-
The geochemistry of Lower Proterozoic mafic tofelsic igneous
rocks, Rombak Window, North Nor-wayARE KORNELlUSSEN & EDWARD W.
SAWYER
Korneliussen, A. & Sawyer, E.W. 1969: The geochemistry 01
Lower Proterozoic mal ic to tetsicigneous rocks, Rombak Window,
North Norway. Nor. geol . unders. Buff. 415, 7-21.
The supracrustal sequence 01 the Rombak Basement Window, cons
isting 01 volcanic rocks, peli-tic sediments , greywackes with
minor amounts 01 carbonate rocks and quartz ites, was intrudedby
mal ic dykes, malic to intermediate plutons and a variety 01
granitoid batholiths c. 1.6-1.7 Gaago. The region has experienced
amphibolite-grade metamorphism, followed by retrogression
togreensch ist facies along Caledonian shear-zones.
On the basis of their petrographic ano geochem ical
characteristics the volcanic rocks can bedivided into 3 suites: (1)
high-Mg basalts : (2) malic to leisic voicanites with lair ly high
potass iumcontents and with calc-alkaline affinities ; and (3)
low-potass ium, calc-alkaline felsic volcanites.
Based on major element geochemis try the evolution of the potas
sic voicani tes is interpre ted tohave been controneo , in the case
01 malic -intermediate varieties, by early fractionation of
Fe,Mg-rich minerals , and by plagioclase crystallisation for the
lelsic varieties. Suites 2 and 3 are similarto associated granites
and granodiorites in their chemical composit ion.
It is concluded that the volcano-sedimentary and intrusive rocks
were formed in an Lower Prate-rozoi c mature magmatic arc
environment at the southern margin of a continent composed predomi
-nantly 01 Archaean tona litic granitoid rocks and Lower
Proterozoic greenstone terranes .
Are Kornefiussen, Geological Survey of Norway. P.B. 3006 - Lade,
N-7002 Trondneim, Norway.£dward Sawyer, Sciences de la Terre.
Universite du Quebec a Chicoutimi, Chicoutimi, Quebec.G7H 2Bl ,
Canada.
IntroductionThe Rombak Basement Window is situatednear the
southern margin of the ArchaeanDomain (Pharaoh & Pearce 1984,
Ohlanderet al. 1987) of the Baltic Shield (Fig. 1), Thewindow conta
ins Lower Proterozoic suprac-rust al sequences consisting of
turbidites andmafic to felsic volcanites that have been in-truded
by numerous, large, felsic to mafic plu-tons . The Proterozoic
rocks of the windoware surrounded by the allochthonous Caledon i-an
nappe comp lexes (Gustavson 1974 a & b,Tull et al. 1985), and
locally by a thin sequenceof autochthonous sediments belonging to
theLate Proterozoic to Cambrian Dividal Group(Vogt 1942, Gustavson
1974 a, Birkeland 1976).
On a regional scale, the Archaean of theBaltic Shield consists
principally of felsic tointermediate, partly tonalitic gneisses
withsubordinate greenstone belts (Witschard 1984,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 erupt ion of large
volu-mes of basaltic (and some komat iitic) magmain these rift s
(Gaal & Gorbatsc hev 1987). Innorthern Sweden , supracrustal
sequencessouth of both the Lower Proterozoic greensto-ne terranes
and the Archaean craton are domi-nated by volcanites that show a
cont inuouscompositional range from maf ic to felsic ty-pes, and
that have ages between 1.9 Ga and1,8 Ga (Fritsch & Perdahl
1987).
The purpose of this paper is to describe thegeochem istry of
volcanic rocks that are partof the Lower Proterozoic supracrustal
sequen -ces exposed in the Rombak Window. Thecompositional
characteristics of the volcanites ,and of spatially assoc iated
plutonic rocks, arethen discussed in the context of an
evolvingmagmatic arc located above a subductionzone that is
postulated to have existed in theregion at some time between 1.9
and 1,7 Ga.
-
8 Are Korneliussen & Edward W.Sawyer GU - BULL. 415.
1989
r::::::lE2J
IEEEI
N
11
Caledonian and upper Proterozoicto Cambrian autochthonouscover
success ions
Prote rozoic int rusionsmainly granites (17 50 - 1900 Ma )
Early Proterozo ic supracr ustalswith fels ic- intermediate
volcanites
Early Prote rozoic supracrus alswith mafic volcanites
ITII] Lapland granulite beltD Archaean base mentSimplified from
a tec onic map compiledby the geological surveys of Finland .
Norwayand Sweden. Nordkalott project 1986.
Fig. 1. Major geolog ical units of the northern part of the
Baltic Shield in Norway. Sweden and Finland. Simplif ied from
atectonic map compiled by the geolog ical surveys of Finland.
Sweden and Norway. Nordka lott Project 1986.
Geologic setting of the RombakBasement Window
Age relationsAt presen t, very few rocks in the RombakWindow
have been dated . Romer (this vol-ume) has obta ined an age of 2.3
Ga (Rb-Sr)
for a suite of high-Mg . low-K,O basalts in theRuvssot-Sjangeli
area. The relationship betwe-en the Ruvssot-Sjangeli supracrustal
belt andthe other belts in the western part of the win-dow is not
clear because the two regions areseparated by a major ,
N-S-trending shearzone that is well exposed at Muohtaguobla
-
NGU • BULL. 415. 1989 The geochemistry of Lower Proterozic rocks
9
S '
S
ROMBAK WI NDOW
D M aflc --: intermediatevctc am t eso Felsic vorcamt esr:'J
Pyroclastic rock s.~ undlllerentia led
D Gramte
Fig. 2. Generalized geological map of the Aombak Windowbased on
Sawyer & Korne liussen (this volume). Locat ionsmentioned in
the text: S - se roai. G - Gautelis, TB -tonalitic basement, N -
Norddal, SH - Stasjonsholmen , M- Muohtaguobla, MTZ - Muotaguobla
Tectonic Zone, AS- Ruvssot-S jangeli. K - Klubbvatnet , R -
Rombaksbotn,C - Cainhavarre.
Fig. 3. Volcanic and sedimentary units of the Serda
lenSupracrustal Belt.
(Fig. 2). However, by analogy with volcanicrocks of similar
compos ition and textur e fromdated supracrustal sequences of
northernSweden (Fritsch & Perdahl 1987, Widenfalket al. 1987),
supracrustal belts west of theMuohtaguobla Tectonic Zone probab ly
haveages between 1.91 and 1.88 Ga. All the suprac-rustal sequences
of the Rombak BasementWindow have been extens ively intruded
bylarge plutons cons isting predom inant ly of gra-nite, but also
including syenite, dior ite andgabbro . Granites have been dated at
1.78 and1.69 Ga (Rb-Sr) by Gunner (1981) and Heier& Compston
(1969), respectively.
LithologyA distinct feature of the Rombak Window isthe pattern
of N-S trending linear supracrustalbelts (fig. 2) preserved between
extensive regi-ons of younger plutonic rocks (Vogt 1942,Gustavson
1974a & b, Birkeland 1976, Robynet al. 1985, Korneliussen et
al. 1986 a & b).Small rafts and inclusions of the
supracrustalrocks are locally abundant in the plutons . Allof the
supracrustal rocks and the Early Protero-zoic pluton ic rocks of
the Rombak BasementWindow are metamorphosed at least underPT-cond
itions of the lower amphibolite facies.
The rocks within the window are variablydeformed and show a
generally N-S-trending,more-or-less vert ical fol iation. The
contactsbetween the supracrusta l belts and the sur-round ing gran
ites are commonly sheared . Wit-hin the supracrustals, practically
undeformedvolcan ic and sedimentary rocks with well -preserved
primary textures are common.
The rock types present, and their relativeproportions, vary cons
iderably from one sup-racrustal belt to the next across the
RombakWindow . The Serda len Supracrustal Belt in thesouthwestern
part of the window (Fig. 2) iscomposed mainly of predominantly
porphyr itic,mafic, intermediate and felsic volcanites. Seve-ral
units of mafic/intermediate amygdaloidalvolcanites together with
felsic volcanites havebeen identified (Fig. 3); locally, thin units
ofsediment separate distinct, mappable volcanicunits. Debris flows
are interbedded with theflows, particularly on the southern side of
thebelt. Clast size in the debris flows varies fromunder 1 dm to
0.5 m. and indicates a high-energy environment of deposition. The
lower-most (eastern) telsic volcanite unit in the S0r-
-
10 Are Korneliussen & Edward W. Sawyer
dalen Supracrustal Belt is K-feldspar-bearingand closely
resembles volcanites at Cain-havarre.
The Stasjonsholmen-Rombak SupracrustalBelt conta ins a thick
sequence of graded pe-lite-greywacke turb idites, with tuff itic
layers inplaces. Amygdaloidal lavas with associateddebris flows are
developed at Klubbvatnet inthe central to northern part of the
Stasjonshol-men-Rombak Supracrustal Belt (Robyn et al.1985).
In the Muohtag uobla area mafic and interme-diate lavas
(containing acicular plagioclasephenocrysts), telsic tuff s,
pelites and graphiticschists, are complexly intermixed with
cross-bedded quartz ites and conglomerates belong-ing to the
Dividal Group . The comp lexity ofoutcrop pattern in this area is
of tecton ic ori-gin (Romer & Boundy 1988), since the
regionprobab ly represents a Caledonian imbricationzone
(terminology of Butler 1982) within theRombak Window.
In the eastern part of the window the Ruvs-sot-Sjangeli
supracrustal sequence containsmafic and ultramafic volcanites,
fine-grainedbiotite schists , greywackes and
silicate-bandedcarbonates (Romer 1988), and generally res-embles a
greensto ne assoc iation. The maficlultramafic volcanic rocks occur
as amphiboli-tes (locally pillowed) and serpentinites, someof which
contain up to 28% MgO.
At Gautelis (fig. 2) the supracrustal sequen-ce is dominated by
a turb idite sequence, butthin hor izons of tuff itic mafic and
felsic vol-canites , cong lomerates and debris flow s arelocally
developed (Skonseng 1985). Pebblesin the scattered conglomeratic
horizons con-sist of fine- to coarse-grained tonalite
andgranodiorite that resemble a nearby body oftonalite (called the
Gautelis Tonalite Comp-lex). The status of this complex is
important,as it might represent older (perhaps Archae-an) basement.
It is overlain by a basal cong lo-merate conta ining clasts derived
from the tona-lite, and a dolomitic carbonate indicating plat-form
sedimentat ion, followed by the turb iditesequence.
The individual volcanic units within the tur-biditic pelites and
greywackes in different partsof th e window range in t h ickness
from a few
centimetres to approximately 10 m, and arein general tuffitic.
In contrast, the thick vol-canite (up to 1 km) success ions are
dominant-ly lava flows. This is clearly indicated by thepresence of
amygdules in some cases (Klubb-
GU . BULl. 4 5. 1989
vatnet and Serdalen), and of delicate needle-shaped plagioclase
phenocrysts (Muohtaguob-la and Serdalen) in others. Flow
structuresare preserved in some rhyolitic flows from
theStasjonsholmen area. Our interpretation isthat the volcanites
were erupted adjacent toa deep basin that was periodically
receivingturbidite flows. Explosive volcanic eruptionsformed ash
which spread out over a largearea. Where waterlain, the ash formed
tuffitichorizons intercalated with the turbidites. Adominance of
felsic over mafic volcanic pebb-les in the debris flows in Serdalen
may indi-cate a larger volume of felsic volcanic materi-al near to
the volcanic centres.
The oldest intrusive rocks known in theRombak Basement Window
are those of themedium- to coarse-grained Gautelis TonaliteComplex.
The Gautelis Tonalite Complex andthe over lying conglomerate,
dolomitic car-bonate and turbidite sequence are intruded bya swarm
of mafic dykes. These are in turnintruded by the numerous large
plutons datedat about 1.78 Ga (Rb/Sr) by Gunner (1981).Minor mafic
to felsic dykes cut the plutonsand are of unknown age, although
Gunner(1981) presents some evidence that they maybe 1.3 Ga old
(Rb/Sr).
MetamorphismThe Rombak window, at least in its central,western
and southwestern parts , has beenmetamorp hosed under amphibolite
facies con-ditions (P 6kb, T 575°C; Sawyer 1986). Eviden-ce for
this is the widely preserved progrademineral zonation patterns
found in the interme-diate and mafic volcanites. The age of
thisprograde metamorphism has not been clearlyestab lished, but is
probably Lower Proterozo-ic. A greenschist-facies metamorphism
hasoverprinted the rock s of the window to varyingdegrees ; in most
places its effects are minor ,or even absent. However, in the
Muohtaguob-la area the greenschist-facies metamorphismhas virtually
obliterated all evidence of theearlier higher temperature event.
The intensityo f re t ro g re s s ion in the Muohta g uob la a rea
is
spat ially related to the Caledonian defo rmat ionthat has
imbricated Lower Proterozoic andDividal rocks (ct. Romer &
Boundy 1988);hence the greenschist-facies metamorphismis likely to
be of Caledonian age.
-
NGU • BULL. 415. 1989 The geochemistry ofLower Proterozic rocks
11
Table 1 (b). Major and trace element abundances inselected
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
GeochemistryRepresentative major and trace element ana-lyses of
extrusive and intrusive rocks from theRombak Basement Window are
given in Table1. The major oxides were determined by XRFusing fused
glass beads. The trace elementsV to Nb were determined by XRF using
pres-sed powder pellets. The rare earths (REE),and Cs, Th, U, Ta
and Hf were determinedby instrumental neutron activation analysis.
Acomplete list of all analysis is available fromA. Korneliussen on
request. For the elementvariation diagrams presented below,
analysesare recalculated to an anhydrous basis.
Alteration processes involving relativelymobile elements such as
Na20 and K20, can-not be excluded. Analyses of rocks from
shearzones in sardaten indicate some mobility ofcertain elements,
but this appears to be relati-vely minor (these results are not
included inthis paper). There is a fairly good consistencybetween
plots presented below involving ele-ments which are generally
accepted to beamong 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-
S102Al203Fe203"nO"90CaDHa20K20Tl02P205i.i ,SUM
VSeceerNIeuInPbRbs-BalrVNb
csThUTaHf
taeeNdSmEuTbVbtu
63.601B.'1z.zz
.06
.582.055.506.23.B'
.251.29
101.53
186
ndndndnd'0nd
107206
11002697
.76...ndnd
.6'2138173.702.80
•• 1.65.10
61.7818.5'3.12
.06
.7'2.0B5.106.11
.79
.2B
.6599.25
26ndndndndnd.01371
3802300
299
nd
203B183.303.20
.39
.66
.11
69.591'.233.82
.06
.371.573.105.76
.53
.12
.6399.78
296
nd13
713.720
2261B6B96322.516
639B'610.701.201.20'.50
.6B
68,'813.934.94
.07
.761.'64.104.99
.56
.10
.75100.14
3088
139
6.6223
22B116822337
5117
3.1021.807.741.'58.60
61.20132.0052.508.841.101.204.19
.71
71.8711.192.80
.02
.15
.672.504.80
.17
.01•• 5
100.62
ndndndndndnd3426
2591769.
8287935
'.7125.70
7.302.19
17.50
86.00191.0073.301•• 30
.12I.B'6.631.0'
76.3511.653.2B
.03
.05
.351.606.83.23.01.24
100.61
ndndndndndnd5238
3007446
6178332
1352209218.30
.292.107.601.18
75.7013.011.27
.02
.291.366.171.03
.22
.03
.5399.62
13•10nd237
1225
290B861891717
.3510.BO'.901.625.05
'2.607B.5025.904.04
.7B
.5B2.25
.32
n.n13.B7
1.75.04
1.441.924.702.1.
.26
.05
.9'99.84
2883462
221169
3331356
1BO1918
1.5610.102.192.0'' ...
3'.906B.5023.'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].55102 48.57 45.55
49.57 54.25 58.83 57.94 54.99 49.99 54.26
50.50 76.76A1ZO) 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 .25 ... • 17 •20 .r , .re
.rs .01 .0' .OS .04",0 6.48 6.89 6.22 5.58 4.26 6.25 5.95 .OB .40
1.54 .42C.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 1.74Na20 ".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
-
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 byalteration is likely to occur, though it is as-sumed that
the igneous trend in these plotsis real since the interpretation of
the majorand trace element plots is relatively consistent.
Extrusive rocksMajor elements: A plot of (Na20 + K20) versusSi02
(Fig. 4) for the volcanites of the Rombaksupracrustal belts shows
that the 2.3 Ga Ruvs-sot-Sjangeli volcanites are more mafic
andcontain less alkalis than volcanites from westof the
Muohtaguobla Tectonic Zone. Threeof the Ruvssot-Sjangeli samples
clearly repre-
* Ruvssot-Sjangelio Gautelis+ Serdalen (mafic-interm.lx 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
intermediatevolcanites from the serdaren, Muohtaguoblaand Rombak
areas plot across the boundarybetween the alkaline and subalkaline
fields.For rocks with >66 % Si02 the (Na20 + KP)versus Si02 plot
is not a useful means of dis-tinguishing between alkaline and
subalkalineseries. However, Fig. 4 shows that the vol-canites from
west of the Muohtaguobla Tecto-nic Zone, Le. the Serdalen
mafic-intermediateand felsic volcanites and the
Stasjonsholmen,Cainhavarre and Muohtaguobla felsic volcani-tes in
the Norddal area, form a continuousrange in Si02 contents from 50
to 78 %, witha preponderance of andesitic compositions.
The Na20 versus K20 plot (Fig. 5) illustratesthree important
compositional differences with-in the Rombak volcanites: (a) The
Ruvssot-Sjangeli extrusives are K20-deficient and havevariable, but
low, Na20 contents; (b) the Gaute-lis felsic volcanites from within
the GautelisTonalite Complex have a higher Na20/K20 rati-os than
the other volcanites from west of theMuohtaguobla Tectonic Zone;
and (c) withinthe Sordalen, Stasjonsholmen, MuohtaguobJaand Rombak
volcanites the mafic members
x ~o 0
60 70Si02
*2 3 456 789
KzO
9
8 -,,/7 ~'\.o
Naz06 0~~'\.
+ ..+
....5 ..+oq.
+ +* + +4 + --. + + •++ x
x+ +
3+ .t+ x+
+ •• •
2+ •+ • •
1
80
•x
'. xx
x
•
50
+ /-If. + /
/+ +/ t
+ ++ ...+ ~~ i -.c
i+ + -+to ++ /* +~ +/ *,. +
r~/ *//0"'?"
40
o 12N
~ 10+o 8
NCO 6z
4
2
Fig. 4. (Na,O+K,O)versus SiO, plot for the Rombak
Windowvolcanites. A - alkaline, SA - subalkaline.
Fig. 5. Na,O versus K,O plot for the Rombak Window vor-canites.
Symbols as in Fig. 4.
-
NGU· BULL. 415.1989
have higher Na20/K20 ratios than the associa-ted felsic
volcanites.
On the basis of Figs. 4 and 5 the volcanitesof the Rombak Window
supracrustal sequen-ces are divided into three principal types:
(1)The RS (Ruvssot-Sjangeli)-type; low-K20 maficto ultramafic
subalkaline extrusives from the2.3 Ga supracrustal belt in the
Ruvssot-Sjange-li area. (2) The G (Gautelis)-type; low-K20,
high-Na20 rhyodacitic to rhyolitic volcanites withinthe Gautelis
Tonalite Complex. (3) The SN(Serdal-Norodalj-type: a suite of mafic
to fel-sic, generally K20-rich extrusives that are cha-racteristic
of the Serdalen-Norddalen area, butoccur widely in the supracrustal
belts westof the Muohtaguobla Tectonic Zone.
The three types of volcanites are shown ona
(Na20+K20)-FeOtot-MgO plot (Fig. 6). Someof the SN-type volcanites
were clearly classi-fied as alkaline on Fig. 4, and Fig. 6
confirmsthat the SN volcanltes cannot be part of atholeiitic trend,
but belong to either the alkali-ne suite or the calc-alkaline suite
defined byIrvine & Baragar (1971). Thus, on the basisof major
elements alone the largest group ofvolcanic rocks in the Rombak
Window (theSN-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)
areclassified directly as belonging to the calc-alkaline suite and
the RS-rocks as tholeiitic(see below), though the mafic member of
theRS-type plot near to the tholeiitic/calc-alkalineboundary.
FeOtot
D. SN-type volcanites
* RS-type votcamteso Gr-type votcanites
+ Hafic to felsic intrusions• Gautetis tonalite
MgO
Fig. 6. The Rombak Window suite of volcanic and intrusiverocks
plotted in an AFM diagram.
The geochemistry of Lower Proterozic rocks 13
Trace elements: Chondrite-normalised REEpatterns for the Rombak
volcanites are shownin Fig. 7. All the samples, except those
fromthe ultramafic rocks of the RS group (Fig.7a), have similar REE
patterns that are en-riched in the light rare earths (LREE),
buthave essentially unfractionated heavy rareearths (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 resemblesthe REE patterns
of calc-alkaline mafic andandesitic magmas (e.g. McBirney et al.
1987,Meen & Eggler 1987, Gill 1981), but contrastswith the
smooth REE patterns characteristicof alkali basalts and andesites
(e.g. Eiche etal. 1987, Lanphere & Frey 1987, Frey 1981,Gill
1981). Thus, the REE patterns suggestthat the Rombak mafic to
felsic volcanitesbelong to the calc-alkaline suite.
In general, the felsic rocks have higher totalREE contents than
the more mafic rocks. Thechange in REE abundance is accompanied bya
change in the Eu anomaly present, as isdemonstrated by the telsic
members of theSN-type volcanites (Fig. 7b). The samples withthe
highest total REE contents have largenegative Eu anomalies, whereas
the sampleswith low total REE contents have positive Euanomalies.
This feature is here ascribed tolow-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 to4
times chondritic values. This type of patternis interpreted as
indicating that these rockswere derived from a LREE-depleted
mantle.The REE pattern and low Zr content of theseultramafic rocks
resembles Type I (also knownas aluminium undepleted) komatiites
(Sun &Nesbitt 1978, Jahn et al. 1982), but becausethe
Ruvssot-Sjangeli samples are Ti-depletedthey also have some
affinities with boniniticmagmas. 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 traceelements
with a strong affinity for the silicatemelt - the hygromagmatophile
elements (HYG)
-
14 Are Korneliussen & Edward W.Sawyer NGU - BULL. 415.
1989
VOLCANITES
INTRUSIONS1000.-..--..--..--................................,.........,.........,........,........,...........-..............-r...,......,......,......,........,........,...............,.......,.......,.......,.......,.......,......,......,.---,1000
2'i::"0c~ 100U
.x:oo
er:10
RS- ultramallc R22.3l
A
............ --..::·::t:::>~~::~~~.~~ ...:...
DMafic intrusions
....,....'" -----
,,"..•~..::::::::::::::~:..~~~.~~.~.....-;a
100
10
SN - felsic volcanites
---••••••••• .,===,.....c
100
10
1
-
NGU - BULL. 415, 1989 Thegeochemistry ofLower Proterozic rocks
15
1000',.--------------------, of Frey & Gordon (1974) - are
arranged inorder of increasing D values (mineral/liquidpartition
coefficients) for partial melting undermantle conditions of low
PH20 and Po2. Theabundance of HYG-elements in the Rombaksamples is
then normalised to the values foundin primordial mantle (Le.
undepleted mantle)using the mantle values of Wood (1979). Com-pared
to anororogenic basalts (Wood 1979)the Rombak basaltic andesites
(representedby the average Rombak Window basaltic ande-site - ARA)
are characteristically enriched inthe more HYG-elements and display
a distinctnegative Ta-Nb anomaly (Fig. 8a). Thus, it isinferred
that the Rombak Window basalticandesites are not of a anorogenic
type. Incontrast, when compared to orogenic (or
sub-duction-related) andesites (Fig. 8b) a strongsimilarity in
HYG-element contents is obser-ved, suggesting a similar origin. The
relativeenrichment of the large ionic Iithopile (L1L)elements such
as Cs, Rb, K and Ba in sub-duction-related rocks is considered to
be theresult 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 ofTa, Nb and Ti
in the subduction-related igne-ous rocks is attributed to the
retention of aTa-Nb-rich refractory titanium oxide phase athigh
PH20 and P02 conditions in the overlyingmantle wedge (Best 1975,
Hawkesworth etal. 1977, Sun 1979).
A
c
T1
........
Ta Nb
\ .... ARA
\........
ARSI\. ,
v
.......
AAA
..•....•...•.....•.....,..,ISLi ..
./ MORS.'
10
-'"u
~ 10
100
QJ.,c:ee
::;;-.;'El 100oE~
Fig. 8. The composition of some Rombak Window rocksnormalised to
the primordial mantle. The elements havebeen arranged after the
scheme of Wood (1979) in theorder of increasing calculated bulk
partition coefficient formantle mineralogies, Le. the more
'incompatible' elementsto the left in the diagram,(a) Comparison of
average Rombak Basement Windowbasaltic andesite (ARA) with selected
mafic lavas fromanorogenic tectonic environments. ARA is the
average ofthe six mafic-intermediate units M1 to M6 from the
serdatprofile (Fig. 3). SEA - a basanite from Victoria, SE
Austra-lia; AZ - Azores basalt; ISL - an Icelandic basalt. MORB-
normal mid ocean ridge basalt. Dataafter Wood et al. 1979.(b)
Rombak Window andesite (ARA) compared to orogenicandesites (52-56 %
SiO,). The apparent similarity sug-gests that the Rombak Window
andesites are of orogenictype, 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 minormalic plutons represented by the average of 6
analysedsamples from sercat, Gautelis and Stasjonsholmen (ARBI:see
REE-plotsof the individual samples in Fig.7d)and ARA.
Intrusive rocksSome workers (e.g. McCarthy & Groves
1979,Tindle & Pearce 1981) have pointed out thatmany granitic
plutons are predominantly accu-mulations of crystals, and do not
necessarilyrepresent melt compositions; thus comparisonwith
volcanic rocks is not straightforward. Forthe purposes of this
study our primary pointin documenting the compositional
characteris-tics of the Rombak Window plutonics is toshow their
close compositional similarity withthe SN-volcanites.Major
elements: On the (Na20+K20)-FeOl0l-MgO plot (Fig. 6) the intrusive
rocks generallyplot along a calc-alkaline trend similar to
theSN-type volcanites. Many mafic dykes andminor mafic plutons are,
however, iron en-riched compared with the mafic SN-type
vol-canites, and they plot on the tholeiite side ofthe
tholeiitic-/calc-alkaline boundary. A corre-
-
16 Are Korneliussen & Edward W.Sawyer
sponding phosphorus and titanium enrichmenttor these rocks
(ARBI) is shown in Fig. 8c.
Trace elements: Figs. 7 d-t show the chondri-te-normalized REE
patterns ot mafic dykesand minor plutons and felsic plutonic
rocksfrom 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. TheREE patterns
therefore resemble those of thecalk-alkaline rocks in the area. In
general, theRombak intrusive rocks have REE pattern ofsimilar shape
and level as the SN-type vol-canites which they intrude.
On the mantle-normalised hygromagmato-phile element diagram,
Fig. 8c, the RombakWindow mafic dykes and minor plutons areenriched
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.
DiscussionSeveral Lower Proterozoic volcanic terranesin North
America and on the Baltic Shield beara striking resemblance to
modern arc systemsin lithological and geochemical
characteristics(Condie 1987, Vivallo & Claesson 1987,
amongothers). A common problem is the bimodalityin the volcanic
successions, with a rarity ofandesites; in proper arcs the volcanic
suitesshow a continuous evolution from mafic tofelsic including
large volumes of andesite. Abimodality, however, can be explained
by aritting of the volcanic arc (Condie 1987, Vival-10&
Claesson 1987) and is not at all contradic-tory to a hypothesis
that modern-style platetectonics were active in the Lower
Proterozo-ic. It is particularly interesting to observe thata
convincing ophlcllte complex has been desc-ribed from northeast
Finland (Kontinen 1987).giving the best evidence so far that
modernplate-tectonic processes were active in theLower Proterozoic.
Thus, an interpretation ofthe rocks in the Rombak Window in the
con-text of modern plate tectonics is relevant.
On the basis of major and trace elementsand REE data the
ultramafic rocks of theRuvssot-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.tes IS~·tY;:lel
V' f.lsic vole ISN-type)
I;) Fttsic vole IG-type)
~ ""aflc and ultra,"T'Iaflcvotcaotes (RS-ty:>e)
• Icoat.te , Gal.OtelLs
X Maflc mtrus 0'15
Fig. 9. Mafic and felsic volcanic and intrusive rocks plottedin
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 extensivevolcanites: the
SN-type. Various discriminationdiagrams have been proposed to
classify thetectonic settings of volcanic and plutonic rocksby
means of their geochemistry, and thesehave been applied to the
Rombak Windowvolcanites. The Th-Hf-Ta concentrations ofmafic to
felsic volcanic and intrusive rocksfrom the Rombak Window are
plotted in thediagram (Fig. 9) of Wood et al. (1979), whichhas the
advantage of being able to distinguishthe tectonic settings of both
mafic and felsicmagma types. The Rombak igenous rocksplot well
within the field D in Fig. 9, which isthe field for magma suites
formed along de-structive plate margins, Le.
subduction-relatedmagmas. On the TiO, versus Zr plot of Pearceet
al. (1981) the indicated tectonic setting ofthe Rombak volcanites
and intrusives withSiO, contents
-
NGU·BULL.415,1989 Thegeochemistry ofLower Proterozic rocks
17
originating beneath the thickest crust. Thus,the SN-type
volcanic rocks of the RombakWindow could represent magmas
extrudedthrough a thick crust, and on the log (CaOI(Na,O + K,O))
versus SiO, diagram of Brown(1982) they do indeed plot on the
«increasingarc maturity»side of normal calc-alkaline
ande-sites.
A major problem in the interpretation of theearliest Proterozoic
evolution of this region isthe paucity of precise
age-determinations. Theconglomerate which overlies the Gautelis
To-nalite Complex (with the G-type volcanites) isitself overlain by
a dolomitic carbonate that isin turn overlain by the greywacke
sequence.The nature of the carbonate-greywacke con-tact is not yet
known. The Gautelis Tonalitecomplex represents the local basement
andcould be of either Archaean or Lower Protero-zoic age. In either
case the conglomerate indi-cates an erosional period that was
followedby platform carbonate sedimentation. TheGautelis
greywacke-tuffite sequence indicatesthe later formation of
sedimentary basins thatreceived sediment derived in part from
calc-alkaline volcanic rocks (SN-type). The tectonicsetting of 'the
sedimentary basin was near toeither a volcanic island or a magmatic
arc si-ted on continental crust, since thick piles ofvolcanic rocks
indicate a position proximal tothe volcanic centres; a more dtstal
position forthe greywackes and pelites is indicated by thethin
interbedded tufts.
Sawyer & Korneliussen (this volume) haveshown that the
tectonic setting in which thegreywackes (turbidites) formed can be
inferredfrom their composition by determining thepossible
source-rock types. The turbidites fromRombaksbotn 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 ofLower 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 parentalmagma originated by the partial melting of amantle
source. As indicated by the REE pat-terns (Fig. 7b) of the felsic
SN-volcanites, frac-tional crystallization has been a major
factorin 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-typeo Felsle vole.•G-type+ Granitess Tonallte,
Gautells
/I • __,
I ".,:+.... :+\' ,+':,j' : ..
-
1BAre Korneliussen & Edward W. Sawyer NGU· BUll. 415.
1989
Fig. 12. Plot of log (CaO/(Na,O+K,O)) against SiO, for vol-canic
and intrusive rocks in the Rombak Window (RS-typeexcluded). The
trends for volcanic suites from modernmagmatic arcs and the field
indicating the range for nor-mal calc-alkaline andesites are from
Brown (1982). T-SS -Tonga-S.Sandwich; NZ - New Zealand; NG - New
Guinea.
enrichment of the L1L-elements in the basalticandesites
indicates a subduction-related mag-ma origin. Thus three stages are
envisioned:(1) The parental magma may have originatedin the mantle
wedge above the related sub-duction zone, followed by (2) crystal
fractiona-tion by Fe-Mg silicates to generate fractiona-ted,
gradually more siliceous, intermediatemagmas; and finally (3) a
late stage of magmaevolution in which the felsic magmas wereformed,
and which was, to a large extent,controlled by the fractional
crystallization ofplagiocfase, presumably at shallow crustal
lev-els. During this process significant interactionwith crustal
rocks is likely to have occurred.In modern geological settings
magmatic rockssimilar to those of the Rombak Window potas-sic
calc-alkaline suite are believed to haveoriginated beneath a
thickened crust. Thick-ening may result from magma injection
into,or magma extrusion onto the crust; in eithercase potassic
cale-alkaline volcanites repre-sent a late-stage mature or
'continentised'stage of magmatic arc development (Fig. 12).Since no
rocks with distinct alkaline REE pat-tern were found ;n the Rombak
Window thereis no reason to suppose that the magmaticarc reached
the stage of rifting.
The Ruvssot turbidites are more mafic thanthose from Gautelis or
Rombaksbotn, andappear to contain neither continent-derived,
nor fractionated volcanic material; thus, Saw-yer &
Korneliussen (this volume) proposed anintraoceanic setting.
Furthermore, they wereable to determine that the Ruvssot
turbiditescontain material that could have been derivedfrom RS-type
volcanites. The komatiitic affinityof the ultramafic members of the
AS-typevolcanites and their associated low-K mafiecalc-atkaune
pillow basalts that show only slightLREE-enrichment is consistent
with a primi-tive, intra-oceanic island arc setting for thevolcanic
rocks of eastern Rombak BasementWindow.
Thus, the eastern part of the Rombak Base-ment Window contains
the remnants of theearly stages of an intra-oceanic arc
voleanismwhich occurred at about 2.3 Ga. In contrast,the western
part of the window contains theremains of a younger, but pre-1.78
Ga, ma-ture volcanic arc located on continental crustperhaps of
Archaean, but probably of Protero-zoic age. The present close
spatial relation-ship of these two terranes may be due to eit-her
Lower Proterozoic collision, or crustalshortening of the Lower
Proterozoic crustduring the Caldonian orogeny.
It is interesting to note that in northernSweden, the
supracrustal rocks south of theArchaean Domain (Fig. 1) are
dominated by acontinuous series of mafic to telsic voleanitesof
continental affinity (Fritsch & Perdahl 1987).Their age is 1.9
Ga based on U-Pb dating ofzircons (Ski6ld 1988). In the Skellefte
district(300 km south of Kiruna) this province givesway southwards
to a somewhat older, predomi-nantly felsic volcanic province of
marine affini-ty (Claesson 1985, Wilson et al. 1987). It hasbeen
suggested that the Skellefte provincecould be related to the
northward-directedsubduction of oceanic crust under a
LowerProterozoic continent (e.g. Hietanen 1975,Wilson et al. 1987),
and that the continentalprovince north of the Skellefte district
repre-sents a continent-based magmatic arc whichdeveloped after the
marine magmatic arc (Wie-denfalk et al. 1987).
In contrast to the Skellefte calc-alkaline!tholeiitic volcanic
suite which shows a bimoda-lity with a scarity of andesitic rocks
(Vivallo1987), the dominant type of volcanic rock inthe Rombak
Window (the SN-type) shows acontinuous evolution from calc-alkaline
basaltto rhyolite with a large proportion of andesite.The bimodal
character of the Skellefte vol-canic suite led Vivallo (1987) to
suggest that
8070% Si02
6050
Normal fieldfor talt-alkaline 0 Volcanites, SN-typeande.lt.. 0
Volcanites, G-type
............ J'. • Mane to acidic intrusions+ 0 '1'(' ~S •
Gautelis tonalite~+ o. 0"""'" I
++ '0 o-tC ......+ .:-...::. .........+:0 """ ......
+ ~,+ ......
o /.O\>~:".~!'.~ ; ;- ": '0:e '
-
NGU - BULL. 415. 1989
the Skellefte volcanic arc was dominated byextensional forces
during long periods, probab-ly producing incipient rifting. There
is no rea-son to suppose that the Rombak Windowmagmatic arc reached
the stage of rifting. Itis possible to make a tentative
correlationthat could be tested by precise age determina-tions,
that the Gautelis Tonalite Complex inthe Rombak Basement Window is
equivalentto the Jorn tonalitic complex (Claesson 1985)and its
associated felsic volcanic rocks in theSkellefte district of
Sweden. The younger grey-wacke-volcanite sequence (SN-type) may
thenhave formed in the Rombak equivalent of thecontinent-based
magmatic arc of northernSweden. If such a correlation is correct,
thenthe Gautelis Tonalite Complex has an age ofapproximately 1.9
Ga, and the greywackesand calc-alkaline SN-type potassic
volcanismare somewhat younger.
ConclusionsThe 2.3 Ga Ruvssot-Sjangeli intra-oceanicvolcanic arc
rocks and associated turbiditesequence are the oldest rocks found
in theeastern part of the Rombak Window, whereasin the south rocks
of the Gautelis TonaliteComplex (unknown age) are the oldest.
TheGautelis Tonalite Complex is overlain by abasal conglomerate and
a sequence of dolomi-tic carbonates indicating platform
sedimentati-on. After carbonate deposition a sequence ofturbidites
and potassic arc-related volcanites(the SN-type) developed,
presumably at about1.9 Ga. The composition of these
calc-alkalinevolcanic rocks ranges from that of basalt torhyollte,
but is predominantly andesitic, and isconsistent with the arc being
of a mature,continentised-type resting upon tonalitic conti-nental
crust. At present it is not known whet-her the Ruvssot-Sjangeli and
Gautelis rocksformed a continuous basement to the car-bonates, or
whether the sequence was assem-bled during a Lower Proterozoic
collision eventprior to 1.9 Ga.
The similarities in major element and traceelement composition
between the potassicarc-related volcanites and the 1.78 Ga
intru-sive rocks (Figs. 6, 7, 8 & 12) indicate a rela-ted
source. Thus, the abundant plutonic rocksin the Rombak Window
could, perhaps, repre-sent a later and deeper stage of mantle
activi-ty as the volcanic arc thickened.
The geochemistry ofLower Proterozic rocks 19
AcknowledgementsMany geologists have been involved in ore
prospecting andother geological investigations in the Rombak
BasementWindow since 1983. We are especially indebted to ArneGmnlie
(NGU) and Erik Skonseng (Univ. of Tromse) forhaving participated in
NGU's fieldwork. and Rolf Romer(Univ. of Lule{t). Frank-Dieter
Priesemann and Jan IngeTollefsrud (both Folldal Verk A/S). and Soye
Flood (nowGeologiske Tjenester a.s.• previously ARea Norway
Inc.)for having participated in many stimulating discussions aswell
as excursions in the field. E.W. Sawyer would like tothank NTNF for
a postdoctoral research fellowship (1985-1986) held at NGU,
Trondheim, without which participationin this project would not
have been possible. Finally. wewould like to thank G. Gaal and an
anonymous reviewerfor constructive reviews on the manuscript.
-
20 Are Korneliussen & Edward W. Sawyer
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