the solund–stavfjord ophiolite complex and associated rocks,...
TRANSCRIPT
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
212 H. FURNES AND OTHERS
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
Solund-Stavfjord Ophiolite Complex 213
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
214 H. FURNES AND OTHERS
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
Solund-Stavfjord Ophiolite Complex 215
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.
Solund-Stavfjord Ophiolite Complex 217
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
218 H. FURNES AND OTHERS
) 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
La Ce Nd Sm Eu Gd Tb Ho Tm Yb Lu
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.
Solund-Stavfjord Ophiolite Complex 219
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
220 H. FURNES AND OTHERS
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)
La Ce Nd Sm Eu Gd Tb Ho Tm Yb Lu
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
Solund-Stavfjord Ophiolite Complex 221
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,
222 H. FURNES AND OTHERS
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).
Solund-Stavfjord Ophiolite Complex 223
References
ANDERSEN, T. B., SKJERLIE, K. P. & FURNES, H. 1990. TheSunnfjord Melange, evidence of Silurian ophioliteaccretion in the west Norwegian Caledonides. Journalof the Geological Society of London 147, 59-68.
BALLARD, R. D. & MOORE, J. G. 1977. Photographic Atlasof the Mid-Atlantic Ridge Rift Valley. New York:Springer. 114 pp.
BOYLE, J. F. Geological implications of mixed oceanic-metalliferous and continental sediments from theSolund-Stavfjord Ophiolite Complex, West Norway.Norsk Geologisk Tidsskrift (in press).
BREKKE, H. & SOLBERG, P. O. 1987. The geology of Atloy,Sunnfjord, western Norway. Norges GeologiskeUndersokelse, Bulletin 410, 73-94.
BRUNFELT, A. O. & STEINNES, E. 1969. Instrumental ac-tivation analyses of silicate rocks with epithermalneutrons. Analytica Chimica Ada 48, 13-24.
BRUNFELT, A. O. & STEINNES, E. 1971. A neutron activationscheme developed for the determination of 42 elementsin Lunar material. Talanta 18, 1197-208.
BRYHNI, I. & LYSE, K. 1985. The Kalvag Melange,Norwegian Caledonides. In The Caledonide Orogen-Scandinavia and Related Areas (eds D. G. Gee andB. A. Sturt), pp. 417-26. New York: Wiley.
CANN, J. R. 1970. Rb, Sr, Y, Zr and Nb in some ocean floorbasaltic rocks. Earth and Planetary Science Letters 10,7-11.
COISH, R. A. 1977. Ocean floor metamorphism in the BettsCove Ophiolite, Newfoundland. Contributions to Min-eralogy and Petrology 60, 255-70.
COLEMAN, R. G. 1977. Ophiolites. Berlin: Springer. 229pp.CURRAY, J. R., MOORE, D. G., LAWVER, L. A., EMMEL,
F. J., RAITT, R. W., HENRY, M. & KIECKHEFER, R.1979. Tectonics of the Andaman Sea and Burma. InGeological and Geophysical Investigations of ContinentalMargins (eds J. Watkins, L. Montadert and P. Dicker-son), pp. 189-98. American Association of PetroleumGeologists Memoir 29.
CURRAY, J. R., EMMEL, F. J., MOORE, D. G. & RAITT, R. W.1982. Structure, tectonics, and geological history of thenortheastern Indian Ocean. In The Ocean Basins andMargins, Volume 6, The Indian Ocean (eds A. E. M.Nairn and F. G. Stehli), pp. 339-450. New York:Plenum Press.
DUNGAN, M. A., VANCE, J. A. & BLANCHARD, D. P. 1983.Geochemistry of the Shuksan greenschists and blue-schists, North Cascades, Washington: variably frac-tionated and altered metabasalts of oceanic affinity.Contributions to Mineralogy and Petrology 48, 153-69.
DUNNING, G. R. & PEDERSEN, R. B. 1988. U/Pb ages ofophiolites and arc-related plutons of the NorwegianCaledonides: implications for the development ofIapetus. Contributions to Mineralogy and Petrology 98,13-23.
FLANAGAN, F. J. 1973. 1972 values for international geo-logical reference standards. Geochimica et Cosmo-chimica Ada 37, 1189-200.
FURNES, H. 1972. Meta-hyaloclastite breccias associatedwith Ordovician pillow lavas in the Solund area, westNorway. Norsk Geologisk Tidsskrift 52, 385-407.
FURNES, H. 1973. Variolitic structures in Ordovician pillowlava and its possible significance as an environmentalindicator. Geology 1, 27-30.
FURNES, H. 1974. Structural and metamorphic history of theLower Palaeozoic metavolcanics and associatedsediments in the Solund area, Sogn. Norges GeologiskeUndersokelse, Bulletin 302, 33-74.
FURNES, H., PEDERSEN, R. B., CANN, J. R., BOYLE, J. F.,STILLMAN, C. J. & SUTHREN, R. J. 1986. Solund-Stavfjorden ofiolittkompleks og overliggendesedimenter-vulkanitter: implikasjoner og tektoniskrnijo. 17e Nordiska Geologmotet 1986, Helsingfors(Abstract), 42.
FURNES, H., PEDERSEN, R. B., SUNDVOLL, B., TYSSELAND,M. & TUMYR, O. 1989. The age, petrography, geo-chemistry and tectonic setting of the late CaledonianGasoy Intrusion, west Norway. Norsk GeologiskTidsskrift (in press).
FURNES, H. & SKJERLIE, F. J. 1972. The significance ofprimary structures in the Ordovician pillow lavasequence of Western Norway in an understanding ofmajor fold pattern. Geological Magazine 109, 315-22.
FURNES, H., SKJERLIE, F. J. & TYSSELAND, M. 1976. Platetectonic model based on greenstone geochemistry in theLate Precambrian-Lower Palaeozoic sequence in theSolund-Stavfjorden area, West Norway. NorskGeologisk Tidsskrift 56, 161-86.
GALE, G. H. 1975. Ocean floor-type basalts from the GrimeliFormation, Stavenes Group, Sunnfjord. NorgesGeologiske Undersokelse 319, 47-58.
GRENNE, T. & ROBERTS, D. 1983. Volcanostratigraphy anderuptive products of the Jonsvatn greenstone form-ation, central Norwegian Caledonides. NorgesGeologiske Undersokelse 387, 21-38.
HART, R. A. 1970. Chemical exchange between sea waterand deep ocean basalts. Earth and Planetary ScienceLetters 9, 269-79.
HART, S. R., ERLANK, A. J. & KABLE, E. J. D. 1974. Seafloor basalt alteration: some chemical and Sr-isotopiceffects. Contributions to Mineralogy and Petrology 44,219-30.
HASKIN, L. A., HASKIN, M. A., FREY, F. A. & WILDEMAN,T. R. 1968. Relative and absolute abundances of therare earths. In Origin and Distribution of the Elements(ed. L. H. Ahrens), pp. 889-912. Oxford: PergamonPress.
HLA MAUNG. 1987. Transcurrent movements in the Burma-Andaman Sea region. Geology 15, 911-12.
HOLM, P. E. 1985. The geochemical fingerprints of differenttectonomagmatic environments using hygromagmato-phile element abundances of tholeiitic basalts andbasaltic andesites. Chemical Geology 51, 303-23.
KOLDERUP, N.-H. 1921. DerMangeritsyenit undumgebendeGesteine zwischen Dalsfjord und Stavfjord in Sondfjordim westlichen Norwegen. Bergen Museum Arbok1920-21,(5).
KOLDERUP, N.-H. 1928. Fjellbygningen i kyststraket mellomNordfjord og Sognefjord. Bergen Museum Arbok 1928,Naturvitenskapelige rekke Nr. 1.
LANGMUIR, C. H. & BENDER, F. J. 1984. The geochemistryof oceanic basalts in the vicinity of transform faults:observations and implications. Earth and PlanetaryScience Letters 69, 107-27.
LUDDEN, J. N. & THOMPSON, G. 1979. An evaluation of thebehaviour of the rare earth elements during theweathering of sea-floor basalts. Earth and PlanetaryScience Letters 43, 85-92.
LUDDEN, J., GELINAS, L. & TRUDEL, P. 1982. Archean
224 Solund-Stavfjord Ophiolite Complex
metavolcanics from Rouyn-Noranda district, AbitibiGreenstone Belt, Quebec. 2. Mobility of trace elementsand petrogenetic constraints. Canadian Journal of EarthScience 19, 2276-87.
MILNES, A. G. & KOESTLER, A. G. 1985. Geological structureof Jotunheimen, southern Norway (Sognefjell-Valdrescross-section). In The Caledonide Orogen - Scandinaviaand Related Areas (eds D. G. Gee and B. A. Sturt),pp. 457-74. New York: Wiley.
MITCHELL-THOME, R. C. 1982. The geological settings andcharacteristics of the Atlantic islands. Ada GeologicaAcademiae Scientiarum Hungaricae 25, 395-420.
MOORE, J. G. 1965. Petrology of deep-sea basalt nearHawaii. American Journal of Science 263, 40-53.
MOORES, E. M. 1982. Origin and emplacement of ophiolites.Reviews of Geophysics and Space Physics 20, 735-60.
PADFIELD, T. & GRAY, A. 1971. Major element rock analysesby X-ray fluorescence - a simple fusion method. N. V.Phillips analytical equipment FS35, Eindhoven.
PEARCE, J. A. 1980. Geochemical evidence for the genesisand eruptive setting of lavas from Tethyan ophiolites.Proceedings of the International Ophiolite Symposium,Nicosia 1979, 261-72.
PEDERSEN, R. B. 1986. The nature and significance of magmachamber margins in ophiolites: examples from theNorwegian Caledonides. Earth and Planetary ScienceLetters 11, 100-12.
PEDERSEN, R. B., FURNES, H. & DUNNING, G. 1988. SomeNorwegian ophiolite complexes reconsidered. NorgesGeologiske Undersokelse, Special Publications 3, 80-85.
PEDERSEN, R. B. & HERTOGEN, J. Magmatic evolution of theKarmoy Ophiolite Complex, SW Norway-Relationships between MORB-IAT-boninitic-calc-al-kaline and alkaline magmatism. Contributions to Min-eralogy and Petrology (in press).
RAY, K. K., SENGUPTA, S. & VAN DEN HUL, H. J. 1988.Chemical characters of volcanic rocks from Andamanophiolite, India. Journal of the Geological Society ofLondon 145, 393-400.
REUSCH, H. 1903. Forsteininger i fjeldet pa Froyen. Naturen7, 160.
SAUNDERS, A. D. 1983. Geochemistry of basalts recoveredfrom the Gulf of California during Leg 65 of the DeepSea Drilling Project. In Initial Reports of the Deep SeaDrilling Project Leg 65 (eds B. T. R. Lewis, P. Robinson
et al.), pp. 591-621. Washington: U.S. GovernmentPrinting Office.
SAUNDERS, A. D., FORNARI, D. J., JORON, J-L., TARNEY, J. &TREUIL, M. 1982. Geochemistry of basic igneous rocks,Gulf of California, Deep Sea Drilling Project Leg 64. InInitial Reports of the Deep Sea Drilling Project Leg 64(eds J. R. Curray, D. G. Moore et al.), pp. 595-642.Washington: U.S. Government Printing Office).
SKJERLIE, F. J. 1969. The pre-Devonian rocks in theAskvoll - Gaular area and adjacent districts, westernNorway. Norges Geologiske Undersokelse, Bulletin 258,325-59.
SKJERLIE, F. J. 1974. The Lower Palaeozoic sequence of theStavfjord district, Sunnfjord. Norges GeologiskeUndersokelse, Bulletin 302, 1-32.
SKJERLIE, K. P., FURNES, H. & JOHANSEN, R. J. 1989.Magmatic development and tectonomagmatic modelsfor the Solund-Stavfjord Ophiolite Complex, WestNorwegian Caledonides. Lithos 23, 137-51.
STAUDIGEL, H. & HART, S. R. 1983. Alteration of basalticglass: mechanism and significance for the oceaniccrust-seawater budget. Geochimica el CosmochimicaAda 47, 37-50.
SKJERLIE, K. P. & FURNES, H. in press. Evidence for a fossiltransform fault in the Solund-Stavfjord OphioliteComplex, west Norwegian Caledonides. Tectonics.
TARNEY, J., WOOD, D. A., SAUNDERS, A. D., CANN, J. R. &VARET, J. 1980. Nature of mantle heterogeneity in theNorth Atlantic: evidence from deep sea drilling.Philosophical Transactions of the Royal Society ofLondon A 297, 179-202.
THOMPSON, R. N., MORRISON, M. A., HENDRY, G. L. &PARRY, S. J. 1984. An assessment of the relative roles ofcrust and mantle in magma genesis: an elementalapproach. Philosophical Transactions of the RoyalSociety of London A 310, 549-90.
VOLPE, A. M., MACDOUGALL, D. & HAWKINS, J. W. 1988.Lau Basin basalts (LBB): trace element and Sr-Ndisotopic evidence for heterogeneity in backarc basinmantle. Earth and Planetary Science Letters 90, 174-86.
WOOD, D. A., JORON, J-L. & TREUIL, M. 1979. A re-appraisal of the use of trace elements to classify anddiscriminate between magma series erupted in differenttectonic settings. Earth and Planetary Science Letters45, 326-36.