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Introduction to Metamorphism (C (Chapter 21) Metamorphic rocks from the middle of the crust – Quad Creek area, MT. Image: Darrell Henry Metamorphism - general IUGS SCMR definition IUGS-SCMR d finiti n off metamorphism t rphi “Metamorphism p is a subsolidus process leading to changes in mineralogy and/or texture (for example grain size) and often in chemical composition in a rock. Folded marble in the Campolungo Regions of the Alps. lps. Image mage source: Darrell Henry • These changes g are due to physical p and/or chemical conditions that differ from those normally occurring at surfaces of planets and in zones of cementation and diagenesis below this surface. • They may coexist with partial melting.” Metamorphism - general Li it off metamorphism Limits t rphi Low-temperature limit grades into diagenesis g • Boundary is somewhat arbitrary • Diagenetic/weathering processes - indistinguishable from metamorphic • Metamorphism begins in range of 100-150°C for more unstable types of protolith Image: Winter (2001) • Some zeolites considered diagenetic and others metamorphic Metamorphism - general Li it off metamorphism Limits t rphi High-temperature limit grades into mel;ting g ; g • Over melting range that solids and liquids coexist. At what point in melting process does it become "igneous"? • Xenoliths and restites considered in igneous realm because melt is dominant, but distinction is vague. Image: Winter (2001) • Migmatites ("mixed rocks") are gradational Metamorphism - general M t Metamorphic rphi agents nt and nd change h n Metamorphic grade: general i increase i degree in d off metamorphism hi without specifying exact relationship between temperature p and p pressure Temperature: typically most important factor • Estimated ranges of oceanic and continental steady-state geotherms h to depth d h off 100 k km using upper and lower limits based on heat flows measured near surface. Image: Winter (2001) Metamorphism - general Increasing temperature has several effects 1. Promotes recrystallization leading to increased grain size. • Larger surface/volume ratio of mineral results in lower stability • Increasing temperature eventually overcomes kinetic barriers to recrystallization, y and fine aggregates gg g coalesce to larger g grains g Image: Winter (2001) Metamorphism - general Increasing temperature has several effects 2) Drive reactions that consume unstable mineral(s) and produces new minerals that are stable under new conditions 3) Overcomes kinetic barriers that might otherwise preclude attainment of equilibrium Image: Winter (2001) Metamorphism - general Increasing g Pressure • Stress is applied force acting on rock (over particular crosssectional area) • Strain is response of rock to applied stress (= yielding or deformation)) Lithostatic pressure is uniform stress (~hydrostatic) Deviatoric stress = unequal pressure in different directions Deviatoric stress can be resolved into three mutually perpendicular stress (s) components: • σ1 - maximum principal stress • σ2 - intermediate p principal p stress • σ3 - minimum principal stress Metamorphism - general Increasing Pressure Deviatoric stresses come in three principal types: • Tension: σ3 is negative, and resulting strain is extension, or pulling apart • Compression: σ1 is dominant, results in folding or more homogenous flattening • Shear motion occurs along planes at an angle to σ1 Image: Winter (2001) Metamorphism - general Increasing Temperature and Pressure "Normal" g gradients may be perturbed in several ways, typically: • High T/P geotherms - in areas of plutonic activity i i or rifting if i • Low T/P g geotherms in subduction zones Image: Winter (2001) Metamorphic field gradients (estimated P-T P T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas. Metamorphism - general Fluids Evidence for the existence of a metamorphic fluid: • Fluid inclusions • Fluids are required for hydrous or carbonate phases • Volatile-involving g reactions occur at temperatures and pressures that require finite fluid pressures Pfluid means total fluid pressure - sum of partial pressures of each component (Pfluid = pH2O + pCO2 + …) May also consider mole fractions of components, which must sum to 1.0 (XH2O + XCO2 + … = 1.0) Metamorphism - general Types yp of Metamorphism p Classification of metamorphism based on setting ( ) Contact (1) Metamorphism Image: Winter (2001) Temperature distribution within 1-km thick vertical dike and in country rocks (initially at 0οC) as function of time. Curves are labeled in years. The model assumes an initial intrusion temperature of 1200oC and cooling by conduction only. Metamorphism - general Types yp of Metamorphism p Classification of metamorphism based on setting (1) Contact Metamorphism • Adjacent to igneous intrusions • Result of thermal (and possibly metasomatic) effects of hot magma intruding cooler shallow rocks •O Occur over wide id range off pressures, including very low P Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. Image: Winter (2001) • Contact metamorphic aureole Metamorphism - general Types yp of Metamorphism p Classification of metamorphism based on setting (1) Contact Metamorphism Contact metamorphic aureole Size and shape of aureole controlled by: Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. Image: Winter (2001) • Size of pluton • Shape of pluton • Orientation of pluton • Depth and metamorphic grade prior to intrusion • Temperature p - mafic magmas g • Composition are hotter than silicic magmas • Fluids and permeability Metamorphism - general Types yp of Metamorphism p Classification of metamorphism based on setting (1) Contact Metamorphism Spotted phyllite in which small porphyroblasts of cordierite develop in a preexisting phyllite. phyllite Image: Winter (2001) Contact metamorphic aureole Most easily recognized where pluton is introduced into shallow p rocks in static environment • Rocks near pluton are often hi h high-grade d rocks k with ith iisotropic t i fabric: hornfelses (or granofelses) - relict textures and structures are common • Polymetamorphic rocks are common, usually ll representing ti orogenic event followed by contact one Metamorphism - general Types yp of Metamorphism p Contact metamorphism example: Skiddaw Pluton, UK Ordovician Skiddaw Slates (English Lake District) intruded by several granitic bodies Intrusions are shallow, and contact effects overprinted on an earlier low-grade regional orogenic g metamorphism p Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. Image: Winter (2001) Metamorphism - general Types yp of Metamorphism p Contact metamorphism example: Skiddaw Pluton, UK • Intermediate zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + quartz andalusite + q Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. Image: Winter (2001) • Cordierite-andalusite slate from intermediate zone of Skiddaw aureole. Metamorphism - general Types yp of Metamorphism p Contact metamorphism example: Skiddaw Pluton, UK • Inner zone: Thoroughly recrystallized • Lose foliation Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. Image: Winter (2001) • Andalusite-cordierite schist from inner zone of Skiddaw aureole. Note the chiastolite cross in andalusite Metamorphism - general Types yp of Metamorphism p Classification of metamorphism based on setting (2) Regional Metamorphism: metamorphism that affects large body of rock, and thus covers great lateral extent Th Three principal i i l types: • Orogenic metamorphism • Burial metamorphism • Ocean-floor metamorphism Metamorphism - general Types yp of Metamorphism p Regional Metamorphism Orogenic Metamorphism is type of metamorphism associated with convergent plate margins Dynamo-thermal, involving one or more episodes of orogeny with combined elevated l t d geothermal th rm l gradients r di nt and deformation (deviatoric stress) Foliated rocks are a characteristic product Metamorphism - general Types yp of Metamorphism p Regional Metamorphism Orogenic Metamorphism Metamorphism M hi • often continues after major deformation ceases • associated with intruding plutons • batholiths commonly in highest grade region • Most orogenic belts have several episodes of deformation and metamorphism, creating more complex polymetamorphic pattern • T Transitional iti l cases can be b ttermed d regional-contact metamorphism Metamorphism - general Types yp of Metamorphism p Regional Metamorphism Burial metamorphism = low-grade metamorphism in sedimentary basins due to burial. • Southland Syncline in New Zealand: thick hi k pile il ((> 10 k km)) off M Mesozoic i volcaniclastics • Mild deformation and no igneous intrusions discovered • Fine-grained, high-temperature phases,, g p glassyy ash: veryy susceptible p to metamorphic alteration • Metamorphic effects attributed to increased pressure and temperature due to burial Image: Winter (2001) Metamorphism - general Types yp of Metamorphism p Regional Metamorphism Burial metamorphism • Range from diagenesis to formation of zeolites, prehnite, pumpellyite, laumontite, etc • Coombs ((1961)) also proposed hydrothermal metamorphism, caused by hot H2Orich fluids and usually involving metasomatism t ti Image: Winter (2001) Metamorphism - general Types yp of Metamorphism p Regional Metamorphism Burial metamorphism p = low-grade g metamorphism p in sedimentaryy basins due to burial. • Burial B i l metamorphism hi occurs iin areas that h h have not experienced significant deformation or orogeny • Restricted R i d to llarge, relatively l i l undisturbed di b d sedimentary piles away from active plate margins e g The Gulf of Mexico? e.g. • Well into metamorphic range, and the weight of the overlying sediments sufficient to impart a foliation at depth Metamorphism - general Types yp of Metamorphism p Regional Metamorphism Ocean-Floor Metamorphism p affects the oceanic crust at ocean ridge spreading centers • Wide range of T at relatively low P • Considerable metasomatic alteration notably loss of Ca and Si and gain of Mg and Na • Changes correlated with exchange between basalt and hot seawater • Example E pl off h hydrothermal dr th r l metamorphism t rphi • Highly altered chlorite-quartz rocks distinctive high high-Mg, Mg, low low-Ca Ca composition http://www.whoi.edu/page.do?pid=10655 Metamorphism - general Types yp of Metamorphism p Impact Metamorphism (dynamic) Impact metamorphism ("shock metamorphism") occurs at meteorite impact craters • dominant type of metamorphism t rphi on n Moon. Metamorphism - general Types yp of Metamorphism p Fault-Zone Metamorphism (dynamic) – primarily Metamorphism - general Progressive Nature of Metamorphism Prograde metamorphism: changes in rock that accompany p y increasing g metamorphic p grade g • High grade metamorphic rocks progress through sequence of mineral assemblages g rather than hopping pp g directlyy from an unmetamorphosed rock to the current metamorphic rock • Prograde g reactions are endothermic and driven byy increasing gT • Devolatilization is easier than reintroducing volatiles • Preserved zonal distribution of metamorphic rocks suggests that each rock preserves conditions of peak metamorphic grade (T) • Geothermometry indicates that the mineral compositions commonly preserve the maximum temperature Metamorphism - general Progressive Nature of Metamorphism Retrograde: decreasing grade as rock cools and recovers from a metamorphic or igneous event • All rocks that we now find must also have cooled to surface conditions • Retrograde metamorphism is of only minor significance Image: Winter (2001) At what point on its cyclic P-T-t path did its present mineral assemblage last equilibrate? Metamorphism - general Types of Protoliths Lump common types of sedimentary and igneous rocks into six chemically based-groups 1 Ultramafic - very high Mg, 1. Mg Fe, Fe Ni, Ni Cr Chromite-bearing serpentinite 2. Mafic - high g Fe,, Mg, g, and Ca 3. Shales (pelitic) - high Al, K, Si g Ca,, Mg, g, CO2 4. Carbonates- high 5. Quartz - nearly pure SiO2. Garnet amphibolite 6. Quartzo-feldspathic p - high g Si,, Na,, K, Al Metamorphism - general Types of Protoliths Lump common types of sedimentary and igneous rocks into six chemically based-groups 1 Ultramafic - very high Mg, 1. Mg Fe, Fe Ni, Ni Cr Garnet-staurolite schist 2. Mafic - high g Fe,, Mg, g, and Ca 3. Shales (pelitic) - high Al, K, Si g Ca,, Mg, g, CO2 4. Carbonates- high 5. Quartz - nearly pure SiO2. 6. Quartzo-feldspathic p - high g Si,, Na,, K, Al Layered vesuvianite-diopside-grossular carbonate-silicate rock Metamorphism - general Types of Protoliths Lump common types of sedimentary and igneous rocks into six chemically based-groups 1 Ultramafic - very high Mg, 1. Mg Fe, Fe Ni, Ni Cr 2. Mafic - high g Fe,, Mg, g, and Ca 3. Shales (pelitic) - high Al, K, Si g Ca,, Mg, g, CO2 4. Carbonates- high 5. Quartz - nearly pure SiO2. 6. Quartzo-feldspathic p - high g Si,, Na,, K, Al Granitic gneiss Metamorphism - general Example: Orogenic Regional Metamorphism of the Scottish Highlands George g Barrow (1893, ( , 1912) - SE Highlands of Scotland Caledonian orogeny ~ 500 Ma • Regional g metamorphic p map of Scottish Highlands, showing zones of minerals that develop with increasing metamorphic grade. • Barrow studied both intercalated pelitic and mafic rocks Image: Winter (2001) Metamorphism - general Example: Orogenic Regional Metamorphism of the Scottish Highlands Subdivide area into metamorphic zones within metapelites, based on appearance of new mineral as metamorphic grade increased. Sequence of zones and typical metamorphic mineral assemblage g are: • Chlorite zone. Pelitic rocks are slates or phyllites • chlorite, chlorite muscovite, muscovite quartz and albite Image: Winter (2001) Metamorphism - general Example: Orogenic Regional Metamorphism of the Scottish Highlands • Bi Biotite i zone. Slates Sl give i way to phyllites and schists • biotite, chlorite, muscovite, quartz and albite quartz, • Garnet zone. Schists • conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase g Image: Winter (2001) Metamorphism - general Example: Orogenic Regional Metamorphism of the Scottish Highlands • Staurolite zone. Schists • staurolite, biotite, muscovite, quartz, garnet, and plagioclase. Some chlorite may persist • Kyanite zone. Schists • kyanite, y , biotite,, muscovite,, quartz, plagioclase, and usually garnet and staurolite zone. Schists and • Sillimanite zone gneisses • sillimanite, biotite, muscovite, qquartz,, plagioclase, p g , garnet, g , and perhaps staurolite. Image: Winter (2001) Metamorphism - general Example: Orogenic Regional Metamorphism of the Scottish Highlands • An isograd represents first appearance of particular metamorphic index mineral in field as one progresses up metamorphic hi grade (Barrow’s definition) • When one crosses an isograd, g , such as the biotite isograd, one enters the biotite zone • Zones thus have same name as isograd that forms low-grade boundary of that zone Image: Winter (2001) • Because classic isograds are based on first appearance of a mineral, and not its disappearance, an index mineral may still be stable in higher grade zones Metamorphism - general Example: Orogenic Regional Metamorphism of the Scottish Highlands A variation occurs in the area just j to the north of Barrow’s, in the Banff and Buchan district Image: Winter (2001) Metamorphism - general Example: Paired metamorphic belts - Japan NW belt ("inner" belt, inward,, or awayy from trench)) is Ryoke-Abukuma Belt • Low P/T Buchan-type yp of regional orogenic metamorphism • Dominant meta-pelitic sediments, and isograds up to the sillimanite zone have been mapped • High-temperature-low-pressure belt, and granitic plutons are common Image: Winter (2001) Metamorphism - general Example: Paired metamorphic belts - Japan Outer belt, called the Sanbagawa g Belt • It is high-P/low-T • Only reaches garnet zone in pelitic rocks • Basic rocks are more common than in Ryoke belt, however, and in these glaucophane may be developed • Rocks are commonly called blueschists Image: Winter (2001) Metamorphism - general Example: Paired metamorphic belts - Japan suggests that h 600oC isotherm, for example, could be as deep as 100 km in the trench-subduction zone area, and as shallow as 20 km beneath volcanic arc Image: Winter (2001) Metamorphism - general Examples: Paired metamorphic belts - Japan Miyashiro (1961, 1973) suggested d that coeval occurrence • metamorphic belts, an outer, high-P/T belt, and an inner, lower-P/T belt • ought to be common occurrence in subduction zones, either modern or ancient Image: Winter (2001)