Structural and Metamorphic History of the Engebøfjellet Eclogite and the Exhumation of the Western Gneiss Region, Norway

NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 53

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

Alvar Braathen & Muriel Erambert

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.

Introduction extension of the thrust welt, through successive stages (Andersen & Jamtveit, 1990; Engvik & Andersen, 2000; Large-scale folds (corrugations) parallel to tectonic Labrousse et al., 2004; Hacker et al., 2010) of horizontal transport on extensional detachments are documented shortening, vertical flattening and subhorizontal for both oceanic and continental core complexes (e.g., stretching, and large-magnitude extensional shearing Cann et al., 1997; Osmundsen and Andersen, 2001; and exhumation of the footwall of the Nordfjord–Sogn Jolivet et al., 2004; Tani et al., 2011), but their formation Detachment (NSD). Alternatively, or additionally, as and importance in crustal-scale processes are still proposed by Terry et al. (2000), thrust stacking at deep- debated. There is a link to orogenic processes at deep- crustal levels concurrent with transcurrent movements crustal levels and especially eclogitised (or serpentinised) played a major role in addition to the extension in the dense crustal roots, which are seen as fundamental for northwesternmost part of the WGR. Other generally the potential energy of the crust and its buoyancy (e.g., proposed mechanisms for exhumation of subducted Ahrens & Schubert, 1975; Austrheim, 1991; Dewey et crust include thrusting accompanied by erosion (Avigad, al., 1993). Eclogites are rare at the Earth’s surface, hence 1992) or upper-plate extension (Jolivet et al., 1994), and the possibility to study exhumed lowermost crust is buoyancy forces driving combined reverse and normal limited to a few areas, with western Norway as a classical faulting (Chemenda et al., 1997). province (Fig. 1). There, eclogites are present both in Caledonian nappes (e.g., Bergen Arcs) and in the sub- In this contribution we examine the eclogitisation and Caledonian parautochthonous basement (Western unroofing models for western Norway (e.g., Johnston Gneiss Region; WGR). According to models proposed by et al., 2007; Hacker et al., 2010). Our extensive dataset Andersen et al. (1994) and Dewey et al. (1993), eclogites is from the large Engebøfjellet Eclogite body, and formed at extreme depths (>100 km) beneath overriding surrounding rocks, the former mapped in great detail thrust sheets near the centre of the Caledonian orogen. (more than 15 km of core have been drilled) by DuPont Subsequent unroofing of the eclogites occurred during and the Geological Survey of Norway (NGU) because of 54 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY a Lofoten b basement 9° 11° window 64°

Saltfjellet basement window Trondheim CENTRAL and Børgefjellet MTFC WESTERN basement NORWAY window Central Norway basement window M ? zone ? ? Molde ? UHP røndelag 5° Møre-T Western Gneiss Fault Complex Region (WGR) N NSD WGR ? ? NSD 62° zone HP Lærdal-Gjende D fault complex Fig. 2a Norway Sweden S Jo

150 km Jo BA Bergen

Permian magmatic rocks Devonian sediments Crystalline allochthons Oslo Outboard allochthons Sediments, allochthon

Sediments, autochthon 5° Stavanger Crystalline autochthon 59° Extensional shear zones 11° 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.

its ore-grade rutile. Data on (1) lithologies, (2) structures, mylonitised rocks that relate to both the footwall and the (3) petrography and (4) mineral chemistry form the hanging wall units (Andersen & Jamtveit, 1990; Wilks & basis for discussion of the (a) structural evolution and Cuthbert, 1994; Braathen et al., 2004). (b) P–T-(t) path, both contributing to the evaluation of the WGR eclogitisation and unroofing history related to Beneath the detachment, the WGR consists of various either regional transpressional-transtensional tectonics deep-crustal orthogneisses and migmatitic gneisses and localised lower-crust instability. intercalated with supracrustal rocks. It contains numerous enclaves of meta-anorthosite, metagabbro/ amphibolite, ultramafic rocks and, in the west and Geological setting northwest, subordinate bodies of eclogite (Milnes et al., 1997). The eclogite protoliths are generally The mountain-ridge Engebøfjellet is locatetabled along gabbroic, but some eclogite-facies rocks are dioritic or the north coast of the Førdefjord in the Sunnfjord granodioritic (e.g., Korneliussen et al., 1998; Engvik et region, WGR (Fig. 1B). Rocks of the Sunnfjord region al., 2000). The ages of the WGR protoliths vary, but are can be conveniently divided into two major units, below mainly Mesoproterozoic (e.g., Kullerud et al., 1986; and above the regional NSD zone (Norton, 1986). This Skår, 1998; Austrheim et al., 2003; Krogh et al., 2011). major extensional shear zone is characterised by various NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 55

Typically, mineral assemblages in country-rock gneisses apparently intrude the Hegreneset complex. All rocks in the western part of the WGR are amphibolite facies are deformed, and a pronounced foliation is generally or, locally, granulite facies (e.g., Krogh & Carswell, parallel to transposed lithological contacts. The overall 1995; Korneliussen et al., 1998; Hacker et al., 2010). A structure in the region is that of a broad E–W trending temperature gradient from <550°C in the southwest and double-plunging antiform (Fig. 2), the Førdefjord (Sunnfjord) to c. 800°C in the northwest (Nordfjord and antiform, which is tighter in the west near Engebøfjellet. Møre) is recorded for eclogite-facies relics (Labrousse et This regional structure probably relates to or was al., 2004; Hacker et al., 2010). A concurrent increase in enhanced during formation of the synclines hosting the pressure is found from Sunnfjord with peak pressure at c. Devonian basins. The map pattern also shows that the 18 kbar (e.g., Cuthbert et al., 2000) to the Nordfjord area antiform was decapitated by the later, openly folded, NSD with P >28 kbar, as indicated by the presence of coesite- zone (Fig. 2, cross-section X–X”). bearing UHP eclogites (Smith, 1984; Wain, 1997) and of microdiamonds (Dobrzhinetskaya et al., 1995). Lithology and structure at Engebøfjellet The Caledonian rocks in the hanging wall of the NSD zone can be divided into nappes of pre-Caledonian The eclogite lens consists of several rock types (Fig. 3A), basement with an overlying cover sequence, exotic of which two are massive. A ferrogabbroic eclogite (or nappes, and unconformable, Devonian, supradetachment ‘ferro-eclogite’) forms a wide core and several smaller basins of low-grade, coarse-clastic sediments (e.g., bodies along the southern margin of the main lens. It can Andersen & Jamtveit, 1990; Furnes et al., 1990; be divided into a darker garnet- and rutile-rich (truly Osmundsen & Andersen 2001). The initial Scandian ferrogabbroic) and a Fe- and Ti-poorer ‘intermediate’ thrusting was towards the SE, whereas late-Caledonian type. A leucogabbroic eclogite (or ‘leuco-eclogite’) deformation included a reversal in transport towards dominates in the west. These massive rock types have the western hinterland, due to extension of the orogenic more foliated counterparts. The well-foliated and layered welt (Andersen, 1998; Fossen, 2000, 2010). The Devonian margin of the ferro-eclogite, some metres wide, defines basins are interpreted to have formed during the a transition to more strained parts of the leuco-eclogite. extension prior to, or during, folding of the Devonian This foliated rock is present in a broad area around the sediments and other rocks into a series of E–W-trending ferro-eclogite, as illustrated in the cross-sections. Other and south-verging synclines and anticlines (Roberts, foliated rocks include (i) quartz-phengite gneiss, (ii) 1983; Chauvet & Seranne, 1994; Krabbendam & Dewey, marginal rocks surrounding the major eclogite body, 1998; Braathen, 1999; Osmundsen & Andersen 2001; with eclogite lenses and layers in a sheared quartz- Sturt & Braathen, 2001; Osmundsen et al., 2006; Johnston mica matrix, and (iii) a unit with alternating dm- to et al., 2007). m-thick eclogite, amphibolite and quartz-rich layers, all commonly retrograded. Unit (iii) is well developed north The present position of high-pressure rocks in western of the main body. Rocks found around the main eclogite Norway shows that the WGR was exhumed from are amphibolites, dioritic gneiss and granitic augen gneiss significant crustal depths during the late stages of that include numerous lenses of retrograded eclogite. the Caledonian orogeny (e.g., Andersen & Jamtveit, 1990; Andersen et al., 1994; Hacker et al., 2010). This The overall preservation of the eclogite parageneses is decompression may have been initiated during upper fairly good in both the massive rock types and their foliated and middle crust extension at 410–400 Ma (Andersen, counterparts (except iii). In the huge volume represented 1998; Fossen & Dallmeyer, 1998; Fossen & Dunlap, 1998), by the central part of this body, retrogression is limited contemporaneous with, or closely following, the UHP / mainly to the amphibolite-facies shear zones (Fig. 3A), HP eclogitisation of the lower crust at around 415 to 400 where near complete recrystallisation occurred, and Ma (Mørk & Mearns, 1986; Terry et al., 2000; Carswell et adjacent areas with local static retrogression (see Table 1). al., 2003; Root et al., 2004, Young et al., 2007; Glodny et al., 2008; Kylander-Clark et al., 2009; Krogh et al., 2011). Macro- and mesoscopic overprinting structural relationships are found throughout the study area. A The Engebøfjellet Eclogite is an elongated lens with structural evolution linked to metamorphic development an E–W long axis. Its length exceeds 3 km, whereas its designated D1 to D6 (Fig. 4) can be established based on width is at most approximately 1 km. In map-view refolding of folds and cross-cutting foliations/fractures. the body is asymmetric with a sigmodal shape (Figs. The Engebøfjellet Eclogite is bounded by amphibolitic

2, 3A). Surrounding rocks have been divided into the D3-sheared and foliated country-rock gneisses (Fig. 3A) Hegreneset and Helle complexes by Korneliussen et al. characterised by a well-developed fabric with a mineral

(1998). The Hegreneset complex comprises tonalitic stretching lineation (L3, for mineralogy, see below). to dioritic and granodioritic mica-bearing gneisses, Throughout the area, D3 structures have a consistent and mafic to ultramafic rocks, some eclogitised like the orientation, conforming to regional observations (e,g., Engebøfjellet eclogite. The Helle complex consists mostly Engvik and Andersen, 2000; Osmundsen and Andersen, of migmatitic granitic to granodioritic gneisses that 2001). The S3-foliation strikes E–W and dips steeply, generally to the north or subordinately to the south. The 56 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY

Upper plate rocks HEGRENESET COMPLEX HELLE COMPLEX Phyllonite and Eclogite Granitic gneiss blastomylonitic gneiss Grey gneiss, amphibolite, Granitic to granodioritic gneiss Sheared- metagabbro, eclogite retrograded gneiss Anti/syncline Y X Z

Førdefjord Engebøfjellet X' Y' Z' X'

X" Y" Z"

0 5 10 Kilometers A

appro N ordfjord-S B detachement z x. Horizontal : vertical = 1:1 base

3 Km og n one

X” X’ X

sealevel

Engebø- fjellet Eclogite shear zone ? ? Y" Y Y' sealevel

?

Z" Z

sealevel Z'

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).

L3 stretching lineation has a moderate to subhorizontal Cross-cutting relationships are exemplified by two plunge either to the east or to the west, as is also the case internal foliations (D1 & D2) of the eclogite lens, which for the F3-folds. formed prior to the surrounding foliation (D3): they are NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 57

LEGEND Trend of fabric/foliation: Ferro-eclogite Lithological contact gite Eclo Leuco-eclogite S1 Road / coastline Amphibolite S2 or S3 stream + sheared eclogite Qtz-Phe gneiss S3 Dioritic + granitic gneiss D3 + D4 main shear zones B C A

D 12

14

B’ D’ C’ 16 15

N A’ D” C” B” 1 km A D'' D 300 B

200

Drill 100 hole

? 0

? metres above sealevel ? ? D' C'' C

400

Drill 300 hole

200 Drill 100 hole

0 ? metres above sealevel C' ? ? ?

B'' B' B

400 Drill hole 300

? 200 ? 100

0

metres ? Figure 3. (A) Bedrock map of the Engebøfjellet Eclogite and above sealevel country rocks, simplified from Korneliussen et al. (1998). ? Sampling locations (arrows), from E to W: 14 (quarry - sam- A' A ples E7A, E7B, E10, E11, ME38/96A), 12 (E2), 15 (ME39/96), 16 (ME1/97, ME2/97). Other samples (labelled Ex/x) are from 300

200 Drill drillcores, see Korneliussen & Erambert (1997). Inset: outline hole

of the first-order eclogite lens, with deduced sense of shear. (B) 100

Cross-sections of the Engebøfjellet Eclogite and country rocks. 0 The profiles are located in Fig. 3A. The rock distribution in the

metres subsurface was constructed using both drillhole data and down- above plunge projection of observed structures. Since most folds and sealevel cut-off lines have steep plunges that change along strike, the LEGEND Dioritic gneis method is regarded as uncertain: deeper portions of the cross- Granitic augen gneis Ferro-eclogite Horizontal : vertical = 1:1 Amphibolite sections are interpretative, although consistent with drillhole Leuco-eclogite and gravimetric data. The margin of the first-order eclogite lens 300 m Qtz-Phe gneiss Tectonic contact Tectonic contact, inferred Sheared eclogite shows lateral changes in rock composition: this boundary is out­ Primary(?) lithological contact Partly retrograded eclogite Internal fabric/foliation lined be­tween the various cross-sections. lenses in Qtz-Phe matrix 58 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY ferrogabbroic sample ferrogabbroic transition D3 to D4 D3 to transition Grt included in Amp cores; transition D3 transition cores; Amp in Grt included D4 to +symplectites (after Omp?, reaction 1) reaction Omp?, (after +symplectites Bt + Ms in E7A; Bt only in E10 in only Bt E7A; + Ms in Bt Ab included in helicitic Grt; minor Chl II minor Grt; helicitic in included Ab post-D1 Omp porphyrobl. post-D1 Omp (E3/33.9;E103/74.5A) reaction (1) complete; Dol+Qz =Tlc+Cal Dol+Qz (1) complete; reaction Ab in felsic aggregates in felsic Ab Grt poikiloblasts; Dol-rich Grt poikiloblasts; reaction (1) complete; (4a)/(4b) incipient (1) complete; reaction Vein in ME38/96:Qz,Amp,Dol,Pyr,Rt,Al in Vein n,Omp reaction (1) complete reaction Comments Comments/Other reactions*** Comments/Other E103/74.5B and C, E7B C, E103/74.5B and E7/074.0 E11, E8/163.2 (rims + shear bands) + shear E8/163.2 (rims E11, E11, E8/163.2 (cores) E11, E103/74.5B and C, E7B C, E103/74.5B and E7A, E10 E7A, E8/22.9 E3/33.9, E8/35.9, E5, E5, E8/35.9, E3/33.9, E103/74.5A,(E7B) E1/164.2 ME39/96, E3/101.9 ME39/96, E4/81.9 E103/103.3 A and B A and E103/103.3 E4/72.9 E3/159.8, E4/97.3, E4/130.3, E2, E2, E4/130.3, E4/97.3, E3/159.8, ME1/97 E1/101.9, ME2/97,ME38/96, E8/229.0 E4/65.9 Selected samples Selected samples Bt+Ep+Amp+Pl+Rt+Py+Qz Amp(Act)+Ep+Qz+Ab+Chl+Cal+Ilm+T tn+Mag Amp(Mg-Hbl)+Czo/Ep+Pl+Qz+Bt+Py Amp(Mg-Hbl)+Grt+Czo/ Ep+Pl+Qz+Rt+Py Amp(Mg-Hbl)+Czo/ Ep+Grt+Pl+Qz+Rt+Py±Ph±Pg Grt+Amp(Fe- Prg)+Ep+Pl+Ms+Bt+Qz+Ilm+Ap Grt+Ph+Pg+Dol+Qz+Rt (+Ab) Grt+Ph+Pg+Dol+Qz+Rt Grt+Amp(Bar)+Omp+Pg+Ph+Czo+Qz+R t+Py±Dol,Ab (1), (3), (4a), (4b), (5), (6) (5), (4b), (4a), (3), (1), Grt+Amp(Bar)+Ph+Pg+Zo/ Czo+Qz+Ab+Rt+Py Amp+Ep+Pl+Ilm+Qz+Py+Cal+Ap Grt+Omp+Amp(Bar)+Dol+Qz+Rt+Py+ Czo/Ep+Ph (1), (3), (4a), (4b), (5), (6) (5), (4b), (4a), (3), (1), Grt+Omp+Amp(Bar)+Rt+Py ± Grt+Omp+Amp(Bar)+Rt+Py Qz,Ph,Czo/Ep,Dol,Ap (1), (2), (3), (5), (6) (5), (3), (2), (1), Paragenesis * Paragenesis Main reactions or product assemblage*** Texture/assemblages thin shear bands fillingfracture/vein foliated foliated foliated foliated, coarse-grainedfoliated, foliated relic Ecl/coronitic D4 Ecl/coronitic relic coronitic/ pseudomorphic coronitic/ symplectites D4 coarse-grained relic Ecl/coronitic D4 Ecl/coronitic relic foliated, fine grainedfoliated, relic Ecl/coronitic D3 Ecl/coronitic relic Texture Bulk composition leucogabbroic leucogabbroic leucogabbroic leucogabbroic layered (felsic/mafic) layered Qz-rich leuco-eclogite leucogabbroic layered Qz-rich layered leucogabbroic ferrogabbroic ferrogabbroic ferrogabbroic/felsic layer ferrogabbroic/felsic ferrogabbroic+ intermediate ferrogabbroic+ ferrogabbroic/felsic layer ferrogabbroic/felsic Bulk composition Summaryof metamorphic parageneses WGR.at Engebøfjellet, Sunnfjord, Pseudomorphic replacements: (1) Omp = Amp + Pl ± Cpx II (2) Grt(1) Omp = Amp + Pl ± replacements: Pseudomorphic (+Omp) = Grt+ Pl ± Ep (4a) Grt (3) Ph = Bt II(rim) + Amp Pl (4b) Grt = Amp + Pl ± Ep in mafic layer + Pl ± Amp in felsic layer = Ep + Bt (6) Rt = Ilm ± Ttn (5) Amp I (Bar) = II + Pl Retrogression stage attributed to D3 or D4 according to stability of GrtD4 according to stability to D3 or stage attributed Retrogression Symbols according to Whitney & Evans (2010), in addition Bar = barroisite. & Evans (2010), according to Whitney Symbols * ** *** Eclogites with advanced coronitic retrogression and unfoliated amphibolites: unfoliated and retrogression coronitic advanced with Eclogites stage** Retrogression Greenschist facies D5 Greenschist Amphibolite facies D4 Amphibolite Grt-amphibolite facies D3 facies Grt-amphibolite D4 Eclogite-facies D1 + D2 Eclogite-facies D3 Table 1. Table parageneses: Equilibrated stage Metamorphic

Notes: NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 59

Stage Observed structures Characteristics

F1-fold Eclogite facies horizontal shortening(?); D1 isoclinal folding, S1 sub-penetrative fabric + banding

Omp needles post- S1 Eclogite facies D1 static growth of (D2?) prismatic Omp

cleavage Eclogite facies coaxial vertical S1= So F2 shortening (restored D2 position); tight to isoclinal folds, S1 penetrative foliation S2- in high-strain zones, foliation cleavage in folds Garnet amphibolite F3-fold facies

non-coaxial D3 shortening; tight + sheath folds w/ cleavage, cleavage S3-shear zones sheared fold-limbs; major rotational strain during sinistral shear.

Amphibolite facies continued non-coaxial deformation (?); D4 m-wide S4-shear band shear zones

Greenschist facies

N-S shortening and/or D5 E-W extension; tension joints vertical extension fractures

N Lower greenschist facies? (Ep+Qtz+Ank) N-S shortening on D6 map view of fault system conjugate strike-slip faults and bisecting normal faults Figure 4. Summary diagram of the structural and metamorphic evolution of the Engebøfjellet Eclogite. 60 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY

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 F fold-hinge zones surrounded by well- Figure 5. (A) Isoclinal F2 fold of the S1 foliation in ferro-eclogite. 2 Arrow points to the D5 fractures (dashed line). (B) Isoclinal F2 fold foliated, sheared-out limbs. This is well expressed by a of the S1 foliation in sheared ferrogabbroic eclogite. S1 is indicated by distinct zone of quartz-phengite gneiss, 5–10 m wide, the dotted line. (C) Partly retrograded eclogite lens in S3-foliated amp- which forms a marker band that is isoclinally folded hibolite. The rim of the preserved eclogite is outlined. The sigmodal within the foliation. The central mass of ferro-eclogite shape of the lens and associated pressure shadows indicate rotational contains a well-preserved D1-fabric (cross-section C–C”). sinistral shear. 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). truncated, or sheared and transposed, and retrograded Towards the boundary of the ferro-eclogite, the D1-fabric along the margin of the first order lens. Another is progressively deformed into tight to isoclinal, elliptical example is the relationship between the two eclogitic F2 folds. A well-developed foliation (S2) is found in fold fabrics. Internally, the eclogite lens consists of massive limbs; fold hinges develop a crenulation or spaced S2 to S1-foliated bodies of ferro-eclogite and leuco-eclogite cleavage. Mesoscopic F2-folds in the central part of the bounded by steep D2 shear zones (locally D3 reactivated). first-order eclogite lens plunge steeply NW and SE. They Massive parts reveal a lithological layering that is either plot along a vague great circle that strikes ESE–WNW isoclinally folded, or totally transposed into the eclogitic and dips steeply north (Fig. 6F). Near the boundary to

S1-foliation (Fig. 5). The N–S subvertical S1-fabric (Fig. the leuco-eclogite, a penetrative S2 fabric predominates. NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 61

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.

The S2 fabric dips NNE and SSW, whereas the L2 In the west (cross-section D–D”, Fig. 3), a distinct F3 fold- stretching lineation has a moderate plunge to the WNW, pair of the S1-S2 foliations in the ferro-eclogite and the or subordinately to the ESE to SE (Fig. 6D, E). contact towards the leuco-eclogite is present. This is well

displayed by the S1 fabric, which plot along a great circle. 62 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY

This circle defines a fold axis that plunges moderately epidote; carbonate is in some cases a major constituent. ENE, in accordance with orientations of parasitic folds These eclogites, especially the ferro-eclogites, are

(Fig. 6C). The macroscopic F3 fold-structure is tight to generally fine-grained (with garnet size ≤0.5 mm). isoclinal and cylindrical, with an elliptical fold shape Notable exceptions (e.g., coarse-grained ferro- or leuco- and an axial surface steeply inclined to the NNE. A eclogites with centimetric poikilitic or helicitic garnet, crenulation cleavage is well developed in the knee of the Table 1 and Fig. 7D) are typically found at the contacts fold. The fold-pair is terminated by an S3 shear-zone to between ferro- and leuco-eclogites. Eclogite-facies veins the north. (quartz-rich, with variable amounts of garnet, carbonate, amphibole, omphacite, pyrite, apatite and allanite) are Two types of structure have no map-scale expression. abundant in the eastern part of the body. Their internal

One type is the very common tensile joints/fractures (S5) foliation is parallel to the eclogite facies foliation (S1 or that cross-cut all fabrics (S1 to S4) of the first-order lens S2) of the wall-rock. Segregations in pressure shadows of (Fig. 5A, B). The fractures are normally 0.1 to 1 cm wide eclogite boudins and along axial planes of F1 or F2 folds and <3 m long. They have a consistent N–S, subvertical contain quartz, omphacite and rutile. orientation throughout the Engebøfjellet Eclogite (Fig.

6K); however, similar fractures in eclogite lenses of the The two eclogite-facies ‘events’, D1 and D2, recognised southern limb of the Førdefjord anticline strike NW– in the field are not easily distinguished from one

SE. These fractures thus constitute a marker that may be another in thin-section. The S1 and S2 foliations, and the folded on a regional scale (see discussion). Mesoscopic F2-related cleavage, are defined by the shape orientation brittle faults (D6) truncate the D5 fractures. The faults of omphacite, amphibole, clinozoisite, white micas, oxide show cm-scale separation of marker beds or bands, and strings, quartz ribbons and apatite (Fig. 7C). Elongated net-slip is suggested from slip-lines on the fault surfaces. omphacite and amphibole form wedge-shaped lenses,

Two types of faults are present; conjugate strike-slip surrounded by a finer grained matrix, especially along S2. faults and bisecting normal faults. The strike-slip fault, all This suggests dynamic recrystallisation during eclogite- with steep dips, strike NNE–SSW (dextral) and NNW– facies shearing. In some leuco-eclogites, late omphacite

SSE (sinistral). The normal faults strike N–S and dip porphyroblasts (up to 3–4 cm in length) grew over S1 steeply east or west (Fig. 6L). (Fig. 7B). They are randomly oriented and commonly

found near F1 fold-hinges. Such overgrowth is not Petrography documented for S2 (Fig. 4), thus this omphacite seems to post-date S1 and pre-date S2. The lack of orientation Eclogite-facies parageneses and fabrics suggests growth under static conditions. Similarly, in

All rocks in the Engebøfjellet eclogite body underwent intermediate and leuco-eclogites, post-D1 (post-D2?) eclogite-facies metamorphism and recrystallisation; amphibole porphyroblasts are common. only ghost magmatic textures may be found. The degree of textural equilibration of the eclogites Subhedral garnet has a poikilitic core and generally an depends on their D1 and D2 deformational history. In inclusion-poor rim. In coarse-grained ferro-eclogites unstrained leucogabbroic rocks (massive leuco-eclogite), (Table 1, Fig. 7D), poikiloblast cores include amphibole, microcrystalline aggregates (Ph + Pg + Ab + Zo + carbonate, clinozoisite, rutile and pyrite; the irregular Qz; abbreviations for minerals according to Whitney overgrowth contains omphacite in addition to the & Evans, 2010) are presumably pseudomorphs after previous phases. Helicitic garnet in coarse, quartz- plagioclase. Small granular amphibole and minor mica rich, leuco-eclogites contains quartz, carbonate, rutile, replaced mafic phases. Garnet coronas separating felsic amphibole and albite (albite is found only here and and mafic domains highlight former grain boundaries of within felsic aggregates in leuco-eclogites). the parent gabbro (Fig. 7A). With increasing strain, the felsic aggregates recrystallise then disappear (foliated Amphibolite-facies parageneses and fabrics leuco-eclogites). Ferro-eclogites are generally completely In D3 shear zones and shear bands, as well as along recrystallised. the axial planes of F3 folds, major minerals are green amphibole, garnet, clinozoisite-epidote, plagioclase, Eclogite-facies parageneses comprise garnet, omphacite, ilmenite and quartz, and subordinate white mica, biotite, amphibole, phengite, paragonite, clinozoisite/epidote, carbonate and chlorite. All minerals are anhedral to quartz, carbonate, rutile, apatite and pyrite (Table 1). subhedral and fine grained. Their long axes are oriented

Accessory minerals are allanite and zircon. Ferro- parallel to the S3 foliation (Fig. 7E). Distinct planar eclogites are usually rich in garnet (from c. 25 to 55% banding, platy mosaics of quartz, wings around garnet modal), rutile (true ferrogabbroic rocks:>3 to c. 10%), and a well defined orientation of individual mineral clinopyroxene and amphibole, with minor epidote, grains all suggest mineral growth under dynamic phengite and quartz. Leuco-eclogites are rich in phengite conditions during shear deformation. For D4, similar and paragonite, clinozoisite and often in amphibole and kinematics are established as for D3, with the same quartz. In most eclogites, volatile-bearing phases are minerals and textures occurring along the S4 foliation. abundant: amphibole (up to c. 40%), mica, clinozoisite/ However, garnet and white mica disappear. In this case, NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 63

A B

X C D

X'

E F

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. too, there are indications of mineral growth during magnetite and titanite. The adjacent wall-rock shows deformation (Fig. 7F). retrogression of omphacite, blue-grey amphibole and garnet to symplectitic green amphibole, epidote, plagioclase and magnetite, as well as transformation of Greenschist-facies fracturing rutile to ilmenite or more rarely titanite.

The D5 tensile fractures contain fibrous amphibole (actinolite), albite, epidote, quartz, calcite, chlorite, The final stage of deformation (brittle faults of the D6 64 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY

stage) did not result in any significant recrystallisation of ferro-eclogites, and increase in XMg from ferro- and the eclogites and nearby rocks. Mineral growth is limited intermediates eclogites to leuco-eclogites (Fig. 9). to fracture walls, commonly coated with fibrous epidote Rims are generally higher in Al (AlIV and often AlVI) or quartz and locally with brown ankerite. 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) Mineral chemistry increases with pressure (Graham et al., 1989), this could reflect a pressure increase during early ferro-eclogite

Some 30 samples of eclogites, equilibrated D3 and D4 crystallisation, before a decrease during leuco-eclogite amphibolites and D5 fractures (Table 1) were selected for (with post-D1 – or post-D2? - amphibole porphyroblasts) electron microprobe study, covering the range in textural, crystallisation (e.g., head of the P–T loop). Inclusions in mineralogical and chemical variations observed within garnet poikiloblasts from coarse ferro-eclogites range the Engebøfjellet Eclogite (see Korneliussen & Erambert, from barroisite to magnesio-katophorite and taramite, 1997). Samples with static retrogression were excluded, a range largely accounted for by re-equilibration of since they are not well suited for geothermobarometry. inclusion rims with host garnet. Profiles consisting of Analytical procedures and recalculation methods are the largest inclusion cores (to avoid disturbance due to described in the Appendix. Selected mineral analyses are rim re-equilibration) were analysed across garnet to presented in Table 3. test whether their compositional evolution matched the ‘prograde’ zoning of the garnet. Barroisites within

Garnet in eclogites is almandine-rich, with XMg (=Mg/ Mn-rich garnet cores,however, are similar to those (Mg + Fe)) increasing from layered quartz-rich eclogites near or within the overgrowth (e.g., coexisting with

(Alm61.4-68.8 Sps0.5-7.3 Prp5.7-22.4 Grs12.2-18.4 Adr2.4-7.4) via ferro- omphacite), both plotting generally mid-way within eclogites (fine- and coarse-grained: Alm56.3-69.2 Sps0.2-7.3 the compositional range for all inclusions (see Fig. Prp7.1-19.6 Grs7.8-25.7 Adr0.0-8.8) and intermediate eclogites 9). Moreover, this reversal in amphibole composition (Alm51.8-57.3 Sps0.9-1.9 Prp15.9-23.3 Grs17.6-23.4 Adr0.1-5.2) to leuco- may follow opposite trends along different profiles eclogites (Alm46.6-60.8 Sps0.5-1.8 Prp14.6-30.3 Grs10.4-30.9 Adr0.0- within a single garnet. Amphibole in recrystallised D3 5.8). Rims have generally the highest Mg, Fe and lowest amphibolites is a ferro-pargasite or ferro-tschermakite. Mn content. Poikiloblasts in ferro-eclogites (Fig. 7D) Amphibole in D4 amphibolites is a magnesio- show a bell-shaped Mn profile, with Mg increasing and hornblende. In both D3 and D4 samples, zoning towards Ca and Fe decreasing towards the rims (Fig. 8). This the rim corresponds to an increase in Al (mostly AlIV),

‘prograde’ zoning is not paralleled by a clear evolution in Ca and usually NaA, and a decrease in Si, NaM4 and XMg, inclusion mineralogy: all are similar to matrix eclogitic a continuation of the trend already observed in eclogites. phases, although omphacite is found only near or within Some porphyroclast cores in D4 samples that have the overgrowth. A similar zoning is found in helicitic relatively high NaM4 (Fig. 9) and include garnet are likely garnet. Garnet in amphibole-poor D3 amphibolite layers relics of the D3 stage (Table 1). Fibrous amphibole in D5 overlaps in composition with eclogite-facies garnet in veins is a ferro-actinolite. 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 Foliation phengite in all eclogites and small crystals in lower Fe and Mg are recorded in amphibole-rich layers felsic segregations in leuco-eclogites has a Si core value

(Alm53.9-67.3 Sps0.3-3.2 Prp6.2-13.1 Grs14.8-30.6 Adr2.3-8.0). Rims of 6.62–6.89 apfu, decreasing to 6.36 apfu towards the tend to be more homogeneous than cores, reflecting a rim. Phengitic muscovite in S3 (sample E7A) has lower tendency towards chemical equilibrium. 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

Clinopyroxene is an omphacite (Morimoto et al., 1988) celadonite component. S3 biotites (samples E10 and E7A) tending towards aegirine-augite in ferro-eclogites. Its have Al = 2.84–3.29 apfu and XMg = 0.43–0.56, and no compositional range is Jd15.4-39.7 Aeg10.0-30.6 CaTs 0.0-2.8 in systematic zoning. Later biotites in these samples (in D4 ferrogabbroic and intermediate eclogites to Jd36.0-45.7 shear planes with biotite ± chlorite) show no systematic Aeg4.0-19.3 CaTs 0.0-3.0 in leuco-eclogites. Higher Jd values difference in composition from S3 biotites, although cannot be discounted due to possible overestimation they tend to plot at the low Al and high Mg end of the of Fe3+ (see below). No systematic zoning and no compositional range. compositional difference between inclusions and matrix omphacites was recorded. Pyroxene compositions in Clinozoisite/epidote has a Fe content (Ps= 100 Fe3+/ eclogite-facies veins and their wall-rock are similar. (Fe3+ + Al)) increasing from Ps14-18 in foliated leuco-

eclogites to Ps16-20 in intermediate eclogites and Ps20-27 Amphibole in eclogites is generally a barroisite (Leake in ferro-eclogites. In felsic pseudomorphs in leuco- et al., 1997). Core composition of matrix amphibole eclogite, it is a zoisite/clinozoisite (Ps04). Inclusions do mirrors bulk composition, with on average higher not differ in composition from matrix grains, even in Al in leuco-eclogites, layered quartz-rich eclogites garnet porphyroblasts from ferro-eclogites. Epidote in and intermediate eclogites than in fine-grained recrystallised D3 amphibolites has Ps12-24 and Ps15-17 in NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 65

C

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). 66 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY

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 VI 3+ in these amphiboles: tschermak (+ferri-tschermak; monitored by Al +Fe ), edenite (Na+KA) and glaucophane (NaM4). Note particularly the decrease in NaM4 from eclogites to amphibolites following decrease in P (see text).

D4 amphibolites (of leucogabbroic composition). Yellow Temperature estimates 2+ epidote in D5 veins is Ps31-38. Considering Fe and Mg partitioning between garnet and clinopyroxene, the complex growth zoning of garnet

Carbonate in eclogites is a dolomite/ankerite (XMg = 0.58 and the erratic zoning in clinopyroxene make it unlikely to 0.87). Carbonate in D3 and D4 amphibolites, as well that core compositions could be in equilibrium. Thus, as in D5 veins, is calcite. Plagioclase in felsic aggregates only analyses at stable grain boundaries (matrix grains or from leuco-eclogites is albite (An02-05), as are inclusions inclusion-host pairs) have been used. A major problem 3+ in garnet from coarse quartz-rich eclogite (An01). is the accuracy of Fe estimates for Na-rich pyroxene: 3+ Plagioclase in D3 amphibolites is An16-33, and An12-20 in D4 assuming stoichiometry, the calculated Fe /Fet of 0.33 amphibolites. In D5 veins, it is albite An04. Chlorite in D5 to 0.95 results in a spread in Kd and temperatures of veins is a clinochlore with Si = 5.94–6.04 apfu, Al = 4.04– 290–623°C (Ellis & Green, 1979; Table 2). The highest 3+ 4.06 apfu and XMg = 0.53. Fe values are unrealistic, as in the nearby Naustdal eclogite, whose mineralogy is similar to the present ferro-/intermediate eclogites, from which a wet chemical 3+ P–T evolution analysis of omphacite gave a Fe /Fet ratio of c. 0.6 (Binns, 3+ 1967). Assuming Fe /Fet = 0.6 in clinopyroxene from Eclogite-facies development: D1 and D2 Engebøfjellet and P = 15 kbar, temperature estimates range from 535 to 645°C (Ellis & Green, 1979) with a No significant variation in mineral compositions could peak around 590°C for ferro-/intermediate eclogites. be correlated to variations in fabrics associated to the two, Temperature estimates using Powell (1985) and Krogh- successive, eclogite-facies deformation phases. Hence, the Ravna (2000a) are systematically lower by c. 25°C and P–T estimates below are taken as representative of both 80°C, respectively (Table 2). All these temperatures may 3+ the D1 and the D2 phases. be too low, if the assumed Fe /Fet ratio is still too high for these ferro-eclogites. NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 67 9.8-9.9 10.7-11.4 10.4-11.3 11.4-12.7 10.4-11.2 9.4-10.2 E10 617-1034 K-Rb 2+ 9.8-11.0 11.0-11.9 10.3-11.7 11.5-12.7 10.5-11.7 9.7-10.5 E7A at T =600 °C at in Cpx = 0.6 t 8.8-8.9 10.1-10.7 8.9-9.6 10.5-11.6 10.1-10.3 9.2-9.9 E10 714-984 G&P84 / F e 3+ F e 8.7-9.8 10.4-11.2 neglected in Ph in neglected 8.7-10.0 15.5-18 P (kbar) 10.5-11.7 10.2-11.3 9.4-10.2 E7A T(°C) T =500°C at in ferro- and intermediatein ferro- and (see text): eclogites F e-Mg partitioningGrt between and Cpx : T estimates (E&G79) assuming 3+ = Fet for Graham & Powell (1984) (=G&P84) & Powell Graham for = Fet P6 P5 P4 P3 P2 P1 2+ P (kbar) 600 T(°C) 575-623 (4) 534-621 (4) 2.5-4.9 566-657 (6) 315-541 (30) Kd(K-Rb) T(°C) (n) T(°C) 588-677 (11) K-Ra 420-517 (30) in Grt - For Amp, Fe Amp, Grt in - For 2+ 0.453-0.511 0.493-0.503 XA,Na 0.600-0.694 Mg apfu Ph Mg apfu 648-718 5.2-6.6 G&H 82 5.3-8.2 1.9-3.8 4.4-7.0 378-601 Kd 3.7-5.6 Kd(G&P84) P85 512-623 in M1,M2,M3 sites for Krogh-Ravna (2000b) (=K-R b) (see calculation procedure in K-R b) in (2000b) (=K-R b) (see procedure calculation Krogh-Ravna for M1,M2,M3 sites in 2+ and Hellman (1982; equation 3) = G&H82. Fe 3) = G&H82. equation (1982; Hellman and and Fe Krogh-Ravna (2000a) =K-Ra - XMg, XMn, XCa in Grt calculated with estimated Fe estimated Grt in with calculated XCa XMn, (2000a) =K-RaKrogh-Ravna - XMg, 1996) and Fet for Ph and Cpx and Ph for Fet 1996) and 0.277-0.446 0.384-0.401 XA,0 6.654-6.788 Si apfu Ph 0.259-0.280 599-674 0.244-0.261 T(°C) K&R 78 0.246-0.297 0.261 X Grs 0.264 0.25-0.33 401-623 293-622 Grt XCa 0.165-0.274 XCa Grt XCa E&G79 (c) calibration for basaltic systems: Krogh & Råheim Krogh (1978 ) = K&R78 basaltic systems: for (c) calibration (a) calibrations of Ellis and Green (1979) = E&G79, Powell (1985) = P85 Powell (1979) = E&G79, of Green Ellis and (a) calibrations Fe (b) calculated T(°C) (n) (for P= 15 kbar) T(°C) (d) using simple Mg-Ca mixing model for Grt (Newton & Haselton, 1981, see reference in Waters & Martin, 1993, 1993, & Martin, Waters in see reference 1981, & Haselton, Grt (Newton for model Mg-Ca mixing simple (d) using (1990) & Spear Kohn P6, P5 and (1989); & Spear Kohn 2 in model P4, P3 and 1; P2,model (e) P1 and 535-646 0.112-0.166 0.114-0.156 XM2,Mg 4.278-4.398 Al apfu Ph Al apfu 15 0.013-0.25 0,03 P (kbar) 0.017-0.20 0.04-0.05 X Sps 0,07 0.02-0.03 11.3-52.3 11.8-153 Grt XMn <0.1 Kd 3-4 XMn Grt XMn (> 8 kbar) 7-8 P (kbar) P (kbar) 9.5-21.2 0.238-0.309 0.193-0.265 XM2,Fe 0.403-0.439 Mg apfu Cpx Mg apfu 4.9-7.3 0.101-0.117 0,12-0,14 Kd 0.106-0.127 0,12 X Prp 0,11 0.196-0.281 0.14-0.18 0.196-0.267 XMg Grt 0.12-0.32 X Ca Grt T(°C) 300 ca. XMg Grt ca. 525-540 ca. 500-525 T(°C) Grt Mn in Cpx = 0.6 : Cpx in t 0.497-0.533 0.425-0.494 XM2,Al 0.502-0.544 Ca apfu Cpx 0.579-0.615 0,43 0.576-0.609 0.42-0.53 X Alm 0.42-0.47 0.002-0.023 0.006-0.016 XMg Amp 0.43-0.64 X XMg Chl 0.53 /Fe 3+ Grt Mg assuming Fe assuming 0.436-0.469 0.456-0.507 X T1,Al 1.410-1.464 Ca apfu Grt Ca apfu 0.157-0.281 XMg Grt 0.35-0.46 Grt: 0.116-0.300 0.236-0.314 An18-26 An19-29 An12-15 Pl X XMg Amp XMg Amp (K-Rb) Ep 31-38 Amp 3+ Cpx Mg 0.32-0.36 0.37-0.39 X Mg 0.940-1.190 Mg apfu Grt Mg apfu An26 0.62-0.76 near overgrowth XMg Ph An21-26 intermediate grt core 0.30-0.38 Pl 0.732-0.925 0.843-0.985 2.73-3.01 1.7-1.9 2.49-2.89 Alt, Amp XMg Amp XMg Amp (Fet) XFe 0.69 X Al content in Amp coexisting with Ep + Pl + Qz (Plyusnina, 1982) in Amp coexisting with Ep + Pl Al content E10 E7A Amp: E10 E7A Grt-Amp-Pl-Qz geobarometers (Kohn & Spear, 1989, 1990) (e) Grt-Amp-Pl-Qz geobarometers (Kohn & Spear, rims E7A E11 E10 Amphibole coexisting with Ep + Ab + Chl + Qz (Triboulet, 1992) Amphibole coexisting with Ep + Ab Chl Qz (Triboulet, XMg Amp F e-Mg partitioning between Grt(Graham & Powell, 1984; Krogh-Ravna, 2000b) (b) and Amp 0.45 D3 D3 D4 rims Grt-Cpx-Ph barometer (Waters & Martin, 1996) (d) 1993, Grt-Cpx-Phbarometer (Waters F e-Mg partitioningGrt between and Ph (c) all eclogite types Amp inclusion cores inclusion Amp F e-Mg partitioningGrt between (b) & Powell, 1984) and Amp (Graham Matrix mineral (rims) mineral Matrix + Grt poikiloblasts Ferro-eclogites + Ferro-eclogites intermediate eclogites Leuco-eclogites F e-Mg partitioning between Grt and Cpx (a) Pressure and temperature estimates on eclogites and amphibolites from Engebøfjellet from amphibolites and eclogites on estimates temperature and Pressure acies f acies D5 acies D4 (D1 + D2) ECLOGITE f f A mp h ibolite G reensc h ist Table 2. Table 68 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY 4.35 0.03 0.04 1.86 6.69 3.32 0.66 0.18 0.13 0.01 0.00 0.70 2.78 0.37 0.31 10.88 49.93 27.57 14.11 95.70 Ph 37 ME1/97 0.00 1.86 6.97 0.02 0.00 0.03 6.98 9.07 2.02 1.29 0.04 0.01 1.12 4.43 0.18 0.22 0.00 2.40 0.00 46.59 10.51 19.18 15.73 97.19 in overgrowth 35 0.00 1.32 7.19 0.03 0.03 0.04 7.33 2.45 1.08 0.18 0.02 1.14 3.77 0.19 7.58 0.23 0.00 2.15 0.00 11.13 49.72 17.42 15.54 97.44 38 interm.Grt E103/103.3 0.00 1.89 7.56 0.06 0.09 0.03 6.92 8.59 1.91 1.21 0.18 0.02 1.21 4.19 0.18 0.50 0.00 2.43 0.01 46.42 10.75 19.50 15.70 97.96 Amp inclusions (core) in Grt 26 in Grt core 0.01 1.43 8.06 0.02 0.01 0.04 7.23 2.58 1.11 0.04 0.00 1.27 3.90 0.20 8.22 0.17 0.04 1.93 0.00 11.77 49.10 15.68 15.62 97.19 30 core E103/103.3 0.00 1.90 6.56 0.02 0.05 0.04 7.20 2.66 1.35 0.03 0.00 1.00 4.91 0.20 0.22 0.00 1.35 0.01 12.55 50.62 11.34 11.35 15.53 97.82 19 core E8/35.9 matrix Amp matrix 0.01 2.30 8.48 0.02 0.00 0.05 6.84 2.54 1.02 0.15 0.02 1.31 3.63 0.29 0.17 0.04 1.43 0.00 11.77 13.47 11.84 47.26 15.53 97.10 21 E3/33.9 rim w Grt 0.00 0.45 0.00 0.00 0.00 2.00 8.19 0.43 0.50 0.00 0.00 0.50 7.25 0.01 4.09 0.07 0.01 0.04 0.08 0.00 4.00 13.24 10.92 56.78 100.56 2.2 E3/33.9 rim w.Grt 0.00 0.38 0.00 0.00 0.00 1.99 7.49 0.40 0.51 0.05 0.00 0.50 7.39 0.04 9.07 7.45 0.03 0.01 0.15 0.07 0.00 4.00 12.98 55.79 100.31 26 ME1/97 rim w. Cpx rim w. 0.00 0.31 0.00 0.00 0.00 1.99 8.10 0.44 0.43 0.00 0.00 0.57 6.09 0.00 7.33 8.66 0.09 0.03 0.13 0.13 0.00 4.00 14.69 54.99 99.99 2.5 Cpx E4/97.3 rim w.Grt 0.00 3.87 7.99 0.00 0.04 0.00 5.99 2.74 0.65 0.00 0.80 0.11 1.36 0.00 0.00 0.03 0.00 0.13 3.87 0.00 37.67 20.65 30.13 16.00 5 100.05 overgrowth 0.00 3.83 8.70 0.01 0.00 0.00 6.00 2.42 0.58 0.00 1.11 0.15 1.49 0.01 0.01 0.08 0.01 0.14 3.79 0.00 37.67 20.39 29.51 16.00 99.93 6 interm. E103/103.3 Grt porphyroblast 0.00 3.80 9.32 0.02 0.09 0.00 5.98 1.84 0.44 0.02 3.23 0.43 1.59 0.05 0.00 0.15 0.00 0.23 3.49 0.01 37.63 20.29 28.05 16.00 100.65 10 core 0.01 3.90 8.37 0.01 0.00 0.00 6.01 6.13 1.41 0.01 0.80 0.10 1.38 0.04 0.01 0.07 0.04 0.07 3.09 0.00 38.98 21.46 24.55 16.00 100.45 2.1 E3/33.9 rim w. Cpx rim w. 0.00 3.92 8,42 0.00 0,00 0.00 6.01 5,11 1.19 0.00 0,48 0.06 1.41 0,00 0,01 0,00 0,00 0.05 3.35 0.00 38,53 21,31 26,06 16.00 99,93 25 Grt ME1/97 rim w. Cpx rim w. 0.00 3.86 9,09 0.01 0,00 0.00 5.97 3,81 0.89 0.00 0,46 0.06 1.53 0,01 0,00 0,07 0,03 0.19 3.48 0.00 37,99 20,84 27,97 16.00 100,29 2.3 E4/97.3 rim w. Cpx rim w. Microprobe analysesMicroprobe of mineralsin eclogites andamphibolites. 3 3 t 2 2 O t al O O 2 2 O 2 3+ 2+ 2 S ample Sum K K To Fe Cr Na Al CaO Ti NiO Ni Si MgO Mg Na TiO Cr MnO Al FeO Mn Ca Table 3. Table ECLOGITES SiO Fe NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 69

Partitioning of Mg–Fe2+ between phengite and garnet above 550°C (Maresch, 1977) and most likely below rims indicates temperatures of 599–674° (Krogh & 700°C (e.g., medium-T eclogites, Mottana et al., 1990). Råheim, 1978) and 648–718° (Green & Hellman, 1982) A lower limit of 500–550°C (at P = 15–17 kbar) is also for 15 kbar. These estimates might be too high, since indicated by the absence of lawsonite within felsic Fe3+ in phengite was neglected. Partitioning of Mg– aggregates in leuco-eclogites (e.g., above the reaction Fe2+ between matrix amphibole and garnet rims gives curve Lws + Jd = Zo + Pg + Qz Holland, 1979). 2+ 588 to 677°C (all eclogite types) assuming Fe = Fet in amphibole (Graham & Powell, 1984). Since garnet with Pressure estimates similar ‘prograde’ zoning and inclusion pattern has The highest Jd content is recorded in omphacites been used to determine the prograde path of eclogites from leuco-eclogites (Jd45) where this mineral coexists in the Sunnfjord and Nordfjord areas of the WGR with quartz. In the absence of coexisting plagioclase (e.g., Cuthbert & Carswell, 1990), we attempted to (albite is found only within felsic aggregates in leuco- trace a possible temperature change during growth of eclogites, e.g., not in equilibrium with omphacite), this garnet poikiloblasts (e.g., Figs 7D, 8) using amphibole indicates a minimum pressure of c. 15 kbar at 600°C inclusion cores (rims have re-equilibrated with adjacent (Holland, 1980). The presence of albite (An05) in fine- garnet) and corresponding garnet zones (based on grained pseudomorphs from leuco-eclogites suggests garnet zoning). Resulting temperature estimates vary that pressure never exceeded 16–17 kbar (for T ≤600°C) from 566–657°C (‘high-Mn’ garnet core) to 534–621°C although the possibility of disequilibrium breakdown of (intermediate zone) and 575–577°C (near/within plagioclase (i.e., persistence at higher pressure) cannot be overgrowth, with omphacite inclusions). No systematic ruled out. The garnet-clinopyroxene-phengite barometer change in temperature is then found, and all estimates (Waters & Martin, 1993 + updated calibration, 1996) 2+ are in the range obtained on matrix eclogite minerals. gives 15.5–18.0 kbar at 600°C assuming Fe = Fet in clinopyroxene (or 16–18.5 kbar at 550°C). However, the The spread in calculated temperature for the garnet- estimated pressure decreases drastically (ΔP = 2–3 kbar) 2+ clinopyroxene thermometer and uncertainty on the when Fe is used instead of Fet. 3+ Fe /Fet ratio do not allow precise estimates. However, considering all estimates above, a peak temperature Support for these pressure estimates can be drawn from of around 600°C seems likely during eclogitisation. comparison with the experimental study of Poli (1993) This conclusion is supported by the actual eclogite- on the amphibolite-eclogite transition (at 550–650°C facies mineralogy. The presence of barroisite, not of and 8–26 kbar) for a basaltic composition close to the glaucophane or edenite-pargasite, indicates temperatures bulk compositions of the leuco-eclogites at Engebøfjellet 0.07 0.00 3.00 0.01 2.01 0.00 0.58 0.02 1.00 0.10 0.00 0.04 0.00 1.92 0.01 0.00 7.99 37.16 21.16 14.82 22.23 96.14 Ep 10 E7/074.0 overgrowth 0.03 0.00 5.96 0.00 4.18 0.01 0.15 0.07 0.00 0.04 4.62 0.03 5.13 0.02 0.01 0.00 28.17 16.74 26.10 16.25 87.56 19.95 9 Chl interm. E7/074.0 0.24 0.07 7.82 0.02 0.25 0.00 1.40 0.49 9.68 0.13 0.01 2.71 0.06 2.17 1.96 0.07 0.01 52.09 21.61 12.20 97.94 15.08 2 core Amp E7/074.0 ACIES VEINS F 1.59 0.29 6.96 0.03 1.74 0.00 0.18 0.30 1.29 0.02 3.08 1.78 0.44 0.05 48.15 10.22 10.66 14.28 11.51 97.18 15.39 14 E11 Amp rim rim w. Cpx rim w. 1.52 0.52 6.12 0.02 2.90 0.00 0.31 5.29 0.20 0.00 0.26 2.57 0.04 1.21 1.88 0.45 0.10 39.94 16.02 22.06 11.45 97.31 15.55 E10 43* Amp rim rim w. Cpx rim w. 0.04 0.00 6.00 0.01 3.90 0.00 0.57 2.93 9.40 0.08 0.00 0.09 3.63 0.08 0.69 1.59 0.01 0.00 38.10 20.99 28.30 16.00 100.43 44 E10 Gt rim rim w. Cpx rim w. Continued. 3 3 t 2 2 O t al O O 2 2 O 2 3+ 2+ 2 S ample K To K Sum Si Ti Al Cr Fe Table 3. Table D3 & D4 AMP H IBOLITES GREENSC IST SiO * : Structural formula= 15 cations- (Na+K) on 23 oxygens and Sum TiO Cr MnO Na MgO CaO Fe Mn Al FeO Mg Ca Na 70 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY

(Korneliussen et al., 1998). The foliated eclogite (E8/35.9, Stage D5 (Greenschist-facies fracturing) Table 1), poor in Mn and K and with an equilibrated The composition of amphibole coexisting with Ab + Chl paragenesis of Grt + Omp + Amp (barroisite) + Pg + Ep + Qz gives 300°C and 3–4 kbar (Sample E7/074.0, + Ep (Ps17-18) + Qz + Rt + Py, is a good natural Table 2) using the empirical calibration of Triboulet equivalent to the experimental material. The presence (1992). These estimates are broadly consistent with of paragonite is consistent with P ≥17 kbar (at the the expected stability field of such a calcite-bearing experimental T = 650°C). Amphibole cores in E8/35.9 assemblage in a Fe-bearing system (Liou et al., 1985; Cho have AlVI, NaM4 and Ca apfu near 1.05, 0.95 and 1.00, & Liou, 1987). 3+ respectively (structural formula recalculated to Fe /Fet = 0.2 as recommended by Poli). This composition in an omphacite- and paragonite-bearing assemblage indicates Discussion crystallisation at pressure between 15 and 18 kbar (at 650°C). Moreover, the lack of kyanite in eclogites from Basis for the exhumation model Engebøfjellet indicates a maximum pressure of c. 20 kbar (for 600°C) for leuco-eclogites containing paragonite and Deformation and metamorphism of the Engebøfjellet omphacite near Jd50, as well as a maximum temperature Eclogite and the surrounding rocks are separated into of 750°C (at 15 kbar) deduced from the stability of Pg stages D1 to D6 (Fig. 4). Several aspects are crucial for + Zo + Qz + Ab in pseudomorphs from leuco-eclogite kinematic analyses of the stages: On the map-scale, the (Holland, 1979). truncating nature of shear zones and/or marginal, large drag-related folds may indicate the separation across the Exhumation path: D , D and D zone, or overall vergence. On the meso- and micro-scale, 3 4 5 shear-sense indicators may be used to confirm the local

Stage D3 (Garnet amphibolite facies) shear, such as: asymmetrical shear folds, composite ductile Close attainment of equilibrium is indicated by the shear fabrics, outcrop-scale stacked units and foliation/ relative homogeneity of rim compositions in D3 shear- bedding duplexes, winged porphyroclasts and shear- zone amphibolites. Several geothermobarometers have related veins and fractures (e.g., Hamner & Passchier, been calibrated that can be applied to such parageneses 1991). In addition, sense-of-slip along brittle faults can be (Grt + Amp + Ep + Pl + Ilm + Qz ± Bt ± Ms), although deduced by slip-related lineations (see Petit, 1987). the absence of reliable solid-solution models for amphibole and mica renders their use hazardous. The D1 stage is expressed as isoclinal folds of the primary For example, the garnet-amphibole geothermometer layering, and a dominant eclogitic S-tectonite type (Graham & Powell, 1984) gives unrealistically high transposition foliation. A subhorizontal E–W orientation temperatures (714–984°C, using Fet in amphibole, of the F1 axes (Fig. 6A) may indicate N–S shortening, but Table 2), probably because the D3 amphiboles with segmentation of the eclogite lens into individually rotated 3+ high Fe and CaM4 values are not in the recommended and deformed bodies during superimposed deformation compositional range for this calibration. Even higher undermines a reliable interpretation. The following temperatures and greater spread (617–1034°C, using eclogitic D2 stage (Fig. 4) reveals steeply plunging 2+ Fe in amphibole) is obtained with the calibration of tight to isoclinal folds of S1, with an associated spaced Krogh-Ravna (2000b) although it should be better suited cleavage. More highly strained parts show a penetrative, to the present calcic amphiboles. Using the Am-Pl-Czo/ LS- to SL-foliation and a distinct WNW–ESE-oriented Ep-Qz geothermobarometer of Plyusnina (1982), rim stretching lineation. This foliation represents the compositions of adjacent amphibole and plagioclase in dominant fabric in the Engebøfjellet Eclogite. Kinematic

D3 amphibolites (samples E7A and E10, Table 2) indicate indicators in the D2 high-strain zones reveal both 525–540°C (and >8 kbar). Kohn & Spear (1989, 1990) dextral and sinistral shear-senses for separate zones, calibrated several geobarometers on coexisting Grt + respectively. The general lack of asymmetry combined Hbl + Pl + Qz, and rim compositions of adjacent Gt-Pl- with the structural orientations indicates subhorizontal Amp give at 600°C quite consistent estimates of 9.4–12.7 (in present position, see below) coaxial N–S shortening kbar for both samples E7A and E10 (or 8.7–11.6 kbar at during the D2 phase. 500°C, Table 2).

The amphibolite-facies S3 foliation represents the main Stage D4 (Epidote amphibolite facies) fabric in the country rocks around the eclogite body. In Application of Plyusnina (1982) to rim compositions the eclogite, this stage is characterised by folds of S1/S2, of adjacent amphibole and plagioclase coexisting with with a spaced cleavage. Distinct, metre-wide, LS-type epidote and quartz in samples E11 and E8/163.2 results shear zones (S3) were generated in the fold limbs, or in 500–525°C and 7–8 kbar. The pressure estimates reactivated the S2 foliation. Abundant D3 shear-sense for D3 and D4 consistently bracket the experimentally indicators are consistent with non-coaxial sinistral shear determined garnet-out boundary for leucogabbroic along the subhorizontal E–W stretching lineation (Figs compositions (P = 9 kbar, Poli, 1993). 5D, 6G). The D4 stage is revealed by localised metre-wide shear zones. Shear-sense indicators are similar to those of NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 71

the D3 stage, i.e., subhorizontal sinistral shear; hence, the Considering the similarity of P–T estimates obtained by D4 phase may represent a progressive continuation of the ‘classical’ geothermobarometry for all these localities and D3 stage. Engebøfjellet, it is likely that recalculation of our analyses with THERMOCALC would similarly lead to higher

The N–S subvertical orientation of the D5 fractures and pressure estimates of around 22–23 kbar for D1 + D2. their tensile nature suggest N–S shortening or E–W However, such high pressure estimates will be in conflict extension. Faults of the subsequent subgreenschist-facies with observed phase relations at Engebøfjellet and

D6 stage divide into two populations: subvertical NNW– absolute constraints provided by experimental reaction SSE and NE–SW-striking strike-slip faults, bisected boundaries: a maximum limit for the pressure of 17–18.5 by N–S striking, moderately to steeply east- and west- kbar at T= 600–700°C indicated by the persistence of dipping normal faults. Kinematic indicators suggest albite in leuco-eclogites (reaction boundary Jd-Ab-Qz; formation of the faults during N–S shortening, which Holland, 1980). Again, because of the similarity of the triggered E–W extrusion/extension on conjugate shear older P–T estimates, it is not likely that eclogitisation fractures (see Braathen 1999, and his description of at Engebøfjellet would have happened at a much lower fracture population 2). pressure than for the other Sunnfjord eclogite bodies. In mafic eclogites, with mostly divariant mineral

The P–T domains for the various stages (D1–D5) assemblages, both classical thermobarometry and P–T are plotted in Fig. 10. Peak conditions for eclogite grid calculations using thermodynamic databases (e.g., facies at Engebøfjellet are c. 600°C and 15–18 kbar THERMOCALC) are fraught with the same problems 3+ (undifferentiated D1 and D2). These estimates are similar (estimation of Fe content and poorly constrained to those previously proposed for eclogites in this inner solid-solution models for Fe3+ and Na-bearing pyroxene, Sunnfjord area and in accordance with the general amphiboles and micas). Considering this, we have tried to metamorphic gradient for eclogite-facies metamorphism better constrain the P–T estimates by direct comparison in the WGR (Krogh & Carswell, 1995). When the same with experimental work (as outlined in 6.1 above). geothermobarometric methods are used, our P–T Improved solid-solution models for these minerals estimates for D1+D2 at Engebøfjellet are compatible might in the future help to resolve the discrepancy to those obtained on various Sunnfjord eclogites by between ourestimates and the newer P–T estimates for other authors. However, newer P–T estimates using the Sunnfjord eclogites (cf., Diener & Powell, 2012, and THERMOCALC (or similar thermodynamic database references therein). methods) indicate significantly higher pressures (although similar or somewhat higher temperatures) for Preferred estimates on retrograde shear-zone the eclogite-facies event. At Kvineset in inner Sunnfjord, amphibolites are c. 525–550°C and 9–12 kbar (D3 the P–T conditions for the eclogite facies of 517–561°C stage) and 500–525°C and 7–8 kbar (D4). Greenschist- and 16–17 kbar estimated by Cuthbert at al. (2000) facies fracturing (D5) occured at c. 300°C and 3–4 kbar. using Grt-Cpx and Grt-Cpx-Ph geothermobarometry The resulting retrograde P–T path is similar to the one have been recalculated by Labrousse et al. (2004) to proposed by Krogh & Carswell (1995) for the nearby 561–613°C and 20–22 kbar using THERMOCALC. In Naustdal eclogite, although at slightly lower temperature. the Dalsfjord area, some 25 km south of Engebøfjellet, In particular, no increase in temperature is recorded Foreman et al. (2005) estimated a peak temperature of during initial decompression, a notable difference from 537–633°C and pressure of 17±2 kbar on the Drøsdal the path established for Naustdal. The only other P–T eclogite body using, respectively, Grt-Amp thermometry path defined with some detail in the Sunnfjord area and Grt-Cpx-Ph barometry, but higher T=720–830°C has been obtained on the high-pressure Hyllestad mica and P=19–21 kbar using THERMOCALC. In the schists (Hacker et al., 2003; Labrousse et al., 2004). It is same area, Engvik & Andersen (2000) estimated that consistent with our P–T data for D3 and D4; the whole eclogitisation at Vårdalneset occurred at 677°C and 16 P–T path for Engebøfjellet (to D5) showing possibly a ± 2 kbar using Grt-Cpx thermometry and Grt-Cpx-Ph slight shift to lower temperature. Our P–T estimates for barometry, while Engvik et al. (2007) recalculated these D1+ D2 would fit nicely as a higher P extension to the P–T conditions to 635°C and 23 kbar using Grt-Omp-Ky- path drawn by Hacker et al (2003) for these mica schists Ph-Qz thermobarometry and THERMOCALC. In the but differ drastically from their P–T estimates for the Solund–Hyllestad–Lavik area, preferred P–T estimates Lavik eclogites from the same area, as discussed above. by Hacker et al. (2003) for the Lavik mafic eclogites are 700°C and 23 kbar, using THERMOCALC. It is to be No indisputable record of the prograde path was noted that THERMOCALC recalculation of the peak found. Even ‘prograde’ zoned garnet poikiloblasts fail to metamorphic conditions for the high-pressure Hyllestad demonstrate an amphibolite-facies precursor. The lack mica schists ( consisting of Grt + Ph + St + Ky + Cld of correlation between garnet zoning and compositional +Qz+ Rt) to 575°C and 16 kbar by Hacker et al. (2003) trends for amphibole inclusions (see also the lack of differ very little from the original estimates of 570–600°C trend in temperature estimates, although this argument and 15 kbar obtained by Chauvet et al. (1992). is rather frail) may indicate that these strongly zoned garnets grew entirely under eclogite-facies conditions 72 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY

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.

(with a possible correlation: core = D1, rim/matrix = defining the NSD zone (e.g., Braathen et al., 2004), and D2?); the garnet zoning being controlled by chemical in rocks that have not been involved in the deformation fractionation rather than by drastic P–T changes. If this of this detachment. Since the tight antiform is truncated is true for those rocks found near lithological contacts by the openly folded overlying detachment, this regional and most susceptible to react under metamorphic events folding occurred prior to formation of the NSD zone. prior to eclogite metamorphism, this suggests that most This touches on a general discussion as to how the NSD of the present ferrogabbroic rocks may have undergone zone should be defined as, for instance, Marques et al. direct transformation from metastable gabbro to eclogite, (2007) argued for an increased vertical shortening with as this is undoubtedly the case for the leuco-eclogites increasing metamorphic conditions (increased depth with pseudomorphic gabbroic texture. towards the east) in the overall zone separating the WGR from the upper plate (see below). Exhumation model The variation in the orientation of D5 subvertical In order to evaluate the kinematic data of the fractures may be used as a guide to relatively young Engebøfjellet Eclogite in a regional perspective, the initial folding. They strike N–S in the Engebøfjellet eclogite, orientation of the various fabrics has to be established. i.e., in the northern limb of the Førdefjord antiform, The Engebøfjellet Eclogite is located in the northern limb and NW–SE in the southern limb (Fig. 6J). This minor of a regional E–W-trending antiform, which is slightly difference indicates that the region was significantly double-plunging and tighter to the west. This structure folded prior to D5 fracturing and subsequent minor is located in the lower plate, beneath distinct mylonites folding. In conclusion, the D5 fractures were probably NORWEGIAN JOURNAL OF GEOLOGY Tectonic evolution of Engebøfjellet Eclogite 73 generated during N–S shortening and/or E–W extension, Such a slow thermal response most likely relates to and could be explained as cross-fold fractures that were extremely rapid uplift, linked to the period between our slightly folded during progressive strain. stages D1/D2 and D4. Geochronology of the region can constrain this event, and includes: (i) a 405±5 Ma age for

Since no unique temporal strain marker of certain eclogitisation (D2?) (Terry et al., 2000; Root et al., 2004, Devonian age can be found in the lower plate, Glodny et al, 2008) (ii) a titanite cooling (c. 500–550°C) reconstructions are debatable. However, if the north age of c. 395 Ma (Tucker et al., 1987; Terry et al., 2000), limb of the antiform is rotated to horizontal (rotation (iii) 40Ar/39Ar white-mica ages of around 400 Ma (<400 axis 265/0, rotation plane 265/70; i.e., disregarding the Ma under, and >400 Ma above the NSD) in the Sunnfjord 40 39 slight westerly plunge), the D1 to D4 fabrics restore to region (e.g., Andersen, 1998), and (iv) a Ar/ Ar biotite near-horizontal attitudes (Fig. 6H, I). In this case, the D2 age of 395 Ma obtained from a pegmatite in a top-W eclogitic fabric records coaxial, near-vertical shortening shear zone in the Dalsfjord area (Eide et al., 1999). and approximately horizontal E–W stretching, followed When these ages are applied to the Engebøfjellet area, by D3 and D4, amphibolitic, top-W, sub-horizontal shear. they indicate that roughly 30 km of unroofing occurred It is noteworthy that this restoration reveals a good fit in <10 Myr, between 405 and 395 Ma. Farther north, with the Dalsfjord eclogites (Engvik & Andersen, 2000). in the Nordfjord and Møre area, even more substantial If valid, one implication is that the lower plate was unroofing took place (Terry et al., 2000). The second subjected to amphibolite -facies top-W shear prior to P–T segment (D4 to D5) was characterised by cooling regional tight folding, the folds later being decapitated by from c. 525°C to 300°C and lower (brittle conditions), the NSD zone. associated with a fairly small drop in pressure (3–4 kbar, from 7 to 3 kbar or lower). According to Eide et al. 40 39 As discussed by Braathen (1999), the following D6 brittle (1999), using Ar/ Ar data (feldspar, phengite, biotite), a faults correlate with a later stage of N–S contraction and phase of rapid cooling took place in Sunnfjord in Early E–W extension/extrusion. This fracture system can relate Carboniferous time (c. 360 Ma), which may well apply to regional folding as conjugate shear and cross-fold to our D6 stage. This brackets the time span of this P–T fractures. Since the fault system has an equal character segment to a period of c. 30 Myr. in both the upper and the lower plates, it suggests that regional uniformity was established at this time; a latest Comparison with previous models for the WGR Devonian – Early Carboniferous age has been indicated (discussed in Braathen, 1999). The prolonged tectonic scenario for Engebøfjellet shows similarities with the proposed setting of nearby areas. Based on the kinematics and structural restoration of the Along Dalsfjord to the south, Engvik & Andersen (2000) Engebøfjellet region, the following sequence of events envisage that two eclogite-facies fabrics were formed can be envisaged (Fig. 10): (i) D1 phase of eclogitisation by bulk horizontal shortening and vertical stretching of uncertain origin, (ii) D2 phase of eclogitisation, of the deep crust, related to plate convergence, followed consistent with vertical flattening and horizontal E–W by rapid decompression during orogenic extensional stretching, (iii) amphibolite-facies, highly ductile, top-W collapse. There is a clear similarity in estimated pressure shear (D3-D4) in the WGR, (iv) regional E–W folding for the Dalsfjord (Engvik & Andersen, 2000; Foreman (D5), (v) truncation of fold(s) by renewed top-W shear, et al., 2005) and Engebøfjellet eclogites (if estimated this time on the more discrete NSD zone, (vi) regional by the same method; see discussion above); both E–W folding of the NSD plus upper- and lower-plate reveal two eclogite fabrics related to the same pressure rocks (D6), and (vii) truncation of the folded NSD zone regime (c. 18 kbar). The estimated P–T domain for the by narrow, low-angle, (semi-)brittle faults at the base of retrograde amphibolite fabric (c. 550°C and 10 kbar) is the upper-plate rocks (e.g., Braathen, 1999; Braathen et also strikingly similar, thus we suggest that the crustal al., 2004). Some authors argue that these late faults are position of these units was more or less identical through gently folded (e.g., Torsvik et al., 1997; Krabbendam & the D1(?)-D2-D3 stages. Northwards, in the Molde region, Dewey, 1998). This evolution in a general sense mimics i.e., marginal to the Møre–Trøndelag Fault Complex that proposed by Johnston et al. (2007). However, the (MTFC), cooling ages on titanite from the WGR (c. 402– overall link between regional folding and detachment 395 Ma; see Labrousse et al., 2004; Hacker et al., 2010) development remains enigmatic: did the extensional and monazite U–Th–Pb ages of eclogites (407–395 detachment(s) repeatedly lock during transpression and Ma; Terry et al., 2000) suggest that regional uplift and unlock (reactivate) during transtension, or did it/they homogeneous cooling occurred in a very short period of experience progressive development? Our observations time. Interestingly, titanite from isoclinally down-folded support a step-wise evolution, as illustrated in Fig. 10. Caledonian nappes is not reset, revealing an Ordovician age, implying that a major extensional detachment existed In linking the regional P–T path with the tectonic model between the WGR and the nappes prior to the intense outlined above, two main P–T segments appear. First, folding (Tucker et al., 1997). Furthermore, Robinson significant decompression from c. 18 to 7 kbar occurred (1995) concluded that the folding was contemporaneous under near-isothermal conditions (c. 600°C to 525°C). with or followed by extension in a sinistral shear field. 74 A. Braathen & M. Erambert NORWEGIAN JOURNAL OF GEOLOGY

According to Terry et al. (2000), early UHP eclogites (407 Cann, J.R., Blackman, D.K., Smith, D.K., McAllister, E., Janssen, B.J.R., Ma) were thrust upward and retrograded to HP eclogite, Mello, S., Avgerinos, E., Pascoe, A.R. & Escartin, J. 1997: Corrugated slip surfaces formed at ridge-transform intersections on the Mid- before becoming incorporated in amphibolite-facies Atlantic Ridge. Nature 385, 329–332. top-W and sinistral shear at approximately 395 Ma. Carswell, D.A., Tucker, R.D., O’Brien, P.J. & Krogh, T.E. 2003: Coesite micro-inclusions and the U/Pb age of zircons from the Hareidland Eclogite in the Western Gneiss Region of Norway. Lithos 67, 181–190. Conclusions Chauvet, A. & Seranne, M. 1994: Extension-parallel folding in the Scandinavian Caledonides: implications for late-orogenic To conclude, our results show similarities to the processes. Tectonophysics 238, 31–54. Chauvet, A., Kienast, J.R., Pinardon, J.L. & Brunel, M. 1992: Petrological tectonic framework established both to the south and constraints and PT path of Devonian collapse tectonics within the to the north. In particular, the D2 eclogite-facies and Scandian mountain belt (Western Gneiss Region, Norway). Journal D3-D4 amphibolite-facies setting for Engebøfjellet is of the Geological Society of London 149, 383–400. consistent with that in Dalsfjord, where eclogitic coaxial Chemenda, A., Matte, P. & Sokolov, V. 1997: A model of Palaeozoic as well as top-W shear structures are retrograded by obduction and exhumation of high-pressure/low-temperature amphibolitic top-W shear zones (Engvik & Andersen, rocks in the southern Urals. Tectonophysics 276, 217–227. 2000). Northwards, folding plays a major role in the Cho, M. & Liou, J.G. 1987: Prehnite-Pumpellyite to Greenschist Facies Transition in the Karmutsen Metabasites, Vancouver Island, B.C. post-eclogite tectonic scenario (e.g., Terry et al., 2000; Journal of Petrology 28, 417–433. Labrousse et al., 2004; Hacker et al., 2010), as we have Cuthbert, S.J. & Carswell, D.A. 1990: Formation and exhumation of also found for the Engebøfjellet Eclogite. medium-temperature eclogites in the Scandinavian Caledonides. In Carswell, D.A. (ed.): Eclogite Facies Rocks, Blackie, pp. 181–203. Cuthbert, S.J., Carswell, D.A., Krogh-Ravna, E.J. & Wain, A. 2000: Acknowledgements. We are thankful to T.B. Andersen and A. Engvik Eclogites and eclogites in the Western Gneiss Region, Norwegian for their very constructive reviews; E. Eide, A. Korneliussen, J. F. Caledonides. Lithos 52, 165–195. Molina, P. T. Osmundsen, P. Robinson, M. Terry and D.A. 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