The Stabben dyke

BR/ANA.STURT, HELGEASKV/K & BJ0RN SUNDVOLL NG U-B UL L 43 2 ,1 9 97 - PAGE 71

A probable Permian hydrothermal alteration age for a quartz syenite dyke from Stabben, Nordmere

BRIAN A.STURT,HELGE ASKVIK&BJ0RN SUNDVOLL

St u rt,RA.,Askvik, A.& Sundvoll, B.1997:A pro ba b le Permianhydr otherm al altera tio nag e fora quartzsyenitedyke fro mSta b ben , Nordmo re.Norges geologiskeundersekelse Bulletin432,71-75.

ARb-Srwho le-rock iso chr onfro m a quartz-syenite dyke atSta b be n,Nord more,hasprovi de danage of 263±6 Ma.

This could betakento ind icatea crystallisationagefor the dyke, i.e.,Early Permian.The age,how ever,doesnotcon- formwithanearlier pub lishedage for asimilardyk e on thenearb yislandofTustna,orwit ha publishedMiddle/Late Carbonifero uspalaeomagnetic po le for theStabb endyke.Itmaybe that theisochronis dat ing hyd rot herm al alter- atio nof the dyke.The presenceofsuchdykes wit h adeepmantle magm at ic source,alo ngtheMore-Trondelag Faul t Zone,emphasisesthe fundam ent al nat ure of thefaul t and ind icatesthat a periodoftension al condit io ns pert ain ed durin gthetime ofemplacementofthedykes.

Bria nA.Stu rt&Bjern Sundvoll', Norg esqeol oqiskeundersekelse,PostBox 3006-Lode, N-7002Trondheim,Norway.

He/geAskvik,Geol ogisklnstitutt,UniversitetetiBergen,N-5007 Bergen,Norway.

'Permanen t address:Minera /ogisk-ge%giskMuseum,Sarsg t.1,N-056205/0,Norway.

Introduction

The More-Tronde lag Fault Zone (MTFZ) is a major, NE-SW- trend ing str uct ure transect ing theCaledon ides of Cent ral Norway (Gabrielsen & Ramb erg 1979, Gronli e & Roberts 1989), and is regardedasa fault-zone wit halong andcom- plexhistory (Grenlieetal. 1994).The occurrenceofrare, un- defo rmed and unmeta mor phosed,quartz syenite dykesin the Nordmore segment of the MTFZ has been repo rted by Raheim (1974),Askvik & Rokoengen (1985) and Sturt &

Torsvik (1987).The presence of such intrusions within the general settingof the MTFZis of interestas it indicatestensi- onal condit ions along the fault zone either before or at the timeoftheiremplacement.

A quart z syenite dykeon theisland ofTust na(Fig.1)has yielded aRb-Srisochron of297±8Ma (Ra heirn 1974),and this hasbeentakento indicate anintr usionagefor the dyke.

In reality,however, theslopeoftheisochron is cont rolled by a singlebiotiteanalysis and hence the age mustbe interpre-

ted to representa minimum age for the dyke.Raheimsho- wed that the mineralogy and geochemistry ofthe Tustna dyke is very similar tothatofdykesoftheno rdmarkit e-eke- rite series in theOsloGraben.Accordingto Raheim (1974) theTustnadykeis apparent lyolder than thenordmarkit e- ekrite dykes which have yieldeda whole-rock Rb-Sr isochron age of276±7 Ma (Heier &Compston 1969).Sturt&Torsvik (1987) made apalaeomagnet ic study of asimilar quartz sye- nite dyke,onthe nearby island of Stabben(Flq.l),and using the published age of the Tustna dyke produced a pole of as- sumed Mid/LateCarboniferous age.

In ordertoconfirm ofthe age of intr usionof the quartz syenite dykes,thepresent aut hors decidedto make anex- tensive collect ionof materialfrom theStabbendyke which is excellentlyexposed atsea-levelon the sout heaste rncor- ner of theislandof Stabb en.From these samples8 werese- lected wit ha reasonable rangein Rb-Srvaluestoform the basis,hopefully,for adefinit ive isochron.

Fig 1.Localit y map.

NGU-BUL L432,1997- PAGE 72 BR/ANA. STURT,HELGEASKV/K

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Fig 2.Essentially concordantcon- tact between quartz

syeni te dyke and country -rock banded gneisses, SE Stabben .

The Stabben dyke

TheStabbendyke intrudes the Precambriancountryrocks at the sout heastern and southwestern corners,respectively,of theisland s of Stabbenand Ert vaqo ya(UTM648042-672044) (Fig.1).The dyke,whi ch is c.3.5 m thick,can be traced for som e 1200malo ng the southern shore of Stabben. It then passesbeneath sea-leveland reappears as two dykes, one up to 2 m and another<50 cm in thickness,on the south- western corner of Ert vaqeya,These dykes can be traced along strikefor a further1300 m.The total exposed length of thedykes is thus >2500 m.The dykes trendat c.N80oE,paral- lel to a faultalong Vinjefjord.

The maindyke is essent iallyconcorda nt with the foliat i- on of the country-rock gneisses(Fig.2), tho ug h it may be strong ly discordantwhere it follows prominentfracturesor smallfaults(Fig.3) .The contacts arealsocom m o nly displa- ced along small faults,indicating tectonicactivityafterdyke emplacement.The dyke rock has a characteristicpalebrow- nish-redcolourthough in placesit is bleached adjac ent to joi ntsurfaces.Thelatt er aresometimesdecoratedbyspora- dicpyriteand calcitecrystals,indicat ingsomepost-crystalli- sationhydrothermalalte rat ion.

The dykerockconsists essentiall yofscatteredalkalifeld- spar(5.4%)and sporadic mafic phenocrysts(2.9%)in a fine- grained alkali feld spar (80.4%) matrix.In thin-sect ion it is seen to have a porphyrit ic to glom eroporphyrit ic text ure (Fig.4)wit h phenocrysts and glomeroporphyrit icaggregates of alkali feldsp ar(up to 5 mm)and smaller phenocrysts of quartz(8.8%)and an alteredpyribole(nowchlo rit e)setina finer grained matr ix.Raheim (1974) has shown that the Tustna dykebearsaegerine-aug ite both asphenocrysts and

Fig 3. Strong lydiscordant contactbetweenquart zsyenit edyke and count ry-ro ck gneisses,wherethe dykeint rudes alonga smallfault .

BR/ANA.STURT, HELGE ASKV/K&BJ0RN SUNDVOLL

Fig4.Glom eroporphyriticaggre gate ofalkali feldsp ar in fine-g rained matrix. Scalebar

=

inthe mat rix.Unfort unately,this mineralisnot preservedin the chlorite aggregates in the material examined from Stabb en.The mat rix consists mainly of sub hedra lalkali feld- spar with lesser amountsof anhedra lquartz,opaques,euhe- dral apatite and aggregates of chlorite,andthe tabular feld- spar shows a fairly well marked,parallelorientation.

The chilled marginsof the dyke consist ofabout 5%scat- tered glomeroporphyritic feldspar (diamet er 3-5 mm),20%

idiomorphic K-feldspa rmicrocrysts(0.1 x0.3 mm),1-2%sub- idiomorphic quartz,a few idiomorphic apatites (up to 1 mm in length),some pseudomorphs after pyribole and a few grains ofidiomorphic opaque minerals.Theseareall setin a very fine-grain edmatrix.

Mean Mol.p. Tustna Oslo

Si02 66.05 1.099 67.19 67.80

Ti02 0.32 0.004 0.61 0.24

AI203 17.11 0.168 15.88 16.10

Fe20₃ 1.78 0.011 2.33 1.41

FeO 0.96 0.013 0.77 0.65

MnO 0.04 0.001 0.19 0.12

MgO 0.22 0.005 0.23 0.28

CaO 1.27 0.023 0.60 0.72

Na20 6.41 0.103 6.14 6.50

Kp 5.05 0.054 5.54 4.80

P20S 0.09 0.001 0.02 0.07

CO2 0.02

L.0. 1. 0.30 0.68 0.07

SUM 99.60 100.18 98.96

Ta b le 1. Mean,major element,chemi calcompositio n ofthe quartz syenitepor p hyry dykefrom Sta bben-Ertvaqoya(Sana lyses.thispaper).

The ta blealsoshowsthe similarity betw ee n the compositionof the Stabben-Ertvaqoyadyke and that on Tustn a,and with the nordmarkite- ekerit e seriesof theOslo Graben(Raheim 1974).

NGU-BULL 432, 1997 -PAGE 73

The feldspar phenocryst and ground mass feldspars are bot h unmixed to an antiperthite with irregular exsolution patches,blebsand stringsof microclinein albite.This isindi- cated by extinction anglesand relief under the polarising microscope and confirmed by microprobeand electron mi- croscope analyses.The patches,blebs and strings range in diameterup to 100 microns.

Chemical analyses of the dyke rock are given in Table 1.

According to the modalcomposition of the rock the alkalies must be almostentirely sited in thealkali feldspar,though nat urallysome of the Na would have resided in any former aegerine augite.The mean major element analyses of the rock have beenrecalculated to cation proport ions giving a Na:Kratio of0.103:0.054(Table 1).Elect ron-microp robe ana- lyses indicateCa2+,togivean averagevalueof0.005based on 24oxygens.Thisindicatesaprim ary alkalifeldsparcom- position close to Or34(mlnlmurnlx b., with anegligi ble An component.

Thefeldspars areturbidand clouded byminut ehaema- tit e flakes,and a formerpyribole is nowpseud om orphed by chlorit e. Secondary calcite and sporadic radiatingaggrega- tesofan unidentifiedyellowish-greenmineral with low bire- fringence are alsopresent. Thesefeat ures attest toa secon- daryhyd rot hermal alteration ofthe dyke.

Rb-Sr geochronology

The analytica lwork was carriedout atthelaborato ry of iso- tope geology at the Mineralogical-Geological Museum, University of Oslo.lsochron calculationsfollowed the proce- dures of York(1969) usingthe values 1.42 xl0.¹¹a' and 6.54 x 10-" a-, for the decayconstant of 8'Rb(Steiger&Jager 1977).The_notationfollows De Paulo & Wasserburg(1976a) using UR values for Rb-Sr reported by De Paulo &

Wasserburg (1976b).

The quartz syenite dyke from Sta bben gave an age of 263±6Ma wit han MSWD of1.62 andaninitia l 8'Sr/86Srra- tioof 0.7102±6. Isoto pic analysis showed thedykerocksto have £Sr(T)=+84.8and ENd(T) =-10.1 (Fig.5).Thesevalues indicate that the syenitemagmahasbeen stro ng lycontami- nated by crustal material and shows little evide nce of an

Sample ppm Rb ppm Sr 8'RbfB6Sr 8'SrfB6Sr

lA 148.7 59.25 7.2824 .73670±3

lC 137.9 35.13 11.4085 .75282±3

IF 136.2 37.27 10.6149 .74996±3

lG 145.8 44.01 9.6253 .74671±3

1J 135.4 58.57 6.7052 .73541±3

lM 63.6 40.89 4.5107 .72714±3

2A 137.8 31.88 12.5696 .75724±3

-

2B 148.0 29.68 14.5011 .76433±3

Table2. Rb-Srcon tents and isot op ic data

NGU-BULL 432, 199 7 - PAG E74 BR/ANA.STURT,HELGEASKV/K

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.780 r-

I-

.760 I-

I-

en

E£ .740 f---

~

(f)r-- eo

I-

.720

QuartzSyenite Dyke Stabben,Nordmme

T

=

MSWD

=

E

= +

.700 ^I

5

I

10 87Rb/86Sr

1

15 Fig 5.lsochron

diagram for the 5tabbenquartz syenite.

eventual mantle origin.The analytica l data are shown in Tab le 2.

Discussion

The authors submit that the isochron age of 263 ±6 Ma could,at first sight,be considered to provide an age forthe crystall isat ion ofthe Stabbenquartzsyenitedyke. It is,howe- ver, sig nificant lyyounger thanthe age publishedbyRaheim (1974)forthe quartzsyenitedyke from Tustna,and whichis likely to represent a minimumage for its emplacement. The petrography and geochemistry of the Stabben and Tustna dykesare virtually identical,except forthe more marked hy- drothermal alteration of the Stabben dyke,and it islikely thattheywere coeval. Sturt&Torsvik (1987). moreover,ob- tain ed a Late Carboniferous palaeomag net ic pole for the Stabbendyke,andanexaminat ionofaddit ional palaeomag- net ic data from this same dyke hasconfirmed this result (T.Torsvik, pers.comm.1996).These observations thus do not lend supportto an interpretation of the isochron as an age of crystallisation.Indeed,theisochron may, in fact,be dating the hydrothermal alteration ofthe dyke.

The isochron age of theStabben quartzsyenitedyke re- portedinthispaper fitswell wit h the isochron recor ded for the nordmarkite-ekeritedykesbyHeier&Com pston(1969),

and with theag esof syenit ic rocksin the north ern part of the Oslo Graben (e.g.,the 0yang en syeniteat266±8 Ma)re- portedin Sundvo ll &Larsen(1990).lt is alsoofinterest that theisochron age of thisdyke isalmostidentical with the Rb- Sr isochron age for the Torpaklint syenite dyke,inScania,at 261±6 Ma(Sundvoll & Larsen 1993)and is distinctlyyoung- er thanthe meenaite syenite dykes ofthe Osloarea at 290.:!:

11 and 285±7Ma(5undvoll&Larsen 1993).The presence of dykes ofsyenitic composition,which apparent ly have ape- trological affinity to the magmatic province of the Oslo Graben,is of consid erab le interestandim p lies a deep mant - le source for theparent magma. As pointedoutabove,ho- wever,the£ valu esforSr andNdshow thatthe syenitemag- mawhich gaverise totheStab ben dyke had been strongly contaminated bycrustalmaterial.Th is deep magmaticsour- ce emphasisesthe fundam ental nature of the MTFZ. The Walls BoundaryFault in Shetland,whichisconside red to be a correlat ive struct ure to the MTFZ,hasfor examp le been shown to penet rate the crust/mantle boundary (McBride 1995).

The positi on oftheStabb en dykein timeand spaceis of considerab le interestand str ongly suggests that tensional condit ions existedalong thispart of the MTFZduring Late Carboniferous/EarlyPerm iantimes.Gronlie&Torsvik(1989) concludedfrom thepalaeom agneticsignat ure of minerali-

BRIANA.STURT, HELGEASKVIK&BJ0RN SUNDVOLL

sed open-net wo rk breccias and thorium-enriched carbona- te veins along theMTFZ in innerTrondheimsfjord,that these were formed during the Permian in an extensional regime.

Palaeomagnetic studies and Rb-Srdataon biotite (256 Ma) also indicate aPermian age for alamprophyre dykefrom Ytterey (Torsvi k etaI.1988).Thus,there seemstobe accum u- lati ngevide nce that, foratleastpart of Permo-Carbo niferous times, the MTFZwas operating as an extensionalstructure.

Thereremains an enigma ,however,concerningthe lack offit betweentheisochron age presented here,and the Late Carboniferous palaeomagnetic pole position (Stu rt&Torsvik 1987) which fits well with the Rb-Sr age published by Riiheim (1974) for the Tustna dyke .It is plannedto collect samples of the Tustna dyke to obtainboth palaeomagnetic andnew geochronologi cal datatoad dress this issue;and U- Pbzirconanalyseswill beobt ained from both theStabb en and theTustnadykes inordertotryto resolve the issue of the initial crystallisa tionageof the dyke system.

Ackno wledgements

We wish tothank a number of our colleagues,part icularly Dr.Trond Torsvik,forinterestingdiscussions concerningthe Stabbendyke.NGUis thanked for providing theresou rces necessary for carryin g out the study.Dr.ArneRaheim is thankedforsuggesti ng improvement s in his reviewof themanuscript and for his kind offerto carryout thezircon datingof the dykes.

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Manuscript received June1996;revisedmanuscript accepted Apri/1997.