GeolCarp_Vol70_No6_471

GEOLOGICA CARPATHICA

, DECEMBER 2019, 70, 6, 471–482

doi: 10.2478/geoca-2019-0027

www.geologicacarpathica.com

Decompressional equilibration of the Midsund granulite

from Otrøy, Western Gneiss Region, Norway

JOHANNA HOLMBERG

, MICHAŁ BUKAŁA

, PAULINE JEANNERET

, IWONA KLONOWSKA

1, 2

and JAROSŁAW MAJKA

1, 2,



Department of Earth Sciences, Uppsala University, Villavägen 16, 752 36 Uppsala, Sweden;



jaroslaw.majka@geo.uu.se

Faculty of Geology, Geophysics and Environmental Protection, AGH University of Science and Technology, Mickiewicza 30,

30-059 Kraków, Poland

(Manuscript received June 10, 2019; accepted in revised form October 24, 2019)

Abstract: The Western Gneiss Region (WGR) of the Scandinavian Caledonides is an archetypal terrain for high-pressure

(HP) and ultrahigh-pressure (UHP) metamorphism. However, the vast majority of lithologies occurring there bear no,

or only limited, evidence for HP or UHP metamorphism. The studied Midsund HP granulite occurs on the island of Otrøy,

a locality known for the occurrence of the UHP eclogites and mantle-derived, garnet-bearing ultramafics. The Midsund

granulite consists of plagioclase, garnet, clinopyroxene, relict phengitic mica, biotite, rutile, quartz, amphibole, ilmenite

and titanite, among the most prominent phases. Applied thermodynamic modelling in the NCKFMMnASHT system

resulted in a pressure–temperature (P–T) pseudosection that provides an intersection of compositional isopleths of

(Mg/Mg+Fe) in garnet, albite in plagioclase and X

(Na/Na+Ca) in clinopyroxene in the stability field of melt +

plagioclase + garnet + clinopyroxene + amphibole + ilmenite. The obtained thermodynamic model yields P–T conditions of

1.32–1.45 GPa and 875–970 °C. The relatively high P–T recorded by the Midsund granulite may be explained as an effect

of equilibration due to exhumation from HP (presumably UHP) conditions followed by a period of stagnation under HT

at lower-to-medium crustal level. The latter seems to be a more widespread phenomenon in the WGR than previously

thought and may well explain commonly calculated pressure contrasts between neighboring lithologies in the WGR and

other HP–UHP terranes worldwide.

Keywords: Scandinavian Caledonides, thermodynamic modelling, granulite facies metamorphism, decompression.

Introduction

A multitude of recent studies have shown that continental

crust can be subducted to mantle depths during collisional

orogenies (see Gilotti 2013 and references therein). This pro-

cess results in densification of the subducting continental plate

due to formation of mineral assemblages that are stable under

high- to ultrahigh-pressure (HP–UHP) conditions. Hence, both

mafic and felsic protoliths that are present in the subducting

plate should be transformed into eclogites and HP–UHP gneis-

ses that contain index minerals typical for eclogite facies con-

ditions. However, it is known from both field observations and

through petrological studies (e.g., Young & Kylander-Clark

2015) that the majority of the rock volume in HP–UHP ter-

ranes shows little to no evidence for metamorphism under

eclogite facies conditions. This phenomenon generally results

from either overstepping of metamorphic reactions (e.g., Castro

& Spear 2017) or from pervasive re-equilibration of eclogitic

assemblages during exhumation (e.g., Engvik et al. 2018).

A combination of both is also possible. Thus, the commonly

observed plagioclase-bearing mineral assemblages could either

have been effectively metastable during the peak metamor-

phism, because the kinetic barrier for the formation of jadeite

+ quartz at the expense of plagioclase has not been overcome,

or have been formed during retrogression. In turn, there

may be various reasons for the aforementioned phenomena.

The most prominent reasons include the variability in the sub-

duction and exhumation rates, availability of fluids, and the

rock bulk chemistry (e.g., Hacker et al. 2003).

Here, we report results from a granulite that was sampled

on the island of Otrøy in the Western Gneiss Region (WGR),

Scandinavian Caledonides, which is a well-known locality for

exposures of UHP eclogites and ultramafic rocks (e.g.,

Kylander- Clark et al. 2007; van Roermund 2009a, b). We demon-

strate through petrography, coupled with phase equilibrium

thermodynamic modelling, that the bulk chemical composi-

tion of the examined granulite allowed for the stabilization of

the plagioclase-bearing assemblage under relatively high pres-

sure. We suggest that the observed mineral assemblage is

an effect of decompressional equilibration during exhumation

from HP (presumably even UHP) conditions, associated with

limited partial melting. Notwithstanding the above considera-

tions, the studied Midsund granulite provides additional

evidence for a complex subduction–exhumation history

recorded by continental crust of the WGR in the Scandinavian

Cale donides.

Geological setting

The Scandinavian Caledonides (Fig. 1A) formed as a result

of Ordovician closure of the Iapetus Ocean and subsequent

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MIDSUND GRANULITE FROM OTRØY, WESTERN GNEISS REGION, NORWAY

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, 2019, 70, 6, 471–482

including folding, resulted in the juxtaposition and tight inter-

folding of the nappes and basement rocks, particularly along

the western coast of Norway (Fig. 1A, B; Gee 1980; Krill

1980; Tucker 1986; Robinson 1995; Braathen et al. 2000;

Robinson & Hollocher 2008; Gee et al. 2010). Throughout

the Scandinavian Caledonides, the occurrence of numerous

windows in the allochthonous nappes exposes the parautoch-

thonous Baltica basement rocks (Fig. 1A; Roberts & Gee 1985;

Roberts 2003). The WGR, representing the hinterland of the

Scandinavian Caledonides, is one of those tectonic windows

and corresponds to the deepest exposed structural level (Gee

1975) and the largest exposure of parautochtonous basement

(Fig. 1B). The basement of the WGR consists mainly of

granitic, granodioritic, and tonalitic gneisses of Proterozoic

age (Fig. 1B) that were formed c. 1690–1620 Ma (Brueckner

1972; Carswell & Harvey 1985; Harvey 1985; Tucker et al.

1990; Skår 2000; Austrheim et al. 2003; Corfu et al. 2014;

DesOrmeau et al. 2015; Young 2017). The basement rocks

were subsequently: (1) overprinted by granulite-facies meta-

morphism related to the Sveconorwegian orogeny at c. 1100–

900 Ma that was associated with pluton and dyke emplacement

and local migmatization (Skår & Pedersen 2003; Røhr et al.

2004, 2013; Tucker et al. 2004; Root et al. 2005; Glodny et al.

2008; Kylander-Clark et al. 2008; Krogh et al. 2011; Corfu et

al. 2014; DesOrmeau et al. 2015); and (2) reworked during the

Caledonian orogeny (e.g., Fossen 2010). High-grade meta-

morphism, reaching HP–UHP conditions, occurred during

the final stages of the Scandian collision at c. 420 to 400 Ma

(e.g., Kylander-Clark et al. 2008; Hacker et al. 2010; Krogh et

al. 2011). This was followed by a lower-pressure (1.5–0.5 GPa),

granulite/amphibolite facies overprint at 400–385 Ma (Terry

et al. 2000; Kylander-Clark et al. 2007, 2008; Walsh et al. 2007;

Krogh et al. 2011; DesOrmeau et al. 2015; Hacker et al. 2015;

Holder et al. 2015).

The UHP rocks (eclogites and quartzofeldspathic gneisses)

crop out at three discrete areas within the WGR. These are

the southern (Nordfjord), central (Sørøyane), and northern

(Nordøyane) UHP domains (e.g., Hacker et al. 2010; Butler

et al. 2013; Smith & Godard 2013; DesOrmeau et al. 2015).

Overall, these UHP rocks show a northeasterly increase in

the peak UHP metamorphic conditions from 750 °C and

3.5 GPa in the Nordfjord UHP domain (e.g., Cuthbert et al.

2000; Young et al. 2007; Smith & Godard 2013; Des -

Ormeau et al. 2015; Butler et al. 2018) to 820–850 °C and

3.8–3.9 GPa in the Nordøyane UHP domain (Fig. 1B; e.g.,

Terry et al. 2000; Carswell et al. 2006; DesOrmeau et al.

2015). Even higher P–T estimates (850–950 °C and 5.5–

6.5 GPa) have been suggested for orthopyroxene eclogites

and garnet websterites hosted by gneisses of the northern

WGR in the Moldefjord region (including Otrøy island of

the Nordøyane UHP domain; Fig. 1B; Vrijmoed et al. 2006,

2008; Scambelluri et al. 2008; Spengler et al. 2009; van

Roermund 2009a, b). The Nordøyane UHP domain extends

from the islands of Nordøyane to the nearby mainland, and

includes the nor thernmost UHP rocks exposed in the WGR

(Butler et al. 2013).

The sample discussed in this paper was obtained from the

Proterozoic Baltica basement exposed on Otrøy in the north-

western part of the WGR in the Nordøyane UHP domain

(Fig. 1B, C). A simplified geologic map of the island is shown

in Figure 1C. Otrøy is characterized by E–W to NNE–SSW

trending belts of basement rocks. These belts consist of inter-

layered migmatitic or augen orthogneisses and well layered

dioritic-gneisses (resembling the so-called Ulla Gneiss sensu

Terry & Robinson 2004) with abundant lenses of eclogites that

preserve eclogite facies mineral assemblages to various

extents (Carswell et al. 2006), garnet peridotites, and minor

eclogitized gabbros (van Roermund 2009b). The dominant

ENE–WSW striking amphibolite facies foliation is subver-

tical, commonly associated with a well-developed subhori-

zontal E–W amphibolite-facies lineation (Spengler 2006).

In the northwestern part of the island, three occurrences of

Caledonized UHP Mg–Cr garnet-peridotite bodies are incor-

porated within the basement gneisses (van Roermund & Drury

1998; Brueckner et al. 2002; Carswell et al. 2006; van

Roermund 2009b).

The southern part of the island is separated from the nor-

thern part by the Midsund–Åkra shear zone (Fig. 1C). This major

shear zone is a greenschist-facies ductile shear zone with

a dominant strike-slip sense of displacement associated with

a well-developed subhorizontal lineation (Terry & Robinson

2004). The rocks belonging to the southern part form an assem-

blage of metasediments (Blåhø Nappe mica schists), metaba-

sites and orthogneisses. An extensive part of the basement

along the southern shores consist of homoge neous amphibo-

lite boudins and finely laminated, pinkish feldspathic gneiss

with deformed diabase dykes (Hollocher et al. 2007).

Analytical procedures

The mineral chemistry data presented below was deter-

mined using a JXA8530F Jeol Hyperprobe Field Emission

Electron Probe Microanalyser (FE-EPMA) at the Department

of Earth Sciences, Uppsala University, Sweden. The measure-

ments were carried out using a 15kV accelerating voltage and

a 10nA beam current. The counting times were 10 s for the

peaks and 5 s for the background. Both mineral and synthetic

oxide standards were used to calibrate the Kα lines for all

measured elements. The raw data was corrected according to

the PAP routine.

Results

Field observations

The Proterozoic Baltica basement exposed in an abandoned

quarry located in Midsund is shown on Figs. 1C & 2A.

The rocks exposed in this quarry consist mainly of migmatitic

banded dioritic gneiss (Fig. 2B) that are interlayered with

granitoidic augen-orthogneiss (Fig. 2C). These gneisses are

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MIDSUND GRANULITE FROM OTRØY, WESTERN GNEISS REGION, NORWAY

GEOLOGICA CARPATHICA

, 2019, 70, 6, 471–482

both the foliated augen-orthogneiss (Fig. 2E) and the migma-

titic banded dioritic gneiss (Fig. 2F, G).

Petrography and mineral chemistry

The Midsund granulite exhibits a grano- to nematoblastic

texture and is dominated by garnet, clinopyroxene, plagioclase

and quartz (Fig. 3A, B). A weak foliation is marked by less

abundant amphibole and biotite, minor apatite, rutile, and

titanite that occur sporadically in the matrix (Fig. 3A, B). Even

less abundant are phengite, ilmenite, zircon and K-feldspar.

Garnet forms subhedral to anhedral porphyroblasts and is

generally homogeneous in its composition in the range of

Alm

47–49

Grs

31–35

Prp

17–18

Sps

0.8–1.5

(Table 1, Fig. 3C). The only

exception in this pattern is the thin outermost rim that shows

a sharp increase of X

Alm

(up to 56 mol. %) and X

(from 72 to

75 %) that is accompanied by a simultaneous decrease of X

Grs

(down to 23 mol. %), while X

Prp

and X

Sps

remain relatively

consistent across the whole profile (Fig. 3C, D). Garnet con-

tains scarce inclusions of sodic clinopyroxene, phengite,

quartz, apatite, rutile, ilmenite and zircon, which presumably

represents a mineral assemblage entrapped during the pro-

grade to peak metamorphism. Seldom composite inclusions of

phengite partly replaced by biotite, and inclusions of amphi-

bole and plagioclase were found in fissured garnet grains,

indicating their retrograde origin. Rarely, multi-phase inclu-

sions, somewhat resembling melts (see e.g., Ferrero et al.

2015), composed of K-feldspar, biotite, plagioclase and/or

pure albite are also found in the garnet (Fig. 3E).

Clinopyroxene of diopside composition rarely occurs as elon-

gated inclusions (not exceeding 200 µm) in garnet (Table 1,

Fig. 3D). Most commonly, clinopyroxene forms poikilitic inter-

growths with plagioclase, quartz and amphibole in the matrix

(Fig. 3B, F), and its composition shows a broad range, i.e.

=79–93 mol. %, X

=6–18 mol. % (X

=0.07–0.21) and

Aeg

=1–4 mol. % (Table 1). Matrix clinopyroxene is generally

well preserved and is only locally replaced by amphibole

(Fig. 3F).

Phengite has only been found as inclusions in garnet and

partially replaced by biotite (Fig. 3G). The Si content varies

between 3.17 and 3.20 a.p.f.u. (Table 2).

Plagioclase occurs in four textural positions. Most com-

monly, it forms subhedral grains (up to 600 µm) evenly

distributed in the matrix (Fig. 3H), hereinafter referred to

as matrix plagioclase. It is a Na-rich variety with X

=75–

78 mol. %, X

=17–22 mol. % and X

=3–5 mol. % (Table 3).

Plagioclase is also a major constituent of poikilitic inter-

growths with clinopyroxene (Fig. 3B, F). The chemical com-

position of this plagioclase corresponds to X

=75–77 mol. %,

=19–21 mol. % and X

=2–4 mol. % (Table 3). Sporadically,

plagioclase also forms intergrowths with biotite and mono-

mineral inclusions in garnet.

Amphibole predominantly forms euhedral to subhedral, uni-

formly oriented elongated grains in the matrix. They contain

abundant quartz and apatite inclusions, and is rarely observed

to have partly replaced poikiloblastic clinopyroxene (Fig. 3F),

but it also forms intergrowth with quartz. In both textural

positions, amphibole has a pargasitic composition (Table 2).

Biotite mainly forms subhedral flakes (with sizes varying

from 50 µm to 1.5 mm) located in the matrix. Less common

larger flakes (> 500 µm) are typically oriented (sub-)parallel

to amphibole (Fig. 3B). However, the orientation of some

grains is at a high-angle to the amphiboles. Biotite is also

found as inclusions partly replacing phengite in garnet

(Fig. 3G).

Quartz is an abundant mineral that is evenly distributed in

the matrix (Fig. 3A, B). Quartz grains often display polygonal

textures with straightened grain boundaries (e.g., Humphreys

& Hatherley 1995) and distinctive triple junctions at the grain

boundaries (e.g., Kruhl 2001). These preserved textures are

indi cative of the high-temperature GBAR (grain boundary

area reduction) process.

In summary, based on the above-mentioned petrographic

description, it is inferred that the studied rock underwent

HP metamorphism, which was followed by a lower pressure

granulite facies overprint. The latter resulted in the stabi-

lization of the peak temperature assemblage composed of

garnet + plagioclase + clinopyroxene + ilmenite ± amphibole

± melt.

Pressure–temperature estimates

Isochemical phase diagrams have been calculated using

the Perple_X 6.8.5 software package (Connolly 1990, 2005).

The internally consistent thermodynamic database by Holland

& Powell (2011) has been used for calculations in the Na

O–

CaO–K

O–FeO–MgO–MnO–Al

–SiO

–H

O–TiO

(NCKFMMnASHT) system. The granulite chemical composi-

tion was determined by areal-EDS analyses performed in

the same electron microprobe laboratory as the WDS mea-

surements. Results are presented in Figure 4. To estimate

the minimum fluid content required to produce melt within

the granulite-facies assemblage observed in the thin section,

the P–X

H2O

pseudosection was modelled. A pure H

O fluid

was assumed for the modelling. The pseudosection was calcu-

lated in a P–T space of 1–2 GPa and 650–1100 °C and

a constant H

O content of 0.40 wt. % (Fig. 4). Solution models

for garnet, biotite and ilmenite are from White et al. (2007),

white mica and melt from White et al. (2014), clinopyroxene

from Green et al. (2007), plagioclase from the ternary feldspar

model of Holland & Powell (2003), and amphibole from

Green et al. (2016) have been selected.

Compositional isopleths of X

=Mg/Mg+Fe) in gar-

net, albite content in plagioclase and X

=Na/Na+Ca) in

clinopyroxene have been combined to estimate P–T condi-

tions of equilibration. Isopleths intersect within the field con-

taining the phase assemblage of melt + plagioclase + garnet +

clino pyroxene + amphibole + ilmenite (Fig. 4). The P–T space

encompassed by the intersection of the isopleths shows that

the granulite assemblage observed in the studied sample has

formed at HP and HT conditions of 1.32–1.45 GPa and

875–970 °C.

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MIDSUND GRANULITE FROM OTRØY, WESTERN GNEISS REGION, NORWAY

GEOLOGICA CARPATHICA

, 2019, 70, 6, 471–482

Mineral

Grt

Cpx

Position

Rim

Cor

Rim

Pbl

Incl

SiO

37.72

38.10

38.30

38.13

38.34

37.86

37.76

37.61

37.46

37.77

51.39

51.32

51.13

50.63

50.96

50.69

50.02

49.1

51.13

51.63

0.06

0.02

0.08

0.12

0.02

0.09

0.1

0.06

0.09

0.01

0.22

0.34

0.28

0.33

0.32

0.36

0.35

0.33

0.31

21.61

21.79

21.82

21.85

21.84

22.20

21.76

21.40

21.56

21.50

7.17

3.88

7.60

5.20

7.84

6.63

6.95

6.22

6.10

5.65

CaO

8.55

10.38

11.37

12.14

12.20

12.55

13.01

11.30

10.64

9.99

18.88

21.83

18.71

21.1

19.16

19.74

21.83

22.09

21.95

22.32

MgO

4.82

4.62

4.69

4.85

4.75

4.51

4.58

4.84

4.92

4.94

9.92

11.56

9.71

10.89

10.02

9.63

10.94

10.76

11.19

12.02

MnO

0.93

0.86

0.68

0.48

0.53

0.40

0.39

0.72

0.70

0.73

0.09

0.1

0.12

0.06

0.09

0.14

0.08

0.00

0.04

FeO

26.21

23.81

22.89

22.41

22.38

22.47

22.10

23.22

22.88

23.87

8.77

9.89

9.23

9.43

9.19

9.99

9.63

9.35

8.15

7.43

0.00

0.06

0.01

0.05

0.07

0.09

0.04

0.02

0.04

2.84

0.96

2.58

1.37

2.36

1.89

1.01

0.95

1.29

1.26

0.00

0.01

0.00

0.01

0.00

0.01

0.00

0.01

0.03

0.00

0.02

Total

99.99

99.81

99.87

100.09

100.16

100.20

99.79

99.22

98.38

98.98

99.37

100.05

99.54

99.1

100.15

99.18

100.96

99.02

100.21

100.89

12.00

6.00

2.95

2.97

2.95

2.96

2.92

2.93

2.94

2.95

2.97

1.90

1.92

1.90

1.88

1.91

1.85

1.89

0.00

0.01

0.00

0.01

0.00

0.01

0.00

0.01

1.99

2.00

1.99

2.02

1.99

1.97

2.00

1.99

0.31

0.17

0.33

0.23

0.34

0.29

0.30

0.28

0.27

0.24

0.72

0.87

0.95

1.01

1.04

1.08

0.95

0.90

0.84

0.75

0.87

0.74

0.85

0.76

0.79

0.86

0.89

0.87

0.88

0.56

0.54

0.56

0.55

0.52

0.53

0.56

0.58

0.55

0.64

0.54

0.61

0.55

0.54

0.60

0.62

0.66

0.06

0.04

0.03

0.05

0.00

1.71

1.55

1.49

1.45

1.43

1.52

1.51

1.57

0.20

0.27

0.25

0.24

0.30

0.24

0.22

0.23

0.19

3+*

0.07

0.04

0.05

0.04

0.01

0.05

0.07

0.02

0.04

0.00

0.01

0.00

0.01

0.00

0.01

0.20

0.07

0.19

0.10

0.17

0.14

0.07

0.09

0.00

Total

8.00

4.00

Jad

17.48

5.78

17.55

8.68

15.87

13.79

6.63

5.89

8.91

8.03

78.61

92.79

80.47

89.54

82.10

85.58

92.17

92.51

90.28

90.58

Aeg

3.91

1.43

1.97

1.77

2.04

0.63

1.20

1.60

0.81

1.39

0.21

0.07

0.20

0.1

0.18

0.15

0.07

0.09

24.69

25.70

26.75

27.84

27.45

26.35

26.98

27.81

27.71

26.95

73.09

70.69

68.32

71.01

69.68

64.23

71.28

73.30

73.02

77.94

Alm

56.13

51.52

49.22

47.58

47.60

47.82

46.69

49.35

49.75

51.66

Grs

23.46

28.78

31.33

33.02

33.25

34.22

35.22

30.77

29.64

27.70

Prp

18.40

17.82

17.98

18.36

18.01

17.1

17.25

18.34

19.07

19.06

Sps

2.02

1.88

1.47

1.04

1.14

0.86

0.84

1.55

1.59

Jad = Al/(Al+Fe

)×(100

−

Di), Di = (Ca+Fe

+Mg)/(Ca+Fe

+Mg)+(2×Na)×100,

Aeg = Al/(Al+Fe

)×(100

−

Di), XNa = Na/(Na+Ca), XMg = Mg/(Fe

+Mg)×100,

Alm = Fe

/(Fe

+Mg+Ca+Mn)×100,

Grs = Ca/(Ca+Fe

+Mg+Mn)×100, Prp = Mg/(Mg+Fe

+Ca+Mn)×100, Sps = Mn/(Mn+Fe

+Mg+Ca)×100, *Fe

has been estimated based on char

ge balance; Pbl — poikiloblast; Inlc — inclusion in garnet

Table 1:

Representative microprobe analyses of garnet and clinopyroxene.

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HOLMBERG, BUKAŁA, JEANNERET, KLONOWSKA and MAJKA

GEOLOGICA CARPATHICA

, 2019, 70, 6, 471–482

Discussion and final remarks

Garnet grains commonly record valuable information

regarding the evolution of a rock. Even though the garnet in

the Midsund granulite is nearly completely homogenized and

is inclusion-poor, it still provides insight into the prograde to

peak-pressure metamorphic evolution of the rock. Rare inclu-

sions of sodic clinopyroxene and phengitic mica are inter-

preted to have formed during the prograde to near peak-pressure

stages of the P–T path. The peak-pressure conditions were

impossible to estimate for the studied rock due to severe post-

peak pressure processes that lead to the obliteration of

the peak assemblage. However, it can be speculated that they

were not significantly different from the P–T conditions for

Scandian re-equilibration that was calculated for the nearby

Ugelvik garnet websterite (c. 3–4 GPa at 800–900 °C, their

M3 assemblage, Spengler et al. 2009) or the peak P–T condi-

tions estimated for an UHP eclogite (c. 3–4 GPa at 750–850 °C,

Kylander-Clark et al. 2007) located in close proximity to

the Midsund granulite.

Mineral

Position

Incl

Mtx

SiO

48.13

47.63

48.72

49.07

41.01

42.25

40.79

41.56

TiO

1.21

1.23

1.00

1.06

1.42

1.78

2.09

1.85

30.76

31.18

31.94

31.81

13.82

12.91

13.34

13.37

CaO

0.16

0.14

0.12

0.13

11.47

11.38

11.56

11.44

MgO

3.00

2.54

2.21

2.45

9.81

10.66

10.18

10.25

MnO

0.01

0.00

0.10

0.03

0.08

0.06

0.03

FeO

2.95

2.43

1.90

2.16

16.10

15.79

15.61

15.59

0.10

0.14

1.87

1.93

1.91

1.85

10.38

10.74

10.66

1.59

1.56

1.63

1.50

Total

97.28

96.58

97.19

98.03

97.12

98.34

97.17

97.44

11.00

23.00

3.18

3.17

3.20

6.21

6.28

6.16

6.23

0.06

0.05

0.16

0.20

0.24

0.21

2.40

2.45

2.47

2.45

2.47

2.26

2.38

2.36

0.01

1.86

1.81

1.87

1.84

0.30

0.25

0.22

0.24

2.22

2.36

2.29

0.00

0.01

0.00

0.01

0.00

0.16

0.14

0.10

0.12

2.04

1.82

1.88

1.84

3+*

0.00

0.14

0.10

0.11

0.01

0.02

0.55

0.56

0.54

0.88

0.91

0.89

0.31

0.30

0.31

0.29

Total

7.00

7.01

6.97

6.98

15.82

15.74

15.80

15.72

*Fe

has been estimated based on charge balance; Mtx — matrix; Inlc — inclusion in garnet

Mineral

Position

Mtx

Pbl

SiO

63.94

63.65

62.95

62.64

62.20

62.99

62.83

62.36

63.16

62.51

21.67

21.91

22.03

22.04

22.56

21.94

22.60

23.01

22.08

22.59

CaO

3.42

3.70

3.89

3.86

4.49

3.97

4.42

4.68

3.90

4.46

8.66

9.01

9.08

8.90

8.57

9.15

8.56

8.73

8.77

8.69

0.87

0.72

0.69

0.64

0.56

0.58

0.59

0.40

0.64

0.54

Total

98.56

98.99

98.64

98.08

98.38

98.63

99.00

99.18

98.55

98.79

8.00

2.86

2.84

2.83

2.82

2.80

2.83

2.81

2.78

2.83

2.80

1.14

1.15

1.17

1.20

1.16

1.19

1.21

1.17

1.19

0.16

0.18

0.19

0.22

0.19

0.21

0.22

0.19

0.21

0.75

0.78

0.79

0.78

0.75

0.80

0.74

0.76

0.05

0.04

0.03

0.02

0.04

0.03

Total

4.97

4.99

5.01

5.00

4.99

5.01

4.98

5.00

4.98

4.99

XAn (Ca)

16.99

17.73

18.40

18.63

21.73

18.71

21.44

22.33

19.00

21.41

XAb (Na)

77.86

78.14

77.72

75.04

78.02

75.14

75.38

77.30

75.50

XOr (K)

5.15

4.13

3.89

3.66

3.23

3.27

3.42

2.29

3.71

3.09

Mtx — matrix; Pbl — forming poikilitic intergrowths with clinopyroxene

Table 2: Representative microprobe analyses of phengite and amphibole.

Table 3: Representative microprobe analyses of plagioclase.

480

HOLMBERG, BUKAŁA, JEANNERET, KLONOWSKA and MAJKA

GEOLOGICA CARPATHICA

, 2019, 70, 6, 471–482

felsic gneisses in the study region. Hence, the results obtained

in this study are in agreement with the general, regional trend

of a temperature gradient increase towards this part of the

WGR as suggested e.g. by Cuthbert et al. (2000). As noted by

Engvik et al. (2018), the WGR was traditionally thought to be

a relatively cold UHP terrane. Their results, coupled with

these presented herein and other observations of granulitized

eclogites from Nordøyane (e.g., Larsen et al. 1998; Butler et

al. 2013) and farther northeast from northernmost Vestranden

in the Roan area (Möller 1988), show that this part of the

WGR must have undergone regional scale HT re-equilibration

of the eclogitized orogenic root during the terminal stages of

the Scandian collision. This, in turn, broadens our understan-

ding of the Caledonian orogenic cycle and shows that the

Scandinavian Caledonides may share more similarities with

other hot orogens, such as the Bohemian Massif (but see also

e.g., O’Brien & Rötzler 2003; Medaris et al. 2018), than pre-

viously envisioned. It also shows a clear need for further

petrological exploration of the garnet–clinopyroxene–plagio-

clase rocks that are within both the Parautochthon and alloch-

thons of the Scandinavian Caledonides in order to deliver

comprehensive data on the evolution of this mountain belt.

Acknowledgements: Simon Cuthbert and Dirk Spengler

are thanked for their constructive reviews. Chris Barnes is

acknowledged for help with linguistic correction, and Marian

Janák as well as Milan Kohút for editorial works. This research

was supported by the National Science Centre (Poland)

funded research project CALSUB no. 2014/14/ST10/00321.

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