Polyphase tectonothermal evolution of exotic caledonian nappes in Troms, Norway: Evidence from 40Ar39Ar mineral ages

Lithos, 29 (1992) 19-42 Elsevier Science Publishers B.V., Amsterdam

19

Polyphase tectonothermal evolution of exotic Caledonian nappes in Troms, Norway: Evidence from 4°Ar/39Ar mineral ages R.D. Dallmeyer" and A. Andresen b aDepartment of Geology, University of Georgia, Athens, G.4 30602 USA bDepartment of Geology, University of Oslo, PB 1047 Blinden, N-0316 Oslo, Norway (Received October 21, 1991~revised and accepted February 10, 1992 )

LITHOS

0

ABSTRACT Dallmeyer, R.D. and Andresen, A., 1992. Polyphase tectonothermal evolution of exotic Caledonian nappes in Troms, Norway: Evidence from 4°Ar/39Ar mineral ages. Lithos, 29:19-42. 4°Ar/39Ar incremental-release ages have been determined for hornblende and muscovite from several high-level, exotic nappes in northern Troms County, Norway. Within the Nordmannvik Nappe and upper tectonic levels of the Lyngen Nappe Complex (Balsfjord Group) both minerals record plateau and isotope correlation ages between c. 427 and 417 Ma. These indicate relatively rapid post-metamorphic cooling which is interpreted to have occurred during Scandian translation of exotic tectonic elements to higher crustal levels along the Baltoscandian margin. Muscovite within various structural levels of the Tromso Nappe Complex (Uppermost Allochthon) yields plateau ages of 427-410 Ma which also record post-Scandian metamorphic cooling. However, hornblende within the Tromso Nappe Complex records significantly older isotopic correlation ages which range between c. 481 Ma and 432 Ma. The 4°Ar/39Ar spectra display no evidence of partial Scandian rejuvenation, and the isotope correlation ages are therefore interpreted to date cooling following preScandian tectonothermal activity. These results indicate that early Paleozoic tectonothermal events affected outboard exotic tectonic elements at approximately the same time as the "early" Caledonian orogenesis previously described for nappe units with initial Baltic palinspastic affinities (e.g., Sere Nappe Complex).

Introduction The Scandinavian Caledonides are characterized by an internally imbricated succession o f nappes which were emplaced eastward onto the Baltic Craton during the Late Silurian-Early D e v o n i a n (Fig. 1). The nappe complexes have been traditionally discussed in terms of four principal tectonic units which include the Lower, Middle, U p p e r and Uppermost Allochthons (e.g., Roberts and Gee, 1985). The Lower and Middle Allochthons together with

Correspondence to: R.D. Dallmeyer, Department of Geology, University of Georgia, Athens, GA 30602 USA

lower structural levels o f the U p p e r Allochthon (Sere N a p p e C o m p l e x ) appear to have had initial Baltic palinspastic affinities (Dallmeyer and Gee, 1986 ). The remainder of the U p p e r Allochthon and the U p p e r m o s t Allochthon are comprised o f structural elements initially derived from non-Baltic palinspastic settings (e.g., Dallmeyer and Gee, 1986; Stephens and Gee, 1989 ). Exotic structural units o f the U p p e r Allochthon include ophiolites, island arc complexes, and basins related to rifted ensimatic island arc complexes (e.g., Stephens and Gee, 1989). The U p p e r m o s t Allochthon (Fig. 1 ) comprises an assembly o f extremely heterogeneous nappe complexes which are dominated by schists, marbles, calc-

0024-4937/92/$05.00 © 1992 Elsevier Science Publishers B.V. All rights reserved.

20

silicate units, granitic rocks and gneisses (of Precambrian and Caledonian age). It locally contains ophiolitic and ensimatic lithologic associations and eclogites (e.g., Krogh et al., 1990). A polymetamorphic evolution has been documented for both native Baltic and structurally overlying, outboard exotic terranes in the Scandinavian Caledonides (e.g., Sturt et al., 1978; Dallmeyer and Gee, 1986; Dallmeyer, 1988; Stephens and Gee, 1989). An early Paleozoic event (late Cambrianmiddle Ordovician) locally transformed rift-facies dolerites and pillow lavas within Baltic Successions to eclogites at c. 505 Ma (Mork et al., 1988). This was followed by relatively rapid post-metamorphic uplift (Dallmeyer and Gee, 1986). Tectonothermal activity in the late Silurian-early Devonian ("Scandian" orogeny) culminated in structural emplacement of the Caledonian nappe complexes into their present relative tectonic positions on the Baltoscandian craton. A regionally extensive unconformity locally separates marginal-basin ophiolires (c. 495-475 Ma) from overlying upper Ordovician (Ashgillian) and younger Caledonian sequences (Thon, 1985; Dunning and Pedersen, 1988; Steltenpohl et al., 1990), and clearly documents pre-Scandian orogenic activity. The lack of fossil control together with a paucity of isotopic age determinations has hampered resolution of the tectonothermal evolution recorded within northern sectors of the Scandinavian Caledonides. Binns (1978) and Andresen et al. ( 1985 ) suggested that those high-grade nappes which are structurally imbricated with lower grade tectonic units could represent an early Caledonian metamorphic "basement" on which lower grade "cover" sequences were deposited. These workers suggested that the apparent "basement-cover" associations were likely dismembered during late Caledonian (Scandian) orogenic activity. On the other hand, Barker (1989 ) argued that there is no evidence for pre-Scandian tectonothermal activity in the Troms Caledonides (except for the possible exception of eclogite-bearing horizons in the Troms~ Nappe Complex). A reconnaissance 4°Ar/39Ar mineral dating program has been carried out in north Troms County to provide a general calibration for tectonothermal events. Results of this initial study are presented here, and they provide clear evidence for a complex, polyphase metamorphic evolution similar to

R.D. DALLMEYER AND A. ANDRESEN

that which has been previously documented in other sectors of the Scandinavian Caledonides (e.g., Dallmeyer, 1988 ).

Geologic setting Comprehensive reviews of the regional tectonostratigraphy of the Troms Caledonides have been presented by Binns (1978), Andresen et al. ( 1985 ) and Barker (1989). Therefore, only those relationships which are critical in interpretation of the new 4°Ar/39Ar results are reviewed here. Exposure of the Caledonian nappes in Troms is controlled by the regional, post-metamorphic, northeast-trending Ofoten synform (Fig. 2). A thin veneer of autochthonous/parautochthonous, very low-grade metasedimentary cover rocks (Dividal Group) unconformably overlies variably retrogressed crystalline basement rocks exposed within the Mauken Window. These are structurally overlain by an internally imbricated series of nappe complexes which display both Baltic and exotic affinities (Figs. 2 and 3). Most Baltic structural elements comprise the Kalak Nappe Complex (typically correlated with the Middle Allochthon group of Scandinavian nappes). The Kalak Nappe Complex exposed in Troms is structurally overlain (in ascending tectonic order) by several exotic structural units (Figs. 2 and 3) which include the: Vaddas Nappe, K~fjord Nappe, Nordmannvik Nappe, Lyngen Nappe Complex and Tromso Nappe Complex. Lower portions of the Vaddas Nappe are represented by interbedded marble and schist which are conformably overlain by a quartzite-dominated sequence (Padget, 1955 ). Upper levels of the Vaddas Nappe comprise a sequence of marble and mafic metavolcanic rocks which locally display pillow structures. Poorly preserved Silurian fossils have been described from marbles within this interval (Binns and Gayer, 1980). A nearly monomict quartzite conglomerate marks the base of an overlying series of schists and impure psammites which likely represent distal metaturbidites (Andresen, 1988). These are overlain conformably by an extensive sequence of greenschist and interlayered massive amphibolite which northward includes the Vaddas Gabbro. Metamorphic grade within the Vaddas Nappe ranges from upper greenschist to

EXOTIC CALEDONIANNAPPES IN TROMS, NORWAY

TERRANE SCANDINAVIAN

0

100

200

I

I

I

21

MAP CALEDONIDES

f~c3. ++-,+÷*÷*.+,÷÷ . . . . ÷ .

km

t

[,

+

÷

.

.

+

+

. ÷

Ofoten~, .

++~÷

'+.+++++ ~.÷,+++

+++÷+÷+

+.÷÷++*÷,

i

+**++++÷

PERMIAN DEVONIAN ÷

÷

+

LAURENTIAN MIOGEOCLINE (?) ;Uppe,~ost

~och~or,,

EUGEOCLINAL TERRANES(Keh nappes) ~upper p a r t O' Upper A h o c ~ t h o r ,

Seve nappes ~,.... p,,to~J~p,r..och~hoo~ BALTOSCANDIAN Undifferentiated ~,d~,, ~n~Lo**. ~.,o~h,hon,

MIOGEOCLINE/ PLATFORM

Sedimentary cover

BALTOSCANDIAN

Au~och~o~

ParaotoCh~hon,

Basement detormed

n ~esl~rn

PLATFORM area~,

Fig. 1. Generalized tectonostratigraphic terrane map of the Scandinavian Caledonides (adapted from Dallmeyer and Gee, 1986 ) ]ocating the areas of North Troms and Ofoten.

22

R.D. DALLMEYER AND A. ANDRESEN

IIIJllll !11Vaddas& K~fjord Nappes

[

• Tromso NappeComplex

~

Kalak Nappe Complex (M~lselv Nappe)

~ v

LyngenNappe Complex Including LyngenOphiolite

~

Dividal Group

~

NorclmannvikNappe

~

Precambrian Basement

SSZ

SvanfleUet SrlearZone

Fig. 2. Generalizedgeologicalmap of north Troms indicating 4°Ar/39Arsample locations (adapted from Andresen et al., 1985). Trace of the Ofoten synform is indicated: SSZ= Svant]ellet Shear Zone. lowermost amphibolite facies and appears to have developed in association with a single tectonothermal event. The Vaddas Nappe is separated from the structurally overlying Khfjord Nappe by the Cappis Thrust (Zwaan and Roberts, 1978). Lower structural levels of the I ~ 0 o r d Nappe are represented by marble, metapsammite and garnet-mica schists. Upper sectors of the nappe contain mylonitic gneisses with locally boudinaged amphibolite layers. Syntectonic and synmetamorphic granitic bodies occur locally (Padget, 1955; Quarnardel, 1978 ). Metamorphic grade typically is middle amphibolite facies throughout the Kht]ord Nappe. Textural

characteristics appear to reflect a single metamorphic event. The Nordmannvik Nappe contrasts with all structurally underlying tectonic units because of its record of a polyphase metamorphic evolution and its characteristically high metamorphic grade (e.g., Bergh and Andresen, 1985). The Nappe is dominated by polymetamorphic rocks which include pervasively mylonitic mica schists, amphibolitiebearing gneisses, marbles and local ultramafic lenses. Binns ( 1978 ), Andresen et al. ( 1985 ) and Bergh and Andresen ( 1985) have suggested that an "early" Caledonian metamorphism locally reached granulite facies (Elvevold, 1988). They concluded that

23

EXOTIC CALEDONIANNAPPES IN TROMS, NORWAY OFOTEN

NORTHERN TROMS

~ ~.~-iijjjjjjj Group Salangen O o

u~

(Bogen Grl

El

D-y v

~Z

UPPER

..~/

v

v

Supergroup v eve ~ evl

4 V V V V V V x~ V V V V V V V J V V V V V V \ •

A

Lyngen Ophiolite

(Evenes Gr}

Gratangseidet A m a f j t oomp2ex

(©se thrust)

•

Nordmannvik

Narvik

A

nappe

nappe ^ ^

^

^

ALLOCHTHON

Ea

^

complex ^ ^

K~fiord nappe ,

A

Hc~gtind nappe

~z

Abisko nappe complex

MIDDLE ALLOCHTHON

A • • Rautas nappe complex • A A •

LOWER ALLOCHTHON

Dividal Group -Oc~

^

^

^

Vaddas nappe

(Keli na p pe s )~......~ ~ P 9 e ..~.s~. ~' . O(3-

^

AUTOCHTHON

Kalak • nappe complex A(M&lselv nappe)

"Divi;al :rou:

+ + ÷ + + + . +÷+++++++~++*+

Fig. 4. Generalized tectonostratigraphic correlations between north Troms and Ofoten.

•, , ~

~o.

Dividal Gr.

Fig. 3. Generalized tectonostratigraphic section of north Troms indicating the relative structural level of the 4°Ar/39Ar sample locations.

the Nordmannvik Nappe may have constituted a metamorphic basement during Silurian deposition of protoliths for the low-grade metasedimentary rocks which presently comprise the Balsi~ord Group (Lyngen Nappe Complex). The Lyngen Nappe Complex is represented by three distinctive lithotectonic units. Lower structural levels are comprised of conglomerate-bearing schist and metasandstone which are tectonically overlain by the Lyngen Ophiolite. The Upper Ordovician-Silurian Balst~ord Group (Olaussen, 1977;

Bjorlykke and Olausson, 1981 ) overlies the gabbro along a tectonically disrupted depositional contact. Lower levels of the Balsfjord Group contain polymict conglomerates which include clasts derived from the Lyngen Ophiolite (Minsaas and Stun, 1985). Middle levels of the Balsi~ord Group are represented by various schists, metaconglomerates and significant marble horizons which are locally fossilbearing. An overlying quartzite-mica schist sequence (Malangen Schist and Quartzite) together with higher structural levels of the Lyngen Nappe Complex (including the Breivikeidet carbonate sequence) appears to be separated from the underlying fossil-bearing sequences by a pre- to synmetamorphic thrust (Olsen, 1982; Velvin, 1984; Opheim, 1986). Several workers (e.g., Minsaas and Stun, 1985 ) have suggested that initial obduction and deformation of the Lyngen Ophiolite was associated

24

R.D. DALLMEYERANDA. ANDRESEN

Table i 40Ar/39Ar analytical data for incremental heating experiments on hornblende concentrates from Caledonian nappe complexes, Troms, Norway.

Release (40Ar/39Ar)* (36Ar/39Ar)* (37Ar/39Ar)C temp (°C)

39Ar % of total

%40Ar nonatmos.

36Arca %

Apparent Age (Ma)**

+

Nordmannvlk Nappe Complex Sample 5:

J - 0.010105

700 800 880 910 930 955 980 i000 1015 1030 1045 1060 1080 Fusion

53.41 32.91 30.97 26.74 26.49 26 81 26 75 26 72 26 32 26 26 26 44 26 54 26 34 26.08

0.08507 0.02260 0.01993 0.00696 0.00309 0.00306 0.00299 0.00367 0.00189 0.00210 0.00193 0.00237 0.00325 0.00185

2.781 2.806 5.254 5.923 6.337 6.198 6.187 6.573 6.412 6.207 6.295 6.380 6.280 6.457

2.56 1.38 1.67 1,97 4.34 10.94 11.70 17.16 4.62 4.71 12.42 12.62 8.06 5.84

53.34 80.38 82.33 94.07 98.46 98.46 98.53 97.90 99.81 99.51 99.74 99.27 98.24 99.87

0 89 3 38 7 17 23 16 55 82 55 i0 56 27 48 76 92.05 80.29 88.90 73.32 52.51 94.78

457. i + 428. i + 414.8 + 409.9 + 423 5 + 428 0 + 427 5 + 424 8 + 426 2 + 424 3 + 427 7 + 427 4 + 420 5 + 423 1 +

i0.9 ii.0 4.1 6 8 33 2 5 2 1 0 9 1 5 1 9 32 2 6 1.4 2.1

Total

27.40

0.00550

6.179

I00.00

97.05

62.15

426.1 +

2.6

425.8 +

2.1

Total without 700-910°C

Sample 4: 700 800 880 900 910 930 940 950 960 970 980 990 I010 1030 1050 1070 Fusion Total

92.42

J = 0.009955 126.96 86.05 34.52 28.27 26.06 25.86 27.45 25.58 25.40 25.39 24.51 25.34 24.62 26.03 25.91 25.81 25.71

0.24174 0.18287 0.02490 0.02015 0.00491 0.00431 0 00473 0 00638 0 00539 0 00513 0 00521 0 00506 0 00518 0 00439 0 00453 0.00464 0.00458

11.039 8.786 16.054 14.122 14.443 15.308 15.779 14.954 15.609 15.729 15.449 15.767 15.961 15.960 15.828 15.841 15.754

2.83 1 45 3 01 2 71 3 76 6 95 8 21 6 63 4 76 3 34 3 58 5 37 4 94 13 01 ii 79 9.41

29.88

0.01515

15.389

I00.00

8.25

44.43 38 02 82 40 82 93 98 86 99 81 99 50 97 30 98 65 98 98 98 76 99.08 98.97 99.92 99.72 99.60 99.64

98 85 94.96 92.91 93.54

395.4 419.2 416,7 414.7 413.5

95.91

81.12

1 1 17 19 80 96 90 63 78 83 80 84

24 31 53 06 00 57 69 71 82 35 64

83 83 81

808 5 511 0 453 9 381 5 415 3 416 1 437 8 402 8 405 3 406.4

+ + + + + ± + + + I +

42.1 38 6 16 0 22 1 7 3 4 6 2 8 3 3 4 1 3 5

392.9

+ + + + + +

3,0 3.2 3.8 2.3 3.0 2,6 2.7

426.6 +

5.4

406.1 + i

I

25

EXOTIC CALEDONIAN NAPPES IN TROMS, NORWAY

Sample

15A:

J - 0.010025

700 800 880 910 930 950 970 985 i000 1015 1030 1060 Fusion

46.06 31.83 27.08 26.62 26.49 26.54 26.59 26,67 26.69 27.02 26.84 26.73 26.75

0.06261 0.01224 0.00197 0.00125 0.00114 0.00109 0.00129 0.00159 0.00124 0.00142 0.00118 0.00185 0.00137

600 731 704 795 821 817 788 785 811 887 951 153 098

0 90 i 05 13 90 18 49 17 83 Ii 88 8 99 6 96 6 .43 4 .66 2 .80 2 .39 3 .73

Total

26.92

0.00203

2.796

I00.00

Total without

60 89 98 99 99 99 99 99 99 99 99 98 99

I0 05 63 43 56 62 39 05 45 28 56 88 39

98 86

0 3 37 60 67 70 58 47 61 55 68 46 61

70 85 39 58 30 48 74 54 73 12 04 23 44

57.25

98.06

700-800°C

442.2 451.7 428.6 425.3 423.8 424.8 424.7 424.6 426.3 430.4 428,9 424.7 427.0

+

14.9

+ + + +

3.3 1.3 I.i 0.9

+ +

i.i 1.3

+ + +

1.8 1.6 3.6

+ + +

4.0 1 3 1 3

426.2 +

1 5

425.8 +

1 0

Lyngen Nappe Complex BalsfJord Sample 700 800 860 880 900 930 960 990 I010 1030 1050 1070 Fusion

Ii:

Group J - 0.009382 5.773 4 342 7 911 9 215 9 897 12 042 i0 146 i0 921 Ii 794 12 164 12 087 ii 648 I0 222

3,19 4.30 5.32 3.48 3 38 16 68 3 I0 3 91 7 67 15 I0 18 88 9 53 5 46

84.37 90.01 92.62 98.35 98.84 99.16 99.22 99.93 99.60 99.80 99.30 99.80 99.57

2.64 8.66 19.31 58.82 68.64 79.89 78.43 98.28 89.62 94.99 83,29 94.87 87.37

32.44

0.00630

10.888

i00,00

98.18

76.21

28

Total Total

28 70 28 92

0.05945 0.01364 0.01114 0.00426 0.00392 0.00410 0.00352 0.00302 0.00358 0.00348 0.00395 0.00334 0.00318

109.46 36 91 36 09 31 85 31 86 29 54 29 50 29 II 28 65 28 62

without

82

700-990°C

56.63

1128 6 490 6 494 2 467 1 469 5 440 9 440 2 437 9 430 7 431.2 431.9 432.2 433.9

+ + + + + + + + + + + + +

8.3 8.1 4.5 4.6 4.4 2.5 4.1 4.0 1,3 1.4 1.6 1.6 1.9

464.4 +

2.5

431.8 +

1.5

26 Sample

R.D. DALLMEYERAND A. ANDRESEN I:

J - 0.009452

700 800 860 880 900 920 940 960 980 I000 1020 1040 1060 1080 Fusion

74.08 39.56 34.76 30.58 31.45 3].94 30.27 28.55 28.56 28.83 29.26 30.13 30.02 29.68 28.55

0.10780 0.03387 0.01297 0.00440 0.00591 0.00644 0.00464 0.00307 0.00263 0.00301 0.00352 0.00432 0.00436 0.00356 0.00375

2.938 3.880 6.534 8.750 9.030 8.961 8 843 8 601 8 789 8 835 8 736 8 659 8 576 8 525 8 194

Total

32.34

0.00983

8.240

Troms~ Nappe Lower Sample

Tectonic 2A:

3 4 4 ii 12 5 3 2 3 13 5 17 5 2 2

77 01 97 58 68 43 96 71 93 i0 77 77 70 38 25

57.31 75.47 90.47 98 03 96 73 96 28 97 80 99 22 99 74 99 35 98 82 98 05 97 99 98 74 98 40

0.74 3.12 13.71 54.06 41.54 37.82 51.87 76.22 91.05 79.72 67.42 54.49 53.55 65 07 59 36

609 5 449.4 471, i 452. i 458.2 462.5 447.3 429.9 432.2 434.4 438.0 446.4 444.7 443 2 426 7

+ + + + + + + + + + + + + + +

i00.00

95.29

52 07

453 3 +

9 2 7 1 3 5 3 6 1 9 i 9 2 5 2 2 2 1 2 1 1 9 1.0 3.1 1.9 3.2 2.4

Complex

Unit

J - 0.009505

700 800 860 890 910 930 945 960 975 990 i010 1040 1070 ii00 Fusion

58 51 42 31 35 74 30 93 29 II 29 Ii 28 95 28 86 28 74 28 00 28 33 29 02 28 67 28 54 28.48

0.07417 0.02977 0.01885 0.00602 0.00262 0.00250 0.00342 0.00396 0.00243 0.00278 0.00299 0.00242 0.00249 0.00239 0.00236

7.408 3.179 4.950 7.280 9.095 8.038 8.470 7.934 7.797 8.248 8.415 8.473 8.350 7.862 7.917

1.66 3.42 2.58 1.88 7.41 6.37 5.83 9.01 4.33 2.49 4.53 18.56 16.00 8.95 6.99

63.55 79.80 85.51 96.12 99.83 99.66 98.84 98.13 99.66 99.42 99.24 99.86 99.75 99.72 99.76

2.72 2 90 7 14 32 91 94 47 87 29 67 28 54 49 87 15 80 83 76.46 95.24 91.07 89.35 91.23

548.1 503.0 461.4 450.8 442.3 441.3 436.1 432.0 436 2 425 4 429 3 441 0 435 8 433 8 433 2

Total

29.96

0.00529

7.980

i00.00

97.82

78.20

441.5

+ + + + + +

+

16 2 6 9 3 5 2 9 3 6 2 2 2 1 3 1 i I 2 6 2 7 1.0 0.7 1.5 2.3

+

2.3

+ + + + + + + +

EXOTIC CALEDONIANNAPPES IN TROMS, NORWAY

Sample

14;

27

J - 0.009875

700 800 860 880 895 910 935 955 975 995 i010 1025 1045 1070 Fusion

58.01 30.37 30.33 29.94 28.96 28.51 28.72 28.69 28.42 28 87 28 51 28 66 28 43 28 34 28 26

0.06273 0 00730 0 00735 0 00690 0 00420 0 00254 0 00267 0 00287 0 00245 0.00351 0.00272 0.00323 0.00246 0.00218 0.00212

2.782 5.692 7.242 6.865 7.288 7.341 7.421 7.577 7.661 7.655 7.481 7.416 7.411 7.405 7.533

11.06 13.03 13.99 4.38 3.47 3.68 3.68 4.61

Total

29.60

0.00510

7 217

I00.00

Total

Sample

without

8:

700°C

2 8 9 4 5 3

08 16 31 85 91 73

8 05

68 42 94 39 94 74 95 Ol 97 72 99 42 99 31 99 15 99 60 98 52 99 27 98.73 99.52 99.80 99.90

1.21 21.22 26.80 27.06 47.20 78.75 75.69 71.84 85.19 59.41 74.84 62 47 81 93 92 26 96 64

597 5 451 1 452 5 448 4 446 4 447 1 449 6 448 5 446 6 448 5 446.4 446.4 446.4 446,2 445.6

97.48

61 06

451.4 +

2.0

448.3 +

1.9

97.92

+ + + + + + + ± + + + + + + +

6 1 2 5 3 1 1 9 3 1 1 8 1 7 1 5 1.2 2.2 2.0 1.5 1.4 1.7 2.1

J = 0.010105

700 800 880 955 980 995 i010 1025 1040 1055 1080 Fusion

69.65 36.63 26.80 28.79 28.38 28.97 28.68 28.82 28.78 28.78 28.78 28.81

0 09149 0 01228 0 00479 0 00424 0 00266 0 00411 0.00360 0.00376 0.00392 0.00408 0.00404 0.00369

3 608 7 832 9 116 9.044 8.869 9.011 8.781 8.721 8.640 8.621 8.628 8.620

0.63 0.92 2.69 6.69 6.17 8.43 28.25 15.40 9.29 4.43 11.89 5.19

61 59 91 80 97 43 98 15 99 72 98 29 98 73 98 55 98.37 98.19 98.24 98.60

1.07 17.35 51.76 57.99 90.59 59.69 66.26 63.06 59.92 57.43 58.07 63.51

650.8 529.9 424.6 455.5 456.0 458.5 456.2 457.4 456.1 455.4 455.5 457.5

Total

29.03

0.00443

8.735

i00.00

98.23

62.82

457.5 +

1.2

456.5 +

i.i

Total

without

700-880°C

95.76

+ + + + + + + + + + + +

16. i 5 6 1 8 2 5 2 3 2 3 0 9 0 5 1.4 1.5 1.4 2.4

28

R.D. DALLMEYERAND A. ANDRESEN

Tromsdalstlnd Sample

i0:

Complex

J - 0.009611

8.068 8.063 8.036 7.927

1 72 4 33 II 39 5 07 7 06 3 83 1 53 4 13 ii 26 18 42 8 51 5 73 3.33 4.78 8.91

59.57 88.04 96.85 96.12 99.08 96 62 98 i0 97 97 98 96 99 64 99 28 99 68 99 34 99 33 99 37

0.77 12.44 40.48 35.20 69 81 37 46 53 32 51 39 67 26 85 87 74 62 86 89 75 96 75.49 76.25

685 2 450 3 450 0 471 7 493 7 501 7 488.7 488,4 488.6 484.5 485.6 483.5 485.3 484.6 486.4

+ + + + + + + + + + + + + + +

ii.i 3.0 2.7 16 1.5 3.6 3.5 2.4 1.7 1.4 2.2 2.6 2.3 2.5 2.4

8,149

i00.00

97 55

63.90

484. I +

2.2

485.8 +

1.7

700 800 860 880 900 920 940 960 980 i000 1015 1030 1045 1080 Fusion

80.55 33.36 30.28 32.18 32.88 34 34 32 82 32 84 32 53 32 Ol 32 21 31 93 32 17 32 12 32 25

0 11106 0 01540 0 00539 0 00649 0 00334 0 00624 0 00447 0 00459 0 00344 0 00264 0.00302 0.00253 0.00289 0.00290 0.00283

3 7 8 8 8 8 8 8 8 8

Total

33.01

0.00601

Total without

Sample

12:

139 044 027 399 569 600 769 679 519 342

8 288

700-920°C

65.07

J - 0.009800

700 800 830 850 865 880 895 910 925 940 955 970 990 1015 1040 Fusion

83.53 45 73 41 64 34 32 31 50 30 12 30 28 29 17 28.06 27.58 27.97 27.45 27.23 27.60 27.72 27.68

0.07618 0.01753 0.00414 0.00432 0.00305 0.00382 0.00493 0.00256 0.00344 0.00302 0.00525 0.00388 0.00300 0.00250 0.00278 0.00232

4.133 5.725 8.308 8.949 9.365 9.286 8.783 8.817 9.154 9.106 8 684 8 954 9 194 9 070 8 766 8 363

Total

32.38

0.00727

8.620

4 29 5 72 2 99 4 42 5 56 9 70 4.42 4.32 9.14 8.87 2.95 5.40 10.99 13.19 4.46 3.57 I00.00

73.44 89.67 98.65 98.35 99.51 98.71 97 50 99 81 98 97 99 40 96 82 98 43 99 43 99 94 99 56 99 93

1.48 8.88 54.55 56.29 83.55 66.19 48.45 93 61 72 28 81 98 45 01 62 80 83 30 98 71 85 71 98.11

851.0 611.2 613.0 518.1 485.7 463.7 460.7 455.0 436.4 431.4 433. i 425.9 426.8 433.8 434.0 434.7

94.47

70.04

477.1 +

+ + + + + + + + ± + + + + + + +

6.3 3.1 3.4 3 2 3 3 3 6 4 0 3 7 2 0 1 9 2.4 2.5 2,2 1.7 2.6 1.9 2.8

29

EXOTIC CALEDONIANNAPPES IN TROMS, NORWAY

Sample

16

575 650 700 735 765 790 810 830 850 865 880 895 915 945 Fusion Total

J -

0.006991

443 61 75 33 62 23 51 57 42 97 39 59 38 37 38 34 38 30 37 88 37 49 37 50 39 35 41 97 42.43

1.19463 0.14462 0.09352 0.04710 0 01832 0 00888 0 00612 0 00452 0 00431 0 00353 0 00333 0 00523 0 00966 0.01510 0.00899

5 221 3 955 6 064 7 345 7 539 7 256 7 032 6 997 6.917 6.913 6.740 6.439 8.451 9.919 6.895

0.52 0.67 0.97 1.16 1.94 4.87 10.79 19 84 23 08 16 59 8 82 4 86 2 15 1 95 1 79

41.20

0.01377

6.994

i00.00

95 98

+

8.6 3.5 2.0

21.94

890 9 374 3 397 2 429 4 428 7 422 5 418 1 422 5 422 7 420.8 419.2 416.4 419.0 430.9 449.9

+ + + + + + +

1.7 2 9 3 4 4 3 6 9 7 2 17 1

41.75

423.4 +

6 3

421.0 +

2.4

+ +

6.3 2 1

+ + +_ + + + + + + _+

2 1 2 1 1 2 2 1 1 1

+ +

1.4 1.6

0.12 0.78 1 85 4 46 ii 77 23 37 32 87 44 23 45.89 55.98 57.81 35.24 25.01 18.79

79.11

Total without 575-790°C and 895°C-fusion

Sample 17:

20.51 43.66 56.35 74.14 88.80 94.82 96.74 97.96 98.10 98.69 98.79 97.23 94.46 91.28 95.02

65.0 45.2 26.2 19.9

J - 0.009555

700 800 830 850 865 880 890 905 920 940 960 980 I000 1025 Fusion

72.84 32.90 31 23 30 75 30 81 30 90 31 13 31 14 30 41 29 82 29 56 29 79 29 84 29 79 29 89

0.10134 0.00972 0.00584 0.00366 0.00290 0.00264 0.00203 0.00189 0.00144 0.00179 0.00251 0.00205 0.00157 0.00237 0.00125

1.860 4.566 5.774 4.988 4.709 4 576 4 491 4 467 4 378 4 251 4 234 4 297 4 367 4 736 4 576

1.72 3.73 4.84 8.69 10.14 9.10 8.40 5.82 5.93 11.04 6.45

Total

31.23

0.00440

4.520

8.44 4.20 3.63

59.08 92.37 95.94 97.76 98.43 98.65 99.21 99.34 99 74 99 35 98 62 99 ii 99 61 98 91 99 97

0.50 12.78 26.89 37.04 44.13 47.18 60.10 64.31 82.70 64.54 45.86 57.08 75.89 54 40 99 28

622 0 461 1 455 7 456 8 460 2 462 3 467 7 468 3 460 2 450.8 444.3 449.4 452.0 448.7 454.2

+

2.0

i00.00

97.91

54 34

459.5 +

1.6

7.88

*measured. Ccorrected for post-irradiation decay of 37Ar +[40Ar tot.

+ 103.2

+ + + + + +

(36Aratmos)

(35.1 day i/2-1ife).

(295.5)] / 40Artot.

**calculated using correction factors of Dalrymple et al. (1981); two sigma, intralaboratory errors.

0 8 2 8 6 3 4 0 2 0

30

R.D. DALLMEYERAND A. ANDRESEN

15A

~o~

L~

I

500

425.8 ± 2.1

I

45o

I

I

I

i

t

l

425.8 ± 1.0

< 400

,

350

,

,

20

,

,

,

40

,

60

,

J

8O

1 O0

20

40

60

80

100

20

40

60

80

100

C U M U L A T I V E % 39Ar R E L E A S E D

Fig. 5.4°Ar/39Ar apparent age and apparent K / C a spectra for hornblende concentrates from the Nordmannvik Nappe, Troms. Two sigma, intralaboratory uncertainties indicated by vertical width of bars. Calculated plateau ages are listed and plateau increments are delineated. Experimental temperatures increase from left to right.

500F

6

422.2 + 1.3

45O

10

11

1

400

,

350

LU 500 (.9 <

15B i

i

i

"~

500

I

|

I

I

i

I

i

I

I

424.8 _+1.0

450

4 3 1 8 -+ 1 . 5

400

350 20

CUMULATIVE

40

60

80

100

%39Ar RELEASED

o

20

40

6o

8o

loo

o

20

40

6o

8o

loo

C U M U L A T I V E %39Ar RELEASED

Fig. 6.4°Ar/39Arage spectrafor muscoviteconcentratesfrom the NordmannvikComplex, Troms. Data plotted as in Fig. 5.

Fig. 7 . 4 ° A r / 3 9 A r apparent age and apparent

with "early" Caledonian tectonothermal activity. Deformation and metamorphism of the Balsi~ord Group has been associated with late Silurian/early Devonian (Scandian) orogenesis. The highest structural elements in Troms are represented by the heterogeneous Tromso Nappe Complex (Andresen et al., 1985) which is composed of three distinct tectonostratigraphic units which include: (1) a Lower Tectonic Unit which includes various gneisses, amphibolites, schists and metaigneous rocks; (2) the Skattora Gneiss which

is dominated by amphibolitic gneisses intruded by numerous anorthositic dikes; and (3) the Tromsdalstind Complex which is comprised of garnetmica schist, quartzofeldspathic gneisses, calc-silicate gneiss, eclogite-bearing marbles, kyanite-garnet mica schists and biotite-microcoline gneiss. Variably serpentinized ultramafic bodies occur locally. A relatively late ductile thrust fault separates the Tromsdalstind Sequence from the Skattora Gneiss and locally excises all lower structural levels of the Tromso Nappe Complex.

K/Ca spectra

for

hornblende concentrates from the Balds0ord Group (Lyngen Nappe Complex), Troms. Data plotted as in Fig. 5.

31

EXOTIC CALEDONIAN NAPPES IN TROMS, NORWAY

500t

plexes in Ofoten and north Troms has long been used (e.g., Binns, 1978; Boyd, 1983; Barker, 1986; Steltenpohl et al., 1990) to support tectonostratigraphic correlations between (Fig. 4): ( 1 ) the Balsfjord Group and the Salangen Supergroup; (2) the Lyngen Ophiolite and the Gratangseidet mafic complex (Gratangseidet Nappe of Barker, 1986): and (3) various high-grade metamorphic units which structurally underlie the mafic complexes. Description of an erosional unconformity between the Salangen Supergroup and underlying Lillevik Dike Complex (with associated mafic schists and metaigneous rocks) supports this correlation (e.g., Steltenpohl et al., 1990). In addition, Steltenpohl and Andresen ( 1991 ) mapped mafic rocks associated with the Lillevik Dike Complex around the closure of the Ofoten Synform and into mafic schists of the Gratangseidet Nappe described by Barker (1986). In agreement with Boyd (1983) and Barker ( 1986 ), Steltenpohl and Andresen ( 1991 ) proposed that a regional ductile thrust fault (~se thrust) separates these lower grade rocks from the higher-grade units comprising the Narvik Nappe Complex along the eastern limb of the Ofoten Synform (Fig. 4 ). The Upper Allochthon exposed in north Troms appears to include (in ascending tectonostratigraphic order: Fig. 4) the Vaddas, K~fjord and Nordmannvik Nappes, and the Lyngen Nappe Complex. Units directly correlative with the regionally extensive Seve Nappe Complex exposed in northern and southern sectors of the Scandinavian

3 417.4 _+ 1.2

450 [

400 t - -

A 3~ol

.

.

.

.

.

.

.

.

.

U.I

(5 500 <

13

450

423.6 + 1.4

I

4OO 35O

20

40

60

80

100

CUMULATIVE %39Ar RELEASED Fig. 8.4°Ar/39Arapparent age spectra for muscoviteconcentrates from the BaldsfjordGroup (LyngenNappe Complex), Troms. Data plotted as in Fig. 5. Tectonostratigraphic correlations between Ofoten and north Troms

Anderson et al. (1992) reported results of an integrated petrological/geochronological research program carried out in upper level Caledonian nappes exposed in the Ofoten region (Fig. 1 ). To facilitate comparison of their 4°Ar/aqAr results with those described herein a brief discussion of tectonostratigraphic correlations is presented. The occurrence of extensive marble sequences together with distinctive metaconglomerate/diamictite intervals unconformably overlying mafic com10

2A

14

01

500

-4-

i

I

I

I

I

q

I

I ~ I I L I I I p

d

450

<

456.5 + 1.1

400 35O

20

40

60

80

100

20

40

60

448.3 _+ 1.9

80

100

20

40

60

80

100

CUMULATIVE % 39Ar RELEASED

Fig. 9. 4°Ar/39Arapparent age and apparent K/Ca spectra for hornblende concentrates from the LowerTectonic Unit of the Tromso Nappe Complex, Troms. Data plotted as in Fig. 5.

32

R.D. D A L L M E Y E R A N D A. A N D R E S E N

~

~°°I

2B

450!~

410.0+ 1.0

350

I

Kvernmo Nappes in the area of Gratangen: Barker, 1986).

Previous geochronology

0

20

40

60

80

100

CUMULATIVE %39Ar RELEASED Fig. 10. 4°Ar/39Ar apparent age spectrum for a muscovite concentrate from the Lower Tectonic Unit of the Tromso Nappe Complex,Troms. Data plotted as in Fig. 5.

[

17

12

[

I

!

qj -

lo[

10

:~00

+

~

,

i

,

i

i

,

I

421 0E + 2 4

450

~

16

< 400 150

20

40

60

80

CUMULATIVE

I

[-

4858= 1 7

L

100

0

20

40

60

80

100

% 39Ar R E L E A S E D

Fig. 11.4°Ar/39Arapparent age and apparent K/Ca spectra for hornblende concentrates from the TromsdalstindComplex of the Tromso Nappe Complex,Troms. Data plotted as in Fig. 5. Caledonides have not been identified in the Ofoten-north Troms area. However, similarities in both metamorphic grade and lithologic characteristics suggest correlations between (Fig. 4): ( 1 ) the Vaddas and Hogtind Nappes (parts of the regionally extensive K61i Nappe Complex); and (2) the K~l]ord-Nordmannvik Nappes and the Narvik Nappe Complex (including the Gronfjellet and

There has been little geochronologic data reported from the Troms Caledonides. Dangla et al. (1978) and Quarnardel et al. (1978) presented several poorly-defined, Rb-Sr whole-rock "errorchrons" of c. 440-450 Ma for foliated felsic intrusive units in the KMjord Nappe. Griffin and Brueckner (1985) reported a poorly-defined SmNd mineral isochron age of 598 4- 107 Ma for eclogite within the Tromso Nappe Complex. However, the analyzed sample was extensively retrogressed and Krogh et al. (1990) questioned the geological significance of the 598 Ma age. Krogh et al. (1990) presented a seven-point, Rb-Sr whole-rock isochron age of 433 ___11 Ma for fine-grained biotitemicrocline metagranite within the Lower Structural Unit of the Tromso Nappe Complex. They also listed K-Ar ages between 448 Ma and 436 Ma for three amphibole concentrates from units of the Skattora Gneiss and the Tromsdalstind Complex. Lindstrom (1988) presented an eight-point Rb-Sr whole-rock isochron age of 432 _ 7 Ma for syntectonic granite within upper portions of the Balsfjord Group. Lindstrom (1988) also reported a Rb-Sr whole-rock isochron age of 492 _+5 Ma for a pyroxene-bearing metaigneous rock within the Nordmannvik Nappe. This suggests that a pre-Scandian tectonothermal history must be at least locally recorded. 4°Ar/39Ar mineral ages have been reported from Caledonian nappes and the structurally underlying basement rocks within the Western Gneiss Terrane exposed on the islands of Senja and Kvaloy in western Troms (Fig. 2: Cumhest et al., 1983; Clark et al., 1985; Cumhest and Dallmeyer, 1985; Dallmeyer, 1991). Northeast of the Svanfjellet Shear Zone (fig. 2), intracrystalline argon systems in basement rocks record no Caledonian rejuvenation and preserve a complex Precambrian evolution. Southwest of the Svan0ellet Shear Zone, intracrystalline argon systems have been variably rejuvenated by several Caledonian tectonothermal events. Muscovite throughout southwest Senja (in Caledonian nappe units and retrogressed basement rocks) records 4°Ar/39Ar plateau ages ofc. 380-390

EXOTIC CALEDONIANNAPPES IN TROMS, NORWAY

33

TABLE 2 36Ar/4°vs. 39Ar/4°Ar isotope correlations from incremental-heating experiments on hornblende concentrates from Caledonian nappe complexes, Troms, Norway Sample

Isotope Correlation Age (Ma)*

4°Ar/36Ar Intercept**

MSWD

Increments Included +

% of Total 39Ar

Calculated 4°Ar/39Ar age (Ma)***

Normanvik Nappe Complex 5 15A

421.6 +_1.6 421.2 + 1.4

383.8 _+27.2 742.7 _+49.3

1.81 1.92

930-fusion 880-fusion

92.42 98.06

425.8 _+2.1 425.8 _+1.0

1416.6+ 112.4 915.9 _+81.2

0.91 0.23

1010-fusion 880-1080

56.63 85.01

431.8_+ 1.5 no plateau defined

864.4 _+62.1 352.7 _+13.9 434.7_+ 47.3

1.69 0.44 0.82

910-fusion 800-fusion 955-fusion

90.47 97.92 95.76

no plateau defined 448.3 _+1.9 456.5 _+1.1

537.2_+43.2 1177.2 _+41.7 419.1 _+18.6 2009.1 _+210.5

0.15 0.84 0.54 2.21

960-fusion 880-1040 810-880 850-1025

65.07 74.32 79.11 86.09

485.8_+ 1.7 no plateau defined 421.0_+2.4 no plateau defined

Lyngen Nappe Complex Bals0ord Group 11 423.8+2.8 1 427.4 _+3.3

Tromso Nappe Complex Lower Tectonic Unit 2A 431.7 _+1.9 14 444.7 _+1.4 8 452.0_+ 2.6 Tromsdalstind Complex 10 481.1 _+1.8 12 422.8 _+2.2 16 419.4_+2.1 17 428.7 _+3.9

*Calculated using the inverse abscissa intercept (4°Ar/agAr ratio ) in the age equation. **Inverse ordinate intercept. ***Table 1.

5OO

7 418.4 _+1.4

450

40O

350 IJ.I

,

,

,

,

,

500

L,

,,

,

9

< 450

I

427.2

_+ 1 . 0

i

|

4O0

35C

.

0

.

20

.

.

.

.

40

.

6O

80

100

CUMULATIVE %39Ar RELEASED Fig. 12.4°Ar/39Ar apparent age spectra of muscovite concentrates from the Tromsdalstind Complex, T r o m s o N a p p e Complex, Troms. Data plotted as in Fig. 5.

Ma. Biotite records similar ages in Caledonian nappe units. It is complicated by extraneous argon components in retrogressed basement lithologies. Anderson et al. (1992) reported 4°Ar/39Ar incremental-release ages for hornblende and muscovite from various Caledonian allochthons exposed in the Ofoten area (Figs. 1 and 4). Most of the muscovite concentrates yielded internally concordant age

spectra; however, most hornblende concentrates were characterized by internally discordant age spectra with poorly-defined isotope correlations. Hornblende from amphibolite in two structural units of the Narvik Nappe Complex (Kvernmo and Dyroy Nappes) displayed generally concordant release spectra which yielded well-defined isotope correlation ages of 419.5 + 1.6 Ma and 417.6 + 2.1 Ma. Muscovite from the Kvernmo Nappe recorded a 427.0 + 2.0 Ma plateau age. Muscovite within the structurally underlying Hogtind Nappe (lowermost nappe of the Upper Allochthon exposed in Ofoten) yielded a 394.9+0.9 Ma plateau age. Muscovite within the Fossbakken Nappe of the Middle Allochthon (broadly correlative with the M~lselv Nappe in north Troms: Fig. 4) yielded a 390.0+0.9 Ma plateau age. The highest tectonostratigraphic levels of the Upper Allochthon exposed in the Ofoten area are represented by the Salangen Supergroup which is tectonically separated from the structurally underlying Narvik Nappe Complex by an out-of-sequence thrust fault Ose Thrust). Muscovite within the Smatinden and Gratangen Nappes of the Salangen Supergroup yielded plateau ages of 373.4 + 0.9 Ma and 388.4 + 0.9 Ma. Muscovite from the structurally highest unit in the Ofoten area (Niigen

34

R.D. DALLMEYERAND A. ANDRESEN

Table 3. 40Ar/39Ar analytical data for incremental heating experiments on muscovite concentrates from Caledonian nappe complexes, Troms, Norway.

Release (40Ar/39Ar)* (36Ar/39Ar)* (37Ar/39Ar) c temp (°C)

39Ar % of total

%4OAr 36Arca non% + atmos.

Apparent Age (Ma)**

Nordmannvik Nappe Complex Sample 6:

J = 0.010025 + + + + + +

2 1 1 1 1 1

+ + +

1.8 1.6 1.7

0.18

422.2 +

1.3

91.83 95.49 97.86 98.75 98.16 97.52 97.70 98.22

0.03 0.05 0.08 0.16 0.14 0.08 0.09 0.09

451 432 424 422 425 424 428 429

+ + + + + + + +

1.8 i.I 1.0 0.7 1.0 2.1 1.3 1.3

97.53

0.I0

426.9 +

I.I

424.8 +

1.0

650 730 770 800 830 880 940 i010 Fusion

28.15 27.03 26.99 26.68 26.65 26.90 26.89 26.76 26.55

0.00642 0.00188 0.00233 0.00145 0.00149 0.00210 0.00175 0.00152 0.00081

0.015 0.012 0.010 0.009 0.008 0.011 0.0!0 0.009 0.013

3.31 7.46 12.30 15.17 10.50 8.27 13.75 19.23 i0.01

93.25 97.93 97.43 98.38 98.33 97.67 98.05 98.30 99.09

0 07 0 18 0 ii 0 16 0 15 0 14 0 15 0 17 0.42

421.4 424.6 422.1 421.4 420.7 421.7 423.1 422.2 422.3

Total

26.84

0.00180

0.010

i00.00

98.00

Sample 15B:

J = 0.009901

600 700 740 760 780 800 830 Fusion

31.24 28.63 27 37 26 97 27 37 27 48 27 69 27 66

0.00862 0.00435 0.00196 0.00112 0.00169 0.00229 0.00213 0.00165

0.010 0.007 0.006 0.007 0.009 0.007 0.007 0.005

3.70 14.80 24.76 21.37 16.23 4.84 4.79 9.52

Total

27.66

0.00233

0.007

I00.00

Total without 600-700°C

81.50

0 1 3 2 5 5 1 7

2 5 5 5 5 9

35

EXOTIC CALEDONIANNAPPES IN TROMS,NORWAY

Lyngen Nappe Complex BalsfJord Group Sample

3:

J - 0.010110

0.00006

0 038 0 020 0 006 0 007 0 005 0 008 0 006 0 008 0.006 0.006 0.008

0.82 3.79 5.34 11.89 16.99 8.86 7.30 11.21 11.99 18.13 3.68

88 41 98 94 99 29 98 95 99 37 99 31 99 30 99 43 99 57 99 54 99 91

0.09 0.58 0.26 0.21 0.27 0.37 0.29 0.44 0.48 0.45 3.72

426.2 417.9 416.8 417.5 417.5 417.7 419.3 416.7 416.8 417.3 421.5

+ + + + + + + + +

3.5 2.5 2.3 1.5 1.8 1.9 1.8 1.6 1.4 1.5 2.1

0.00063

0.007

i00.00

99.28

0.49

417.6 ±

1.4

417.4 +

1.2

600 700 765 780 790 815 845 880 930 980 Fusion

29.82 26.06 25,89 26,03 25.92 25.95 26.06 25.85 25.82 25.86 26.06

0 01169 0 00092 0 00060 0 00090 0 0O054 0 00059 0 00060 0.00048 0.00036 0.00038

Total

25.96

95.50

Total without 600°C and fusion

Sample

13:

+

+

J = 0.010105

600 650 690 715 735 765 795 825 860 920 Fusion

28.47 28.32 26.92 26.56 26.24 26.24 26.40 26.40 26.34 26.62 26.31

0.00629 0.00081 0.00087 0.00114 0.00017 0.00062 0.00085 0.00079 0.00044 0.00099 0.00012

0.022 0.018 0.011 0.010 0.007 0.008 0.011 0.010 0.009 0.010 0.013

2 22 5 34 4 61 9 O0 ii 67 14 55 12 48 6 83 10.61 12.22 10.48

93.45 99.13 99,02 98.72 99.79 99.29 99.03 99.10 99.49 98.88 99.84

0.09 0,59 0.34 0.23 1.19 0.35 0.36 0.36 0.57 0.27 2.82

429.6 450.5 430,2 423.9 423.5 421,6 422.8 423.2 423.8 425.4 424.7

+ + + ± + + + + + + +

5.2 2.9 3.3 1.5 1.6 1.6 1.9 1.6 1.5 1.4 1.7

Total

26 55

0.00078

0.010

100.00

99.13

0.72

425.4 +

1.5

423,6 ±

1.4

Total without

600-690°C

87.83

36

R.D, DALLMEYER AND A, ANDRESEN

Troms~ Nappe Complex Lower Tectonic Unit Sample 2B:

J - 0.009622

600 650 690 720 750 780 810 840 870 900 950 Fusion

27 27 27 26 26 26 27 26 26 26 26 27

47 61 46 84 67 98 02 93 83 82 90 03

0.00664 0.00275 0,00267 0.00122 0.00098 0.00118 0,00139 0.00167 0.00107 0.00148 0.00093 0,00049

0.042 0.063 0.048 0.046 0.046 0.041 0,031 0.029 0.029 0.032 0.029 0,i02

3.12 5.07 ii.01 12.48 23.41 6.83 5,73 7.22 6,01 4.68 11.68 2.75

Total

26.97

0.00156

0.042

I00.00

Total without 600°C and fusion

92 85 97 05 97 12 98 65 98 90 98 69 98 47 98 16 98 81 98 36 98 96 99.48

0.17 0.62 0.49 1,03 1.27 0.94 0.61 0.47 0.72 0,59 0.85 5.70

395.9 413.8 412,0 409.4 408.0 411.4 411.1 408.8 409.9 408.1 411.3 415.1

98.29

0.98

409.7 +

1.6

410 0

1.0

94.12

+ + + + + + + + + + + +

3.2 1.5 1.9 1 6 1 3 1 9 1 3 1 8 1 9 1 6 i,i 2.1

Tromsdalstind Complex Sample 7:

J - 0.010065

600 650 690 715 735 765 795 825 860 920 Fusion

29.04 26.68 26.29 26.28 26.38 26.45 26.27 26.45 26.33 26.13 26,34

0.01015 0.00241 0.00144 0.00107 0.00144 0.00150 0,00126 0.00137 0.00136 0.00091 0.00064

0.023 0.004 0.010 0.007 0.017 0.007 0.006 0.005 0.007 0.006 0.007

1.91 3 95 7 09 9 82 9 15 I0 79 12 85 12 15 12.82 16.62 2.85

Total

26.38

0.00146

0.008

i00.00

Total without 600°C and fusion

95.24

+ + + +

89,66 97.31 98.36 98.78 98.37 98.31 98.57 98.45 98.45 98.95 99.26

0.06 0.04 0.19 0.18 0.31 0.13 0.12 0.i0 0 14 0 19 0 29

419 9 418 7 417.3 418.7 418.6 419.4 417.7 419.9 418.2 417.3 421.5

9 8 .3 6

0 16

418.5

1.7

418.4 +

1.4

+ + + + + +

3.9 2.9 2.1 1 9 1 7 1 7 1 5 1 7 1 3 1.2 2.9

37

EXOTIC CALEDONIAN NAPPES IN TROMS, NORWAY

Sample 9:

J - 0.009702 + + + +

3.5

+ +

0.9 1.0

+ + + + +

1.2 I.i

0.9

0.07

430.1 435.0 432.6 427.7 424.7 425.8 427.5 428.8 428.5 428.9 424.6

0.07

427.6 +

1.2

427.2 +

1.0

0.009 0.007

2.78 3.90 8.44 10.31 14.77 15.83 8.21 10.57 5,48 7.78 11.93

90.67 96.75 95.15 95.27 97.16 97.68 96.46 95.27 95.19 95.47 97.14

0.05 0.09 0.05 0.05 0,09 0,09 0,07 0.04 0,05 0,05

0.008

I00.00

96.18

600 650 690 715 735 765 795 825 860 900 Fusion

30.61 29.06 29.36 28.95 28.17 28.10 28.58 29.03 29.04 28.98 28.17

0.00965 0.00318 0.00480 0.00462 0.00269 0.00218 0.00341 0.00463 0.00471 0.00442 0.00271

0.018 0.010 0.009 0.009 0.008 0.007 0.009 0.007

Total

28.68

0.00371

0.008

Total without 600-650°C

93.32

1.8 1.9 1.2

1.0 I.i

measured. Ccorrected for post-irradiation decay of 3TAr (35.1 day i/2-1ife). +[40Artot.

(36Aratmos.)

(295.5)] / 40Artot

.

**calculated using correction factors of Dalrymple et al. (1981); two sigma, intralaboratory errors.

Nappe Complex: Uppermost Allochthon; Fig. 4) yielded a muscovite plateau age of 392.6 _+1.8 Ma.

Analytical techniques The techniques used during 4°Ar/39Ar analyses of the mineral concentrates generally followed those described in detail by Dallmeyer and Gil-Ibarguchi (1990). Optically pure (> 99%) mineral concentrates were wrapped in aluminum-foil packets, encapsulated in sealed quartz vials and irradiated in the U.S. Geological Survey TRIGA reactor. Variations in the flux of neutrons along the length of the irradiation assembly were monitored with several

mineral standards, including MMhb- 1 (Samson and Alexander, 1987 ). The samples were incrementally heated until fusion in a double-vacuum, resistance heated furnace. Temperatures were monitored with a direct-contact thermocouple and are controlled to 1 ° C between increments and are accurate to + 5 ° C. Blank-corrected isotopic ratios were adjusted for the effects of mass discrimination and interfering isotopes produced during irradiation using empirically determined factors. Apparent 4°Ar/39Ar ages were calculated from the corrected isotopic ratios using the decay constants and isotopic abundance ratios listed by Steiger and J~iger (1977). Intralaboratory uncertainties are reported and have been calculated by statistical propagation of

38

uncertainties associated with measurement of each isotopic ratio (at two standard deviations of the mean) through the age equation. Interlaboratory uncertainties are c. -+ 1.25-1.50% of the quoted age. Analysis of the MMhb-1 monitor indicates that apparent K/Ca ratios may be calculated through the relationship 0.518 (_+0.005) Xa9Ar/37Ar corrected. Analyses of the amphibole concentrates have been plotted on 36Ar/4°Ar vs. 39Ar/4°Ar isotope correlation diagrams using the regression techniques of York (1969). Total-gas ages have been computed for each sample by appropriate weighting of the age and percent 39Ar released within each temperature increment. A "plateau" is herein considered to be defined if the ages recorded by two or more contiguous gas fractions, each representing > 4% of the total 39Ar evolved and characterized by generally similar apparent K/Ca ratios (and together constituting > 50% of the total quantity of 39Ar evolved) are mutually similar within a + 1% intralaboratory uncertainty.

Results

Twelve hornblende and seven muscovite concentrates have been analyzed from samples collected within various Caledonian nappe complexes exposed in the Troms region. Sample locations are indicated in Fig. 2 and coordinates are listed in the Appendix. The 4°Ar/39Ar analytical data are listed in Tables 1 and 3 and are portrayed as age spectra in Figs. 5-12. 36Ar/4°Ar vs. 39Ar/4°Ar isotope correlations are listed in Table 2. Apparent K/Ca ratios are relatively small and display considerable intrasample variations in the hornblende analyses. Apparent K/Ca ratios recorded by increments evolved from the muscovite concentrates are very large with considerable associated uncertainties. For this reason, and because they do not display any significant and/or systematic intrasample variations, they are not shown with the muscovite age spectra. Considerable variation in apparent age is displayed by most gas fractions evolved at low experimental temperatures from the hornblende concentrates. These are matched by fluctuations in apparent K/Ca ratios which suggest experimental evolution of argon from compositionally distinct, and relatively non-retentive, phases. These could be represented by: ( 1 ) very minor, optically undetect-

R.D. DALLMEYER AND A. ANDRESEN

able mineralogical contaminants in the amphibole concentrates, (2) petrographically unresolvable exsolution or compositional zonation within constituent amphibole grains, (3) minor chloritic replacement of amphibole, and/or, (4) intracrystalline inclusions. Intermediate- and high-temperature gas fractions evolved from the hornblende concentrates are characterized by generally similar apparent K/Ca ratios. This suggests experimental evolution of argon occurred from compositionally uniform populations of intracrystalline sites during these portions of the analyses.

Nordmannvik Nappe Hornblende concentrates were prepared from samples of blastomylonitic garnet amphibolite collected at three locations in the Nordmannvik Nappe (4, 5 and 15A). Concentrates from samples 5 and 15A record intermediate- and high-temperature plateaux (Fig. 5) which define ages of 425.8 +2.1 Ma (5) and 425.8_+ 1.0 Ma (15A). 36Ar/4°Ar vs. 39Ar/4°Ar isotope correlations of the plateau data are well-defined (MSWD < 2.0) with inverse ordinate intercepts only slightly larger than the 4°Ar/36Ar ratio in the present-day atmosphere (Table 1 ). These indicate no significant intracrystalline contamination with extraneous argon components. Using inverse abscissa ratios (4°Ar/39Ar) in the age equation yields plateau isotope correlation ages of 421.6_+ 1.6 Ma (5) and 421.2_+ 1.4 Ma (15A). Because calculation of isotope correlation ages does not require assumption of a 4°Ar/a6Ar ratio, they are considered geologically significant. They are interpreted to date the last cooling through temperatures required for intracrystalline retention of radiogenic argon. Harrison (1981) suggested that temperatures of c. 500_+25°C are appropriate for hornblende argon retention at cooling rates likely to characterize most geologic settings. The hornblende concentrate from sample 4 displays an internally discordant age spectrum (Fig. 5 ) and isotope correlations are not defined by any combinations of the analytical data. The total-gas age therefore has uncertain geologic significance. Muscovite concentrates were prepared from two samples of mylonitic, garnet-bearing schist collected within the Nordmannvik Nappe (6 and 15B). Both concentrates display well-defined plateaux (Fig. 6) which define ages of 422.2+ 1.3 Ma (6)

39

EXOTIC CALEDONIAN NAPPES IN TROMS, NORWAY

and 424.8 + 1.0 Ma ( 15B ). These are interpreted to date the last cooling through temperatures required for intracrystalline retention of argon. Detailed experimental evaluation of these temperatures has not been carried out. However, using the preliminary experimental data of Robbins ( 1972 ) in the diffusive equations of Dodson (1973) suggest temperature of c. 375°C may be appropriate. These are comparable with those suggested for muscovite on the basis of empirical composition with isotopic systems within other mineral species (e.g., Wagner et al., 1977; J~iger, 1979).

Lyngen Nappe Complex Hornblende concentrates were prepared from two samples collected within the Balst]ord Group (upper tectonic levels of the Lyngen Nappe Complex). These included hornblende garbenschiefer ( 11 ) and mylonitic amphibolite ( 1 ). Both concentrates display internally discordant 4°Ar/39Ar age spectra (Fig. 7). Apparent ages defined by low-temperature gas fractions evolved from the concentrate from sample 11 systematically decrease to define an intermediate- and high-temperature plateau of 431.8 + 1.5 Ma. This type of spectra discordancy is characteristic of samples contaminated with significant intracrystalline extraneous argon (e.g., Dallmeyer and Rivers, 1983 ). Isotope correlation of the plateau data yields an inverse ordinate intercept very much larger than the 4°Ar/36Ar ratio in the present-day atmosphere and therefore confirms extraneous argon contamination. Using the inverse abscissa intercept in the age equation yields a plateau isotope correlation age of 423.8 + 2.8 Ma. The concentrate from sample 1 displays a complex release spectrum which does not correspond to a plateau. However, the 880-1080°C increments (representing c. 85% of the gas evolved from the sample) yields an isotope correlation age of 427.4 + 3.3 Ma. Both isotope correlation ages are considered geologically viable, and are interpreted to date the last cooling through appropriate argon retention temperatures in this portion of the Lyngen Nappe Complex. Muscovite concentrates were prepared from two samples of mylonitic, argillaceous quartzite collected in the BalsI]ord Group (3 and 13 ). Both concentrates display well-defined plateaux (Fig. 8) corresponding to ages of 417.4+ 1.2 Ma (3) and

423.6 + 1.4 Ma ( 13 ). These are interpreted to date cooling through c. 375 ° C.

Tromso Nappe Complex Lower Tectonic Unit Hornblende concentrates were prepared from three samples of garnet-bearing amphibolite collected within the Lower Tectonic Unit of the Tromso Nappe Complex (2A, 8 and 14). Concentrates from samples 8 and 14 record intermediate- and hightemperature plateaux (Fig. 9) of 456.5 +1.1 Ma and 448.3 + 1.9 Ma. Isotope correlations of the plateau data suggest only slight intracrystalline contamination with extraneous argon and yield plateau isotope correlation ages of 452.0 + 2.6 Ma (8) and 444.7 + 1.4 Ma (14). Intermediate- and high-temperature gas fractions evolved from sample 2A do not rigorously define a plateau (Fig. 8); however, the 910°C-fusion increments (representing c. 90% of all the gas evolved from the concentrate) yield a well-defined isotope correlation corresponding to an age of 431.7 + 1.9 Ma. Muscovite was concentrated from a sample of schist collected at location 2B within the Lower Tectonic Unit of the Tromso Nappe Complex. The concentrate displays a plateau age of 410.0 + 1.0 Ma (Fig. 10).

Tromsdalstind Complex Samples of mylonitic amphibolite were collected at locations 10, 12, 16 and 17 within upper structural levels of the Tromso Nappe Complex (the Tromsdalstind Complex). Hornblende concentrates from the four samples yield markedly contrasting results (Fig. 11 ). That from sample 16 defines a plateau of 421.0 + 2.4 Ma. The plateau data yield an isotope correlation corresponding to an age of 419.4 + 2.1 Ma. The concentrate from sample 10 is characterized by a much more complex release spectrum in which apparent ages fluctuate markedly in both low- and initial intermediate-temperature portions of the analysis (Fig. 11 ). However, the remaining intermediate- and high- temperature increments (representing c. 65% of the total gas evolved from the sample) record mutually similar apparent ages which correspond to a plateaux of 485.8 + 1.7 Ma. Isotope correlation of the plateau data yields an age of 481.1 + 1.8 Ma (Table 1 ). The hornblende concentrates separated from samples 12

40

and 17 display internally discordant release spectra that do not define plateaux (Fig. 11 ). However isotope correlation of most intermediate- and hightemperature increments are well-defined (Table 2), and yield geologically meaningful ages of 422.8 + 2.2 Ma (12) and 428.7+3.9 Ma (17). Muscovite concentrates were prepared from two samples of garnet-bearing schist collected at locations 7 and 9 within the Tromsdalstind Complex. These record well-defined plateaux (Fig. 12 ) which define ages of 418.4+ 1.4 Ma (7) and 427.2+ 1.0 Ma (9).

Interpretation Interpreting the geologic significance of the 4°Ar/ 39Ar results depends upon calibration of the Paleozoic time-scale (e.g., Palmer, 1983; Harland et al., 1989). Snelling (1985) and Kunk et al. (1985) suggested that the Ordovician-Silurian boundary (base of the Llandovery) is c. 435-440 Ma. This together with a 420 Ma calibration of the Ludlow (Wyborn et al., 1982) is used for interpretation of the 4°Ar/39Ar results from Troms. Hornblende and muscovite within the Nordmannvik Nappe and within upper tectonic levels of the Lyngen Nappe Complex (Bals00rd Group) record similar 4°Ar/39Ar plateau and 36Ar/4°Ar vs. 39Ar/4°Ar isotope correlation ages which range between c. 431 Ma and 410 Ma. These indicate relatively rapid post-metamorphic cooling through c. 500 °C and 375 ° C which likely occurred as a result of tectonic translation of nappe units to higher structural levels during east-vergent, Scandian thrusting. Combined with the late Ordovician-Silurian paleontologic age of the Bals00rd Group, the 4°Ar/39Arresults indicate a very narrow time interval between deposition, deformation, metamorphism and cooling through 500-375 °C. Muscovite within various structural levels of Tromso Nappe Complex records 4°At/39Ar plateau ages which range between c. 427 Ma and 410 Ma. These are generally similar to those recorded by muscovite and hornblende in structurally underlying nappe complexes and suggest that final imbrication of the various Troms nappes likely occurred while all units were maintained above c. 375 ° C. This is consistent with the ductile deformational mechanisms suggested by fabric characteristics.

R.D. DALLMEYER AND A. ANDRESEN

Hornblende within the Tromso Nappe Complex records 36Ar/a°Ar vs. 39Ar/4°Ar isotope correlation ages which suggest a significant pre-Scandian thermal evolution. Within lower tectonic levels isotope correlation ages range between c. 452 Ma and 432 Ma. There is no evidence of partial rejuvenation in any of the 4°Ar/39Ar spectra, and the 452-432 Ma age range is therefore interpreted to reflect diachronous cooling following a tectonothermal event which occurred prior to 452 Ma. Within upper structural levels of the Tromso Nappe Complex (Tromsdalstind Complex) hornblende at three localities was completely rejuvenated during Scandian tectonothermal activity and records 423-429 Ma cooling ages. However, at another location in the Tromsdalstind Complex a pre-Scandian (c. 481 Ma) cooling age is recorded.

Geologic significance The c. 431-410 Ma hornblende and muscovite cooling ages recorded in the north Troms region are slightly older than the c. 400-380 Ma dates reported by Anderson et al. (1992) from generally similar tectonostratigraphic levels exposed in the Ofoten area. The exotic nappe sequences exposed in both areas display a clear record of early to middle Silurian tectonothermal activity which most likely was associated with eastward thrust transport to higher crustal levels along the Baltic margin. The younger 4°Ar/39Ar cooling ages recorded in the Ofoten area suggest either a delay in Scandian tectonic transport and/or maintainence of initially deeper crustal levels. The regional tectonic significance of these contrasts is uncertain. The 4°Ar/a9Ar mineral ages recorded in north Troms clearly document the generally regionally penetrative nature of Scandian tectonothermal activity. However, pre-Scandian ages are locally preserved within hornblende at several locations with the Tromso Nappe Complex. These are regionally significant because they imply that early Paleozoic orogenic activity affected exotic terrane elements of the Scandinavian Caledonides. The relationship of these events to the "early" Caledonian tectonothermal record locally preserved in structurally underlying nappe units comprised of Baltic protoliths (e.g., Seve Nappe Complex) awaits future more detailed field and geochronologic work.

EXOTIC CALEDONIAN NAPPES IN TROMS, NORWAY

Acknowledgments Field work was supported in part by grants from the Norwegian Marshall F u n d ( R D D ) a n d the N A V F (AA: 47.31.035).

Appendix Sample

Mapsheet ( 1:50,000)

Grid reference (UTM)

1 2 3 4 5 6 7 8 9 I0 1t 12 13 14 15 16 !7

Helgoy Reinoy Ullsfjord Lyngen Stor0ord Sto~ord Tromso Tromso Malangseidet Malangseidet Lenvik Lenvik Malangseidet Malangseidet Takvatn Tran~y Malangseidet

474 687 410 433 493 385 745 236 671 020 667 007 209 220 266 183 208 034 212 010 012 986 014 924 259 977 238 041 215 707 597 567 209 086

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