the solund–stavfjord ophiolite complex and associated rocks,...

Geol. Mag. 127 (3), 1990, pp. 209-224. Printed in Great Britain 209

The Solund-Stavfjord Ophiolite Complex and associatedrocks, west Norwegian Caledonides: geology, geochemistry

and tectonic environmentH. FURNES*, K. P. SKJERLIE*, R. B. PEDERSEN*, T. B. ANDERSENf, C J. STILLMANJ,

R. J. SUTHREN§, M. TYSSELAND* & L. B. GARMANN**Geologisk Institutt, Avd. A, Allegt. 41, 5007 Bergen, Norway

t Institutt for Geologi, P.O. Box 1047, 0316 Blindern, Oslo 3, Norway% Department of Geology, Trinity College, Dublin 2, Ireland

§ Department of Geology, Oxford Polytechnic, Oxford OX3 0BP U.K.

(Received 18 May 1989; accepted! November 1989)

Abstract - Metabasalts of the Upper Ordovician Solund-Stavfjord Ophiolite Complex of thewesternmost Norwegian Caledonides, show N- to E-MORB affinity, with high Th/Ta (or Nb) ratiosgiving evidence of subduction influence. The Solund-Stavfjord Ophiolite Complex is overlain by aheterogeneous assemblage of sedimentary and volcanic rocks, the Stavenes Group, of which theHeggoy Formation of metasandstones and phyllites conformably overlies the metabasalts of theSolund-Stavfjord Ophiolite Complex. The Heggoy Formation contains, in places, abundantmetabasalt pillow lavas and minor intrusions, geochemically similar to those of the Solund-StavfjordOphiolite Complex, and basic metavolcaniclastites of island arc tholeiite (IAT) composition. Thisindicates that the Solund-Stavfjord Ophiolite Complex and Heggoy Formation developed in amarginal basin between a continental margin and an active subduction system, for which the present-day Andaman Sea may provide a realistic model. The other magmatic rocks of the Stavenes Group,showing both calc-alkaline and alkaline affinities, are less well time-constrained, but they are thoughtto represent an advanced stage of the island arc development, and ocean island build-up, respectively.

1. IntroductionThe area between Solund and Bremanger (Fig. 1)forms part of the westernmost NorwegianCaledonides. Tectonostratigraphic units comprisingvarious types of gneiss, continental margin sediments,an ophiolite complex associated with volcanic andsedimentary cover rocks, and sedimentary as well astectonic melanges, can be distinguished. The firstpetrographic and tectonostratigraphic studies werethose by Kolderup (1921, 1928). Later work bySkjerlie (1969, 1974) and Gale (1975) added con-siderable information, in particular on the greenstonecomplexes. An extensive review of the general geologyof the Stavfjorden area is summarized by Brekke &Solberg (1987), who divided the tectonostratigraphyinto lower, middle and upper tectonic units. Amodified tectonostratigraphic division has subse-quently been presented by Andersen, Skjerlie & Fumes(1990), and a brief description of the tectonic units(Fig. 1) is given below.

The lower tectonic unit comprises the VevringComplex of eclogite-bearing gneisses belonging to theWestern Gneiss Region of Precambrian age, and theAskvoll Group which consists of low- to medium-grade sedimentary, volcanic and plutonic rocks.

The middle tectonic unit, consisting of the DalsfjordSuite and the Hoyvik and Herland groups, is separated

from the lower tectonic unit by the extensionalKvamshesten Fault. The Dalsfjord Suite is composedof various syenitic to charnockitic orthogneisses,granites and gabbros (Kolderup, 1921), and has beencorrelated with similar rocks of the Jotun Nappe(Milnes & Koestler, 1985). The Hoyvik Group of pre-Silurian age, locally resting with a primary deposi-tional contact on the rocks of the Dalsfjord Suite,consists mainly of meta-arkoses and quartzites whichexperienced polyphasal deformation and metamor-phism in the upper greenschist - lower amphibolitefacies prior to the deposition of the Herland Group.The Herland Group (Brekke & Solberg, 1987) ofSilurian age, redefined by T. Berg (unpub. Cand.Scient. thesis, Univ. Bergen, 1988) and Andersen,Skjerlie & Furnes (1990), consists of two fossiliferousformations: the Sjoralden Formation of basal con-glomerates, quartzites, meta-arkoses and graphiticblack shales; and the overlying Brurastakken Form-ation of conglomerates, metasandstones, shales andmarbles.

The upper tectonic unit comprises the Solund-Stavfjord Ophiolite Complex and its cover of meta-sediments and metavolcanites.

A recent research programme of mapping, geo-chemical, geochronological, structural and sedimento-logical studies has been concentrated on the Solund-Stavfjord Ophiolite Complex and its cover and

210 H. FURNES AND OTHERS

TECTONOSTRATIGRAPHY

? KalvagMelange

O _O-^O_ <W3 W -,0

Dalsfjord SuiteFault i I i—i I I I I i

Askvoll Gp.Tectoniccontact

Western Gneiss Region

Devonian

x i Granodiorite

Gabbronorite/diorite

Y j Kalvag Melange

SOLUND - STAVFJORD OPHIOLITE COMPLEX (SSOC)AND COVER SEQUENCE (THE STAVENES GROUP)

Pillow lava,metavolcaniclastitesMetagreywacke, meta-volcaniclastites,lavasMetagreywacke, phyllite/with Las & intrusions Heggay Format.on

Metagabbro, sheeted dykes ) .pillow lava, metahyaloclastite) b5»UO

«| SunnfjordMelange

Herland Group

Heyvik Group

+ ] Dalsfjord SuiteAskvoll Group

W Gneiss Region

Figure 1. Simplified geological map of the Solund-Bremanger area, with the stratigraphy/tectonostratigraphy of the variousrock complexes.

substrate. The Solund-Stavfjord Ophiolite Complexis the youngest dated ophiolite complex in theScandinavian Caldedonides, based on a U-Pb zircondate of 443 + 3 Ma (Dunning & Pedersen, 1988). TheSunnfjord Melange, occurring between the upper andmiddle tectonic units, has largely tectonic boundariesbut in one crucial area is seen to overlie strati-

graphically the Herland Group (Fig. 1) with a deposi-tional contact. The melange developed duringophiolite obduction and thus provides a terrane linkbetween the Solund-Stavfjord Ophiolite Complexand the continental margin (Andersen, Skjerlie &Fumes, 1990). Fundamental for the interpretation ofthe tectonic environment of formation of the Solund-

Solund-Stavfjord Ophiolite Complex 211

Stavfjord Ophiolite Complex is the continental affinityof its sedimentary cover, and the MORB and IATcharacter of the intercalated volcanites/intrusionsand volcaniclastics, respectively. On the basis of theabundant geochemical data and our present knowl-edge of the field relationships between the Solund-Stavfjord Ophiolite Complex and associated rocks, wewill argue that the most appropriate geotectonicmodel is provided by the present-day Andaman Sea.

2. Geology of the Solund-Stavfjord OphioliteComplex and associated rocks

In the area between Solund and Flora (Fig. 1), therelationship between the Solund-Stavfjord OphioliteComplex and its cover can be demonstrated in anumber of places. The tectonostratigraphy of theSolund-Stavfjord Ophiolite Complex and the rocksbetween Kinn and Bremanger (Fig. 1) is uncertain,and can only be inferred. Brekke & Solberg (1987)included all the pre-Devonian strata of the uppertectonic unit in the Stavenes Group. In this context weretain this group name, but exclude the ophiolite, andfurther subdivide the remaining rocks into formal andinformal units. The ophiolite complex has in a numberof papers been referred to as the Solund-StavfjordOphiolite Complex (Furnes et al. 1986; Pedersen,Furnes & Dunning, 1988; K. P. Skjerlie, unpub. Cand.Scient. thesis, Univ. Bergen, 1988; Andersen, Skjerlie& Furnes, 1990; Skjerlie, Furnes & Johansen, 1989),and is now proposed as a formal name. The extensivesedimentary and volcanic sequence, which can bedemonstrated to overlie the Solund-StavfjordOphiolite Complex, will be given the formal nameStavenes Group. The Stavenes Group is divided intothe Heggoy Formation, which comprises the sedimentsand tholeiitic volcanic rocks conformably overlyingthe Solund-Stavfjord Ophiolite Complex, and theHersvik and Smelvjer units, which are volcanic/volcaniclastic and sedimentary sequences seen in theHersvik and Smelvaer/Moldvser areas (Fig. 1). Theyhave been given informal names since their precisestratigraphic positions are unknown, and theirchemistries are of calc-alkaline and alkaline affinities,respectively.

In the Bremanger area, a sedimentary melange, theKalvag Melange, is in a strongly sheared contact withquartzites which are correlated with the Hoyvik Group(Fig. I).

2.a. The Solund-Stavfjord Ophiolite Complex

The Solund-Stavfjord Ophiolite Complex mostlycomprises sheeted dykes and volcanic rocks, i.e. theupper part of the standard ophiolite pseudo-stratigraphy (e.g. Coleman, 1977). In a few places,however, and best preserved on the island of Tviberg(Fig. 1), high-level isotropic gabbro and diorite occur.

The gabbro is typically varitextured, with grain sizeranging from fine to pegmatitic (Fig. 2 a), and mayshow diffuse as well as sharp transitions to diorite(Pedersen, 1986; K. P. Skjerlie, unpub. Cand. Scient.thesis, Univ. Bergen, 1988). Faintly laminated meta-gabbro occurs in Solund (Slotteneset area, Fig. 1).Sheeted dykes (Fig. 2 b) are particularly well displayedon some of the southwestern islands in Solund (Fig. 1).Individual dykes range in thickness from a fewcentimetres up to c. 2 m (mostly commonly < 1 m). Acharacteristic feature of the Solund-StavfjordOphiolite Complex is the high proportion of volcanicrocks, which in the Solund and Staveneset areas(Fig. 1) comprise non-amygdaloidal pillow lavas andmeta-hyaloclastite breccias (Fig. 2c, d) (Furnes, 1972,1973, 1974; Furnes & Skjerlie, 1972; Furnes, Skjerlie& Tysseland, 1976). On the islands of Vasrlandet andAlden (Fig. 1), the volcanic succession is dominatedby sheet flows (Fig. 2e) and fossil lava lakes (Skjerlie,Furnes & Johansen, 1989). A composite profile of theSolund-Stavfjord Ophiolite Complex is shown in theleft-hand part of Figure 2.

An important tectonic feature of the Solund-Stavfjord Ophiolite Complex is the presence of abroad shear zone (c. 500 m) on the island of Tviberg(Fig. 1), in which serpentinite bodies were emplacedcontemporaneously with and prior to the last phasesof magmatic activity. This tectonic zone is consideredto have originated at the oceanic stage as part of atransform fault (K. P. Skjerlie, unpub. Cand. Scient.thesis, Univ. Bergen, 1988; Skjerlie & Furnes, inpress), which subsequently became the site whereobduction initiated, with the contemporaneous for-mation of the Sunnfjord Melange (Andersen, Skjerlie& Furnes, 1990).

2.b. The Stavenes Group

2.b.l. The Heggoy Formation

The sedimentary cover to the Solund-StavfjordOphiolite Complex has its largest extent in the areabetween Heggoy and Eikefjord (Fig. 1), and primarycontacts with the metavolcanites can be seen both onStaveneset and in Solund. The best preserved andmost important locality is on Slotteneset in Solund(Fig. 1), where a c. 3 m thick, dark green to blackschist rests with a primary conformable contact onpillow lava. The dark schist, composed mainly ofchlorite and magnetite, with subordinate garnet, pyriteand graphite, is rich in Fe, Mn, Cu, V, Zn and P,suggesting formation at an active spreading ridge(Boyle, in press). Intercalated with these sediments arefine laminae and beds of pale grey siltstone, composedof quartz, white mica and albite, with or withoutcalcite.

The cover metasediments are well preserved on theisland of Heggoy (Fig. 1), where a c. 1000 m thicksequence of predominantly calcareous metagreywackerests directly upon the sheeted dyke complex of the

GEO 127


Solund-Stavfjordencomposite profile

^7} Metasandstone, phyllite3 Sill= ^ Sheet flowsT | Lava laked | Pillow lava

A| Meta hyaloclastiterjJJ Sheeted dykes•+ | Dioritev /"c'/j Gabbro (massive or

varitextured)

Figure 2. Composite profile of the Solund-Stavfjord Ophiolite Complex, with photographs showing its components (a-e), andassociated cover sediments (0 of the Heggoy Formation, (a) Varitextured metagabbro grading into metadiorite; southwestTviberg. (b) Sheeted mafic dyke complex; Oldra. (c) Metahyaloclastite breccia; Oldra. (d) Slightly deformed pillow lava;Oldra. (e) Numerous submarine, massive sheet flows, interbedded with pillow lava; Alden. (0 Thin- to thick-bedded, littledeformed quartz-rich metasandstone of the Heggoy Formation; Tryggoy.

Solund-Stavfjord Ophiolite Complex. The meta-greywacke, hosting numerous intrusive bodies andpillow lava horizons (Fig. 3), is dominantly fine- tomedium-grained, thin- to thick-bedded, light to dark

greenish-grey metagreywacke (Fig. 2f) composed ofquartz, albite, and minor rock fragments of greenstoneand quartzite set in a matrix of white mica, chlorite,epidote and variable amounts of calcite. More


THE STAVENES GROUPHeggoy Formation (HF) Hersvik Unit (HU) Smelvaer Unit (SU)

m1000

800-

600-

400-

200-

0-1

Heggoy

Fault

Fault

Explanation (HF)

Phyllite

Metasandslone withphyllite interbeds

Pillow lava (Tholeiitic)

Basic intrusive sheets(Thol.)

Metasandstone withgreen volcaniclastiteinterbeds (Thol.)

Slotteneset

Devonian

«•'»'*•;*'

Basementunknown

Fault

Dykes & gabbroof the SSOC

-unconformityBasementunknown

Pillow lavaof the SSOC

Explanation (HU)

> • «• • Conglomerate

Metasandstone

with cgl. interbedswith green volcani-clastite interbeds

Massive lava &intrusions(calc-alcaline)

Explanation (SU)

Green volcaniclastiteswith chert interbeds

Massive lava

Pillow lava /with volcaniclastiteinterbeds

All magmatic componentsof alkaline composition

Figure 3. Volcanic and sedimentary development of the Stavenes Group, shown by composite stratigraphical logs of theHeggoy Formation and Hersvik and Smelvaer units.

comprehensive petrographic descriptions of the meta-greywacke are provided by Skjerlie (1974) and Furnes(1974). The metagreywackes occur as thick mon-otonous sequences, or alternate with dark greyphyllite, beds of quartzite, minor marbles, andgreenish-grey to dark green metavolcaniclastic rocks(Fig. 3, Slotteneset profile). These lithologies may allshow gradational as well as sharp boundaries to eachother.

The occurrence of metabasalts within the meta-sediments is highly variable. Thus in the Tryggoy area,on Heggoy and on the northern part of the StavenesPeninsula (Fig. 1), pillow lava, massive lava andminor intrusions occur abundantly, whereas furthernorth and north-northeast (in the area between Svanoyand Eikefjord, Fig. 1) only sporadic occurrences oflava can be found.

Due to Caledonian deformation, it is only possibleto reconstruct the sequence in a few places, such as onHeggoy, where younging directions can be observedin the graded-bedded metasandstone and inter-calations of pillow lava. The relationships between themetasediments and the metabasalts of the Solund-Stavfjord Ophiolite Complex are indicated in Figure 3.

2.b.2. The Hersvik Unit

The rocks of the Hersvik Unit (Fig. 1) were previouslydivided into three groups (the Hersvik, Mjelteviknesetand Arneset groups; Furnes, 1974). With new datashowing a coherent geochemical development of thevolcanogenic rocks throughout the sequence of theHersvik area (Fig. 1), we now find it unjustified tosustain this subdivision.

15-2


GranodioriteGabbronorite - dioriteChert / distal turbiditeConglomerateIgnimbriteMetasandstone (shallow marine)Brecciated metasandstone /metapelite

Figure 4. Simplified geological map of the Kalvag Melange (from Bryhni & Lyse, 1985) and the Gasoy Intrusion (from Furneset al. 1989). Photographs (see map for location) of typical block lithologies of the melange are. (a) Storm wave-generated bed(tempestite) of quartz-arenitic metasandstone. Note the sharp base and top, and the symmetrical ripples at the top of the bed.(b) Brecciated strata representing a debris flow, (c) Lower part of rhyolitic ignimbrite flow unit. Note the well-developed, large-scale eutaxitic structure in the upper part, and homogeneous (due to extremely strong welding) lower part of the deposit, (d)Thin-bedded chert deposit (mainly turbidites) of deep-marine origin, (e) Portion of a sequence of polymict mass-flowmetaconglomerates, between black meta-shales and thick metachert slump/turbidite deposits, representing submarineresedimentation of foreshore gravels.

The lower part of the sequence consists pre-dominantly of metagreywacke with abundant lavaflows and minor intrusions. Interbedded with themetagreywacke are beds of dark green meta-volcaniclastites and conglomerates (with a dominanceof quartzite pebbles), which both increase in abun-dance up-sequence (Fig. 3). A fuller description of thevarious lithologies has been given by Furnes (1974).

2.b.3. The Smelvoer Unit

The Smelvaer Unit is dominated by metabasalticvolcanic rocks. On the island of Smelvsr (Fig. 1),pillow laval predominates, but there are minoroccurrences of massive lava flows, intercalated withmetachert and brownish-green metavolcaniclastites(Fig. 3). The pillow lavas commonly show drain-outstructures (Ballard & Moore, 1977; Grenne & Roberts,1983), and some pillows have a moderate content of

amygdales, indicating eruption at a relatively shallowwater depth (e.g. Moore, 1965). The westernmost andnorthernmost islands of the Smelvaer Unit (Fig. 1)consist nearly exclusively of strongly foliatedyellowish-green to dark green volcaniclastic meta-sediments, interbedded with dark metachert (up to30 cm thick) and graphite-bearing black schist. Minorbodies of metagabbro, in some cases coarse-grained topegmatitic, intrude the metavolcaniclastites. An ex-posed contact between the metavolcanic/meta-volcaniclastic rocks of the Smelvaer Unit and thesurrounding rocks has not been identified.

2.c. The Kalvag Melange

A petrographic description of the various componentsof the Kalvag Melange (Figs 1, 4) and a discussion ofthe environment in which it formed, has been given byBryhni & Lyse (1985). The melange, most likely


representing an olistostrome, has a matrix of meta-pelite and quartz-rich metasandstone, hosting olisto-liths of different lithologies ranging in size up to morethan 2 km. Some olistoliths are composed of shallow-marine metasandstones (Fig. 4a) showing variousstages of disintegration due to syn-sedimentary slump-ing (Fig. 4b). Associated with the metasandstones aredisrupted layers of rubbly, most probably subaerial,aa lava which may also occur as individual fragmentssurrounded by the metapelite/metagreywacke matrix.Strongly to slightly welded ignimbrite (Fig. 4c) occursas > 300-m-long olistoliths in the melange. Blocks ofdeep-water distal turbidites interbedded with chert(Fig. 4d) are well represented on the western andsouthern parts of Froya (Fig. 4). A spectacularolistolith ( > 200 m long) of a coarse, unsorted andpolymict conglomerate (Fig. 4e), in association withdistal turbidites/chert and ignimbrite, occurs on thewestern part of Froya. Within this conglomerate,which contains well rounded pebbles and boulders ofmetagabbro, greenstones, quartz porphyry, chert,quartzite and rounded to angular fragments ofmetapelite and metagreywacke, are beds of coarse- tofine-grained metasandstone. In a sheared contact withthis olistolith is black shale, from which Reusch (1903)reported the occurrence of Silurian graptolites.

The melange is intruded by two plutons (Figs 1, 4),one of granodioritic and the other of gabbronorite/dioritic composition, as well as by several thin felsicdykes. Mineral separates (plagioclase, clinopyroxeneand apatite) from a sample of diorite from thesyntectonic gabbronorite/diorite intrusion haveyielded a Sm-Nd age of 380 + 26 Ma (Fumes et al.1989). The geochemical composition of this intrusionis transitional between calc-alkaline and tholeiitic(Fumes et al. 1989).

2.d. Undifferentiated rocks

The western part of Skorpa (Fig. 1) and the neigh-bouring islands and skerries consist of a light grey,garnetiferous, mica-bearing gneiss, with minor thinlayers and lenses of amphibolite. At most localities thegneiss has a porphyroclastic texture with augendevelopment. On the western side of Batalden (Fig. 1)the dominant rock type is metabasalt (greenstonesgrading into amphibolites) of MORB composition(H. Fumes, unpublished data), containing minorbodies of metagabbro and layers of a light grey, mica-rich, quartz schist.

These gneisses and metabasalts appear as isolatedoccurrences, and it is not yet possible to deduce towhich part of the well-established tectonostratigraphy(in the Atloy area, Fig. 1) they belong. They will notbe considered further in the discussion which follows.

3. Geochemistry

A large number of geochemical analyses of themetabasalts and metavolcaniclastites from the variousabove-mentioned volcanic complexes have been car-ried out by XRF. For this account we have reportedfull analyses only of representative sample, for whichthe rare earth and other trace elements have also beendetermined by instrumental neutron activation analy-ses. The results are presented in Table 1.

3.a. Analytical methods

Major oxides and the trace elements V, Cr, Rb, Sr, Yand Zr were determined by X-ray fluorescence. Theglass-bead technique of Padfield & Gray (1971) wasused for the major elements, and pressed powderpellets for the trace elements using internationalbasalt standards for calibration and Flanagan's (1973)recommended values. The REE together with Hf, Ta,Th, U, Sc and Co were determined by instrumentalneutron activation, using international standards forcalibration. The gamma-ray activities were measuredwith a large Ge(Li) detector. Methods are describedby Brunfelt & Steinnes (1969, 1971). Instrumentalprecisions for trace elements in this account are asfollows: better then or c. ± 5 % : Sm, Tb, Ta, Th, Y,Zr, Sr, Sc, Cr, V; c. ± 5 - 1 0 % : La, Eu, Yb, Hf, Co;c. +10-15%: Ce, Nd, Ho, Tm, U, Rb, Ni. Thecomplete analytical procedures are available onrequest (M.T. for XRF and L.B.G. for INAA). Fortwo of the samples (83-MS-7 and H43) all the traceelements were determined by ICP-MS at MemorialUniversity, Newfoundland.

3.b. Alteration effects

Since only minor and trace elements have been used incharacterizing the rocks, only the behaviour of theseparticular elements during alteration and low-grademetamorphism will be discussed here. The elementsTi, Y, Zr, Hf and Ta are reported to remain stable(Cann, 1970; Hart, 1970; Hart, Erlank & Kable,1974; Coish, 1977; Ludden, Gelinas & Trudel, 1982;Staudigel & Hart, 1983). The behaviour of Th is lesswell known, but Wood, Joron & Treuil (1979) havereported that the Th/La ratio remains stable inaltered rocks. All studies concerning the behaviour ofREE during various types of alteration have shownthat HREE can be regarded as immobile. Thebehaviour of LREE, however, is debatable; someauthors (e.g. Ludden & Thompson, 1979) havedocumented some mobility, whereas others (e.g.Dungan, Vance & Blanchard, 1983) have reported nomobility. Because the greenstones discussed in thispaper generally show smooth REE patterns (Figs 5-7),we believe that their compositions largely reflect thatof the original magma.

Tabl

e 1.

Repr

esen

tativ

e m

ajor

and

tra

ce e

lem

ent a

naly

ses

from

the

Sol

und-

Stav

fjord

Oph

iolit

e Co

mpl

ex a

nd a

ssoc

iated

roc

ksto O

N

Ref.

no.

Roc

k

SiO

2Ti

O2

A1 2

O3

Fe2O

3Fe

OM

nOM

gOC

aON

a 2O

K2O

LOI5

Tota

lSc V C

rC

oN

iR

bSr Y Zr N

bLa C

e Nd

Sm Eu Gd

Tb Ho

Tm Yb Lu Hf

Ta Th U

Solu

nd-S

tavf

jord

O

phio

lite

Com

plex

1 D

47.4

22.

2313

.10

2.59

8.71

0.18

6.57

10.0

53.

210.

060.

193.

9399

.24

—25

0 — — —14

0 41 115 — 4.

3711 17 5.

25 1.15

— 1.37

1.74

— 3.57 — — 0.20

0.57

0.60

2 PL

48.4

82.

3414

.41

3.37

9.17

0.17

6.34

8.56

3.02

0.13

0.23

3.45

99.3

7

—19

6 — — 117

1 51 138 — 6.

4725 19 6.

34 1.79

7.00 1.62

2.26 — 4.42

0.31 — 0.26

0.44

0.91

3 D

48.6

81.5

514

.38

2.40

8.06

0.16

7.47

11.3

83.

700.

080.

13 1.25

99.2

4

—29

1 — — —13

7 30 81— 3.50

12— 4.00 1.40

— 0.90 1.30

— 2.90

0.30 — 0.10

0.30

0.40

4 D

49.3

61.6

615

.60

2.69

6.51

0.16

6.89

10.9

03.

470.

240.

202.

2599

.93

— —39

2 — — 324

0 31 111 — 4.

9013 16 4.

702.

00 — 0.90 — — 2.90

0.50 — 0.30

0.60

0.50

5 D

49.8

62.

4013

.25

3.04

9.76

0.21

6.02

10.8

42.

740.

150.

25 1.35

99.8

7

—34

6 — — 111

0 43 122 — 5.

4018 13 6.

302.

006.

001.

402.

00 — 4.60

0.60 — 0.20

0.30

0.40

6 GS

49.5

61.0

817

.01

7.32

6.66

0.27

3.98

9.01

3.24

0.13

0.08 1.70

100.

1332

.10

415 38 30

.20

16 852

1 23 13— 3.20

10 9 3.60 1.30

— 0.70 1.20

0.50

2.80

0.60 1.40

0.03

0.39

0.87

Heg

goy

7 GS

47.6

40.

9216

.21

3.21

8.55

0.15

4.58

8.52

3.21

0.61

0.08

5.30

98.9

8

34.8

035

0 15 44.1

024 23 10

3 23 62— 4.10

12— 3.50 1.20

5.00

0.60 — 0.50

2.50

0.40 1.90

0.06

0.66 1.50

Form

atio

n

8 MG

50.5

71.6

214

.51

2.29

8.29

0.18

7.11

11.4

22.

060.

280.

17 1.20

99.6

836

.00

280

263 39

.60

56 13 144 29 120 — 4.

4012 11 4.

20 1.40

— 0.90 1.40

0.60

2.70

0.40

3.00

0.17

0.14

0.88

9 S

49.7

72.

3513

.96

2.52

9.40

0.23

8.64

10.1

72.

880.

230.

392.

4210

1.04

45.2

741

828

6 55.0

073 1

156 46 178 1.6

54.

6516 16 5.

53 1.76

6.50 1.29

1.81

0.76

4.68

0.63

2.81 — 0.19

0.07

Her

svik

1 in

il\j

HI

i

10 ML

52.7

01.4

615

.49

7.25

3.72

0.15

4.33

5.20

4.96

0.57

0.39

2.50

99.4

629

.14

—98

— —12 653 35 247 13

.11

34.5

572 34 7.

302.

207.

25 1.20

1.46

0.53

2.94

0.40

2.78 — 4.49 1.06

11 PL

57.1

02.

4513

.82

2.78

8.24

0.22

2.13

5.63

4.79

0.36

0.85

0.70

99.0

7

14.5

085 4 15

.80

6 531

3 62 415 —

50.6

011

8 64 16.0

03.

6015

.00

1.90

2.40 1.00

4.80 —

10.3

04.

105.

60 1.80

Smel

vaer

Uni

t

12 ML

49.6

83.

7715

.32

2.63

10.4

40.

185.

127.

634.

190.

450.

652.

0010

1.97

27.6

041

8 6 31.9

09 7

176 51 326 —

34.0

068 35 10

.30

2.70

11.0

01.6

02.

100.

804.

10 — 7.80

2.70

3.10 1.40

13 PL

48.0

02.

7116

.15

4.15

5.72

0.08

5.20

11.9

81.5

50.

310.

30 1.90

98.0

5

31.3

032

310

5 38.1

052 9

372 31 144 —

23.6

052 25 6.

70 1.90

— 0.90 1.20

0.50

2.50 — 4.00 1.00

1.20

1.10

14M

eG

50.6

42.

9715

.33

1.58

9.43

0.17

5.71

6.17

4.37

0.84

0.55

2.30

100.

0630

.20

501 73 31

.70

33 25 236 51 286 —

36.3

068 31 8.

802.

3010

.00

1.40

1.80

0.80

4.10 — 6.00

3.30

3.90 1.10

15 GrC

54.2

90.

9215

.33

1.61

8.73

0.21

5.62

5.72

2.82 1.71

0.11 1.7

098

.77

30.1

024

6 61 33.7

022 66 15

2 32 66— 3.70

13 9 3.30 1.20

5.00

0.70 1.00

0.50

2.50

0.50 1.30

0.04

0.31 1.10

16 GrC

49.5

61.8

714

.67

1.95

10.1

40.

346.

619.

062.

550.

490.

14 1.70

99.0

836

.10

337

239 40

.40

72 11 162 41 128 — 4.

6015 13 4.

70 1.30 — 1.00

1.60

0.70

3.70

0.70

3.10

0.17

0.21 1.1

0

Kal

vag

17 GrC

50.9

72.

1416

.88

3.16 5.96

0.12 5.00

11.51 1.8

70.

380.

292.

2099

.59

42.0

033

934

7 26.2

043 8

205 40 138 — 5.

5016 13 4.

90 1.30

— 1.10

1.70

0.70

3.90 — 3.33

0.17

0.31 1.20

Mel

ange

18 QpC

75.8

30.

2511

.68

1.68

1.37

0.06

0.76 2.31

4.21

0.33

0.06

2.50

101.

0510

.40

0 10 5.20

2 11 94 19 69— 3.40

9 — 2.30

0.90 — 0.50

0.90

0.50

2.50 — 2.50

0.10 1.20

1.00

19 PD

54.4

31.4

720

.67

1.50

4.59

0.07

3.42

2.86

4.78 1.41

0.26

3.20

99.6

1

15.9

015

1 20 22.3

018 62 893 28 313 —

23.5

044 24 5.

30 1.60

6.00

0.60

0.70

0.30 1.50

— 4.60

0.99

5.40

2.20

20 RL

51.6

32.

6821

.84

1.30

2.81

0.12

2.08

10.0

30.

536.

320.

53 1.35

101.

22

31.9

029

123

8 46.5

069 12

571

5 48 367 —

23.0

054 33 9.

602.

80 — 1.20

1.30

0.50

2.20

0.20

5.90

2.80

3.20

4.00

X c 73 Z m z oA

bbre

viat

ions

: D

= d

yke;

PL

= pi

llow

lav

a; G

S =

gree

nsch

ist;

MG

= m

icro

gabb

ro;

S =

sill;

ML

= m

assi

ve la

va; M

eG =

met

agab

bro;

GrC

= g

reen

ston

e cl

asts

; QpC

= q

uart

z po

rphy

ry c

last

s;PD

= p

orph

yriti

c dy

ke;

RL

= ru

bbly

lav

a. F

ield

num

bers

of

sam

ples

cor

resp

ondi

ng t

o re

fere

nce

num

bers

: 1

= So

l; 2

= So

3; 3

= S

f3; 4

= S

f5; 5

= S

f6; 6

= 8

6-SF

-129

; 7 =

86-

SF-1

65; 8

= 8

6-SF

-15

3; 9

= 8

3-M

S-7;

10

= H

43;

11 =

84-

SF-5

; 12

=84

-SF-

ll;

13 =

84-

SF-1

6; 1

4 =

86-S

F-24

; 15

= 8

7-SF

-I4;

16

= 87

-SF-

15;

17 =

87-

SF-1

6; 1

8 =

87-S

F-27

; 19

= 8

7 :SF

-59;

20

= 87

-SF-

63.


1 2 Jeg

o

Oldra, dykes & pillows

CfP

200 400Zr-

Alden, dykes & pillows

2-

200Zr-

400

Tviberg, dykes

i200 400

Zr-

oO 20-O

10-

Solund

La Ce Nd Sm Eu Gd Tb Ho

• So 1• So 3

Yb Lu

Stavfjorden area40-,

20-

10-

La Ce Nd Sm Eu Gd Tb Ho Yb Lu

a S I3 (Staveneset)• Sf 5 (Tviberg)+ Sf 6 (Alden)

Th Ta Ce P Zr Sm Ti Y Yb Cr

Figure 5. Geochemical data from the Solund-Stavfjord Ophiolite Complex, showing TiO2-Zr relationships (a, b, c), andrepresentative samples from the Solund (Oldra) and Stavfjorden (Alden and Tviberg) areas, showing REE (d, e) and traceelement (f, g) patterns. Chondrite data from Haskin et al. (1968). MORB values of Ta, Ce, P, Zr, Hf, Sm, Ti, Y, Yb, Sc andCr from Pearce (1980), and Th from Tarney et al. (1980).

3.c. Metabasalts of the Solund-Stavfjord OphioliteComplex

Representative major and trace element analyses ofmetabasalts from the Solund-Stavfjord OphioliteComplex are given in Table 1. Figure 5 a, b, c showsthe TiO2-Zr relationships of dykes and pillow lavas.The Oldra and Alden metabasalts, and in particularthe samples from Oldra, are enriched in TiO2 and Zrrelative to average MORB (e.g. Pearce, 1980), whereasthe Tviberg samples show a much larger spread inthese elements. REE analyses of the metabasalts fromSolund (Oldra) and Stavfjorden (Alden and Tviberg)are shown in Figure 5d, e. A characteristic pattern ofall samples is their upward convex pattern. TheSolund and Alden samples show significant to slightnegative Eu anomalies, respectively, which are notseen in the Staveneset and Tviberg samples. MORB-normalized trace element diagrams are shown inFigure 5f, g. The samples from Solund, Alden andStaveneset show patterns which are similar to, orsomewhat enriched relative to, average MORB but

with slight to pronounced negative Ta anomalies. Thesample from Tviberg shows a slightly different trendwith a continuous and gradual increase in the MORB-normalized trace element values from Yb (less thanMORB) through to Th (c. 3 x MORB).

3.d. Metabasalt lavas, intrusions and metavolcaniclastitesof the Stavenes Group

Representative analyses of the metabasaltic intrusions,lavas and volcaniclastites (greenschists) are shown inTable 1. The geochemistry of the lavas/intrusions andthe volcaniclastic rocks are here described separately.

3.d.1. The Heggey Formation

the TiO2-ZrMetabasalt lavas and intrusions. Indiagram (Fig. 6a) the majority of the samples plot onthe same trend as the metabasalts of the Solund-Stavfjord Ophiolite Complex (Fig. 5 a, b, c). The TiO2

and Zr data show a large spread, comparable to thosefrom Tviberg (Fig. 5 c), but the majority plot within


) The Heggoy Formation4-1 Pillow lava & intrusions 4T 0 0 volcaniclastites

400

30-

o

La Ce Nd Sm Eu Gd Tb Ho Tm Yb Lu

• 86-SF-153> • 83-MS-7

.3-1pa=e

Th Ta Ce P Zr Hf Sm Ti Y Yb Sc Cr(Nb)

t D D

0 200

Z r — •400


D 86-SF-129• 86-SF-165

Th Ta Ce P Zr Hf Sm Ti Y Yb Sc Cr

Qi -

The Hersvik Unit

2- D D

0 200 400Z r — •

La Ce N d Sm Eu Gd Tb Ho Tm Yb Lu

D H43

Th Nb Ce P Zr HI Sm Ti Y Yb Sc Cr

The Smelvaer Unit

La Ce Nd Sm Eu Gd Tb Ho Tm Yb

Th Ta Ce P Zr HI Sm Ti Y Yb Sc Cr

Figure 6. Geochemical data of metabasalt pillow/massive lava, volcaniclastites and intrusions of the Stavenes Group (theHeggoy Formation, and Hersvik and Smelvaer units). In the TiO2-Zr diagram of the Hersvik Unit (g) the fields of the HeggoyFormation (HF) (a, d) are indicated. Squares with crosses: massive lava; open squares: volcaniclastites. In the Nb-Zr diagramof the Smeh/Tr Unit (j) the fields of the Solund-Stavfjord Ophiolite Complex (SSOC), Heggoy Formation (HF) and HersvikUnit (HU) are shown. Chondrite and MORB data as in Figure 5.


the more restricted (enriched MORB type) range:TiO2 c. 1.7-2.5 wt%, and Zr c. 100-200 ppm, i.e.approximately the same as the field defined by theAlden samples (Fig. 5 b). Two samples, however, plotaway from the majority of the samples in the TiO2-Zrdiagram, and are characterized by relatively low TiO2and high Zr concentrations, i.e. more akin to calc-alkaline magmas. Representative samples of theMORB type (83-MS-7, 86-SF-153) show a rather flatREE pattern, slightly depleted in the LREE andHREE (Fig. 6b), and in the trace element diagram(Fig. 6 c) they define a flat MORB-comparable patternwhich may show a negative Nb anomaly.

Basic metavolcaniclastic rocks. With the exceptionof a few samples, the majority of the metabasalticgreenschists intercalated with the metagreywackeshave considerably lower TiO2 and Zr contents(Fig. 6d) than those of the lavas and intrusions(Fig. 6a). The flat REE patterns (Fig. 6e), combinedwith the generally low abundance of incompatibleelements, with the exception of Th, and the pro-nounced negative Ta anomalies (Fig. 6f), show thatthese rocks share the characteristic geochemicalsignature of island arc tholeiites (e.g. Wood, Joron &Treuil, 1979; Holm, 1985).

3.d.2. The Hersvik Unit

The metabasalt lavas and most of the basic meta-volcaniclastites of the Hersvik Unit have lower TiO2and higher Zr contents than those of the HeggoyFormation (Fig. 6g). The REE pattern of a rep-resentative metabasalt samples (Table 1) shows aLREE enrichment (Fig. 6h), and in the trace elementdiagram (Fig. 6i) the most incompatible elements (e.g.Th) show the highest MORB-normalized values, witha marked negative Nb anomaly as well as minornegative Hf and Ti anomalies. These features aretypical of calc-alkaline magmas (e.g. Thompson et al.1984).

3.d.3. The Smelvctr Unit

The geochemical compositions of some representativesamples of the metabasaltic volcanigenic and intrusiverocks of the Smelvaer Unit are shown in Table 1.Compositionally they differ from those of the Solund-Stavfjord Ophiolite Complex, the Heggoy Formationand the Hersvik Unit. This is particularly welldemonstrated with respect to their generally high Nbconcentrations (Fig. 6j), but also in their LREE-enriched REE patterns (Fig. 6 k), and continuousenrichment in their trace elements from Yb to Th(Fig. 61). Geochemically they are thus to be classifiedas alkaline magmatic products (e.g. Thompson et al.1984). This, combined with their field characteristicsas submarine volcanites, would be compatible withthe evolution of an ocean island complex.

3.e. Magmatic rocks of the Kalvag Melange

The geochemical affinities of some of the magmaticrocks occurring as (1) pebbles in the conglomerateblocks of the melange, and (2) intercalated lava andintrusive rocks, are described. Their geochemicalcompositions are shown in Table 1.

3.e.l. Pebbles in the conglomerate blocks

Metabasalt pebbles occur abundantly, and analyses ofthree samples are shown in Table 1. They exhibit a flatREE pattern (Fig. 7 a) and trace element diagramsshow a typical MORB and IAT character, in the lattercase with negative Ta anomalies (Fig. 7 b).

Quartz porphyry is another dominant clast type.The REE pattern of one of these pebbles shows adepleted nature, and the chondrite-normalized REEpattern is flat (Fig. 7c). Other trace element concen-trations are, with the exception of Th, lower thanthose of MORB (Fig. 7d). These characteristicsindicate that the quartz porphyry pebbles representderivation from a strongly depleted IAT parent (e.g.Holm, 1985).

3.e.2. Lavas and intrusions

The REE patterns and trace element diagrams of thebasic lava interbedded with the shallow-marine meta-sandstone occurring as blocks in the melange (87-SF-63), and dykes cutting it (87-SF-59), show an alkalineaffinity with an enrichment in the most incompatibleelements such as LREE, Th and Ta (Fig. 7e, f)-

4. Tectonic environment

The tectono-magmatic environment in which theSolund-Stavfjord Ophiolite Complex and associatedrocks formed must be consistent with the followinggeochemical and geological features:

(a) The metabasalts of the Solund-StavfjordOphiolite Complex show N- to E-MORB affinitieswith a clearly detectable influence from a subductionzone, as demonstrated by a moderate to strongdepletion in Ta and enrichment in Th, relative toaverage MORB;

(b) The Solund-Stavfjord Ophiolite Complex isoverlain by quartz-rich, continentally-derived meta-sediments which contain pillow lavas, metavolcanic-lastites and intrusions of MORB, IAT, calc-alkalineand alkaline compositions (the Stavenes Group);

(c) The presence of the Kalvag Melange, whichcontains olistoliths of (i) shallow-marine meta-sandstones with associated subaerial lavas, (ii) con-glomerate containing a variety of magmatic (MORB,IAT, calc-alkaline) and sedimentary (chert, sandstone)clasts, and (iii) ignimbrite;

(d) The evidence for a transform fault, which may


3 0 n

10-

O 87-SF-14• 87-SF-15X 87-SF-16

GreenstoneClasts

10-1

10—i

coO 10-

oocc

100-1

30-

10-

|O 87-SF-27|

QuartzPorphyryClast

O 87-SF-59• 87-SF-63

lava (•)dyke (o)


mccO

Oo

cc

1 -

Th Ta Ce P Zr Hf Sm Ti

Figure 7. REE and trace element diagrams of greenstone and quartz-porphyry clasts from a conglomerate block (Fig. 4e);metabasalt lava and a dyke of the Kalvag Melange. Chondrite and MORB data as in Figure 5.

have acted as the obduction surface and therebydeveloped as a tectonic melange during accretion ofthe Solund-Stavfjord Ophiolite Complex and sedi-mentary cover onto the continental margin.

4.a. Models4.a.l. Gulf of California

Field relations such as the quartz-rich metasedimentsoverlying the MORB-type pillow lavas of the Solund-Stavfjord Ophiolite Complex, hosting pillowed meta-basalts and intrusions (Figs 2, 3), also of MORB-type(Fig. 6a, b, c), provide strong evidence that theSolund-Stavfjord Ophiolite Complex developed inthe near vicinity of a continental margin (cf. Moores,1982). Such a development appears comparable tothat of the present-day Gulf of California, whereyoung to recent N-MORB at the East Pacific Rise areintercalated with sandstone, siltstone, or claystone(Saunders et al. 1982; Saunders, 1983). However, thegeochemical signature of the metabasalts of theSolund-Stavfjord Ophiolite Complex, giving supportfor subduction influence (Figs. 5f, g) and, even moresignificantly, the presence of typical I AT (Fig. 6d, e, f)and calc-alkaline lavas and metavolcaniclastites(Fig. 6g, h, i) of the cover sequence to the Solund-Stavfjord Ophiolite Complex, does not correspond toa Gulf of California setting.

4.a.2. Andaman Sea

A more appropriate environment in which the rocksof the Solund-Bremanger area may have formed isthought to be represented by the area between Burmaand Sumatra, i.e. the Andaman Sea region of theIndonesian Arc system. In this region, obliquesubduction of the Indian Plate beneath the northward-moving Burma Plate has, since mid-Miocene times,resulted in pull-apart opening of the Andaman Seaand generation of oceanic crust along several ridge-transform systems (Curray et al. 1979, 1982; Fig. 8 a).Thus the Andaman Sea ocean crust developed adjacentto the continental margin of the Malay Peninsula tothe east and a subduction system to the west andsouthwest. The latter consists of an inner activevolcanic arc (Sumatra), a forearc region with activevolcanoes (extending into Burma), and an outer ridgerepresenting a well-developed accretionary prism(Fig. 8 a) in which ophiolite fragments occur (Ray,Sengupta & van den Hul, 1988).

The relationship between continentally-derivedsediments and the pillow lava/intrusions from theactive spreading ridge may here, in principle, beidentical to that in the Gulf of California, but thebasalt geochemistry would contain a subduction zonesignature. In the Andaman Sea model, the MORB ofthe ocean crust may or may not have a geochemical


A: Andaman Sea Region B: Andaman Sea model applied to the Solund-Bremanger Region

Present time SagaingBURMA [si, Fault

IndianPlate

Line of separation

Forearc Basin

A Volcano

I j Continental Crust

500km

Time:~440 Ma

edge ofcontinental crust

•a

2

\'°iSpreading

ax'Transform

fault

Time:post~440 Mapre~380 Ma

Figure 8. The Solund-Stavfjord Ophiolite Complex and associated rocks reconstructed by using the present-daytectonomagmatic evolution of the Andaman Sea as a model. In (Bl) we have indicated geochemical effects from subductionactivity upon the metabasalts of the Solund-Stavfjord Ophiolite Complex. We further indicate the beginning stage of subaerialisland arc volcanoes for supplying air-borne IAT tuffaceous material to the near-continent spreading centre of theSolund-Stavfjord Ophiolite Complex and its cover sequence (the Heggoy Formation). For explanation of the Dalsfjord Suite,the Hoyvik and Herland groups, see Figure 1 and the text. For explanation of the Solund-Stavfjord Ophiolite Complex andthe Heggoy Formation, see Figures 2 and 3, respectively. In (B2) we present a speculative and uncertain model in which werelate the Hersvik Unit to an evolved, near-continent island arc, and the Kalvag Melange to have received material from aneroded, but still active, mature island arc, with the possibility of also receiving material supply from the erosion of anaccretionary prism. The Smelvrer Unit is proposed to have developed as an oceanic island, distant enough from the continentalmargin and island arc not to receive quartz-dominated sediments, and for the magmatic products not be influenced bysubduction processes, respectively. Further age determinations of these three sequences are needed in order to refine this model.For explanation of the Smelvaer and Hersvik units see Figure 3. For explanation of the Kalvag Melange see Figure 4.

subduction-related fingerprint, and the volcaniclasticrocks intercalated with the sediments might be derivedfrom the ash-fall of subduction-generated volcanoes.On the basis of geological relationships and geo-chemistry, we tentatively suggest that the Solund-Stavfjord Ophiolite Complex and associated sedi-mentary cover with lavas, intrusions and volcanic-lastites of the Heggoy Formation could have de-veloped in a tectonic environment comparable to thatof the Andaman Sea (Fig. 8 b).

The subaerial calc-alkaline lavas and volcanic-lastites of the Hersvik Unit and the alkaline volcanicrocks of the Smelvar Unit are more difficult to fit intothe model, because of their uncertain stratigraphicrelationship to the Solund-Stavfjord Ophiolite Com-plex and the Heggoy Formation, but they are suitable

analogies (Fig. 1). Basalts generated close to transformfaults are often enriched in incompatible elements(e.g. Langmuir & Bender, 1984), and alkaline oceanicislands may develop over mantle plumes on oceaniccrust (e.g. Mitchell-Thome, 1982). An importantfeature of the magmatic development of the KarmoyOphiolite Complex, southwest Norway, is the evol-ution towards alkaline magmatism, with a concomi-tant lowering of the eNd values relative to theMORB/IAT products (Pedersen & Hertogen, inpress). Similar features have recently been discoveredin basalts from the Lau Basin (Volpe, MacDougall &Hawkins, 1988). It is therefore interesting to note thatthe metabasalts of the Smelvaer Unit and the Solund-Stavfjord Ophiolite Complex have eNd (T = 430 Ma)values of c. 5 and 8, respectively (R. B. Pedersen,


unpublished data), This could well indicate that theSmelvasr Unit represents a late stage of ocean islandgrowth in the back-arc basin within which theSolund-Stavfjord Ophiolite Complex developed(Fig. 8 b).

The subaerial calc-alkaline metabasalts and meta-volcaniclastites, associated with metagreywackes andconglomerate beds of the Hersvik Unit, indicateevolution in a mature arc setting near a continentalmargin (Fig. 8 b).

The Kalvag Melange cannot, on the basis of fieldrelations, be directly connected to the Solund-Stavfjord Ophiolite Complex and its cover sequence.It is cut by a transitional tholeiitic/calc-alkalineintrusion (the Gasoy Intrusion), dated as 380 + 26 Ma(Furnes el al. 1989), which is thus at least 36 Mayounger than the oldest known part of the Solund-Stavfjord Ophiolite Complex, dated to 443 + 3 Ma(Dunning & Pedersen, 1988). Based on the geo-chemistry of conglomerate pebbles and the environ-ment of formation of the various lithologies, wesuggest that the Kalvag Melange represents materialderived from an active, mature arc near a continent,containing an exposed basement of ophiolitic rocks.Alternatively, the ophiolitic metabasalt pebble ma-terial may be derived from an accretionary prism, asindicated in Figure 8 b.

Occurring between the Solund-Stavfjord OphioliteComplex and the continental-type sediments of theHerland Group, is the Sunnfjord Melange (Fig. 1), adeposit which was first initiated in a transform setting(K. P. Skjerlie, unpub. Cand. Scient. thesis, Univ.Bergen, 1988; Skjerlie & Furnes, in press), and laterdeveloped into an obduction melange (Andersen,Skjerlie & Furnes, 1990). In the Andaman Sea thereare several transform faults that are parallel to the arcand subparallel to the continental margin. The mostextensive fault, the Sagaing Fault (Hla Maung, 1987),defines the active boundary between two differentterranes, i.e. the continental margin, and the oceanfloor of the Andaman Sea (Fig. 8 a). This tectonicfeature may be a modern analogue to the early stagein the development of the Sunnfjord Melange, whichappears to have received material from both theoceanic (Solund-Stavfjord Ophiolite Complex) andcontinental (Herland and Hoyvik groups) environ-ments.

5. Summary

The Solund-Stavfjord Ophiolite Complex of lateOrdovician age (U-Pb zircon age of 443 + 3 Ma)consists, from bottom to top, of the followingcomponents: (1) Varitextured, massive or faintlylaminated metagabbro, (2) a sheeted dyke complex,and (3) a thick sequence of pillow lavas, meta-hyaloclastites and massive metabasalt units which insome cases represent lava lakes, in other cases sheet

flows. The Solund-Stavfjord Ophiolite Complex isconformably overlain by a sequence of quartz-richmetasandstones, phyllites and basic metavolcanic-lastites (the Heggey Formation), hosting metabasaltintrusions and pillow lavas. The geochemistry of themetabasalts of the Solund-Stavfjord Ophiolite Com-plex and the Heggoy Formation are of N- to E-MORB composition, with positive evidence of asubduction-related influence, indicated by high Th/Ta(or Nb) ratios, and the basic metavolcaniclastites aregeochemically similar to IAT. These geological andgeochemical features are best explained by theSolund-Stavfjord Ophiolite Complex having formedin a marginal basin near enough to a continentalmargin for sandstones to be deposited at the activespreading ridge, and subsequently become intrudedand interlayered by MORB and island-arc-influencedbasalts. Contemporaneously, IAT volcanites, prob-ably representing tuffs from an emerging island arc,became interbedded with the continentally-derivedmetasediments. This tectonomagmatic development isin many ways similar to that of the present-dayAndaman Sea region of the Indonesian Arc system.

Calc-alkaline metabasalt lavas and volcaniclastites,interbedded with metasandstones and quartz-pebbleconglomerates (the Hersvik Unit), probably reflect amore advanced stage in the development of the islandarc system, in the proximity of a continental margin.A sedimentary melange, the Kalvag Melange, con-sisting of pebble material of MORB, IAT and quartzporphyry, as well as olistoliths of rhyolitic ignimbrite,shallow-marine metasandstone with interbedded alka-line lava, may also have developed in connectionwith a mature island arc/accretionary prism, prior to380 + 26 Ma (the age of a gabbronorite intruding themelange). Pillow lavas and associated metavolcani-clastites of alkaline composition, the Smelvaer Unit,probably developed on a Solund-Stavfjord OphioliteComplex basement as an oceanic island. A tentativemodel for these three, poorly age-constrained rockunits, as representing part of an evolved arc system(the Hersvik Unit and Kalvag Melange) and oceanicisland development (the Smelvaer Unit), is presented(Fig. 8 b).

Acknowledgements. Financial support for this study hasbeen provided through grants (D.41.31.147) from theNorwegian Research Council for Science and the Hu-manities. R.J.S. acknowledges financial support for field-work from Oxford Polytechnic. We express our thanks toF. J. Skjerlie for many useful discussions about the generalgeology of the area, J. Boyle, J. R. Cann and J. Malpas fortheir contributions in mapping minor parts of the HeggoyFormation and the Solund-Stavfjord Ophiolite Complex atan early stage of the project, and D. Roberts and ananonymous reviewer for constructive comments to an earlyversion of the manuscript. E. Lier, J. Ellingsen and E.Irgens prepared the illustrations. This work is publicationno. 77 in the International Lithosphere Project (ILP).


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