207
Geol. Mag. 128 (3), 1991, pp. 207-226. Printed in Great Britain
Melt-enhanced deformation during emplacement of gabbro
and granodiorite in the Sunnhordland Batholith,
west Norway
TORGEIR B.ANDERSEN, PETER NIELSEN, ERLING RYKKELID & HANNE S0LNA
Geological Institute, University of Oslo, P.O. Box 1047, 0316 Blindern Oslo 3, Norway
(Received 6 September 1990; accepted 15 November 1990)
Abstract - The Caledonian Sunnhordland Batholith comprises calc-alkaline plutons that have been
assigned to three units according to their relative age and composition: a gabbro-diorite unit, a
granodiorite unit and a later granodiorite-granite unit. The batholith was emplaced into an envelope
including ophiolite and island-arc complexes, sediments and volcanites of early Ordovician age that
were developed in a zone of plate convergence. Continued convergence resulted in the formation of
a mature magmatic arc and a thickened crust; the late granitoids (unit 3), which commenced their
crystallization at pressures around 6 to 7 kb, rose as permitted diapiric intrusions. The ingress and
ascent of the magmas in this setting is considered to have been facilitated by the presence of major
shear zones developed in relation to plate convergence. In this model, plastic instabilities were formed
in an otherwise elastic middle and upper crust. Non-coaxial deformation was accelerated by the
emplacement of magmas and the formation of abundant partial melts in water-rich sediments of the
envelope. The deformation, which was accelerated by magma and melt lubrication in aureoles,
controlled both the shape and internal structure in the gabbro and granodiorite plutons.
1. Introduction
This paper discusses the emplacement of syn-tectonic
plutons in the Caledonian Sunnhordland Batholith of
west Norway (Andersen & Jansen, 1987), and describes structures which are interpreted to be the result
of syn-magmatic deformation phenomena both within
plutons as well as in their migmatitic aureoles
(Andersen, 1989). It is suggested that the deformation
was enhanced during emplacement of the plutons as a
result of melt lubrication.
In a crustal segment under the influence of a major
deviatoric stress field, the presence of melts and fluids
will be an important factor in controlling the
localization and the kinematics of shear zones (Hollister & Crawford, 1986). Zones of displacement
initiated at elevated temperatures, or in magmatic
rocks above their solidus may, however, be difficult to
recognize because of post-kinematic crystallization/
recrystallization and annealing of the fabric. Hence, in
order to identify high-Tor syn-magmatic shear zones,
the rock must possess an anisotropy already at an
early stage in its deformation history, which may
record strain by structural and/or textural modification. If deformation has occurred in the presence
of melts, the rocks must record the strain after
complete crystallization for the strain to be recognized.
Blumenfeld & Bouchez (1988) and Paterson, Vernon
& Tobisch (1989) have discussed and reviewed some
important textural criteria for recognizing pre-solidus
deformation in magmatic rocks. Phenocryst tiling
(Den Tex, 1969), shape-preferred orientations (SPO)
of primary minerals and shape-controlled differential
rotation of rigid particles in a viscous matrix described
in relation to deformation of conglomerates by Gay
(1968) and demonstrated experimentally by Ildefonse
& Fernandez (1988), are important criteria. Where
small melt fractions are present, this requirement may
be satisfied as deformation will cause frequent
interaction of the solid particles, and favour a
segregation of the melt from the solid (Wickham,
1987). In a body of magma, or in zones where high
degrees of partial, or near complete melting have
occurred, record of deformation will be wholly
dependent on the type of flow which has affected the
strain markers (Ferguson, 1979). Such sites may,
however, represent zones of high strain in syn-tectonic
igneous complexes, and it is likely that the strain will
not be recognized until a major part of the magma has
crystallized. In this situation, the early deformation
can be identified only from displacement of structures/contacts in the envelope, or by the shape and
time-dependent spatial arrangement of plutons in a
major shear zone (Hutton, 1982, 1988a, b; Castro,
1986).
High content of melt in a zone will most likely result
in a near ideal Newtonian behaviour at geological
strain rates (Ferguson, 1979), and the strain across
such a zone will increase linearly with time given a
constant stress and viscosity and have the form:
where e = strain, cr = stress, t = time and v = viscosity. Van der Molen & Paterson (1979) showed on
GEO 128
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208
T. B. ANDERSEN AND OTHERS
SIMPLIFIED GEOLOGICAL MAP,
SUNNHORDLAND BATHOLITH, W. NORWAY
The Batholith:
Unit I, gabbro, diorite, minor ultramafics.
Unit II, Reksteren granodiorite.
Unit III, granodiorite and granites.
The Envelope:
o
O7T
«o°o o °
O 00 0
Dyvikvagen Gp.
AUSTEVOLL
Up.Ordovician to
Lr. Silurian in age.
Partly younger
than the batholith
REKSTEREN
Undifferentiated
Caledonian rocks
Bimodal volcanics
Mid.to Lr. Ordovician
in age.
Basal unconformities
where marked.
High-grade contact
metamorphic
sediments. Migmatites
adjacent to gabbro.
Probably Lr.to Mid. Ordovician
in age.
Ophiolite and Island-arc lithologies
locally cut by qtz-diorites.
Lr. Ordovician and older.
Sunnhordland fault
and other late faults.
—•—
Syn-magmatic shear zones
active during emplacement of
Unit I & II in the batholith.
10km
ense of shear on syn-magmatic shear zones
Figure 1. Simplified geological map of the Sunnhordland Batholith, west Norway (from Andersen, 1989).
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Melt-enhanced deformation
the basis of experimental data that the most rapid
change in the mechanical properties of a partially
molten granite occurs at 30 to 40 % melt, referred to
as the critical melt fraction (CMF). The experimental
data were in general agreement with Arzi's (1978)
definition of the rheological critical melt percentage
(RCMP).
2. Regional setting of the Sunnhordland Batholith
The Sunnhordland Batholith (Andersen & Jansen,
1987) is a composite, synorogenic, intrusive complex
which was emplaced into Lower Ordovician ophiolite-island arc lithologies, bimodal volcanites and
sediments representing the one of the most complete
sequences of convergent margin volcanics in the
Norwegian Caledonides. These envelope rocks described by Brekke et al. (1984) and Nordas et al.
(1985) were formed during an extended period of plate
convergence that commenced in early Ordovician
time, in an intra-oceanic setting (Pedersen, Furnes &
Dunning, 1988). The ensimatic complexes were
accreted to a continent or formed part of a mature
island arc prior to the development of the bimodal
volcanic and sedimentary rocks of the Siggjo and
Kattnakken groups (Lippard, 1976; Nordas et al.
1985), and the batholith post-dates these complexes
(Andersen & Jansen, 1987). The Sunnhordland Batholith is of I-type, and is considered to represent the
products of a continued convergence which resulted in
the formation of a mature magmatic arc (Andersen &
Jansen, 1987). The syn-deformational magma emplacement described in this paper referes only to the
gabbros and granodiorites which make up the earlier
units of the Sunnhordland Batholith; the later granite
plutons are post-deformational and their Rb-Sr
isochron ages indicate that they were emplaced
following a Middle—Upper Ordovician unconformity
which is present along the length of the Norwegian
Caledonides and corresponds with a break in magmatic activity of some 10-40 Ma.
The plutons in the Sunnhordland Batholith have
been assigned to three units according to their relative
age and composition. The plutons of the older Unit 1
(gabbros and diorites) and Unit 2 (one major
granodiorite pluton) were emplaced syn-tectonically,
while the youngest intrusions in Unit 3 (granodiorite
and granite plutons) are permitted intrusions (Andersen & Jansen, 1987; Andersen, 1989). The age of the
Sunnhordland Batholith is constrained by the dating
of the envelope, including precise U-Pb ages on
zircons from early primitive island arc volcanites and
the bimodal volcanites of Siggjo and Kattnakken
groups, which give U-Pb ages in the range 475-495 Ma
(Pedersen & Dunning, 1991). The U-Pb ages overlap with previously published Rb-Sr whole rock
ages from, these rocks on B0mlo (464 +16 Ma and
209
535 ±45 Ma, Furnes et al. 1983). A concordant U-Pb
age of 472 ± 2 Ma from the Vardafjellet Gabbro
(Unit 1) in the batholith on B0mlo (Fig. 1), has
been reported by Pedersen & Dunning (1991).The
geochronological studies show that the Vardafjellet
Gabbro was emplaced shortly after the deposition of
the sediments and volcanites of the Siggjo and
Kattnakken groups. Rb-Sr whole rock isochrons
from granites of Unit 3 have given 430±10Ma
(Andersen & Jansen, 1987) and 430 ± 6 Ma (Fossen &
Austrheim, 1988). The structural relationships of the
batholith show that it is pre-orogenic with respect to
the nappe transport during the Scandian phase. A
younger age, however, cannot be ruled out for the
Rolvsnes granodiorite on north Bomlo and the Droni
granite in Austevoll (Fig. 1), as the deformation
assigned to the main thrusting event during the
Scandian phase cannot be shown to have affected
these plutons. The batholith forms an integral part of
the allochthonous outboard terranes in the Upper
Allochthon of the Scandinavian Caledonides.
3. Level of crystallization of the SB
As pointed out by Zen (1989), it is important for the
discussion of emplacement models to have knowledge
of the level at which the plutons have crystallized.
3.a. Unit 1, gabbros and diorites
The oldest plutons in the Sunnhordland Batholith
(Unit 1) comprise mainly gabbros and diorites. The
gabbros in the eastern part of the batholith, on Stord
and Tysnesoy (Fig. 1), have not been studied in detail.
The Stolmen Gabbro and the Vardafjellet Gabbro
(Fig. 1), however, have been mapped in detail. Both
plutons have high-grade migmatitic aureoles where
they intrude metasediments.
3.a.l. The Vardafjellet Gabbro
The Vardafjellet Gabbro intrudes volcanites, sedimentary rocks and the ophiolite-island arc complexes
on Stord and Bomlo (Fig. 2). Outside the aureole of
the Vardafjellet Gabbro, stilpnomelane co-exists with
white mica, and the regional metamorphism affecting
these rocks did not exceed the low-J part ( « 400 °C)
of the greenschist facies (K. G. Amaliksen, unpub.
Cand. Real, thesis, Univ. Bergen, 1983; H. Brekke,
unpub. Cand. Real, thesis, Univ. Bergen, 1983; J.
Nordas, unpub. Cand. Real, thesis, Univ. Bergen,
1985). The rocks on Stord and Bomlo have apparently
remained at an elevated structural position throughout
the Caledonian orogeny.
The Bremnes Migmatite Complex (Fig. 2) formed
in the roof of the Vardafjellet Gabbro on Bomlo
(Andersen, 1989; P. E. Nielsen, unpub. Cand. Scient.
15-2
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Deformed layering
Foliated
gabbro/diorite
Non-layered gabbro,
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VARDAFJELLET GABBRO (VG)
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ROLVSNES GRANODIORITE (RG
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volcanics. sediments.
Basal cgl.
limestone/shale
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SUNNHORDLAND BATHOLITH
Figure 2. Geological map of the Vardafjellet Gabbro and its envelope, Bomlo and Stord, west Norway.
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GEOLOGICAL MAP, VARDAFJELLET GABBRO &
ENVELOPE, SUNNHORDLAND, W. NORWAY
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Melt-enhanced deformation
thesis, Univ. Oslo, 1990). The migmatites developed
from sandstones, pelites and calcareous sediments and
possibly also from felsic volcanics/volcaniclastics of
the Siggjo Group (J. Nordas, unpub. Cand. Real,
thesis, Univ. Bergen, 1985). The protoliths of the
Bremnes Migmatite Complex are correlated with the
low-grade sedimentary rocks interlayered with volcanites in the upper part of the Siggjo Group (Fig. 2).
The precise U-Pb dating of the Vardafjellet Gabbro
and the Siggjo Group (Pedersen & Dunning, 1991)
suggests that the sedimentary protoliths of the
Bremnes Migmatite Complex were < 5 million years
old at the time of the migmatitization. The migmatites
include biotite-cordierite-sillimanite-bearing diatexites and metatexites, which locally are garnetiferous.
Calibration of the contact metamorphism in the
Bremnes Migmatite Complex gives temperatures of
approximately 750 °C (P. E. Nielsen, unpub. Cand.
Scient. thesis, Univ. Oslo, 1990). Cordierites from the
Bremnes Migmatite Complex, determined by microprobe analyses, have 58 % Mg and 42 % Fe. Holdaway & Lee's (1977) geobarometer, assuming P =
PH 0 and T = 750 °C, indicates pressures close to 4 kb,
corresponding to a crystallization depth for the
Vardafjellet Gabbro in the order of 15 km. If PH 0 <
P total , the estimated pressure would be reduced
(Holdaway & Lee, 1977). The estimate of 4 kb or less
is in good agreement with low-grade regional metamorphic assemblages in the area.
3.a.2. The Stolmen Gabbro
The contact metamorphism in the metasediments
adjacent to the Stolmen Gabbro in Austevoll (Fig. 1)
has been studied by E. Rykkelid (unpub. Cand. Scient.
thesis, Univ. Bergen, 1987). Formation of melts by
anatexis was abundant in the sediments, and a
migmatitic high-grade aureole formed in these lithologies. Cordierite has not been identified in the aureole
in Austevoll. The P-T estimates carried out by E.
Rykkelid (unpub. Cand. Scient. thesis, Univ. Bergen,
1987) on preserved early mineral assemblages in the
aureole and the mineralogy of the gabbro indicate
that metamorphism associated with the emplacement
of the Stolmen Gabbro occurred at temperatures in
excess of 750 °C (760-810 °C); the pressure was
estimated to be between 4 and 6 kb. The P-T path
(Fig. 3) for the metasedimentary rocks in Austevoll
indicates that these rocks were depressed to deeper
levels after emplacement of the Stolmen Gabbro.
The metamorphism in the aureole around the
Stolmen Gabbro and the Vardafjellet Gabbro, as well
as the lack of corona textures developed between
olivine and plagioclase in the presence of hydrous
phases such as phlogopite and hornblende in the
gabbros (Espensen, 1978; Griffin & Heier, 1973),
indicate that the Unit 1 gabbros crystallized relatively
high in the crust. Although the P-T estimates (Fig. 3)
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211
200
400
600
800 C
Figure 3. /'-rpath from the metasediments in the aureoles
in the Mckster area, Austevoll (modified from E. Rykkelid,
unpub. Cand. Scient. thesis, Univ. Bergen, 1987) and the
Bremnes Migmatite Complex on Bomlo, 1,2 and 3 represent
the approximate P-T conditions during the intrusion of
Units 1 to 3 in the batholith at Austevoll.
are inherently uncertain, the relative variation calculated by E. Rykkelid (unpub. Cand. Scient. thesis,
Univ. Bergen, 1987) between early and late mineral
assemblages based on the same geothermometers and
barometers, are thought to reflect a significant trend
in the P-T path for the rocks in Austevoll. In the
Bremnes Migmatite Complex at Bomlo, there is no
evidence of a higher-P overprint on the contactmetamorphic assemblages, and this is in agreement
with the low-grade regional metamorphism of the
area (Fig. 3).
3.b. Unit 2, the Reksteren Granodiorite
Unit 2 comprises the Reksteren Granodiorite pluton,
which forms an elongate body of approximately
35 km exposed length and a preserved maximum width
of 10 km (Fig. 1). The northern part of the pluton is
cut by a late extensional fault, while the intrusive
contact is preserved along its southern margin
(Andersen & Jansen, 1987). The contact relationships
have been described in detail by H. Solna (unpub.
Cand. Scient. thesis, Univ. Oslo, 1989), and show that
the Reksteren Granodiorite was emplaced after the
Stolmen Gabbro (Unit 1) and similar gabbros on
Tysnesoy (Fig. ip-'Contemporaneously with the
intrusion of the Reksteren Granodiorite, partial melts
with a composition close to the minimum melt
composition for a granite system formed in metasediments in Austevoll, and Unit 1 gabbros were
amphibolitized adjacent to the granodiorite. Thermobarometric calibrations for this event indicate that the
high-grade aureole around the Stolmen Gabbro had
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212
T. B. ANDERSEN AND OTHERS
Figure 4. (a) Photomicrograph of gabbro from Vardafjellet on Bemlo. Minerals include plagioclase, clinopyroxene and olivine,
with minor biotite and hornblende. The magmatic foliation is defined by laths of plagioclase. (b) Photomicrograph of banded
migmatite from the Bremnes Migmatite Complex. The laminae are defined by quartz-feldspathic leucosome and biotitesillimanite aggregates. Note that thefibroliticsillimanite overgrows the orientated biotite crystals randomly. The porphyroblast
with high relief in upper left corner is garnet.
cooled to approximately 680-700 °C, at a pressure
between 5 and 6 kb (Fig. 3). Subsequent to the
intrusion of the Reksteren Granodiorite, kyanite
became the stable Al2SiO5 polymorph in the wall-rock
enclave in Austevoll, and the P-T estimates indicate
that the depression of the area to deeper crustal levels
continued after emplacement of the Unit 2 (Fig. 3).
This is in contrast with the B0mlo-Stord area where
stratigraphical relationships show that the Vardafjellet
Gabbro was uplifted and eroded prior to the deposition of the Ashgillian to early Llandoverian
sediments of the Dyvikvagen Group (Ryan & Skevington, 1976; Thon, Magnus & Breivik, 1981). An
unconformity, previously not recognized, is locally
preserved between conglomerates of the Utslettefjell
Formation in the Dyvikvagen Group and inverted
rocks of the Vardafjellet Gabbro. This demonstrates
that the southern part of the batholith, in contrast to
the Austevoll area (see above), underwent uplift and
unroofing in middle-late Ordovician time. The metamorphism superimposed on the aureole at Bremnes is
diaphthoretic, and has no record of a P-increase (P. E.
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Nielsen, unpub. Cand. Scient. thesis, Univ. Oslo,
1990).
3c. Unit 3, granodiorite and granite
Unit 3 comprises four major plutons of granodioritic
to monzogranitic composition (Andersen & Jansen,
1987). A common feature of the late granitic plutons
of the Sunnhordland Batholith is the occurrence of
primary magmatic epidote (Andersen & Jansen, 1987).
Magmatic epidote co-existing with a silica melt of
granitoid composition has been suggested to be
dependent on a high PH^, high oxygen fugacity and a
high Plotal (Naney, 1983)! Zen & Hammarstrem (1984)
and a recent review by Zen (1989) suggest that a
crystallization pressure in excess of 6 kb is required
for the mineralogy of the granitoids in Unit 3
(Andersen & Jansen, 1987). The crystallization of the
Unit 3 granitoids in the Sunnhordland Batholith may
have commenced at depths in the order of 20 to 25 km.
This is supported by P-T estimates from a garnetplagioclase-kyanite-bearing assemblage adjacent to
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Melt-enhanced deformation
the Dreni granite in Austevoll, which gave 570 °C at
7 kb (Fig. 3), and a T-P estimate of 600 ± 50 °C at
9 ± 2 kb from kyanite-bearing rocks adjacent to
granites north of Bjornafjorden (Fig. 1) (Fossen,
1988). No record of a P increase has, however, been
found in the metasediments adjacent to the granodiorite on Bomlo. This suggests that the Rolvsnes
granodiorite, which also contains primary epidote,
reached its solidus at lower pressures, at a depth where
a mush of early phases such as epidote, hornblende
and biotite was carried diapirically to the final site of
crystallization.
4. Synmagmatic deformation
The margins of the plutons in Units 1 and 2 represent
high-strain zones where metasedimentary rocks form
the envelope, and locally also where the plutons make
contact with meta-igneous rocks. In many of the
marginal zones, deformation was apparently enhanced
by the presence of melts, either derived by partial
melting in rocks of favourable composition or
introduced from the intruding magma. Melts of both
types occur adjacent to Unit 1 gabbros, while enhanced
deformation due to the ingress of magma was most
significant during emplacement of the Reksteren
Granodiorite (Unit 2).
4.a. Vardafjellet Gabbro
4.a.J. Deformation along the margin
The Vardafjellet Gabbro has a medium- to finegrained marginal zone of non-layered gabbro and
meta-diorite (Fig. 2). The gabbro is net-veined by
leucocratic, granitoid material in a zone of variable
thickness (50-100 m) adjacent to the Bremnes Migmatite Complex which forms the roof of the pluton.
Deformation of the marginal zone has locally resulted
in hybridization of the two melts, giving rise to dioritic
and quartz-bearing gabbros. In most places, however,
the differences in viscosity and solidus temperatures
prevented mixing of the two melts, and a gneissic
banding formed. Chilled margins of the gabbro in
contact with granitoid veins are commonly observed.
At outcrop, the gneissic structure is most prominent
on E-W surfaces, where if forms a prominent element
in a L > S fabric with a subhorizontal E-W stretching
lineation. At the scale of the individual minerals and
mineral aggregates, a combined LPO (lattice-preferred
orientation) and SPO (shape-preferred orientation)
fabric is defined by primary plagioclase crystals and of
polygonized aggregates of amphibole in the gabbro
(Fig. 4a). The fabric is most obvious at the scale of
outcrop, because a static annealing of the texture
commonly obscures the fabric when observed in thin
sections.
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213
Intrusions of the mafic magma into anatectic melts
derived from melting of the envelope resulted in the
formation of pillowy and amoeboidal, fine-grained
gabbro bodies engulfed in granitoid material. Backveining of leucocratic material in the mafic rocks is
commonly observed. The mafic bodies usually have
an elongate shape, with the long axes oriented E-W.
Internally, the mafic material is without strong fabric
and shows little sign of deformation. The elongation is
a result of early stretching, probably developed
contemporaneously with the formation of the mafic
protrusions.
Adjacent to the gabbro, the Bremnes Migmatite
Complex is characterized by irregularly banded
diatexites formed by near complete anatexis (Fig. 2).
The diatexites contain metasedimentary inclusions,
most commonly metasandstones, calc-silicate schists
and mica schists, and locally an irregular and
discontinuous banding is present. The banding is
chiefly defined by variations in the content of biotite
and quartz-feldspathic material. The foliation clearly
formed at an early stage, most probably as a result of
viscous flow. A preferential segregation of neosomal
material with an igneous texture (Wickham, 1987),
from biotite-fibrolite laminae which internally have
annealed to develop a SPO fabric (Fig. 4 b) is
characteristic of the banding. In these zones the mica
is usually replaced by fibrolitic sillimanite during
solid-state recrystallization. In zones with abundant
metasedimentary inclusions, the xenoliths are usually
dominated by one lithology, either quartzite (Fig. 5 a),
knobby mica schist (Fig. 5 b) or calc-silicate schist
(Fig. 5 c), indicating that they represent disrupted,
melt-resistant horizons which were interlayered with
more fusible material. Disruption of such layers to
produce angular to subangular inclusions probably
occurred during near viscous flow of the anatectic
matrix, at a time when the strain rate in the least
viscous material was sufficiently high to enable brittle
failure to take place in the more melt-resistant layers.
The concentration of xenoliths of one lithology in
zones probably represents a ghost stratigraphy,
indicating that the flow in the migmatite complex was
dominantly laminar, with limited exchange of the
solid material vertically.
Away from the gabbro contact, the Bremnes
Migmatite Complex consists chiefly of metatexites
(Mehnert, 1968), where transitional contacts may be
observed between zones which contained variable
melt fractions. In Figure 5 a, a gradual increase in
leucocratic neosome is shown in a metatexitic migmatite. There is a clear negative correlation between
the intensity of the fabric in the rock expressed by the
foliation intensity, and the amount of neosome that
was developed. The detailed mapping by P. E. Nielsen
(unpub. Cand. Scient. thesis, Univ. Oslo, 1990) shows
that the most intensely foliated material occurs in
areas with a relatively low content of neosome.
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214
T. B. ANDERSEN AND OTHERS
Figure 5. Bremnes Migmatite Complex, (a) Preserved quartz-rich metapsammite (left) with transitional zone to highly
mobilized migmatite (right). Note the intensely foliated contact zone (arrowed) where the neosome constitutes approximately
20-30% of the rock. See text for discussion. Lens cap for scale, (b) Lineation in migmatitic mica schist defined by lenticular
aggregates of biotite, muscovite and sillimanite. (c) Dextral shear zone (looking towards the NE) where the shear sense
(arrowed) can be determined from the asymmetrical drag of calc-silicate xenoliths. Note the lack of fabric in the migmatitic
matrix parallel to the shear zone. Hammer for scale, 40 cm. (d) Shear bands (looking towards the north) parallel to melt-rich
zones, with asymmetrical inclusions and boudins of solid material. The asymmetry indicates a dominantly sinistral (arrowed)
sense of shear. Hammer for scale, 40 cm.
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Melt-enhanced deformation
Intensely banded, E-W striking, zones with shear
planes (C-planes) which are almost indistinguishable
from planes with preferred shape-fabric orientation
(S-planes) are developed in wide areas in the metatexite, particularly in quartz-rich metapsammites
where little neosome has formed because of the
composition (Fig. 5d). A variety of shear sense
indicators are frequently present in the zones which
contained abundant solid material during the synkinematic high-grade metamorphism (Fig. 5d), and
indicate a dominantly E-W sinistral sense of shear
(Nielsen & Andersen, 1990). Locally, however, zones
of dextral displacement can be observed (Fig. 5 c).
The reduction in the shear strength of a rock at
increasing melt-solid ratio suggests that under the
same deviatoric stress field, displacements with a
similar polarity and at higher strain rates would also
be accommodated in nearly completely melted rocks
with lower viscosity. In the model for the emplacement
of the Vardafjellet Gabbro, the diatexites along the
roof of the gabbro are thought to have constituted a
layer with low shear strength in which considerable
displacement was accommodated. The deformation,
however, is not easily recognized in the diatexites
because of the lack of synkinematic strain markers.
The southern contact of the Vardafjellet Gabbro on
Bomlo (Fig. 2) represents the original floor of the
pluton. The contact is inverted, and dips steeply
towards the south and southwest. A fine- to mediumgrained marginal facies, usually less than 100 m wide,
is developed in the gabbro. The gabbro makes contact
with igneous rocks, mainly of basic composition,
belonging to the Lykling ophiolite, the Geitung island
arc and the Siggjo Group (Nordas et al. 1985). The
development of partial melts in these rocks was
insignificant because of their composition. Locally,
hornfelsed metasediments interlayered with volcanics
of the Geitung Unit (K. G. Amaliksen, unpub. Cand.
Real, thesis, Univ. Bergen, 1983) occur in the aureole,
and a thin basal conglomerate of the Siggjo Group is
preserved locally along the contact zone (Fig.--2).The~
conglomerate comprises material derived from the
ophiolite and island arc lithologies. In its type section
(J. Nordas, unpub. Cand. Real, thesis, Univ. Bergen,
1985), the basal conglomerate and lavas of the Siggjo
Group are virtually undeformed. Adjacent to the floor
of the Vardafjellet Gabbro, however, the pebbles are
strongly flattened and have an oblate shape (P. E.
Nielsen, unpub. Cand. Scient. thesis, Univ. Oslo,
1990). The plane of flattening is sub-parallel to the
contact of the pluton. Similarly, epidote-quartz-filled
vesicles in the Siggjo volcanites are deformed and
define oblate amygdales adjacent to the gabbro. The
increase in the strain in the zone is ascribed to
deformation during the initial phase of the emplacement of the gabbro, as outcrop scale fabrics in the
metasediments and volcanics are overprinted by a
215
post-kinematic hornfels texture in which cummingtonite is a characteristic mineral.
On Stord, the northeastern margin of the Vardafjellet Gabbro is preserved as an intrusive contact with
a thin, usually < 5-m-wide, chilled margin along the
contact (Fig. 2). The chilled zone consists of microgabbro which has a transitional contact to the
medium- to coarse-grained layered gabbro and is netveined by leucocratic material. The contact dips
towards the N and NE (Fig. 2). Well preserved
cumulate layering preserved in a number of localities
along the contact shows that the layering and the
contact to the envelope are inverted. The contact
originally was the floor or a gently inclined wall of the
pluton at the time when the density-graded cumulates
formed.
In spite of the inversion, the contact and the
cumulate sequence adjacent to the contact are little
deformed. The wall rocks of the actual contact are
hornfelses. A few tens of metres away from the
contact, however, fine-grained porphyritic metavolcanites have an L > S fabric with an orientation
sub-parallel to the contact, and a shallowly plunging
stretching lineation with azimuth around 300° (Fig.
2).
4.a.2. Deformation of the cumulate layering
The cumulates of the Vardafjellet Gabbro are well
preserved in a number of areas, particularly on Bomlo
and along the northeastern margin on Stord, and the
primary mineralogy is locally unaltered. On Bomlo,
the way-up in the cumulates, shown by structures such
as density grading and erosional features, is to the
north. The strike of the layering changes gradually
from around 100° in the west to c. 165° near
Stokksundet (Fig. 2). The layering is subvertical or
steeply inverted. At the scale of outcrop, structures
which can be related to early, pre-solidus instability
and deformation of the cumulate sequence are
common.
In the type locality at Vardafjellet on Bamlo (Fig.
2), the cumulate sequence has a vertical thickness of
approximately 750 m. The layered part of the gabbro
gets thinner towards the west and thickens to the east.
In most of the sequence, which comprises the least
deformed parts of the pluton, structures characteristic
of pre-solidus disturbance of the layering are common.
These include erosional features, with local unconformities (Fig. 6 a), slump structures including isoclinal
folding of layering and early faults (Fig. 6e). Slump
structures and erosional features have been described
from many anorogenic layered gabbros; in the
Vardafjellet Gabbro, however, these structures are
common and occur at all levels in the layered sequence.
The pre-solidus faulting was associated with brecciation, particularly of the darker, pyroxene-rich parts
of the compositional layers. The early, pre-solidifica-
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216
T. B. ANDERSEN AND OTHERS
Figure 6. For legend see facing page.
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Melt-enhanced deformation
tion origin can be identified in the field as the faults
and breccias have undeformed gabbro as 'fault-rock'
matrix (Fig. 6e). A common observation at the scale
of outcrop in the type area is a mineral SPO fabric
(Fig. 7a). The fabric, which is sub-parallel to parallel
with the layering, has a pronounced sub-horizontal,
linear component and is defined by polygonized
aggregates of the pyroxene, olivine and plagioclase. In
detail, the igneous texture is replaced by a polygonized
granoblastic texture which was formed by high-7',
sub- or syn-solidus crystallization under stress (Fig.
7 b, c). As there is now evidence for post-emplacement
high-71 metamorphism in the area, this can only be
related to the emplacement of the gabbro. The igneous
minerals are commonly replaced by serpentine, amphibole, chlorite and saussurite aggregates during static
retrograde metamorphism. In relation to several of
the early faults described above, the SPO fabric had
formed already prior to the brecciation, as it occurs
with a random orientation in rotated blocks in breccia
(Fig. 6e), clearly demonstrating its pre-solidus origin.
Extensional veins (000°/90) of leucogabbro, are
very common in the type area (Fig. 6a, b). The veins
are usually < 3 cm wide, and can in many cases be
traced several metres along strike. The veins have an
orientation normal to the L > S fabric, and both the
fabric and the extensional veins are most common in
the middle and upper part of the cumulate sequence.
The E-W extension is also illustrated by boudinage,
which most commonly affects layers with an original
pyroxenitic composition, but also plagioclase-rich
layers (Fig. 6a, b).
Mobilization of leucocratic gabbro and disruption
of mafic layers are commonly observed, and an L > S
structure defined by primary minerals in the gabbro
is locally developed. The L-component has an E-W
sub-horizontal orientation, and is usually observed in
the field as a preferred orientation of dark mineral
aggregates. The S-component is generally steep to
subvertical. In many areas this structure is parallel or
sub-parallel to the layering; locally, however, it can be
oblique. This indicates an origin where the early
crystallized mineral phases acquired an orientation
after the partly crystallized matrix changed its mech-
anical properties from those of a near ideal viscous
fluid with suspended solids, to a solid-like behaviour
where stress was transmitted by interaction of solid
grains (Arzi, 1978).
On Stord, the strike of the layering changes from
around 045° to become sub-parallel with the contact
along the NW-SE trending margin of the pluton (Fig.
2). The folds defined by the deformed layering
approach a reclined orientation, and face towards the
northwest (Fig. 2). Like the L > S structure described
above, the folding of the cumulates was initiated
before the gabbro had crystallized completely. The
coast sections along Stokksundet on Stord (Fig. 2)
provide spectacular examples of pre-solidus deformation of the cumulates in the Vardafjellet Gabbro.
In Figure 6 c, highly deformed olivine gabbro cumulates, with primary mineralogy, are truncated by
mobilized olivine gabbro. The pyroxene-rich cumulates (Fig. 6d) are commonly strongly disturbed and
folded in disharmonic folds. Locally, however, a
systematic sinistral sense of shear (Fig. 2) can be
observed where the deformed cumulates, providing
strain markers, are deflected into zones of higher
shear strain (Fig. 60- The evidence of pre-solidus
deformation of the Vardafjellet Gabbro, can be
summarized in the following six points:
(1) Cumulates have been mobilized and form dykes
and veins, some of which intrude parallel to the trend
of the axial surface of folds near the northeastern
margin on Stord.
(2) Primary minerals, best shown by dark minerals
in a leucogabbro (Fig. 7 a), have locally acquired a
preferred orientation which may be oblique to the
igneous layering.
(3) Slumping and disruption of the layering is
common, particularly in the central and eastern parts
of the pluton (Fig. 6c, d, f)(4) Rootless veins of gabbro representing mobilized
remnant magma intrude already folded and sheared
cumulates (Fig. 6 c, e). Intermediate to trondhjemitic
veins and dykes which may represent late differentiates
cut the already deformed layering.
(5) In areas not affected by later retrograde
recrystallization, the formation of the early structures
Figure 6. Vardafjellet Gabbro. (a) Banded cumulates (looking to the west) at type locality near Vardafjellet. Note local
unconformable contacts and the N-S trending vertical extensional plagioclase-rich veins. The veins are normal to the layering
and particularly common in the plagioclase-rich cumulate layers. Hammer for scale, 30 cm. (b) Detail of extensional
plagioclase veins in the deformed plagioclase-rich layers, (c) Deformed olivine gabbro with dark, highly deformed pyroxenerich cumulates. Note the truncation and drag of the deformed layers by a more isotropic olivine gabbro. This demonstrates
the pre-solidus origin for the deformation of the cumulates. The primary mineralogy at this locality, on a small island off the
coast of Stord in Stokksundet, is preserved. Pencil for scale, arrowed, is 15 cm. (d) Highly deformed pyroxene-rich cumulates
in olivine gabbro. Note the linear fabric parallel to the elongate, white, quartzitic xenolith. Later quartz-diorite veins with dark
amphibolitic alteration rims in the gabbro, truncate the earlier deformed cumulates. Same locality as Figure 6 c. Note-book
for scale, 17 cm long, (e) Angular fragments of foliated gabbro cumulates in a matrix of undeformed gabbro. Note that the
internal fabric in the fragments is sharply truncated by the undeformed matrix. See text for discussion. From type locality near
Vardafjellet. Hammer-head for scale, 13 cm. (f) Sheared cumulates of olivine gabbro, Stokksundet, Stord. Note the sinistral
offsets of the dark, pyroxene-rich layer in the upper and lower part of the photograph.
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T.B.ANDERSEN AND OTHERS
a
e ;?
Figure 7. For legend see facing page.
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219
Melt-enhanced deformation
was not associated with retrogression or alteration of
the primary igneous minerals (Fig. 7 b, c).
(6) The geometry of the early structures in the
cumulates (Fig. 6f) is consistent with the dominantly
sinistral shear that affected the aureole and the margins
of the pluton.
4.b. Stolmen Gabbro
The Stolmen Gabbro and its envelope have been
described in detail by E. Rykkelid (unpub. Cand.
Scient. thesis, Univ. Bergen, 1987). The present
description summarizes some of the results from that
study.
The Stolmen Gabbro intruded a highly dismembered ophiolite complex and metasediments of
unknown age in the Austevoll area (Fig. 8). The
structural and metamorphic development of the
migmatitic aureole shows that it was deformed
contemporaneously with the emplacement of the
gabbro (Rykkelid, 1986). E. Rykkelid (unpub. Cand.
Scient. thesis, Univ. Bergen, 1987) has calculated
shear strains up to 16 based on deformed pebbles in a
conglomerate adjacent to the gabbro, and argued for
an average shear strain > 10 in the migmatites. It is,
however, difficult to quantify the shear strains that
were associated with the emplacement of the gabbro v.
the strains that post-date the crystallization of the
Stolmen Gabbro. Common observations of synmigmatization kinematic indicators in the metatexites
and a penetrative stretching lineation plunging at 20°
to 30° towards 240-270° document an E-W sinistral
sense of shear along the margins of the gabbro (Fig.
8). The structures in the migmatites are very similar to
those described above from the Bremnes Migmatite
Complex. E. Rykkelid (unpub. Cand. Scient. thesis,
Univ. Bergen, 1987), Salna & Andersen (1988) and
H. Solna (unpub. Cand. Scient. thesis, Univ. Oslo,
1989) have shown that the displacement on the shear
zone continued with the same polarity during the
emplacement of the Reksteren Granodiorite.
The internal structure of the Stolmen Gabbro is
significantly different to that described above from the
Vardafjellet Gabbro. The layered part of the Stolmen
Gabbro is dominated by originally horizontal sills or
sheet intrusions, of which the thicker units (max.
200 m) may show internal cumulate layering. A
normal sill thickness is approximately 1.5 to 3 m, and
the total maximum thickness of the Stolmen Gabbro
is approximately 2 km. The gabbro sheets and sills are
separated by thin zones of leucodiorite and granite
(Fig. 9). The present orientation of the sills and sheets
is generally E-W with steep to vertical dips (Fig. 8).
The evidence for co-existence of dioritic to granitic
and mafic magmas is abundant, and gravity-induced
flame and load structures occur repeatedly at the base
of the gabbro sills (Fig. 9). The mafic rocks have finegrained chilled margins towards the leucodiorites and
granites (Fig. 9). The load structures and the densitygraded cumulates show consistent way-up to the
south, indicating that the wall-rocks in the Mogster
area originally formed the floor of the gabbro (Fig. 9).
The roof is marked by a zone of penetrative
deformation which is exposed only on the southernmost headlands of Selbjorn (Figs 1, 8). The
formation of an originally near-horizontal sheeted
complex shows that vertical extension affected the
area during the emplacement of the Stolmen Gabbro,
and a model for this will be discussed below.
4.c. The Reksteren Granodiorite
The Reksteren Granodiorite (Figs 1, 8) is an approximately 30 km long, elongate pluton, where the
northern margin is truncated by a late, south-dipping
extensional fault (Andersen, 1989; H. Solna, unpub.
Cand. Scient. thesis, Univ. Oslo, 1989), previously
interpreted as a thrust (Andersen & Jansen, 1987).
Along the southern margin, which makes contact with
gabbros of Unit 1, the original intrusive relationships
are preserved.
Internally, the Reksteren Granodiorite is characterized by a vertical to steeply inclined compositional
banding which is parallel to the long axis of the
pluton, and the common occurrence of late, idiomorphic, K-feldspar megacrysts (Andersen & Jansen,
1987). The banding has many similarities with the
'regular' banding in the Main Donegal Granite
(Pitcher & Berger, 1972). The origin of the banding is
ascribed to deformation at the crystal mush stage
(Andersen & Jansen, 1987; H. Selna, unpub. Cand.
Scient. thesis, Univ. Oslo, 1989), an interpretation
which is broadly similar to that for the banding in the
Main Donegal Granite (Berger, 1971; Hutton, 1982).
An early L > S mineral fabric, parallel with the
steeply inclined banding, defined by SPO of primary
minerals in the granitoid, is locally preserved. The
early fabric is, however, usually overprinted by a
concordant secondary fabric, which makes it difficult
to interpret the kinematics related to the formation of
the early fabric. The formation of the secondary fabric
was accompanied by penetrative sericitization/
Figure 7. Vardafjellet Gabbro. (a) Detail of shape-preferred orientation (SPO) fabric defined by the primary minerals in the
gabbro. Ball-pen for scale is parallel to the fabric, (b) Foliation with granoblastic polygonal texture defined by bands of
plagioclase and clinopyroxene. Note that the foliation is defined by SPO fabric of the mineral aggregates. Plane-polarised light;
from type locality near Vardafjellet. (c) Granoblastic polygonal texture in banded olivine gabbro. Olivine crystal is arrowed.
Crossed nicols; from type-locality near Vardafjellet.
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T. B. ANDERSEN AND OTHERS
SIMPLIFIED GEOLOGICAL MAP,
REKSTEREN - AUSTEVOLL,
SUNNHORDLAND, W.NORWAY
Legend:
Sunnhordland Batholith:
Unit 1
Unit 2
Unit 3
Gabbro
Granodiorite
Envelope:
Granite
Undillerentiated
=
..REKSTEREN
Intersection ol
dilational dykes
_r- _ i - j~ _r* Melt-lubricated shear zones
•*^ ^
Stretching lineation
Sense ol shear
-$-
Dilational dykes
—y—
Way-up in gabbro
Eq.area plot ol poles to extended
and lolded •
dykes, SW. Reksteren. \
s t r e tching
lineation
Figure 8. Simplified geological map of the ReksterenAustevoll area. Stereogram (equal area) shows poles to late,
folded and extended dykes in the contact zone of the
Reksteren Granodiorite on south Reksteren. See text for
discussion of the stereogram.
saussuritization and the development of a S-C
foliation, the asymmetry of which is consistent with
an E-W sinistral shear sense (Fig. 8).
4.c.l. Deformation in the contact zone along the S-margin
The S-margin of the Reksteren Granodiorite is
characterized by a large number of variably megacrystic granitoid sheets interleaved with the amphibolitized gabbro of the country rock. Where the sheets
coalesce, they form the main body of the Reksteren
Granodiorite pluton (Andersen & Jansen, 1987). The
contact zone represents a high-strain zone, where
several sub-parallel, anastomosing shear zones with
variable shear strains and relative age occur. The
foliation in the contact zone strikes E-W and is
generally steeply inclined with dips to the north.
The evidence for syn-intrusive deformation at the
scale of outcrop in the contact zone is of several
categories. A very large number of granitoid veins and
dykes were emplaced into the contact zone at various
stages in its displacement history. This provides near
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ideal examples of progressive deformation where the
granitoids are successive time markers in the strain
history (Fig. 10 a). The relationships can be studied on
the wave-washed coast exposures in a number of
localities.
A majority of the dykes were initially intruded at a
high angle to the sub-horizontal E-W stretching
lineation and formed dilational dykes in the compressional field of the incremental strain ellipsoid
(Ramsay, 1967). Evidence of early shortening of the
dykes may locally be preserved within the older dykes,
and is very commonly observed in relation to the
youngest and least deformed dykes (see also fig. 4, p.
169 in Andersen & Jansen, 1987). During progressive
rotational deformation the dykes that already were
intruded underwent an anticlockwise rotation into the
field of finite and infinitesimal extension (Ramsay,
1967). Examples of different stages of the progressive
rotational deformation of granitoid dykes are shown
in Figure 10. The youngest dykes are leucocratic and
generally comprise composite pegmatite-aplites. In an
attempt to identify the general shape of the minimum
late, finite strain ellipsoid affecting the rocks during
the emplacement of the late dykes, the method
described by Talbot (1970) was applied. The orientations of 23 folded, 19 boudinaged and 8 young
dilational dykes were measured on south Reksteren.
The data were plotted in an equal-area stereographic
projection and thefieldsof extension and compression
were defined (Talbot, 1970). The distribution of the
poles to folded and stretched dykes (Fig. 8) defines a
strain ellipsoid with X/Y(a) « 1.3 and Y/Z (b) « 1.8.
The intersection of the late dilational dykes is close to
the y-axis of the strain ellipsoid, and measurements of
the stretching lineation coincide with the J-axis (Fig.
8). The observed consistent anticlockwise rotational
strain indicates that the strain in the zone was chiefly
related to sinistral simple shear. The oblate shape of
the strain ellipsoid (k < 1), however, indicates that the
finite strain was a result of simple and pure shear. In
addition, the ingress of an unquantified volume of
granitoid material into the zone from which the strain
was studied, implies a positive volume change SV =
(Kj+ Vo)/Vo, suggesting that the component of pure
shear was significant during the terminal stages of
strain history in the contact zone.
H. Solna (unpub. Cand. Scient. thesis, Univ. Oslo,
1989) documents in detail a large number of kinematic
indicators in the S-contact zone of the Reksteren
Granodiorite, and a representative selection of asymmetrical structures is shown in Figure 11. These
generally indicate an E-W sinistral sense of shear
throughout the strain history, and include anticlockwise rotation of early dykes (Fig. 11 a), asymmetrical boudinage of dykes rotated into the field of
extension (Fig. 10a, b; Fig. lla, b,d), consistent
observations of sinistral displacement on local zones
of high shear strains, fold vergence and asymmetrical
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Melt-enhanced deformation
221
SCHEMATIC MODEL FOR THE SYN-DEFORMATIONAL
EMPLACEMENT OF THE STOLMEN GABBRO,
AND ITS SUB-HORIZONTAL LAYERING
SCALE:
1 km
DIFFERENTIAL
STRESS
APART ON RELEASING-BEND
METATEXITES
GRANITIC
MATERIAL
SHEAR-BANDS AND OTHER
SHEAR-SENSE INDICATORS IN
THE MIGMATITIC AUREOLE
GABBRO
W/CHILLED
MARGINS
MODEL FOR HORIZONTAL GABBRO
SHEETS. DIAMETER 2 M
LOAD
\
STRUCTURES
Figure 9. A restored vertical model (modified from the unpublished Cand. Scient. thesis of E. Rykellid, Univ. Bergen, 1987)
for the emplacement and formation of the horizontal layering in the Stolmen Gabbro. A stress-% melt diagram, redrawn from
Van der Molen & Paterson (1979), illustrates that diatexites with melt contents above the critical melt fraction (CMF) in the
proximal parts of the aureole will sustain smaller differential stresses than the more distal parts of the aureole. See text for
further discussion of the model.
tails on rotated K-feldspar megacrysts (Fig. 11 c). The
tail asymmetry, however, is not completely consistent,
as evidence of both sinistral and dextral shear may be
present in the same locality. This suggests that the
deformation, already at an early stage in the strain
history affecting porphyritic granodiorite sheets which
are cut by the later dykes (Fig. 11 e), took place in a
shear zone where N - S shortening occurred normal to
the shear direction.
As mentioned above, the deformation continued
after complete crystallization of the Reksteren Gra-
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nodiorite. This produced a secondary fabric, associated with saussuritization and sericitization of
feldspar. The S-C relationships developed during the
solid-state deformation also indicate an E-W sinistral
sense of shear (Fig. 10c). Late sinistral shears also
affected the metasedimentary rocks in the Austevoll
area.
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T.B.ANDERSEN AND OTHERS
5. Emplacement models for syn-tectonic plutons in
the Sunnhordland Batholith
5.a. The Vardafjellet Gabbro
C.
Figure 10. Southern contact zone of the Reksteren Granodiorite. (a) A shear zone (looking to the north) along the
southern margin of the Reksteren Granodiorite on the
Reksteren. Note that successive, syn-tectonic, granitic dykes,
originally intruded at a high angle to the stretching direction,
have been folded, rotated and progressively sheared in a
sinistral, anticlockwise direction according to their relative
age with respect to the deformation in the sinistral shear
zone. A lens cap for scale is arrowed, (b) Strong shear fabric
(looking to the north) cut by a syn-tectonic granitic dyke
which has been rotated anticlockwise into the field of
extension. Note the extensional quartz-filled veins in the
dyke. Locality south Reksteren. Lens cap for scale is
arrowed, (c) Detail of the strong fabric (looking to the
north) in a sinistral shear zone on south Reksteren. Note the
late sinistral shear band which offsets the granodioritic
banding. Ball-pen for scale is 5 cm long.
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Above, evidence of syn-magmatic deformation both
internally, along the margins and in the aureole of the
Vardafjellet Gabbro have been presented. A reconstruction of the syn-tectonic emplacement model for
the Vardafjellet Gabbro is presented in Figure 12. It
will be noticed from the model (Fig. 12) that the
restored E-W vertical section closely corresponds to
the present map of the pluton and its envelope. The
differences in the orientation of the bedding in the
Siggjo and Kattnakken groups and the original
horizontal cumulate layering in the pluton, show that
the youngest rocks in the envelope already were tilted
when the gabbro intruded. In the model, the Vardafjellet Gabbro originally formed a sheet-like body
intruded at a relatively high level ( < 4 kb) in the crust.
The volcanic and sedimentary rocks of the Siggjo and
Kattnakken groups were tilted, and the cumulates in
the gabbro had started to form. Before the gabbro had
crystallized completely, it was penetrated by an Edipping reverse shear zone. The eastern part of the
gabbro was transported westward onto the less
deformed western part. The cumulates were strongly
deformed in the central and eastern parts of the
pluton presently exposed on Stord and the coastal
areas along Stokksundet, where the displacement was
accommodated. At this stage, the partly crystallized
cumulates were strongly deformed and remnant
magma was mobilized, and the 'chaotic zone' was
formed (Fig. 12). The deformed cumulates provide
qualitative strain markers, and the preservation of a
primary igneous mineral assemblage shows that the
deformation took place before the gabbro had reached
its solidus. Deformation was also concentrated in the
partially melted metasedimentary rocks in the roof of
the gabbro presently forming the Bremnes Migmatite
Complex. Observations of kinematic indicators from
both the interior of the gabbro and in the migmatites
of the envelope are generally consistent with a sinistral
sense of shear. Local occurrences of dextral shear
zones in the Bremnes Migmatite Complex and
Vardafjellet Gabbro, as well as the development of
oblate pebbles and vesicles in the Siggjo Group on
Bemlo (see above) indicate that a component of
contemporaneous vertical flattening affected the footwall of the shear zone.
The regional extent of the shear zone to which the
deformation of the Vardafjellet Gabbro can be related
is not known, as the northwestward continuation is
truncated by the late Rolvsnes Granodiorite, and in
the southeast the shear zone is cut by the late NE-SW
trending Sunnhordland Fault (Fig. 1).
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Melt-enhanced deformation
Figure 11. For legend see page 224.
GEO 128
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T.B.ANDERSEN AND OTHERS
S.b. The Stolmen Gabbro
The Stolmen Gabbro comprises an originally subhorizontal sheeted intrusion, which indicates that it
was formed in an area which underwent vertical
extension. In the model, the vertical extension occurred by pull-apart on a flat releasing-bend segment
of an inclined reverse shear zone which also acted as
a conduit for the mafic magma (Fig. 9). The magmatic
pressure, together with the pull-apart geometry of the
shear zone, opened the space in which the sheeted sill
complex crystallized. With analogy to pull-apart
basins located along major wrench fault systems
(Crowell, 1974), releasing-bend geometries have also
been envisaged to have created the space for magmas
of granitoid composition in the South Amorican
Shear Zone (Guineberteau, Bouchez & Vigneresse,
1987) and for the Strontian Granite located along the
Great Glen Fault of Scotland (Hutton, 1988 a). In the
present case, however, the sheeted sill complex forming
the Stolmen Gabbro indicates that the extension was
vertical. The syn-emplacement rotational strains with
a sinistral sense of shear in the aureole at both the base
and the top of the gabbro are consistent with a
W-dipping reverse shear zone.
5.c. The Reksteren Granodiorite
Consanguinity of deformation and emplacement of
the Reksteren Granodiorite is documented from both
the internal structures in the pluton and the relationships between intrusion and deformation along the
southern contact zone of the pluton. The pluton was
emplaced in a zone which underwent sinistral shear
(Fig. 8). The shear zone was part of a relatively longlived system of shear planes which also controlled the
shape and deformation pattern of the earlier Stolmen
Gabbro and its envelope. The deformation affecting
the partly crystallized magma resulted in segregation
of potassium-rich fluids from biotite-plagioclase-rich
domains and the regular, steeply inclined banding was
formed. The remnant magma crystallized as layers
and lenses characterized by a higher content of Kfeldspar. The idiomorphic megacrysts may locally be
up to 15 cm long.
Details of the deformation in the marginal zone of
the pluton have been outlined above. The sinistral
syn-emplacement rotational strains were accompanied
by N-S flattening. It is suggested that the regional
shear stress was accompanied by a ballooning effect
(Ramsay, 1989) caused by the magma pressure from
the late phases of the intruding granodiorite. The late
N-S shortening in the marginal zone is indicated by
both sinistral and dextral asymmetrical tails around
deformed feldspar megacrysts and conjugate minor
shear zones. The flattening is also indicated by the
X—Y and Y—Z ratio of the late finite strain ellipsoid
defined by the deformation of the youngest granitoid
dykes in the zone. The late, solid-state S-C texture
developed under retrograde metamorphism (Fig. 10 c)
shows that the regional stress field resulted in limited
and localized sinistral rotational strains in the area
after the Reksteren Granodiorite had crystallized.
An unknown factor in the development of the
system of sinistral shear zones affecting the magmatic
arc in the Austevoll-Reksteren area is the orientation
of the shear zones during the emplacement of the
Reksteren Granodiorite. There are no available
indicators of the palaeo-horizontal during the emplacement of this pluton. Hence, it is not known when
the steep dip of the southward younging internal
layering in the Stolmen Gabbro was established, and
whether the rotation of the layering in the Stolmen
Gabbro occurred prior to, during or after the
emplacement of the Reksteren Granodiorite.
6. Conclusions
The studies in the Sunnhordland Batholith show that
the early deformation related to the emplacement of
plutons in this complex occurred in relation to shear
zones. The displacement on these shear zones was
controlled by the increased ductility which accompanied the introduction of magmas and partial melts
(Van der Molen & Paterson, 1979) in an active
tectonic region. Releasing-bend geometries on the
syn-emplacement shear zones constitute a particularly
important mechanism by which space for intruding
magmas in an area under influence of horizontal
compression and vertical extension may be created, as
demonstrated from the Stolmen Gabbro in Austevoll.
The detailed mapping of gabbroic and granodiorite
intrusions and their envelopes in the Sunnhordland
Batholith shows that the shape of the plutons as well
Figure 11. Contact zone of the Reksteren granodiorite, south Reksteren. (a) Sinistral shear zone (looking to the north) with
several syn-tectonic granitic dykes and veins. Note the asymmetrical structures in the shear fabric and the progressive rotational
deformation that has affected the dykes and veins. Note also the late, folded, thin vein which still has an orientation in the
field of compression, but which is sinistrally offset on local shear bands parallel to the main fabric. Lens cap for scale is
arrowed, (b) Asymmetrical quartz-filled boudin neck and solid-state shear bands, both showing sinistral displacement (looking
to the north), (c) K-feldspar megacryst with asymmetrical tails indicating anticlockwise, sinistral shear (looking to the north),
(d) Two examples of asymmetrical boudinage of granitic dykes on south Reksteren. The boudinage of both dykes is consistent
with a sinistral sense of shear (looking to the north). Lens cap for scale is arrowed, (e) Highly deformed bands with K-feldspar
megacrysts showing tail and augen geometry. Note that the tail asymmetry is difficult to interpret kinematically. The pencil
for scale, 14 cm, is arrowed.
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225
Melt-enhanced deformation
PRE-SOLIDUS DEFORMATON GEOMETRY,
VARDAFJELLET GABBRO
SUNNHORDLAND
BATHOL1TH. W. NORWAY
|SHEAR PLANE]
INTERNAL "CHAOTIC" ZONE,
MAINLY NON-LAYERED GABBRO
1 km
A MAJOR SHEAR-2
IN THE BREMNES
MIGMATITE COMPLEX
MARGINAL ZONE
OF THE GABBRO
BASAL CGL
OF THE SIGGJO GP
NOT PRESERVED
BELOW LINE
Figure 12. A restored model for the pre-solidus deformation geometry of the Vardafjellet Gabbro. See text for discussion of
details.
as their internal structure can be attributed to synemplacement deformation. Early deformation in
crystallizing magmas may be difficult to recognize and
interpret kinematically. Nevertheless, the deformation
in the aureoles may provide additional evidence for
high-T syn-emplacement deformation. Observations
of displacement in marginal parts of the Reksteren
Granodiorite and in the aureole of the Vardafjellet
Gabbro at Bremnes indicate that the strain in both
areas was chiefly the result of rotational deformation.
However, observations of conjugate shears, and other
indications offlatteningdiscussed above, suggest that
the finite strains include a component of pure shear.
The flattening is possibly an effect of the magma
pressure during positive dilation (ballooning), which
resulted in shortening normal to the shear zones.
Increase in pore fluid pressures or introduction of
magma or melts in the relatively brittle middle/upper
crust will have a profound effect on the rheology and
are likely to enhance localized deformation in areas
subjected to deviatoric stresses. In zones where the
viscosity is significantly reduced, in relation to
crystallizing plutons and migmatitic aureoles, the
strain rate will increase accordingly, and such zones
will form plastic instabilities (Ord & Hobbs, 1989) in
an otherwise elastic crust.
the Norwegian Research Counsil for Science and the
Humanities (NAVF). An unpublished field map of central
parts of Stord by H. Brekke and Chr. Stillman was
generously made available to the authors, and greatly
helped the mapping of the Vardafjellet Gabbro on Stord.
The first author would also express his thanks to his coauthors for all the good effort put into their theses from the
area. The manuscript has benefitted from critical reading by
Prof. A. Andresen, Prof. H. Furnes and Dr D. Roberts, and
by a constructive review by Prof. Chr. Stillman. This study
is No. 121 in the Norwegian ILP-project.
Acknowledgement. Financial support for this study has been
provided through grants (440.89/021, 440.90/007) from
BREKKE, H., FURNES, H., NORDAS, J. & HERTOGEN, J. 1984.
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