Structural and metamorphic history of the Engebøfjellet Eclogite … · gabbroic, but some eclogite-facies rocks are dioritic or granodioritic (e.g., Korneliussen et al., 1998; Engvik

53

IntroductionLarge-scale folds (corrugations) parallel to tectonic transport on extensional detachments are documented for both oceanic and continental core complexes (e.g., Cann et al., 1997; Osmundsen and Andersen, 2001; Jolivet et al., 2004; Tani et al., 2011), but their formation and importance in crustal-scale processes are still debated. There is a link to orogenic processes at deep-crustal levels and especially eclogitised (or serpentinised) dense crustal roots, which are seen as fundamental for the potential energy of the crust and its buoyancy (e.g., Ahrens & Schubert, 1975; Austrheim, 1991; Dewey et al., 1993). Eclogites are rare at the Earth’s surface, hence the possibility to study exhumed lowermost crust is limited to a few areas, with western Norway as a classical province (Fig. 1). There, eclogites are present both in Caledonian nappes (e.g., Bergen Arcs) and in the sub-Caledonian parautochthonous basement (Western Gneiss Region; WGR). According to models proposed by Andersen et al. (1994) and Dewey et al. (1993), eclogites formed at extreme depths (>100 km) beneath overriding thrust sheets near the centre of the Caledonian orogen. Subsequent unroofing of the eclogites occurred during

extension of the thrust welt, through successive stages (Andersen & Jamtveit, 1990; Engvik & Andersen, 2000; Labrousse et al., 2004; Hacker et al., 2010) of horizontal shortening, vertical flattening and subhorizontal stretching, and large-magnitude extensional shearing and exhumation of the footwall of the Nordfjord–Sogn Detachment (NSD). Alternatively, or additionally, as proposed by Terry et al. (2000), thrust stacking at deep-crustal levels concurrent with transcurrent movements played a major role in addition to the extension in the northwesternmost part of the WGR. Other generally proposed mechanisms for exhumation of subducted crust include thrusting accompanied by erosion (Avigad, 1992) or upper-plate extension (Jolivet et al., 1994), and buoyancy forces driving combined reverse and normal faulting (Chemenda et al., 1997).

In this contribution we examine the eclogitisation and unroofing models for western Norway (e.g., Johnston et al., 2007; Hacker et al., 2010). Our extensive dataset is from the large Engebøfjellet Eclogite body, and surrounding rocks, the former mapped in great detail (more than 15 km of core have been drilled) by DuPont and the Geological Survey of Norway (NGU) because of

Braathen, A. & Erambert, M.: Structural and metamorphic history of the Engebøfjellet Eclogite and the exhumation of the Western Gneiss Region, Norway. Norwegian Journal of Geology, Vol 94, pp. 53–76. Trondheim 2014, ISSN 029-196X.

Crustal-scale extensional denudation and exhumation of crustal roots in mountain belts have a complex evolution, as can be documented for the Engebøfjellet Eclogite of the Western Gneiss Region, Norway. This eclogite is situated in a large antiform below the major extensional Nordfjord–Sogn Detachment, with an unroofing history attesting to six events; (Phases D1–2) complete HP eclogitisation of a gabbroic complex during vertical shortening and horizontal E–W stretching. (D3–4) Retrogression along amphibolite- and greenschist-facies structures signifying highly ductile top-W shear. (D5–6) Following folding, earlier structures were cut during top-W shear on the Nordfjord–Sogn Detachment, prior to renewed regio-nal E–W folding. The P–T path points to rapid near-isothermal decompression from c. 17 to 7 kbar (D1 to D4), followed by a period of cooling from 525°C to ≤300°C (brittle conditions; D5 and D6) during a pressure decrease of 3–4 kbar. During extension, exhumation of the crust within a growing major antiform parallel to tectonic transport was caused by either (i) repeated locking and unlocking of extensional detachment(s) that alterna-ted with folding from transpression-transtension events on the major Møre–Trøndelag Fault Complex, or (ii) progressive folding interacting with extensional shear zones along crustal boundary layers.

Alvar Braathen, University Centre in Svalbard, Box 156, N–9171 Longyearbyen, Norway. Department of Geosciences, University of Oslo, P.O.Box 1047 Blindern N–0316, Oslo , Norway. Muriel Erambert, Department of Geosciences, University of Oslo, P.O.Box 1047 Blindern N–0316, Oslo , Norway.

E-mail corresponding author (Alvar Braathen): [email protected]

Published October 20. 2014.

Alvar Braathen & Muriel Erambert

NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite

Structural and metamorphic history of the Engebøfjellet Eclogite and the exhumation of the Western Gneiss Region, Norway

54 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY

mylonitised rocks that relate to both the footwall and the hanging wall units (Andersen & Jamtveit, 1990; Wilks & Cuthbert, 1994; Braathen et al., 2004).

Beneath the detachment, the WGR consists of various deep-crustal orthogneisses and migmatitic gneisses intercalated with supracrustal rocks. It contains numerous enclaves of meta-anorthosite, metagabbro/amphibolite, ultramafic rocks and, in the west and northwest, subordinate bodies of eclogite (Milnes et al., 1997). The eclogite protoliths are generally gabbroic, but some eclogite-facies rocks are dioritic or granodioritic (e.g., Korneliussen et al., 1998; Engvik et al., 2000). The ages of the WGR protoliths vary, but are mainly Mesoproterozoic (e.g., Kullerud et al., 1986; Skår, 1998; Austrheim et al., 2003; Krogh et al., 2011).

its ore-grade rutile. Data on (1) lithologies, (2) structures, (3) petrography and (4) mineral chemistry form the basis for discussion of the (a) structural evolution and (b) P–T-(t) path, both contributing to the evaluation of the WGR eclogitisation and unroofing history related to either regional transpressional-transtensional tectonics and localised lower-crust instability.

Geological settingThe mountain-ridge Engebøfjellet is locatetabled along the north coast of the Førdefjord in the Sunnfjord region, WGR (Fig. 1B). Rocks of the Sunnfjord region can be conveniently divided into two major units, below and above the regional NSD zone (Norton, 1986). This major extensional shear zone is characterised by various

a

Lærdal-Gjendefault complex

Sw

ed

en

No

rway

? ?

?? ?

?

NSD

Møre-Trøndelag

Fault Complex

Saltfjelletbasementwindow

Børgefjelletbasementwindow

Central Norwaybasement window

Western GneissRegion (WGR)

Lofoten basementwindow

Trondheim

Oslo

Stavanger

Bergen

59°11°

5°

9°

64°11°

5°

62°

Jo

Jo

WGR

MTFC

HP zone

UHP zone

BA

NSD

Sediments, allochthon

Outboard allochthons

Crystalline allochthons

Devonian sediments

Permian magmatic rocks

Sediments, autochthon

Crystalline autochthon

Extensional shear zones

b

Fig. 2a

Molde

CENTRAL andWESTERNNORWAY

M

D

N

S

150 km

Figure 1. (A) Geological outline of West and Central Norway, modified from Braathen et al. (2000). (B) Simplified bedrock map of western South Norway. The box locates the study area. UHP and HP zones of eclogite-facies metamorphism. BA – Bergen Arc; D – Dalsfjord, M – Møre region, MTFC – Møre–Trøndelag Fault Complex; N – Nordfjord region, NSD – Nordfjord–Sogn detachment, S – Sunnfjord Region.

55NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite

apparently intrude the Hegreneset complex. All rocks are deformed, and a pronounced foliation is generally parallel to transposed lithological contacts. The overall structure in the region is that of a broad E–W trending and double-plunging antiform (Fig. 2), the Førdefjord antiform, which is tighter in the west near Engebøfjellet. This regional structure probably relates to or was enhanced during formation of the synclines hosting the Devonian basins. The map pattern also shows that the antiform was decapitated by the later, openly folded, NSD zone (Fig. 2, cross-section X–X”).

Lithology and structure at EngebøfjelletThe eclogite lens consists of several rock types (Fig. 3A), of which two are massive. A ferrogabbroic eclogite (or ‘ferro-eclogite’) forms a wide core and several smaller bodies along the southern margin of the main lens. It can be divided into a darker garnet- and rutile-rich (truly ferrogabbroic) and a Fe- and Ti-poorer ‘intermediate’ type. A leucogabbroic eclogite (or ‘leuco-eclogite’) dominates in the west. These massive rock types have more foliated counterparts. The well-foliated and layered margin of the ferro-eclogite, some metres wide, defines a transition to more strained parts of the leuco-eclogite. This foliated rock is present in a broad area around the ferro-eclogite, as illustrated in the cross-sections. Other foliated rocks include (i) quartz-phengite gneiss, (ii) marginal rocks surrounding the major eclogite body, with eclogite lenses and layers in a sheared quartz-mica matrix, and (iii) a unit with alternating dm- to m-thick eclogite, amphibolite and quartz-rich layers, all commonly retrograded. Unit (iii) is well developed north of the main body. Rocks found around the main eclogite are amphibolites, dioritic gneiss and granitic augen gneiss that include numerous lenses of retrograded eclogite.

The overall preservation of the eclogite parageneses is fairly good in both the massive rock types and their foliated counterparts (except iii). In the huge volume represented by the central part of this body, retrogression is limited mainly to the amphibolite-facies shear zones (Fig. 3A), where near complete recrystallisation occurred, and adjacent areas with local static retrogression (see Table 1).

Macro- and mesoscopic overprinting structural relationships are found throughout the study area. A structural evolution linked to metamorphic development designated D1 to D6 (Fig. 4) can be established based on refolding of folds and cross-cutting foliations/fractures. The Engebøfjellet Eclogite is bounded by amphibolitic D3-sheared and foliated country-rock gneisses (Fig. 3A) characterised by a well-developed fabric with a mineral stretching lineation (L3, for mineralogy, see below). Throughout the area, D3 structures have a consistent orientation, conforming to regional observations (e,g., Engvik and Andersen, 2000; Osmundsen and Andersen, 2001). The S3-foliation strikes E–W and dips steeply, generally to the north or subordinately to the south. The

Typically, mineral assemblages in country-rock gneisses in the western part of the WGR are amphibolite facies or, locally, granulite facies (e.g., Krogh & Carswell, 1995; Korneliussen et al., 1998; Hacker et al., 2010). A temperature gradient from <550°C in the southwest (Sunnfjord) to c. 800°C in the northwest (Nordfjord and Møre) is recorded for eclogite-facies relics (Labrousse et al., 2004; Hacker et al., 2010). A concurrent increase in pressure is found from Sunnfjord with peak pressure at c. 18 kbar (e.g., Cuthbert et al., 2000) to the Nordfjord area with P >28 kbar, as indicated by the presence of coesite-bearing UHP eclogites (Smith, 1984; Wain, 1997) and of microdiamonds (Dobrzhinetskaya et al., 1995).

The Caledonian rocks in the hanging wall of the NSD zone can be divided into nappes of pre-Caledonian basement with an overlying cover sequence, exotic nappes, and unconformable, Devonian, supradetachment basins of low-grade, coarse-clastic sediments (e.g., Andersen & Jamtveit, 1990; Furnes et al., 1990; Osmundsen & Andersen 2001). The initial Scandian thrusting was towards the SE, whereas late-Caledonian deformation included a reversal in transport towards the western hinterland, due to extension of the orogenic welt (Andersen, 1998; Fossen, 2000, 2010). The Devonian basins are interpreted to have formed during the extension prior to, or during, folding of the Devonian sediments and other rocks into a series of E–W-trending and south-verging synclines and anticlines (Roberts, 1983; Chauvet & Seranne, 1994; Krabbendam & Dewey, 1998; Braathen, 1999; Osmundsen & Andersen 2001; Sturt & Braathen, 2001; Osmundsen et al., 2006; Johnston et al., 2007).

The present position of high-pressure rocks in western Norway shows that the WGR was exhumed from significant crustal depths during the late stages of the Caledonian orogeny (e.g., Andersen & Jamtveit, 1990; Andersen et al., 1994; Hacker et al., 2010). This decompression may have been initiated during upper and middle crust extension at 410–400 Ma (Andersen, 1998; Fossen & Dallmeyer, 1998; Fossen & Dunlap, 1998), contemporaneous with, or closely following, the UHP / HP eclogitisation of the lower crust at around 415 to 400 Ma (Mørk & Mearns, 1986; Terry et al., 2000; Carswell et al., 2003; Root et al., 2004, Young et al., 2007; Glodny et al., 2008; Kylander-Clark et al., 2009; Krogh et al., 2011).

The Engebøfjellet Eclogite is an elongated lens with an E–W long axis. Its length exceeds 3 km, whereas its width is at most approximately 1 km. In map-view the body is asymmetric with a sigmodal shape (Figs. 2, 3A). Surrounding rocks have been divided into the Hegreneset and Helle complexes by Korneliussen et al. (1998). The Hegreneset complex comprises tonalitic to dioritic and granodioritic mica-bearing gneisses, and mafic to ultramafic rocks, some eclogitised like the Engebøfjellet eclogite. The Helle complex consists mostly of migmatitic granitic to granodioritic gneisses that


Cross-cutting relationships are exemplified by two internal foliations (D1 & D2) of the eclogite lens, which formed prior to the surrounding foliation (D3): they are

L3 stretching lineation has a moderate to subhorizontal plunge either to the east or to the west, as is also the case for the F3-folds.

Z"Z'

Z

?

YY'

Y"

sealevel

sealevel

FørdefjordEngebøfjellet

Upper plate rocks

Phyllonite andblastomylonitic gneiss

Sheared-retrograded gneiss

HEGRENESET COMPLEX

Eclogite

Grey gneiss, amphibolite,metagabbro, eclogite

HELLE COMPLEX

Granitic gneiss

Granitic to granodioriticgneiss

X

X'X'

X"

Y

Y'

Y"

0 5 10 Kilometers

Z

Z'

Z"

A

B

Anti/syncline

approx. base

Nordfjord-Sogn

detachement zone

3 Km

XX’X”

Engebø-fjellet

Eclogite ? ?

sealevel

Horizontal : vertical = 1:1

shear zone

Figure 2. (A) Bedrock map of the Førdefjord region. For details, see Korneliussen et al. (1998). Lines of cross-sections are indicated. (B) N–S cross-sections of the Førdefjord region. The two lower sections (Y–Y” and Z–Z”) are modified from Korneliussen et al. (1998).


Figure 3. (A) Bedrock map of the Engebøfjellet Eclogite and country rocks, simplified from Korneliussen et al. (1998). Sampling locations (arrows), from E to W: 14 (quarry - sam-ples E7A, E7B, E10, E11, ME38/96A), 12 (E2), 15 (ME39/96), 16 (ME1/97, ME2/97). Other samples (labelled Ex/x) are from drillcores, see Korneliussen & Erambert (1997). Inset: outline of the first-order eclogite lens, with deduced sense of shear. (B) Cross-sections of the Engebøfjellet Eclogite and country rocks. The profiles are located in Fig. 3A. The rock distribution in the subsurface was constructed using both drillhole data and down-plunge projection of observed structures. Since most folds and cut-off lines have steep plunges that change along strike, the method is regarded as uncertain: deeper portions of the cross-sections are interpretative, although consistent with drillhole and gravimetric data. The margin of the first-order eclogite lens shows lateral changes in rock composition: this boundary is out-lined be tween the various cross-sections.

1 km

ND”

D

D’

C

C’

C”

B

B’

B”

A’

A

14

12

1516

Ferro-eclogite

Leuco-eclogite

Amphibolite+ sheared eclogite

Qtz-Phe gneiss

Dioritic + granitic gneiss

LEGEND

D3 + D4 main shear zones

Lithological contact

Road / coastline

stream

S1

S2 or S3

S3

Trend of fabric/foliation: etigolcE

A

0

100

200

300

Drillhole

metresabove sealevel

A' A

0

100

200

300

400

B'' B' B

?

?

?

?

Drillhole


0

100

200

300

400

C

C'

C''

Drillhole

Drillhole

?

?

?

?


0

100

200

300

D

D'

D''

Drillhole

?

?

?

?


LEGEND

Ferro-eclogite

Leuco-eclogite

Qtz-Phe gneiss

Sheared eclogite

Partly retrograded eclogitelenses in Qtz-Phe matrix

Dioritic gneis

Granitic augen gneis

Amphibolite

Tectonic contact

Tectonic contact, inferred

Primary(?) lithological contact

Internal fabric/foliation

300 m

Horizontal : vertical = 1:1

B


Tabl

e 1.

Sum

mar

y of m

etam

orph

ic pa

rage

nese

s at E

ngeb

øfjel

let, S

unnf

jord

, WG

R.

Equi

libra

ted

para

gene

ses:

Met

amor

phic

sta

geBu

lk c

ompo

sitio

nTe

xtur

ePa

rage

nesi

s *

Sele

cted

sam

ples

Com

men

tsEc

logi

te-f

acie

s D1

+ D

2fe

rrog

abbr

oic+

inte

rmed

iate

folia

ted,

fine

gra

ined

Grt

+Om

p+A

mp(

Bar)

+Rt+

Py ±

Q

z,Ph

,Czo

/Ep,

Dol

,Ap

E3/1

59.8

, E4/

97.3

, E4/

130.

3, E

2,

ME1

/97

ME2

/97,

ME3

8/96

, E1/

101.

9,

E8/2

29.0

Vein

in M

E38/

96:Q

z,A

mp,

Dol

,Pyr

,Rt,A

ln,

Om

pfe

rrog

abbr

oic

coar

se-g

rain

edG

rt+O

mp+

Am

p(Ba

r)+D

ol+Q

z+Rt

+Py+

Czo

/Ep+

PhE1

03/1

03.3

A a

nd B

Grt

poi

kilo

blas

ts; D

ol-r

ich

leuc

ogab

broi

cco

roni

tic/ p

seud

omor

phic

Grt

+Am

p(Ba

r)+P

h+Pg

+Zo/

Czo

+Qz+

Ab+

Rt+P

yM

E39/

96, E

3/10

1.9

Ab

in fe

lsic

aggr

egat

es

leuc

ogab

broi

cfo

liate

dG

rt+A

mp(

Bar)

+Om

p+Pg

+Ph+

Czo

+Qz+

Rt+

Py±D

ol,A

bE3

/33.

9, E

8/35

.9, E

5,

E103

/74.

5A,(E

7B)

post

-D1

Om

p po

rphy

robl

.(E

3/33

.9;E

103/

74.5

A)

Qz-

rich

leuc

o-ec

logi

tefo

liate

d, c

oars

e-gr

aine

dG

rt+P

h+Pg

+Dol

+Qz+

Rt (+

Ab)

E8/2

2.9

Ab

incl

uded

in h

elic

itic

Grt

; min

or C

hl II

Grt

-am

phib

olite

faci

es D

3la

yere

d (fe

lsic/

maf

ic)

folia

ted

Grt

+Am

p(Fe

-Pr

g)+E

p+Pl

+Ms+

Bt+Q

z+Ilm

+Ap

E7A

, E10

Bt

+ M

s in

E7A

; Bt o

nly

in E

10

leuc

ogab

broi

cfo

liate

dA

mp(

Mg-

Hbl

)+C

zo/

Ep+G

rt+P

l+Q

z+Rt

+Py±

Ph±P

gE1

03/7

4.5B

and

C, E

7B+s

ympl

ectit

es (a

fter O

mp?

, rea

ctio

n 1)

leuc

ogab

broi

cfo

liate

dA

mp(

Mg-

Hbl

)+G

rt+C

zo/

Ep+P

l+Q

z+Rt

+Py

E11,

E8/

163.

2 (c

ores

)G

rt in

clud

ed in

Am

p co

res;

tran

sitio

n D

3 to

D4

Am

phib

olite

faci

es D

4le

ucog

abbr

oic

Am

p(M

g-H

bl)+

Czo

/Ep+

Pl+Q

z+Bt

+Py

E11,

E8/

163.

2 (r

ims +

shea

r ban

ds)

tran

sitio

n D

3 to

D4

leuc

ogab

broi

cth

in sh

ear b

ands

Bt+E

p+A

mp+

Pl+R

t+Py

+Qz

E103

/74.

5B a

nd C

, E7B

Gre

ensc

hist

faci

es D

5fr

actu

re/v

ein

fillin

gA

mp(

Act

)+Ep

+Qz+

Ab+

Chl

+Cal

+Ilm

+Ttn

+Mag

E7/0

74.0

fe

rrog

abbr

oic

sam

ple

Eclo

gite

s w

ith a

dvan

ced

coro

nitic

retro

gres

sion

and

unf

olia

ted

amph

ibol

ites:

Retro

gres

sion

sta

ge**

Bulk

com

posi

tion

Text

ure/

asse

mbl

ages

Mai

n re

actio

ns o

r pro

duct

ass

embl

age*

**Se

lect

ed s

ampl

esC

omm

ents/

Oth

er re

actio

ns**

*D

3 fe

rrog

abbr

oic/

felsi

c la

yer

relic

Ecl

/cor

oniti

c D

3(1

), (2

), (3

), (5

), (6

)E4

/65.

9re

actio

n (1

) com

plet

e

D4

ferr

ogab

broi

c/fe

lsic

laye

rre

lic E

cl/c

oron

itic

D4

(1),

(3),

(4a)

, (4b

), (5

), (6

)E4

/72.

9re

actio

n (1

) com

plet

e; (4

a)/(

4b) i

ncip

ient

ferr

ogab

broi

csy

mpl

ectit

es D

4A

mp+

Ep+P

l+Ilm

+Qz+

Py+C

al+A

pE4

/81.

9la

yere

d Q

z-ri

chre

lic E

cl/c

oron

itic

D4

(1),

(3),

(4a)

, (4b

), (5

), (6

)E1

/164

.2re

actio

n (1

) com

plet

e; D

ol+Q

z =Tl

c+C

al

Not

es:

* Sy

mbo

ls ac

cord

ing

to W

hitn

ey &

Eva

ns (2

010)

, in

addi

tion

Bar =

bar

rois

ite.

**

Re

trogr

essi

on s

tage

attr

ibut

ed to

D3

or D

4 ac

cord

ing

to s

tabi

lity

of G

rt

***

Pseu

dom

orph

ic re

plac

emen

ts: (1

) Om

p =

Am

p +

Pl ±

Cpx

II

(2) G

rt (+

Om

p) =

Grt

II(rim

) + A

mp

+ Pl

(3

) Ph

= Bt

+ P

l ± E

p (4

a) G

rt =

Am

p +

Pl ±

Ep

in m

afic

laye

r (4

b) G

rt =

Ep +

Bt +

Pl ±

Am

p in

felsi

c la

yer

(5

) Am

p I (

Bar)

= A

mp

II +

Pl (

6) R

t = Il

m ±

Ttn


CharacteristicsObserved structuresStage

D1

post-D1

(D2?)

D2

D3

D4

D5

D6

horizontal shortening(?);isoclinal folding,sub-penetrativefabric + banding

coaxial vertical shortening (restored position);tight to isoclinal folds,penetrative foliationin high-strain zones, cleavage in folds

non-coaxialshortening;tight + sheath foldsw/ cleavage,sheared fold-limbs;major rotational strainduring sinistral shear.

continued non-coaxialdeformation (?);m-wide shear zones

N-S shortening and/orE-W extension;vertical extensionfractures

N-S shortening onconjugate strike-slip faultsand bisectingnormal faults

F1-fold

S1

Omp needles

S1

S1= So

S1

S2-foliation

F2

cleavage

S3-shear zones

F3-fold

cleavage

S4-shear band

tension joints

map viewof fault system

N

Eclogite facies

Eclogite facies

static growth of prismatic Omp

Eclogite facies

Garnet amphibolitefacies

Amphibolite facies

Greenschist facies

Lower greenschist facies?

(Ep+Qtz+Ank)

Figure 4. Summary diagram of the structural and metamorphic evolution of the Engebøfjellet Eclogite.


6B) has variable mode, mineralogy and preferred mineral orientation. Near the margins of the massive to S1-foliated bodies there is a clear angular relationship between the two eclogitic fabrics. Superimposed NW–SE-oriented D2 structures occur as open to tight F2-folds of the D1-fabric, with development of axial-plane cleavage. With increasing strain, F2-folds become isoclinal and are commonly sheared out to lenses or boudins, whereas the S1-foliation in the fold limbs is entirely transposed into the penetrative S2-foliation. In the latter case, a mineral stretching lineation (L2) is developed.

At the macro-scale, the incremented D2 and D3 strain is displayed as a predominant subvertical orientation of sheared lithological contacts (Fig. 3B). In more detail, starting in the east (cross-section A–A”, Fig. 3A, B), the eclogite foliation (S1) was folded during D3 into a F3 synform-antiform pair, both folds with elliptical shape. They verge slightly to the south in that they have open to tight inter-limb angles and the axial surfaces are steeply inclined to the north. In detail, folding of the S1 foliation resulted in steeply dipping S1 orientations with strike varying from E–W to N–S (Fig. 6C). A vaguely defined F3-fold axis plunges steeply to the north, whereas mesoscopic parasitic F3 folds have a moderate to subvertical plunge to the east (Fig. 6G). Near the fold-hinges, a spaced cleavage, subparallel to the axial surface, is developed. D3 shear zones in the fold-limbs modify the fold geometry and are accompanied by retrogression of the eclogite into garnet amphibolite. A metre-wide shear zone, striking WNW–ESE (Fig. 6J), deviates from this pattern by cutting the fold-pair and the other amphibolite-facies shear zones, hence it probably post-dates this deformation. This D4 shear zone, with a weak, subhorizontal, mineral stretching lineation, caused retrogression of the eclogites and garnet amphibolites.

Farther west (cross-section B–B”, Fig. 3), steeply north-dipping shear zones (D2 ± D3) divide the area into several bodies. The northern sheared eclogite contains mesoscopic F2 fold-hinge zones surrounded by well-foliated, sheared-out limbs. This is well expressed by a distinct zone of quartz-phengite gneiss, 5–10 m wide, which forms a marker band that is isoclinally folded within the foliation. The central mass of ferro-eclogite contains a well-preserved D1-fabric (cross-section C–C”). F1-folds of the locally preserved (but metamorphosed) magmatic layering (=S0) are consistent with significant strain and transposition, seen as a foliation that parallels the limbs and axial surfaces of F1 folds (Fig. 5A, B). Towards the boundary of the ferro-eclogite, the D1-fabric is progressively deformed into tight to isoclinal, elliptical F2 folds. A well-developed foliation (S2) is found in fold limbs; fold hinges develop a crenulation or spaced S2 cleavage. Mesoscopic F2-folds in the central part of the first-order eclogite lens plunge steeply NW and SE. They plot along a vague great circle that strikes ESE–WNW and dips steeply north (Fig. 6F). Near the boundary to the leuco-eclogite, a penetrative S2 fabric predominates.

truncated, or sheared and transposed, and retrograded along the margin of the first order lens. Another example is the relationship between the two eclogitic fabrics. Internally, the eclogite lens consists of massive to S1-foliated bodies of ferro-eclogite and leuco-eclogite bounded by steep D2 shear zones (locally D3 reactivated). Massive parts reveal a lithological layering that is either isoclinally folded, or totally transposed into the eclogitic S1-foliation (Fig. 5). The N–S subvertical S1-fabric (Fig.

Figure 5. (A) Isoclinal F2 fold of the S1 foliation in ferro-eclogite. Arrow points to the D5 fractures (dashed line). (B) Isoclinal F2 fold of the S1 foliation in sheared ferrogabbroic eclogite. S1 is indicated by the dotted line. (C) Partly retrograded eclogite lens in S3-foliated amp-hibolite. The rim of the preserved eclogite is outlined. The sigmodal shape of the lens and associated pressure shadows indicate rotational sinistral shear.


The S2 fabric dips NNE and SSW, whereas the L2 stretching lineation has a moderate plunge to the WNW, or subordinately to the ESE to SE (Fig. 6D, E).

In the west (cross-section D–D”, Fig. 3), a distinct F3 fold-pair of the S1-S2 foliations in the ferro-eclogite and the contact towards the leuco-eclogite is present. This is well displayed by the S1 fabric, which plot along a great circle.

Figure 6. Stereoplots of structural data presented in equal-area, lower-hemisphere stereonets. (A) S1 foliation (poles) in the eastern part of the Engebøfjellet Eclogite. Star indicates the general fold axis (as in plot b and c), based on the distribution of the foliation along a great circle. (B) S1 foliation (poles) of the central part of the Engebøfjellet Eclogite. Dots are recorded F1 fold axes. (C) S1 foliation (poles) from the western part of the Engebøfjellet Eclogite. (D) L2 lineation (triangles) and S2 foliation (squares; poles) from the central and eastern parts of the Engebøfjellet Eclogite. (E) L2 lineation (triangles) and S2 foliation (squares; poles) from the western part of the Engebøfjellet Eclogite. (F) F2 fold axes from the entire Engebøfjellet Eclogite. (G) L3 lineation (triangles), S3 foliation (squares; poles) and F3 fold axes (cicles) from the entire Engebøfjellet Eclogite and surrounding wall rocks. (H) Summary plot of average structural orientations for the D1, D2 and D3 structures. (I) Plot of restored (by rotation; axis 265/0, plane 265/70) D1, D2 and D3 structures in ’h’. (J) D4 foliation and L4 lineation from a shear zone near the eastern end of the Engebø-fjellet Eclogite. (K) Poles to D5 fractures from the entire Engebøfjellet Eclogite (filled squares). (L) Slip-linear plot of D6 brittle faults.


epidote; carbonate is in some cases a major constituent. These eclogites, especially the ferro-eclogites, are generally fine-grained (with garnet size ≤0.5 mm). Notable exceptions (e.g., coarse-grained ferro- or leuco-eclogites with centimetric poikilitic or helicitic garnet, Table 1 and Fig. 7D) are typically found at the contacts between ferro- and leuco-eclogites. Eclogite-facies veins (quartz-rich, with variable amounts of garnet, carbonate, amphibole, omphacite, pyrite, apatite and allanite) are abundant in the eastern part of the body. Their internal foliation is parallel to the eclogite facies foliation (S1 or S2) of the wall-rock. Segregations in pressure shadows of eclogite boudins and along axial planes of F1 or F2 folds contain quartz, omphacite and rutile.

The two eclogite-facies ‘events’, D1 and D2, recognised in the field are not easily distinguished from one another in thin-section. The S1 and S2 foliations, and the F2-related cleavage, are defined by the shape orientation of omphacite, amphibole, clinozoisite, white micas, oxide strings, quartz ribbons and apatite (Fig. 7C). Elongated omphacite and amphibole form wedge-shaped lenses, surrounded by a finer grained matrix, especially along S2. This suggests dynamic recrystallisation during eclogite-facies shearing. In some leuco-eclogites, late omphacite porphyroblasts (up to 3–4 cm in length) grew over S1 (Fig. 7B). They are randomly oriented and commonly found near F1 fold-hinges. Such overgrowth is not documented for S2 (Fig. 4), thus this omphacite seems to post-date S1 and pre-date S2. The lack of orientation suggests growth under static conditions. Similarly, in intermediate and leuco-eclogites, post-D1 (post-D2?) amphibole porphyroblasts are common.

Subhedral garnet has a poikilitic core and generally an inclusion-poor rim. In coarse-grained ferro-eclogites (Table 1, Fig. 7D), poikiloblast cores include amphibole, carbonate, clinozoisite, rutile and pyrite; the irregular overgrowth contains omphacite in addition to the previous phases. Helicitic garnet in coarse, quartz-rich, leuco-eclogites contains quartz, carbonate, rutile, amphibole and albite (albite is found only here and within felsic aggregates in leuco-eclogites).

Amphibolite-facies parageneses and fabricsIn D3 shear zones and shear bands, as well as along the axial planes of F3 folds, major minerals are green amphibole, garnet, clinozoisite-epidote, plagioclase, ilmenite and quartz, and subordinate white mica, biotite, carbonate and chlorite. All minerals are anhedral to subhedral and fine grained. Their long axes are oriented parallel to the S3 foliation (Fig. 7E). Distinct planar banding, platy mosaics of quartz, wings around garnet and a well defined orientation of individual mineral grains all suggest mineral growth under dynamic conditions during shear deformation. For D4, similar kinematics are established as for D3, with the same minerals and textures occurring along the S4 foliation. However, garnet and white mica disappear. In this case,

This circle defines a fold axis that plunges moderately ENE, in accordance with orientations of parasitic folds (Fig. 6C). The macroscopic F3 fold-structure is tight to isoclinal and cylindrical, with an elliptical fold shape and an axial surface steeply inclined to the NNE. A crenulation cleavage is well developed in the knee of the fold. The fold-pair is terminated by an S3 shear-zone to the north.

Two types of structure have no map-scale expression. One type is the very common tensile joints/fractures (S5) that cross-cut all fabrics (S1 to S4) of the first-order lens (Fig. 5A, B). The fractures are normally 0.1 to 1 cm wide and <3 m long. They have a consistent N–S, subvertical orientation throughout the Engebøfjellet Eclogite (Fig. 6K); however, similar fractures in eclogite lenses of the southern limb of the Førdefjord anticline strike NW–SE. These fractures thus constitute a marker that may be folded on a regional scale (see discussion). Mesoscopic brittle faults (D6) truncate the D5 fractures. The faults show cm-scale separation of marker beds or bands, and net-slip is suggested from slip-lines on the fault surfaces. Two types of faults are present; conjugate strike-slip faults and bisecting normal faults. The strike-slip fault, all with steep dips, strike NNE–SSW (dextral) and NNW–SSE (sinistral). The normal faults strike N–S and dip steeply east or west (Fig. 6L).

Petrography

Eclogite-facies parageneses and fabricsAll rocks in the Engebøfjellet eclogite body underwent eclogite-facies metamorphism and recrystallisation; only ghost magmatic textures may be found. The degree of textural equilibration of the eclogites depends on their D1 and D2 deformational history. In unstrained leucogabbroic rocks (massive leuco-eclogite), microcrystalline aggregates (Ph + Pg + Ab + Zo + Qz; abbreviations for minerals according to Whitney & Evans, 2010) are presumably pseudomorphs after plagioclase. Small granular amphibole and minor mica replaced mafic phases. Garnet coronas separating felsic and mafic domains highlight former grain boundaries of the parent gabbro (Fig. 7A). With increasing strain, the felsic aggregates recrystallise then disappear (foliated leuco-eclogites). Ferro-eclogites are generally completely recrystallised.

Eclogite-facies parageneses comprise garnet, omphacite, amphibole, phengite, paragonite, clinozoisite/epidote, quartz, carbonate, rutile, apatite and pyrite (Table 1). Accessory minerals are allanite and zircon. Ferro-eclogites are usually rich in garnet (from c. 25 to 55% modal), rutile (true ferrogabbroic rocks:>3 to c. 10%), clinopyroxene and amphibole, with minor epidote, phengite and quartz. Leuco-eclogites are rich in phengite and paragonite, clinozoisite and often in amphibole and quartz. In most eclogites, volatile-bearing phases are abundant: amphibole (up to c. 40%), mica, clinozoisite/


too, there are indications of mineral growth during deformation (Fig. 7F).

Greenschist-facies fracturingThe D5 tensile fractures contain fibrous amphibole (actinolite), albite, epidote, quartz, calcite, chlorite,

magnetite and titanite. The adjacent wall-rock shows retrogression of omphacite, blue-grey amphibole and garnet to symplectitic green amphibole, epidote, plagioclase and magnetite, as well as transformation of rutile to ilmenite or more rarely titanite.

The final stage of deformation (brittle faults of the D6

B

F

C D

A

E

X'

X

Figure 7. Photomicrographs of eclogites and retrograded rocks. Transmitted light. Scale bar = 1 mm. (A) Coronitic leuco-eclogite. Garnet coronas (arrows) between mafic domains (amphibole ± white mica) and microcrystalline felsic domains (Zo + Ph + Pg + Qz + Ab, dark). Sample ME39/96. (B) Recrystallised leuco-eclogite. Omphacite porphyroblasts overprint the eclogite D1 fabrics with euhedral garnet, amphibole, clinozoisite, phengite and paragonite. Incipient retrogression as symplectites around omphacite. Sample E3/33.9. (C) Recrystallised ferro-eclogite. Foliation and lineation (S2 and L2) marked by oriented omphacite and strings of rutile. Sample E2. (D) Garnet poikiloblast in coarse-grained ferro-eclogite. Inclusions (Amp, Dol, Ep, Rt, Omp, Py) are similar to matrix phases. Sample E103/103.3A. Line XX’ indicates location of compositional profile (see Fig. 8). (E) Layered garnet amphibolite (D3) with zoned amphibole porphyroclasts and foliation of smaller Amp + Bt + Ep + Qz ribbons. Garnet in felsic layers. Sample E10. (F) Amphibolite (D4, or D3 to D4?) with foliation of Amp (Hbl) and Czo/Ep. Sample E11.


ferro-eclogites, and increase in XMg from ferro- and intermediates eclogites to leuco-eclogites (Fig. 9). Rims are generally higher in Al (AlIV and often AlVI) and (Na+K)A and lower in Si, NaM4 and often in XMg. However, in fine-grained ferro-eclogites, NaM4 increases towards the rim. Since NaM4 (glaucophane substitution) increases with pressure (Graham et al., 1989), this could reflect a pressure increase during early ferro-eclogite crystallisation, before a decrease during leuco-eclogite (with post-D1 – or post-D2? - amphibole porphyroblasts) crystallisation (e.g., head of the P–T loop). Inclusions in garnet poikiloblasts from coarse ferro-eclogites range from barroisite to magnesio-katophorite and taramite, a range largely accounted for by re-equilibration of inclusion rims with host garnet. Profiles consisting of the largest inclusion cores (to avoid disturbance due to rim re-equilibration) were analysed across garnet to test whether their compositional evolution matched the ‘prograde’ zoning of the garnet. Barroisites within Mn-rich garnet cores,however, are similar to those near or within the overgrowth (e.g., coexisting with omphacite), both plotting generally mid-way within the compositional range for all inclusions (see Fig. 9). Moreover, this reversal in amphibole composition may follow opposite trends along different profiles within a single garnet. Amphibole in recrystallised D3 amphibolites is a ferro-pargasite or ferro-tschermakite. Amphibole in D4 amphibolites is a magnesio-hornblende. In both D3 and D4 samples, zoning towards the rim corresponds to an increase in Al (mostly AlIV), Ca and usually NaA, and a decrease in Si, NaM4 and XMg, a continuation of the trend already observed in eclogites. Some porphyroclast cores in D4 samples that have relatively high NaM4 (Fig. 9) and include garnet are likely relics of the D3 stage (Table 1). Fibrous amphibole in D5 veins is a ferro-actinolite.

Foliation phengite in all eclogites and small crystals in felsic segregations in leuco-eclogites has a Si core value of 6.62–6.89 apfu, decreasing to 6.36 apfu towards the rim. Phengitic muscovite in S3 (sample E7A) has lower Si (6.45–6.60 apfu in cores) and Mg+Fet (0.79–1.03 apfu) than phengite in eclogites, indicating a decrease in the celadonite component. S3 biotites (samples E10 and E7A) have Al = 2.84–3.29 apfu and XMg = 0.43–0.56, and no systematic zoning. Later biotites in these samples (in D4 shear planes with biotite ± chlorite) show no systematic difference in composition from S3 biotites, although they tend to plot at the low Al and high Mg end of the compositional range.

Clinozoisite/epidote has a Fe content (Ps= 100 Fe3+/(Fe3+ + Al)) increasing from Ps14-18 in foliated leuco-eclogites to Ps16-20 in intermediate eclogites and Ps20-27 in ferro-eclogites. In felsic pseudomorphs in leuco-eclogite, it is a zoisite/clinozoisite (Ps04). Inclusions do not differ in composition from matrix grains, even in garnet porphyroblasts from ferro-eclogites. Epidote in recrystallised D3 amphibolites has Ps12-24 and Ps15-17 in

stage) did not result in any significant recrystallisation of the eclogites and nearby rocks. Mineral growth is limited to fracture walls, commonly coated with fibrous epidote or quartz and locally with brown ankerite.

Mineral chemistrySome 30 samples of eclogites, equilibrated D3 and D4 amphibolites and D5 fractures (Table 1) were selected for electron microprobe study, covering the range in textural, mineralogical and chemical variations observed within the Engebøfjellet Eclogite (see Korneliussen & Erambert, 1997). Samples with static retrogression were excluded, since they are not well suited for geothermobarometry. Analytical procedures and recalculation methods are described in the Appendix. Selected mineral analyses are presented in Table 3.

Garnet in eclogites is almandine-rich, with XMg (=Mg/(Mg + Fe)) increasing from layered quartz-rich eclogites (Alm61.4-68.8 Sps0.5-7.3 Prp5.7-22.4 Grs12.2-18.4 Adr2.4-7.4) via ferro-eclogites (fine- and coarse-grained: Alm56.3-69.2 Sps0.2-7.3 Prp7.1-19.6 Grs7.8-25.7 Adr0.0-8.8) and intermediate eclogites (Alm51.8-57.3 Sps0.9-1.9 Prp15.9-23.3 Grs17.6-23.4 Adr0.1-5.2) to leuco-eclogites (Alm46.6-60.8 Sps0.5-1.8 Prp14.6-30.3 Grs10.4-30.9 Adr0.0-

5.8). Rims have generally the highest Mg, Fe and lowest Mn content. Poikiloblasts in ferro-eclogites (Fig. 7D) show a bell-shaped Mn profile, with Mg increasing and Ca and Fe decreasing towards the rims (Fig. 8). This ‘prograde’ zoning is not paralleled by a clear evolution in inclusion mineralogy: all are similar to matrix eclogitic phases, although omphacite is found only near or within the overgrowth. A similar zoning is found in helicitic garnet. Garnet in amphibole-poor D3 amphibolite layers overlaps in composition with eclogite-facies garnet in ferrogabbroic samples (core analyses: Alm60.1-68.4 Sps1.1-1.7 Prp11.2-14.6 Grs14.4-23.9 Adr1.7-5.7). Higher Ca and on average lower Fe and Mg are recorded in amphibole-rich layers (Alm53.9-67.3 Sps0.3-3.2 Prp6.2-13.1 Grs14.8-30.6 Adr2.3-8.0). Rims tend to be more homogeneous than cores, reflecting a tendency towards chemical equilibrium.

Clinopyroxene is an omphacite (Morimoto et al., 1988) tending towards aegirine-augite in ferro-eclogites. Its compositional range is Jd15.4-39.7 Aeg10.0-30.6 CaTs0.0-2.8 in ferrogabbroic and intermediate eclogites to Jd36.0-45.7 Aeg4.0-19.3 CaTs0.0-3.0 in leuco-eclogites. Higher Jd values cannot be discounted due to possible overestimation of Fe3+ (see below). No systematic zoning and no compositional difference between inclusions and matrix omphacites was recorded. Pyroxene compositions in eclogite-facies veins and their wall-rock are similar.

Amphibole in eclogites is generally a barroisite (Leake et al., 1997). Core composition of matrix amphibole mirrors bulk composition, with on average higher Al in leuco-eclogites, layered quartz-rich eclogites and intermediate eclogites than in fine-grained


Figure 8. Garnet compositions in atomic proportions of Fet+Mn–Mg–Ca in (A) eclogites (cores and rims) and (B) amphibolites D3 (+ relict Grt in amphibolites D4) compared with compositional fields for eclogite-facies garnets. (C) Zoning profile in garnet porphyroblast from coarse-grained ferro-eclogite (location of profile, see Fig. 7D).

C


Temperature estimatesConsidering Fe2+ and Mg partitioning between garnet and clinopyroxene, the complex growth zoning of garnet and the erratic zoning in clinopyroxene make it unlikely that core compositions could be in equilibrium. Thus, only analyses at stable grain boundaries (matrix grains or inclusion-host pairs) have been used. A major problem is the accuracy of Fe3+ estimates for Na-rich pyroxene: assuming stoichiometry, the calculated Fe3+/Fet of 0.33 to 0.95 results in a spread in Kd and temperatures of 290–623°C (Ellis & Green, 1979; Table 2). The highest Fe3+ values are unrealistic, as in the nearby Naustdal eclogite, whose mineralogy is similar to the present ferro-/intermediate eclogites, from which a wet chemical analysis of omphacite gave a Fe3+/Fet ratio of c. 0.6 (Binns, 1967). Assuming Fe3+/Fet = 0.6 in clinopyroxene from Engebøfjellet and P = 15 kbar, temperature estimates range from 535 to 645°C (Ellis & Green, 1979) with a peak around 590°C for ferro-/intermediate eclogites. Temperature estimates using Powell (1985) and Krogh-Ravna (2000a) are systematically lower by c. 25°C and 80°C, respectively (Table 2). All these temperatures may be too low, if the assumed Fe3+/Fet ratio is still too high for these ferro-eclogites.

D4 amphibolites (of leucogabbroic composition). Yellow epidote in D5 veins is Ps31-38.

Carbonate in eclogites is a dolomite/ankerite (XMg = 0.58 to 0.87). Carbonate in D3 and D4 amphibolites, as well as in D5 veins, is calcite. Plagioclase in felsic aggregates from leuco-eclogites is albite (An02-05), as are inclusions in garnet from coarse quartz-rich eclogite (An01). Plagioclase in D3 amphibolites is An16-33, and An12-20 in D4 amphibolites. In D5 veins, it is albite An04. Chlorite in D5 veins is a clinochlore with Si = 5.94–6.04 apfu, Al = 4.04–4.06 apfu and XMg = 0.53.

P–T evolution

Eclogite-facies development: D1 and D2

No significant variation in mineral compositions could be correlated to variations in fabrics associated to the two, successive, eclogite-facies deformation phases. Hence, the P–T estimates below are taken as representative of both the D1 and the D2 phases.

Figure 9. Amphibole compositions in eclogites, D3 and D4 amphibolites and D5 veins. Structural formula on 13 cations minus Ca,Na, K (see appen-dix). Compositional domains for cores of eclogite-facies amphiboles outlined for comparison. The three plots illustrate the three main substitutions in these amphiboles: tschermak (+ferri-tschermak; monitored by AlVI +Fe3+), edenite (Na+KA) and glaucophane (NaM4). Note particularly the decrease in NaM4 from eclogites to amphibolites following decrease in P (see text).


Tabl

e 2.

Pre

ssure

and

tem

pera

ture

estim

ates

on

eclo

gite

s and

am

phib

olite

s fro

m E

ngeb

øfjel

let

ECLO

GIT

E fA

CIE

S (D

1 +

D2)

fe-M

g pa

rtitio

ning

bet

wee

n G

rt an

d C

px (a

)fe

-Mg

parti

tioni

ng b

etw

een

Grt

and

Cpx

:

T es

timat

es (E

&G

79)

assu

min

g fe

3+/f

e t in

Cpx

= 0

.6in

ferr

o- a

nd in

term

edia

te e

clog

ites

(see

text

):

T(°C

) (n)

(fo

r P=

15 k

bar)

XM

g Cpx

XM

g Grt

XM

n Grt

X C

a G

rtK

dE&

G79

P85

K-R

a

Ferr

o-ec

logi

tes +

in

term

edia

te e

clog

ites

0.73

2-0.

925

0.11

6-0.

300

0.00

2-0.

023

0.19

6-0.

281

11.3

-52.

340

1-62

337

8-60

131

5-54

1 (3

0)as

sum

ing

Fe3+

/Fe t in

Cpx

= 0

.6 :

9.5-

21.2

535-

646

512-

623

420-

517

(30)

Leuc

o-ec

logi

tes

0.84

3-0.

985

0.23

6-0.

314

0.00

6-0.

016

0.19

6-0.

267

11.8

-153

293-

622

fe-M

g pa

rtitio

ning

bet

wee

n G

rt an

d A

mp

(Gra

ham

& P

owel

l, 19

84) (

b)X

Mg

Am

pX

Mg

Grt

XM

n G

rtXC

a G

rtK

dT(

°C) (

n)M

atri

x m

iner

al (r

ims)

0.43

-0.6

40.

12-0

.32

<0.1

0.16

5-0.

274

3.7-

5.6

588-

677

(11)

Grt

poi

kilo

blas

ts +

grt c

ore

0.42

-0.4

70,

110,

070.

264

4.4-

7.0

566-

657

(6)

Am

p in

clus

ion

core

sin

term

edia

te0.

42-0

.53

0,12

0.04

-0.0

50.

261

5.3-

8.2

534-

621

(4)

near

ove

rgro

wth

0,43

0,12

-0,1

40,

030.

244-

0.26

15.

2-6.

657

5-62

3 (4

)

fe-M

g pa

rtitio

ning

bet

wee

n G

rt an

d Ph

(c)

T(°C

)X

Mg

PhX

Mg

Grt

Kd

P (k

bar)

K&

R 78

G&

H 8

2al

l ecl

ogite

type

s0.

62-0

.76

0.15

7-0.

281

4.9-

7.3

1559

9-67

464

8-71

8

Grt-

Cpx

-Ph

baro

met

er (W

ater

s &

Mar

tin, 1

993,

199

6) (d

)

Mg

apfu

Grt

Ca

apfu

Grt

Ca

apfu

Cpx

Mg

apfu

Cpx

Al a

pfu

PhSi

apfu

Ph

Mg

apfu

Ph

T(°C

)P

(kba

r)ri

ms

0.94

0-1.

190

1.41

0-1.

464

0.50

2-0.

544

0.40

3-0.

439

4.27

8-4.

398

6.65

4-6.

788

0.60

0-0.

694

600

15.5

-18

AM

PhIB

OLIT

E fA

CIE

S D

4

fe-M

g pa

rtitio

ning

bet

wee

n G

rt an

d A

mp

(Gra

ham

& P

owel

l, 19

84; K

rogh

-Rav

na, 2

000b

) (b)

XM

g A

mp

(Fet

)XM

g Am

p (K-

Rb)

XM

g G

rtX

Mn

Grt

XCa

Grt

Kd(

G&

P84)

Kd(

K-R

b)T(

°C)

G&

P84

K-R

bD

3ri

ms

0.30

-0.3

80.

35-0

.46

0.14

-0.1

80.

02-0

.03

0.25

-0.3

31.

9-3.

82.

5-4.

971

4-98

461

7-10

34

Grt-

Am

p-Pl

-Qz

geob

arom

eter

s (K

ohn

& S

pear

, 198

9, 1

990)

(e)

P (k

bar)

at T

=50

0°C

at T

=60

0 °C

PlG

rt:

X A

lmX

Prp

X S

psX

Grs

E7A

E10

E7A

E10

E7A

An2

1-26

0.57

6-0.

609

0.10

6-0.

127

0.01

7-0.

200.

246-

0.29

7P1

9.4-

10.2

9.2-

9.9

9.7-

10.5

9.4-

10.2

E10

An2

60.

579-

0.61

50.

101-

0.11

70.

013-

0.25

0.25

9-0.

280

P210

.2-1

1.3

10.1

-10.

310

.5-1

1.7

10.4

-11.

2P3

10.5

-11.

710

.5-1

1.6

11.5

-12.

711

.4-1

2.7

Am

p:X

Mg

X T

1,A

lX

M2,

Al

XM

2,Fe

XM

2,M

gX

A,0

XA

,Na

P48.

7-10

.08.

9-9.

610

.3-1

1.7

10.4

-11.

3E7

A0.

37-0

.39

0.45

6-0.

507

0.42

5-0.

494

0.19

3-0.

265

0.11

4-0.

156

0.38

4-0.

401

0.49

3-0.

503

P510

.4-1

1.2

10.1

-10.

711

.0-1

1.9

10.7

-11.

4E1

00.

32-0

.36

0.43

6-0.

469

0.49

7-0.

533

0.23

8-0.

309

0.11

2-0.

166

0.27

7-0.

446

0.45

3-0.

511

P68.

7-9.

88.

8-8.

99.

8-11

.09.

8-9.

9A

l con

tent

in A

mp

coex

istin

g w

ith E

p +

Pl +

Qz

(Ply

usni

na, 1

982)

(a) c

alib

ratio

ns o

f Elli

s and

Gre

en (1

979)

= E

&G

79, P

owel

l (19

85) =

P85

K

rogh

-Rav

na (2

000a

) =K

-Ra

- XM

g, X

Mn,

XC

a in

Grt

cal

cula

ted

with

est

imat

ed F

e2+

(b) c

alcu

late

d Fe

2+ in

Grt

- Fo

r Am

p, F

e2+ =

Fet

for G

raha

m &

Pow

ell (

1984

) (=G

&P8

4)

and

Fe2+

in M

1,M

2,M

3 sit

es fo

r Kro

gh-R

avna

(200

0b) (

=K-R

b) (

see

calc

ulat

ion

proc

edur

e in

K-R

b)

(c) c

alib

ratio

n fo

r bas

altic

syst

ems:

Kro

gh &

Råh

eim

(197

8 ) =

K&

R78

an

d H

ellm

an (1

982;

equ

atio

n 3)

= G

&H

82. F

e3+ n

egle

cted

in P

h (d

) usin

g sim

ple

Mg-

Ca

mix

ing

mod

el fo

r Grt

(New

ton

& H

asel

ton,

198

1, se

e re

fere

nce

in W

ater

s & M

artin

, 199

3,

1996

) and

Fet

for P

h an

d C

px (e

) P1

and

P2,m

odel

1; P

3 an

d P4

, mod

el 2

in K

ohn

& S

pear

(198

9); P

5 an

d P6

, Koh

n &

Spe

ar (1

990)

Alt,

Am

pPl

T(°C

)P

(kba

r)D

3E7

A2.

73-3

.01

An1

8-26

ca. 5

25-5

40(>

8 k

bar)

E10

2.49

-2.8

9A

n19-

29

D4

E11

1.7-

1.9

An1

2-15

500-

525

7-8

GRE

ENSC

hIS

T fA

CIE

S D

5

Am

phib

ole

coex

istin

g w

ith E

p +

Ab

+ C

hl +

Qz

(Trib

oule

t, 19

92)

XM

g A

mp

XFe

3+ A

mp

EpX

Mg

Chl

T(°C

)P

(kba

r)0.

450.

6931

-38

0.53

ca. 3

003-

4


Tabl

e 3.

Micr

opro

be a

naly

ses o

f min

eral

s in

eclo

gite

s and

am

phib

olite

s.

ECLO

GIT

ES

Grt

Grt

porp

hyro

blas

tC

pxm

atrix

Am

pA

mp

incl

usio

ns (c

ore)

in G

rtPh

SAM

PLE

E4/9

7.3

ME1

/97

E3/3

3.9

E103

/103

.3E4

/97.

3M

E1/9

7E3

/33.

9E3

/33.

9E8

/35.

9E1

03/1

03.3

E103

/103

.3M

E1/9

7

2.3

252.

110

65

2.5

262.

221

1930

2638

3537

rim w

. Cpx

rim w

. Cpx

rim w

. Cpx

core

inte

rm.

over

grow

thrim

w.G

rtrim

w. C

pxrim

w.G

rtrim

w G

rtco

reco

rein

Grt

core

inte

rm.G

rtin

ove

rgro

wth

SiO

237

,99

38,5

338

.98

37.6

337

.67

37.6

754

.99

55.7

956

.78

47.2

650

.62

49.1

046

.42

49.7

246

.59

49.9

3

TiO

20,

070,

000.

070.

150.

080.

030.

090.

030.

070.

170.

220.

170.

500.

230.

220.

37

Al 2O

320

,84

21,3

121

.46

20.2

920

.39

20.6

57.

339.

0710

.92

13.4

711

.34

8.22

10.7

57.

5810

.51

27.5

7

Cr 2O

30,

030,

000.

040.

000.

010.

000.

030.

010.

010.

040.

000.

040.

000.

000.

00

FeO

t27

,97

26,0

624

.55

28.0

529

.51

30.1

38.

667.

454.

0911

.84

11.3

515

.68

19.5

017

.42

19.1

82.

78

MnO

0,46

0,48

0.80

3.23

1.11

0.80

0.00

0.05

0.00

0.15

0.03

0.04

0.18

0.18

0.04

0.13

MgO

3,81

5,11

6.13

1.84

2.42

2.74

8.10

7.49

8.19

11.7

712

.55

11.7

78.

5911

.13

9.07

3.32

NiO

0,00

0,00

0.00

0.09

0.00

0.04

0.00

0.00

0.00

0.00

0.05

0.01

0.09

0.03

0.00

CaO

9,09

8,42

8.37

9.32

8.70

7.99

14.6

912

.98

13.2

48.

486.

568.

067.

567.

196.

970.

03

Na 2O

0,01

0,00

0.04

0.05

0.01

0.00

6.09

7.39

7.25

3.63

4.91

3.90

4.19

3.77

4.43

0.70

K2O

0,00

0,01

0.01

0.00

0.01

0.00

0.00

0.04

0.01

0.29

0.20

0.20

0.18

0.19

0.18

10.8

8

ToTa

l100,29

99,93

100.45

100.65

99.93

100.05

99.99

100.31

100.56

97.10

97.82

97.19

97.96

97.44

97.19

95.70

Si5.

976.

016.

015.

986.

005.

991.

991.

992.

006.

847.

207.

236.

927.

336.

986.

69

Ti0.

010.

000.

010.

020.

010.

000.

000.

000.

000.

020.

020.

020.

060.

030.

020.

04

Al

3.86

3.92

3.90

3.80

3.83

3.87

0.31

0.38

0.45

2.30

1.90

1.43

1.89

1.32

1.86

4.35

Cr

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.01

0.00

0.00

0.00

Fe3+

0.19

0.05

0.07

0.23

0.14

0.13

0.13

0.15

0.04

Fe2+

3.

483.

353.

093.

493.

793.

870.

130.

070.

081.

431.

351.

932.

432.

152.

400.

31

Mn

0.06

0.06

0.10

0.43

0.15

0.11

0.00

0.00

0.00

0.02

0.00

0.00

0.02

0.02

0.01

0.01

Mg

0.89

1.19

1.41

0.44

0.58

0.65

0.44

0.40

0.43

2.54

2.66

2.58

1.91

2.45

2.02

0.66

Ni

0.00

0.00

0.00

0.01

0.00

0.00

0.00

0.00

0.00

0.00

0.01

0.00

0.01

0.00

0.00

Ca

1.53

1.41

1.38

1.59

1.49

1.36

0.57

0.50

0.50

1.31

1.00

1.27

1.21

1.14

1.12

0.00

Na

0.00

0.00

0.01

0.02

0.00

0.00

0.43

0.51

0.50

1.02

1.35

1.11

1.21

1.08

1.29

0.18

K0.

000.

000.

000.

000.

000.

000.

000.

000.

000.

050.

040.

040.

030.

040.

031.

86

Sum

16.00

16.00

16.00

16.00

16.00

16.00

4.00

4.00

4.00

15.53

15.53

15.62

15.70

15.54

15.73

14.11


above 550°C (Maresch, 1977) and most likely below 700°C (e.g., medium-T eclogites, Mottana et al., 1990). A lower limit of 500–550°C (at P = 15–17 kbar) is also indicated by the absence of lawsonite within felsic aggregates in leuco-eclogites (e.g., above the reaction curve Lws + Jd = Zo + Pg + Qz Holland, 1979).

Pressure estimatesThe highest Jd content is recorded in omphacites from leuco-eclogites (Jd45) where this mineral coexists with quartz. In the absence of coexisting plagioclase (albite is found only within felsic aggregates in leuco-eclogites, e.g., not in equilibrium with omphacite), this indicates a minimum pressure of c. 15 kbar at 600°C (Holland, 1980). The presence of albite (An05) in fine-grained pseudomorphs from leuco-eclogites suggests that pressure never exceeded 16–17 kbar (for T ≤600°C) although the possibility of disequilibrium breakdown of plagioclase (i.e., persistence at higher pressure) cannot be ruled out. The garnet-clinopyroxene-phengite barometer (Waters & Martin, 1993 + updated calibration, 1996) gives 15.5–18.0 kbar at 600°C assuming Fe2+ = Fet in clinopyroxene (or 16–18.5 kbar at 550°C). However, the estimated pressure decreases drastically (ΔP = 2–3 kbar) when Fe2+ is used instead of Fet.

Support for these pressure estimates can be drawn from comparison with the experimental study of Poli (1993) on the amphibolite-eclogite transition (at 550–650°C and 8–26 kbar) for a basaltic composition close to the bulk compositions of the leuco-eclogites at Engebøfjellet

Partitioning of Mg–Fe2+ between phengite and garnet rims indicates temperatures of 599–674° (Krogh & Råheim, 1978) and 648–718° (Green & Hellman, 1982) for 15 kbar. These estimates might be too high, since Fe3+ in phengite was neglected. Partitioning of Mg–Fe2+ between matrix amphibole and garnet rims gives 588 to 677°C (all eclogite types) assuming Fe2+ = Fet in amphibole (Graham & Powell, 1984). Since garnet with similar ‘prograde’ zoning and inclusion pattern has been used to determine the prograde path of eclogites in the Sunnfjord and Nordfjord areas of the WGR (e.g., Cuthbert & Carswell, 1990), we attempted to trace a possible temperature change during growth of garnet poikiloblasts (e.g., Figs 7D, 8) using amphibole inclusion cores (rims have re-equilibrated with adjacent garnet) and corresponding garnet zones (based on garnet zoning). Resulting temperature estimates vary from 566–657°C (‘high-Mn’ garnet core) to 534–621°C (intermediate zone) and 575–577°C (near/within overgrowth, with omphacite inclusions). No systematic change in temperature is then found, and all estimates are in the range obtained on matrix eclogite minerals.

The spread in calculated temperature for the garnet-clinopyroxene thermometer and uncertainty on the Fe3+/Fet ratio do not allow precise estimates. However, considering all estimates above, a peak temperature of around 600°C seems likely during eclogitisation. This conclusion is supported by the actual eclogite-facies mineralogy. The presence of barroisite, not of glaucophane or edenite-pargasite, indicates temperatures

Tabl

e 3.

Con

tinue

d.

D3

& D

4 A

MPh

IBO

LITE

S &

GRE

ENSC

hIS

T fA

CIE

S V

EIN

S

E10

E10

E11

E7/0

74.0

E7/0

74.0

E7/0

74.0

SAM

PLE

4443

*14

29

10

Gt r

imA

mp

rimA

mp

rimA

mp

Chl

Ep

rim w

. Cpx

rim w

. Cpx

rim w

. Cpx

core

inte

rm.

over

grow

th

SiO

238

.10

39.9

448

.15

52.0

928

.17

37.1

6Ti

O2

0.08

0.20

0.30

0.13

0.00

0.10

Al 2O

320

.99

16.0

210

.22

1.40

16.7

421

.16

Cr 2O

30.

000.

000.

010.

040.

00Fe

Ot

28.3

022

.06

10.6

621

.61

26.1

014

.82

MnO

0.57

0.31

0.18

0.49

0.15

0.58

MgO

2.93

5.29

14.2

89.

6816

.25

0.02

CaO

9.40

11.4

511

.51

12.2

00.

0722

.23

Na 2O

0.04

1.52

1.59

0.24

0.03

0.07

K2O

0.00

0.52

0.29

0.07

0.00

0.00

ToTa

l100.43

97.31

97.18

97.94

87.56

96.14

Si6.

006.

126.

967.

825.

963.

00Ti

0.01

0.02

0.03

0.02

0.00

0.01

Al

3.90

2.90

1.74

0.25

4.18

2.01

Cr

0.00

0.00

0.00

0.00

0.01

0.00

Fe3+

0.09

0.26

1.00

Fe2+

3.

632.

571.

292.

714.

62M

n0.

080.

040.

020.

060.

030.

04M

g0.

691.

213.

082.

175.

130.

00C

a1.

591.

881.

781.

960.

021.

92N

a0.

010.

450.

440.

070.

010.

01K

0.00

0.10

0.05

0.01

0.00

0.00

Sum

16.00

15.55

15.39

15.08

19.95

7.99

* : S

truct

ural

form

ula

on 2

3 ox

ygen

s an

d Su

m c

atio

ns- (

Na+

K) =

15


(Korneliussen et al., 1998). The foliated eclogite (E8/35.9, Table 1), poor in Mn and K and with an equilibrated paragenesis of Grt + Omp + Amp (barroisite) + Pg + Ep (Ps17-18) + Qz + Rt + Py, is a good natural equivalent to the experimental material. The presence of paragonite is consistent with P ≥17 kbar (at the experimental T = 650°C). Amphibole cores in E8/35.9 have AlVI, NaM4 and Ca apfu near 1.05, 0.95 and 1.00, respectively (structural formula recalculated to Fe3+/Fet = 0.2 as recommended by Poli). This composition in an omphacite- and paragonite-bearing assemblage indicates crystallisation at pressure between 15 and 18 kbar (at 650°C). Moreover, the lack of kyanite in eclogites from Engebøfjellet indicates a maximum pressure of c. 20 kbar (for 600°C) for leuco-eclogites containing paragonite and omphacite near Jd50, as well as a maximum temperature of 750°C (at 15 kbar) deduced from the stability of Pg + Zo + Qz + Ab in pseudomorphs from leuco-eclogite (Holland, 1979).

Exhumation path: D3, D4 and D5

Stage D3 (Garnet amphibolite facies)Close attainment of equilibrium is indicated by the relative homogeneity of rim compositions in D3 shear-zone amphibolites. Several geothermobarometers have been calibrated that can be applied to such parageneses (Grt + Amp + Ep + Pl + Ilm + Qz ± Bt ± Ms), although the absence of reliable solid-solution models for amphibole and mica renders their use hazardous. For example, the garnet-amphibole geothermometer (Graham & Powell, 1984) gives unrealistically high temperatures (714–984°C, using Fet in amphibole, Table 2), probably because the D3 amphiboles with high Fe3+ and CaM4 values are not in the recommended compositional range for this calibration. Even higher temperatures and greater spread (617–1034°C, using Fe2+ in amphibole) is obtained with the calibration of Krogh-Ravna (2000b) although it should be better suited to the present calcic amphiboles. Using the Am-Pl-Czo/Ep-Qz geothermobarometer of Plyusnina (1982), rim compositions of adjacent amphibole and plagioclase in D3 amphibolites (samples E7A and E10, Table 2) indicate 525–540°C (and >8 kbar). Kohn & Spear (1989, 1990) calibrated several geobarometers on coexisting Grt + Hbl + Pl + Qz, and rim compositions of adjacent Gt-Pl-Amp give at 600°C quite consistent estimates of 9.4–12.7 kbar for both samples E7A and E10 (or 8.7–11.6 kbar at 500°C, Table 2).

Stage D4 (Epidote amphibolite facies)Application of Plyusnina (1982) to rim compositions of adjacent amphibole and plagioclase coexisting with epidote and quartz in samples E11 and E8/163.2 results in 500–525°C and 7–8 kbar. The pressure estimates for D3 and D4 consistently bracket the experimentally determined garnet-out boundary for leucogabbroic compositions (P = 9 kbar, Poli, 1993).

Stage D5 (Greenschist-facies fracturing)The composition of amphibole coexisting with Ab + Chl + Ep + Qz gives 300°C and 3–4 kbar (Sample E7/074.0, Table 2) using the empirical calibration of Triboulet (1992). These estimates are broadly consistent with the expected stability field of such a calcite-bearing assemblage in a Fe-bearing system (Liou et al., 1985; Cho & Liou, 1987).

Discussion

Basis for the exhumation model

Deformation and metamorphism of the Engebøfjellet Eclogite and the surrounding rocks are separated into stages D1 to D6 (Fig. 4). Several aspects are crucial for kinematic analyses of the stages: On the map-scale, the truncating nature of shear zones and/or marginal, large drag-related folds may indicate the separation across the zone, or overall vergence. On the meso- and micro-scale, shear-sense indicators may be used to confirm the local shear, such as: asymmetrical shear folds, composite ductile shear fabrics, outcrop-scale stacked units and foliation/bedding duplexes, winged porphyroclasts and shear-related veins and fractures (e.g., Hamner & Passchier, 1991). In addition, sense-of-slip along brittle faults can be deduced by slip-related lineations (see Petit, 1987). The D1 stage is expressed as isoclinal folds of the primary layering, and a dominant eclogitic S-tectonite type transposition foliation. A subhorizontal E–W orientation of the F1 axes (Fig. 6A) may indicate N–S shortening, but segmentation of the eclogite lens into individually rotated and deformed bodies during superimposed deformation undermines a reliable interpretation. The following eclogitic D2 stage (Fig. 4) reveals steeply plunging tight to isoclinal folds of S1, with an associated spaced cleavage. More highly strained parts show a penetrative, LS- to SL-foliation and a distinct WNW–ESE-oriented stretching lineation. This foliation represents the dominant fabric in the Engebøfjellet Eclogite. Kinematic indicators in the D2 high-strain zones reveal both dextral and sinistral shear-senses for separate zones, respectively. The general lack of asymmetry combined with the structural orientations indicates subhorizontal (in present position, see below) coaxial N–S shortening during the D2 phase.

The amphibolite-facies S3 foliation represents the main fabric in the country rocks around the eclogite body. In the eclogite, this stage is characterised by folds of S1/S2, with a spaced cleavage. Distinct, metre-wide, LS-type shear zones (S3) were generated in the fold limbs, or reactivated the S2 foliation. Abundant D3 shear-sense indicators are consistent with non-coaxial sinistral shear along the subhorizontal E–W stretching lineation (Figs 5D, 6G). The D4 stage is revealed by localised metre-wide shear zones. Shear-sense indicators are similar to those of


Considering the similarity of P–T estimates obtained by ‘classical’ geothermobarometry for all these localities and Engebøfjellet, it is likely that recalculation of our analyses with THERMOCALC would similarly lead to higher pressure estimates of around 22–23 kbar for D1 + D2. However, such high pressure estimates will be in conflict with observed phase relations at Engebøfjellet and absolute constraints provided by experimental reaction boundaries: a maximum limit for the pressure of 17–18.5 kbar at T= 600–700°C indicated by the persistence of albite in leuco-eclogites (reaction boundary Jd-Ab-Qz; Holland, 1980). Again, because of the similarity of the older P–T estimates, it is not likely that eclogitisation at Engebøfjellet would have happened at a much lower pressure than for the other Sunnfjord eclogite bodies. In mafic eclogites, with mostly divariant mineral assemblages, both classical thermobarometry and P–T grid calculations using thermodynamic databases (e.g., THERMOCALC) are fraught with the same problems (estimation of Fe3+ content and poorly constrained solid-solution models for Fe3+ and Na-bearing pyroxene, amphiboles and micas). Considering this, we have tried to better constrain the P–T estimates by direct comparison with experimental work (as outlined in 6.1 above). Improved solid-solution models for these minerals might in the future help to resolve the discrepancy between ourestimates and the newer P–T estimates for the Sunnfjord eclogites (cf., Diener & Powell, 2012, and references therein).

Preferred estimates on retrograde shear-zone amphibolites are c. 525–550°C and 9–12 kbar (D3 stage) and 500–525°C and 7–8 kbar (D4). Greenschist-facies fracturing (D5) occured at c. 300°C and 3–4 kbar. The resulting retrograde P–T path is similar to the one proposed by Krogh & Carswell (1995) for the nearby Naustdal eclogite, although at slightly lower temperature. In particular, no increase in temperature is recorded during initial decompression, a notable difference from the path established for Naustdal. The only other P–T path defined with some detail in the Sunnfjord area has been obtained on the high-pressure Hyllestad mica schists (Hacker et al., 2003; Labrousse et al., 2004). It is consistent with our P–T data for D3 and D4; the whole P–T path for Engebøfjellet (to D5) showing possibly a slight shift to lower temperature. Our P–T estimates for D1+ D2 would fit nicely as a higher P extension to the P–T path drawn by Hacker et al (2003) for these mica schists but differ drastically from their P–T estimates for the Lavik eclogites from the same area, as discussed above.

No indisputable record of the prograde path was found. Even ‘prograde’ zoned garnet poikiloblasts fail to demonstrate an amphibolite-facies precursor. The lack of correlation between garnet zoning and compositional trends for amphibole inclusions (see also the lack of trend in temperature estimates, although this argument is rather frail) may indicate that these strongly zoned garnets grew entirely under eclogite-facies conditions

the D3 stage, i.e., subhorizontal sinistral shear; hence, the D4 phase may represent a progressive continuation of the D3 stage.

The N–S subvertical orientation of the D5 fractures and their tensile nature suggest N–S shortening or E–W extension. Faults of the subsequent subgreenschist-facies D6 stage divide into two populations: subvertical NNW–SSE and NE–SW-striking strike-slip faults, bisected by N–S striking, moderately to steeply east- and west-dipping normal faults. Kinematic indicators suggest formation of the faults during N–S shortening, which triggered E–W extrusion/extension on conjugate shear fractures (see Braathen 1999, and his description of fracture population 2).

The P–T domains for the various stages (D1–D5) are plotted in Fig. 10. Peak conditions for eclogite facies at Engebøfjellet are c. 600°C and 15–18 kbar (undifferentiated D1 and D2). These estimates are similar to those previously proposed for eclogites in this inner Sunnfjord area and in accordance with the general metamorphic gradient for eclogite-facies metamorphism in the WGR (Krogh & Carswell, 1995). When the same geothermobarometric methods are used, our P–T estimates for D1+D2 at Engebøfjellet are compatible to those obtained on various Sunnfjord eclogites by other authors. However, newer P–T estimates using THERMOCALC (or similar thermodynamic database methods) indicate significantly higher pressures (although similar or somewhat higher temperatures) for the eclogite-facies event. At Kvineset in inner Sunnfjord, the P–T conditions for the eclogite facies of 517–561°C and 16–17 kbar estimated by Cuthbert at al. (2000) using Grt-Cpx and Grt-Cpx-Ph geothermobarometry have been recalculated by Labrousse et al. (2004) to 561–613°C and 20–22 kbar using THERMOCALC. In the Dalsfjord area, some 25 km south of Engebøfjellet, Foreman et al. (2005) estimated a peak temperature of 537–633°C and pressure of 17±2 kbar on the Drøsdal eclogite body using, respectively, Grt-Amp thermometry and Grt-Cpx-Ph barometry, but higher T=720–830°C and P=19–21 kbar using THERMOCALC. In the same area, Engvik & Andersen (2000) estimated that eclogitisation at Vårdalneset occurred at 677°C and 16 ± 2 kbar using Grt-Cpx thermometry and Grt-Cpx-Ph barometry, while Engvik et al. (2007) recalculated these conditions to 635°C and 23 kbar using Grt-Omp-Ky-Ph-Qz thermobarometry and THERMOCALC. In the Solund–Hyllestad–Lavik area, preferred P–T estimates by Hacker et al. (2003) for the Lavik mafic eclogites are 700°C and 23 kbar, using THERMOCALC. It is to be noted that THERMOCALC recalculation of the peak metamorphic conditions for the high-pressure Hyllestad mica schists ( consisting of Grt + Ph + St + Ky + Cld +Qz+ Rt) to 575°C and 16 kbar by Hacker et al. (2003) differ very little from the original estimates of 570–600°C and 15 kbar obtained by Chauvet et al. (1992).


(with a possible correlation: core = D1, rim/matrix = D2?); the garnet zoning being controlled by chemical fractionation rather than by drastic P–T changes. If this is true for those rocks found near lithological contacts and most susceptible to react under metamorphic events prior to eclogite metamorphism, this suggests that most of the present ferrogabbroic rocks may have undergone direct transformation from metastable gabbro to eclogite, as this is undoubtedly the case for the leuco-eclogites with pseudomorphic gabbroic texture.

Exhumation model

In order to evaluate the kinematic data of the Engebøfjellet Eclogite in a regional perspective, the initial orientation of the various fabrics has to be established. The Engebøfjellet Eclogite is located in the northern limb of a regional E–W-trending antiform, which is slightly double-plunging and tighter to the west. This structure is located in the lower plate, beneath distinct mylonites

defining the NSD zone (e.g., Braathen et al., 2004), and in rocks that have not been involved in the deformation of this detachment. Since the tight antiform is truncated by the openly folded overlying detachment, this regional folding occurred prior to formation of the NSD zone. This touches on a general discussion as to how the NSD zone should be defined as, for instance, Marques et al. (2007) argued for an increased vertical shortening with increasing metamorphic conditions (increased depth towards the east) in the overall zone separating the WGR from the upper plate (see below).

The variation in the orientation of D5 subvertical fractures may be used as a guide to relatively young folding. They strike N–S in the Engebøfjellet eclogite, i.e., in the northern limb of the Førdefjord antiform, and NW–SE in the southern limb (Fig. 6J). This minor difference indicates that the region was significantly folded prior to D5 fracturing and subsequent minor folding. In conclusion, the D5 fractures were probably

Figure 10. Model of structural evolution and P–T path for Engebøfjellet. For deformation stages D1 to D6, see Fig. 4 and text. Arrows indicate the orientation of the shortening or stretching axis. UC – upper crust; LC – lower crust; DT – detachment; F – fold.


Such a slow thermal response most likely relates to extremely rapid uplift, linked to the period between our stages D1/D2 and D4. Geochronology of the region can constrain this event, and includes: (i) a 405±5 Ma age for eclogitisation (D2?) (Terry et al., 2000; Root et al., 2004, Glodny et al, 2008) (ii) a titanite cooling (c. 500–550°C) age of c. 395 Ma (Tucker et al., 1987; Terry et al., 2000), (iii) 40Ar/39Ar white-mica ages of around 400 Ma (<400 Ma under, and >400 Ma above the NSD) in the Sunnfjord region (e.g., Andersen, 1998), and (iv) a 40Ar/39Ar biotite age of 395 Ma obtained from a pegmatite in a top-W shear zone in the Dalsfjord area (Eide et al., 1999). When these ages are applied to the Engebøfjellet area, they indicate that roughly 30 km of unroofing occurred in <10 Myr, between 405 and 395 Ma. Farther north, in the Nordfjord and Møre area, even more substantial unroofing took place (Terry et al., 2000). The second P–T segment (D4 to D5) was characterised by cooling from c. 525°C to 300°C and lower (brittle conditions), associated with a fairly small drop in pressure (3–4 kbar, from 7 to 3 kbar or lower). According to Eide et al. (1999), using 40Ar/39Ar data (feldspar, phengite, biotite), a phase of rapid cooling took place in Sunnfjord in Early Carboniferous time (c. 360 Ma), which may well apply to our D6 stage. This brackets the time span of this P–T segment to a period of c. 30 Myr.

Comparison with previous models for the WGR

The prolonged tectonic scenario for Engebøfjellet shows similarities with the proposed setting of nearby areas. Along Dalsfjord to the south, Engvik & Andersen (2000) envisage that two eclogite-facies fabrics were formed by bulk horizontal shortening and vertical stretching of the deep crust, related to plate convergence, followed by rapid decompression during orogenic extensional collapse. There is a clear similarity in estimated pressure for the Dalsfjord (Engvik & Andersen, 2000; Foreman et al., 2005) and Engebøfjellet eclogites (if estimated by the same method; see discussion above); both reveal two eclogite fabrics related to the same pressure regime (c. 18 kbar). The estimated P–T domain for the retrograde amphibolite fabric (c. 550°C and 10 kbar) is also strikingly similar, thus we suggest that the crustal position of these units was more or less identical through the D1(?)-D2-D3 stages. Northwards, in the Molde region, i.e., marginal to the Møre–Trøndelag Fault Complex (MTFC), cooling ages on titanite from the WGR (c. 402–395 Ma; see Labrousse et al., 2004; Hacker et al., 2010) and monazite U–Th–Pb ages of eclogites (407–395 Ma; Terry et al., 2000) suggest that regional uplift and homogeneous cooling occurred in a very short period of time. Interestingly, titanite from isoclinally down-folded Caledonian nappes is not reset, revealing an Ordovician age, implying that a major extensional detachment existed between the WGR and the nappes prior to the intense folding (Tucker et al., 1997). Furthermore, Robinson (1995) concluded that the folding was contemporaneous with or followed by extension in a sinistral shear field.

generated during N–S shortening and/or E–W extension, and could be explained as cross-fold fractures that were slightly folded during progressive strain.

Since no unique temporal strain marker of certain Devonian age can be found in the lower plate, reconstructions are debatable. However, if the north limb of the antiform is rotated to horizontal (rotation axis 265/0, rotation plane 265/70; i.e., disregarding the slight westerly plunge), the D1 to D4 fabrics restore to near-horizontal attitudes (Fig. 6H, I). In this case, the D2 eclogitic fabric records coaxial, near-vertical shortening and approximately horizontal E–W stretching, followed by D3 and D4, amphibolitic, top-W, sub-horizontal shear. It is noteworthy that this restoration reveals a good fit with the Dalsfjord eclogites (Engvik & Andersen, 2000). If valid, one implication is that the lower plate was subjected to amphibolite -facies top-W shear prior to regional tight folding, the folds later being decapitated by the NSD zone.

As discussed by Braathen (1999), the following D6 brittle faults correlate with a later stage of N–S contraction and E–W extension/extrusion. This fracture system can relate to regional folding as conjugate shear and cross-fold fractures. Since the fault system has an equal character in both the upper and the lower plates, it suggests that regional uniformity was established at this time; a latest Devonian – Early Carboniferous age has been indicated (discussed in Braathen, 1999).

Based on the kinematics and structural restoration of the Engebøfjellet region, the following sequence of events can be envisaged (Fig. 10): (i) D1 phase of eclogitisation of uncertain origin, (ii) D2 phase of eclogitisation, consistent with vertical flattening and horizontal E–W stretching, (iii) amphibolite-facies, highly ductile, top-W shear (D3-D4) in the WGR, (iv) regional E–W folding (D5), (v) truncation of fold(s) by renewed top-W shear, this time on the more discrete NSD zone, (vi) regional E–W folding of the NSD plus upper- and lower-plate rocks (D6), and (vii) truncation of the folded NSD zone by narrow, low-angle, (semi-)brittle faults at the base of the upper-plate rocks (e.g., Braathen, 1999; Braathen et al., 2004). Some authors argue that these late faults are gently folded (e.g., Torsvik et al., 1997; Krabbendam & Dewey, 1998). This evolution in a general sense mimics that proposed by Johnston et al. (2007). However, the overall link between regional folding and detachment development remains enigmatic: did the extensional detachment(s) repeatedly lock during transpression and unlock (reactivate) during transtension, or did it/they experience progressive development? Our observations support a step-wise evolution, as illustrated in Fig. 10.

In linking the regional P–T path with the tectonic model outlined above, two main P–T segments appear. First, significant decompression from c. 18 to 7 kbar occurred under near-isothermal conditions (c. 600°C to 525°C).


According to Terry et al. (2000), early UHP eclogites (407 Ma) were thrust upward and retrograded to HP eclogite, before becoming incorporated in amphibolite-facies top-W and sinistral shear at approximately 395 Ma.

ConclusionsTo conclude, our results show similarities to the tectonic framework established both to the south and to the north. In particular, the D2 eclogite-facies and D3-D4 amphibolite-facies setting for Engebøfjellet is consistent with that in Dalsfjord, where eclogitic coaxial as well as top-W shear structures are retrograded by amphibolitic top-W shear zones (Engvik & Andersen, 2000). Northwards, folding plays a major role in the post-eclogite tectonic scenario (e.g., Terry et al., 2000; Labrousse et al., 2004; Hacker et al., 2010), as we have also found for the Engebøfjellet Eclogite.

Acknowledgements. We are thankful to T.B. Andersen and A. Engvik for their very constructive reviews; E. Eide, A. Korneliussen, J. F. Molina, P. T. Osmundsen, P. Robinson, M. Terry and D.A. Carswell are acknowledged for discussions and critical reading of the manuscript. Participants to the NGU-Dupont Rutile Project headed by A. Korneliussen are thanked for various help. We thank NGU, UNIS and UiO for funding his work on this manuscript. Permission to publish was granted by Dupont (R. McLimans) and ConocoPhillips Norge (J. Egeland)..

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Appendix

Analytical methods

Minerals were analysed on a CAMECA Camebax Microbeam electron microprobe at the Mineralogical Museum, Oslo. Analytical conditions were accelerating voltage 15 kV, counting time 10 s on peak. Beam current of 10 or 20 nA and focused beam were used for analysis of garnet, pyroxene, amphibole, epidote and sphene, 10 nA and raster size of 10 by 10 μm for feldspar, micas and carbonate. Natural and synthetic silicates and oxides were used as standards, and analyses corrected according to the PAP procedure. Structural formulae and Fe3+ were calculated assuming stoichiometry on 24 oxygen and 16 cations for garnet, 6 oxygen and 4 cations for pyroxene (e.g., Morimoto et al., 1988), 12.5 oxygen and Fet as Fe3+ for epidote minerals, and on 8, 22 and 28 oxygen, respectively, for plagioclase, micas and chlorite. Among the possible recalculation schemes for amphibole (e.g., Leake et al., 1997), only those on 23 oxygen (Fet = Fe2+) and on total cations excluding Ca,Na,K= 13 (and for some calcic amphiboles, sum cations excluding Na,K= 15) satisfied crystallochemical constraints for most amphiboles. The recalculation scheme on 13 cations was chosen when plotting amphibole compositions, corresponding here to the maximum allowed Fe3+ and NaM4 apfu; relative compositional trends are, however, little affected by the recalculation scheme adopted. Abbreviations for mineral names are taken from Whitney & Evans (2010) with the addition of Bar (barroisite), Aeg (aegirine) and CaTs (Ca-tschermak component)

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