U-Pb zircon ag e of the Meklevatnet granodiorite, Gjersvik Nappe , Central Norwegian Caledonides
DAVIDROBERTS& ROBERT D.TUCKER
Roberts,D.&Tucker,R.D.1991:U-Pb zirconageofthe Moklevat netgranodiorite,GjersvikNap- pe,Central Norw egianCaledon ides. Nor.geol.unders.Bull.421,33-38.
The Moklevatnetgranodio rite in the GjersvikNappe has yieldedaU-Pb zirconage of 456 ± 2Ma from three, concordant, multigra infractions.This can alsoberegarded asa maximumage,Early MiddleCaradoc,for the oldest sediments in the unconformably overlying LimingenGroup.Taken in conjunctio nwithisotopic datareportedearlier,the zirconage for theMoklevatnetplutonindica- tes thatthe magmatic activityrepresentedin theGjersvikNappe extend ed over a periodof 25to 30 millionyears.
DavidRoberts,Norgesgeologis ke undersokelse,Postboks 3006-Lade,N-7002Trondheim,Norway.
Robert D.Tucker,Jack Satter lyGeochronologyLaboratory,RoyalOntarioMuseum,100Queen's Park,Ont. M5S 2C6,Canada.
In troduction
Of the severalK61iNappesintheUpperAlloch- thon of the Caledonides in Nord-Trendelaq, Central Norw ay, the Gjersvik Nappe has the most voluminousigneouscomponent, compri- singa thick bimodal volcanicassemblage and coeval to younger plutonic bodies (Halls et aI.1977,Lutro 1979,Reinsbakke n1980).Lying unconformably upon the eroded magmatic complex is a sequence of low-grade polymict conglomerates,calcareous sandstones,phyll- ites and thin volcanites, the Limingen Group, ofunknown buthitherto assumed Late Ordovi- cian/Early Silurian age.
While the age of the sub-Limingen mag- matic complex has generally been assumed to be Early toMiddle Ordovician (Gale& Rob- erts1974,Hallset a1. 1977),no isotopic dating had beenavailableuntil Kullerud et al. (1988) produceda U-Pb zircon ageof 483 ± 4Ma for a foliated concordant trondhjemite body. In thiscontributionwe report a U-Pb zirconage from one ofthe youngest but most extens ive plutons in thismagmaticcomplex - the Mekls- vatnet granodiorite.
Geological setting
The Meklevatnet granodiorite is adominating element in the geology of the Grong region (Roberts 1989) and conspicuous even at 1:1 million scale (Sigmond et a1. 1984). Covering an area of close to 400 km2 it is situated in the southernmost part of the Gjersvik Nappe where it has been variously described astrend-
hjemite, quartz diorite or granodiorite (Gale 1975, Halls et al.1977, Kollung 1979)(Fig.1).
Gale (1975)referred to the body informally as the Sanddelatrondhjemite.However,sincethe valley Sanddela is located south of the ex- posed southern limit of the pluton, we prefer a more appropriateand acceptablegeograph- ic designation;and suggest the name Mekle- vatnet. Also, it can be demonstrated that, in the Meklevatnet area,the rock is a granodio- rite rather than a trondhjemite (see below).
Fieldrelationships of the plutonic members of the igneous complex in this southern part of the Gjersvik Nappe have been described and discussed mainly by Gale (1975) and Halls et al. (1977). Gabbros intrude the vol- canitesbut these are in turn cut by,and enclo- sed as xenolithswithinthe granodiorite,which thus appears to be the youngest plutonicbody in this area.The gabbro and granodiorite are, however,considered to be broadly cogenetic.
The ratio of rafts of gabbro, volcanites and some schists to the enclosing granodiorite in some areas is quite high, so much so that Gale (1975) considered these localities to be situated within the roof zone of the pluton.
Thegranodiorite displaysa variablebutlocal- lypervasive foliation whichparallelsthe schis- tosity in both the older volcanites and the younger sediments. This is regarded as a metamorphic fabricdatingto the Late Silurian, Scandian orogenesis. No older,pre-granodio- rite,foliate structure has beenrecorded from the rocks of the Gjersv ik Nappe. The south-
34 DavidRobert s & RobertD.Tucker NGU·BULL.421,1991
11 /
... ARKO SE A D t.rrrunqen
... CO GLO ERATE Group
GRA ODIORIT E GASB RO
E TS
D KERA OPHYRE.
o
,
Fig.l.Simplified geologicalmapof the southwestern partof theGjersvikNappe,showingthe location(circledcross)of the dated granodio rite samplenear M0kl evatnet. Thegranod iorite extendsover a larger area further east where itis direct contactwiththe overlyingbasalsediments ofthe Limingen Group(Gale1975.Kollung197 9).Intheinset map.GN=Gjer- svik appe,HNC= HelgelandNappe Complex and GOC= Grong·OldenCulmination.
ern boundary of the granodiorite north of Sandde la(Fig.1), in contact with volcanic and sedimentary rocks. is locally tectonised, in places quite strongly, a feature arising from competenc ycontrastsbetween lithologies rath- er than major thrust -faulting.
A previous attempt at dating the body, by the Rb-Sr method, using 3 granodiorite sam- ples together with one sample of assumed cogeneticgabbro,produced an isochron age of 433 ± 10 Ma with an initial Sr ratio of 0.70427 (Rc!lheim et al.1979).This wasinterpre- ted as probab ly representing a deformational and low-grade metamorphicevent;and,thus, a minimumage for emplacementof thepluton .
Petrograp hy and geochemistry
The granodior iteat the sampledlocality,c.500 m southeast of the lake Meklevatnet (UTM 8765/5390,map-sheet 1823IV Grong) is med- ium grained(2-5 mm),weaklyfoliated and pin- kish-grey to greenish-greyin colour.Itis gene- rally equigranu lar, although some feldspars reachup to 8mminsize.Plagioclase (oligocla- se.c.An.j,quartz andK-feldspa rare themajor minerals,the plagioclase showing moderately advanced saussuritisation. Accesso ry phases
Table1.Majorelements(W1.%).sometraceelements(ppm.
exceptSrand BainW1.%)andCIPW normsfor the grano- diorite samplefrom near Meklevatnet.
SiO, AI,o. T,O, FeO Fe,o. MgO
64.64 17.01 .36 .57 2.42 .69
CaO Na,O K,O MnO P,O,
3.73 5.09 2.61 .06 .14
Nb Zr Y Sr Rb V Ba
7 136 16 -.14 77 46 .12
q or ab an di hy mt hm if ap
16.2 16.6 43.1 15.3 1.2 1.7 1.1 1.7 .7 .3
are epidote, hornblende,biotite,zircon,sphe- ne,apatite, chlorite and opaques. The green colorationstems from epidotisation,notedfirst byGale (1975).
A c.l kg specimen taken from the c.50 kg of sample materialcollectedfrom theMeklevat- net locality has been analysed for major and trace elements. The sample was broke n into small pieces before crushing, to ensure that there were no xenoliths present.Of the major elements, the high AI,O) and Na,O, and low MgO (Table 1) stand out in relation to values for average granodiorites (Hatch et aI.1951).
Another characteristic isthatof highcontents
NGU-BULL.421,1991 U-Pb zirconage, Moklevatnetgranodiorite 35
Fig.2.NormativeAn-Ab-Ordiagramshowingthe composi- tion of theMeklevatnst granodioritefromthesampleloca- lity.Diagramfrom Barker(1979).
of Sr and Ba, which is a feature common to a number of calc-alkaline plutons elsewhere in the Norwegian Caledonides (Nordgulen et al. 1988). On a plot of normative Or,Ab and An (modified by Barker (1979) from O'Connor (1965)), the Meklevatnet pluton falls in the granodiorite field (Fig. 2); this is a trait con- firmed by other diagrams. On a normative quartz-orthoclase-albite diagram, not shown here,the sample falls close to the calc-alka- line trend line.
Analytical methods and results
Mineralseparationwas performed using con- ventional magnetic- and density-separation procedures. Zircon fractions were prepared by hand-picking generally clear, crack-free grains, and abrading them after the method of Krogh (1982) to increase the degree of concordancy. Zircon dissolution,spiking with an enrichedllllPb_"'U tracer solution,and sepa- ration of U and Pb for mass spectrometric analysis followed the method of Krogh (1973). The isotopes of U and Pb were purified,elu-
ted andloaded together ona Refilament with Si-gel and H,PO. and measured in a VG 354 mass spectrometer in the temperature range 1500° - 1630°C witheach runlasting approxi- mately 2 hours.
Zircon analyses were corrected for U blanks of 1-2 pg and Pb blanksof10 pg orless; any remaining common-Pc was assumed to have the isotopic composition given by the model of Stacey & Kramers (1975) at themeasured age of the rock. Total common-Pb abundan- ces range between 10.5 and 13.1 pg and measured l<l6pb/"" Pb ratios range between 30,914 and 12,960 (Table 2), indicating that the uncertainty in theinitial common-Pb compo- sition has a negligible effect upon the age calculation.
Further details regarding the chemical,mass spectrometric and data reduction procedures are summarised in Tucker et al. (1991). Decay constants and isotope abundance ratios are those of Jaffey et al. (1971), recommended bythe InternationalUnionof GeologicalScien- ces (lUGS) ( Steiger & Jaqer 1977). Cited ages are the average 207Pb/l<l6pb ages and, because all analyses are concordant,the un- certainty of the age was obtained by estima- tingthat positionof concordia which is overlap- ped significantly by the error ellipses (Fig.3, calculated after the method of Ludwig 1980).
Age errors are given at the 95% confidence level.
Three multigrain fractions of coarse (>200 urn) short-prismatic zircon were selected for isotopic analysis, including tips broken from prismatic grains, middle parts of prismatic grains,and whoieshort-prismaticgrains.The- se parts were selected and analysed separate- ly to test for the presence or absence of in- herited zircon which is known to be a com- mon componentin short-prismaticzircon popu- lations.No inheritance (or secondary Pb-Ioss) is indicated by the results, however, as all
Or An
Ab
Table2.U-Pb zircondata.
Fraction wt. U Pb Pb Th/U 206Pbl 206Pbl 207Pbl 207Pbl 207Pbl
comm. rad. 204Pb 238u 235u 206Pb 206Pb
(l1g) (ppm) (pg) (ppm) (M a)
1 + 100,t-p,A 118 378 11.9 29.7 .58 17,157 .07335± 28 0.5678±23 .05614±8 457.9 2 -:200,s-p,A 100 371 13.1 29.3 .61 12,960 .07326±30 0.5663±25 .05607±9 454.9
3 + 100,mp,A 254 281 10.5 22.1 .60 30,914 .07311±26 0.5656±21 .05611±8 456.7
Thezirconsare clear,transparent grains. fromleast paramagneticfractionsofFrantzseparates,free of cracksandinclusi- onsunlessotherwise specified.+100 =>150I1m,-200 = <75I1m;sop=short-prismatic (lengthto width<3),mp=middle partsof prismatic(length towidth >3)grains;top= tips from prismaticgrains;A-abraded.
36 DavidRoberts& RobertD.Tucker NGU-BULL.421.1991
(Gale&Roberts 1974,Hallset al.1977,Reins- bakken 1980), tending towards calc-alkaline compositions higher up,has extended over a period of 25 to 30 million years, from c.
Middle Arenig to Early MiddleCaradoc(Tuc- ker et al.1990).Such a timespanis compara- ble to that of island arc/basin systems in the western Pacific Ocean(Crawford et al. 1981),
The 456 ± 2 Ma age for the granodiorite can also be regarded as a maximum age for the sediments of the Limingen Group. The duration of the hiatusbetweenmagmatismand uplift,erosion and sedimentat ion isimpossible to determine; yet one may infer this to have been comparatively short.Acceptinga Middle Carado c or younger age for the Limingen Group sediments makes them time equiva- lents of the uppermost parts of the Lower Hovin and succeeding Upper Hovin Groups of the western Trondheim region (Roberts 1975, Hallset al.1977).The basalconglomera- te of the Limingen Group may thus occupy a comparable stratigraphicpositionto that of the Stokkvola Conglomerate . Higher parts of the Kjelhauqen Group in the Meraker Nappe of eastern Trendelaq (Wolff 1979)may be chro- nostratigraphic equivalents of the Limingen Group.Another unit oflikelysimilarageis the Blasj6 Phyllite in the Stikke Nappe in lower parts of the MiddleK61i (Claessonet al.1988).
Although noregional metamorphicevent has been report ed from the Gjersvik Nappe prior to Limingen Group deposition, Halls et al.
(1977) have described features which they ascribe to pervasive sea-floor metamorphism.
The penetrative foliation in the granodiorite and coplanar schistosityin the Limingenmeta- sediments,on the otherhand,are considered to date to the Scandian orogenesis.
Thisscenario differs somewhat from that in theadjacent Helgeland Nappe Complex (HNC), above and to the west ofthe Gjersvik Nappe.
This exotic terrane has a history of igneous activity ranging from Late Cambrian to Early Silurian time,and includes a significant tecto- noth ermalevent,withthrusting,inthe Ordovi- clan(Nissen 1986,Nordgulen& Schouenborg 1990, 0.Nordgu len pers.comm.1990) in addi- tion to the later Scandian deformation and metamorphism.In the HNC the Heilhornet Plu- ton has yielded a U-Pb zircon age of 444
±11 Ma (Nordgulen & Schouenborg 1990) whichthus overlaps,withinerror,withtheage of the Meklevatnet granodiorite.The initialSr ratio of the Heilhornetgranite(c.0.7070),how-
.57 .5
Moklevatnctgra nodioritc 456 :!:2Ma
.56 450
.075 465
:Jcc
;:'j
--
.o 46 0
.074 "~
N
.073
.071"=_--'-_ _'-,-_ --'-_ _ '-=-_--'-_ _L-...J
.55 .072
three analysesare concordant(Fig.3)indicating an emplacement age of the granodiorite of 456 ± 2 Ma.
Fig.3. U·Pb concorcra diagram of zircon analyses (1, 2, 3,Table2)from the Moklevatnetgranodiorite.
Discussion
The age of 456 ± 2 Ma forthe Meklevatnst granodiorite complements the reported U-Pb zircondate of 483 :!:4 Ma from an albitetrond- hjemite in the northern part of the Gjersv ik Nappe(Kullerud et aI.1988).The 483 Ma trend- hjemite is located in the stratigraph icallylower part of the magmatic assemblage(Lutro 1979).
Field relationsh ips show that the Mekievatnet granod iorite is the youngest major piuton wit- hin this nappe, and that in these southern areas we are close to the roof of the intrusion (Gale1975).Moreover,Gale (1975),has descri- bed an erosional and weathering surface of the granodiorite in one area where the con- tact withthe basal conglomerate of theLimin- gen Group is marked by «a few metres of a clastic 'trondhjemitic sediment' which...cont- ains abundant clastic feldspar and quartz».In other areas the basalpart of the conglomer- ateconsistslargely of angularblocks of grano- diorite.
Accepting that we have dated perhaps the youngest major memberof thethick volcano- pluton ic complex, then the evolution of this ensimatic arc- and rift-related assemblage
NGU-BULL.421.1991
ever, is somewhat higher than that of the Meklsvatnet body (0.70427; or 0.70414 minus theonegabbro sample).Allinall,the geologi- cal development andstructuralhistoriesof the Helgeland and Gjersvik Nappes indicate that these tectonic units represent disparate terr- anes.
Conclusions
The M0klevatnet pluton in the southern and tectonostratigraphically highest part of the Gjersvik Nappe is a granodioritewith a calc- alkaline signature. Three fractions of zircon chosenforU-Pb datinggave concordant analy- ses that indicate a crystallisation age of the granodioriteof 456 ± 2 Ma;l.e.,Early-Middle Caradoc.This can be regarded as a maximum age for the oldest sediments of the uncon- formably overlying Limingen Group.
Magmatic activity in the Gjersvik Nappe covered atime span of some25to 30million years. Unlike the situationinthe suprajacent HNC, no evidence has been reported in the Gjersvik Nappe ofanysignificant tectonother- mal activity intheOrdovician.These twojuxta- posed nappes wouldthusrepresent disparate terranes,with different histor ies of geological andtectonometamorphic development,brought together during Scandian continent-continent collision.
Acknowledgeme nts
We would like to thank 0ysteinNordqulen,MikeStephens and Brian Sturt for their helpful comments on an early version of themanuscript.
U-Pbzirconage,Moklevatnetgranodiorite 37
References
Barker,F.1979:Trondhjemite:definition,environmentand hypothesesoforigin.InBarker,F. (eo.)Trondhjemites, ascites andrelatedrocks.ElsevierSci.Pub!.Co.Ams- terdam.1-12.
Claesson, S., Stephens, M.B.& Klingspo r,I. 1988: U-Pb zircondating of tetsic intrusions,Middle KoliNappes.
centralScandinavianCaledonides.Nor.Geol.Tidsskr.
67.89-97.
Crawford,A.J.,Beccaluva,L.&Serri,G.1981:Tectonomag- maneevolutionof the WestPhilippine-Mariana region and the originof boninites.Earth Planet.Sci.Lett.54, 346-356.
Gale,G.H.1975:Geologyand SUlphidemineralization in the Sanddola-Gaizervann area, Trondelag.Nor.geo/.un- oers.Unpubl.Report 1293,65pp.
Gale,G.H.&Roberts,D.1974:Traceelement geochemistry of NorwegianLowerPalaeozoicbasicvolcanicsandits tectonicimplications.EarthPlanet.Sci.t.ett.22,380-390.
Hails,C.,Reinsbakken,A.,Ferriday,I.,Haugen,A.&Ran- kin,A.1977:Geologicalsetting oftheSkorovasorebo- dy withinthe ailochthonous volcanicstratigraphyofthe Gjersvik nappe,Central Norway. Inst. Min. Metall. &
Geol.Soc.London,Special Paper 7,128-151.
Hatch,F.H.,weas ,A.K.&wens,M.K.1951:Thepetrology
oftheigneousrocks.ThomasMurby &Co.,London,
469pp.
Jaffey, A.H.. Flynn, K.F., Glendenin,I.E.•Bentley, W.C. &
Essling, A.M. 1971: Precision measurements of half- lives and specific activities of '''u and "'U. Physical Rev.Sec.C:NuclearPhysics 4,1889-1906.
Koilung,S.1979:Stratigraphyand majorstructures of the Grong District,Nord-Trondelag.Nor.geol.unders.354, 151.
Krogh.T.E.1973:Alow contaminationmethod for hydrother- mal decompositionof zirconand extractionof U and Pb forisotopicage determinations.Geochim.Cosmo- chim.Acta46,485-494.
Krogh,T.E.1982:Improved accuracy ofU-Pb zirconages by creation of moreconcordant systems using anair abrasion technique. Geochim. Cosmochim. Acta 46, 637-649.
Kuilerud, K., Stephens, M.B. & Claesson, S. 1988: Age constraintsonexoticarc-basincomplexes and tectonic implications, centralScandinavianCaledonides. (Exten- ded abstract)Geol.Foren.Stockh.Fom.110,402-403. Ludwiq, K.R. 1980: Calculation of uncertainties of U-Pb
isotopic data.EarthPlanet.Sci.t.ett.46.212-220. Lutro, O. 1979:Thegeology of the Gjersvik area. Nord·
Trondelag, Central Norway. Nor. geol. unders. 354, 53-100.
Nissen, A.L. 1986: Rb-Sr age determination of intrusive rocks in the southeastern part of the Bindal Massif, Nord-Trondelag,Norway.Nor.geol.unders.Bull.406, 83-92.
Nordgulen, 0.& Schouenborg, B. 1990:The Caledonian Heilhornet Pluton.north-centralNorway:geologicalset- ting.radiometricage andimplicationsforthe Scandinavi- an Caledonides.J.Geol.Soc.London 147,439-450.
Nordgulen,0.,Anderse n,T.B.& Gautneb,H.1988:Granito- idsincompositebatholiths intheNorwegianCaledoni- des, a review. (Abstract) Geot. Assoc. Canada- Min.
Assoc.Canada,Ann. Meeting. Progr. with Abstracts, StJohn's,Newfoundland,May 1988.A91.
O'Connor,J.T.1965:Aclassificationfor quartz-richigneous rocks based onfeldspar ratios.U.S.Geot.SurveyProf.
Paper 525-B,79-84.
38 DavidRoberts &RobertD.Tucker
Reinsbak ken.A.1980:geology of the Skorovass Mine:a volcanogenic massive sulphidedepos it in the Centr al Norwegian Caledonides. Nor. geol. unders. 360, 123-154.
Roberts,D.1975:TheStokkvola conglomerate-arevised stratigraphicalposition.Nor.Geol.ttasst«.55,361-371, Roberts, D.1989: Tectonostratigraphy withinthe area of
1:250,000map-shee t'Grong',Nord-Tranoetaq,Central Norway.(Extendedabstract) Geol.Foren.Stockh.F6rh.
111,404-407.
Rllh eim,A.,Gale,G.H.& ROberts,D.1979:Rb,Sr ages of basement gneissesandsupr acrustalrocks of the Grong area,Nord-Trendelaq .Norw ay.Nor.gtiol.unders.354, 131-142.
Sigmond,E.M.O.,Gustavson,M.&Robert s,D.1984:Berg- grunnskartover NorgeM-1:1million.Nor.geol.unders.
Stacey,J.S.& Kramer s,J.D.1975:Approximation of terrestri- allead isotope evolutionbyatwo-stage model.Earth Planet.Sci.Let!.26,207-221.
NGU-BULL.421,1991
Steiger,R.H.& Jager,E.1977:Subcommission on geochro - nology:convention onthe use of decay constants in geo-and cosmochronology.EarthPlanet.Sci.t.ett.36, 359-362.
Tucker, RD., Krogh, T.E., Ross, RJ.Jr. &Williams,S.H.
1990:Time-scalecalibrationbyhigh-precision U-Pbzir- con datingofinterstratified volcanicashesintheOrco vi- cian and Lower Silurian stratotypes of Britain. Earth Planet.Sei.Lett. tOO,51-58.
Tucker, RD., Krogh,T.E. & Rllheim,A. 1991: Proterozoic evolution and age-province boundaries in thecentral part of theWestern GneissRegion,Norway.Results of U-Pbdatingofaccessory mineralsfrom Trondheims - fjordto Geiranger.InGower,C.F.,Rivers,T.&Ryan,B.
(eds.) Mid-Proterozoie Laurentia-Baltiea.Geol.Assoc.
Canada Spec.Paper 38,149-173.
WolH,F.C.1979:Beskrivelse tiIde berggrunnsgeologiske kartbladene Trondheim og Ostersund 1:250,000. Nor.
geol.unders.353,77pp.
Manuscript receivedApril 1991; final typescript accepted June 199 1
