Geol. Mag. 137 (3), 2000, pp. 235–255. Printed in the United Kingdom © 2000 Cambridge University Press
235
Pre-Caledonian granulite and gabbro enclaves in the Western
Gneiss Region, Norway: indications of incomplete transition at
high pressure
M. KRABBENDAM*†, A. WAIN*‡ & T. B. ANDERSEN§
*Department of Earth Sciences, University of Oxford, Oxford OX1 3PR, UK
§Institut for Geologi, University of Oslo, Blindern 0316, Oslo, Norway
(Received 15 April 1999; accepted 17 February 2000)
Abstract – The Western Gneiss Region of Norway is a continental terrane that experienced
Caledonian high-pressure and ultrahigh-pressure metamorphism. Most rocks in this terrane show
either peak-Caledonian eclogite-facies assemblages or are highly strained and equilibrated under lateCaledonian amphibolite-facies conditions. However, three kilometre-size rock bodies (Flatraket,
Ulvesund and Kråkenes) in Outer Nordfjord preserve Pre-Caledonian igneous and granulite-facies
assemblages and structures. Where these assemblages are preserved, the rocks are consistently unaffected by Caledonian deformation. The three bodies experienced high-pressure conditions
(20–23 kbar) but show only very localized (about 5 %) eclogitization in felsic and mafic rocks, commonly related to shear zones. The preservation of Pre-Caledonian felsic and mafic igneous and granulite-facies assemblages in these bodies, therefore, indicates widespread (~ 95 %) metastability at
pressures higher than other metastable domains in Norway. Late-Caledonian amphibolite-facies retrogression was limited. The degree of reaction is related to the protolith composition and the interaction
of fluid and deformation during the orogenic cycle, whereby metastability is associated with a lack of
deformation and lack of fluids, either as a catalyst or as a component in hydration reactions. The three
bodies appear to have been far less reactive than the external gneisses in this region, even though they
followed a similar pressure–temperature evolution. The extent of metastable behaviour has implications for the protolith of the Western Gneiss Region, for the density evolution of high-pressure
terranes and hence for the geodynamic evolution of mountain belts.
1. Introduction
In many eclogite-bearing terranes of continental affinity, eclogite occurs as small pods or boudins surrounded by felsic or pelitic gneiss or schist that show
low-pressure assemblages, ranging from greenschist- to
granulite-facies (see, for example, Heinrich, 1982;
Coleman & Wang, 1995). The close occurrence of such
different metamorphic assemblages may be explained
in three ways. First, it has been suggested that eclogite
pods were tectonically emplaced into country rock that
never experienced high-pressure conditions (Smith,
1988). Tectonic emplacement of high-density eclogite
into low-density gneiss is, however, mechanically
implausible and is generally not accepted (Cuthbert,
Harvey & Carswell, 1983; Cuthbert & Carswell, 1990).
In our study area (the Western Gneiss Region of
Norway) there is no field evidence, such as steep shear
zones adjacent to eclogite, to support this hypothesis
(Krabbendam & Wain, 1997).
† Author for correspondence: maarten@earth.monash.edu.au; present address: Australian Crustal Research Centre, Department of
Earth Sciences, Monash University, Melbourne, VIC 3800,
Australia
‡ Died May 2000
Secondly, the country rock experienced eclogitefacies conditions but behaved metastably, retaining
pre-orogenic assemblages. Metastability at high pressure has been demonstrated in several terranes. In the
Bergen Arcs, southwest Norway, eclogitization is
restricted to shear zones and reaction fronts adjacent
to fluid channels of various scales (Austrheim, 1987;
Austrheim, Erambert & Engvik, 1997). Similar relationships are now being recognized in the Western
Gneiss Region (Engvik, Austrheim & Andersen, 2000,
and this paper). Outside the Caledonides, metastability at high pressure has been documented in the
Zermatt–Saas Fee Zone (Wayte et al. 1989) and in the
Sesia Zone (Koons, Rubie & Frueh-Green, 1987), both
in the Alps.
Lastly, the country rock equilibrated at high-pressure
conditions but eclogite-facies assemblages re-equilibrated extensively to low-pressure assemblages, whilst
mafic eclogites retained their high-pressure assemblages
(Griffin & Carswell, 1985; Cuthbert & Carswell, 1990).
Based on work in the Adula Nappe in the Alps,
Heinrich (1982) proposed a mechanism whereby retrogression from eclogite-facies assemblages is favoured in
pelitic rocks, as it involves dehydration reactions that
are crossed from the low-temperature side during
decompression. Retrogression of mafic rocks, on the
236
other hand, requires fluid infiltration to produce amphibolite-facies assemblages. The mechanism of Heinrich
(1982) is probably also applicable to quartzo-feldspathic gneisses, since felsic eclogite-facies assemblages
contain significant amounts of hydrous minerals, such
as zoisite and phengite (Austrheim & Griffin, 1985;
Koons, Rubie & Frueh-Green, 1987; Black, Brothers &
Yokoyama, 1988).
Whether country rocks in a high-pressure terrane
equilibrated at high-pressure conditions (and subsequently retrogressed) or did not develop eclogite-facies
assemblages has a number of metamorphic and tectonic implications. Firstly, a failure to recognize widespread metastability or extensive retrogression may
lead to incorrect interpretations of the metamorphic,
tectonic and geochronological evolution and of the
tectono-stratigraphic architecture of a high-pressure
terrane. Secondly, metamorphic reactions are not just
the result of a particular tectonic evolution of an orogen, but may also play an active role during orogenesis
(Austrheim, Erambert & Engvik, 1997). Extensive
eclogitization is increasingly invoked in geodynamic
modelling. For instance, Dewey, Ryan & Andersen
(1993) proposed a model whereby eclogitization of an
orogenic root can result in very thick (≥ 120 km) crust
and hence explain ultrahigh-pressure metamorphism
in the Western Gneiss Region. Le Pichon, Henry &
Goffé (1997) describe a model where the eclogite to
granulite transition may be responsible for the uplift of
the Tibetan Plateau. On the basis of mass balance calculations and structural restorations of the Alps,
Butler (1986) and Laubscher (1990) suggested that
large masses of eclogitized continental crust had
returned into the mantle underneath the Alps. To
assess whether extensive eclogitization as modelled by
these studies is plausible it is necessary to estimate the
degree of eclogitization in former orogenic root zones
with reasonable accuracy.
In this paper we present lithological, petrological and
structural data of three kilometre-size bodies within the
Western Gneiss Region in southwest Norway. The
Flatraket, Ulvesund and Kråkenes bodies contain relics
of igneous assemblages and three different metamorphic assemblages: granulite-, eclogite- and amphibolitefacies that occur in mafic, intermediate and felsic rocks.
We describe the spatial and temporal relationships
between the different assemblages and structures that
show that the granulite-facies assemblages pre-date the
eclogite-facies metamorphism. The data demonstrate
that the bodies were low-strain zones and behaved
metastably during the Caledonian high-pressure event.
We compare the three bodies with the surrounding
gneisses and suggest that they were subjected to similar
pressure–temperature conditions during Caledonian
time but responded differently to these conditions.
These different responses can be explained by a difference in Pre-Caledonian metamorphic assemblages in
different domains within the Western Gneiss Region.
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
An earlier paper (Krabbendam & Wain, 1997)
describes in detail the quartzo-feldspathic gneisses
surrounding these three bodies and focuses on the lateCaledonian evolution of this area. A detailed documentation of the mineral chemistry and pressure–
temperature conditions recorded in the Flatraket Body
is found in Wain (A. Wain, unpub. Ph.D. thesis, Univ.
Oxford, 1998) and will be presented elsewhere (Wain,
Waters & Austrheim, unpub. data). Here, we will focus
on the essential field relationships, structures and mineralogy of all three bodies.
2. Geological setting
2.a. The Western Gneiss Region
The Western Gneiss Region is a large basement window in west Norway, overlain by a series of
Caledonian nappes. The Western Gneiss Region is
mainly composed of Proterozoic quartzo-feldspathic
gneisses with broadly granodioritic to granitic compositions (Tucker, Krogh & Råheim, 1990) and minor
anorthosite, ultramafic rocks, metasediments and
mafic rocks (see, for example, Bryhni, 1989), reworked
during the Scandian phase of the Caledonian orogeny
(see, for example, Kullerud, Tørudbakken & Ilebekk,
1986). Eclogites occur in an area of some 25 000 km2
(Griffin et al. 1985), with ultrahigh-pressure (coesitebearing) eclogites occurring in the far northwest, in an
area at least 100 km long but of unknown width
(Smith, 1984, 1988; Wain, 1997). Most preserved
eclogite-facies rocks are mafic, although evidence of
felsic eclogite-facies rocks is increasingly being found
(Wain, 1997; Engvik, Austrheim & Andersen, 2000).
The gneiss surrounding the eclogite is predominantly
at amphibolite-facies grade (Bryhni & Andréasson,
1985; Andersen, Osmundsen & Jolivet, 1994;
Krabbendam &Wain, 1997) although in several places
granulite-facies assemblages have been recorded
(Griffin et al. 1985; Straume & Austrheim, 1999).
The Scandian Phase of the Caledonian orogeny
was responsible for (ultra)high-pressure metamorphism in the Western Gneiss Region and has been
dated at 420–400 Ma (Griffin & Brueckner, 1980,
1985; Gebauer et al. 1985; Mørk & Mearns, 1986).
Cooling ages in southwest Norway indicate rapid
exhumation between c. 410 and 385 Ma (Berry et al.
1994; Andersen, 1998; Fossen & Dallmeyer, 1998).
Exhumation took place by east–west extension.
This extension resulted in strong non-coaxial deformation along the extensional Nordfjord-Sogn
Detachment. Near-coaxial extensional to constrictional strain under granulite- and amphibolite-facies
conditions occurred at structural deeper levels within
the Western Gneiss Region (Andersen & Jamtveit,
1990; Krabbendam & Wain, 1997; Andersen, 1998;
Krabbendam & Dewey, 1998; Straume & Austrheim,
1999).
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
2.b. Outer Nordfjord area
The outer Nordfjord area (Fig. 1) is situated in the
northwestern part of the Western Gneiss Region. The
bulk of the area is composed of layered, micaceous
237
quartzo-feldspathic gneisses with about 10 % mafic
pods. In general, eclogites are predominantly mafic and
are preserved as boudins within felsic gneisses. Other
rock types in outer Nordfjord include thin layers of
quartzite, anorthosite and ultramafic bodies (Bryhni,
Figure 1. Geological map of outer Nordfjord area, showing geological setting of the Kråkenes, Flatraket and Ulvesund bodies.
Metamorphic zonation after Wain (1997); additional data after Bryhni (1966).
238
1966; Lappin, 1966; Krabbendam & Wain, 1997). Most
quartzo-feldspathic gneisses equilibrated under amphibolite-facies conditions and were highly strained during late-Caledonian extension (Krabbendam & Wain,
1997; Krabbendam & Dewey, 1998). In the area north
of Nordfjord, however, there are several kilometre-size
bodies that contain felsic, intermediate and mafic rocks
and experienced little or no Caledonian deformation.
These bodies preserve either granulite-facies assemblages or igneous textures and minerals. Three such
bodies have been studied in some detail and are the
subject of this paper: the Flatraket, Ulvesund and
Kråkenes bodies (Figs 2, 4, 5). These three bodies are
bounded on all exposed sides by layered quartzo-feldspathic gneiss containing eclogite pods, typical of the
Western Gneiss Region (Fig. 1). Their contacts are subparallel with the main extensional foliation of the
enclosing gneisses (Figs 2, 4, 5). The Flatraket and
Ulvesund bodies occur within the ‘mixed’ high-pressure–ultrahigh-pressure zone (Wain, 1997), with both
high-pressure and ultrahigh-pressure eclogites occurring in the direct vicinity (Fig. 1). Coesite-eclogite pods
occur structurally below and above the Flatraket Body
(Fig. 2). Recent work by Austrheim & Engvik (2000)
demonstrates that the Kråkenes Gabbro Body was
affected by eclogite-facies metamorphism. No coesiteeclogites have, however, been observed on Vågsøy, so
the relationship of the Kråkenes Body with the ultrahigh-pressure rocks is, as yet, unclear.
3. Lithologies and assemblages
Amphibolite, eclogite and granulite-facies assemblages were distinguished mainly on the basis of mineralogical and textural criteria. Clinopyroxene and
garnet with distinct mineral chemistries occur in both
granulite and eclogite-facies assemblages. In the following discussion, ‘augite’ refers to unaltered granulite-facies calcic clinopyroxene (< 3 % jadeite) and
‘omphacite’ refers to clinopyroxene compositions
that have been partially reset (< 10 % jadeite) or have
equilibrated under eclogite-facies conditions (~ 40 %
jadeite). In rocks where mineral chemistry has not
been measured, the term clinopyroxene is used, and
the facies association was determined from textural
relationships.
3.a. Amphibolite-facies gneisses
The dominant rock type, comprising about 80 % of the
outer Nordfjord area (Fig. 1), is layered micaceous
quartzo-feldspathic gneiss. Locally, kilometre-size bodies of relatively uniform granitic augen gneiss occur (Fig.
1). The gneisses have the amphibolite-facies assemblage
of plagioclase + quartz + K-feldspar + biotite + epidote
± hornblende ± white mica ± sphene ± garnet. A welldeveloped foliation and lineation is defined by the
amphibolite-facies assemblage, indicating pervasive
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
deformation under late-Caledonian amphibolite-facies
conditions (Krabbendam & Wain, 1997). Eclogite-facies
assemblages are not commonly preserved in these
gneisses.
3.b. Eclogite and eclogite-facies gneisses
Eclogite occurs throughout the quartzo-feldspathic
gneisses as pods, usually less than 50 m across. These
pods generally possess an amphibolite rim foliated
parallel to the enclosing gneisses. The foliated amphibolite-facies rims and external amphibolite-facies
gneisses show compatible pressure–temperature conditions, suggesting they both re-equilibrated during
exhumation (A. Wain, unpub. Ph.D. thesis, Univ.
Oxford, 1998). The layering and amphibolite-facies
foliation of the enclosing gneisses commonly wraps
eclogite pods, truncating eclogite-facies structures.
Long tails of amphibolite stretch from the eclogite
into the enclosing gneisses. In metre-scale pods, the
core consists of the least retrogressed eclogite with
increasing retrogression towards the margin. In larger
(> 10 m) mafic bodies, however, several ‘fresh’ eclogite
zones occur, separated by a network of amphibolitefacies alteration and shear zones.
Eclogite-facies material also occurs within layered
packages, up to 1 km thick, including mafic eclogite, eclogite-facies garnet-phengite gneiss and metaanorthosite (Wain, 1997). Both high-pressure (P ~ 20–24
kbar) and ultrahigh-pressure eclogites (P ≥ 28 kbar)
occur in all settings (Wain, 1997). Felsic eclogite-facies
mineralogies or textures are only preserved in amphibolite-facies low-strain zones associated with mafic eclogite
pods, similar to observations of Engvik, Austrheim &
Andersen (2000). In domains of amphibolite-facies high
strain, the felsic eclogites have been recrystallized to foliated biotite–plagioclase–quartz gneiss with relict garnet
(A. Wain, unpub. Ph.D. thesis, Univ. Oxford, 1998).
3.c. Megacrystic gneiss and mafic dykes in the Flatraket
Body
The core of the Flatraket Body (Fig. 2) is mainly composed of weakly deformed gneiss of quartz-monzonitic composition. The rock, hereafter referred to as
megacrystic gneiss, is characterized by large (3–10 cm)
ovoidal megacrysts of K-feldspar (Bryhni, 1966;
Lappin, 1966; Fig. 6a). Plagioclase (oligoclase–andesine) occurs as smaller ovoids (5–10 mm). The finegrained matrix is composed of plagioclase, quartz,
garnet, biotite and hornblende and/or augite. In
weakly deformed gneiss, the mafic minerals (biotite,
augite and hornblende) occur as irregular clusters,
locally separated from plagioclase by coronas of garnet. Much of the megacrystic gneiss is sheared and has
the amphibolite-facies assemblage of K-feldspar +
plagioclase (oligoclase) + quartz + garnet + biotite
± hornblende ± zoisite (Table 1). However, some K-
–
Qtz + Kfs + Grt + Cpx
Qtz + Kfs + Grt + Cpx
Fsp +Grt + Cpx ± Qtz
Kfs + Qtz + Grt ± Cpx ± Pl
Grt + Pl + Cpx ± Qtz
–
–
–
–
–
–
–
–
Pl + Cpx + Ol ± Bt
–
–
Kråkenes Gabbro
Gabbro, undeformed
Gabbro, deformed
Megacrystic gneiss
Mineral abbreviations after Kretz (1983); Phe = phengite. *Austrheim & Engvik (2000).
Opx + Grt coronas?
–
–
Kfs + Grt + Qtz ± Bt
Kfs + Pl + Grt + Qtz + Ilm + Bt ± Aug ± Hbl
Kfs ± Grt
Kfs + Qtz + Grt ± Aug ± Ilm
Kfs + Pl + Aug + Grt ± Qtz ± Ilm
Pl + Grt + Aug + Qtz + Ilm ± Bt ± Hbl ± Kfs
Grt + Aug + Pl ± Bt ± Ilm
Grt + Aug + Pl ± Bt ± Kfs ± Ilm
Pl + Grt + Aug ± Opx ± Bt ± Ilm ± Kfs
Pl?
–
Kfs megacrysts
Kfs megacrysts
–
–
± Opx?
Pl + Cpx ± Bt
–
± Opx?
–
Flatraket Body
Megacrystic gneiss, undeformed
Megacrystic gneiss, sheared
Layered garnetiferous gneiss, undeformed
Layered garnetiferous gneiss, deformed
Intermediate gneiss
Mafic dykes, layers and pods, undeformed
Mafic dykes, layers and pods, deformed
Meta-anorthosite, undeformed
Meta-anorthosite, deformed
Granulite-facies
Ulvesund Body
Felsic gneiss, including megacrystic
gneiss, undeformed
Felsic gneiss, including megacrystic
gneiss, sheared
Layered garnetiferous gneiss, undeformed
Layered garnetiferous gneiss, deformed
Intermediate gneiss/pods, undeformed
Intermediate gneiss/pods, undeformed
Mafic layers and pods, undeformed
Mafic layers and pods, deformed
Igneous
Rock type; state of deformation
Grt + Omp*
–
–
–
–
–
Qtz + Zo + Grt + Cpx ± Rt
Qtz + Zo + Grt + Cpx ± Ky
Grt + Omp ± Qtz ± Ph ± Zo
Grt + Cpx ± Qtz ± Ph ± Zo
Qtz + Ph ± Rt?
–
–
Qtz + Omp + Grt ± Rt
Qtz + Zo + Omp ± Rt
Qtz + Cpx + Grt ± Zo ± Ky ± Rt
Grt + Omp ± Qtz ± Zo ± Ky ± Ph ± Rt
Grt + Cpx ± Qtz ± Zo ± Ky ± Phe
–
Zo + Ky + Qtz ± Phe ± Grt ± Omp ± Rt
Eclogite-facies
Table 1. Mineral assemblages within the Flatraket, Ulvesund and Kråkenes bodies, separated by rock-type and state of deformation
Hbl coronas?
Pl + Hbl + Bt + Grt ± Spn ± Ilm
Kfs + Pl + Qtz + Bt + Ms + Zo + Ky
Kfs + Pl + Qtz + Bt ± Ms ± Hbl ± Ep
Kfs + Pl + Qtz +Ep + Bt ± Hbl
Kfs + Pl + Qtz +Hbl + Bt ± Ep
Qtz + Pl + Bt ± Epi ± Spn
Qtz + Pl + Bt ± Epi ± Grt
Hbl, Fsp, Bt, Ep, (symplectite)
Hbl + Fsp + Bt ± Ep
Kfs + Pl + Qtz + Bt + Ms + Ep
Kfs + Pl + Qtz + Bt + Hbl ± Czo ± Ms
Kfs + Pl + Qtz + Bt + Hbl ± Czo ± Ms
Kfs + Pl + Qtz + Bt ± Hbl ±Czo ± Ms
Kfs + Pl + Qtz + Bt ± Hbl ± Zo ± Ms
Kfs + Qtz + Pl + Hbl + Bt ± Grt
Amp, Pl, Bt, Ep (symplectite)
Amp, Pl, Bt, Ep (symplectite)
Amp + Bt + Ep
Pl + Qtz + Amp + Czo ± Mrg ± Kfs
Amphibolite-facies
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
239
240
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
presence of the assemblage garnet + omphacite +
quartz + zoisite ± rutile ± white mica indicates partial
or complete equilibration at eclogite-facies conditions
(Table 1). Several mafic dykes and pods are strongly
altered with symplectite, very fine-grained feldspar
and turbid clinopyroxene present. In these cases it is
ambiguous whether the plagioclase and clinopyroxene
are relics of a granulite-facies assemblage or if plagioclase is associated with retrogression from an eclogitefacies assemblage. The different assemblages are
difficult to distinguish in the field and many dykes that
superficially may look like eclogite are mafic granulites
(cf. Bryhni, 1966).
3.d. Meta-anorthosite and eclogite near Seljeneset
On the eastern side of the Flatraket Body, metaanorthosite occurs, interlayered with eclogite, studied
in some detail by Cotkin, Valley & Essene (1988) and
Cotkin (1997). The meta-anorthosite is layered with
pale-grey and pale-green layers and commonly contains pink or white clots about 1 cm across. The pale
layers are predominantly composed of plagioclase
with granoblastic texture, zoisite, quartz, blue-green
amphibole and accessory biotite and rutile. The palegreen layers contain the same minerals but more
amphibole and zoisite and less plagioclase. Thin layers
of almost pure zoisite also occur. The pink clots are
aggregates of coarse margarite in a very fine matrix of
symplectitic feldspar and micas. The white clots are
aggregates of quartz and plagioclase, free of other
minerals. The current assemblage of plagioclase +
quartz + amphibole + zoisite + margarite ± biotite ±
K-feldspar suggests amphibolite-facies equilibration.
Cotkin, Valley & Essene (1988) suggested that the
current assemblage evolved from an eclogite assemblage of kyanite + zoisite + quartz; the possibility of
metastability at eclogite-facies conditions was, however, not discussed.
Figure 2. Geological map of the Flatraket Body and surroundings. For location see Figure 1.
3.e. The margins of the Flatraket Body
feldspar, garnet and plagioclase appear to represent
relics of an older granulite-facies assemblage and
augites are locally pseudomorphed by amphibole.
Numerous mafic to intermediate dykes cut the
megacrystic gneiss (Figs 2, 6a). Primary, intrusive contacts with the surrounding gneiss are commonly preserved although minor shearing along dyke margins is
common. Some mafic and intermediate dykes preserve
a granulite-facies assemblage of garnet + augite + plagioclase ± quartz ± K-feldspar ± biotite (Table 1, Fig.
7e), commonly with a well-defined granoblastic texture. Augite aggregates are commonly rimmed by garnet. In other dykes, the absence of plagioclase and the
The margin of the Flatraket Body is best exposed along
a stream section on the western side. To the west and
south, the megacrystic gneiss is bounded by layered
garnetiferous felsic gneiss that lacks K-feldspar
megacrysts (Figs 2, 3). This rock contains a coarsegrained (0.1–3 mm) relict granulite-facies assemblage
of plagioclase + quartz + garnet + biotite ± K-feldspar
± augite ± kyanite (Table 1, Fig. 7c). A finely spaced
(1–3 mm) layering, composed of alternating felsic and
mafic layers, is commonly developed. This layering is
cross-cut by the later amphibolite-facies fabric (Figs
3, 6b) and is also at high angles to the rare eclogitefacies shear zones. In most samples, 30 % or more of
the rock is extremely fine-grained (< 10–50 µm) and
composed of feldspar (at least in part plagioclase),
amphiboles, biotite and zoisite. These fine-grained
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
Figure 3. Geological map of the western contact zone of the Flatraket Body. For location see Figure 2.
241
242
domains represent a retrogressive transition towards an
amphibolite-facies assemblage from an earlier granulite-facies assemblage. Generally, the amount of very
fine-grained material increases with stronger subsequent deformation so that in high-strain zones most
feldspar is extremely fine-grained with finely dispersed
mica and zoisite. At the highest strains this developed
into a mylonitic fabric, with a pervasive amphibolitefacies assemblage of plagioclase (oligoclase) + quartz
+ biotite (2) ± K-feldspar ± hornblende ± epidote ±
white mica (Table 1).
Along the stream section, several 10–30 cm wide
mafic layers occur concordant with the compositional
layering of the surrounding felsic garnetiferous gneiss
(Figs 3, 6b). The centre of some layers contains an
eclogite-facies assemblage of garnet + omphacite +
quartz ± rutile ± phengite. Towards the rim, increasing
retrogression to a symplectite of feldspar + green
amphibole + biotite is responsible for the dark, greencoloured margin (Fig. 6b, Table 1). Other mafic layers
have a granulite-facies assemblage of garnet + augite
+ plagioclase (andesine) in the centre, but have an
eclogite-facies assemblage of garnet + omphacite +
quartz ± rutile developed along the margins or in narrow zones within the layer.
Pods of dark-pink, fine-grained rock of dioritic to
gabbroic composition occur in several places along the
stream section. The largest of these pods, Pod 916
(Fig. 3), contains locally relics of an igneous texture
with ophitic, lath-like plagioclase (Fig. 7b). Most of
the pod, however, shows the granulite-facies assemblage garnet + clinopyroxene + plagioclase ± biotite,
with a relict granoblastic texture and remnants of garnet coronas separating feldspar and clinopyroxene.
This assemblage has been strongly affected by amphibolite-facies retrogression, as evidenced by turbid
clinopyroxene and feldspar recrystallized to a very fine
grain-size with very finely dispersed zoisite, biotite and
hornblende needles. A sample from the margin of this
pod contains medium to fine-grained garnet and
clinopyroxene, in a matrix of fine omphacite + quartz
+ biotite + zoisite, but without any feldspar. This
assemblage is interpreted as eclogite-facies. Thus, this
mafic pod contains igneous, granulite-facies, eclogitefacies and amphibolite-facies assemblages. The eclogitization only affected the margin of the pod.
Near the first waterfall in the stream section (Fig. 3), a
weakly deformed pod comprises white to pale-pink
coronitic meta-anorthositic gneiss with large (~ 1 cm)
garnet coronas. This pod contains a well-preserved granulite-facies assemblage of garnet + plagioclase (andesine) + augite ± K-feldspar ± biotite ± quartz with a
well-developed granoblastic texture. Orthopyroxene
occurs as armoured relics within coronas of augite and
garnet. Garnet occurs preferentially along the contacts
of plagioclase and ferro-magnesian minerals (Fig. 7d).
Similar rocks occur in a road cut near the southern margin of the Flatraket Body.
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
Near the first waterfall, several narrow (~ 5–10 cm)
felsic eclogite-facies shear zones occur (Fig. 3). The
mylonitic matrix of these shear zones has the assemblage of kyanite + zoisite + quartz + phengite ±
omphacite (Fig. 7.f,g). Micro-lithons within the
mylonite have a more ferro-magnesian composition
and have the assemblage of garnet + omphacite ±
quartz ± phengite. One of the shear zones deforms
(and hence post-dates) the margins of the granulitefacies coronitic meta-anorthosite described in Section
3.d. The felsic eclogite-facies shear zones are generally
associated with mafic layers that also possess a sheared
eclogite-facies fabric.
In the drain that branches off the stream (Fig. 3), a
trail of small (≤ 1 m) mafic eclogite pods and slivers
occurs in a matrix of pale green, felsic to intermediate
rock. In thin section, quartz + garnet + clinopyroxene ±
rutile ± phengite suggest an eclogite-facies assemblage.
3.f. The Ulvesund Body
Most of the Ulvesund Body comprises medium finegrained garnetiferous felsic gneiss, resembling the garnetiferous felsic gneiss in the Flatraket stream section.
A finely spaced (1–4 mm) layering of alternating felsic
and garnet + amphibole or pyroxene-rich layers is
commonly present. Most of the felsic gneisses are
deformed and contain amphibolite-facies assemblages
(Table 1). This amphibolite-facies assemblage commonly occurs as a very fine-grained matrix surrounding coarse porphyroclasts of garnet ± K-feldspar ±
clinopyroxene ± quartz (Fig. 7h). This is similar to
textures of amphibolitized granulites in the Flatraket
body, and so is interpreted as representing remnants of
a granulite-facies assemblage. In some cases it is
ambiguous, however, whether these porphyroclastic
assemblages represent remnants of a granulite-facies
assemblage or an eclogite-facies assemblage. On the
west coast of Ulvesund, leucocratic patches and anastomosing zones occur, suggesting partial melting.
Felsic gneiss with ovoidal K-feldspar megacrysts
occurs throughout the Ulvesund Body (Fig. 4). In
general, the megacrysts are more recrystallized and
more intensely deformed than at Flatraket.
Mafic dykes or pods are less common than at
Flatraket. Most mafic pods are small and have wellequilibrated assemblages of garnet + clinopyroxene ±
quartz ± phengite ± rutile ± zoisite, which is interpreted as eclogite-facies. On Hanekammen (Fig. 4),
some eclogite pods are particularly fresh; some have
well-developed eclogite-facies fabrics, others appear
undeformed. About 1 km north of Måløy, an eclogite
contains a well-developed layering of garnet-rich and
pyroxene/symplectite-rich layers. This eclogite layering
is strongly folded. Locally, felsic rocks, closely associated with mafic eclogite, have assemblages of quartz +
zoisite + white mica + garnet or quartz + white mica
that are interpreted as eclogite-facies assemblages.
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
243
Figure 4. Geological map of the Ulvesund Body and surroundings. A–A′ and B–B′ are lines of cross-section in Figure 9. For
location see Figure 1.
Bodies of medium- to fine-grained, dark-pink intermediate rocks, commonly associated with mafic rocks,
occur on the summit of Hanekammen and on the west
coast of Ulvesund (Fig. 4). These intermediate rocks
preserve the best evidence of granulite-facies assemblages within the Ulvesund Body: garnet + augite +
plagioclase ± K-feldspar ± quartz occur as coarse porphyroclasts, commonly with relict granoblastic textures, although fine-grained domains of feldspar +
amphibole + biotite ± epidote/zoisite indicate that
partial amphibolitization occurred in all rocks.
3.g. The Kråkenes Body
The core of the Kråkenes Body consists of a 250 m
wide sheet of gabbro, bounded on either side by recrystallized megacrystic felsic gneiss (Fig. 5). Where undeformed, the gabbro at Kråkenes preserves an ophitic
texture that is clearly visible in the field. Locally,
10–50 cm widely spaced igneous cumulate layering
occurs; this layering dips 60–70° to the north–northeast. In these domains, the igneous assemblage is plagioclase + clinopyroxene + olivine ± biotite ± ilmenite.
Plagioclase crystals show igneous (euhedral, lath-like)
habit in a radial or ophitic texture (Fig. 7a).
Partial transition to metamorphic assemblages is
evident from coronas in all samples studied (Table 1).
Observed reaction textures include (Fig. 7a): garnet
coronas around plagioclase; red biotite coronas
Figure 5. Geological map of the Kråkenes Body and
surroundings. For location see Figure 1.
244
Figure 6. Field photographs. (a) Poorly deformed megacrystic gneiss (top) with mafic dyke, Flatraket Body. The
dyke–gneiss contact is not deformed. Coastal outcrop southwest of Naveneset. Hammer is 40 cm long. (b) Mafic layers
with pale eclogite-facies core and dark symplectitic margins
(arrows), within garnetiferous layered gneiss with relics of
granulite-facies assemblages. The folding and shearing of the
layers is associated with late amphibolite-facies fabrics.
Western contact zone of the Flatraket Body. Notebook is
20 cm long. Plan view, top to the south.
around opaque minerals; sphene mantling rutile
and/or ilmenite; fine fibrous radial orthopyroxene surrounding or replacing olivine or igneous clinopyroxene and thin garnet coronas, surrounding the above
orthopyroxene coronas. The igneous clinopyroxene is
coarse and commonly strongly turbid, whereas newgrown pyroxenes are clear but fine-grained. Locally, a
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
third shell of hornblende ± biotite coronas surrounds
garnet and orthopyroxene coronas, suggesting a multistage corona development. All samples with ophitic
relics exhibit corona textures; all these samples are
undeformed. Similar corona textures have been
observed in meta-dolerites and meta-gabbros elsewhere in southwest Norway (Griffin & Heier, 1973;
Mørk, 1985, 1986).
Within the gabbro, fractures occur that resemble
pseudotachylite. In these fractures, euhedral garnet,
low-Al orthopyroxene and omphacite have been
observed (Austrheim & Engvik, 2000), suggesting that
the Kråkenes gabbro experienced eclogite-facies conditions at pressures of 24–29 kbar and temperature of
c. 700 °C (Austrheim & Engvik, 2000). The preservation of abundant igneous plagioclase, however, indicates that the body largely failed to equilibrate at
eclogite-facies conditions.
To the south, a 100–200 m wide zone of strongly
altered and hydrated meta-gabbro occurs. Here, the
igneous and corona textures are obliterated by the
growth of amphibolite-facies assemblages with hornblende, plagioclase, biotite, garnet and zoisite. All
deformed meta-gabbro has been totally recrystallized
to amphibolite-facies assemblages. Locally, however,
total recrystallization occurred without deformation.
To the south, the meta-gabbro is bounded by felsic
gneiss. In places, the margin of the felsic gneiss contains friable, schistose layers consisting predominantly
of coarse biotite, indicating hydration under lowgrade metamorphic conditions. Further south, the felsic gneiss becomes progressively more deformed with
flattened relics of K-feldspar megacrysts, ultimately to
give way to a layered gneiss, similar to gneisses elsewhere in the Western Gneiss Region, except for a
higher content of K-feldspar. This transition is gradual over about 200 m. Within the felsic gneiss, a
doleritic dyke occurs, now strongly altered but with
relict ophitic textures similar to the main gabbro body.
North of the gabbro, felsic megacrystic gneiss
occurs, similar in appearance to the megacrystic gneiss
at Flatraket, albeit more recrystallized at amphibolite
facies. A narrow (~ 30 cm) shear zone separates the
gabbro and the megacrystic gneiss. The lack of a strong
mylonitic fabric in this shear zone suggests limited displacement implying that the gabbro and the megacrystic gneiss probably crystallized close to each other.
The Kråkenes gabbro has yielded a Sm–Nd age of
1258 ± 56 Ma (E. W. Mearns, unpub. Ph.D. thesis, Univ.
Aberdeen, 1984, reported in Kullerud, Tørudbakken &
Ilebekk, 1986). This age suggest that the gabbro is MidProterozoic in age so that it was emplaced prior to the
Caledonian high-pressure event. The gabbro underwent
very little reaction under these conditions and behaved
even more metastably than the meta-gabbros described
by Mørk (1985, 1986). Whether or not the gabbro experienced granulite-facies conditions remains an open
question.
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
4. Structural analysis
4.a. Evidence for low strain
Within the pervasively deformed quartzo-feldspathic
gneisses of outer Nordfjord, low-strain zones are more
exceptional than high-strain zones (Krabbendam &
Wain, 1997). In the Kråkenes Gabbro, the absence of
any strain is evident from the very good preservation
of igneous cumulate layering, ophitic textures and
coronas. Within the megacrystic gneiss at Kråkenes
and Flatraket, low strain is indicated by the preservation of the near-spherical K-feldspar megacrysts.
Many mafic dykes at Flatraket preserve near-original
contacts, also indicating low strain.
4.b. Structures within the Flatraket Body
Within the Flatraket body, K-feldspar megacrysts are
near-spherical in about 30 % of the body, indicating
low strain. Much of the megacrystic gneiss is weakly
to moderately deformed, with K-feldspar megacrysts
aspect ratios of about 1 : 2–3. Both L > S and S > L
fabrics occur. The L > S fabric is subvertical in many
places, whereas the S > L fabric is moderately to
steeply east-dipping. The matrix surrounding the
megacrysts shows a moderately developed amphibolite-facies foliation and it appears that the matrix has
taken up more strain than the megacrysts themselves.
These structures affect relatively large domains (up to
100 m across) in a homogeneous way and cross-cut the
mafic dykes or are deflected around them.
In several places, distinct 1–2 m wide mylonite zones
cross-cut the megacrystic gneiss and the older structures described above. These mylonite zones are characterized by a progressive flattening and rotation of
megacrysts. In the centre of the mylonite zones,
megacrysts are destroyed and replaced by a finely
spaced (< 1 mm) mylonitic layering and well-defined
amphibolite-facies foliation. Most shear zones are
sub-parallel to the boundary of the Flatraket Body
and the dominant foliation outside the body. The lineations, however, are shallow north–northwest or
south–southeast plunging, highly discordant to the
dominant east–west plunging lineations outside the
Flatraket Body (compare Fig. 8b and 8c).
Shear sense is best indicated by the progressive rotation of the shear fabric into the shear zone. Of the 20
shear zones where shear sense has been determined, 10
are dextral and 10 are sinistral. This suggests that the
bulk strain regime was near-coaxial during the development of the mylonite zones, resulting in an anastomosing network of shear zones with opposite shear
senses.
4.c. Structures in the Flatraket stream section
The stream section at Flatraket (Fig. 3) provides a
well-exposed section from typical Western Gneiss
245
Region quartzo-feldspathic gneiss through a complicated transition zone to the megacrystic gneiss in the
core of the Flatraket Body.
The quartzo-feldspathic gneiss northeast of the
drain (Fig. 3) and further outside the Flatraket Body
shows an east dipping, well-developed finely spaced
layering and parallel mica-foliations. The dominant
lineation, defined by biotite ± epidote ± chlorite is
down-dip to the east (Fig. 8c). Minor tight to isoclinal
folds have axial surfaces parallel with the dominant
foliation and fold axes with plunge directions varying
between northeast to southeast. These structures were
formed under (low) amphibolite-facies conditions and
are similar to elsewhere in the outer Nordfjord area
(Krabbendam & Wain, 1997; Krabbendam & Dewey,
1998).
The eclogite-facies shear zones near the first waterfall (described in Section 3.e, Fig. 7f) dip steeply to the
east. Eclogite-facies lineations plunge gently (5–10°) to
the north or south, highly discordant to the eastplunging amphibolite-facies lineations outside the
Flatraket body (Fig. 8c).
Above the second waterfall (Fig. 3), clear overprinting relationships occur between two sets of planar
structures (Fig. 6b). The older finely spaced layering,
defined by modal variation in granulite-facies minerals, was probably formed during the granulite-facies
event. Mafic layers with eclogite-facies assemblage in
their centres (see Section 3.e) are parallel with this layering. Where younger deformation is at its lowest, the
layering has an east–west strike and a moderate
southerly dip (Fig. 3). The fine layering and the mafic
layers are folded into open to close, north–south
trending folds, with a steeply east dipping, amphibolite-facies axial planar fabric defined by hornblende
and biotite. The limbs of these folds are strongly
deformed and locally amphibolite-facies high-strain
zones have obliterated the older layering (Fig. 6b).
4.d. Structures within and adjacent to the Ulvesund Body
Only the lower part of the Ulvesund Body is preserved; this part has a scoop-shaped geometry (Fig. 9).
Planar structures inside the Ulvesund are represented
by a finely spaced compositional layering in garnetiferous felsic gneiss and a poorly defined layer-parallel
mineral fabric of probable granulite-facies grade. The
mineral fabric cuts and deforms leucocratic melt
patches, suggesting that partial melting in the garnetiferous felsic gneiss occurred before or coeval with granulite-facies metamorphism. Along the western shore
of Ulvesund, the layering is folded by recumbent and
upright folds. Locally, these folds have amphibolitefacies axial surface fabrics.
The felsic gneiss within the Ulvesund Body has, in
general, been deformed and recrystallized to a higher
degree than in the Flatraket Body, commonly with
highly flattened and recrystallized megacrysts. On
246
Figure 7. For legend see facing page.
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
Hanekammen and north of Måløy, the megacrysts
show a L >> S fabric with aspect ratios up to 1 : 5 : 20.
Planar structures (layering and foliations) are flatlying or gently inclined for a significant part of the
body, sub-parallel to foliations outside the body. (Figs
4, 8d, e, f). Lineations within the Ulvesund body, however, are consistently north–northwest or south–southeast oriented. This is highly discordant to the lineations
outside the Ulvesund body (Figs 4, 8d, e, f).
All studied contacts of the Ulvesund Body show
evidence of intense deformation, with 10–50 m wide
amphibolite-facies mylonite and ultramylonite zones.
Highly non-cylindrical folds and sheath folds with
amphibolite-facies axial planar fabrics are locally
developed. Top-to-the-west shear sense indicators
occur both north and south of the body. South of the
body, the gneissic layering and amphibolite-facies foliation dip gently northwards, and east–west trending
late-Caledonian folds have a northward vergence (Figs
4, 9). North of the body, these structures are mirrored
with gently south-dipping gneissic layering and southward verging gently inclined folds. Structurally below
the body, that is, along Sørpollen, the gneissic layering
and fold axial surfaces are sub-horizontal or gently
east-dipping (Figs 1, 8f). The high-strain zones along
the margins of the Ulvesund Body can be interpreted
as compensating for the relatively low strain within the
body during late-Caledonian extension.
5. Summary of lithological, metamorphic and
structural observations
The structural, lithological and metamorphic observations presented above can be summarized as follows.
All rocks within the three bodies are of igneous origin.
247
However, igneous textures are preserved only where
little or no deformation occurred (e.g. in the core of
the Kråkenes gabbro and Pod 916 and in some dykes).
These domains were low strain zones during all subsequent tectonic and metamorphic events.
After magmatic crystallization, the rocks in the
Flatraket and Ulvesund bodies show evidence of transitions to three metamorphic assemblages: granulite-,
eclogite- and amphibolite-facies. In many places, these
transitions are only partial or absent. Only rarely (e.g.
in Pod 916 in the Flatraket stream section) do four different assemblages occur within the same rock body.
In all rock types, partial or complete retrogression to
amphibolite-facies assemblages is evident.
Granulite-facies metamorphism was associated
with deformation and possible partial melting in the
garnetiferous gneisses in the Flatraket stream section
and at Ulvesund. No deformation, however, was associated with the granulite-metamorphism that affected
the megacrystic gneiss. All felsic gneisses, where unaffected by subsequent eclogite or amphibolite-facies
metamorphism, show evidence for granulite-facies
equilibration. This suggests that the granulite-facies
metamorphism was widespread in all felsic gneisses of
the Flatraket and Ulvesund bodies, regardless of
deformation.
Granulite-facies metamorphism of mafic rocks was
less widespread, as indicated by the preservation of
igneous textures and assemblages in the Kråkenes
Gabbro. Well-equilibrated granulite-facies assemblages, however, occur in several mafic dykes and intermediate layers, especially in the Flatraket Body. Most
mafic rocks that carry a granulite-facies assemblage
are relatively undeformed (Table 1). In dioritic granulite, anorthositic granulite and the megacrystic gneiss
Figure 7. Photomicrographs. Abbreviations: Am = amphibole, Bt = biotite, Cpx = clinopyroxene, Gr = garnet, KF = Kfeldspar, Ol = olivine, Pl = plagioclase, Qz = quartz. (a) Igneous texture in Kråkenes Gabbro: euhedral plagioclase laths with
ophitic texture; olivine crystals are surrounded by a corona of radial, fibrous orthopyroxene (1) and a dark corona of fine garnet
(2) around it. Locally a hydrous reaction rim rich in amphiboles (3) occurs. Sample MK169, roadcut east of Kråkenes
Lighthouse, KP902848. Scale bar is 1 mm. Cross-polarized light. (b) Igneous relics in gabbroic Pod 916. Relic igneous texture
indicated by euhedral plagioclase. Garnet may have been formed by partial granulite-facies transition; fine aggregates of amphibole and biotite indicate partial retrogression and hydration at amphibolite-facies conditions. Sample MK423, Flatraket stream
section, LP030078. Scale bar is 1 mm. Cross-polarized light. (c) Granulite-facies relics in megacrystic gneiss. Coarse plagioclase
± quartz domains are separated by garnet coronas from mafic domains, now containing biotite, amphibole and relict pyroxene.
Sample MK40, 60 m east of quarry, Flatraket, LP033777. Scale bar is 1 mm. Cross-polarized light. (d) Granulite-facies assemblage in anorthositic gneiss, Flatraket. Clinopyroxene and plagioclase are separated by garnet coronas. Plagioclase has a granoblastic texture, although recrystallized in the top left corner. MK164, Flatraket stream section, LP029078. Scale bar is 1 mm.
Cross-polarized light. (e) Granulite-facies assemblage in mafic dyke. Assemblage: garnet + clinopyroxene + plagioclase ±
quartz. MK116, coastal outcrop along Nordpollen, east of quarry, Flatraket, LP03727780. Scale bar is 0.25 mm. Cross-polarized light. (f) Eclogitic meta-anorthosite with relict corona texture. Mylonitic, felsic eclogite-facies assemblage of quartz + kyanite + zoisite + phengite ± pseudomorphs after omphacite surround ferromagnesian microlithons with the eclogite mineralogy of
omphacite (+ quartz, phengite) enclosed in garnet. Sample AW 98, Flatraket stream section. Scale bar 1 mm. Plane-polarized
light. (g) Close-up of felsic eclogite-facies mylonite from AW98. Fine-grained (~ 100 µm) acicular zoisite (dark needles) and
kyanite (stumpy prisms) in a quartz matrix define a strong eclogite-facies fabric. Sample AW 98, Flatraket stream section. Scale
bar is 0.1 mm. Plane-polarized light. (h) Granulite-facies relics in fine-grained amphibolite-facies matrix, Ulvesund Body. Felsic
gneiss with coarse clinopyroxene, garnet and K-feldspar ± quartz as relics of a granulite-facies assemblage, in a matrix of recrystallized feldspar with very fine grained amphibole and biotite (1) or zoisite and mica (2). MK343, western shore of Ulvesund,
roadcut 300 m north of Måløy, KP97047432. Scale bar is 0.5 mm. Plane-polarized light.
248
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
Figure 8. Stereographic plots of structural data. Dots are foliation poles and open squares are lineations within the Flatraket
and Ulvesund bodies. Open circles are foliation poles and diamonds are lineations outside the bodies. Great circles are π planes
of planar data; large grey squares are poles to π planes.
within the Flatraket body, granulite-facies pressure–
temperature conditions were estimated at 9–11 kbar,
700–880 °C (A. Wain, unpub. Ph.D. thesis, Univ.
Oxford, 1998; Wain, Waters & Austrheim, unpub.
data).
Eclogitization of felsic gneisses appears to be rare
(less than 2 % by volume of the rock) and is only
observed where these rocks were deformed under
eclogite-facies conditions (Fig. 7f, g), or in felsic rocks
intimately associated with mafic eclogite. All felsic
eclogites possess a hydrous eclogite-facies assemblage
rich in phengite and zoisite (Table 1). The best evidence for eclogite-metamorphism of felsic rocks
occurs in the Flatraket stream section (Section 3.e).
Pressure–temperature conditions of eclogite-facies
assemblages in the shear zones were estimated at
20–23 kbar, 650–800 °C (A. Wain, unpub. Ph.D. thesis,
Univ. Oxford, 1998; Wain, Waters & Austrheim,
unpub. data).
About half of the mafic dykes and layers in the
Ulvesund and Flatraket bodies are affected by eclogitization (Table 1). However, only very localized eclogitization occurred in the Kråkenes Gabbro (Austrheim &
Engvik, 2000). Deformation accompanying eclogitization is not so obvious in mafic rocks, although tight
eclogite-facies folds occur locally. Most mafic eclogites
contain only accessory hydrous eclogite-facies minerals, although some contain zoisite and/or phengite as
main constituents of their high-pressure assemblage
(Table 1). In the dioritic pods and layers, eclogitization
is restricted to their margins, whereas the cores of
these bodies preserve igneous or granulite-facies
assemblages. Overall, eclogitization affected less than
5 %, by volume, of the three bodies.
Amphibolite-facies retrogression has affected all
rock types to varying degrees (Table 1). Pervasive
amphibolitization, however, is mostly restricted to
those rocks that have undergone substantial deformation; in these rocks, a medium- to coarse-grained
amphibolite-facies assemblage occurs. Granulitefacies relics, however, are commonly still present in
domains that have undergone substantial deformation. In these rocks, amphibolite-facies retrogression
and deformation resulted in a very fine matrix of
amphibolite-facies minerals, suggesting incomplete
recrystallization under amphibolite-facies conditions.
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
249
Figure 9. North–south cross-section across the Ulvesund Body. Two transects (A–A′ and B–B′) are superimposed; see Figure 4
for location. Note opposite vergence of folds in the granodioritic gneiss outside the body.
The eclogite- and granulite-facies structures are
highly discordant to the strongly developed lateCaledonian amphibolite-facies structures outside the
bodies. This indicates that the Kråkenes, Flatraket and
Ulvesund bodies were low strain zones during the lateCaledonian amphibolite-facies extensional deformation. These observations suggest that equilibration at
certain pressure–temperature conditions is highly
dependent on deformation and fluid availability and
that older metamorphic assemblages are easily
destroyed by later deformation and fluid-assisted
reaction kinetics.
6. Discussion
The observations of different igneous and metamorphic assemblages preserved in the low strain zones of
the Flatraket, Ulvesund and Kråkenes bodies raise the
following issues: (1) the temporal relationship between
granulite and eclogite-facies metamorphism; (2) the
metamorphic evolution of the bodies and whether or
not the occurrence of granulite-facies assemblages
imply metastable behaviour at high pressures; (3) the
causes of this metastable behaviour, the role of deformation and the importance of fluid infiltration; (4) the
relationship between the three bodies and the surrounding gneisses; (5) the protoliths of the Western
Gneiss Region as a whole and the role of the protolith
during orogenesis.
6.a. Temporal relationship of granulite and eclogite:
metamorphic evolution
The close spatial occurrence of granulite-facies and
eclogite-facies assemblages in the Flatraket and
Ulvesund bodies can be explained in different ways,
each of which would have important implications for
the tectonic evolution of the Western Gneiss Region.
Three possibilities can be envisaged: (1) the granulites
were isofacial to the eclogites, (2) the granulites pre-date
the eclogites or (3) the granulites post-date the eclogites
and were related to late-Caledonian exhumation.
If the granulites are isofacial with associated eclogites, as suggested elsewhere in the Western Gneiss
Region by Krogh (1980), bulk rock composition, fluid
composition or water activity would control the development of granulite (plagioclase-bearing) or eclogite
(plagioclase-free) assemblages. However, this possibility can be discounted, as eclogitization affected both
basic and felsic rocks. The presence of aqueous fluid
will lower the pressure of plagioclase-out reactions,
especially at high X An (Wayte et al. 1989). However,
the equilibrium pressures of eclogite-facies shear
zones are 20–23 kbar (at 650–800 °C) which is significantly higher than equilibrium conditions of the granulite-facies assemblages (9–11 kbar, 700–850 °C: A.
Wain, unpub. Ph.D. thesis, Univ. Oxford, 1998; Wain,
Waters & Austrheim, unpub. data). These calculations
are not dependent on fluid composition, and also indicate pressures that are significantly higher than plagioclase-out reactions for all plagioclase compositions
(Wayte et al. 1989). Therefore, granulite-facies assemblages would indicate metastability at high pressures if
they pre-dated eclogite-facies metamorphism.
Three lines of evidence suggest that granulite-facies
metamorphism pre-dated eclogite-facies metamorphism. Firstly, eclogite-facies shear zones in felsic rocks
deform or cross-cut granulite-facies rocks or structures.
Secondly, the mafic Pod 916 in the Flatraket stream
section shows all transitions from an igneous assemblage, via granulite and eclogite to amphibolite-facies
assemblages (Section 3.e). Granulite-facies metamorphism affected most of the pod whereas eclogitization
only occurs along the margin. Finally, several mafic
layers in which the bulk of the layer has a granulitefacies assemblage show very localized eclogitization in
thin, distinct zones. The reverse situation of a narrow
zone of granulite in a more pervasive eclogite is not
observed.
The preservation of widespread granulites during
250
subsequent eclogite-facies metamorphism implies a
high degree of metastability at eclogite-facies conditions within the bodies described here, because only
few rocks display eclogite-facies assemblages.
The three bodies described in this paper, therefore,
underwent the following overall evolution (Fig. 10):
intrusion, granulite-facies metamorphism, eclogitefacies metamorphism (peak-Caledonian) and finally
amphibolite-facies metamorphism (late-Caledonian).
The pressure–temperature estimates of the eclogiteand granulite-facies assemblages (see Section 5) are
based on the work of A. Wain (unpub. Ph.D. thesis,
Univ. Oxford, 1998; Wain, Waters & Austrheim,
unpub. data). The retrograde pressure–temperature
path of the Flatraket body and the surrounding
gneisses remained within the kyanite field (Cotkin,
Valley & Essene, 1988; Cuthbert, 1991; M. Dransfield,
unpub. Ph.D. thesis, Univ. Oxford, 1994; A. Wain,
unpub. Ph.D. thesis, Univ. Oxford, 1998). The eclogite-facies conditions are similar to those estimated for
non-coesite-bearing eclogites embedded in the surrounding gneisses (Wain, 1997), suggesting that the
pressure–temperature evolution of the three bodies
was similar to at least some (that is, probably not the
coesite-bearing eclogites) of the rocks outside the bodies (but see discussion in Section 6.e). The Caledonian
pressure–temperature evolution of the three bodies
was thus roughly similar to the pressure–temperature
Figure 10. Intrusive, metamorphic and P–T–t evolution of
the Flatraket, Ulvesund and Kråkenes bodies. Pressure–temperature conditions are approximate. Age of the Flatraket
megacrystic gneiss after Lappin, Pidgeon & van Breemen
(1979); age of the Kråkenes Gabbro after E. W. Mearns
(unpub. Ph.D. thesis Univ. Aberdeen, 1984, reported in
Kullerud, Tørudbakken & Ilebekk, 1986).
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
evolution proposed for the Western Gneiss Region as a
whole (see, for example, Dunn & Medaris, 1989; Wain,
1997). Nevertheless, different rocks clearly responded
differently to the same changes in pressure and temperature and the extent of reaction is strongly linked
to deformation and fluid availability (see next section).
Other uncertainties in the history as depicted in
Figure 10 include the pressure–temperature path
between intrusion and granulite-facies metamorphism
and between granulite- and eclogite-facies metamorphism, the intrusion ages of the felsic and mafic rocks
and the age and nature of the granulite-facies metamorphism. Further work is required to solve these
problems.
6.b. Metastability at high-pressure: the role of fluid and
deformation
Experiments and natural examples suggest that survival of relatively dry plagioclase-bearing assemblages
outside their stability field (in the study area at pressures above 20 kbar) is associated with sluggish reaction kinetics caused by limited availability of a free,
hydrous fluid phase (Ahrens & Schubert, 1975; Rubie,
1986; Wayte et al. 1989; Austrheim, Erambert &
Engvik, 1997). As a result, the availability of water
exerts a strong control on the extent of plagioclase
breakdown towards high-pressure assemblages. The
role of fluids in metamorphism is threefold: fluids can
be components in metamorphic reactions, they can be
catalysts during reactions and different fluid composition may affect the stability of certain mineral assemblages (Austrheim, Erambert & Engvik, 1997). As we
have no data on fluid composition we will restrict ourselves to the first two points (although the scarcity of
calc-silicate minerals in all assemblages suggest low
X CO , so that the role of different fluid compositions
2
may not have been significant).
Generally, mafic eclogites contain only minor
hydrous minerals. Felsic eclogites, on the other hand,
contain significant amounts of hydrous minerals such
as zoisite and phengite (Koons & Thompson, 1985;
Austrheim & Griffin, 1985; Black, Brothers &
Yokoyama, 1988; Bousquet et al. 1997). This suggests
that incomplete equilibration of mafic eclogites is
mainly the result of lack of fluids as a catalyst,
whereas incomplete equilibration of felsic eclogites is
also the result of lack of fluids as a component. Thus,
for eclogitization to occur in dry rocks, fluid infiltration is required for all compositions, but the amount
of water required may be greater for felsic compositions than for mafic compositions.
Within the Ulvesund and Flatraket bodies, 30–40 %
of the mafic rocks were eclogitized whereas only 2 % of
the felsic rocks did so. In the felsic and intermediate
eclogite-facies rocks, hydrous assemblages (those containing substantial phengite and zoisite) dominate. The
eclogitized mafic rocks, on the other hand, contain
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
both hydrous and (near)-anhydrous assemblages.
These observations can be explained by the fact that
felsic granulite requires more fluid infiltration to equilibrate than mafic granulite and dolerite. Eclogitization
of felsic rocks is focused in shear zones and the contacts between mafic and felsic rocks; these settings
probably provided easy pathways for fluid access.
The above analysis, however, does not explain the
extensive metastable behaviour of the Kråkenes
Gabbro. Clearly, processes controlling eclogitization
are more complex than sketched above. Fluid infiltration into dry rocks not only catalyses and controls
metamorphic reactions, but also enhances deformation (see, for example, Carter et al. 1990). Deformation
enhances both fluid infiltration and metamorphic
reactions (see, for example, Brodie & Rutter, 1985). If
metamorphic reactions result in reaction softening
(‘reaction-enhanced ductility’) and involve a negative
volume change, it enhances both deformation and
fluid infiltration (Klaper, 1990; Rubie, 1990).
Consequently, fluid infiltration, deformation and
metamorphic reactions can be linked into a vicious circle of mutual enhancement (Austrheim, Erambert &
Engvik, 1997). This mutual enhancement is expected
to result in strongly localized zones of fluid infiltration, strain and metamorphic reaction (Carter et al.
1990), leaving large domains untouched. This implies
that, to explain metastable behaviour, (lack of) deformation can be as important as (lack of) fluid. One possible explanation of the metastable behaviour of the
Kråkenes Gabbro is that it was a large, uniform and
mechanically competent body that escaped virtually
all Precambrian as well as Caledonian deformation.
Fluids were unable to pervasively infiltrate into the
body even though fluids may have been abundantly
available in the direct vicinity (see next section).
Fluid infiltration is enhanced by deformation and it
is interesting that all felsic eclogites in our study have
well-developed fabrics, whereas mafic eclogites occur
both with and without eclogite-facies fabrics. Eclogitefacies re-equilibrium of felsic granulite was, apparently, only possible when fluid infiltration was
combined with deformation.
Concluding, this study confirms that metamorphic
reactions are not only a function of pressure, temperature and rock compositions but are also highly dependent on fluid availability and deformation.
6.c. Relationship with amphibolite-facies gneisses: fluid
sources
The data presented here show that, within the three
bodies, Caledonian eclogitization affected only about
5 % of the rocks, whereas 95 % of the rock retained their
pre-Caledonian assemblages under high-pressure conditions. Whilst these bodies display similar features of
eclogitization and amphibolitization to the granulites of
the Bergen Arcs (Austrheim & Griffin, 1985), rocks
251
external to these bodies are quite different in character.
In large parts of the northwestern part of the Western
Gneiss Region, late-Caledonian amphibolite-facies
assemblages dominate with about 5–10 % Caledonian
peak-pressure assemblages preserved, mainly as mafic
eclogite pods, but with very little evidence of PreCaledonian assemblages (Bryhni & Andréasson, 1985;
Andersen, Osmundsen & Jolivet, 1994; Krabbendam &
Wain, 1997). Two (end-member) scenarios can be envisaged to explain these differences:
(1) The protolith of the Western Gneiss Region
gneisses comprised relatively anhydrous igneous and
granulite-facies rocks, but was somehow hydrated during orogenesis. Hydration was pervasive in ~ 90 % of
the terrane, except for small remnants which behaved
metastably and retained pre-Caledonian granulitefacies assemblages. This scenario requires large volumes of externally derived fluid.
(2) Before the Caledonian Orogeny, the Western
Gneiss Region was composed of both hydrous amphibolite-facies domains and domains comprising anhydrous granulites and igneous rocks. This is a common
feature of many exposed high-grade gneiss terranes
(see, for example, Fountain & Salisbury, 1981). During
the Caledonian, the hydrous amphibolite-facies
domains may have experienced extensive eclogitization. The anhydrous igneous and granulite-facies
rocks domains behaved largely metastably, with only
very localized eclogitization. During exhumation, felsic eclogite (formed in the hydrous domains) readily
retrogressed (as in the model of Heinrich, 1982), but
mafic eclogite and granulite-facies domains did not
equilibrate at amphibolite-facies conditions. Fluids
migrating within the Western Gneiss Region resulted
in local eclogitization of granulitic and gabbroic bodies during burial and in hydration of the margins of
many mafic eclogites during exhumation and of the
margins of the Kråkenes Gabbro. However, no largescale, pervasive fluid infiltration into the Western
Gneiss Region is required.
We favour the second explanation for several reasons.
Firstly, mineral inclusions (mainly hornblende and epidote, also plagioclase) in prograde zoned garnets in
eclogites suggest pre-Caledonian amphibolite-facies
equilibration (Bryhni & Griffin, 1971; Krogh, 1982; A.
Wain, unpub. Ph.D. thesis, Univ. Oxford, 1998; Engvik
& Andersen, in press). Secondly, mafic rocks outside the
three bodies show a high degree of eclogitization
(> 90 %), whereas the mafic pods and dykes within the
three bodies show a medium degree (30–40 %) of eclogitization and virtually no reaction in the Kråkenes
Gabbro. In the Nordfjord–Stadlandet area, no igneous
or granulitic relics have been found in more than 100
pods studied (see also Smith, 1988; M. Dransfield,
unpub. Ph.D. thesis, Univ. Oxford, 1994; A. Wain,
unpub. Ph.D. thesis, Univ. Oxford, 1998). This suggests
that eclogitization was more pervasive outside the three
bodies. Thirdly, pervasive fluid infiltration on a vast
252
scale (affecting possibly as much as 250 000 km3 rock) is
unlikely, even if a source can be envisaged. Fluid infiltration in conjunction with metamorphic transitions
has a great tendency to localize itself (see Section 6.d)
which can explain the features observed in the Bergen
Arcs and in the three studied bodies here, but cannot
explain the field relationships outside the three bodies.
Here it is worth noting that stable isotope data from the
Dabie Shan ultrahigh-pressure terrane in China show
that no stable isotope equilibration occurred across
lithological boundaries, suggesting that no large-scale
fluid infiltration occurred, neither during burial, nor
during exhumation (Rumble, 1998). Finally, in all three
bodies, amphibolite-facies retrogression is more widespread than eclogitization, even though the former may
require more fluid. This suggests that fluid infiltration
into the three bodies was more widespread during
exhumation than during burial or at peak-pressure conditions. This is corroborated by a fluid inclusion study
in gneisses further north that suggest an influx of aqueous fluid during exhumation and retrograde metamorphism (Larsen, Eide & Burke, 1998). Increased activity
of H2O-rich fluids during exhumation is satisfactorily
explained by dehydration of felsic or pelitic eclogites
(Heinrich, 1982). An external source of fluids, such as a
subducting oceanic slab, as forwarded by Larsen, Eide
& Burke (1998), may have been operating during burial
but is unlikely to have been present during exhumation.
6.d. Granulite-facies relics elsewhere in the Western Gneiss
Region
If our scenario of both hydrous and anhydrous
domains prior to the Caledonian is correct, it is important for the tectonic and metamorphic evolution of the
Western Gneiss Region as a whole to constrain the ratio
of these two domains. To date, incomplete eclogitization of rocks in the Western Gneiss Region is described
in mafic rocks in the north by Mørk (1985, 1986) and on
Bårdsholmen in Sunnfjord, where small occurrences of
felsic granulite occur that experienced partial eclogitization (Austrheim & Engvik, 1997; Engvik, Austrheim &
Andersen, 2000). Further evidence of partial eclogitization may be found in the following places. Abundant
megacrystic rapakivi-like augen gneiss occurs north
of Moldefjord (150 km northwest of the study area)
and contains garnet coronas and rare symplectitic
clinopyroxene. Geochemically, these rocks are very similar to the megacrystic gneiss at Flatraket (Harvey,
1983; Carswell & Harvey, 1985). Eclogites occur within
these gneisses with very similar field relations to those
observed in Flatraket (Griffin & Carswell, 1985). These
features suggest that metastable behaviour of these
augen gneisses is a real possibility. Well-preserved felsic
granulites which are partially eclogitized occur in Gulen
(unpublished observation by T.B. Andersen and H.
Austrheim) and near Lavik (Skår, pers. comm., 1998),
some 100 km south of the study area. Two kilometre-
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
size bodies of rock near Hjørundfjord (70 km northwest of the study area) have been mapped as ‘charnockite’ (Bryhni, 1991). The undeformed state of these
bodies may indicate that these bodies also behaved
metastably under eclogite-facies conditions. Further
possible granulite-facies relics in the Western Gneiss
Region are briefly mentioned in Griffin et al. (1985).
Further north in the Western Gneiss Region, lateCaledonian decompression occurred at higher temperatures and resulted in granulite- rather than
amphibolite-facies metamorphism (see, for example,
Dunn & Medaris, 1989; Straume & Austrheim, 1999).
This implies that not all granulite-facies rocks in the
Western Gneiss Region are necessarily metastable PreCaledonian relics.
6.e. Implications for the tectonic and metamorphic evolution
of the Western Gneiss Region
In conjunction with the findings of Engvik, Austrheim
& Andersen (2000), this study shows that at least certain parts of the Western Gneiss Region were at granulite-facies prior to the Caledonian and behaved
metastably during the Caledonian high-pressure
event. The tectono-metamorphic history of the three
bodies, which may be applicable to wider parts of the
Western Gneiss Region, is summarized in Figure 10.
A problem considering the pressure–temperature
evolution of the three bodies is the proximity of
coesite-bearing ultrahigh-pressure eclogites. Two
coesite-bearing eclogites indicating P ≥ 28 kbar occur
in the immediate vicinity of the Flatraket Body (Fig.
1), one less than 200 m structurally below the
megacrystic gneiss and another one structurally above
on the east coast of Nordpollen (Wain, 1997). Wain
(1997) explained the present proximity of ultrahighpressure and high-pressure rocks by tectonic juxtaposition, although a kinetic explanation cannot be ruled
out. The absence of amphibolite-facies high strain
zones between the coesite-bearing eclogites and the
Flatraket body and other non-coesite-bearing eclogites (also in areas of good exposure) indicates that any
juxtaposition must have occurred prior to amphibolite-facies conditions, that is, in an early phase of
exhumation (Krabbendam & Wain, 1997; A. Wain,
unpub. Ph.D. thesis, Univ. Oxford, 1998). Together
with the fact that the Western Gneiss Region may have
been composed of quite different lithologies with different Pre-Caledonian metamorphic grade, this suggests that relatively large tectonic movements have
occurred within the Western Gneiss Region. The lateCaledonian extensional structures and pervasive
amphibolite-facies re-crystallization largely obliterated the structural evidence of these movements.
A further implication of this study concerns density
changes and their effect on the mechanics of collisional tectonics. Dewey, Ryan & Anderson (1993) presented a model whereby extensive eclogitization in the
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
Western Gneiss Region resulted in increasing density
and decreasing buoyancy of the entire Caledonian
orogenic crust. They proposed that by this mechanism
a crustal thickness of 120 km could be achieved by
collisional tectonics. The validity of this model is
highly dependent on the actual density changes during
the Caledonian Orogeny. If metastability within the
Western Gneiss Region proves to be widespread
(rather than restricted to small domains) significant
density changes may not be plausible.
7. Conclusions
The Flatraket, Ulvesund and Kråkenes bodies occur
within the mixed high-pressure–ultrahigh-pressure
zone within the Western Gneiss Region and are
surrounded by quartzo-feldspathic gneisses which
experienced late-Caledonian strain and equilibration.
The Flatraket, Ulvesund and Kråkenes bodies preserve igneous and granulite-facies metamorphic
assemblages, textures and structures that pre-date the
Caledonian Orogeny. The three bodies behaved mainly
as low strain zones during the Caledonian. The three
bodies show very localized eclogitization in felsic and
mafic rocks eclogites (about 5 %), commonly related to
shear zones or lithological contacts, and have experienced high-pressure conditions (20–23 kbar), possibly
up to 24–29 kbar (Austrheim & Engvik, 2000). The
preservation of the felsic and mafic igneous and granulite-facies assemblages in these bodies, therefore,
indicates widespread (~ 95 %) metastability at pressures that are higher than other areas in Norway where
metastability has been demonstrated. This study confirms that metamorphic reactions are not only a function of pressure, temperature and rock compositions
but are also highly dependent on fluid availability and
deformation, and metastability in the studied rocks
occurred where there was a lack of both fluid and
deformation. The contrasting behaviour of the three
bodies with respect to the surrounding gneisses can be
explained by difference in metamorphic grade of the
respective protoliths and we suggest that, before the
Caledonian Orogeny, the Western Gneiss Region comprised both hydrous amphibolite-facies domains and
anhydrous igneous and granulite-facies domains.
Acknowledgements. Håkon Austrheim, John F. Dewey and
Dave Waters are thanked for many discussions on eclogites
and eclogitization in West Norway. Ian Cartwright is
thanked for a critical reading of the manuscript. Critical
reviews by Håkon Austrheim and an anonymous reviewer
improved the manuscript. NERC funding for MK and AW
and funding from NFR and Norsk Agip (TBA) is gratefully
acknowledged.
Note added in proof
Alice Wain was tragically killed in a car accident on May
3rd, 2000, while carrying out fieldwork in Scotland. We
mourn her loss. Alice was a dedicated and enthusiastic geologist. She loved the rocks and scenery of West Norway. We
dedicate this paper to her memory. MK & TBA.
253
References
AHRENS, T. J. & SCHUBERT, G. 1975. Gabbro-eclogite reaction rate and its geophysical significance. Reviews of
Geophysics and Space Physics 13, 383–400.
ANDERSEN, T. B. 1998. Extensional tectonics in the
Caledonides of southern Norway, an overview.
Tectonophysics 285, 333–51.
ANDERSEN, T. B. & JAMTVEIT, B. 1990. Uplift of deep crust
during orogenic extensional collapse: a model based on
field studies in the Sogn-Sunnfjord region of Western
Norway. Tectonics 9, 1097–1111.
ANDERSEN, T. B., OSMUNDSEN, P. T. & JOLIVET, L. 1994.
Deep crustal fabrics and a model for extensional collapse of the southwest Norwegian Caledonides. Journal
of Structural Geology 16, 1191–1203.
AUSTRHEIM, H. 1987. Eclogitization of lower crustal granulites by fluid migration through shear zones. Earth and
Planetary Science Letters 81, 221–32.
AUSTRHEIM, H. & ENGVIK, A. K. 1997. Fluid transport,
deformation and metamorphism at depth in a collision
zone. In Fluid flow and transport in rocks (eds B.
Jamtveit and B. W. D. Yardley), pp. 123–38. London:
Chapman & Hall.
AUSTRHEIM, H. & ENGVIK, A. K. 2000. Fluid driven highand ultrahigh pressure metamorphism, Caledonides of
W. Norway. Geonytt 1–2000, 35–6.
AUSTRHEIM, H., ERAMBERT, M. & ENGVIK, A. K. 1997.
Processing of crust in the root of the Caledonian continental collision zone: the role of eclogitisation.
Tectonophysics 273, 129–53.
AUSTRHEIM, H. & GRIFFIN, W. L. 1985. Shear deformation
and eclogite formation within the granulite-facies
anorthosites of the Bergen Arcs, Western Norway.
Chemical Geology 50, 267–81.
BERRY, H. N., LUX, D. R., ANDRESEN, A. & ANDERSEN, T.
B. 1994. Argon 40–39 dating of rapidly uplifted high
pressure rocks during late-orogenic extension in southwestern Norway. Geological Society of America:
Abstracts with Programs 25, 6, A–477.
BLACK, P. M., BROTHERS, R. N. & YOKOYAMA, K. 1988.
Mineral parageneses in eclogite-facies meta-acidites in
northern New Caledonia. In Eclogites and eclogitefacies rocks (ed. D. C. Smith), pp. 271–89. Amsterdam:
Elsevier.
BOUSQUET, R., GOFFÉ, B., HENRY, P., LE PICHON, X. &
CHOPIN, C. 1997. Kinematic, thermal and petrological
model of the Central Alps: Lepontine metamorphism
in the upper crust and eclogitisation of the lower crust.
Tectonophysics 231, 105–27.
BRODIE, K. H. & RUTTER, E. H. 1985. On the relationship
between deformation and metamorphism with special
reference to the behavior of basic rocks. In
Metamorphic reactions. Kinetics, textures and deformation (eds A. B. Thompson and D. C. Rubie), pp. 138–79.
New York: Springer Verlag.
BRYHNI, I. 1966. Reconnaissance studies of gneisses, ultrabasites, eclogites and anorthosites in Outer Nordfjord,
Western Norway. Norges geologiske undersøkelse
Bulletin 241, 1–68.
BRYHNI, I. 1989. Status of the supracrustal rocks in the
Western Gneiss Region, S Norway. In The Caledonide
Geology of Scandinavia (ed. R. A. Gayer), pp. 221–8.
London: Graham & Trotman.
BRYHNI, I. 1991. Sykkylven. Berggrunnsgeologisk kart 1219
IV, scale 1:50 000 (geological map). Norges geologiske
undersøkelse.
254
BRYHNI, I. & ANDRÉASSON, P. G. 1985. Metamorphism in the
Scandinavian Caledonides. In The Caledonide Orogen –
Scandinavia and related areas (eds D. G. Gee and B. A.
Sturt), pp. 763–82. Chichester: John Wiley & Sons.
BRYHNI, I. & GRIFFIN, W. L. 1971. Zoning in eclogite garnets from Nordfjord, west Norway. Contributions to
Mineralogy and Petrology 32, 112–25.
BUTLER, R. W. H. 1986. Thrust tectonics, deep structure and
crustal subduction in the Alps and Himalayas. Journal
of the Geological Society, London 143, 857–73.
CARSWELL, D. A. & HARVEY, M. A. 1985. The intrusive history and tectonometamorphic evolution of the Basal
Gneiss Complex in the Moldefjord area, west Norway.
In The Caledonide Orogen – Scandinavia and related
areas (eds D. G. Gee and B. A. Sturt), pp. 843–57.
Chichester: John Wiley & Sons.
CARTER, N. L., KRONENBERG, A. K., ROSS, J. V. &
WILTSCHKO, D. V. 1990. Control of fluids on deformation of rocks. In Deformation mechanisms, rheology and
tectonics (eds R. J. Knipe and E. H. Rutter), pp. 1–13.
Geological Society of London, Special Publication no.
54.
COLEMAN, R. G. & WANG, X. 1995. Overview of the geology
and tectonics of UHPM. In Ultrahigh pressure metamorphism (eds R. G. Coleman and X. Wang), pp. 1–32.
Cambridge University Press.
COTKIN, S. J. 1997. Igneous and metamorphic petrology of
the eclogitic Seljeneset meta-anorthosite and related
jotunites, Western Gneiss Region, Norway. Lithos 40,
1–30.
COTKIN, S. J., VALLEY, J. W. & ESSENE, E. J. 1988. Petrology
of a margarite-bearing meta-anorthosite from
Seljeneset, Nordfjord, western Norway: Implications
for the P–T history of the Western Gneiss Region during Caledonian uplift. Lithos 21, 117–28.
CUTHBERT, S. J. 1991. Evolution of the Devonian Hornelen
Basin, Western Norway: new constraints from petrological studies of metamorphic clasts. In Developments in
sedimentary provenance studies (eds A. C. Morton, S. P.
Todd and P. D. W. Haughton), pp. 343–60. Geological
Society of London, Special Publication no. 57.
CUTHBERT, S. J. & CARSWELL, D. A. 1990. Formation and
exhumation of medium-temperature eclogites in the
Scandinavian Caledonides. In Eclogite facies rocks (eds
D. A. Carswell), pp. 180–203. Glasgow: Blackie.
CUTHBERT, S. J., HARVEY, M. A. & CARSWELL, D. A. 1983.
A tectonic model for the metamorphic evolution of the
Basal Gneiss Complex, Western South Norway. Journal
of Metamorphic Geology 1, 63–90.
DEWEY, J. F., RYAN, P. D. & ANDERSEN, T. B. 1993. Orogenic
uplift and collapse, crustal thickness, fabrics and metamorphic phase changes: the role of eclogites. In
Magmatic processes and plate tectonics (eds H. M.
Prichard, T. Alabaster, N. B. W. Harris and C. R.
Neary), pp. 325–43. Geological Society of London,
Special Publication no. 76.
DUNN, S. R. & MEDARIS, L. G. 1989. Retrograded eclogites
in the Western Gneiss region, Norway, and thermal evolution of a portion of the Scandinavian Caledonides.
Lithos 22, 229–45.
ENGVIK, A. K. & ANDERSEN, T. B. In press. Evolution of
Caledonian deformation fabrics under eclogite and
amphibolite facies at Vårdalsneset, Western Gneiss
Region, Norway. Journal of Metamorphic Geology.
ENGVIK, A. K., AUSTRHEIM, H. & ANDERSEN, T. B. 2000.
Structural, mineralogical and petrophysical effects on
M . K R A B B E N DA M , A . WA I N & T. B . A N D E R S E N
deep crustal rocks of fluid-limited polymetamorphism,
Western Gneiss Region, Norway. Journal of the
Geological Society of London 157, 121–34.
FOSSEN, H. & DALLMEYER, R. D. 1998. 40Ar/39Ar muscovite dates from the nappe region of southwestern
Norway: dating extensional deformation in the
Scandinavian Caledonides. Tectonophysics 285, 119–33.
FOUNTAIN, D. M. & SALISBURY, M. H. 1981. Exposed crosssections through the continental crust: implications for
crustal structure, petrology and evolution. Earth and
Planetary Science Letters 56, 263–77.
GEBAUER, D., LAPPIN, M. A., GRUNENFELDER, M. &
WYTTENBACH, A. 1985. The age and origin of some
Norwegian eclogites: a U–Pb zircon and REE study.
Chemical Geology 52, 227–47.
GRIFFIN, W. L., AUSTRHEIM, H., BRASTAD, K., BRYHNI, I.,
KRILL, A. G., KROGH, E. J., MØRK, M. B. E., QVALE, H.
& TØRUDBAKKEN, B. 1985. High-pressure metamorphism in the Scandinavian Caledonides. In The
Caledonide Orogen – Scandinavia and related areas (eds
D. G. Gee and B. A. Sturt), pp. 783–802. Chichester:
John Wiley & Sons.
GRIFFIN, W. L. & BRUECKNER, H. K. 1980. Caledonian
Sm–Nd ages and a crustal origin for Norwegian eclogites. Nature 285, 319–21.
GRIFFIN, W. L. & BRUECKNER, H. K. 1985. REE, Rb–Sr and
Sm–Nd studies of Norwegian eclogites. Chemical
Geology 52, 249–71.
GRIFFIN, W. L. & CARSWELL, D. A. 1985. In situ metamorphism of Norwegian eclogites: an example. In The
Caledonide Orogen – Scandinavia and related areas (eds
D. G. Gee and B. A. Sturt), pp. 813–22. Chichester:
John Wiley & Sons.
GRIFFIN, W. L. & HEIER, K. S. 1973. Petrological implications of some corona structures. Lithos 6, 315–35.
HARVEY, M. A. 1983. A geochemical and Rb–Sr study of the
Proterozoic augen orthogneisses on the Molde peninsula, west Norway. Lithos 16, 325–38.
HEINRICH, C. A. 1982. Kyanite-eclogite to amphibolite
facies evolution of hydrous mafic and peltitic rocks,
Adula Nappe, Central Alps. Contributions to
Mineralogy and Petrology 81, 30–8.
KLAPER, E. M. 1990. Reaction-enhanced formation of
eclogite-facies shear zones in granulite-facies
anorthosites. In Deformation mechanisms, rheology and
tectonics (eds R. J. Knipe and E. H. Rutter), pp. 167–74.
Geological Society of London, Special Publication no.
54.
KOONS, P. O., RUBIE, D. C. & FRUEH-GREEN, G. 1987. The
effects of disequilibrium and deformation on the mineralogical evolution of quartz diorite during metamorphism in the eclogite facies. Journal of Petrology 28,
679–700.
KOONS, P. O. & THOMPSON, A. B. 1985. Non-mafic rocks in
the greenschist, blueschist and eclogite facies. Chemical
Geology 50, 3–30.
KRABBENDAM, M. & DEWEY, J. F. 1998. Exhumation of
UHP rocks by transtension in the Western Gneiss
Region, Scandinavian Caledonides. In Continental
transpressional and transtensional tectonics (eds R. E.
Holdsworth, R. A. Strachan and J. F. Dewey), pp.
159–81. Geological Society of London, Special
Publication no. 135.
KRABBENDAM, M. & WAIN, A. 1997. Late-Caledonian structures, differential retrogression and structural position
of (ultra)high-pressure rocks in the Nordfjord-
Pre-Caledonian granulite and gabbro enclaves in the Western Gneiss Region, Norway
Stadlandet area, Western Gneiss Region. Norges geologiske undersøkelse Bulletin 432, 127–39.
KRETZ, R. 1983. Symbols for rock-forming minerals.
American Mineralogist 68, 277–9.
KROGH, E. J. 1980. Compatible P–T conditions for eclogites
and surrounding gneisses in the Kristiansund area,
Western Norway. Contributions to Mineralogy and
Petrology 75, 387–93.
KROGH, E. J. 1982. Metamorphic evolution of Norwegian
country-rock eclogites, as deduced from mineral inclusions and compositional zoning in garnets. Lithos 15,
305–21.
KULLERUD, L., TØRUDBAKKEN, B. O. & ILEBEKK, S. 1986. A
compilation of radiometric age determinations from
the Western Gneiss Region, South Norway. Norges
geologiske undersøkelse Bulletin 406, 17–42.
LAPPIN, M. A. 1966. The field relationships of basic and
ultrabasic masses in the basal gneiss complex of
Stadlandet and Almklovdalen, Nordfjord, southwestern Norway. Norsk Geologisk Tidsskrift 46, 439–96.
LAPPIN, M. A., PIDGEON, R. T. & VAN BREEMEN, O. 1979.
Geochronology of basal gneisses and mangerite syenite
of Stadlandet, west Norway. Norsk Geologisk Tidsskrift
59, 161–81.
LARSEN, R. B., EIDE, E. A. & BURKE, E. A. J. 1998.
Evolution of metamorphic volatiles during exhumation
of microdiamond-bearing granulites in the Western
gneiss Region, Norway. Contributions to Mineralogy
and Petrology 133, 106–21.
LAUBSCHER, H. 1990. The problem of the Moho in the Alps.
Tectonophysics 182, 9–20.
LE PICHON, X., HENRY, P. & GOFFÉ, B. 1997. Uplift of Tibet:
from eclogites to granulites – implications for the Andean
Plateau and the Variscan belt. Tectonophysics 273, 57–76.
MØRK, M. B. E. 1985. A gabbro to eclogite transition on
Flemsøy, Sunnmøre, western Norway. Chemical
Geology 50, 283–310.
MØRK, M. B. E. 1986. Coronite and eclogite formation in
olivine gabbro (Western Norway): reaction paths and
garnet zoning. Mineralogical Magazine 50, 417–26.
MØRK, M. B. E. & MEARNS, E. W. 1985. Sm–Nd isotopic
systematics of a gabbro–eclogite transition. Lithos 19,
255–67.
255
RUBIE, D. C. 1986. The catalysis of mineral reactions by
water and restrictions on the presence of aqueous fluid
during metamorphism. Mineralogical Magazine 50,
399–415.
RUBIE, D. C. 1990. Mechanisms of reaction-enhanced
deformability in minerals and rocks. In Deformation
processes in minerals, ceramics and rocks (eds D. J.
Barber and P. G. Meredith), pp. 262–95. London:
Unwin Hyman.
RUMBLE, D. 1998. Stable isotope geochemistry of ultrahighpressure rocks. In When Continents Collide:
Geodynamics and Geochemistry of Ultrahigh-Pressure
Rocks (eds B. R. Hacker and J. G. Liou), pp. 241–59.
Dordrecht: Kluwer Academic Publishers.
SMITH, D. C. 1984. Coesite in clinopyroxene in the
Caledonides and its implication for geodynamics.
Nature 310, 641–4.
SMITH, D. C. 1988. A review of the peculiar mineralogy of
the “norwegian coesite-eclogite Province”, with crystalchemical, petrological, geochemical and geodynamical
notes and an extensive bibliography. In Eclogites and
eclogite-facies rocks (ed. D. C. Smith), pp. 1–206.
Amsterdam: Elsevier.
STRAUME, A. K. & AUSTRHEIM, H. 1999. Importance of
fracturing during retro-metamorphism of eclogites.
Journal of Metamorphic Geology 17, 637–52.
TUCKER, R. D., KROGH, T. E. & RÅHEIM, A. 1990.
Proterozoic evolution and age-province boundaries in
the central part of the Western Gneiss region, Norway;
results of U–Pb dating of accessory minerals from
Trondheimsfjord to Geiranger. In Mid-Proterozoic
Laurentia-Baltica (eds C. F. Gower, T. Rivers and B.
Ryan), pp. 149–73. Geological Association of Canada,
Special Paper no. 38.
WAIN, A. 1997. New coesite-eclogite occurrences in
western Norway: the nature of an ultrahigh pressure
province in the Western Gneiss Region. Geology 25,
927–30.
WAYTE, G. J., WORDEN, R. H., RUBIE, D. C. & DROOP, G. T.
R. 1989. A TEM study of disequilibrium plagioclase
breakdown at high pressure: the role of infiltrating
fluid. Contributions to Mineralogy and Petrology 101,
426–37.