Unravelling the internal architecture of the Alnö alkaline

35 Pages·2017·22.7 MB·English

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Unravelling the internal architecture of the Alnö alkaline and
carbonatite complex (central Sweden) using 3D models of gravity
and magnetic data
MagnusAnderssonandAlirezaMalehmir
DepartmentofEarthSciences,UppsalaUniversity,Villavägen16,SE-75236,Uppsala,Sweden.
Correspondenceto:MagnusAndersson([email protected])
Abstract. The Alnö complex in central Sweden is one of the largest alkaline and carbonatite ring-shaped intrusions in the
world. Presented here is the 3D inversion of ground gravity and aeromagnetic data that confirms some of the previous ideas
aboutthe3Dgeometryofthecomplexbutalsosuggeststhatthecomplexmaycontinuelaterallyfurthertonorththanearlier
expected. The gravity and aeromagnetic data show the complex as (i) a strong positiver Bouguer anomaly, around 20 mGal,
5 one of the strongest gravity gradients observed in Sweden, and (ii) a strong positive magnetic anomaly, exceeding 2000 nT.
Magnetic structures are clearly discernible within the complex and surrounding area. Petrophysical measurements (density,
bulk magnetic susceptibility, and magnetic remanence) were used to constrain the 3D inversion. Both gravity and magnetic
inversion models suggest that dense (> 2850 kg/m3) and magnetic (> 0.05 SI) rocks extend down to about 3.5–4 km depth.
Previousstudieshavesuggestedasolidifiedmagmareservoiratthisapproximatedepth.Theinversionmodelsfurthersuggest
10 thattwoapparentlyseparateregionswithintheintrusionwithgravityandmagnetichighsarelikelyconnectedatdepth,starting
from800–1000m,implyingacommonsourcefortherocksobservedinthesetworegions.Themodellingoftheaeromagnetic
data indicates that a more than 3 km wide ring-shaped magnetic high in the bay that can be a hidden part of the complex,
linkingasatelliteintrusioninSöråkeronthenorthernsideofthebaytothemainintrusionontheAlnöIsland.Whiletherimof
theringmustconsistofhighlysusceptiblerockstosupportthemagneticanomaly,thecentrehasarelativelylowmagnetisation
15 andisprobablymadeupoflow-susceptiblewall-rocksormetasomatisedwall-rocksdowntoabout2km.Belowthisdepththe
3Dsusceptibilitymodelshowshighermagneticsusceptibilityvalues.Fromtheseobservationsthesolidifiedmagmachamber
isinterpretedtoextendfurthertonorththanhaspreviouslybeensuggested.
1 Introduction
Althoughalkalineandcarbonatiterocksmakeupasmallportionofthecrust’svolume,theyarefoundinallcontinents,various
20 geologicalsettingsandemplacedduringvariousgeologicalperiods,fromArchaeantothepresent-day(TreimanandSchedl,
1983; Le Bas, 1987; Berger et al., 2009). The only carbonatite volcano that have been recorded to be active in the modern
timeistheOldoinyoLengailocatedintheEastAfricanRiftinnorthernTanzania(e.g.,Dawson,1962;Dawsonetal.,1990;
WoolleyandChurch,2005;MattssonandVuorinen,2008).TheAlnöcomplex,locatedtothesouthofKlingefjärdenbayon
thenorthernpartofAlnöIsland(Figure1),wasoneofthefirstcarbonatiteandalkalinelocalitiestobedescribedandhassince
1
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thenbeenextensivelystudied(e.g.,Högbom,1895;vonEckermann,1948;Kresten,1980,1990;Vuorinen,2005;Meertetal.,
2007;Skeltonetal.,2007).Despiteitsuniquegeologyandglobalsignificance,theprojectionofsurfacegeologicaldatahave
untilrecentlybeenthemainsourceofinformationtoshedlightonthedeeperstructures(e.g.,vonEckermann,1948;Kresten,
1980, 1990; Mattsson et al., 2014). Kresten (1980) suggested the presence of a shallow magma chamber on the basis of 2D
5 projections using measurements of dip and dip direction of alkaline and carbonatite sheet intrusions and concluded that the
Alnö intrusion was emplaced in two stages. Boulders with carbonatite breccia and accretionary lapilli fragments have been
found at Sälskär skerries (von Eckermann, 1960), and was suggested by Kresten (1990) to relate to surface or near-surface
volcanicactivity.
The complex is characterised by a strong gravity (> 20 mGal) and magnetic high on the northern part of the Alnö Island.
10 Howevergeophysicalanomaliesarealsoobservedatitsimmediatemarginstothenorth,onsomesmallerislandsandonthe
northern side of the Klingefjärden bay. The bay also features a ring-shaped magnetic structure (Figure 2b). There is still no
concreteevidence(e.g.,fromdeepboreholes)connectingthestructuresobservedontheAlnöIslandwiththemagneticring-
shaped structure in the bay. Using a combination of reflection seismic data, surface geological studies and the presence of
largeaccretionarylapilliblocksfoundinSälskärskerries(Figure1)Anderssonetal.(2013)alsosuggestedthepossibilityof
15 a shallow fossil magma chamber at around 3–4 km depth, indicated by an anomalous region of low and diffuse reflectivity
onstacked2Dreflectionseismicdata.Acaldera-stylevolcanismwasinterpretedtoformthecomplexwithastrongexplosive
volcanismoccurringattheendofthemagmaemplacement,likelysomewhereinthebaywherethemagneticring-structureis
observed(Figure2b).
Aspartofaninterdisciplinarygeophysicalstudyinvolvingreflectionseismics(Anderssonetal.,2013),anisotropyofmag-
20 neticsusceptibility(Anderssonetal.,2016)andpotentialfieldmethods(thisstudy),newsurfacegravityandmagneticdatawere
collectedtoprovideinformationaboutthedeeperinteriorofthecomplexandwiththeaimtotestifashallowmagma-chamber
can be verified independently by 3D modelling of potential field data over the complex. We propose that by estimating the
thicknessoftheintrusionitispossibletoindirectlyinferthelocationofthemagmachamber.Tothebestofourknowledge,the
currentstudyandrecentstudiesconductedovertheAlnöcomplexarethemostdetailedgeophysicalinvestigationsconducted
25 onanycarbonatitecomplex.Thisillustratesthepotentialofanintegratedgeological,petrophysicalandgeophysicalapproach
tobetterunderstandtheinternalarchitectureofsuchintrusions.TheAlnöintrusiononthemainislandandtheinferredvolcanic
activity in the bay would put Alnö among the few unique geological sites where intrusive and extrusive activities have been
recordedatthesamelocality(cf.WoolleyandChurch,2005).
Significant physical property contrasts (e.g., density or susceptibility) must exist between adjacent lithologies in order to
30 delineate them using gravity and magnetic modelling methods. These criteria, as illustrated later in this paper, exist for the
Alnörocktypesjustifyingtheapplicationofhigh-resolutionmodellingtoinvestigatethevariouslithologicalentitiesforming
thecomplex.Successfulcasestudies(cf.Oldenburgetal.,1997;DutraandMarangoni,2009)exist,whichshowtheinversion
of potential field data (e.g., gravity and magnetic) in many different geological settings. Inversion can also be conducted in
conjunctionwithreflectionseismicdatainordertoreducetheambiguitywheninterpretingtheresults(e.g.,RoyandClowes,
35 2000;Malehmiretal.,2009;Hedinetal.,2013).AlthoughAnderssonetal.(2013)used2.5Dforwardmodellingofthegravity
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(cid:13)
Easting (km)
622 623 624 625 626 627 628 629 630
3 3
3 3
9 9
6 6
Söråker
2 43 2
3 3
69 50 37 69
43
30 Klingefjärden bay
31 25 31
69 52 69
21
51
0 0
93 Alnö1 93
Northing (km) 69236924692569266927692869296 60˚6F35˚0e1700n˚˚ Aln10˚ö3240˚20˚ 30˚AAK3ll04nn"o"""""˚"0"""""""˚l"öö""""a""""""""""""""""""""""""""""""""""""""22""""""""""""""7""""""""""""0""""""""""""""˚""""""""""""""""""""""6"""""""""""""5""""""""""""""˚"""""""""""4"""""""""""""6""0""""""""""""0"˚""""""""""""˚"""""""6""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""AA""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""ll""""""""""""""""""""""nn"""""""""""""""Ss""""""""""""""""""""""""""""""""""""""""""""""""""""""""k"""öö""""""""ä""""""""""""""""""""""""""""""""e""""""""""""""""""""""""""""""""""""""""""l"""33"""""""""""s"""""""""""""r"""""""""""""""""""""""""""""""""""""""r"""k"""""""""""""""""i""""""""""""""eä""""""""""""""""""""""""""""""""sr""""""""""""""""""" """""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""AA""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""9""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""ll"""""""""""""""""""""""nn"""""""""""""""""""""""""""""""""""""öö""""""""""""""""""""""""""""""""""""""""""""""""22"""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""Hh"""""""""""""""""""""""""""""o"""""""""ö""""""""""""""""""""l""""""""r"""m"""""""""""""""""""""""n""""""""""""""""""""""""""""""""""""""""""""""""""""""""""""ie"""""n""""AA""""n"""""g"""""""""""""ll"s""""""""nn"""""""""-""""""""""""""AA"öö""""""""""""""""33""B"ll"""""""nn"""""""""""o"""""""""""""""""öö"""""""""""""""t"""""""""h""1"""11""""""""n3"""""""""""""i""""""a""""""3"""""n""""0"""""""1"" """"""S"""8"""""""""""""""e"""""""""""""a 69236924692569266927692869296
622 623 624 625 626 627 628 629 630
Rock sampling
Fenite Carbonatite Pyroxenite Granite
site
Ijolite Nsyeepnhiteeline Migmatite """"""""""""""""""""""""""""""""""S(Aelinsöm1i-cA llinnöe3s) 37Water depth
Figure1.GeologicalmapoftheAlnöalkalineandcarbonatitering-complex,centralSweden.Samplesforpetrophysicalmeasurementswere
takenfromrepresentativerocktypesintheintrusionarea.Blacklinesshowavailablereflectionseismicprofiles(Alnö1,2,and3)fromthe
studyarea(Anderssonetal.,2013).TheinsetmapofnorthernEuropeshowsthelocationoftheAlnöcomplex,thecoevalFencomplexin
southernNorwayandtheswarmofalkalineandcarbonatiteintrusionsinnorthernFinlandandnorthwesternRussia.Thegeologicalmapis
kindlyprovidedbytheGeologicalSurveyofSweden.ThecoordinatesystemisSWEREF99TM(UTMzone33N).
datafromthestudyareaalongsidethereflectionseismicprofilestoaidintheinterpretation,itisclearthatthecomplexshapeof
thelithologicalunitsandtheir3Dgeometry,bothatthesurfaceandmostlikelyatdepth,justifyafull3Dmodelling.Therefore,
3
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Easting (km)
(a) 616 620 624 628 634
6936 6936
AArreeaa ffoorr
iinnvveerrssiioonn
6932 6932
KKlliinnggeeffjjäärrddeenn
bbaayy
m)6928 6928
ng (k AAllnnöö
hi ccoommpplleexx
Nort6924 6924
6920 6920
6916 6916
616 620 624 628 634
-35-32 -28 -25 -22 -19 -16 -13 -10 -7 mGal
Easting (km)
(b) 616 620 624 628 634
6936 6936
MMaaggnneettiicc rriinngg--
6932 ssttrruuccttuurree 6932
m)6928 6928
g (k
Northin6924 AcAcoollnnmmööpp lleexx 6924
6920 6920
AAlluummiinniiuumm
ssmmeelltteerr RRööddöönn
rraappaakkiivvii
6916 iinnttrruussiioonn 6916
616 620 624 628 634
50578 506065062650646 50668507035077050971 nT
Figure2.Gravitydata.(a)BougueranomalymapoftheAlnöarea.“Plus”signsindicatethelocationsofthegravitystations.Thegravity
stationsareabout100mapartinsideandabout1kmapartoutsidetheintrusionarea.(b)Aeromagneticmapofthearea,theflightlinesrun
north-southwith200mdistancebetweenprofiles,atanominalflight-heightof60m.Themagneticanomalyinthesouth-easterncorner
ofthemapisrelatedtoanolderrapakivigraniteintrusionoutcroppingatRödönIsland.Asemi-circularanomalywithabout2kmextent
(low-magneticinsidehigh-magneticarea).Theanomalyinthesouthernpartofthemapisrelatedtoanaluminiumsmelter.Inbothmaps
watersurfacesareindicatedwithpalecolour.Therectanglewithadashedlineshowstheareaofinterest,thedataforinversionwereextracted
fromthisarea.
the 3D modelling of gravity and magnetic data from Alnö is the major focus of this study with the main objectives of (1)
estimatingthedepthextentoftheintrusionintheAlnöIslandinordertoindependentlyverifytheinterpretationoftheshallow
fossil magma chamber, (2) studying the petrophysical properties of different geological units on the island and using them
asconstraintsintheinversionandinterpretation,and(3)providinginformationaboutthepotentiallinkbetweentheintrusion
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andthemagneticring-structureinthebay,whichcanbeanimportantpieceofthepuzzleoftheemplacementofthecomplex.
Samplesfromrepresentativerocktypeswerecollectedandtheirpetrophysicalpropertiesmeasured;inthisstudywepresent
density, magnetic susceptibility and natural magnetic remanence (NRM). Gravity and magnetic data were inverted with the
helpofaprioriinformationobtainedfromthepetrophysicalmeasurements.Weshowhowthemodellingresultssuggesttwo
5 separateregionswithintheintrusionthatareassociatedwithgravityandmagnetichighs.Theyarelikelyconnectedatdepthsat
800–1000mbelowthesurface,implyingacommonsourcefortherocksobservedinthesetworegions.Themodellingsuggests
thatamorethan3kmwidering-shapedmagneticanomalyobservedadjacenttothemainintrusion,intheKlingefjärdenbay,
iscausedbyabowl-shapedregionofhighlymagneticmaterial.Thecentreofthebodyishosttoaregionofmaterialswitha
lowmagneticsusceptibilitythatextenddowntoabout2kmdepth.Thehighlymagneticfloorofthe“bowl”maybeconnected
10 withtheonshoreintrusionatthisdepth.
2 Geologicalbackgroundandearlierstudies
2.1 Geologicalsetting
TheAlnöcarbonatitecomplex(Figure1)wasemplacedintoPalaeoproterozoicmigmatiticcountryrockat584±7Ma(Meert
etal.,2007).Carbonatiteswithvariablegrainsizesandcompositionsoccurasdykesandsheetintrusions(Figure3a,b)(von
15 Eckermann, 1948; Kresten, 1979, 1990; Morogan and Lindblom, 1995). Carbonatites are igneous rocks that contain more
than 50 volume percent of calcite or other carbonate minerals (Le Maitre et al., 2002). The Alnö complex also contains a
wide variety of alkaline silicate igneous rocks (e.g., nepheline syenite and ijolite), which are rich in feldspathoid minerals
(e.g., nepheline) but lack quartz (Vuorinen, 2005). Pyroxenite occurs as lenses and small intrusions (Figure 3a). Alnöite, a
mafic alkaline silicate rock that belongs to the kimberlite family and is melilite-rich, occurs as dykes and diatremes in the
20 Alnöcomplexandinthesurroundingcountryrock(vonEckermann,1948).However,theyaretosmalltobeindicatedonthe
geological map. The intrusion is surrounded by fenite, which is country rock that has undergone metasomatic alteration by
CO -rich solutions from the intruding carbonatite and alkaline silicate rocks (Morogan and Woolley, 1988; Morogan, 1989;
2
Skelton et al., 2007). The degree of alteration varies with distance from the intrusion, from nearly unaltered approximately
500mawaytoastronglyfenitisedvarietywithnofreequartzremainingclosetotheintrusion(MoroganandWoolley,1988;
25 Kresten,1990).Thefluid-rockinteractiontypicallyaddsNa2O,K2O,CaO,MgOandFeOandreducesSiO2 content(Kresten
andMorogan,1986;Vuorinen,2005).
Thesubsurfacegeologicalstructuresinthecomplexwereinitiallyinferredfromsurfacegeologicalmappingofdipanddip
direction of the alkaline silicate rocks and the carbonatite sheet intrusions (von Eckermann, 1948; Kresten, 1980, 1990). A
dome-shapedmagmachamberwiththeroofat~1.5kmbelowthecurrentlandsurfacewassuggestedtohavesuppliedsteeply
30 dippingradialdykesandshallowtomoderatelydippingcarbonatitecone-sheets(Kresten,1980,1990).The“Sälskärbreccia”
has been described from boulders found close to the Sälskär skerries (Figure 3c, d); they contain fragments of carbonatites
andmeliliticlapilliinacarbonatitematrix(Kresten,1990).Thebouldersareassumedtobemoreorlessinsitualthoughno
outcropwithbrecciaarefound(Kresten,1990).Thebrecciaisinterpretedtobetheproductofexplosivecarbonatiticvolcanism
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Figure3.(a)Coarse-grainedcarbonatiteinoutcrop.Amorecompetentpyroxenitedykeshowsboudinagestructure.Ayoungerfine-grained
carbonatitedyke(brown)cross-cutstheboudinage.(b)Afine-grained(dark)carbonatitedykecross-cutsacoarse-grainedcarbonatitedyke.
(c)AboulderofbrecciainthewaterbetweenthewesternandeasternSälskärskerries.Thebouldercontainsfragmentsofcarbonatitesand
meliliticlapilliinacarbonatitematrix.Theboreholemarksare2.5cmindiameter.(d)Asamplefromtheboulderin(c),showingcarbonatite
fragmentsandlapillis.
(Kresten,1990;Anderssonetal.,2013).About500msurfaceerosionisestimatedtohaveoccurredinthestudyarea(Kresten,
1990)sincethetimeofintrusion.
Small-scalepetrophysicaldataareavailablefromtheAlnöIsland.Forexample,Piper(1981)andMeertetal.(2007)focussed
onpalaeomagneticpropertiesandnaturalremanentmagnetisationmeasuredonorienteddrillcores;emphasiswasgiventothe
5 emplacement time and the palaeogeographic position of Baltica during the Neoproterozoic era. Malehmir et al. (2013) and
Anderssonetal.(2013)publishedmeasurementsofultrasonicvelocitiesanddensitiesofacollectionofsamplesfromanumber
oflocalitiesinAlnö.
2.2 Reflectionseismicsandanisotropyofmagneticsusceptibility
The seismic study by Andersson et al. (2013) focussed on the data acquisition, processing and interpretation of three high-
10 resolutionreflectionseismicprofilescrossingtheAlnöcomplex(Figure1);oneabout9kmlong(Alnö1)andtwoprofilesthat
wereabout4kmlongeach(Alnö2andAlnö3).Theacquisitionwasperformedwithanominalsourceandreceiverspacingof
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10m.AVIBSIST™hammerwasusedtogeneratetheseismicsignal(Anderssonetal.,2013)with3-5sweepsateachsource
location. A diffuse reflection pattern in the centre of the longest profile (Alnö1) was interpreted to represent a shattered and
sunkencentralcalderablockthatextendsdowntoabout3kmbelowthepresentdaylandsurface.Belowthecalderablocka
seismically transparent zone was suggested to relate to an up to 1 km thick solidified saucer-shaped magma chamber; today
5 justapluton.Theseismicallytransparentzonesuggestsamorehomogenousrock,likelyamixtureofcarbonatiteandalkaline
rock,whichdifferfromtherocksabovethemagmachamber.Themaximumdepthcontinuationofthecomplex(includingthe
pluton) was estimated to about 4 km below the surface using 2.5D forward gravity modelling (Andersson et al., 2013). An
initialup-domingindicatedfromtheseismicreflectivitypatternthatbeginswithflatlyingreflectorsfarfromtheintrusionat
about4kmdepth,whichbecomegraduallymoresteeplydippingreflectorsclosertothesurfaceandtheintrusion,wasargued
10 tofavourthelocationofthefossilmagmachamber.
Arecentlyconductedstudy focussingontheanisotropyofmagnetic susceptibility(AMS)ofcarbonatitedykesrevealed a
patternofthemagneticfabricthatfollowstheorientationofthedykes(Anderssonetal., 2016).AMSresultsarecommonly
presentedassusceptibilityellipsoidsanditwasfoundthatmostoftheAMSmeasurementsshowedoblateellipsoidshape.The
dominanceofoblateshapewasinterpretedtorepresentre-orientationordeformationofmagnetitegrainsduetodykeclosure
15 atalatestageofmagmatransport,probablyduetopressuredropinthedyke(Anderssonetal.,2016).
3 Dataandmethods
3.1 Petrophysicalmeasurements
TheabovementionedAMSstudywasthemainpurposetocollectdrillcores,buttheybecameavailabletothisstudybecause
AMS measurements are conducted with a weak applied magnetic field which is non-invasive. The sample set constitutes a
20 collectionofblocksamplesanddrillcores(25.4mmindiameterandupto~10cmlong)fromthemainrocktypesavailable
in outcrops in the Alnö Island and the small islands north of it (see Figure 1 for locations). The core samples were drilled
with a handheld gasoline-powered drill. All drill cores and some of the block samples were oriented with a sun compass.
Thesamplesweremeasuredfortheirdensityandbulksusceptibilities(k );somesampleswerealsomeasuredfortheirnatural
m
remanentmagnetisation(NMR).Thedensitymeasurementspresentedhereareanextensionofthedatasetthatwaspresentedin
25 Anderssonetal.(2013).Intotal207samplesweremeasuredfordensity,morethan250samplesforbulkmagneticsusceptibility
and40samplesforremanentmagnetisation.Elasticwavevelocitiesatultrasonicfrequencywerealsomeasuredearlierbutnot
shownnordiscussedhere(seeAnderssonetal.,2013;Malehmiretal.,2013).Table1showsasummaryofthephysicalproperty
measurementsfordifferentrocktypesfromtheAlnöcomplex,whereasthedataispresentedinthesupplementaryinformation.
3.1.1 Densityandmagneticsusceptibilitymeasurements
30 Prior to the preparation of sub-samples (21 mm long) suitable for AMS measurements (Andersson et al., 2016), the core
samplesweremeasuredfortheirdensities.Theweightofeachsamplewasmeasuredinairandsubsequentlyinwaterallowing
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106
Trend 3
105 Trend 2
104
)
I
S
µ
(103
m
k
Trend 1
102 Fine-grained carbonatite
Coarse-grained carbonatite
Fenite
Ijolite
101 Nepheline syenite
Pyroxenite
Migmatite
Alnöite
100
2500 2600 2700 2800 2900 3000 3100 3200 3300 3400
Density (kg/m3)
Figure4.Densityversusmagneticsusceptibility.Scatterplot,withverticalerrorbars(onestandarddeviation),showingthedensityversus
mean bulk susceptibility (km) data measured in this study. The density of the samples (1 inch drill cores) were first measured and then
subsequentlycutinto21mmlongsub-samplesandmeasuredforkm.Threetrendsareevident,whicharediscussedinthetext.Atotalof207
measurementsareplotted.
thedensitytobecalculatedusingArchimedes’principle.Thebulksusceptibilitiesofthesub-samplesweremeasuredwithan
Agico KLY-2 Kappabridge™ at Lund University. Some of the samples were measured with an AC Bridge at the Geological
Survey of Sweden (SGU). The magnetic field used was 48 A/m inside the coil system for the AC Bridge (Puranen et al.,
1993);thisvaluewaslaterusedtocalculatetheisolinesforKönigsbergerratioshowninthispaper.Figure4presentsdensity
5 versusbulkmagneticsusceptibilitymeasurementsforallthesamplesandsub-samples.Afocuswasgiventothecarbonatites
(fine-andcoarse-grained),astheywerethemostinterestingrocktypeinthecomplexfortheAMSstudy.Thisisreflectedin
themeasurements.Mostsamplesshowrelativelyhighmagneticsusceptibility,especiallysomeofthecarbonatites,nepheline
syenitesandpyroxenites.Pyroxenite,ijoliteandnephelinesyeniteshowmuchhigherdensitythanthewallrocks(migmatite
andfenite).
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5
10
Q = 0.1
Q = 1
4
10
)
I Q = 10
S
µ
(

m
k Fine-grained carbonatite
3 Coarse-grained carbonatite
10 Fenite
Ijolite
Nepheline syenite
Pyroxenite
Migmatite
Alnöite
2
10
1 2 3 4
10 10 10 10
M (mA/m)
r
Figure5.Remanentmagnetisation(Mr)versusbulksusceptibility(km).IsolinesindicateKönigsbergerratios(Q)of0.1,1,and10.Afew
rocksamplesshowhighremanentmagnetisation(Q 1);remanentmagnetisationdominatesoverinducedmagnetisationforthesesamples.
≥
Atotalof40measurementsareplotted.
3.1.2 Naturalremanentmagnetisation
Naturalremanentmagnetisation(NRM)wasmeasuredwitharemanencemeter(BartingtonMAG-03MLS™fluxgatemagne-
tometer) at SGU on a selection of block samples. The remanence meter has a lower limit of detection of 10 mA/m, with an
errorof 5mA/m.Sampleswithrelativelyweakremanencethereforehaveconsiderableuncertainty(50%forasamplewith
±
5 aremanenceof10mA/m),butinthecaseofhigherremanencetheerrorisusuallynegligible.Sampleswithremanencelower
than 10 mA/m were excluded from the study. Figure 5 presents the NRM measurements as a function of bulk susceptibility
suggestingthatmostofthesampleshaverelativelylowremanentmagnetisation,essentiallywithaKönigsbergerratioofless
than1.
3.2 Gravityandmagneticdata
10 Groundgravityandmagneticdatahavebeencollectedsince2010inconjunctionwiththereflectionseismicstudies(Andersson
etal.,2013)andinfollow-upfieldcampaignstoprovideamuchwiderareaandbettercoverageforthecurrentstudy.While
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SolidEarthDiscuss.,doi:10.5194/se-2017-3,2017
ManuscriptunderreviewforjournalSolidEarth
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c Author(s)2017.CC-BY3.0License.
(cid:13)
ground magnetic data were also measured, they are not used in the modelling due to their limited areal coverage. Instead,
high-resolutiontotal-fieldaeromagneticdatawereusedforthispurpose.
3.2.1 Acquisitionandpreparationofgravitydata
Morethan400gravitydatapointswereacquiredduringtheyears2010to2012usingaLacosteRomberg™modelGgravimeter.
5 Mostpointsweremeasuredonland,butafewonthesea-ice(shallow-water)northoftheislandduringwinter2011.Thesame
base-stationandinstrumentwereusedforallthefieldcampaigns.Standardprocessingofthedatawasapplied(tidalcorrection,
latitudecorrection,terraincorrection)andtheBougueranomalywascalculatedwiththeassumptionofabackgrounddensity
of 2670 kg/m3. These measurements were shifted and merged with additional gravity data points provided by SGU (Figure
2a).Intotal610gravitydatapointswereused,coveringthemaincomplexandthesurroundings.Weestimateabout0.1mGal
10 dataerrorinthepreparationofthedatauntilcompleteBouguerresponse;henceweusedanRMSmisfitof0.2mGalduring
theinversion(Malehmiretal.,2016).
TheBouguergravityanomalyisnotonlyabout20mGalhigheroverthecomplexthanforthebackgroundrocks(migmatites),
italsoshowsastronghorizontalgradienttowardsthecentreofthecomplex.Thisisbyfarthemostnoticeablegravitygradient
anomalyobservedinSweden.Thegravityanomalyclearlyillustratestheextentofthecomplex,especiallyonlandandinthe
15 southernandeasternpartsofthestudyareawhereitshowsthestrongestgradient(Figure2a).
3.2.2 Preparationofmagneticdata
The aeromagnetic data (Figure 2b) used in this study were collected using a high-resolution caesium vapour magnetometer.
The lines were flown in north-south directions at 60 m nominal altitude, with 16 m sampling spacing and 200 m flight line
distance. The aeromagnetic data were delivered with ground clearance information from radar altitude measurements. The
20 groundclearanceandelevationdatawereaddedtogethertoobtainthecorrectlocationsabovesealevelforthemeasurements.
Thedatasetwasdecimatedtoagridof100mby100m;thisprovidedadatasetcontaining18620datapointsfortheinversion.
Errorinthesetypesofdataisestimatedtobeontheorderof1–5nT(afterallthenecessarycorrections).
Theaeromagneticdatauniformlycoverthecomplexonlandbutalsooverthesea,revealingsmaller-scalestructures.Similar
tothegravitydata,magneticdataclearlyshowtheextentofthecomplex,amagnetichighinthecentreofthecomplexonthe
25 mainislandandinadditionaring-shapedmagneticanomalyatthemarginofthecomplexinthebay.Twosmallerbutnoticeable
magnetic anomalies are also observed in the southern part of the study area, one generated from a major industrial complex
(analuminiumsmelter),andoneinthesouth-easterncornerfromtheRödönrapakivigraniteintrusion,whichhasbeendated
to~1500Ma(Welin,1994;Lundqvistetal.,1990;Andersson,1997).ThemagneticanomalyovertheAlnöcomplexisonthe
orderof2000nT(Figure2b),whichmakesthecomplexstandingoutfromthesurroundingmigmatiticrocks.
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