PLATE TECTONIC MODEL BASED ON GR EENSTONE GEOCHEMISTRY IN THE LATE PRECAMBRIAN - LOWER PALAEOZOIC SEQUENCE IN THE SOLUND-STAVFJORDEN AREAS, WEST NOR WAY HARALD FURNES, FINN J. SKJERLIE & MAGNE SSELAND Fues, H., Skjerlie, F. J. & sseland, M.: Plate tectonic model based on greenstone geochistry in the Late Precamban-Lower Palaeozoic se- quence in the Solund-Stavfjorden areas, wt Noay. Norsk Geologisk Tids- skrift, Vol. 56, pp. 161-186. Oslo 1976. A plate tectonic model for the Caledonian eugeosynclinal sequence in the Solund and Stavfjorden districts of west Norway has been based on green- stone geochemistry, sediment deposition, and stctural pattems. The thick metabasalts at the base of the sequence are geochemically similar to modem ocean floor tholeiitic basalts, and relate to the late Precambrian - Cambrian opening of the lapetus ocean. Contemporaneously with the formation of oceanic cst, thick piles of sediments accumulated on an Atlantic-type margin to the Baltic continent. During the closing of lapetus an ensimatic island are was formed upon the wedge of earlier sediments, and basic Javas of tholeiitic, calc-alkali, and alkaline affinities were successively erupted. Steepening of an easterly dipping Benioff zone with time, and associated back-are spreading, is the interetation of the spatiallcompositional relation- ships of the plate-margin Javas and the Upper Ordovician tectonic evolution including uplift, thrusting, and subsequent redepition of the pre-existing sequence. H. Fues, F. J. Skjerlie & M. Tysse/and, Geologisk institutt, Avd. A, J. Frielesgt. l, 5014 Bergen, Norway. In a series of recent studies, attention has been directed at Caledonian magmatism attempts at a renstruction of the evolution of the northem part of the protAtltic related to the Norwegian Caledonides (Gale & Roberts 1972, 1974, Ramsay 1973, Fues & Færseth 1975, Robins & Gardner 1975, Gale 1975). In the Stavfjorden and Solund districts of western Norway, thick eugeo- synclinal suences of Lower Palaeozoic age occur within the Stavfjord Anticline (Fig. 1). In a series of earlier papers (Skjerlie 1969, 1974, Fues 1972, 1973, 1974, Fues & Skjerlie 1972), the stratigraphy, lithology, and structure of both districts have been dealt with separately. Integration of the results reveals that the geological history in both areas is similar in most respes. Greenstones occur at a number of stratigraphic levels in the suence of this region and appear to have significantly different geochemical character- istics. This implies a progressive evolution in terms of the paeogeographic
Transcript
PLATE TECTONIC MODEL BASED ON
GREENSTONE GEOCHEMISTRY IN THE LATE
PRECAMBRIAN - LOWER PALAEOZOIC
SEQUENCE IN THE SOLUND-STAVFJORDEN
AREAS, WEST NORWAY
HARALD FURNES, FINN J. SKJERLIE & MAGNE TYSSELAND
Furnes, H., Skjerlie, F. J. & Tysseland, M.: Plate tectonic model based on greenstone geochemistry in the Late Precambrian-Lower Palaeozoic sequence in the Solund-Stavfjorden areas, west Norway. Norsk Geologisk Tidsskrift, Vol. 56, pp. 161-186. Oslo 1976.
A plate tectonic model for the Caledonian eugeosynclinal sequence in the Solund and Stavfjorden districts of west Norway has been based on greenstone geochemistry, sediment deposition, and structural pattems. The thick metabasalts at the base of the sequence are geochemically similar to modem ocean floor tholeiitic basalts, and relate to the late Precambrian - Cambrian opening of the lapetus ocean. Contemporaneously with the formation of oceanic crust, thick piles of sediments accumulated on an Atlantic-type margin to the Baltic continent. During the closing of lapetus an ensimatic island are was formed upon the wedge of earlier sediments, and basic Javas of tholeiitic, calc-alkali, and alkaline affinities were successively erupted. Steepening of an easterly dipping Benioff zone with time, and associated back-are spreading, is the interpretation of the spatiallcompositional relationships of the plate-margin Javas and the Upper Ordovician tectonic evolution including uplift, thrusting, and subsequent redeposition of the pre-existing sequence.
H. Furnes, F. J. Skjerlie & M. Tysse/and, Geologisk institutt, Avd. A, J. Frielesgt. l, 5014 Bergen, Norway.
In a series of recent studies, attention has been directed at Caledonian magmatism in attempts at a reconstruction of the evolution of the northem part of the proto-Atlantic related to the Norwegian Caledonides (Gale & Roberts 1972, 1974, Ramsay 1973, Furnes & Færseth 1975, Robins & Gardner 1975, Gale 1975).
In the Stavfjorden and Solund districts of western Norway, thick eugeosynclinal sequences of Lower Palaeozoic age occur within the Stavfjord
Anticline (Fig. 1). In a series of earlier papers (Skjerlie 1969, 1974, Furnes
1972, 1973, 1974, Furnes & Skjerlie 1972), the stratigraphy, lithology, and
structure of both districts have been dealt with separately. Integration of the
results reveals that the geological history in both areas is similar in most
respects. Greenstones occur at a number of stratigraphic levels in the sequence of
this region and appear to have significantly different geochemical character
istics. This implies a progressive evolution in terms of the palaeogeographic
162 H. FURNES, F. J. SKJERLIE & M. TYSSELAND
setting of the volcanicity which the authors consider to be important in re
construction based on plate-tectonic theory. The main purpose of this account is to discuss the geochemistry of the different greenstones from both
the Solund and Stavfjorden districts. These data, combined with the present knowledge of the lithostratigraphy and structural pattern of the Stavfjord
are considered sufficient for a local reconstruction of the sequence of plate boundary events in the light of present-day knowledge of global tectonics
and magma generation.
Geological setting The area under discussion is located between Sognesjøen and Askrova on the western coast of Norway (Fig. 1) and contains rocks ranging in age from Precambrian to Devonian. The rock units dealt with in this account com
prise the following:
The Stavfjord Anticline. - This rock sequence comprises a thick metabasalt
pile as the stratigraphically lowest unit, and is succeeded by metasediments with intercalated basic lavas as the stratigraphically highest units.
The Askvoll Group. - Consisting of low grade metamorphilic sediments, in
fault contact with the Precambrian basement rocks.
The Dalsfjord Nappe. - Consisting of mangeritic rocks, and being in thrust contact with the underlying Askvoll Group and the overlying Stavfjord Anticline.
Undifferentiated Precambrian basement. - Consisting of a variety of gneisses and gabbroic intrusions.
Stavfjord Anticline LITHOLOGY
In the Solund and Stavfjorden districts of west Norway (Fig. 1), sequences of eugeosynclinal volcanic and sedimentary rocks with thickness of at
least 2700 and 400 m, respectively, form the Stavfjord Anticline (Furnes
1974, Skjerlie 1974). They show the effects of Caledonian polyphase deformation and low-grade metamorphism.
On a lithostratigraphic basis, the sequence in the Solund district can be
divided into the four groups (Furnes 1974), similar to those of the Stav
fjorden district (Skjerlie 1969). The assumption of Skjerlie (1974) that the
rocks on Håsteinen ( called the Hås teinen Group) are equivalent to the low
grade metamorphic sediments of the Askvoll Group is rejected in this ac-
PLATE TECTONIC MODEL FROM SOLUND-STAVFJORDEN AREAS 163
• -'Ryggsteinen
' .. Moldvær "
" Håstcon'l" '\_
Z"'
�S�OVA
o�
Stav fjorden district
Fig. l. Geological map of the Stavfjorden and Solund districts.
the
10
count, and it is considered more likely that they belong to the Precambrian basement.
Mainly in broad outline, but partly also in detail, the sequences in both parts
of the Stavfjord Anticline (Fig. 1) are similar both with respect to lithology
and structural pattern (Furnes & Skjerlie 1972), and can be correlated as
shown on Fig. 2.
The Oldra and Stavenes Groups are predominantly composed of a thick succession of metabasalts, though tuffaceous rocks occur to the NW in the
Stavfjorden district. Pillow lavas and metahyaloclastites dominate, parti
cularly in the lowest part of the sequence. Estimation of the volume of amygdales and the occurrence of variolitic structure in the pillows of the
Solund district led to the conclusion that they were formed under consider-
164 H. FURNES, F. J. SKJERLIE & M. TYSSELAND
ST AVFJORDE N district
- Limes tone c.. -::l
o a:: o � f-- Meta ark ose, meta subarkose -::..:: > � � Quarts-mica schist J: ooooooooc Conglomerate c.. � Limestone ::l
o o<l] a:: o o 'VV Greenstone z 4: Quarts se hist, mica schist, _ ...J
c.. ::l l/" o a:: o ... .................. pooooo 4:
0000� a::
000 00 c ...J
.................. ... o ........................ oaoao 00000
Fig. 2. Generalized lithostratigraphic correlation of the Stavfjorden and Solund districts.
able depth of water, probably in the order of 2500 ro (Furnes 1973, 1974). Above the pillow lava/hyaloclastite sequence, a thick pile of mostly massive
greenstones, interpreted as non-pillowed submarine lava flows, occurs in
both districts.
In the Stavfjorden district, Skjerlie (1974) assigned the pillow lava and
PLATE TECTONIC MODEL FROM SOLUND-STAVFJORDEN AREAS 165
massive metabasaltic part of the Stavenes Group to the Grimeli Formation,
which contains in its lower part minor amounts of calcareous metagrey
wackes. Above the Grimeli Formation a sequence of metatuffs and volcano
genic sediments assigned to the Moldvær Formation, occur on the islands
of Svanøy, Askrova, Moldvær and Ryggsteinen (Fig. 1). The boundary be
tween the Grimeli and Moldvær Formations is transitional; thin layers of
metatuff do occur in the lavas of the Grimeli Formation, for example at
Smelvær and Stavenes. The tuffaceous material gradually disappears south
wards, and in the Solund district, no traces of tuffs are to be found in the
Oldra Group.
Gabbroic intrusives of varying size occur abundantly both in the Oldra and
Stavenes Groups, and in the latter serpentinites are also present.
The Hersvik and Lower Herland Groups are situated stratigraphically
above the Oldra and Stavenes Groups, and separated from them by a thin
horizon of phyllite or mica schist. The dominant rock type of this part of the
sequence is calcareous metagreywacke, together with occasional thin con
glomeratic lenses and layers. In the Stavfjorden district the metasediments
form the Hatteli Formation (Skjerlie 197 4). In the middle to upper parts of the Hersvik and Lower Herland Groups
a revival of basaltic to andesitic volcanism is marked by a number of green
stone horizons from 3 to 200 m in thickness, intercalated with the metasedi
ments. In the Stavfjorden district, a greenstone unit at least 200 m thick and
situated at the highest part of the Lower Herland Group has been called the
Holten greenstone (Skjerlie 1974). The greenstones of the Hersvik and
Lower Herland Groups are massive and usually plagioclase phyric. Their
amygdale content is highly variable, and in the absence of any intrusive
contacts, it is most likely that they represent lava flows.
The Mjelteviknes and Upper Herland Groups are situated unconformably
upon the Hersvik and Lower Herland Groups. This is clearly demonstrated in the Stavfjorden district where the polymict Stubseid conglomerate, up to
200 m thick, rests with a primary discordance on different lithological units of the Lower Herland Group (Skjerlie 1974). Within this conglomerate the
Brurstakken Limestone Member (Skjerlie 1974) contains fossils of Lower
Ordovician to Lower Silurian age. An equivalent, thick conglomerate exists
in the Solund districts at the base of the Mjelteviknes Group (Furnes 1974). The further sedimentological development of the Mjelteviknes and Upper
Herland Groups is characterized by fine-grained sediments, mostly chlorite/
sericite schists in the Solund district, and quartz-mica schists in the Stavfjor
den district. Traces of basaltic volcanism are represented by a thin layer
of greenstone. In the Stavfjorden district, layers of limestone occur within
the stratigraphically highest strata of the group (Skjerlie 1974). One of them,
on Atløy, contains a fauna of rugose corals and crinoid stems of Upper
Ordovician or Lower Silurian age (Skjerlie 1969, 1974). Towards the tap
of the Upper Herland Group the lithology gradually changes from a quartz
mica schist to a coarse grained meta-arkose characteristic of the highest rock
166 H. FURNES, F. J. SKJERLIE & M. TYSSELAND
units of the Stavfjorden Anticline, the Høyvik Group (Skjerlie 1969, 1974). The latter is subdivided into the Hovden Conglomerate situated at the base,
and the Øyravatn Formation. Equivalent rocks are thought to exist in the Hersvik area of the Solund
district; they have been called the Arnes Group (Furnes 1974).
AGE RELATIONS
Based on the traditional assumption that thick greenstone sequences within the Norwegian Caledonides are of Lower Ordovician age, the Oldra and Stavenes Groups have previously been thought of as being of Arenig age (Skjerlie 1969, 1974, Furnes 1974). However, as no fossiliferous basement
to the sequence has ever been found, its maximum age cannot be established.
In this account, we discuss later the possibility of a Cambrian to Lower
Ordovician age. Similarly, no fauna exists in the sedimentary rocks of the Hersvik and
Lower Herland Groups, and the supposed Mid-Ordovician age (Skjerlie
1969, 1974, Furnes 1974) which followed on the supposition of a Lower Ordovician age for the underlying volcanic rocks cannot in fact be supported,
though it may be correct. An Upper Ordovician maximum age for the Upper Herland Group can be
established on fauna! evidence. In the Solund district, the Mjelteviknes Group is only the lithostratigraphic equivalent.
The rocks of the Høyvik Group, though unfossiliferous, are post-Upper
Ordovician and pre-Devonian, and are probably of Silurian age. This is
indicated by palaeontological evidence in the upper part of the Upper Herland Group (Skjerlie 1974).
The Askvoll Group Skjerlie (1969) described the Askvoll Group as a parautochthonous sequence of low-grade metamorphic sediments at !east 5000 m thick lying upon metamorphic rocks containing assemblages typical of the almandine
amphibolite facies and including eclogites. In this account, only the upper
low-grade metamorphic rock sequence will be assigned to the Askvoll
Group. The reason for this restriction is the pronounced change in lithology
and metamorphic grade across major faults (Fig. l) of the lower and upper parts of the former Askvoll Group which suggests quite different age rela
tions. As no fossils are known from this sequence, an absolute age for it
cannot be established, and only a Iithological correlation is possible. Even
with the uncertainties implicit in such a correlation, we think there is some
justification for suggesting that the low-metamorphic sediments and tuffs of
the Askvoll Group formed contemporaneously with both the Stavenes- and Lower Herland Groups. The higher grade metamorphic rocks are almost cer
tainly considerably older; they have been assigned to the Precambrian (Fig.
1). Further reasons for these proposals will be discussed later in this paper.
PLATE TECTONIC MODEL FROM SOLUND-STAVFJORDEN AREAS 167
The Dalsfjord Nappe 'The Dalsfjord Nappe, consisting of mangeritic rocks, rests with thrust contact
upon the Askvoll Gro up and beneath the Stavfjord Nappe. The mangerites have
not yet been dated, but the type rocks in the Bergen area give a metamorphic age of 1064 ± 24 my (Pringle & Sturt 1972, Sturt et al. 1975). A Precambrian
age for the mangeritic rocks of the Dalsfjord Nappe is therefore supposed.
GEOCHEMISTRY OF THE GREENSTONES
Table l shows the major and trace element geochemistry of the greenstones from both areas under discussion.
The major elements, excepting sodium which was analysed by atomic absorption, have been analysed by X-ray fluorescence methods (XRF), using glass beads prepared according to the method of Padfield & Gray (1971).
Twenty international standards and the recommended values of Flanagan
(1973), refined by least squares procedures and matrix corrections, were used for calibration. Ferrous iron was determined titrimetrically employing potassium dichromate. Trace elements were determined by XRF using pressed powder pellets. Twelve international standards and Flanagan's (1973) recommended values, refined by least squares procedures, were used for
calibration.
The Oldra and Stavenes Groups
Metabasalts from the Oldra and Stavenes Groups are very uniform in their major element geochernistry. As seen from the AMF diagram (Fig. 3a) they clearly define a tholeiitic trend with a marked iron enrichment at constant alkali
F F • OldraondStavenes Groups O Holten greenstones
x Hersvik greenstones O Leknessund greenstones
Ti O,
AL--------------------------------------1M �ho�----------------------�P,� Fig. 3. AMF (A = Na20 + K20, M = MgO, F = FeO + 0.8998 Fe203) and Ti02 - K20 - P205 variation diagrams. (a) Metabasalts from the Oldra and Stavenes Groups, (b) Metabasalts and meta-andesites from the Hersvik and Lower Herland Groups. (c) Metabasalts from the Oldra and Stavenes Groups. The field boundary separating tholeiitic (above) from calc-alkaline (below) compositions in (a) and (b) is after Irvine & Barager (1971).
168 H. FURNES, F. J. SKJERLIE & M. TYSSELAND
content. The REE pattern, characterized by depletion of the light rare-earth
elements (Fig. 7b ), confirms the tholeiitic nature of these metabasalts.
Although tholeiitic basalts are dominantly generated at oceanic ridges,
continental basalts of tholeiitic affinities cover large areas. Pearce et al.
(197S) compiled a large number of analytical data on both oceanic and con
tinental tholeiites, and by means of a Ti02 - P 205 - K20 diagram were a ble to discriminate between the two types. The metabasalts from the Oldra and Stavenes Groups plot in this discrimination diagram well inside the field for
ocean tholeiites (Fig. 3c).
In many respects, tholeiites generated at a mid-oceanic ridge are very similar to low-K tholeiites generated in the early stages of island are evolution (Jakes & Gill 1970). By the use of variation diagram where the ratio total
Fe as FeO/MgO is plotted against FeO or Ti02, Miyashiro (1973, 197S) has shown that with increasing FeO/MgO ratio, there is a rapid enrichment of FeO and Ti02 in abyssal tholeiites, whereas in the case of low-K tholeiites,
FeO is more slowly enriched, and Ti02 is constant. The Oldra and Stavenes Groups metabasalts plot in these diagrams close to the trend of abyssal tholeiites (Fig. 4a). Although the major element geochemistry of basalts may change considerably during hydrothermal alteration and low grade metamorphism (Vallance 1960, 1969, 1973, Herrmann et al. 1974, Hart et al.
1974), a consistent ocean floor tholeiitic trend has been established (Figs.
3a, c, 4a). As a consequence of the conditions under which tholeiitic, calc-alkali, and
alkaline basalts are generated, pronounced primary differences in the ab
solute concentrations, or ratios, of the 'incompatible' elements exist (e.g. J akes & Gill 1970, Green 1971, Pearce & Cann 1971, 1973, Kay & Gast 1973, Chazen & Vogel 1974). This has enabled various authors to establish discrimination diagrams for genetically different types of basalt on the basis of trace-element combinations (Pearce & Cann 1971, 1973), and by chondrite-normalized rare-earth element abundances (J akes & Gill 1970, Kay & Gast 1973). Further, since alteration processes do not significantly alter the ratios or concentrations of these elements (Frey et al. 1968, Philpotts et al. 1969, Cann 1970, Herrmann et al. 1974), they have been applied in
the recognition of the primary character of greenstones (White et al. 1971,
Bickle & Nisbet 1972, Pearce & Cann 1973, Gale & Roberts 1974, Herrmann
In terms of such trace-element combinations as Ti-Zr, Ti-Zr-Y and Ti
Zr-Sr, the Oldra and Stavenes greenstones group within the fields for oceanfloor tholeiites (Fig. Se, e). The greenstones of the Oldra Group in Solund
are comparatively rich in Ti02, and considerably richer than their equivalents
in the Stavfjorden district (Fig. 4a, Sa). In Fig. Sa they plot above the oceanfloor basalt field. From the Ti02 values alone, they would appear to be more akin to alkali basalts. However, on all other geochemical grounds they are
typical tholeiites, with low concentrations of incompatible elements (Table 1).
It has recently been shown by Nisbet & Pearce (1973) that the amount of
a
b
6
l
l
;· l
,.
PLATE TECTONIC MODEL FROM SOLUND-STAVFJORDEN AREAS 169
l CArrH
l
l l
15 % FeO
3 10
2
50 l A
��L.__M ;:::;-.--:--l
l l
/ . ... .
' ------M
s�--�----�--� ------Am
o 1 2 3 o 1 FeO/MgJ
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60 %Si�
o 1 FeO/MifJ
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10
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Fig. 4. SiO! - FeO/MgO, FeO - FeO/MgO and TiO! - FeO/MgO variation diagrams Oxides as wt %, total iron as FeO. The field boundaries separating the tholeiitic (TH) and calc-alkaline (CA) series, and the trend lines for the abyssal tholeiites (A), Macauley Island are tholeiitic series (M), and the Amagi calc-alkaline series (Am), are after Miyashiro (1975). Symbols as in Fig. 3. (a) Metabasalts from the Oldra and Stavenes Groups. In the TiO! - FeO/MgO diagram
fields, O and S show the plots from the Oldra and Stavenes Groups, respectively. (b) Metabasalts and meta-andesites from the Hervik and Lower Herland Groups.
Ti02 in ocean-floor tholeiites is closely related to the rate of spreading, and
increases with a higher spreading rate. As a corollary of such an empirically
determined relationship, the Oldra Group greenstones could be tentatively
regarded as abyssal tholeiite generated at a time when spreading was fast.
In the Stavfjorden district variations in the Ti02-content of the metabasalts
170 H. FURNES, F. J. SKJERLIE & M. TYSSELAND
20000 ® 20 000 ®
15000 15000
lO
z, ppm
@ ®
l l
..
15() 200
r�oo
l @ {;!)-. D .(l • o
'c ·�·· � • o
o c o' �.o
B
z, Y•3 Zr
Fig. 5. Ti - Zr, Ti - Y - Zr, and Ti - Y - Sr discrirninant diagrams (after Pearce &
Cann 1973). In the Ti - Zr diagram: ocean floor type basalts (fields B and D), calc
alkaline basalts (field C), and low-potassium tholeiites of island arcs (fields A and B). In the Ti - Zr - Y diagram 'within-plate' basalts (field D), low potassium tholeiites of island arcs (fields A and B), ocean floor type basalts (field B), and calc-alkaline basalts (field C). In the Ti - Zr - Sr diagram: Ocean floor type basalts (field C), low potassium tholeiites of island arcs (field A), and calc-alkaline basalts (field B). 0: Oldra Group, S: Stavenes Group. Symbols as in Fig. 3.
can clearly be related to stratigraphy. In the lowest part of the sequence, along the axial plane trace of the Stavfjord Anticline (Fig. 1), the Ti02
values of the greenstones are consistently higher than of those stratigraphi
cally higher in the sequence. The correlation between Ti02-content and
distance from the axial plane trace of the Stavfjorden Anticline is shown in
Fig. 6. It is most likely that this feature represents a primary feature of the
basalt magmatism (Nisbet & Pearce 1973), which could mean that the
spreading rate was continuously decreasing during the time of formation
of these basalts. Another possibility could be that the Ti02-content reflects
different degrees of differentiation (Fig. 4a).
On the Ti-Zr-Sr diagram, some of the plots also fall outside the MORB
field (Fig. Se) as a result of relatively low Sr values. Sr is, however, the most
mobile of the trace elements used in this account for basalt discrimination
(Hart 1974, Pearce & Cann 1973), and tends to be depleted in varying
amounts consequent upon hydrothermal alteration.
Both on the basis of major and trace-element abundances it can safely be
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2
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Fig. 6. Relationship between Ti02 values and distance from axial plane trace of the Stavfjord Anticline of the Stavenes peninsula. Note the continuous decrease in Ti02 of the metabasalts stratigraphically upwards in the Stavenes Group.
concluded that the greenstones of the Oldra and Stavenes Groups represent
tholeiites of ocean-floor type; the equivalents of those being generated today
at actively spreading oceanic ridges by a high degree of partial melting in the
upper mantle. After a study of greenstones of the Stavenes Group, Gale
(1975) has reached the same conclusion. The differences in geochemistry
between the Oldra and Stavenes Groups include consistently higher concen
trations of the elements Ti, Y, and Zr in the former (Table 1). The higher Ti may be related to a higher spreading rate, or crystal fractionation on ascent to the surface (Nisbet & Pearce 1973). The elements Y and Zr
sympathetically follow Ti, as shown by Cann (1970). Within the Stavenes Group of the Stavfjorden district, metatuffs are inter
calated with the metabasalt pile at Stavenes and Smelvær. On the islands
of Moldvær, Ryggsteinen, Svanøy, and Askrova, metatuffs are by far the
most dominating rocks (Fig. 1 ). The geochemistry of these are shown in
Table 1. Although the geochemistry of the tuffs, especially the major-element
geochemistry, is likely to have changed considerably during hydrothermal
alteration and low grade metamorphism (Soyles & Mannheim 1975), elements
such as K and P are so much higher than in the intercalated tholeiitic meta
basalts that it seems more likely that they are alkali basaltic tuffs. What
effects syn-transport differentiation of the tuffs will have upon their geo
chemistry, is unknown. The alkali nature of these tuffs are more clearly seen
from the REE pattern (Fig. 7b ), showing an enrichment of the light rare
earths compared to the strongly depleted abundances of the associated meta
basalts. Any secondary process whereby the La-content of the metatuffs
could be enriched by a factor of almost 10 relative to the associated tholeiitic
metabasalts is difficult to envisage. Even though the tuffs to same extent
.!!
� �
PLATE TECTONIC MODEL FROM SOLUND-STAVFJORDEN AREAS 173
1 00
10
o Holten greens tones o Leknessund greenstones
1 � x Hersvik greenstones fl. Meto-ondesites
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Fig. 7. Chondrite-normalized rare-earth distributions in metabasalts, meta-andesites and tuffs from the Solund and Stavfjorden districts (chondrite data from Frey et al. 1968).
(a) Means of metabasalts and meta-andesites from the Hersvik and Lower Herland Groups.
(b) Means of metabasalts from the Oldra and Stavenes Groups.
are contaminated by extraneous material, such an effect upon the light rareearth elements is considered negligible, since these elements are preferentially
enriched in a melt phase and not in any crystalline phases (Kay & Gast 1973). It is supposed that the high light rare-earth contents are primary and reflect the alkaline nature of the parent of the tuffs.
The low Y INb ra ti os of the tuffs (ca. 0.5-0.9) are also strongly suggestive
of an alkaline nature (Pearce & Cann 1973), and contrast with the much
high er ratios of the tholeiitic metabasalts (ca. 5-6). Since both Y and Nb
are amongst the most immobile elements upon any alteration (Pearce &
Cann 1973, Lambert & Holland 1974), it seems very unlikely that the Y/Nb
ratio could have been changed secondarily by a factor of between 5 and 12.
The Hersvik and Lower Herland Groups
Although spatially confined to the same area, the basic lavas of the Hersvik
and Lower Herland Groups show a marked diversity in their geochemical
trends, and differ significantly from those of the Oldra and Stavenes Groups.
In the Hersvik and Lower Herland Groups, lavas of tholeiitic, calc-alkaline,
174 H. FURNES, F. J. SKJERLIE & M. TYSSELAND
and possibly alkaline affinities are represented (Table 1). On the AMF
diagram two trends may be seen (Fig. 3b). The meta-andesites from Hersvik
(Fig. 1) seem to define a typical calc-alkaline trend, whereas the Leknessund
and Holten greenstones (Fig. 1) may define a trend transitionally between that
of typical tholeiitic and calc-alkaline series. This is also reflected in Fig. 4b.
A somewhat confused scatter emerges in the Si02 -, FeO and Ti02 - FeO/
MgO plots (Fig. 4b). The Hersvik-andesites which on Fig. 3b define a strong
calc-alkaline trend, plot on the boundary between the fields of calc-alkaline
and tholeiitic lavas in the Si02 - FeO/MgO diagram (Fig. 4b). However, in
strong contrast to typical tholeiites, the plots define a very steep trend, i.e.
a nearly constant FeO/MgO ratio despite increasing Si02, characteristic of
calc-alkaline suites. A more difficult phenomenon to explain is the con
sistently high Ti02 content of the calc-alkaline lavas, as shown in the Ti02
- FeO/MgO diagram on Fig. 4b. Although individual points define an ap
proximately horizontal trend, i.e. constant Ti02 at increasing FeO/MgO
ratios, the Ti02 values are on the average much higher than for modem
low-K tholeiites and calc-alkaline lavas, which typically contain less than
This is also reflected in the Ti-Zr plot of Fig. 5b, where both Ti and Zr in
the Hersvik meta-andesites are consistently too high, and plot outside the
fields of recent calc-alkaline suites and low-K tholeiites. The Y and Nb
values for the Hersvik meta-andesites, averaging 40 ppm and 17 ppm re
spectively, would also appear to be relatively high, compared to modem
calc-alkaline andesites (Jakes & White 1972). However, when dealing with
ratios of the elements mentioned above, such as Ti-Y-Zr and Ti-Y-Sr (Fig.
Sd, f), the Hersvik meta-andesites are almost totally confined to the calcalkaline fields. The REE pattem (Fig. 7a) of these rocks is also compatible with a calc-alkaline trend for the Hersvik meta-andesites. The alkaline nature
of the metabasalts from the Hersvik area is indicated by the high concentrations of elements such as Ti, K, P, and the trace elements (Tab le 1, Fig.
Sb, 7a).
As stated earlier the Leknessund and Holten greenstones show character
istics of tholeiitic basalts. This is reflected in the major element plots (Fig.
3b, 4b), and the depleted nature of their light REE (Fig. 7a). In the discrim
ination diagrams Ti-Zr, Ti-Y -Sr, they mostly occupy the fields of ocean
floor basalts (Fig. 5b, d, f).
Both on major- and trace-element data, it is evident that during deposition
of the Hersvik- and Lower Herland Groups, different types of basaltic lavas
were available, and in the following the genesis of these melts will be dis
cussed.
Although showing tholeiitic affinities, and on discrirnination diagrams
plotting in the fields of ocean-floor basalts, it nevertheless would seem dif
ficult, if not impossible, to regard the Leknessund- and Holten greenstones
as true ocean-floor basalts generated at a spreading ridge. The overall litho
logical development of the Hersvik- and Lower Herland Groups, where the
PLATE TECTONIC MODEL FROM SOLUND-STAVFJORDEN AREAS 175
tholeiitic basalts are associated with thick sequences of calcareous metagreywackes and conglomerates, is incompatible with a formation at an oceanic
ridge. In an earlier paper (Furnes 1974) the Leknessund greenstones were regarded as time equivalent to those of the Oldra Group. In this paper this assumption is rejected on the basis of the sedimentary rocks they are as
sociated with. Generation related to an early stage of island-are development, such as
released H20 from amphibolite in the subducted oceanic crust, causes partial melting to occur in the pyrolite wedge above the Benioff zone, and subsequent differentiation under high PH2o of such a liquid would produce a tholeiitic magma (Ringwood 1974). This mechanism might be possible and would be
in general agreement with the geochemical data of the greenstones, but as their stratigraphy relative to the calc-alkaline and alkaline lavas is uncertain, such a conclusion cannot be confidently established.
Another mechanism of producing tholeiitic magmas related to an island
are development could be by active spreading and related volcanicity in the back are region. Such phenomena seem to be well documented (Karig 1971,
1972, 1973, 1974) and Hart et al. (1972) have shown that the Mariana trough basalts, generated by back-are spreading, come from depleted mantle similar to that feeding the oceanic spreading ridges. The same authors (Hart
et al. 1972) emphasize that the distinction between are tholeiites, midoceanic ridges basalts, and inter-are basalts is hardly detectable on a geochemical basis.
Of the two possibilities outlined regarding the genesis of the Leknessundand Holten greerrstones, i.e. either early island are tholeiites or inter-are
tholeiitic basalts generated by back-are spreading, it is, at present, impossible to tell which of the two would be the most likely. lf the latter possibility is applied, it would probably imply that the tholeiites are time equivalent with,
or even postdate the calc-alkaline and alkaline lavas in the Hersvik area. Alkaline magmatism in island arcs is a common phenomenon at an evolved
stage of the are, as, for instance, the J apanese Palaeozoic geosynclinal basalts (Tanaka & Sugisaki 1973). Two alternative hypotheses pertaining to the origin of basalt types can be quoted. One hypothesis (e.g. Kuno 1960, Kushiro & Kuno 1963) maintains that the compositions of the principal basaltic magmas are determined by the depth in the mantle at which partial mel ting occurs rather than by subsequent fractionation processes. The other (e.g. Green
& Ringwood 1964, 1967) asserts that a 'primitive magma' produces derivative liquids of other types. Prior to these authors, Tilley (1950) in studies on Hawaiian volcanic rocks postulated derivation of alkali olivine basalt from tholeiitic magma by fractional crystallization. Subsequently, Yoder & Tilley
(1962) and Green & Ringwood (1964) experimentally demonstrated that the low pressure 'thermal divide' between alkali basalts and olivine tholeiites
was absent at high pressure. Later, Green & Ringwood (1967) demonstrated that at pressures corresponding to depths of 35-70 km, separation of aluminous enstatite from olivine tholeiite magma, regarded as primary liquid,
176 H. FURNES, F. J. SKJERLIE & M. TYSSELAND
produces a direct fractionation trend from olivine tholeiites through olivine
basalt to alkali olivine basalt. It thus seems most plausible that the alkali basalts in the Hersvik area are
genetically unrelated to the associated calc-alkaline meta-andesites, and either represent an alkali basaltic liquid generated by a small degree of partial melting, or a high pressure fractionation product from an olivine tholeiitic parent.
Experimental studies have shown that partial fusion of mafic rocks at high pressures can yield iron-poor magmas of intermediate to acidic composition (e.g. Green & Ringwood 1968, Holloway & Bumham 1972, Green 1972). This has led petrologists to propose that island-are calc-alkaline suites are derived from, and owe their distinctive geochemical characteristics to, equilibrium with underthrust lithosphere (e.g. Taylor 1969, Jakes & White 1970, Fitton 1971, Green 1972). However, since the derivation of island-are calcalkaline andesites from parent eclogites having the initial composition of oceanic crust would involve a high degree of partial melting (20-40 % ),
there are discrepancies in the trace-element concentrations of the product (Gill 1974). To maintain the low concentrations of, for example, Ti, Zr, Y, Nb, and REE, the accessory minerals rutile or other Fe - Ti oxides, sphene, perovskite and zirkon would have to be refractory. According to Gill (1974) this is possible, but rather unlikely.
Nicholls & Ringwood (1973) have proposed a three-stage model for the evolution of some island-are magmas, involving reaction of acidic magmas
derived from the subducted plate with overlying peridotite to give wet garnet pyroxenite which subsequently can yield calc-alkaline magmas upon diapiric uprise and partial fusion.
As outlined above, andesites may form in many ways. Apparently one way of explaining the high concentrations of minor and trace elements (Ti, Zr, Y, Nb) in the Hersvik meta-andesites could be by an unfractionated partial
melt of eclogite where accessory minerals such as rutile, sphene, perovskite, and zirkon entered the melt, rather than behaving as refractory phases.
The Askvoll Group
Eight samples of metatuff of the Askvoll Group have been analysed. These
metatuffs are clearly volcanic derivates of two different types: (l) tholeiitic tuffs, and (2) andesitic tuffs (Table 2). As mentioned earlier, the majorelement geochemistry of metamorphosed tuffaceous rocks may be rather
misleading; it is believed that particularly the rare-earth elements are nearly
immobile. Fig. 8 shows the REE pattern of the metatuffs, and clearly one type shows
tholeiitic characteristics with relative depletion of the light rare earth, whereas
the other type is quite strongly enriched in the same elements. The latter
feature, combined with the low Ti02 values (Table 2), most likely relates to calc-alcaline andesitic volcanism.
PLATE TECfONIC MODEL FROM SOLUND-STAVFJORDEN AREAS 177
took place at stable continental margins. The sediments deposited nearest to
the continent, and upon continental crust, possibly built up the thick sequence
assigned to the Askvoll Group (Fig. 9A). Those deposited further to the
west, resting upon oceanic crust, formed the lower part of the Hersvik- and
Lower Herland Groups of the Solund and Stavfjorden districts, respectively.
The metagreywackes of the Askvoll and Lower Herland Groups in the
Stavfjorden district are compositionally similar, the main difference between
them being their basement rocks.
Many configurations of the contact between continental and oceanic crust
may exist, from gradational to abrupt. Microcontinents may exist within the
oceanic plate, representing detached fragments of the main continent
Fig. 9. Successive stages depicting the development of the Caledonian orogen in the Solund-Stavfjorden districts of western Norway. A. Early event in the rupture of continent and expansion of an ocean (lapetus). This stage marks the early formation of the Oldra and Stavenes Groups (oceanic crust), and the Askvoll Group (continent derived sediments). Precambrian to Cambrian time? B. Late evolution in the expansion of lapetus, with oceanic islands evolving on the plate, giving alkali basaltic Javas and tuffs. Continuous pelagic sedimentation on the evolving oceanic plate. Atlantic type sedimentation supplying material to the Askvoll, Hersvik, and Lower Herland Groups. C. Early stage of island-are evolution with tholeiitic volcanicity. Early or Middle Ordovician? Dl. More evolved stage of the island-are system, with calc-alkaline and alkaline volcanicity. Steepening of Benioff zone, and probable associated marginal basin spreading (as a consequence of steepening Benioff zone). Initial thrusting of the eugeosynclinal pile (Atlantic and back are basin types), including a slice of the Precambrian continental margin (i.e. the Dalsfjord Nappe). D2. Alternative model to Dl, with an oceanward migration of the Benioff zone, rather than steepening of it. E. Stage of substantial erosion from the emplaced thrust sheet (the Dalsfjord Nappe) and faultedlthrusted (?) blocks of the island-are/back-are sequence. Long period of sedimentation starting with deposition of the Stubseid conglomerate. Volcanicity nearly ceased. Upper Ordovician to Lower Silurian time. F. Middle Silurian continental collision resulting in the polyphase deformation and further thrusting. The model shows a schematic development of the Stavfjord Anticline.
In the Precambrian basement there is a series of basic dykes which may
relate to the incipient stage of the opening of the lapetus ocean (F. J. Skjerlie,
unpublished data). Hence it is tentatively suggested that the boundary be
tween the Baltic continental crust and the oceanic crust of lapetus may be
transitional in character, as indicated on Fig. 9 B. At some distance from
the continental margin, possibly related to fracture zones or hot spots, oceanic
islands were formed (Fig. 9 B). Such volcanism is documented by alkali
basaltic tuffaceaus material resting upon tholeiitic basalts. Subsequent to the
change from being a stable Atlantic type continental margin to become
an active margin, an ensimatic island are was initiated at some distance from
the continent (Fig. 9 C). At which time the island are commenced its growth
is uncertain, but an Early Ordovician age would probably be realistic. Vol
canism related to island-are growth can be dated as early as Arenig!Lower
Llanvirn in the Welsh Basin (Jeans 1973). Whether this stage represents the
change from oceanic expansion to contraction of the lapetus is uncertain, as
subduction obviously can go on without contraction of the ocean basin, e.g.
in the Caribbean (Malfait & Dinkelman 1972).
In the Solund district both tholeiitic, calc-alkaline, and alkaline basalts of
the Hersvik Group are spatially confined to the same area, without any ap
parent tectonic zones between the various types of basalts. A continuous
section through the Hersvik area (Fig. 1) shows that alkaline basalts are
stratigraphically situated below and above calc-alkaline lavas. No stratigraph
ical relationship of the above-mentioned lavas to the tholeiitic lavas of the
supposedly same group in the Leknessund area can be seen. However, the
time-related volcanism in active island arcs (Jakes & White 1969, 1972, Jakes & Gil! 1970, Miyashiro 1972, 1974, Mitchell & Bell 1973) might indirectly suggest that the tholeiitic lavas in the Leknessund area pre-dated the calcalkaline and alkaline lavas of the Hersvik area. On the other hand, if the
Leknessund greenstones are related to tholeiitic volcanism resulting from
back are spreading (Hart et al. 1972), they may be syngenetic or even post
date the calc-alkaline and alkaline lavas. The Holten greenstones in the Stavfjorden district only show tholeiitic affinities, and as no lavas of the
calc-alkali or alkaline suites have been found, this might possibly mean that
the volcanism of the evolving island are was at an earlier stage of evolution in
this area than in the Solund area.
At !east two mechanisms could account for the generation of melts at
progressively greater depths (from tholeiitic to alkaline basalts), but erupting
at the same place. One would imply a steepening of the Benioff zone with
time (Fig. 9 Dl), the other an oceanward (westward) migration of it (Fig.
9 D2). In most modem island arcs which overlie Benioff zones dipping more
than about 35° there has been extension of marginal basins (Karig 1971,
1972, 1973, 1974, Mitchell 1973, Stepanov 1974). This phenomenon of
back-are spreading is apparently cyclic and relates to changes in the inclina
tion of the Benioff zone, spreading taking place only when inclination is steep
PLATE TECTONIC MODEL FROM SOLUND-STAVFJORDEN AREAS 181
(Bracey & Ogden 1972). The dip of the subducted plate is again related to
the relative rate of convergence, and an inverse dip-rate relationship has been
shown to exist (Luyendyk 1970). Hence, since the relationship as outlined
above seems to be a general phenomenon in such tectonic settings, it is con
sidered most plausible, albeit not proved, that steepening (perhaps in a cyclic
manner) of the Benioff zone, associated back-are spreading, and magmatism
occurred as outlined on Fig. 9 Dl.
The subsequent events would most probably also favour such a situation.
As seen from Fig. 9 Dl, incipient thrusting of basement rocks (the Dalsfjord
Nappe) is indicated at this stage of evolution.
Armstrong & Dick (1974) postulated that a high thermal gradient is a
necessary condition of the origin of overthrust sheets. In the behind-the-arc
tectonic environment where marginal spreading takes place, high heat flow
characterizes a broad region, as for instance in the centre of the Japan Sea
(Rikitake 1967). Detachment will occur when the lithosphere is subject to
tectonic forces that result in a shear stress between the thinned brittle litho
sphere and its ductile infrastructure. A failure threshold has thus been ex
ceeded, and as a relief from the stress, the crystalline sheet (the Dalsfjord
Nappe) started to move away from its soft substrate in a southeasterly direc
tion, to its present position above the sediments of the Askvoll Group.
Skjerlie (1969) assumed that the thrusting of the Dalsfjord Nappe took
place in late Middle Ordovician time, and that this orogenic event correlates
with the Ekne disturbance. However, we do not know with certainty the age
of the sediments of the Askvoll Group over which the Dalsfjord Nappe has
been thrusted, and hence not the minimum age of thrusting. It is therefore
important in this context to consider the Caledonian orogenic/metamorphic
events which pre-date the Middle-Upper Silurian orogenesis.
Sturt et al. (1967) indicate the possible existence of an important early
phase of Caledonian orogeny in north Norway about 500 m.y. ago, on the basis of K-Ar age determinations on alkaline rocks. This conclusion was further substantiated on the basis of Rb-Sr age for the emplacement of the Hasvik gabbro (Pringle & Sturt 1969, Sturt & Taylor 1972).
Bryhni et al. (1971) suggest two important maxima of Caledonian orogenic!metamorphic events occurring 524 and 488 m.y. ago on the basis of
40Ar/39Ar amphibole and biotite dates from the Fjordane Complex of west
Norway.
Metamorphic mineral ages of Precambrian rocks from the Bergen area
give results of 500-590 m.y., representing an incomplete reset age related
to the early Caledonian-Grampian event (Sturt et al. 1975).
As a result of comprehensive geochronological investigations in the Dal
hurst & Pidgeon 1973), there is clear evidence of an Early Caledonian de
formation and metamorphism taking place about 500 m.y. ago.
Since this Early Ordovician orogenic event obviously has been of great
182 H. FURNES, F. J. SKJERLIE & M. TYSSELAND
importance in the Caledonides, it is tempting to suggest that the movements
of the Dalsfjord Nappe might be related to this period of orogenesis.
A marked unconformity exists between the rock sequence described so far,
and the Upper Herland/Mjelteviknes Groups of the Stavfjorden and Solund
districts. At the time of deposition of these groups, the Dalsfjord Nappe had
been emplaced and was subject to erosion, as was also part of the island-are/
back-are eugeosynclinal sequence (Fig. 9 E). This is witnessed by a thick
polygenic conglomerate, the Stubseid conglomerate (Skjerlie 1974), sitting
on a substrate of stratigraphically different rock units of the Lower Herland
Group, and consisting of fragments of these rocks. The deposition of the
above-mentioned conglomerate probably took place in lower Upper Ordo
vician time, since faunal evidence from near the top of the Upper Herland
Group indicates an Upper Ordovician or Lower Silurian age. At this time
the volcanic activity must have nearly ceased, as only traces of metabasalt
can be found. The fine-grained sediments of the Upper Herland Group, in
total ca. 500 m. thick, become progressively more quartz-rich upwards. A
change in sedimentation from deposition of dominantly fine-grained quartz
schists to coarse-grained arkose, partly conglomeratic, marks the transition
from the Upper Herland Group to the HØyvik Group, a documentation of
further substatial uplift of the continent. A minimum thickness of the Høyvik
Group, of Silurian age (Skjerlie 1974), is 500 m.
The final stage of contraction and collision of the North America-Green
land continent with the Baltic Shield in Middle Silurian time (Gale & Roberts
1974) resulted in the polyphase deformation and thrusting of the eugeosyn
clinal sequence (Fig. 9 F). A part of the Precambrian basement must have
been affected by these movements, as high-grade metamorphic rocks on the island Håsteinen (Fig. 1) constitute a deep level of the Stavfjord Anticline.
Acknowledgements. - The authors are much indebted to Prof. B. A. Sturt and Dr. B. Rabins for constructive criticism and comments on the manuscript. The illustrations were kindly prepared by S. Meling Karlsen and E. Irgens.
May 1975
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