Regional Metamorphism sensu lato: metamorphism that affects a large body of rock, and thus covers a great lateral extent Three principal types:  Orogenic metamorphism  Burial metamorphism  Ocean-floor metamorphism
  24. 24. The Types of Metamorphism Orogenic Metamorphism is the type of metamorphism associated with convergent plate margins • Dynamo-thermal: one or more episodes of orogeny with combined elevated geothermal gradients and deformation (deviatoric stress) • Foliated rocks are a characteristic product
  25. 25. The Types of Metamorphism Orogenic Metamorphism Figure 21.6. Schematic model for the sequential (a  c) development of a “Cordilleran-type” or active continental margin orogen. The dashed and black layers on the right represent the basaltic and gabbroic layers of the oceanic crust. From Dewey and Bird (1970) J. Geophys. Res., 75, 2625-2647; and Miyashiro et al. (1979) Orogeny. John Wiley & Sons.
  26. 26. The Types of Metamorphism Orogenic Metamorphism
  27. 27. The Types of Metamorphism Orogenic Metamorphism • Uplift and erosion • Metamorphism often continues after major deformation ceases  Metamorphic pattern is simpler than the structural one • Pattern of increasing metamorphic grade from both directions toward the core area From Understanding Earth, Press and Siever. Freeman.
  28. 28. The Types of Metamorphism Orogenic Metamorphism • Polymetamorphic patterns • Continental collision • Batholiths are usually present in the highest grade areas • If plentiful and closely spaced, may be called regional contact metamorphism
  29. 29. The Types of Metamorphism Burial metamorphism • Southland Syncline in New Zealand: thick pile (> 10 km) of Mesozoic volcaniclastics • Mild deformation, no igneous intrusions discovered • Fine-grained, high-temperature phases, glassy ash: very susceptible to metamorphic alteration • Metamorphic effects attributed to increased temperature and pressure due to burial • Diagenesis grades into the formation of zeolites, prehnite, pumpellyite, laumontite, etc.
  30. 30. The Types of Metamorphism Hydrothermal metamorphism • Hot H2O-rich fluids • Usually involves metasomatism • Difficult type to constrain: hydrothermal effects often play some role in most of the other types of metamorphism
  31. 31. The Types of Metamorphism Burial metamorphism occurs in areas that have not experienced significant deformation or orogeny • Restricted to large, relatively undisturbed sedimentary piles away from active plate margins  The Gulf of Mexico?  Bengal Fan?
  32. 32. The Types of Metamorphism Burial metamorphism occurs in areas that have not experienced significant deformation or orogeny • Bengal Fan  sedimentary pile > 22 km • Extrap.  250-300oC at the base (P ~ 0.6 GPa) • Passive margins often become active • Areas of burial metamorphism may thus become areas of orogenic metamorphism
  33. 33. The Types of Metamorphism Ocean-Floor Metamorphism affects the oceanic crust at ocean ridge spreading centers • Considerable metasomatic alteration, notably loss of Ca and Si and gain of Mg and Na • Highly altered chlorite-quartz rocks- distinctive high-Mg, low-Ca composition • Exchange between basalt and hot seawater • Another example of hydrothermal metamorphism
  34. 34. The Types of Metamorphism  Impact metamorphism at meteorite (or other bolide) impact craters  Both correlate with dynamic metamorphism, based on process Fault-Zone and Impact Metamorphism  High rates of deformation and strain with only minor recrystallization
  35. 35. (a) Shallow fault zone with fault breccia (b) Slightly deeper fault zone (exposed by erosion) with some ductile flow and fault mylonite Figure 21.7. Schematic cross section across fault zones. After Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London.
  36. 36. Prograde Metamorphism • Prograde: increase in metamorphic grade with time as a rock is subjected to gradually more severe conditions  Prograde metamorphism: changes in a rock that accompany increasing metamorphic grade • Retrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous event  Retrograde metamorphism: any accompanying changes
  37. 37. The Progressive Nature of Metamorphism A rock at a high metamorphic grade probably progressed through a sequence of mineral assemblages rather than hopping directly from an unmetamorphosed rock to the metamorphic rock that we find today
  38. 38. The Progressive Nature of Metamorphism Retrograde metamorphism typically of minor significance • Prograde reactions are endothermic and easily driven by increasing T • Devolatilization reactions are easier than reintroducing the volatiles • Geothermometry indicates that the mineral compositions commonly preserve the maximum temperature
  39. 39. Types of Protolith Lump the common types of sedimentary and igneous rocks into six chemically based-groups 1. Ultramafic - very high Mg, Fe, Ni, Cr 2. Mafic - high Fe, Mg, and Ca 3. Shales (pelitic) - high Al, K, Si 4. Carbonates - high Ca, Mg, CO2 5. Quartz - nearly pure SiO2. 6. Quartzo-feldspathic - high Si, Na, K, Al
  40. 40. Why Study Metamorphism? • Interpretation of the conditions and evolution of metamorphic bodies, mountain belts, and ultimately the state and evolution of the Earth's crust • Metamorphic rocks may retain enough inherited information from their protolith to allow us to interpret much of the pre-metamorphic history as well
  41. 41. Orogenic Regional Metamorphism of the Scottish Highlands • George Barrow (1893, 1912) • SE Highlands of Scotland - Caledonian Orogeny ~ 500 Ma • Nappes • Granites
  42. 42. Barrow’s Area Figure 21.8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.
  43. 43. Orogenic Regional Metamorphism of the Scottish Highlands • Barrow studied the pelitic rocks • Could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased
  44. 44. The sequence of zones now recognized, and the typical metamorphic mineral assemblage in each, are: • Chlorite zone. Pelitic rocks are slates or phyllites and typically contain chlorite, muscovite, quartz and albite • Biotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albite • Garnet zone. Schists with conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase • Staurolite zone. Schists with staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persist • Kyanite zone. Schists with kyanite, biotite, muscovite, quartz, plagioclase, and usually garnet and staurolite • Sillimanite zone. Schists and gneisses with sillimanite, biotite, muscovite, uartz, plagioclase, garnet, and perhaps staurolite. Some kyanite may also be present (although kyanite and sillimanite are both polymorphs of Al2SiO5)
  45. 45. • Sequence = “Barrovian zones” • The P-T conditions referred to as “Barrovian-type” metamorphism (fairly typical of many belts) • Now extended to a much larger area of the Highlands • Isograd = line that separates the zones (a line in the field of constant metamorphic grade)
  46. 46. Figure 21.8. Regional metamorphic map of the Scottish Highlands, showing the zones of minerals that develop with increasing metamorphic grade. From Gillen (1982) Metamorphic Geology. An Introduction to Tectonic and Metamorphic Processes. George Allen & Unwin. London.
  47. 47. To summarize: • An isograd represents the first appearance of a particular metamorphic index mineral in the field as one progresses up metamorphic grade • When one crosses an isograd, such as the biotite isograd, one enters the biotite zone • Zones thus have the same name as the isograd that forms the low-grade boundary of that zone • Because classic isograds are based on the first appearance of a mineral, and not its disappearance, an index mineral may still be stable in higher grade zones
  48. 48. A variation occurs in the area just to the north of Barrow’s, in the Banff and Buchan district • Pelitic compositions are similar, but the sequence of isograds is:  chlorite  biotite  cordierite  andalusite  sillimanite
  49. 49. The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite  sillimanite at the sillimanite isograd only above this pressure Figure 21.9. The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).
  50. 50. Regional Burial Metamorphism Otago, New Zealand • Jurassic graywackes, tuffs, and volcanics in a deep trough metamorphosed in the Cretaceous • Fine grain size and immature material is highly susceptible to alteration (even at low grades)
  51. 51. Regional Burial Metamorphism Otago, New Zealand Section X-Y shows more detail Figure 21.10. Geologic sketch map of the South Island of New Zealand showing the Mesozoic metamorphic rocks east of the older Tasman Belt and the Alpine Fault. The Torlese Group is metamorphosed predominantly in the prehnite-pumpellyite zone, and the Otago Schist in higher grade zones. X-Y is the Haast River Section of Figure 21-11. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill.
  52. 52. Regional Burial Metamorphism Otago, New Zealand Isograds mapped at the lower grades: 1) Zeolite 2) Prehnite-Pumpellyite 3) Pumpellyite (-actinolite) 4) Chlorite (-clinozoisite) 5) Biotite 6) Almandine (garnet) 7) Oligoclase (albite at lower grades is replaced by a more calcic plagioclase)
  53. 53. Regional Burial Metamorphism Figure 21.11. Metamorphic zones of the Haast Group (along section X-Y in Figure 21-10). After Cooper and Lovering (1970) Contrib. Mineral. Petrol., 27, 11-24.
  54. 54. Paired Metamorphic Belts of Japan Figure 21.12. The Sanbagawa and Ryoke metamorphic belts of Japan. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill and Miyashiro (1994) Metamorphic Petrology. Oxford University Press.
  55. 55. Paired Metamorphic Belts of Japan
  56. 56. Figure 21.13. Some of the paired metamorphic belts in the circum-Pacific region. From Miyashiro (1994) Metamorphic Petrology. Oxford University Press.
  57. 57. Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK • Ordovician Skiddaw Slates (English Lake District) intruded by several granitic bodies • Intrusions are shallow • Contact effects overprinted on an earlier low-grade regional orogenic metamorphism
  58. 58. Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK • The aureole around the Skiddaw granite was sub- divided into three zones, principally on the basis of textures: • Unaltered slates • Outer zone of spotted slates • Middle zone of andalusite slates • Inner zone of hornfels • Skiddaw granite Increasing Metamorphic Grade Contact
  59. 59. Figure 21.14. Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. After Eastwood et al (1968). Geology of the Country around Cockermouth and Caldbeck. Explanation accompanying the 1-inch Geological Sheet 23, New Series. Institute of Geological Sciences. London.
  60. 60. Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK • Middle zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + andalusite + quartz Figure 21.15. Cordierite- andalusite slate from the middle zone of the Skiddaw aureole. From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London. 1 mm
  61. 61. Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK Inner zone: Thoroughly recrystallized Lose foliation Figure 21.16. Andalusite-cordierite schist from the inner zone of the Skiddaw aureole. Note the chiastolite cross in andalusite (see also Figure 22- 49). From Mason (1978) Petrology of the Metamorphic Rocks. George Allen & Unwin. London. 1 mm
  62. 62. Contact Metamorphism of Pelitic Rocks in the Skiddaw Aureole, UK • The zones determined on a textural basis • Prefer to use the sequential appearance of minerals and isograds to define zones • But low-P isograds converge in P-T • Skiddaw sequence of mineral development with grade is difficult to determine accurately
  63. 63. Contact Metamorphism and Skarn Formation at Crestmore, CA, USA • Crestmore quarry in the Los Angeles basin • Quartz monzonite porphry intrudes Mg-bearing carbonates (either late Paleozoic or Triassic) • Burnham (1959) mapped the following zones and the mineral assemblages in each (listed in order of increasing grade):
  64. 64. • Forsterite Zone:  calcite + brucite + clinohumite + spinel  calcite + clinohumite + forsterite + spinel  calcite + forsterite + spinel + clintonite • Monticellite Zone:  calcite + forsterite + monticellite + clintonite  calcite + monticellite + melilite + clintonite  calcite + monticellite + spurrite (or tilleyite) + clintonite  monticellite + spurrite + merwinite + melilite • Vesuvianite Zone:  vesuvianite + monticellite + spurrite + merwinite + melilite  vesuvianite + monticellite + diopside + wollastonite • Garnet Zone:  grossular + diopside + wollastonite
  65. 65. Contact Metamorphism and Skarn Formation at Crestmore, CA, USA An idealized cross-section through the aureole Figure 21.17. Idealized N-S cross section (not to scale) through the quartz monzonite and the aureole at Crestmore, CA. From Burnham (1959) Geol. Soc. Amer. Bull., 70, 879- 920.
  66. 66. Contact Metamorphism and Skarn Formation at Crestmore, CA, USA 1. The mineral associations in successive zones (in all metamorphic terranes) vary by the formation of new minerals as grade increases This can only occur by a chemical reaction in which some minerals are consumed and others produced
  67. 67. Contact Metamorphism and Skarn Formation at Crestmore, CA, USA a) Calcite + brucite + clinohumite + spinel zone to the Calcite + clinohumite + forsterite + spinel sub-zone involves the reaction: 2 Clinohumite + SiO2  9 Forsterite + 2 H2O b) Formation of the vesuvianite zone involves the reaction: Monticellite + 2 Spurrite + 3 Merwinite + 4 Melilite + 15 SiO2 + 12 H2O  6 Vesuvianite + 2 CO2
  68. 68. Contact Metamorphism and Skarn Formation at Crestmore, CA, USA 2) Find a way to display data in simple, yet useful ways If we think of the aureole as a chemical system, we note that most of the minerals consist of the components CaO-MgO-SiO2-CO2-H2O (with minor Al2O3)
  69. 69. Zones are numbered (from outside inward) Figure 21.18. CaO-MgO-SiO2 diagram at a fixed pressure and temperature showing the compositional relationships among the minerals and zones at Crestmore. Numbers correspond to zones listed in the text. After Burnham (1959) Geol. Soc. Amer. Bull., 70, 879-920; and Best (1982) Igneous and Metamorphic Petrology. W. H. Freeman.
  70. 70. Figures not used Figure 21.4. A situation in which lithostatic pressure (Plith) exerted by the mineral grains is greater than the intergranular fluid pressure (Pfluid). At a depth around 10 km (or T around 300oC) minerals begin to yield or dissolve at the contact points and shift toward or precipitate in the fluid-filled areas, allowing the rock to compress. The decreased volume of the pore spaces will raise Pfluid until it equals Plith. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
  71. 71. Figures not used Figure 21.5. Temperature distribution within a 1-km thick vertical dike and in the country rocks (initially at 0oC) as a function of time. Curves are labeled in years. The model assumes an initial intrusion temperature of 1200oC and cooling by conduction only. After Jaeger, (1968) Cooling and solidification of igneous rocks. In H. H. Hess and A. Poldervaart (eds.), Basalts, vol. 2. John Wiley & Sons. New York, pp. 503-536.

Editor's Notes

  • Rocks as chemical systems (Ch. 5)
    -> a particular assemblage of coexisting phases (thermodynamic equilibrium and the phase rule)
    A basaltic composition can be either:
    Melt
    Cpx + plag ( olivine, ilmenite…)
    Or any combination of melt + minerals along the liquid line of descent
    If uplifted and eroded  surface, will weather  a combinations of clays, oxides…
    Between these is the realm of metamorphism
    We shall see that the chemistry of a basalt can  a number of different mineral assemblages between the conditions under which plag + cpx and clays + oxides are stable
    The mineralogy is dependent upon P, T, and X
  • Deformation alone does not count: requires crystallization or recrystallization
  • The boundary is somewhat arbitrary
    Diagenetic/weathering processes are indistinguishable from metamorphic
    Some zeolites are considered diagenetic and others metamorphic – pretty arbitrary
    Metamorphism begins in the range of 100-150oC for the more unstable types of protolith

    Marked by the formation of minerals such as laumontite, analcime, heulandite, carpholite, paragonite, prehnite, pumpellyite, lawsonite, glaucophane or stilpnomelane
  • If we heat a metamorphic rock until it melts, at what point in the melting process does it become “igneous”?
    Xenoliths, restites, and other enclaves are considered part of the igneous realm because melt is dominant, but the distinction is certainly vague and disputable
    We may all recognize a melt, but we may not be so good at recognizing the solid products crystallized from one
    Small, elongate, fairly coarse-grained and cross-cutting segregations of granitoid material in gneisses:
    Thin dikes of melt or precipitates from fluids, or fluid-enhanced recrystallization along fluid-filled fractures?
    The distinction between a silicate-saturated aqueous fluid and a fluid-saturated silicate melt
  • Continental geotherm is higher than oceanic due to concentration of radioactive (LIL) elements
  • 1) Promotes recrystallization  increased grain size
    Larger surface/volume ratio of a mineral  lower stability
    Fine aggregates coalesce to larger grains
    Especially for fine-grained and unstable materials in a static environment (shear stresses often reduce grain size)
    2) Drives reactions that consume unstable mineral(s) and produces new minerals that are stable under the new conditions
    3) Overcomes kinetic barriers that might otherwise preclude the attainment of equilibrium
    Disequilibrium is relatively common in sediments and diagenesis
    Mineral assemblages are usually simpler at higher grades and the phase rule is applicable
  • Temperature rarely increases without an accompanying increase in pressure (geothermal gradients)

    Most disturbances are transient and eventually return to “normal”
  • Fig. 21-1 = estimates of metamorphic temperature-pressure relationships from ancient orogenic belts
    Based on P-T estimates for rocks exposed at the surface in these areas along a traverse from lowest to highest metamorphic conditions: metamorphic field gradients – not same as geotherms
  • We may thus refer to “high-grade” rocks or “low-grade” rocks from any area depicted in Fig. 21-1
    Consider pressure as a modifier, in the sense that temperature can increase along any number of pressure-varied paths
    High T/P paths (low P) favor the formation of low-density metamorphic minerals as temperature rises
    Low T/P paths (high P) favor denser minerals
  • Stress is an applied force acting on a rock (over a particular cross-sectional area)
    Strain is the response of the rock to an applied stress (= yielding or deformation)
    Deviatoric stress can be maintained only if application keeps pace with the tendency of the rock to yield
    This occur most often in orogenic belts, extending rifts, or in shear zones. (i.e. generally at or near plate boundaries)
  • Deviatoric stress affects the textures and structures, but not the equilibrium mineral assemblage
    Strain energy may overcome kinetic barriers to reactions
  • In shear motion occurs along planes at an angle to s1
    May occur as slip along spaced cleavages or as flow
    Distinguishing shear from flattening may be hard
  • Metamorphic fluids dominated by H2O, but CO2 may also be present in rocks with a significant carbonate
    CH4, S, and N2 may be minor components, as well as dissolved species, notably alkalis and halides

    Nature and source of fluids? Are they present at the higher metamorphic grades?
    Fluids can be meteoric, juvenile magmatic, subducted material, trapped sedimentary brines, or degassing of the mantle
    The motion of fluids may transport various chemical species over considerable distances- metasomatism

    Gradients in temperature, pressure, and fluid composition across an area are the norm
    As a result, zonation in the mineral assemblages constituting the rocks that equilibrate spanning these gradients
    Along a traverse in an eroded metamorphic area cross from non-metamorphosed rocks through zones of progressively higher metamorphic grade (or through zones reflecting metasomatic composition gradients)
  • Dynamic Metamorphism when deviatoric stress is dominant and deformation + recrystallization is the main process
    Thermal Metamorphism when temperature in a near-static stress field is the main agent
    Dynamo-thermal Metamorphism when both temperature and deformation are prevalent
  • Plutons can rise and transmit heat to the shallow crust, so may occur over a wide range of pressures, including very low
  • The foliated phyllite formed during a regional event and the later ovoid “spots” are minerals that grew during the contact event
    Overprint may be due to:
    Lag time between the creation of the magma at depth during T maximum, and its migration to the lower grade rocks above
    Plutonism may reflect a separate phase of post-orogenic collapse magmatism (Chapter 18)
  • A minor type of contact metamorphism characterized by very high temperatures at very low pressures, generated by a volcanic or subvolcanic body
    Also developed in xenoliths
    Pyrometamorphism may be accompanied by various degrees of partial melting
  • Island arcs, active continental margins, and continental collision zones
    Most studies focus on orogenic belts, and the term, “regional metamorphism” is often used synonymously with “orogenic metamorphism”
  • (a) = the incipient stages of subduction

    (b) “orogenic welt” created by compression, crustal thickening, thrust stacking of oceanic slices, and addition of magmatic material from below
    Underthrusting in the forearc migrates trenchward, adding successive slabs to the base of the outer welt (tectonic underplating)
    Heat added by rising plutons, magmatically underplated magma, and induced mantle convection

    Temperature increases both downward and toward the axial portion of the welt where plutons concentrated
  • Temperature increases both downward and toward the axial portion of the welt where plutons concentrated
  • Uplift and erosion results in exposure of the metamorphic rocks
    Heat dissipates slowly, so the metamorphism often continues after major deformation ceases
    When this occurs, the metamorphic pattern is simpler than the structural one
    Folding and thrusting are often complex, but the metamorphic pattern may be a simple thermal dome, centering on the metamorphic/igneous core where heat input, thickening, and uplift are the greatest
    Exposed surface pattern of increasing metamorphic grade from both directions toward the core area
  • Most orogenic belts have several episodes of deformation and metamorphism, creating a more complex polymetamorphic pattern
    Continental collision involves interaction of a continental arc with a continental mass having a “passive” margin and an apron of sediments extending from the continental shelf. Such collisions will usually produce even more complex structural, magmatic, and metamorphic patterns
    Although batholiths are usually present in the highest grade areas of regional terranes, the metamorphism isn’t considered contact metamorphism because it develops regionally, and the pattern of metamorphic grade does not relate directly to the proximity of the igneous contacts
    Contact metamorphism typically occurs locally within regional terranes
    In many cases intrusive rocks may be plentiful and closely spaced, so that it is difficult or impossible to distinguish regional metamorphism from overlapping contact aureoles. Spear (1993) calls such situations regional contact metamorphism
  • Burial metamorphism = for low-grade metamorphism in sedimentary basins due to burial
    A term coined by Coombs (1961) for low-grade metamorphism that occurs in sedimentary basins due to burial by successive layers
    Coombs worked in the Southland Syncline in New Zealand, where a thick pile (> 10 km) of Mesozoic volcaniclastics had accumulated
  • Coombs (1961) also proposed hydrothermal metamorphism
    Many areas of burial metamorphism are also hydrothermal fields, suggesting an elevated geothermal gradient and fluids may be important elements in at least some cases of burial metamorphism
  • It is thus restricted to large, relatively undisturbed sedimentary piles away from active plate margins
    The Gulf of Mexico may represent a modern example
    Bengal Fan is another, fed by the Ganges and Brahmaputra rivers, has the form of a sedimentary wedge accumulating along a passive continental margin
  • It is thus restricted to large, relatively undisturbed sedimentary piles away from active plate margins
    The Gulf of Mexico may represent a modern example
    Bengal Fan is another, fed by the Ganges and Brahmaputra rivers, has the form of a sedimentary wedge accumulating along a passive continental margin

    Seismic data in Bengal Fan  sedimentary pile > 22 km
    Extrapolating the low geothermal gradient at the surface (18-22oC/km)  250-300oC at the base (P ~ 0.6 GPa)
    Conditions are well into the metamorphic range, and the weight of the overlying sediments  sufficient compression to impart a foliation to the metamorphic rocks forming at depth
    It may be impossible to distinguish a hand specimen of a rock retrieved from these depths with one from the lower grade regions of an orogenic belt

    End: Typical examples of burial and regional metamorphism are easily recognized today, but transitional types are common
  • Variety of metamorphic minerals in ocean-floor rocks, representing a wide range of temperatures at relatively low pressure
    Alteration concentrated along vein systems, presumably associated with hydrothermal activity (note black smokers)
    Seawater penetrates down ubiquitous fracture systems, where it becomes heated, and leaches metals and silica from the hot basalts
  • “Fault” includes zones of distributed shear that can be up to several kilometers across
    Impact metamorphism (also called shock metamorphism) occurs at meteorite (or other bolide) impact craters
    Both fault-zone and impact metamorphism correlate with dynamic metamorphism, based on process
  • Schematic cross-section across fault zones
  • Metamorphic rocks usually maintain equilibrium as grade increases
    High-grade metamorphic rock probably progressed through a sequence of mineral assemblages as it adjusted to increasing temperature and pressure, rather than hopping directly from un-met to the metamorphic rock that we find today
    If a metamorphosed sedimentary rock experienced a cycle of increasing metamorphic grade, followed by decreasing grade, at what point on this cyclic P-T-t path did its present mineral assemblage last equilibrate?
    The zonal distribution of metamorphic rock types preserved in a geographic sequence of increased metamorphic grade suggests that each rock preserves the conditions of the maximum metamorphic grade (temperature) experienced by that rock during metamorphism
  • Retrograde is usually detectable by observing textures, such as the incipient replacement of high-grade minerals by low-grade ones at their rims
  • Chemistry of the protolith is the most important clue toward deducing the parent rock
    1. Ultramafic rocks. Mantle rocks, komatiites, or cumulates
    2. Mafic rocks. Basalts or gabbros, some graywackes
    3. Shales (or pelitic rocks). Fine grained clastic clays and silts deposited in stable platforms or offshore wedges.
    4. Carbonates. Mostly sedimentary limestones and dolostones. Impure carbonates (marls) may contain sand or shale components
    5. Quartz rocks. Cherts are oceanic, and sands are moderately high energy continental clastics. Nearly pure SiO2.
    6. Quartzo-feldspathic rocks. Arkose or granitoid and rhyolitic rocks. High Si, Na, K, Al
    Categories are often gradational, and cannot include the full range of possible parental rocks
    One common gradational rock type is a sand-shale mixture:psammite
    Other rocks: evaporites, ironstones, manganese sediments, phosphates, laterites, alkaline igneous rocks, coal, and ore bodies
  • George Barrow (1893, 1912): one of the first systematic studies of the variation in rock types and mineral assemblages with progressive metamorphism
    Caledonian orogeny, ~500 Ma ago
    Deformation was intense: rocks folded into a series of nappes
    Numerous granites also intruded toward the end of the orogeny, after main regional metamorphism
  • Barrow noted significant and systematic mineralogical changes in the pelitic rocks
    He found that he could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increased (which he could correlate to increased grain size)
    The new mineral that characterizes a zone is termed an index mineral
  • This sequence of zones now recognized in other orogenic belts, and is now so well established in the literature that the zones are often referred to as the Barrovian zones
    Tilley, Kennedy, etc. confirmed Barrow’s zones, and extended them over a much larger area of the Highlands
    Tilley coined the term isograd for the line that separates the zones
    An isograd, then, is meant to indicate a line in the field of constant metamorphic grade
    Really = the intersection of the isogradic surface with the Earth’s surface
  • Later we shall see broader categories: metamorphic facies
    Barrovian zones have become the norm to which we compare all other areas of regional metamorphism
    OK practice, but we shouldn’t let these zones constrain our thinking or our observations
    Other zones may be important and useful locally
    A chloritoid zone is prevalent in the Appalachians (X)
  • The molar volume of cordierite is also quite high, indicating that it too is a low-pressure mineral
    The geothermal gradient in this northern district was higher than in Barrow’s area, and rocks at any equivalent temperature must have been at a lower pressure
    This lower P/T variation has been called Buchan-type metamorphism. It too is relatively common
    Miyashiro (1961), from his work in the Abukuma Plateau of Japan, called such a low P/T variant Abukuma-type
    Both terms are common in the literature, and mean essentially the same thing
  • Voluminous Permian through Jurassic sedimentation and volcanism  graywackes, tuffs, and some volcanics in a deep trough that was metamorphosed in the Cretaceous
    The fine grain size and immature nature of the material makes it highly susceptible to metamorphic alteration, even at low grades
    The “type locality” of burial metamorphism
  • The isograds mapped at the lower grades are listed below and are well represented in the Haast River section (Fig. 21-11 - next frame)
  • Orogenic belts typically proceed directly from diagenesis to chlorite or biotite zones
    The development of low-grade zones in New Zealand may reflect the highly unstable nature of the tuffs and graywackes, and the availability of hot water, whereas pelitic sediments may not react until higher grades
  • Shikoku and Honshu in Japan: a pair of parallel metamorphic belts are exposed along a NE-SW axis parallel to the active subduction zone
    These belts are of the same age, suggesting that they developed together
    The NW belt (“inner” belt, inward, or away from the trench) is the Ryoke (or Abukuma) Belt
    Low P/T Buchan-type of regional orogenic metamorphism
    Dominant meta-pelitic sediments, and isograds up to the sillimanite zone have been mapped
    A high-temperature-low-pressure belt, and granitic plutons are common
    Outer belt, called the Sanbagawa Belt
    It is of a high-pressure-low-temperature nature
    Only reaches the garnet zone in the pelitic rocks
    Basic rocks are more common than in the Ryoke belt, however, and in these glaucophane is developed (giving way to hornblende at higher grades)
    Rocks are commonly called blueschists
    Two belts are in contact along their whole length across a major fault zone (the Median Line)
    Ryoke-Abukuma lithologies are similar to seds derived from a relatively mature volcanic arc
    Sanbagawa lithologies more akin to the oceanward accretionary wedge where distal arc-derived sediments and volcanics mix with oceanic crust and marine sediment
  • Fig. 16-15 suggests that the 600oC isotherm, for example, could be as deep as 100 km in the trench-subduction zone area, and as shallow as 20 km beneath the volcanic arc
  • Miyashiro (1961, 1973) noted the paired nature of the Ryoke-Sanbagawa belts, and suggested …
    Coeval metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T belt ought to be a common
    Called these paired metamorphic belts
    May be separated by 100-200 km of less metamorphosed and deformed material (“arc-trench gap”) or closely juxtaposed (Ryoke-Sanbagawa)
    In the latter cases the contact is commonly a major fault
    Most of these belts are quite complex, and are not always coeval
  • First effects (1-2 km from contact) = 0.2 - 2.0 mm sized black ovoid “spots” in the slates
    At the same time, recrystallization -> slight coarsening of the grains and degradation of the slaty cleavage
    Spots were probably cordierite or andalusite, since re-hydrated and retrograded back to fine aggregates of mostly muscovite
    Both cordierite and andalusite occur at higher grades, where they are often partly retrograded, but not farther out
    Spots that we now see in most of the spotted slates are probably pseudomorphs
  • Cordierite forms ovoid xls with irregular outlines and numerous inclusions, in this case of biotite, muscovite, and opaques
    The biotite and muscovite inclusions often retain the orientation of the slaty cleavage outside the cordierites
    This indicates that the growing cordierite crystals enveloped aligned micas that grew during the regional event
    Excellent textural evidence for the overprint of contact metamorphism on an earlier regional one
    Micas outside the cordierites are larger and more randomly oriented, suggesting that they formed or recrystallized during the later thermal event
    Andalusites have fewer inclusions than cordierite, and many show the cruciform pattern of fine opaque inclusions known as chiastolite
  • Both andalusite and cordierite are minerals characteristic of low-pressure metamorphism, which is certainly the case in the Skiddaw aureole, where heat is carried up into the shallow crust by the granites
    The rocks of the inner zone at Skiddaw are characterized by coarser and more thoroughly recrystallized textures
    Same mineral assemblage as the middle zone
    Some rocks are schistose, but in the innermost portions the rock fabric loses the foliation, and the rocks are typical hornfelses
  • The zones determined on a textural basis
    A more modern approach conform to the regional example above, and use sequential appearance of minerals and isograds to define the zones
    First new mineral in most slates is biotite, followed by the approximately simultaneous development of cordierite and andalusite
    Perhaps the textural zonation is more useful in some cases
    Comrie: hotter-
    Orthopyroxene occurs in pelitic and quartzo-feldspathic rocks only at the very highest grades of contact and regional metamorphism, grades that may not be reached prior to melting in many instances
    Typical mineral assemblages = hypersthene + cordierite + orthoclase + biotite + opaques
    Some very interesting silica-undersaturated rocks also occur in the inner aureole
    Contain such non-silicate high-temperature phases as corundum and Fe-Mg spinel
    Tilley noted that the low-silica rocks occur only in the inner aureole, and attributed their origin to loss of SiO2 into the diorite
    Better explanation is that SiO2 (and H2O) were scavenged by granitic partial melts formed in the sediments adjacent to the contact with the hot diorite
  • In this progression we can see the sequential development of index minerals, such as clinohumite, followed by forsterite, clintonite, monticellite, melilite, spurrite/tillyite, merwinite, vesuvianite, diopside, wollastonite, and finally grossular garnet
  • The list of zones is at first quite confusing, and again serves to illustrate a common problem faced by petrologists (and probably all scientists)
    We can collect quality data, but can become overwhelmed by the quantity at times, and it is often difficult to recognize meaningful patterns

    Two approaches are helpful in this case:
  • If this is so, one ought to be able to relate minerals in the lower and next higher zone by a balanced chemical reaction
  • For example, the step from the first zone to the second zone is (a)

    When we address isograds as reactions we can then turn to what variables are involved
    In the present case, we discover than the majority of these prograde reactions consume SiO2
    Since quartz is not found in the Crestmore aureole, the SiO2 must be added in the form or dissolved silica in hydrothermal fluids
    We can thus conclude that diffusion of silica from the monzonite into the country rocks must play a critical role in the aureole development
  • Addressing the list of mineral assemblages in the zones at Crestmore can be bewildering
    Find a way to display…


    What if we graphically plot the minerals on a triangular CaO-MgO-SiO2 diagram?
  • Silica-saturated water escaping from the porphry permeates the silica-free marbles and a gradient in silica content results due to the diffusion
    Silica reacts with the carbonates to produce skarns consisting of Ca-Mg silicates, while CO2 is liberated by the reactions
    The porphyritic nature of the pluton thus supports the idea of fluid release
    The zones at Crestmore could have formed at constant temperature, and reflect a diffusion gradient in SiO2 only
    This is probably not the case, however, since temperature should also increase toward the pluton
    Only by knowing the pressure-temperature stability ranges of the minerals, and the pressure-temperature dependence of the reactions relating them, could we fully understand the processes at Crestmore

×