Geological Society of America Special 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. Andersen Department of Geology, University of Oslo, P.O. Box 1047 Blindern, 0316 Oslo, Norway P. Agard Laboratoire de Tectonique UMR 7072, UPMC T26E1 case 129, 4, place Jussieu 75252 Paris cedex 05, France R. Hébert Departement des Sciences de la Terre UMR 7072, Université de Cergy-Pontoise, Le Campus Bat I, 95031 Cergy cedex, France H. Maluski Laboratoire de Géochronologie UMR 5573, Université Montpellier II, Place Eugène Bataillon, 34095 Montpellier cedex, France U. Schärer Géochronologie–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, dened 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 continental 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 Nordfjord-Sogn Detachment Zone. The rst stage of deformation is coeval with reequilibration from maximum pressure conditions around 2.8 GPa, 650 °C (THERMOCALC 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] 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
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L. Labrousse et al. 500-km-long N-S prole, 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 detachment 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 afnities 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 geometry, and the structure of the crustal or lithospheric elements involved in the exhumation. Structural and petrological analyses in the Dora Maira massif (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 metamorphic 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 exhumation 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 structural 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-granulitic assemblages in the Flatraket area and even primary igneous mineralogy (Krabbendam et al., 2000). Meter-scale mac 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; Grifn et al., 1985). Eclogitic paragenesis described in surrounding felsic gneisses
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Figure 1. Location map of the different areas and localities in the Western Gneiss Region. Simplied 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 Detachment Zone.
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 “internal” eclogites are also associated with kilometer-scale ultramac bodies incorporated into the continental material before or during 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 denition of an “UHP Province” from Nordfjord to Moldefjord (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 Andersen, 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 during 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 Detachment 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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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). Postcollisional 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 conrming the latest estimations of the beginning 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 northwestward polarity both in the peak metamorphic conditions recorded by eclogites (Krogh, 1977; Grifn et al., 1985) and in the Caledonian imprint lead to asymmetrical models with continental subduction 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).
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Pressure-temperature-time deformation history of ultra-high pressure rocks 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 southeastward 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 (Krabbendam 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 (Krabbendam and Dewey, 1998). Most of these models consider the Western Gneiss Complex 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; Grifn 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 (Krabbendam 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 Greenland 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 lithosphere (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-
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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 Western 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 trajectories 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 EW 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 lithological 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 4. Field evidence of stretching and constriction. A, B. Detailed photograph and outcrop sketch in Bortnepollen. Augen gneiss shows a penetrative E-W stretching lineation and a discrete foliation. C. Layered gneiss outcrop photograph in Flatraket showing constrictive folding parallel to E-W stretching direction. D, E, F, G. Photographs and outcrop sketches in the Nordfjordeid region showing folding of early veins (C) and N-S late veins (D) perpendicular to stretching direction.
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as well as from Caledonian prograde and retrograde metamorphism, 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 Flatraket 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 lithologies (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 pressure 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 5. Field evidence of shear criteria. A, B. Photograph and outcrop sketch of top-to-west shear-bands in a garnet-bearing gneiss in Breidteigelva. C, D. Photograph and outcrop sketch of E-W trending sheath folds in migmatitic gneiss in the Stadlandet-Flatraket high shear zone. E. outcrop sketch of sinistral shear criteria in Nordfjordeid.
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Figure 6. Field evidence of boudinage at any scale in Verpeneset. A. Outcrop sketch. B. Photograph of asymmetrically truncated metabasic layers preserving eclogitic paragenesis. C. Photograph of a truncated quartz-rich layer. Asymmetry of boudins and deection of markers indicate a dextral sense of shear. D. Synthetic block-faulting of a more competent amphibolitic gneiss layer.
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deection of markers in the boudins is synthetic to the sense of rotation of the blocks (Grasemann and Stüwe, 2000). The asymmetry 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 kinematic 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 retrogression of eclogites to amphibolites. The three eastern units have been sheared toward the west over the Flatraket deeper unit
A
B
⃪ ⇿ ỹ ⓿ ⇿ ␦⑊ ⅋⋿ ῾ ᥏ ⇿ ⃪ ⣿ ␦ ⅋⑊
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F
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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).
Pressure-temperature-time deformation history of ultra-high pressure rocks during E-W stretching, folding, and retrogression in the amphibolite 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 concordant 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 sigmoidal 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 exceeding 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 NordfjordSogn 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 (Torsvik 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 synextensional 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 Nordfjord area are the L-tectonite fabrics preserved in the eclogitic cores of metabasic lenses (Andersen et al., 1994; Bascou et al.,
165
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 Kristiansund 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 stretching 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 viscosity 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 calculation process has been chosen. The latest version of the THERMOCALC software (Holland and Powell, 1998) was used to calculate equilibrium conditions with uncertainties, condence index, and correlation factors for the mac and pelitic associations of eclogite lenses and surrounding gneisses. Sodic-calcic amphibole, plagioclase, and sodic clinopyroxene symplectites growing at omphacite grain boundaries in eclogites (Waters,
A ᙶ
ᝓ
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ݏ ֊⅋
B C
⋿ ␦⑊␦ ▟
⃪⑊ ⋿␦
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ޱ ֊⅋
൯ Ↄ⇿ ⑊ ⇿ ⅋ ⇿ ⃪ ⃪⓿ ⇿ Ↄ
E ᝓ
F
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G
ᙶ
ᝓ
⇿ ⃪ϻ⚡ ⑊ ⋿␦⃪ ⃪⃪ ⓿ ⇿⃪ Ↄ⓿ ⇿ Ↄ ⋿ỹ ⅋⓿ ⓿▟ Ↄ ⑊ Ↄ⚡ ⃪ ⋿ ⃪Ↄ ⑊῾ ⑊ ⓿ ⇿ ⑊ ⓿ ➾
ۿ ֊⅋
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 Teyssier, 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.
Pressure-temperature-time deformation history of ultra-high pressure rocks 2002) were used to determine the retrogression conditions of eclogites, based on hornblende-plagioclase thermometry (Holland and Blundy, 1994) and clinopyroxene-plagioclase barometry (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 Fe2O3 (Fe), MnTiO3 (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 estimates 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 twosite 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).
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outer Sunnfjord eclogitic area (Engvik et al., 2000), and amphibole-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 published 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.
Pressure-Temperature Path for the Southern Western Gneiss Complex
P-T Estimates in the Nordfjord Area
P-T estimates for different units of the southern Western Gneiss Complex: the Hyllestad area (Chauvet et al., 1992), the
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)
fit
590 550 500
1.3 0.7 0.6
707 ± 115 627 ± 35 575 ± 28
2.48 ± 0.38 0.96 ± 0.13 0.84 ± 0.1
1.58/1.96 1.19/1.61 1.03/1.61
677 ± 48 495 ± 15
1.6 ± 0.2 1.5
615 ± 22 525 ± 46
2.27 ± 0.1 2.34 ± 0.22
0.14/1.61 0.59/1.73
Original
Recent*
561 ± 74 567 ± 72 613 ± 79 561 ± 87
2.22 ± 0.40 2.27 ± 0.39 2.09 ± 0.38 2.20 ± 0.42
1.47/1.96 1.43/1.96 1.33/1.96 1.67/1.96
Hyllestad region (Chauvet et al, 1992; ) EC1 GN MS
Grt1+Cpx6+Phe10 Grt2+Bt4+Plg6+Ms8 Grt1+Bt3+Plg5+Ms7
Outer Sunnfjord (Engvik et al., 2000) V5B B8
Grt2+Cpx1.2+Phe2+Ky+T Grt1.1b+Cpx2.1+Phe2.7
Inner Sunnfjord (Krogh, 1980a, 1980b) T (°C) Kvi193 Kvi194 Kvi195B Kvi196
Grt+Omp+Phe†
P (GPa)
540 ± 35 1.25 ± 0.25
T (°C)
P (GPa)
561 517 – 558
1.64 1.56 – 1.56
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 confidence index of the estimate versus the critical value for 95% confidence. Analysis numbers refer to cited references. *Cuthbert et al., 2000. † Amphibole excluded for inadequate activity models.
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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 temperature 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, condence index, and the number of independent reactions used compared to the number of end-members. For calculations with the gneiss, end-members with condence values higher than 2.5 have been eliminated so that P-T estimates have a condence index lower than the critical value for the condence 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 agreement 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 apyr × agros2 in garnet, maximum jadeite content in omphacite, and maximum Si content in phengite 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 uncertainties 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 HPUHP 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 structural units of Stadlandet, Vanylven, and Volda.
Pressure-temperature-time deformation history of ultra-high pressure rocks
Hbl + Cpx + Plg
A
169
B Hbl
Qtz
Grt
Grt
Plg
Qtz
Bt + Plg Phe
Bt
C
D Ky
Grt
Plg Ms Qtz
Chl
Chl
Bt
Qtz
Figure 10. Plane polarized light microphotographs of thin sections; scale bars are 100 µm wide. A. Eclogite sample NOL243 (Table 2) from Nordfjordeid 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 HPUHP zone toward the east. The pressure difference between the Bryggja and Lefdal sites (Cuthbert et al., 2000) is conrmed by this study. The scatter of these estimates on a P-T diagram (Fig. 11B) is interpreted as reecting 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-katophorite end-members [Leake et al. {1997}]) (Fig. 10A), sodic clinopyroxene (Jd20–30), 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 estimates 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).
27 0.24 0.40 0.33 0.01 26 0.36 0.56 0.08 Phe 25 0.60 0.03 0.37 – q, H2O
q, H2O
T (°C) P (GPa)
P-T estimate
†
600–650 0.94–1.00
q, H2O
79 0.22 0.24 0.50 0.01 80 0.54 0.37 0.09 Phe 84 0.57 0.06 0.36 –
650 Fixed 1.0
0.03
0.13–0.35
550–620 0.87–1.26
–
661 98 2.20 0.35 –0.44 1.32/1.96 3/11
q, H2O
10 0.23 0.18 0.56 0.01 9 0.55 0.32 0.13 Phe 6 0.67 0.16 0.17
05°17.97! E
NOL295 Bortnepollen 61°51.40! N
0.3
–
–
773 61 1.95 0.21 –0.27 1.10/1.42 8/16
128 0.119 0.463 0.400 0.010 127 0.39 0.49 0.12 Bt 126 0.16 0.72 0.12 129 Mg kat q, H2O
05°17.07! E
NOL125 Stadlandet 62°04.55! N
Mg = kat
594 64 2.45 0.15 –0.195 1.51/1.73 4/12
q, H2O, Ky
46 0.13 0.36 0.51 0.01 38 0.46 0.39 0.15 Phe 37 0.56 0.04 0.40
05°16.63! E
NOL20 Venøy 61°59.00! N
576 34 3.15 0.13 –0.13 1.50/1.73 4/12
q, H2O, Ky, Tlc
73 0.17 0.45 0.36 0.01 78 0.33 0.62 0.05 Phe 77 0.45 0.00 0.55
05°11.51! E
TP01 Verpeneset 61°53.65! N
Fe-ed
655 640 35 59 2.04 2.21 0.17 0.25 –0.26 –0.49 1.25/1.45 0.22/1.96 8/16 3/13
59 0.21 0.27 0.51 0.01 62 0.55 0.36 0.10 Phe 58 0.58 0.06 0.36 60 ed q, H2O
05°02.59! E
05°11.12! E
15 0.25 0.40 0.33 0.01 14 0.35 0.56 0.08 Phe 13 0.48 0.00 0.52 –
F01 Vågsøy 61°55.85! N
NO2 Drage 62°05.95! N
650 736 112 76 2.86 2.60 0.39 0.23 –0.3 –0.38 1.87/1.96 0.79/1.96 3/11 3/11 3/97 758 29.9 Retrogression assemblage Amph Major end-member Prg Cpx Jd 0.2 Pl An 0.1–0.35
T (°C) sdT P (GPa) sdP correlation Fit/sigfit (95%) Nr/Nem Previous estimates*
THERMOCALC results
Grt Grs Pyr Alm Sps Omp Jd Di Hed Mica Ms/An Tri/Phl Cel/East Amp Major end-member Others
Eclogitic assemblage
Sample Location Coordinates
q, H2O
23 0.20 0.26 0.53 0.01 30 0.50 0.36 0.13 Phe 24 0.67 0.04 0.29
579–589 0.91–1.32
0.13
0.31
Prg, Mg = tar
717 794 72 84 1.87 1.93 0.28 0.28 –0.55 –0.51 1.02/1.45 0.51/1.96 3/11 3/11
q, H2O
34 0.21 0.26 0.52 0.01 40 0.50 0.37 0.13 Phe 36 0.64 0.04 0.32
05°17.97! E
NOL299 Bortnepollen 61°51.40! N
TABLE 2. ANALYZED PARAGENESIS AND PRESSURE-TEMPERATURE ESTIMATES FOR MICA-BEARING ECLOGITES IN THE NORDFJORD AREA
(continued )
674 103 2.84 0.37 –0.25 1.64/1.96 4/13 UHPM-70 759 29.2
q, H2O
104 0.25 0.28 0.46 0.01 83 0.50 0.41 0.09 Phe 101 0.52 0.05 0.43
05°23.28! E
NOL220 Totland 61°55.70! N
TABLE 2. ANALYZED PARAGENESIS AND PRESSURE-TEMPERATURE ESTIMATES FOR MICA-BEARING ECLOGITES IN THE NORDFJORD AREA (continued) Sample
NOL218
Location
Krokkenaken
Levdal
?
?
Austefjord
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
Coordinates
NOL228
NOL215
NOL243
NOL360
Eclogitic assemblage Grt Grs Pyr Alm Sps Omp Jd Di Hed Phe Ms Tri Cel Amp Major end-member Others
41 0.27 0.25 0.46 0.01 42 0.46 0.44 0.10 40 0.60 0.03 0.37 –
60 0.27 0.25 0.45 0.01 61 0.47 0.42 0.11 59 0.40 0.02 0.58 –
24 0.22 0.19 0.57 0.02 26 0.59 0.25 0.16 23 0.64 0.05 0.31 –
09 0.20 0.20 0.58 0.02 20 0.56 0.28 0.16 11 0.55 0.063 0.39 –
27 0.18 0.30 0.51 0.00 26 0.44 0.43 0.13 83 0.54 0.00 0.46
32 0.22 0.28 0.48 0.01 34 0.45 0.43 0.12 31 0.48 0.03 0.49
59 0.25 0.25 0.48 0.01 60 0.44 0.44 0.12 58 0.48 0.04 0.48
q, H2O
q, H2O
q, H2O
q, H2O
q, H2O
q, H2O
q, H2O
5 0.20 0.31 0.47 0.01 3 0.48 0.40 0.12 4 0.57 0.02 0.41
22 0.19 0.33 0.46 0.08 23 0.48 0.41 0.11 19 0.73 0.07 0.20
Zo, Ky, q, H2O
THERMOCALC results T (°C) sdT P (GPa) sdP correlation Fit/sigfit (95%) Nr/Nem Previous estimates* Retrogression assemblage Amph Major end-members Cpx Jd Pl An P-T estimate† T (°C) P (GPa)
716 628 71 71 2.52 2.85 0.24 0.24 –0.34 –0.15 0.23/1.96 1.63/1.96 3/11 3/11 UHPM-6 730 29.8
588 626 84 58 2.34 2.04 0.38 0.25 –0.52 –0.49 1.43/1.96 0.60/1.96 3/11 3/11 UHPM-24 646 22.1
756 73 2.16 0.27 –0.41 1.42/1.96 3/11
– Mg = tar, Prg
Mg-tar
0.3
0.44
0.2
0.2
600–650 0.94–1.00
600–640 0.45–0.73
642 608 58 54 2.59 2.68 0.24 0.24 –0.26 –0.28 0.41/1.96 0.21/1.96 3/11 3/11
612 684 42 58 2.59 2.07 0.11 0.13 –0.14 –0.56 1.01/1.49 0.81/1.54 7/16 6/15
–
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 aH 0 = 1 with THERMOCALC 3.1 (Powell and Holland, 1988; Holland and Powell, 1998). Fig/sigfit is the 2 confidence index for the estimates versus its value for 95% confidence. 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).
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TABLE 3. ANALYZED PARAGENESIS AND PRESSURE-TEMPERATURE ESTIMATES FOR AMPHIBOLITES AND GNEISS IN THE NORDFJORD AREA Sample Lithology Location Coordinates
NOL130 Amphibolite Stadlandet 62°06.30! N
NOL322 Amphibolite Breidteigelva 62°08.20! N
NOL128 Gneiss Drage 62°05.95! N
NOL301 Gneiss Davik 61°54.00! N
NOL298 Migmatite Bortnepollen 61°51.40! N
NOL307 Migmatite Bortnepollen 61°51.95! 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
Assemblage Grt Grs Pyr Alm Sps Pl An Phe Ms Tri Cel Bt An Phl East Chl Clin Ames Sud Amph Major end-member Epidote Zo AlSi THERMOCALC results T (°C) sdT P (GPa) sdP correlation Fit/sigfit (95%) Nr/Nem members eliminated
74 0.26 0.21 0.51 0.02 80 0.26 –
31 0.18 0.20 0.56 0.06 27 0.35 –
–
29 0.38 0.48 0.14 –
–
NOL424 Mylonitic gneiss Gurskøy 62°12.80! N 05°34.01! E incl.
Matrix 12 0.06 0.15 0.69 0.08 10 0.36 –
–
–
67 0.15 –
–
165 0.21 168 0.64 0.13 0.23 162 0.46 0.41 0.13 160 0.80 0.19 0.01 –
04 0.07 0.16 0.70 0.07 06 0.22 –
66 0.49 0.34 0.17 68 0.81 0.19 0.00 –
05 0.45 0.34 0.20 02 0.63 0.37 0.00 –
11 0.42 0.36 0.21 13 0.62 0.38 0.00 –
03 0.22 0.23 0.54 0.01 05 0.29 07 0.65 0.04 0.31 04 0.40 0.38 0.22 –
104 0.27 0.23 0.47 0.01 107 0.46 105 0.54 0.04 0.42 108 0.39 0.43 0.18 –
–
79 Prg 77 0.83 –
28 Mg-tar 0.85 –
06 0.8 –
84 0.95 –
170 0.7 –
69 0.7 –
–
–
Ky
Sil
683 30 1.32 0.11 0.908 1.12/1.61 5/12 Fe-act
613 18 0.86 0.07 0.70 1.28/1.49 7/15 Amph
689 47 1.13 0.15 0.94 11.17/1.54 6/15 Phl
720 45 1.43 0.16 0.94 0.52/1.61 5/14 Phl, Grt
421 38 0.4 0.15 0.89 1.43/1.54 6/15 Ames
420 38 0.54 0.16 0.91 1.49/1.61 5/13 Ames
550 19 0.52 0.13 0.28 1.32/1.61 5/12 Pyr
519 17 0.6 0.16 –0.23 0.98/1.96 3/10 Pyr, Phl (continued )
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 amphibolitefacies 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 independent 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 peninsula 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 plagioclase 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
Pressure-temperature-time deformation history of ultra-high pressure rocks
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TABLE 3. ANALYZED PARAGENESIS AND PRESSURE-TEMPERATURE ESTIMATES FOR AMPHIBOLITES AND GNEISS IN THE NORDFJORD AREA (continued) Sample Location Coordinates Assemblage Grt Grs Pyr Alm Sps Pl An Phe Ms Tri Cel Bt An Phl East Chl Clin Ames Sud Amph Major end-member Epidote Zo AlSi THERMOCALC results T (°C) sdT P (GPa) sdP correlation Fit/sigfit (95%) Nr/Nem members eliminated
NOL425 Gneiss Gurskøy 62°12.80! N 05°34.01! E 43 0.07 0.12 0.69 0.12 41 0.35 –
NOL321 NO007 Gneiss Pegmatite Breidteigelva Breidteigelva 62°08.20! N 62°08.20! N 05°42.03! E 05°42.03! E –
NOL381 Gneiss Kalvatnet 61°58.75! N 06°19.40! E
NOL206 Migmatite Krokkenaken 61°55.00! N 05°21.21! E
NOL358 Gneiss Austefjord 62°04.45! N 06°07.37! E
NOL314 Migmatite Vanylven 62°08.30! N 05°25.22! E
NOL315 Migmatite Vanylven 62°11.25! N 05°27.67! E
112 0.22 0.05 0.67 0.04 122 0.33 116 0.88 0.06 0.06 115 0.44 0.38 0.18 114 0.58 0.39 0.03 –
87 0.33 0.08 0.53 0.04 98 0.35 96 0.57 0.02 0.41 –
50 0.11 0.27 0.58 0.03 48 0.74 –
97 0.77 0.15 0.08 –
52 0.39 0.40 0.21 51 0.59 0.33 0.08 –
66 0.09 0.18 0.59 0.14 67 0.30 68 0.86 0.06 0.08 69 0.33 0.45 0.22 71 0.54 0.32 0.14 –
24 0.08 0.07 0.45 0.39 27 0.23 25 0.74 0.08 0.18 26 0.46 0.35 0.18 –
42 0.38 0.41 0.21 –
01 0.07 0.14 0.73 0.05 04 0.37 03 0.93 0.02 0.05 02 0.43 0.33 0.24 –
–
–
96 0.25 93 0.71 0.09 0.20 91 0.50 0.32 0.17 92 0.74 0.26 0.00 –
–
–
–
–
–
–
–
–
Sil
Sil/Ky
–
Sil
–
–
Ky
–
574 22 0.57 0.2 0.07 1.61/1.61 5/12 0
586 18 0.56 0.04 0.97 0.89/1.49 7/15 0
433* 75 0.33* 0.26 – – 3–4/18 –
566 15 0.76 0.04 0.53 1.17/1.45 8/16 0
563 15 0.61 0.07 0.60 0.80/1.61 4/12 Clin
585 20 0.63 0.14 0.11 1.27/1.61 5/12 0
597 11 0.72 0.08 0.63 1.06/1.73 9/17 0
623 56 0.66 0.19 0.76 1.60/1.61 5/13 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 aH 0 = 1 with THERMOCALC 3.1 (Powell and Holland, 1988; Holland and Powell, 1998). Fig/sigfit is the confidence index 2 for the estimates versus its value for 95% confidence. 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 calculations from the retrograde symplectites in the eclogites, conrming the hypothesis of a common P-T history at that point at least. The gneisses apparently equilibrated at temperatures immediately below the hydrated solidus for granitoids (Huang and Wyllie, 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 underwent 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.
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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 retrograde 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 conrm 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 decompression from eclogite to amphibolite domains, whereas Moldefjord shows early isothermal stages of exhumation. The P-T paths for Nordfjord and Moldefjord share the point 780 °C and 1.8 GPa (Grifn et al., 1985; Terry et al., 2000b) obtained by different studies. These conditions are considered representative of eclogite facies equilibration in the HP units of northern Western Gneiss Region (Terry et al., 2000a). The late thermal reequilibration is
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).
Pressure-temperature-time deformation history of ultra-high pressure rocks 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 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 reect different exhumation processes operating at the same time in different levels of the orogen. Material involved in continental subduction 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 Northwestern Gneiss Complex assisted by pervasive partial melting. 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 Norway 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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