penetrative cleavage in the Gaissa Nappe Complex, northern Norway

BJ0RN SUNDVOLL&DAVID ROBERTS NGU-BULL 441, 2003- PAGE 51

A likely Early Ordovician age for the regional,

penetrative cleavage in the Gaissa Nappe Complex, northern Norway

B J0 RN SUN DVOLL & DAVID R OBERTS

Sundvoll,B.&Rob ert s,D.2003:AlikelyEarlyOrdovici anagefortheregional, penetrativecleavageintheGaissa NappeComp lex,northernNorway.Norgesgeologiskeundersekeise Bulletin 441,51-59.

Pervasively cleaved mudst ones and c1aystonesfrom two different parts of the GaissaNappeComplexin Finnmark weresubjected to Rb-Srwhole-rockanalyticalinvestigation.Althoughneitherofthe two groupsof sampl esyielded atrue isochron,thedatado support the conceptof a probable ageat arou nd480-500Ma,as can bedi.~cer~edfrom the'isochron fits'ofvarious combinations ofthe analyticaldata.Togetherwit h therelativelyhigh Sri Sriniti al ratios,thisresultstronglysuggest sthat a Sr-isotopicresetting eventoccurredat around480-500Ma,aneventwhich is considered to reflect anoverall,pervasivemetamo rphicimprint on the rockformat ionsof the Gaissa Nappe Comp lex.

Earlierradiometricdat ing fromthissameallocht hon has yieldedambiguo us results,somepointingtoFinnmarkian (Late Camb rian-EarlyOrdovici an)deformationwhereasot herdatahaveindicated a scandian(Late Silurian-Ea rly Devonian)age.The Rb-Sranalyticaldata reportedherethustendto favoura likely Finnmarkian ageforthe verylow- grademetamorp hism.Asthe youngestfossiliferousrocks in the Gaissa thrustsheetareof EarlyTremadocage,the cleavage-forming eventis inferred tohave occurredinTremadoc orpossiblyArenigtime.

Bjorn Sundvoll.GeoloqiskMuseum,NHM,Universiteti Oslo,Postboks 1172,N-0318Oslo,Norway.

DavidRoberts,GeologicalSurvey ofNorway, N-7491Trondheim,Norway.

Introduction

Rock complexes of Neoproterozoic to Early Palaeozoic ag e dominate the Caledonides of Finnmark, northern Norw ay.

The nappes o

f

the metamorphic alloc

htho n

ext end over a

dist an ce of

som e 370 km from Kvcenangen i n

th

e sout

hwest

to

Varan ger Penin

sula in

th

e

no rt heast, and

overlie a gener- al

ly

thin, parautochthonous to autochthonou s sedi

mentary

sequence that was deposited upon a penep

laned su rface of

Palaeoproterozoic and Archaean crysta

lline

rock s.

The preci se ages of the thick metasedimentary succes- sions in the different parts of the Caledonide tectonostr atig- raphy are poorly known. Fossils are comparatively rare, occ

urr ing

only in

th

e

ver yhigh est and low est of the nap pes.

Likewise, our knowledg e of

th

e exact timing of Caledo nian orogenic deformation is far from perfect. Although isotopic dating investigation s have been made in cert ain areas, the data, in most cases

,

are not sufficiently robust to allow us to state categorically that, for examp

le,

metamorphi sm occur red at j

ust

one sp ecific time. Only in t

he

hig hest allocht hon in t

his

area, the Mageroy Nappe, where there are fossili

ferous

me

tasedimentary

rocks of

Lat e Ordovician to

Early Silur ian age, can we be sure that, in this particular case, we are dealing with

Scandian (Silu rian

to Early/Mid Devonian) orogenic deformation. Else where, a case has bee n presented for suggesting that it is pos sible to id entify an earlier phas e of

Lat e

Cambrian/

Early

Ordovician

t

ect on oth erm al activity, known as

t

he

Finnmarkian

(Ramsay

&

Sturt 1976, Sturt et al. 1978), in so m e of

th

e nappes.

In thi s contribution, we present R b- Sr

who

le-ro ck

analyt-

ical data fro m th e Gaissa Nappe Complex,

which is

a part of the

Low

er Allochthon of Scandina vian Caledonid e t ect on o- st ratig rap hy (Robert s & Gee 1985). Th e rocks sam pl ed are pen etrative ly cleaved mud stones and c1aystone s fr om t w o widely separate areas, in so uthern Porsang erfjord and in th e

Vestertana di

strict of inne rmo st Tanafjord. These particu lar rock s w ere coll ect

ed

wi t h the aim of t rying to det erm ine t he age of this pervasive m et amorph ic fab ric. A prelim inar

y,

unpublished report on th ese results has been presented earlier (Sund vo ll

&

Roberts 2002

).

General geology

The tectono

stratigraphy of th e Caledon id es in F

innma

rk has been touched upon in the

int rod uct io ns

to several papers on diverse topics, but so m e of the ma in co ntri bution s to th e ov erall architecture and structu re of the pr incipa l napp e comple

xes

are those of Sturt et al.

(1975), Williams

et al.

(1976), Chapman et al. (1985), Ramsay et al.

(1985), Robert s

(1985), Tow nsend et al. (1986) and Gayer et al. (198S, 1987).

Four

m

aj or nap

pes

or nappe complexes are recognised,

from botto m to

t

op: (1) Gaissa Nappe Complex,

(2)

Laksefjord Nappe Complex, (3) Kaiak Nappe Complex, (4)

Maq erey Nappe. With th e exception of the Mage roy Napp e,

these various nappe complexes occur fai rly w ide ly across

th

e main

land

of Finn mark, moreso the Kaiak Nappe

Complex w

hic h

exte

nds

southwards into t he

neig h bo uring

coun

ty

of

Tro m

s (Zw aan & Roberts 1978, Zw aan 1988). The

NGU-BULL 441,20 0 3 -PAGE 52 BJ0RN SUNDVOLL

s

^DAVIDROBERTS

70'N 30'E

N

Mageroy appe

KaiakNappe Complex

Gaissa NappeComplex Laksefjord

Nappe Complex

Parautochthon and Autochthon

_J..

~~

o 0

~

o 0

CJ . .

~

Nordkinn

Svecokarelianrocks intectonic windows

AutochthonousPrecambrian crystallinerocks(undifferentiated) V777l BarentsSea Region

~ allochthon

100 km

50

Fig . 1~~[J/:}..~123°E

o

er' :~~rm

vr

Fig.1.Sim pli fiedtectonostratigraphy oftheCaledonides ofFinn mark;modifiedfrom Roberts(1985).The approxim ate locationsofthetwosampli ng traverses in theGaissa Napp eComplex areshow n,marked bythe short,thick linesandthelettersGSand SB.Detailsof sample location s,grid references and form ati onnames are giveninTable2.

Mageroy Nappe occurs exclusively on the island of Mageroya in northweste rn Finnmark

(Fig.

1).

Briefly,

the Gaissa Nappe

Complex

(also termed

infor-

mallytheGaissaThrustB

elt, e.g. Gayer et al. 1987,Tow

nsend

et al. 1989)

isdomin ated by

low-grade, Neopro

terozo ic to

earliest Ordovician,

metasedim entary rocks,

w

hilst th

e

Laksefjord and

Kaiak Nappe C ompl exes are com

posed

mainly of

greenschist- to

amph ibolite-facies, late Mesoproterozoic

to Neoproterozoic,

metased imentary sue- cessions w ith or wi t hout basal plin

ths

of older Precambrian crystalline rocks

(St urt et al. 1978,

Ramsay et al. 1985).

Although this basement/cov er con tact is generall y the locus of high shear strains, wi t h mylon ite development, a part ly

tectonised unconformity

has been identified

in one of the

Kaiak thrust sheets

(Ramsay

& Sturt 1977). The lithostrati- graphic succession of the Kaiak Nappe Complex also hosts the mag

matic

rocks of the extensive

S

eiland Igneous Province

(R

o

bins 1996).The Mageroy Nappe

comp

rises

an amp hibol ite-facies metasedim ent ary succession of Ordo- S

ilurian

age which is cut by

div

erse

plutoni c rocks varyi ng in comp osition from mafic/

ultramafic

to granit ic

(Andersen

1984,Roberts

&

And ersen 1985, Robins

1998).

The Mageroy Nappe and th

e KaiakNappe Complex

carry thick mylonit es, and locally ultram ylon ite s,along their soles.

My lonit es also occur along the basal thrust to the Laksefjord Nappe Compl ex, but the thickness

is far

less.The floor th

rust to the Gaissa

Nappe Comp lex is genera lly characterised by a 3-5 cm-th ick ultracatacl asit e, rarely prot omylo nite.

Promin ent st retching lineati ons wit hin

and above the

thrust zones to all these napp es generally deno te

a

translation sense bet ween southeast and east-southeast.

Nappemove-

ment

was

po

lyphase in the case of

the

K

aiak

Nappe

Comp lex,

wit h early

d uct ile SE t

ranslation

superceded by a more brittle, ESE-direct ed transport

(Gayeret al. 1987).A

po lyph ase, internal deformation history

is also

recogn

ised in

all these nappe com plexes, though in the case of

th

e Gaissa this is mu ch

less

complicated t

han in

the other allo chthon s

(Townsend

et al. 1986).

In th e Kaiak Nappe Comp lex,there is also evide nce fro

m

isoto pic dat ing

(Daly

et al. 1991, Reginiussen et al. 1995,

Kirkland & Daly 2003) of a pre-Caledonian, Late Riphean

(>850 Ma) tec

tonothermal

event, at least in parts of t he

high er thrust

sheet s. Wheth er this affects th e

regi

onally

ext ensive, S ore y Group succession in

its

ent

irety

or only

BJ0RN SUNDVOLL&DAVIDROBERTS NGU-BULL 441, 2003 - PAGE 53

partl y, or is confined to basem ent rock s of the old er Eidv aqeid Seq uence

(R

ice 1990), is a mat te r w hic h has yet to be resolved. A comp arabl e, but undated, pre-Caledonia n tectonoth ermal event has also been reported from on e of the nap pes in Tro ms

(Zwaan

& van R oerm und 1990).

Table 1.Lithostratigraphy of theRiph ean to LowerOrdovician (Tremadoc) successionoccurringwit hin the Gaissa NappeComplex,Finnmark.Division int o formatio ns fortheDigermul en,Vestertana andTanafjordenGroupsis basedlargely on Banks etal.(1971),Siedlecka&Siedlecki(1971)andWilliams (1976).Subdivisionsofthe AiroaiviGroup are fro mTow nsend etal.(1989).

See also the 1:250,000 map-sheets 'Vadso' (Siedlecki 1980) and

'Honninqsvaq'(Roberts 1998);and the 1:500,000 bedrock map-sheet of

Finnm ark county(Siedlecka&Robert s1996).

Gaissa Nappe Complex

Over most of it s 200 km str ike exte nt, the Gaissa Napp e Compl ex lies tect onically abo ve th

e

thin ,Vend ian to Low er Cambrian, con densed sequence of th e Dividal Group

(Feyn

1967, 1985).ln the east, to w ard sVaranger Penin sula,the pre- cise path of the basal thrust plan e is unc ert ain, and differ ent solut io ns have been prop osed (cf. R ob ert

s

1985, Towns end 1986, R ice 1994, Sygnabere 1997).This unc ert ainty relates to the fact that the Gaissa and the subjacent parautochth on share a com mo n Iith ostrat igraph y i n th e sout hwestern part of Varange r P eninsula . In th is area,and west of th e T ana rive r, bedr oc k exposure is also comparatively poor.

Bet w een innermo st Laksefj ord and Langfj ord, in western Tanafjord, the Laksefjord Nappe Com plex, or its basal, base- m ent slice,the Kunes Napp e (Rice 2001),tecton ostratig raph - ically overli es th e Gaissa. To th e sout hw est, the Laksefj ord Nappe Compl ex is excised and st rongly mylonitised rocks of the Kaiak Nap pe Complex th ere lie directly up on diverse for- mations of th e Gaissa Napp e Compl ex (Gayer et al. 1987, Robert s 1998).

Group Formation

Rb-Sr dating

Sampling and lithologies

With the purpo se of trying to determine the met am or ph ic age of th e rock s of th e Gaissa Nappe C ompl ex, s am pling was carried out in tw o areas: (1) along th e main road bet w een inn er Vestert ana and the sole th rust to th e Laksefj ord Nappe Com plex, east of lfjord, over a distance of roughly 20 kilome- tres; (2) along th e road and coast north and sout h of Sta b bursnes,west ern and inner part of P orsang erfj ord,a dis- tance of 12-13 km

(F

ig. 1).Sam pling was concen trated on the most pelitic and thus better cleaved roc k typ es in several form ation s.Mesoscopic and large-scale fold s are ubi quitou s The lithostratigraphy of th e Gaissa alloc ht hon is sho w n in Table 1. Formation s of the Late Riphean Tanafjorden

Group

occur throughout the nappe com plex, but are more extens ively exposed in th e sout hwes t. The oldest unit, the Airoai vi Group

(Tow nsend

et al. 1989), is exposed in the sout hwes te rn most part of t he nappe, west of Lakselv. The Vend ian to basal Cambrian Ves

ter tana Gro up

is p resent from sout h of Laksefjord northeastw ard s to th e sout hwestern parts of Varanger Penin sula, whe reas t he youn gest unit, the Cambrian to Low er Tremadoc Digermul

Group (Read

ing 1965, Banks et al. 1971, Nikol aisen & Henning smoen 1985), occurs onl y on th e Digermul P enin sula.

Alth ou gh the various form at ion s of these groups can be follow ed over distances of 200 km or more, there are facies variat io ns and ot her change

s w

hich have given rise to a slightl y different st rat ig raphic nom encl ature in part s of the succession from Porsangerfjord to Varange r

(Sied lecki 1980,

Ri ce & T own send 1996, Rober ts 1998). In the Porsangerfjord dist rict , for exam ple, the Porsang er F ormat ion at the top of th e Tanafjord en Group ove rlies th e S ta bbursdal Form ati on.

On Varang er, th e upper m ember of the Grasdal F orm at ion is equiva lent to th e P or sang er Formatio n,and the low er mem

-

ber of th e Grasdal correlates wi t h th e Stabb ursdal.

In sout hweste rn areas

,

in th e P or sangerfj ord district, the Gaissa Napp e Com plex is comp osed of a series of wester ly dipping, imb ricate th rust sheets, separated into fou r struc- tural zo nes, inclu di ng t w o duplexes (T ow nsend et al. 1986).

N-S to NNE -SSW-trending fold

s

wi t h a

steep, west-dipping ,

axial pl anar cleavage are quite comm on

(Williams 1979).

Metam orph ic gr ade is generally in anchizone

(Rice et

al.

1989) but reaches up to epizo ne j ust beneat h the ov

erlyi ng

basal Kaiak thrust. A somewhat simpler struc t ural picture o btai ns some 200 km farth er to th e nort heast in th e inne r T anafjord-SW Varange

r

Penin sula regi on . There, im bricati on

is

far less evide nt but mesoscopi c folds are still co mmon, with axest rending betw een N-Sand NE-SW The ESE-verging folds are open to close, asym met ric and carry a pervasive axial-p lanar cleavage of mainl y anchizo ne m et amor ph ic gr ade (Will iams 1979,T own send et al. 1986, Rice et al. 1989).

In interb eds of mud ston e typ e in m ost part s of the Gaissa Nappe Complex, the cleavage can best be describ ed as an incip ient slaty cleavage.

Biostratigraphic age

Vendian LowerCambrian LowerTrem ado c Middle/UpperCamb rian LowerCambrian

Up perRiph ean Berlogaissa

Kistedalen Duol basg aissa

Breid vik a Stap po g iedde } Morten snes Nybo rg Smalfjord

Porsanger Stabbursdal Hangl ecaerro Vagg e Gam asfjellet Dakkovarr e St ange ne s Gronneset

Ab ba rdasras'sa Nav'kaoaivi Adnevarr i Bal'dn eras'sa Vestert an a

Digermulen

Tanafj ord en

Airoaivi

NGU-BULL 441,2003 - PAGE54 BJ0RNSUNDVOLL &DAVID ROBERTS

Fig.2.(a)Tightlyspacedcleavagein peliticbeds in multilayered sandsto ne andc1aysto ne/mudstoneofthe Nybo rg Form ation,attheroad-cut localit ies G5.1-3(see Table2).The steeplywest- di pping cleavage isaxial planar tocomm on, open totig ht ,C.N-5-trending fold s.Photo takenbyChristophe Pascal,looking sout h.(b)Typicalcharacter of thepervasivecleavage, axialplanarto prominentfolds,ininterlayered maroon- vio letmudstone,green- ish-greyc1aystoneand greysiltstoneof the 5tabburdalFormation,westernshoreof Porsangerfjord,near localit ies5B.1 and2 (Table2);looking approx- imately north-northeast.Disaggregationstructuresin theupper partof the photosign ify that the clasticframewo rk of the sedim enthas lost cohesion atsomest age during its deformationhistory,possiblyinvolving partialliquefaction and tectonic dewateringintheveryearly phaseof the fold-and cleavage-formingevent .

in

both areas,and thecleavage isaxialplanar tothese folds (Fig.2).

Table 2. Sample numbers, formation names, rock characteristics and map/gridreferences. All map-sheetsare the latest available editions,with bluecoordinates.SamplesGS.l-S,1:50,000 map-sheet 2235I 'Smalfjo rd',3- NORedition,printed 10/94:samples GS.7-15,1:50,000map-sheet2235IV 'Ifjordfjellet',3-NOR edition,printed 7/91;samples SB.1 -9,1:50,000 map- sheet 2035 III'Lakselv',4-NORedit ion, printed2/94.

Sample

NumberFor mation

Maroon,cleavedmudstone 2460 9465 Grey,cleaved claystone 2460 9465 Red.brown,cleaved mudstone 24059425 Grey/maroon,cleavedmud stone 22109305 Maroon,cleavedmudstone 20308765 Darkgrey/black,cleaved mudstone 20858765 Darkgrey, cleavedc1aystone 20758295

A list of

the samples used in this study, their formation

affil iations,

general feat ures and grid references

is given in Tab le 2.Detailed geology at 1:50,000

scale is availab le on

ly

for map-sheet

'Lakselv' (Robert s & Davidsen 1992).

For a m

ore

regional compilation of the geology the

read er is

referred to the 1:250,000 map-sheet

'

Honninq svaq' (Roberts 1998).

Analytical techniques

Rb

and

Sr concentrationsw

ere dete

rm ined

by

isotope dilu- tion technique

using a combined spike of enr

iched87Rb and

"Sr, as described by Boelrijk(1968).

The uncertainty of the

method is estimated t o 0.5% in the Rb/Sr ratios. All isotopic measurements were carried out on a VG354 TIMS

inst ru-

ment at

the Mineral ogi cal-Geological

Museum,University of

Oslo, follow ing methods similar to those described

by Jacobsen & Heier

(1978).

During the period of analysis the NBS987 Sr-isotopic standard yielded an average of 0.71024

± 3.lsochron calculations

were carried

out accord

ing

to the method described by York

(1969). The calculations were

based on

the value of

1.42

x

10

"

a' for the decay of

8'

Rb

(Steiger

& Jaqer

1977).All results

are given as 2crerrors.

Map reference 29651510 2960151 0 29551510 29701510 26501455 2135 1350 16001665 1020 1575 10201575 07301630 Characteristics

Nyborg Green-g reyslaty mudstone Nyborg Green-greyslaty mud/c1ayston e Nyborg Green-greyslaty mudstone Mortensnes Grey,cleavedtillite matrix Nyborg Red/maroon slaty mudstone 5tappogiedde Grey-green,cleavedc1aystone Breidvik Red/maroon,cleaved mud stone Breidvik Red/maroon,cleavedmudstone Breidvik Green,cleavedmud/c1aystone Breidvik Green-gre y,slate/phyllite

5tabbursdal 5tabbursdal 5tabbursdal 5tabbursdal 5tabbursdal 5tabbursdal Dakkovarre G5.1

G5.2 G5.3 G5A G5.s G5.7 G5.10 G5.12 G5.13 G5.15

5B.1 5B.2 5B.3 5B.5.

58.7 58.8 5B.9

BJ0RN SUNDVOLL&DAVID ROBERTS NGU-BULL441,2003 - PAG E55

o.ss

"

~ O~

(f)

<0

"

CO

o.es

~

(f)

"

r-,CO

Of:.-

" " ^•

0,75

0,7

0 5 10 15 20 25 ~O ~5 40

87Rb/86S r

"

"

"

"

"

Fig.3.The analysedsamples plottedon a 87Sr/86Sr ve rsus"RbrSr diagram.(a)The sam plese ries'GS:

from the northeas tern part of the Gaissa Nap pe Com plex(cf.Fig.1). (b) Thesamplese ries'SB'from thewesternpartof theGaissaNappe Com plex.The analyticaldat a and iso chr on calculation s areshow n in Tables3 and4,respect ively.

40

~5

87Rb/86Sr

15 20 25

10 5

"

0~ 5

o~

en

<0 co _{0f:.- 5}

~ (f) r-,co _of:.-

0,75

0,7 0

Table3.Rb andSr analyticaldata.(Rb and Srconce ntrat ionerrorsapprox.

Results

^1.5%).

The results of th e elementa l and isotopic determination s

^SampleRb(ppm) Sr(ppm) Rb/Sr "Rbl"Sr "Sr/"Sr SE

and th e calculat ed Rb

/Sr

and correspondi ng

8'

Rb/

86

S r rat ios

^{GS1 243.49 27.66 8.084 25.980 .91125 ±.00003}

are present ed in Table 3.The results of the isochron calcula-

^{GS2 170.60 31.00 5.505 16.141 .84400 ±.00003} GS3 204.73 29.70 6.893 20.246 .86354 ±.0000 5

tions are listed in T able 4.The data are also plotted in Figs.3 a

_{GS4 142.79 112.83 1.266 3.680 .75885 ±.00003}

&

b.

^{GS5 218.95 65.44 3.346 9.768 .80013} ±.00003

As can be seen from Table 4,cases 1 and 3,neith er of the

^{GS7 165.95 42.11 3.941 11.493 .790 15 ±.00003}

two groups of samples yielded any tru e isochron, i.e., wit h a

^{GSlO 124.33 38.41 3.237 9.440 .79055 ±.00003} GS12 213.07 48.35 4.407 12.893 .82193 ±.00003

MSWD of

<2.5.There are no

clear-cut indications or reasons

_{GS13 150.41 25.85 5.820 17.094 .86324 ±.00003}

for leaving anyo ne or more sam ples out of the calculations.

GS15 129.99 27.17 4.784 14.007 .830 16 ±.00003

The semi-linear relat

ionship of the data as

se

en

in Figs.3 a

& ^{SB1 258.40 20.36 12.690 37.722 .98833 ±.00003}

b, probably reflects random secondary effect s on th e Rb/Sr

^{SB2 233,00 30.56 7.625 22.471 .89746 ±.00003} SB3 218.06 53.10 4.107 12.014 .82 162 ±.00003

syst em in the mud stone and clayston e rocks. How ever, th e

_{SB5 186.61 65.10 2.866 8.364 .79487 ±.00003}

clo se and tru e isochron ages indicated by cases 2, 4 and 5

5B7 184.61 83.87 2.201 6.065 .7802 8 ±.00003 (Table

4), and th e relati vely high

8'

Sr/

⁸⁶

Sr initi al rat ios (0.732-

^{SB8 224.07 81.21 2.759 8.065 .81390 ±.00003}

0.735), suggest an event of S r-isotop ic resetting at around

^{SB9 148.65 100.17 1.484 4.318 .76521 ±.00003}

480-500 Ma, which is thought to reflect an overall, pervasive

_Table4.lsochroncalculations.

metamorphic imprint on the rock form ation s from w hich

_{Case Sa mp les Age error} ("Sr/"Sr)1 error MSWD

the sample s are taken.

1 allGS samples 480 ±60 .7319 ±.007 125

2 allexce pt

Discussion

^{GS3,G S7,GS13 475 ± 32 .7334 ±.004 25}

Gaissa Nappe Comp lex

3 allSB samples 548 ±128 .7339 ±.014 226

Although not definitive, the result s of this Rb-Sr analytical

_{4 allexceptSB8 495 ±22 .7351 ±.002 9}

investigation st rongly suggest that resetting of th e S r iso-

^{5 allexceptSB1, SB8 512 ±10 .7336 ±.001 .1}

(87Sr/86Sr)1=calculated initialratio of87Sr/86Sr

NGU-B ULL 441,20 03 - PAGE 56

tope system occurred at around 480-500 Ma. Moreover , the very

hig h initialB7Sr/86Sr ratio further suggests that th

is reset- ting can, in all probability, be ascribed to a pervasive met a- morphic imprint on the rock formations of th e Tanafjorden and Vestertana Groups.This calls for an assessment of earlier isotopic studie s from th is particular nappe complex.

The earliest radiometric work on the Gaissa is that of Pringle (1973; and in Sturt et al. 1975) w ho repo rted a Rb-Sr whole-rock isochron age of 504 ± 7 Ma (recalculat ed to a 1.42 decay constant ),interpreted as the age of cleavage for- mation in pelites of th e Stappog iedde Formation (V estert ana Group). This 'isochron' was subsequent ly rejected by Dallmeyer & Reuter (1989) and Dallmeyer et al.

(1989) on the grounds that complete isotopic equilibrati on was probably not achieved, and thus detrital Rb -Sr systems were not fully rejuvenated.

This st udy was follo wed by that of Dallmeyer (1988b), who reported on the

4OArj39Ar

geochronolog y of w hole-rock, cleaved pelite samples from four undetermined locat ions withi n the Gaissa Nappe Comple x. He not ed th at spect ra of samples taken from the hing e zones of cleavage-related folds gave

«well-defined plateau

x.. ...which correspond to ages of 474 and 482 Ma.» These ages w ere interpreted by Dallme yer (1988b) to approximat ely date cleavage forma - tion at these locations, a metamorph ic event which this same autho r considered was probably related to late-stage F innmarkian tectonothermal act ivity.

Subsequently, Dallmeyer et al. (1989) carried out a joint

4°Ar/39Ar and K-Ar study on five samples of penetratively

cleaved pelite s from Gaissaformations, and th en came to a different conclusion, namely that the earliest metamor- phism had a maximum age of c. 440 Ma. Although whole- rock,4

°Ar/39

Ar , increment al-release,total-ga s ag esfor the five sampl es ranged bet ween c. 474 and 550 Ma, more reliance was placed on one sample taken from th e south western part of the Gais sa Nappe Comple x, in Porsangerfjord, w here the

<0.5 mm size

fraction of illite gave a K-Ar apparent age of 440.2 ± 9.4 Ma. In this same paper (Dallmeyer et al. 1989), another apparent age of 463.6 ± 9.9 Ma,and also in the

<0.5

mm size-fract ion range, from the northeastern part of th e Gais sa Nappe Comple x,was not di scussed.

P reference for the one KlAr apparent age at the expense of the ot her K-Ar and

4OAr/39

Ar data was influ enced by th e w ork of Reuter (1987) in the Rheinisches Schiefergebirge in Germany,w ho had d emon strated that only the very smallest size fraction yielded ages that were compatible wi t h th e geo logical con straints in that area. Coarser size fract ion s we re fou nd to retai n a detrital memory, and thu s gave imprecise ,'mixed' ages.Dallme yer et al. (1989) the refor e pre- ferred to reject the

4°Ar/39

Ar analyses;and, at the same tim e, Dallmeyer

's (1988b)

own earlier w ork and conclusions from rocks of the Gaissa Nappe Complex were also rej ected.They also argued that an Early Silur ian metamorphic age w as more consistent wit h

«t hat of the S

candian orog enesis recorded wit hin late Proterozoic through Earl y S ilurian

BJ0RNSUNDVOL L&DAVID ROBERTS

auto cht honous/-parautochthon ous metasedim entary sequ ences expo sed in the centr al and so ut hern Scandinavian Caledon ides.» However, this contention, that low -grade metamo rphism and deformat ion of S candian age wi thin th e Lower Allochthon in southern areas must also be S cand ian 1000 km farth er north,is clearly open to deb ate.

In a palaeomagnetic stu dy of low-grade se dimentary ro cks from t he parautocht hon and G aissa Nappe C omplex of sout hwestern Va ranger Peninsula, Bylund

(1994)

observed that tw o of three samples from sites within t he Gaissa along the shores ofTanafj ord yield ed data

which indi-

cated that a remagnetization had occurred in these rocks

in

E arly Ordovician time. The impl

ications

of these observa- tio ns were not discussed fu rt her.

Field evidence from basal part s of the Gaissa Nappe Compl ex shows that thrusting po st-dated the peak of meta- morph ism and cleavage generation. One sa mple of ultra cat- aclasite from th e sole thrust

(from

Handelsbukt, c. 12 km E NE of Lakselv) has been dated by the K-Ar method, and has given an age of 39 1 ± 9 Ma

(J.G.Mit

chell, unpubl. data, pers.

comm. 1989; reported in R ob ert s & Sundvoll 1990,wi th per- mission).This Mid Devonian age is com parable to dates gen- erated from top-ESEshear bands affectin g mylonites at the base of th e Kaiak Nappe Compl ex

(see below).

Laksefjord and Kaiak Nappe Comp lexes

In order to fully appreciate the data presente d

in

this contri

-

bution, it is relevant to consid er some of the pub lished iso- topic data from the suprajacent Laksefjord and Kaiak Nappe Compl exes.

The earliest attem pt at dating the penetrative slaty cleavage in pelites of the Friarfjord F ormat ion,

Laksefjord Nappe Comp lex,

is that of Pring le

in S

turt et al.

(1978).

Sa mples taken from the slate quarry at Friarfjord generated a Rb -Sr wh ole-r ock isochro n age of 492 ± 45 Ma

(recalcu-

lated using a 1.42 decay constan t),w hich w asinterp reted to date th e peak of metamorph ism.The rocks of the Laksefj ord Nappe Complex were t hus considered by Sturt et al.

(1978)

to have been involved in F in nmarkian orogen

ic

deforma- tion

.Subsequently,

Dallmeyer et al.(1989) sampled from the sa me slate quarry and obt ained a

4OAr/39

Ar total

-gas

age of 485 ± 9 Ma.

Several isotopic dat ing studies carried out w

ithin

the supracrustal and mag matic rocks of the Kaiak

Nappe Comp lex

have provid ed evid ence for eit her Scandi an or Finnmarkian metam orphic event s

,

or both. E arly

Kl

Ar and Rb/Sr s t udies in West F innmark

(S

t urt et al. 1967

,

Pringle &

S tu rt 1969) laid a basis for recognising an early-Caledonian,

Finnmarkian event in rocks of the Ka

iak

Nappe Complex

(S

turt et al. 1978). Alt houg h these early data,and th eir inter-

pretat ion, were shrouded in con troversy

(Krill & Zwaan 1987,

Binns 1989,T ownsend & Gayer 1989), subseque nt and more

precise

4OAr/39

Ar and Sm-Nd dati ng from different areas in

north ern Norway and in the correlative S eve Nappe s of

nort hern S weden

(Dallmeyer 1988a,

Dallmeyer et al. 1988,

BJ0RN SUNDVOLL&DAVID ROBERTS

1991, M e rk et al. 1988, Essex et al. 1997) confirm ed this important, latest Cambrian to earlie st Ordovician, o rogenic phase. Zw aan & van R oermund 1990) also recogn ised this early, am phib o lit e-facies and initia l th rust ing event in th e Cor rovarre Nap pe ofTroms and sugg ested possibl e correla- tio ns wi t h th e eclog it e-be aring Seve Napp es of no rthern Sweden.

In an invest igation invo lvi ng R b-Sr who le-rock and th in- slab dat ing of mylonites from the Kaiak Thru st Zone near Berselv, easte rn Porsangerfjord, o ne set of 11 sam p les yiel de d an isoch ro n age of 479 ± 15 M a (M SWD - 2.26) (Rob erts & Sund vol l 1990). In ano t her sam ple series t aken from close to th e base of th e t hru st zone,two separate sub- set s of th in slabs of top-ESE, shear- bande d my lonit es gave isochron ages of 385 ± 29 (MSWD - 2.51) and 380 ± 22 (MSWD - 3.04) Ma, respectively. Th is th in -slab dat ing ind i- cates resetti ng of th e Rb-Sr isot o pe system s on a local scale in Mid Devonian time, broadly coeval w it h th e lat est phase of ESE-directed nappe translation. Although the 479 ± 15 Ma isochron d at e could be t aken to ind icat e du ctil e myl onite gen erat ion at aro und the t ran sit ion fro m Tremadoc t o Arenig time, Roberts & Sund vo ll (1990) wer e careful to point out possibl e complications related t o th e Rb-Sr isoto pe sys- tem w it h these particular sampl es.

In northwestern Varanger Penin sula, a

4°Ar1'9Ar

st udy on th e penetrative cleavage in lower to mid greenschist-facie s phyllites of th e Berlevaq Formation in the Tanahorn Nappe, considered to represent a part of the Kaiak Nappe Compl ex (Levell & Roberts 1977), has yielded an age of c. 460 Ma (Frank & Rice 1999, Rice & F rank , in press). This metamor- phism and deformation is regard ed by these authors as rep- resenting a late st age of the Finnmarkian tectonothermal event. Samples taken from beneath the Tanahorn Napp e in formations of the Barents Sea Region (Fig. 1) gave a pre-470 Ma age for this same cleavage, whereas a lat er, spaced, crenulation cleavage yielded an age of c. 443 Ma (Rice &

Frank, in press).There are thus clear indications that the main syn-metamorphic, SE-directed fold deformation in the Tanahorn Nappe and it s footwall is pre-Scandian and of Ordovician age.

In sum m ary, there is evidence, both in the literature and from ongoing research , that part s of th e metamorphic allochthon of Finnm ark wer e invo lve d in early -Caledon ian (Lat e Cambrian to Early Ordovician ) orogenic deform ation, particularly at the Middle and lowermost Upper Allochthon level s represented by th e Laksefjord and Kaiak Nappe Complexes. In th e Gaissa Nappe Complex, th e evidence has been am big uous. While early radiometric work provided ind icat io ns of a Finnm arkian event, Dall m eyer and cowo rk- ers rejecte d th ese data, th ereby den ying the existe nce of an Early Ordovician tectonothermal event in th e metasedim en- tary rocks of the Gaissa Nappe Comple x.

The Rb-Sr analyt ical data presented here there fo re brin g us back to square one. Whil e we prefer to kee p an op en mind in thi s particula r instance, in vie w of th e interpretative

NGU-BU L L441, 2003 - PA GE 57

problems associa te d wit h th e Rb-Sr w ho le-roc k d ating method, it is pertin ent to note that the st rong indication s of a metamorphic event at aro und 480-500 M a fit w ell wit h th e bio stratig raphic evid ence from th e youngest rocks in th e nappe, th e Dig ermul Group, w hich ext end up into th e Early Tremadoc. With th e C ambrian-O rdovici an bou nd ary now def in ed at c. 490 M a and the base Areni g at c. 480 Ma

(David

ek et al. 1998, Land ing et al. 2000), deformatio n in the Gaissa Nappe Complex could conceiva bly have started sho rt ly after d epo siti on of th e Berlogaissa Form at ion (Reading 1965). Alt hou gh paleocur rent data are lacking f rom most of t he Dig ermul Group, Read in g

(1965)

has no ted th e sudde n incursio n of scour-a nd-fi ll cross-bedd ing close t o th e expose d to p of th e Berlogaissa

.Speculative

ly,this may be indicati ng t he onset of a fluvial reg ime as an emergent t op ography develop ed in th e west and no rth w est ahead of a rising Fin nm arkian mountain chai n. A d et ailed sedimento- log ical study of th e th ree form atio n s of t he Dig erm ul Group wo uld be a prereq uisite for takin g th ese ideas further.

Conclusions

Sampl es of p erva sively cleaved mud st on es and c1aystones fro m t wo different part s of th e Gaissa Nappe Co m plex in Finnmark we re subje ct ed to Rb-Sr analyti cal investigat io n w it h th e aim of tr ying t o det erm in e th e met amo rphic age of th e rocks in th is th ru st sheet . Althoug h neit her of the t

wo

groups o f samp les yield ed a true isoch ro n, th e data as a whole d o sup po rt the con cept of a pro bab le isochro n age at around 480-500 Ma, as can be discerned f ro m t he

'

isochro n f its' of variou s combination s of th e analyt ical data . Toget her w it h the relatively high

8'Sr/⁸⁶

Sr initial ratios, this result st ro ng ly suggests th at a Sr-isotop ic resetti ng event occurred at around 480 -500 Ma. Thi s eve nt is th ou ght to reflect an ov erall , perva sive met amorphic imp rint on th e rock fo rma- tions of th e Gaissa Napp e Co mplex.

Previou s rad io me t ric d ating evid ence from the Gaissa Nappe Complex has given am bi g uo us results, som e point- ing to Finnmark ian deform at ion w hereas ot her data have ind icated a Scandian age.The Rb-Sr analytic al data reported here do tend to favour a likely Finnma rkian age fo r t he very low-g rade met am orphi sm; and th is fit s with dive rse isotopic data obtai ned from th e overlyi ng nappe complexes,as

wel

l as w it h th e biostrat igraphi c evide nce w he re the you ngest rocks in th e Gaissa Nappe Complex are of Early Tremadoc age.

Acknowledgements

We are most grateful to Professors Donald Ramsay and To rgeir Ande rsenfortheir helpfu land con structivereview sof themanu script.

Thoughtful and important suggest ions byProfessorArne Raheim and Dr.BoukeZwaan also ledtoimprovementsin thefinaltypescript. Dr.

ChristophePascalkindlyprovid ed areplacem entfieldphot ograph for Figure2a.

NGU-BULL 441,2003 - PAGE 58

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