Tectonophysics, Elsevier
178 (1990) 67-87
Science Publishers
67
B.V.. Amsterdam
- Printed
in The Netherlands
Age relations among Oslo Rift magmatic rocks: implications for tectonic and magmatic modelling B. SUNDVOLL,
E.-R.
NEUMANN,
Mineralogisk-Geologisk (Received
October
B.T. LARSEN
* and E. TUEN
**
Museum, Sarsgt. 1, N-0562 Oslo 5 (Norway)
2,1989;
revised version accepted
November
15,1989)
Abstract Sundvoll,
B., Neumann,
implications
E.-R.,
for tectonic
of Europe-Geophysical, This paper method
presents
and structural
B.T. and Tuen,
Geological a compilation
on magmatic
evolution
Larsen,
and magmatic
of 67 published
units in the subaerial
and syenitic
basaltic
and intermediate
composition and highest
The plateau
caldera
collapses.
and 266-243 273-241 NNE
volcanism
Ring-dykes
Ma in the Vestfold
has been estimated
estimate
magmas
the magmatism Oslo rifting
was succeeded and central
Ma in the Akershus to l-2
appears
started
to be initiated
The main faulting
intrusions
intrusive
GS, respectively.
adresses:
by movement
activity
295 Ma and lasted Graben
and graben volcanoes,
initial
to about
formation
the first 20-25
of
lavas of
275 Ma, with (GS) (one flow
occurred
during
this
most of which later underwent
were emplaced ratios
of plateau
Segment
give ages in the range 268-266
A rate of propagation
was the intrusion
Ma in the Vestfold
in the periods
278-268
of the rift from SSW towards and type of magmatic Ma of the magmatic
products, period.
fault zones which were reactivated
and the we
Some of
during
the
of formation
tensile stress in the elastic part of the crust and uniform, simple shear of the lithosphere (e.g. Neugebauer, 1983, 1987; Spohn and Schubert, 1983;
of continental
Statoil
A/S,
P.O.Box
300,
Turcotte and Emerman, 1983; Wernicke, 1985; Zuber and Parmentier, 1986). Each of these models has specific consequences with regard to the evolutionary history of continental rifts. Their validity may therefore be tested by comparing the tectonic and magmatic history predicted by each model with those observed in different continental rifts. Of particular importance is the relative timing of magmatic and tectonic events.
N-4001
Olje-direktoratet,
N-4000
Stavanger,
Nor-
way.
0040-1951/90/$03.50
to the tectonomagmatic
magmatic
central
along Precambrian
Norway.
adresses:
by the Rb-Sr
event.
The mechanisms
** Present
178: 67-87.
most major rock types
300 Ma. Extrusion
complexes
into the crust only during
rifts are still hotly debated. Among the most discussed models are convective heating of the lithosphere, buoyantly driven diapiric penetration,
Stavanger,
age determinations
The data include
recorded
activity
in these calderas
Introduction
* Present
Tectonophysics,
at about about
cm y- ‘. On the basis of 87Sr/ssSr
to have been emplaced
Crust
Graben.
Oslo Rift
of these data with respect
sediments
by a stage of bimodal
GS. Large composite
and Akershus
rocks:
rate from 295 to 285 Ma in the Vestfold
per 250,000 years and 0.30 km3 per 10’ years). period.
magmatic
Rift Zones in the Continental
radiometric
Norway.
years. The earliest
Paleozoic
(rhomb-porphyries)
extrusion
among
and crust, are discussed.
60 million Lower
(Editor),
Oslo-Horn
and unpublished
part of the rift. The implications
sills in the pre-rift
volumes
Evidence:
Oslo Rift in southeast
in the mantle
The Oslo Rift was active for about basaltic
E., 1990. Age relations
In: E.-R. Neumann
and Geochemical
rocks in the continental
of the rift, and processes
largest magma
modelling.
0 1990 - Elsevier
Science Publishers
B.V.
B. SUNDVOLL
68
In this connection,
a paleorift
has the ad-
on-land structure called the Oslo Graben
ET AL.
(Ram-
vantage over an active rift in that it has completed
berg, 1976) but geophysical data have shown that
its evolutionary cycle. A disadvantage is that many
the rift continues to the SSW into the Skagerrak
paleorifts are disturbed by later deformation, so that it may be difficult to unravel their
Sea and is probably terminated by the Tomquist line (Ramberg and Smithson, 1975; Pegrum, 1984;
tectonomagmatic
Ro et al., 1990).
histories in detail.
The Permo-Carboniferous ern
Norway
paleorift
is an
Inside the Oslo Graben, the Precambrian
Oslo Rift in south-
example
which has remained
of
a continental
undisturbed
after
base-
ment is overlain by two formations of sedimentary rocks. A Lower Cambrian-Silurian,
pre-rift
se-
the rifting event. Studies of this rift over many
quence, up to 2000 m thick, was folded during the
decades have, furthermore,
Caledonian orogeny (Henningsmoen,
resulted in an impres-
sive volume of geological, petrological, ical and geophysical
geochem-
data. Models for its forma-
tion and evolutionary history have been presented by Oftedahl
(1960),
Ramberg
(1976),
Ramberg
Cambro-Silurian
deposits
overlain by a SO-200
are
1978). These
unconformably
m thick sequence of sedi-
mentary rocks of Upper Carboniferous
age (the
Asker Group) which includes shallow-marine
de-
and Larsen (1978), Russell and Smythe (1983), Schonwandt and Petersen (1983), and Ramberg
posits (Olaussen, 1981). The Asker Group comprises agglomerates and tuffs in its upper part,
and Morgan (1984). Although these models give an outline of the rift history, they are very generalized. The time-space development is not given
and is conformably overlain by lavas belonging to the Oslo rifting event (Dons and Gyory, 1967). In
in sufficient detail to be used in geophysical modelling. For example, the models do not constrain
addition to the areas covered by Paleozoic sediments and igneous rocks (Oslo Region), the Oslo Graben system includes parts of the Precambrian
the start and end of important tectonic or magmatic stages or events with absolute age determinations; numerous important details are lacking.
fold block (Ramberg and Larsen, 1978). The latter may be an uplifted horst (Schonwandt and Peter-
A large number of recent radiometric age determinations, mainly by the Rb-Sr method (e.g.
sen, 1983). The alkaline Sarna complex (Sweden) of similar age as the Oslo Rift, is situated in the
Neumann et al., 1985, 1986; Tuen, 1985; Rasmussen et al., 1988; Sundvoll and Larsen, 1990;
continuation
Sundvoll et al., in prep.) have added considerably to our knowledge about the magmatic and tectonic development of the Oslo Rift. It is the aim of this paper to present a compilation of age determinations on extrusive and plutonic Rift, some of which are hitherto to discuss the implications of respect to the tectonomagmatic rift. Similar data on dykes and lished separately.
rocks in the Oslo unpublished, and these data with evolution of the sills will be pub-
Geological and petrological relations The Oslo Rift (Fig. 1) is located in the 1600-800 Ma old Sveconorvegian Province in the southwestern part of the Fennoscandian Shield (Berthelsen, 1980). The most conspicuous part of the rift is the
terrain west and east of the Oslo Region, i.e. the Kongsberg block, the Bamble block and the Ost-
of the main rift trend, some 100 km
to the north-northeast. It has been proposed that the Sarna intrusion is related to the Oslo Rift magmatism (Bylund and Patchett, 1977). A series of
prominent
Precambrian
lineaments
(shear
zones, thrust faults, fault breccias) in southeastern Norway terminates in the area now occupied by the Oslo Rift, e.g. the Kristiansand-Porsgrunn, Meheia-Adal, 0ymark (and Dalsland) and Mjosa-Vanern lineaments (Fig. 1) (e.g. Starmer, 1978, 1985a, b; Berthelsen, 1980; Falkum and Petersen, 1980). These lineaments have been reactivated more than once and may have influenced the location and development of the Oslo Rift. Gravity data (Ramberg, 1976; Wessel and Huseby, 1985) imply the existence of a “pillow” of high-density material in the crust under the Oslo Rift. According to Neumann et al. (1986) this “pillow” represents a combination of mafic
AGE
RELATIONS
AMONG
OSLO
RIFT
MAGMATIC
69
ROCKS
LEGEND
CALEDON~AN NAPPES
CAMBRO-SILURIAN SEDIMENTS
Fig. 1. Geological pers. commun.). BB = Bamble Zone;
map of Oslo Graben VGS = Vestfold
block;
Brummundal;
and adjacent Segment;
KFZ = Kristiansand-Porsgnmn
OF2 = Oslofjorden
MZ = Mjesa-VPnem
Graben
Zone;
Fault
Zone;
0FZ
Fault
= Oymark
ROFZ = Rena-Own
H = Hurdal;
areas (based on maps by Larsen, AGS = Akershus
N = Nordmarka;
Fault
Zone; Fault
Zone;
S = Skrim;
Graben
Segment;
1975; Ramberg 0B = Ostfold
and Larsen, block;
1978; K.Y. Buer,
KB = Kongsberg
MFZ = Meheia-Ada1
Fault
Zone;
RFZ = Randsfjorden
DFZ = Daisland
Fault
Zone;
LFZ = Loten
Zone;
FL = Vestfold .SJ = Siljan;
lava plateau; E = Eikeren;
KL = Krokskogen HU = Hurum;
Fault
lava plateau;
F = Finnemarka.
block; Fault Zone; B =
B. SUNDVOLL
cumulates
and residues
after
anatectic
melting
shape of their feldspar phenocrysts.
ET AL.
The RP lavas
during the Oslo rifting event. The Oslo Graben
are numbered according to stratigraphic
has been subdivided into two asymmetric graben
with RP, at the base. According to the TAS clas-
segments (GS)
sification
termed the Vestfold GS and the
of Le Bas et al. (1986),
position,
most RP lavas
Akershus GS (Fig. 1). The Vestfold GS is bordered
are latites. The lavas become increasingly
to the east, the Akershus GS to the west by major
and silicic towards the top of the lava sequences
faults (Fig. 1). Ramberg
(e.g. Oftedahl, 1978a; Andresen, 1985). Three areas
and Larsen (1978) pro-
posed that to the northeast Oslo Graben
is bordered
and southwest,
by flexures.
the
Swensson
of RP lavas are of major importance
(Fig. 1): the
Vestfold
flows),
plateau
(a
separate
Krokskogen
borders (shown in Fig. 1).
Brummundal area (4 flows) (Oftedahl,
activity appears to be a
complex of sills of microsyenites (earlier referred to as ‘maenaites’) and camptonites intruding the Lower Paleozoic sediments (Brargger, 1898; S&her, 1947). The main period of magmatism started with the eruption of basaltic lavas termed B, basalts (Ramberg and Larsen, 1978). In the south-
plateau
40
(1990), however, has found faults also along these The oldest ma~atic
evolved
(about
20 flows)
the
and the
1952, 1953,
1978a; Larsen, 1978). Small, do~faulted blocks of lava outside the main occurrences imply that the present outcrops represent the erosion remnants of a once much more extensive lava cover which probably extended onto the flanks of the rift (Bragger, 1931; Sterrmer, 1935). In the Vestfold and Krokskogen areas basaltic lava-flows also
western part of the Oslo Graben (Skien) B, consists of a 1500 m thick sequence of nephelinites,
occur locally within the RP-sequence.
basanites
the Oslo Rift and their assumed extensive lateral extent, the RP lavas have been interpreted as
and
ankaramites
(Segalstad,
Anthony et al., 1989). In the Horten-Moss along the central
Oslofjord,
1979; area
B, is made up by
mildly alkaline and subalkaline basalts and trachybasalts. Strong, local variations in thickness (150-1~ m) and interfinge~ng lavas belonging to different compositional series suggest that in this area the basaltic lavas originated from three different eruption centres, one of which was situated on the Oslofjorden Fault (Tollefsrud, 1987; Overli, 1985; Schou-Jensen and Neumann, 1988). North of Oslo (Nordmarka) the B,-se-
Because of
the presence of large RP dykes inside and outside
plateau lavas extruded from fissures (Oftedahl, 1967). However, so far it has not been possible to tie any lava to a specific “feeder dyke”. The regional lava cover is cut by a series of circular structures bordered by ring-dykes and/or ring-faults (Fig. 1) (e.g. Oftedahl, 1953; Segalstad, 1975). In agreement with their interpretation as caldera remnants, we use the term caldera for these circular structures (Oftedahl, 1953; Sorensen, 1975; Ramberg and Larsen, 1978). Some of
quence consists of a single, aphyric flow of tholei-
these calderas
itic composition,
rows. One of these marks the axis of the Vestfold
on average about 25 m thick
define
two roughly
north-south
(Weigand 1975). In the northern part of the Oslo
GS. The other is located along the extension of the
Graben B, basalts are absent.
Oslofjorden
On the basis of lava stratigraphy and petrographic classification, Oftedahl (1952) and
Zone (RZ), that is at the offset between the Vestfold and Akershus GS (Oftedahl, 1953; Larsen,
Holtedahl (1953) proposed that the B, volcanism started in the Skien area and progressed northwards with time. However, as no marker horizon
1978; Ramberg and Larsen, 1978). RP lavas and b&modal extrusives (basalts and trachyrhyolitic lavas and ignimbrites) are often found inside and close to calderas (Oftedahl, 1953, 1978b; Soren-
exists which may be used as a basis for stratigraphic correlation between the lavas in the different areas, this problem can only be solved by radiometric age determinations. The B,-basalt sequence is overlain by numerous lava flows of intermediate composition termed rhomb-porphyry
(RP) lavas because of the rhomb
Fault towards the Randsfjorden
Fault
sen, 1975; Weigand, 1975; Ramberg and Larsen, 1978). In order to differentiate between the regional lavas and lavas inside the calderas, we use the terms plateau lavas and caldera lavas. A group of subvolcanic rocks, often associated with explosion breccias, occurs in the northern
AGE
RELATIONS
and
central
breccias
part
are
cambrian 1931;
AMONG
OSLO
grouped
MAGMATIC
of the Oslo
also
terrain
Dons,
RIFT
found
Region.
in
the
Ramberg
these subvolcanic
1918; and
the presence
of similar calderas.
of the
breccias the
also occur
Lindum
Oftedahl,
(1978)
distribution
subvolcanic
isolated in
Bragger,
Larsen
However,
breccia
Pre-
rocks with the caldera
stage on the basis of their spatial some
Explosion
adjacent
(Werenskiold,
1952).
71
ROCKS
within
the
rock some
within (e.g.
granite,
Two sets of observations imply that significant topographic contrasts arose during the period of RP volcanism as the result of increasing vertical displacement along major and minor faults: (1) The thicknesses of the lowermost RP lavas (RP,-RP,) are uniform over large areas, whereas the thicknesses in the upper part of the RP stratig-
particularly area.
adjacent fault blocks (e.g. Fjerdingstad, 1983). This is
well documented
for the Krokskogen
(2) Thick sequences of fanglomerates and continental sandstones dominated by basalt and RP lava fragments have been deposited locally within the graben; along
the
the main Oslofjorden
occurrences Fault
by later intrusions
field relations
among
(Tuen,
the syenitic
1985). The
to alkali-granitic
rocks are very complex.
and
1957).
raphy vary between Larsen, 1978, 1979;
morphosed
Analytical procedure
of these
plutons
Drammen
Tuen, 1985; Rasmussen et al., 1988). The larvikites in the Hurdal area are strongly meta-
form
islands
(Bragger,
1900;
Rosendahl, 1929; Strarmer, 1935), and deposits in a “paleo-canyon” in the upper part of the RP sequence at Krokskogen (Larsen, 1978, 1979). A series of composite intrusive complexes dominate the central part of the Oslo Graben. Both in volume and area1 extent the plutonic rocks are more important than the extrusive ones (Fig. 1). The intrusive complexes range in composition from larvikites (the intrusive equivalents to the RP lavas) to syenitic-granitic complexes (e.g. Saether,
All analytical laboratory
work has been carried
out at the
for geochronology
and isotope
geology
at the Mineralogical-Geological
Museum,
Univer-
sity of Oslo, during
the period
1975 to 1987. The
analytical procedures used, including performance tests on instruments, have been published elsewhere (Jacobsen and Heier, 1978). The analytical results and validity discussion of the samples specially prepared for this paper are published separately (Sundvoll and Larsen, 1990). A summary of the general results in that paper is presented
below.
All isochron calculations and age data quoted have been performed or recalculated using the decay constants (1977).
recommended
by Steiger and Jager
Available age determinations on Oslo Rift rocks are compiled in Table 1. For the locations of the individual nal papers
units,
the reader is directed
presenting
to the origi-
the data. The data of Tuen
(1985) on intrusions from the Hurdal area have been regrouped and recalculated in accordance with new field evidence, and some analysis of poor quality have been omitted. The average age resolution (2~ error) is about 5 million years, thus the various episodes discussed below cannot be defined with a smaller time limit than 5 million years (Sundvoll and Larsen, 1990).
1962; Czamanske, 1965; Neumann, 1978, 1980; Ihlen et al., 1982; Andersen, 1984). There appears
The results are listed in Table 1, and the geographic distribution of the age determinations is shown in Figs. 2-4.
to be a regional tendency for the larvikites to be cut by, and thus be older than the syenites and the
Results
alkali granites. Biotite granites were emplaced during both the early and the late part of the pluton emplacement period. Some syenites have chemical affinities with the larvikites, whereas others seem to form separate geochemical trends and are believed to have a hybrid origin (ðer, 1962; Nystuen, 1975b; Gaut, 1981; Andersen, 1984;
Plateau lams Among the plateau lavas in Vestfold, only RP lavas and trachytes have been dated so far. Ignoring the unsatisfactory result from RP, + *, they give an age range of 294 rt: 6 (RPz and RP,) to 283 f 8
B. SUNDVOLL
12
TABLE
ET AL.
1
Rb-Sr
age determinations
and “Sr/‘%r
initial ratios.
For each graben
within each of which the rocks are listed from south to north and/or
segment
the rocks are divided
west to east. The plateau
into 4 tectonomagmatic
lavas are listed from bottom
groups to top of
stratigraphy.
Age Ofa)
Rock unit a
-
MSDW
(“Sr/‘%r),
b
Method
’
Nd
Reference
Vestfold Graben segment Plateau lavas RP,+z
Vestfold
291 + 18
0.70392 f
14
0.69
6
6
Rp2+3
Vestfold
294+
6
0.70392 f
4
0.20
min.
6
6
T,
Vestfold
288 *
4
0.70650 f
8
0.99
min.
6
6
R”,,
Vestfold
288 + 7
0.70409 f
8
2.07
min.
3
6
T2
Vestfold
285 *
7
0.71010 f
10
0.01
mm.
3
6
RP,,
Vestfold
284 f 10
0.70418 f
12
1.07
3
6
RP26
Vestfold
283 f
8
0.70401 +
8
0.36
mm.
3
6
268 +
5
Caldera laws and intrusions 0.70470 + 50
1.58
w.r.
4
1
269 zb 6
0.70555 +
52
1.10
w.r.
6
6
w.r.
15
1
Sande quartz porphyry
268 f
3
0.70710 * 40
2.23
w.r.
6
1
R-lava
214 *
3
0.70390 *
40
0.76
w.r.
5
6
Ramnes
granite
Hillestad
syenite
Sande larvikite,
no isochrons
nordmarkite
Drammen
Glitrevann
gramte
267 f
4
0.70519 k
52
1.78
w.r.
6
6
Glitrevann
syenite
266 *
5
0.70614 5
36
1.72
w.r.
6
6
271+
3
0.70391 *
5
1.31
w.r.
20
1
w.r.
6
6
w.r.
4
6
5
6
Plutonic intrusions Larvik larvikite Hedrum
plagi-foyaite
no isochron
Kvelde foyaite
216 + 6
0.10464 k
18
1.90
Bolreme
larvikite
281+
4
0.70401 +
5
0.38
Siljan nordmarkite
210+
4
0.70420 +
30
0.35
w.r.
Nordagutu
278 + 7
0.7044
+
2
1.46
w.r.
7/8 6
4
Skrim larvikite
278 + 5
0.70392 +
8
0.71
w.r.
14/17
6
Eikeren
271 k
2
0.70530 + 60
2.25
w.r.
17
6
267 *
4
0.70539 & 90
2.09
4
6
268+
3
0.70466 f
12
1.46
w.r.
9/10
6
Snaukollen
265 + 11
0.70390 f
10
1.2
w.r.
4
Dignes
266 f
0.70380 f
5
0.63
mm.
granite alkali granite
Drammen
granite
Finnemarka
granite
6
Gabbroic necks 6
Other gabbros
no isochrons
4 21
Akershus Graben segment Plateau laoas B, Krokskogen
291 f
8
0.70528 +
6
0.91
w.r.
14
6
RP, Krokskogen
292 + 20
0.70398 +
14
0.54
WI.
12
6
RPz~+~
Krokskogen
RP, Krokskogen
290 *
4
0.70388 f
4
0.26
w.r.
7
6
288 +
9
0.70424 +
10
0.44
min.
4
6
RP, Krokskogen
284+
7
0.70402 f
6
0.32
mm.
6
6
RP, Krokskogen
281 f
6
0.70417 f
10
0.76
mm.
3
6 6
RP, , Krokskogen
276 + 6
0.70446 k
8
0.99
min.
6
RP,2 Krokskogen
278 +
8
0.70462 k
4
0.22
min.
7
6
278 + 12
0.70446 f
12
0.37
mm.
3
6
RP13c
StorflHten
Storflaten RPr,,+a RP, Brummundal
278 f
5
0.70408 f
6
0.36
min.
6
6
219 +
9
0.70660 f
20
0.96
min.
8
6
e
AGE
RELATIONS
TABLE
AMONG
OSLO
RIFT
MAGMATIC
73
ROCKS
1 (continued)
Rock unit a
(%/%r)
Age (Ma)
,
MSDW
Method
b
’
N d
Reference
’
Caldera laws and intmions Baerum syenite porphyry
243 +
3
0.70681 k
36
1.25
w.r.
7/11
6
Kampen
2741t
6
0.70672 f
16
1.61
min.
3
6 6
syenite porphyry
212 + 6
0.70571 +
6
0.19
min.
7
270 + 4
0.70644 +
6
1.24
min.
5
6
B-lava, Oyangen
280 *
7
0.70440 f
5
1.20
3
6
Stubdaf
268 f
Oppkuven
syenite porphyry
Heggelia
syenite porphyry syenite, Oyangen
Ringkollen Stryken
syenite, 0yangen
syenite porphyry
5
0.70396 f
9
0.08
w.r.
0.70404 f
56
0.20
w.r.
5/6 3
6
268 + 5 271 +
0.70421 +
6
0.38
w.r.
8
6
3
6
Plutonic intrusions Tryvann
granite
241 5
3
0.70570 +
30
1.44
w.r.
6
6
Grefsen
syenite, Nittedal
255 f
4
0.70472 f
18
1.09
w.r.
8/10
6
Nit&al
nordmarkite
252 +
3
0.70540 f
30
1.37
w.r.
6
1
263+
3
0.70560 + 10
2.17
w.r.
10
1
0.70400 +
0.87
mm.
6
6
Holterkollen Kjeldsas
granite
273 + 4
larvikite
10
Slottet larvikite
no isochron
w.r.
10
6
Kampehaug
no isochron
w.r.
3
6
7/8 6
6
Oyangen
syenite (Nordmarka)
Stor0yungen Gjaerdingen Harestua
syenite
granite monzonite/syenite
nordmarkite
Grua granite Oyangen
(Hurdal)
Brennhaugen
syenite/granite
alkalinegranite
Buraskollen
8
0.70399 +
14
0.61
w.r.
4
0.70560 f
60
0.92
w.r.
256 + 5
0.70402 f
14
0.64
w.r.
3
0.70448 f
12
2.07
w.r.
5/7 7
6
252 + 262 +
3
0.70570 + 50
1.85
w.r.
6
6
Mistberget
alkalinegranite
Gvemtjem
granite
Bergevann
aplo granite
0.70542 +
9
1.70
w.r.
8
1
0.70660 *
40
0.35
w.r.
4/6 5
6
6/7 6
6
w.r.
248 f
4
0.70535 + 14
0.76
w.r.
256 f
6
0.70544 f 116
1.44
w.r.
(0.71293 f 215)
3.37
w.r.
54
0.70
w.r.
9
1.52
w.r.
(249 f 10) 249 + 6
monzonite
Other Hurdal
0.70491* 0.70510 +
264 + 13
monzonite
no isochron
Nordliskampen
alkali granite
Nordliskampen
syenite/granite
6
245 of: 4
no isochron
syenite
6
251 + 2
granite
Fjellsjskampen
Hoversjo
266 f 263 f
(248 * 6) (248 & 5)
3/6 6
2 2 2 2
4/6 15
2
w.r.
6
2
(0.70456 5
58)
2.71
w.r.
(0.70477 ?
24)
3.25
w.r.
10/12
2 2
Rotjem
granite
253 f 10
0.70565 + 35
0.57
w.r.
6
6
Kroken
syenite.
248 *
4
0.70940 f 244
2.33
w.r.
6
2
Skurven
alkalinegranite
2
267 +
3
0.70603 f
90
0.92
w.r.
6
Hersjo granite
263k
5
0.70570 + 30
0.47
w.r.
5/7
6
Toten syenite
2545
8
0.70490 *
20
1.27
w.r.
5/7
2
282 f 14
0.70549 +
10
0.85
8
5
Areas outside the Oslo Graben S&na cancrinite a B = basalt,
syenite
RP = rhomb-porphyry
lava, T = trachyte,
R = rhyolite.
The locations
of the different
rock complexes
are shown in Figs.
l-4. b MSDW=
[“X*/(N-2)],
’ N = number d
of samples;
where 5/6
means
X = (87Sr/86Sr)measured 5 of 6 analyzed
samples
- (“Sr/*%r)isochron. are used to define the isochron.
w.r. = whole rock, min. = minerals.
e 1 = Rasmussen et al. (1987); Patchett
(1977); 6 = Sundvoll
2 = Tuen (1985); and Larsen
(1990).
3 = Neumann et al. (1985);
4 = Jacobsen and Raade (1975);
5 = Bylund
and
B. SUNDVOLL
74
ET AL.
LEGEND PRECAMBRIAN GNEISSES CAMBAO~SILURIAN SEDIMENTS FEN
CARBONATITE
LAVAS CLASTIC
& PYRO ROCKS
MONZONITES +++++ y-:: III
EARLY SYENITE
GABBRO
111111M11 FAULTS __ ZONES
GRANITES 8
NECKS
8 FAULT-
Fig. 2. Geological map showing age relations in the Vestfold GS. Ages from Table 1; geological data from Larsen (1975) and Buer (1990).
R = Ramnes;
SJ = Siljan; SK = Skrim; VL = Vestfold lava plateau; H = Hillestad caldera; E = Eiieren; D = Drammen; G = Glitrevann; F = Finnemarka; KL = Krokskogen lava plateau; N = Nordmarka.
Ma (RP,,), that is an extrusion period of 10 million years. The RP lavas rest on the B,-basalt sequence which, in Vestfold, comprises up to 39
S = Sande;
flows (IZlverli, 1985; Tollefsrud, 1987). This implies that the B, volcanism here started well before 294 Ma. A biotite K-Ar age, recalculated to 277 f 5
AGE
RELATIONS
AMONG
OSLO
RIFT
MAGMATIC
ROCKS
LEGEND
PRECAMBRIAN
GNEISSES
L. PALEOZOIC
SEDIMENTS
PLATEAU
LAVAS
MONZONITE SYENITE ALKALI SYENITE GRANITE LATE
GRANITE
CALDERA
CALOERA
5
8
EXTRUSIVES
INTRUSIONS
10
1
15km
4
Fig. 3. Geological map showing age relations in the southwest part of the Akershus GS. Ages (in Ma) from Table 1; geological data from Srether (1962) and Larsen (1978). Gabbroic intrusions are shown in black. K = Kampen caldera; 0 = Oppkuven caldera; H = Heggefi caldera;
S = Svarten caldera;
0 = Oyangen caldera;
B = Baxum caldera;
complex. Other abbreviations as in Fig. 1.
N = Nittedal caldera;
SY = Stryken ring
B. SUNDVOLL
76
Ma, on a basaltic
lava northwest of Horten has
ET AL.
Plutons
been reported by Neumann (1960). This age most probably reflect secondary contact metamorphism from intrusive
activity
related
to the Hillestad
The large composite plutons also show different age ranges for the Vestfold and the Akershus GS. In the Vestfold GS the plutons were emplaced
caldera nearby. GS) lava
during a relatively short period. The oldest rocks
to 276 + 6 Ma
a total time span of at least 15 million
are found in the composite larvikite-foyaite intrusive complex in the Larvik-Tonsberg area in the
years. Field relations suggest that RP lavas (termed RP13-17) encountered within the calderas are
This complex consists of 10 intrusive units (Peter-
In the Krokskogen
area (Akershus
ages range from 291 &-8 Ma (B,) (RP,,),
southernmost
part of the Vestfold
GS (Fig.
2).
younger than the ones of the adjacent lava plateau
sen, 1978a, b; Neumann,
(ðer,
numbered from I (oldest) to X (youngest). Units I (Bolzeme larvikite) and III-VI (larvikite) give 281
1962; Larsen, 1978). However, RP lavas
from the Storflaten area (0yangen caldera) yield ages of about 278 Ma, which does not indicate any significant age difference from those in the upper part of the Krokskogen area (RP,, and RP,,). In the Brummundal area (Fig. 1) the uppermost lava flow, RP,, gives an age of 279 f 9 Ma. The occurrence of relatively thick beds of sandstone between some of the flows (Rosendahl, 1929) suggests that the period of eruptions may have lasted a few million years.
1980) which have been
+ 4 Ma and 277 f 3 Ma, respectively. The Kvelde foyaite which cuts unit X, is dated to 276 + 6 Ma. Although the age differences are not significant, arr intrusion period of a few million years is suggested. The larvikite in the Skrim area and a biotite granite west of that area (Nordagutu) are of similar age (278 f 5 and 278 + 7 Ma, respectively). The youngest ages are obtained
for the granitic
intrusions
Finnemarka)
(Eikeren,
Drammen,
in
Central volcanoes and calderas
the central and northern part of the Vestfold GS (271 k 2 to 267 f 4 Ma). The total intrusive period
Lavas and silicic intrusions (ring-dykes and central domes) associated with 12 calderas (5 from
is thus about 15 million years, with a northwards shift in the intrusive activity with time. Only two ages are reported from the northern
the Vestfold GS and 7 from the Akershus GS) have been dated. Intrusions associated with the Vestfold caldera chain show uniform ages (269 f 6 to 266 + 5 Ma) (Fig. 2). A rhyolitic lava in the Drammen caldera is somewhat older (274 k 3 Ma). The Kampen, Oppkuven, Heggelia, Svarten, 0yangen (Fig. 3) chain of calderas at the intersection between the Vestfold
and the Akershus GS
give ages of 280 k 7 Ma (basalt), and 274-268 Ma (ring-dykes and central intrusions). The Bazrum caldera
at the southern
end of the Kampen-
0yangen chain (Fig. 3) is considerably younger. Its youngest ring-dyke is dated at 243 k 3 Ma. The only established caldera within the Akershus GS is the Nittedal caldera (Fig. 3). Its age is limited by those of the Holterkollen granite (263 + 3 Ma) and the Grefsen syenite (255 f 4 Ma). In both graben segments the age determinations are consistent with a change in volcanic style with time from plateau volcanism to central volcanoes which developed into calderas.
part of the Vestfold GS: the Drammen granite (267 + 4 Ma, mineral age) and the Finnemarka granite (268 k 3 Ma). However, preliminary whole-rock data on the Drammen
granite (Sund-
~011, 1978) suggest an older age of emplacement, the mineral age probably represents age (Sundvoll and Larsen, 1990). Also in the Nordmarka
a secondary
area of the Akershus
GS (Fig. 3) the oldest ages are found in the larvikitic and related rocks (e.g. Kjelsis larvikite: 273 + 4 Ma; the 0yangen
syenite:
266 + 8 Ma).
The larvikite phase was followed by a phase of emplacement of syenites and biotite granites (e.g. the Gjrerdingen syenite: 256 f 5 Ma; the Grefsen syenite in the Nittedal caldera: 255 + 4 Ma; the Holterkollen biotite granite: 263 + 3 Ma; the Grua biotite granite: 262 + 3 Ma; Table 1). The intrusion period was terminated by emplacement of alkali syenites (nordmarkite) and alkali granites (e.g. the Harestua nordmarkite: 252 + 3 Ma, the
AGE
RELATIONS
AMONG
OSLO
RIFT
MAGMATIC
ROCKS
m
m :.:....~~~_li.::::;~,~ y;<:,;, I’::. 2.:;
x
. . . . . I:;:::.:.:.:1
x
<<<<-c-c <<<<< *.*.*.*.-.* :.:.:.-.*.. . . .*.*.*.*.-.
..:;.: ..,.:. ~ ,.:,. ,. .. ..... ..~ ~ . . .... . :... ;:...)
:.i’:;:.:.: ,:....:.:::..:::; ::: ..
+4-b+*+
x
x
7 xxxxxxxxx
6
10
8
9
ii+++++++++ ii>>>>>>>>>> ++++++++++++*>>>>>>>>>>>
iiiiii>iiF >>>>>>>>> >>>>>>>>.
++++++4+++++++.>>>>>>>>>>
++++++++++++++ >>>>>>>>’ >>>>>>>~.~~++++++++++++++
f.
%
>>>>>>>>> >>>>>>>>
-
/
m --
l>>>,>>>>>>>>:
Fig. 4. Geological map showing age relations in the Hurdal area in the central part of the Akershus GS. Age determinations Table 1; geological data from Nystuen (1975b), Skurven; 4 = lavas;
FK = Fjellsjekampen. 5 = monzonites;
Legend:
Schenwandt
1 = Precambrian
6 = early biotite
granites;
and Petersen (1983), and Tuen (1985).
gneisses;
7 = granite;
10 = alkali granites; I1 = alkali granite porphyries;
2 = Cambro-Silurian 8 = early syenite
sediments;
NK = Norliskampen;
from SK =
3 = gabbros and mafic sills;
and alkali granite;
I2 = late composite alkali syenite-granite
9 = pyroclasic
complexes.
rocks;
B. SUNDVOLL
78
Nittedal
nordmarkite:
252 rt 3
Ma,
and
ET AL.
the
Tryvann granite near Oslo: 241 rfr3 Ma; Table 1). The radiometric age determinations
are consistent
with field indications of relative ages (Fig. 3). In the northernmost part of the Akershus GS, the Hurdal area (Fig. 4 ), the field relations are even more complex than in the Nordmarka (Nystuen,
1975a,
b;
Schonwandt
area
and Petersen,
1983). In general the age data are consistent with the field observations
(Fig. 4). The oldest rocks
are altered larvikites and biotite granites, together with syenites
north
and east of Norliskampen
(Hoversjo monzonite: 264 + 13 Ma, Toten syenite: 254 _t 8 Ma, Skurven alkaline granite: 267 + 3 Ma,
a
4 t
3 2
_
b
? ,
?-_.____-_?
Hersjo biotite granite: 263 ~fr:5 Ma; Table 1). The youngest rocks (the Fjellsjokampen alkali-syenite: 248 + 4 Ma, Brennhaugen per-alkaline granite: 245 f 4 Ma; Table 1) occur in the southern and western part of the area. The semicircular Norliskampen alkali granite (248 rf: 6 Ma) encircling the Norliskampen central complex and the associated Norliskampen syenite-granite dykes (248 + 5 Ma) suggest a caldera collapse at about 250 Ma. However, the time resolution is not sufficient to test the model of Schonwandt and Petersen (1983) of repeated collapses of a caldera centered at N.orliskampen. In the Akershus GS the total emplacement period is about 32 million years. Unlike the Vest-
240
Ma
Fig. 5. Number of age determinations plotted against age for the Vestfold (A) and the Akershus GS (B). Data from Table 1. I = period of basaltic plateau lavas; 2 = period of RP eruptions; 3 = period of central volcanoes and caldera formation; 4=intrusions
of large plutons (a: iarvikites, h: syenites and granites).
fold GS, the Akershus GS does not show any systematic, geographical shift in ages on a regional scale. Discussion
Temporal
evolution of the Oslo Rift
Age determinations on magmatic rocks in the Oslo Rift show a clear pattern of changing emplacement style and composition with time. There are, furthermore, significant differences between the Vestfold and the Akershus GS, this is demonstrated in Fig. 5. The earliest magmatism associated with the Oslo Rift is represented by the syenitic and mafic sills which intruded the Lower Paleozoic sediments. Their ages, 304-294 Ma (Sundvoll et al., in prep.), date the beginning of the magmatism to the period of deposition of the
middle to upper part of the Asker Group, that is Middle Westph~an-Stepha~an (Olaussen, 1981). Age dete~nations on B,-basaltic lavas are too few and unreliable to resolve the stratigraphic relationship between the different occurrences. The available data imply, however, that B, volcanism started 291 + 8 Ma ago at Krokskogen, and somewhat earlier in Vestfold. The RP volcanism started more or less simultaneously, 294-291 Ma ago in the Vestfold and Krokskogen areas, in the Brummundal area somewhat later (285-280 Ma). The obtained lava ages may be used to date important periods of tectonic activity. As stated above, the B,-sequence seems to be connected with fissures and faults, some of which later developed into master faults and/or graben segment offsets. The Skien basalts thus rest on the presumed offset
AGE
RELATIONS
AMONG
OSLO
RIFT
MAGMATIC
19
ROCKS
between the Vestfold GS and the adjacent graben
ficult to date because they rarely cut older lavas or
segment in the Skagerrak
intrusions.
quence of Moss-Horten the Oslofjorden
Sea, the thickest
se-
basalts are found along
fault zone north of Moss (Fig. l),
An exception
is the fault at Brum-
mundal which cut, and must be younger than, RP lavas dated to 279 + 9 Ma.
extruded near
The change from plateau volcanism to calderas
the offset between the Vestfold and the Akershus
has been interpreted as a change in eruptive style
and the Kols&s basalt (~okskogen)
GS. This suggests that the major graben-forming
from fissure-fed
faults were active, or existed as zones of weakness,
large central volcanoes and to mark the end of the
from the start of the B, volcanism
period of maximum extensional
some time
1953; Ramberg
before 294 Ma. Surface
erosion
and shallow intrusions
flood lavas to eruptions
from
stress (Oftedahl,
and Larsen, 1978;
Ramberg
and
have
Morgan, 1984). The oldest lava associated with the
obliterated much of the RP lava cover. However, estimated variations in thicknesses of the lava remnants (Larsen, 1978; Ramberg and Larsen,
central volcano stage in the Vestfold GS is about 283 Ma (Sundvoll and Larsen, 1990), the caldera
1978) suggest that also these extrusions were fed by fissures close to, and parallel to, the master faults. Vertical displacement along the Oslofjorden Fault is verified by fanglomerates
resting on
RP lavas which probably correspond to RP, (Stormer, 1935). This indicates that the Vestfold
collapses (as indicated by ring-dykes and central intrusions) are dated to 269-266 Ma. This means that this stage lasted about 15 million years in the Vestfold GS. In the Akershus GS the calderas formed at intervals from 280 to 243 Ma, that is over a total period of about 35 million years. The lifetime of individual central volcanoes appears to
GS was being formed about 292-290 Ma. Our data furthermore date the increased faulting activity after RP, time in the Krokskogen area (implied by differences in lava stratigraphy in adjacent fault blocks and large deposits of fanglomerates) to about 285 Ma (Table 1). The change in tectonic style at this stage strongly suggests an increase in tensional stress which resulted in block
have been 5-10 million years (Sundvoll and Larsen, 1990). The change from plateau volcanism to central volcanoes/calderas appears also to have been accompanied by a migration of the magmatic activity towards the central parts of the Oslo Graben. A similar inward ovation of the magmatic activity with time has been observed in the
faulting within the graben segments as well as movement along the master fault zones. A similar
The last magmatic stage in both graben segments is the emplacement of large felsic plutons. The period of emplacement started earlier and lasted for a shorter period in the Vestfold than in
conclusion was reached for the Moss area by Schou-Jensen and Neumann (1988). Before RP,time vertical movement along faults appear to have been compensated by the extrusion of RP (regional RP lavas). After RP,-time extension and faulting exceeded the extrusion rate and topographic contrasts arose (local RP lavas). Towards the end of the RP period, the lavas again assume a more regional character_ This probably marks the end of the period of maximum faulting. The RP lavas thus represent a useful marker for the peak of rifting event, i.e. the maximum tensional regime as depicted by Ramberg and Morgan (1984). An alternative interpretation of the change in tectonic style is a “jump” in the graben developement from the Vestfold to the Akershus GS (Larsen, 1978). The master faults of the Akershus GS are dif-
Kenya Rift (Williams, 1982).
the Akershus GS (281-267 and 273-241 Ma, respectively). Both graben segments show a general shift in composition with time from larvikites through syenites
and granites
to alkali
syenites
and alkali granites. In the Akershus GS, the field relationships between the subvolcanic rocks and dated plutons imply that the former were emplaced
at several
stages throughout the period of pluton emplacement. The subvolcanic rocks at Stryken (Fig. 3) have been dated to 271 f 3 Ma. In the Hurdal area, subvolcanic rocks were emplaced during the period 263 + 5 Ma (Hersje granite) to 248 f 4 Ma (Fjellsjokampen syenite). Ramberg and Larsen (1978) may, however, be right in their assumption that some of the subvolcanic rocks and structures
80
B. SUNDVOLL
ET AL.
are related to central volcanoes later destroyed by intrusions. K-Ar
dates on some late diabase dykes from
the Oslo area (Akershus GS) have given ages of
300
around 220 Ma (Dons, 1972). However, these ages are probably
too low because
determinations
of Ar loss. Age
on very young granitic
m
dykes in
the same area (B. Sundvoll, unpublished data) fall within the age range of the rocks presented in this paper. The termination of the magmatic activity in
200
the Oslo Rift thus seems to be not much later than 240 Ma.
RP extrusion
,___-----~-----~ ,
rates
The intensity of the volcanic activity differed along the Oslo Graben. In Fig. 6 the ages of RP plateau lavas are plotted against stratigraphic position. This diagram shows that eruption
i
KROKSKOGEN LRVA-PLATEAU ,x20 flows)
100
frequency decreased northyards along the rift axis. In the Vestfold area the minimum average erup-
BRUMMUNDAL
tion rate is one flow per 250,000 years, and in the Krokskogen
area the rate is approximately
one
flow per 800,000 years. In the Brummundal area we have no direct control on the rate, but the two lowermost flows are here separated from the upper two by a relatively thick sandstone (lo-50 m) indicating a long pause in the volcanic activity. The eruption rate from the Vestfold area is of the same magnitude as that estimated for the Kenya Rift Miocene plateau phonolites in the Uasin Gishu
area (Lippard, 1973a; Goles, 1976). The age relations also allow us to make
obtained
estimates
with respect to lava production
rates.
300
290
280
270
Ma
260
Fig. 6. Age (shown with 20 error bars) plotted versus stratigraphic position (m) for RP lava flows from Vestfold ( x
),
Krokskogen (O), and Brummundal (W). Dashed lines represent interpolated average evolution trends. Stratigraphic data from Ramberg and Larsen (1978).
If we accept an original RP volume of 4000 km3 and assume a distribution between the Vestfold and the Akershus GS of 3:1, the production rate was about 0.30 km3 per lo3 years in the Vestfold
Ramberg (1976) and Ramberg and Larsen (1978) estimated the original lava volume to be 10,000
area, and about 0.07 km3 per lo3 years in the Krokskogen area. We associate the spatial dif-
km3, of which about 40% was made up by RP lavas. However, recent data on uplift rates in southern Scandinavia from fission track ages on
ference in RP lava production rate with decreasing
apatites (Zeck et al., 1988), suggest that the erosion in the Oslo Rift since Permian time may have been at least a factor of two higher than that used to estimate the original lava volume. Although the major part of any “missing” cover probably consisted of sediments and silicic lavas, an original volume of RP lavas of 4000 km3 should be regarded as a conservative estimate.
tensional stress northwards along the Oslo Graben. The average rate for the whole Oslo Graben is 0.35 km3 per lo3 years. Data on Miocene flood phonolites from Kenya (Lippard, 1973a, b) imply a similar rate for the Uasin Gishu area as for the Oslo Graben (0.25 km3 per lo3 years). However, data by Williams (1982) and Baker et al. (1971) imply that the total average production rate in Kenya is much higher than in the Oslo Rift, possibly by an order of magnitude.
AGE
RELATIONS
AMONG
OSLO
RIFT
MAGMATIC
81
ROCKS
0.7040, that is close to the mantle signature. After
Mantle interaction with the crust
280 Ma, the rocks show a clear trend of increasing initial ratios, mantle signatures are only present in
Table 1 shows a wide range in initial 87Sr/86Sr ratios
among
the
Oslo
Rift
magmatic
the highly fractionated
rocks
larvikites.
The Akershus
GS shows a wide range in initial ratios for the
(0.70392 f 4 to 0.7101 + 1). On the basis of Smand trace element data it has been
whole period of magmatic activity. However, the
found that most of the basaltic and the monzonitic
majority ot the rock units older than 275 Ma show
rocks (RP
initial Sr isotopic ratios below 0.7045,
Nd, Rb-Sr
lavas and larvikites)
originated
in a
this group
mildly depleted mantle source (initial 87Sr/86Sr =
includes most basaltic rocks in the Akershus GS.
0.7039).
After about 265 Ma, both the average initial ratio
The
basaltic
magmas
formed
magma
chambers in the lower crust. This, in turn, led to
and the lower limit of the range increase
anatectic
decreasing age. We interpret Fig. 7 to reflect the
melting
in the
Precambrian
country
rocks. Initial Sr isotopic ratios significantly
relative
above
importance
of
mantle-derived
with versus
anatectic melts. During the main magmatic period
0.7039, which are typical of the syenitic and granitic rocks, imply formation by anatexis in the lower crust, or a significant crustal component. In
(300-280 and 300-275 Ma in the Vestfold and the Akershus GS, respectively) heat and material were
situ contamination of some rock complexes in the upper crust has led to a scatter in the 87Rb/86Sr87Sr/86Sr ratios (“no isochrons” in Table 1) (Neu-
transported from the mantle into the crust. This period, which lasted about 20 million years in the Vestfold GS and 25 million years in the Akershus GS, also represent the period of maximum tensional stress. About 280 and 275 Ma ago the magmatism became dominated by melts contain-
mann et al., 1986, 1988, 1990; Rasmussen et al., 1988). When initial Sr isotopic ratios are plotted against age (Fig. 7) some interesting relations
ing a large crustal component (syenitic and granitic melts). The mantle source had apparently become
emerge. In the Vestfold GS most magmatic rocks older than 280 Ma show initial ratios close to
inactive,
but mantle-derived
magmas
were still
.70
x x
a?sr h!
.70
.7c
.70
300
Fig. 7. Initial *‘Sr/*%r
290
280
270
260
250
Ma
240
ratios plotted against age for the Vestfold GS (dots), the Akershus GS (crosses) and the S&ma alkaline
complex, Sweden (circle). Gabbros are indicated by triangles. The dashed lines mark the lower limits of the ranges of initial ratios obtained for magmatic rocks from the Vestfold GS (V) and the Akershus GS (A). Arrows marked 1 and 2 indicate the suggested transition from the plateau lava to the central volcano stage in the Vestfold and the Akershus GS, respectively. Data from Table 1.
82
B. SUNDVOLL
undergoing
fractional
crystallization
in magma
calderas show a northeastward Vestfold
rocks such as larvikites. This period which lasted
Oyangen chain and the Nittedal
about 15 and 35 million years in the Vestfold and
Hurdal complex.
Akershus
large
represents
the after-
plutons
chain,
younging from the
chambers in the lower crust giving rise to evolved
GS, respectively,
axial
ET AL.
through
the
Kampen-
caldera,
to the
The onset of emplacement
shows
a northward
of
progression
maths of the rifting activity. The transition from
through the Vestfold GS into the Akershus GS.
the “mantle melting” to the “aftermath”
style:
Finally, the isotopic data imply that mantle-derived melts ceased to be available under the Vest-
migrated towards the central
fold GS while such melts still invaded the lower
period is
also reflected in a change in magmatectonic (a) the magmatism
part of the graben, and (b) fissure eruptions and normal faults gave way to central volcanoes, ringfaults (connected
with caldera collapses)
plosive volcanism. Mantle-derived have
been
present
somewhat
and ex-
melts appear to later
under
the
crust under the Akershus GS. We have made a rough estimate of the rate of northward propagation
of the magmatic activity,
on the basis of the initiation of the main magmatic period, that is the B,-basalt
volcanism.
A mini-
Akershus than under the Vestfold GS. This is in agreement with the proposed northwards propa-
mum estimate for the difference in the starting dates of B, volcanism between Vestfold and
gation of the rift (arrows marked I and 2 in Fig. 7).
Krokskogen is 5 million years, over a distance of about 60 km. This gives a minimum rate of 1.2 cm
Graben propagation
y-i. If the Sama magmatism is related to the Oslo rifting event (Table l), it means a time difference
Ramberg and Larsen (1978) proposed that graben segments in the Oslo Graben are off-set relative to one another in an en echelon manner (Fig. 1). Rift jumps or off-sets of graben segments
Krokskogen-SIma of a distance of 210 km, Rate estimates based fall within this range.
about 10 million years over that is a rate of 2.1 cm y-l. on other magmatic periods The estimated rate of l-2
cm y-l
are a common feature to all types of rifts, passive or active (Wood, 1983). Some rift jumps are un-
is within the range of Cenozoic plate spreading rates and hot-spot movements (Molnar and Stock, 1987).
idirectional, as Trough-Cameroon
exemplified by the Benue Line (Fitton, 1980) and the
The proposed northward progression of the rifting activity may have taken place as en echelon
Kenya Rift (Wood, 1983), but most jumps are oblique, forming en echelon patterns. Rift propa-
rift jumps (as proposed by Ramberg and Larsen, 1978). An alternative model is propagation pro-
gation controlled
by non-uniform
motion of the
moted by changes in the directions
of the prin-
affected lithosphere over the mantle magma source
cipal stress axes (proposed
(hot spot, mantle plume) has been proposed as a
commun.). The latter model seems to agree better with both the observed difference in the tensional stress axis between the Vestfold GS (E-W) and
mechanism for such jumps (Fitton, 1980; Hey et al., 1980), but evidently rifts formed mainly by horizontal shear motion (pull-apart basins) also display “jumps” (i.e. the Dead Sea Rift; Garfunkel, 1981). The age determinations presented above suggest a northwards shift with time of the Oslo Rift magmatism (Table 1, Fig. 5). The B,-basaltic
by K.Y.
Buer, pers.
the Akershus GS (ESE-WNW) (Ramberg and Spjeldnres, 1978) and the non-linear developement of the central volcanoes in the northern part of the Vestfold segment and the southern part of the Akershus segment.
se-
quence started earlier in the central Vestfold than in the Krokskogen area, the age difference is, however, unknown. The RP lavas in Brummundal are younger than the first RP lavas in Krokskogen; a fair estimate based on age determinations and lava stratigraphy is about 5 million years. The
Episodic magmatism / reactivation Another important result of the age determinations is that the magmatic activity lasted much longer in the Akershus than in the Vestfold GS,
AGE
RELATIONS
AMONG
OSLO
and that mantle-derived
RIFI
MAGMATIC
83
ROCKS
magmas in the Akershus
in the underlying crust. The emplacement
of the
GS were available for a longer period of time (Fig.
Hurdal central complex (265-245
5). The data also suggest that whereas the magma-
been triggered by movement along the Mylonite
tism in the Vestfold
zone (Figs. 1, 4). Neugebauer
GS had uniform
intensity
Ma) may have
(1987)
has shown
over most of the active period, the magmatism in
that a magma body formed in the mantle or crust
the Akershus GS varied considerably
will, during ascent, split into several minor bodies
in intensity.
Explosive volcanism furthermore represents a sig-
whose time of emplacement
nificant
may differ by several million years.
GS,
part of the magmatism
but appears
in the Akershus
to be more infrequent
in the
into the upper crust
Also other groups of intrusions form conspicu-
Vestfold GS.
ous linear
A possible explanation for the (suggested) episodicity of the magmatism in the Akershus GS is that melting was triggered by specific tectonic
Hadeland (Fig. 3) define a curved line which continues into a unique group of syenites in the
events. Precambrian the Oslo Rift
shear zones east and west of
(e.g. the Oslofjorden
Fault
Zone
(FZ), the 0ymark FZ, the Mjosa-Xneren Zone, the Porsgruxu-Kristiansand FZ, and the MeheiaAda1 FZ) were reactivated during the Oslo rifting event
(e.g.
Ramberg
and
Larsen,
1978;
Schsnwandt and Petersen, 1983; Starmer 1985a, b; Buer, 1990; Swensson, 1990). Stress release along these fault zones may have brought the hot
Nordmarka
trends. The small gabbros
area, the Grefsen
necks
at
syenites of Saether
(1962). The gabbros at Hadeland have not been dated, but two of the Grefsen syenites at Nordmarka, Gjerdingen
and Grefsen, give similar ages
of about 255 Ma. The hypothesis that magmatism
was triggered
by movement along major fault zones should to be tested by additional age determinations.
Summary of conclusions
lithosphere under the Oslo Rift above the solidus, thus causing episodic melting. Episodic recurrence of magmatic and tectonic activity is known both from the Rio Grande Rift
Age relations in the Oslo Rift imply that the magmatic developement may be divided into four
(Chapin, 1979) and the Kenya Rift (Baker et al., 1971; Baker and Wohlenberg, 1971). In the Rio
distinct phases: (1) B,-basalt volcanism, (2) RP fissure eruptions, (3) central volcano and caldera
Grande Rift the origin for this episodicity has been considered external to the rift itself (Eaton, 1979). Central volcanoes resting on the extensions of
formation, and (4) emplacement of large plutons. These phases correspond roughly to the stages 2-5 in the tectonomagmatic model of Ramberg and Larsen (1978). The length of each phase is differ-
Precambrian
ent in the Vestfold
fault zones into the Oslo Rift may
and the Akershus
Graben
represent juvenated
magmas formed at the times of retension. Some of the calderas along the
segments. A summary of the tectonomagmatic development is given in Table 2, the main conclu-
extension
of the Oslofjorden
sions are listed below: - The Oslo rift was initiated at about 300 Ma (Late Carboniferous time) and was magmatically active for about 60 million years.
Fault show a con-
centrated period of activity at 270-265 Ma (Kampen, Oppkuven, Heggelia, Oyangen) (Table 1, Fig. 3). A later period of activity along the same zone is indicated by the Brerum caldera (243 k 3 Ma) and the Tryvann granite (241 f 3 Ma). The formation of the Nittedal caldera (Fig. 3) suggests movement along the 0ymark FZ about 255-252 Ma. Although the relation between this northsouth trending chain of calderas and major fault zones is not obvious, the density of north-south trending faults across the Vestfold lava plateau strongly suggests a weakness zone of similar trend
- The earliest recorded magmatic activity was the intrusion of sills and dykes of basaltic to syenitic composition. - The main period of magmatism started about 295 Ma with the extrusion of plateau lavas of basaltic (Bl) and intermediate composition (RP lavas). The period of plateau volcanism lasted to about 275 Ma with the greatest magma production in the period 295-285 Ma.
84
B. SUNDVOLL
TABLE 2
volcanoes
Graben segment developement: comparision between the Vest-
270-245 Ma. - The main intrusive period started with em-
fold and Akershus Graben Segments Age (Ma)
Vestfold
Akershus
were
placement
subjected
of larvikites
to
and ended
collapses
with alkali
syenites and alkali granites. This phase lasted from
240
no further record _____--------Latest intrusions
245
Brerum caldera
-
Late alkali syenites
rived magmas were available
250
caldera
ET AL.
280 to 267 Ma in the Vestfold, and from 273 to 241 Ma in the Akershus GS. “Sr/*%r
initial ratios imply that mantle-deunder the Vestfold
and granites
GS from 300 to about 280 Ma, under the Akershus
(Reactivation?)
GS from 300 to about 275 Ma. In both graben
255
Early syenites
segments emplacement of highly fractionated mantle-derived melts and anatectic melts con-
260
Nittedal caldera Early granites
265
no further record _____--------Late intrusions
(Reactivation?)
270
Caldera collapses
Caldera collapses, monzonite intrusion (Nordmarka)
275
Monzonite and early
RP vol$anism Main graben formation Main graben formation RP vol&nism RP volcanism Basaltic volcanism
295 Basaltic volcanism 300
The tectonic and magmatic activity appear to
sent the northern
development
290
-
have migrated with time from SSW to NNE. The Sarna alkaline complex, dated to 282 k 14 Ma and the northeast tip of the Oslo Graben,
Axial central volcano
285
derived to crustally derived magmas is accompanied by a migration of the magmatic activity towards the central axis of the rift.
situated about 100 km to the north-northeast
granite intrusions
280
tinued for a long time (about 15 and 35 million years, respectively). The transition from mantle-
of
may repre-
end of the tectonomagmatic
event. The rate of northward propagation
is esti-
mated to l-2 cm y-l, that is similar to plate movements and hot-spot spreading rates. - The much longer period of magmatic activity in the Akershus than in the Vestfold GS is considered to reflect recurrent movement along Precambrian fault zones, leading to partial melting in the hot lithosphere under the Oslo Graben. Acknowledgements
Sill intrusions
Sill intrusions
and sedimentation
and sedimentation
Estimated rates of RP extrusions are about one flow per 250,000 years and 0.3 km3 per lo3 years in the Vestfold GS, and one flow per 800,000 years and 0.07 km3 per lo3 years for the Krokskogen lava plateau in the southern part of the Akershus GS. - The main faulting activity and graben formation started shortly prior to 285 Ma. The period of central volcanoes started about 280 Ma in Vestfold GS and a few million years later in the Akershus GS. The ages of ring-dykes and central intrusions indicate that these central
T. Enger and B. Nilssen are gratefully acknowledged for their enduring help with the analytical work. This paper has improved through constructive criticism by H. Sorensen and L.A.J. Williams. The work was made possible through grants from the National Research Council for Science and the Humanities (NAVF). Norwegian ILP Contribution
No. 96.
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