Age relations among Oslo Rift magmatic rocks: implications for tectonic and magmatic modelling

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.

References Andersen, T., 1984. Crystallization history of a Permian composite

monzonite-alkali

syenite

pluton

in

the

Sande

cauldron, Oslo rift, Southern Norway, Lithos, 17: 153-170.

AGE

RELATIONS

Andresen,

AMONG

OSLO

RIFT

MAGMATIC

P., 1985. A geochemical

investigation

pose of a ~he~cal-strati~ap~c sequence

of rhomb

graben.

Cand.

mantle

region

Geochim.

of the Kenya B.H.,

1971. Berthelsen,

of Oslo, Oslo, 109

Williams,

the Oslo

Acta, 53: 106’7-1076. and evolution

Miller,

J.A.

and

Fitch.

of the

F.J.,

Kenya

rift

A., 1980. Towards

a palinspatic

tectonic

analysis

Int. Geol. Congr.,

of

Paris 1980,

M&m. Bur. Rech. GCol. Min., 108: 5-21. Bragger,

W.C., 1898. Die Eruptivgesteine

III. Das Ganggefolge I: Mat.-Naturv. Bragger,

ds Kristianiagebietes,

des Laurdalits.

Skr. Vidensk.

W.C., 1900. Conglomerates

Oslo Region. zwischen

from

1931.

Die

Randsfjord

formation

38: 29-64

Sperillen,

in the

(in Norwegian).

Explosionsbreccie

und

I. On

the island-chain

a new sedimentary

Nyt Mag. Naturv.,

W.C.,

bei

Nor.

Sevaldrud

Geol.

Tidsskr.,

G. and Patchett,

isotopic

evidence

western

central

Chapin,

C.E.,

summary.

Lithos,

Evolution

Rio

(Editor),

Grande

Rio

American

complex,

Oslo Region,

rift-a

Grande

Geophysical

Rift: Union,

aspects

Oslo area, Norway.

of the Finnmarka

J. Geol., 73: 293-322.

on the igneous rock complex

XI. Compound

volcanic

neck, igneous

feltgruppen,

Diabase

4: 29-31

from

Stig,

219 Ma.

Nytt

Oslo-

G.P.,

1979. A plate-tectonic

Riecker

(Editor),

tism. American

model

in the western Rio Grande Geophysical

United

lavas and

for late Cenozoic States.

Rift: Tectonics Union,

Tectono-

of the biotite

Geol.

Unders.,

magmas

on the origin

Rift, Kenya,

367:

in the Rift System.

G., 1978. Sedimentary

the Oslo

Region

(Editors),

The

Excursions.

lavas.

Oslo

Paleorift:

Hey, R., Duennebier,

In: R.E.

and Magma-

Washington,

D.C., pp.

7-32.

Lithos,

rocks

In: J.A.

Dons

associated

with

B.T. Larsen

and

Guide

to

337: 17-24.

F.K. and Morgan, ridges.

concem9: 1-8.

and

A Review

Nor. Geol. Unders.,

of phono-

and inferences

W.J., 1980. Propagat-

J. Geophys.

Res., 85: 3647-

3658. Holtedahl,

O., 1953. Norges

geologi.

Nor. Geol.

Unders.,

164:

1118 pp. Ihlen,

P.M., Tronnes,

tion,

wall-rock

associated

R. and Vokes, alteration

and

with the Drammen

F.M.,

granite

In: A.M. Evans (Editor),

with Acid Magmatism.

1982. Mineraliza-

zonation

of ore

deposits

in the Oslo Region,

Metallization

Associated

J. Wiley and Sons, New York, N.Y.,

pp. 111-136. S.B.

tematics

and

Heier,

Norway.

Lithos,

Jacobsen,

1978.

rocks,

Rb/Sr

isotope

Kongsberg

sector,

syssouth

11: 257-276.

S.B. and Raade,

Tidsskr., Larsen,

K.S.,

in metamorphic

G., 1975. Rb-Sr

granite,

Oslo region,

whole rock dating Norway.

Nor. Geol.

55: 171-178.

B.T., 1975. Geology

and tectonics

map

of the Oslo rift zone

1:250,000). Institute,

Report

Project

for

the

No. 74628 (un-

published). Larsen,

B.T., 1978. Krokskogen

Guide

(Editors),

to Excursions.

Norges

B.T., 1979. Volcanism,

the Permian

lava area.

tectonics

area. Cand.

Le Bas, M.J., Le Maitre,

S.J.,

1973a.

in

University

of

in Norwegian). A. and Zanettin,

of volcanic

diagram.

Plateau

and

337: 143-162.

with the emphasis

Real. Thesis,

classification

on the total alkali-silica

and

and sedimentation

R.W., Streckeisen,

B., 1986. A chemical

A Review

GeoI. Unders.,

Oslo rift: five contributions

on the Krokskogen

Lippard,

In: J.A. Dons

The Oslo Paleorift:

Oslo, Oslo, 169 pp. (unpublished;

spreading

Sea leaky

Nor.

1976. Some constraints

Henningsmoen,

Larsen, sediments,

in the Kols%s area W of Oslo. Nor. Geol. Tidsskr.,

crustal

J. Petrol.,

phonolite

lava

rocks based 27: 745-750. flows,

Kenya.

Geol. Mag., 110: 543-549. T. and Petersen,

J.S., 1980. The Sveconorwegian

genie belt, a case of Late-Proterozoic Rundsch., Fitton,

G.G.,

lites from the Gregory

Geotechnical

(in Norwegian). E., 1967. Permian

129 pp.

kinematics.

and petrography

Region.

Norwegian

47: 57-17.

Falkum.

Goles,

B.T. Larsen

Dons, J.A. and Gyery,

Eaton,

of the Oslo

dykes,

Kl. 1952, 2: 96

1972.

Graben.

Oslo,

of the Dead

to plate

A., 1981. Field relations

granites

tectonic

area, Oslo. Skr.

PP. Dons, J.A.,

of mag-

the Oslo

80: 81-108.

(including

Nor. Vidensk.-Akad.

Oslo, I: Mat.-Naturv.

petrology

of Oslo,

structure

in relation

of the

and fault zones in the Ullem-Huseby%sen

faults

(rift)

physics, Gaut,

University

Z., 1981. Internal

transform

and

cauldron,

in Norwegian).

of the Nordagutu

1965. Petrologic

J.A., 1952. Studies

Thesis,

(unpublished;

Jacobsen

D.C., pp. 1-5.

G.K.,

igneous

complex,

10: 73-79.

of the

R.E. Riecker

and Magmatism.

Czamanske,

and Rb-Sr

for the age of the Sarna alkaline

1979.

Washington

Dons,

P.J., 1977. Palaeomagnetic

Sweden.

In:

Tectonics

Real.

Norway.

11: 281-346. Bylund,

Cand.

in the Nittedal

ing rifts on midocean

in the Oslo Region,

the rhomb-po~hy~-~nglomerates

Bragger,

Selsk.,

Kl. 1897, 6: 377 pp,

Revlingen-Sestrene,

V., 1983. Geochemistry

rocks

ing basaltic

11: 191-215.

the Baltic Shield. 6th Colloq.

matic

39-64.

229: 538-542.

Geochronology

Tectonophysics,

1989. Un-

from

J., 1971. Structure

L.A.J.,

and

E.-R.,

for nephelinites

Rift valley. Nature,

Sequence

volcanics.

Oslo

Fjerdingstad,

Garfunkel,

Cosmochim.

Baker, B.H. and Wohlenberg, Baker,

in the central

T.V. and Neumann,

source

Rift, Norway.

of the lower

in Norwegian).

E., Segalstad,

usual

lavas

Scient. Thesis, University

pp. (unpublished; Anthony,

with the pur-

identification

porphyry

85

ROCKS

J.G.,

migrating

oro-

plate-collision.

Geol.

69: 622-647. rift

system

Kenya Molnar,

1980. The Benue

Lett., 51: 132-138.

Lippard,

trough

in West

and Cameroon

Africa.

Earth

line-a

Planet.

Sci.

S.J., 1973b. rift. Lithos,

The petrology

of phonolites

P. and Stock, J., 1987. Relative

the Pacific,

Atlantic

ceous time. Nature,

from

the

6: 217-234. and

Indian

327: 587-591.

motions

Oceans

of hotspots

in

since late Creta-

B. SUNDVOLL

86

Neugebauer,

H.J.,

rifting.

1983.

Mechanical

Tectonophysics,

Neugebauer,

aspects

Neumann,

Planet.

E.-R.,

plex in the Permian and I.B. Ramberg Continental Neumann,

of the Larvik

Petrology

Rifts. Reidel,

Dordrecht,

E.-R., 1980. Petrogenesis E.-R.,

positional Neumann,

Larsen,

Pallesen,

larvi-

in the Oslo

P., 1986. Mass

after

anatexis

in the

Res., 91: 11629-11640.

G.R. and Tuen,

geochemistry Norway.

Andresen

Geochim.

Cosmochim.

Acta,

upper mantle

from

basalts

E.-R.

Neumann

Crust

B. and 0verli, beneath

(Editor),

Rift

Zones

of Europe-Geophysical, Evidence:

89-107

Oslo

R.M.,

Petersen,

J.S.,

Graben.

In:

and Geochem-

Tectonophysics,

ages of Norwegian

rocks. Nor. Geol. Tidsskr., Nystuen,

178:

the Oslo region, Hurdal

Mat.-Naturv. Nystuen,

J.P.,

and

67: 330-342.

Petersen,

J.S.,

XXV. Diatremes

the Hurdal

and volcanic

Oslo, I:

Plutonic

area,

Oslo

region.

Nor.

Geol.

Unders.,

C., 1952. Studies on the igneous Region,

XII.

The

Oslo, I: Mat.-Naturv. Oftedahl,

XIII. The cauldrons.

on the igneous

of the

rock complex

and related

of the

rocks.

Skr.

Kl. 1957, 4: 21

In: 0.

rocks

HoltedahI

and structures

(Editor),

of Norway.

208: 298-343. Oslo-Gebiete.

nach

Lava-Stratigra-

Geol. Rundsch.,

57: 203-

218. C., 1978a.

plateau.

Geotherm.

Cauldrons

In: E.-R.

of the Permian

Oslo

rift. J.

Res., 3: 343-371.

C., 1978b. Origin

of the magmas

Neumann

E.-R.,

history

T., Sundvoll,

Mineral.

Evidence:

178: l-10

in the

J.I. and Myhre, A.M., 1990.

offshore Neumann

B.,

processes

Mag., 52: 293-307.

continuation

(Editor),

Oslo-Horn

of the Oslo

Rift Zones

Crust of Europe-Geophysical,

in the

Geological Graben.

and

Tectono-

(this issue).

1929.

Brumunddal.

Rifts.

and silicic magmatism

L.M., Faleide,

In: E.-R.

H.,

of

I.B. Ramberg

of Continental

Andersen,

Norway.

Graben-the

Geochemical

Russell,

M.J.

graben and

Volcanol.

and Geophysics

and

V. and Stabel, A., 1988. Petrogenetic

Continental

of

pp. 167-194.

Ro, H.-E., Stuevold,

Rosendahl,

coast

The

Nor.

porphyry-sandstone

Geol.

Tidsskr.,

sequence

10:

367-438

in (in

Norwegian).

C., 1967. Magmen-Entstehung

pbie im slidlichen

Geology

of the Oslo

interpre-

the southeastern

Neumann

with intermediate

Graben.

characteristics

rifts. Proc. 27th Int.

N., 1978. The tectonic

In: E.-R.

Oslo rift, south-east Skagerrak

Nor. Geol.

Geol. Sot. Am. Bull., 86: 769-774.

E., Neumann,

Fjerdingstad,

physics,

Nor. Geol. Unders.,

Oftedahl,

Rasmussen,

to Excursions.

along

I.B. and Spjeldnms,

associated

rock complex

Oslo, I: Mat.-Naturv.

C., 1960. Permian

Region.

Oftedahl,

and Skagerrak.

Tectonics

of the

evoluThe Oslo

S.B., 1975. Geophysical

structure

(Editors),

Vidensk.-Akad.

(Editors),

325: 193

7: 165-216.

of crustal

Norway

pp.

of the Oslo Graben

of continental

I.B. and Smithson,

317:

Skr. Nor. Vidensk.-Akad.

XVI. On ignimbrite

Nor. Vidensk;Akad.

rock complex Nor.

and Geo-

Dordrecht,

P., 1984. Physical

trends

Region.

Kl. 1953, 3: 108 pp.

C., 1957. Studies

Oslo Region, PP. Oftedahl,

Skr.

and Guide

I.B. and Morgan,

and evolutionary

the Oslo

Kl. 1952, 3: 64 pp.

Oslo, I: Mat.-Naturv. Oftedahl,

lavas.

C., 1953. Studies on the igneous

Oslo Region,

Oftedahl,

A Review 337: 55-13.

Reidel, Dordrecht,

Oftedahl,

Petrology

Reidel,

a

In: E.-R.

B.T., 1978. Tectonomagmatic

in

intrusions

(Editors),

B.T. Larsen

Geol.

ring-complexes:

plutonism.

interpretation

and

in

Larvikite-Lardalite its evolution.

plutonic

Rifts.

Zone

I.B. and Larsen, In: J.A. Dons

Ramberg,

and subvolcanic

Sci-

64: 39-68.

rocks. Nor. Geol. Unders.,

Unders.,

tation

Skr. Nor. Vidensk.-Akad.

Cand.

igneous

Paleorift:

rocks in the

l-21. Oslo

PP. Ramberg,

the and

of rift-zone

I.B., 1976. Gravity

and associated

of

Norway,

Composite

characteristic

Ramberg,

Kl., N. Ser., 33: 39 pp. 1975b.

1978b.

structural

of

rock complex

study of the

of the Tomquist

Structure

Rundsch.,

Geol. Congr.,

on the igneous

area, Oslo region.

minerals

40: 173-191.

J.P., 1975a. Studies

1978a.

Oslo-Region,

Ramberg,

H., 1960. Apparent

Norway.

Oslo Region.

Sea. Nor. Geol. Tidsskr.,

Complex,

tion.

(this issue).

Neumann,

North

211-229.

evidence

and petrological

1984. The extension

the Norwegian

52: 1997-

in the Continental

Geological

Oslo-Horn

Palaeozoic

Southern

of Oslo, Oslo, 108 pp. (unpublished;

of Continental

Rift.

in Upper

southern

and I.B. Ramberg

Norway:

in the Permo-Carboniferous

B,-level,

Neumann

P.E., 1990. A mildly

southeast

Reidel,

in Norwegian).

Ramberg, E.-R., Sundvoll,

depleted

ical

basalts,

chemistry

2007. Neumann,

incursion

of the Oslo Region,

province,

E., 1988. Sr, Nd and

of the Oslo rift igneous

Horten

Pegrum,

B., 1985. Com-

intrusions

and residues

J. Geophys.

E.-R, Tilton,

Pb isotope southern

gabbroic

Marine

rocks

ent. Thesis, University

21: 498-531.

Sundvoll,

S. and

of cumulates

Oslo graben. Neumann,

of

18: 35-59.

E.-R.,

estimates

among

S., 1981.

Overli, P.E., 1985. A stratigraphic

In: E.-R. Neumann and Geochemistry

of the Oslo Region

B.T. and

Olaussen,

Rifts.

Geol. Mag., 118: 281-288.

ring-com-

pp. 231-236.

rocks. J. Petrol.,

variations

rift. Lithos,

Annu.

of Continental

pp. 191-208.

sedimentary

Oslo rift, Norway. (Editors),

kites and associated Neumann,

thinning.

Sci., 15: 421443.

1978. Petrogenesis

and Geochemistry

Dordrecht,

H.J., 1987. Models of lithospheric

Rev. Earth

Petrology

of continental

94: 91-108.

ET AL.

and

lithospheric

physics,

Smythe,

in relation

rifting

and I.B. Ramberg

lava

(Editors),

1983.

Origin

of the Oslo

in the North

Atlantic.

orogeny Tectono-

94: 457-472.

‘ðer, E., 1947. Preliminary of the Vestfold

D.K.,

to the Hercynian-Alleghenian

report

the Fen area. Nor. Geol. Tidsskr.,

on the results of studies in 27: 66-73

(in Norwegian).

AGE

RELATIONS

Saether,

AMONG

OSLO

E., 1962. Studies

Oslo Region,

General

Oslo, I: Mat.-Naturv.

Tidsskr.,

T.V., major

province.

Oslo Region:

in the Oslo region, 1988. Volcanic

subsidences,

in the Skien

T.V., 1979.

southwestern

rocks

Region,

alkaline

Norway.

southern

Lithos,

Cauldron Norway.

rocks,

Geol.

of

Unders.,

T. and Schubert,

lithosphere: volcanism

a

G., 1983. Convective

mechanism

on Eath,

for

Venus

thinning

rifting

and Mars.

and

of the

mid-plate

Tectonophysics,

94:

of the Bamble

Serie

I., 1978. The major

between

Sondeledfjord

Telemark). Starmer, the

and

Kilsfjord

Nor. Geol. Unders.,

I.C., 1985a.

Proterozoic

and

Touret

Bamble

(Editors),

of the south

sectors.

Atlantic Dordrecht,

Provinces,

pp. 259-290.

NATO

Starmer,

I.C., 1985b.

The geology

and

the evolution

Nor. Geol. Unders.,

nology:

Convention

Stormer, part

with remarks

on younger

and

Vol.

158.

of the Kongsberg Kongsberg

district

Sector,

south

on Geocbroin geo- and

rocks.

to the geology

the rhomb-po~hy~ tectonics.

B., 1978.

Rb/Sr-relationship

In:

Neumann

Dordrecht,

E.-R.

and Geochemistry pp. 181-184.

and

of the southern conglomerate

Nor. Geol. Tidsskr., in the Oslo LB. Ramberg

of Continental

rocks

Norway:

along

the Nesodden

a reactivated (Editor),

Evidence:

178: 51-65

in the

Geologicai Graben.

and

Tectono-

(this issue).

at Holmestrand, Thesis,

Precambrian

Rift Zones

Oslo-Horn

J., 1987. A stratigraphic

B,-basalt

and petrological

southern

University

of Oslo,

Oslo Oslo,

study

Region.

of

Cand.

133 pp. (unpub-

lished; in Norwegian). Tuen,

E., 1985. A geochemical Hurdal.

study of acid intrusive Cand.

Scient.

of Oslo, Oslo, 108 pp. (unpublished; Turcotte,

D.L. and Emerman,

tive and passive

rifting.

University

in Norwegian).

Tectonophysics,

XXIV.

rocks at

Thesis,

S.H., 1983. Mechanisms

P.W., 1975. Studies

the Oslo region,

of av-

94: 39-50.

on the igneous

Geochemistry

rocks. Skr. Nor. Vidensk.-Akad.

rock complex

of

of the Oslo basaltic

Oslo, I: Mat.-Naturv.

W., 1918. Explosion-pipes

Tidsskr., Wemicke,

5: 99-104

IU.,

lithosphere.

Wessel, P. and Huseby, Williams,

L.A.J.,

tinental

15:

igneous (Editors),

Rifts.

normal

Can. J. Earth

simple shear of the Sci., 22: 108-125.

Am.

Geophys.

gravity

high

142: 1.5-26.

aspects

In: G. Palm&on

Rifts.

Nor. Geol.

E., 1985. The Oslo Graben Teetonophysics,

1982. Physical

rifts.

at Lysaker.

(in Norwegian).

B., 1985. Uniform-sense

continental

of magmatism

(Editor), Union,

in con-

Continental Geodyn.

and

Ser.,

8:

193-222. Wood, C.A., 1983. Continental Zeck, H.P., Andriessen, Rasmussen, em part straints.

Petrology

Region,

B., in prep. The age

rift jumps.

Tectonophysics,

94:

529-540.

Sci. Lett., 36: 359-362.

43-113. Sundvoll,

physics,

Oceanic

401: 35-58.

Planet.

L., 1935. Contribution of the Oslofjord:

Tobi

Series,

on use of decay constants Earth

of

Crust in the

K.H. and Jagerr, E., 1977. Su~o~~ion

cosmochronology.

(in

stress regime.

Cataclastic

and taphrogenesis.

of the entire

Unders.

in the Permo-carbonifer-

Crust of Europe-Geophysical,

Werenskiold,

Norwegian

In: A.C. ASI

systematics

Geol.

N. Ser., 34: 38 pp.

The Deep Proterozoic

Reidel,

Steiger,

and

by the major and mega-tectonics

North

Norway.

(Aust-Agder

338: 37-58.

The evolution

as revealed

Kongsberg

J.L.R.

Tectonics

phase

shear zone. In: E.-R. Neumann

Weigand.

67-90. Starmer,

Oslo

Nordliskampen,

321: 67-86. Sphon,

Fault,

Scient.

12: 221-239.

in the Permian Nor.

isotope

Nor.

B.T. and Wandaas,

E., 1990.

Tollefsrud,

of the Skien basaltic

R., 1975. The Ramnes

the Oslo

of the Oslo

rocks.

magmatic

Geochemical

ring-structures,

55: 321-337.

Petrology

Oslo Region,

part

B.T., 1990. Rb-Sr

ous Oslo rift and its related

Continental

1975. Cauldron faults

B., Larsen,

of the incipient Swensson,

the mafic lavas. Nor. Geol.

Nor. Geol. Tidsskr.,

SegaIstad, Sorensen,

rifting

94: 609-631. E.-R.,

B. and Larsen,

in the Oslo Rift magmatic press).

48: 289-308.

Segalstad, and

occurrences

Sundvoll,

Sundvoll,

J.S., 1983. Continental

E. and Neumann, central

of the

of the igneous

IU., N. Ser., 1: 184 pp.

Tectonophysics,

on Jeleya,

rock complex

of Oslo. Skr. Nor. Vidensk-Akad.

and po~hy~-molybdenum Schou-Jensen,

87

ROCKS

investigation

H.K. and Petersen,

Norway.

MAGMATIC

of the igneous

XVIII.

rocks in the area north Schonwandt,

RlFT

Reidel,

Zuber,

P.A.M.,

Hansen,

B.L., 1988. Paleozoic

of the Fennoscandian Tectonophysics, model

Sci. Lett., 77: 373-383.

Shield-fission

P.K. and

of the southtrack

con-

148: 61-66.

M.T. and Parmentier,

ing: a dynamic

K., Jensen,

paleo-cover

E.M., 1986.

Lithospheric

for rift morphology.

Earth

neckPlanet.