Metamorphic rocks

• High T & P change in solid state
• Examples

– Limestone à marble (still calcite)
– Shale (very fine grained) grows large micas

• Three major causes:

– Internal heat of earth (includes volcanism)
– Weight of overlying rock: pressure
– Horizontal pressure from tectonism

• Basic divisions

– High and low grade metamorphism

• General trend is low à high
• Retrograde metamorphism is high à low
 

Physical and chemical factors

• Temperature

– Heat breaks chemical bonds
– Recrystallization and formation of new minerals
– New minerals may have specific orientation
– New minerals useful as geothermometers

• Pressure

– Confining pressure: surrounds rock uniformly
– Directed pressure: exerted in specific direction
– Orientation of mineral commonly perpendicular to directed pressure

• E.g. micas flat sides perpendicular to pressure

– New mineral useful as geobarometers

• Chemical changes

– Introduction or removal of chemical components
– Examples:

• Hydrothermal addition from magma, called metasomatism, add Pb, Zn, Cu
• Loss of volatiles, e.g. CO2 from carbonate rocks
• Water very important, although minor component

Kinds of Metamorphism

• Several types, depend on circumstances

– Regional
– Contact
– Deformation
– Hydrothermal
– Burial

• Regional Metamorphism

– Widespread geographically
– High T and/or P over large areas
– Moderate T and high P at subduction zones
– High T and low P at volcanic arcs
– High T and high P at base of high mountains

• Contact metamorphism

– Thin region surrounding intruded magma
– Largely high T
– Contact metamorphism of extrusive magmas is very thin
– Occurs  near igneous locations – mid-ocean ridges,  convergent margins, hot spots

• Deformation Metamorphism

– Occurs where faults break rocks
– Called cataclastic metamorphism
– Mylonite: type of rock from cataclastic
– Generally very fine grained
 

• Hydrotherml metamorphism

– Commonly associated with mid-ocean ridges
– Change in chemical composition because of reactions with water and basalt

• Burial metamorphism

– Changes in sediments following deep burial
– Beyond the 250º C cut-off for diagenesis

Metamorphic Textures

• Size, shape and orientation of new minerals

– Very distinctive in metamorphic rocks
– Commonly used for classification
– Major distinction:

• Foliated or not foliated

• Foliation

– Flat or wavy planes in the rocks
– May parallel bedding, or be at an angle
– Commonly caused by lining up of platy minerals (e.g. micas) or long minerals (e.g. amphiboles)
– They line up perpendicular to main direction of force

• Called “preferred orientation”
 
 

• Slate: common example of foliation

– Foliation called “cleavage”
– Differs from mineral cleavage
– Caused by breaking of rock along planes of weakness
– Uses: roof tiles, billiard tables, blackboards
 

Foliated Rocks

• How foliated rocks are classified:

– Nature of foliation (how good?)
– Size of crystals (how big?)
– Segregation into light and dark bands (layering)
– Metamorphic grade (how high T and P?)

• Slate

– Lowest grade
– Small grain size, indistinguishable with naked eye
– Originate from shales (also basalts)

• Phyllite

– Similar to slates, but slight higher grade metamorphism
– Slightly larger crystals
– Glossy sheen on surface from large minerals
– Still good cleavage, but wavy surfaces
 

• Schist

– Platy minerals now large enough to see
– Coarse wavy foliation, called “schistosity”
– Common type minerals

 

• Gneiss

– High grade metamorphism
– Now can have large grain sizes
– More granular than platy minerals
– Separated into different banded layer, light and dark minerals (foliation)
• Layers of light quartz and dark feldspars
– Not easily split
 

Non-foliated Rocks

• Composed mostly of equant minerals (same lengths in all dimensions)

– E.g. cubes & spheres

• Result from contact, regional, hydrothermal, or burial (equal confining pressure)
• Most defined on their chemical composition

– Exception is hornfels: from high T contact metamorphism.
– No directed stress

• Quartzite

– Very hard rocks
– Nearly all quartz

• Marbles

– Recrystallized limestone or dolomite from high T and P
– Example: Carrara marble, Italian sculptors

• Argillite

– Low grade metamorphism of mudstones
– Insufficient P and T to arrange platy minerals
– Large percentage of other rounded grains
 

• Greenstones

– From volcanic rocks
– Contain much chlorite (green clay mineral)
– Result from reactions with water (hydrothermal)
– Much of oceanic crust is greenstone

• Amphibolite

– Medium to high grade metamorphism of mafic volcanics
– Mostly amphiboles, some plagioclase feldspars

• Granulite

– Highest grade metamorphims
– Almost to the melting stage
– Medium to coarse grained, equant grains

• Pophyroblasts

– Very large crystals in metamorphic rocks
– Grow quicker than new “matrix” minerals
– Change composition of matrix
– Compare with phenocrysts (large crystals in igneous rocks)

Regional Metamorphism & Metamorphic Grade

• Fine distinction of extent of metamorphism

– Beyond “high” and “low”
– Based on mineral composition of metamorphic rocks
– Complication

• New minerals depend on precursor rock
• E.g. shale and basalt generate very different suites of minerals

• Isograds: zones of levels of metamorphism

– Defined on basis of index minerals
– Index minerals: characteristic minerals that form under specific P and T

• For example presence of chlorite separates slates from schists

– Separate regions separated on maps by lines called isograds

• Metamorphic facies

– Way to group metamorphic rocks according to

• Precursor rock
• Level of P and T
 
 

Contact Metamorphic zones

• Contact aureoles

– The metamorphic grade in contact metamorphic zones
– Decrease away from intrusion
– Depend on the composition of the rock being intruded

• Link to tectonics

– Blueschist: high P, low T of subduction zones
– Greenschist: moderate P and T of ocean crust
– Hornfels: high T & low P of island arcs

• Regional metamorphic facies

– Result from collisions and mountain building
– Deep burial, high pressure