A new occurrence of gabbro in the Oslo Rift, South Norway

LONEPEDERSEN&HENNINGS0RENSEN NGU-BULL 44 1,2003 - PAGE33

A new occurrence of gabbro in the Oslo Rift, South Norway

LONEPEDERSEN&HENNINGS0 RENSEN

Pedersen,L.& Sor ensen,H,2003:Anew occurrenceofgabbrointheOsloRift, Sout h Norw ay,Norqesgeologiske undersekelseBull etin44 I, 33-38.

Xenolit hsmadeupof anassem bla geofgabb ro,dioriteand bio titemonzonite insyeniteandgrani teandveined by thesero ckshavebeenfo un dwithina sma llareaatthesout hend oflakeMykleinthe sout hernpartof theOslo Rift.

Thexenolit hs mo st probab ly repr esent theremainsof aformergabbroicneck similartothose occurring elsewhere in the OsloRift. TheMyk le occurrenceislo cated about SOkmtothe south andinprolo ngatio n of alinearbeltofgab- broi cnecksinthenorthernpart of the rift zone.Gab b ro,dio rit e andbiotitemonzoni te may representremnantsof a composite neck, or the biotitemonz onite and perh ap s also thediorite maybe gabb romod ified byinfi ltr atio nof flu- ids fromthe enclos ingsyenit esand grani tes. Xeno lithsof larvikit ein xenolithsofbiotitemonzoni teindicatethat gabb roic rocks inthisregion werelaterthanlarvikite andolde rthansyenite and granite.

LonePedersen,Valdem arsgade28/th,OK-I665,Kob enha vn\I,Danma rk.

Henning Serensen,Geol ogi sklnstitut, KebenhavnsUniversitet,0sterVoldgade10,OK- I350KebenhavnK,Danmark.

.,c..

.~Oslo

'fSd region

~ "

~MYk,eif ; /

J

Plutonic and volcanic rocks

Precambrian basement and Lowe r Palaeozo ic sedimentary rocks

[11 [11

Introduction

Gabbroic rocksmakeupabo ut 0.3%of theareaoccupiedby plutoni c igneousrocksin the OsloRift(Barth 1945, Oftedahl 1960).Fifteen gabbroicnecks ranginginsize from about300 m to except ionally 3 km are aligned along trends N-Sand NNE-SSW,parallelto promin entfaults in the rift(Larsenetal.

1978,Neumannet al. 1985), andare located nearthemar- ginsof therift zone(Fig.1).Mostofthemare intruded into the Precamb rianbasement and itscoverof Palaeozoic sedi- mentaryrocks(Neumann etal. 1985,fig.1).Rarexenolit hs of gabbroicrockshavebeenobserved in the plutonic rocksof the rift (Larsen et al. 1978). the most promi nent example beingthe syenit ic ringdykeof theGlit revanncauldron.One of thesexenolit hsiscomp osit e,and iscomp osed ofbasalt (local term B₃) as well asthe overlying rhomb porph yry (RP₁₃) (OftedahI1960, fig. 102).The basaltisint rudedby gab- bro.One occurrenceatEiangenin thesout hwestern partof the rift islocated at thecont act betweenlarvikit eand the Precamb rianbasementtothe west of the rift(Neumannet al. 1985,fig.1).

Thegabbroicnecks arecomposedofgabbro,pyroxenit e, monzoniteand other rocks andcommonly have a concen- tric internalst ructure(Neumann etal.1985).Theyare consid- eredto representfeederchannels tocentralvolcanoes.

During mapping of the Siljan map-sheet in thesout h- western partoftherift,xenolithsconsistingof gabbroicand related rockswere observed in syenite and granit eonthe sout hwest and southeast coastsof lake Mykle.This occur- renceislocatedabo ut15 kmtotheeast of theEiangengab- broandabout 50 km tothesouth of thenearest occurrence

Fig.1.Sket chmapshow ingthedistributio n of gabbroic ro cks inthe Oslo Rift(basedonNeumannetal.1985).

)Il:

Gabbro

~

Mykle gabbro

- ---- Majo r faults

NGU-BUL L 441.20 0 3 -PAG E34 LONEPEDERSEN&HENNING S0RENSEN

[:=:=:=:=:=:::] Larvikil e

~

[ :. t t:,

1 < :>:: :::::1

~ Nel -v eined

~ complex

r<:'~_~'-" '.':~;'

< ,j

N ^Nepheline

syenite

Fig . 2.Simpl ified geological map of the Mykle area.

to the north andin prolongat ionoftheline ofgabbronecks located along thewest borderof the rift (Fig.1).A detailed descripti on has beenpresented inanunpublisheddisserta- tion (Pedersen 1994). The present paper gives a brief account ofthe geolog y,petrology andgeochemist ryof this occurrence.

General geology

Thearea around lakeMykleis dominated by granit ic rocks (Fig. 2) that have intruded a large massif of larvikite.

Pet rographically, larvikite comprises augite syeniti c to augit emonzonit icrocks.Roof pendantsof larvikite overlie the granit ein the western partof the granite massif.Around the sout hern end of lake Mykle a succession of intrusive phaseshasbeendistinguishedin thecontact zone between

graniteand larvikite.From the oldest to the youngestthey are:

1. Larvikite.

2. Gabb ro/diorite(Pedersen 1994) and nepheline syenite (Andersen&Sorensen 1994)found asxenoli t hs insyen- ite and granit e.Xenoli ths of larvikite and intersecting dykes show that these rocks are younger than the larviki te. The age relationship between gabbro and nephe linesyenite isunknown.

3. Porphyrit ic syenite int rudin g the gabbro and cut by syenitesandgranit es(Petersen &Sorensen 1997).lt sage relati on shiptothe nepheline syeni teisnot known.

4. Several phases of quartz alkali-feldspar syenite (nord- markite).

5. Several phases of granite, the youngest being the

LONEPEDERSEN &HENNINGS0RENSEN NGU-BULL441,2003 - PAGE 35

74 8 8 2 4 4

<1

<1 23238 77173

Gab b ro Diorite Biotitemonzonite leucodolerite 7628 7 81791

ite and biotite monzonite occur along the southeast coast of Mykle and also1 km to the south on the southcoast of the lake(Fig. 2).Thexenolit hs are enclosed in porphyriticsyenite (Fig.3),nordmarkiticsyeniteand granite and range insize from a few cm to more than 10 m. The xenolithsarein all stages of disintegration into the enclosing rocks and are intricate ly veined by several generations of syenite (Fig. 4) and granite.Xenoliths of biotite monzon it e in the southern part of the area contain xenoliths of larvikite.About 1km to the south ofthe xenolit harea,larvi kit e is inte rsected by a dyke of leucodolerite, petrographicallysimilarto thegabbro ofthexenolit hs.This may support theview that thegabbro andaccompa nyi ng rocks areyoungerthanthe larvi kit e.

Petrography

The rocks of the mafic xenolit hs vary petrographically from true gabbro to diorite,monzodior it e and monzonite.The rocks commonly contain plates ofbiotite up to 1 cm insize.

Thisis especially the case adjacent to int ersecti ng veinsof syenite and granite.The matrix of the rocks is fine- to medium-grained,and the grain sizeis1-5mm.Thegabbro and dior iteare equigranularand show distinctdolerit ictex- ture(Fig.5);the biotite monzonite isheter ogran ula r(Fig.6).

The colour indexis rather low,about 30inthe gabbro, c.40in the dioriteand20-30inthe monzodiorite/monzonite (Table 1).

The plagioclaseof the gabbro hascoresoflab radorite and rim s of oligoclase and in some casesalkali feldspar.

There are interstitial grains of orthopyroxen e (W0₂_3En61_ 67Fs30-37) overgrown by Ca-rich clinopyroxene whic h also occu rsastwo typesofintersti t ialgrains,one brown (W04' - 4,En4G-4,Fs13.5-14) and the ot her green (W043-44En37-49Fs16_18) in thin-sect io n (mi neraldata from Pedersen 1994).The brown aug itic variety was earlie r thanthe green diopsidi c variety.

The brown variety has 0.4-1.2%Ti0₂and 1.5-2.7%A1₂₀3,the green variety 0.2%Ti0₂and 0.6-1%AIP3'Magnet it e (up to 30%ulv6spi nel)andilme nite (Ilm90_99Hel_1O)form indepen- dentgrains.Accessory mineralsare allanite,pyrit eandchal- copyrite.The Fe-Tioxides areoften rimmed bybio t it e, the clinopyroxeneby actinolitic amphibol e.

The plagioclase of the dio rite can have cores of

Table1.Modalanalysesofgabbro,dioriteand biotitemonzonite,theMykle area,South Norway.

Composite xenolithsof gabbro and accompanyingdior- coarse-grained peralkaline granite termed ekerite(Bonin

& Serensen 2003).In some contacts between larvikite and granite,net-veined complexes occur (Morogan &

Serensen1994).

Fig.3.Gabb roxenolithinporphyri t icsyenite,sout heastcoastof Mykle.

plagi ocl ase 63 57 30

alkali feldspar 7 2 38

c1ino pyroxene 15 13 3

orthopyroxene 4

~

Gabbro/

Porphyritic

1=======1

Sye nite

amphibole 4 12

Diorite

Syenite

Opaques 4 5 5

Fig. 4.Sketchmapof relatio nshipsbetweengabbro/diorite,porph yriti c Apatite 1 1 1

syeniteandsyenite,westcoastofMykle. Titan it e 1 <1

NGU-BU L L441,2003 -PAGE 36 LONEPEDERSEN

s

Fig.S.Photo ofwholethin-section of gab- bro.Sample no.72286,crossed polars. The lengthofthe thin-section is 3 cm.

Fig.6.Photo of whole thin-sect ion ofbiotite monzonite showi ng heterogeneous tex- ture.Sample no.817SS,crossed polars.The lengthof thethin-sect ionis 3 cm.

labradorite, generallymore sodic than the plagioclaseof the gabbro,and haverimsofoligoclase andalkalifeldspar.There are brown (W04S_48En38.s_4,Fsll_d and green (W046-48En37- 40Fs13.s-1S)grains of clinopyroxene.Thebrownvariety has1% Ti02and 1.8-3.7%AbO"the green variety 004-1%Ti02and 004-1%A1203.Pargasit ic to edenitichorn blende forms rims on c1inopyroxeneand also occurs as independentcrystals.

There are independent grains of magnetite (0-2 % ulvospin el)andilme nite(1ImgOHe_lO).The diorite is general ly more st rongly altered than the gabb ro.The plagiocl ase is altered to epidoteand albitic plagioclase,themafic minerals to actinolite and chlorite,ilmeni te to titanite and alkali feldspartosericite.

The monzodiorite/monzonite is more heterogeneous than the gabbroand diorite(Fig.6)and contains abundant biotite.Partsof therocks haveadolerit ictextu re,otherpart s a more granulartexture.The cores ofthe plagioclase can consistoflabradorite,but thecrystalsaredominatedby a moresodic plagioclase.Alkali feldsparformsrims on plagio-

c1ase andalsooccursas interstitialgrains.Diopsidicpyrex- ene(Wo43-46En32-4l s1S.s-21.S)occurs as inter stitialgrains which may be rimmed by amphib ol e and biot ite. Inde penden t amphi bo legrainsvaryin composit ion from magnesiohorn- blend e to actino litic hornblende. Magn etite (0-2 % ulvospin el).i1menite (IIm₈₈_96He4

_ d ,

Themonzonite/mon zod iorite containsfine-grainedgrey xenolit hs up to 10 cm across.They consist of plagioclase, c1ino pyroxene,apatiteandFe-Tioxides and havelarge flakes of biotite.

The dykeof leucodol erite to the south of the occur- rencesofgabbro xenolithsconsistsof plagioclase forminga pronounced trachyt ic texture, c1inopyroxene (Wo₄₆.sEn37.s FS16)wit h0.5-0.7%Ti02and 104-1.8%AIP3'amp hibolevary- ing from pargasite to hastingsite,alkalifeldspar,mag netite and ilmenite.Thecolourindexisaround 15.

LONEPEDERSEN&HENNfNGS0RENSEN

Whole-rock chemistry

The outcro psof the gabbro are so weathered that it was impossible to collect samplesfresh and large enough for chemical analyses.Chemical analyses ofdiorite and monzo- diorit e/monzonite are presented in Table2. Majorelem ents were analysed byXRF on fusedglassdiscs at theGeologi cal SurveyofDenmark and Greenland,traceelements by XRF analysis on powder mounts at the Geological Institute, University ofCopenhagen,anduranium bythedelayed neu- tron methodbyTracechemAlS,Cop enh agen.

The diorit e may be interpr eted as a metasomatically modi fiedgabbroand the monzodiorite/monzoniteasafur- ther stage in the modificationofthe gabbroor perhaps as differe ntia tes of agabb roic magma.Thus,thecontent s of (aO,MgO,Ni,Cr,Vand normative

an,

ne

Table 2.Chemicalanalysesofdiorite andbiot itemonzonite from the Myklearea,SouthNorwayandC1PWweight norm s (m ajoreleme ntsin weightpercent,trace elements inppm).

Dior ite Biot ite mo nzoni te

Sam ple 81791 81755 77173

Si02 48.41 51.99 52.96

Ti02 2.84 2.42 2.06

AI203 16.19 15.97 16.84

Fe203 3.99 3.29 3.22

FeO 6.96 6.33 5.40

MnO 0.18 0.17 0.16

MgO 4.96 3.62 3.00

CaO 6.36 5.85 6.19

Na20 4.54 4.48 4.66

K20 2.31 2.81 3.0 1

P20S 0.65 0.72 0.75

Volatiles 1.48 1.51 1.21

To t al 98.88 99. 16 99.46

Or 14.03 17.10 18.11

Ab 36.12 41.2S 40.14

An ^17.44 15.58 15.73

Ne 3.44 0 0

Di 8.49 7.60 8.51

Hy 0 6.76 4.37

01 10.75 3.21 2.30

mt 4.29 3.53 5.05

1/ 4.06 3.46 4.02

ap 1.40 1.52 1.77

Rb 95 114 122

Ba 925 910 909

Sr 1110 990 1170

Pb 2 5 13

La 61 lOO 87

Ce 119 202 185

Nd 56 93 84

y _{43 68 57}

Zr 288 724 551

Nb 63 91 76

Th 8 17 22

U 4.0 5.0 5.3

Zn 112 118 103

Cu 63 91 76

Ni 57 31 26

Se 15 16 16

V 224 163 154

Cr 20 10 10

Ga 21 22 26

NGU-BULL441,20 0 3 - PAG E37

conte nts ofK,O,normative or,ap,andofmost traceelement s increasefrom dioriteto monzonite.

The chemicalcompositionof the diorite is moreevolved than mostanalyses fromotherbasiccomplexesin theOslo Riftbutit is rat hersimilar to analyses reported from basic necks at Son stebyfl akeneand Dignes(Neumann etal.1985, table 3)and to analysesof sorkedalite from Kjelsas (Bose 1969).

The chemical composit ion of the monzodiorite/mon- zoniteis more evolved thanthe rocks analysed by Neumann etal.(1985, table 3)butit is similartothatoflarvikite1 from the Sandecauldro n(Andersen1984)andof leucocrat ic kjel- sasite from Kjelsas (Bose 1969).The monzodiorite/mon- zonite has a lessevolved chemical comp osition than the larvikitesof therift zone;cf.the compilati on ofanalyses by Neumann(1980).

Discussion

The scattered xenolit hs ofgabbro and related rocks at the southernend oflake Mykleprobably representthe remains of a former gabbr oic neck similar to the occurrences described from elsewherein the province(Neumannet al.

1985).The occurrencemay beinterpretedas a continua tion of one of the linear belt s of gabbroic neckstowards the south (Fig.1).In contrast to the occurrencesfarther north, which intru dethe Precambrian basement and its cover of Palaeozoic sedim ent aryrocks,the Mykle occurrence forms xenolit hs insyenit icand granitic pluton ic rocks.Xenoli t hsof gabbroic rocksin pluton ic rockshave been described from otherplaces in the OsloRift ,the most significant example occurring in the syenitic ring dyke of the Glit revann caul- dron (Oftedahl 1953) which is located in the above-men- tioned lin ear belt and ending in the Mykle occurrence.

According to Neumann et al.(1985, fig.1).gabbroic rocks intruding Palaeozoic sediments at Son st ebyfl akene are in contact wit hintr usiverocks.Therelati onshipbetween gab- broand the other int rusionsis not described.The map of the Glitrevann cauldron (Oftedahl 1953, 1960) indicates an occurrenceof gabbro in the northern partofthecauldronin contact wit h Palaeozoic sedimentary rocks and rhomb por- phyry lavas. Thisoccurrence was described by Barth(1945, fig.13).Neither ofthe descriptions includes information on the relati onships betweentheserocks.

Barth (1945,p.67) suggested thatan occurrenceof tree- tolit e at Kjelsasin thenorthern part of the Oslo Rift may have been form ed by remelting and assimilat ion of a 'buried'gabb roicneckby alarvikit ic magma. According to Bose(1969)thisrock,serkedalite,is associatedwit h kjelsasite tow ard s which it has developed gradual contacts.Kj elsasit e may bedescribed asa plagioc lase-rich variety of larvikit e.

Thegenesisof the sorkedalit e is not clear.

It isnot known whetherxenolit hs ofgabbroicrocksare ofwidespread occurrencein theplutonicrocksof the Oslo Rift. A largepart ofthearea is coveredby forestsand out- cropsare scarceandare mainly restrictedtoshores of lakes,

NGU-BULL44 1, 20 0 3 -PAGE 38

fjordsandrivers androadcuts.TheMykle occurrenceshows that xenoliths of gabbroicrocks should belookedforalsoin other partsof therift zone.

The spatial associati on of gabbro,diorite and biot ite monzonite indicat esthattheserocks arecloselygenet ically associated. In consequence of this, the occurrence of larvikite xenoliths in biotite monzonite at Mykle indicates thatlarvi kit e,in thisplace, is older thanthegabbr o andits associated rocks.This,to ourknowledge,providesthe first observation of the age relationship between gabbro and larvi kit ein theOsloprovince.It brackets the emplacemen t of thegabb robetween thatof thelarvikite andthatofthe syenitic rocks. It should,however,be point edout that map- ping of the Siljan map-sheet has demonstrated several stagesofemp lacementof larvikit e and occurrences of fine- grained larvikite containing cm-large basic xenolit hs (unpublished observat ions). In addition,the large Larvik larvikite massif was form ed by a succession of intrusion s (Pet ersen 1977).However,larvikite expo sures in theMykle areaare so few and poor that it has beenimpossible to dis- tinguish separate intrusive phases and to decide if the larvikite,from which thexenolit hs in biotite monzonitehave been derived, could be an early orlate memberof a succes- sionof intrusion s.

Age determinations of Oslo Rift rocks presented by Sundvoll et al.(1990) give gabbro ages of about 265Ma whereas thepluton ic rocks of the weste rn part of the rift give agesof 281to 267 Ma andthe syeniteofthe Glitrevann cauldron 266 Ma.However,the nordmarkit icsyenit eof the Silj an distric t, which is definitely younger than the lake Mykle gabbro,is reportedto be 270Ma old.Theage relations therefore have to be further exploredand constrained.

The association of gabb ro, diorite and monzonite at Myklemay representdifferentpartsof a zoned necksimilar to the necks described byNeumann et al.(1985) and others.

Itmay,how ever, alsorepresentstages inthe transformat ion of the original gabbro into diori ticand monzoniticrocks asa result of reactions wit h fluids released from theenclosing syenitic and granitic melts.Bot h views are in accordance with inform ation obtai nedonthechemistryand mineralogy of theserocksas described in detailbyPedersen(1994)and briefly outlined in the presentpaper(Tables 1,2). However, the occurre nceof patches wit h dolerit ic texture inthehet- erogeneous monzoniticrocks(Fig.6) and the occurrenc e of two coexisting c1inopyroxenes indi cate that these rocks prob ably arerecrystallised, whichmay support the viewthat the monozonitic rocks rich in biotite are secondary after gabbro/diorite.

Conclusions

Larvikite in the Mykle area was intruded by gabbroicrocks.

These were intr uded and engulfed by later syenitic and granit icbodies.Xenolit hs of gabbroshouldbeloo ked for in the pluton icrocksof the Oslo Rift,and especiallytheobser- vation thatthe gabbro at Mykle appearsto beyounger than

LONEPEDERSEN &HENNI NGS0RENSEN

larviki te should encourage the search for gabb roic intru- sions in thelargelarvi kitemassifs of theriftzone.

Acknowledgements

Thefieldwor k of L.P. and H.S.was supportedbythe Geological Survey of Nor way and the Danish Natu ralScience ResearchCouncil. During a st udy visit at the Geological Museum,University of Oslo, L.P.was intro- ducedto the gabbroic rocks of the Oslo province by Professor E.-R.

Neum ann.Professor Tom Andersen, Universityof Oslo,offered valuable adviceduring the planning andexecutionof the fieldwork.Major ele- mentrock analyseswere providedby the Chemical Laboratory of the Geolog ical Survey of Greenland (now the Geological Survey of Denm ark and Greenland),trace element analyses by the Geological Institute, UniversityofCopenhagen (J.c.Bailey),and microprobe analy- ses ofmineralswere carried outat Geological Instit ute, University of Cop enhagen (J. Ronsbo). Britta Munch and Ole Bang Berthelsen, Geologi calInstitute, UniversityofCopenhage n,offered valuable assis- tance wit h the preparat ionofillust rations.We are grateful for construc- tive criticism of the manuscriptfrom Drs.Odd Nilsenand Brian Robins.

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