Volcanic rocks on Jeløya, central Oslo Region: the … · Volcanic rocks on Jeløya, central ......

Volcanic rocks on Jeløya, central Oslo Region: the mafic lavas

ERIK SCHOU-JENSEN & ELSE-RAGNHILD NEUMANN

Schou-Jensen, E. & Neumann, E.-R.: Volcanic rocks on Jeløya, central Oslo Region: the mafic lavas. Norsk Geologisk Tidsskrift, Vol. 68, pp. 289-308. Oslo 1988. ISSN 0029-196X.

Jeløya is situated in the southeastern part of the Permo-Carboniferous Oslo graben. The oldest tectonomagmatic activity in this area was movements along the major N-S trending fault east of Jeløya. This was followed by alternating volcanism and movement along NNW-SSE trending faults. The volcanism resulted in a 800-1500 m thick sequence of basaltic rocks (81) belonging to the earliest eruptions in the Oslo rift (starting about 300 Ma ago). These basalts are overlain by rhomb-porphyry lavas. The basaltic sequence in northern Jeløya is dominated by pyroclastic rocks with a few lava flows, further south by la vas, mainly alkali basalts to trachybasalts. All the lavas have a somewhat evolved character. The basaltic rocks may be divided into three groups: (l) TiOrrich basalts originating from a chemically distinct parent magma, (2) moderately Ti02-rich lavas which crystallized under relatively reducing conditions and show a trend from silica-undersaturation towards silica-oversaturation, and (3) moderately TiOrrich basalts which crystallized under more oxidizing conditions and show no clear tendency for increasing silica activity during crystallization. The three rock groups probably represent three contemporaneous eruption centers. One of these was situated just east of the northernmost tip of Jeløya, and gave rise to the pyroclastic rocks and lavas of series (3) in the northern and south central parts of the istand. Another, which gave rise to TiOrrich basalts, lay west of Jeløya.

E. Schou-Jensen, Københavns Universitet, Geologisk Museum, Øsier Vo/dgade 5-7, 1350 København K, Denmark; E.-R. Neumann, Universitetet i Oslo, Mineralogisk-geologisk museum, Sarsgt. l, 0562 Oslo 5, Norway.

In Permo-Carboniferous time, southeast Norway was subjected to an intracontinental rifting episode which led to the formation of the Oslo graben. The rifting was accompanied by extensive magmatism which lasted from about 300 to about 240 Ma ago. The diversity of the magmatic products is typical of intracontinental rifts. The oldest magmatism is represented by a series of basalts called B1. Younger lavas comprise basalts, rhomb-porphyry lavas, ignimbrites and rhyolites. Magmatism ended with the emplacement of large monzonitic to granitic batholiths (Oftedahl 1952; Ramberg & Larsen 1978; Sundvoll 1978; Larsen & Sundvoll 1982; Tuen 1985). The B1 basalts show a general pattern of decreasing thickness and alkalinity from south to north. The Skien basalts in the southernmost part of the rift (Fig. la) consist of nephelinites, ankaramites and basanites with a minimum thickness of 1500 m (Segalstad 1979). The Horten-Holmestrand B1 sequence in eastern Vestfold is 170-180 m thick, and is made up of mildly silica-undersaturated to silica-saturated ankaramites, alkali basalts and trachybasalts (Øverli 1985; Tollefsrud 1987).

Only a single ftow of quartz tholeiite represents B1 at Krokskogen, north of Oslo (Larsen 1978). A geochemical survey of basalts in the Oslo Region has been presented by Weigand (1975). For a review of the tectonomagmatic evolution and B 1 volcanism, the reader is referred to Ramberg & Larsen (1978).

Remnants of the once more continuous lava cover are now found in different parts of the rift. One of these remnants makes up a large part of the island of Jeløya in the Oslofjord (Fig. 1). The stratigraphy of the Paleozoic sediments and volcanic rocks on Jeløya was first established in the geological quadrangle map l: 100,000 Moss, published by Brøgger & Schetelig (1926). The first detailed description of the la vas was presented by Brøgger (1931, 1933). More recently the Jeløya Javas have been briefty mentioned by Oftedahl (1952) and Weigand (1975). The latter published three whole rock analyses of Jeløya basalts. During the period 1968-1974, Jeløya and the neighboring islands were remapped for the Norwegian Geological Survey by E. Schou-Jensen. This map ( scale l : 37 ,000) was published by Larsen et al.

290 E. Schou-Jensen & E. -R. Neumann

in 1978. Little is known about the Jeløya basalts, although they make up an important part of the early basalt series (B1) in the central Oslo Region. It is the aim of this paper to describe the geology and stratigraphy of the basalts and associated

NORSK GEOLOGISK TIDSSKRIFT 68 (1988)

Fig. la. Geological map of the Oslo graben, based on maps by Oftedahl (1960) and Ramberg & Larsen (1978). The location of Jeløya is indicated by an arrow.

rocks, to describe the compositional character and stratigraphic variations among the basalts, to make a preliminary comparison between the Jeløya basalts and early basalts in other parts of the Oslo Region, and to discuss the implications


Fig. lb. Geological map of Jeløya based on map by Schou-Jensen (in Larsen et al. 1978). The sampling profiles are shown as dotted lines marked with the profile number at the stratigraphically lowest end.

JELØYA �O Strike ond dip

'v("-. Foults \ \\\

r-1 Quoternory L__j marine deposifs

PERMIAN � Rhomb·porphyry � conglomerote

lntrusive roclcs • Gabbro

-t!JP' Trochytic felsite

Volconic rocks � Rhomb-porphyry � with oggl. beds

�"'\ Augite-Piogioclase� -! porphyritic lova

EJ � lgnimbritic loyer

r��?��J;J' :�;:','::!;' rocks/

1:::::::1 Oebris flow deposits

DOWNTONIAN

(::.:·.:-::-:·.:·.:-:·.:·.) Ringerike sondstone

N

t o 2

Volcanic rocks, Jeløya 291

30'

Revlingen

292 E. Schou-Jensen & E.-R. Neumann

of these results for our understanding of the earliest tectonomagmatic activity in the central Oslo Region.

Geology Jeløya is the !argest of the islands in the central part of the Oslofjord. It is situated dose to the major fault along the southeastern margin of the Oslo graben (Fig. la), that is, the subaerial part of the Oslo rift. Jeløya consists of several faulted blocks with a general di p of 20--30° to SE-ESE in the northern and central parts of the island, and s-ssw in the south em part (Fig. lb).

Sedimentary rocks Ringerike sandstone. - The oldest rocks at Jeløya are gra y to reddish, fine- to medium-grained sandstones with ftuviatile structures such as large-scale trough crosscbedding, intraformational conglomerates, ripple marks, and parting lineation with paleocurrent direction towards SSE. These rocks are exposed in the northwestern part of Jeløya and on the island of Bile (Fig. lb), while limestone has been sampled from the ftoor of the Oslofjord just NW and N of Jeløya. The sandstones dip 20° to ESE, and have an estimated minimum thickness of 425 m. They belong to the Holmestrand formation in the Ringerike group (Turner 1974). Cephalaspids found in the sandstone suggest a lower Downtonian age (Heintz 1934). The limestone in the fjord just NW of Jeløya is likely to be of Silurian age.

Debris flows. - Above a weak discordance, the Downtonian sandstones are overlain by a number of debris ftows which can be followed from Bevøya (north of Jeløya) to the central part of Jeløya, where the standstones are downfaulted below sea level. The debris ftows above the sandstones consist of megabreccias with fragments of sandstone (Ringerike type) up to 25 m in diameter. The matrix consists of a gra y to reddish, fine-grained, relatively soft sandstone, generally with dis tinet ftow structures around the fragments. Several of the sandstone fragments show plastic deformation. They are norrnally imbricated or arranged in large fold structures. In addition to standstone fragments, the breccia carries scattered fragments of conglomerate with numerous rounded quartzite pebbles and occasional angular fragments of altered limestone (similar to limestones of the Tanum forrnation) (Olaussen 1981).


The conglomerate fragments include a 'trachybasalt lava of the same petrographic type as that seen in the basaltic series of Jeløya. These fragments represent the earliest volcanism in the area.

The strong deformation of the sandstone in steep cliffs on the northern tip of Jeløya was interpreted by Brøgger (1931) as the result of tectonic movements associated with a volcano N of Jeløya. The same type of deformation of the uppermost part of the Ringerike sandstone can, however, be followed all along the sandstonedebris ftow contact. The extent of this deformation, together with structural analysis of the tectonized fragments, indicate large-scale displacements of the sandstone from NE in the northern part, and from ESE in the central part of the island. The spectacular ftow structures in the matrix of the breccia show that the movement must have taken place in a wet environment. The often strongly deform ed sandstone fragments indicate that at !east some of the sandstone was unconsolidated when the displacement started. However, undisturbed beds of tuff with volcanic bombs with no sign of erosion are also found within the breccia. On the basis of these observations we find it most likely that the breccia represents a series of lower Permian landslides with mud ftows deposited in the eastern part of Vestfold graben in connection with movements along the major fault along Mosse-sundet east of Jeløya, during the initial phases of the volcanic activity. The undulating upper contact of the breccia is believed to be due partly to loading, partly to paleotopography.

Volcanic rocks The volcanic rocks on top of the brecciated sandstone are dominated by tuffs and volcanic breccias with a few basalt ftows. Large load east structures in the sandstone are accompanied by sandstone 'dikes' with turbulent ftow structures in the overlying tuffs and agglomerates. These show that the unconsolidated surface of the debris ftows was deformed during or after deposition of the volcanic rocks.

The lava ftows, which are most prevalent in the central and southern parts of Jeløya, are, together with the tuffs and volcanic breccias in the north, assigned to the lower basalt series (B1) because of their position directly above sedimentary rocks dominated by Downtonian sandstone (Brøgger 1931).

NORSK GEOLOGISK TIDSSKRIFT 68 (1988) Volcanic rocks, Jeløya 293

PROFILE l

N s

PROFILE 11

NNW SSE

E Ti-baaalt � Trachybasalt nmnn lgnimbrite � Augite-p4agioclase basalt � Plagioclase basalt

� AVfyrte basalt � Agglomerate/volcanic breccia � Glacial deposits

PROFILE llla NNW SSE

� Augite-plagkK:Iase basalt m Plagioclase basalt D Aphyric basalt

� Agglomefato/IIO!canic broccia � Rhomb porphyry lava llilliT] Felsite

Fig. 2. Sample profiles I to V in southwestern and central Jeløya. For exact locations see Fig. lb. Sample locations are marked by sample numbers. Numbers along the vertical axes indicate elevation above sea level.


NNW

'" "'"

PROFILE lllb

PROFILE IV

�ti]] Ti-basalt � Trachybasalt - lgnimbrite

� Augite-plagioclase basalt � Plagioclase basalt

� Aphyric basalt � Agglomerate Jvolcanic breccia

Fig. 2 (contd.).


SSE

1100


PROFILE V NW

1111 Agglomeratef\IOicanlc breccla � Augite-plagloclase basaH

Fig. 2 (contd.).

Pyroclastic rocks. - Pyroclastic rocks predominate on northern Jeløya. Together with subordinate amounts of augite- and feldsparphyric lava fl.ows, they have an estimated thickness of 800-1000 m. The lower part of the sequence seems to consist mostly of intercalating lava fl.ows. The upper part comprises a diversity of chaotic agglomerates, volcanic breccias, stratified to unstratified lapilli tuffs, agglomerates with rotated bombs and volcanic breccias with angular fragments up to l m in diameter. These pyroclastic deposits, which dominate the sequence east of the fault which runs N-S along Jeløya, are interpreted as mainly the result of explosive eruptions. In the northernmost part of the island, the pyroclastic rocks are cut by numerous branching, irregular felsitic intrusions which often show autobrecciation due to rapid cooling. The large volumes of pyroclastic rocks on northern Jeløya and the large size of the tephra (both decreasing southward), together with the presence of irregular sills, suggest close proximity to a volcanic vent.

Basa/tie lavas. - The basaltic lava series in the central and southern part of Jeløya consists of 1-15 m thick fl.ows which often show ropy pahoehoe surfaces and pahoehoe toe structure. The lavas are usually rich in amygdules, especially in the upper part of each fl.ow. The amygdules generally

SE

� Plagloclase baaalt lililiiJ Felsite

consist of calcite, but epidote, chlorite and zeolites also occur. The fl.ows are intercalated with agglomerates and volcanic breccias up to lO m in thickness. The latter often display a sandy, red matrix. A 2-3 m thick trachytic ignimbrite overlain by plagioclase porphyry la vas can be followed almost to the east coast as an excellent marker horizon in the B1 stratigraphy of Jeløya (see profiles in Fig. 2). A correlation of lava fl.ows from one fault block to the next is otherwise difficult. Many of the lavas are separated by thin layers of sandy siltstone. The scarcity of sediments in the lava sequence, and the lack of erosional features suggest that the lava fl.ows have been deposited within a relatively short time span. The total thickness of the B1 lava series on Jeløya is estimated from constructed profiles (Fig. 2) to be 800-1500 m.

Rhomb-porphyry lavas. - In the southern part of Jeløya the B1 basalt series is overlain by several fl.ows of rhomb-porphyry (RP) lava, 1 -25 m thick (Fig. 1 }. Also these are separated by thick units of volcanic breccias and agglomerates of similar composition as the rhomb-porphyry lavas (E. Schou-Jensen, unpublished data). The agglomerates show cowdung bombs, up to 30 cm in diameter. The total thickness of the RP units is about 850 m. The RP fl.ows in the lower half of


the sequence are rich in feldspar phenocrysts and resemble RP1 at Krokskogen (Oftedahl 1952). RP flows in the upper half of the sequence have scattered phenocrysts and resemble RP2 at Krokskogen. However, the suggested correlation between RP Javas on Jeløya and Krokskogen has not yet been tested by geochemistry.

Gabbroic rocks ( former! y called 'Oslo essexite'). - A gabbro occurrence on the island of Bevøya, north of Jeløya, intrudes sandstone breccia and was interpreted by Brøgger (1931) as a small parasitic volcanco to 'der Grosse Hurumvulkan' and the source of at !east some of the B1 Javas on Bevøya and Jeløya. A more recent view is that the gabbro intrusions in the Oslo graben represent a younger stage of the Oslo rift volcanism (Dons 1952; Oftedahl 1952, 1953; Ramberg & Larsen 1978; Neumann et al. 1985).

Petrography The basaltic Javas at Jeløya may be divided into groups which are readily recognized in the field:

• aphyric basalt • augite (cpx) basalt • augite-plagioclase ( cpx-plag) basalt • plagioclase (plag) basalt • trachybasalt • ignimbrite.

The alternation between these rock types in different parts of the island is shown in Fig. 2. The last two rock types make good marker horizons, while the first four types alternate with each other in the different profiles.

The aphyric lava flows (A in Table l) consist of a microcrystalline groundmass (grain size :S 0.4 mm) consisting of plagioclase, clinopyroxene, Fe-Ti oxides, pseudomorphs after olivine, and biotite, with scattered microphenocrysts (1-2 mm long) of plagioclase and clinopyroxene. The plagioclase is often preferentially oriented in a flow texture.

The cpx basalt is porphyritic to glomerophyric, with abundant phenocrysts of zoned augite (1-9 mm in diameter), and less abundantly, pseudomorphs after olivine (0. 5-1 0 mm in diameter). Phenocrysts of plagioclase (1 -2 mm) and titanomagnetite ( <3 mm) are generally also present in minor amounts, but are abundant in sample ESJ-51. The augite phenocrysts often contain


inclusions of olivine and devitrified glass, and are frequently arranged in clusters. In the TiOz-rich basalt (TIB in Table l) (see below), augite phenocrysts are relatively equidimensional, whereas in the TiOz-poorer basalt they are prismatic. The groundmass consists of plagioclase, pyroxene and altered olivine, with minor amounts of biotite, Fe-Ti oxides, copper, devitrified glass, and secondary calcite and sphene.

The cpx-plag basalt is characterized by abundant euhedral to subhedral phenocrysts of tabular, sericitized plagioclase (1-20 mm long), augite (1-7 mm in diameter), olivine pseudomorphs and titanomagnetite (s3 mm in diameter). The plagioclase phenocrysts are partly subparallel to one another, partly intergrown in 'starshaped' clusters. The augite phenocrysts show concentric or hourglass zoning and also often form clusters. The dense, microcrystalline groundmass consists of plagioclase, clinopyroxene, altered olivine and Fe-Ti oxides.

The plag basalt at Jeløya may be divided into two subgroups, namely macroporphyritic and microporphyritic plag basalt. The macroporphyritic plag basalt (Fig. 3a) is porphyritic to glomerophyric with two generations of plagioclase phenocrysts. The older generation is represented by abundant, sericitized plagioclase grains, 5-25 mm long, often showing undulatory extinction, generally arranged in starshaped clusters. The younger generation consists of 1 -3 mm long microphenocrysts. Scattered phenocrysts (s3 mm) of augite, pseudomorphs after olivine, and titanomagnetite are also present. The groundmass consists of plagioclase, clinopyroxene, pseudomorphs after olivine, Fe-Ti oxides, apatite, biotite and some devitrified glass. The microporphyritic plag basalt has abundant phenocrysts of tabular, highly sericitized plagioclase (1-4 mm long) arranged in a flow texture, and pseudomorphs after olivine. Scattered clusters of clinopyroxene phenocrysts (1-2 mm in diameter) are also found. The dense groundmass consists of plagioclase, pyroxene abundant Fe-Ti oxides, biotite, devitrified glass, and secondary sphene and haematite.

The trachybasalt (TR in Table l) is characterized by abundant tabular phenocrysts of plagioclase, up to 50 mm long. The plagioclase is highly sericitized and often shows oscillatory zoning. Scattered phenocrysts of titanomagnetite, and microphenocrysts of augite, plagioclase, apatite and somewhat altered olivine are also present.


Fig. 3. Photomicrographs of different basalt types exposed at Jeløya: (a) macrophyric p1ag basalt (sample 170) with phenocrysts of plagioclase, pseudomorphs af ter olivine and Fe-Ti oxides (plain light); (b) ignimbrite (sample 108) (crossed nicols). The black line is 5 mm long.

The fine-grained groundmass consists of clinopyroxene, feldspar, Fe-Ti oxides, apatite. Secondary phases include biotite, chlorite, carbonites, scattered sulphides, Fe-Ti oxides and sphene.

The ignimbrite (Fig. 3b) (IGN in Table l) found in the basalt sequence at Jeløya and Gullholmen consists of rock fragments (1-20 mm lang), flattened pumice fragments, and crystal fragments of feldspar, augite, magnetite, quartz and same biotite (less than l mm) in a dense matrix. Chlorite, oxides and quartz in a spherulitic texture, and circularly arranged oxide dust, which is probably a ghost feature after perlitic cracks, indicate that the matrix represents devitrified glass. Subparallel, flattened pumice fragments give a flow texture.

Compositional relations

Analytical procedure The concentrations of major elements and Sr have been determined for 128 samples of Jeløya basic volcanic rocks at the chemical laboratory of the Geological Survey of Greenland. In addition, the concentrations of Cu, Ni and Zn have been determined by atomic absorption on selected samples. Sampling profiles and stratigraphic positions of the analyzed samples are indicated in Figs. l and 2. Compositional data are listed in Table l. Norm calculations are based on the ratio Fe3+ / Fe2+ + Fe3+ = 0. 18. Clinopyroxene and plagioclase phenocrysts were analyzed on a Cameca

Table

l. C

onc

entr

atio

ns o

f m

ajo

r el

emen

ts (

in w

t.%

), s

elec

ted

trac

e el

emen

ts (

in p

pm

), s

ize

of

ph

eno

crys

ts (

in m

m)

and

no

rmat

ive

(CIP

W)

qu

artz

an

d n

eph

elin

e in

Jel

øya

bas

alti

c ro

cks.

Su

bseq

uen

t sa

mp

les

wit

h t

he

sam

e la

st t

wo

dig

its

are

fro

m t

he s

ame

lava

flo

w.

Par

enth

eses

in

dica

te v

ery

low

co

ncen

trat

ions

of

ph

eno

crys

ts.

Ch

arac

teri

stic

lav

as a

re m

ark

ed:

TIB

= T

i-b

asal

t, T

R =

tr

ach

ybas

alt,

IG

N =

ig

nim

brit

e, A

= ap

hyri

c.

Si0

2 T

i02

Al2

03

Fe2

03

FeO

M

nO

MgO

C

aO

Na2

0

K20

Profi

le I

ES

J-10

1 E

SJ-

002

ES

J-10

2 E

SJ-

004

ES

J-00

5 E

SJ-

006

ES

J-106

E

SJ-

008

ES

J-10

8 E

SJ-

111

ES

J�12

E

SJ�

13

ES

J-11

4 E

SJ-

115

ES

J-11

6 E

SJ�

l7

ES

J-11

8 E

SJ-

119

ES

J�20

E

SJ�

l E

SJ-

122

ES

J�3

ES

J-12

4 E

SJ-

125

ES

J-12

6 E

SJ-

129

ES

J�30

E

SJ�

31

ES

J-13

2 E

SJ-

133

Profi

le Il

ES

J-15

0 E

SJ�

51

ES

J�52

E

SJ-

153

ES

J-15

4

49.3

4 50

.06

50.2

4 50

.65

49.8

1 50

.20

50.2

2 62

.31

62.5

1 48

.60

48.6

3 49

.52

47.8

8 46

.22

49.7

5 49

.42

50.9

9 47

.98

47.9

3 47

.13

46.7

6 47

.53

47.2

1 50

.39

50.00

47

.84

48.1

7 47

.53

47.8

4 50

.36

43.1

3 42

.00

49.9

5 49

.64

57.9

9

3.17

15

.52

3.15

15

.61

3.16

15

.40

3.09

15

.74

3.09

15

.72

3.13

15

.81

3.19

15

.34

0.92

17

.87

0.93

17

.77

3.41

12

.34

3.05

10

.33

3.39

12

.90

3.52

12

.99

3.12

10

.48

3.14

12

.20

3.06

12

.01

2.87

11

.02

3.07

11

.43

3.06

u

.n

3.64

13

.!!4

3.77

14

.05

3.69

13

.99

3.83

14

.03

3.36

14

.20

3.37

14

.40

2.69

11

.62

2.59

11

.32

2.60

11

.07

2.58

11

.06

3.31

14

.50

8.16

1.

74

0.16

6.

43

3.31

0.

16

5.92

4.

04

0.21

6.

97

2.71

0.

15

6.45

3.

11

0.22

5.

20

3.84

0.

22

6.24

3.

46

0.21

3.

75

0.24

0.

10

3.86

0.

14

0.10

7.

80

5.54

0.

20

7.21

5.

69

0.18

6.

62

5.62

0.

00

6.88

5.

50

0.21

9.

04

4.77

0.

21

6.90

5.

06

0.20

6.

04

5.81

0.

17

7.98

4.

05

0.12

7.

25

. 5.0

3 0.

18

7.67

4.

78

0.16

5.

97

6.57

0.

22

6.50

6.

52

0.20

6.

05

6.48

0.

12

6.23

6.

48

0.23

5.64

5.

68

5.31

6.

33

6.23

6.

34

5.16

5.68

0.

17

5.86

0.

17

6.76

0.

21

5.69

0.

18

5.96

0.

19

5.63

0.

18

6.59

0.

18

4.82

8.

65

9.89

4.

37

0.12

5.

05

9.04

9.

19

3.05

15

.84

7.36

3.

25

15.9

2 8.

29

1.08

18

.22

3.04

5.05

0.

17

2.35

0.

18

1.65

0.

21

1.28

0.

16

3.00

6.

65

2.95

6.

39

3.00

6.

85

2.87

5.

90

3.30

6.

19

2.99

6.

67

2.88

6.

75

0.50

1.

15

0.48

1.

21

5.79

8.

71

7.30

11

.70

4.%

9.

80

5.79

8.

94

7.72

11

.21

6.58

9.

92

7.87

9.

37

7.39

9.

36

8.54

9.

07

8.01

8.

67

6.33

7.

17

5.19

8.

49

5.49

7.

45

5.02

7.

98

3.87

6.

94

3.76

7.

32

7.62

11

.14

7.69

12

.66

7.94

12

.28

8.09

12

.26

3.84

7.

62

8.75

12

.37

9.27

11

.79

3.29

4.

65

3.45

4.

65

1.81

1.

15

2.95

2.

86

3.45

3.

24

2.82

2.

94

3.19

7.

91

9.15

2.

75

2.24

2.

91

2.92

2.

07

3.00

2.

87

2.58

2.

68

2.55

2.

68

2.92

2.

46

2.81

3.

14

2.99

2.

48

2.42

2.

35

2.20

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76

1.65

1.

30

3.26

3.

67

3.42

5.20

5.

40

4.66

5.

18

5.15

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24

4.90

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2.37

2.

60

1.60

2.

18

2.33

2.

23

1.50

1.

50

1.49

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17

2.56

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12

2.34

3.

62

2.93

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26

3.05

1.

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1.24

1.

56

1.62

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86

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M

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98.9

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83

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1.84

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88

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10

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98.9

1 1.

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98.5

2

Sr 1711

15

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1559

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cp

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28

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3.28

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92

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76

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68

1.75

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50

1.36

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28

1.05

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13

3.28

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91

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4.77

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1.59

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52

6.52

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99

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44

3.56

3.

65

2.60

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96

3.84

2.

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0.

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0.20

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20

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19

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24

0.24

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24

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15

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15

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4.55

6.

60

4.19

5.

57

6.50

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13

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45

7.48

10

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5.13

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43

3.01

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3.71

6.

25

3.19

5.

66

7.03

7.

87

6.67

8.

26

3.94

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91

3.34

5.

86

5.22

11

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3.69

8.

23

4.17

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22

2.47

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74

4.60

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99

3.50

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32

4.25

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30

3.98

4.

56

6.74

8.

79

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17

3.92

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82

3.78

8.

69

3.96

8.

49

3.69

8.

30

4.26

6.

92

3.98

6.

75

5.62

8.

91

6.61

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12

7.49

8.

85

7.06

8.

03

6.97

8.

42

8.70

11

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7.64

8.

57

7.49

9.

34

7.67

9.

47

8.47

8.

81

3.58

5.

20

4.58

4.

77

2.19

2.

62

2.33

2.

55

4.99

2.90

4.

55

2.82

2.

71

4.06

6.

19

2.73

2.

73

3.69

5.46

4.

21

5.06

4.

54

4.24

3.

47

3.26

3.

89

4.14

3.

94

4.57

4.

93

4.28

2.

84

2.61

1.

95

2.22

3.

41

1.73

1.

85

2.71

2.

03

1.63

4.

63

1.62

2.

07

2.09

2.

28

2.65

3.

48

2.75

4.42

2.

82

2.73

3.

07

2.24

0.

81

2.00

2.

08

2.79

4.20

3.

39

3.35

3.

80

3.71

1.

75

2.5

2 1.

80

1.76

1.

56

1.32

1.

62

2.52

1.

40

2.04

2.

76

2.86

1.

81

1.85

3.

15

2.27

3.

08

3.50

3.

63

0.41

3.

24

99.7

2 0.

45

2.81

99

.62

0.44

2.

79

100.0

3 0.

44

2.01

99

.81

0.41

2.

12

100.4

2 0.

51

2.62

99

.87

0.48

2.

55

99.8

7

0.50

1.

90

99.5

4 0.

45

3.86

99

.82

0.42

2.

98

99.7

2 0.

45

2.55

100

.21

0.42

6.

87

100.0

1 0.

43

4.83

99

.91

0.40

11

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99.9

2 0.

31

8.81

99

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0.34

5.

43

100.1

6

0.53

0.

52

0.48

0.

50

0.51

0.

44

0.56

0.

55

0.66

0.

56

0.53

0.

47

0.40

0.

49

0.43

0.

41

0.43

0.

43

0.30

0.

46

0.35

0.

38

0.37

0.

44

1.91

99

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3.57

99

.76

1.84

99

.61

1.85

99

.27

3.18

99

.35

2.78

100

.18

2.13

99

.89

2.40

100

.22

2.38

100

.32

2.23

99

.18

2.46

99

.14

3.12

100

.09

2.79

99

.60

2.02

99

.19

2.10

100

.39

2.38

100

.41

2.14

99

.84

2.02

99

.70

1.77

100

.08

2.56

99

.55

1.99

99

.83

2.05

99

.89

2.89

99

.97

1.73

99

.81

1470

15

84

458

494

462

562

859

1524

53

1 81

0 10

66

461

654

371

258

308

732

883

842

852

707

1218

11

96

1002

964

1043

12

04

859

596

724

454

700

496

845

711

851

603

806

23

23

13

12

20

25

33

29

16

19

99

17

19 7 24

20

35

33

23

248

<l 2 2 4 3

<l 2 2 2 2 2

<l 2 2 2 2 2 2 5 3

176

161 23

5

1-4

1-4

1-10

:s

l

1-8

1-8

1-3

1-3

1-3

18

(1)

23

17

15

:sl

91

1-8

61

1-2

114

1-2

115

:sl

104

:sl

143

:Sl

14

1-2

90

1-2

112

1-5

146

1-8

157

1-6

1-6

1-6

1-10

1-

10

1-2

1-2

(1-5)

2

2-15

2-

15

1-5

1-5

1-5

:s5

5--15

5-

-15

1-5

5--15

1-

5 1-

5 1-

5

1-6

1-5

1-6

5--10

5-

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5--15

5-

-15

5--15

5-

-30

5--30

5-

-30

1-15

1-

5 1-

5 1-

5

3.1

3.3

2.8

2.0

2.5

3.5

0.4

2.0

A

0.6

4.1

4.7

14.2

IGN

2.4

A 1.

8 A

1.5

4.9

2.7

4.0

3.5

2.0

4.7

8.3

4.3

1.9

2.0

0.4

0.8

2.5

0.6

l. O

1.

3 0.

5 A

0.9

A

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�

Table

l (co

ntd.)

.

Si0

2 T

i02

Al20

3 F

eP3

FeO

M

nO

M

gO

CaO

N

a20

K

20

P20

, H

20

SU

M

Sr

ES

J-08

3 E

SJ-

183

ES

J-18

4 E

SJ-

185

ES

J-18

6 E

SJ-

187

ES

J-18

8 E

SJ-

089

ES

J-19

0 E

SJ-

191

ES

J-09

2 E

SJ-

192

ES

J-09

3 E

SJ-

194

ES

J-19

5 E

SJ-

096

ES

J-09

7 E

SJ-

098

Profi

le IV

E

SJ-

501

ES

J-60

1 E

SJ-

701

ES

J-31

4 E

SJ-

208

ES

J-20

9 E

SJ-

212

ES

J-21

3 E

SJ-

215

ES

J-21

6 E

SJ-

316

ES

J-21

7 E

SJ-

317

ES

J-31

8 E

SJ-

219

ES

J-22

0 E

SJ-

221

ES

J-32

2 E

SJ-

223

ES

J-22

4

45.8

3 46

.35

46.6

9 46

.78

47.7

2 48

.03

45.7

4 48

.64

48.1

1 44

.64

46.4

1 46

.11

47.3

2 44

.97

48.9

3 45

.56

45.7

2 45

.67

45.4

9 43

.18

44.2

3 47

.87

55.5

2 58

.84

48.4

9 47

.97

48.9

4 48

.40

49.9

3 48

.97

48.3

4 48

.66

48.4

3 48

.86

48.3

1 49

.46

47.5

5 49.

73

3.35

13

.34

11.1

1 3.

42

13.2

2 10

.69

2.92

13

.10

10.5

6 2.

61

15.7

8 10

.36

2.79

14

.52

11.2

4 2.

98

13.0

1 11

.66

3.03

14

.20

7.36

2.

84

15.1

3 8.

30

2.75

15

.00

11.7

3 3.

63

12.3

5 8.

77

2.82

12

.26

10.6

2 2.

85

12.2

5 10

.99

2.69

16

.06

9.03

2.

79

14.6

7 10

.26

2.

89

16.6

0 11

.90

3.1

7 14

.07

10.8

9 3

.01

11.4

9 10

.84

3.17

11

.41

12.0

2

4.60

10

.71

4.70

11

.03

4.60

10

.81

3.1

2 15

.97

1.40

18

.35

1.06

18

.36

2.92

12

:24

3.07

15

.35

3.14

15

.50

2.99

11

.98

2.94

11

.79

3.15

14

.34

3.2

0 14

.40

3.4

4 14

.48

3.45

14

.22

2.94

11

.68

3.4

0 13

.62

3.53

14

.20

3.14

13

.15

3.0

7 16

.29

7.58

7.

68

8.19

7.

99

1.8

4 2.

97

6.51

6.

87

5.59

5.

56

7.88

7

.99

6.8

9 6.

39

6.0

9 7.

75

6.65

5.

31

5.08

5.

50

2.8

4 0.

17

3.3

1 0.

21

2.36

0.

15

1.38

0.

14

1.17

0.

12

1.54

0.

16

5.40

0.

18

3.65

0.

18

0.62

0.

17

5.72

0.

24

2.50

0.

18

2.32

0.

20

2.72

0.

14

2.55

0.

16

0.43

0.

16

2.83

0.

16

3.56

0.

26

2.90

0.

18

4.37

4.

14

4.51

1.

96

2.81

1.

13

4.48

4.

27

5.26

5.

10

4.05

3.

74

4.92

5.

47

5.90

4.

58

5.51

5.

47

6.35

4.

16

0.17

0.

18

0.18

0.

14

0.16

0.

11

0.19

0.

18

0.13

0.

16

0.14

0.

12

0.12

0.

13

0.14

0.

14

0.20

0.

16

0.15

0.

12

6.44

8.

45

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NORSK GEOLOGISK TIDSSKRIFf 68 (1988)

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Camebax microprobe at the Mineralogisk-geologisk museum, Oslo. Representative analyses are presented in Table 2.

Compositional character Most of the Jeløya basaltic rocks are olivinehypersthene normative or have small amounts of nepheline (:s l4%) in the norm, a few samples have normative quartz (Table 1). The majority of the rocks thus classify as olivine tholeiites or alkali basalts. The basaltic ftows with very large plagioclase phenocrysts found near the base of Profiles I, Il and IV (TR in Table l) are also very rich in alkalies (NazO + K20 � 8. 0 wt.%) and fall within Middlemost's (1980) classification of trachybasalts; the ignimbrite(s) (IGN in Table l) classifies as trachyte.

The K20/Na20 ratios of the analyzed Javas vary strongly (0. 13--2. 1). Variations in Na20 and K20 contents are also observed within single flows (Table l) (0-28% deviation from the mean). These observations suggest some mobilization of alkalies by seritization of feldspar and other postmagmatic processes. Also Cu shows an irregular behavior. Variable Cu, including very high values (13--376 ppm) found in the trachybasalts in Profile I (samples ESJ-101 to 106) (Table 1), suggests postmagmatic mobilization of Cu. However, because of the systematic compositional variations with respect to most elements found to exist among the basalts, we do not believe that chemical modification through secondary processes is a serious problem.

All the Jeløya basaltic rocks have an evolved character with MgO :s 9. 4 wt. %, Mg/(Mg + Fe1) ratios (mg) in the range 0.58-{).32, and Ni contents below 29 ppm (Table 1). Similarly low mg has been observed among basalts and gabbros elsewhere in the Oslo Region (Larsen 1978; Segalstad 1979; Neumann 1980; Neumann et al. 1985; Øverli 1985). The evolved character reflects a general tendency for considerable crystallization (or other postmelting processes) to have taken place before extrusion.

The mildly alkaline to olivine tholeiitic nature of the Jeløya basaltic rocks is similar to that of the B1 basalts in eastern Vestfold. The Jeløya basalts thus confirm the general pattern of decreasing alkalinity among B1 basalts from south to north in the Oslo Region.

302 E. Schou-Jensen & E.-R. Neumann NORSK GEOLOGISK TIDSSKRIFr 68 (1988)

Table 2. Representative analyses of clinopyroxene and plagioclase phenocrysts. C= core, R =rim.

ESJ-101 (TR) ESJ-111 ESJ-51 (TIB) ESJ-66 ESJ-262

cpx p lag cpx c R c R c

Si02 50.46 48.79 56.33 52.01 49.69 52.68

Ti02 1.98 2.70 0.19 0.73 1.79 0.97

Al203 3.31 4.80 27.48 2.47 3.54 1.27

Fe O, 7.97 8.41 0.41 5.84 8.07 4.34

Mn O 0.23 0.31 0.00 0.11 0.13 0.06

Mg O 13.58 12.70 0.03 16.18 14.40 17.28

Ca O 21.19 20.84 9.28 21.29 21.42 22.74

Na20 0.63 0.64 5.49 0.35 0.38 0.30

K20 n.d. n.d. 1.00 n.d. n.d. n.d. Cr203 0.03 0.00 n.d. 0.12 0.07 0.29 SUM 99.38 99.19 100.21 99.12 99.48 99.92

Stratigraphic and geographic variations Each profile shows considerable compositional variations, including a range in MgO concentrations (Table l, Figs. 4, 5). It is therefore difficult to correlate the individual lava fl.ows on a chemical basis. However, three groups of lavas are readily distinguished from the others on the basis of their modal and chemical compositions. Two of these are exposed at or near the base of Profiles l, Il and IV, that is, the trachybasalts with large plagioclase phenocrysts (TR), and the overlying ignimbrite (IGN). In Profiles Il and IV Ti02-rich cpx basalts (TIB) are exposed below the trachybasalts. TIB continue below sea leve!; the total thickness is not known.

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P20s

Fig. 4. Ti0,-P205 relations among the Jeløya basaltic rocks. Samples from Profile I are represented by large dots, samples from the other profiles by small dots. TIB = TiO,-rich basalts, TR = trachybasalts, IGN = ignimbrites, cpx1 =representative core of clinopyroxene in lavas from Profile l, cpx2 = core of cpx from TiO,-rich basalt. Dashed lines indicate crystallization trends. See text for further explanation.

cpx p lag cpx p lag R c R c R

49.09 52.44 49.98 53.80 52.80 49.56 54.92 2.34 0.66 1.75 0.18 0.68 1.75 0.11 3.21 2.65 3.93 28.84 2.12 3.63 27.87 7.01 5.38 7.90 0.47 7.07 9.47 0.56 0.18 0.17 0.21 0.02 0.18 0.22 0.04

15.04 16.78 14.01 0.04 17.16 14.31 0.13 21.96 20.74 21.67 11.34 19.60 20.32 9.96 0.49 0.53 0.46 5.03 0.42 0.42 4.99

n.d. n.d. n.d. 0.51 n.d. n.d. 0.52 0.05 0.41 O.Ql 0.06 0.24 0.01 n.d.

99.37 99.76 99.92 100.27 100.27 99.69 99.15

The TIB have 4.6-5.1 wt.% Ti02 and are the most MgO-rich basalts in Jeløya. No other analyzed Jeløya rock has above 4. 0% Ti02• The TIB are also richer in P205, but have lower contents of alkalies than other basalts with similar MgO contents (Table l, Fig. 4).

Their large plagioclase phenocrysts and high

16 14 12

12 10

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.

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Fig. 5. Compositional variations within each of the Jeløya sample profiles. +: Ti02-rich basalts (TIB), •: trachybasalts (TR), j.: ignimbrite, e: all other sampl�s.

Profile V Profilelll

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P205 contents (�1. 0%) set the trachybasalts in Profiles l, Il and IV apart from other Jeløya Javas (Table l, Fig. 4). They are also characterized by high K20 and Sr contents (above 4.8%, and up to 1880 ppm, respectively).

The ignimbrite overlying these trachybasalts has Si02:56-62 and MgO < 2%, and is very rich in alkalies. This unit has the same stratigraphic position in Profiles l, Il and IV, and is regarded as the same unit. The observed compositional differences (particularly with respect to Si02, Na20 and K20 contents) between different samples are regarded as being partly due to the presence of foreign material, partly the result of secondary processes. A trachytic ignimbrite is also found at the base of Profile Illb. As we have no control of the stratigraphic position of this ignimbrite, its correlation with that in Profiles l, Il, and IV is questionable.

The rest of the samples (including all samples in Profile V), referred to below as 'other samples', are not readily separated in to distinct units. However, an attempt to divide the Javas in to groups on the basis of their crystallization histories is presented below.

Each profile shows trends of variation of the major elements with decreasing MgO among samples other than TIB, TR and IGN (Fig. 5). In Profile Ill these trends are poorly defined; in the other profiles they are relatively well defined. Among the 'other samples' in Profiles l, Il and IV (Fig. 5), Al203 increases, and CaO decreases with decreasing MgO, whereas Fe01 and Ti02 increase in the range 9 to 5% MgO, and decrease in the range 5 to 2% MgO; P205 shows a maximum at about 4% MgO. These trends may be explained as the res ult of crystallization of olivine + clinopyroxene, followed by Fe-Ti oxide and finally apatite at 5 and 4% MgO, respectively.

This crystallization model has been tested grapl;tically for Profile I in Fig. 6, which includes mineral data from Table 2. Each stage of crystallization is represented by a liquid line of descent ( dashed), backtracked towards the area of 'crystal extract'. The method has been explained in detail by Cox et al. (1979, pp. 145-175). Fig. 6 confirms that the rocks have been through two stages of crystallization, I and Il. Regression lines for stage I (Fig. 6a, b, c and Fig. 4) are transformed into an equilateral triangle in Fig. 7a. (The relative distances within each triangle are retained during the transformation.) The regression lines seem to converge at the diopside-olivine tieline with a

A

25

20

'"' � 15

10

15

10


o l 10

cao

/oplag

' .... ....... .... .. ....

•-:· ....... .. ..

15

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10

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10 20

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15 CaO

30 Mg O

20

o cpx

20

o l

40

Fig. 6. Two-element variation diagrams for Jeløya Profile I ( excluding the ignimbrite) showing liquid lines of descent (dashed) for a two-stage crystaUization sequence; each line is backtracked towards the area of 'crystal extract'. The 'crystal extract' of Stage I is assumed to Iie within a triangle represented by the cores of phenocryst from relatively mafic samples; as no fresh olivine was found, the composition of olivine is inferred to be Fo85• See text for further explanation.

cpx : o l ratio similar to that representative of the fo -di liquid cotectic at low pressure (<lO kbar) in the forsterite-diopside-anorthite system (Fig. 7b ), as given by Presnall et al. (1978). The second stage of crystallization clearly includes a Ti-Fe rich phase (titanomagnetite).


A a Ti02-CaO b AI203-Ca0 c FeOt-MgO d Ti02-P205


concentrations up to 1880 ppm are observed in la vas with numerous large plagioclase phenocrysts (e.g. TR samples ESJ-1 01 to 1 06, and the samples ESJ-55, 156, 201 and 185). Similarly, whereas the aphyric lavas have Ni contents below 11 ppm, up to 29 ppm Ni is found in some of the rocks with abundant phenocrysts of augite and pseudomorphs after olivine (e.g. samples ESJ-116 to 119, 150 and 151).

Di

The TIB samples clearly fall short of the trends of the 'other samples' and cannot be related to these through fractional crystallization or accumulation (Figs. 4, 5). The marked differences in Ti02 between TIB and 'other' rocks most likely reftect different source rock compositions, or difFo. ferent degrees of partial melting.

8

Fig. 7. The three-phase triangles with backtracked Iiquid lines of descent for crystallization Stage I (dashed) from Fig. 6, transformed into equilateral form (7a). Data from Fig. 4 are also included. The Iiquidus phase relations in the forsteritediopside-anorthite system at l atm and lO kbar (Presnall et al. 1978) are shown for comparison (7b).

This result is at some variance with the fact that plagioclase phenocrysts are observed in some of the rocks with MgO contents above 7% (e.g. samples ESJ-132, 159 and 216) (Table 1). This apparent discrepancy between petrographic observations and petrogenetic modeling suggests that crystallization mainly took place at a depth where the stability field of clinopyroxene was somewhat expmtded at the expense of plagioclase (>5 kbar), whereas the observed phenocryst assemblage formed at shallow depth. A similar two-stage crystallization history is also suggested for Oslo rift gabbros (Neumann et al. 1985).

Some scatter in the data points is interpreted as the result of accumulation. This is supported by the trace element data. The aphyric rocks (marked A in Table l) have Sr contents between 460 and 930 ppm (Tab le l, Fig. 5), whereas Sr

Systematic differences between profiles are exhibited by the elements Fe01 and Ti02. In con trast to Profiles I, Il and IV, the northernmost profile, V, shows very constant Ti02 contents s3% over the whole range of MgO concentrations. This implies that crystallization of Fe-Ti oxides in the samples of this profile started at a MgO concentration above 8%. Early crystallization of magnetite is favored by relatively oxidizing conditions (e.g. Kushiro 1979). The 'other' rocks of Profile V therefore cannot have evolved in the magma chambers which gave rise to 'other samples' in Profiles l, Il and IV.

The lower part of Profile Ill (IIIa) resembles Profile V in showing only Ti02 values below 3% (Table l, Fig. 5). Otherwise, Profile Ill shows a wide scatter in data points. This scatter most likely reftects interfingering of lavas from the two eruption systems represented by 'other samples' in Profiles l, Il, IV and V. A separation into two different trends is also observed when aphyric and near aphyric rocks are projected into the Q-NeFo and Mg0-Fe01-(Na20 + K20) systems (Fig. 8). (Profile V does not contain aphyric or near aphyric rocks.)

The data thus suggest that the basalt sequence at Jeløya includes Javas from at least three eruption centers. The la vas from each eruption system appear partly to be concentrated in specific fault blocks (Profiles I, Il, IV and Profiles Il, V), partly to occupy different stratigraphic positions within a single fault block (high-Ti02 and moderateTi02 basalts in Profile Il).


Ne • Profile l • Profile llla • Profile lllb • Profile IV

NORSK GEOLOGISK TIDSSKRIFf 68 (1988)

FeQt

Weight%

Fig. 8. (}-Ne-Fo and Mg0-Fe0,-(Na20 + K20) relations among aphyric and near aphyric Jeløya basaltic rocks. Dotted lines indicate crystallization trends.

Summary and conclusions Above a number of debris ftows of PermoCarboniferous age (Tanum formation), Jeløya exhibits a thick sequence of basaltic rocks (B1) overlain by rhomb-porphyry lavas. In northem Jeløya, B1 consists mainly of pyroclastic rocks such as agglomerates and volcanic breccias, interfingering with basalt flows. There the estimated thickness is 800-1000 m. In central and southem Jeløya, the B1 series is mainly made up of basaltic lavas with a total thickness of 800-1500 m.

The Jeløya B1 sequence differs markedly from that in eastem Vestfold by its large proportion of pyroclastic rocks and great thickness. The eastem Vestfold sequence is only 170-180 m thick, and does not include any significant pyroclastic unit. Furthermore, within Jeløya, the size of tephra and the relative importance of pyroclastic rocks increase from the central and southem parts, towards the northem part of the island, which also exhibits numerous irregular sills. On the basis of these differences it is concluded that an important eruption center was situated dose to northem Jeløya. A magnetic anomaly just east of the Kongshavn bay (Fig. lb) could represent this 'Jeløya volcano'.

The lava sequence consists of silica-undersaturated to silica-saturated augite- and plagioclase-phyric alkali basalts to trachybasalts. Through their mildly alkaline and evolved nature (mg:0.58--0.32), the Jeløya favas confirm the

pattem of decreasing alkalinity from south to north among the oldest basalts in the Oslo Region. Three units at the base of the lava sequence in the southwestem (Profiles I and Il in Fig. l) and east central (Pro file IV) parts of Jeløya are readily distinguished from the other lavas on the basis of their modal and chemical character. These are Ti02"rich basalts, plagioclase-phyric trachybasalts, and a trachytic ignimbrite.

Many of the observed compositional differences among the Jeløya basaltic lavas may be attributed to fractional crystallization of olivine and clinopyroxene, followed by Fe-Ti oxides, apatite, and finally plagioclase. Also, accumulation has taken place. Furthermore, the data imply that the B1 series at Jeløya cannot be cogenetic, but consists of interfingering lavas from three eruption centers.

Most apparent is the compositional difference between the Tm (Ti02"rich basalts) at the base of Profiles Il and IV and the other rocks. A series of similar Ti02-rich cpx basalts, overlain by aphyric Ti02"rich basalts, are found at the base of the lava section in eastem Vestfold (the HortenHolmestrand area) (Tollefsrud 1987; Øverli 1985). It has been shown that the TIB cannot be cogenetic with the other basalts at Jeløya. Their parent magma must originate in a Ti02"rich source or from a small degree of partial melting. The TIB most likely originate from an eruption center which also gave rise to the numerous flows of Ti02"rich basalts in eastem Vestfold. Available


data are so far inconclusive with respect to the geographic position of this eruption center, but it was probably located west of Jeløya.

A comparison between the moderately Ti02-rich Javas (referred to above as 'other samples') suggests that these rocks too, may be subdivided into two different evolutionary series. One series, represented by Profile V in the northem part of the island, appears to have maintained silicaundersaturation during crystallization, and crystallized under somewhat oxidizing conditions which brought magnetite to the liquidus at a relatively early stage (MgO �8% ) . The Javas of northem and central Jeløya directly overlie pyroclastic rocks believed to be deposited on the ftanks of the 'Jeløya volcano'. It is therefore likely that the Javas of Profile V also represent extrusion products from this volcano.

The other series, mainly found in southwestem and east central Jeløya (Profiles l, Il and IV), defines a trend from mild silica-undersaturation toward silica-oversaturation in the Q-Ne-Fo system and appears to have crystallized under more reducing conditions, suppressing the crystallization of magnetite to a relatively advanced stage of fractionation (MgO = 5% ) . Profile Ill in the south central part of the island appears to contain Javas of the northem type at the base (IIIa), and a mixture of both types in the upper part of the sequence (IIIb). The magmas of both series must thus have existed contemporaneously, and consequently originate from separate eruption centers. The reason why these magma chambers show different trends of evolution should be further investigated.

The debris flows underlying the B1 sequence all over Jeløya are interpreted as the result of landslides caused by movements along the major fault east of Jeløya. The differences in lava stratigraphy between different fault blocks imply that block movements occurred during the period of eruptions. These inferences signify that the oldest tectonomagmatic activity during the rifting event in this area was movements along the major N-S trending fault east of Jeløya, followed by alternating volcanism and movements along NNWSSE and NE-SW trending fault blocks. Due to these movements a topography was maintained that restricted the, areal extent of each lava flow.

Acknowledgements.- We thank Steinar Skjeseth for his enthusiastic encouragement, and for many constructive discussions. We also thank Lotte Melchior Larsen and Asger Ken Pedersen, whose constructive criticism has greatly improved the


manuscript. We are grateful to Grønlands Geologiske Undersøkelse for making the chemical analyses.

Manuscript received September 1987

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