Precambrian Research, 22 (1983) 149--155
Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
149
Discussion
AGE RELATIONSHIPS IN THE PROTEROZOIC HIGH-GRADE GNEISS
REGIONS OF SOUTHERN NORWAY: DISCUSSION AND COMMENT
D. WEIS* and D. D E M A I F F E
Laboratoires Associgs de G~ologie --Pdtrologie, Universite" Libre de Bruxelles, Avenue
F.D. Roosevelt, 50 -- B.1050 Brussels (Belgium)
(Received August 24, 1982; accepted December 13, 1982)
Field and R~heim (1979a, b, 1980, 1981) provided interesting data on the
possibility of resetting Rb--Sr whole-rock isochrons by a very low- to lowgrade thermal event subsequent to a first high-grade metamorphism. In
some cases, isochrons, which appear very good from a statistical point of
view, nevertheless yield meaningless apparent ages, intermediate between
the ages of the two metamorphisms.
The data of Field and Raheim (1981) for the Arendal chamockitic gneisses of southern Norway are quite convincing from both petrographical and
geochronological viewpoints: a granulite facies metamorphism dated at
1500 Ma (by Rb--Sr whole-rock isochron) has been disturbed by a subsequent low-grade event at ca. 1000 Ma. Field and R~heim (1981) extrapolate
their interpretation of the Arendal data to most of the other geochronological data of southern Norway. They claim that:
" . . . the main high-grade gneiss-forming event may have been pre-Grenvillian (that is pre-Sveconorwegian) in all sectors (of southern Norway)
and there may have been no regional high-grade reworking during the
period 1.2--0.9 Ga ago."
With regard to the Rogaland and Vest--Agder provinces, it is obvious from
the numerous geochronological data obtained in different laboratories that
the granulite-facies metamorphic event is of Sveconorwegian (Grenvillian)
age, that is between 1200 and 900 Ma.
The situation is as follows:
(1) all the U--Pb data on zircons (more than 30 fractions) from augen
gneisses and granitic gneisses define discordia chords whose upper intercepts
with Concordia give agee at ~ 1050--1000 Ma (Versteeve, 1975; Pasteels and
Michot, 1975; Wielens et al., 1981);
(2) large massifs of anorthosites, mangerites and charnockites syn- or
late-tectonically intruded into the surrounding gneisses (Michot, 1960;
Michot and Michot, 1969; De Waard et al., 1974) yield Rb--Sr and U--Pb
emplacement ages between 980 and 900 Ma. The magmatic structures (i.e.,
*FNRS Senior Research Assistant.
0301-9268/83/$03.00
© 1983 Elsevier Science Publishers B.V.
150
ophitic structures, igneous layering, orthocumulate t e x t u r e s , . . . ) a r e well
preserved for most of the magmatic rocks and it is evident that the measured
ages really represent the crystallization age (Pedersen et al., 1978; Pasteels
et al., 1979).
(3) Rb--Sr and K--At ages measured on primary brown biotites belonging
to the granulite facies mineral assemblages are close to 870 Ma (Verschure et
al., 1980), interpreted as cooling ages after the last phase of the Sveconorwegian metamorphism.
The only visible secondary alteration in the metamorphic cover sequence
is the incipient development of very low-grade mineral assemblages (prehnite
+ pumpellyite) in the Flekkefjord--Tonstad area. To the north-east, nearer
the Caledonian front, greenschist-facies assemblages are observed, representing replacement of brown biotites by green biotite and titanite. These
assemblages are the result of a low-grade metamorphic event which can be
related, in view of the field relations, to the effects of Caledonian orogenesis
on the Sveconorwegian rocks. This is confirmed by a Caledonian age of ca.
400 Ma on the green biotites (Verschure et al., 1980). In general agreement,
the lower intercepts of some discordia chords give ages between 410 and
310 Ma (Wielens et al., 1981) which might be interpreted as the result of
episodic lead loss from Sveconorwegian zircons during the Caledonian event.
In conclusion, in the Rogaland province, it appears that high-grade metamorphism is really of Sveconorwegian age (ca. 1000 Ma) and the incipient
superimposed low-grade event is of Caledonian age (ca. 400 Ma).
WHAT ABOUT THE POSSIBILITY OF A PRE-SVECONORWEGIAN HIGH-GRADE
METAMORPHISM?
For the hypothesis of a pre-Sveconorwegian (1500 Ma? ) high-grade
event, as postulated by Field and R~theim (1981), it is highly improbable
that different zircon fractions would define linear arrays giving the observed
~ 1000 Ma in the concordia diagram if they had suffered two lower-grade
thermal events and two accompanying lead losses, one of Sveconorwegian
age (ca. 1000 Ma) and one of Caledonian age (ca. 400 Ma). Indeed, in that
case, the zircon data points would plot in a triangle defined by the three
ages ~ 1500 Ma (?) to ~ 1000 Ma to ~ 400 Ma, but not on straight lines.
Nevertheless, very few zircons, particularly those from metasedimentary
rocks (garnet gneisses, quartzites . . . . ), yield 2°~Pb/2°6Pb ages significantly
older than ~ 1200 Ma (Pasteels and Michot, 1975). In the Rb--Sr diagram,
the garnetiferous migmatite data points scatter between two reference lines
corresponding to ~ 1500 Ma and ~ 1000 Ma (Wielens et al., 1981). These
ages, slightly higher than the Sveconorwegian ages, could result from a preSveconorwegian metamorphic event or from an incomplete resetting and/or
isotopic rehomogenization of the Rb--Sr and U--Pb systems during the
Sveconorwegian granulite-facies metamorphism. In such rocks, it is indeed
obvious that at least a fraction of the zircon population is of detrital origin
151
and could have retained a part of the previously accumulated radiogenic
lead even after a high-grade metamorphism. In our view, these data do not
constitute convincing evidence for the existence of a well-defined high-grade
metamorphic event significantlyolder (i.e.,~ 1500 Ma) than the Sveconorwegian orogenesis in the Rogaland province.
Pb--Pb isochron age studies of whole-rock gneisses m a y date a highgrade event. Indeed, it is well known (Lambert and Heier, 1968; Moorbath
et al., 1969; Sighinolfi, 1971; Heier, 1973) that granulite-faciesmetamorphism induces regional U depletion resulting in low U/Pb ratios. If these
rocks behaved as closed systems to U and Pb since metamorphism, the Pb
isotopic composition of whole rocks could give the age of the U depletion
and, thus, the age of the high-grade metamorphism (Moorbath et al., 1969;
Gray and Oversby, 1972; Beckinsale et al., 1980). In general, lower grade
metamorphic events, post-dating the main granulite facies phase do not significantly disturb the Pb--Pb systematics of whole rocks.
Eleven granulite-faciesgneisses of different lithologiesfrom the Rogaland
complex have been analysed for their Pb isotopic compositions. The results,
together with Th, U and Pb concentrations are reported in Table I. Except
the two granitic gneisses which have normal U concentrations (5--7 ppm,
samples J C D 73-48 and J C D 72-111), most of the samples appear variably
depleted in U although not so severely depleted as the granulite-facies
Lewisian gneisses (mean U content, 0.24 ppm; Moorbath et al., 1969).
The U content is quite variable; consequently there is a wide scatter in the
23SU/2°4Pb values, from 0.8 to 4.7.
In the 2°7Pb/2°4pb v. 20~ pb/204pb diagram (Fig. la),the data points define a
very good straight line ( M S W D = 0.55) which, if interpreted as a secondary
isochron, yields an age of 1359 + 120 M a (2o). In the classicalinterpretation of the Pb--Pb isotope data, this age corresponds to a regional U depletion occurring during a granulite-faciesmetamorphism. This age m a y be correlated with the firstmetamorphic phase of the Sveconorwegian orogenesis
(the Nil phase of Maijer et al., 1981), as also m a y the age of the poorly defined U--Pb zircon ages slightly exceeding 1200 M a discussed above and in
Pasteels and Michot (1975). In the 2°SPb/2°4Pb v. 2°6Pb/2°4pb diagram (Fig.
lb), the scatter of the data points indicates that under granulite-facies
conditions the behavior of Th is quite different from that of U, as pointed
out by Moorbath et al. (1969) and Gray and Oversby (1972). The highgrade metamorphic event at 1000 M a has not disturbed the Pb--Pb systematics in whole rocks since, in such a short time interval (300 Ma) and in a
relatively U-depleted environment, there was nearly no radiogenic lead accumulation.
The possibilitythat the isochron represents a "transposed palaeo-isochron"
(Griffin et al., 1978) has also been examined. Following this model, the
rocks should have been formed at ~ 1200 M a and became U-depleted at
900 Ma. The main characteristicsof the Pb isotopic data in such a situation are (Moorbath and Taylor, 1981):
c~
TABLE I
L e a d i s o t o p i c c o m p o s i t i o n a n d U, T h a n d Pb c o n c e n t r a t i o n s o f g r a n u l i t e - f a c i e s g n e i s s e s o f S o u t h R o g a l a n d
Sample c
Lithology
2°~Pb/2°4Pb ± Io
2°TPb/2°4Pb + Io
2°sPb/2°4Pb +- Io
Pb a
(ppm)
Ub
(ppm)
Th b
(ppm)
23'U/2°4Pb
DD
DD
DD
DD
PA
F i n e n o r i t i c gneiss
O p x g r a n i t i c a u g e n gneiss
G r a n i t i c gneiss
G a r n e t - - c o r d i e r i t e gneiss
A u g e n gneiss
18.453
17.417
19.701
18.016
17.192
17.189
18.195
18.553
19.618
17.938
17.920
19.832
15.587
15.481
15.681
15.548
15.450
15.448
15.550
15.604
15.670
15.544
15.538
15.694
37.593
36.765
39.963
37.260
37.086
37.088
37.813
37.704
40.202
36.816
37.704
39.760
7.9
22.7
42.3
19.4
40.3
0.6
<0.3
-0.5
0.6
2.1
0.35
-1.4
6.6
4.7
0.8
-1.6
0.9
25.7
---
0.5
1.7
2.6
6.9
1.1
7.4
5.9
4.1
2.6
52
4.5
56
1.2
------
354-1/1
622-1/1
390-1/1
212-1/3
66/L
PA 70/B
J C D 78-34
JCD 72-158
J C D 73-48
J C D 80-30
J C D 72-111
A u g e n gneiss
L e u c o g r a n i t i c gneiss
G r a n u l i t i c gneiss
B i o t i t e granitic gneiss
L e u c o g r a n i t i c gneiss
Porphyritic granitic
gneiss
± 0.013
± 0.012
± 0.026
± 0.019
± 0.030
± 0.018
± 0.016
± 0.015
± 0.016
± 0.018
± 0.011
± 0.016
+-0 . 0 1 5
± 0.013
+-0 . 0 2 6
-+0 . 0 2 4
± 0.031
± 0.020
± 0.018
± 0.016
± 0.018
± 0.019
± 0.014
± 0.015
± 0.039
-+0 . 0 3 4
+ 0.074
± 0.065
+ 0.089
+-0 . 0 5 7
+_0 . 0 4 7
± 0.045
± 0.052
+-0 . 0 3 6
± 0.037
± 0.042
---
a Concentrations determined by X-ray fluorescence s p e c t r o m e t r y .
b C o n c e n t r a t i o n s d e t e r m i n e d b y n e u t r o n a c t i v a t i o n a n a l y s i s (J. H e r t o g e n , K . U . L . ) .
c All s a m p l e s w e r e a n a l y s e d in t h e O x f o r d I s o t o p e L a b o r a t o r y . L e a d w a s s e p a r a t e d b y a n e l e c t r o d e p o s i t i o n m e t h o d ( A r d e n a n d Gale, 1 9 7 4 )
a n d a n a l y s e d o n a M i c r o m a s s 5 4 E m a s s s p e c t r o m e t e r . A n a l y s e s w e r e c o r r e c t e d f o r m a s s f r a c t i o n a t i o n . D e c a y c o n s t a n t s u s e d in t h e calculat i o n s : k~3au = 1 . 5 5 1 2 5 × 1 0 - 1 ° y - 1 ; k23su = 9 . 8 4 8 5 × 1 0 - 1 ° y -~ ( J a f f e y et al., 1 9 7 1 ) .
153
208 Pb/20G Pb
7Z-111 .,..,.~,.J_
38!
4
PA ~ I B
B0-30
78 -3~,
~ 035t~-1/1
37.5
e.0212-113
~)PA 66/L
3'7
0622-1/1
73-48
,,'
36.5
I
15.'7
I
I
b
I
0390-111
78-3~
035~-1/1 ,( /
73-48 0212 1/3
~'
00-30.~' B - z O ~
15.6
15.5
I
20"/Pb 120~.Pb
~
15.~
1359 _.t120M0
MSWO 0.55
PA 70/B
mean
PA 66/L
15.3
17
I
17.5
I
18
I
1B.5
I
19
ZO" error
I
19.5
a
Z06 Pbl?O.~Pb
Fig. 1. (a) =or pb/204 Pb v. 2o~ pb/204 Pb diagram for the granulite-facies gneisses of the Rogaland province. (b) 20s pb/204 Pb v. 20~ pb/204 Pb diagram.
(1) high Pz value for the source-region of these rocks on the basis of a
single-stage evolution model;
(2) radiogenic and variable Pb isotopic compositions.
This "palaeo-isochron" model does not seem to be appropriate to the
Rogaland gneisses Pb data, because:
(i) these gneisses are not all orthogneisses, they can thus not be comagmatic;
(ii) the pz calculated value is 8.07 which is not especially high compared
with the mantle value at this time (~ 8.9, Zartman and Doe, 1981);
(iii) the range of Pb isotopic compositions is large, but not very large
(the highest 2°6Pb/2°4pb ratio is 19.83).
154
As far as the Rogaland province is concerned, the main conclusions are:
(a) the Sveconorwegian orogenic event produced high-grade (granulite
facies) metamorphism closely defined at ~ 1000 Ma from U-Pb zircon data.
An earlier metamorphic phase occurred at 1300--1200 Ma as indicated by
whole-rock Pb--Pb dating.
(b) there is no obvious pre-Sveconorwegian high-grade event comparable
to the ~ 1500 Ma age reported for the Arendal area.
(c) the very low- to low-grade retrogression of the granulite facies rocks
is due to Caledonian (ca. 400 Ma) orogenesis.
ACKNOWLEDGEMENTS
Prof. S. Moorbath from Oxford University critically read a first draft of
the present paper and made helpful suggestions. All the measurements of
isotopic compositions were done at the isotope laboratory of the Oxford
University and Dr. P. Taylor and R. Goodwin have given technical assistance.
Dr. J. Hertogen is thanked for measuring the Th and U concentrations by
N.A.A. Dr. J.C. Duchesne provided some samples and discussed various
aspects of this work. One of the authors (D.W.) was supported by the F.N.
R.S. (National Fund of Scientific Research) which also included the stay
she made at Oxford University.
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