Age and composition of crystalline basement rocks on the Norwegian continental margin: offshore extension and continuity of the Caledonian-Appalachian orogenic …

Journal of the Geological Society

doi: 10.1144/0016-76492010-136 2011; v. 168; p. 1167-1185Journal of the Geological Society

Trond Slagstad, Børre Davidsen and J.Stephen Daly Appalachian orogenic belt

−continental margin: offshore extension and continuity of the CaledonianAge and composition of crystalline basement rocks on the Norwegian

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Journal of the Geological Society, London, Vol. 168, 2011, pp. 1167–1185. doi: 10.1144/0016-76492010-136.

1167

Age and composition of crystalline basement rocks on the Norwegian continental

margin: offshore extension and continuity of the Caledonian–Appalachian

orogenic belt

TROND SLAGSTAD1*, BØRRE DAVIDSEN 1 & J. STEPHEN DALY 2

1Geological Survey of Norway, 7491 Trondheim, Norway2UCD School of Geological Sciences, University College Dublin, Belfield, Dublin 4, Ireland

*Corresponding author (e-mail: [email protected])

Abstract: Twenty-two wells on the Norwegian continental margin have penetrated underlying basement. We

present U–Pb zircon, whole-rock geochemical, and Sm–Nd and Rb–Sr isotopic data from nine wells in the

North Sea and Norwegian Sea with relevance to the offshore continuation of the Norwegian Caledonides, and

their correlation throughout the Caledonian–Appalachian orogenic belt. Palaeozoic magmatism in the North

Sea can be divided into two groups. The older group consists of 460 Ma calc-alkaline granites with evolved

isotopic compositions, correlative with similar rocks in the Uppermost Allochthon. The younger group

consists of a 430 Ma dacite and a 421 Ma leucogabbro, with less evolved isotopic compositions. In the

Norwegian Sea, isotopically evolved granitic magmatism at 437 Ma and more juvenile dioritic magmatism at

447 Ma are correlative with magmatism in the Bindal and Smøla–Hitra Batholiths in the Uppermost

Allochthon. Metasedimentary basement rocks from the North Sea and Norwegian Sea, dominated by Late

Palaeoproterozoic and Mesoproterozoic grains, resemble rocks found in the Caledonides of Scotland,

Greenland and Svalbard. The new data, along with studies elsewhere along the belt, suggest that similar rocks

may exist along much of the orogen.

Supplementary material: U–Pb isotopic data are available at http://www.geolsoc.org.uk/SUP18471.

Offshore drilling for hydrocarbons is technically challenging and

expensive. Oil companies therefore strive to confine their drilling

operations to hydrocarbon-bearing sedimentary rocks overlying

crystalline basement. For this reason, cores of crystalline base-

ment rock from continental margins are infrequent. Nevertheless,

such cores exist from 22 wells on the Norwegian continental

margin (Slagstad et al. 2008), which allow investigations into the

nature of this basement. Here we present U–Pb zircon ages, and

geochemical and isotopic (Sm–Nd and Rb–Sr) data from cores

taken from nine of these wells (Fig. 1) that have particular

relevance to the offshore extent of the Scandinavian Caledonides

and the continuity of the Caledonian–Appalachian orogenic belt.

The Scandinavian Caledonides have an along-strike length of

nearly 2000 km, and form part of a once extensive, but now

fragmented, early Palaeozoic orogenic belt that stretched from

the southeastern USA through Newfoundland, the British Isles,

Scandinavia and Greenland, north to the Barents Sea (Fig. 2).

The orogen formed in response to convergence between several

continents and microcontinents, of which Laurentia, Baltica and

Avalonia were key components (e.g. van Staal et al. 1998). The

Scandinavian Caledonides are generally considered to consist of

four major, allochthonous nappe complexes, referred to as the

Lower, Middle, Upper and Uppermost Allochthons, overlying

Parautochthonous and Autochthonous Baltican rocks (Roberts &

Gee 1985). The allochthons are distinguished based on structural

and stratigraphical relationships, tectonometamorphic and tecto-

nomagmatic history, and lithology. The nappes were emplaced

towards the SE, along generally NW-dipping thrusts, during the

Siluro-Devonian Scandian phase of the Caledonian orogeny (e.g.

Roberts 2003). Locally, in the Uppermost Allochthon, west-

vergent structures are preserved (Roberts et al. 2002b; Yoshinobu

et al. 2002), and are interpreted to represent an Ordovician

Taconian orogenic event on the Laurentian margin of the Iapetus

ocean. Overall, the tectonostratigraphical level increases west-

wards; thus, the samples described here from the Norwegian

continental margin most likely derive from the structurally high-

est nappes, most likely representing the Upper or Uppermost

Allochthon. These allochthons are generally interpreted to derive

from the Iapetan oceanic realm (including microcontinents) or

the continental margin of Laurentia, and much of their pre-

Scandian evolution probably took place outboard of or on the

Laurentian margin (Yoshinobu et al. 2002; Barnes et al. 2007;

Roberts et al. 2007).

In addition to providing information concerning the extent of

onshore geological units onto the continental margin, the data

provide information about the continuity of the Caledonian

orogenic belt between Norway and the British Isles, and further

south into the Appalachian orogenic belt. It should be kept in

mind, however, that there is little control on the extent of the

various units sampled, or their structural relationships to sur-

rounding units. The discussion therefore relies on age, geochem-

ical and isotopic correlation with potential onshore equivalents,

where such information is available.

Analytical methods

Geochemical and Sm–Nd/Rb–Sr isotopic data

Geochemical data were obtained using standard X-ray fluores-

cence (XRF) and laser ablation inductively coupled plasma mass

spectrometry (LA-ICP-MS) techniques at the Geological Survey

of Norway (NGU). XRF analyses were performed on fused

Fig. 1. Simplified geological map of south and central Norway, modified after Solli & Nordgulen (2006), with locations and ages of samples discussed in

the text.

T. SLAGSTAD ET AL .1168

lithium tetraborate discs for the major and minor elements,

whereas the trace elements were determined on pressed powder

pellets. REE and several trace elements (Nb, Hf, Ta, Th, U) were

analysed by LA-ICP-MS following Flem et al. (2005), using the

same glass discs from which XRF major element data were

obtained. Whole-rock major- and trace-element data are pre-

sented in Table 1. Sm–Nd and Rb–Sr isotopic data (Table 2)

were obtained at University College Dublin.

Geochronology

Zircons were separated from nine samples from the North Sea

and Norwegian Sea, using standard techniques including Wilfley

table, heavy liquids, magnetic separation and final hand picking

under a binocular microscope. Zircons from eight samples were

analysed by secondary ion mass spectrometry (SIMS) on the

Cameca IMS 1270 system at the Nordsim laboratory in Stock-

holm, and one sample of metasediment was analysed by LA-

ICP-MS at NGU. Prior to SIMS analysis, the grains were

mounted in epoxy and polished to approximately half grain

thickness. Cathodoluminescence (CL) images were obtained

using a scanning electron microscope. LA-ICP-MS age determi-

nations were performed on whole grains mounted on double

adhesive tape. These grains were ablated twice, first at low

energy removing potential rims, followed by a second ablation

with an energy chosen to achieve optimal counting statistics. The

age obtained from the latter ablation is taken as the age of the

grain. Isoplot 3.00 (Ludwig 2003) was used to calculate and plot

the U–Pb isotopic data from both SIMS and LA-ICP-MS

analyses.

SIMS. The analytical method, data reduction and error propaga-

tion of the results have been outlined by Whitehouse et al.

(1999) and Whitehouse & Kamber (2005). The analyses were

conducted with an O2 beam of c. 2–4 nA and a spot size of 15–

20 �m, and U–Pb ratios were calibrated to the Geostandard

91500 reference zircon with an age of 1065 Ma (Wiedenbeck et

al. 1995). The error on the U/Pb ratio includes propagation of

the error on the day-to-day calibration curve obtained by regular

analysis of the reference zircon. A common-Pb correction was

applied using the measured 204Pb counts and a present-day

isotopic composition (Stacey & Kramers 1975), where significant204Pb counts were recorded. This procedure avoids negative

common Pb corrections where background counts are greater

than low levels of 204Pb.

LA-ICP-MS. The instrumentation used at NGU consists of a

Finnigan MAT Element 1 single collector high-resolution sector

ICP-MS system, in this case supplied by a Finnigan MAT

266 nm (Nd:YAG) UV-laser. During analysis the laser is operated

in Q-mode, at a frequency of 10 Hz. Depending on the sample,

the energy used on samples typically varies from about 0.01 mJ

to 1.0 mJ, tuned to give the optimum counting statistics. To

minimize elemental fractionation, the zircon crystals are ablated

in a 80 �m 3 60 �m raster. The sample aerosol is transported

from the sample chamber in He gas, and introduced in the ICP-

MS instrument as a mixture of He and Ar gas. The data are

acquired in a time-resolved counting scanning mode for 60 s.

Masses 202Hg, 204(Hg + Pb), 206Pb, 207Pb, 208Pb, 232Th and 238U

are measured. Monitoring 202Hg and assuming a 202Hg/204Hg

ratio of 4.36 (natural abundance) corrects the interference of204Hg on 204Pb. A 60 s delay is performed after each zircon

analysis, and a 60 s gas blank is acquired at regular intervals.

The measured isotope ratios are corrected for element- and

mass-bias effects using the Geostandard 91500 reference zircon

(Wiedenbeck et al. 1995), normally based on 15–30 analyses

during one analytical session. The data reduction is performed

using MS Excel spreadsheets with Visual Basic macros devel-

oped in-house.

Geochronological, geochemical and isotopic data

North Sea

16/1-4 (1937 m), leucogabbro. Sample 16/1-4 (1937 m) is a

medium-grained leucogabbro (Fig. 3a), consisting mainly of

strongly saussuritized plagioclase, amphibole and biotite, with

accessory, secondary calcite in thin veins or as small crystal

aggregates. The only evidence of deformation is a slight kinking

observed in many biotite laths. The leucogabbro is calc-alkaline

with a moderately fractionated REE pattern (La/YbN ¼ 26) and

small negative Eu anomaly (Eu/Eu*N ¼ 0.88) (Fig. 4). The

sample displays a strong negative Nb anomaly in the primitive-

mantle normalized diagram, and plots in the volcanic arc field in

the Yb–Ta tectonic classification diagram of Pearce et al. (1984)

(Fig. 4d).

The zircons from sample 16/1-4 vary from stubby to elongate

biprismatic, 100–300 �m, with well-developed crystal faces.

They display internal oscillatory zoning, and in some cases

sector zoning (Fig. 5a), typical of magmatic zircons (see Corfu

et al. 2003). Some grains display a narrow CL-bright rim (0–

5 �m), and occasionally CL-bright patches may occur within the

grains. The zircons are transparent to pink to light brown.

Inclusions and fractures are common.

Nine SIMS analyses from nine grains yield a concordia age of

421 � 3 Ma, interpreted to represent the crystallization age of

the leucogabbroic magma (Fig. 6a). One analysis yields a206Pb/238U age of 374 Ma and is interpreted to have lost radio-

genic Pb. This analysis was excluded from the calculation.

Fig. 2. Map showing the extent of the Caledonian–Appalachian orogenic

belt in the North Atlantic region. Areas within the belt that are referred

to in the text are indicated.

BASEMENT ON THE NORWEGIAN CONTINENTAL MARGIN 1169

Table1.Geochem

icaldata

ofinvestigatedsamples

Wel

l:1

6/1

-41

6/3

-21

6/4

-12

5/7

-1S

16

/5-1

66

09/7

-11

6/6

-16

30

6/1

0-1

63

06/1

0-1

64

07/1

0-3

Dep

th(m

):1

93

72

01

7.7

29

08.6

35

54

.31

92

9.3

19

45.8

20

59.7

31

58.5

31

59.2

29

72.1

Ro

ckty

pe:

Leu

cog

abb

roG

ran

ite

Gra

nit

eQ

tz-r

ich

san

dst

on

eG

ran

ite

Alt

ern

atin

gsi

ltst

one

–sa

nd

sto

ne

Dac

ite

Dio

rite

Dio

rite

Gra

nit

e

Lat

itu

de

(N):

5885

195

5.2

00

5884

791

2.8

00

5883

891

8.3

30

5981

893

5.2

30

5883

895

3.6

60

6682

495

6.4

90

5884

290

6.0

00

638992

6.3

20

638992

6.3

20

648691

1.6

60

Lo

ng

itu

de

(E):

281

795

6.1

20

284

793

4.7

00

28

89

17

.030

28

169

05

.370

282

993

9.6

90

98191

4.9

10

285

494

4.0

00

681

994

1.4

50

681

994

1.4

50

781

891

1.4

30

SiO

24

9.0

57

0.1

57

1.4

67

7.6

66

8.6

67

5.0

26

5.8

35

3.9

75

9.9

97

2.6

7A

l 2O

31

6.0

11

6.0

51

5.0

17

.40

16

.71

11

.11

15

.86

17

.36

17

.04

14

.06

Fe 2

O3

7.3

91

.97

1.6

74

.27

2.6

00

.68

3.9

87

.88

5.7

21

.98

TiO

21

.40

0.2

40

.18

0.2

80

.30

0.6

20

.62

0.8

10

.58

0.2

8M

gO

7.4

40

.84

1.0

10

.19

1.0

70

.28

2.5

24

.71

3.8

50

.34

CaO

5.2

82

.39

1.1

40

.14

2.3

53

.56

0.7

54

.48

3.0

51

.18

Na 2

O3

.29

4.1

74

.08

1.3

83

.41

0.3

24

.59

5.1

64

.62

3.2

7K

2O

3.3

72

.71

3.7

53

.22

3.4

83

.33

2.7

21

.55

1.9

35

.31

Mn

O0

.14

0.0

60

.05

0.0

20

.05

0.0

30

.04

0.1

30

.10

0.0

2P

2O

50

.58

0.1

00

.11

0.0

20

.09

0.0

60

.24

0.2

00

.17

0.0

8L

OI

4.8

41

.12

1.1

82

.56

1.4

34

.76

2.2

63

.13

2.6

30

.59

To

tal

98

.81

99

.80

99

.64

97

.14

10

0.1

69

9.7

79

9.4

09

9.3

79

9.6

89

9.7

9Z

r2

04

16

41

14

26

01

72

31

41

75

71

12

62

08

Y2

51

19

67

28

14

13

12

5S

r9

57

67

14

06

20

65

63

50

52

62

35

71

23

01

Rb

14

18

11

15

96

10

51

13

78

40

55

19

0C

r3

45

,1

0,

10

,1

0,

10

,1

05

3,

10

13

,1

0V

17

92

01

43

52

14

59

31

95

12

81

2B

a1

28

98

16

68

61

33

80

98

04

97

10

47

31

35

34

78

4G

a1

41

1,

10

,1

01

4,

10

12

11

,1

01

4Z

n1

43

42

61

19

49

12

69

10

98

93

3N

i1

21

,5

,5

5,

5,

55

022

22

,5

Co

30

66

,5

,5

51

32

21

7,

5N

b1

3.3

13

.31

3.5

6.3

01

3.0

10

.61

1.5

7.7

56

.57

8.7

6L

a5

4.3

34

.72

1.6

17

.82

8.2

24

.82

4.8

24

.01

7.4

79

.7C

e1

24

59

.63

8.9

39

.35

3.3

49

.24

9.5

45

.73

5.8

19

3P

r1

5.4

6.3

04

.19

4.0

65

.41

5.7

05

.41

4.7

73

.76

15

.2N

d6

7.8

25

.21

7.4

16

.62

0.9

25

.62

2.1

19

.51

6.0

58

.2S

m1

1.8

4.3

73

.57

2.9

13

.53

5.3

64

.12

3.5

73

.04

9.0

5E

u2

.86

1.1

20

.86

0.7

60

.99

1.1

41

.07

1.0

80

.86

1.2

1G

d7

.21

2.8

92

.56

2.2

12

.33

4.7

62

.92

2.7

02

.23

4.4

6T

b1

.00

0.4

60

.43

0.3

00

.33

0.8

90

.46

0.4

40

.39

0.4

2D

y4

.59

2.4

72

.17

1.7

61

.76

5.5

92

.68

2.3

42

.29

1.5

7H

o0

.79

0.5

10

.43

0.3

30

.33

1.2

00

.52

0.5

00

.45

0.2

4E

r1

.95

1.3

81

.00

1.0

30

.86

3.4

71

.47

1.3

31

.20

0.5

1T

m0

.26

0.2

30

.16

0.1

60

.14

0.5

40

.23

0.2

00

.19

0.0

7Y

b1

.50

1.3

60

.95

0.9

70

.93

3.4

91

.34

1.1

71

.08

0.4

2L

u0

.21

0.1

90

.16

0.1

90

.15

0.5

70

.20

0.1

90

.19

0.0

9H

f4

.08

3.8

63

.50

8.5

34

.41

10

.74

.23

1.7

53

.23

6.5

3T

a0

.57

0.7

70

.78

0.5

80

.92

0.7

90

.73

0.6

00

.44

0.3

8T

h2

.83

8.7

07

.99

3.6

51

1.1

7.8

41

0.3

4.6

67

.08

58

.3U

8.3

72

.62

2.2

60

.96

1.0

72

.34

2.7

72

.68

1.9

62

.65

LO

I,lo

sson

ignit

ion.


The leucogabbro yields a Nd depleted mantle model age (TDM)

of 1008 Ma (�Nd ¼ �1.25) and an initial 87Sr/86Sr ratio of 0.707

(Table 2, Fig. 4e).

16/6-1 (2059.7 m), dacite. Sample 16/6-1 (2059.7 m) is a fine-

grained, undeformed, grey volcanic rock with abundant pheno-

crysts of plagioclase (Fig. 3b). The plagioclase phenocrysts are

,1–2 mm and are blocky to angular prismatic, typically with

albite twinning. The phenocrysts are strongly saussuritized, but

many of the larger crystals contain less altered cores. The matrix

consists of fine-grained plagioclase and slightly larger biotite,

with small amounts of quartz. The phenocrysts are unoriented

and the rock does not appear deformed. The sample also contains

a small (c. 4 cm) xenolith of slightly coarser grey rock. The

dacite is calc-alkaline, with a moderately fractionated REE

pattern (La/YbN ¼ 13) and a small, negative Eu anomaly (Eu/

Eu*N ¼ 0.90) (Fig. 4). In the primitive mantle-normalized dia-

gram, the dacite displays a marked negative Nb anomaly and

small, negative P and Ti anomalies, and plots in the volcanic arc

field in tectonic discrimination diagrams (Fig. 4d).

The zircons from sample 16/6-1 are biprismatic, 75–200 �m,

with well-developed, slightly rounded crystal faces. They are

transparent and colourless to pink. Inclusions occur in some

grains. The zircons have a well-developed oscillatory zoning

(Fig. 5b), and a few have brighter cores and/or brighter growth

zones in the core. One crystal (number 5) has a core with an

internal diffuse zoning that is truncated by the surrounding rim.

Ten SIMS analyses of oscillatory-zoned zircons yield a

concordia age of 430 � 6 Ma (Fig. 6b), interpreted to represent

the crystallization age of the porphyritic magma. Two zircon

cores yield 206Pb/238U ages of 491 and 1100 Ma. We interpret

the oscillatory-zoned zircons to have crystallized from the

porphyritic magma or lava at 430 Ma, whereas the cores may be

inherited from the magma source or represent grains entrained in

the magma during ascent and emplacement.

The dacite yields a TDM of 1052 Ma (�Nd ¼ �1.49), and an

initial 87Sr/86Sr ratio of 0.706 (Table 2, Fig. 4e).

16/3-2 (2017.7 m), granite. Sample 16/3-2 (2017.7 m) is a

medium-grained, unfoliated granite (Fig. 3c) consisting of

plagioclase, K-feldspar and quartz, locally with development of

myrmekite. Irregular, small grains of biotite and amphibole are

the only mafic minerals, and the granite also contains some

muscovite and minor epidote. The granite is calc-alkaline, with a

moderately fractionated REE pattern (La/YbN ¼ 18) and nearly

negligible Eu anomaly (Eu/Eu*N ¼ 0.91) (Fig. 4). In the primi-

tive mantle-normalized diagram, the granite displays marked

negative Nb, P and Ti anomalies, and plots in the volcanic arc

field in the tectonic discrimination diagrams of Pearce et al.

(1984).

The zircons from sample 16/3-2 are stubby biprismatic, 100–

300 �m, with well-developed crystal faces. They are pink to

brownish, and may have an iron-oxide stained surface. The

grains locally contain inclusions and fractures. Internally, the

zircons typically contain large, CL-bright, rounded cores with

(locally truncated) oscillatory or irregular zoning, surrounded by

thick, oscillatory-zoned rims (Fig. 5c). Given the high abundance

of cores in the zircons, we consider the cores most likely to be

inherited, whereas the rims represent zircon crystallized from the

magma.

Eight SIMS analyses of oscillatory-zoned rims yield a con-

cordia age of 456 � 7 Ma, interpreted to date the crystallization

age of the granitic magma (Fig. 6c). Fifteen analyses of zircon

cores yield 207Pb/206Pb ages ranging from 1044 to 3605 Ma (Fig.Table2.Sm–NdandRb–Srisotopic

data

Sam

ple

Dep

thR

ock

typ

eA

ge

(Ma)

Sm

Nd

14

7S

m/1

44N

d1

43N

d/1

44N

dt D

M(M

a)� N

dR

b*

Sr*

Rb

/Sr

87R

b/8

6S

r8

7S

r/8

6S

r(8

7S

r/8

6S

r)t

NorthSea

TS

11

6/1

-41

93

7L

euco

gab

bro

42

0.8�

2.9

12

.58

73

.09

0.1

04

00

.512

31

91

00

8�

1.2

51

41

95

70

.14

70

.42

60

.70

92

45

0.7

06

7T

S6

16

/6-1

20

59

.7D

acit

e4

29

.8�

5.5

3.6

62

0.5

70

.10

76

0.5

12

31

11

05

2�

1.4

97

85

26

0.1

48

0.4

29

0.7

08

99

80

.706

4T

S2

16

/3-2

20

17

.7G

ran

ite

45

6.4�

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Fig. 3. Sample photographs. (a) Leucogabbro, well 16/1-4 in the North Sea. (b) Dacite, well 16/6-1 in the North Sea. (c) Granite, well 16/3-2 in the North

Sea. (d) Granite, well 16/4-1 in the North Sea. Inset core photograph from NPD website; the circle indicates cross-cutting relationship between granite and

overlying metasediments. (e) Granite, well 16/5-1 in the North Sea. (f) Metasandstone, well 25/7-1S in the North Sea. (g) Granite, well 6407/10-3 in the

Norwegian Sea. (h) Diorite, well 6306/10-1 in the Norwegian Sea. (i) Metasediment, well 6609/7-1 in the Norwegian Sea.


Fig. 4. Geochemical plots of the investigated samples (metasedimentary rocks omitted). (a) molar Al2O3/(CaO + Na2O + K2O) v. Al2O3/(Na2O + K2O),

after Maniar & Piccoli (1989). (b) AFM diagram, with fields for calc-alkaline and tholeiitic compositions after Irvine & Baragar (1971). (c) Primitive

mantle-normalized diagram, with normalization factors from Sun & McDonough (1989). (d) Yb v. Ta discrimination diagram after Pearce et al. (1984).

(e) �Nd v. initial 87Sr/86Sr ratio for samples presented here, with fields indicating isotopic compositions for major granitoid batholiths in the Norwegian

Caledonides. Data presented in Table 2; references to other data are presented in the text.


6d), interpreted to reflect inheritance from the source or assimi-

lated wall rocks.

The granite yields a Nd depleted mantle model age (TDM) of

1664 Ma (�Nd ¼ �10.67), and an initial 87Sr/86Sr ratio of 0.709

(Table 2, Fig. 4e).

16/4-1 (2908.6 m), granite. Sample 16/4-1 (2908.6 m) is a fine-

grained granite (Fig. 3d), consisting mainly of K-feldspar,

plagioclase and quartz. Biotite is the only mafic mineral and the

rock also contains minor amounts of muscovite. The granite

appears undeformed except for undulose extinction in quartz.

Overlying the granite is a fine-grained sedimentary rock consist-

ing of alternating layers of immature quartzite and dark grey

siltstone. The granite clearly cuts the layering in the overlying

sedimentary rock (Fig. 3c inset). Like the granite, the sedimen-

tary rock appears undeformed except for undulose extinction in

quartz. The granite is calc-alkaline, with a moderately fractio-

nated REE pattern (La/YbN ¼ 16) and small, negative Eu

anomaly (Eu/Eu*N ¼ 0.82) (Fig. 4). In the primitive mantle-

normalized diagram, the granite displays marked negative Nb, P

and Ti anomalies, and plots in the volcanic arc field in the

tectonic discrimination diagrams of Pearce et al. (1984).

The zircons from sample 16/4-1 are rather similar to those

found in sample 16/3-2; they are pink to brown, stubby

biprismatic, 100–300 �m, with well-developed crystal faces, and

typically contain large (often CL-bright) cores surrounded by

Fig. 5. Examples of analysed zircon

crystals. (a) 16/1-4; (b) 16/6-1; (c) 16/3-2;

(d) 16/4-1; (e) 16/5-1; (f) 25/7-1S; (g)

6407/10-3; (h) 6306/10-1; (i) 6609/7-1.


Fig.6.

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thick, oscillatory-zoned rims (Fig. 5d). Some of the cores

themselves have oscillatory zoning that is commonly truncated,

locally by a CL-brighter layer. As above, the rims most probably

represent zircon crystallized from the magma, whereas the cores

could be inherited from the source of the granitic magma or

represent assimilated zircons during ascent and/or emplacement.

Eight SIMS analyses of eight oscillatory-zoned rims yield a

concordia age of 460 � 8 Ma, interpreted to date the crystallization

of the granitic magma (Fig. 6e). Seven analyses of zircon cores

yield 207Pb/206Pb ages ranging from 942 to 2719 Ma (Fig. 6f).

The granite yields a TDM of 1831 Ma (�Nd ¼ �10.58), and an


16/5-1 (1929.3 m), granite. Sample 16/5-1 (1929.3 m) is a fine-

grained, unfoliated, dark red granite (Fig. 3e), which consists

mainly of K-feldspar, plagioclase and quartz. Biotite is the only

mafic mineral and the rock contains minor amounts of muscovite

and accessory epidote. The granite appears undeformed, except

for moderately undulating extinction in quartz. The granite is

calc-alkaline, with a moderately strongly fractionated REE

pattern (La/YbN ¼ 22) and no Eu anomaly (Eu/Eu*N ¼ 1) (Fig.

4). In the primitive mantle-normalized diagram, the granite

displays marked negative Nb, P and Ti anomalies, and plots in

the volcanic arc field in the tectonic discrimination diagrams of

Pearce et al. (1984).

The zircons from sample 16/5-1 are similar to those found in

samples 16/3-2 and 16/4-1, albeit slightly smaller. The zircons

are prismatic, 100–200 �m, with well-developed external crystal

faces, and commonly contain large, rounded CL-bright cores,

surrounded by thick, oscillatory-zoned rims (Fig. 5e). We inter-

pret the cores to be inherited from the source of the granitic

magma or assimilated during ascent and/or emplacement,

whereas the rims most probably represent zircon crystallized

from the magma.

Twelve SIMS analyses of oscillatory-zoned rims yield a

concordia age of 463 � 6 Ma, interpreted to date the crystal-

lization age of the granitic magma (Fig. 6g). Six analyses of

zircon cores yield 207Pb/206Pb ages ranging from 1040 to

1877 Ma (Fig. 6h).

The granite yields a TDM of 1590 Ma (�Nd ¼ �10), and an


25/7-1S (3554.3 m), metasandstone. Sample 25/7-1S (3554.3 m)

is a faintly banded, grey metasandstone (Fig. 3f) consisting of

fine-grained quartz with minor plagioclase and K-feldspar layers

alternating with more feldspar-rich, very fine-grained layers.

Muscovite is abundant as oriented laths, up to a few millimetres

long. Subhedral pyrite is rather common and appears to be

associated with the feldspar-rich layers. The metasandstone is

criss-crossed by numerous calcareous veins.

The zircons from sample 25/7-1S are slightly to moderately

abraded or rounded, with shapes ranging from imperfect bipris-

matic (some are multifaceted), via rounded (abraded) biprismatic

to rounded elongate grains. The crystals are colourless to faint

yellowish, and commonly have fractures or contain inclusions.

The zircons range in size from 90 to 200 �m and are typically

oscillatory-zoned (Fig. 5f), and in some cases also display sector

zoning (see Corfu et al. 2003). The zoning suggests a magmatic

origin for the zircons. The internal zoning is cut by the grain

surface in some cases, demonstrating sedimentary transport and

abrasion. Some grains contain homogeneous zones or patchy,

convoluted domains, possibly owing to recrystallization. A thin

(typically 2–10 �m), CL-bright rim is present in about half the

grains.

Figure 6k shows a cumulative probability plot based on 93

LA–ICP–MS analyses of zircon grains that are ,10% discordant.

The main age group, ranging from 1000 to 1800 Ma, comprises

several populations, at 1000–1150 Ma, c. 1350 Ma and 1600–

1650 Ma, with a subordinate peak at 1500 Ma. In addition there

are a few data points at c. 1900 and c. 2800 Ma.

Norwegian Sea

6407/10-3 (2972.1 m), granite. Sample 6407/10-3 is a fine-

grained granite (Fig. 3g), consisting mainly of K-feldspar,

plagioclase and quartz. Biotite is the only mafic mineral,

occurring as small (,1 mm), dispersed grains. Muscovite is an

accessory phase. The granite appears undeformed except for

undulose extinction in quartz. The granite is calc-alkaline, with a

strongly fractionated REE pattern (La/YbN ¼ 137) and negative

Eu anomaly (Eu/Eu*N ¼ 0.52) (Fig. 4). In the primitive mantle-

normalized diagram, the granite displays marked negative Nb

and P anomalies, and plots in the syncollisional and volcanic arc

field in the tectonic discrimination diagrams of Pearce et al.

(1984).

The zircons from sample 6407/10-3 are commonly stubby

biprismatic, 100–300 �m, in some cases multifaceted and in

others with irregular or poorly developed crystal faces that give

the grains a rounded appearance. The grains are clear, colourless

to yellowish, commonly with an iron-oxide stained surface. A

few grains contain inclusions. The zircons are oscillatory-zoned

and some of the grains have CL-bright cores or CL-brighter

domains within the grains (Fig. 5g); however, such cores are not

nearly as ubiquitous as in the granite samples from the North

Sea. The cores typically display oscillatory zoning that in some

cases appears to be in crystallographic continuity with the

surrounding oscillatory-zoned rim. The cores and rims of the

zircons may therefore represent episodic growth in a magma of

varying composition, or alternatively the cores could represent

grains entrained in the magma during ascent and/or emplace-

ment. The two analysed cores yield ages indistinguishable from

the darker, oscillatory-zoned margins in other grains, lending

support to the first interpretation.

Six SIMS analyses of oscillatory-zoned zircon (including the

two ‘cores’) yield a concordia age of 437 � 4 Ma (Fig. 6i),

interpreted as the crystallization age of the granitic magma. One

analysis, with excessive common Pb (12% f206), was excluded.

The granite yields a TDM of 1215 Ma (�Nd ¼ �5.24), and an


6306/10-1 (3158.5 + 3159.2 m), diorite. Sample 6306/10-1 is a

medium-grained diorite (Fig. 3h), mainly consisting of strongly

saussuritized plagioclase and amphibole, with accessory, second-

ary calcite in thin veins. The rock appears undeformed apart

from a slight kinking observed in some plagioclase grains where

albite twinning is preserved. The diorite is calc-alkaline, with a

moderately fractionated REE pattern (La/YbN ¼ 12–15) and no

Eu anomaly (Eu/Eu*N ¼ 0.97–1.02) (Fig. 4). In the primitive

mantle-normalized diagram, the diorite displays a marked nega-

tive Nb anomaly, and plots in the volcanic arc field in the

tectonic discrimination diagrams.

The zircons from sample 6306/10-1 are mostly stubby bipris-

matic, 100–200 �m, with well-developed crystal faces, and

internal oscillatory or parallel zoning, typically combined with

sector zoning (Fig. 5h). The grains are colourless to smoke

coloured and clear. Inclusions may occur locally, whereas

fractures are more abundant.

Eight SIMS analyses from eight grains yield a concordia age


of 447 � 4 Ma (Fig. 6j), interpreted as the crystallization age of

the dioritic magma.

The diorite yields a TDM of 870 Ma (�Nd ¼ 1.42), and an initial87Sr/86Sr ratio of 0.705 (Table 2, Fig. 4e).

6609/7-1 (1945.8 m), metasandstone or metasiltstone. Sample

6609/7-1 is a laminated metasediment consisting of ,1 cm, light

orange layers dominated by very fine-grained quartz (sandstone

or quartzite) with some muscovite, and thinner grey layers

dominated by very fine-grained quartz and abundant muscovite

(siltstone) (Fig. 3i). Weak undulatory extinction of quartz attests

to some deformation. The rock is jointed with calcite-filled veins,

and also contains dispersed calcite within the quartzitic matrix.

The zircons from sample 6609/7-1 are rather small, typically

between 50 and 100 �m. The majority of the grains are rounded

equidimensional to elongate, but sub-rounded biprismatic (in

some cases multifaceted) grains also occur. In general, the

zircons display oscillatory zoning (Fig. 5i), in some cases

combined with sector zoning. Many grains represent abraded

fragments of larger crystals, reflecting sedimentary transport and

abrasion. A few grains have a distinct core, but there is no

evidence of metamorphic growth of zircon in the rock.

Twenty-seven SIMS analyses yield 207Pb/206Pb ages between

1056 and 2127 Ma. Figure 6l inset shows a cumulative prob-

ability plot of analyses that are ,10% discordant. The dominant

populations are 1600–1750 Ma, 1500 Ma and 1100–1200 Ma.

Discussion

The Palaeozoic Caledonian–Appalachian orogenic belt formed

in response to closure of the Iapetus ocean and Siluro-Devonian

collision between Baltica, Laurentia and Avalonia (e.g. van Staal

et al. 1998). The belt thus incorporates the pre-collisional

histories of three continents, of which Baltica and Laurentia are

the most important here. The Laurentian margin was active

during the Late Cambrian–Ordovician, with development several

arc–back-arc and ophiolite complexes that were accreted to the

Laurentian margin during the Mid-Ordovician Taconian orogeny

(e.g. Zagorevski et al. 2006). In contrast, the Baltican margin

appears to have been largely passive prior to onset of the Late

Silurian Scandian orogeny (e.g. Roberts 2003). As discussed in

the introduction, the rocks discussed here most probably repre-

sent vestiges of magmatic complexes formed on or near the

Laurentian margin. To place the studied samples in a wider

geological context, we present a summary of the pre-collisional

evolution of the exotic Upper and Uppermost Allochthons of the

Scandinavian Caledonides, and the larger Caledonian–Appala-

chian orogenic belt.

Magmatic and tectonic evolution of the Upper andUppermost Allochthons, Norwegian Caledonides

Figure 7 shows a compilation of published U–Pb zircon ages,

interpreted to be crystallization ages, from the Norwegian

Caledonides (see also Bingen & Solli 2010, for a comprehensive

compilation and discussion of the available geochronological

data). It is generally accepted that the Upper and Uppermost

Allochthons record a Neoproterozoic to Ordovician magmatic

and tectonometamorphic history, along both the Laurentian and

Baltican margin (e.g. Meyer et al. 2003; Barnes et al. 2007;

Roberts et al. 2007).

The Bindal Batholith intruded between 482 and 424 Ma

(Nordgulen et al. 1993; Yoshinobu et al. 2002; Nissen et al.

2006; Barnes et al. 2007). Lithologically, the batholith spans the

compositional range from gabbro to granite, and isotopic studies

have shown that the rocks formed by partial melting of a variety

of crustal sources with variable mantle and crustal contributions

(Nordgulen & Sundvoll 1992; Birkeland et al. 1993; Barnes et

al. 2004, 2005). Nordgulen & Sundvoll (1992) and Birkeland et

al. (1993) documented highly variable isotopic compositions for

the Bindal Batholith granitoids, with �Nd ranging from �3 to �9

and initial 87Sr/86Sr ratios varying between 0.705 and .0.715

(Fig. 4e). Nordgulen & Sundvoll (1992) identified a consistent

lithological and geographical distribution in initial 87Sr/86Sr

ratios, with granitoids to the SE yielding the lowest ratios and

tourmaline granities and anatectic granitoids to the west yielding

the highest ratios. The available isotopic and geochemical data

suggest that the plutons constituting the Bindal Batholith formed

from variable and heterogeneous sources, including upper man-

tle, lower and upper crustal rocks in an evolving active-margin

setting (Nordgulen & Sundvoll 1992; Birkeland et al. 1993;

Barnes et al. 2003, 2004, 2007).

The Nesaa Batholith is located in the Upper Allochthon

Gjersvik Nappe, near the southeastern margin of the Uppermost

Allochthon (Meyer et al. 2003). The Nesaa Batholith consists of

two intrusive complexes, the largely gabbroic Grøndalsfjell

Intrusive Complex, dated at 458 � 3 Ma (Meyer et al. 2003), and

the dominantly quartz monzodioritic Møklevatnet Complex,

dated at 456 � 2 Ma (Roberts & Tucker 1991). The Nesaa

Batholith is characterized by an arc-like geochemistry, and a

relatively restricted range of Nd and Sr isotopic compositions

with �Nd between 2 and 3.5 (one outlier at �3.5) and initial 87Sr/86Sr ratios of 0.704 (Meyer et al. 2003) (Fig. 4e). The isotopic

data suggest more juvenile compositions than those observed in

the Bindal Batholith, and Meyer et al. (2003) interpreted the

Nesaa Batholith as an early phase of the more voluminous Bindal

Batholith. They argued for formation of the batholiths in an

active continental margin, intruding the Gjersvik Nappe after

accretion of the latter onto the Laurentian margin.

In central Norway, the gabbroic to granitic Smøla–Hitra

Batholith (e.g. Gautneb & Roberts 1989; Lindstrøm 1995) repre-

sents an intrusive complex that is probably correlative with the

Bindal Batholith (Tucker et al. 2004; Roberts et al. 2007). The

batholith intruded Early Ordovician, low-grade metasediments

that have Laurentian faunal affinities (Bruton & Bockelie 1979).

Tucker et al. (2004) dated several parts of the Smøla–Hitra

Batholith to between 446 � 4 and 440 � 3 Ma. Gautneb &

Roberts (1989) and Lindstrøm (1995) regarded the batholith as

part of a continental magmatic arc, based on geochemical data.

The isotopic data from the Smøla–Hitra Batholith suggest

variable mantle and crustal input, with �Nd ranging from �6.1 to

3.6 and initial 87Sr/86Sr from 0.704 to 0.710 (Lindstrøm 1995;

Nordgulen et al. 1995) (Fig. 4e).

In SW Norway, only the Middle and Upper Allochthons are

exposed and most of the reported magmatic U–Pb ages are older

than c. 470 Ma and related to ophiolite complexes. There are,

however, several notable exceptions that have been dated by only

the Rb–Sr or Sm–Nd method. These include the gabbroic to

granitic Sunnhordland Batholith, south of Bergen, and the related

Krossnes Granite, which have been dated at 430 � 10 and

430 � 6 Ma, respectively (Rb/Sr whole-rock isochrons, Andersen

& Jansen 1987; Fossen & Austrheim 1988); the Kattnakken

volcanic series (Lippard 1976) dated at 445 � 5 Ma (Rb/Sr

whole-rock isochron, Priem & Torske 1973) and cut by a dolerite

dyke swarm at 435 � 5 Ma (Ar–Ar maximum age, Lippard &

Mitchell 1980); and the bimodal Siggjo Group (Nordas et al.

1985), including andesitic and rhyolitic lavas, dated at

468 � 23 Ma and 464 � 16 Ma, respectively (Rb/Sr whole-rock


Fig. 7. Simplified geological map of Norway (modified after Solli & Nordgulen 2006) showing published U–Pb age data from Caledonian magmatic

rocks of the Norwegian Caledonides.


isochron, Furnes et al. 1983). These data suggest that the absence

of rocks younger than c. 470 Ma in West Norway may be more

apparent than real, but require confirmation from U–Pb zircon

analyses. The Sunnhordland Batholith and related Krossnes

Granite yield relatively low initial 87Sr/86Sr of 0.7056 and 0.7066

(Fig. 4e), respectively, and the structural, geochemical and

isotopic investigations of the Sunnhordland Batholith are compa-

tible with formation in a continental magmatic arc (Andersen &

Jansen 1987).

Tectonomagmatic evolution of the Caledonian–Appalachian orogen

There is ample evidence that tectonically and temporally similar

processes, including Early and Middle Ordovician arc–back-arc

magmatism and accretion, took place along the length of what

later became the Caledonian–Appalachian orogen (e.g. van Staal

et al. 1998; Roberts 2003). Although along-strike events, such as

Taconian and Grampian orogenesis, may not be direct correla-

tives (e.g. van Staal et al. 1998), a brief discussion of these

processes is helpful for placing the rocks described here in a

wider tectonic and magmatic context.

Early Ordovician ophiolites and ophiolite fragments are strung

out along the entire length of the Caledonian–Appalachian

orogen, and are generally considered an integral part of the

Taconian and time-equivalent Grampian orogenic events along

the SE margin of Laurentia (Pedersen & Furnes 1991; Cawood

& Suhr 1992; van Staal et al. 1998). The ophiolites are

commonly interpreted to have formed by rifting along the

Laurentian margin to form peri-continental microcontinents

separated from Laurentia by small, ophiolite-floored ocean basins

(Cawood et al. 1995; van Staal et al. 1998; Waldron & van Staal

2001; Hibbard et al. 2007). Closure of these ocean basins

resulted in arcs forming on the various microcontinent blocks,

ophiolite obduction and Taconian orogenesis, recorded in differ-

ent segments of the Caledonian–Appalachian orogen, such as

New England (Karabinos et al. 1998), Newfoundland (Waldron

& van Staal 2001), Scotland (Kinny et al. 1999; Friend et al.

2000) and the Uppermost Allochthon of the Norwegian Caledo-

nides (Barnes et al. 2007; Roberts et al. 2007). In contrast, the

Greenland Caledonides has no record of Taconian orogenesis,

and in the northern parts carbonate sedimentation persisted into

Early Silurian times (Higgins et al. 2004), just prior to Scandian

orogenesis.

Figure 8 shows probability density plots of Caledonian

magmatism in Scandinavia, East Greenland and Svalbard, the

British Isles and the Newfoundland Appalachians. All segments

show extensive Late Ordovician to Early Silurian magmatism

between c. 445 and 425 Ma, related to final closing of the Iapetus

ocean and initiation of continent–continent collision. In contrast,

the Greenland–Svalbard segment lacks the Early Ordovician (c.

480–460 Ma) magmatic activity that is seen in the British and

Irish, Newfoundland, and Norwegian segments, although less

pronounced in the latter. It is possible that studies (and thus

available data) from Greenland are biased towards processes

related to crustal anatexis and associated magmatism, which took

place at c. 435–425 Ma (e.g. Kalsbeek et al. 2001); however,

considering the lack of ocean-derived rocks in the Greenland

Caledonides (e.g. Gee et al. 2008), the absence of this largely

oceanic magmatic event is not surprising. Observations from the

various segments constituting the Caledonian–Appalachian

orogen suggest that late orogenic, Siluro-Devonian extension was

accompanied by sinistral megashear (e.g. Soper et al. 1992;

Strachan et al. 1992; Dewey & Strachan 2003). Thus, as

Fig. 8. Probability density plots showing published age data from

Caledonian magmatic rocks in the Caledonides of Norway, East

Greenland and Svalbard, Britain and the Newfoundland Appalachians.


discussed by Roberts et al. (2007), restoring the Scandinavian

Caledonides southwestward to their likely pre-megashear posi-

tion would place Norway and the Upper and Uppermost Alloch-

thons closer to the northern Appalachians than Greenland.

In Newfoundland, the Andean-type Notre Dame arc formed on

Laurentian continental crust between c. 490 and 455 Ma (van

Staal et al. 1998, 2007). The arc-related rocks range in composi-

tion from tonalite to granite, and zircon xenocrysts and Sm–Nd

isotopes (with �Nd values between 2.6 and �13.5) suggest signifi-

cant contributions from old crustal and/or subcontinental litho-

sphere material (Whalen et al. 1997; van Staal et al. 1998, 2007).

Simultaneously with formation of the Notre Dame Arc, the

Annieopsquotch accretionary tract developed. Zagorevski et al.

(2006) described the Annieopsquotch accretionary tract as a

series of west-dipping, eastward-younging thrust slices containing

remnants of ophiolitic and arc–back-arc complexes that devel-

oped intermittently in response to eastward retreat of a single,

west-dipping subduction zone just outboard of the Dashwoods

microcontinent. They interpreted these complexes to represent

peri-Laurentian terranes accreted to the Laurentian margin during

Ordovician closure of the Iapetus ocean. Geochronological,

geochemical and isotopic data show that several arcs formed and

were accreted to the continental margin between c. 473 and

460 Ma. The arc rocks range from basaltic to rhyolitic, are

tholeiitic to calc-alkaline, and have variable Sm–Nd isotopic

compositions with �Nd ranging from +9 to �10. The isotopic

compositions reflect variable input from Laurentian continental

crust, indicating that the arcs formed on a substrate of previously

rifted continental crust (Dashwoods ribbon continent). Likely

equivalents of the Notre Dame Arc and Annieopsquotch accre-

tionary tract in the British Isles have been described from the

Midland Valley (Midland Valley arc, Bluck 1984) and possibly

the Northern Belt of the Southern Uplands (van Staal et al. 1998).

Flowerdew et al. (2005, 2009) discussed the Sm–Nd and Rb–

Sr whole-rock, and Hf-in-zircon isotopic compositions of c.

470 Ma tonalitic to granitic rocks intruding metasedimentary

rocks of the Slishwood Division in NW Ireland. �Nd values range

from �6 to �9 and initial 87Sr/86Sr from 0.715 to 0.720, and �Hf

is �7.7. The magmatic rocks are interpreted to have formed by

subduction at or outboard of the Laurentian margin, and the

isotopic compositions suggest that Palaeoproterozoic crust repre-

sents a significant magma source. Based on these data, Flowerdew

et al. interpreted the Slishwood Division to represent a micro-

continental block situated outboard of the Laurentian margin.

Timing and nature of magmatism in the North Sea andNorwegian Sea

The samples from the North Sea fall into two groups; an older

group consisting of three granites (16/3-2, 16/4-1 and 16/5-1)

yielding ages between 456 and 463 Ma, and a younger group

consisting of leucogabbro and dacite (16/1-4 and 16/6-1) yielding

ages of 421 and 430 Ma, respectively. The older granite group

displays arc-like geochemical characteristics, and a narrow range

of Palaeoproterozoic TDM between 1590 and 1831 Ma (�Nd of

�10) and an initial 87Sr/86Sr ratio of 0.709. The Palaeoproter-

ozoic model ages are consistent with abundant inherited zircon

cores of Mesoproterozoic, Palaeoproterozoic and Archaean age,

and the combined geochemical and isotopic data point towards

formation in an arc situated on Proterozoic crust at c. 460 Ma.

As discussed above, a similar setting as been proposed for

numerous Ordovician rock complexes elsewhere in the Caledo-

nian–Appalachian orogen, from Newfoundland to Norway, that

formed outboard of the Laurentian margin. In particular, the

ages, geochemical and isotopic compositions of this group of

rocks are similar to those displayed by the Bindal Batholith in

the Uppermost Allochthon. Interestingly, the oldest inherited

core, dated at 3605 Ma (from 16/3-1), is older than the oldest

known rock in Baltica (Mutanen & Huhma 2003), consistent

with a Laurentian affinity. These data suggest that rocks similar,

and probably correlative, to the Uppermost Allochthon underlie

the continental margin as far south as the North Sea.

Although the younger samples (leucogabbro and dacite) also

display arc-like compositions, their TDM values are younger and

their �Nd values are higher than those of the older plutonic rocks.

Initial 87Sr/86Sr ratios are also lower, at 0.706–0.707. The ages

of the leucogabbro and dacite are indistinguishable from the Rb/

Sr ages of the Sunnhordland Batholith and Krossnes granite, and

the initial 87Sr/86Sr ratios are identical. Andersen & Jansen

(1987) interpreted the Sunnhordland Batholith to have formed in

a continental arc at c. 430 Ma. The new data from the North Sea

are consistent with this interpretation. The leucogabbro

(421 � 3 Ma) and dacite (430 � 6 Ma) overlap in age with the

main phase of the Scandian orogeny elsewhere in the Scandina-

vian Caledonides. It is therefore uncertain whether this magma-

tism reflects the last stages of oceanic subduction prior to ocean

closure and continental collision in this part of the Caledonides

or represents melting of older crust during orogenesis. Although

inherited zircon is absent from the leucogabbro and sparse in the

dacite, an inherited grain from the latter, dated at 491 Ma,

suggests input of Palaeozoic material. The age of this inherited

grain is similar to the ages of ophiolite complexes in the

Scandinavian Caledonides (see compilation by Slagstad 2003),

British and Irish Caledonides (Shetland ophiolite, 492 Ma, Spray

& Dunning 1991) and Newfoundland (Bay of Islands ophiolite,

484 Ma, Jenner et al. 1991). A likely interpretation is that the

420–430 Ma magmatism is related to the early stages of the

Scandian orogenic event, following earlier, Taconian thrusting

that emplaced the ophiolite fragments (including arc–back-arc

assemblages) onto older Laurentian crust. The isotopic data from

the 420–430 Ma rocks, indicating a continental component, are

also consistent with such an interpretation. This situation might

be analogous to that inferred from the Bindal Batholith in central

Norway, where Barnes et al. (2007) interpreted mafic to felsic

magmatism of this age to have a previously strongly reworked

Palaeozoic source.

The two samples of granite (6407/10-3) and diorite (6306/10-1)

from the Norwegian Sea are intermediate in age (437 and 447 Ma,

respectively) between the two age groups identified in the North

Sea. Although their geochemical composition is rather similar to

that of the samples from the North Sea, their isotopic composi-

tions are not. The granite yields a Mesoproterozoic TDM of

1215 Ma (�Nd of �5) and initial 87Sr/86Sr ratio of 0.707, whereas

the diorite is more juvenile with a TDM of 870 Ma (�Nd of 1.5) and

initial 87Sr/86Sr ratio of 0.705. The ages and isotopic compositions

overlap with major magmatic complexes in the Upper and Upper-

most Allochthons, including the Bindal and Hitra–Smøla Bath-

oliths, suggesting that rock units correlative with these

Allochthons underlie much of the mid-Norwegian margin.

Sedimentary successions within the Caledonian orogenicbelt

Samples of metasedimentary rock were obtained from well 25/7-

1S in the North Sea and well 6609/7-1 in the Norwegian Sea

(Fig. 1). The samples display very similar age distributions

characterized by dominantly Late Palaeoproterozoic and Meso-

proterozoic zircons, with rare Palaeoproterozoic grains. The age


distributions observed in these two samples strongly resemble

those observed in samples from the Kalak Nappe Complex in

Finnmark, North Norway (Kirkland et al. 2007), the North-

western Terrane of the Svalbard Caledonides (Pettersson et al.

2009), the Moine Supergroup in the Scottish Caledonides (Friend

et al. 2003; Cawood et al. 2004; Kirkland et al. 2008a) and the

Krummedal–Smallefjord sequence in the East Greenland Cale-

donides (Kalsbeek et al. 2000; Watt et al. 2000). The latter

probably correlates with the Brennevinsfjorden Group in the

Svalbard Caledonides (Johansson et al. 2005). Cawood et al.

(2007) and Kirkland et al. (2007) discussed the similarities

between these and other sedimentary successions within the

Caledonian orogen. In Finnmark, Kirkland et al. (2007) identified

two major metasedimentary successions: the Svaerholt Succes-

sion, deposited between 980 and 1030 Ma, and the Sørøy

Succession deposited between 840 and 910 Ma. The zircon

populations in these successions are dominated by Late Palaeo-

proterozoic and Mesoproterozoic grains, similar to the samples

investigated here, but the Sørøy Succession contains a significant

Neoproterozoic population, which is not observed in our sam-

ples. Kirkland et al. (2007) argued that the metasediments

comprising the two successions were deposited in successor

basins, located in the developing Grenville(–Sveconorwegian)

Orogen. Cawood et al. (2010) presented an alternative model in

which these metasedimentary rocks, and correlative sedimentary

rocks from East Laurentia and Baltica, were deposited along the

Laurentian margin facing the Late Mesoproterozoic Asgard Sea.

Cawood et al. (2010) hypothesized that the Asgard Sea devel-

oped in response to southward movement and clockwise rotation

of Baltica with respect to Laurentia. In contrast to Kirkland et al.

(2007) and Cawood et al. (2010), Roberts (2007) argued, based

on palaeocurrent data from northern Finnmark, that the sedi-

ments were derived from a south to southeasterly, Fennoscandian

source. Although no potential sources of the Mesoproterozoic

zircons are known from northern Fennoscandia, Roberts (2007)

suggested that the concealed basement underneath the Caledo-

nian nappes in NW Finnmark might contain such sources.

The new data presented from the Norwegian and North Seas

suggest that these sediments, sourced from the same area as

several other metasedimentary units in the Caledonian orogen,

may be found along much of the length of the orogen, from

Scotland to northern Norway and Svalbard. These two samples

therefore strengthen the impression that geological units, whether

magmatic or sedimentary, within tectonostratigraphically higher

nappes of the Scandinavian Caledonides can be correlated with

similar units in other parts of the Caledonian–Appalachian

orogenic belt. Sample 25/7-1S (metasediment) from the North

Sea is located in the same general area as seemingly voluminous

c. 460 Ma arc-related granites (the latter c. 60 km farther south).

The 460 Ma granites are a feature that appears diagnostic of the

Laurentian margin, and, although the relationship to the metase-

diment is unconstrained, the data from the North Sea appear to

support a Laurentian affinity. However, the possibility of tectonic

juxtaposition of originally unrelated assemblages clearly cannot

be ruled out.

Conclusions

The new geochronological data from basement samples from the

North Sea and Norwegian Sea provide a unique glimpse into the

constitution of the Norwegian continental margin. The crystalline

basement consists of magmatic and metasedimentary rocks that

can be correlated with rocks in tectonostratigraphically high (i.e.

Upper and Uppermost Allochthons) nappe complexes within the

Scandinavian Caledonides. The data also provide an improved

correlation between the Scandinavian Caledonides and more

southerly segments of the Caledonian–Appalachian orogenic

belt. In particular, metasedimentary rocks that have detrital

zircon populations that resemble metasediments in the northern

Caledonides in Finnmark and the Moine Supergroup in Scotland

can be found continuously along the length of the orogen. The

new data also show that the Ordovician Taconian magmatic

event, a major orogenic event along the Laurentian Iapetus

margin, is more widespread in the Norwegian Caledonides than

hitherto recognized.

D. Bering and C. Magnus from the Norwegian Petroleum Directorate

helped obtain the investigated samples. Ø. Skar and T. Røhr are thanked

for technical assistance and discussion of the data. We thank Michael

Murphy (University College Dublin) for assistance with Sm–Nd and Rb–

Sr analyses. We thank C. Kirkland and an anonymous reviewer for

constructive comments. The work formed part of the Statoil-sponsored

KONTIKI project (NGU Project 306600). The Nordsim facility is operated

under an agreement between the research funding agencies of Denmark,

Norway and Sweden, the Geological Survey of Finland and the Swedish

Museum of Natural History. This is NORDSIM Contribution 273.

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Received 13 August 2010; revised typescript accepted 24 February 2011.

Scientific editing by Chris Clark.


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• Special Publication 355

The SE Asian Gateway: History and Tectonics of the Australia-Asiacollision

Edited by R. Hall, M. Cottam and M. E. J. Wilson

Collision between Australia and SE Asia began in the Early Miocene and reduced the former wide ocean between them toa complex passage which connects the Pacific and Indian Oceans. Today, the Indonesian Throughflow passes through thisgateway and plays an important role in global thermohaline flow, and the region around it contains the maximum globaldiversity for many marine and terrestrial organisms. Reconstruction of this geologically complex region is essential forunderstanding its role in oceanic and atmospheric circulation, climate impacts, and the origin of its biodiversity.

The papers in this volume discuss the Palaeozoic to Cenozoic geological background to Australia and SE Asia collision,and provide the background for accounts of the modern Indonesian Throughflow, oceanographic changes since theNeogene, and aspects of the region’s climate history.


Growth and Collapse of the Tibetan PlateauEdited by R. Gloaguen and L. Ratschbacher

Despite agreement on first-order features and mechanisms, critical aspects of the origin and evolution of the TibetanPlateau, such as the exact timing and nature of collision, the initiation of plateau uplift, and the evolution of its heightand width, are disputed, untested or unknown. This book gathers papers dealing with the growth and collapse of theTibetan Plateau. The timing, the underlying mechanisms, their interactions and the induced surface shaping, contributingto the Tibetan Plateau evolution are tightly linked via coupled and feedback processes. We therefore present cross-disciplinary contributions which allow insight into the complex interactions between lithospheric dynamics, topographybuilding, erosion, hydrological processes and atmospheric coupling. The book is structured in four parts: early processes inthe plateau formation; recent growth of the Tibetan Plateau; mechanisms of plateau growth; and plateau uplift, surfaceprocesses and the monsoon.


Slope TectonicsEdited by M. Jaboyedoff

Usually geomorphology, structural geology and engineering geology provide descriptions of slope instability in quitedistinctive ways. This new research is based on combined approaches to providing an integrated view of the operativeslope processes. ‘Slope Tectonics’ is the term adopted here to refer to those deformations that are induced or fullycontrolled by the slope morphology, and that generate features which can be compared to those created by tectonicactivity. Such deformation can be induced by the stress field in a slope which is mainly controlled by gravity, topographyand the geological setting created by the geodynamic context.

The content of this book includes slope-deformation characterization using morphology and evolution, mechanicalbehaviour of the material, modes of failure and collapse, influence of lithology and structural features, and the roleplayed by controlling factors. The contributions cover broad aspects of slope tectonics that attempt to underline amultidisciplinary approach, which should create a better framework for studies of slope instability.


Kinematic Evolution and Structural Styles of Fold-and-Thrust Belts Edited by J. Poblet and R. J. Lisle

Fold-and-thrust belts occur worldwide, have formed in all eras of geological time, and are widely recognized as the mostcommon mode in which the crust accommodates shortening. Much current research on the structure of fold-and-thrustbelts is focused on structural studies of regions or individual structures and on the geometry and evolution of theseregions employing kinematic, mechanical and experimental modelling. In keeping with the main trends of currentresearch, this title is devoted to the kinematic evolution and structural styles of a number of fold-and-thrust belts formedfrom Palaeozoic to Recent times. The papers included in this book cover a broad range of different topics, from modellingapproaches to predict internal deformation of single structures, 3D reconstructions to decipher the structural evolution ofgroups of structures, palaeomagnetic studies of portions of fold-and-thrust belts, geometrical and kinematical aspects ofCoulomb thrust wedges and structural analyses of fold-and-thrust belts to unravel their sequence of deformations.

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