Presentation on theme: "Ch. 21 An Introduction to Metamorphism"— Presentation transcript:
1Ch. 21 An Introduction to Metamorphism Metamorphism is the transformation of rock by temperature and pressureMetamorphic rocks are produced by transformation of:Sedimentary and Igneous rocks, and by the further alteration of other metamorphic rocksKyanite, Sillimanite, and Andalucite, greenschist, amphibolite, glaucophane
2Metamor-phism occurs between about 10 and 50 km of depth Sedimentary0 kmrockMetamorphicrockIgneousSedimentrock10 kmMetamor-phism occurs between about 10 and 50 km of depth~200ºCSedimentaryrockThe rocks don’t meltMetamorphismIncreasing depthand temperature50 kmMelting~800ºC
3metamorphic rocks are seen when erosion Glaciers exposed theRockyMountainsCanadianShieldNorthCascadesBlackHillsAppalachianMountainsBest US exposuresin New Englandand the SouthGrandCanyonLlanoUpliftUsually buried deep,metamorphic rocks are seen when erosionremoves covering rocks, and in the coresof mountains
4What causes metamorphism? 1. Temperature IncreaseGreater Temps at depthRadioactive IsotopesIntruding MagmaFriction Between Moving Bodies of Rock
5Metamorphic Agents and Changes Temperature: typically the most important factor in metamorphismContinental geotherm is higher than oceanic due to concentration of radioactive (LIL) elementsContinental geotherm is higher than oceanic due to concentration of radioactive (LIL) elementsLESSON: It gets hotter faster with depth in continental lithosphereFigure 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,
6Metamorphic Agents and Changes Increasing temperature has several effects1) Promotes recrystallization increased grain size2) Drives reactions3) Overcomes phase stability barriers1) Promotes recrystallization increased grain sizeLarger surface/volume ratio of a mineral lower stabilityIncreasing temperature eventually overcomes kinetic barriers to recrystallization, and fine aggregates coalesce to larger grainsEspecially for fine-grained and unstable materials in a static environment (shear stresses often reduce grain size)2) Drive reactions that consume unstable mineral(s) and produces new minerals that are stable under the new conditionsHeating to conditions outside the stability range of some mineral(s) may cause a reaction to take place that consumes the unstable mineral(s) and produces new minerals that are stable under the new conditions3) Overcomes kinetic barriers that might otherwise preclude the attainment of equilibriumDisequilibrium is relatively common in sediments and diagenesisMineral assemblages are usually simpler at higher grades and the phase rule is applicable
7What causes metamorphism? 2. Pressure (stress) = Force/meters2Increases with depthPressure can be applied equally in all directions or differentiallyAll Directions = “Confining Pressure”also called “lithostatic pressure”Differential = “Directed Pressure”
8Origin of pressure in metamorphism Confining pressure aka “lithostatic”(due to burial)(Convergent Margin)
9Source: Kenneth Murray/Photo Researchers Inc. Directed Pressure causes rocks to become folded, and minerals to reorient perpendicular to the stress: “foliation”Original bedding is obliteratedSource: Kenneth Murray/Photo Researchers Inc.
10FoliationMinerals Recrystallize Perpendicular to the Directed PressureIf the minerals are flat, such as sheet-like Micas, their parallel orientation gives a layered look; layering unrelated to the original bedding in the parent rock.
11Main factors causing metamorphism 3. Parent rockMetamorphic rocks typically have the same chemical composition as the rock they were formed from.Different minerals, but made of the same atoms.Exception: when hot water is involved.
12Types of ProtolithLump the common types of sedimentary and igneous rocks into six chemically based-groups1. Ultramafic - very high Mg, Fe, Ni, Cr2. Mafic - high Fe, Mg, and Ca3. Shales (pelitic) - high Al, K, Si4. Carbonates- high Ca, Mg, CO25. Quartz - nearly pure SiO2.6. Quartzo-feldspathic - high Si, Na, K, AlChemistry of the protolith is the most important clue toward deducing the parent rock1. Ultramafic rocks. Mantle rocks, komatiites, or cumulates2. Mafic rocks. Basalts or gabbros, some graywackes3. 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 components5. 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, AlCategories are often gradational, and cannot include the full range of possible parental rocksOne common gradational rock type is a sand-shale mixture:psammiteOther rocks: evaporites, ironstones, manganese sediments, phosphates, laterites, alkaline igneous rocks, coal, and ore bodiesChemistry of the metamorphic rock is the most important clue toward deducing the parent rock
13Metamorphic Settings Most is Dynamothermal Four types of metamorphic settings:1. Contact metamorphism – due heat from adjacent rocks2. Hydrothermal metamorphism – chemical alterations from hot, ion-rich water3. Regional metamorphism -- Occurs in the cores of mountain belts and subduction zones (Converging Margins) . Makes great volumes of metamorphic rock. Includes:a. Burial Metamorphism – e.g. Burial of sediments deeper than 10 km – non-foliatedb. Dynamothermal Metamorphism – Directed pressure in Plate Tectonic Processes – foliated4. Dynamic. Smearing of minerals as faults slipMost is Dynamothermal
14Contact metamorphism Produced mostly by local heat source Baking due to nearby MagmaEffect strongest in rocks in immediate contact
15Hydrothermal Metamorphism Hot water facilitates metamorphic reactions by allowing movement of atoms and ionsAtoms in the rock may be added or removed by the waterDue circulation of water near MagmaImportant at mid-ocean ridge, or near continental volcanoes
16MetasomatismMetamorphism by mobile fluids, e.g. H2O incl. salines, silica, CO2The composition changesEvidence for the existence of a metamorphic fluids: Fluid inclusionsFluids are required for hydrous H2O OH- or carbonate CO3-- phasesEx: The fluids expelled from the subducting slab migrate upwards and form hot springs, mud- volcanoes or serpentinite Diapirs within the forearc system. Fluids or melts released at greater depth will hydrate and metasomatise the mantle rocks of the hanging plate (i.e. the mantle wedge). The source of melt for the volcanic arcs located above the subduction zones is this hydrated mantle wedge above the subducted slab.
173. Regional MetamorhismMost Dynamothermal metamorphism occurs along convergent plate boundariesExample 1: In Subduction ZonesExample 2: Continent-Continent CollisionsCompressional stresses deforms plate edgeContinents CollideMajor Folded Mountain Belts: Alps, Himalayas, and Appalachian Mts.
18Dynamothermal Metamorphism, Before collision, in subduction zone Sediments are “unconsolidated”. They will fold if pushed.
19Dynamothermal Metamorphism, After continental collision Felsic continental materials and sediments are buoyant, they have low densityThey float, cannot be subducted, so they get squashed.
20Orogenic Regional Metamorphism of the Scottish Highlands George Barrow studied the metapelites in NE Scotland from 1884 to 1900.He could subdivide the area into a series of metamorphic zones, each based on the appearance of a new mineral as metamorphic grade increasedBarrow noted significant and systematic mineralogical changes in the pelitic rocksHe 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
21Figure 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.
22The 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 albiteBiotite zone. Slates give way to phyllites and schists, with biotite, chlorite, muscovite, quartz, and albiteGarnet zone. Schists with conspicuous red almandine garnet, usually with Biotite, chlorite, muscovite, quartz, and Albite or OligoclaseStaurolite zone. Schists with Staurolite, Biotite, Muscovite, Quartz, garnet, and plagioclase. Some Chlorite may persistKyanite zone. Schists with Kyanite, Biotite, Muscovite, Quartz, plagioclase, and usually garnet and StauroliteSillimanite zone. Schists and gneisses with Sillimanite, Biotite, Muscovite, Quartz, plagioclase, garnet, and perhaps Staurolite. Some Kyanite may also be present. Kyanite and Sillimanite are both polymorphs of Al2SiO5)
24Metamorphic Grade and Index Minerals Certain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form
25Index Minerals in metamorphic rocks Note Temperature gradientIndex Minerals in metamorphic rocksCertain minerals, called index minerals, are good indicators of the metamorphic conditions in which they form580oC220oCStable temps vary based on pressure and other species present460oC690oCNote Quartz and Feldspar are not index minerals: Why?
26Here is an internally heated pressure vessel at the AMNH With these you can study, for example:1. the temperature and pressure conditions at which polymorphs change from one form to another.2. the reactions of minerals with fluids (for example salty or alkaline water) at high temperatures and pressures.3. the conditions necessary to change one assemblage of minerals to another
27Thermometers and Pressure Gauges SillimaniteKyanitePolymorphs of Al2SiO5Note other minerals in systemAndalusite
28 New England Dynamothermal Metamorphism 7_21CANADANew EnglandDynamothermalMetamorphismMAINECANADAAugustaU.S.A.Caused by C-C collision M-PMost of AppalachiansMontpelierNEWHAMPSHIREVERMONTConcordATLANTICOCEANBostonAlbanyMASSACHUSETTSNEW YORKR.I.HartfordBinghamtonProvidenceCONNECTICUTUnmetamorphosedyeallLowgradeChlorite/muscovite zonePENNSYLVANIAvBiotite zoneScrantonLongIslandMediumgradeGarnet zonetfiStaurolite zoneNEWrNewarkHigh gradeSillimanite zoneJERSEYIncreasing pressure and temperatureDIAGENESISLOW GRADEINTERMEDIATE GRADEHIGH GRADEMELTINGChlorite and muscoviteBiotiteGarnetStauroliteSillimanite
29Metamorphic Facies in Subduction Zones Metamorphic grade or Facies: A group of minerals that form in a particular P-T environment. Can be used to deduce T-P conditions of formationWe can look at minerals in Metamorphic Rocks and determine where they formed.Subducted Water, water from hydrated minerals, facilitates metamorphic reactions by allowing movement of atoms and ions
31Greenschist Hand Sample Chlorite is commonly found in igneous rocks as an alteration product of mafic minerals such as pyroxene, amphibole, and Biotite. In this environment chlorite may be a retrograde metamorphic alteration mineral of existing ferromagnesian minerals, or it may be present as a Metasomatism product via addition of Fe, Mg, or other compounds into the rock mass.Greenschist Thin SectionChl-Ep
33RINGWOOD, A. E. 1974. The petrological evolution of island arc systems RINGWOOD, A.E The petrological evolution of island arc systems. Journal of the Geological Society, London 130,Remember solid + water = liq(aq) and LeChatelierDramatic lowering of melting point of peridotiteRingwood, (1974) suggested that island arc lavas (IAT), which are basaltic, could be related to dehydration of the hydrated ocean crust (amphibolite) as it transforms to dense eclogite at depths of ~150km. The hydrous fluids rise up into the peridotite mantle wedge, promoting partial melting. Melting occurs at much lower temperatures in the presence of water.EclogiteEclogite: red to pink garnet (almandine-pyrope) in a green matrix of sodium-rich pyroxene (omphacite)Figure 10-4 or 10-5 (2nd ed). Dry peridotite solidus compared to several experiments on H2O-saturated peridotites.
341. There are many reactions suggested for the origin near subduction zones of glaucophane, which is a characteristic component of metabasite blueschistsbasalticbasalticgreenschistamphiboliteWater out2. Gibbs Free Energy calcs are an obvious check, and this drives research programs to provide the needed numbers experimentally in the presence of water, etc. from wet subducted ocean lithosphereeclogite
35Non-Foliated Rocks 1: Quartzite Field Geologists are grateful for quartzite. It doesn’t foliate, so you can see folds. Mudrocks foliate; much harder to map.Sample ofquartziteThin sectionof quartzite
37Sandstone: grains and cement 7_18FractureSandstone: grains and cementFractureQuartzite: grains interlock
38Common metamorphic rocks Nonfoliated rocks (cont.)MarbleCoarse, crystallineParent rock usually limestoneComposed of calcite crystalsFabric can be random or oriented
39Parent rock usually limestone Composed of calcite crystals Marble (nonfoliated)MarbleCoarse, crystallineParent rock usually limestoneComposed of calcite crystalsFabric can be random or oriented
40Change in metamorphic grade with depth Mudstones are sediments, can be squashed by burial and/or in continent-continent collisionsChange in metamorphic grade with depthMudstone is most common sedimentary rock. When metamorphosed, rocks reveal grade:Increasing Directed Pressure and increasing Temps =>
41Common metamorphic rocks Foliated rocksSlateVery fine-grainedExcellent rock cleavage, often perp. to originalMade by low-grade metamorphism of shale
42Example of slate Slate Very fine-grained Excellent rock cleavage, often perp. to originalMade by low-grade metamorphism of shale
43Phyllite also lacks visible mineral grains Grade of metamorphism between slate and schistMade of small platy mineralsGlossy sheen with rock cleavageComposed mainly of muscovite and/or chlorite
44Common metamorphic rocks Foliated rocksSchistMedium- to coarse-grainedComprised of platy minerals (micas)The term schist describes the textureTo indicate composition, mineral names are used (such as mica schist)
45Schist Foliated rocks Schist Medium- to coarse-grained Comprised of platy minerals (micas)The term schist describes the textureTo indicate composition, mineral names are used, e.g. Garnet Schist
46Common metamorphic rocks Foliated rocksGneissMedium- to coarse-grainedBanded appearanceHigh-grade metamorphismComposed of light-colored feldspar layers with bands of dark mafic minerals
47Gneiss displays bands of light and dark minerals Foliated rocksMedium- to coarse-grainedBanded appearanceHigh-grade metamorphismComposed of light-colored feldspar layers with bands of dark mafic minerals
48What are metamorphic textures? Texture refers to the size, shape, and arrangement of mineral grains within a rockEx: Foliation – planar arrangement of mineral grains within a rock, perpendicular to the directed pressure, common near convergent margins
49Development of lineation or foliation due to directed pressure
51Migmatites- When Partial Melting Starts Heat the rock, when the minerals with the lowest melting points (Qtz, Feldspar) at that pressure melt then recrystallize, we get separate bands of Metamorphic and Igneous rock
52Chapter 22: A Classification of Metamorphic Rocks Metamorphic rocks are classified on the basis of texture and composition (either mineralogical or chemical)Unlike igneous rocks, which have been plagued by a proliferation of local and specific names, metamorphic rock names are surprisingly simple and flexibleMay choose some prefix-type modifiers to attach to names if care to stress some important or unusual textural or mineralogical aspects
53Foliation: any planar fabric element Lineation: any linear fabric elements
54CleavageThe property of a rock to split along a regular set of sub-parallel, closely-spaced planesany type of foliation in which platy Phyllosilicates are aligned (parallel) but are too fine grained to see without a microscope
55SchistosityA preferred orientation of mineral grains or grain aggregates produced by metamorphic processesAligned minerals are coarse grained enough to see with the unaided eyeThe orientation is generally planar, but linear orientations are not excluded
56Gneissose AKA gneissic structure Segregated into mafic and felsic layers by metamorphic processesGneissose rocks usually have visible grains
57b. Phyllite: a rock with a schistosity in which very fine phyllosilicates (sericite and/or chlorite), although rarely coarse enough to see unaided, cause a reflective foliation surface.a. Slate: compact, very fine-grained, metamorphic rock with a well-developed cleavage. Freshly cleaved surfaces are dullabFigure Examples of foliated metamorphic rocks. a. Slate. b. Phyllite. Note the difference in reflectance on the foliation surfaces between a and b: phyllite is characterized by a satiny sheen. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
58Foliated Metamorphic Rocks Schist: metamorphic rocks in which the foliated minerals are coarse enough to see easily in hand specimen.Figure 22-1c. Garnet muscovite schist. Muscovite crystals are visible and silvery, garnets occur as large dark porphyroblasts. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
59Foliated Metamorphic Rocks Gneiss: a metamorphic rock displaying gneissose structure. Gneisses are typically layered (also called banded), generally with alternating felsic and darker mineral layers. Gneisses may also be lineated, but must also show segregations of felsic-mineral-rich and dark-mineral-rich concentrations.Figure 22-1d. Quartzo-feldspathic gneiss with obvious layering. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.
60Non-Foliated Metamorphic Rocks Granofels: a comprehensive term for any rock with no preferred orientationAn outdated alternative to granofels is granulite, but this term is now used to denote very high grade rocks (whether foliated or not), and is not endorsed here as a synonym for granofels.Granofels(ic) texture is then a texture characterized by a lack of preferred orientationAn outdated alternative to granofels is granulite, but this term is now used to denote very high grade rocks (whether foliated or not), and is not endorsed here as a synonym for granofels.Hornfels is a subset, preferably for fine grained, contact metamorphosed rocks
61Non-Foliated Metamorphic Rocks Marble: a metamorphic rock composed predominantly of calcite or dolomite. The protolith is typically limestone or dolostone.
62Non-Foliated Metamorphic Rocks Quartzite: a metamorphic rock composed predominantly of quartz. The protolith is typically sandstone.
63Metamorphic Rock Facies Greenschist/Greenstone: a low-grade metamorphic rock that typically contains chlorite, actinolite, epidote, and albite. Note that the first three minerals are green, which imparts the color to the rock. Such a rock is called greenschist if foliated, and greenstone if not. The protolith is either a mafic igneous rock or a graywacke.
64Metamorphic Rock Facies Amphibolite: a metamorphic rock dominated by hornblende + plagioclase. Amphibolites may be foliated or non- foliated. The protolith is either a mafic igneous rock or graywacke.
65Serpentinite: an ultramafic rock metamorphosed at low grade, so that it contains mostly serpentine.
66Blueschist: a usually blue, usually glaucophane bearing metamorphosed mafic igneous rock or mafic graywacke. This term is even applied to non-schistose rocks.
67Eclogite: a green and red metamorphic rock that contains the green clinopyroxene Omphacite and pink garnet Pyrope. The protolith is typically basaltic. It forms via the dehydration of Amphibolite at about 150km down a subduction zone.
68Granulite: medium to coarse–grained metamorphic rocks that have experienced high-temperature metamorphism, composed mainly of feldspars sometimes associated with quartz and anhydrous ferromagnesian minerals, with granoblastic (anhedral phaneritic equi-granular) texture and gneissose to massive structure.Metagabbro showing granoblastic textureNote how the interlocking plagioclase grains in this rock meet at ~120 degree triple junctions. This feature is characteristic of granoblastic texture.
69Skarn: a contact metamorphosed and silica- metasomatized carbonate rock containing calc-silicate minerals, such as grossular, epidote, tremolite, vesuvianite, etc. Tactite is a synonym.
70Migmatite: a composite silicate rock that is heterogeneous on the 1-10 cm scale, commonly having a dark gneissic matrix (melanosome) and lighter felsic portions (leucosome). Migmatites may appear layered, or the leucosomes may occur as pods or form a network of cross-cutting veins.
71Additional ModifiersPorphyroblastic means that a metamorphic rock has one or more metamorphic minerals that grew much larger than the others. Each individual crystal is a porphyroblastSome porphyroblasts, particularly in low- grade contact metamorphism, occur as ovoid “spots”e.g. spotted hornfels, or spotted phyllite
72AugenSome gneisses have large eye-shaped grains (commonly feldspar) that are derived from pre- existing large crystals by shear (as described in Section 23.1). Individual grains of this sort are called auge (German for eye), and the (German) plural is augen. An augen gneiss is a gneiss with augen structure (Fig ).
73Additional Modifying Terms: Other modifying terms that we may want to add as a means of emphasizing some aspect of a rock may concern such features as grain- size, color, chemical aspects, (aluminous, calcareous, mafic, felsic, etc.). As a general rule we use these when the aspect is unusual. NOT a calcareous marble or mafic greenschist, as these are redundant, as is a fine grained slate.
74Additional Modifying Terms: Ortho- a prefix indicating an igneous parent, andPara- a prefix indicating a sedimentary parentThe terms are used only when they serve to dissipate doubt. For example, many quartzo- feldspathic gneisses could easily be derived from either an impure arkose or a granitoid rock. If some mineralogical, chemical, or field-derived clue permits the distinction, terms such as orthogneiss, paragneiss, or orthoamphibolite may be useful.