GEO 303 Lecture Notes - Lecture 9: Facies, Biotite, Schist

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10 May 2016
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Geo Lecture Chapter 9: Metamorphic Rocks
Next lecture: Ch 11
Study the power points, book supports them, test focuses on book
*Evaporates vs. Precipitates: Evap form by thermal evaporation, leave
concentrated ions in mineral. Precipitates involves other process that alters
those crystallization.
Lecture:
Definitions
oRecrystallization of a prior rock (could be all three)
oMetamorphism causes recrystallization
oTemperature and pressure source of change
oAfter metamorphism, they’re a different rock
oMost important thing: NO MELTING (melt=igneous)
Solid state transformation
Processes
oMetamorphism: high pressure process
Digenesis: pressure isn’t enough to turn into a metamorphic
rock
High temp without pressure is possible (volcanoes and dikes)
oIngredients of Metamorphism:
Elevated temperature
Elevated pressure
Shear stress
Tiny amounts of H20
In pore spaces
In crystal framework of hydrous minerals
oOccurs in a closed chemical system
Elements aren’t lost, they are arranged in different ways
oMetamorphic rocks have parents
Composition matters!
Degree at which it occurs matters!
Increasing the amount can further change the mineral
Classification
oContact
In contact with something that produces it, changed mainly by
heat due to contact with magma
The heating and degree of contact metamorphism will
decrease away from the source
Rocks will depend upon
Temp/heating/distance from intrusion
Parent mineralogy
oRegional
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A type of metamorphism that occurs over broad areas-typically
associated with deformation under pressure during an
orogenic event
Convergent stress creates pressure resulting in regional
metamorphism
Rocks may also be buried causing them to increase in
temperature as well
High temp, pressure, shear stress over a large region
Index minerals define metamorphic zones
oDynamic
Occurs in narrow zones associated with faults. High shear
stresses and frictional heating result in highly localized rock
metamorphism
Localized high shear stress at faults and folds
Alignment of crystals=metamorphic foliation/lineation
Deformation of grains
Degree of metamorphism
oGrade
Degree of metamorphism
Relies on index mineral, based on the appearance
Low temps vs. high temps and what’ll form
The same chemistry will produce different minerals depending
upon the range of temperature and pressure
oFacies
Minerals assemblage indicating grade
Set of metamorphic mineral assemblages formed under a
similar range of P and T
Rocks containing these facies can be attributed to specific
metamorphic conditions
oTexture
Non-foliated
Texture is massive crystalline, composed of single
mineral
Ex: marble, quartzite
Foliated:
Type of foliation (slaty cleavage, gneissic layering)
Mineralogy and facies
Ex: slate, schist, gneiss
Foliation that you produce has nothing to do with the
original bedding
oCan also tell us about the degree of
metamorphism
Why foliation?
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oDifferentiated stress in a certain direction
oRounded grains get flattened
oMinerals will crystalize in preferred orientation
Development: starting with a shale with planar bedding
oStress will cause new sheet silicate minerals to
form perpendicular to the direction of stress
oSize of the crystals will increase with degree of
metamorphism, eventually achieving near planar
foliation especially with biotite and muscovite
oHigher metamorphism will create distinct bands
o
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