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Ch 25 Metamorphic Facies

Ch 25 Metamorphic Facies

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Ch 25 Metamorphic Facies

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  1. Ch 25 Metamorphic Facies

  2. V.M. Goldschmidt (1911, 1912a) contact metamorphosed pelitic, calcareous, and psammitichornfelses Oslo s. Norway fewer than six major minerals in the aureoles around granitoid intrusives first to note that the equilibrium mineral assemblage of a metamorphic rock could be related to Xbulk Aluminous pelites contained Al-rich minerals, such as cordierite, plagioclase, garnet, and/or an Al2SiO5 polymorph Calcareous rocks contained Ca-rich and Al-poor minerals such as Di, Wo, and/or amphibole http://www.weizmann.ac.il/ICS/booklet/20/pdf/bob_weintraub.pdf

  3. Victor Moritz Goldschmidt- Oslo Certain mineral pairs (e.g. anorthite + enstatite) were consistently present in rocks of appropriate composition, whereas the compositionally equivalent pair (diopside + andalusite) was not If two alternative assemblages are X-equivalent, we must be able to relate them by a reaction In this case the reaction is simple: MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5 En An Di Als

  4. Metamorphic Facies PentiiEskola (1914, 1915) Orijärvi, S. Finland Rocks with K-feldspar + cordierite at Oslo contained the compositionally equivalent pair biotite + muscovite at Orijärvi Eskola: difference must reflect differing physical conditions Finnish rocks (more hydrous and lower volume assemblage) equilibrated at lower temperatures and higher pressures than the Norwegian ones

  5. Metamorphic Facies Oslo: Kfs + Mg-Crd Orijärvi: Phlo + Ms + Qtz Reaction: 2 KMg3AlSi3O10(OH)2 + 6 KAl2AlSi3O10(OH)2 + 15 SiO2 Phlo Ms Qtz = 3 Mg2Al4Si5O18 + 8 KAlSi3O8 + 8 H2O CrdKfswater (missing at Oslo)

  6. Metamorphic Facies PentiiEskola (1915) developed the concept of metamorphic facies: “In any rock or metamorphic formation which has arrived at a chemical equilibrium through metamorphism at constant temperature and pressure conditions, the mineral composition is controlled only by the chemical composition. We are led to a general conception which the writer proposes to call metamorphic facies.” Eskola ‘s dual basis facies : Xbulk & mineralogy A metamorphic facies is a set of repeatedly associated metamorphic mineral assemblages

  7. Metamorphic Facies Eskola aware of the P-T implications and correctly deduced the relative temperatures and pressures of facies he proposed Modern lab results: can now assign relatively accurate temperature and pressure limits to individual facies

  8. Metamorphic Facies Eskola (1920) proposed 5 original facies: Greenschist Amphibolite Hornfels Sanidinite Eclogite Easily defined on the basis of mineral assemblages that develop in mafic rocks

  9. Metamorphic Facies In his final account, Eskola (1939) added: Granulite Epidote-amphibolite Glaucophane-schist (now called Blueschist) ... and changed the name of the hornfels facies to the pyroxene hornfels facies

  10. Metamorphic Facies Fig. 25-1The metamorphic facies proposed by Eskola and their relative temperature-pressure relationships. After Eskola (1939) Die Entstehung der Gesteine. Julius Springer. Berlin.

  11. Metamorphic Facies Several additional facies types have been proposed. Most notable are: Zeolite Prehnite-pumpellyite ...resulting from the work of Coombs in the “burial metamorphic” terranes of New Zealand Fyfe et al. (1958) also proposed: Albite-epidote hornfels Hornblende hornfels

  12. Fig. 25-2.Temperature-pressure diagram showing the generally accepted limits of the various facies used in this text. • The limits are approximate and gradational, because the reactions vary with rock composition and the nature and composition of the fluid phase • The 30oC/km geothermal gradient is an example of an elevated orogenic geothermal gradient. Metamorphic Facies

  13. Metamorphic Facies Table 25-1. The definitive mineral assemblages that characterize each facies (for mafic rocks).

  14. Metamafic and pelitic grade correspondence Fig. 25-9.Typical mineral changes that take place in metabasic rocks during progressive metamorphism in the medium P/T facies series. The approximate location of the pelitic zones of Barrovian metamorphism are included for comparison. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  15. Miyashiro extended the facies concept to encompass broader progressive sequences: facies series A traverse up grade through a metamorphic terrane should follow one of several possible metamorphic field gradients (Fig. 21-1, next slide), and, if extensive enough, cross through a sequence of facies Facies Series

  16. Metamorphism of Mafic Rocks Mineral changes and associations along T-P gradients characteristic of the three facies series Hydration of original mafic minerals generally required If water unavailable, mafic igneous rocks will remain largely unaffected, even as associated sediments are completely re-equilibrated Coarse-grained intrusives are the least permeable and likely to resist metamorphic changes Tuffs and graywackes are the most permeable and so subject to metamorphic changes.

  17. Metamorphism of Mafic Rocks The principal mineral changes are due to the breakdown of the two most common basaltic minerals: plagioclase and clinopyroxene Plagioclase: More Ca-rich plagioclases become progressively unstable as T lowered General correlation between temperature and maximum An-content of the stable plagioclase At low metamorphic grades only albite (An0-3) is stable The An-content of plagioclase jumps as grade increases More Anorthite content (= more calcic) plagioclases are stable in the upper amphibolite and granulitefacies

  18. Metamorphism of Mafic Rocks Clinopyroxene breaks down to a number of mafic minerals: (less hot) chlorite, actinolite, hornblende (hotter), etc. depending on T & P

  19. Greenschist, Amphibolite, and GranuliteFacies The greenschist, amphibolite and granulitefacies constitute the most common facies series of regional metamorphism on Mafic rocks

  20. GreenschistFacies ACF diagram The most characteristic mineral assemblage of the greenschistfacies is: chlorite + albite + epidote + actinolite quartz Fig. 25-6.ACF diagram illustrating representative mineral assemblages for metabasites in the greenschist facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  21. GreenschistFacies Greenschist®amphibolitefacies transition involves two major mineralogical changes 1. Albite ® oligoclase (increased Ca-content with temperature across the peristerite gap) 2. Actinolite® hornblende (amphibole accepts increasing aluminum and alkalis at higher Ts) Both transitions occur at approximately the same grade, but have different P/T slopes Exist exsolution lamellae on cooling in the peristerite miscibility gap, ~An5-An18

  22. AmphiboliteFacies ACF diagram Typically two-phase: Hbl-Plag Amphibolites are typically black rocks with up to about 30% white plagioclase Like diorites, but differ texturally Garnet in more Al-Fe-rich and Ca-poor mafic rocks Clinopyroxene in Al-poor-Ca-rich rocks Fig. 25-7.ACF diagram illustrating representative mineral assemblages for metabasites in the amphibolite facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  23. Amphibolite to GranuliteFacies Mafic rocks generally melt at higher temperatures If water is removed by the earlier melts the remaining mafic rocks may become depleted in water Hornblende decomposes and orthopyroxene + clinopyroxene appear

  24. Hornblende => orthopyroxene + clinopyroxene Low water Fig. 26-19.Simplified petrogenetic grid for metamorphosed mafic rocks showing the location of several determined univariant reactions in the CaO-MgO-Al2O3-SiO2-H2O-(Na2O) system (“C(N)MASH”). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  25. GranuliteFacies • Critical meta-basite mineral assemblage is orthopyroxene + clinopyroxene + plagioclase + quartz • Garnet, minor hornblende and/or biotite may be present • mineralogy plagioclase and pyroxene similar to basalt • The texture gneissic, and grain shapes decussate or polygonal, no resemblance to the original basalt Granulitefacies characterized by a largely anhydrousmineral assemblage

  26. GranuliteFacies Origin of granulitefacies: general agreement on two points 1) Granulites represent unusually hot conditions Temperatures > 700oC (geothermometry has yielded some very high temperatures, even in excess of 1000oC) Average geotherm temperatures for granulitefacies depths should be in the vicinity of 500oC, suggesting that granulites are the products of crustal thickening and excess heating

  27. GranuliteFacies 2) Granulites are dry Rocks don’t melt due to lack of available water Granulitefaciesterranes represent deeply buried and dehydrated roots of the continental crust Fluid inclusions in granulitefacies rocks of S. Norway are CO2-rich, whereas those in the amphibolitefacies rocks are H2O-rich

  28. Fig. 25-2.Temperature-pressure diagram showing the generally accepted limits of the various facies used in this text. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  29. Mafic Assemblages of the High P/T Series: Blueschist and Eclogite Facies Mafic rocks (not pelites) develop definitive mineral assemblages under high P/T conditions High P/T geothermal gradients characterize subduction zones Mafic blueschists are easily recognizable by their color, and are useful indicators of ancient subduction zones. Formation requires abundant water. The great density of eclogites: subducted basaltic oceanic crust becomes more dense than the surrounding mantle Begins to sink , extended necks thin and break off pieces

  30. BlueschistFacies The blueschistfacies is characterized in metabasites by the presence of a sodic blue amphibole stable only at high pressures (notably glaucophane) The association of glaucophane + lawsonite(CaAl2Si2O7(OH)2·H2O) is diagnostic of the high pressure. Albite breaks down at high pressure by reaction to jadeite (a pyroxene) + quartz: NaAlSi3O8 = NaAlSi2O6 + SiO2 (reaction 25-3) AbJdQtz Jadeite without quartz would be stable into the albite field at lower pressure =>low pressure blueschist Specimen: Jadeite + Glaucophane w/o quartz

  31. Blueschist and Eclogite Facies Fig. 25-10.ACF diagram illustrating representative mineral assemblages for metabasites in the blueschist facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  32. Eclogite Facies Glaucophane + Paragonite = Pyrope + Jadeite + Qtz +H2O Glaucophane Na2Mg3Al2Si8O22(OH)2 Paragonite is NaAl2(AlSi3O10)(OH)2 Pyrope is Mg3Al2(SiO4)3 Jadeite is NaAlSi2O6 Eclogite facies: mafic assemblage omphacitic pyroxene + pyrope-grossular garnet Fig. 25-11.ACF diagram illustrating representative mineral assemblages for metabasites in the eclogite facies. The composition range of common mafic rocks is shaded. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  33. BTW Omphacite compositions are intermediate between calcium-rich augite and sodium-rich jadeite “Non-quad” pyroxenes Jadeite Aegirine NaAlSi2O6 NaFe3+Si2O6 0.8 Omphacite aegirine- augite Na# = Na / (Na + Ca) Ca-Tschermack’s molecule 0.2 CaAl2SiO6 Augite (Ca, Na) (Fe, Al) Si2O6 Diopside-Hedenbergite Ca(Mg,Fe)Si2O6

  34. Pressure-Temperature-Time (P-T-t) Paths The facies series concept suggests that a traverse up grade through a metamorphic terrane should follow a metamorphic field gradient, and may cross through a sequence of facies (spatial sequences) Progressive metamorphism: rocks pass through a series of mineral assemblages as they continuously equilibrate to increasing metamorphic grade (temporal sequences) Are the temporal and spatial mineralogical changes the same?

  35. Staurolite (St) as relict within poikiloblastic andalusite (And). Pressure-Temperature-Time (P-T-t) Paths Metamorphic P-T-t paths may be addressed by: 1) Observing partial overprints of one mineral assemblage upon another The relict minerals may indicate a portion of either the prograde or retrograde path (or both) depending upon when they were created Muscovite + chlorite + quartz + staurolite = andalusite / sillimanite + biotite + H20 Staurolite (St) as relict within poikiloblastic andalusite (And).

  36. Pressure-Temperature-Time (P-T-t) Paths Metamorphic P-T-t paths may be addressed by: 2) Apply geothermometers and geobarometers to the core vs. rim compositions of chemically zoned minerals to document the changing P-T conditions experienced by a rock during their growth

  37. Pressure-Temperature-Time (P-T-t) Paths Classic view: regional metamorphism is a result of deep burial or intrusion of hot magmas Plate tectonics: regional metamorphism is a result of crustal thickening and heat input during orogeny at convergent plate boundaries (not simple burial)

  38. Chapter 26: Metamorphic Reactions • Isograds are reaction • lines

  39. 1. Phase Transformations Isochemical phase transformations (the polymorphs of SiO2 or Al2SiO5 or graphite-diamond or calcite-aragonite The transformations depend on temperature and pressure only Aragonite is the stable CaCO3 polymorph commonly found in blueschist facies terranes

  40. 1. Phase Transformations Independent of other minerals present, fluids, etc. Andalusite -> Sill as T and P increase regardless of other phases Stau, Mus, Qtz

  41. 1. Phase Transformations Small DS for most polymorphic transformations  small DG between two alternative polymorphs, even several tens of degrees from the equilibrium boundary  little driving force for the reaction to proceed ® common metastable relics in the stability field of other Coexisting polymorphs may therefore represent non-equilibrium states (overstepped equilibrium curves) Staurolite poikiloblast

  42. Some solid solutions are unstable at lower T, and exsolve as T falls. Classic example K-spar and Ab form a solid solution at 1000C but separate into Microcline + Albite -> Perthitic Texture 2. Exsolution Figure 6-16. T-X phase diagram of the system albite-orthoclase at 0.2 GPa H2O pressure. After Bowen and Tuttle (1950). J. Geology, 58, 489-511. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  43. 3. Solid-Solid Net-Transfer Reactions Differ from polymorphic transformations: involve solids of differing composition Material must diffuse from one site to another for the reaction to proceed NaAlSi2O6 + SiO2 = NaAlSi3O8 • JdQtzAb MgSiO3 + CaAl2Si2O8 = CaMgSi2O6 + Al2SiO5 • En An Di And 4 (Mg,Fe)SiO3 + CaAl2Si2O8 = • Opx An Plag • (Mg,Fe)3Al2Si3O12 + Ca(Mg,Fe)Si2O6 + SiO2 • GntCpxQtz Reaction curves typically pretty straight DS and DV change little

  44. 3. Solid-Solid Net-Transfer Reactions If minerals contain volatiles, but the volatiles conserved in the reaction so that no fluid phase is generated or consumed For example, the reaction: Mg3Si4O10(OH)2 + 4 MgSiO3 = Mg7Si8O22(OH)2 Talc Enstatite Anthophyllite involves hydrous phases, but conserves H2O It may therefore be treated as a solid-solid net-transfer reaction

  45. 4. Devolatilization Reactions For example the location of the reaction line on a P-T phase diagram of the dehydration reaction: KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2O MuscQtzKfld Sill Water depends upon the partial pressure of H2O (pH2O)

  46. 4. Devolatilization Reactions Here the equilibrium curve represents equilibrium between the reactants and products under water-saturated conditions (pH2O = PLithostatic) P-T phase diagram for the reaction Ms + Qtz = Kfs + Al2SiO5 + H2O showing the shift in equilibrium conditions as pH2O varies (assuming ideal H2O-CO2 mixing). Calculated using the program TWQ by Berman (1988, 1990, 1991). After Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  47. KAl2Si3AlO10(OH)2 + SiO2 = KAlSi3O8 + Al2SiO5 + H2OMs QtzKfs Sill water Suppose H2O is withdrawn from the system at some point on the water-saturated equilibrium curve: pH2O < Plithostatic According to Le Châtelier’s Principle, removing water at equilibrium will be compensated by the reaction running to the right, thereby producing more water This has the effect of stabilizing the right side of the reaction at the expense of the left side So as water is withdrawn the Kfs + Sill + H2O field expands slightly at the expense of the Mu + Qtz field, and the reaction curve shifts toward lower temperature • Pfluid < PLith by drying out the rock and reducing the fluid content • Pfluid = PLith, but the water in the fluid can become diluted by adding another fluid component, such as CO2 or some other volatile phase

  48. CaCO3 + SiO2 = CaSiO3 + CO2 (26-6) • Cal Qtz Wollastonite 4. Decarbonization Reactions Figure 26-5. T-XCO2 phase diagram for the reaction Cal + Qtz = Wo + CO2 at 0.5 GPa assuming ideal H2O-CO2 mixing, calculated using the program TWQ by Berman (1988, 1990, 1991). Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Figure 26-1. A portion of the equilibrium boundary for the calcite-aragonite phase transformation in the CaCO3 system. After Johannes and Puhan (1971), Contrib. Mineral. Petrol., 31, 28-38. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  49. 5. Continuous Reactions Occur when F  1, and the reactants and products coexist over a temperature (or grade) interval Fig. 26-9.Schematic isobaric T-XMg diagram representing the simplified metamorphic reaction Chl + Qtz  Grt + H2O. From Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  50. 6. Ion Exchange Reactions • Reciprocal exchange of components between 2 or more minerals • MgSiO3 + CaFeSi2O6 = FeSiO3 + CaMgSi2O6 • Enstatite + Hedenbergite = Ferrosilite + Diopside • Expressed as pure end-members, but really involves Mg-Fe (or other) exchange between intermediate solutions • Basis for many geothermobarometers http://www.springerlink.com/content/ghr3426g33686522/