Download Metamorphism and metamorphic rocks

Metamorphismand
metamorphicrocks
GEOL115
AlexanderLusk
Outline:
•  Metamorphicrocks
–  Defini>onandmajortypesofmetamorphism
–  Rockcycle
•  Metamorphicprocesses
•  Deforma>onanddevelopmentofafolia>on/
schistosity/cleavage
•  Progressive(prograde)metamorphicseries
•  Environmentsofmetamorphism
•  Metamorphicfacies
Metamorphic
rocks
Ifpre-exis>ngrocks(e.g.igneous,
sedimentary,metamorphic)are
buriedwithinintheEarth,new
mineralscangrowinthesolidrock
throughrecrystalliza>onandthe
rockcandeforminresponseto
tectonicforces.
Thisrockmayhavestartedoff
asasedimentofmudorclay,
whichlaterbecomeasedimentary
rocksuchasmudstoneorshale…
Whatmineralsarepresent?
Recalltherockcycle:
Recallwheredifferentrocktypesform:
Typesofmetamorphism
Regional metamorphism: as rocks are
buried deeper and deeper within the earth
they are subject to increasingly higher
pressures and temperatures, which result in
recrystallization. The directed pressures
(stresses) produce planar structures called
foliation.
Contact metamorphism: rocks within the
crust that are intruded by magma are
recrystallized by the magma’s high
temperatures. These rocks are commonly
not deformed (foliated).
Processesinvolved:
1.  Neomineraliza-on–growthofnewminerals
replacingmineralsthatbecomeunstable.
2.  Graingrowth–growthofneworexis>ng
mineralgrains.
3.  Deforma-on–createsaschistosityor
folia2ondefinedbytheorienta>onofplaty
orelongatemineralgrains.
Neomineraliza>on
Porphyroblasts:
Porphyroclasts:
Graingrowth:
Exis>ngcrystalsgrow:
Quartz
overgrowths
Original quartz
grain
Rockdeforma>on:
Cleavageandschistosity
Folia-on–repe>>ve
layeringandparallel
alignmentofplaty
(flat)minerals,suchas
micas,byshearing
and/ordifferen>al
pressure
ExamplesofFolia>on
Slatyrockcleavage–low
grademetamorphism
ExamplesofFolia>on
Phylli-ctexture–intermediategrademetamorphism
ExamplesofFolia>on
Schistosity–intermediatetohighgrade
metamorphism
ExamplesofFolia>on
Gneissicbanding–highgrademetamorphism
MetamorphicTextures
Environmentsofmetamorphism:
1. Subduc(onzones
2. Con(nentalcollisionzones
3. Magma(cenvironments
TectonicseZngsofmetamorphism
Rockcycleinsubduc>onzonecontext
contact
metamorphism
regional
metamorphism
Subduc>onzones:
•  Sedimentaryandvolcanicrocksfromtheoceanfloor
carrieddownsubduc>onzonesexperience
substan>alincreaseinpressure,modestincreasein
temperature.
•  Rocksarecommonlybrokenanddisrupted:tectonic
melange
•  Highpressure/lowtemperaturemetamorphism:
–blueschistfacies
–eclogitefacies
Tectonicmelange–Californiacoastranges
Microscopicviewofblueschist–
CatalinaIslands
Collisionalenvironments
•  Wherecrustisthickenedbycon>nentalcollision
rocksbecomeburiedandheated;maystartto
melt.Regionalmetamorphism
•  Rocksbecomeduc>leandstarttoflow
•  Sedimentaryrocksareconvertedintoslateor
schist;
–abundantwell-orientedmica
–porphyroblastsofmineralslikegarnet
•  Ifrocksstarttomelt,producegneiss.
–abundantquartzandfeldspar,strongfolia>on
Magma>cenvironments
•  Largeintrusions(plutons)riseintothecrust,heat
surroundingrocks.
•  Temperatureincreases,butpressuredoesnot.Contact
metamorphism
•  Li^leornodeforma>on
•  Neomineraliza2on,butnofabrics
–hornfels
•  Magma>cenvironmentsinclude:
–  mid-oceanridges
–  plutonsrelatedtosubduc>onzonemagma>sm
–  plutonsrelatedtocollision-relatedmel>ng.
Zoneofcontactmetamorphism
Contact metamorphic rocks in the roof of a granite pluton, Sierra
Nevada batholith
MetamorphicFacies
Mineralassemblagesindica2veofacertain
rangeofP-Tcondi2ons
Usingmineralstogetarock’shistory
Garnetasataperecorder
Green“island”:1Gagarnet
growth.
White:450Magarnetgrowth
aeerearliergarnetcrackedup.
Red:360Ma
garnetgrowth.
Every>methatAfricaandNorthAmericacollidedthisgarnetgrewandrecordedtheP-T
condi>ons.
Andalusitereplacedby
kyaniteandstaurolite.
KyaniteisthehighP
formofAndalusite.
Kyaniteisbeing
deformed.Thisrock
recordsburialand
deforma>onofthe
rock.