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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.