Metamorphic rocks from the middle of the crust Quad Creek area, MT. Image: Darrell Henry

Transcription

1 Introduction to Metamorphism (Chapter 21) Metamorphic rocks from the middle of the crust Quad Creek area, MT. Image: Darrell Henry

2 IUGS-SCMR SCMR definition n of metamorphism Folded marble in the Campolungo Regions of the Alps. Image source: Darrell Henry 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 surfaces of planets and in zones of cementation and diagenesis below this surface. They may coexist with partial melting.

3 Limits it of metamorphism Low-temperature limit grades into diagenesis Boundary is somewhat arbitrary Diagenetic/weathering processes - indistinguishable from metamorphic Metamorphism begins in range of C for more unstable types of protolith Some zeolites considered diagenetic and others metamorphic

4 Limits it of metamorphism High-temperature limit grades into mel;ting 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. Migmatites ("mixed rocks") are gradational

5 Metamorphic agents and change Metamorphic grade: general increase in degree of metamorphism without specifying exact relationship between temperature and pressure Temperature: typically most important factor Estimated ranges of oceanic and continental steady-state geotherms to depth of 100 km using upper and lower limits based on heat flows measured near surface.

6 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, and fine aggregates g coalesce to larger grains

7 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

8 Increasing 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 principal p stress σ3 - minimum principal stress

9 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

10 Increasing Temperature and Pressure "Normal" gradients - may be perturbed in several ways, typically: High T/P geotherms - in areas of plutonic activity ii or rifting ifi Low T/P geotherms - in subduction zones Metamorphic field gradients (estimated P T conditions along Metamorphic field gradients (estimated P-T conditions along surface traverses directly up metamorphic grade) for several metamorphic areas.

11 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 P fluid 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)

12 Types of Metamorphism Classification of metamorphism based on setting (1) () Contact Metamorphism Temperature distribution within 1-km thick vertical dike and in country rocks (initially at 0 ο C) as function of time. Curves are labeled in years. The model assumes an initial intrusion temperature of 1200 o C and cooling by conduction only.

13 Types of Metamorphism 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 Occur over wide range of pressures, including very low P Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. Contact metamorphic aureole

14 Types of Metamorphism Classification of metamorphism based on setting Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. (1) Contact Metamorphism Contact metamorphic aureole Size and shape of aureole controlled by: Size of pluton Shape of pluton Orientation of pluton Depth and metamorphic grade prior to intrusion Temperature Composition - mafic magmas are hotter than silicic magmas Fluids and permeability

15 Types of Metamorphism Classification of metamorphism based on setting Spotted phyllite in which small porphyroblasts of cordierite develop in a preexisting phyllite. (1) Contact Metamorphism Contact metamorphic aureole Most easily recognized where pluton is introduced into shallow rocks in static environment Rocks near pluton are often high-grade h rocks with isotropic i fabric: hornfelses (or granofelses) - relict textures and structures are common Polymetamorphic rocks are common, usually representing orogenic event followed by contact one

16 Types of Metamorphism 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 metamorphism Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK.

17 Types of Metamorphism Contact metamorphism example: Skiddaw Pluton, UK Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. Intermediate zone: slates more thoroughly recrystallized, contain biotite + muscovite + cordierite + andalusite + quartz Cordierite-andalusite slate from intermediate zone of Skiddaw aureole.

18 Types of Metamorphism Contact metamorphism example: Skiddaw Pluton, UK Geologic Map and cross-section of the area around the Skiddaw granite, Lake District, UK. Inner zone: Thoroughly recrystallized Lose foliation Andalusite-cordierite schist from inner zone of Skiddaw aureole. Note the chiastolite cross in andalusite

19 Types of Metamorphism Classification of metamorphism based on setting (2) Regional Metamorphism: metamorphism that affects large body of rock, and thus covers great lateral extent Three principal i types: Orogenic metamorphism Burial metamorphism Ocean-floor metamorphism

20 Types of Metamorphism 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 geothermal gradients and deformation (deviatoric stress) Foliated rocks are a characteristic product

21 Types of Metamorphism Regional Metamorphism Orogenic Metamorphism Metamorphism 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 iti l b t d Transitional cases can be termed regional-contact metamorphism

22 Types of Metamorphism Regional Metamorphism Burial metamorphism = low-grade metamorphism in sedimentary basins due to burial. Southland Syncline in New Zealand: thick pile (> 10 km) of Mesozoic volcaniclastics Mild deformation and no igneous intrusions discovered Fine-grained, high-temperature phases, glassy ash: very susceptible to metamorphic alteration Metamorphic effects attributed to increased pressure and temperature due to burial

23 Types of Metamorphism 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 ti

24 Types of Metamorphism Regional Metamorphism Burial metamorphism = low-grade metamorphism in sedimentary basins due to burial. Burial metamorphism occurs in areas that have not experienced significant deformation or orogeny Restricted to large, relatively l undisturbed d sedimentary piles away from active plate margins e.g. The Gulf of Mexico? Well into metamorphic range, and the weight of the overlying sediments sufficient to impart a foliation at depth

25 Types of Metamorphism Regional Metamorphism Ocean-Floor Metamorphism 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 of hydrothermal r metamorphism Highly altered chlorite-quartz rocks - distinctive high-mg, low-ca composition

26 Types of Metamorphism Impact Metamorphism (dynamic) Impact metamorphism ("shock metamorphism") occurs at meteorite impact craters dominant type of metamorphism on Moon.

27 Types of Metamorphism Fault-Zone Metamorphism (dynamic) primarily

28 Progressive Nature of Metamorphism Prograde metamorphism: changes in rock that accompany increasing metamorphic grade High grade metamorphic rocks progress through sequence of mineral assemblages rather than hopping directly from an unmetamorphosed rock to the current metamorphic rock Prograde reactions are endothermic and driven by increasing T 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

29 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 At what point on its cyclic P-T-t path did its present mineral assemblage last equilibrate?

30 Types of Protoliths Chromite-bearing serpentinite Garnet amphibolite Lump 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

31 Types of Protoliths Lump common types of sedimentary and igneous rocks into six chemically based-groups 1. Ultramafic - very high Mg, Fe, Ni, Cr Garnet-staurolite schist 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 Layered vesuvianite-diopside-grossular carbonate-silicate rock

32 Types of Protoliths Lump 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 Granitic gneiss

33 Example: Orogenic Regional Metamorphism of the Scottish Highlands George Barrow (1893, 1912) - SE Highlands of Scotland - Caledonian orogeny ~ 500 Ma Regional metamorphic map of Scottish Highlands, showing zones of minerals that develop with increasing metamorphic grade. Barrow studied both intercalated pelitic and mafic rocks

34 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 are: Chlorite zone. Pelitic rocks are slates or phyllites chlorite, muscovite, quartz and albite

35 Example: Orogenic Regional Metamorphism of the Scottish Highlands Biotite i zone. Slates give way to phyllites and schists biotite, chlorite, muscovite, quartz, and albite Garnet zone. Schists conspicuous red almandine garnet, usually with biotite, chlorite, muscovite, quartz, and albite or oligoclase

36 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, biotite, muscovite, quartz, plagioclase, and usually garnet and staurolite Sillimanite zone. Schists and gneisses sillimanite, biotite, muscovite, quartz,,plagioclase, garnet, and perhaps staurolite.

37 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 grade (Barrow s definition) When one crosses an isograd, 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 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

38 Example: Orogenic Regional Metamorphism of the Scottish Highlands A variation occurs in the area just to the north of Barrow s, in the Banff and Buchan district

39 Example: Paired metamorphic belts - Japan NW belt ("inner" belt, inward, or away from trench) is Ryoke-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 High-temperature-low-pressure belt, and granitic plutons are common

40 Example: Paired metamorphic belts - Japan Outer belt, called the Sanbagawa 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

41 Example: Paired metamorphic belts - Japan suggests that 600 o C 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

42 Examples: Paired metamorphic belts - Japan Miyashiro (1961, 1973) suggested 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