Journal of the Geological Society, London, Vol. 157, 2000, pp. 121–134. Printed in Great Britain.
Structural, mineralogical and petrophysical effects on deep crustal rocks of
fluid-limited polymetamorphism, Western Gneiss Region, Norway
ANE K. ENGVIK 1 , HÅKON AUSTRHEIM 1 & TORGEIR B. ANDERSEN 2
Mineralogical–Geological Museum, University of Oslo, Sarsgate 1, N-0562 Oslo, Norway
(e-mail: ane.engvik@toyen.uio.no; hakon.austrheim@toyen.uio.no)
2
Department of Geology, University of Oslo, PB. 1047 Blindern, N-0316 Oslo, Norway
(e-mail: t.b.andersen@geologi.uio.no)
1
Abstract: The Proterozoic banded granulite facies complex of Bårdsholmen, Western Gneiss Region,
Norway (T=815–845C) was locally transformed to eclogite (T=455–510C, P>12 kbar) and amphibolite
facies rocks (T=460C) during the Caledonian continental collision. The granulite complex consists of
mafic two-pyroxene granulite and leucocratic orthopyroxene+garnet-bearing layers alternating on a scale
from 1 cm to 10 m. The granulite facies rocks change to eclogite facies rocks over centimetre-scale
distances along well defined fluid-infiltration fronts. The mafic granulite was transformed to
omphacite+garnet-rich eclogites and the leucocratic rocks were converted to quartz+phengite-rich
assemblages with minor garnet and local omphacite. Melange-like lithologies consisting of mafic lenses of
eclogite surrounded by felsic material represent an advanced stage in the process of converting deep crust
to eclogite facies. During amphibolitization this melange-like lithology evolves to a rock where
amphibolite lenses and layers are surrounded by granitoid gneiss, a lithology typical of the Western Gneiss
Region. The Bårdsholmen locality illustrates the profound control exerted by fluids on the timing of
metamorphism, the structural make up and petrophysical properties such as density and rheology of
crustal root zones. Fluid-induced metamorphism will therefore exert control on the attributes of orogenic
belts such as topography and Moho depth and influence the dynamics of collision zones by controlling the
time of orogenic collapse and the buoyancy of the subducted crust. We suggest that orogens may develop
differently depending on the fluid budget.
Keywords: Western Gneiss Region, eclogite, fluid phase, density, rheology.
In modern-day continental collision zones the Moho deepens
to more than 50–75 km under the Alps and the Himalayas
respectively. The P–T conditions prevailing in such root zones
are within the stability field of eclogite. Formation of eclogite
causes a reduction or the complete disappearance of feldspars
leading to drastic changes in rock properties such as density
and rheology. In response to heating, exhumation or changing
fluid composition and availability, eclogite-facies rocks formed
in crustal root zones may react to form feldspar-bearing
granulite- or amphibolite-facies rocks, resulting in a reduction
in density. These mineralogical changes will influence the
geophysical signature of the lowermost crust. Because our
conception of the deep crust in modern mountain belts is based
largely on interpretation of geophysical signatures, it is important to describe the effect of metamorphism on petrophysical
properties in exposed deep-crustal provinces.
Geodynamic models for collision orogens have highlighted
the active role of the metamorphic processes in the lower crust
(e.g. Richardson & England 1979; Austrheim 1991; Dewey
et al. 1993; Bousquet et al. 1997; Le Pichon et al. 1997; Ryan &
Dewey 1997). Most models are based on assumptions regarding the rate of metamorphic reactions, degree of metamorphic
transformation and the resulting petrophysical properties of
the lowermost crust. Notably, in many geodynamic models it is
assumed that the metamorphism takes place as a function of P,
T and time (t) and that reactions take place whenever a
reaction boundary is passed. The many outlined P–T–t-loops
for collision orogens, which only can be determined if disequilibrium exists, suggest that widespread equilibration is incom-
plete and indeed uncommon. It has been claimed that the
transformation of granulite and gabbro to eclogite also is
dependent on fluid availability (Ahrens & Schubert 1975;
Austrheim 1998; Rubie 1991; Zhang & Liou 1997). Earth
science is in the initial stages of understanding the complicated
processes by which the lower crust undergoes metamorphic
transformations. In particular, little is known about fluid
sources and their budget, and the behaviour of intermediate to
felsic rocks that are likely to constitute abundant parts of the
deep crust. In this paper we present a detailed study from a
banded Proterozoic granulite facies complex in western
Norway, containing both felsic and mafic rocks. The granulites
were partly eclogitized and amphibolitized during the Caledonian orogeny and the subsequent extensional exhumation
of the high-pressure rocks. The disequilibrium conditions
expressed by the mineral assemblages allow some assessment
of the effect that the mineralogical changes have on petrophysical properties and determination of the role of fluid
in metamorphic reactions in the deep crust. Our results
are discussed in relation to geodynamic models of collision
orogens.
General geology of the Western Gneiss Region and
Dalsfjord area
The Western Gneiss Region of SW Norway represents an
exhumed part of the root zone of the Caledonian orogen
(Andersen et al. 1991). The Western Gneiss Region is well
121
122
A. K. ENGVIK ET AL.
Fig. 1. Geological map of the Dalsfjord
area with the location of Bårdsholmen.
Modified after Kildal (1970), Cuthbert
(1985) and Andersen et al. (1991).
Additional mapping by E. Eide, A. K.
Engvik, M. Erambert and Ø. Skår.
known for the common presence of eclogite, coesite-bearing
eclogite and garnet peridotite, which are included in ortho- and
paragneisses, gabbros, amphibolites, anorthosites and ultramafic rocks (e.g. Bryhni 1966; Griffin et al. 1985; Smith 1984).
The rocks are of Mesoproterozoic (900–1800 Ma) age
(Kullerud et al. 1986; Tucker et al. 1987; Skår 1998), and
experienced high to ultra-high pressure conditions during the
Silurian continental collision between Baltica and Laurentia
(Griffin et al. 1985; Andersen et al. 1991). Eclogite-facies
metamorphism has been dated by U–Pb and Sm–Nd methods,
and the best defined isochrons are in the range of c. 400–
420 Ma, although some older ages (c. 440 Ma) have been
reported (Griffin & Brueckner 1980, 1985; Gebauer et al. 1985;
Tucker in Lutro et al. 1997). The eclogites are variably
overprinted by amphibolite facies assemblages which dominate
the gneisses. The retrogression is interpreted to be caused by
exhumation of the deep crust during late-orogenic extension of
the Caledonian orogen (Andersen & Jamtveit 1990).
Conventional P–T estimates of the eclogites indicate that
pressures of 25 kbar or more were reached in the northwest
parts of the Western Gneiss Region (Jamtveit 1987). The
identification of coesite- and diamond-bearing eclogite and
gneisses in this area indicate pressures of more than 30 kbar
during the Caledonian orogeny, implying subduction of
continental crust to more than 90 km depth (Smith 1984;
Dobrzhinetskaya et al. 1995; Wain 1997). Previous estimates of
Tmax of the eclogite facies metamorphism increase in the
northwesterly direction from about 600C in the Sogn–
Sunnfjord area and eastern part of the Western Gneiss Region,
to more than 750C in the northwest (Griffin et al. 1985).
Recent P–T estimates from the Vårdalsneset eclogite body on
the north side of Dalsfjorden (Fig. 1) suggest a pressure of
c. 162.5 kbar at a temperature of 68525C (Engvik et al.
1997). This estimate is different from those of Krogh (1980)
who calculated P–Tmax conditions ranging from 510–525C
and 122.5 kbar to 63035C and 152.5 kbar in the
Sunnfjord region.
In the region around Dalsfjorden (Fig. 1), the rocks of the
Western Gneiss Region are mainly granitoid gneisses and
amphibolites (Cuthbert 1985). In addition, metagabbros,
chlorite harzburgites, pyroxenites and different types of
eclogites occur in variable quantities. Most of the rocks on the
northern side of Dalsfjorden are mylonites formed by penetrative top-to-the-W rotational deformation in the Nordfjord–
Sogn Detachment during the late-orogenic extension of the
Caledonides (Norton 1986; Andersen & Jamtveit 1990). The
rocks in the hanging wall of the detachment constitute the
Caledonian allochthons, unconformably overlain by Devonian
supradetachment basins (Andersen et al. 1990).
The Bårdsholmen locality: protolith of the Western
Gneiss Region-gneisses and eclogites
The small island of Bårdsholmen (c. 180 by 70 m) is located in
the south of the Dalsfjord (Fig. 1). The central part of the
island is forested but spectacular exposures occur in a 10–15 m
wide belt all along the shore. The rocks vary from gabbroic to
granitic, locally approaching pure quartz. These rocks
presently have widely different metamorphic assemblages
ranging from granulites to eclogites and amphibolites
(Fig. 2). Prior to the Caledonian orogenic cycle the protoliths
constituted a banded leucocratic–mafic granulite complex.
From the detailed studies presented below, we can demonstrate the relative age and progressive evolution of the
structure, texture and mineral assemblages of this complex.
Our observations are consistent with reworking of
Proterozoic crust in the root zone of the orogen during the
Caledonian collision.
The banded granulite of Bårdsholmen
The granulite complex is best preserved along the northeast
shore section of the island. The complex consists of alternating
leucocratic and mafic layers. Individual layers can be up to
10 m thick, but layering on centimetre to decimetre scale is
more common (Fig. 3a). The normally steeply dipping granulite banding strikes northeast, at a high angle to the late
Caledonian amphibolite-facies gneisses and mylonites, which
FLUID-LIMITED POLYMETAMORPHISM
123
Fig. 2. Geological map of Bårdsholmen
showing the distribution of granulite-,
eclogite- and amphibolite-facies
equivalents of mafic and leucocratic/felsic
lithologies. The Caledonian eclogite- and
amphibolite-facies reworking is pervasive
on the eastern part of the island, but is
more localized within the Proterozoic
granulite facies complex on the western
part.
generally have an east–west-striking foliation in the Dalsfjord
area.
Mafic granulite. The mafic granulite (Fig. 4a) is medium
grained and equigranular. Plagioclase, enstatite and diopside
grains are subhedral with concave grain boundaries commonly
forming triple-points. Pargasite and biotite occur locally in
bands, whereas ilmenite and apatite occur with a relatively
high modal abundance in the rock (Table 1). The mafic
granulites from Bårdsholmen, even in their most pristine form,
show some sign of eclogitization. Notably, the plagioclase is
clouded with tiny inclusions, which have been identified as
garnet, kyanite, epidote, white mica and amphibole using the
electron microprobe.
Leucocratic granulite. The leucocratic granulites vary in grainsize from typically 1 mm up to 2 cm in pegmatitic parts. The
modal composition is variable, but dominated by quartz,
oligoclase, orthoclase with local garnet, enstatite and biotite
(Table 1). Apatite, zircon, monazite, Fe+Zn-spinel, ilmenite
and Fe-oxide occur as accessories. Parts of the leucocratic
rocks have granitic compositions, whereas other parts approach pure quartz in some layers. Larger quartz grains
display undulose extinction and embayed grain boundaries.
The feldspars are often perthitic. Fluid and mineral inclusions
appear both scattered and in trails, and quartz displays a
characteristic blue colour in outcrop. Varying modal proportions of mafic and leucocratic phases define the banding of
the granulite complex. Biotite shows crystal lattice-preferred
orientation whereas plagioclase, enstatite, ilmenite, iron oxide
and garnet show shape-preferred orientations with long-axes
parallel to the banding. The larger quartz and orthoclase
grains also commonly show a shape-preferred orientation
parallel to the banding of the complex.
Eclogitization of the banded granulite
The banded granulites are overprinted by eclogite- and subsequent amphibolite-facies mineralogies. Partial to complete
static eclogitization is visible along traces of fractures, whereas
complete eclogitization associated with ductile deformation
occurs in larger areas. Major parts of the exposures on
Bårdsholmen and the adjacent mainland are characterized by
lenses of eclogite in a matrix of quartz-rich felsites (Fig. 2).
This structurally complex association is here referred to as
‘eclogite-facies melange’. The term felsite is used for the
quartz–phengite-rich rocks in the area. The transformation of
the granulites to eclogite- and amphibolite-facies equivalents
can be studied in series of stages interpreted to represent
evolutionary stages (Fig. 3). The initial stages of the eclogitization took place along fractures which are seen as sharp
borders (Fig. 3b) where the rocks change colour. The mafic
and leucocratic granulites transform to eclogite and quartzrich felsites, respectively, as described below. At intermediate
stages of transformation to the eclogite-facies melange, eclogitized mafic layers have been folded and disrupted (Fig. 3c–d)
and locally interfolded with felsites (Fig. 3e–f). The eclogite
lenses vary in size from less than 10 cm to more than 3 m.
Felsites with abundant content of phengite and epidote are
commonly foliated, and disrupted eclogite lenses ‘float’ in the
more ductile felsite. The eclogitization of the mafic rocks
described here is in many respects similar to descriptions from
other areas in the Western Gneiss Region (Mørk 1985) and
Bergen Arcs (Austrheim & Mørk 1988). On Bårdsholmen,
however, it is also possible to study the leucocratic rocks of the
protolith complex through the eclogitization and subsequent
amphibolitization processes.
Eclogitization of mafic granulite. The colour change observed
in outcrop when passing across an eclogitization front in the
mafic granulite (Fig. 3b) is associated with fractures commonly
oriented obliquely to the protolith banding. The colour
change is tied to clear reaction textures forming eclogite-facies
minerals at the expense of the granulite-facies mineralogy
and is inferred to have been triggered by introduction of
fluids. This interpretation is based on growth of hydrous
minerals as well as the sharp appearance of the eclogitization
124
A. K. ENGVIK ET AL.
FLUID-LIMITED POLYMETAMORPHISM
125
Fig. 4. Photomicrographs of mafic rocks showing transformations from granulite to eclogite and amphibolite. Abbreviations after Kretz (1983).
(a) Mafic granulite with plagioclase, pyroxene, ilmenite and apatite. Note the clouding of plagioclase which is due to incipient growth of kyanite,
epidote, white mica and amphibole. Sample BA22/94, scale bar 0.4 mm. (b) Growth of garnet and omphacite at the boundary between
plagioclase–ilmenite and plagioclase–pyroxene in the mafic granulite. Tiny mineral inclusions of epidote, white mica, amphibole and kyanite
cause saussuritization of plagioclase. Sample B33/93, scale bar 0.4 mm. (c) Dynamically recrystallized eclogite with garnet, omphacite and white
mica. Orientation of elongated omphacite define a lineation in the eclogite. Garnet is concentrated in a layer giving the rock a millimetre-scale
banding paralleling the lineation. Sample B8/93, scale bar 0.4 mm. (d) Amphibolite with very fine-grained symplectitic intergrowth of hornblende
and albite. A foliation is defined of oriented biotite, epidote and hornblende. Sample B2/93, scale bar 0.2 mm.
reaction front. The description below is based on a traverse
across such a transition zone starting in the granulite. Mineral
reactions based on textural observations are summarized in
Table 2.
(1) The initial breakdown of the plagioclase is seen as
saussuritization of the original plagioclase grains (Fig. 4a).
Fine-grained euhedral garnet nucleates as coronas on the grain
boundaries between plagioclase and ilmenite. Locally pargasite
breaks down, whereas biotite appears to be stable.
(2) At the reaction front defined by colour change, the
coronas of garnet around ilmenite coarsen (Fig. 4b). The
growth of garnet consumed Fe from ilmenite, while Ti from
ilmenite reacts to form rutile. Omphacite grew as coronas
around and at the expense of the granulite-facies pyroxenes.
Consumption of plagioclase is advanced with abundant
growth of mineral inclusions comprising epidote, white mica,
kyanite, garnet and amphibole.
(3) In the eclogite, the coronas of garnet and omphacite
coarsen at the expense of original plagioclase and pyroxene.
Domains with fine-grained omphacite, epidote or phengite
appear in statically recrystallized eclogite and replace former
plagioclase and pyroxene domains.
The completely recrystallized eclogites found in the eclogite
melange consist mainly of euhedral almandine-rich garnet
and subhedral omphacite (Table 1). In addition the eclogites
contain variable amounts of epidote, phengite, paragonite,
Fig. 3. Field pictures of the banded granulite-facies complex undergoing eclogite- and amphibolite-facies reworking. (a) Banded granulite-facies
complex composed of alternating mafic and leucocratic bands. The steeply-dipping NE–SW foliation is in outcrop defined by strictly parallel
mafic bands alternating with the more dominant leucocratic bands. (b) Eclogitization of mafic and leucocratic granulite along a fracture. In
outcrop, the granulite–eclogite transition can be seen as a sharp boundary taking place on a centimetre-scale (dashed line). The leucocratic part
characterized by large blue quartz crystals on the left, becomes white during recrystallization to a quartz-rich felsite (right). (c) Folding and
disruption of an eclogite layer in quartz-rich felsite. (d) Boudinage of an eclogite layer (lower part) in quartz-rich felsite shows the development
of the eclogite-facies melange. (e) Lenses of eclogite in quartz-rich felsites constituting the eclogite-facies melange. (f) Folded eclogite layer with
quartz-rich felsite interfolded in the eclogite. A modally banding of the eclogite constitute an internal foliation in the eclogite. (g) Progressive
amphibolitization during flattening of the eclogite-facies melange. The eclogite lenses and melange are deformed during retrogression to
amphibolite in granitoid gneiss (from upper right towards lower left). (h) Highly attenuated amphibolite bands with tight isoclinal folds in the
granitoid gneiss.
126
A. K. ENGVIK ET AL.
Table 1. Modal content of selected samples of mafic granulite, leucocratic granulite, eclogite and quartz-rich felsite, based on counting of 1000 points in
each thin section
Mafic granulite
Rock type
Sample
Main minerals:
Opx
Cpx
Bt
Amph
Grt
Qtz
Pl
Kfs
Pth
Wmica
Ep
Accessory minerals:
Rt
Ap
Zrn
Cl-rich Amph
Opq
Leucocratic granulite
Eclogite
Felsite
B27/93
BA14/94
BA22/94
B25/93
B29/93
BA15/94
B17/93
B3/93
BA5/94
BA23/94
B5/93
SF21B
6.6
2.0
15.0
28.8
—
—
43.3
—
—
—
—
25.4
3.0
0.3
—
—
0.5
64.5
—
—
—
—
19.0
5.6
0.5
—
—
—
59.0
—
—
—
—
—
—
—
—
—
17.8
24.2
57.7
—
—
—
—
—
—
—
7.0
30.4
—
—
61.7
—
—
x
—
—
—
x
x
x
x
—
—
—
—
33.0
0.1
—
51.3
7.1
—
—
—
—
3.5
—
45.5
5.7
0.5
32.7
0.4
—
—
—
12.0
0.8
—
31.0
8.4
1.6
45.8
3.1
—
—
—
1.8
2.8
—
—
—
—
—
74.7
—
0.9
—
18.5
5.5
—
—
—
—
—
59.5
—
—
—
35.2
5.3
—
x
x
—
x
x
—
—
—
x
—
—
0.8
—
—
3.5
—
0.9
—
—
5.4
—
3.8
0.3
—
11.8
—
0.3
—
—
—
—
0.1
—
—
0.8
—
x
x
—
x
5.0
—
—
—
—
1.8
0.6
—
—
0.2
3.9
—
—
—
1.5
0.2
—
0.2
—
—
—
—
—
—
—
x
—
x
x
—
Abbreviations after Kretz 1983. In addition: Amph, amphibole; Opq, opaque phases; Wmica, white mica; Pth, perthite.
Table 2. Mineral reactions in the mafic and leucocratic/felsic lithologies at the granulite–eclogite- and eclogite–amphibolitefacies transformations, based on mineral textures (equations not balanced)
Granulite facies
Eclogite facies
Amphibolite facies
Mafic lithology
An+Ilm
Px+Pl
Pl+Fe+Mg+fluid
<
<
<
Leucocratic/felsic lithology
Coarse, blue Qtz
Or+Fe+Mg+fluid
An+Fe+fluid
Opx+fluid
An+Ilm
Grt+Rt
Omp
Ky+Ep+Wmica+Grt+Amph
Omp+fluid
<
Hbl+Ab
<
<
<
<
<
white, saccharoidal fine-grained Qtz
Phg+qtz
Ep+Qtz
Cum+Fe-oxide+Qtz+Grt
Grt+Rt
Phg
Grt+fluid
<
<
Or+Bt+fluid
Amph+Bt
Abbreviations after Kretz 1983. In addition: Amph, amphibole; Phg, phengite; Wmica, white mica.
quartz, barroisite, biotite, rutile and apatite. In the part of the
complex that records ductile deformation during eclogitization, the eclogite displays a banding defined by garnet and
omphacite rich layers, which is commonly folded (Fig. 3f). The
eclogites are commonly equigranular with grain size varying
from 0.2 to 1 mm. Orientation of omphacite, barroisite and
epidote locally define a lineation (Fig. 4c), in which omphacite
and barroisite crystals are up to 5 mm in length.
Eclogite facies transformation of leucocratic granulite. The high
P alteration of the leucocratic granulites is generally difficult to
recognize and to distinguish from amphibolite-facies alteration
because mafic phases such as garnet and omphacite that distinguish eclogite facies metamorphism are rare. However, fractures similar to those causing eclogitization of mafic granulites
are associated with the transformation of leucocratic layers
(Fig. 3b) where they cause decolourization of blue quartz. The
colour change is apparently associated with grain-size reduc-
tion of quartz during the eclogitization (Table 2; Fig. 5c). The
feldspars in quartz-rich rocks break down to phengite and
epidote (Table 2; Fig. 5d). Similar to the mafic granulites,
ilmenite and iron oxide often develop thin coronas of garnet at
contacts with the plagioclase (Fig. 5b). Growth of new garnet
on granulite garnet in pegmatites occurs locally where the
granulite garnet is in contact with ilmenite (Fig. 5a), apparently
controlled by the availability of Fe from ilmenite or other
Fe-oxides. Locally omphacite (Table 1) is formed during
eclogitization of the leucocratic granulite. Grains of Zn-spinel
have reaction coronas of corundum while enstatite starts to
break down to cummingtonite. Small grains of Cl-rich amphibole occur in the leucocratic granulite. The Cl-rich amphibole
is also found as very fine-grained inclusion trails in larger
quartz-grains (Austrheim & Engvik 1997).
Within the eclogite-facies melange lithologies, mafic eclogite
lenses are enclosed in felsic material rich in phengite and
quartz. The quartz–phengite-rich rocks were formed from the
FLUID-LIMITED POLYMETAMORPHISM
leucocratic granulites and represent the end product of the
process where all the earlier phases have been consumed or
recrystallized. The quartz-rich felsites, which have a white to
grey colour depending on the amount of phengite, range in
composition from nearly pure quartz to those containing up to
35% phengite and 5% epidote (Table 1). In addition garnet,
omphacite, dark green symplectites after omphacite, rutile,
biotite, Cl-rich amphibole are present in variable amounts
(Fig. 5e–f). The significant modal variation of the felsite is due
to the large modal variation of the leucocratic protolith. The
rock appears equigranular with grain size of 0.2–0.5 mm.
Quartz has embayed grain boundaries and saccharoidal texture. Phengite-rich parts of the felsites are strongly foliated,
defined by the orientation of phengite, epidote and rods of
finer-grained quartz. The increased content of hydrous minerals indicates addition of fluids during transformation of the
original dry leucocratic granulite.
Amphibolite facies reworking of the eclogite-facies
melange lithology
On the eastern side of Bårdsholmen (Fig. 2) the eclogite-facies
melange is retrograded and reworked to amphibolite facies
gneisses with bands and lenses of mafic material (Fig. 3g–h).
The eclogite lenses are converted to greenish-grey amphibolites
by retrograde deformation related to exhumation of the complex. The most deformed lenses are found as highly attenuated
limbs of isoclinal folds and bands in the gneiss.
Lenses and bands of amphibolite. The mineralogy of the amphibolites comprises magnesiohornblende, albite and oligoclase,
biotite, epidote and titanite. The amphibolites contain very
fine-grained hornblende–albite symplectites (Fig. 4d). Titanite
occurs in very fine-grained aggregates. Biotite, epidote and
locally hornblende appear as sub- to euhedral porphyroblasts
with a grain size of 0.2–0.5 mm. The foliation is commonly
defined by the preferred orientation of biotite and the larger
hornblende crystals.
Granitoid gneiss. The granitoid gneiss surrounding and interlayered with the amphibolites is normally fine-grained with a
saccharoidal texture and is composed of quartz, albite, oligoclase and orthoclase with some biotite, epidote, apatite and
titanite. Oriented biotite occurs together with the quartz and
feldspar as fine-grained euhedral crystals up to 1 mm across
which define the foliation of the gneiss (Fig. 5h). Phengite with
rims of orthoclase, oriented biotite, some chlorite and oligoclase occurs as inclusions in fine-grained (about 0.5 mm)
quartz. Pseudomorphs after phengite composed of these
minerals frequently occur as inclusions in the quartz. Biotite
in the pseudomorphs is oriented parallel with the fabric defined
by phengite (Fig. 5g). These textures indicate that phengite
breaks down to biotite and orthoclase (Table 2), a process
assumed to occur during dynamic amphibolitization related to
decompression of the complex. The formation of biotite
gives the gneiss a darker grey colour than the eclogite-facies
equivalent.
Mineral chemistry
Mineral analyses have been performed on a Cameca Camebax electron
microprobe at the Mineralogical-Geological Museum, University of
Oslo. The analyses were carried out by wavelength dispersive spectrometry and the data reduction was done with Cameca PAP software
package. Quantitative analyses were carried out with an accelerating
127
voltage of 15 kV. Sample current was 20 nA and counting time for
each element was 20 s for point analyses of garnet and pyroxene. For
minor elements the counting time was increased. Amphiboles, micas,
epidotes and feldspars were analysed by 10 nA with a defocused beam.
Synthetic and natural minerals and oxides were used for standards.
The mineral chemistry data can be obtained from the Society Library
or the British Library Document Supply Centre, Boston Spa,
Wetherby, West Yorkshire LS23 7BQ, UK as Supplementary
Publication No. SUP 18139 (5 pages).
Garnets
All analysed garnets are rich in almandine component. The
two generations of garnet found in the felsic rocks, however, have different compositions. The anhedral granulitefacies garnet has a low grossular component of Alm67–78
Gross2–3Pyr17–31Spess2. The second generation contains a
markedly higher grossular content of 10–15. The composition
of garnet in the mafic eclogites is in the range of Alm60–70
Gross21–28Pyr14–17Spess1–4. A weak zonation pattern exhibits
an increase in grossular content (c. 3–4) and decrease in
spessartine and pyrope from core to rim.
Pyroxenes
The pyroxenes are classified after Morimoto (1988), and Fe3+
has been calculated based on ideal structural formula and
charge balance after Neumann (1976). Orthopyroxene in
both the mafic and leucocratic granulite has the composition
En52–58Fs40–45 and Wo <2. The clinopyroxene in the mafic
granulite is diopsidic with Wo46En34–37Fs13–17. The Al2O3
content is moderate with Al(VI) of 0.02–0.04 a.f.u. The
clinopyroxene found in the eclogites varies in composition, but
is generally rich in the aegerine component (Ae9–18 and
jadeite34–45) and is classified as omphacite. Clinopyroxene of
the eclogite-facies felsite is also an omphacite, but has higher
aegerine component (up to Ae31) and a jadeite31–36. However,
the calculated aegerine content should be treated with caution
because the calculation of Fe3+ is sensitive to errors in the
analysis and to non-stoichiometry, and therefore also affects
calculation of the jadeite component. The chemistry of
clinopyroxenes from the Kvineset eclogite (Krogh 1980) resembles that of Bårdsholmen, whereas omphacites of the
nearby Vårdalsneset eclogite contain less iron and have lower
aegerine and higher jadeite contents (Engvik & Andersen
unpublished data).
Sheet silicates
White mica in the felsites, eclogites and gneisses has a moderate phengite component corresponding to a Si content of
6.50–6.86 a.f.u. Na content in phengite from felsite and gneiss
is <0.1 while the phengites in eclogites show higher Na content
up to 0.3 a.f.u. FeO/(FeO+MgO) varies from 0.39 to 0.62, the
highest values found in the felsite and gneiss. In the eclogite,
paragonite also occurs in equilibrium with phengite.
Biotite is present in the granulites, amphibolite, gneisses,
and locally also in the eclogite and felsite. The FeO/
(FeO+MgO) ratio in mafic granulite and eclogites overlap and
are lower than in the corresponding felsic rocks. The biotites in
the leucocratic granulite and the felsite have a Cl content of
0.1–0.3 a.f.u. Biotite of the mafic granulite and eclogite has a
lower Cl content of 0.04 a.f.u., whereas the chlorine is below
0.01 a.f.u. in the amphibolite-facies rocks.
128
A. K. ENGVIK ET AL.
FLUID-LIMITED POLYMETAMORPHISM
129
Table 3. P–T estimates of the metamorphic events at Bårdsholmen
Lithology
Geothermobarometer
Granulite facies
Mafic granulite1
opx-cpx (T)
K=0.115–0.123
K=0.115–0.123
Eclogite facies
Eclogite2
Wood & Banno (1973)
Wells (1977)
Lindsley (1983)
812–816
838–846
700–800
grt-cpx (T)
Felsite3
Xjad (Pmin)
grt-cpx (T)
Xjad (Pmin)
Siphengite (Pmin)
Ellis & Green (1979)
Powell (1985)
Holland (1980)
Ellis & Green (1979)
Powell (1985)
Holland (1980)
Massonne & Schreyer (1987)
Kd =21.2–28.6
Kd =21.2–28.6
XJd =0.42–0.44
Kd =21.8–26.3
Kd =21.8–26.3
XJd =0.31–0.35
Siphengite =3.35–3.39
480–511
457–487
Assumed 500
375–427
347–399
Assumed 500
Assumed 500
amph-plag-epidot (T)
Plyusnina (1982)
CaPl =0.02–0.03
460–465
Amphibolite facies
Amphibolite4
Reference
Parameter
T (C)
P (kbar)
Assumed
Assumed
12
Assumed
Assumed
12
10
15 kbar
15 kbar
15 kbar
15 kbar
Analyses from sample 1B27, 2B8, 3B21B and SF24, 4B1A.
Amphiboles
The amphiboles are classified according to Leake (1997). The
green amphibole locally found in the mafic granulite is a
pargasite. The blue amphibole which is present in the leucocratic granulite and felsite is Cl-rich and varies between
hastingsite, ferropargasite and ferroedenite. The Cl content of
the blue amphibole varies up to 1.0 a.f.u. Some of the eclogite
lenses have barroisite in the eclogite-facies assemblage. The
green amphibole forming by retrogression of omphacite in the
amphibolite can be classified as magnesiohornblende.
P–T estimates
Granulite-facies event
A temperature estimate of the granulite-facies event is
attempted based upon rim analyses of equilibrated diopside
and enstatite of the mafic granulite (Table 3). The opx–cpx
thermometer of Wood & Banno (1973) and Wells (1977) gives
temperatures of about 815C and 840–845C respectively.
Graphical projections of diopside and enstatite after Lindsley
(1983) indicate a temperature of 700–800C.
eclogites give temperatures of 375 and 425C (Ellis & Green
1979) and 350 and 400C (Powell 1985) for two garnet–
pyroxene pairs. However, the estimated temperatures depend
on calculated Fe3+ which is, as discussed above, sensitive to
error and uncertainty in the mineral chemical analyses and
the stoichiometry of the pyroxene. Eclogites with Fe-rich
omphacite commonly have a high Fe3+ /Fe2+ ratio and a low
estimated T (e.g. Krogh 1980). A minimum P estimate of the
eclogite is calculated based on Xjd in clinopyroxene after
Holland (1980) and gives Pmin around 12 kbar assuming a
temperature of 500C. A Pmin of 10 kbar is obtained for the
quartz+phengite-rich felsite, based on the Siphg barometer of
Massone & Schreyer (1987). For comparison, the Kvineset
eclogite, which has similar clinopyroxene chemistry, gave an
estimated temperature of 54035C and a pressure of
12.52.5 kbar (Krogh 1980). Previous P–T estimates from the
Vårdalsneset eclogite body on the north side of Dalsfjorden
(Fig. 1), which is less Fe-rich, suggest a temperature of
68525C and a pressure of about 162.5 kbar (Engvik et al.
1997).
Amphibolite-facies event
Eclogite-facies event
P–T estimates of the eclogite-facies event are calculated from
equilibrated rim compositions of garnet and omphacite in
contact with each other by applying the geothermometers of
Ellis & Green (1979) and Powell (1985). Calculated concentrations of Fe2+ are used and the results are given in Table 3.
Temperature estimates for the eclogite, assuming P=15 kbar,
range between 480–510C and 455–490C for the Ellis & Green
(1979) and Powell (1985) thermometers, respectively. The felsic
T estimates of the amphibolite facies event are based upon
analyses of amphibole and plagioclase (Plyusnina 1982) (Table
3). A large variation in the chemistry of the amphibole (see
Supplementary Publication details on p. 127) causes variable
estimates of pressure, while the CaPlag indicates a temperature
of 460–465C for the amphibolite. For comparison, a higher
temperature estimate of 56444C is achieved for retrograde
amphibolites of the nearby Vårdalsneset eclogite (Engvik &
Andersen unpublished data).
Fig. 5. Photomicrographs of leucocratic/felsic lithologies showing transformations from granulite- to eclogite- and amphibolite-facies
paragenesis. Abbreviations after Kretz (1983). (a) Anhedral garnets in leucocratic granulite. The granulite facies garnet has an overgrowth of
secondary garnet, in connection with rutile. Sample B29/93, scale bar 0.2 mm. (b) A thin corona of garnet around ilmenite of the contact with
feldspar in the leucocratic granulite. The growth of garnet extracts Fe from the ilmenite producing residual rutile. Sample B29/93, scale bar
0.1 mm. (c) Recrystallization of coarse, granulitic quartz crystals with undulating extinction to very fine-grained quartz with saccharoidal
texture. Sample SF21A/95, crossed nicols, scale bar 0.4 mm. (d) Quartz-rich felsite with intergrowth of oriented phengite and epidote in
saccharoidal, very fine-grained quartz. Sample BA23/94, scale bar 0.4 mm. (e) Quartz-rich felsite with phengite, garnet and symplectites after
omphacite. Sample SF24/95, scale bar 0.4 mm. (f ) Omphacite and skeletal garnet in quartz-rich felsite. Omphacite grains are surrounded by
symplectites. Sample SF21B/95, scale bar 0.2 mm. (g) Symplectite of oriented biotite and orthoclase after phengite in granitoid gneiss. B1/93,
scale bar 0.2 mm. (h) Foliation in the granitoid gneiss is defined by the orientation of biotite. B1/93, scale bar 0.4 mm.
130
A. K. ENGVIK ET AL.
Table 4. Density measurements of samples of the different rock types at
Bårdsholmen
Metamorphic
facies
Granulite
facies
Rock
type
Mafic granulite
Leucocratic
granulite
Eclogite
facies
Eclogite
Felsite
Amphibolite
facies
Amphibolite
Granitoid gneiss
Density
(g cm 3)
Mean
density
B27/93
3.17
3.16
BA1/94
BA8/94
BA9/94
BA14/94
BA22/94
B25/93
3.14
3.23
3.21
3.07
3.15
2.54
2.65
B30/93
B32/93
BA16/94
BA17/94
B3/93
2.60
2.70
2.71
2.69
3.41
3.46
B8/93
B11/93
B15/93
B17/93
B45/93
BA5/94
BA6/94
BA13/94
B4/93
B5/93
B12/93
B18/93
B39/93
B1/93
(mafic)
B2/93
B6/93
B1/93 (felsic)
B22/93
B23/93
3.55
3.52
3.51
3.53
3.51
3.39
3.30
3.41
2.64
2.65
2.64
2.62
2.66
3.00
Sample
2.98
2.84
2.65
2.65
2.59
Rheology
The close field relationships between the rock types described
above show that the different rocks have experienced the same
P, T and regional stress conditions. However, the rocks of
different composition and metamorphic facies responded differently to these applied stresses. The mafic and leucocratic
granulites which have been preserved behaved as rigid rock
bodies that fractured under eclogite facies conditions (Fig. 3a).
Lenses and bands of mafic eclogite are deformed and show
intense boudinage and folding (Fig. 3d–f). They are enclosed in
the quartz-rich felsites, suggesting that the felsites were weaker
than their mafic boudins. The amphibolitization was apparently associated with further competance reduction of the
rocks, resulting in strongly deformed granitoid gneisses with
highly flattened mafic lenses (Fig. 3g–h).
Summary and discussion
2.64
2.94
2.63
Petrophysical properties
Density
The density of samples of mafic and felsic rocks from the
granulite, eclogite and amphibolite facies were determined to
assess the effect of metamorphism on the density of the deep
crust. The densities are achieved by measuring the weight of
hand samples in air and water. The results (Table 4) show that
the density of the mafic granulites ranges between 3.10 and
3.20 g cm 3 with an average of 3.16 g cm 3. The densities of
eclogitized equivalents are between 3.30 and 3.55 g/cm 3
(average 3.46 g cm 3), resulting in an average density increase
of 9.5%. Because all of the granulites of Bårdsholmen show
incipient growth of eclogite-facies minerals including garnet,
their measured densities will be higher than pristine granulite
and, consequently, 9.5% represents a minimum increase. The
amphibolites range in density from 2.85 to 3.00 g cm 3 with
an average of 2.94 g cm 3. This implies a density reduction of
15% as the eclogites back-reacted to amphibolites. The leucocratic granulites have densities between 2.55 and 2.70 g cm 3
reflecting their quartz- and feldspar-rich composition. The
eclogitized and amphibolitized equivalents are within the same
range (2.60–2.65 g cm 3).
The rocks at Bårdsholmen allow the study of relationships
between mafic and leucocratic Proterozoic protoliths and their
Caledonian eclogite- and amphibolite-facies equivalents. A traditional interpretation of the P–T estimates and field relationships presented above, together with information and age
dating from the area, can be used to construct the following
tectonic scenario for the rock units at Bårdsholmen. A Proterozoic granulite-facies complex, previously metamorphosed in the
middle crust at T=815–845C, was subducted and underwent
partial eclogitization at T=455–510C, P>12 kbar, during the
Caledonian orogeny. Other P estimates from eclogites near
Bårdsholmen show that the area may have been subducted to
pressures of 162.5 kbar (Engvik et al. 1997) corresponding to
approximately 60 km. These conditions correspond to those
prevailing in crustal root zones of recent collision zones. Following the eclogitization, the complex experienced amphibolitefacies metamorphism at T=460–465C during exhumation.
The Proterozoic granulite-facies complex
The mineralogy, petrography, fabric, mineral chemistry and
P–T estimates indicate that granulite-facies metamorphism
affected the protolith of the complex. Garnet and pyroxene
changed composition from granulite to eclogite suggesting a
break between the two events. Zircons from the leucocratic
rocks of Bårdsholmen, dated by 207Pb/206Pb single grain
evaporation methods, gave an age of 100010 Ma (Gromet &
Andersen 1994). This is in accordance with other studies in
western Norway showing emplacement of granites and highgrade metamorphism during the Sveconorwegian event (e.g.
Austrheim & Mørk 1988; Skår 1998; Bingen & van Breemen
1998). The crust that was subducted during the Caledonian
orogeny already contained high-grade mineral assemblages
and structures. However, the rocks were not completely anhydrous as both biotite and pargasite were locally present,
although in small amounts, in the mafic granulites. Many parts
of the subducted crust were compositionally inhomogeneous
down to the metre scale. Similar rock complexes constitute
large areas of continental shields and are likely to be commonly recycled in continental collision zones. We therefore
argue that the processes affecting the rocks examined in this
study may be of general validity for crustal recycling orogens.
Eclogite-facies reworking of the granulite-facies complex
As described above, eclogitization of mafic protoliths is characterized by formation of garnet, omphacite and variable
FLUID-LIMITED POLYMETAMORPHISM
131
eclogitization of granulites described from the Bergen Arcs
(Austrheim & Mørk 1988). When fluids are introduced at
eclogite-facies conditions, the mafic rocks react to form eclogite
whereas the leucocratic rocks recrystallize to quartz+phengiterich felsites. Rheological contrasts between mafic and weaker
felsic rocks produced the complex deformation products of the
chaotic structured eclogite facies melange. The melange fabric
is progressively modified in a gently inclined foliation.
Gneiss-forming processes during amphibolitization
Fig. 6. Schematic model for the development of the Proterozoic
banded granulite-facies complex.
amounts of white mica, kyanite, epidote, amphibole and rutile,
at the expense of plagioclase, pyroxene and ilmenite. On
Bårdsholmen, however, the leucocratic rocks of the protolith
complex are also transformed under eclogite-facies conditions
comprising formation of phengite, epidote and quartz by breakdown of feldspars. In addition to preserved garnet and symplectites after omphacite, omphacite is also locally preserved
in the eclogite-facies felsites. Phengite and symplectites after
omphacite and phengite have previously been described as
possible relicts of eclogite facies in gneisses from WGR (Mysen
& Heier 1972; Cuthbert 1985; Krogh 1980; Griffin 1987).
The progressive transformation of the granulites during
eclogite and subsequent amphibolite-facies events is summarized in Fig. 6. Parts of the protolith were preserved with the
original mineralogy and northeast-striking steep banding
(Fig. 6, 1) in spite of subduction to 60 km depth. The partial
eclogitization of the complex can be explained by the introduction of fluids along fractures (Fig. 6, 2). This pattern of
eclogitization of an anhydrous protolith is similar to the
Most of the eclogites in the Western Gneiss Region were
formed from crustal rocks by metamorphic reactions during
the Caledonian orogeny (e.g. Mørk 1985; Cuthbert & Carswell
1991). The discussion whether the eclogites of the Western
Gneiss Region was introduced from great depths or metamorphosed ‘in situ’ (e.g. Lappin & Smith 1978; Griffin 1987),
is partly based on the apparent lack of eclogite-facies
parageneses in the gneisses surrounding eclogite lenses. The
mineralogy of the eclogite-facies felsites and the field relationships described above demonstrates that the felsic and mafic
parts followed the same P–T evolution. The tendency of felsic
rocks to re-equilibrate during retrogression (Heinrich 1982)
indicates that major parts of the amphibolite-facies gneisses of
the Western Gneiss Region were originally in metamorphic
equilibrium during the eclogite-facies event. Hornblende–
albite symplectites of the amphibolites are a characteristic
texture resulting from the retrogression of omphacite (Mysen
& Heier 1972). Pseudomorphs after phengite described here
and by other authors (Krogh 1980; Griffin et al. 1985) suggest
also that the gneisses were in eclogite facies prior to the
amphibolite-facies overprint.
The amphibolitization of the rocks on Bårdsholmen and
adjacent areas can be used to demonstrate the evolution of
amphibolite-facies banded gneisses, which are the most common lithologies in the Western Gneiss Region. As illustrated
in Fig. 6(3), the deformation of the eclogite-facies melange
lithology under amphibolite-facies conditions by near coaxial
flattening produced a prominent gneissic fabric. Most areas
along Dalsfjorden are dominated by east–west-striking foliation interpreted as having evolved during the progressive
exhumation of the deep crust. The characteristic structures and
textures found in many of these gneisses including lenses of
amphibolites, indicate that they have been formed from
the eclogite-facies-melange-type lithologies. This lithology is
particularly well preserved at Brådsholmen (Fig. 2), but is
commonly observed in variably retrograded and structurally
modified conditions in the adjacent area along Dalsfjorden
(Fig. 1) as well as in other parts of the Western Gneiss Region
(e.g. Bryhni 1966).
The role of fluid during eclogitization and
amphibolitization: consequences for P–T conditions
recorded and rheology
It has been shown that transformation of the granulites to
eclogites at high pressures is largely controlled by the availability
of fluids. Fluids can have many roles during the metamorphism
of the deep crust: e.g., fluids are components in metamorphic
reactions, they are needed to overcome sluggish reaction kinetics
and they may enhance deformation and thereby speed up
reaction kinetics (Austrheim et al. 1997). In the case of the
felsic rocks, fluids are obviously needed as a component in
the reactions converting the K-feldspar to phengite. Phengitic
132
A. K. ENGVIK ET AL.
micas contain on average 8 wt% K2O and 5 wt% H2O. Granite
with 4 wt% K2O contained in K-feldspar will require more
than 2 wt% H2O to convert K-feldspar to phengite. Additional
H2O is required to form epidote from the anorthite component
in plagioclase. The required amount of fluid is not present in
the granulites and must have been introduced from external
sources. External fluids are also necessary to convert the mafic
lithologies, although these rocks already locally contain some
hydrous phases. As described above, pargasite in the mafic
granulites breaks down during eclogitization, whereas biotite
appears to be stable. Eclogitization fronts are related to fractures cross-cutting the layering in the granulites, and former
modally dominated plagioclase domains within mafic granulite
are extensively replaced by epidote and phengite. The observed
fractures are interpreted as fluid channels and the eclogitization
of mafic rocks together with the leucocratics, must be caused by
fluid from an external source.
The presence of Cl-rich amphiboles (Austrheim & Engvik
1997) and brine inclusions with a low Cl/Br ratio suggests that
the fluid was enriched in Cl and Br (Svensen et al. 1999). The
enrichment indicates that water was consumed and that the
fluid was used up during the metamorphic reactions. Similar
enrichment has also been demonstrated from the Lofoten
Islands of northern Norway (Kullerud 1996; Markl & Bücher
1998). In the rocks from Bårdsholmen, Cl-rich amphiboles are
found on the border between granulite and eclogite. The
incomplete conversion of the granulite protolith to eclogite
during subduction is interpreted as being caused by this lack of
fluid to enable the reaction to go to completion.
The retrogression of eclogite to amphibolite is dominated by
the reaction of omphacite to amphibole and albite, which is a
fluid-consuming reaction. More fluid than that already contained in the eclogite seems to be required to complete this
extensive retrogression. The retrogression within the small
eclogite lenses of the eclogite-facies melange lithologies could
proceed in a closed system when supplied with the necessary
fluid from the breakdown of phengite in the surrounding felsic
lithology as suggested by Heinrich (1982). Contrary to the
source and behaviour of the fluid during eclogitization, internal recycling of fluids within the rock complex seems realistic
during amphibolite-facies reworking.
The sources of the fluids will have different influences on the
timing of the metamorphism, the relationship between metamorphism and the thermal evolution of the subducting slab. In
the case where fluid is derived from subducted rocks, it will
become active as a function of temperature in the slab. The
timing of metamorphism will then depend on transport from
the site of fluid production to the site of metamorphism, and
on the dehydration temperature of the minerals present in the
source rocks. However, if the fluid is added from an external
source the metamorphism may be disconnected from the
thermal evolution of the slab and metamorphism would have
occurred at the moment the fluid became active in the rock.
This does not necessarily correspond to maximum P–T, nor to
the moment at which various reaction boundaries are passed.
The consequence of this is that Pmax and Tmax experienced by
a rock body is not necessarily recorded by its mineral assemblages. P–T estimates for the eclogites of Bårdsholmen are
markedly lower than those calculated for the neighbouring
Vårdalsneset eclogite where Engvik et al. (1997) obtained
temperatures of 68525C at pressures of 162.5 kbar.
However, the Bårdsholmen results overlap with other eclogites
occurring in the area, such as Sande and Kvineset, described
by Krogh (1980). The range of P–T conditions calculated for
the eclogites located relatively close together in the Sunnfjord
region may indicate that they were eclogitized as the fluid
reached the rocks at different times along the P–T–t path.
However, the different P–T estimates can also be artefact of
calculation due to the varying content of Fe3+ in omphacite. It
is possible that the differences in calculated P–T could be due
to tectonic stacking. Larger tectonic movement zones within
the Western Gneiss Region are not recorded in the area,
although a tectonic contact with unknown displacement is
indicated further south (Cuthbert 1985). A survey of incompletely metamorphosed eclogite terrains (Austrheim 1998)
shows that overstepping of reaction boundaries can occur in
the range of 10 kbar at temperatures as high as 800C.
The above descriptions imply that metamorphic processes
have strong effects on the rheology of the deep crust. The
relative strength decreases from the stronger granulites through
mafic eclogites, quartz-rich felsites to the weakest banded
amphibolite-facies gneisses. It is not possible to ascribe these
rheological changes to differences in pressures, temperatures
or regional stress variation because this suite of granulites,
eclogites and amphibolites have obviously experienced the
same P–T–t-path during the Caledonian orogeny. A high fluid
pressure in the subducted crust could have enhanced brittle
fracturing under eclogite facies conditions, and channelled fluid
into fractures thereby initiating the eclogitization of the dry
granulites. The presence of fluid inclusions in omphacites
(Svensen et al. 1999) and frequent occurrences of hydrous
minerals in the eclogite-facies rocks, suggest that formation of
the eclogite-facies melange occurred under the presence of
fluids, favouring hydrolytic weakening (Griggs 1967). Wetted
grain or interphase boundaries could allow for effective
grain-boundary sliding with grain-shape accommodation by
dissolution–precipitation creep (Stöckhert & Renner 1998).
This process is favoured by increasing pressure (Laporte &
Watson 1991) and low solute content in the fluid (Brenan 1991;
Lee et al. 1991). Svensen et al. (1999) documented that the fluid
phase during eclogitization on Bårdsholmen was brines with a
high Cl/Br ratio. Introduction of fluids into the leucocratic
granulite caused reduction in grain size of the quartz and
produced sheet silicates, characteristic of the eclogite-facies
felsites and amphibolite-facies banded gneisses, which are the
most deformed lithologies. Jadeite and garnet have been shown
to be much stronger minerals than quartz (Stöckhert & Renner
1998), thus explaining the rheological differences between the
eclogite and felsite of the eclogite facies melange.
Bartley (1982) found that hydrolytic weakening and
reaction-induced ductility were the most important mechanisms for preferred ductile deformation in a shear zone. In the
case of Bårdsholmen, we can not distinguish between hydrolytic weakening, deformation along wetted grain boundaries,
reaction-induced ductility, grain-size reduction, production of
weak minerals or a new unstrained mineral assemblage as the
main cause of the rheological change. As discussed by
Austrheim & Engvik (1998), the mechanism by which the rock
competance is changed will be important for the duration of
the weakening; reaction-enhanced ductility will be temporary,
whereas mineralogical changes, hydrolytic weakening and
wetted grain boundaries will cause long-lasting rheological
changes. It is important to realize that, whatever explanation
we prefer for the observed weakening, it is induced by the fluid
activity either directly through its presence or indirectly
through the fluid controlled reactions. Introduction of fluids
into the dry granulites activated the metamorphic reactions,
and appears to have been the most important factor control-
FLUID-LIMITED POLYMETAMORPHISM
ling the process of metamorphic transitions, deformation and
rheological weakening.
Fluid induced metamorphism and geodynamics of
collision zones
Most geodynamic models for collision zones (Andersen et al.
1991; Dewey et al. 1993; Bousquet et al. 1997; Le Pichon et al.
1997; Ryan & Dewey 1997) assume that the metamorphism
takes place as a function of P, T and time. The Bårdsholmen
example together with other granulite facies and igneous rocks
undergoing eclogitization, show that unreacted and reacted
rocks are found together and that the availability of fluid
controls whether they react or not. Continental crust with
common metastable assemblages recording high-pressure
metamorphism occurs throughout the Western Gneiss
Region and in the Bergen Arcs of the South Norwegian
Caledonides (e.g. Austrheim & Mørk 1988) as well as in the
Alps (Pennacchioni 1996) and China (Zhang & Liou 1997).
Austrheim et al. (1997) view fluid-induced eclogitization as
a rapid process that may be associated with seismic activity.
At the same time, fluid-induced metamorphism controls the
rheology and density of the deep crust; these are regarded as
important input parameters in geodynamic modelling.
The following findings are of general relevance to crustal
geodynamic models.
(1) It may be possible to bring crustal rocks to great depth
without their reacting to form eclogite. This is in accordance
with the deep crustal roots of the Himalayas, where the crustal
thickness is doubled. However, this is only possible if the crust
subducted is dry.
(2) An increase in density caused by eclogitization will take
place only when fluid is introduced. An average Bårdsholmentype crust of intermediate composition may not acquire
mantle-like physical properties even if completely eclogitized.
To obtain mantle densities it would not only be necessary to
completely eclogitize the crust, but the protolith would also
have to be more mafic.
(3) The change in structural evolution depends on fluid
introduction which suggests that the disintegration of the
existing structure depends on the addition of fluid. Andersen &
Jamtveit (1990) interpreted eclogite lenses of different sizes
surrounded by amphibolite as relicts of eclogites deformed and
amphibolitized during flattening and exhumation connected to
the late-orogenic extension of the Caledonian orogen. The
model is consistent with the occurrence of smaller and larger
eclogite lenses in amphibolites and gneisses in the Dalsfjord
area. However, the formation of the eclogite facies melange
described in this work shows that eclogite lenses also can be
formed under disruption of crust during the eclogitization
process in the root zone of an orogen.
(4) Fluid-induced eclogitization will weaken the rocks, and
the collapse of the orogen may be fluid controlled. Austrheim
(1998) found that introduction of fluid will weaken the crust
more than a temperature increase of 100C.
(5) The back-reaction of the mafic eclogites cannot be a
function of uplift alone, because it will also depend on fluid
transport. It may, however, proceed if the fluid is produced in
the neighbouring rocks.
The Bårdsholmen locality illustrates the profound control
exerted by fluids on the timing of metamorphism, the structural make up and petrophysical properties of crustal root
zones. Fluid induced metamorphism will therefore exert control on the attributes of orogenic belts such as topography and
133
Moho depth. It will also influence the dynamics of collision
zones by controlling the time of orogenic collapse and the
buoyancy of the subducted crust. The Bårdsholmen example
highlights the role of fluids during deep crustal metamorphism
and suggests that orogens may develop differently depending
on the fluid budget.
We are grateful to M. Erambert, H. Svensen, E.R. Neumann and E.
Eide for help and discussions during this work and to E.A. Dunworth
for correcting the English. J. M. Bartley is thanked for helpful comments to an early version of the paper. Suggestions and comments by
S. Cuthbert and one anonymous reviewer are gratefully acknowledged.
This work has been supported by the Norwegian Research Council to
project 107603/410 ‘Fluid induced metamorphism and Geodynamic
processes’ and Norwegian Geological Society to project 1900.05
‘Rutile provinces in Norway’.
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Received 3 March 1999; revised typescript accepted 2 July 1999.
Scientific editing by Rob Strachan.