SIMONA.WELLlNGS&BRIANA.STURT NGU - BU LL 434 ,1 9 9 8- PAGE 10 9
Tectonic setting of the Tronfjell Massif: further evi dence for pre-Scandian orogenesis in the Trondheim Na ppe Complex, Central Norway
SIMONA.WELLlNGS&BRIANA. STURT
Wellings,SA & Stu rt,BA1998:Tectonicsetting oftheTronfjellMassif:furth er evidence for pre-Scandianorogenesis in theTrondh eimNap peComplex,Centr al Norway.Norgesgeologiske undersekelse Bulletin434,109 -115.
TheTronfj ellMassif consistsofalayeredmaficintrusion, contain ing dunites andoliv ine-bearing tonoriti c gabbros, lyingwithin metasedimentaryrocks of theHummelfjellGrou p.Theinit iallymore orlessflat form of theintrusionis nowbowl-like,due tosett ling of theintrusion unde r its ownweight.Largeareas ofrock were affectedbyupper gre- enschist-facies deformation duringthe Scandianorogeny. Rareareas of hornfels,wrappedand cross-cut by a Scandian-agefoliation,preserve anannealedtecto nicfabri c which,togetherwiththeinferredpre-int rusion formof struct ures in the metasedi mentaryrocks,demonstrate apre-Scandianorogenicevent.This isin agreementwit h the latest tecto nost ratig raphic modelforthe area,which placestheHummelfjell Grou p wit hinthe HeidalGrou p of the Trondh eim Nappe Complex.
SAWellings',DepartmentofEarth Sciences,ParksRoad, Oxford,OX1 3PR,England.
BAStutt,GeologicalSurveyofNorway,PostBox3006-Lade,7002Trondheim,Norway.
IPresentaddress- 57aBlenheim Road,Caversham,Reading,RG47RT,England.
Introduction
The Tronfjell Massif lies wit hi n the Trondheim Nap pe Complex (TNC) of the Upper Allochthon of the Central NorwegianCaledonides(Fig.1).The massifconsists of a lay- ered mafic int rusion surrounded by metasedimentswhich have a complexhistory of metamorphi sm and deformation. Detailed mapping has revealed further evid ence for pre- Scandian metamorphism and deformation within rocks of theTNC and enabled us to place this int rusion within a tee- tonostratigraphicframework.
Regional geology
Recentwork has established a new tectonostratigraphy for
the sou t hern part ofthe TNC,recognisingtwodistinct rock suit es separatedbyamajorunconformit y(St urtetal.1991, 1995,1997, Bje rkqard & Bjorly kke 1994) (Fig. 1).Theoldest unit(HeidalGroup)experienced aphaseof deformatio nand metamorph ism prior to an EarlyOrdo vician oroge nesisas- sociate dwit h theobductionofthe Vagamo ophi ol ite. Post- orogenic sedime ntat ion and vulcanismare repr esented by theSelGroupwhichis separatedfrom theolderrocks bya major unconformity. All unitswere affe cte d by mid-Siluria n Scandian orogenesiswhich involved polyphase deform a- tion, including the formation of a major recumbent fold structure (Je nndalen syncline). Earlier, theTronfjell Massif was placed within the Hummelfj ell Group,a unit of the
Fig.1.Mapshowingthetecton ostratigraphy of the sout hern porti on of theTrondheimNappeComplex, afte r Sturtetal.(1997). Thelocation of Figure2is out lined.
\
\
Trondhei Thrust contact
!
Fault
T
N
I
HovringenGneiss, , "
JotunNappe
r
Trondheim NappeComplex
D •
D
SelGroupandVagttmoopntoute.undividedD
HeidalGroupNGU-BU LL 434,1998 - PAG E110
Remsklepp Nappeof theMiddleAllochthon(Nilsen&Wolff 1989).Tracingofunit s from their type-localities in the Otta area (St urt etal.1997)sugg est s that the rocksassigned to the Hummelfj ell Group (which form the carapace of the Tro nfjellMassif)are a direct correlat ive of theHeidalGro up.
Struct ures previouslyint erprete d as thrusts separat ingthese rocks from the TNC are, in our opin ion ,st rat ig raphiccon- tacts,thus placing the Hummelfjell Groupwithin theTNC.
Given this,thetectonic setting of theTronfjell massif may shed light on thetiming of the polyorogenic sequence of events ment ion edabove.
Local geology
The stu dyarea wascoveredearlier bythe regionalwork of Holmsen(1943) and Holmsen &Holmsen (l9S0)inthe east and Marlo w (1935)in the west. The only mod ern workon theint rusion itselfis that of Kleine-Hering (1969)and Dreyer (1975). Thework of Klei ne-Hering coveredalargearea and form edpart of amastersthesis.Thispart icularcoverage of Tro nfj ell waslarge ly superceded bythe more detailed and sub stant ial doctoral thesis of Dreyer, whodied in 1978.The study areais covered by theNGU1:250,000bed rock map Roros (Nilsen&Wolff 1989).TheTronfjellint rusion lies wit hin a metasedimentary rock succession wit h tuffaceou s and 'greenstone' horizons also present. Bedthicknesses are vari- able, in particularthose wit hin a prominentgreenston eunit (Fig.2). Som e unit s host sulp hidedepos itsofvolcanicorig in (Dreyer1975).
Pet rology and field relationships
Dreyer (1975) divided theTronfj ellint rusion into three,al- most concentriczones(Fig.2).He defin edthezones in term s of facies typ es and rocktext ures and wasable to confi rm this division with whole-rock analyses. Oursubseq uent re- mapping ofthe intrusion has confirmedthistripar ti tedivisi- on, whichis as foll ow s:-
1. Low erzone(e.250-7300m thick). Thisis seen everywhere above thecontactwit h thecountry rocks.It is a fine- to med ium -g rained« 2 mm)massivegabbro.Irreg ular lay- ers of dunite and pod-like blue-blackquartz xenoliths occu r locally.Thiszone isinterp rete d asa cont aminated chill-zoneformed by intera cti on bet wee nthe intrusion andit smetasedi m entary country rocks.
2. Transitional zone(e.750m thick). A sharp contactat the top of thelower zoneleadsint o medium-to fine-grained
« 2 mm)olivi negabb ro, associated wit hmetre-todeci- metre -scalepods and lenses of dunitic material.
3. Upperzone (>7000m thick). A gradationalcontactover tensofmetr es leadsinto the high estexpo sed levels of the intr usion which consistof coarse-grained « 2mm) olivinegabbro wit h minor layers of troctoliteand dunit e.
Mafic sheetsarelocally abundant in the country rocks im- mediately beneat h the contac t with the Tronfjell intr usion, especially on thenorthern side.These sheet sare usually al-
S/MONA.WELLlNGS&BR/ANA.STURT
tered,butthey resemble the rocks of the lower zone intex- ture andmineralogy.
Primary magmatic fabrics
Modal layeringis commonin the upper zone, whereitis de- fined by a variation in the modes of plag ioclase relat iveto mafic phases or,more rarely,c1inopyroxeneoikocr ystsrelat i- vetocumulus phases. Although disruptionof the layering is rare,slumps and trough-like structures were found which sug gest thatthe intrusion has not beeninverted.
An igneouslam inat ionis commonin rocksof the up per zone.This may be the onlyfabricpresent,butin som ecases it is associatedwit h parallelmodallayering . Clinopyroxene oikocrysts commonlycontain unoriented plag ioclase lath s yet are surrounded by flattened lat hs defin ing anign eou s lamina tion. These relationships suggest that thisfabric is due to compaetion of a crystal mush, post-dating the grow t h of the oikocrysts.
Stronger evidence for deformation of the crystalmush is seen within the transit iona lzone. Isolate d pods ofdun ite are interpreted as boudinsof an origina llycont inuo ussheet . The pods contain a crude magmaticfabricdefined byalig - nedpyroxene grains. In one case a vein or layer of pyroxene, obliqu eto thisfabric,has been foldedwit hin one plane and boudinag ed withi nanot her. Olivine-poor gabbro surroun- ding the podcontains a strong magmatic fabric defined by an alignment of centimetre-scaleplag ioclase-rich patches. This fabricis parallel to other magmaticstructuresinthe in- trusion and is interpreted as the consequence of compacti- on flat tening,which perhaps occurred at the timeof formati- on of thebow l-likeshape ofthe massif.
The pod- or sheet-like bodies of dunite which occur throug hout the intrusion,especially within the lower and transitional zones,sometimes containsmall patches ordis- continuous veins of chromite.
Alteration
The TronfjellMassif is cross-cut in many places byareas of greenschist-facies altera tion . These vary from discrete cm- wide planes or zones of alterationto a mappable areaoflar- gely altered rocks(Fig.2). Thisalteration occurred at upper greenschist-facies conditions,andis associated wit h the bre- akdow n ofplagioclaseand mafic minera lsto chlorite,epido- te,albite and amphibo le.Alteration is commonly associated wit hdeformation alongdiscrete shear zones which can be seen tooff set primary igneous boundaries.Moving into the- se shear zon es, unalt ered gab bro passes via undeformed metagabb ro into fin e-grained amphi bole-richrocks.A zone of hyd rous alterationupto 10 m thickis seen along the mar- gin softheintrusion. The large area of alterat io nalong the nor thwestern side of the intru sion is associated wit h cross- cutting veins of felsicmelt,the emplacementof which was associated wit h hydr ous fluids and associated alteration (Kanaris-S ot irou&Angu s 1979).
As a large layeredbody, theTron fjellintrusion might bear
S/MONA.WELLlNGS &BR/ANA.STURT NGU-BULL 43 4 ,199 8 - PAGE 111
D
Undivided HeidalGroup-
Tectonic structuresD
Tronfj elllower zone;----.-
(transpose d bedd ing) olivine-freegabbroD
Heidal Groupgree nstones+ - - Magm atic structures D
Tronfjolivineellgauppbbroer zone;
D D
Tronfj ell transiti onzone; ~ AlteredTronfj ellrocksAreasof preser vedhornfels dunite andgabbro
\
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/
IOOOm B Ikm
N ~O Om'-.. r: J """"
(09 \
\ \
»:
(-: »
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/ " .r-
4 ~~~
6 '-J4q
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-,
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Fig.2 .Geolog icalmap of the Tronfjellarea,modifiedafter Welling s (1996a)and Dreyer (1975).
NGU-BULL 434, 1 99 8 - PAGE 112
comparison withsome of the othermafic bodies in the TNC, suchas the Fong en-Hyllingen Complex (Wilson et al. 1985).
Com pared wit h these, the Tronfjell body is unusual in containing chromite and in having a thick basal layerof con- taminated massivegabbro.
Timing of emplacement
Thelatest stratig rap hiccorrelat io ns in the area, in particular those based on a tracin g of the Sel-Heidal unconformity from type localitites in the Otta area(St urt et al.1991),sug- gest a polyorogenic history for the country rocks surroun - ding the Tronfjell int rusion.Synorogenicintrusions mayact as a useful golden spike,representinga datable event which constitutes a markerwit hin atectonomet amorp hic successi- on(Rogersetal. 1989). Graniti cbodies have typically been used butrecent workon a synorogenicmafic intrusions has shown thatthese bodies may be equally useful in thisre- spect(Well ings 1998).
Scandian orogenesis
Bj erkqard & Bjorlykke (1994) described metasedim enta ry rocksof the Se lGroup from the nearbyare a of Folldal(Fig 1).
They conside red that uppergreenschist-faciesfab rics asso- ciated with E-W-t rendingmineralIineations were products of the Scandian orogeny. Similar fabrics and lineationsare recognisedthroughout the areaof the Tronfjell massif.
Within the Tronfjellcountry rocks the main schistosity is generally defined bychlorit e or white mica andis associated with an E-W-trendingmineral alignment or stretchinglinea- tion(Fig.3a). Closeto the country rock/ int rusion contact, thisfoliat io n is locally more stronglydevelope d and in pla- ces it is associated with boudinage of mafic sills. Areasof highly prolatefabrics are seen along the western edge of the intrusion. Raresense-of-shear indi cators, such as shear- bands in well foliated rocks,give a top-to -the -eastsenseof shear movement.
Within the intrusion itself,such fabrics are associated wit halte rat ionof the igneou smineralogy andare variablein orientat ion (Fig.3c), especially wherethe defor mati on took place in narrow« 10 m thick)zones,but a general pattern of c.E-W lineations is evident. This dominant E-Wlinear grain stronglysuggests thatthe Tronfjell intrusion was affected by theScandian orogeny andwas therefore emplacedintoit s hostrocks beforethis event.
Pre-Scandian structures
Xenoli ths within the int rusion are generally rounded and have been comp let ely recrystalli sed, destroying any pre- intrusion fabricswhich they may have contained.
Small areas just belowthe westerncontact of the intru si- onexpose undefo rmedhorn fels.These hornfelsesare both wrapped and cross-cut by Scand ian fabrics,but they also preserv eevid encefor anearlier phaseofdeformat ion.Fig.4 dep icts a samp le ofhorn felsicmaterial showi nga folded tec-
S/MONA.WELUNGS&BR/ANA.STURT
*
+ ++ ++
+ ++ + t
+
~.~+~ ~
+ l' + ++fT+
+ +
+ +
+
+
+
++ ++
+ +
+ +
+
Fig.3. Ste reog raphicprojectio nsof stru ctural data from the Tronfjell massif.(a)Data from the countryrocks.Dots are lineations and crosses are poles to foliatio n. (b) Data taken from unaltered gabbro of the Tronfjell intrusion. Dots aremagmatic lineation s, crossesarepoles to magma ticplanarfabrics. (cl Data fromdeformed metagabbroofthe Tronfje ll int rusion.Dots arelineations,crosses are poles to foliations.
SIMONA.WELLlNGS&BR/ANA.STURT NG U- B U L L434,1 9 98 - PAGE 113
Fig.4.Phot ograph of thepoli shed surface of ablock of hornfels, showing a folded folia- tion.Thewidt hof the photog raphis equiva- lent to 15cm.
tonicfabric. Structu res wit hinthehornfelsesare comp lex, with folding common. There is evid enceformultiple deve- lopment of tectonic fabrics,but thelimited outcrop hinde rs a detailed study of these early fabrics.Under the microscope the foldsand folded fabric in the hornfelses described abo- ve pre-date the contact metamorphismand are now part of a totally undeformed recrystallised mine ral aggregate.
These hornfelses are preserved inan area where the angle between pre-intrusionfabricsand the contact is relative ly high.Wherethe contact with the underlying rocks is roughly parallel,any pre-intrusion fabricshavebeen complete lyde- stroyed by contact-parallelScandian deformation. In these areas,both pre- and post-intrusionfoliationsare parallel to the boundarybut are now indistinguishable.West of the in- trusion,the fabrics far fromthecontact are discordant,tren- ding atahigh ang letothe contact.The swingof these folia- tionsinto parallelismwit hthecontact is likelyto be there- sultof Scandian shear deformat ion.Thevariation in discor- dance between the contactand the country rock fabricand the dramatic variat ion in the thicknessofthe Heidal Group greenstone horizon, to the south and east of theintrusion, providefurther evidence for pre-intrusion deformation.This provides compelling evidencein supportof the notion that rocks of the TNC have been involve d in polyorogenic Caledon ian deformat ion (Guezou 1978, Lagerblad 1983, Sturtetal. 1991).
The Tronfje ll Massif is undated, but thi swork con str ains it s ageas pre-Scandian and post polyphasal deformation / metamorphismof the countryrocks of the HeidalGroup. It is not possible,at thisstage,to placetheintrusion into a con- textof Sel Group sedimentationand subsequent deformati- on.The 0yungen gabbro (Wellings 1996b),which bears manysimilarit ies to the Tronfjellintrusion,is emplaced into rocksof the SelGroup.Indeed, it appears to post-date the Scandian D1 schistosi tybut to pre-date Scandian D2 which producedthe main regional foliation.It is tempting to consi- der the Tronfjell and 0yungen intru sionsas being of the
same age as the petrologically similar Fongen-Hyllingen Comp lex (Wilson 1985). This latter has been dated at 426+~ Maby U-Pbage determinat ionson zircon (Wilson et al. 1983). It is hoped that dating of the Tronfjell and 0yungen bodies will be available in the nearfuture.
E mplacement of the intru sion
Maficint rusions are generallysheet-like bodies (Pet raske et al.1978) and the Tronfjell intrusionis no except ion. The cru- de parallelism of the base, internalcontacts and magmatic planar structuressuggests that the intrusion took advant- age of a flat-lyingplaneofweakness(met amorphicfoliat ion) during emplacement. The Fongen-Hyllingen intru sion to the north grew by forcing itselfalong bedding,thickening by forcing its roof and base apart and growing in areaby wedging along bedding (Wilsonet al. 1987). Areasto the east of theTronfje ll intr usion show abundan t sheeting of gabbro just below the contact. This is not seen in western areas,except for in one sheeted xenolit h. This absence of sheeting in the west may be related to the inferred pre- Scandian angular discordance between the contactand the transposed bedding.Therelat ively small thicknessof the lo- werzone in this area may suggest that the angular discor- danceinhibitedexpansion of the intrusionat thispoint.
Recentwork by one of us(SAS) has suggested thatlarge mafic int rusions inthis southern part of theTNC are largely foundwit hin the Sel Group.However,thiswouldreq uire the magma to havepassed throughboth Sel and Heidal rocks.
The fact that the only major intrusion (t he Tronfjellintrusion) recorded within theHeidal Group,onthe eastern limb ofthe Jenndalen Syncline,coincides withan area of flat-lyingtrans- posedbedding suggestsa structuralcontrol upon emplace- ment. It is suggest ed that the presence of flat-lyi ng discont- inuities was a vita lfacto rcont rolling theviabilityof empla- cing a largemafic int rusion. For magma ascending from depthin post-Se]Group/pre-Scandiantimes,such discontin-
NGU-BULL 434,199 8 - PAGE 114
uities were found in abundance only within the Selsedi- ments. Intrusion growth within the Heidal Group would be inhibited by the lack of suitable discontinuities and magma would perhaps have tended to pond within the Sel. The change in rock properties(such as densityor viscosity)at the transit ion from Heidal metasediments to unmetamor- phosed Selsedimentaryrocksmightalsohave acted toinhi - bitupward dyke propagation and aided the pondingof rna- fic magma. Such a model would explain the distribution of mafic intrusions within the southern TNC. Magma would still have passed though the Heidal Group,however,and should have left some evidenceof it s presence,perhapsin the form of small sheets or dykes. Small mafic bodies are common in these rocks,although a large number are un- doubtedly not of this age.
Controls on the form of the Tronfjell intrusion Both the contact with country rocks and planar structures wit hin the Tronfjellintrusionare parallelbut nowforma sau- cer shape(Fig. 2).A plotof magmatic structures (Fig. 3b)de- monstrates that this sauceris unlikely to be related tolater folding,butinstead appears to be due to the int rusion sin- kingunder its own weight. This mechanism has been invo- ked to explain the form of other maficint rusions(e.g. Loney
&Himme lberg 1983). If this sinking was activeduringcrys- tallisation it may be responsible for some of the magmatic deformationoutlined above;however,sincethe majo rity of magmat ic structures are folded around the struct ure,they largely formed before thissink ing.
Metamorphism
The country rocks in this area are a mixtureof metapelites, and meta-igneous rocks.The greenstones have not been studiedin detail, but they generally contain greenschist-faci- es assemblages of green amphibole,plagioclase and epido- te group minerals.Meta-pelit icassemblageshave been stu- died in more detail.
Evidence for pre-intrusion metamorphism is largely ob- scured by later events,but complex garnet morphologies suggest up to three phases of growth,ofwhich theearliest pre-da t e d emplacement ofth e intrusion. Contact- m e t a m o r- phic minerals are alsopoorlyrepresented, large garnetsand rare sillimaniteclose« 50 m)to theint rusion beingtheonly definite examples . Xenoliths preserve blue quartz contai- ning sub-microscopic exsolved needles (mi neralogy un- known)but are extensively overprinted by laterwhite mica and chlorite. Most studiedsamplesof metapeliteare exten - sivelyoverprinted by Scandianmetamorphism ,manifested in features ranging from the growth of new garnet rims on garnets through to pervasive recrystallisation at greenschist facies. A common assemblagein metapelitesis quartz-pia- gioclase-biotite-white mica-amphibole and epidote. This metamorphic picture is consistent with the polyorogenic nature of the Hummelfjell/Heidal metasediments and sug-
SIMONA.WELLlNGS&BRIANA.STURT
gests thatthe contact metam or phismdid not affecta large volumeof rock.
Discussion and conclusions
The recogni ti on that the Tronfjell Massif was emplacedprior to the mainScandian event but post-dates pre-Selorogene- sis furt her strengt henscorrelation with other mafic intru- sions wit hin the TNC(Wilson1985).Thiscorrelation will be tested by an ongoingprogramme ofdatingand, if correct,is of interest for two reasons: firstly, it strongl ysuggest sthat the Tronfje ll Massif may becorrelated with,and therefore shed sfurther lig ht upon, other intrusions farth ernorth;se- condly,the Tronfje ll Massifisthe only large mafic intrusion so far recogn ised wit hin the Heidalport ion of theTNC,provi- dingvitalevidencethat mafic magma passedthroughthese rocks.The fact that all such bodies so fardescribed within the TNC are found wit hin count ryrocks wit h flat-lying fab- rics suggests a struct uralcont rol upon their emplacement.
Tectoni c fabrics withinrecrystalli sedhornfels adj acent to the Tronfje ll intrusion provide corroborating evidence in support of the tectonost rat ig raphic model of St urt et al.
(1991,1995,1997) and Bj erkqard & Bjerlykke (1994). This confi rmat ionthat rocks of the Heidal Group were affected by two tectonometamo rp hic events,the second related to the Scand ian oroge ny, is in agreemen t with thismodel. A similar bipar t itesubdivision has been recogni sedinthe nor- thern part of theTNC (e.g.Lagerbl ad 1983,Tietzsch-Tyler 1989).Itis suggestedthat polyorogenicrock packagesmay be more commo n withinmountainbelt s thaniscurrently re- alised;structu ralst ud iesof intrusionsand their immediate surroundingsprovidea powerfu lmeansof recogn isingsuch packages.
Acknowledgements
Fieldworkfor SAW was made possiblebyfunding from GU andtheEU LeonardodaVinciprogram m e, whichisgratefullyacknow ledg ed.This work has benefitted from discussions wit h Donald M.Ramsay,Terj e Bjerkqardand Lars-PetterNilsson.The man uscri ptwas greatly im pro- ved by reviews by RichardWilsonand Odd Nilsen,and addit iona lco m - ments by the edito r.
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Manuscriptreceived July1997;revised manuscriptaccepted July 1998.
