AnInvestigationofCharacteristicsoftheMagmaChamberRepresentedbytheCapeAnn

PlutonicSuiteAaronBeltzer,DepartmentofGeosciences,UniversityofMassachusetts,Amherst,MA01003

Overthecourseofseveralweeksinthesummerof2015meandmyfieldpartner,PaulSouthard,didareconnaissanceoftheCapeAnnPlutonineasternMassachusetts.Ourgoalwastotestseveralmainquestions.

• HowwelldoestheplutonsgeometryholduptothemodelthatWilliamDennenproducedwhilemappingfrom1972-1975?

• CouldthestyleofintrusionoftheCapeAnnPlutonbesimilartotheMaficandSilicicLayeredIntrusionmodelofWiebeandCollins(1998)?

• Whatabouttheroleofwater–isthereavariationinwatercontentwithinthepluton?• IsthisplutonrelatedtotheCoastalMaineProvince(alongstriketotheN)?

Wechosetomapandperformourfieldworkinseveralwell-known(geologicallyfamous)locationsontheislandofCapeAnn:Lane’sCove,Andrew’sPoint,andRafe’sChasm.AtLane’sCovenumerouscontactsbetweenCapeAnngraniteandK-feldsparquartzsyeniteareexposed.Severallargerenclave-likemaficbodiesconsistofaplagioclase-phyricgroundmasswithlargepurpleplagioclasecrystals(seeFig.1)andglomerocryst“rafts”scatteredthroughout.Thesefieldrelationsprovideevidencefortransportofdeepsourcedmaficmaterialupthroughthedominantlyfelsicpluton.AtAndrew’spoint,severalmilesnortheastofLane’sCove,thereisalargeexposureofsinglefeldspargranite.Thisvariationofgranite(singlefeldspar),alongwithexposuresofpegmatiticbluequartzcrystals,indicatesthatthereisasignificantvariationinwatercontentwithintheplutonitself.FurthertotheSouth,theRafe’sChasmarea’sgranitehostsa2.5mwidecompositedike(describedbyRoss,2014),consistingofaphyricbasalticmarginsandanintermediateinteriorrichinmaficenclavesandK-feldsparphenocrysts.Fieldrelationsareconsistentwithassimilationoffelsicmagmabybasalticmagma,withresorbedK-feldsparphenocrystsprovidingevidenceoftheassimilationevent(Ross,2015).Justeastofthisdike(seeFig.2)isapreviouslyundescribedrockunitthatappearstobeaveryshallow(hypabyssal)intrusiveunitoraroofpendantofrhyolite.Theunit(~20mwide)isporphyritic,witheuhedralresorbedmeltinclusion-bearingquartzphenocrystsandlarge(upto0.5cm)K-feldsparphenocrystsinaveryfine-grainedmatrixinterpretedasdevitrifiedglass(seeFig3).Finally,andperhapsthemostinterestingofallvarietiesofgranitethatwemappedisarapakivigranite(anotherpreviouslyundescribedunit)thatoutcropsatCoolidgePoint(seeFig.4).Bothfullsectionandhigh-resolutionmicroprobemapswereproducedfortheplutonicrocksofCapeAnn.Thescaleandvarietyoftexturesindicativeofmagmainteractionanddisruptionofcumulatelayerssuggestthattheplutonmightbetheresultofrepeatedinfluxesofmaficmagmaintoadominantlyfelsicchamber,consistentwiththeMASLImodelofWiebeandCollins(1998),andsimilartocontemporaneousplutoniccomplexesonthecoastofMaine.

Figure1.Contactbetweenplag-phyricmaficenclaveandhostgranite.Notethelargepurpleplagioclasemegacrystandthesharpcontactbetweenthemaficandfelsicrocks.

Figure2.GeologicSketchMapoftheRafe’sChasmArea(Gloucester),CapeAnn,MA

Figure3.Thinsection(sampleCa-9b)ofRafe’sChasmvolcanicunitdisplayingresorbedmelt-inclusionbearingquartzphenocrystsandlargeK-feldsparphenocrsystsinafinegrainedmatrixinterpretedasdevitrifiedglass.

Figure4.CaKαWDSmapofCoolidgePointrapakivigranitesampleCa-15x.Notethecalciumrichrimssurroundingthemoresodiccoresinthefeldsparphenocrysts.Thesecoresmayhavegrownviadepressurization(unlikelyinthecaseofthisparticularplutonasitlikelyremainedatdepthforasignificantperiodoftime),achangeinwatercontent(likelyafactorbecauseoftheevidenceforwatervariationwithinthepluton),orviaaninfluxofmaficmagma(likelyforthisplutonbecauseoftheobservedevidenceforcontaminationofthefelsicmagmabydeeperlevelmafics).