209
Geol. Mag. 127 (3), 1990, pp. 209-224. Printed in Great Britain
The Solund-Stavfjord Ophiolite Complex and associated
rocks, west Norwegian Caledonides: geology, geochemistry
and tectonic environment
H. 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 the
westernmost Norwegian Caledonides, show N- to E-MORB affinity, with high Th/Ta (or Nb) ratios
giving evidence of subduction influence. The Solund-Stavfjord Ophiolite Complex is overlain by a
heterogeneous assemblage of sedimentary and volcanic rocks, the Stavenes Group, of which the
Heggoy Formation of metasandstones and phyllites conformably overlies the metabasalts of the
Solund-Stavfjord Ophiolite Complex. The Heggoy Formation contains, in places, abundant
metabasalt pillow lavas and minor intrusions, geochemically similar to those of the Solund-Stavfjord
Ophiolite Complex, and basic metavolcaniclastites of island arc tholeiite (IAT) composition. This
indicates that the Solund-Stavfjord Ophiolite Complex and Heggoy Formation developed in a
marginal basin between a continental margin and an active subduction system, for which the presentday 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 thought
to represent an advanced stage of the island arc development, and ocean island build-up, respectively.
1. Introduction
The area between Solund and Bremanger (Fig. 1)
forms part of the westernmost Norwegian
Caledonides. Tectonostratigraphic units comprising
various types of gneiss, continental margin sediments,
an ophiolite complex associated with volcanic and
sedimentary cover rocks, and sedimentary as well as
tectonic melanges, can be distinguished. The first
petrographic and tectonostratigraphic studies were
those by Kolderup (1921, 1928). Later work by
Skjerlie (1969, 1974) and Gale (1975) added considerable information, in particular on the greenstone
complexes. An extensive review of the general geology
of the Stavfjorden area is summarized by Brekke &
Solberg (1987), who divided the tectonostratigraphy
into lower, middle and upper tectonic units. A
modified tectonostratigraphic division has subsequently 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 Vevring
Complex of eclogite-bearing gneisses belonging to the
Western Gneiss Region of Precambrian age, and the
Askvoll Group which consists of low- to mediumgrade sedimentary, volcanic and plutonic rocks.
The middle tectonic unit, consisting of the Dalsfjord
Suite and the Hoyvik and Herland groups, is separated
from the lower tectonic unit by the extensional
Kvamshesten Fault. The Dalsfjord Suite is composed
of various syenitic to charnockitic orthogneisses,
granites and gabbros (Kolderup, 1921), and has been
correlated with similar rocks of the Jotun Nappe
(Milnes & Koestler, 1985). The Hoyvik Group of preSilurian age, locally resting with a primary depositional contact on the rocks of the Dalsfjord Suite,
consists mainly of meta-arkoses and quartzites which
experienced polyphasal deformation and metamorphism in the upper greenschist - lower amphibolite
facies prior to the deposition of the Herland Group.
The Herland Group (Brekke & Solberg, 1987) of
Silurian age, redefined by T. Berg (unpub. Cand.
Scient. thesis, Univ. Bergen, 1988) and Andersen,
Skjerlie & Furnes (1990), consists of two fossiliferous
formations: the Sjoralden Formation of basal conglomerates, quartzites, meta-arkoses and graphitic
black shales; and the overlying Brurastakken Formation of conglomerates, metasandstones, shales and
marbles.
The upper tectonic unit comprises the SolundStavfjord Ophiolite Complex and its cover of metasediments and metavolcanites.
A recent research programme of mapping, geochemical, geochronological, structural and sedimentological studies has been concentrated on the SolundStavfjord Ophiolite Complex and its cover and
210
H. FURNES AND OTHERS
TECTONOSTRATIGRAPHY
?
Kalvag
Melange
O _O-^O_ < W 3 W -,0
Dalsfjord Suite
Fault i I i—i I I I I i
Askvoll Gp.
Tectonic
contact
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,
metavolcaniclastites
Metagreywacke, metavolcaniclastites,lavas
Metagreywacke, phyllite/
with L a s & intrusions
Heggay Format.on
Metagabbro, sheeted dykes ) .
pillow lava, metahyaloclastite) b5»UO
« | SunnfjordMelange
Herland Group
Heyvik Group
+ ] Dalsfjord Suite
Askvoll Group
W Gneiss Region
Figure 1. Simplified geological map of the Solund-Bremanger area, with the stratigraphy/tectonostratigraphy of the various
rock complexes.
substrate. The Solund-Stavfjord Ophiolite Complex
is the youngest dated ophiolite complex in the
Scandinavian Caldedonides, based on a U-Pb zircon
date of 443 + 3 Ma (Dunning & Pedersen, 1988). The
Sunnfjord Melange, occurring between the upper and
middle tectonic units, has largely tectonic boundaries
but in one crucial area is seen to overlie strati-
graphically the Herland Group (Fig. 1) with a depositional contact. The melange developed during
ophiolite obduction and thus provides a terrane link
between the Solund-Stavfjord Ophiolite Complex
and the continental margin (Andersen, Skjerlie &
Fumes, 1990). Fundamental for the interpretation of
the tectonic environment of formation of the Solund-
211
Solund-Stavfjord Ophiolite Complex
Stavfjord Ophiolite Complex is the continental affinity
of its sedimentary cover, and the MORB and IAT
character of the intercalated volcanites/intrusions
and volcaniclastics, respectively. On the basis of the
abundant geochemical data and our present knowledge of the field relationships between the SolundStavfjord Ophiolite Complex and associated rocks, we
will argue that the most appropriate geotectonic
model is provided by the present-day Andaman Sea.
2. Geology of the Solund-Stavfjord Ophiolite
Complex and associated rocks
In the area between Solund and Flora (Fig. 1), the
relationship between the Solund-Stavfjord Ophiolite
Complex and its cover can be demonstrated in a
number of places. The tectonostratigraphy of the
Solund-Stavfjord Ophiolite Complex and the rocks
between Kinn and Bremanger (Fig. 1) is uncertain,
and can only be inferred. Brekke & Solberg (1987)
included all the pre-Devonian strata of the upper
tectonic unit in the Stavenes Group. In this context we
retain this group name, but exclude the ophiolite, and
further subdivide the remaining rocks into formal and
informal units. The ophiolite complex has in a number
of papers been referred to as the Solund-Stavfjord
Ophiolite 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 extensive
sedimentary and volcanic sequence, which can be
demonstrated to overlie the Solund-Stavfjord
Ophiolite Complex, will be given the formal name
Stavenes Group. The Stavenes Group is divided into
the Heggoy Formation, which comprises the sediments
and tholeiitic volcanic rocks conformably overlying
the Solund-Stavfjord Ophiolite Complex, and the
Hersvik and Smelvjer units, which are volcanic/
volcaniclastic and sedimentary sequences seen in the
Hersvik and Smelvaer/Moldvser areas (Fig. 1). They
have been given informal names since their precise
stratigraphic positions are unknown, and their
chemistries are of calc-alkaline and alkaline affinities,
respectively.
In the Bremanger area, a sedimentary melange, the
Kalvag Melange, is in a strongly sheared contact with
quartzites which are correlated with the Hoyvik Group
(Fig. I).
2.a. The Solund-Stavfjord Ophiolite Complex
The Solund-Stavfjord Ophiolite Complex mostly
comprises sheeted dykes and volcanic rocks, i.e. the
upper part of the standard ophiolite pseudostratigraphy (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 size
ranging from fine to pegmatitic (Fig. 2 a), and may
show diffuse as well as sharp transitions to diorite
(Pedersen, 1986; K. P. Skjerlie, unpub. Cand. Scient.
thesis, Univ. Bergen, 1988). Faintly laminated metagabbro occurs in Solund (Slotteneset area, Fig. 1).
Sheeted dykes (Fig. 2 b) are particularly well displayed
on some of the southwestern islands in Solund (Fig. 1).
Individual dykes range in thickness from a few
centimetres up to c. 2 m (mostly commonly < 1 m). A
characteristic feature of the Solund-Stavfjord
Ophiolite Complex is the high proportion of volcanic
rocks, which in the Solund and Staveneset areas
(Fig. 1) comprise non-amygdaloidal pillow lavas and
meta-hyaloclastite breccias (Fig. 2c, d) (Furnes, 1972,
1973, 1974; Furnes & Skjerlie, 1972; Furnes, Skjerlie
& Tysseland, 1976). On the islands of Vasrlandet and
Alden (Fig. 1), the volcanic succession is dominated
by sheet flows (Fig. 2e) and fossil lava lakes (Skjerlie,
Furnes & Johansen, 1989). A composite profile of the
Solund-Stavfjord Ophiolite Complex is shown in the
left-hand part of Figure 2.
An important tectonic feature of the SolundStavfjord Ophiolite Complex is the presence of a
broad shear zone (c. 500 m) on the island of Tviberg
(Fig. 1), in which serpentinite bodies were emplaced
contemporaneously with and prior to the last phases
of magmatic activity. This tectonic zone is considered
to have originated at the oceanic stage as part of a
transform fault (K. P. Skjerlie, unpub. Cand. Scient.
thesis, Univ. Bergen, 1988; Skjerlie & Furnes, in
press), which subsequently became the site where
obduction initiated, with the contemporaneous formation 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-Stavfjord
Ophiolite Complex has its largest extent in the area
between Heggoy and Eikefjord (Fig. 1), and primary
contacts with the metavolcanites can be seen both on
Staveneset and in Solund. The best preserved and
most important locality is on Slotteneset in Solund
(Fig. 1), where a c. 3 m thick, dark green to black
schist rests with a primary conformable contact on
pillow lava. The dark schist, composed mainly of
chlorite and magnetite, with subordinate garnet, pyrite
and 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 are
fine laminae and beds of pale grey siltstone, composed
of quartz, white mica and albite, with or without
calcite.
The cover metasediments are well preserved on the
island of Heggoy (Fig. 1), where a c. 1000 m thick
sequence of predominantly calcareous metagreywacke
rests directly upon the sheeted dyke complex of the
GEO 127
H. FURNES AND OTHERS
212
Solund-Stavfjorden
composite profile
^7} Metasandstone, phyllite
3
Sill
= ^ Sheet flows
T | Lava lake
d | Pillow lava
A | Meta hyaloclastite
rjJJ Sheeted dykes
• + | Diorite
v /"c'/j Gabbro (massive or
varitextured)
Figure 2. Composite profile of the Solund-Stavfjord Ophiolite Complex, with photographs showing its components (a-e), and
associated cover sediments (0 of the Heggoy Formation, (a) Varitextured metagabbro grading into metadiorite; southwest
Tviberg. (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, little
deformed quartz-rich metasandstone of the Heggoy Formation; Tryggoy.
Solund-Stavfjord Ophiolite Complex. The metagreywacke, hosting numerous intrusive bodies and
pillow lava horizons (Fig. 3), is dominantly fine- to
medium-grained, thin- to thick-bedded, light to dark
greenish-grey metagreywacke (Fig. 2f) composed of
quartz, albite, and minor rock fragments of greenstone
and quartzite set in a matrix of white mica, chlorite,
epidote and variable amounts of calcite. More
Solund-Stavfjord Ophiolite Complex
213
THE STAVENES GROUP
Heggoy Formation (HF)
m
1000
Hersvik Unit (HU)
Smelvaer Unit (SU)
Heggoy
Devonian
Explanation (HF)
Phyllite
800Fault
Metasandslone with
phyllite interbeds
Pillow lava (Tholeiitic)
Basic intrusive sheets
(Thol.)
600Fault
Metasandstone with
green volcaniclastite
interbeds (Thol.)
«•'»'*•;*'
Basement
unknown
400Slotteneset
-unconformity
Basement
unknown
200Fault
Explanation (HU)
> • «
• • Conglomerate
Metasandstone
0-1
with cgl. interbeds
with green volcaniclastite interbeds
Dykes & gabbro
of the SSOC
Pillow lava
of the SSOC
Massive lava &
intrusions
(calc-alcaline)
Explanation (SU)
Green volcaniclastites
with chert interbeds
Massive lava
Pillow lava /
with volcaniclastite
interbeds
All magmatic components
of alkaline composition
Figure 3. Volcanic and sedimentary development of the Stavenes Group, shown by composite stratigraphical logs of the
Heggoy Formation and Hersvik and Smelvaer units.
comprehensive petrographic descriptions of the metagreywacke are provided by Skjerlie (1974) and Furnes
(1974). The metagreywackes occur as thick monotonous sequences, or alternate with dark grey
phyllite, beds of quartzite, minor marbles, and
greenish-grey to dark green metavolcaniclastic rocks
(Fig. 3, Slotteneset profile). These lithologies may all
show gradational as well as sharp boundaries to each
other.
The occurrence of metabasalts within the metasediments is highly variable. Thus in the Tryggoy area,
on Heggoy and on the northern part of the Stavenes
Peninsula (Fig. 1), pillow lava, massive lava and
minor intrusions occur abundantly, whereas further
north and north-northeast (in the area between Svanoy
and Eikefjord, Fig. 1) only sporadic occurrences of
lava can be found.
Due to Caledonian deformation, it is only possible
to reconstruct the sequence in a few places, such as on
Heggoy, where younging directions can be observed
in the graded-bedded metasandstone and intercalations of pillow lava. The relationships between the
metasediments and the metabasalts of the SolundStavfjord Ophiolite Complex are indicated in Figure 3.
2.b.2. The Hersvik Unit
The rocks of the Hersvik Unit (Fig. 1) were previously
divided into three groups (the Hersvik, Mjeltevikneset
and Arneset groups; Furnes, 1974). With new data
showing a coherent geochemical development of the
volcanogenic rocks throughout the sequence of the
Hersvik area (Fig. 1), we now find it unjustified to
sustain this subdivision.
15-2
214
H. FURNES AND OTHERS
Granodiorite
Gabbronorite - diorite
Chert / distal turbidite
Conglomerate
Ignimbrite
Metasandstone (shallow marine)
Brecciated metasandstone /
metapelite
Figure 4. Simplified geological map of the Kalvag Melange (from Bryhni & Lyse, 1985) and the Gasoy Intrusion (from Furnes
et 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, largescale 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-flow
metaconglomerates, between black meta-shales and thick metachert slump/turbidite deposits, representing submarine
resedimentation of foreshore gravels.
The lower part of the sequence consists predominantly of metagreywacke with abundant lava
flows and minor intrusions. Interbedded with the
metagreywacke are beds of dark green metavolcaniclastites and conglomerates (with a dominance
of quartzite pebbles), which both increase in abundance up-sequence (Fig. 3). A fuller description of the
various lithologies has been given by Furnes (1974).
2.b.3. The Smelvoer Unit
The Smelvaer Unit is dominated by metabasaltic
volcanic rocks. On the island of Smelvsr (Fig. 1),
pillow laval predominates, but there are minor
occurrences of massive lava flows, intercalated with
metachert and brownish-green metavolcaniclastites
(Fig. 3). The pillow lavas commonly show drain-out
structures (Ballard & Moore, 1977; Grenne & Roberts,
1983), and some pillows have a moderate content of
amygdales, indicating eruption at a relatively shallow
water depth (e.g. Moore, 1965). The westernmost and
northernmost islands of the Smelvaer Unit (Fig. 1)
consist nearly exclusively of strongly foliated
yellowish-green to dark green volcaniclastic metasediments, interbedded with dark metachert (up to
30 cm thick) and graphite-bearing black schist. Minor
bodies of metagabbro, in some cases coarse-grained to
pegmatitic, intrude the metavolcaniclastites. An exposed contact between the metavolcanic/metavolcaniclastic rocks of the Smelvaer Unit and the
surrounding rocks has not been identified.
2.c. The Kalvag Melange
A petrographic description of the various components
of the Kalvag Melange (Figs 1, 4) and a discussion of
the environment in which it formed, has been given by
Bryhni & Lyse (1985). The melange, most likely
Solund-Stavfjord Ophiolite Complex
representing an olistostrome, has a matrix of metapelite and quartz-rich metasandstone, hosting olistoliths of different lithologies ranging in size up to more
than 2 km. Some olistoliths are composed of shallowmarine metasandstones (Fig. 4a) showing various
stages of disintegration due to syn-sedimentary slumping (Fig. 4b). Associated with the metasandstones are
disrupted layers of rubbly, most probably subaerial,
aa lava which may also occur as individual fragments
surrounded by the metapelite/metagreywacke matrix.
Strongly to slightly welded ignimbrite (Fig. 4c) occurs
as > 300-m-long olistoliths in the melange. Blocks of
deep-water distal turbidites interbedded with chert
(Fig. 4d) are well represented on the western and
southern parts of Froya (Fig. 4). A spectacular
olistolith ( > 200 m long) of a coarse, unsorted and
polymict conglomerate (Fig. 4e), in association with
distal turbidites/chert and ignimbrite, occurs on the
western part of Froya. Within this conglomerate,
which contains well rounded pebbles and boulders of
metagabbro, greenstones, quartz porphyry, chert,
quartzite and rounded to angular fragments of
metapelite and metagreywacke, are beds of coarse- to
fine-grained metasandstone. In a sheared contact with
this 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 felsic
dykes. Mineral separates (plagioclase, clinopyroxene
and apatite) from a sample of diorite from the
syntectonic gabbronorite/diorite intrusion have
yielded a Sm-Nd age of 380 + 26 Ma (Fumes et al.
1989). The geochemical composition of this intrusion
is transitional between calc-alkaline and tholeiitic
(Fumes et al. 1989).
215
3. Geochemistry
A large number of geochemical analyses of the
metabasalts and metavolcaniclastites from the various
above-mentioned volcanic complexes have been carried out by XRF. For this account we have reported
full analyses only of representative sample, for which
the rare earth and other trace elements have also been
determined by instrumental neutron activation analyses. The results are presented in Table 1.
3.a. Analytical methods
Major oxides and the trace elements V, Cr, Rb, Sr, Y
and Zr were determined by X-ray fluorescence. The
glass-bead technique of Padfield & Gray (1971) was
used for the major elements, and pressed powder
pellets for the trace elements using international
basalt standards for calibration and Flanagan's (1973)
recommended values. The REE together with Hf, Ta,
Th, U, Sc and Co were determined by instrumental
neutron activation, using international standards for
calibration. The gamma-ray activities were measured
with a large Ge(Li) detector. Methods are described
by Brunfelt & Steinnes (1969, 1971). Instrumental
precisions for trace elements in this account are as
follows: 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. + 1 0 - 1 5 % : Ce, Nd, Ho, Tm, U, Rb, Ni. The
complete analytical procedures are available on
request (M.T. for XRF and L.B.G. for INAA). For
two of the samples (83-MS-7 and H43) all the trace
elements were determined by ICP-MS at Memorial
University, Newfoundland.
3.b. Alteration effects
2.d. Undifferentiated rocks
The western part of Skorpa (Fig. 1) and the neighbouring islands and skerries consist of a light grey,
garnetiferous, mica-bearing gneiss, with minor thin
layers and lenses of amphibolite. At most localities the
gneiss has a porphyroclastic texture with augen
development. On the western side of Batalden (Fig. 1)
the dominant rock type is metabasalt (greenstones
grading into amphibolites) of MORB composition
(H. Fumes, unpublished data), containing minor
bodies of metagabbro and layers of a light grey, micarich, quartz schist.
These gneisses and metabasalts appear as isolated
occurrences, and it is not yet possible to deduce to
which part of the well-established tectonostratigraphy
(in the Atloy area, Fig. 1) they belong. They will not
be considered further in the discussion which follows.
Since only minor and trace elements have been used in
characterizing the rocks, only the behaviour of these
particular elements during alteration and low-grade
metamorphism will be discussed here. The elements
Ti, 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 less
well known, but Wood, Joron & Treuil (1979) have
reported that the Th/La ratio remains stable in
altered rocks. All studies concerning the behaviour of
REE during various types of alteration have shown
that HREE can be regarded as immobile. The
behaviour of LREE, however, is debatable; some
authors (e.g. Ludden & Thompson, 1979) have
documented some mobility, whereas others (e.g.
Dungan, Vance & Blanchard, 1983) have reported no
mobility. Because the greenstones discussed in this
paper generally show smooth REE patterns (Figs 5-7),
we believe that their compositions largely reflect that
of the original magma.
—
196
—
—
1
171
51
138
—
6.47
25
19
6.34
1.79
7.00
1.62
2.26
—
4.42
0.31
—
0.26
0.44
0.91
48.48
2.34
14.41
3.37
9.17
0.17
6.34
8.56
3.02
0.13
0.23
3.45
99.37
47.42
2.23
13.10
2.59
8.71
0.18
6.57
10.05
3.21
0.06
0.19
3.93
99.24
—
250
—
—
—
140
41
115
—
4.37
11
17
5.25
1.15
—
1.37
1.74
—
3.57
—
—
0.20
0.57
0.60
2
PL
1
D
—
291
—
—
—
137
30
81
—
3.50
12
—
4.00
1.40
—
0.90
1.30
—
2.90
0.30
—
0.10
0.30
0.40
48.68
1.55
14.38
2.40
8.06
0.16
7.47
11.38
3.70
0.08
0.13
1.25
99.24
3
D
49.36
1.66
15.60
2.69
6.51
0.16
6.89
10.90
3.47
0.24
0.20
2.25
99.93
—
—
392
—
—
3
240
31
111
—
4.90
13
16
4.70
2.00
—
0.90
—
—
2.90
0.50
—
0.30
0.60
0.50
4
D
—
346
—
—
1
110
43
122
—
5.40
18
13
6.30
2.00
6.00
1.40
2.00
—
4.60
0.60
—
0.20
0.30
0.40
49.86
2.40
13.25
3.04
9.76
0.21
6.02
10.84
2.74
0.15
0.25
1.35
99.87
5
D
47.64
0.92
16.21
3.21
8.55
0.15
4.58
8.52
3.21
0.61
0.08
5.30
98.98
34.80
350
15
44.10
24
23
103
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
32.10
415
38
30.20
16
8
521
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
7
GS
49.56
1.08
17.01
7.32
6.66
0.27
3.98
9.01
3.24
0.13
0.08
1.70
100.13
6
GS
36.00
280
263
39.60
56
13
144
29
120
—
4.40
12
11
4.20
1.40
—
0.90
1.40
0.60
2.70
0.40
3.00
0.17
0.14
0.88
50.57
1.62
14.51
2.29
8.29
0.18
7.11
11.42
2.06
0.28
0.17
1.20
99.68
8
MG
Heggoy Formation
45.27
418
286
55.00
73
1
156
46
178
1.65
4.65
16
16
5.53
1.76
6.50
1.29
1.81
0.76
4.68
0.63
2.81
—
0.19
0.07
49.77
2.35
13.96
2.52
9.40
0.23
8.64
10.17
2.88
0.23
0.39
2.42
101.04
9
S
29.14
—
98
—
—
12
653
35
247
13.11
34.55
72
34
7.30
2.20
7.25
1.20
1.46
0.53
2.94
0.40
2.78
—
4.49
1.06
52.70
1.46
15.49
7.25
3.72
0.15
4.33
5.20
4.96
0.57
0.39
2.50
99.46
10
ML
1\j inil
HI i
Hersvik
14.50
85
4
15.80
6
5
313
62
415
—
50.60
118
64
16.00
3.60
15.00
1.90
2.40
1.00
4.80
—
10.30
4.10
5.60
1.80
57.10
2.45
13.82
2.78
8.24
0.22
2.13
5.63
4.79
0.36
0.85
0.70
99.07
11
PL
27.60
418
6
31.90
9
7
176
51
326
—
34.00
68
35
10.30
2.70
11.00
1.60
2.10
0.80
4.10
—
7.80
2.70
3.10
1.40
49.68
3.77
15.32
2.63
10.44
0.18
5.12
7.63
4.19
0.45
0.65
2.00
101.97
12
ML
31.30
323
105
38.10
52
9
372
31
144
—
23.60
52
25
6.70
1.90
—
0.90
1.20
0.50
2.50
—
4.00
1.00
1.20
1.10
48.00
2.71
16.15
4.15
5.72
0.08
5.20
11.98
1.55
0.31
0.30
1.90
98.05
13
PL
Smelvaer Unit
30.20
501
73
31.70
33
25
236
51
286
—
36.30
68
31
8.80
2.30
10.00
1.40
1.80
0.80
4.10
—
6.00
3.30
3.90
1.10
50.64
2.97
15.33
1.58
9.43
0.17
5.71
6.17
4.37
0.84
0.55
2.30
100.06
14
MeG
30.10
246
61
33.70
22
66
152
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
54.29
0.92
15.33
1.61
8.73
0.21
5.62
5.72
2.82
1.71
0.11
1.70
98.77
15
GrC
36.10
337
239
40.40
72
11
162
41
128
—
4.60
15
13
4.70
1.30
—
1.00
1.60
0.70
3.70
0.70
3.10
0.17
0.21
1.10
49.56
1.87
14.67
1.95
10.14
0.34
6.61
9.06
2.55
0.49
0.14
1.70
99.08
16
GrC
42.00
339
347
26.20
43
8
205
40
138
—
5.50
16
13
4.90
1.30
—
1.10
1.70
0.70
3.90
—
3.33
0.17
0.31
1.20
50.97
2.14
16.88
3.16
5.96
0.12
5.00
11.51
1.87
0.38
0.29
2.20
99.59
17
GrC
10.40
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
75.83
0.25
11.68
1.68
1.37
0.06
0.76
2.31
4.21
0.33
0.06
2.50
101.05
18
QpC
Kalvag Melange
15.90
151
20
22.30
18
62
893
28
313
—
23.50
44
24
5.30
1.60
6.00
0.60
0.70
0.30
1.50
—
4.60
0.99
5.40
2.20
54.43
1.47
20.67
1.50
4.59
0.07
3.42
2.86
4.78
1.41
0.26
3.20
99.61
19
PD
31.90
291
238
46.50
69
125
715
48
367
—
23.00
54
33
9.60
2.80
—
1.20
1.30
0.50
2.20
0.20
5.90
2.80
3.20
4.00
51.63
2.68
21.84
1.30
2.81
0.12
2.08
10.03
0.53
6.32
0.53
1.35
101.22
20
RL
Abbreviations: D = dyke; PL = pillow lava; GS = greenschist; MG = microgabbro; S = sill; ML = massive lava; MeG = metagabbro; GrC = greenstone clasts; QpC = quartz porphyry clasts;
PD = porphyritic dyke; RL = rubbly lava. Field numbers of samples corresponding to reference numbers: 1 = Sol; 2 = So3; 3 = Sf3; 4 = Sf5; 5 = Sf6; 6 = 86-SF-129; 7 = 86-SF-165; 8 = 86-SF153; 9 = 83-MS-7; 10 = H43; 11 = 84-SF-5; 12 = 8 4 - S F - l l ; 13 = 84-SF-16; 14 = 86-SF-24; 15 = 87-SF-I4; 16 = 87-SF-15; 17 = 87-SF-16; 18 = 87-SF-27; 19 = 87 : SF-59; 20 = 87-SF-63.
Hf
Ta
Th
U
Lu
Sc
V
Cr
Co
Ni
Rb
Sr
Y
Zr
Nb
La
Ce
Nd
Sm
Eu
Gd
Tb
Ho
Tm
Yb
LOI 5
Total
SiO 2
TiO 2
A12O3
Fe 2 O 3
FeO
MnO
MgO
CaO
Na 2 O
K2O
Ref. no.
Rock
Solund-Stavfjord Ophiolite Complex
Table 1. Representative major and trace element analyses from the Solund-Stavfjord Ophiolite Complex and associated rocks
to
o
z
m
Z
73
c
X
ON
217
Solund-Stavfjord Ophiolite Complex
12J
eg
Tviberg, dykes
Alden, dykes & pillows
Oldra, dykes & pillows
2-
CfP
o
200
200
400
400
Zr-
Zr-
Zr-
i
200
400
Stavfjorden area
Solund
40-,
o
O 20O
20-
10-
10-
La Ce
Nd
Sm Eu Gd Tb
Ho
Yb Lu
La Ce
Sm Eu Gd Tb
Nd
• So 1
• So 3
Yb Lu
Ho
a S I 3 (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 TiO 2 -Zr relationships (a, b, c), and
representative samples from the Solund (Oldra) and Stavfjorden (Alden and Tviberg) areas, showing REE (d, e) and trace
element (f, g) patterns. Chondrite data from Haskin et al. (1968). MORB values of Ta, Ce, P, Zr, Hf, Sm, Ti, Y, Yb, Sc and
Cr from Pearce (1980), and Th from Tarney et al. (1980).
3.c. Metabasalts of the Solund-Stavfjord Ophiolite
Complex
Representative major and trace element analyses of
metabasalts from the Solund-Stavfjord Ophiolite
Complex are given in Table 1. Figure 5 a, b, c shows
the TiO 2 -Zr relationships of dykes and pillow lavas.
The Oldra and Alden metabasalts, and in particular
the samples from Oldra, are enriched in TiO 2 and Zr
relative to average MORB (e.g. Pearce, 1980), whereas
the Tviberg samples show a much larger spread in
these elements. REE analyses of the metabasalts from
Solund (Oldra) and Stavfjorden (Alden and Tviberg)
are shown in Figure 5d, e. A characteristic pattern of
all samples is their upward convex pattern. The
Solund and Alden samples show significant to slight
negative Eu anomalies, respectively, which are not
seen in the Staveneset and Tviberg samples. MORBnormalized trace element diagrams are shown in
Figure 5f, g. The samples from Solund, Alden and
Staveneset show patterns which are similar to, or
somewhat enriched relative to, average MORB but
with slight to pronounced negative Ta anomalies. The
sample from Tviberg shows a slightly different trend
with a continuous and gradual increase in the MORBnormalized trace element values from Yb (less than
MORB) through to Th (c. 3 x MORB).
3.d. Metabasalt lavas, intrusions and metavolcaniclastites
of the Stavenes Group
Representative analyses of the metabasaltic intrusions,
lavas and volcaniclastites (greenschists) are shown in
Table 1. The geochemistry of the lavas/intrusions and
the volcaniclastic rocks are here described separately.
3.d.1. The Heggey Formation
Metabasalt lavas and intrusions. In the TiO 2 -Zr
diagram (Fig. 6a) the majority of the samples plot on
the same trend as the metabasalts of the SolundStavfjord Ophiolite Complex (Fig. 5 a, b, c). The TiO 2
and Zr data show a large spread, comparable to those
from Tviberg (Fig. 5 c), but the majority plot within
218
H. FURNES AND OTHERS
)
The Heggoy Formation
4-1 Pillow lava & intrusions
4T 0 0 volcaniclastites
t
D D
0
400
200
400
Zr—•
30-
o
La Ce
Nd
Sm Eu Gd Tb
Ho
Tm Yb Lu
La Ce
Nd
Sm Eu Gd Tb
• 86-SF-153
• 83-MS-7
>
Ho
Tm Yb Lu
D 86-SF-129
• 86-SF-165
pa=e
.3-1
Th
Ta Ce P Zr
(Nb)
Hf Sm Ti
Y Yb Sc Cr
Th Ta Ce P Zr
The Smelvaer Unit
The Hersvik Unit
2-
Hf Sm Ti Y Yb Sc Cr
D D
Q
i0
200
Zr—•
400
La Ce
La Ce
Nd
Sm Eu Gd Tb
Ho
Nd
Sm Eu Gd Tb
Ho
Tm Yb
Tm Yb Lu
D H43
Th Nb Ce P Zr HI Sm Ti
Y Yb Sc Cr
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 (the
Heggoy Formation, and Hersvik and Smelvaer units). In the TiO 2 -Zr diagram of the Hersvik Unit (g) the fields of the Heggoy
Formation (HF) (a, d) are indicated. Squares with crosses: massive lava; open squares: volcaniclastites. In the Nb-Zr diagram
of the Smeh/Tr Unit (j) the fields of the Solund-Stavfjord Ophiolite Complex (SSOC), Heggoy Formation (HF) and Hersvik
Unit (HU) are shown. Chondrite and MORB data as in Figure 5.
Solund-Stavfjord Ophiolite Complex
the more restricted (enriched MORB type) range:
TiO 2 c. 1.7-2.5 wt%, and Zr c. 100-200 ppm, i.e.
approximately the same as the field defined by the
Alden samples (Fig. 5 b). Two samples, however, plot
away from the majority of the samples in the TiO 2 -Zr
diagram, and are characterized by relatively low TiO2
and high Zr concentrations, i.e. more akin to calcalkaline magmas. Representative samples of the
MORB type (83-MS-7, 86-SF-153) show a rather flat
REE pattern, slightly depleted in the LREE and
HREE (Fig. 6b), and in the trace element diagram
(Fig. 6 c) they define a flat MORB-comparable pattern
which may show a negative Nb anomaly.
Basic metavolcaniclastic rocks. With the exception
of a few samples, the majority of the metabasaltic
greenschists intercalated with the metagreywackes
have considerably lower TiO 2 and Zr contents
(Fig. 6d) than those of the lavas and intrusions
(Fig. 6a). The flat REE patterns (Fig. 6e), combined
with the generally low abundance of incompatible
elements, with the exception of Th, and the pronounced negative Ta anomalies (Fig. 6f), show that
these rocks share the characteristic geochemical
signature of island arc tholeiites (e.g. Wood, Joron &
Treuil, 1979; Holm, 1985).
219
3.e. Magmatic rocks of the Kalvag Melange
The geochemical affinities of some of the magmatic
rocks occurring as (1) pebbles in the conglomerate
blocks of the melange, and (2) intercalated lava and
intrusive rocks, are described. Their geochemical
compositions are shown in Table 1.
3.e.l. Pebbles in the conglomerate blocks
Metabasalt pebbles occur abundantly, and analyses of
three samples are shown in Table 1. They exhibit a flat
REE pattern (Fig. 7 a) and trace element diagrams
show a typical MORB and IAT character, in the latter
case with negative Ta anomalies (Fig. 7 b).
Quartz porphyry is another dominant clast type.
The REE pattern of one of these pebbles shows a
depleted nature, and the chondrite-normalized REE
pattern is flat (Fig. 7c). Other trace element concentrations are, with the exception of Th, lower than
those of MORB (Fig. 7d). These characteristics
indicate that the quartz porphyry pebbles represent
derivation from a strongly depleted IAT parent (e.g.
Holm, 1985).
3.e.2. Lavas and intrusions
3.d.2. The Hersvik Unit
The metabasalt lavas and most of the basic metavolcaniclastites of the Hersvik Unit have lower TiO 2
and higher Zr contents than those of the Heggoy
Formation (Fig. 6g). The REE pattern of a representative metabasalt samples (Table 1) shows a
LREE enrichment (Fig. 6h), and in the trace element
diagram (Fig. 6i) the most incompatible elements (e.g.
Th) show the highest MORB-normalized values, with
a marked negative Nb anomaly as well as minor
negative Hf and Ti anomalies. These features are
typical of calc-alkaline magmas (e.g. Thompson et al.
1984).
3.d.3. The Smelvctr Unit
The geochemical compositions of some representative
samples of the metabasaltic volcanigenic and intrusive
rocks of the Smelvaer Unit are shown in Table 1.
Compositionally they differ from those of the SolundStavfjord Ophiolite Complex, the Heggoy Formation
and the Hersvik Unit. This is particularly well
demonstrated with respect to their generally high Nb
concentrations (Fig. 6j), but also in their LREEenriched REE patterns (Fig. 6 k), and continuous
enrichment in their trace elements from Yb to Th
(Fig. 61). Geochemically they are thus to be classified
as alkaline magmatic products (e.g. Thompson et al.
1984). This, combined with their field characteristics
as submarine volcanites, would be compatible with
the evolution of an ocean island complex.
The REE patterns and trace element diagrams of the
basic lava interbedded with the shallow-marine metasandstone occurring as blocks in the melange (87-SF63), and dykes cutting it (87-SF-59), show an alkaline
affinity with an enrichment in the most incompatible
elements such as LREE, Th and Ta (Fig. 7e, f)-
4. Tectonic environment
The tectono-magmatic environment in which the
Solund-Stavfjord Ophiolite Complex and associated
rocks formed must be consistent with the following
geochemical and geological features:
(a) The metabasalts of the Solund-Stavfjord
Ophiolite Complex show N- to E-MORB affinities
with a clearly detectable influence from a subduction
zone, as demonstrated by a moderate to strong
depletion in Ta and enrichment in Th, relative to
average MORB;
(b) The Solund-Stavfjord Ophiolite Complex is
overlain by quartz-rich, continentally-derived metasediments which contain pillow lavas, metavolcaniclastites and intrusions of MORB, IAT, calc-alkaline
and alkaline compositions (the Stavenes Group);
(c) The presence of the Kalvag Melange, which
contains olistoliths of (i) shallow-marine metasandstones with associated subaerial lavas, (ii) conglomerate 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
10-1
30n
O 87-SF-14
• 87-SF-15
X 87-SF-16
10-
Greenstone
Clasts
10—i
c
o
O
|O 87-SF-27|
10-
Quartz
Porphyry
Clast
o
o
cc
m
cc
O
1O
o
cc
100-1
30O 87-SF-59
• 87-SF-63
10-
lava (•)
dyke (o)
La Ce
Nd
Sm Eu Gd Tb
Ho
Tm Yb Lu
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 thereby
developed as a tectonic melange during accretion of
the Solund-Stavfjord Ophiolite Complex and sedimentary cover onto the continental margin.
4.a. Models
4.a.l. Gulf of California
Field relations such as the quartz-rich metasediments
overlying the MORB-type pillow lavas of the SolundStavfjord Ophiolite Complex, hosting pillowed metabasalts and intrusions (Figs 2, 3), also of MORB-type
(Fig. 6a, b, c), provide strong evidence that the
Solund-Stavfjord Ophiolite Complex developed in
the near vicinity of a continental margin (cf. Moores,
1982). Such a development appears comparable to
that of the present-day Gulf of California, where
young to recent N-MORB at the East Pacific Rise are
intercalated with sandstone, siltstone, or claystone
(Saunders et al. 1982; Saunders, 1983). However, the
geochemical signature of the metabasalts of the
Solund-Stavfjord Ophiolite Complex, giving support
for subduction influence (Figs. 5f, g) and, even more
significantly, 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 SolundStavfjord Ophiolite Complex, does not correspond to
a Gulf of California setting.
4.a.2. Andaman Sea
A more appropriate environment in which the rocks
of the Solund-Bremanger area may have formed is
thought to be represented by the area between Burma
and Sumatra, i.e. the Andaman Sea region of the
Indonesian Arc system. In this region, oblique
subduction of the Indian Plate beneath the northwardmoving Burma Plate has, since mid-Miocene times,
resulted in pull-apart opening of the Andaman Sea
and generation of oceanic crust along several ridgetransform systems (Curray et al. 1979, 1982; Fig. 8 a).
Thus the Andaman Sea ocean crust developed adjacent
to the continental margin of the Malay Peninsula to
the east and a subduction system to the west and
southwest. The latter consists of an inner active
volcanic arc (Sumatra), a forearc region with active
volcanoes (extending into Burma), and an outer ridge
representing a well-developed accretionary prism
(Fig. 8 a) in which ophiolite fragments occur (Ray,
Sengupta & van den Hul, 1988).
The relationship between continentally-derived
sediments and the pillow lava/intrusions from the
active spreading ridge may here, in principle, be
identical to that in the Gulf of California, but the
basalt geochemistry would contain a subduction zone
signature. In the Andaman Sea model, the MORB of
the ocean crust may or may not have a geochemical
221
Solund-Stavfjord Ophiolite Complex
A: Andaman Sea Region
Present time
S againg
BURMA [si,
Fault
B: Andaman Sea model applied to the Solund-Bremanger Region
Time:
~440 Ma
Time:
post~440 Ma
pre~380 Ma
edge of
continental crust
•a
2
\'°i
Spreading
ax'
Transform
fault
Indian
Plate
Line of separation
Forearc Basin
A
I
Volcano
j Continental Crust
500km
Figure 8. The Solund-Stavfjord Ophiolite Complex and associated rocks reconstructed by using the present-day
tectonomagmatic evolution of the Andaman Sea as a model. In (Bl) we have indicated geochemical effects from subduction
activity upon the metabasalts of the Solund-Stavfjord Ophiolite Complex. We further indicate the beginning stage of subaerial
island arc volcanoes for supplying air-borne IAT tuffaceous material to the near-continent spreading centre of the
Solund-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 and
the Heggoy Formation, see Figures 2 and 3, respectively. In (B2) we present a speculative and uncertain model in which we
relate the Hersvik Unit to an evolved, near-continent island arc, and the Kalvag Melange to have received material from an
eroded, but still active, mature island arc, with the possibility of also receiving material supply from the erosion of an
accretionary prism. The Smelvrer Unit is proposed to have developed as an oceanic island, distant enough from the continental
margin and island arc not to receive quartz-dominated sediments, and for the magmatic products not be influenced by
subduction 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 volcaniclastic
rocks intercalated with the sediments might be derived
from the ash-fall of subduction-generated volcanoes.
On the basis of geological relationships and geochemistry, we tentatively suggest that the SolundStavfjord Ophiolite Complex and associated sedimentary cover with lavas, intrusions and volcaniclastites of the Heggoy Formation could have developed in a tectonic environment comparable to that
of the Andaman Sea (Fig. 8 b).
The subaerial calc-alkaline lavas and volcaniclastites of the Hersvik Unit and the alkaline volcanic
rocks of the Smelvar Unit are more difficult to fit into
the model, because of their uncertain stratigraphic
relationship to the Solund-Stavfjord Ophiolite Complex and the Heggoy Formation, but they are suitable
analogies (Fig. 1). Basalts generated close to transform
faults are often enriched in incompatible elements
(e.g. Langmuir & Bender, 1984), and alkaline oceanic
islands may develop over mantle plumes on oceanic
crust (e.g. Mitchell-Thome, 1982). An important
feature of the magmatic development of the Karmoy
Ophiolite Complex, southwest Norway, is the evolution towards alkaline magmatism, with a concomitant lowering of the eNd values relative to the
MORB/IAT products (Pedersen & Hertogen, in
press). Similar features have recently been discovered
in basalts from the Lau Basin (Volpe, MacDougall &
Hawkins, 1988). It is therefore interesting to note that
the metabasalts of the Smelvaer Unit and the SolundStavfjord Ophiolite Complex have eNd (T = 430 Ma)
values of c. 5 and 8, respectively (R. B. Pedersen,
222
unpublished data), This could well indicate that the
Smelvasr Unit represents a late stage of ocean island
growth in the back-arc basin within which the
Solund-Stavfjord Ophiolite Complex developed
(Fig. 8 b).
The subaerial calc-alkaline metabasalts and metavolcaniclastites, associated with metagreywackes and
conglomerate beds of the Hersvik Unit, indicate
evolution in a mature arc setting near a continental
margin (Fig. 8 b).
The Kalvag Melange cannot, on the basis of field
relations, be directly connected to the SolundStavfjord Ophiolite Complex and its cover sequence.
It is cut by a transitional tholeiitic/calc-alkaline
intrusion (the Gasoy Intrusion), dated as 380 + 26 Ma
(Furnes el al. 1989), which is thus at least 36 Ma
younger than the oldest known part of the SolundStavfjord Ophiolite Complex, dated to 443 + 3 Ma
(Dunning & Pedersen, 1988). Based on the geochemistry of conglomerate pebbles and the environment of formation of the various lithologies, we
suggest that the Kalvag Melange represents material
derived from an active, mature arc near a continent,
containing an exposed basement of ophiolitic rocks.
Alternatively, the ophiolitic metabasalt pebble material may be derived from an accretionary prism, as
indicated in Figure 8 b.
Occurring between the Solund-Stavfjord Ophiolite
Complex and the continental-type sediments of the
Herland Group, is the Sunnfjord Melange (Fig. 1), a
deposit which was first initiated in a transform setting
(K. P. Skjerlie, unpub. Cand. Scient. thesis, Univ.
Bergen, 1988; Skjerlie & Furnes, in press), and later
developed into an obduction melange (Andersen,
Skjerlie & Furnes, 1990). In the Andaman Sea there
are several transform faults that are parallel to the arc
and subparallel to the continental margin. The most
extensive fault, the Sagaing Fault (Hla Maung, 1987),
defines the active boundary between two different
terranes, i.e. the continental margin, and the ocean
floor of the Andaman Sea (Fig. 8 a). This tectonic
feature may be a modern analogue to the early stage
in the development of the Sunnfjord Melange, which
appears to have received material from both the
oceanic (Solund-Stavfjord Ophiolite Complex) and
continental (Herland and Hoyvik groups) environments.
5. Summary
The Solund-Stavfjord Ophiolite Complex of late
Ordovician age (U-Pb zircon age of 443 + 3 Ma)
consists, from bottom to top, of the following
components: (1) Varitextured, massive or faintly
laminated metagabbro, (2) a sheeted dyke complex,
and (3) a thick sequence of pillow lavas, metahyaloclastites and massive metabasalt units which in
some cases represent lava lakes, in other cases sheet
H. FURNES AND OTHERS
flows. The Solund-Stavfjord Ophiolite Complex is
conformably overlain by a sequence of quartz-rich
metasandstones, phyllites and basic metavolcaniclastites (the Heggey Formation), hosting metabasalt
intrusions and pillow lavas. The geochemistry of the
metabasalts of the Solund-Stavfjord Ophiolite Complex and the Heggoy Formation are of N- to EMORB composition, with positive evidence of a
subduction-related influence, indicated by high Th/Ta
(or Nb) ratios, and the basic metavolcaniclastites are
geochemically similar to IAT. These geological and
geochemical features are best explained by the
Solund-Stavfjord Ophiolite Complex having formed
in a marginal basin near enough to a continental
margin for sandstones to be deposited at the active
spreading ridge, and subsequently become intruded
and interlayered by MORB and island-arc-influenced
basalts. Contemporaneously, IAT volcanites, probably representing tuffs from an emerging island arc,
became interbedded with the continentally-derived
metasediments. This tectonomagmatic development is
in many ways similar to that of the present-day
Andaman Sea region of the Indonesian Arc system.
Calc-alkaline metabasalt lavas and volcaniclastites,
interbedded with metasandstones and quartz-pebble
conglomerates (the Hersvik Unit), probably reflect a
more advanced stage in the development of the island
arc system, in the proximity of a continental margin.
A sedimentary melange, the Kalvag Melange, consisting of pebble material of MORB, IAT and quartz
porphyry, as well as olistoliths of rhyolitic ignimbrite,
shallow-marine metasandstone with interbedded alkaline lava, may also have developed in connection
with a mature island arc/accretionary prism, prior to
380 + 26 Ma (the age of a gabbronorite intruding the
melange). Pillow lavas and associated metavolcaniclastites of alkaline composition, the Smelvaer Unit,
probably developed on a Solund-Stavfjord Ophiolite
Complex basement as an oceanic island. A tentative
model for these three, poorly age-constrained rock
units, as representing part of an evolved arc system
(the Hersvik Unit and Kalvag Melange) and oceanic
island development (the Smelvaer Unit), is presented
(Fig. 8 b).
Acknowledgements. Financial support for this study has
been provided through grants (D.41.31.147) from the
Norwegian Research Council for Science and the Humanities. R.J.S. acknowledges financial support for fieldwork from Oxford Polytechnic. We express our thanks to
F. J. Skjerlie for many useful discussions about the general
geology of the area, J. Boyle, J. R. Cann and J. Malpas for
their contributions in mapping minor parts of the Heggoy
Formation and the Solund-Stavfjord Ophiolite Complex at
an early stage of the project, and D. Roberts and an
anonymous reviewer for constructive comments to an early
version of the manuscript. E. Lier, J. Ellingsen and E.
Irgens prepared the illustrations. This work is publication
no. 77 in the International Lithosphere Project (ILP).
Solund-Stavfjord Ophiolite Complex
223
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