The geochemistry of Lower Proterozoic mafic to felsic igneous rocks, Rombak Window, North Nor- way

The geochemistry of Lower Proterozoic mafic to felsic igneous rocks, Rombak Window, North Nor- way

ARE KORNELlUSSEN &EDWARDW.SAWYER

Korneliussen,A. &Sawyer,E.W. 1969:The geochemistry 01Lower Proterozoic malic to tetsic igneous rocks,Rombak Window,Nort hNorw ay. Nor.geol.unders. Buff.415,7-21.

Thesupracrustal sequence 01 theRombak BasementWindow,consisting 01 volcanicrocks,peli- tic sediments,greywackes with minor amounts 01 carbonate rocks and quartzites ,wasintruded by mal icdykes,malic tointermediate plutons and avariety 01granitoidbatholithsc.1.6-1.7Ga ago.The region has experienced amphibolite-g rade metamor phism,followed byretrogression to greenschist faciesalongCaledonian shear-zones.

Onthebasis of their petrographic anogeochemical characteristicsthevolcanic rocks can be divided into 3 suites:(1) high-Mg basalts:(2) malic toleisic voicaniteswithlairlyhighpotassium contents and withcalc-alkaline affinities;and (3) low-potass ium,calc-alkalinefelsicvolcanites.

Based on major elementgeochemistrythe evolutionof thepotassicvoicanitesis interpre ted to have been controneo, in the case 01 malic-intermediate varieties, by early fractionation of Fe, Mg-richminerals,and by plagioclasecrystallisationfor the lelsic varieties. Suites 2 and 3 are similar toassociated granites andgranodioritesintheirchemicalcomposition.

It is concluded that thevolcano-sedimentaryandintrusiverockswere form edin anLowerPrate- rozoicmature magmaticarc environmen tat the southernmargin of a continentcomposed predomi- nantly 01 Archaean tonalitic granitoidrocksand Lower Proterozoicgreens tone terranes.

Are Kornefiussen,GeologicalSurveyofNorwa y.P.B.3006- Lade,N-7002Trondneim,Norway.

£dward Sawyer,Sciences dela Terre. Universitedu Quebec

a

Chicoutimi,Chicoutimi,Quebec.

G7H2Bl,Canada.

Introduction

The Rombak Basement Window is situated near the southern margin of the Archaean Domain (Pharaoh & Pearce 1984, Ohlander et al. 1987) of the Baltic Shield (Fig. 1), The window contains Lower Proterozoic suprac- rust alsequences consisting of turbidites and mafic to felsic volcanites that have been in- truded by numero us,large,felsicto maficplu- tons. The Proterozoic rocks of the window aresurrounded bythe alloc hthono usCaledoni- an nappe complexes (Gustavson 1974 a & b, Tullet al.1985),andlocallyby a thin sequence of autochthonous sediments belonging to the Late Proterozoic to Cambrian Dividal Group (Vogt 1942,Gustavson1974a,Birkeland1976).

On a regional scale, the Archaean of the Baltic Shield consists principally of felsic to intermediate, partly tonalitic gneisses with subordinate greenston ebelts(Witschard1984,

Gaal & Gorbatschev 1987, Ohlander et al.

1987), In the earliest Proterozoic (c. 2.4 Ga)

the Archaean craton was fragmented by epi- sodes of rift ing,and greenstone terranes for- med by the submar ine eruption of large volu- mes of basaltic (and some komatiitic) magma in these rifts (Gaal & Gorbatsc hev 1987). In northern Sweden, supracrustal sequences south ofboth the Lower Proterozoicgreensto- ne terranesandthe Archaeancraton are domi- nated by volcanites that show a continuous compositional range from mafic to felsic ty- pes,and that have ages between 1.9Ga and 1,8 Ga(Fritsch & Perdahl 1987).

Thepurpose of thispaper is to describethe geochemistry of volcanic rocks that are part ofthe LowerProterozoic supracrustal sequen- ces exposed in the Rombak Window. The compositional characteristics ofthevolcanites, and of spatiallyassociated pluto nic rock s,are then discussed in the context of an evolving magmatic arc located above a subductio n zone that is postulatedto have existed in the regionat some timebetween1.9 and 1,7Ga.

8 Are Korneliussen& EdwardW.Sawyer GU -BULL.415.1989

r::::::l E2J

IEEEI

N

11

Caledonianand upper Prot erozoic to Cambrian autocht honous cover success ions Prote rozoic int rusions mainly granit es(1750-1900 Ma) Early Proterozoicsupracrustals with felsic-inter mediate volcanites

Early Proteroz oic supracrus als with mafic volcanites

ITII]

Lapland granulite belt

D

^{Archaean basement}

Simplified from a teconicmap compiled bythe geologicalsurveys of Finland.Norway and Sweden.Nordkalott project 1986.

Fig.1.Major geologicalunitsof the northern part of the BalticShieldin Norw ay.SwedenandFinland.Simplifiedfrom a tectonic mapcompiled bythe geolog ical surveysofFinland.Swedenand Norwa y.Nordkalott Project1986.

Geologic setting of the Rombak Basement W indow

Age relations

At present, very few rocks in the Rombak Windo w have been dated. Romer (this vol- ume)has obtained an age of 2.3 Ga(Rb-Sr)

for a suite of high-Mg.low-K,O basaltsin the Ruvssot-Sjangeliarea.The relationshipbetwe- entheRuvssot-Sjangeli supracrustalbelt and the other belts in the westernpart of thewin- dow is not clear because thetwo regions are separated by a major, N-S-trending shear zone that is well exposed at Muohtaguo bla

NGU•BULL.415.1989 The geochem istryofLower Proterozicrocks 9

S' S

ROMBAK WI NDOW

D

^Maf lc --: intermediate vctcam t es

o

Felsic vorcamtes

r:'J Pyroc la sti crock s.

~ undlllerentia led

D

^Gramte

Fig.2.Generalizedgeologicalmap oftheAombakWindow based on Sawyer & Korneliussen (this volume).Locations mentioned in the text: S - seroai.G - Gautelis,TB - tonaliticbasement,N- Norddal,SH- Stasjonsholmen,M - Muohtaguobla,MTZ- Muotaguobla Tectonic Zone,AS - Ruvssot-Sjange li.K- Klubbvatnet,R- Rombaksbotn, C - Cainhavarre.

Fig. 3. Volcanic and sedimentar y units of the Serdalen SupracrustalBelt.

(Fig. 2). However, by analogy with volcanic rock sof similarcompos itionand textur e from dated supracr ustal sequences of northern Sweden (Fritsch & Perdahl 1987, Widenfalk et al. 1987), supracr ustal belts west of the Muohtaguobla Tectonic Zone probably have ages between 1.91and 1.88Ga.All the suprac- rustal sequences of the Rombak Basement Window have been extensively intruded by large plutons consistingpredominant ly of gra- nite, but also including syenite, dior ite and gabbro.Graniteshave beendated at 1.78and 1.69 Ga (Rb-Sr) by Gunner (1981) and Heier

& Compston (1969),respectively.

Lithology

A distinct feature of the Rombak Window is the patternofN-S trendinglinearsupracrustal belts (fig.2)prese rved betw eenextensiveregi- ons of younger pluto nic rock s (Vogt 1942, Gustavson 1974a & b,Birkeland 1976,Robyn et al. 1985, Korneliusse n et al. 1986 a & b).

Small rafts and inclusionsof thesupracrustal rocks are locally abundant in the plutons.All of the supracrustalrock sand theEarlyProte ro - zoic pluton ic rocks of the Rombak Basement Window are metamorphosed at least under PT-conditionsof thelower amphibolitefacies.

The rocks within the window are variably defor medandshow a generally N-S-trending, more-or-less vertical foliation. The contacts between the supracrustal belts and the sur- roundinggranitesare commonly sheared.Wit- hin the supracrustals, practically undeformed volcanic and sedimentary rocks with well- preserved primary textures are commo n.

The rock types prese nt, and their relative proportions, vary considerablyfrom one sup- racrustal beltto the next across the Rombak Window.The SerdalenSupracrustalBeltin the southwestern part of the window (Fig. 2) is composedmainlyofpredominantly porphyr itic, mafic,intermediate and felsic volcanites. Seve- ral units of mafic/intermediate amygdaloidal volcanites together with felsicvolcaniteshave been identified (Fig. 3); locally, thin units of sedimentseparate distinct,mappablevolcanic units. Debris flow s are interbedded with the flow s,particularlyon the southern side of the belt. Clast size in thedebrisflow svariesfrom under 1 dm to 0.5 m. and indicates a high- energyenvironment of deposition.The lower- most (eastern) telsic volcanite unit in the S0r-

10 Are Korneliussen&Edward W.Sawyer

dalen Suprac rustal Belt is K-feldspar-bearing and closely resembles volcanites at Cain- havarre.

The Stasjons holmen-Ro mbak Supracrusta l Belt contains a thick sequence of graded pe- lite-greywacke turbidites,with tuffiticlayersin places. Amygdaloidal lavas with associated debris flows are developed at Klubbvatnet in thecentraltonorthernpartof the Stasjonshol- men-Rom bak Supracrustal Belt (Robyn et al.

1985).

IntheMuohtaguoblaarea maficandinterme- diate lavas (containing acicular plagioclase phenoc rysts),telsictuffs,pelites andgraphitic schists,are complexlyintermixed with cross- bedded quartzitesandconglomerates belong- ing to the Dividal Group. The complexity of outcrop pattern in this area is of tectonicori- gin (Romer & Boundy 1988),since the region probablyrepresents a Caledo nian imbrication zone (terminology of Butler 1982) within the Rombak Windo w.

Inthe eastern part of the window theRuvs- sot-Sjangeli supracrus tal sequence contains mafic and ultrama fic volcanites, fine-grained biotiteschists,greywackesandsilicate-banded carbo nates (Romer 1988), and generally res- embles a greensto ne association.The maficl ultramaficvolcanic rock s occur as amphiboli- tes (locally pillowed) and serpentinites, some of which contain up to 28% MgO.

At Gautelis(fig. 2)the supracrustalsequen- ce is dominated by a turbidite sequence, but thin horizons of tuffitic mafic and felsic vol- canites, conglomerates and debris flow s are locally developed (Skonseng 1985). Pebbles in the scatte red conglomeratic horizons con- sist of fine- to coarse-grained tonalite and granodiorite that resemble a nearby body of tonalite (called the Gautelis Tonalite Comp- lex). The status of this complex is important, as it might represent older (perhaps Archae- an)basement. Itis overlainby a basalconglo- merate containingclastsderivedfrom thetona- lite,and a dolomitic carbonate indicating plat- form sedimentation, followed by the turbidite sequence.

The individual volcanic units within the tur- biditicpelitesand greywackesin differentparts

of th e window range in thick nes s from a few

centimetres to approximately 10 m, and are in general tuffitic. In contrast, the thick vol- canite(up to 1 km)successionsaredominant- ly lava flows. This is clearlyindicated by the prese nce ofamygdulesinsomecases (Klubb-

GU.BULl.45.1989

vatnet and Serda len),and of delicate needle- shapedplagioclasephenocrysts(Muohtaguob- la and Serdalen) in others. Flow structures are preservedinsomerhyolitic flows from the Stasjonsholmen area. Our interpretation is that the volcanites were erupted adjacent to a deep basin that was periodically receiving turbidite flows. Explosive volcanic eruptions form ed ash which spread out over a large area.Where waterlain,theash formed tuffitic horizons intercalated with the turbidites . A dominanceof felsic over maficvolcanicpebb- les in the debris flows in Serdalen may indi- cate a larger volumeof felsicvolcanic materi- al near to the volcanic centres.

The oldest intrusive rocks known in the Rombak Basement Window are those of the medium- to coarse-grained Gautelis Tonalite Complex.TheGautelisTonalite Complex and the overlying conglomerate, dolomitic car- bonate and turbiditesequenceareintrudedby a swa rm of mafic dykes. These are in turn intruded by the numerouslarge plutonsdated at about 1.78 Ga (Rb/Sr) by Gunner (1981).

Minor mafic to felsic dykes cut the plutons and are of unknown age, although Gunner (1981)presents some evidence that theymay be 1.3 Ga old (Rb/Sr).

Metamorphism

The Rombak window, at least in its central, western and southwestern parts, has been metamorphosedunderamphibolitefaciescon- ditions(P 6kb,T575°C;Sawyer 1986).Eviden- ce for this is the widely preserved prograde mineralzonation patternsfoundintheinterme- diate and mafic volcanites. The age of this prog rade metamor phism has not been clearly established,butis probably Lower Proterozo- ic. A greenschist-facies metamorphism has overprinted therocksofthe windowtovarying degrees;in most places its effects areminor, or evenabsent. However,inthe Muohtaguob- la area the greensc hist-facies metamorphism has virtually obliterated all evidence of the earlierhighertemperature event.Theintensity

of re t r o g re s sion in the Muoht a g uo bla area is

spatiallyrelatedto the Caledonian deformation that has imbricated Lower Proterozoic and Dividal rocks (ct. Romer & Boundy 1988);

hence the greensc hist-facies metamor phism is likely to be of Caledonian age.

NGU • BULL. 415. 1989 The geochemistry of Lower Proterozic rocks 11

Table 1 (b). Major and trace element abundances in selected tetslc volcanic rocks.

MuOhUg. (SN) Ca1nhav. (SN) StasJonsh. iSH) Gautel1s (G) Sample K301.3 K302.3 K2S'.3

KID'.,

^{KlOl.' K269.3 Kl01.5 Kl03.5}

od • not detected; - . not determ1ned

Geochemistry

Representative major and trace element ana- lyses of extrusive and intrusive rocks from the Rombak Basement Window are given in Table 1. The major oxides were determined by XRF using fused glass beads. The trace elements V to Nb were determined by XRF using pres- sed powder pellets. The rare earths (REE), and Cs, Th, U, Ta and Hf were determined by instrumental neutron activation analysis. A complete list of all analysis is available from A. Korneliussen on request. For the element variation diagrams presented below, analyses are recalculated to an anhydrous basis.

Alteration processes involving relatively mobile elements such as Na20 and K20, can- not be excluded. Analyses of rocks from shear zones in sardaten indicate some mobility of certain elements, but this appears to be relati- vely minor (these results are not included in this paper). There is a fairly good consistency between plots presented below involving ele- ments which are generally accepted to be among the least mobile, Le. Th, Hf, Ta, Nb, Y, Zr, Ti and the REE. This indicates an insigni- ficant degree of element mobility during altera-

S102 Al203 Fe203

"nO

"90 CaD Ha20 K20 Tl02 P205 i.i , SUM V Sece erNI eu In Pb Rbs-

Ba lr V Nb

csTh U Ta Hf eeta Nd Sm Eu Tb Vb tu

63.60 1B.'1

z.zz.06 .58 2.05 5.50 6.23

.B'

_.25

1.29 101.53 18

6 nd nd nd nd '0 nd 107 206 1100 26 9 7 .76

...

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nd 21 38 17 3.70 2.80

•• 1 .65 .10

61.78 18.5' 3.12 .06

.7'

2.0B 5.10 6.11 .79 .2B .65 99.25 26 nd nd nd nd nd .0 13 71 380 2300 29 9 nd

20 3B 18 3.30 3.20 .39 .66 .11

69.59 1'.23 3.82 .06 .37 1.57 3.10 5.76 .53 .12 .63 99.78 29

6 nd 13 7

.713 20 226 1B6 B96 322.5

16

63 9B '6 10.70

1.20 1.20 '.50 .6B

68,'8 13.93 4.94 .07 .76 1.'6 4.10 4.99 .56 .10 .75 100.14 30

8 8 13

6.9 62 23 22B 116 822 337 51 17 3.10 21.80 7.74 1.'5 8.60 61.20 132.00 52.50 8.84 1.10 1.20 4.19 .71

71.87 11.19 2.80 .02 .15 .67 2.50 4.80 .17 .01

•• 5 100.62 nd nd nd nd nd nd 34 26 259 176 9.

828 79 35 '.71 25.70 7.30 2.19 17.50 86.00 191.00 73.30 1•• 30 .12 I.B'6.63 1.0'

76.35 11.65 3.2B .03 .05 .35 1.60 6.83 .23 .01 .24 100.61 nd nd nd nd nd nd 52 38 300 74 46 617 83 32

135 220 92 18.30

.29 2.10 7.60 1.18

75.70 13.01 1.27 .02 .29 1.36 6.17 1.03 .22 .03 .53 99.62 13

•

10 nd 2 3 7 12 25 290 B86 189 17 17 .35 10.BO

'.90 1.62 5.05 '2.60 7B.50 25.90 4.04 .7B .5B 2.25 .32

n.n

13.B7 1.75 .04 1.44 1.92 4.70 2.1.

.26 .05

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99.84 28

8 3 4 6 2 22 11 69 333 1356 1BO 19 18 1.56 10.10 2.19

'

2.0'

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3'.90 6B.50 23.'0 3.49 .78

•• 2 1.63

.23

RUV$sot(RS) sereeten(5"') Maflclntruslons ~ ~

Sample RI.] R22.3 .1

.,

^.3

_••

"

^{.5 AR'} Sample ^{~ ~ ~}1:511.J 11:75.4 U33.4 1C152.3 1:273.3 1:268.3 ARBI ^{~ ~}1C55.]K5J6.J UotO.S !Cl4].5 5102 48.57 45.55 49.57 54.25 58.83 57.94 54.99 49.99 54.26

50.50 76.76

A1ZO) 9.88 7.49 11.92 14.10 16.18 16.38 17.80 17.91 15.72 5102 46.59 47.81 48.38 47.79 55.27 "9.72 71.2" 67.29 71.9"

Fe203 13.04 10.63 9.39 7.65 7.35 6.30 7.92 9.64 8.0" ,t,1203 16.03 16.U 16.32 13.53 14.29 lot.94 15.25 12.47 13.93 14.91 15.27

.'0 .09 .16 .IS .15 .13 .11 .08 .13 .13 ^{Fe203 12.63} 12.12 12.36 14.71 10.83 12.19 12.47 1.36 3.54 4.47 1.31

.,0 7.75 20.56 11.19 6.19 3.20 1.94 1.74 3.75 4.67 ,"0_{",0 6.48.25 6.89}

...

_6.22^{• 17} _5.58^{• 20} _4.26^{.r ,} _6.25^.re _5.95^{.rs .01}_.OB ^.0'_{.40 1.54}^{.OS .04}_.42

C.O 6.86 8.85 9.47 6.18 5.35 5.06 4.48 7.83 6.40 cec 8.47 7.93 9.67 9.21 6.15 8.8B B.39 .72 1.06 3.60 1.74

Na20 ".10 .50 2.63 4.05 3.59 4.60 4.90 3.97 3.96 Ha20 3.20 3.40 2.30 2.70 2.70 2.50 2.80 3.30 2.50 4.50 6.59

<10 .17 .DJ 2.69 3.91 3.12 3.78 4.36 2.B4 3.45 "0 2.07 2.39 1.20 1.37 3.04 1.45 1.92 5.06 6.45 2.12 1.52

ncz 1.30 .15 .se .89 .95 1.31 1.73 1.72 1.25 TlO2 1.68 1.60 1.)4 2.10 1.65 1. ..9 1.64 .10 .47 .54 .'7

P205 .11 .01 .

.,

.38 .24 •• 0 .75 .68 .48 P205 .32 .29 .40 .83 .52 .J5 .47 .0' .r2 .r , .OS

r.i , 6.86 4.28 1.41 .89 .B6 .58 .38 .91 I.L. .es 1.05 .90

... ...

^{J.31 .}

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113 168 157 121 115 .09

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^{15 1313 1550 721 1233 1567 1806 1365 , 21 19 21 34 .0}

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^{83JJ 15}

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^{11119 23524 1902' 41529 41915 19730 25215 e,Nb}

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7.6C1 ^{14 17 11}1."C1

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od 12 U 11 24 27 15 18 .61

" ^{.25 .93 2.00 1.06 7.58 11.6C1}

e, ,d lB.20 B.B5 7.11 12.72 U .51 .32 .85 .58 1.48 1.84

"

^{.12 .22 .J5 .21 1.13 .75}

Th .JJ 5.33 12.20 8.27 7.47 Hr 1.60 1.90 3.04 2.18 4.57 3.19

u ne

."

^{1.7B 3.B4 2.14}

To .07 .55 .83 .82 .64 L. 1J 9.91 18.50 32

..

^{24.50 23.65 6J 105 30.60 0.50}

Ht .

.,

2.50 5.33 4.05 3.66 e. 25 21.50 39.30

" ..

^{52.20 48.50 '0' 172 62.90 84.40}

" ^{17 11.00 14.00 J7}

..

^{20.70 23.95 4l 77 20.60 26.30}

La 14 .65 48.00 66.80 35.03 91 105 10' 74.52

..

^{4.7C1 3.07 4.01 8.90 9.60 4.58 5.81 10.30 13.10 J.94 3.87}

C. 15 ,d 97.50 131.00 71.93 155 187 175 135.55 E. 1.80 1.2" 1."5 2.20 2.00 1.44 1.69 .17 1.40 .98 .64

Nd 16 od 33.30 39.50 27.20

"

^as

..

^{55.89 TO}_" 1.70^.51 1.76^.ee 2.12^{.75 1.00}2.50 3.30^.» 2.44

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2.30^{.77 1.80}7.40 ^1.503.70 ' 1.44^.58 1.00^.40

Srn 3.40 .43 6.29 6.77 6.11 11.5 15.4 14.B 9.98 Lu .1. .'1 ^{.l1 .50 .50 .32 .J9 1.23 .51 .18} .u

E' .97 .1' 1.11 1.50 1.30 1.' 3.7 3.' 2.54

Tb .17 .14 .zs .54 .83 .97 1.20 1.20 .92 nd-notlletuted; -_notdehrmlned

Vb 1.40 1.12 1.10 2.28 2.62 2.60 2.60 3.3 2.47

tu .26 .14 .23 .35 .37 .45 .45 .67 .42

nd_notdetected; - - not detelllllned

Table t (a). Major and trace element abundances in Table 1 (c). Major and trace element abundances in

selected malic volcanic rocks. selected intrusive rocks.

12 Are Korneliussen&Edward W. Sawyer NGU-BULL.415.19a9

tion as far as these elements are concerned.

For the plots involving the more mobile ele- ments Na20 and K20 some scatter caused by alteration is likely to occur, though it is as- sumed that the igneous trend in these plots is real since the interpretation of the major and trace element plots is relatively consistent.

Extrusive rocks

Major elements: A plot of (Na₂₀ +K₂₀₎versus Si02(Fig. 4) for the volcanites of the Rombak supracrustal belts shows that the 2.3 Ga Ruvs- sot-Sjangeli volcanites are more mafic and contain less alkalis than volcanites from west of the Muohtaguobla Tectonic Zone. Three of the Ruvssot-Sjangeli samples clearly repre-

* Ruvssot-Sjangeli

o

Gautelis

+

Serdalen (mafic-interm.l

x

Serdalen

(f

elsicl

• Stasjonsholmen

• Rombaken ... Muohtaguobla

• Cainhavarre

sent liquid compositions (2 samples with >28%

MgO are probably cumulates) and are subalka- line. In contrast, the mafic and intermediate volcanites from the serdaren, Muohtaguobla and Rombak areas plot across the boundary between the alkaline and subalkaline fields.

For rocks with >66 % Si02 the (Na2 0 + KP) versus Si02plot is not a useful means of dis- tinguishing between alkaline and subalkaline series. However, Fig. 4 shows that the vol- canites from west of the Muohtaguobla Tecto- nic Zone, Le. the Serdalen mafic-intermediate and felsic volcanites and the Stasjonsholmen, Cainhavarre and Muohtaguobla felsic volcani- tes in the Norddal area, form a continuous range in Si0₂contents from 50to 78 %, with a preponderance of andesitic compositions.

The Na20 versus K20 plot (Fig. 5) illustrates three important compositional differences with- in the Rombak volcanites: (a) The Ruvssot- Sjangeli extrusives are K20-deficient and have variable, but low, Na₂₀contents; (b) the Gaute- lis felsic volcanites from within the Gautelis Tonalite Complex have a higher Na_20/K20rati- os than the other volcanites from west of the Muohtaguobla Tectonic Zone; and (c) within the Sordalen, Stasjonsholmen, MuohtaguobJa and Rombak volcanites the mafic members

x ~

o 0

60 70

Si02

*

2 3 4 5 6 7 8 9 KzO

9

8 _,/

^-,

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Na _z0

6

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+

..

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^{+ +}

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^{+ --. + +} ++ x

•

x+ +

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^{+ .t+}+ ^x+

•

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2

+ ⁺

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80

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x ' . xx

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50

+ /

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+ +/

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+ ++ ...

+ ~~

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i+ + -+to +

+

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~/r

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//0"'?"

40

o ¹²

~

N

10

o + 8

N

CO 6

z

4 2

Fig. 4. (Na,O+K,O) versus SiO, plot for the Rombak Window

volcanites. A - alkaline, SA - subalkaline. Fig. 5. Na,O versus K,O plot for the Rombak Windowvor- canites. Symbols as in Fig. 4.

NGU· BULL. 415.1989

have higher Na_{20/K 20} ratios than the associa- ted felsic volcanites.

On the basis of Figs. 4 and 5 the volcanites of the Rombak Window supracrustal sequen- ces are divided into three principal types: (1) The RS (Ruvssot-Sjangeli)-type; low-K₂₀mafic to ultramafic subalkaline extrusives from the 2.3 Ga supracrustal belt in the Ruvssot-Sjange- li area. (2) The G (Gautelis)-type; low-K20,high- Na20 rhyodacitic to rhyolitic volcanites within the Gautelis Tonalite Complex. (3) The SN (Serdal-Norodalj-type: a suite of mafic to fel- sic, generally K_20-richextrusives that are cha- racteristic of the Serdalen-Norddalen area, but occur widely in the supracrustal belts west of the Muohtaguobla Tectonic Zone.

The three types of volcanites are shown on a (Na20+K 20)-FeOtot-MgO plot (Fig. 6). Some of the SN-type volcanites were clearly classi- fied as alkaline on Fig. 4, and Fig. 6 confirms that the SN volcanltes cannot be part of a tholeiitic trend, but belong to either the alkali- ne suite or the calc-alkaline suite defined by Irvine & Baragar (1971). Thus, on the basis of major elements alone the largest group of volcanic rocks in the Rombak Window (the SN-type) cannot be classified with certainty, but the predominance of andesitic compo- sitions favours a calc-alkaline affinity. In con- trast, the G-type rocks (three samples) are classified directly as belonging to the calc- alkaline suite and the RS-rocks as tholeiitic (see below), though the mafic member of the RS-type plot near to the tholeiitic/calc-alkaline boundary.

FeOtot

D.SN-type volcanites

*

RS-type votcamtes oGr-type votcanites

+Hafic to felsic intrusions

• Gautetis tonalite

MgO

Fig. 6. The Rombak Window suite of volcanic and intrusive rocks plotted in an AFM diagram.

The geochemistry of Lower Proterozic rocks 13

Trace elements: Chondrite-normalised REE patterns for the Rombak volcanites are shown in Fig. 7. All the samples, except those from the ultramafic rocks of the RS group (Fig.

7a), have similar REE patterns that are en- riched in the light rare earths (LREE), but have essentially unfractionated heavy rare earths (HREE). The mafic RS-type volcanite (Fig. 7a) differs somewhat from either the SN- or the G-type volcanites (compare Figs. 7a, b, and c) in its lower La/SmN ratio. Neverthe- less, the LREE-enriched patterns of the SN-, G- and mafic RS-type volcanites resembles the REE patterns of calc-alkaline mafic and andesitic magmas (e.g. McBirney et al. 1987,

Meen & Eggler 1987, Gill 1981), but contrasts

with the smooth REE patterns characteristic of alkali basalts and andesites (e.g. Eiche et al. 1987, Lanphere & Frey 1987, Frey 1981, Gill 1981). Thus, the REE patterns suggest that the Rombak mafic to felsic volcanites belong to the calc-alkaline suite.

In general, the felsic rocks have higher total REE contents than the more mafic rocks. The change in REE abundance is accompanied by a change in the Eu anomaly present, as is demonstrated by the telsic members of the SN-type volcanites (Fig. 7b). The samples with the highest total REE contents have large negative Eu anomalies, whereas the samples with low total REE contents have positive Eu anomalies. This feature is here ascribed to low-pressure tractionation of feldspar (probab- ly plagioclase) in the parental magma.

The REE pattern for the ultramafic extru- sives of the RS-type (Iowermost curve on Fig.

7a) is LREE-depleted, and ranges from 1 to 4 times chondritic values. This type of pattern is interpreted as indicating that these rocks were derived from a LREE-depleted mantle.

The REE pattern and low Zr content of these ultramafic rocks resembles Type I (also known as aluminium undepleted) komatiites (Sun &

Nesbitt 1978, Jahn et al. 1982), but because the Ruvssot-Sjangeli samples are Ti-depleted they also have some affinities with boninitic magmas. Boninite series volcanites, however, range from 52 to 68% Si0₂(Bloomer & Haw- kins 1987).

In order to examine the compositional variati- ons of a number of trace elements simultane- ously, normalised element plots ('spider- grams') are used (Fig. 8). In Fig. 8 the trace elements with a strong affinity for the silicate melt - the hygromagmatophile elements (HYG)

14 Are Korneliussen&Edward W. Sawyer NGU - BULL. 415. 1989

VOLCANITES INTRUSIONS

1000.-..--..--..--...,...,...,...,...,...-...-r...,...,...,...,...,...,......,...,...,...,...,...,...,...,.---,1000

'i::

2

"0

~c 100 U .x:o

er:o 10

RS-ultramallcR22.3l

A

...

... --

.. ::·::t:::>~~::~~~.~~ ^...:...

D

Mafic intrusions

....,....

' " --- ,,"

..•~..::::::::::::::~:..~~~.~~.~...-;a 100

10

SN - felsic volcanites

---•••••••••.,===,...

c

100

10 1<536.3 1<55.3

E

Acidic intrusions

8

SH

lA ... $

.:." .

...

···t 10

.x:o

er:o

--

Q)^~

"0 C

~100 U

c F

--

Q) ' ' :

"0 C

~100

U .x:o

er:o 10

Gautelis (G-type) volcanites Gautelis tonalite 100

10

LaCe Nd SrnEu Tb YbLu LaCe Nd SrnEu Tb Yb Lu

Fig. 7. Chondrtte-norrnalisec REE patterns for Rombak volcanites and intrusive rocks (normalising factors from Taylor &

Gorton, 1977). (A) RS-type mafic and ultramalic extrusives and SN-type malic and intermediate volcanltes. (B) SN-type telslc volcanites. SH - Stasjonsholmen, C - Cainhavarre, M - Muohtaguobla, (C)G-type volcanites. (0) Malic dykes Irom seroat (dashed) and Gautelis (stippled) and small mafic plutons Irom Norddal (solid lines). (E) Two granitic batholites from Serdal (solid lines); one from the western side of the S0rdal supracrustal belt (KS5.3) and the other from the eastern side (KS36.3). (F)ronauucrocks from the Gautelis tonalite complex. Note that the Gautelis tonalite complex exhibits HREE frac- tionation and has a nearly smooth REE pattern compared with the G-type volcanites, suggesting that they may not be related.

NGU - BULL. 415, 1989 Thegeochemistry of Lower Proterozic rocks 15

1 0 0 0 ' , . - - - , of Frey & Gordon (1974) - are arranged in order of increasing D values (mineral/liquid partition coefficients) for partial melting under mantle conditions of low PH2⁰ and Po2. The abundance of HYG-elements in the Rombak samples is then normalised to the values found in primordial mantle (Le. undepleted mantle) using the mantle values of Wood (1979). Com- pared to anororogenic basalts (Wood 1979) the Rombak basaltic andesites (represented by the average Rombak Window basaltic ande- site - ARA) are characteristically enriched in the more HYG-elements and display a distinct negative Ta-Nb anomaly (Fig. 8a). Thus, it is inferred that the Rombak Window basaltic andesites are not of a anorogenic type. In contrast, when compared to orogenic (or sub- duction-related) andesites (Fig. 8b) a strong similarity in HYG-element contents is obser- ved, suggesting a similar origin. The relative enrichment of the large ionic Iithopile (L1L) elements such as Cs, Rb, K and Ba in sub- duction-related rocks is considered to be the result of the dehydration, or incipient melting, of subducted lithosphere enriching the overly- ing mantle wedge (Hanson 1977, Best 1975, Hawkesworth et al. 1977). The depletion of Ta, Nb and Ti in the subduction-related igne- ous rocks is attributed to the retention of a Ta-Nb-rich refractory titanium oxide phase at high PH2⁰ and P02 conditions in the overlying mantle wedge (Best 1975, Hawkesworth et al. 1977, Sun 1979).

A

c

T1 ...

Ta Nb

\ .... ARA

\...

ARSI\. ,

v

...

AAA

..•....•...•...•...

,..,

ISLi ..

./ MORS.'

10

-'"_u

~ 10

100

.,

QJ

c:ee ::;;

-.;'El100 oE

~

Fig. 8. The composition of some Rombak Window rocks normalised to the primordial mantle. The elements have been arranged after the scheme of Wood (1979) in the order of increasing calculated bulk partition coefficient for mantle mineralogies, Le. the more 'incompatible' elements to the left in the diagram,

(a) Comparison of average Rombak Basement Window basaltic andesite (ARA) with selected mafic lavas from anorogenic tectonic environments. ARA is the average of the six mafic-intermediate units M1 to M6 from the serdat profile (Fig. 3). SEA - a basanite from Victoria, SE Austra- lia; AZ - Azores basalt; ISL - an Icelandic basalt. MORB - normal mid ocean ridge basalt. Data after Wood et al. 1979.

(b) Rombak Window andesite (ARA) compared to orogenic andesites (52-56 % SiO,). The apparent similarity sug- gests that the Rombak Window andesites are of orogenic type, i.e. subduction-related. RP - K-rich Series of Vol- canic Roman Province. Mediterranean; M - Mediterranean (excluded K-rich Series of Roman Province): WSA - Wes- tern (Andean) South America: NWP - North-Western Paci- fic. Andesite data after Ewart (1982).

(c) Comparison of Rombak Window mafic dykes and minor

malic plutons represented by the average of 6 analysed samples from sercat, Gautelis and Stasjonsholmen (ARBI:

see REE-plotsof the individual samples in Fig. 7d) and ARA.

Intrusive rocks

Some workers (e.g. McCarthy & Groves 1979, Tindle & Pearce 1981) have pointed out that many granitic plutons are predominantly accu- mulations of crystals, and do not necessarily represent melt compositions; thus comparison with volcanic rocks is not straightforward. For the purposes of this study our primary point in documenting the compositional characteris- tics of the Rombak Window plutonics is to show their close compositional similarity with the SN-volcanites.

Major elements: On the (Na20+K20)-FeOl0l- MgO plot (Fig. 6) the intrusive rocks generally plot along a calc-alkaline trend similar to the SN-type volcanites. Many mafic dykes and minor mafic plutons are, however, iron en- riched compared with the mafic SN-type vol- canites, and they plot on the tholeiite side of the tholeiitic-/calc-alkaline boundary. A corre-

16 Are Korneliussen&EdwardW.Sawyer

sponding phosphorus and titanium enrichment tor these rocks (ARBI) is shown in Fig. 8c.

Trace elements: Figs. 7 d-t show the chondri- te-normalized REE patterns ot mafic dykes and minor plutons and felsic plutonic rocks from the Rombak Window. The Rombak Win- dow intrusive rocks have REE patterns of simi- lar shape, specifically LREE-enriched and with- out significant fractionation of the HREE. The REE patterns therefore resemble those of the calk-alkaline rocks in the area. In general, the Rombak intrusive rocks have REE pattern of similar shape and level as the SN-type vol- canites which they intrude.

On the mantle-normalised hygromagmato- phile element diagram, Fig. 8c, the Rombak Window mafic dykes and minor plutons are enriched in the L1L elements in a manner simi- lar to the SN-type extrusive rocks. Further- more, they also have prominent negative Ta- Nb and Ti anomalies, indicative of subduction- related magmas.

Discussion

Several Lower Proterozoic volcanic terranes in North America and on the Baltic Shield bear a striking resemblance to modern arc systems in lithological and geochemical characteristics (Condie 1987, Vivallo & Claesson 1987, among others). A common problem is the bimodality in the volcanic successions, with a rarity of andesites; in proper arcs the volcanic suites show a continuous evolution from mafic to felsic including large volumes of andesite. A bimodality, however, can be explained by a ritting of the volcanic arc (Condie 1987, Vival- 10&Claesson 1987) and is not at all contradic- tory to a hypothesis that modern-style plate tectonics were active in the Lower Proterozo- ic. It is particularly interesting to observe that a convincing ophlcllte complex has been desc- ribed from northeast Finland (Kontinen 1987).

giving the best evidence so far that modern plate-tectonic processes were active in the Lower Proterozoic. Thus, an interpretation of the rocks in the Rombak Window in the con- text of modern plate tectonics is relevant.

On the basis of major and trace elements and REE data the ultramafic rocks of the Ruvssot-Sjangeli area are shown to be compa- rable to komatiites, and the SN·, G- and ma- fie RS-type volcanites all belong to the catc- alkaline suite. Potassic andesite is the most

NGU - BULL. 415. 1989

Hfl3

~ !'!~,le -:ntl"'m~!atl"

vorcao.tesIS~·tY;:lel V'f.lsic vole ISN-type)

I;) Fttsic vole IG-type)

~""aflcand ultra,"T'Iaflc votcaotes (RS-ty:>e)

• Icoat.te , Gal.OtelLs

X Maflc mtrus 0'15

Fig. 9. Mafic and felsic volcanic and intrusive rocks plotted in the Th-Hf-Ta discrimination diagram after Wood et a!.

(1979) and Wood (1980). Field A - N-type MORB; field B - E-type MORB; field C - within-plate basatts: field 0 - magma series at destructive plate margins.

common rock type within the most extensive volcanites: the SN-type. Various discrimination diagrams have been proposed to classify the tectonic settings of volcanic and plutonic rocks by means of their geochemistry, and these have been applied to the Rombak Window volcanites. The Th-Hf-Ta concentrations of mafic to felsic volcanic and intrusive rocks from the Rombak Window are plotted in the diagram (Fig. 9) of Wood et al. (1979), which has the advantage of being able to distinguish the tectonic settings of both mafic and felsic magma types. The Rombak igenous rocks plot well within the field D in Fig. 9, which is the field for magma suites formed along de- structive plate margins, Le. subduction-related magmas. On the TiO, versus Zr plot of Pearce et al. (1981) the indicated tectonic setting of the Rombak volcanites and intrusives with SiO, contents <56 % is transitional from 'arc' to 'within-plate' (Fig. 10); the MORB possibility is excluded for lithological reasons. The same transitional character is shown on the Rb ver- sus (Y+Nb) plot (Fig. 11) for the SN-type vol- canites and the Rombak granites. However, the Rb-poor Gautelis Tonalite Complex and G-type volcanites plot well within the 'volcanic arc' field.

From Table 1 and Figs. 5 and 7 it can be seen that many of the SN-type volcanites are high-K calc-alkaline andesites. Recently, Gill (1981) and Meen (1987) have discussed the

NGU·BULL.415,1989 The geochemistry of Lower Proterozic rocks 17

originating beneath the thickest crust. Thus, the SN-type volcanic rocks of the Rombak Window could represent magmas extruded through a thick crust, and on the log (CaOI (Na,O+ K,O)) versus SiO, diagram of Brown (1982) they do indeed plot on the «increasing arc maturity» side of normal calc-alkaline ande- sites.

A major problem in the interpretation of the earliest Proterozoic evolution of this region is the paucity of precise age-determinations. The conglomerate which overlies the Gautelis To- nalite Complex (with the G-type volcanites) is itself overlain by a dolomitic carbonate that is in turn overlain by the greywacke sequence.

The nature of the carbonate-greywacke con- tact is not yet known. The Gautelis Tonalite complex represents the local basement and could be of either Archaean or Lower Protero- zoic age. In either case the conglomerate indi- cates an erosional period that was followed by platform carbonate sedimentation. The Gautelis greywacke-tuffite sequence indicates the later formation of sedimentary basins that received sediment derived in part from calc- alkaline volcanic rocks (SN-type). The tectonic setting of 'the sedimentary basin was near to either a volcanic island or a magmatic arc si- ted on continental crust, since thick piles of volcanic rocks indicate a position proximal to the volcanic centres; a more dtstal position for the greywackes and pelites is indicated by the thin interbedded tufts.

Sawyer & Korneliussen (this volume) have shown that the tectonic setting in which the greywackes (turbidites) formed can be inferred from their composition by determining the possible source-rock types. The turbidites from Rombaksbotn and Gautelis formed in an ac- tive marginal basin setting adjacent to a matu- re volcanic arc that was, in the case of Gaute- lis, probably located on a tonalitic crust of Lower Proterozoic or Archaean age. An Ande- an-type setting is proposed. From a considera- tion of the geochemistry of the SN-type vol- canites it is possible to elaborate on the histo- ry of that magmatic arc.

The high MgO content in some of the SN- type volcanites indicates that the parental magma originated by the partial melting of a mantle source. As indicated by the REE pat- terns (Fig. 7b) of the felsic SN-volcanites, frac- tional crystallization has been a major factor in the evolution of the calc-alkaline volcanites.

A negative Ta-Nb anomaly (Fig. 8b) and the

1000

WPG

ORG

Zr

- ---,

_-,

\

\

\

\

\

\

100

+ Intrusions

<>Volcanites

Syn-COLG

VAG

<> Felsle vole.• SN-type oFelsle vole.• G-type

+Granites s Tonallte, Gautells

I/ • __,

I ".,:+.... :+

\ ' ,+

':,j' : ..<0 e ~ ,.-~~ ~~~~~ +

/' "'" ~~ + -1:" <>.::::::--..::::::-. WJTHIN- . : /MORB/.'<> <> o<>\~\ PLATE

/ / ~ ^{• I \ LAVAS}

I I ,

I »>: _ _ I \

, _ _ / <, ARCLAVAS I \

\ I \

\ 1 \

\ \ \

\ \ \

10 100

Fig. 10. Volcanites and malic dykes and minor plutons (SiO,

<56%) plotted on the TiO,-Zr diagram after Pearce (1980).

10...-~--~'--'---"""--"'"

Fig. 11. Volcanic and intrusive rocks (SiO, ) 56%) plotted in the Rb-(Y+Nb) discrimination diagram 01Pearce et al.

(1984). Syn-COLG - syn-collisional granites. WPG - wit- hin plate granites, ORG - ocean ridge granites, VAG - volcanic arc granites.

origin of such rocks and noted that they often have a definite spatial and temporal associati- on with low-K calc-alkaline andesites and high- potassium andesites of alkaline affinity (shos- honites). Meen (1987) has proposed that the observed transition from low-K calc-alkaline to high-K calc-alkaline to shoshonitic andesites away from the trench is related to the depth at which fractional crystallization takes place in a magmatic arc: the more potassic rocks

1000r---.---...,.:--...

-~

Rb

Ti02