Porphyritic syenite at Lake Mykle, the Oslo Rift a possible derivative of larvikite

TOMANDERSEN,ROBERT FREI,HENNING S0RENSEN&NIELS LANGAGER WESTPHAL ^NGU-BULL 442 , 2004 - PAGE 23

Porphyritic syenite at Lake Mykle, the Oslo Rift a possible derivative of larvikite

TOM ANDERSEN,ROBERT FREI,HENNINGS0RENSEN&NIELSLANGAGERWESTPHAL

Andersen, T.,Frei,R.,Sorensen,H.&West phai,N.L.2004:The occurrenceof porphyriticsyeniteat Lake Mykle,the OsloRift-apossiblederivativeof larvikite.Norgesgeologiske undersokelseBulletin442, 23- 28.

NewRb-Sr andSm- Nd isot op edata onporphyrit icsyenite fromlakeMyklein the sout hern partoftheOslo Rift con- firm aclosecomposit ionalandgenet icrelationship bet weenthisrockand the associated larvikite.Theporphyritic syent itehasiniti al87Sr/86Srat280Ma between0.7044and0.7048 ,and ENdbetween+1.56and+2.52.This patte rn of variation is com pati ble wit h contaminat ion of a mantl e-derived parent magma wit h small amo unts of Precam briancalc-alkalinegneissesin thedeepcrust.

TomAndersen,Institutt forgeofag,Universiteteti Oslo,Postboks1047Blindern,N-0316Oslo,Norge;Robert Frei and Henning Serensen,Geologisklnstitut,KobenhavnsUniversitet,0ster Voldgade10,DK-1350 KobenhavnK,Danmark;Niels L.Westphal,McerskOlieogGasAlS,Esplanaden 50,DK-1263 KebenhavnK,Danmark.

Introduction

An occurrence of porphyritic syenit elocated at the so ut h end of lake Mykle,on the 1:50 000 geologicalmap-sheet 1713 I Siljan in the southwestern part of the Oslo Rift,was described in some detailby Petersen& Sorensen(1997).This syenitecontainsphenocrystsof plag ioclasealmostidentica l tothe feldsparofthesurrounding larvikite,whichis a mon-

zonitic plutonicrock.The matrixofthesyenit e is very close in composition to nordmarkitic syenit e which intrudesthe porphyritic syenite.Based on petrography and majo rand trace element data, Pet ersen & Sorensen (1997) proposed that the porphyri tic syenite was derived from a larvi kit ic melt by fractionation processes involving plagioclase, c1inopyroxene,Fe-Tioxidesand possiblyapat ite.

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Fig.1.Geologicalmapof theLakeMyklearea.Sample locati on s: Stars:Porphyriticsyenite.Diamon ds:Larvikite.

NGU-BULL442 , 20 0 4 - PAG E24 TOMANDERSEN,ROBERTFREI,HENNING S0 RENSEN

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Fig.2.(a)Two larvikite xenolithsinastate ofdisagg regationin porp hy- ritic syenite.Southeast coast of Lake Mykle. (b) Porphyritic syenit e (right)intrudedby syenite dyke.Southwest coast of Lake Mykle.(c) Dyke ofporphyriticsyenitewithsharp cont acts againstlarvikit e.South endof Lake Mykle.

Rb-Srand Sm-Nd isoto pedata forthepor phyriti csyenite arepresentedanddiscussed in thepresent paperwit h the aim ofconstraining themode of origin of the porphyri t ic syenite.

Field relationships

Theporph yritic syenite forms adom e-shapedbody measur- ing 4.5x2 km(Fig.1),and the exposedverticalthicknessis about 150 m.Towardsthe south andwest, theporphyritic syenite isin contact wit h larvi kite, whichalso overli esthe

syenite,asdemonstrated by the presence of larvikite roof pendantsin theporphyriticsyenite.Towardsthenorthand east it is in cont act with granites (including ekerite) and minor bod ies of nord markit ic syenite. Cont acts between larvi kite and porphyritic syenite are generally sharp and there arenochillzonesintheporphyriti c syenitein contact wit h larvikite.Locally,however,there is a transition from larvikiteintoporphyrit icsyeniteintheform of anincreasing densityofplagioclasephenocryststow ardthe larviki teover a distance of a few cm.The porphyrit ic syenite contains xenolithsoflarvikit ewhichapp earto disinte grate into clus- tersoffeldspar(Fig.2a).It also intrudesthe larvikitein the form of dykes (Fig.2c).The nordmarkitic syenite and the granites areyoungerthan theporphyrit ic syenite (Fig.2b);

these younger intru sions have oblit erated the original northerncont actofthelarvikitemassif.

Petrography

The porphyritic syenite ischaract erised by evenly distrib- uted phenoc rysts of plagioclase (andesine-oligoclase) which measureupto abou t2cmand makeup 30-70%of the rock. They are rimm ed by crypt operthi tic ternary feldspar.The fin e-grained matrix consists of crypto- or microp erthitic An-poor alkali feldspar,thegrainsof which may have cores of plagioclase,and of edenit ic amphibole, augi ticpyroxene,Fe-Ti-oxides,biotite andabou t5%intersti- tialquartz.Accessoryminerals arezircon, allanite,chevkinite, apatite,fluor iteand calcite(Petersen&Sorensen 1997).

Thelarvikiteofthearea is porphyritic andisdom inated by large grains ofplagiocl ase(50to20%An),which make up about 75% of the rock.The plagio claseisrimmed by microperth itic ternary feldspar. Augitic c1inopyroxene and som e orthopyroxenemakeup about 10%,edeniticamphi- bole5%,Fe-Tioxides5%andaccessoryminerals5%(zir- con,apatit e,biotite,quartz,allanite,chevkinit eandf1uorite).

Thematrixiscomposed ofalkalifeldspar andinte rgrowths of oligoclase and pot ash feldspar. Petersen (1992) distin- guished two minor types of larvi kit e:(1 ) a coarse-grained type with alarger content ofinterstit ial alkali feld spar,and (2) afine-grained typ econt aining subcalcicand pigeonit ic clinopyroxenes and a high-temperatu re plagioc lase. The fine-grained variet y of larvikit e occursas scattered,minor bodies,anditsorig inremainsuncert ain.

The plagioclase phenocrysts ofthe porphyri ti c syenite have rimsofternary feldspar, andarealmostiden tical tothe feldsparof thehostlarvikite.

Major and tra ce element data

Majorand trace elementcomposit ionsof larvikiteand por- phyriticsyenite arepresent ed inTable 1,based ondatafrom Petersen&Sorensen(1997), supplem entedby isoto pe dilu- tion trace elementanalysesfromthepresentstudy(Table 2).

Thelarvikit eanalysesareofthe most abundant variety,with the except ion of 81543which representsthe fine-grained type.Interms of majorelements,there is agradu altransition

TOM ANOERSEN,ROBERTFREI,HENNING S0RENSEN&NIELSLANGAGER WESTPHAL NGU-BULL442 , 20 0 4 - PAGE 25

Tabl e1.Whole-rockmajor(wt%)and trace element(p p m) data of larviki teand porp hyriticsyentiefrom the Myklearea.

81541 81543 81456 81595 81594 81559 81561 81566 81571 81577

LK LK LK LK LK PS PS PS PS PS

Weightpercent oxides

Si02 57.30 60.04 58.62 57.53 59.91 60.74 59.80 58.81 60.52 59.19

Ti02 1.25 1.44 1.36 1.33 1.11 1.33 1.43 1.42 1.30 1.52

AI20 3 18.77 15.82 17.39 18.09 17.64 16.17 16.16 16.36 16.17 16.25

FeP 3 1.43 1.72 1.81 1.46 1.20 1.80 2.02 2.26 1.67 2.15

FeO 3.63 4.29 3.52 4.02 3.55 3.67 3.90 3.73 3.75 3.87

MnO 0.12 0.16 0.13 0.11 0.12 0.15 0.15 0.15 0.14 0.15

MgO 1.36 1.38 1.44 1.50 1.12 1.28 1.47 1.49 1.29 1.57

CaO 5.16 3.20 4.22 5.17 3.56 3.38 3.87 3.80 3.35 4.08

Na20 5.54 4.90 5.48 4.89 5.87 5.08 5.01 5.37 5.13 4.96

K20 3.27 4.82 4.42 3.43 4.14 4.61 4.28 4.68 4.71 4.25

P20S 0.60 0.54 0.55 0.60 0.45 0.47 0.56 0.54 0.45 0.62

Volatiles 0.69 0.69 0.63 1.00 0.75 0.67 0.71 0.68 0.68 0.71

Tota l 99.12 99.00 99.57 99.13 99.42 99.35 99.36 99.29 99.16 99.32

(Na+ K)/AI 0.67 0.84 0.79 0.65 0.80 0.83 0.80 0.85 0.84 0.79

Partsper million

Rb 109 157 144 112 155 211 201 215 218 191

Cs 3.1 3.8 n.d. n.d. 3.7 4.5 n.d. 2.7 n.d. n.d.

Sr 573 296 488 515 435 329 394 379 334 439

Ba 728 1000 915 780 948 627 640 604 582 695

Zr 939 1180 907 750 819 1190 10 10 1300 1300 1040

Hf 24.2 31.1 n.d. n.d. 19.8 33.6 n.d. 31.1 n.d. n.d.

Nb 119 139 137 99 117 165 151 204 133 136

Ta 7.9 9.4 n.d. n.d. 7.8 11.1 n.d. 13.2 n.d. n.d.

La 104 118 111 95 106 138 124 147 139 117

Ce 204 239 223 188 202 273 264 288 278 253

Nd 85 107 98 85 85 127 109 119 115 106

Sm 14.3 20 n.d. 16 14 19.2 20 22 21 20

Eu 4.2 4.4 n.d. n.d. 4.4 3.7 n.d. 3.7 n.d. n.d.

Tb 1.9 2.9 n.d. n.d. 2 2.9 n.d. 3.1 n.d. n.d.

Yb 5.7 8.4 n.d. n.d. 5.7 10 n.d. 9.9 n.d. n.d.

Lu 0.8 1.1 n.d. n.d. 0.8 1.2 n.d. 1.3 n.d. n.d.

Y 57 80 66 59 59 87 83 95 93 n.d.

Th 17 22 21 16 20 38 39 41 38 81

Zn 104 124 101 84 94 106 105 93 94 103

V 45 23 40 52 32 38 55 54 42 55

Sc 8 11.1 8 8 8 9.7 12 10.3 11 13

LK:Larvikit e, PS:Porp hy rit icsyenite.n.d.notdete rm ined

Majorelementsby XRFonfused discsprepa redwit hsodium tet raborate flux atthe Geolo g icalSurveyof Denm ark and Green land; trace elementsby XRFonpressed powde rpelletsat Geologica lInst it ute,Unive rsityof Cop en hagen and byInstrum entalneutr on act ivat ionanalysis By TracechemAlS, Cop en hagen .Rb,Sr,Sm andNddata in italics:Isoto pedilu tio nanalyses(Table2).

fro mlar vikit e overporp hyriticsyeniteto nord markit ic syen- ite, as shownby Harker variationdiagrams(Figs.6 and7in Petersen & Serensen 1997). When compared with the larvikite,nordmarkite and porphyritic syenite are enri ched in Si,K,Rb,Th,Nb,Ta,REE(wit h theexcept io n of Eu),Y,Zr and Hf and relativel ydepleted in AI,Ca,Na,Eu,Srand Ba, thatis in the compon ent sof plagioclase .The two syenit es have practicallyide nt ical REEpatterns; larv ik it e has a weak posi- tiveEu anomaly,whereasthesyenites have veryweak nega- tive anomalies (Fig. 8 in Pet ersen & Sorensen 1997).

Nordmarkiteand porphyr it ic syenite have almost ind ist in- gu ishab leIit hop hileelemen tpatterns,whic hdiffe ron lyinSr (higherintheporphyritic syenite)and Zr,ThandHf (lowerin theporphy ritic syenit e).These differencesreflect differences in modal mine ralo gy; plag io clase is present in the per-

phy rit ic syeni te, whereas zircon is mo re ab unda nt in the no rdm ar kit e (Fig.9 inPetersen& Sorensen 1997).

Sr and Nd Isot ope data

Analyt icalmethods

Rb, Sr,Sm and Nd concentrations of four samples of por- phyritic syenite and three samples of asso ciated larvikit e (Pete rsen&Ser en sen 1997) were analysed by X-rayfluores- cence and by isotope dilutiontherm alionizationmassspec- trom etry at theGeolog icalInstitute, Copen hage n Universit y.

The sam ple powders (200 mg) were at ta cke d follo w ing a sequen tial dissolution first wit h 8N HBr, follo w ed by 14N HN0₃- 32%HF mixtures, in Tefl onbeakers on ahot plate(T= 150°C)for three days.A149Sm-^1SoNd spike was added before-

NGU- BUL L 442,2004 - PAGE26 TOMANDERSEN,ROBERT FREI,HENNING S0RENSEN&NIELSLANGAGERWESTPHAL

Table2.Rb-Srand Sm-Nddata forintrus ive rocks from the Mykle Area.Oslo Rift.

Sample Rb Sr 87Rb/86Sr 87Sr/86Sr 2a Sm Nd 147Sm/'44Nd 143Nd/' 44Nd 2a Sr; ENd

ppm ppm ppm ppm 280Ma 280Ma

Porphyric syenite

81577 191 439 1.301 0.709559 0.000020 20 106 0.1082 0.512556 0.000021 0.70438 1.56

81566 215 379 1.682 0.711482 0.000020 22 119 0.1039 0.512591 0.000019 0.70478 2.40

81559** 221 329 1.945 0.712205 81567** 237 328 2.093 0.712697

81582** 206 426 1.4 0.71001

81571 218 334 1.935 0.712187 0.000020 21 115 0.1054 0.512577 0.000012 0.70448 2.07

81561 201 394 1.492 0.710534 0.000020 20 109 0.1071 0.512603 0.000013 0.70459 2.52

Larvikit e,SLakeMykle

81543 157 296 1.556 0.711099 0.000020 20 107 0.1068 0.512590 0.000012 0.70490 2.28

81594 155 435 1.043 0.707204 0.000020 15 85 0.1002 0.512637 0.000011 0.70305 3.43

81595 112 515 0.625 0.707015 0.000020 16 85 0.1048 0.512609 0.000011 0.70453 2.72

81456** 144 488 0.854 0.707331 81541** 109 573 0.551 0.706017 Larvi kit e,ELakeMykle

76461* 74 709 0.301 0.705987 0.00042 19 105 0.1119 0.512621 0.000010 0.704786 2.70

76465* 90 650 0.399 0.706144 0.00003 16 87 0.1096 0.512615 0.000010 0.704554 2.66

76468* 54 870 0.180 0.705332 0.00003 19 101 0.1136 0.512609 0.000010 0.704612 2.41

77805* 80 664 0.350 0.706097 0.00003 20 108 0.11 13 0.512628 0.000010 0.704702 2.86

77814* 66 725 0.263 0.705598 0.00003 20 108 0.1145 0.512611 0.000012 0.704551 2.41

81236* 77 697 0.320 0.705833 0.00003 19 105 0.1119 0.512602 0.000010 0.704558 2.33

79389* 102 558 0.529 0.706519 0.00003 22 117 0.1133 0.512628 0.000010 0.704411 2.78

Granit e

86044* 82 24 9.820.742261548 0.000009 7.8 44.4 0.1068 0.512622 0.000010 0.70315 2.90

":Analysedat theLaboratory of Isot opeGeology,Mineralogical-GeologicalMuseum,University of Oslo.**: Analyst:P.M.Holm,Copenhagen University.

Samples 76461 to79389 from the larvikiteringstructureeastof lake Mykle(Fig.1)were provided byUffeLarsen.

an age of 28 0 Ma and an in itial 875r/865r of 0.7045 (Fig.3).

One, off-lying lar v ikite sam ple (8 1594) plots significan tly belo w this line,with875r/ 865r2s0Ma=0.7031 .

Larv ikite an d porp h yr iticsyenite show small and over- lap p i n gran ges of1475m/J44Ndand143Nd/'44Nd.At 280 Ma,all sam p les havepositi v e epsilo nNd values,rangingfrom 1.56 hand.5r and REE fracti on s were sep a rate d over15 ml glass

stem colu m ns cha rged with AG SOW cation resi n.

Purificat ion of the 5r fract ion was achieved by a passover micro -col u m n s conta in ing5r5pec™resin .REEs were further separatedoverHDEHP-co at e d bio be ad s(Bio Rad™) loaded in 6 ml glass ste m colu m ns. 5r and Nd isotopes were analysedin dyna m ic mu lti-co l le ct in g routines,5m in a static mode, on aVG 54 5ector IT mass spect rameter.Th e mean value fo r ourinternal JM Ndstanda rd (refe rence d against La Joll a)du ring the period of measurement wa s 0.511115 fo r '43Nd/'44Nd,witha2aexterna lreproducibil ityof±0.000013 (f iv emea sure m ents).The mean875r/865r valueof the NB5 987 5r standardwas 0.710248,witha2aextern alreproduci bility of0.000011(f o u rmea su rem en t s).

Ad ditio n al sam ple s of larvikite an d granite were ana lysed at th e Lab o rato ry of Isotope Geology, Mine ra log ical-Geological Museum, University of Oslo, by meth o d s described by Andersen et al.(20 0 1).

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Fig.3.Rb-Srcorrelationdiagramshowing larvikit e and porphyritic sye- nite, comparedto a 280 Ma reference linewit h aninitia l87Sr/⁸⁶Sr ratio of 0.7045.Circles and squares:Samples from the sout hendof LakeMykle.

Triangles:SamplesfromNEof Lake Mykle.

Results

Rb -5r an d Sm-Ndiso t ope data for selected samplesofpor- phyriti c syeni te,larv ik ite an dgranitefro mtheMyklearea are giveninTab le 2.Larvik iteandporphyrit icsy e n ite show mar - gi nallyoverlapp ingrangesof87Rb/865 r and875r/865r. Neit her of the two rock ty p e s defi ne sta tist ically valid Rb-5r isoch ro ns; th e sam plesofporp hyri t ic sye n it e and a majority ofthe larvi kitesamplesscatterarou nda refere ncelinewith

TOMANDERSEN,ROBERT FREI,HENNING S0RENSEN&NIELSLANGAGER WESTPHAL NGU-B ULL 442 , 20 0 4 - PAGE 27

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Fig.4.Sr and Nd isotopiccompositionof porphyriticsyenite and associ- atedrocksfromthe Mykle area. Black triang les rep resent larvikites from NEof Lake Mykle.Ruled/cross-hat ch ed areas repr esentranges of reg io- nalvariationof larvikiteandsyenite and granite inthe Oslo Rift (data from Neumann etal. 1988).Ranges of Precambriancrusta lcomponents are taken fro m Andersen & Knudsen(2000):MM U:1.15- 1.50 Ga Mafi c underpl at e,GRA:c.0.93Gagranit es,NDC:'Normal deepcrust;i.e.,mode- ratelyL1LE-enrich edrocks in the deepcrust,UCE:Up percrustal rocks east of therift,LSR: Upper crust alrocksoftheTelemarkarea,showi ng elevate d Rb/Srratiosat nor mal Rbconcentra t io ns. TIG: Precambrian (1.6- 1.2Ga)cale-alka line met a-ign eo us rocks,includ in g ton alite,tron d - hj em it e, gran od iorite and their ext rusive eq uivalents (Knudsen &

Andersen1999,Andersen& Knudsen2000 and referencestherein).

The insetis anexpa nsio nof thero cksfrom the Mykle area,showing datapoints wit h 2s erro r bars.

Discussion

Larvikite magmain the Osloigneous provinceis considered to havebeenderivedbyfractiona lcrystallizat ionof basaltic melts formedbypart ial meltingof a mildly depletedsome- what heterogeneous mantle source (Neumann 1980, Neumann et al. 1988, Rasmussen et al. 1988).The same auth orsproposedthat crustalcontaminati onwas involved in the formation of the syenit ic and granitic rocks of the region, whichis alsosuppo rtedby published radiogenic iso- topedata (Andersen&Knudsen 2000).

Based on major and trace-element data, Petersen &

Sorensen(1997) pointed out thatlarvikite,porphyriticsyen- iteand nordmarkitic syenite may have been formed from successive pulses of magma from a common source,and that feldspar fractiona tion playedan impor tant rolein this process. The Harker variation diagrams of Pet ersen &

Serensen (1997)indicat e that fractionation of c1inopyrox- ene, Fe-Tioxides,and perhaps,apatite, was alsoinvolved.The nordmarkitic syenite and the microsyenite at Lake Mykle described by Andersen & Sorensen (2003) mark a more evolved stagethan the porphyriticsyenite.

The overall outcrop pattern(Fig.2),and the presenceof roof pendants of larvikitein the porphyriticsyenite,indicate that the porphyritic syenite is located inside the larvikite massif,but close to its contacts to (younger) granites and nordmarkiticsyenite.

Thecontact relation ships and the presenceof disaggre- gated xenolithsoflarvikitein the porphyritic syenitemay be takenas evidence that the plagioclase phenocrysts of the porphyriticsyeniteoriginated by disintegration of larvikit e.

Suchan originforthepor phyrit ic syeniteis,however,hard to reconcilewit h the even distribution of phenocr ysts overthe ent ire volume of the porphyrit ic syenite,including dykes which intersect the larvikite with sharp intrusive contacts (Fig. 2c).

Thesimilarityinstruct ure andcompositi onof the plagio- c1aseof larvikiteand porphyritic syenit e, including theirrim s of ternary feldspar,led Petersen& Serensen(1997)to pro- pose thattheporphyrit ic syenite may have been formed by extended crystallizat ion of larvikit ic melts caused by an increase in HP ,other volatilesand incompatibleelement s in responseto thefract ionat ion of anhydrous minerals such as feldspar,c1inopyroxene,Fe-Tioxidesand possibly apatite . Such evolved melts may have collected in cupola in the upper part of the still hot,solidifying larvikite. This can explain the generally sharp, non-chilled contacts and local transitional phenomena.The even distribution of plagio- c1ase phenocr ysts in the porphyrit ic syenite may then be explained by assumi ng thatthe magma, which formedthe porphyritcsyenite,was at its liquid us at thetimeofemplace- ment,andthatearly nucleationof plagioclasetookplace ina supercoo led melt whichsolidifie d rapid ly to for m the fine- pled to a pronounced increasein initial87Sr/86Sr;see cross- hatched areain Fig.3(Andersen&Knudsen 2000).Thisis not observed inthe porphyritic syenitefrom the Mykle area.

0.760 1.0 0.750

3.0 (8^7Sr/86Sr)

0.720 0.730 0.740

-15

to 2.54for the porphyrit ic syenite,andfrom 2.28to 3.43 for the larvikite(Table 2).Regardlessof composition,themaj or- ityofsamplesfor ma clusterinaENdvs.87Sr/86Sr280Ma diagra m (Fig. 4).Variationsininit ial Srand Nd composition within this cluster are small, but exceed analytical error, and are uncor- related (Fig.4,inset). One sample of larvikite (81594), and one granite show significantlylowertime-corrected87Sr/⁸⁶Sr than the othersamples.This is probablydueto lossof radi- ogenicstrontium during late(postglacial ?)weathering.The granitesample has a highRb/Sr rati o compared to the other samples, and only moderate loss of radiogenicSr from this sample could increase theRb/Srratio enoughtocause sig- nificant overcorrectionfor radiogenicgrowth when recalcu- lated to 280 Ma.

The total ranges of Srand Nd isotopic variationat 280 Ma reported in Table 2 fall within the overall range of variation of larvikitic rocks from the Oslo Rift (Fig. 4, data from Neumann etal. 1988).However,in mostothe rfelsic rocks in the rift,a reductionin initial143Nd/ 144Nd is commonlycou-

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NGU-BULL 442,20 04 - PAGE 28 TOM ANDERSEN,ROBERTFREI,HENNING S0RENSEN

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grained, quart z-b earing, syen itic mat rix (Pet ersen &

Sorensen 1997).

Similarit ies wit h both larvikiticrocks of the Oslo Rift in general,andwit hlarv iki t esand granit icrocksof the Mykle area in particular,arealso hig hlig hte d bytheSrand Nd iso- top edat a(Fig. 4).

Felsicintru siverocksin the OsloReg ionshowanoverall tendency toward s initial87Sr/86Sr rat iosof 0.710 or higher, and near-zero orweakly negative epsilo n-N dvalues(cross- hat ch ed fieldinFig.4).Based ona compilation ofgeochemi - caldataon Precambrianrocksfro m So ut hNo rw ay,Andersen

& Knud sen (2000) def in ed six compositio nally and geo- graphically constrained crustaI end mem ber s, which may haveactedascrustalcontamin antsinthemagmaticsystem ofthe OsloRift.Theregionalvariation inthe felsicint rusive roc ks can beaccount ed forbycontam ination wit h moder- ately L1LE-enriched, Precambrian rocks in the deep crust, represented by the 'Normal Deep Cru st' (NOC) and 'Precam briangranite'(GRA)components inFig.4(Andersen

&Knudsen 2000).Som e gran iticrocks showevide nce ofcon- tamination by st ro ng ly evolved crusta l rockswith low Sr concentrations and correspondingly hig h Rb/Srratios(LSR com ponent in Fig.4).In theMyklelarvikite and porphy ritic syeni t e,a minor shifttowardslowerepsilo n-Ndis not cou- pled to an increa se ininitial87Sr/86Sr, which suggeststhat neit her of thesecrusta lcom ponentscanhavehad anysig- nificant influ enc eontheintermediate andfelsicmagmasof theMykle area.However,the samplesanalysedinthis study overlap in Srand Ndcom positionwit hthe hig hepsilon-N d range of 1.6-1.2Gacale-alkalinegneissesat 280 Ma.Such rocksare wide sp read in the 0stfo ld-Akershus, Kongsb erg and Bamble secto rsin the Precambrian of South No rway (Andersen&Griffin2002),i.e.,adja cent to the southern part of the Oslo Rift.These potential contaminants are repr e- sented bytheTTGcomponent inFig.4,whi ch is constrained by datafrom arange of subduction -relat ed,meta -ign eo us Precam brian rocks (Andersen & Knudsen 2000 and refer- ences therein). From their geog rap hical distribut ion,TIG cont amin ant s aremostlikelytohaveinflue ncedmagmasin thesout he rn part ofthe rift,and thepresentdataarecom- pat ible with moderate amounts of such con t amina ti on in the magmas of theMyklearea. However,the cale-alkaline meta -igneou sPrecambrian rocksare ingene ral met alu mi- noustoperaluminou s,and majorcontam inationwithsuch material isnot likely in rockstrend ing toward sincreasing (Na+K)/AI(Table1).

Conclusions

Sr and Nd isot o p e dataon porphyrit ic syen ite and associ- ate d larvikitein the Mykle area of theOslo Rift confirm the

closegenet ic relat ionsh ipbet weenthe two rocktypes indi - cat ed by petrog rap hy and major and trace elem ent geo- chemistry(Petersen&Soren sen 1997).A lim itedvariationin initialNdisot o piccomposit ion (from epsilon -Nd=+3.4 to+ 1.6 at 280 Ma).wit hout correspo nding variation in intitial 87Sr/86Sr sugg ests that thelarvikite and po rp hyrit icsyen it e mag mas were cont aminat ed by minor amo unts of Precam brian cale-alkaline rocks, sim ilar to sub du ction- related meta-igneousrocksfoundat both side s of thesouth- ern onshor e segmentofthe OsloRift.

Ackno wledgements

Fieldw o rk was supported by Norges geologiske undersokelse, the Dani shNat ural Scienc e Research Co u ncil andtheCarl sberg Foundation. Uffe Larsen kindlyprovided samples of larvikitefromthe area east of Lake Mykle.TorilEng er assisted intheisoto pe laboratoryin Oslo.Britt a Munck and Ole Bang Berth elsen,Geol og ical Institute,University of Copen hag enprovid ed valuableassistance withthe illustra tions.Thanks toPoul Martin Ho lm for Sr isotopeanalyses,and toTo re Prestvik and 0ysteinNordgulenforcriti calcommentsonthe manuscript.

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