Metamorphic rocks
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Metamorphic Rocks. Metamorphism – the process by which heat, pressure and chemical reactions change the mineral composition and/or structure of any type of preexisting rock without melting it

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Metamorphic Rocks

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Metamorphic rocks

Metamorphic Rocks

  • Metamorphism – the process by which heat, pressure and chemical reactions change the mineral composition and/or structure of any type of preexisting rock without melting it

  • Metamorphic process – occurs at temperatures and pressures > those of lithification (~200C) and < those that melt rock into magma (~800C)


Metamorphic process

Metamorphic Process

  • Heat – > depth > temp = geothermal gradient

    • (~72 degrees/mi)

    • Intrusions - temp diff between intrusion and surrounding rock

  • Pressure - > depth > pressure

    • Lithostatic or confining = from all directions

    • Directed – plate boundaries, folding and faulting

      • Changes texture not mineralogy – the shape and arrangement of minerals in a rock

      • Foliation – arrangement of minerals perpendicular to directed pressure

  • Solutions – liquids and/or gases are catalysts for metamorphic processes

    • Water

    • Water-rich minerals – clays and amphiboles

    • Produce the growth of new minerals

    • Can add or remove ions to change the composition of rocks


Types of metamorphism

Types of metamorphism

  • Contact – Rocks surrounding magmas are altered by heat and circulation of hot fluids

    • < meta the further you get away from contact

    • Hi-temp minerals close/Lo temp minerals farther away

    • Coarse-grained close/finer grained father away

    • < water close >water farther away

    • Temperature gradient between intrusion and country rock

    • Size can vary with size and temp of intrusion


Contact metamorphism

Contact metamorphism

  • Subsurface intrusion of molten material creates zone of metamorphism in the surrounding country rock (Image below)

  • Surface extrusion of lava flow creates metamorphism of underlying soil horizon (Image at right – red layer)


Types of metamorphism cont d

Types of metamorphism (cont’d)

  • Regional – two types, affect large areas of crust

    • Burial – deep sedimentary basins

      • > 6mi depth

      • Mostly sed. Rx

      • No foliation

    • Dynamothermal – plate tectonic boundaries

      • Foliated

      • Convergent plate pressure

      • Thickening


Other types of metamorphism

Other types of metamorphism

  • Hydrothermal – chemical alteration by heated water

    • Mostly at divergent plate boundaries

    • Fringes of magmatic intrusions

    • Hi-grade mineral deposits

  • Fault-zone – mylonites Localized along fault planes

    • Frictional heat and pressure

  • Shock – meteorites

    • Very localized

    • Hi-pressure and temp

    • Minerals occur at no other geologic setting (coesite)


Impact breccias popigai impact crater siberia

Impact BrecciasPopigai impact crater, Siberia

  • Some ejecta "clasts" are as big as a house

  • Megabreccias, similar to those found on the lunar highlands, are not uncommon at terrestrial impact structures


Grade of metamorphism

Grade of Metamorphism

  • Metamorphic grade is a general term for describing the relative temperature and pressure conditions under which metamorphic rocks form.

    • Low-grade metamorphism takes place at temperatures between about 200 to 320oC, and relatively low pressure.  Low grade metamorphic rocks are characterized by an abundance of hydrous minerals (minerals that contain water, H2O, in their crystal structure).

      • Clay minerals

      • Serpentine

      • Chlorite

    • High-grade metamorphism takes place at temperatures greater than 320oC and relatively high pressure.  As grade of metamorphism increases, hydrous minerals become less hydrous, by losing H2O and non-hydrous minerals become more common.

      • Muscovite – hydrous mineral that eventually disappears at high grade

      • Biotite – a hydrous mineral that is very stable at high grade

      • Kyanite – a non-hydrous mineral

      • Garnet – a non-hydrous mineral

      • Pyroxene – a non-hydrous mineral


Metamorphic grade

Metamorphic Grade

  • Slate – low meta because it appears just like the parent rock

  • Phyllite – low to moderate grade as platy texture begins to dominate

  • Schist – moderate to high grade and texture is extremely platy and mineralogy begins to change

  • Gneiss – high grade because mineralogy and texture have changed completely

  • Metamorphic zones

    • Systematic change in mineralogy

    • Change occurs as zones

    • Index minerals


Regional metamorphism and index mineral zones

Regional Metamorphism and Index Mineral Zones


Metamorphism and plate tectonics

Metamorphism and Plate Tectonics


Folded mountain belts appalachians

Folded Mountain BeltsAppalachians


Himalayan vs appalachian

Himalayan vs Appalachian


Classifying metamorphic rocks

Classifying Metamorphic Rocks

  • 2 main classes – foliated and non-foliated

  • Foliated from sedimentary rx

    • Slate – derived from shales

      • Red – hematite, green - chlorite, purple – manganese oxides, black – carbon rich

      • Smooth rock cleavage

    • Phyllites – thin wavy foliation

      • Fine grained – micas, chlorites and graphite

      • Shiny

    • Schist –

      • Coarser grained

      • New minerals

      • Same overall composition

    • Gneiss – light and dark bands

      • Mineral differentiation

      • Coarse grained

      • Different composition


Metamorphic textures

Metamorphic Textures

  • Slate – low meta because it appears very similar to the parent rock


Metamorphic textures1

Metamorphic Textures

  • Phyllitic Texture - This texture is formed by the parallel arrangement of platy minerals, usually micas, that are barely macroscopic (visible to the naked eye).


Metamorphic textures2

Metamorphic Textures

  • Schistose Texture This is a foliated texture resulting from the sub-parallel to parallel orientation of platy minerals such as chlorite or micas.


Metamorphic textures3

Metamorphic Textures

  • Gneissic – alignment of minerals and segregation of light and dark minerals (banding) high grade because mineralogy and texture have changed


Non foliated metamorphic rocks

Non-foliated Metamorphic Rocks

  • Quartzite – formed from sandstones by additional growth of quartz crystals between grains

  • Marble – formed from limestone

  • Anthracite – formed from coal

  • Crystal size related to degree of metamorphism

    • > meta > crystals

    • Often related to heat not burial pressure


Foliated rocks from igneous rocks

Foliated Rocks From Igneous Rocks

  • Greenschist –

    • Chlorite, epidote and amphibole

    • Basalt and gabbro

    • Subduction zones

    • Lo-temp, lo-press

  • Blueschist –

    • Hi-press, low temp

    • Glaucophane

    • Subduction zones only

  • Gneiss –

    • Granites and diorites

    • Coarse grained

    • No dramatic change in composition and texture


Non foliated metamorphic rocks from volcanic rocks

Non-foliated metamorphic rocks from volcanic rocks

  • Serpentinite

    • Basic to ultramafic meta

    • Water-rich environment

    • Beautiful but weak ornamental rock

  • Hornfels

    • Low-hi grade contact meta

    • Of no economic importance

  • Eclogite

    • Low temp & high press


Migmatites

Migmatites

Border metamorphic & igneous textures

Full range between gneiss and granite

Lower temp/lighter minerals

Begin to flow

Quartz & feldspar

Higher temp/darker minerals

Biotites & pyroxenes


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