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Labrousse, L., Jolivet, L., Andersen, T.B., Agard, P., Hébert, R., Maluski, H., and Schärer, U., 2004, Pressure-temperature-time deformation history of the exhumation of ultra-high pressure rocks in the Western Gneiss Region, Norway, in Whitney, D.L, Teyssier, C., and Siddoway, C.S., Gneiss domes in orogeny: Boulder, Colorado, Geological Society of America Special Paper 380, p. 155–183. For permission to copy, contact [email protected]. © 2004 Geological Society of America

Geological Society of AmericaSpecial Paper 380

2004

Pressure-temperature-time deformation history of the exhumation of ultra-high pressure rocks in the Western Gneiss Region, Norway

L. Labrousse*L. Jolivet

Laboratoire de Tectonique UMR 7072, UPMC T26E1 case 129, 4, place Jussieu 75252 Paris cedex 05, France

T.B. AndersenDepartment of Geology, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway

P. AgardLaboratoire de Tectonique UMR 7072, UPMC T26E1 case 129, 4, place Jussieu 75252 Paris cedex 05, France

R. HébertDepartement des Sciences de la Terre UMR 7072, Université de Cergy-Pontoise, Le Campus Bat I,

95031 Cergy cedex, France

H. MaluskiLaboratoire de Géochronologie UMR 5573, Université Montpellier II, Place Eugène Bataillon,

34095 Montpellier cedex, France

U. SchärerGéochronologie–Geosciences Azur UMR 6526, Université de Nice—Sophia Antipolis, 06108 Nice cedex 02, France

ABSTRACT

The Nordfjord area, north of the Hornelen Devonian basin in Western Norway, is the southernmost part of the Ultra-High Pressure (UHP) Province, de ned by the occurrence of coesite-bearing and diamond-bearing continental rocks. Compilation of structural, petrological, and chronological data from the area leads to a model for the formation of dome structures at the crustal scale and the behavior of the conti-nental crust during its exhumation from mantle depths. The Nordfjord area appears as a 100 × 50 km dome-shaped boudin affected by at least two deformation stages. A stage of E-W stretching and top-to-west shearing produced several envelopes of migmatitic gneisses bounded by narrow high-strain zones over a core preserving the Precambrian granulite protolith. This dome is affected by the west-vergent Nordfjord Mylonitic Shear Zone on its southern limb during late exhumation under the Nord-fjord-Sogn Detachment Zone. The rst stage of deformation is coeval with reequili-bration from maximum pressure conditions around 2.8 GPa, 650 °C (THERMO-CALC multiequilibrium method) in the coesite stability eld to higher temperature and lower pressure conditions (1.8 GPa, 780 °C). Subsequent retrogression was recorded in the amphibolite facies (0.7 GPa, 580 °C) and in the greenschist facies (0.4 GPa, 420 °C). Dates for these stages yield exhumation velocities higher than 2 mm/yr. 40Ar/39Ar ages in the area, compared to a spectrum of cooling ages along a

*[email protected]

156 L. Labrousse et al.

500-km-long N-S pro le, show that cooling of the northern part of the Western Gneiss Complex is at least 20 Ma younger than in the south. The Western Gneiss Complex is therefore the result of the late juxtaposition of two complexes, the Northwestern Gneiss Complex, characterized by UHP relics, constrictive stretching, partial melting, and doming during a multi-stage exhumation from the deep parts of the orogen, and the Southwestern Gneiss Complex with Devonian basins, a well-developed detach-ment system, and distinct high pressure to medium pressure units stacked together during a single and rapid exhumation stage. The two complexes may represent deep subduction channel dynamics (north) and shallower wedge circulation (south) in the Caledonian orogen. The Nordfjord Mylonitic Shear Zone appears as a major tectonic in the Western Gneiss Complex. Partial melting in the Northwestern Gneiss Complex may have favored the late exhumation of E-W elongated domes such as the Nordfjord crustal-scale boudin and their juxtaposition to the Southwestern Gneiss Complex during top-to-west shearing.

Keywords: ultra-high pressure, exhumation processes, doming, Western Gneiss Region, Caledonides.

INTRODUCTION

During the last two decades, ultra-high pressure (UHP) rocks have been discovered in most of the Alpine, Variscan, and Caledonian orogens (Maruyama et al., 1996; Ernst and Liou, 2000). Occurrences of UHP index minerals such as coesite and diamond (Chopin and Sobolev, 1995) have been described in lithologies of continental af nities such as the pyrope-quartzite of the Dora Maira massif (Chopin, 1984), the paragneisses of the Dabie Shan belt (Okay et al., 1989) and the granodioritic gneisses of the Western Gneiss Region (Smith, 1984; Wain, 1998). The burial of low-density material to mantle depths and subsequent exhumation with incomplete retrogression raise questions about the importance of buoyancy forces in the orogenic wedge. Key parameters to estimate the force balance are the size, the geom-etry, and the structure of the crustal or lithospheric elements involved in the exhumation.

Structural and petrological analyses in the Dora Maira mas-sif (Henry et al., 1993) and in the Moldefjord area in western Norway (Terry et al., 2000b) lead to the conclusion that UHP units were nappes stacked together in lower grade metamor-phic wedges during exhumation. Thermal considerations for preservation of UHP-low temperature (LT) assemblages led to the idea that they must be rapidly exhumed early in the orogeny (Hacker and Peacock, 1995). Extensional doming, concomitant with thermal reequilibration and partial melting, would therefore play a minor role in the exhumation process. Nevertheless, eld evidence in the Dabie Shan belt indicates that doming and partial melting affected the Dabie block and contributed to the exhuma-tion of the UHP paragneisses (Faure et al., 1999; Zhong et al., 1999). Precise chronological data are clearly necessary to relate the timing of thermal reequilibration stages and exhumation of high-grade rocks.

The Nordfjord area in the Western Gneiss Region is a key area for unraveling the structures associated with exhumation of UHP rocks and their relationship with shallower structures. We

present here the results of a multi-method study combining struc-tural analysis, thermobarometry, and geochronology to propose a model in which syn-collisional upward ow in the subduction channel and subsequent extensional doming are responsible for the exhumation of a crustal-scale UHP core, part of the large UHP Province of Western Norway.

GEOLOGICAL SETTING

The Western Gneiss Complex (Milnes et al., 1997) is the deepest exposed unit of the Scandinavian Caledonides (Fig. 1). Silurian to Devonian metamorphism and deformation in this segment of Proterozoic continental crust, contemporaneous with southeastward thrusting onto the Baltic shield, is referred to as the Scandian phase of the Caledonian orogeny. The latest Caledonian tectonic event recorded in the dominant granitic to granodioritic gneisses is regional E-W stretching during their equilibration in the amphibolite facies. This late Scandian extension stage resulted in a 35,000 km2 core-complex structure separated from supracrustal lithologies by the basal Jotunheimen Décollement Zone in the east, the Bergen Arc Shear Zone in the south, and the Nordfjord-Sogn Detachment Zone in the west (Fig. 1). The Møre-Trøndelag Fault Zone in the north separates the Western Gneiss Complex from the Vestranden Gneiss Complex, recently described as an extensional dome (Braathen et al., 2000).

Lithological heterogeneities in the dominant amphibolitic gneiss complex recorded the different stages of their Proterozoic and Caledonian tectono-metamorphic history. Kilometer-scale bodies, preserved from penetrative deformation and refractory to mineralogical reequilibrations, show mid-Proterozoic HP-granu-litic assemblages in the Flatraket area and even primary igne-ous mineralogy (Krabbendam et al., 2000). Meter-scale ma c lenses, in swarms or isolated in the gneissic matrix, commonly preserve eclogitic assemblages in their cores in a 100-km-wide zone along the west coast (Krogh, 1977; Grif n et al., 1985). Eclogitic paragenesis described in surrounding felsic gneisses

Pressure-temperature-time deformation history of ultra-high pressure rocks 157

demonstrate that the whole region has experienced the high pressure stage recorded by those “external” eclogites (Krogh, 1980a; Krabbendam and Wain, 1997; Wain, 1998). The “inter-nal” eclogites are also associated with kilometer-scale ultrama c bodies incorporated into the continental material before or dur-ing the Caledonian subduction (Carswell et al., 1983; Brueckner and Medaris, 2000; Brueckner et al., 2002). North of Nordfjord, more than twenty localities with preserved coesite or coesite pseudomorphs (Smith, 1984; Cuthbert et al., 2000; Wain et al., 2000), occurrences of diamond in gneisses (Dobrzhinetskaya et al., 1995) and in majorite-bearing orogenic peridotite lenses (van Roermund and Drury, 1998; van Roermund et al., 2002) led to the de nition of an “UHP Province” from Nordfjord to Moldef-jord (Krabbendam and Wain, 1997; Wain, 1998; Wain et al, 2000; Cuthbert et al., 2000).

The main extensional event structured the whole crust from the Western Gneiss Complex to higher structural levels of the nappe stack (Andersen, 1998). The main extensional structure in the Western Gneiss Region is the Nordfjord-Sogn Detachment Zone (Norton, 1986), in which the top-to-west movement was contemporaneous with the deposition of the continental Devonian

basins (Hossack, 1984) in its hanging wall. Top-to-the-west shear and E-W folding of the Nordfjord-Sogn Detachment Zone during the deposition of the Devonian basins (Osmundsen and Ander-sen, 2001) resulted in the juxtaposition of the deep gneisses in the core of antiforms with the hanging wall material in synforms. The evolution of structures from the cores of anticlines toward the Nordfjord-Sogn Detachment Zone is therefore representative of the successive deformations experienced by the gneisses dur-ing their exhumation (Andersen et al., 1994). Early coaxial E-W stretching is progressively overprinted by top-to-west shearing when approaching the Nordfjord-Sogn Detachment Zone (Andersen and Jamtveit, 1990). On the Nordfjord-Sogn Detach-ment itself, brittle faults and mylonitic textures show the latest localization of deformation (Andersen et al., 1994).

RECENT SCENARIOS FOR THE EXHUMATION OF THE WESTERN GNEISS COMPLEX AND THE UHP PROVINCE

Recent structural, petrological, and radiochronological data lead to different interpretations of depth-time paths for the

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Figure 1. Location map of the different areas and localities in the Western Gneiss Region. Simpli ed geological contours after Roberts and Gee (1985), coesite occurrences after Wain(1998) and Cuthbert et al. (2000), and diamond occurrence after Dobrzhinetskaya et al. (1995). BASZ—Bergen Arc Shear Zone; L—Lavik; LGFZ—Laerdal-Gjende Fault Zone; NSDZ—Nordfjord-Sogn De-tachment Zone.


exhumation of UHP rocks in Western Norway. The rst stages of exhumation are systematically fast and syn-collisional (Fig. 2). This stage is responsible for 30 km (Wilks and Cuthbert, 1994) to 60 km (Terry et al., 2000a) of vertical motion, and brings UHP to HP rocks to a depth of ~60 km in all the models (Fig. 2). Post-collisional exhumation, due to changing boundary conditions from convergence to divergence, is correlated in all models with decreasing exhumation velocity below 3 mm/yr. Estimates of the timing of the boundary conditions inversion vary from 425 Ma for the earliest (Wilks and Cuthbert, 1994) to 395 Ma for the latest (Milnes et al., 1997). Dates on zircons and monazites from UHP rocks in Western Norway (Terry et al., 2000a; Root et al., 2001) show that burial of continental material was active at least until 400 Ma, thus con rming the latest estimations of the begin-ning of divergence (Milnes et al., 1997).

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The dominant forces driving rocks upward depend on the inferred geometry of the Caledonian orogen at its climax. The widespread extensional structures recorded by the Western Gneiss Complex during retrogression into amphibolites were explained by models of gravitational collapse during thickening of the lithosphere (Andersen and Jamtveit, 1990). The northwest-ward polarity both in the peak metamorphic conditions recorded by eclogites (Krogh, 1977; Grif n et al., 1985) and in the Caledo-nian imprint lead to asymmetrical models with continental sub-duction plunging to the northwest and subsequent eduction of the Western Gneiss Complex (Andersen et al., 1991) as a coherent portion of continental crust. Eduction may have been triggered by a change of buoyancy of the subducting lithosphere due to delamination during convergence (Andersen et al., 1991) or by a creation of free space by plate divergence (Fossen, 1992, 2000).


Syncollisional gravitational collapse would be compatible with the rst hypothesis, whereas late overall extension would feed the second scenario. It is then crucial to constrain the relative timing of extension in the upper levels of the crustal wedge and subduction of continental material. The change from south-eastward thrusting to northwestward extension in the southern part of the Western Gneiss Region and the overlying nappes is constrained by 40Ar/39Ar crystallization ages on syntectonic micas between 402 and 408 Ma (Fossen, 2000). The deposition of the western syn-extensional detrital basins may have begun as soon as the Praguian-Emsian boundary (ca. 409 Ma; Tucker and McKerrow, 1995), and the recent ages on UHP rocks (Terry et al., 2000a; Root et al., 2001) indicate that burial was active until at least 400 Ma. There would therefore be an overlap of ~10 m.y. between the extension period in shallower levels and active subduction at depth.

Sinistral strike-slip between Baltica and Laurentia (Ziegler, 1985; Torsvik et al., 1996) has been recorded in the Western Gneiss Region both by ductile deformation in the gneisses (Krab-bendam and Dewey, 1998) and the geometry of Devonian basins (Osmundsen and Andersen, 2001). This implies a non-cylindrical three-dimensional geometry for the exhumation and extension processes. The sinistral activation of the Møre Trøndelag Fault Zone in the Devonian (Roberts, 1983) would be responsible for a component of constriction (Krabbendam and Dewey, 1998; Terry et al., 2000b) and for the progressive counter-clockwise rotation of stretching direction from Sunnfjord to Moldefjord (Krabben-dam and Dewey, 1998).

Most of these models consider the Western Gneiss Com-plex as a coherent body at least during its retrograde history (Andersen et al., 1991; Wilks and Cuthbert, 1994; Milnes et al., 1997; Fossen, 2000), with continuous gradients from SE to NW in the equilibrium conditions of eclogites (Krogh, 1977; Grif n et al., 1985; Cuthbert et al., 2000), in the intensity of Caledonian reworking, and in the constrictional component of stretching (Krabbendam and Dewey, 1998).

Only scenarios making the distinction of a UHP province within the Western Gneiss Complex (Terry et al., 2000b; Wain et al., 2000) consider it as a composite body. The southern part of the UHP Province in Stadlandet (Fig. 3) is separated from the HP rocks by a large HP-UHP cryptic transition zone (Krab-bendam and Wain, 1997). Terry et al.’s (2000b) model considers the UHP rocks of Moldefjord as a nappe incorporated late into a HP wedge.

The recent description of a large eclogite-bearing gneiss province in the Laurentian basement exposed in the East Green-land Caledonides (Gilotti, 1993) and the discovery of UHP eclogites in these terranes (Gilotti and Ravna, 2002) are new arguments for a wide eclogitic root within a thickened litho-sphere (Ryan, 2001) or for several continental subductions with different senses (Gilotti and Ravna, 2002).

In this context, the study of structures inside the Western Gneiss Complex and the precise relationships between UHP eclogite-bearing gneisses and their surrounding rocks is impor-

tant for determining whether the Western Gneiss Complex must be considered as a coherent unit during the Scandian phase.

FIELD EVIDENCE FOR CRUSTAL-SCALE BOUDINAGE AND MIGMATIZATION DURING EXHUMATION IN THE NORDFJORD AREA

The regional E-W stretching direction observed in the West-ern Gneiss Region (Andersen, 1998; Fossen, 1992) is the rule in the central part of the studied area with only local rotation to N040 on the Nordfjord shores and to N140 on Gurskøy Island (Fig. 1). We have mapped foliation and stretching lineation tra-jectories as well as kinematic indicators in the whole Nordfjord area (Fig. 3). Foliation trajectories (Fig. 3A) indicate a dome structure formed by several units (data from Krabbendam et al., 2000, and this work):

1. A core region (south Stadlandet and Flatraket) where foliation shows an intense folding pattern around kilometric pods of preserved granulites (Krabbendam et al., 2000);

2. Layered metatexites in the Stadlandet, Vanylven, and Volda areas, structurally above the Flatraket core and showing systematic intense E-W stretching and folding of foliation;

3. A kilometer-scale mylonitic shear band limiting the region to the south, interpreted as the ductile expression of the Nordfjord-Sogn Detachment Zone in the gneisses (Krabbendam and Wain, 1997).

Lithological heterogeneities in gneiss led to boudinage in the E-W direction from the centimeter (more silicic layers in the mylonites) to the kilometer scale (pods of granulites in Flatraket area). L-tectonites and folding of foliation along E-W axes (Fig. 4) testify for constriction during extension at all scales (Krabbendam and Dewey, 1998). Shear sense indicators in gneisses such as shear bands, asymmetric boudinage, and drag folds indicate an overall dextral shear in subvertical layers and top-to-west shearing in subhorizontal gneisses. Segregation of partial melt and retrogression of eclogite lenses (Wain, 1997) are coeval with stretching and shearing in the surrounding gneisses. The construction of the E-W elongated 100 × 50 km dome (i.e., boudin at the crustal scale) with core-preserving protolith bodies and migmatized rims thus occurred during decompression from eclogite to amphibolite facies conditions and is correlated to exhumation.

Finite Structure of the Nordfjord Area

The average strike of foliation in gneisses is E-W in the central part of the area with local perturbations and turns to N160 in Stadlandet, N-S in Vanylven, and N140 in Gurskøy, with local perturbations of the foliation trajectories mainly due to lithologi-cal heterogeneities in gneisses. Foliations dip systematically to the east in the peninsulas of Stadlandet, Volda, and Vanylven, as well as in the island of Gurskøy, making the Flatraket region the deepest unit of the area. There, the granulitic bodies of Ulvesund and Flatraket preserved from distributed ductile deformation,

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Figure 3. A. Structural map of the Nordfjord area. Litholo-gies modi ed from Norges Geologiske Undersøking maps (Kildal, 1970; Lutro et al., 1998; Lutro and Tveten, 1998; Tveten et al., 1998); structural data compiled from this study and Bryhni (1966) and Krabbendam and Wain (1997). Coesite occurrences from Wain (1997), Smith (1984), and Cuthbert et al. (2000). UHP, UHP-HP and HP zones limits after Cuth-bert et al. (2000). B. Foliation trajectories for the differ-ent units. C. Interpretative diagram of the crustal-scale boudin structure of the Nor-dfjord area. A larger size ver-sion of this gure is included on the CD-ROM accompany-ing this volume.


as well as from Caledonian prograde and retrograde metamor-phism, induce large folding of the wrapping gneiss (Krabbendam and Wain, 1997) and rotation of the fold axis from E-W to N-S locally (Fig. 3B). This region contrasts with the regularly layered Stadlandet unit, which is directly above the Flatraket core. They are separated by a layer of nely layered gneiss with abundant sheath folds (Fig. 5C, D), mapped as a mylonitic shear zone (Lutro et al., 1998). Sheath folds are markers of intense shear (Cobbold and Quinquis, 1980) along the boundary between Fla-traket and Stadlandet units. A second strip of mylonites (Lutro et al., 1998) corresponds to a break in the foliation trajectories in the Åheim region between the units of Stadlandet and Vanylven. Further east, the Volda peninsula and Gurskøy island unit is the uppermost structural unit with foliation strike turning to N140. On the Nordfjord shores, foliation shows a rotation in strike from N080 in the north to N040 and then back to N090 along the Nordfjord-Sogn Detachment Zone proper, drawing a 10 km thick dextral shear-band interpreted as the ductile expression of the Nordfjord-Sogn Detachment Zone in its footwall (Wain, 1998; Krabbendam and Dewey, 1998).

The stretching lineation is penetrative in most gneiss litholo-gies (Fig. 4A, B). Constrictive strain ellipsoids can be locally deduced from L-tectonites in augen gneiss. The direction of lineation turns from N140 in Gurskøy to N090-080 in the major part of the studied area, following the sigmoidal shape of the foliation pattern in the Nordfjord-Sogn Detachment Zone. The average plunge of the lineation is 10° E and rarely exceeds 30° E. The stretching direction is also indicated by the geometry of sheared quartz veins (Fig. 4D, F). When initially parallel to the stretching direction (e.g., E-W), the earliest veins are thinned and truncated, but are tightly folded when perpendicular to stretching (e.g., N-S).

Rotational Deformation and Instantaneous Strain

The rotational component of deformation is expressed in gneisses by C and C! shear bands (Fig. 5A, B), asymmetric pres-sure shadows around garnets, mica shes, and "- and #-rotated objects. Asymmetric boudinage of more viscous horizons in the gneiss (Fig. 6) is used as a criterion for sense of shear when the

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Figure 7. Field evidence for recording of the late Caledonian deformation by eclogite lenses in the Nordfjord area. A. Outcrop overview in Drage (Stadlandet). B. Photograph of asymmetric metabasic lenses. C. Detail sketch of deviated eclogitic foliation around the boudin rim. D. Detail sketch of asymmetrical boudin. E. Photograph of pegmatitic pressure shadows around metabasic lenses. F. Outcrop sketch showing asymmetric metabasic lenses in a sheared migmatite (Gurskøy).

de ection of markers in the boudins is synthetic to the sense of rotation of the blocks (Grasemann and Stüwe, 2000). The asym-metry of meter-scale inclusions and particularly metabasic lenses (Fig. 7B) is systematically consistent with other shear sense criteria at the outcrop scale and can therefore be used as a kine-matic indicator. In the northern part of the area, rotational criteria are mostly to the west in shallow-dipping, foliated gneisses or

sinistral in the regions of subvertical foliation. The asymmetric amphibolitized rims of metabasic lenses included in the gneisses are concordant with local senses of shear, as observed at the Drage site in Stadlandet (Fig. 7A, B).

Top-to-west shearing is thus contemporaneous with retro-gression of eclogites to amphibolites. The three eastern units have been sheared toward the west over the Flatraket deeper unit


during E-W stretching, folding, and retrogression in the amphib-olite facies. Even though the granulite bodies near Flatraket were preserved from intense stretching and shearing, the attitude of foliation in cross section shows an asymmetric geometry concor-dant with this shearing to the west (Krabbendam and Wain, 1997; Labrousse et al., 2002).

In the southern part of the area, shear sense criteria in the vertically foliated gneisses and mylonites indicate a dextral sense of shear for most of the outcrops. Widespread boudinage of gneisses at any scale is systematically asymmetrical and compatible with an overall dextral shear (Fig. 6). The sigmoi-dal pattern of foliation trajectories allows extrapolation of this dextral shear at the map scale. The Nordfjord Mylonitic Shear Zone is adjacent to the Nordfjord-Sogn Detachment Zone and considered as its ductile expression in its footwall (Krabbendam and Wain, 1997).

Antithetic (i.e., sinistral) senses of shear (Fig. 5E) are observed in the southeastern part of the studied area (Fig. 3A). Those senses of shear might be former top-to-west shears that were subsequently folded along E-W axes or structures related to an early regional sinistral shear structure partly erased by the regional top-to-west and/or dextral shearing.

Late deformation is expressed by vertical, centimeter-scale greenschist veins (Fig. 4F). These veins systematically trend perpendicular to stretching in the gneiss with an average N-S orientation indicating an E-W extension concordant with the ductile strain, with obliquities between the veins never exceed-ing 20°. Brittle deformation is more important in the Nordfjord Mylonitic Shear Zone (Torsvik et al., 1992), with faults, tension gashes, and locally intense brecciation along the Nordfjord-Sogn Detachment Zone. Conjugate strike-slip faults, sometimes organized with orthorhombic symmetry, indicate a direction of extension parallel or subparallel to the stretching direction given by the lineation.

Although Permian and Jurassic reactivation affected the Nordfjord-Sogn Detachment Zone both in the Nordfjord (Tors-vik et al., 1992) and the Sunnfjord regions (Eide et al., 1997), the direction of stretching remained stable during the exhumation of rocks through the brittle-ductile transition.

The nite geometry of the Nordfjord region is thus an E-W stretched crustal-scale boudin with a constrictive core and an overall top-to-west shear of its external envelopes (Fig. 3C). Antithetic shears may have been part of the initial structure but have been erased by the westward Nordfjord Mylonitic Shear Zone now limiting the structure on its southern end. Partly syn-extensional folding of the Nordfjord-Sogn Detachment Zone is responsible for the verticalization of the Nordfjord Mylonitic Shear Zone and the tilting of the whole structure.

Field Evidence for the Timing of Partial Melting

The earliest Caledonian structures preserved in the Nord-fjord area are the L-tectonite fabrics preserved in the eclogitic cores of metabasic lenses (Andersen et al., 1994; Bascou et al.,

2001). They commonly appear as tight, isolated fold hinges in sections perpendicular to the lineation and truncated in boudins in the E-W direction, thus indicating a constrictional stretching. This is representative of the strain during peak conditions or the very rst steps of exhumation in the eclogite facies. No clear evidence of Caledonian prograde structures or fabrics has been preserved in the amphibolite facies gneiss of the Nordfjord area.

The gneisses in the units of Stadlandet, Vanylven, and Volda show widespread partial melting (Labrousse, 2001; Labrousse et al., 2002) with local segregation of melts and layering into melanosome and leucosome (Fig. 8). These metatexitic textures (Brown, 1973) suggest maximum partial melting rates of 20% to 30% (Vanderhaeghe, 2001). No organized collection network of veins, sills, and dikes has been observed in the Nordfjord region, but relationships between leucosome and deformation are clear. The granitic melts commonly concentrated in the pressure shadows of metabasic lenses and amphiboles in the retrogressed rims of the eclogitic lenses are in equilibrium with the pegmatitic minerals (Fig. 7E, F and 8C, D). The main partial melting event is thus post-eclogitic and synchronous with retrogression in the amphibolite facies. Nevertheless, several felsic eclogite-facies veins associated with eclogite pods throughout the Western Gneiss Region show partial melting textures, and trondhjemitic leucosomes have been described in eclogites from the Kristian-sund area (Cuthbert, 1995, 1997). Partial melting could therefore have begun early in the decompression history. Boudinage of gneiss is associated with collection of pegmatitic liquids between the boudins, and melanosomes regionally show the same stretch-ing direction as the unmolten gneiss. Layered migmatites show the same E-W folding as unmolten gneiss, and pegmatitic veins are common in the axial plane schistosity of these E-W axis folds (Fig. 8G, H). Dilatational top-to-west shear-bands collected the melts in partially molten gneiss (Fig. 8E, F). A limited but pervasive partial melt therefore assisted E-W stretching, E-W folding, and top-to-W shearing in the wrapping envelopes of the Nordfjord crustal-scale boudin.

This partial melting of the gneiss increased the viscosity contrasts responsible for boudinage and lowered the bulk viscos-ity of subducted material while amphibolitization enhanced the density contrasts and promoted exhumation.

THERMOBAROMETRIC CONSTRAINTS ON EXHUMATION CONDITIONS IN THE WESTERN GNEISS COMPLEX

In order to compare P-T estimates for various lithologies in one outcrop and for different outcrops, the following calcula-tion process has been chosen. The latest version of the THER-MOCALC software (Holland and Powell, 1998) was used to calculate equilibrium conditions with uncertainties, con dence index, and correlation factors for the ma c and pelitic associa-tions of eclogite lenses and surrounding gneisses. Sodic-calcic amphibole, plagioclase, and sodic clinopyroxene symplectites growing at omphacite grain boundaries in eclogites (Waters,

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G

H

Figure 8. Field evidence for synkinematic partial melting in the Nordfjord area. A, B. Photograph and outcrop sketch of the most mature migmatite texture observed in the Nordfjord area (Stadlandet). The metatexite-diatexite textures join is coeval with 40% partial melt (Vanderhaeghe and Teys-sier, 2001). C, D. Photograph and outcrop sketch in the Breidteigelva site (Volda) showing collection of pegmatoid liquids between the boudins of a truncated metabasic layer. E, F. Photograph and outcrop sketch in Gurskøy showing the collection of liquids in a dilatational shear-band. G, H. Outcrop photography and detail sketch in Gurskøy showing collection of uids in veins parallel to axial schistosity of an E-W trending fold.


2002) were used to determine the retrogression conditions of eclogites, based on hornblende-plagioclase thermometry (Hol-land and Blundy, 1994) and clinopyroxene-plagioclase barom-etry (Perchuk, 1992). Electron microprobe analysis (EMPA) was performed at University Pierre et Marie Curie (CAM Paris), using a Cameca instrument (Camebax and SX 50; 15 kV, 10 nA beam conditions; wavelength dispersion mode). Standards used were Fe

2O

3 (Fe), MnTiO

3 (Ti), diopside (Mg, Si), orthoclase

(K, Al), albite (Na) and anorthite (Ca). The analytical spot size diameter was routinely set at 3 µm, keeping the same analyst, standards, and beam current. Reproducibility was checked from one working session to another. Precision on alkali contents esti-mates is better than 2%.

The activity models chosen are displayed in AX software (Holland and Powell, 1996a, 1996b). Most P-T estimates use garnet end members, whose activities are calculated with a two-site mixing model for the ideal part and symmetric formalism for the non-ideal part. Average uncertainties for temperature and pressure estimates are 68 °C and 0.24 GPa for the eclogites facies parageneses in metabasites and 30 °C and 0.13 GPa for amphibolites facies parageneses in gneiss. This compares with the estimated accuracy of recent thermobarometric studies in the Nordfjord: 50 °C and 0.15 GPa according to Wain (1998) and 75 °C and 0.2 GPa according to Cuthbert et al. (2000).

Pressure-Temperature Path for the Southern Western Gneiss Complex

P-T estimates for different units of the southern Western Gneiss Complex: the Hyllestad area (Chauvet et al., 1992), the

outer Sunnfjord eclogitic area (Engvik et al., 2000), and amphi-bole-bearing eclogites of the inner Sunnfjord (Krogh, 1980b) were recalculated using the techniques described above. The association used for the Kvineset eclogites in inner Sunnfjord is a combination of the IIB dataset and the compositions of garnet rims (Krogh, 1980b). Table 1 show the original P-T estimates and their present recalculations.

Our temperature recalculations are in agreement with pub-lished values, especially for the Kvineset eclogites, recently recalibrated by Cuthbert et al. (2000). Apart from the EC1 assemblage for Hyllestad eclogites, all temperature estimates are below 650 °C. Pressure estimates for eclogitic associations show a systematic offset of 0.5 GPa toward higher values. Gneiss and micaschists from Hyllestad indicate equilibration at 600 °C for pressures of 0.9 GPa after a decompression path along the staurolite-garnet join (Hacker et al., 2003). Apart from two of the THERMOCALC recalculations, all P-T estimates are within a 100 °C interval centered on 600 °C. The different units of the southern Western Gneiss Region thus show different isothermal decompression paths from minimum peak pressures of 1.6 GPa to 0.9 GPa and share the last stage of exhumation and thermal reequilibration from 0.8 GPa and 600 °C. The resulting P-T paths are compatible with the previous estimates for the Sunnfjord area and allow their comparison with the P-T path proposed here for the Nordfjord area.

P-T Estimates in the Nordfjord Area

To determine the P-T evolution of the Nordfjord area, 13 eclogite lenses, two amphibole-bearing gneisses, 10 garnet-bear-

TABLE 1. RECALCULATED PRESSURE-TEMPERATURE ESTIMATES IN THE SOUTHERN WGC

Sample Mineralogical assemblage Previous estimates THERMOCALC estimates

T (°C)

P (GPa)

T (°C)

P (GPa)

fi t

Hyllestad region (Chauvet et al, 1992; )

EC1 Grt1+Cpx6+Phe10 590 1.3 707 ± 115 2.48 ± 0.38 1.58/1.96GN Grt2+Bt4+Plg6+Ms8 550 0.7 627 ± 35 0.96 ± 0.13 1.19/1.61MS Grt1+Bt3+Plg5+Ms7 500 0.6 575 ± 28 0.84 ± 0.1 1.03/1.61

Outer Sunnfjord (Engvik et al., 2000)

V5B Grt2+Cpx1.2+Phe2+Ky+T 677 ± 48 1.6 ± 0.2 615 ± 22 2.27 ± 0.1 0.14/1.61B8 Grt1.1b+Cpx2.1+Phe2.7 495 ± 15 1.5 525 ± 46 2.34 ± 0.22 0.59/1.73

Inner Sunnfjord (Krogh, 1980a, 1980b) Original Recent*

T (°C)

P (GPa)

T (°C)

P (GPa)

Kvi193 561 1.64 561 ± 74 2.22 ± 0.40 1.47/1.96Kvi194 Grt+Omp+Phe† 540 ± 35 1.25 ± 0.25 517 1.56 567 ± 72 2.27 ± 0.39 1.43/1.96Kvi195B – – 613 ± 79 2.09 ± 0.38 1.33/1.96Kvi196 558 1.56 561 ± 87 2.20 ± 0.42 1.67/1.96

Note: Original pressure-temperature estimates in the southern part of the Western Gneiss Complex and values recalculated with the AX and THERMOCALC software. Fit column gives the confi dence index of the estimate versus the critical value for 95% confi dence. Analysis numbers refer to cited references. *Cuthbert et al., 2000. †Amphibole excluded for inadequate activity models.


ing gneisses, and three lower grade gneisses were sampled in the different structural units (Fig. 9). In these samples, the analyzed mineral associations were considered to be texturally equilibrated (Fig. 10). When results show a variation in the same thin section, associations leading to maximum pressure and maximum tem-perature are presented. Table 2 (eclogites) and Table 3 (gneisses) give end-member proportions of the phases used for calculations. THERMOCALC results are given with uncertainties, correlation factors, con dence index, and the number of independent reac-tions used compared to the number of end-members. For cal-culations with the gneiss, end-members with con dence values higher than 2.5 have been eliminated so that P-T estimates have a con dence index lower than the critical value for the con dence angle of 95%.

Estimates of Peak Pressures for the Eclogites

Temperature estimates from garnet-omphacite-phengite associations (Fig.10A), and garnet-omphacite-biotite for NOL125,

range from 575 °C to 795 °C with pressures from 1.9 to 3.2 GPa. Comparison with P-T estimates by Cuthbert et al. (2000) in 4 localities (Table 2) show that both P and T estimates are in agree-ment within their uncertainties. For eclogites NO2, F01, NOL218, NOL228, and NOL360, THERMOCALC maximum pressure estimates come together with minimum temperature estimates. The “max gros” association favored by Cuthbert et al. (2000) composed of maximum a

pyr × a

gros2 in garnet, maximum

jadeite content in omphacite, and maximum Si content in pheng-ite gives hybrid estimates compared with our local equilibrium estimates. As observed by previous authors (Cuthbert et al., 2000; Wain et al., 2000), no clear relation can be deciphered between peak conditions and structures. The 0.4 GPa gap between the UHP and HP eclogites revealed by Wain (1997; 1998) is not seen in this data set, and would be within uncertain-ties anyway. Peak pressures for eclogites NO2, TP01, NOL20, NOL220, NOL218, NOL243, and NOL360 are within the coesite stability eld. Five of them are located within the UHP or the HP-UHP mixed zone. NOL243 and NOL360 estimates together with

Figure 9. Location maps of the samples cited here. Filled circles are for samples with thermobarometric estimates, open circles and/or italic for samples used for 40Ar/39Ar dating (Labrousse, 2001), bold for samples dated with U-Pb and/or Rb-SR methods (Schärer and Labrousse, 2003). Long dashed lines are for the limits of the UHP and UHP-HP zones (Cuthbert et al., 2000) and short dashed lines are for the limits of the struc-tural units of Stadlandet, Vanylven, and Volda.


A B

C D

Grt

Grt

Grt

Phe

Bt

Bt

Hbl

Hbl + Cpx + Plg

Qtz

Qtz

Qtz

Qtz

Bt + Plg

Ky

Plg

Plg

Chl

Chl

Ms

Figure 10. Plane polarized light microphotographs of thin sections; scale bars are 100 µm wide. A. Eclogite sample NOL243 (Table 2) from Nor-dfjordeid with garnet (Grt), omphacite (Omp) and phengite (Phe) belonging to the eclogitic paragenesis. Bt-Plg and Hbl-Cpx-Plg symplectites at grain boundaries represent the retrogression in amphibolite facies. B. Grt-bearing amphibolite sample NOL130 (Table 3) from Stadlandet with Grt, Hbl, and Plg in equilibrium. C. Grt-bearing gneiss sample NOL314 (Table 3) from Vanylven showing Grt, Ky, Bt, and Chl. D. Greenschist facies pegmatite sample NO007 (Table 3) from Breidteigelva, with local phyllosilicate intergrowths of Bt-Ms-Chl in a plagioclase and quartz matrix. Mineral abbreviations after Kretz (1983).

two UHP eclogite occurrences in the Hornindal area (Cuthbert et al, 2000) (Fig. 2) would indicate a continuation of the mixed HP-UHP zone toward the east. The pressure difference between the Bryggja and Lefdal sites (Cuthbert et al., 2000) is con rmed by this study. The scatter of these estimates on a P-T diagram (Fig. 11B) is interpreted as re ecting the equilibration of the different eclogite lenses along a clockwise loop in the eclogite facies eld, with a difference of more than 100 °C between prograde and retrograde sections.

Estimates of Retrogression Conditions for the Eclogites

Decompression conditions of eclogites can be calculated from retrograde symplectites (Waters, 2002) composed of amphi-

boles (intermediate between pargasite, taramite, and Mg-kato-phorite end-members [Leake et al. {1997}]) (Fig. 10A), sodic clinopyroxene (Jd

20–30), and plagioclase (mainly oligoclase). P-T

estimates for such symplectites in six samples are within 550–650 °C and 0.45–1.32 GPa, within the uncertainties of other esti-mates in the same area that lead to temperatures between 600 °C and 700 °C for pressures of 0.7–1.4 GPa (Waters, 2002). These temperature estimates are lower than the 700–800 °C interval deduced from granulite facies symplectites in microfractures in eclogites from Ulsteinvik (Straume and Austrheim, 1999). A similar 100 °C difference between estimates of retrogression conditions from matrix assemblages and microfractures has been described in the Dora Maira pyrope-quartzites and explained by lower water activities in the cracks (Chopin and Schertl, 2000).

TAB

LE 2

. AN

ALY

ZE

D P

AR

AG

EN

ES

IS A

ND

PR

ES

SU

RE

-TE

MP

ER

ATU

RE

ES

TIM

ATE

S F

OR

MIC

A-B

EA

RIN

G E

CLO

GIT

ES

IN T

HE

NO

RD

FJO

RD

AR

EA

Sam

ple

NO

2F

01T

P01

NO

L20

NO

L125

NO

L295

NO

L299

NO

L220

Loca

tion

Dra

geV

ågsø

yV

erpe

nese

tV

enø

yS

tadl

ande

tB

ortn

epol

len

Bor

tnep

olle

nTo

tland

Coo

rdin

ates

62°0

5.95! N

61°5

5.85! N

61°5

3.65! N

61°5

9.00! N

62°0

4.55! N

61°5

1.40! N

61°5

1.40! N

61°5

5.70! N

05°1

1.12! E

05°0

2.59! E

05°1

1.51! E

05°1

6.63! E

05°1

7.07! E

05°1

7.97! E

05°1

7.97! E

05°2

3.28! E

Ecl

ogiti

c as

sem

blag

e

Grt

1527

5979

7346

128

1034

2310

4G

rs0.

250.

240.

210.

220.

170.

130.

119

0.23

0.21

0.20

0.25

Pyr

0.40

0.40

0.27

0.24

0.45

0.36

0.46

30.

180.

260.

260.

28A

lm0.

330.

330.

510.

500.

360.

510.

400

0.56

0.52

0.53

0.46

Sps

0.01

0.01

0.01

0.01

0.01

0.01

0.01

00.

010.

010.

010.

01O

mp

1426

6280

7838

127

940

3083

Jd0.

350.

360.

550.

540.

330.

460.

390.

550.

500.

500.

50D

i0.

560.

560.

360.

370.

620.

390.

490.

320.

370.

360.

41H

ed0.

080.

080.

100.

090.

050.

150.

120.

130.

130.

130.

09M

ica

Phe

13Ph

e 25

Phe

58Ph

e 84

Phe

77Ph

e 37

Bt 1

26Ph

e 6

Phe

36Ph

e 24

Phe

101

Ms/

An

0.48

0.60

0.58

0.57

0.45

0.56

0.16

0.67

0.64

0.67

0.52

Tri/P

hl0.

000.

030.

060.

060.

000.

040.

720.

160.

040.

040.

05C

el/E

ast

0.52

0.37

0.36

0.36

0.55

0.40

0.12

0.17

0.32

0.29

0.43

Am

p–

–60

–12

9M

ajor

end

-mem

ber

edM

g ka

tO

ther

sq,

H2O

q, H

2Oq,

H2O

q, H

2Oq,

H2O

, K

y, T

lcq,

H2O

, Ky

q, H

2Oq,

H2O

q, H

2Oq,

H2O

q, H

2O

TH

ER

MO

CA

LC r

esul

ts

T (°

C)

650

736

655

640

576

594

773

661

717

794

674

sdT

112

7635

5934

6461

9872

8410

3P

(G

Pa)

2.86

2.60

2.04

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1.87

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0.39

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TABLE 2. ANALYZED PARAGENESIS AND PRESSURE-TEMPERATURE ESTIMATES FOR MICA-BEARING ECLOGITES IN THE NORDFJORD AREA (continued)

Sample NOL218 NOL228 NOL215 NOL243 NOL360

Location Krokkenaken Levdal ? ? Austefjord

Coordinates 61°55.90! N 61°55.40! N 61°56.85! N 61°54.95! N 62°04.60! N

05°26.38! E 05°30.13! E 05°28.71! E 06°03.10! E 06°09.96! E

Eclogitic assemblage

Grt 41 60 24 09 27 32 59 5 22Grs 0.27 0.27 0.22 0.20 0.18 0.22 0.25 0.20 0.19 Pyr 0.25 0.25 0.19 0.20 0.30 0.28 0.25 0.31 0.33 Alm 0.46 0.45 0.57 0.58 0.51 0.48 0.48 0.47 0.46 Sps 0.01 0.01 0.02 0.02 0.00 0.01 0.01 0.01 0.08 Omp 42 61 26 20 26 34 60 3 23Jd 0.46 0.47 0.59 0.56 0.44 0.45 0.44 0.48 0.48Di 0.44 0.42 0.25 0.28 0.43 0.43 0.44 0.40 0.41Hed 0.10 0.11 0.16 0.16 0.13 0.12 0.12 0.12 0.11Phe 40 59 23 11 83 31 58 4 19Ms 0.60 0.40 0.64 0.55 0.54 0.48 0.48 0.57 0.73Tri 0.03 0.02 0.05 0.063 0.00 0.03 0.04 0.02 0.07Cel 0.37 0.58 0.31 0.39 0.46 0.49 0.48 0.41 0.20Amp – – – –Major end-memberOthers q, H2O q, H2O q, H2O q, H2O q, H2O q, H2O q, H2O Zo, Ky, q, H2O

THERMOCALC results

T (°C) 716 628 588 626 756 642 608 612 684sdT 71 71 84 58 73 58 54 42 58P (GPa) 2.52 2.85 2.34 2.04 2.16 2.59 2.68 2.59 2.07sdP 0.24 0.24 0.38 0.25 0.27 0.24 0.24 0.11 0.13correlation –0.34 –0.15 –0.52 –0.49 –0.41 –0.26 –0.28 –0.14 –0.56Fit/sigfi t (95%) 0.23/1.96 1.63/1.96 1.43/1.96 0.60/1.96 1.42/1.96 0.41/1.96 0.21/1.96 1.01/1.49 0.81/1.54Nr/Nem 3/11 3/11 3/11 3/11 3/11 3/11 3/11 7/16 6/15Previous estimates* UHPM-6 UHPM-24

730 29.8 646 22.1Retrogression assemblage – –AmphMajor end-members Mg = tar, Prg Mg-tarCpxJd 0.3 0.44PlAn 0.2 0.2P-T estimate†

T (°C) 600–650 600–640P (GPa) 0.94–1.00 0.45–0.73

Note: Sample locations on Figure 9; analysis numbers (bold notations) refer to Labrousse (2001). End-members proportions have been recalculated to 1; abbreviations are from Spear (1993). Activities calculated from chemical analysis with AX (Holland and Powell, 1996a, 1996b). All calculations have been performed for aH20 = 1 with THERMOCALC 3.1 (Powell and Holland, 1988; Holland and Powell, 1998). Fig/sigfi t is the confi dence index for the estimates versus its value for 95% confi dence. Nr/Nem is the number of independent reactions used versus the number of considered end-members. Fe-ed—ferro-edenite; Mg-kat—magnesio-katophorite; Mg-tar—magnesio-taranite; Prg—pargasite; sdP—standard deviation for pressure; all other mineral abbreviations after Kretz, 1983. *refers to Cutherbert et al. (2002) calculations for samples from the same localities. †P-T estimates from amphibole-clinopyroxene-plagioclase sympletctites have been processed by successive iterations of the Hb-plag thermometer (Holland and Blundy, 1994) and a clinopyroxene-plagioclase barometer (Perchuk, 1992).


Estimates of Retrogression Conditions from Gneisses

Several relics of HP parageneses have been described in the surrounding gneiss of eclogites lenses (cf. Krabbendam and Wain, 1997), in the lithologies of cover units, in gneisses of the Western Gneiss Region (Cuthbert et al., 2000), and in discrete shear-bands within the massive granulite bodies (Krabbendam and Wain, 1997; Krabbendam et al., 2000). Preserved coesite or polycrystalline quartz pseudomorphs have been found both in eclogites and in gneisses (Wain, 1997). It is thus admitted that preserved protoliths, granulites, eclogites, and amphibolite-facies and greenschist-facies gneisses equilibrated at different stages of a common P-T history, due to different kinetics of reaction and availability of uids (Austrheim, 1990; Wain, 1998; Krabbendam et al., 2000). The P-T conditions of equilibration of amphibolites (Fig. 10B), amphibolite-facies garnet-bearing gneisses (Fig. 10C), and greenschist-facies gneisses (Fig.10D)

can therefore be used to further constrain the retrograde path of the Nordfjord crustal-scale boudin and the conditions of melting. The parageneses used for the calculations are detailed in Table 3. The uncertainties on the P-T estimates (30 °C and 0.13 GPa) are lower for the gneisses than for the eclogites because the greater number of phases in the gneisses increases the number of inde-pendent reactions.

The P-T estimates fall into three clusters (Fig. 11B). The three highest grade estimates, in the partial melt domain for granitoids (Huang and Wyllie, 1981; Stern and Wyllie, 1981), are obtained for an amphibolite (NOL130) from the Stadlandet pen-insula and two garnet-bearing gneisses from the Stadlandet area (NOL128) and the southern shore of the Nordfjord (NOL301 in Davik). NOL130 and NOL128 share similar garnet and pla-gioclase compositions (Table 3). The amphibole in NOL130 is mainly pargasitic in composition, similar to amphiboles in the retrograde symplectites in eclogites (Table 2). NOL301, with a

TABLE 3. ANALYZED PARAGENESIS AND PRESSURE-TEMPERATURE ESTIMATES FOR AMPHIBOLITES AND GNEISS IN THE NORDFJORD AREA

Sample NOL130 NOL322 NOL128 NOL301 NOL298 NOL307 NOL424Lithology Amphibolite Amphibolite Gneiss Gneiss Migmatite Migmatite Mylonitic gneissLocation Stadlandet Breidteigelva Drage Davik Bortnepollen Bortnepollen GurskøyCoordinates 62°06.30! N 62°08.20! N 62°05.95! N 61°54.00! N 61°51.40! N 61°51.95! N 62°12.80! N

05°20.56! E 05°42.03! E 05°11.12! E 05°30.65! E 05°17.97! E 05°18.62! E 05°34.01! E

Assemblage incl. Matrix

Grt 74 31 03 104 – – 04 12Grs 0.26 0.18 0.22 0.27 0.07 0.06Pyr 0.21 0.20 0.23 0.23 0.16 0.15Alm 0.51 0.56 0.54 0.47 0.70 0.69Sps 0.02 0.06 0.01 0.01 0.07 0.08Pl 80 27 05 107 165 67 06 10An 0.26 0.35 0.29 0.46 0.21 0.15 0.22 0.36Phe – – 07 105 168 – – –Ms 0.65 0.54 0.64Tri 0.04 0.04 0.13Cel 0.31 0.42 0.23Bt – 29 04 108 162 66 05 11An 0.38 0.40 0.39 0.46 0.49 0.45 0.42Phl 0.48 0.38 0.43 0.41 0.34 0.34 0.36East 0.14 0.22 0.18 0.13 0.17 0.20 0.21Chl – – – – 160 68 02 13Clin 0.80 0.81 0.63 0.62Ames 0.19 0.19 0.37 0.38Sud 0.01 0.00 0.00 0.00Amph 79 28 – – – – – –Major end-member Prg Mg-tarEpidote 77 06 84 170 69 – –Zo 0.83 0.85 0.8 0.95 0.7 0.7AlSi – – – – – – Ky SilTHERMOCALC resultsT (°C) 683 613 689 720 421 420 550 519sdT 30 18 47 45 38 38 19 17P (GPa) 1.32 0.86 1.13 1.43 0.4 0.54 0.52 0.6sdP 0.11 0.07 0.15 0.16 0.15 0.16 0.13 0.16correlation 0.908 0.70 0.94 0.94 0.89 0.91 0.28 –0.23Fit/sigfi t (95%) 1.12/1.61 1.28/1.49 11.17/1.54 0.52/1.61 1.43/1.54 1.49/1.61 1.32/1.61 0.98/1.96Nr/Nem 5/12 7/15 6/15 5/14 6/15 5/13 5/12 3/10members eliminated Fe-act Amph Phl Phl, Grt Ames Ames Pyr Pyr, Phl

(continued)


TABLE 3. ANALYZED PARAGENESIS AND PRESSURE-TEMPERATURE ESTIMATES FOR AMPHIBOLITES AND GNEISS IN THE NORDFJORD AREA (continued)

Sample NOL425 NOL321 NO007 NOL381 NOL206 NOL358 NOL314 NOL315Gneiss Gneiss Pegmatite Gneiss Migmatite Gneiss Migmatite Migmatite

Location Gurskøy Breidteigelva Breidteigelva Kalvatnet Krokkenaken Austefjord Vanylven VanylvenCoordinates 62°12.80! N 62°08.20! N 62°08.20! N 61°58.75! N 61°55.00! N 62°04.45! N 62°08.30! N 62°11.25! N

05°34.01! E 05°42.03! E 05°42.03! E 06°19.40! E 05°21.21! E 06°07.37! E 05°25.22! E 05°27.67! E

AssemblageGrt 43 01 – 112 87 50 66 24Grs 0.07 0.07 0.22 0.33 0.11 0.09 0.08Pyr 0.12 0.14 0.05 0.08 0.27 0.18 0.07Alm 0.69 0.73 0.67 0.53 0.58 0.59 0.45Sps 0.12 0.05 0.04 0.04 0.03 0.14 0.39Pl 41 04 96 122 98 48 67 27An 0.35 0.37 0.25 0.33 0.35 0.74 0.30 0.23Phe – 03 93 116 96 – 68 25Ms 0.93 0.71 0.88 0.57 0.86 0.74Tri 0.02 0.09 0.06 0.02 0.06 0.08Cel 0.05 0.20 0.06 0.41 0.08 0.18Bt 42 02 91 115 – 52 69 26An 0.38 0.43 0.50 0.44 0.39 0.33 0.46Phl 0.41 0.33 0.32 0.38 0.40 0.45 0.35East 0.21 0.24 0.17 0.18 0.21 0.22 0.18Chl – – 92 114 97 51 71 –Clin 0.74 0.58 0.77 0.59 0.54Ames 0.26 0.39 0.15 0.33 0.32Sud 0.00 0.03 0.08 0.08 0.14Amph – – – – – – – –Major end-memberEpidote – – – – – – – –ZoAlSi Sil Sil/Ky – Sil – – Ky –THERMOCALC resultsT (°C) 574 586 433* 566 563 585 597 623sdT 22 18 75 15 15 20 11 56P (GPa) 0.57 0.56 0.33* 0.76 0.61 0.63 0.72 0.66sdP 0.2 0.04 0.26 0.04 0.07 0.14 0.08 0.19correlation 0.07 0.97 – 0.53 0.60 0.11 0.63 0.76Fit/sigfi t (95%) 1.61/1.61 0.89/1.49 – 1.17/1.45 0.80/1.61 1.27/1.61 1.06/1.73 1.60/1.61Nr/Nem 5/12 7/15 3–4/18 8/16 4/12 5/12 9/17 5/13members eliminated 0 0 – 0 Clin 0 0 0

Note: Sample locations on Figure 9; analysis numbers (bold notations) refer to Labrousse (2001). End member proportions recalculated to 1; abbreviations from Spear (1993). Activities calculated from chemical analysis with AX (Holland and Powell, 1996a, 1996b). All calculations have been performed for aH20 = 1 with THERMOCALC 3.1 (Powell and Holland, 1988; Holland and Powell, 1998). Fig/sigfi t is the confi dence index for the estimates versus its value for 95% confi dence. Nr/Nem is the number of independent reactions used versus the number of considered end-members. Ames—amesite; Amph—amphibolite; Clin—clinopyroxene; Fe-act—ferro-actinolite; Mg-tar—magnesio-taranite; Prg—pargasite; sdP—standard deviation for pressure; all other mineral abbreviations after Kretz (1983).

lower almandine content in garnet, a higher anorthite content in plagioclase, and higher celadonite in phengite than NOL128 yielded higher P and T values. Calculations with all the end members for NOL301 yielded 764 ± 120 °C for 1.3 ± 0.36 GPa; both grossular and phlogopite have been excluded from calculation. An amphibolite (NOL322) from Volda and the 10 selected garnet-bearing gneisses yielded temperature estimates between 519 °C and 623 °C for pressures between 0.52 and 0.86 GPa. This domain is adjacent to the kyanite-sillimanite join. Two of the samples (NOL321 and NOL424) show both aluminosilicates with clear textural evidence for the reaction of kyanite to sillimanite.

These P-T estimates are in good agreement with the calcula-tions from the retrograde symplectites in the eclogites, con rm-ing the hypothesis of a common P-T history at that point at least. The gneisses apparently equilibrated at temperatures immedi-ately below the hydrated solidus for granitoids (Huang and Wyl-lie, 1981; Stern and Wyllie, 1981).

The last group of P-T estimates at 420–430 °C for pressures between 0.3 and 0.5 GPa is obtained from lithologies that under-went low greenschist overprinting: pegmatite NO007 from Volda and migmatites NOL298 and NOL307 from Bortnepollen (Table 3). These more-hydrated lithologies equilibrated at lower grade than the surrounding gneisses.


Figure 11. Pressure-temperature estimates in the Western Gneiss Region. A. Estimates in the southern Western Gneiss Complex from published analysis (rec.—recalculated in Table 1). B. Estimates for the Nordfjord area (see text for calculation procedure). C. Pressure-temperature for the different areas. (1)—Staurolite-garnet transition in the KFMASH system (Hacker et al., 2003; (2)—Hydrated solidus for a biotite granite (Stern and Wyllie, 1981); (3)—Hydrated solidus for a muscovite granite (Huang and Wyllie, 1981); (a)—Chauvet et al. (1992); (b)—Hacker et al. (2003); THERMOCALC estimates have simply been replotted without recalculations since the same process has been used. (c)— Engvik et al. (2000); (d)—Krogh (1980a, 1980b); (e)—Cuthbert et al. (2000).

As for the eclogites, no clear correlation appears between the scatter of P-T estimates and their geographical location. A denser sampling would have been necessary to determine precise trends.

Conclusion

A composite P-T path can be extrapolated for the entire Nordfjord area (Fig. 11C), with a maximum pressure of 2.8 GPa reached at temperatures between 650 °C and 700 °C. The retro-grade path in the hypersolidus domain is recorded by low-grade eclogites and some gneisses. A maximum temperature value of 780 °C is reached for pressures of ~1.8 GPa, and most of the gneisses reequilibrated at temperatures near 620 °C for pressures of 0.8 GPa. The latest cooling path is constrained by a P-T point

at 420 °C and 0.4 GPa. This path compares with the recalculated path for the Sunnfjord region and a recently published path for the Moldefjord area (Terry et al., 2000b). The peak pressures are intermediate and con rm the northward gradient in metamorphic conditions (Cuthbert et al., 2000). According to our study, the shape of the P-T path for the Nordfjord area is slightly different, with a maximum temperature of 780 °C reached during decom-pression from eclogite to amphibolite domains, whereas Molde -fjord shows early isothermal stages of exhumation. The P-T paths for Nordfjord and Moldefjord share the point 780 °C and 1.8 GPa (Grif n et al., 1985; Terry et al., 2000b) obtained by different studies. These conditions are considered representative of eclog-ite facies equilibration in the HP units of northern Western Gneiss Region (Terry et al., 2000a). The late thermal reequilibration is


different, however: the Moldefjord HP and UHP units crossed the kyanite-sillimanite join at 700 °C (Terry et al., 2000a), whereas the Nordfjord units reached this line at temperatures <600 °C.

CHRONOLOGICAL CONSTRAINTS FOR THE EXHUMATION OF THE WESTERN GNEISS COMPLEX

Timing of the UHP Stage

In the diamond-bearing gneiss of the Moldefjord area, cores of monazites included in high-grossular rims of garnets yielded a 415 ± 6.8 Ma sensitive high-resolution ion microprobe (SHRIMP) age and a 408 ± 6.8 Ma electron microprobe (EMP) age (Terry et al., 2000a). Terry et al. (2000a) interpret these ages as maximum ages for the garnet growth at UHP conditions and give a value of 407 Ma for the maximum age of peak pressures. The 415–400 Ma interval for this UHP stage is therefore a rea-sonable estimate, taking the uncertainties into account.

In the Nordfjord area, SHRIMP analysis combined with multi-step abrasion on zircons from four UHP eclogites yielded clusters of U-Pb ages at 404–407 and 400 Ma (Root et al., 2001). These ages show that recrystallization of zircons must have occurred until 400–395 Ma. This age is interpreted as the time of UHP equilibration. This estimate is in agreement with the 402 ± 2 Ma Pb-Pb recalculated age for UHP in Ulsteinvik (Lutro et al., 1997), less than 40 km to the north of the locali-ties studied by Root et al. (2001). An estimate of the UHP stage at 405–395 Ma is therefore possible for the Nordfjord region. Whether the 5 Ma offset of the two intervals is relevant or not will be discussed further.

Time Constraints on HP Stages and Decompression Paths

In the Moldefjord area, the HP stage in lower units (Terry et al., 2000a) has a 400 ± 16 Ma mixed Sm-Nd age (Mørk and Mearns, 1986) for synkinematic equilibration at 780 °C and 1.8 GPa (Grif n et al., 1985; Terry et al., 2000b). The rims of monazites included in garnets, as well as the monazites analyzed in the matrix of UHP gneiss from Nordøyane, yield a cluster of ages at 394.8 ± 2.3 Ma thought to represent late resetting in response to uid percolation during cooling (Terry et al., 2000a). This age is coeval with concordant U-Pb ages on titanites and an average low intercept age obtained by regression of the regional discordant titanite ages (Terry et al., 2000a) and thought to record cessation of Pb-loss by cooling (Tucker et al., 1987). The composite P-T path for the Nordøyane units (Terry et al., 2000b) places the temperature near 700 °C at maximum pressures of 1.1 GPa (Fig. 11C).

In the Nordfjord area, titanites from the Drage eclogite in Stadlandet give a 389 ± 4 Ma U-Pb age (Schärer and Labrousse, 2003). This age is interpreted as the timing of maximum tem-peratures, 780 °C, at 1.8 GPa, according to the present P-T path for Nordfjord.

Time Constraints on Partial Melting and Cooling

In a kyanite-garnet mylonite from Moldefjord, a cluster of ages at 374.6 ± 2.7 Ma is interpreted as the age of cooling at 500 °C and 0.25 GPa after mylonitization (Terry et al., 2000a). Among the lower intercept U-Pb ages on titanite of the north-ernmost Western Gneiss Complex, two come from migmatite gneisses (Tucker et al., 1987, 1990) at 396 ± 2 Ma and 394 ± 4 Ma. Mixed and whole rock Rb-Sr dating on pegmatites from the Kristiansund area yielded 377 ± 10 Ma and 384 ± 6 Ma for crystallization and cooling of the partial melts (Pidgeon and Råheim, 1972; Råheim, 1977). In the Nordfjord area, a U-Pb age on titanites from a pegmatite in Drage yielded a 375 ± 6 Ma date (Schärer and Labrousse, 2003). This date was considered as the age of crystallization of the pegmatite in the subsolidus eld at 600 °C. The 40Ar/39Ar age spectra in the region (Fig. 12) show ages younger than 378 Ma for migmatites from the northern Nordfjord shore and ages between 390 and 380 Ma for samples within the Nordfjord Mylonitic Shear Zone (Labrousse, 2001), all with uncertainties of ±5 m.y. Temperature estimates on mus-covite-bearing assemblages in ve samples (NOL298, NOL307, NOL314, NOL315, and NOL321 in Table 3) are between 420 and 620 °C, beyond closure temperature estimates for the K-Ar system in micas (Villa, 1998; York, 1984). The ages are thus con-sidered as the age of cooling at 400 °C (Labrousse, 2001)

The age zoning is in agreement with the crustal-scale boudin structure proposed for the area, with ages near 372 Ma for the core and the Stadlandet unit and a cluster of ages at 378 Ma for the structurally shallower migmatized rims of Vanylven and Volda (Labrousse, 2001). A Rb-Sr age on biotites from a pegmatite in Volda dates cooling below 300–350 °C at 357 ± 9 Ma, close to the Devonian-Carboniferous boundary (Schärer and Labrousse, 2003).

Numerous 40Ar/39Ar dating studies in the southern Western Gneiss Complex give a reliable dating of cooling of the lower units of the Nordfjord-Sogn Detachment Zone (Cuthbert, 1991; Chauvet and Dallmeyer, 1992; Berry et al., 1995; Andersen et al., 1998; Fossen and Dunlap, 1998). 40Ar/39Ar ages range between 393 and 411 Ma for muscovite and biotite between 61°N and 61°45!N in the Western Gneiss Complex (Fig. 12). Among those, muscovite ages range between 395 and 403 Ma, giving a time interval for the regional cooling below 400 °C (Ander-sen, 1998). Ages within the Nordfjord-Sogn Detachment Zone itself (Chauvet and Dallmeyer, 1992) give a lower limit for the amphibolite facies extension stage along the Nordfjord-Sogn Detachment Zone of 403 Ma (Hacker et al., 2003).

In order to complete a N-S age pro le along the whole west-ern basement exposure, these data can be compared with data from the Vestranden Gneiss Complex, north of the Møre Trøn-delag Fault Zone. The Vestranden Gneiss Complex is exposed in the footwall of the doubly vergent detachment system of Høy-bakken and Kollstraumen (HDZ and KDZ on Fig. 12) and shows pervasive anatexis during stretching and exhumation (Braathen et al., 2000). U-Pb (Schouenborg et al., 1991) and Rb-Sr

Figure 12. Schematic block di-agram of Western Norway with a N-S pro le of 40Ar/39Ar cool-ing ages on muscovites, Rb-Sr ages, and U-Pb ages on migmatites discussed in the text. P-T paths for the North-western Gneiss Complex (NWGC) and Southwestern Gneiss Complex (SWGC) are compared. M—Moldefjord; N—Nordfjord; S—Sunnfjord; H—Hyllestad; BASZ—Bergen Arc Shear Zone; HDZ—Høy-bakken Detachment Zone; JDZ—Jotunheimen Décolle-ment Zone; KDZ—Kollstrau-men Detachment Zone; MTFZ—Møre Trøndelag Fault Zone; NMSZ—Nordfjord My-lonitic Shear Zone; NSDZ—Nordfjord-Sogn Detachment Zone. Gray boxes are common points to the different regions. UHP, UHP-HP zones limits and partial melting in the Kris-tiansund area from Cuthbert et al. (2000). Vestranden Gneiss Complex (VGC) fold axis and detachment geometry from Braathen et al. (2000).

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(Piasecki and Cliff, 1988) ages on dikes that intruded the sur-rounding gneiss before, during, and after deformation range between 404 Ma and 380 Ma. This interval overlaps with 40Ar/39Ar muscovite ages for the region (Dallmeyer, 1992; Lutro et al., 1997), indicating cooling below 400 °C between 410 and 390 Ma.

Estimation of Exhumation Rates

From the various pressure-time points listed above, exhu-mation rates can be calculated, with a depth-pressure relation-ship deduced from tables in Dziewonski and Anderson (1981). Minimum, mean, and maximum rates can be calculated accord-ing to uncertainties on ages and pressures (Table 4, Fig. 13A). Estimates for the Moldefjord region, including data from Terry et al. (2000a, 2000b), yield exhumation rates decreasing from 8.1 to 4.2 and 1.4 mm/yr. No upper limit can be calculated for the two rst steps, due to the large uncertainty on the timing of HP stage. In the Nordfjord, the different exhumation rates are comparable (2.3 to 4.6 mm/yr) with uncertainties of one order of magnitude. For the Hyllestad-Solund-Lavik area, depths and time values evaluated by Hacker et al. (2003) indicate slightly higher exhumation rates for the Lavik and Hyllestad units than for the UHP province.

DISCUSSION

Comparison of the structural, thermobarometric, and geo-chronologic datasets along the western coast highlights the

differences between the northern and the southern parts of the Western Gneiss Complex. The Nordfjord area, characterized by a crustal-scale boudin structure (Labrousse et al., 2002), and the Nordøyane area, in the Moldefjord, as a folded UHP nappe in HP wedge (Terry et al., 2000b), both show constrictive deformation patterns concordant with a direction of extension turning from NE-SW to E-W southward. Both regions reached the coesite sta-bility eld. The local variations of peak conditions recorded by the eclogites in the northern Western Gneiss Complex and rst in the wide HP-UHP transition zone (Cuthbert et al., 2000; Wain et al., 2000) and the close imbrication of various lithologies (Fig. 3 herein and Figure 2a in Terry et al., 2000b) in the gneisses may be partly explained by turbulent mixing at depth. Partial melting, clearly post-eclogitic in the Nordfjord gneisses (Labrousse et al., 2002), has been described as syn-exhumation in the eclogites of the Kristiansund area (Cuthbert, 1995, 1997).

Timing constraints on exhumation have common points and the ages for maximum pressures, overlapping uncertainties domains, and may represent a common UHP event at 400–410 Ma. A midway stage of reequilibration has been recorded at 395 Ma in the Moldefjord area (Terry et al., 2000a) and at 389 Ma in the Nordfjord area (Schärer and Labrousse, 2003). The post-migmatization cooling of gneisses in the Nordfjord area is coeval with late crystallizations in the gneisses of Moldefjord at 375 Ma (Terry et al., 2000a). The cooling of the northern West-ern Gneiss Complex is thus considerably younger than deposi-tion in the Devonian basins, the youngest preserved strata dated in the Hornelen basin being Eifelian (Bockelie and Nystuen, 1985; Wilks and Cuthbert, 1994), that is, 390 ± 4 Ma (Tucker

TABLE 4. EXHUMATION RATES CALCULATED FROM PRESSURE-TEMPERATURE-TIME POINTS (FIG. 12) FOR THE DIFFERENT REGIONS OF THE WESTERN GNEISS REGION

Stage P (GPa)

dP (GPa)

z (km)

dz (km)

t (Ma)

dt (Ma)

Vmin V(mm/yr)

Vmax

MoldefjordUHP 3.7 0.2 117 6 407 7

0.6

0.5

0.9

8.1

4.2

1.4

–

–

2.3

HP 1.8 0.2 60 6 400 16

MP 1.1 0.1 39 3 395 2

LP 0.25 0.05 11 3 375 3

Nordfjord

UHP 2.8 0.15 90 4 400 51

0.8

0.4

2.7

2.3

4.6

20

11

–

HP 1.8 0.2 60 6 389 4

MP 0.7 0.15 28 6 375 6

LP 0.4 0.1 14 2 372 5

Solund-Hyllestad-Lavik

Lavik – – 80 – 405 56.9

2.5

25

10

–

–Hyllestad – – 50 – 405 5

stacking 0.8 0.1 30 2 403 –

Note: Exhumation rates calculated from pressure-temperature-time points (Fig. 12) for the different regions of the WGC with a pressure-depth relation deduced from the PREM model (Dziewonski and Anderson, 1981). Minimum and maximum velocity values were calculated with uncertainties on time intervals and depths.


Figure 13. A. Depth-time intervals used in Table 4 for calculation of exhumation rates, compared to published scenarios detailed in Figure 2. B. Pressure-tem-perature-time paths for the different areas of the Western Gneiss Complex. H—Hyllestad; L—Lavik; M—Mold-efjord; N—Nordfjord; S—Sunnfjord; SHL—Solund-Hyllestad-Lavik. P-T boxes are 150 °C × 0.3 GPa (i.e., the maximum uncertainties for P-T calcu-lations; Tables 1, 2, and 3). Exhumation rate calculations and pressure-depth conversion are detailed in Table 4. References for ages are discussed in the text.

and McKerrow, 1995). The depth-time evolution proposed for the Nordfjord area is shifted toward younger ages compared to published scenarios (Fig 12A).

The southern Western Gneiss Complex shows drastically different patterns. The most striking feature at the regional scale is the preservation of low-grade rocks in the hanging wall of a continuous detachment zone along the western shore and the

presence of Devonian detrital basins. Inside the footwall of the Nordfjord-Sogn Detachment Zone, different lithotectonic units can still be distinguished, such as the Hyllestad metapelites below the Solund basin (Chauvet et al., 1992; Hacker et al., 2003) and the eclogite massif of the outer Sunnfjord (Engvik et al., 2000). Mixing of the different tectonic units at depth has thus been limited. The distinct P-T estimates for those units


Figure 14. Schematic transect across the Southwestern Caledonides at 410–400 Ma locating the different units of the Western Gneiss Complex in the orogen dynamics. Moldefjord and Nordfjord represent the Northwestern Gneiss Complex involved in the deep subduc-tion channel dynamics; Sunnfjord, Lavik, Hyllestad, and Lifjorden repre-sent the Southwestern Gneiss Complex already exhuming in the upper wedge at that time. 500 °C and 800 °C isotherms and the quartz-coesite transition geom-etries are hypothetical.

(Krogh, 1980a, 1980b; Chauvet et al., 1992; Cuthbert et al., 2000; Engvik et al., 2000; Hacker et al., 2003) show a maximum temperature below 700 °C and maximum pressures below 2.4 GPa. The retrograde paths share a common late segment after a reequilibration stage at 600 °C and 0.8 GPa (Hacker et al., 2003). The southern Western Gneiss Complex thus apparently behaved as a coherent unit during its latest stages of exhumation (Andersen et al., 1991).

Among the numerous 40Ar/39Ar ages in the southern part of the Western Gneiss Region and the overriding nappes, the older dates (415–408 Ma; Fossen, 2000) are related to the south-eastward thrusting stages of the allochthon units. The younger cooling ages (402–394 Ma; Fossen, 2000), associated with northwestward to westward reactivation of the tectonic contacts (Nordfjord-Sogn Detachment Zone and Jotunheimen Décolle-ment Zone), are older than the estimated onset of deposition of the Devonian basins. The northern and southern areas of the Western Gneiss Complex are therefore likely to be two differ-ent units with contrasting pressure-temperature-time-deforma-tion histories (Terry et al., 2000a) rather than a single coherent body ruled by a northwestward gradient of overall Caledonian imprint (Krogh, 1977; Grif n et al., 1985; Krabbendam and Dewey, 1998; Cuthbert et al., 2000). The step shape in the 40Ar/39Ar ages spectrum across the whole Western Gneiss Complex is probably the best clue (Ring et al., 1999) for a sharp tectonic contact between southern and northern compartments along the Nordfjord Mylonitic Shear Zone. The different blocks represent two different levels of the Caledonian orogenic wedge.

The northern Western Gneiss Complex, with UHP relics, high mixing of lithologies, partial melting, boudinage, and con-striction preserves the structures associated with deep dynamics in a subduction channel at mantle depths. The common point of the different P-T paths, at 780 °C and 1.8 GPa (i.e., ~60 km) at different times (400 Ma for the Moldefjord rocks, 389 Ma for the Nordfjord area), suggests the presence of a zone of reequili-bration at the base of the thickened crust (Fig. 14). The southern

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Western Gneiss Complex, with distinct sheared tectonic units of different P-T levels all gathered and exhumed rapidly by the detachment system, is representative of dynamics in the upper levels, with burial and immediate exhumation from HP. These two different circuits for exhumation of HP and UHP rocks have also been recognized in the western Alps (Agard et al., 2001), and several levels of circulation of subducted material are pro-posed for the exhumation of eclogites and blueschists in the Mediterranean belt (Jolivet et al., 2003).

Numerical modeling of continental collision dynamics (Burov et al., 2001) reproduces such geometries and shows that one important control parameter is the eclogitization rate suf-fered by the continental material at depth (Dewey et al., 1993). Scenarios with highly eclogitized material, with low buoyancy and low viscosity, show the development of deep subduction channel dynamics, whereas scenarios with less eclogitized material show exhumation processes dominated by shallower accretionary complex dynamics (Burov et al., 2001).

The partial melting of gneiss and eclogites reported in the northern Western Gneiss Complex (Bryhni, 1966; Cuthbert, 1995; Cuthbert, 1997; Labrousse et al., 2002) is responsible for a signi cant decrease of the bulk viscosity of the crustal mate-rial, even in the metatexitic eld (Vanderhaeghe and Teyssier, 2001). This low-viscosity material may have been an ef cient lubricant in the subduction channel, like muddy serpentinites or marbles in corner ow models (Shreve and Cloos, 1986; Guil-lot et al., 2000), able to preserve large pods of protolith and to exhume dense inclusions such as eclogite and peridotite lenses. Partial melting is also an important feature in the Vestranden Gneiss Complex, where synkinematic pegmatite dikes intrude the detachment systems of Kollstraumen and Høybakken (Braa-then et al., 2000). In this region, where structures and dynam-ics show similarities with metamorphic core complex models (Braathen et al., 2000; Malavieille, 1993), partial melting of the lower granulitic continental crust may have enhanced gravita-tional instabilities and favored early exhumation (prior to 395


Ma according to U-Pb and 40Ar/39Ar ages on dikes) in an oro-gen-parallel dome-detachment system (Braathen et al., 2000). In the northern Western Gneiss Complex, the late exhumation of the deep parts of the subduction channel and their juxtapo-sition to the shallower levels of the southern Western Gneiss Complex may have been favored by gravitational instability due to accumulation of partially molten rocks at depth (Vander-haeghe et al., 1999). Different stages of gravitational collapse would therefore have occurred in the Western Caledonides, syn-collisional orogen-parallel collapse above the granulitic Vestranden Gneiss Complex ( xed boundary collapse mode 1 in Rey et al., 2001), and post-collisional collapse in the Western Gneiss Complex during E-W extension.

Structures in the different gneiss complexes show generic patterns of dome formation in collapse processes. The dome structures are systematically elongated in the stretching direc-tion: NE-SW for the Vestranden Gneiss Complex and E-W for the Nordfjord area. They show constrictive deformation in their inner part. The detachment systems (Nordfjord-Sogn Detachment Zone, Høybakken Detachment Zone, and Koll-straumen Detachment Zone) that separate lower from upper levels of the wedge are systematically folded along axes par-allel to the stretching direction and show constriction at the regional scale during deposition in the basin (Chauvet and Seranne, 1994; Krabbendam and Dewey, 1998; Osmundsen and Andersen, 2001). Such doming patterns have been described in the exhumed lower crust of the Aegean domain (Avigad et al., 2001; Jolivet et al., this volume) where they represent the deepest parts of the orogenic wedge exhumed in a regional con-strictional regime. The middle crust, rst affected by detach-ment zones, doming, and crustal boudinage perpendicular to stretching direction, is progressively affected by folding in the parallel direction and exhumation of the lower crust in the antiforms. The Western Gneiss Complex would show a similar strati cation with three layers: the uppermost crust preserved in the hanging wall of the Nordfjord-Sogn Detachment Zone, the middle levels in the southern Western Gneiss Complex imme-diately below the Nordfjord-Sogn Detachment Zone, and the lower levels in E-W elongated domes in the northern Western Gneiss Complex.

Determination of boundary conditions in the late stages of the Scandian phase reveal an overall sinistral transtensional regime (Krabbendam and Dewey, 1998) that would be respon-sible for the constrictive extension recorded in the different dome structures.

CONCLUSIONS

The southern limit of the UHP-HP transition zone as it has recently been inferred from coesite-bearing eclogite occur-rences (Cuthbert et al., 2000) corresponds in the Nordfjord area to the southern ank of a dome with a crustal-scale boudin structure (Labrousse et al., 2002). It follows the latest and shal-lowest structure developed during the exhumation of the West-

ern Gneiss Complex in this area: the Nordfjord Mylonitic Shear Zone, below the Nordfjord-Sogn Detachment. The UHP eclog-ites appear scattered in the inner and structurally lower parts of a core-and-envelope structure, with preserved granulites in the core and migmatized rims stretched in the E-W direction and sheared to the west. The P-T-t path for this dome structure, elongated in the stretching direction, shows af nities with the other UHP zone of Moldefjord further to the north. Although their maximum depths were different (120 km for the Moldef-jord; Terry, et al., 2000b; versus 90 km for the Nordfjord) they were at that time (410–400 Ma) equilibrated along the same thermal gradient (Cuthbert et al., 2000). Exhumation at veloci-ties higher than 3 mm/yr resulted in an isothermal decompres-sion stage to 60 km depth at different times. This level appears as a stable reequilibration level for ~10 m.y. Post-eclogitic par-tial melting observed in the Western Gneiss Complex from the Nordfjord to the Kristiansund area (Bryhni, 1966; Cuthbert, 1995, 1997; Labrousse et al., 2002) has been dated by indepen-dent radiometric methods between 380 and 370 Ma. 40Ar/39Ar cooling ages in the Nordfjord area show that the different enve-lopes cooled progressively from 378 Ma in the external parts to 373 Ma in the core of the Nordfjord dome.

These P-T-t-strain histories sharply contrast with the evo-lution of the southern part of the Western Gneiss Complex. The footwall of the Nordfjord-Sogn Detachment Zone underwent a short burial episode to depths not exceeding 80 km before a regional cooling between 400 and 405 Ma (Hacker et al., 2003, and references therein). The exhumation of units of Baltic basement and its cover, like the Hyllestad complex, occurred at high velocities (>3 mm/yr) in the wide Nordfjord-Sogn Detachment Zone, resulting in the stacking of tectonic units of eastward increasing grade. The Nordfjord Mylonitic Shear Zone appears as a sharp limit between this compartment and the Northwestern Gneiss Complex, which cooled below 400 °C more than 20 m.y. later.

The contrasting deformation patterns re ect different exhumation processes operating at the same time in different levels of the orogen. Material involved in continental subduc-tion to mantle depths, now exposed in the Northwestern Gneiss Complex, was initially exhumed by syn-collisional upward ow in the subduction channel, whereas upper levels, now exposed in the Southwestern Gneiss Complex, were exhumed in a shallower crustal wedge. The late juxtaposition of these two levels was achieved by extensional doming in the North-western Gneiss Complex assisted by pervasive partial melt-ing. The constrictional regime driven by oblique divergence between Baltica and Laurentia controlled both the E-W folding of the upper levels and the Nordfjord-Sogn Detachment Zone and the exhumation of the lower levels as domes elongated in the stretching direction. The UHP Province of Western Nor-way could be considered as the juxtaposition of crustal-scale structures such as the dome described in the Nordfjord region rather than the continuous border of a coherent crustal block or a nappe stacked in a HP wedge.


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