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PlutonismandIntrusiveIgneousRocksnotesfromthetextbook,integratedwithoriginalcontributions

AlessandroGrippo,Ph.D.

Granite,anintrusiveigneousrock,intheSierraNevadaTuolumneMeadows,YosemiteNationalPark,California

©AlessandroGrippo

review

•  Therockcycle•  TherockcycleandPlateTectonics

Magma

•  Anyrockcanbemeltedundertherightconditionofpressureandtemperature

•  Thatmeltiscalledmagma

•  Magmacancontainsolid,liquid,andgaseousmaterials

Magmavs.Lava

•  Whenmagmasolidifies,itformsigneousrocks

–  IfmagmasolidifiesslowlywithinEarth’ssurfaceitgeneratesintrusiveigneousrocks(e.g.,granite)

–  IfmagmasolidifiesquicklyatEarth’ssurfaceitiscalledlava,anditgeneratesextrusiveigneousrocks(e.g.,basalt)

Howdoweclassifyigneousrocks?

•  Weusecompositionandtextureoftherocks

•  ThechemicalCompositionrefersmainlytotheirironandsilicacontent

•  Texturereferstothesize,shape,andarrangementofmineralgrains(crystals,inthecaseofigneousrocks)

•  Texturalclassification–  Plutonicorintrusiverocks(gabbro-diorite-granite)arecoarse-

grainedandcooledslowlyatdepth–  Grainsarevisibletothenakedeye–  Twotextures:pegmatitic,phaneritic

–  Volcanicorextrusiverocks(basalt-andesite-rhyolite)aretypicallyfine-grainedandcooledrapidlyattheEarth’ssurface

–  Grainsarenotvisibletothenakedeye,orabsent(e.g.obsidian,avolcanicglass)–  Sixtextures:porphyritic,aphanitic,glassy,vesicular,frothy,pyroclastic

•  Compositionalclassification–  Maficrocks(gabbro-basalt)containabundantdark-colored

ferromagnesianminerals–  Intermediaterocks(diorite-andesite)containroughlyequalamounts

ofdark-andlight-coloredminerals–  Felsicrocks(granite-rhyolite)containabundantlight-colored

minerals

Composition•  Rockchemistry,particularlysilica(SiO2)content,determines

mineralcontentandgeneralcolorofigneousrocks–  Ultramaficrockshave<45%silica,byweight,andarecomposed

almost entirelyofdark-coloredferromagnesianminerals•  Mostcommonultramaficrockisperidotite(intrusive)

–  Maficrockshave~50%silica,byweight,andcontaindark-coloredmineralsthatareabundantiniron,magnesiumandcalcium•  Intrusive/extrusivemaficrocks-gabbro/basalt

–  Intermediaterockshavesilicacontentsbetweenthoseofmaficandfelsicrocks•  Intrusive/extrusiveintermediaterocks-diorite/andesite

–  Felsic(silicic)rockshave>65%silica,byweight,andcontainlight-coloredmineralsthatareabundantinsilica,aluminum,sodiumandpotassium•  Intrusive/extrusivefelsicrocks-granite/rhyolite

EightTextures•  Pegmatitic

–  Visiblecrystalsbiggerthan1cminsize•  Phaneritic(Coarse-grained)

–  Visiblecrystalssmallerthan1cminsize(mostcommon)•  Porphyritic

–  Twocrystalsizestogetherinthesamerock•  Aphanitic(Fine-grained)

–  Invisiblecrystals(mostcommon)•  Glassy

–  Nocrystals(e.g.,obsidian)

•  Vesicular–  vesiclesleftintherockbyescapinggases;e.g.,vesicularbasalt

•  Frothy–  avarietyofvesicularthatappliestofelsic,viscousmagmas;e.g.,pumice

•  Pyroclastic,orFragmental–  aftervolcanicexplosions;e.g.,volcanicbrecciaandvolcanictuff

Fig.3.7b

Fig.3.7c

Fig.3.7d

Fig.3.7e

Fig.3.7f

HowMagmaForms•  Heatfrombelow

–  Heatupward(byconductionandconvection)fromtheveryhot(>5000°C)corethroughthemantleandcrust

•  Heatvs.pressure–  Meltingpointofmineralsincreaseswithincreasingpressure–  Inthehottestregionswithintheuppermantleandcrust,pressurecan

belowenoughformeltingtooccur•  Hotwaterunderpressure

–  Waterbecomesincreasinglyreactiveathighertemperatures–  Atsufficientpressuresandtemperatures,highlyreactivewatervapor

canreducethemeltingpointofrocksbyover200°C•  Mineralmixtures

–  Mixturesofminerals,suchasquartzandpotassiumfeldspar,canresultinthemeltingofbothattemperatureshundredsofdegreeslowerthaneithermineralwouldmeltonitsown

Bowen’sReactionSeries

•  Mineralscrystallizeinapredictableorder,overalargetemperaturerange

•  Discontinuousseriesofcrystallization–  Maficminerals(olivine,pyroxene,amphibole,biotite)crystallizein

sequencewithdecreasingtemperature

–  Asonemineralbecomeschemicallyunstableintheremainingmagma,anotherbeginstoform

•  Continuousseriesofcrystallization–  Plagioclasefeldsparformswithachemicalcompositionthatevolves–  (fromCa-richtoNa-rich)withdecreasingtemperature

•  Finalstagesofcrystallization–  Atlowertemperatures,ifthereisstillmagmaleft,coolingwillnot

affectthetwoendtermsoftheseries(biotiteandNa-plagioclase)

–  ItwillformquartzandK-feldspardirectlyoutofmagma

•  Alargevarietyofigneousrocksisproducedbyalargevarietyofmagmacompositions

•  Maficmagmaswillcrystallizeintobasaltorgabbroifearly-formedmineralsarenotremovedfromthemagma

•  Intermediatemagmaswillsimilarlycrystallizeintodioriteorandesiteifmineralsarenotremoved

•  Separationofearly-formedferromagnesianmineralsfromamagmabodyincreasesthesilicacontentoftheremainingmagma

•  Mineralsmeltinthereverseorderofthatinwhichtheycrystallizefromamagma

MagmaEvolution

•  Achangeinthecompositionofamagmabodyisknownasmagmaevolution

•  Magmaevolutioncanoccurbydifferentiation,partialmelting,assimilation,ormagmamixing

Differentiation

•  Differentiationinvolvesthechangingofmagmacompositionbytheremovalofdenserearly-formedferromagnesianmineralsbycrystalsettling

•  Partialmeltingproducesmagmaslessmaficthantheirsourcerocks,becauselowermeltingpointmineralsaremorefelsicincomposition

•  Assimilationoccurswhenahotmagmameltsandincorporatesmorefelsicsurroundingcountryrock

•  Magmamixinginvolvesthemixingofmoreandlessmaficmagmastoproduceoneofintermediatecomposition

IntrusiveRockBodies

•  Intrusiverocksexistinbodiesorstructuresthatpenetrateorcutthroughpre-existingcountryrock

•  Intrusivebodiesaregivennamesbasedontheirsize,shapeandrelationshiptocountryrock

–  Shallowintrusions•  Form<2kmbeneathEarth’ssurface•  Chillandsolidifyfairlyquicklyincool•  countryrock•  Generallycomposedoffine-grainedrocks

–  Deepintrusions:Plutons•  FormatconsiderabledepthbeneathEarth’ssurfacewhenrisingblobs

ofmagma(diapirs)gettrappedwithinthecrust•  Crystallizeslowlyinwarmcountryrock•  Generallycomposedofcoarse-grainedrocks

VolcanicNecks

Shiprock,Farmington,NewMexico

Plutons

SierraNevadabatholith

IgneousActivityandPlateTectonics

•  Igneousactivityoccursprimarilyatorneartectonicplateboundaries

•  Maficigneousrocksarecommonlyformedatdivergentboundaries

–  Increasedheatflowanddecreasedoverburdenpressureproducemaficmagmasfrompartialmeltingoftheasthenosphere

•  Intermediateigneousrocksarecommonlyformedatconvergentboundaries

–  Partialmeltingofbasalticoceaniccrustproducesintermediatemagmas

•  Felsicigneousrocksarecommonlyformedadjacenttoconvergentboundaries

–  Hotrisingmagmacausespartialmeltingofthegraniticcontinentalcrust

•  Intraplatevolcanism–  Risingmantleplumescanproducelocalizedhotspotsandvolcanoes

whentheyproducemagmasthatrisethroughoceanicorcontinentalcrust

–  Hawaiiisanexample

TheEnd