Chapter 21: Metamorphism - Faculty web pages

The Limits of Metamorphism• Low-temperaturelimit grades into diagenesis The boundary is somewhat arbitraryDiagenetic/weathering processes are indistinguishable frommetamorphic Metamorphism begins in the range of 100-150150 o C for the moreunstable types of protolith Some zeolites are considered diagenetic and othersmetamorphic

Metamorphic Agents and Changes• Temperature: : typically themost important factor inmetamorphismFigure 1-9. Estimated ranges of oceanic andcontinental steady-state geotherms to a depth of100 km using upper and lower limits based on heatflows measured near the surface. After Sclater etal. (1980), Earth. Rev. Geophys. Space Sci., 18,269-311.

Metamorphic Agents and ChangesIncreasing temperature has several effects1) Promotes recrystallization → increased grainsize2) Drives reactions that consume unstablemineral(s) and produces new minerals that arestable under the new conditions3) Overcomes kinetic barriers that might otherwisepreclude the attainment of equilibrium

Metamorphic Agents and Changes• Pressure “Normal” gradients may be perturbed in severalways, typically:High T/P geotherms in areas of plutonicactivity or riftingLow T/P geotherms in subduction zones

Figure 21-1. Metamorphic field gradients (estimated P-T conditions along surface traverses directly upmetamorphic grade) for several metamorphic areas. After Turner (1981). Metamorphic Petrology: Mineralogical, Field, andTectonic Aspects. McGraw-Hill.

Metamorphic Agents and Changes• Metamorphic grade: : a general increase indegree of metamorphism without specifyingthe exact relationship between temperatureand pressure

Stress• Lithostatic pressure is uniform stress (hydrostatic)• Deviatoric stress = unequal pressure in differentdirections• Three mutually perpendicular stress (σ)(components:σ 1 is the maximum principal stressσ 2 is an intermediate principal stressσ 3 is the minimum principal stress• In hydrostatic situations all three are equal

Metamorphic Agents and Changes• Stress is an applied force acting on a rockForce/ unit area (e.g., lb/in 2 )• Strain is the response of the rock to an appliedstress (= deformation)

•Deviatoricstress affects the textures and structures,but not the equilibrium mineral assemblage•Strain energy may overcome kinetic barriers toreactions

FluidsMetamorphic Agents and ChangesEvidence: Fluid inclusions Fluids are required for hydrous or carbonatephases Volatile-involving involving reactions

Fluids• P fluid indicates the total fluid pressure, which is the sumof the partial pressures of each component (P fluid = p H2OCO2 + …)p CO2H2O +• May also consider the mole fractions of the components,which must sum to 1.0 (X H2O + X CO2 + … = 1.0)

Metamorphic Agents and Changes• Gradients in T, P, X fluid across an area• Zonation in the mineral assemblages

The Types of MetamorphismDifferent approaches to classification1. Based on principal process or agent Dynamic Metamorphism - mylonites Thermal Metamorphism – contactmetamorphism Dynamo-thermal Metamorphism – regionalmetamorphism

The Types of MetamorphismDifferent approaches to classification2. Based on setting Contact Metamorphism Regional MetamorphismOrogenic MetamorphismBurial MetamorphismOcean Floor Metamorphism Shock Metamorphism

Contact Metamorphism• Adjacent to igneous intrusions• Result of thermal (and possibly metasomatic)effects of hot magma intruding cooler shallowrocks• Occur over a wide range of pressures, includingvery low• Contact aureole

The Types of MetamorphismContact MetamorphismThe size and shape of an aureole is controlled by:The nature of the plutonSizeShapeOrientationThe nature of the country rocksTemperatureCompositionCompositionDepth and metamorphic grade prior to intrusionPermeability

The Types of MetamorphismContact MetamorphismMost easily recognized where a pluton is introduced intoshallow rocks in a static environmentThe rocks near the pluton:hornfels = dense, strongly indurated bakedrocks, very fine-grainedGrain size and metamorphic grade increasesinward toward the intrusion

The Types of MetamorphismRegional Metamorphism : metamorphism that affectsa large body of rock, and thus covers a great lateralextentThree principal types: Orogenic metamorphism Burial metamorphism Ocean-floor metamorphism

The Types of MetamorphismOrogenic Metamorphism - associated with convergentplate margins• Dynamo-thermal, involving one or more episodes oforogeny with combined elevated geothermal gradientsand deformation (deviatoric stress)• Foliated rocks are a characteristic product• Foliation – penetrative parallel alignment of minerals

OrogenicMetamorphismThe Types of MetamorphismFigure 21-6. Schematicmodel for the sequential (a→ c) development of a“Cordilleran-type” oractive continental marginorogen. The dashed andblack layers on the rightrepresent the basaltic andgabbroic layers of theoceanic crust. From Deweyand Bird (1970) J. Geophys. Res.,75, 2625-2647; and Miyashiro et al.(1979) Orogeny. John Wiley &Sons.

The Types of MetamorphismOrogenic Metamorphism• Following metamorphism - uplift and erosion• Metamorphism often continues after majordeformation ceases• Pattern of increasing metamorphic grade fromboth directions toward the core area

The Types of MetamorphismOrogenic Metamorphism• Most orogenic belts have several episodes ofdeformation and metamorphism, creating a morecomplex polymetamorphic pattern• Batholiths are usually present in the highest gradeareas• Continental collision

The Types of MetamorphismBurial metamorphism = for low-grademetamorphism in sedimentary basins due to burial• Metamorphic effects attributed to increased pressure andtemperature due to burial• Range from diagenesis to the formation of zeolites,prehnite and pumpellyite

Burial metamorphism occurs in areas that have notexperienced significant deformation or orogeny• Restricted to large, relatively undisturbed sedimentarypiles away from active plate margins•e.g., Bengal Fan → sedimentary pile > 22 km•Extrapolating→ 250-300o C at the base (P ~ 0.6 GPa)•Well into the metamorphic range, and the weight of theoverlying sediments sufficient to impart a foliation atdepth•Passive margins often become active•Areas of burial metamorphism may thus become areasof orogenic metamorphism

The Types of MetamorphismOcean-Floor Metamorphism affects the oceaniccrust at ocean ridge spreading centers• Wide range of temperatures at relatively lowpressure• Metamorphic rocks exhibit considerablemetasomatic alteration, notably loss of Ca and Siand gain of Mg and Na• These changes can be correlated with exchangebetween basalt and hot seawater

The Types of MetamorphismImpact Metamorphism occurs in areas experiencingrelatively high rates of deformation and strain withonly minor recrystallization• Impact metamorphism (“shock(metamorphism”) ) occurs atmeteorite (or other bolide) impact craters

The Progressive Nature of Metamorphism• Prograde: increase in metamorphic grade with timeas a rock is subjected to gradually more severeconditions changes in a rock that accompany increasingmetamorphic grade• Retrograde: decreasing grade as rock cools andrecovers from a metamorphic or igneous event Changes in a rock that accompany decreasingmetamorphic grade

The Progressive Nature of Metamorphism• A rock at a high metamorphic grade probablyprogressed through a sequence of mineralassemblages rather than hopping directly from anunmetamorphosed rock to the metamorphic rockthat we find todayAll rocks that we now find must also have cooledto surface conditionsAt what point on its cyclic P-T-t t path did itspresent mineral assemblage last equilibrate?

Each rock preserves the conditions of the maximummetamorphic grade (temperature)Prograde reactions are endothermic and easilydriven by increasing T•Heat flow limiting factor•Devolatilizationreactions•Geothermometry- mineral compositionscommonly preserve the maximum temperature

Retrograde metamorphism is exothermic and requiresthe addition of volatiles (H 2 O, CO 2 …)• Limiting factor is lack of fluids• Rarely complete reactions – so good record of them• EvidenceChlorite replacing biotite, , garnetAmphibole replacing pxAlteration of feldspar to sericite

Types of ProtolithCommon types of sedimentary and igneous rockprotoliths:1. Ultramafic - very high Mg, Fe, Ni, Cr2. Mafic - high Fe, Mg, and Ca3. Pelites (mudrocks) - high Al, K, Si4. Carbonates- high Ca, Mg, CO 25. Quartz Ss.- nearly pure SiO 26. Granite - high Si, Na, K, Al

• Interpretation of the conditions and evolution ofmetamorphic bodies, mountain belts, and ultimatelythe evolution of the Earth's crust• Metamorphic rocks may retain enough inheritedinformation from their protolith to allow us tointerpret much of the pre-metamorphic history

Orogenic Regional Metamorphism ofthe Scottish Highlands• George Barrow (1893, 1912)• SE Highlands of Scotland - Caledonian orogeny ~ 500 Ma• Barrow studied the pelitic rocks• Could subdivide the area into a series of metamorphiczones, , each based on the appearance of a new mineral asmetamorphic grade increased• Sequence = Barrovian zones• The P-T P T conditions referred to as Barrovian-typemetamorphism (fairly typical of many belts)• Isograd = line that separates the zones (a line in the field ofconstant metamorphic grade)

Barrow’sAreaFigure 21-8. Regional metamorphicmap of the Scottish Highlands,showing the zones of minerals thatdevelop with increasingmetamorphic grade. From Gillen(1982) Metamorphic Geology. AnIntroduction to Tectonic andMetamorphic Processes. GeorgeAllen & Unwin. London.

The sequence of zones now recognized, and the typicalmetamorphic mineral assemblage in each, are:Chlorite zone: : chlorite, muscovite, quartz and albiteBiotite zone: biotite, , chlorite, muscovite, quartz, and albiteGarnet zone: : red almandine garnet, usually with biotite, chlorite,muscovite, quartz, and albite or oligoclaseStaurolite zone: staurolite, , biotite, muscovite, quartz, garnet, andplagioclase. Some chlorite may persistKyanite zone: kyanite, , biotite, muscovite, quartz, plagioclase, andusually garnet and staurolite Sillimanite zone: sillimanite, biotite, muscovite, quartz,plagioclase, garnet, and perhaps staurolite. Some kyanite may alsobe present.

Figure 21-8. Regionalmetamorphic map of theScottish Highlands,showing the zones ofminerals that developwith increasingmetamorphic grade.From Gillen (1982)Metamorphic Geology. AnIntroduction to Tectonic andMetamorphic Processes.George Allen & Unwin.London.

To summarize:• An isograd represents the first appearance of a particularmetamorphic index mineral in the field as one progressesup 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 formsthe low-gradeboundary of that zone• Since classic isograds are based on the first appearance ofa mineral, and not its disappearance, an index mineralmay still be stable in higher grade zones

Paired Metamorphic Belts of JapanFigure 21-12. The Sanbagawa andRyoke metamorphic belts of Japan.From Turner (1981) MetamorphicPetrology: Mineralogical, Field, andTectonic Aspects. McGraw-Hill andMiyashiro (1994) Metamorphic Petrology.Oxford University Press.

Paired Metamorphic Belts of Japan• The “inner” belt, inward, or away from the trench isthe Ryoke -AbukumaBeltLow P/T regional orogenic metamorphismDominant meta-pelitic sediments, and isograds up to thesillimanite zone have been mappedA high-temperaturetemperature-low-pressurebelt, and graniticplutons are common

Paired Metamorphic Belts of Japan• Outer belt, called the Sanbagawa Belt• High-pressure, low-temperatureOnly reaches the garnet zone in the pelitic rocksMafic rocks are more common than in the Ryoke belt,and in these glaucophane (blue amphibole) is developedRocks are commonly called blueschists

Figure 21-12. The Sanbagawa andRyoke metamorphic belts ofJapan. From Turner (1981)Metamorphic Petrology: Mineralogical,Field, and Tectonic Aspects.McGraw-Hill and Miyashiro (1994)Metamorphic Petrology. OxfordUniversity Press.

Paired Metamorphic Belts of Japan• Two belts are in contact along their whole lengthacross a major fault zone, the Median Line• Ryoke-Abukumalithologies are similar tosediments derived from a relatively mature volcanicarc• Sanbagawa lithologies more akin to the oceanwardaccretionary wedge where arc-derived sedimentsand volcanics mix with oceanic crust and marinesediment

Paired Metamorphic Belts of Japan• Fig. 16-15 15 suggests that the 600 o C isotherm, for example,could be as deep as 100 km in the trench-subduction zonearea, and as shallow as 20 km beneath the volcanic arc

Miyashiro (1961, 1973) suggested that the occurrence of coevalmetamorphic belts, an outer, high-P/T belt, and an inner, lower-P/Tbelt ought to be a common occurrence in a number of subductionzones, either modern or ancientFigure 21-13. Some ofthe pairedmetamorphic belts inthe circum-Pacificregion. From Miyashiro(1994) MetamorphicPetrology. OxfordUniversity Press.