A reconnaissance Rb-Sr study of Precambrian rocks from
the S angeli-Rombak window and the pattem of initial
87
6
Sr/ Sr ratios from northem Scandinavia
J
JOHN D. GUNNER
Gunner, J. D.: A reconnaissance Rb-Sr study of Precambrian rocks from the Sjangeli-Rombak window
and the pattem of initial 87Srf86Sr ratios from northern Scandinavia. No k Geologisk Tidsskrift. Vol. 61
pp. 281-290.
rs
Oslo 1981. ISSN 0029-196X.
Coarse-grained granitic rocks underlie the Caledonian series in the Sjangeli-Rombak window, North·
Norway. A Rb-Sr whole-rock isochron on these granites gives a date of about 1780 Ma. The initial
87 f86
Sr Sr ratio of 0. 700 + 0.006 suggests that the granites were derived from the mantle or from an
isotopically similar region without significant incorporation of sialic material. The distribution of pub­
Iished initial ratios for crystalline rocks in North Norway and Sweden dated between 1845 and 1535 Ma is
consistent with the existence of an eastward-dipping NW-SE trending subduction zone beneath the area
during this period.
J.
D. Gunner, Brathay Field Study Centre, Ambleside, Cumbria,
LA22
OHN, England.
The Sjangeli-Rombak window is an area of pre­
diorite to granite. The dykes comprise aplites
belt south-east of Narvik, North Norway (Fig.
and K-feldspar-rich pegmatites, both having sub­
ordinate contents of biotite and white mica. The
Caledonian rocks within the main Caledonian
1).
There is no complete account of the geology of
the window.
Gustavson's accounts
(1966, 1969, 1972) of the
regional geology include descriptions of the Nor­
wegian part. Parts have been mapped by Vogt
(1941, 1950)
and Birkeland
and by Kulling
(1964)
(1976)
in Norway,
in Sweden. Fig.
2
is a
simplified compilation of their maps. Table
l
supracrustal rocks are mostly dark fine- to me­
dium-grained rocks, generally greenish. Their
textures vary from massive to well-foliated. They
have been subjected to folding and low-grade
metamorphism. Their parent rocks were prob­
ably volcanic or hypabyssal types with composi­
tions ranging from basic to acid.
summarises the main events in the geological
history of the area.
A reconnaissance Rb-Sr study of the coastal
region to the west included analyses of granitic
rocks from the Rombak window (Heier &
Compston
1969)
which suggest that the rocks
were formed at !east 1670 Ma ago. Migmatitic
gneisses from Vesterålen, 100 km north-west of
the Sjangeli-Rombak window, have been dated
at about
2700
Ma "(Griffin et al.
1978).
The present study was undertaken with three
objectives: to clarify the history of the major
Analytical methods
Samples were analysed for Rb and Sr by stan­
dard isotope dilution techniques, using spikes
enriched in 87Rb and 84Sr respectively. Isotopic
analyses were carried out at Cambridge Universi­
ty using an Atlas CH4 mass spectrometer
equipped with peak switching and punched paper
tape digital output. All measured values of the
87Sr/86Sr ratio were corrected for isotopic frac­
tionation to 86Sr/88Sr
=
0.1194.
Isochrons were
f
granitic bodies in the Sjangeli-Rombak window;
fitted by the method of York (1969). A es were
11
calculated using a value of 1.42 · 10" a· , and in
dykes that intrude them; and to date the supra­
citing the literature all Rb-Sr ages have been
unit in the window.
ses of N.B.S. isotope standard
to determine the age of pegmatite and aplite
crustal series which forms the oldest known rock
A total of
25
whole-rock samples were ana­
lysed for this study (Table
(11 samples)
2).
The granitic rocks
vary in composition from grano-
recalculated using this decay constant. Six analy­
987 carried out in
the Cambridge laboratory during the period cov­
ering
this study give
ratio of
0.71053.
a
mean corrected 87Srf86Sr
NORSK GEOLOGISK TIDSSKRIFT 3-4 (1981)
282 J. D. Gunner
Table l. Major geological events in the Sjangeli-Rombak win­
Sjangczli- Rombak
dow
window......._
_
(6) Ca/edonian Tectonic Phase
Large-scale eastward thrusting of the sedimentary pile (See
5). Regional metamorphism, principally affecting·its upper
portions. Precambrian basement locally sheared but largely
unaffected by metamorphisrn.
(5) Depositional Phase
Deposition of Upper Precarnbrian
(?)
and Lower Palaeo­
zoic detrital sedirnents resting on a basal conglornerate,
locally autochthonous.
(4) Minor Jntrusive Phase
Ernplacement of pegrnatite and aplite dykes and sheets.
(3) Major Jntrusive Phase
Ernplacernent of extensive bodies of granite, preceded by
srnaller bodies of syenite and locally gabbro. Incorporation
of supracrustal rocks as inclusioris in granite. Regional
�
c=J
rnetamorphism of rernnants of supracrustal series up to
Post -Caledonian
arnphibolite facies.
Ca\edonian
(2) Fo/ding
PRECAMBRIAN
l!l'E
Fig.
l.
c:=:J
Undifh:rent iatczd
Widespread folding of the suppracrustal series, giving pre­
æ::::::Y]
Volcanics
dominantly steep dips. Axes generally NNE-SSW.
§:[)
Archaoan
(l)
Formation of the Supracrustal Series
Extrusion of volcanic rocks, dorninantly basic. Deposition
Map of Fennoscandia showing distribution of major
of sediments, largely argillaceous, with sorne carbonates.
rock series and locations of main pre-Caledonian windows.
After Wilson
&
Nicholson (1973) and Adamek
&
Wilson
(1979).
Results and interpretation
(2)
Granites
The 11 granite samples form a linear array on the
Rb-Sr isochron diagram (Fig. 3). A straight-line
isochron fitted to the points indicates a date of
1780 ± 85 Ma. The points do not all lie within the
analytical uncertainty of this line. The mean
square of the weighted deviates is
34.1.
Increas­
ing the errors by a factor of six produces a perfect
fit.
The data presented here
(11
samples) and the
results of the previous Rb-Sr isochron study of
granitic rocks from the Sjangeli-Rombak window
(Heier & Compston
1969,
4 samples) do not
differ significantly. Both the dates,
1780
±
85
and
1691 ± 90 Ma, and initial 87Sr/86Sr ratios,
O. 700 ± 0.006 and O. 706 ± 0.002 (computed from
data in Heier & Compston 1969, Table 11), dif­
fer by less than the sum of the quoted errors.
There are three possible interpretations for
these results:
(l)
The rocks were derived from the ultrameta­
morphism of a pre-existing crustal complex
whose overall composition was granitic. This
event caused complete homogenisation of Sr iso­
topes between the rocks analysed, and the data
.records the time elapsed since;
(3)
The rocks
crystallised as granitic intrusives at some time
prior to about 1780 Ma ago, when regional meta­
morphism reset their Rb-Sr systems (cf. Skiold &
Larsson
1978,
Åberg
1978).
The available geological evidence is too limit­
ed to give unequivocal support to one model.
However, the low value of the initial 87Sr/86Sr
ratio together with the relatively high Rb/Sr ra­
tios suggest that the parent material of the gran­
ite, of whichever type, did not reside in the sialic
crust for any significant period before about
1780
Ma ago. It may instead have been derived from
the mantle (or a region with a comparable Sr
isotopic composition) shortly before that time.
The rocks crystallised from a
common parent magma, and the date of
1780 Ma
records the time elapsed since the rocks became
closed to Rb and Sr following crystallisation;
Fig.
2.
Simplified geological rnap of the Sjangeli-Rornbak window,
showing locations of rocks sampled for this study. Geology after
Vogt (1950) and Birkeland (1976).
NORSK GEOLOGISK TIDSSKRIFT 3--4 (1981)
Rb-Sr study in the Sjangeli-Rombak window
283
LEGEN O
c:
a
·c
�
o
"O t:.......ll..
.,
o
u
-
Allochthonous
"
.,E [U
(/)
.
dl
c: ru
3d
N
Autochthonous
Base
c
Ca l
Lowest thrust
of
Caledonian
Granite & syenite
Syenite
a
�
Gabbro
�
Supracrustal rocks
a
a.
•152
SarrØe locality
Ca l
g
139.
152.
e
'
'
'
g
.... '
Cal
O
5
10km
.._____._____.
6SN
284 J. D. Gunner
NORSK GEOLOGISK TIDSSKRIFT�
(1981)
Table 2. Descriptions and Rb-Sr analyses of rocks from the Sjangeli-Rombak window
Average1
Sample Rock type
Rb ppm
Sr ppm
87
Rbf"6Sr
87Srf"6s
r
152
139
138
244
195
355
347
231
182
341
347
165
86.5
492
126
180
112
55.4
123
93.5
117
120
2.70
4.75
0.821
5.70
3.19
9.46
19.2
5.58
5.76
8.72
8.65
0.7564
0.8355
0.7333
0.8348
0.7668
0.9418
1.207
0.8508
0.8462
0.9153
0.9229
grain size
Granites
126
130
131
138
139
141
147
148
151
152
157
Leucocratic granite
m
Leucocratic granite, as dyke cutting syenite
m
Equigranular biotite granite
m
Biotite granite, somewhat foliated
c
Biotite granite, somewhat foliated
c
Biotite granite, somewhat foliated
c
Leucocratic biotite granite
m
Biotite granite, strongly porphyritic
Leucocratic granite, as dyke cutting foliated biotite syenite
c
Biotite granite, weakly foliated
c
Biotite granite, foliated
m-c
Pegmatites
119
120
Quartz-microcline rock,
Quartz-microcline r�k,
� m thick dyke intruding
>2 m thick dyke intruding
syenite
ve
ve
413
53.8
19.0
15.7
87.0
10.3
1.903
0.%83
ve
415
21.3
63.2
1.839
ve
259
18.9
42.7
1.297
ve
123
33.9
10.9
1.013
m
43.0
m
335
f-m
164
greenstone
124
Quartz-microcline rock, dyke intruding
greenstone
129
Quartz-microcline rock, >
2
m thick dyke intruding
foliated syenite
146
Quartz-microcline rock, dyke intruding granite
121
Foliated quartz-feldspar rock,
Aplites
O. 75
m thick sheet
1480
0.085
0.7195
concordant with foliation in greenstone
142
Unfoliated quartz-feldspar rock, dyke intruding granite,
36.3
19.1
1.038
a�sociated with pegmatite dyke
Supracrustal Rocks
101
Greenish black epidote-homblende-feldspar rock, cut by
epidote-bearing veinlets up to
103
104
108
111
2
238
2.02
0.7351
137
0.182
0.7163
59.0
128
446
0.555
1.86
0.523
0.7298
0.7433
0.7111
656
0.129
0.7138
mm thick
Grey epidote-plagioclase rock, interlayered with biotite
8.46
sehist
Dark green epidote-homblende-feldspar rock, unfoliated
Green homblende-feldspar rock
f-m
f-m
Green chlorite-epidote-feldspar rock, somewhat foliated,
f-m
10.9
81.1
79.8
m
28.9
interlayered with chlorite schist
113
Grey quartz-feldspar rock, unfoliated
1f = fine-grained, m
=
medium-grained, c = coarse-grained, ve = very coarse•grained.
Pegmatite and aplite dykes
pattem. No clear conclusions can be drawn, par­
The 7 sample analyses are plotted on a Rb-Sr
isochron diagram in Fig.
4.
The points form a
diffuse band and no isochron can be fitted to
them. If the samples are co-magmatic, reference
isochrons A (1150 Ma) and B (625 Ma) may
indicate maximum and minimum ages for them.
In this case their isotopic systems have been
disturbed, in the Late Precambrian or during
Caledonian events, or both.
Supracrustal series
When plotted on a Rb-Sr isochron diagram (Fig.
5)
the 6 samples show no simple distribution
ticularly since the number of samples is small. If
one assumes that the rocks are volcanic and co­
magmatic, then evidently they have not re­
mained closed systems with respect to Rb and Sr.
Geological evidence (Table
l)
indicates that
the supracrustal series in the Sjangeli-Rombak
window predates the emplacement of the gran­
ites represented by the 1780 Ma isochron. Simi­
lar supracrustal rocks, interpreted as older than
adjacent granites, have been reported from adja­
cent parts of Sweden (Adamek 1975). One might
expect at !east partial open system behaviour in
the supracrustal rocks during the granite em­
placement. The isotopic disturbance may have
NORSK GEOLOGISK TIDSSKRIFT
3-4 (1981)
Rb-Sr study in the Sjangeli-Rombak window
occurred then or during later Sveconorwegian or
Caledonian events.
285
t
87Sr
MSr
l20
J.JQ
Svecofennian Rb-Sr data from
northern Scandinavia
Pattern of initia/87Sr/86Sr ratios
10
Rocks giving Rb-Sr isochron ages between about
2000 and 1500 Ma occur in northern Sweden and
in the pre-Caledonian basement of northern Nor­
way (Heier & Compston
1969, Welin et al. 1970,
1971, Gulson 1972,
Wilson & Nicholson 1973, Skiold 1976 & 1977,
Welin et al. 1977, Griffin et al. 1978, Skiold &
Larsson 1978, Reymer 1979, Råheim et al. 1979,
Skiold 1979, Andresen 1980). Welin (1970) dis­
Brueckner
1971,
o.ao
Welin et al.
�
•'"
070
Q.700•0·006
0 -- -::,�
5 ---.;2�
00�--� 5:-----:,�
Fig. 3. Whole-rock isochron plot for granites from the Sjangeli­
Rombak window.
cussed the evolution of the Svecofennian oro­
geny and Lundqvist
(1979) reviewed the Precam­
(1979) com­
brian of Sweden. Wilson & Sundin
piled radiometric age determinations between
1960
and
1978 for Swedish rocks and minerals,
(1980) discussed the classification of
and Wilson
.,•
Sr
"sr
J.9
Swedish Precambrian granites.
Table
3
lists published results for
40
'".
/
Rb-Sr
isochrons on igneous and gneissic rocks from
northern Scandinavia. Of these,
6 give ages in
2000 to 1800 Ma, 8 give
'synorogenic' ages between 1800 and 1745 Ma,
and 26 give 'post-orogenic' ages between 1745
and 1500 Ma.
the 'primorogenic' range
The geographical distribution of dates shows
no apparent pattem, but the initial ratios of the
syn- and post-orogenic rocks together differ in
the western and eastern parts of the region (Fig.
6). An arcuate transition zone trending N-S in
the Narvik-Tromsø area and more nearly E-W
near Luleå separates these two areas.
In the western area the initial ratios of rocks
from all three age groups vary from 0.700 to
0.706. In the transition zone the majority of the
ratios are in the range
0.700
to
0.708.
87Rb
�·
o.7 - :--__.___...._____--::-:-- --:0
20
40
60
BO - 100
m
Fig. 4. Whole-rock isochron plot for pegmatite (dots) and aplite
(crosses) dykes from the Sjangeli-Rombak window. A and B are
reference isochrons.
t
0·74
"sr
"sr
One rock
•101
unit only, the Guorbavare granite, has a higher
ratio
(0.712).
In the eastem area the majority of
the ratios are in the range
0.707
to
0.716.
•104
0·73
Only
two rock units have lower ratios: the primoro­
genic Paittasjarvi granodiorite
post-orogenic Kiruna volcanics
(0. 703)
(0.705).
and the
In each
on
o103
•113
of the three areas, the post-orogenic rocks gener­
ally have the highest ratios and the primorogenic
rocks the lowest ones.
The rocks listed in Table
3
could have been
derived from thr� types of source region: the
0·7l
87Rb­
.m
0:-
"Sr
:':
1 o· ,.-----...._---:2:':
·o:--
___...____
Fig. 1 Whole-rock isochron plot for supracrustal rocks from
the Sjangeli-Rombak window.
286
l.
D. Gunner
NORSK GEOLOGISK TIDSSKRIFT
3-4 (1981)
Table 3. Rb-Sr whole- rock isochron data for Svecofennian igneous and gneissic rocks, north Norway and north Sweden
Number
lA
lB
2
3
4
5
6
7
8
9
10
11
12
1Age(Ma)
21.R.
1915±145
1884 ±120
1875±20
1825 ±40
1805 ±20
1800±50
1790±50
1780±85
1780±50
1777±90
1772±40
1772±20
1770±70
1747±40
0.7032 ±0.0005
o. 7033±0.0006
o. 7025±0.0007
0.706 ±0.002
0.7032±0.0003
0.7033±0.0006
0.7046±0.0007
0.700 ±0.006
0.707 ±0.005
0.702 ±0.004
0.7050±0.003
o. 7051 ±0.0004
0.7046 ±0.0008
0.707 ±0.002
Notes
Rock unit
Source
Gianodiorite, Lake Paittasjarvi
Skiold
Novijaur granodiorite
Skii:\ld
1979
& Larsson 1978
Skii:\ld & Larsson 1978
Welin et al. 1970
Griffin et al. 1978
Griffin et al 1978
Griffin et al 1978
Juoksjokko granite
Haparanda suite
3
Austvagøy grey mangerite, L-V
Sund-Ølkona mangerite, L-V
Hopen mangerite
&
charnockite, L-V
Granite, Sjangeli-Rombak window
This paper
Lina granite, E. Norrbotten, Sweden
1971
& Compston 1969
Skiold 1977
Skiold 1977
Griffin et al. 1978
Welin et al. 1971
Welin et al.
Granite. Svolvær. L-V
Heier
Degerberg granite, Kalix
Red granite, Kalix
Hamarøy basic mangerite, L-V
Revsund granite, Lycksele
whole-rock
&
mineral isochron
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
1742±43
1742±30
1735±25
1735±30
1733±25
1730±40
1713±40
1710±60
1706±15
1705±30
1698±25
1695±15
1694±73
1690±75
1679±90
0.703 ±0.002
0.705 ±0.002
0.7041±0.0002
0.7050±0.0003
0.705 ±0.001
0.7030±0.0002
0.716 ±0.002
o. 7046±0.0005
0.7040±0.0003
0.7051±0.0003
0.708 ±0.002
0.7043±0.0003
0.705 ±0.003
0.703 ±0.005
0.706 ±0.002
Wilson
Granite, Arvidsjaur
Welin et al.
&
orthogneiss, L-V
Granite, Adak
Steine mangerite, L-V
Granite, Vuolosjarvi-Vakkojarvi
Raftsund mangerite, L-V
Ersfjord granite, W. Troms
Raftsund mangerite, L-V
Hiillniis granite, Skellefte
Hamarøy acid mangerite
&
charnocki-
te, L-V
Griffin et al.
Granitic gneiss, Glomfjord
Wilson
Duobblon acid volcanics, Sorsele
Welin et al.
&
1669±45
1667 ±80
1653±84
0.704 ±0.007
0.706 ±0.004
0.704 ±0.001
1973
Nicholson
1977
Griffin et al. 1978
Griffin et al. 1978
Welin et al. 1977
Griffin et al. 1978
Welin et al. 1971
Griffin et al. 1978
Andresen 1980
Griffin et al. 1978
Welin et al. 1977
S.W. Lofoten mangerite
Lundøy mangerite
l. R.
&
1978
Nicholson
1973
1971
Heier
&
Compston
1969
Gneiss, Red granite etc., L-V
Heier
&
Compston
1%9
Torset granite, Langøy,
Brueckner
Granite, syenite
aplite,
Sjangeli-Rombak window
28
29
30
&
Granitic gneiss, Nasafjiill
calculated
from data in source
L-V
Basement gneiss, Grong
1971
Råheim, Gale
&
Roberts
1979
31
32
33
34
1645±35
1600±90
1570±65
1590±45
0.708
0.714
0.705
0.708
±0.005
±0.005
±0.003
±0.002
35
36
37
38
1590±35
1530±25
1530±35
1503±30
0.712
0.710
0.712
0.713
±0.002
±0.002
±0.003
±0.004
Ledfat granite, Skellefte
Welin et al.
Acid volcanics, Kaska Tjaurek
Welin et al.
mine- Welin et al.
1977
1971
1971
1977
Guorbavare granite, Skellefte
Welin et al.
1977
Sjaunjatuottar syenite
Gulson
Lina granite, E. Norrbotten
Welin et al.
Perthite granite, Masugnsbyn
Gulson
Acid volcanics, Kiruna
Sorsele granite
whole-rock
&
Welin et al.
ral isochron
1
87
Initial Srf'6 ratio
3
mantle, the lower continental crust and the up­
per continental crust.
However, the average
Rb/Sr ratios of these zones place constraints on
the range of initial 87Sr/86Sr ratios that could be
expected in rocks deriving their strontium from
them (Faure & Powell 1972, pp. 43-54).
The low initial ratios of the mangerite series in
Lofoten-Vesterålen (10 of the rock suites in Ta-
ble
3)
1972
1971
1972
Lofoten-Vesterålen
indicate that they did not ongmate by
extensive anatexis of basement rocks in the area.
It has been suggested that they have been de­
rived by magmatic differentiation from a basic
mass emplaced in the lower crust, with limited
contamination by crustal material in some cases_
(Griffin et al. 1978).
The remaining
3,
granitic syites from Lofoten-
NORSK GEOLOGISK TIDSSKRIFf
3-4 (1981)
287
Rb-Sr study in the Sjangeli-Rombak window
Rb-Sr ISOCHRON DATA
Valeanic
Age (Mal
Plutonic
& Gneissic
\]
o
o
D
7
2000-1801
1800-1746
1745-1500
Anolysi� 1
0
I.R.:
0-707
1
�\
\
·"\.
\�
l
l
1
ø
\,.
@
@]
\
V
.,:>
�
L.
\
V
..�
14
�
•
(
\
'>
l
\
\
®�
�
LuleB
30
�
Fig. 6. Distribution of initial 87Srf"6sr ratios (I. R.) listed in Table
Vesterålen (Table 3) have initial ratios compara­
ble to the mangerites, and cannot have originat­
ed by extensive crustal anatexis either.
Four rock suites from the western area outside
Lofoten-Vesterålen are included in Table 3. All
are dominantly granitic. The significance of the
Rb-Sr data from the Sjangeli-Rombak window
has been discussed above. The remaining 3 suites
3.
include gneissic rocks which may have been af­
fected by Caledonian metamorphism (Wilson &
Nicholson 1973). Although the origin of these
gneisses is not definitely established, it has been
suggested that those at Nasafjall may have bad a
simple magmatic origin (Wilson & Nicholson
1973, Discussion). Whether they were derived
from magmatic or sedimentary rocks (Steenken
288 J. D. Gunner
NORSK GEOLOGISK TIDSSKR!Ff
1957) the parent material, like that of the granitic
rocks in the Sjangeli-Rombak window and Lofo­
ten-Vesterålen, cannot have had an extensive
3-4 (1981)
canogenic material to ancient crust would be
lower. So the pattern of initial 87Sr/86Sr ratios
shown by the granitic rock suites from
mainland
previous crustal history.
Scandinavia in Table
acidic. The relatively wide range of initial ratios
tion in a post-2000 Ma subduction system.
All the rocks suites in the eastern area are
can be interpreted in terms of two types of deri­
vation:
(l)
From an isotopically heterogeneous
3,
including those west of
the Caledonian front, is compatible with forma­
Addition of volcanic and sedimentary material
to the top of the crust in the istand are would
From two or
tend to cause upward migration of isotherms.
The consequent upward movement of the zone
crustal rocks of similar age and Rb/Sr ratio, and
of anatexis could have caused incorporation in
not allow the relative merits of these alternatives
with high 87Sr/86Sr ratios from upper levels. This
from isotopically normal mantle is involved, it
initial ratios of the granite with time.
collection of parent materials,
(2)
more isotopically homogeneous sources
(e.g.
mantle-derived magma). The available data do
to be assessed. However, if material derived
the .resulting Svecofennian granites of material
would be consistent with the observed increase in
must have been contaminated by significant pro­
portions of crustal material in the majority of
rock suites. The volcanic rocks from Duobblon
and Kiruna (Welin et al. 1971) and the granites
from Adak and Arvidsjaur (Welin et al. 1977)
could be derived from substantially uncontamin­
ated mantle material.
Except in Lofoten-Vesterålen few rocks older
Lofoten-Vesterålen and western Troms.
-
The
magmatic and metamorphic history of Lofoten­
Vesterålen during the period
2100-1700 Ma is
1978). The pre­
well documented (Griffin et al.
Caledonian spatia! relationship between the Lo­
foten-Vesterålen and western Troms areas and
the Baltic shield is debatable, and the amount of
than 2000 Ma have been dated in northern Scan­
Caledonian crustal shortening between them is
crust into which the Svecofennian intrusives were
part of the same pre-Caledonian plate, the con­
dinavia west of Finland. Thus the nature of the
emplaced is largely unknown, although experi­
mental work suggests that the granites may have
been emplaced at depths less than
tanen
35
km (Hie­
1975).
The rock suites in Table 3 show a general
increase in initial ratio with decreasing radiome­
tric age (Fig. 7). This suggests a change with time
either in the isotopic composition of the source
regions or in the relative proportions of material
contributed by more than one source (Derivation
2, above). An expansion of the source regions to
higher levels (Hietanen 1975) could account for
this, provided that the crust was isotopically
stratified.
not known. However, if the three areas formed
trast between the low Svecofennian initial ratios
west of the Caledonian front and the higher ra­
tios further east is compatible with a north-east­
ward dipping subduction system and lends sup­
port to the models discussed by Lundqvist.
Alternatively, the post-1800 Ma mangerites of
Lofoten-Vesterålen may be derived from a dif­
ferent, tensional, tectonic environment (Griffin
et al.
1978).
Conclusions
l. Granitic rocks
of Svecofennian age are pre­
sent in the Sjangeli-Rombak window, and
were largely unaffected isotopically by Cale­
Tectonic implications
Mainland Scandinavia.
donian events.
-
Lundqvist
(1979)
has
2.
discussed the Proterozoic evolution of Sweden in
ratios of Svecofennian igneous and gneissic
rocks in northern Scandinavia may reflect an
terms of plate tectonic models of the Svecofen­
E-W variation in parent rock type.
nian period proposed by several authors, includ­
ing Hietanen
(1975)
and Rickard
(1978).
The
3.
models incorporate an island are system with a
ping eastwards beneath the pre-Karelian con­
eastward beneath the pre-Karelian continental
material in the island are would be younger than
that further east, where the ratio of recent vol-
This contrast is compatible with plate tectonic
models incorporating a subduction zone dip­
NW-SE trending subduction zone dipping north­
block. In such a system the average age of crustal
An eastward increase in the initial 87Sr/86Sr
tinental block.
4.
lntrusive rocks formed in Lofoten-Vesterålen
during the Svecofennian period have initial
87Sr/86Sr ratios compatible with this model,
NORSK GEOLOGISK TIDSSKRIFT
3-4 (1981)
Rb-Sr study in the Sjangeli-Rombak window 289
+
Initial
87Sr/�r
SIDE OF CALEOONIAN
FRONT
0·715
�CK-TYPE
West
East
o
•
Plutonic
& gnl'issic
&
Volcanic
0·710
0·705
l
Age(Ma)
0·700
1900
Fig.
7.
1800
1700
1600
but petrological considerations may indicate
that they originated in a different environ­
ment.
5. The supracrustal series in the Sjangeli-Rom­
bak window is older than
Acknowledgements.
-
1780
Ma.
sen uraninitesulphide mineralization, Norrbotten County,
Sweden, Sver. geo/. unders. C. 712, 69 pp.
Adamek, P. M. & Wilson, M. R. 1979: The evolution of a
uranium province in northern Sweden. Phil. Trans. R. Soc.
London.
A 291, 355-368.
1980:
Andersen, A.
Members of the British Schools Explor­
291-298.
Birkeland, T.
advice in the field. Laboratory facilities in Cambridge were
1:100.000,
made available by Dr. I. R. Pringle and additional computing
facilities at Oxford by Dr. S. Moorbath. Dr. M. R. Wilson, Dr.
P. N. Taylor and Prof. E. Welin read the manuscript and made
helpful suggestions.
Revised October
Brueckner, H. K.
1980.
1981.
References
Precambrian geochronology of south-east­
em Sweden. Geo/. Foren. Stockholm Forh. 100,
Adamek, P. M.
19
-
125-154.
1975: Geology and mineralogy of the Kopparå-
Norsk Geologisk Tidsskrift 3-4/81
Skjomen,
Berggrunnsgeologisk
10, Nor. geo/.
1971: The age
Faure, G.
&
kart
unders.
of the Torset Granite,
Langøy, northern Norway. Nor. Geo/. Tidsskr. 51,
1972: Strontium
York, 188 pp.
Powell, J. L.
85-87.
Isotope Geology,
Griffin, W. L., Taylor, P. N., Hakkinen, J. W., Heier, K. S.,
Iden, I. K., Krogh, E. J., Malm,
sen, D. E.
1978:
1976:
Sheet N
Springer-Verlag, New
Received October
G.
The age of the Precambrian basement in
western Troms, Norway. Geo/. Foren. Stockholm Forh. 101,
ing Society and T. Birkeland provided valuable assistance and
Åberg,
1500
87
Relationship between ages and initial Srf!6Sr ratios for isochron data listed in Table 3. Bars indicate quoted errors.
&
Tveten, E.
1978:
0.,
Olsen, K. I., Ormaa­
Archaean and Proterowic
crustal evolution in Lofoten-Vesterålen, N. Norway.
Geo/. Soc. Lond.
Gulson, B. L.
J.
135, 629-647.
1972: The
Precambrian geochronology of grani­
tic rocks from northern Sweden. Geo/. Foren. Stockholm
Forh.
94, 229--244.
1966:
Gustavson, M.
The Caledonian mountain chain of the
290
J. D. Gunner
NORSK GEOLOGISK TIDSSKRIFT
southern Troms and Ofoten areas.
1:
Basement rocks and
Caledonian metasediments. Nor. geo/. unders.
M.
Gustavsson,
239,
162 pp.
1969: The Caledonian mountain chain of the
southern Troms and Ofoten areas. I l : Caledonian rocks of
igneous origin . Nor. geo/. unders.
Gustavson,
M.
261,
1 10 pp.
ry. Nor. geo/. unders.
&
283,
56 pp.
age patterns in high-grade metamorphic rocks, north Nor­
49,
the northern Sierra Nevada and the Svecofennian of Finland.
Res. U. S. Geo/. Surv.
3,
lendonberggrund. Sver. geo/. unders. Ba
19,
166 pp.
87 pp .
Råheim, A . , Gale, G . H .
&
Roberts, D . 1979: Rb, S r ages of
Nord-Trøndelag, Norway. Nor. geo/. unders.
354,
131-142.
Reymer, A . P . S. 1979: Age determinations on reworked
crystalline basement of the Grong culmination, Norway.
354,
line. Geo/. Foren. Stockholm Forh.
100,
of late Precambrian rocks in southwestern Sweden. Geo/.
98,
:l-29.
Sweden. Geo/. Foren. Stockholm Forh.
99,
Forh.
92,
43:l-45 1 .
&
b.
Nilsson,
1970: Rb-Sr age
dating of intrusive rocks of the Haparanda suite. Geo/.
92, 336-346.
& Nilsson, b.
Welin, E . , Christiansson, K.
197 1 : Rb-Sr radio­
of Proterozoic intrusives on map sheet Lannavaara, north­
Geo/.
Foren.
Sweden.
l.
Sver. geo/. unders.
Welin, E . , Einarsson,
0.,
C. 666,
31 pp.
Gustafsson , B . , Lindberg, R . , Chris­
tiansson, K . , Johansson, G.
&
Nilsson,
b.
1977: Radiome­
tric ages of intrusive rocks in northern Sweden. Il. Sver.
geo/. unders.
M.
C. 731,
Stockholm Forh.
M.
21 pp.
R. 1980: Granite types in Sweden . Geo/. Foren.
R.
&
102,
167-176.
Nicholson, R . 1973 : The structural setting
and geochronology of basal granitic gneisses in the Caledon­
ides of part of Nordland, Norway.
J.
Geo/. Soc. Lond.
129,
Stockholm
Forh.
Wilson,
M.
R.
&
Sundin, N. O. 1979 : lsotopic age determina­
tions on rocks and minerals from Sweden 1960-1978. Sver.
76-79.
Skiold, T. 1979: U-Pb and Rb-Sr whole rock and mineral ages
Sweden.
1970: Den Svecofenniska orogena zonen i norra
Sverige - en pre1iminiir diskussion . Geo/. Foren. Stockholm
365--387 .
Skiold, T . 1977: Granite ages in the Kalix area, northern
131-137.
198-213.
Nor. geo/. unders.
Wilson,
19-29.
Skiold, T. 1976: The interpretation of the Rb-Sr and K-Ar ages
eastern
21,
Vogt, T. 1950: Narvik, Berggrunnsgeologisk kart, 1 : 1 00.000 .
Wilson,
14:l-149.
Rickard, D. T. 1978: The Svecokarclian anomalous ore lead
Foren. Stockholm Forh.
244.
metric ages of extrusive and intrusive rocks in northern
basement gneisses and supracrustal rocks of the Grong area,
Nor. geo/. unders.
Univ. Amst.
Foren. Stockholm Forh.
Lundqvist, T. 1979: The Precambrian of Sweden. Sver. geo/.
C. 768,
171-176.
Vogt, T. 1941 : Trekk av Narvik-Ofoten traktens geologi . Nor.
Welin, E . , Christiansson, K .
631-645 .
Kulling, O. 1964: Oversikt over norra Norrbottensfjiillens ka­
unders.
100,
Foren. Stockholm Forh .
Steenken, W . F. 1957 : Geology and petrology of the region
Welin, E.
257-283.
Hietanen, A . 1975 : Generation of potassium-poor magmas in
J.
C. 1978 : Age data o n Proterozoic grani­
toids from the Novijaur area of northern Sweden. Geo/.
Geo/. Tidsskr.
Compston, W . 1969: Interpretation of Rb-Sr
way. Nor. Geo/. Tidsskr.
& Larsson,
( 198 1 )
south of Russånes, Saltdal, Norway. Meded. Geo/. Inst.
1972: The Caledonian mountain chain of the
southern Troms and Ofoten areas. Ill: Structures and bisto­
Heier, K. S .
Skiold, T.
:l-4
101 ,
geo/. unders. , Rapp. o. medd.
16,
42 pp.
York, D. 1969: Least squares filting of a straight line with
correlated errors. Earth Planet. Sei. Lett.
5,
32(}..-3 24.