Lithos 52Ž. 2000 165±195 www.elsevier.nlrlocaterlithos

Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides

S.J. Cuthbert a,), D.A. Carswell b, E.J. Krogh-Ravna c, A. Wain d,1 a Geology Section, Department of CiÕil Structural and EnÕironmental Engineering, UniÕersity of Paisley, High Street, Paisley PA1 2BE, UK b Department of Earth Sciences, UniÕersity of Sheffield, Dainton Building, Brookhill, Sheffield S3 7HF, UK c Institute of Geology, UniÕersity of Tromsù, Tromsù 9000, Norway d Department of Earth Sciences, UniÕersity of Oxford, Oxford OX1 3PR, UK

Abstract

The Western Gneiss RegionŽ. WGR marks the outcrop of a composite terrane consisting of variably re-worked Proterozoic basement and parautochthonous or autochthonous cover units. The WGR exhibits a gross structural, petrographic and thermobarometric zonation from southeast to northwest, reflecting an increasing intensity of ScandianŽ. late Palaeozoic metamorphic and structural imprint. Scandian-aged eclogites have been widelyŽ. though for kinetic reasons not invariably stabilised in metabasic rocks but have suffered varying degrees of retrogression during exhumation. In the region between the Jostedal mountains and Nordfjord, eclogites commonly have distinctively prograde-zoned garnets with amphibolite or epidote±amphibolite facies solid inclusion suites and lack any evidence for stability of coesiteŽ high pressurewx HP eclogites. . In the south of this area, in Sunnfjord, eclogites locally contain glaucophane as an inclusion or matrix phase. North of Nordfjord, eclogites mostly lack prograde zoning and evidence for coesite, either as relics or replacive polycrystalline quartz, is present in both eclogitesŽ ultrahigh pressurewx UHP eclogites. and, rarely, gneisses. Coesite or polycrystalline quartz after coesite has now been found in eight new localities, including one close to a microdiamond-bearing gneiss. These new discoveries suggest that, by a conservative estimate, the UHP terrane in the WGR covers a coastal strip of about 5000 km2 between outer Nordfjord and Moldefjord. A ``mixed HPrUHP zone'' containing both HP and UHP eclogites is confirmed by our observations, and is extended a further 40 km east along Nordfjord. Thermobarometry on phengite-bearing eclogites has been used to quantify the regional distribution of pressure Ž.P and temperature Ž.T across the WGR. Overall, a scenario emerges where P and T progressively increase from 5008C and 16 kbar in Sunnfjord to )8008C and 32 kbar in outer Moldefjord, respectively, in line with the distribution of eclogite petrographic features. Results are usually consistent with the silica polymorph present or inferred. The P±T conditions define a linear array in the P±T plane with a slope of roughly 58Crkm, with averages for petrographic groups lying along the trend according to their geographic distribution from SE to NW, hence defining a clear field gradient. This P±T gradient might be used to support the frequently postulated model for northwesterly subduction of the WGC as a coherent body. However, the WGC is clearly a composite edifice built from several tectonic units. Furthermore, the mixed HPrUHP zone seems to mark a step in the regional P gradient, indicating a possible tectonic break and tectonic juxtaposition of the HP and UHP units. Lack of other clear evidence for a tectonic break in the mixed zone dictates caution in this interpretation, and we cannot discount the possibility that the mixed zone is, at

) Corresponding author. E-mail: [email protected] 1 Present address: British Geological Survey, Murchison House, West Mains Road, Edinburgh EH9 3LA, UK.

0024-4937r00r$ - see front matter q 2000 Elsevier Science B.V. All rights reserved. PII: S0024-4937Ž. 99 00090-0 166 S.J. Cuthbert et al.rLithos 52() 2000 165±195 least, partly a result of kinetic factors operating near the HP±UHP transition. Overall, if the WGC has been subducted during the Scandian orogeny, it has retained its general down-slab pattern of P and T in spite of any disruption during exhumation. Garnetiferous peridotites derived from subcontinental lithospheric mantle may be restricted to the UHP terrane and appear to decorate basement-cover contacts in many cases. P±T conditions calculated from previously published data for both relict Ž.Proterozoic lithospheric mantle? porphyroclast assemblages and Scandian Ž. subduction-related? neoblastic assemblages do not define such a clear field gradient, but probably record a combination of their pre-orogenic P±T record with Scandian re-working during and after subduction entrainment. A crude linear array in the P±T plane defined by peridotite samples may be, in part, an artifact of errors in the geobarometric methods. A spatial association of mantle-derived peridotites with the UHP terrane and with basement-cover contacts is consistent with a hypothesis for entrainment of at least some of them as ``foreign'' fragments into a crustal UHP terrane during subduction of the Baltic continental margin to depths of )100 km, and encourages a more mobilistic view of the assembly of the WGC from its component lithotectonic elements. q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Eclogite; Western Gneiss Region; Norwegian Caledonides

1. Introduction boundary to the UHP province, and suggested that this may mark a tectonic boundary to a distinct UHP In order to determine the significance of ultrahigh lithotectonic unit. pressureŽ. UHP metamorphism in deep crustal geo- In spite of the large area of the WGRŽ. Fig. 1 , dynamic processes, it is clearly important to establish UHP eclogite discoveries have generally remained the nature and extent of the UHP lithotectonic units, restricted to the same general area in the early the nature of their boundaries, and their orogenic discoveries, i.e., in outer Nordfjord and Stadlandet setting. The Western Gneiss RegionŽ. WGR in the Ž.Figs. 1±3 of Smith Ž 1984; 1988 . . In the meantime, Norwegian Caledonides is well-known for its spec- the tantalising discovery of rare microdiamonds tacular occurrences of eclogites and garnetiferous Ž.Dobrzhinetskya et al., 1995 from pelitic and peridotites. This extensive high pressureŽ. HP meta- migmatitic gneisses at Fjùrtoft, about 70 km NE of morphic terrane is accessible, very well-exposed and StadlandetŽ. Figs. 1 and 2 indicated that the its orogenic setting has been well-characterised. coesite±eclogite province may be much more exten- Hence, the WGR provides us with an excellent sive. natural laboratory for the examination of the setting, This contribution describes some new discoveries nature and distribution of HP and UHP metamor- of UHP rocks which extend the area of the UHP phism, and the dynamics, physical properties and terrane in the WGR. We also examine the regional behaviour of subducted continental crust. context of UHP metamorphism in relation to the Coesite was discovered in eclogites from the WGR adjacent HP eclogite terranes and mappable lithotec- soon after its discovery in the AlpsŽ Chopin, 1984; tonic units, and finally make some comments on the Smith, 1984. . On the basis of seven scattered loca- possible nature of the boundaries to the UHP terrane lities showing evidence for coesite, SmithŽ. 1988 in terms of tectonic and kinetic factors. established a ``coesite±eclogite province'' in the Stadlandet area of coastal SW NorwayŽ. Fig. 1 . Subsequently, WainŽ. 1997a; b expanded the database of known localities where coesite or polycrystalline 2. Geological setting aggregates after coesite are known to a total of 24 localities and Krabbendam and WainŽ. 1997 were The WGR occupies an area of approximately able to map a boundary zone to the coesite±eclogite 5=10 4 km2 along the coast of southwest Norway province, distinguishing a group of UHP eclogites between Bergen and TrondheimŽ. Fig. 1 . The WGR from a group of normal HP eclogites to the south. contains the lowest exposed structural level in the WainŽ. 1997a; b established a pressure difference of southern Scandinavian Caledonides. It lies within a )4 kbar between HP and UHP eclogites in the large tectonic window, and is surrounded by the S.J. Cuthbert et al.rLithos 52() 2000 165±195 167

Fig. 1. Simplified lithotectonic and metamorphic map of the southern Scandinavian Caledonides after Roberts and GeeŽ. 1985 , showing the distribution of eclogite facies metamorphism. M Ð Malùy;Ê MoÐ Molde; R Ð Romsdal; S Ð Stryn; F Ð Fùrde; D Ð Dalsfjord. Location of diamond gneiss from Dobrzhinetskya et al.Ž. 1995 , locations of garnetiferous ultramafic rocks from Krogh and Carswell Ž. 1995 . Question marks indicate uncertainty over location of UHP terrane or mixed HPrUHP zone. outcrop of the Caledonide allochthon which consists tochthonous or para-autochthonous cover of late Pro- of a thick stack of thrust sheets. To the southeast and terozoic and lower Palaeozoic sediments. The al- east of the allochthon lies the Baltic foreland, mostly lochthon is composed of sheets of Baltic basement consisting of Proterozoic high-grade gneisses, gneisses and their lower Palaeozoic sedimentary metasediments and metavolcanics and their au- cover, overthrust by ophiolitic, arc-related and some- 168 S.J. Cuthbert et al.rLithos 52() 2000 165±195

Fig. 2. Distribution of petrographic types of eclogites in the coastal parts of the WGR between Sognfjord and Molde, also showing the location of cover units, peridotites and microdiamond. Geological units after KildalŽ. 1970 , Robinson Ž. 1995 , Tveten Ž. 1995 , Tveten and LutroŽ 1995a; b . . Eclogite localities from Krogh Ž 1980; 1982 . , Cuthbert Ž. 1985 , Griffin et al. Ž. 1985 , Smith Ž. 1988 , Bailey Ž. 1989 , Chauvet et al.Ž. 1992 and Krabbendam and Wain Ž. 1997 and additional unpublished data of the authors. times continental units with a provenance outboard The contact of the basement gneisses and their of the Baltican marginŽ Roberts and Gee, 1985; autochthonous cover with the base of the allochthon Stephens, 1988. . has had a complex evolution and shows evidence for S.J. Cuthbert et al.rLithos 52() 2000 165±195 169

Fig. 3. Lithotectonic and metamorphic map of the outer Nordfjord±Stadlandet area showing the distribution of petrographic types of eclogite and associated HP and UHP gneisses, and peridotites. Dashed lines mark the limits of the HP and UHP zones, and the extent of the mixed HPrUHP zone. southeast- or eastward-directed thrusting, followed Cuthbert, 1994; Wennberg, 1996. . This extensional by a reversal of motion in a normal sense to the reworking took place in at least two stagesŽ Ander- northwest or westŽ Hossack, 1984; Norton, 1986, sen and Jamtveit, 1990; Cuthbert, 1991; Cuthbert 1987; Chauvet and Brunel, 1988; Chauvet and Ser- and Carswell, 1990; Hartz et al., 1994; Wilks and anne, 1989; Fossen and Rykkelid, 1992; Wilks and Cuthbert, 1994; Milnes et al., 1997. with earlier, 170 S.J. Cuthbert et al.rLithos 52() 2000 165±195 amphibolite facies, ductile shear zones at lower to the northern part of the WGR with the main al- mid-crustal levels being reactivated or cut across by lochthon to the northeast. To the south of Alesund,Ê later, Middle Devonian, brittle normal and strike±slip correlations cannot be made so directly but the com- faults within the cover which accommodated the fill mon association of anorthosites, mangerites, of the late orogenic Old Red Sandstone molasse quartzites, marbles and pelitic schists is typical of the basins now lying along the west coastŽ. Fig. 2 . A autochthonous cover and overlying nappes in the significant element of transtensional or transpres- Jotunheim area southeast of the WGRŽ Strand, 1969; sional deformation played a role during this period, Brueckner, 1977; Bryhni, 1989. which form the especially in the north of the WGRŽ Seranne, 1992; Lower and Middle allochthons. Hence, by analogy Krabbendam and Dewey, 1998; Robinson and Terry, with the known Caledonide autochthon and al- 1998. . The net result of the extensional tectonic lochthonous nappes, we tentatively designate such phases was to bring the WGR rocks up against the assemblages of lithologies outcropping within the cover in the footwall of a system of low-angle WGR as ``cover'' units lying upon aŽ. para- auto- normal faults, so that the WGR rocks now lie within chthonous Baltic basement, although we cannot en- a large metamorphic core complex. Evidence for the tirely rule out the possibility that some anorthosites, earliest, eclogite-facies, subduction and exhumation mangerites and metasediments could be an integral fabrics is sporadic and highly localised, and thus is part of the autochthonous Proterozoic Baltic base- difficult to fit into the regional context. ment. Furthermore, Krabbendam and WainŽ. 1997 The predominant lithologies exposed in the WGR and WainŽ. 1998 have shown that some pelitic schist are broadly granitoid migmatitic and augen or- units described by BryhniŽ. 1966 from Nordfjord are, thogneisses which have a Proterozoic provenance in fact, eclogite-facies equivalents of the common ŽBrueckner, 1972, 1979; Pidgeon and Raheim,Ê 1972; granodioritic basement gneisses rather than sedimen- Skjerlie and Pringle, 1978; Lappin et al., 1979; tary cover. Thus, at this stage, the extent of Caledo- Harvey, 1983; Tucker et al., 1990. . These gneisses nian cover within the Western Gneiss Complex form the Jostedal Complex of BryhniŽ. 1966 , the Ž.WGC remains a matter of debate. Nevertheless, Fetvatn Gneiss of BruecknerŽ. 1977 , the Lùnset mappable units of diverse lithology and origin do gneiss of KrillŽ. 1985 and the Baltica Basement of outcrop in the WGR, some of which have certainly RobinsonŽ. 1995 , and correlate with the Baltic Pro- been emplaced by thrusting or possibly by exten- terozoic basement in the forelandŽ Milnes et al., sional faulting. It follows that the WGR contains an 1997. . They are referred to as ``basement'' here. assemblage of lithotectonic units which has not acted Other common lithologies are meta-anorthosites and as a coherent entity throughout its history. For this metamorphosed pyroxene syeniticrmonzonitic rocks reason, the term ``Western Gneiss Complex'' will be Ž.``mangerites'' , pelitic schists, quartzites, calc-sili- used here to refer to the assemblage of rock units cates and marbles. These diverse lithologies are often occupying the WGR and the term ``Western Gneiss spatially associated and lie within synclinal ``keels'' Region'' will more properly refer to the geographical downfolded into the dominant orthogneissic base- area in which the WGC outcrops, following Milnes mentŽ Muret, 1960; Bryhni, 1966, 1989; Bryhni and et al.Ž. 1997 . Grimstad, 1970; Brueckner, 1977; Krill, 1985; The WGC shows a systematic change in its struc- Tveten, 1995; Robinson, 1995, 1997; Robinson and tural and metamorphic characteristics from the Terry, 1998; Terry and Robinson, 1998. . This latter southeast to the northwest and westŽ. Fig. 1 . In the characteristic assemblage of lithologies is typical of SE, close to the contact with the main allochthon at the Lower and Middle Allochthons, and part of the Jotunheimen, eclogites are absent and Proterozoic Upper AllochthonŽ. Blahù±SurnaÊ nappe in the Scan- structures and parageneses are predominantly well- dinavian CaledonidesŽ. Roberts and Gee, 1985 . Such preserved, being similar to those in the foreland on rocks have been designated as ``supracrustal'' by the SE side of the allochthon, where eclogites are BryhniŽ 1966; 1989 . and Tveten Ž. 1995 . Krill Ž. 1985 , also lackingŽ. Milnes et al., 1988, 1997 . Passing RobinsonŽ. 1995 and Terry and Robinson Ž. 1998 westwards and northwestwards, Caledonide deforma- have directly correlated such supracrustal rocks in tion becomes more pervasive and Proterozoic struc- S.J. Cuthbert et al.rLithos 52() 2000 165±195 171 tures are progressively obliterated. Eclogites are first cies may also have varied in their efficiency and it recognised about 40 km NW of the edge of the may be possible to find evidence preserved for meta- allochthonŽ. NW of the Jostedal mountains and are morphic processes operating at the HP±UHP transi- relatively common throughout the rest of the WGC tion. We return to this theme in Section 6. within both basement and cover rock sequences. The timing of Caledonian metamorphism in the Early geothermometric studies on eclogites indicated WGR is based on a rather limited data set but both a progressive increase in temperature towards the U±Pb zircon and Nd±Sm mineral isochrons for NWŽ. Krogh, 1977; Griffin et al., 1985 consistent eclogites, presumed to approximate to timing of with the pattern of increasing intensity of Caledonide eclogite crystallisation, give late Silurian ages of tectonometamorphic reworking, but the pressure dis- 400±447 Ma with an average of 418 MaŽ Krogh et tribution for peak eclogite facies metamorphism was al., 1974; Griffin and Brueckner, 1980; Griffin and poorly constrained due to a lack of reliable geo- Brueckner, 1985; Mùrk and Mearns, 1986. . K±Ar, barometers. In the extreme NW of the WGR, eclog- Ar±Ar and Rb±Sr mineral ages are mostly younger ites show evidence for partial melting during ex- Ž.410±370 Ma and probably represent cooling ages humationŽ. Cuthbert, 1995, 1997 . The overall pattern during decompressionŽ Lux, 1985; Wilks and Cuth- of increasingly intense Caledonide deformation and bert, 1994; Boundy et al., 1996; see also the compi- metamorphism has been the basis for suggestions lation of Kullerud et al., 1986. . These ages are that the WGC has been subducted towards the NW consistent with structural and stratigraphic evidence Ž.in the present frame of reference during the Cale- for the major ``Scandian'' phase of thrust sheet donian collision between Baltica and Laurentia emplacement and consequent subduction of the WGC ŽGriffin et al., 1985; Cuthbert and Carswell, 1990; occurring during the late SilurianŽ Roberts and Gee, Cuthbert et al., 1983. . 1985. . No indisputably UHP eclogites have yet been Ultramafic rocks are common throughout the dated, so the age of UHP metamorphism is not WGC but garnetiferous ultramafites are only known well-constrained. No age difference is discernable from the region to the north of NordfjordŽ Figs. 1 between eclogites from basement and cover units and 2. where they often occur as pods and sheets within the WGC. However, eclogites are present in lying along basement-cover contactsŽ Brueckner, some units of the main outcrop of the allochthon and 1977; Bryhni, 1989; Bucher-Nurminen, 1991. and these give pre-Scandian ages Ž.)450±505 Ma , attest to considerable crust±mantle tectonic interac- mostly older than those in the WGRŽ Dallmeyer and tion during the Caledonide plate collisionŽ Jamtveit Gee, 1986; Mùrk et al., 1988; Boundy et al., 1996, et al., 1991; Brueckner and Medaris, 1998. . 1997; Essex et al., 1997 but see Bingen et al., 1998 In spite of the general trend of increased rework- for a Scandian U±Pb zircon age in the Bergen Arcs ing towards the northwest, there is evidence for anorthosite nappe. . If cover units within the WGC metastability on a large range of scales during eclog- correlate with those in the main allochthon, it is ite facies metamorphism resulting in the localised possible that some eclogites in the WGC may also be survival of pre-Caledonian parageneses throughout pre-Scandian, or have undergone two phases of the WGCŽ Gjelsvik, 1952; Griffin and Raheim,Ê 1973; eclogite-facies metamorphism. Overall, it is clear Cuthbert, 1985; Mùrk, 1985; Wain, 1997a,c, 1998; that an extensive isotopic dating programme is re- Austrheim, 1998. . Metastability is also manifested in quired to ascertain the timing of HP±UHP metamor- the survival of eclogite facies relics in the WGC, phism in the WGR. which is now predominantly an amphibolite-facies terrane. Clearly, the efficiency of conversion to dense, eclogite-facies parageneses and subsequent retro- 3. Field and petrographic characteristics of eclog- grade metamorphism to lower density parageneses ites in the WGC was highly variable and was probably a complex function of temperature, strain and fluid fluxŽ see Eclogites in the WGC are usually found as lentic- Austrheim, 1998. . In this context, it seems likely that ular pods or tabular layers from decimeters to a few metamorphic transformations within the eclogite fa- tens of meters thick within most of the other litholo- 172 S.J. Cuthbert et al.rLithos 52() 2000 165±195 gies comprising the WGC. A few much larger eclog- 1997; Wain, 1998. ; hence, the apparently incompati- ites occur as large, tabular bodies several kilometers ble mineral facies of eclogites and gneisses may be, in outcrop extent, some of which were originally at least in part, due to differential retrogression. layered, mafic intrusionsŽ Mysen and Heier, 1971; The ``internal'' eclogites, which are found as Cuthbert, 1985; Jamtveit, 1987. . layers and lenses within the masses of mantle-de- Most eclogites have retrograde selvages of amphi- rived dunite or garnetiferous peridotiteŽ.Ž Fig. 2 Lap- bolite against their host gneisses. The gneisses them- pin, 1966, 1974; Carswell, 1974; Medaris, 1980; selves usually have amphibolite facies mineral para- Jamtveit, 1984; Griffin and Qvale, 1985. , will be geneses, so that most eclogites are not co-facial with considered further in Section 5 below. their host rocks. However, Krabbendam and Wain Apart from the essential garnet and omphacite, Ž.1997 and Wain Ž. 1998 have shown that swarms of common matrix phases in the ``external'' eclogites eclogite pods commonly enclose, or are enveloped Ž.those enclosed within schists or gneisses are quartz, by, schists or gneisses with abundant relics of eclog- rutile, zoisiterclinozoisite, phengite, paragonite, ite-facies parageneses including phengite, kyanite, kyanite, orthopyroxene, phlogopitic mica, amphi- zoisite, garnet and quartz"omphacite, or their sym- boles of a wide range of compositions, carbonates plectic breakdown products. Eclogites within such Ž.Ždolomite, calcite, magnesite , pyrite and apatite for masses tend to lack retrogressive amphibolitic sel- a more exhaustive account of the peculiarities of vages and have fabric elements colinear with those eclogite mineralogy in the WGC, see Smith, 1988. . in the host rocksŽ Krabbendam and Wain, 1997; Orthopyroxene is commonly associated with phlogo- Terry and Robinson, 1998; Terry et al., 1999; Wain, pitic mica while kyanite, phengite and zoisite are 1998. . Some such gneisses contain polycrystalline absent from orthopyroxene eclogites. Mineral inclu- quartz after coesiteŽ. Wain, 1997a,b . Krabbendam sion suites and compositional zoning are common in and WainŽ. 1997 and Wain Ž. 1998 have suggested eclogite matrix phases. that these eclogite facies schist or gneiss envelopes While the mineralogy and fabrics of eclogites are metamorphic equivalents of the common gran- vary considerably, even within single bodies, a num- odioriticŽ. basement gneisses. While this may be true ber of features show clear geographic distributions. in some cases, the phengitic schistose varieties often South of the latitude of the southern end of Stadlan- have pelitic compositions and are associated with det, and southeast of a line approximately joining the quartzite and marble, and the curious zoisite-rich inland mouths of the major fjordsŽ. Figs. 1±3 , eclog- gneiss at OtnheimŽ. Fig. 3 has a mineralogy consis- ite garnets are commonly idioblastic and have spes- tent with an anorthositic composition. These litholo- sartine-rich cores containing amphibolite facies in- gies are typical of those designated to cover units. clusion suites and eclogite facies inclusion suites The reason for apparently enhanced survival of near their rimsŽ. Fig. 4 . The change in inclusion eclogite facies felsic rocksŽ. and metabasic eclogites assemblage corresponds to a sharp increase in MgrFe in such units is unclear at present, although Krabben- and is interpreted as representing theŽ. epidote- am- dam and WainŽ. 1997 argue they are preserved in phibolite-facies to eclogite-facies transition during low strain zones that have escaped late-orogenic prograde growth of garnetŽ Bryhni and Griffin, 1971; deformation and recrystallisation. In addition to these Krogh, 1980, 1982; Cuthbert, 1985; Cuthbert and examples, phengite relics or replacement inter- Carswell, 1990. . To the north of outer Nordfjord growths of biotiteqplagioclase"kyanite are rarely, Ž.Figs. 1±3 , eclogite garnets are often more xeno- but widely, found in quartzo-feldspathic gneisses of blastic, lack prograde zoning and have marginal the WGCŽ. Griffin, 1987; Wain, 1998 , possibly indi- retrograde zoning, and have eclogite facies inclu- cating that such HP, non-mafic rocks were once sions suites similar to matrix parageneses. Krogh much more common. Considerations of the effi- Ž.1982 noted that the change from prograde-zoned ciency of hydration vs. dehydration reactions in garnets to unzoned garnets lies approximately along eclogites and gneisses suggest that eclogites should the 7008C isotherm as defined by the regional pattern be less prone to loss of HP parageneses during of garnet±clinopyroxene Fe±Mg geothermometry retrogression than gneissesŽ Heinrich, 1982; Straume, Ž.Krogh, 1977 , and attributed the lack of zoning at S.J. Cuthbert et al.rLithos 52() 2000 165±195 173

Fig. 4. Photomicrographs showing petrographic features of HP and UHP eclogites.Ž. a Prograde-zoned garnet typical of quartz-stable, HP eclogites with a dark core containing hornblendeŽ. Hbl inclusions and ringed by monocrystalline quartz inclusions Ž. Qz , and a paler rim clear of inclusions; from Evja, 3 km SE of Malùy,Ê PPL.Ž. b Relict coesite Ž Cs . surrounded by replacive quartz Ž pallisade textured PCQ . within garnetŽ. Grt from eclogite at Salta, PPL. Ž. c Granular polycrystalline quartz Ž PCQ . inclusion considered to have replaced coesite, in omphaciteŽ. Omp , with adjacent garnet Ž. Grt in eclogite from Bryggja, Nordfjord, CPL. Ž. d Polycrystalline quartz Ž. PCQ inclusions considered to have replaced coesite, within garnetŽ. Grt also containing inclusions of omphacite Ž Omp . ; Fjùrtoftvika, Fjùrtoft, Nordùyane, CPL. higher temperatures to diffusional homogenisation. lowed by a barroisite eclogite stage. In outer Nor- However, WainŽ. 1997a; b; 1998 has claimed that fjord and in RomsdalŽ. 40 km SE of Molde; Fig. 1 , the change in the Nordfjord area from prograde-zoned an early amphibolite or epidote amphibolite stage idioblastic garnets to unzoned xenoblastic garnets was followed by an eclogite stage. corresponds to the HP±UHP transition rather than Retrograde mineral assemblages in eclogites and simply to an increase in temperature. the predominant parageneses in gneisses appear to be The prograde growth of garnets as defined by broadly co-facial, and show a similar pattern to compositional zoning, inclusion suites and matrix prograde assemblages, with epidote±amphibolite in assemblages, shows a systematic spatial variation inner Sunnfjord, epidote amphibolite or amphibolite ŽBryhni and Griffin, 1971; Krogh, 1982; Krogh and in outer Sunnfjord, and amphibolite in Nordfjord, Carswell, 1995; Cuthbert, 1985; Bailey, 1989. . In Stadlandet and coastal Sunnmùre. In the northern inner SunnfjordŽ. Fig. 3 , an early epidote amphibo- parts of the WGR from Molde to Kristiansund, lite stage was followed by a glaucophane or bar- eclogites tend to have retrograded to garnet gran- roisite eclogite stage. In outer Sunnfjord, an early HP ulitesŽ. Krogh, 1981, 1982; Krogh and Carswell, 1995 barroisite-bearing epidote amphibolite stage was fol- and some eclogite bodies contain trondhjemitic par- 174 S.J. Cuthbert et al.rLithos 52() 2000 165±195 tial melt seggregationsŽ. Cuthbert, 1995, 1997 . In the quartzitesŽ. cover unit 1±2 km south of the Bjùrkedal northern extension to the WGR northeast of Trond- anorthositergarnet peridotite massifŽ. Fig. 2 . Evi- heimŽ. Vestranden Gneiss Region or VGR , eclogites dence for coesite is pallisade-textured PCQ included have not been found, but HP granulites are common within garnet. Eclogite with prograde-zoned garnets in basic and felsic lithologiesŽ. Moller,È 1988 . The and no evidence for coesite is also found in Stigedal southwest extension of the VGR, rocks along strike within this metasedimentary cover sequence. At would lie offshore to the NW of Kristiansund and Fjùrtoftvika on the north coast of the island of Moldefjord, and could represent the part of the WGC Fjùrtoft, NordùyaneŽ. Figs. 2 and 4 , a kyanite eclog- where temperatures were highest during retrograde ite with optically unzoned garnets containing om- HPŽ. UHP? metamorphism, and where the efficiency phacite and phengite inclusions also has granular of overprinting of eclogite facies parageneses has polycrystalline quartz included in garnet. This ex- been greatest. Hence, a broad pattern emerges across tends the range of UHP eclogites 55 km northwest of the WGR which is consistent with lowest tempera- the previous northernmost occurrence on Dimnùy, tures in the SE and highest temperatures in the NW HareidlandetŽ. Smith, 1988 . The Fjùrtoftvika eclog- during prograde, peak and retrograde Scandian meta- ite occurs in a basement unitŽ Ulla Gneiss; Terry and morphism. Some variations may be superimposed Robinson, 1998. . Polycrystalline quartz has also been upon this pattern; e.g., WainŽ. 1998 suggests that the reported recently from a kyanite eclogite in a cover exhumation path for eclogites in the Nordfjord area unitŽ. BlahùÊ nappe on Fjùrtoft at Nytun Ž Terry et al., is cooler than that for eclogites further north. 1999. . Interestingly, this cover unit also contains the SmithŽ. 1984; 1988; 1995 , Krabbendam and Wain microdiamond-yielding gneiss reported by Do- Ž.1997 and Wain Ž 1997b; 1998 . have identified co- brzhinetskya et al.Ž. 1995 . The contact of this unit esite or polycrystalline quartzŽ. PCQ aggregates at- with the underlying basement gneisses is marked by tributable to the breakdown of pre-existing coesite at a zone of mylonitic eclogite-facies gneisses contain- a total of 24 localities concentrated in the outer ing bodies of garnetiferous peridotiteŽ Terry and Nordfjord and Stadlandet areaŽ. Fig. 3 , but also in Robinson, 1998. . Thus, these new discoveries signif- HareidŽ. Dimnùy and Hessdalen in Sunnmùre Ž Fig. icantly increase the size of the coesite±eclogite ter- 2. . In the present study, we have confirmed all of rane in the WGR, confirming the suggestion of these occurrences and found a further coesite eclog- SmithŽ. 1995 that his Coesite Eclogite Province ex- ite locality at FlisterŽ. Figs. 2 and 3 . Coesite or PCQ tends along the coast to Fjùrtoft. There appears to be is found exclusively as inclusions in other phases, a close connection between coesite eclogites and including garnet, omphacite, kyanite and zoisite, and garnetiferous peridotites, and with the previously has been found in both eclogites and, rarely, gneisses enigmatic discovery of microdiamond. Ž.Krabbendam and Wain, 1997; Wain, 1997a, 1998 . Krabbendam and WainŽ. 1997 and Wain Ž 1997a; We have also found eight further localities with PCQ b; 1998. have shown that the boundary between the at Maurstad and BryggjaŽ. Figs. 3±5 , Nordfjord: HP and UHP terranes on the north side of Nordfjord north Grytting, Djupedalsvatnet and Sandvikneset Ž.Figs. 3 and 5 is not simple, but is marked by a Ž.Stadlandet ; Hornet and Brautene Ž south of ;10 km wide ``mixed zone'' within which both HP Almklovdalen.Ž ; Stigedalen south of Bjùrkedalsvat- and UHP eclogites are found, sometimes -0.5 km net.Ž.Ž. , and Fjùrtoft island Nordùyane Figs. 2±5 . apart from each other. They mapped the mixed zone The HornetrBrautene and Stigedal localities extend as a SW±NE-trending strip to the east of Nordpollen the known occurrences of probable coesite eclogites which swings westward across Sùrpollen. Based on 40 km eastwards from previous localities. The Hor- currently known localities where evidence for coesite net eclogite is a large, tabular mass 4.5 km long and is well-established, the UHP terrane underlies all of 0.5 km wide in outcrop lying 1.5 km south of the Stadlandet and extends into Almklovdalen. Our more Almklovdalen peridotite massifŽ. Fig. 3 . Evidence extensive survey shows that the southern boundary for coesite is in the form of granular polycrystalline of the mixed zone probably trends E±W parallel to quartz included within garnet. In Stigedalen, eclogite inner Nordfjord and includes the Stigedalen eclog- occurs as meter scale pods within mica schists and ites. The northern boundary lies between VagsùyÊ and S.J. Cuthbert et al.rLithos 52() 2000 165±195 175

Fig. 5. Lithological and thermobarometric map of the Kroken±Maurstad area, Nordfjord showing the distribution of HP and UHP eclogites. Numbers in boxes are calculated P±T conditions using the Krogh-RavnaŽ. 1999 garnet±clinopyroxene Fe±Mg thermometer combined with the Waters and MartinŽ. 1983 garnet±clinopyroxene±phengite geobarometer using the garnet activity model of Newton and Haselton Ž.1981 . PCQspolycrystalline quartz after coesite Ž UHP eclogite . . QPZsquartz eclogite with prograde zoned garnets Ž HP eclogite . . Outcrop of eclogite, eclogite facies gneiss and dunite from Bryhni et al.Ž. 1969 , Krabbendam and Wain Ž. 1997 and the authors' unpublished data.

Stadlandet in the west, but is less certain in the east km2. The existence and positions of the mixed zone due to a paucity of data. The new finding of PCQ at and the UHP terrane are poorly constrained to the Hornet south of AlmklovdalenŽ. Fig. 3 must lie east and northeast of Stigedalen, and inner Sunnmùre. within the mixed zone or UHP zone; until better data However, the association of UHP eclogites with emerge, we tentatively suggest that the presence of garnetiferous peridotites leads us to speculate that prograde-zoned garnets in an internal eclogite in the the UHP terrane may extend as far eastwards as Almklovdalen peridotite massifŽ Griffin and Qvale, Tafjord, so it could conceivably cover an area of up 1985. and north of the Hornet eclogite places the to 8000 km2 . The Romsdal eclogiteŽ. Krogh, 1982 northern boundary of the mixed zone to the north of has prograde-zoned garnet, which may constrain the Almklovdalen in a similar position to that of easternmost limit of the UHP terrane in the northern Krabbendam and WainŽ. 1997 . This would be con- part of the WGR. The total area underlain by UHP sistent with the trace of the gneissic foliation. There eclogites is, on current evidence, considerably smaller are no good data to fix the northern boundary of the than that suggested by Coleman and WangŽ. 1995 mixed zone further east at this stage. Scattered locali- whose estimate of 52,500 km2 encompassed the ties in outer SunnmùreŽ. Dimnùy, Hessdalen and entire WGR, including the recognisable HP eclogite NordùyaneŽ. Fjùrtoftvika and Nytun allow the UHP terrane south of Nordfjord. terrane to be tentatively extended in a strip up to 40 The present database does not reveal a clear km wide along the coast between Stadlandet and relationship between the distribution of petrographic Fjùrtoft, so that the total area underlain by the UHP types of eclogite and lithotectonic units. Cover rocks terrane may be tentatively estimated at about 5000 Ž.quartzites and mica schists in Stigedalen contain 176 S.J. Cuthbert et al.rLithos 52() 2000 165±195 both HP and UHP eclogites, and both types are also coexistence of omphacite with albite in eclogites and found within basement lithologies. In Nordùyane, using the omphacite activity model of Holland UHP eclogite and gneiss occur both in a cover unit Ž.1980 . The resulting apparent trend of increasing P Ž.BlahùÊ nappe; Terry et al., 1999 and in the underly- towards the NW was largely an artifact of the d PrdT ing basement gneissesŽ. Ulla Gneiss . However, the slope of the employed barometer, although the cur- Sñtra and Risberget cover nappes, outcropping along rently known distribution of coesite clearly indicates nearby Moldefjord and lying structurally between the higher pressures in the north of the WGR than in the BlahùÊ nappe and basement, contain eclogites with south. prograde-zoned garnets and no evidence for coesite New geobarometers are now available for the Ž.Robinson, 1995 . determination of absolute pressures in eclogites, and Within the mixed zone, WainŽ. 1998 considered provide an opportunity to obtain a much more reli- that individual eclogite bodies were internally uni- able evaluation of trends in pressure across the WGR. form in petrographic type and that HP and UHP WainŽ. 1997b; 1998 used the garnet±omphacite± types were not found in the same body, and she phengite barometerŽ Waters and Martin, 1983 up- observed that eclogites of the same type appeared to dated 1996; see http:rrwww.earth.ox.ac.ukr; lie along discrete horizons in the gneisses. These davewarresearchrecbar.html. and found a bimodal observations suggest that, even in the mixed zone, distribution of P and T in the Stadlandet Ð outer HP and UHP eclogite bodies have distinct and se- Nordfjord area Ð with a significant pressure gap of parate origins. However, typical prograde-zoned )4 kbar between the two petrographically defined garnets with hornblende inclusions are found in groups of eclogites, and calculated pressures which the ArsheimnesetÊ polycrystalline quartz-bearing, are broadly consistent with the type of silica poly- orthopyroxene eclogiteŽ Lappin and Smith, 1978; morph present. Pressures for UHP eclogites mostly Carswell et al., 1985; Smith, 1988. . At Vetrhus, lay close to, and just above, the coesite±quartz equi- Nordpollen, prograde-zoned garnets and polycrys- librium. This was taken by WainŽ. 1997b; 1998 and talline or multicrystalline quartz inclusions are found Krabbendam and WainŽ. 1997 to indicate a tectonic within eclogite and the enveloping pelitic schists. break between the HP and UHP terranes, with the These examples suggest that an early prograde his- mixed zone being a zone of interfolding or imbrica- tory may, rarely, be preserved in rocks which have tion of the two terranes. witnessed UHP conditions. We have established a more geographically ex- tended database of P±T determinationsŽ. Table 1 , concentrated on the area studied by WainŽ. op. cit. 4. Distribution of pressure ()P and temperature but also including samples from Sunnfjord and ()T Sunnmùre. Mineral phases from the Kvineset eclog- Early attempts to determine the distribution of T iteŽ. Table 1 were analysed on the ARL-EMx elec- across the WGR used garnet±clinopyroxene Fe±Mg tron microprobe at the Central Institute for Industrial exchange geothermometry, and defined a regional Research, Oslo with WDS with matrix corrections increase towards the NW with isotherms trending according to Bence and AlbeeŽ. 1969 , or on the NE±SW parallel to the coast north of MalùyÊ Ž Krogh, ARL-EMX microprobe at the Mineralogy±Geology 1977; Griffin et al., 1985. . The pattern was not Museum, Oslo by EDS with matrix corrections cal- well-defined further south between Fùrdefjord and culated by the program ZAF-4 of StathamŽ. 1976 , Nordfjord due to the large area covered by the both with running conditions of 15 kV, 10 nA. The allochthon and Devonian basinsŽ. Fig. 2 . Placement remaining samples were analysed on either the of isotherms was necessarily highly generalised due Cameca Camebax microprobe at the Mineralogy± to uncertainties in T estimates and scarcity of data Geology Museum, Oslo using WDS at 15 kV and 20 away from the coast. Estimation of P was hampered nA with matrix corrections using the Cameca PAP by the lack of geobarometers available for the com- program, or the JEOL840 SEM at the University of mon Opx-free eclogites. Griffin et al.Ž. 1985 esti- Tromsù using EDS at 20 kV, 3 nA with matrix mated minimum pressures based on the assumed corrections by ZAF. S.J. Cuthbert et al.rLithos 52() 2000 165±195 177

Eclogites with well-equilibrated textures and lack- omphacite and garnetŽ. rims for HP garnets , and ing extensive retrogression or abundant pre-eclogitic highest silica phengites. The similarity of results relics were selected for analysis wherever possible. from these two different sample sets and different Where garnets exhibit prograde zoning, near-rim approaches to selecting phase compositions strength- compositions were used. T and P were calculated ens confidence in our conclusions. using a combination of the updated Grt±Cpx Fe±Mg Selecting the alternative combination of ma- exchange thermometer of Krogh-RavnaŽ. 1999 , and ximum pyrope in garnet, minimum MgrFe in the Grt±Cpx±Phe barometer of Waters and Martin omphacite and minimum MgrFeŽ. and low Si in Ž.1983 . Ferric iron in Grt and Cpx were estimated by phengiteŽ. ``max py'' dataset in Table 1 , nearly all the charge balance method of DroopŽ. 1987 . For the samples fall in the quartz fieldŽ. Fig. 6A and B , at phengite barometer, the procedures recommended by lower pressures than for the ``max gros'' set. These Carswell et al.Ž. 1997 were used and calculations lower apparent pressures may be largely controlled made with the garnet activity models of Berman by the retrograde decrease of Si in phengite and it is Ž.1990 and Newton and Haselton Ž. 1981 . unlikely that this choice of phase compositions was In Fig. 6A and B, results are plotted for two in equilibrium at peak P. choices of phase compositions. For the favoured Temperatures vary widely within both data sets, = 2 combination of maximum apyra gros in garnet, probably partly due to uncertainties in ferric iron maximum jadeite in omphacite and maximum Si in estimation done by charge balance. The UHP group phengiteŽ. ``max gros'' dataset in Table 1 , P±T forms a fairly distinct cluster, except for the Hol- points for UHPŽ.Ž coesite stable and HP quartz sta- mane eclogite. WainŽ. 1997b also calculated a lower ble. eclogites are generally consistent with the posi- T for the Holmane body when the DroopŽ. 1987 tion of the CssQz equilibrium within the uncer- ferric iron calculation was applied, but when calcu- tainty of the thermobarometers. The BermanŽ. 1990 lated on the basis of total Fe as ferrous, it gave a garnet activity modelŽ. Fig. 6A gives slightly higher result more consistent with the rest of its group. pressures, with all UHP eclogites lying on or above We have selected the ``max gros'' dataset using the CssQz equilibrium, but some HP eclogites lie the Newton and Haselton garnet activity model to on the HP side of the CssQz equilibrium. The show the relationships between P, T and the petro- Newton and HaseltonŽ. 1981 garnet activity model graphic and geographic groupingsŽ. Fig. 6C . A gen- gives pressures ;2 kbar lowerŽ. Fig. 6B and all the eral trend emerges in which P and T increase from HP eclogites lie in the quartz field, but two UHP southeast to northwest, with lowest values for the eclogites, from Hornet in Almklovdalen, and Fla- glaucophane eclogites of inner Sunnfjord and highest traket Harbour also lie in the quartz field. For the values for the coesite eclogites of outer Nordfjord, Flatraket Harbour eclogite, WainŽ. 1998 calculated a Stadlandet and Fjùrtoft. Values for the mixed higher pressure in the coesite field for this body, and HPrUHP zone overlap to some extent with those our low result may possibly be again due to retro- outside the mixed zone, but it is perhaps significant grade re-equilibration of phengite to lower Si com- that all but two HP eclogite samples from the mixed positions, as perhaps is also be the case for the zone cluster at slightly higher P than those in the HP Hornet eclogite. In general, the Newton and Haselton zone to the south. A single UHP sample from the garnet activity model seems to give results more BlahùÊ nappe on FjùrtoftŽ. Terry et al., 1999 , close to consistent with the observed type of silica poly- the diamond gneiss, lacks phengite in the matrix, in morph for this data set, and indeed seems to fit better common with many eclogites in the northern WGR, with experimental data and well-calibrated natural but inclusions of omphacite and phengite are present examplesŽ D. Waters, 1999, personal communica- in garnet. Phengite inclusions are retrograded to tion. . biotiteqplagioclase symplectite at margins, possibly WainŽ. 1997b; 1998 calculated a similar P±T resulting in loss of Si; hence, the calculated pressure distribution to the ``max gros'' dataset for the Nord- of 7648C at 28 kbarŽ. Terry et al., 1999 is likely to fjord±Stadlandet area, using a different set of HP be a minimum value. The isopleth for the Grt±Cpx and UHP samples and using average compositions of thermometerŽ. Fig. 6C intersects the quartzscoesite 178 S.J. Cuthbert et al.rLithos 52() 2000 165±195

Table 1 Sample locations, P and T calculations and parageneses of selected eclogites from the WGR GtCp Ð Grt±Cpx Fe±Mg thermometer of Krogh-RavnaŽ. 1999 ; Pphe Berm Ð Grt±Cpx±Phe barometer of Waters and Martin Ž. 1983 using the garnet activity model of BermanŽ. 1990 ; Pphe N and H Ð Grt±Cpx±Phe barometer of Waters and Martin Ž. 1983 using the garnet activity model of Newton and HaseltonŽ. 1981 ; Phe Ð phengite present if marked x; additional phases Ð kyskyanite, glsglaucophane, barsbarroisite, hblshornblende, czosclinozoisite, zoszoisite, dolsdolomite, parsparagonite; PCQ Ð polycrystalline quartzŽ after coesite. present; Cs Ð coesite present. Values in italics for the phengite-free matrix of the Fjùrtoft sample give T for the Grt±Cpx Fe±Mg thermometer of Krogh-RavnaŽ. 1999 at a nominal pressure of 25 kbar. Question marks in UTM Ž map grid reference . column indicates some uncertainty over location of samples collected by other workers. Sample Place name UTM co- Map T Pphe T Pphe Phe Other PCQr ordinates sheet GtCp Berm GtCp N and H phases Cs Inner Sunnfjord HP eclogites Max gros 193 Kvineset 227,292 1217 IV 573 18.7 561 16.4 x Gl ± 194 Kvineset 227,292 1217 IV 528 18.0 517 15.6 x Gl ± 196 Kvineset 227,292 1217 IV 569 17.9 558 15.6 x Bar ± 85 Kvineset 227,292 1217 IV 543 19.3 533 17.0 x Bar ± 56 Kvineset 227,292 1217 IV 483 19.7 472 17.0 x Bar ± Max pyr 193 Kvineset 227,292 1217 IV 540 16.3 528 14.0 x Gl ± 194 Kvineset 227,292 1217 IV 522 17.1 511 14.7 x Gl ± 196 Kvineset 227,292 1217 IV 599 18.2 588 15.9 x Bar ± 85 Kvineset 227,292 1217 IV 515 17.1 504 14.7 x Bar ± 56 Kvineset 227,292 1217 IV 540 16.2 528 14.0 x Bar ±

Outer Sunnfjord HP eclogites Max gros IB-1C Sellevoll ? 1117 II 590 23.2 581 21.2 x Ky Bar ± IB-2C N AsenÊ ? ? 589 22.2 580 20.0 x Ky Bar ± Max pyr IB-1R Sellevoll ? 1117 II 611 21.2 598 18.9 x Ky Bar ± IB-2R N AsenÊ ? ? 604 21.8 594 19.6 x Ky Bar ±

Nordfjord HP eclogites Max gros IB-7C Davik 182,684 1218 IV 644 25.3 636 23.4 x Ky Hbl ± UHPM-33 Skavùypollen 972,710 1118 I 575 22.9 566 20.8 x Ky Czo ± UHPM-36 Halnes 946,719 1118 IV 738 25.2 728 23.3 x Ky Zo Hbl ± UHPM-59 Biskjelneset 995,686 1118 IV 607 22.8 598 20.9 x Ky Czo ± 45r97 Hùgefossen 480,696 1218 I 573 25.6 563 23.4 x Ky Hbl ± UHPM-24 Levdal 167,715 1218 IV 655 24.1 646 22.2 x Ky Dol ± Max pyr IB-7R Davik 182,684 1218 IV 702 23.2 690 21.1 x Ky Hbl ± UHPM-33 Skavùypollen 972,710 1118 I 677 22.5 666 20.4 x Ky ± UHPM-36 Halnes 946,719 1118 IV 687 21.1 673 18.7 x Ky ± UHPM-59 Biskjelneset 995,686 1118 IV 710 20.3 699 18.4 x Ky ± UHPM-24 Levdal 167,715 1218 IV 542 22.1 531 19.8 x Ky ± 45r97 Hùgefossen 480,696 1218 I 537 24.8 527 22.6 x Ky ±

Nordfjord±Stadlandet mixed zone HP eclogites Max gros UHPM-51 S Raudeberg 977,754 1118 I 736 27.0 725 25.0 x Ky Zo ? UHPM-57 N. VagsùyÊ 984,787 1118 I 785 27.9 773 25.7 x Ky ± UHPM-46 MalùyÊ N 969,740 1118 I 760 28.3 748 26.2 x Ky ± UHPM-43 N Oppedal 2 940,772 1118 IV 705 27.4 694 25.3 x Ky Czo ± UHPM-42 N Oppedal 1 938,768 1118 IV 583 25.4 574 23.4 x Ky? ± S.J. Cuthbert et al.rLithos 52() 2000 165±195 179

Table 1Ž. continued Sample Place name UTM co- Map T Pphe T Pphe Phe Other PCQr ordinates sheet GtCp Berm GtCp N and H phases Cs Nordfjord±Stadlandet mixed zone HP eclogites Max gros IB-20 N Oppedal 938,768 1118 IV ± Ky par ± 84r97 Flatrakt Bekk 28,765 1118 I 630 25.0 619 23.4 x Czo ± 89r97 MalùyÊ Fjel 950,727 1118 I 598 26.7 587 24.5 x ± ± UHPM-10 MalùyÊ South 957,723? 1118 IV 606 26.4 597 24.4 x Ky ± TA-1 Kroken 080,704 1118 I 616 26.5 607 24.4 x Hbl dol ± KROKEN Kroken 080,704 1118 I 602 26.0 593 24.1 x Hbl dol ± UHPM-27 Almenning 029,700 1118 I 611 27.1 601 25.0 x Hbl ± ALM-1 Almenning 029,700 1118 I 629 28.7 616 26.1 x Hbl ± Max pyr UHPM-51 S Raudeberg 977,754 1118 I 706 23.8 695 21.9 x Ky Zo ? UHPM-57 N. VagsùyÊ 984,787 1118 I 762 23.0 750 20.9 x Ky ± UHPM-46 MalùyÊ N 969,740 1118 I 676 24.0 665 21.9 x Ky ± UHPM-43 N Oppedal 2 940,772 1118 IV 589 24.9 579 22.9 x Ky Czo ± UHPM-42 N Oppedal 1 938,768 1118 IV 618 22.7 609 20.7 x Ky? ± IB-20 N Oppedal 938,768 1118 IV ± Ky par ± 84r97 Flatrak Bekk 28,765 1118 I 680 19.9 668 17.6 x Czo ± 89r97 MalùyÊ Fjel 950,727 1118 I 586 24.3 575 22.1 x ± ± UHPM-10 MalùyÊ South 957,723? 1118 IV 713 22.1 703 20.2 x Ky ± TA-1 Kroken 080,704 1118 I 601 24.3 591 22.1 x Hbl dol ± KROKEN Kroken 080,704 1118 I 602 20.6 592 18.4 x Hbl dol ± UHPM-27 Almenning 029,700 1118 I 601 25.6 591 23.5 x Hbl ± ALM-1 Almenning 029,700 1118 I 547 22.7 533 19.9 x Hbl ±

Nordfjord±Stadlandet mixed zone UHP eclogites Max gros UHPM-6 Bryggja 136,728 1118 I 739 31.5 730 29.8 x Ky? Zo PCQ UHPM-13 Maurstad 143,728 1118 I 706 30.4 694 28.2 x Ky Zo PCQ UHPM-70 Totland 096,713 1118 I 769 31.0 759 29.2 x Ky Czo PCQ UHPM-76 Holmane 103,712 1118 I 594 28.6 585 26.5 x Ky? Czo PCQ F-13A Flatraket 026,775 1118 I 718 27.4 700 24.4 x Ky Cs UHPM-60 Hornet 247,777 1218 IV 630 26.7 621 24.8 x Ky? PCQ Max pyr UHPM-6 Bryggja 136,728 1118 I 731 30.0 722 28.3 x Ky? Zo PCQ UHPM-13 Maurstad 143,728 1118 I 671 25.1 661 23.2 x Ky Zo PCQ UHPM-70 Totland 096,713 1118 I 765 27.0 756 25.3 x Ky Czo PCQ UHPM-76 Holmane 103,712 1118 I 799 25.8 787 23.8 x Ky? Czo PCQ F-13A Flatraket 026,775 1118 I 654 22.4 636 19.3 x Ky Cs UHPM-60 Hornet 247,777 1218 IV 665 25.1 654 23.1 x Ky? PCQ

Nordfjord±Stadlandet UHP eclogite Max gros 3r97 Drage 015,914 1019 II 768 31.8 758 29.9 x Ky Zo x Max py 3r97 Drage 015,914 1019 II 742 26.9 732 25.0 x Ky Zo x

Hareidlandet and Nordùyane UHP eclogites Max gros Terry et al., Fjortoft 659,561 1220 III 748 25 xKyx 1999 180 S.J. Cuthbert et al.rLithos 52() 2000 165±195 equilibrium at 7608C, 26.5 kbar and the graphites valuesŽ recorded as "1 standard deviation from the diamond equilibrium at 8008C, 33.5 kbar, the former mean. for subsets with a reasonably large sample giving a minimum pressure for this body and the size. Unfortunately, this introduces real geographical latter being consistent with the presence of diamond variation in P and T as an additional source of error in nearby gneisses. but, in the absence of any detailed evaluation of In order to identify any real trends in P and T uncertainties from individual eclogite bodies, we re- across the WGR, it is necessary to evaluate the likely gard our selection of sample subsets as the best uncertainties in the P and T estimates. Uncertainties compromise possible with the data currently avail- due to thermobarometer calibration and microprobe able. Averages and uncertainties for the sample sub- analysis are likely to be about "508C and "1.5 sets are plotted in Fig. 6D. Uncertainty in T is likely kbar for garnet q omphacite q phengite thermo- to be about "758C. Uncertainty in pressure could be barometryŽ. Wain, 1998 . Related uncertainty due to up to "2 kbar, although the spread of P is smaller ferric iron calculation, especially in omphacite, may in the HP eclogites than the UHP eclogitesŽ largely result in even larger errors. Furthermore, an addi- based on samples from outer Nordfjord. . The larger tional source of uncertainty arises due to prograde spread of data for UHP eclogites could be reduced to and retrograde zonation of mineral phases and the that for HP eclogites if those plotting below the difficulties in selecting phase compositions which QzsCs equilibrium are ignoredŽ on the basis that were in equilibrium at the peak metamorphic condi- they have suffered retrogressive re-adjustment of tions recorded by individual specimens. We have phengite composition. . attempted to evaluate the overall uncertainty due to These large uncertainties allow only gross gener- all these factors by inspection of our ``max gros'' alisations to be made about regional trends. Never- dataset. Breaking the dataset into its petrographic and geographic subsets, we have found the range of

Fig. 6. P ±T plots of thermobarometry results from eclogites in the WGR. See Table 1 for data, locations and literature sources. Ž.A Krogh-Ravna Ž 1999 . garnet±clinopyroxene Fe±Mg exchange thermometer combined with the garnet±clinopyroxene±phengite barometer of Waters and MartinŽ. 1983 , using the garnet activity model of BermanŽ. 1990 . Circles Ð HP eclogites; squares Ð UHP eclogites; filled symbols Ð ``max gros'' dataset; open symbols Ð ``max pyr'' datasetŽ see text for explanation of datasets.Ž. . B As for Ž. A , using Newton and Haselton Ž 1981 . garnet activity model for garnet.Ž. C Relationships of P and T with petrographic type of eclogite and geographic area using the Krogh-RavnaŽ. 1999 garnet±clinopyroxene Fe±Mg exchange thermometer combined with Waters and MartinŽ. 1983 garnet± clinopyroxene±phengite barometer, using the ``max gros'' dataset only. Diamonds Ð glaucophane eclogites from Inner Sunnfjord; triangles Ð Outer Sunnfjord HP eclogites; circles Ð Nordfjord± Stad HP eclogites; squares Ð Nordfjord±Stad UHP eclogites. Grey symbols are from the mixed HPrUHP zone. Solid lines are for the graphiteŽ. Gph sdiamond Ž. Dm and quartz Ž. Qz scoesite Ž.Cs equilibria Ž Bundy, 1980; Bohlen and Boettcher, 1982 . . Dashed line is the Krogh-RavnaŽ. 1999 grt±cpx Fe±Mg exchange geothermometer isopleth for the sample from Fjortùft.Ž. D Aver- age P and T of petrographic and geographical groupings, with symbols as in plotŽ. C . Error bars are "1 sd. Dash±dot line is a reference ``geotherm'' of 58Crkm. Error brackets with bar termi- nations are calculated from the dataset. Error brackets without terminating bars are for small sample sets where the applied uncertainty is taken from a larger sample set from an adjacent area. S.J. Cuthbert et al.rLithos 52() 2000 165±195 181 theless, by inspection of Fig. 6D, it is clear that extreme northwesterly position in the WGR. Deter- recorded temperatures in the northwestern, coastal mination of the actual pressure recorded by this UHP terrane of the WGR are significantly higher sample awaits further developments in geobarome- than those from eclogites in the HP terrane in Sunn- try, so the significance of the Fjùrtoft rocks remains fjord and Nordfjord. Pressures show a progressive tantalisingly uncertain. The slope and order of the regional increase from southeast to northwest from trend are consistent with the hypothesis of equilibra- 15 kbar in inner Sunnfjord to about 30 kbar in tion at steadily increasing depths along a geothermal Stadlandet. gradient similar to that of a subduction zone, al- The eclogites from the mixed HPrUHP zone though such an interpretation must be made with show a large overlap in temperature. WainŽ 1997a; b; caution due to the likely transient thermal structure 1998. , using a different sample set, found that pres- of a continental collision zone and the possibility sures and temperatures from eclogites in the mixed that eclogite formation was diachronous, or even zone clustered in a bimodal distribution with a P polycyclic, across the WGC. We also reiterate the difference of )4 kbar between the HP and UHP point that a wide spread of P±T results within some types. HP and UHP eclogites lying only a few groups has been averaged, within which some of the hundred meters apart were found by WainŽ. op. cit. variations may be real. to give pressures differing by up to 8 kbar. Applying the rather cautious error brackets discussed above 5. Orthopyroxene external eclogites and garnetif- Ž."2 kbar , any calculated pressure difference of 4 erous peridotites kbar or less could not be considered to be signifi- cantly different. Inspection of Fig. 5, which uses the Orthopyroxene±clinopyroxene±garnet assem- P±T data set from this study, confirms the large blages suitable for quantitative thermobarometric pressure differences between adjacent eclogite bod- evaluationŽ. see review of Carswell and Harley, 1990 ies; for instance, the Kroken HP eclogite gives a are found in ``external'' Opx eclogites enclosed pressure 6 kbar lower than the Totland UHP eclogite within gneisses, as well as in ``internal'' eclogites less than 2.5 km map distance away or in fact about enclosed within the garnetiferous peridotite bodies 1 km across strike. The Maurstad UHP eclogite gives Žsee review of Krogh and Carswell, 1995 and refer- a pressure 6 kbar higher than the Levdal eclogite, ences therein. . ``External'' Opx eclogites are found also less than 2.5 km awayŽ. 2 km across strike . exclusively from Nordfjord northwardsŽ Carswell et Even within the bounds of uncertainty defined here, al., 1995. . Some are petrographically UHP eclogites these variations in pressure in the mixed zone seem with PCQŽ e.g., Arsheimneset,Ê Sandvikneset, to be real in some cases. As the apparent pressure Otnheim, Hessdalen. and occur within our desig- gradients are much larger than the normal lithostatic nated UHP terrane. Most lack free silica and all lack pressure gradient across likely structural distances phengite and kyanite. between these eclogites, we must infer either signifi- Several cation exchange and net transfer geother- cant crustal shortening between them, or equilibra- mometers and barometers are available for use with tion of adjacent eclogites at different times on a Opx eclogites, offering an alternative to phengite- P±T±t path. based thermobarometry. However, previous esti- Average P and T for each petrographic group mates of absolute P from Opx eclogites based on Al arranged by geographic area lie along a linear trend in Opx in equilibrium with garnet varied widely Ž.Fig. 6D , with a slope of approximately 58Crkm. depending on interpretation of Al zoning patterns in Mixed HPrUHP zone eclogites straddle the CssQz Opx. For example, Lappin and SmithŽ. 1978 esti- equilibrium line, and the group averages lie along the mated P±T conditions of 30±45 kbar at 700±8508C trend in geographic order from south to north with for external Opx eclogites, assuming equilibrium increased P and T. Interestingly, the Grt±Cpx ther- between garnet and low Al Opx cores. Lappin and mometer isopleth for the Fjùrtoft sample intersects Smith's samples all came from the UHP terrane as the diamondsgraphite line on the HP extrapolation defined above and certainly give P±T conditions of the trend, suggesting conditions consistent with its consistent with UHP metamorphism. In contrast, 182 S.J. Cuthbert et al.rLithos 52() 2000 165±195

Table 2 Calculated P±T conditions for garnetiferous peridotites and orthopyroxene external eclogites in the WGR using analyses from previously published and unpublished sources See text for details of calculations. Sample T Ž.8C P Žkbar . Locality Data source MgCr peridotites Ð coresrporphyroclasts ES-4 898 42.3 Flemsùy, Nordoyane Mùrk, unpublished F12 719 21.7 Fjùrtoft Nordoyane Jamtveit et al., 1991 U95 661 27.7 Ugelvik, Otrùy Carswell, 1986 U92 724 25.6 Ugelvik, Otrùy Carswell, 1986 U8 739 22.8 Raudhaugane, Otrùy Carswell, 1986 UCS-7A 708 27.9 Raudhaugane Otrùy Medaris, 1984 U539 697 26.9 Raudhaugane Otrùy Jamtveit et al., 1991 119-111 777 30.3 Sandvika Gurskùy Medaris, 1984 Sandvika 2 637 21.1 Sandvika Gurskùy Krogh-Ravna, upublished data JAMT 947 43.8 Gurskebotn, Gurskùy Jamtveit, 1984 96-3-26A 712 29.8 Aldalen Medaris, 1984 NT-3 870 40.0 Kalskaret, Tafjord Medaris, 1984 T96 882 38.8 Kalskaret Tafjord Jamtveit et al., 1991 T153 832 35.5 Kalskaret Tafjord Jamtveit et al., 1991 SU-1K 781 29.8 Almklovdalen Medaris, 1984 7174 760 26.9 Bjùrkedalen Brastad, 1985 7998 786 24.4 Bjùrkedalen Brastad, 1985

MgCr peridotites Ð rims or neoblasts ES-4 828 29.7 Flemsùy, Nordoyane Mùrk, unpublished F12 719 21.7 Fjùrtoft Nordoyane Jamtveit et al., 1991 U92 662 16.1 Ugelvik, Otrùy Carswell, 1986 U8 736 26.1 Raudhaugane, Otrùy Carswell, 1986 U8 740 30.2 Raudhaugane, Otrùy Carswell, 1986 U8 721 29.6 Raudhaugane, Otrùy Carswell, 1986 UCS-7A 639 27.6 Raudhaugane, Otrùy Medaris, 1984 U539 685 18.7 Raudhaugane, Otrùy Jamtveit et al., 1991 JAMT 659 20.8 Gurskebotn, Gurskùy Jamtveit, 1984 96-3-26A 759 31.1 Aldalen Medaris, 1984 NT-3 790 30.2 Kalskaret Tafjord Medaris, 1984 T96 648 15.1 Kalskaret Tafjord Jamtveit et al., 1991 T153 685 22.0 Kalskaret Tafjord Jamtveit et al., 1991 SU-1K 669 25.4 Almklovdalen Medaris, 1984 7998 785 24.3 Bjùrkedalen Brastad, 1985

FeTi peridotites Ð cores H185 765 26.0 Kolmannskog, Moldefjord Carswell et al., 1983 U125 675 21.0 Raknestangen, Midùy Carswell et al., 1983 E4 675 32.0 Eiksundal, Hareidlandet Carswell et al., 1983

External opx eclogites Ð cores or unzoned grains U 206 733 22.5 Solholm, Otrùy Carswell et al., 1985 U 243 695 22.3 Solholm, Otrùy Carswell et al., 1985 U 19 727 19.6 Skarsh, Otrùy Carswell et al., 1985 KR 4 734 23.0 KvalvagÊ Carswell et al., 1985 KR-3 760 25.8 HjùrungavagÊ Carswell et al., 1985 D171 773 29.3 Liset Stadlandet Lappin and Smith, 1978 D171A 713 35.2 Liset Stadlandet Lappin and Smith, 1978 KR 5 676 25.1 Grytting Stadlandet Carswell et al., 1985 A44 688 30.7 Grytting Stadlandet Carswell et al., 1985 IB-21A 750 33.4 Grytting Stadlandet Krogh-Ravna, upublished data S.J. Cuthbert et al.rLithos 52() 2000 165±195 183

Table 2Ž. continued Sample T Ž.8C P Žkbar . Locality Data source External opx eclogites Ð cores or unzoned grains IB-21B 752 30.8 Grytting Stadlandet Krogh-Ravna, upublished data IB-21C 750 34.2 Grytting Stadlandet Krogh-Ravna, upublished data 8G 776 34.7 Grytting Stadlandet Lappin and Smith, 1978 78r19 734 35.5 Grytting Stadlandet Lappin and Smith, 1978 KR 6 677 29.7 Hellesylt Carswell et al., 1985 V80r19 660 22.9 Hùydalsneset Carswell et al., 1985 C414 774 39.1 Nybù, Sùrpollen Lappin and Smith, 1978 C411A 745 37.9 Nybù, Sùrpollen Lappin and Smith, 1978 C411H 820 47.6 Nybù, Sùrpollen Lappin and Smith, 1978 C411C 740 37.7 Nybù, Sùrpollen Lappin and Smith, 1978 Ly 707 33.5 Lyngenes, Sùrpollen Lappin and Smith, 1978 KR 1 612 15.7 Hornindal Carswell et al., 1985 KR-2A 509 19.5 Kvalneset, Totland Carswell et al., 1985 KR-2B 501 18.0 Kvalneset, Totland Carswell et al., 1985 127M 683 30.3 ? Lappin and Smith, 1978 68.87.1 787 31.0 ? Lappin and Smith, 1978

External opx eclogites Ð rims or neoblasts UHPM 7r97 645 21.7 Otnheimneset, Stadlandet Krogh-Ravna, unpublished KR 5 694 22.3 Grytting Stadlandet Carswell et al., 1985 A44 639 20.5 Grytting Stadlandet Carswell et al., 1985 IB-21A 624 18.0 Grytting Stadlandet Krogh-Ravna, unpublished IB-21B 576 16.0 Grytting Stadlandet Krogh-Ravna, unpublished IB-21C 680 26.5 Grytting Stadlandet Krogh-Ravna, unpublished KR 6 616 18.5 Hellesylt Carswell et al., 1985 KR-2A 607 19.1 Kvalneset, Totland Carswell et al., 1985 KR-2B 589 15.2 Kvalneset, Totland Carswell et al., 1985

Carswell et al.Ž. 1985 calculated much lower P±T two more belts of peridotites stretch from Tafjord to conditions of around 700±7508C and 17±18 kbar Gurskùy and parallel to Moldefjord through Otrùy to using the higher Al rims of Opx grains. It seems Nordùyane. Bucher-NurminenŽ. 1991 noted that clear now that the high Al rims on Opx are a result peridotites ``decorated'' basement-cover contacts, of retrograde growth of amphibole and that higher and there is a particularly strong association of peri- pressures are more likely in some cases. Neverthe- dotites with anorthosite horizons, e.g., at Tafjord less, problems with combination of appropriate grain Ž.Brueckner, 1977 and to the south of Nordfjord compositions in what are often highly retrograded Ž.Bryhni, 1966 . eclogites remain a serious problem. Preserved garnetiferous peridotites are rare and Peridotites and serpentinites occur throughout the restricted to the region north of Nordfjord. Carswell WGRŽ. Fig. 2 . Bryhni Ž. 1966 noted that peridotite et al.Ž. 1983 distinguished two groups of peridotites bodies occur in linear belts or swarms across the based on mineralogy, bulk composition and field Nordfjord area. One of these swarms, stretching relationships. An Fe±Ti-enriched group is associated from VagsùyÊ through Sùrpollen to Nordpollen, was with metamorphosed layered mafic complexes or noted by WainŽ. 1998 and Krabbendam and Wain meta-anorthosites and probably has a Caledonide Ž.1997 to coincide with their mixed HPrUHP metamorphic origin. A Mg±Cr-enriched group is boundary zone, although such swarms of ultramafites probably derived from tectonically introduced litho- are not unique to this zone and are also found in the spheric or sub-lithospheric mantle. Mg±Cr peri- HP zone to the south of Nordfjord. Further north, dotites in different ultramafic belts seem to have 184 S.J. Cuthbert et al.rLithos 52() 2000 165±195 distinctive characteristics. For example, in the south- ern belts, ferrobasaltic eclogites enclosed within peri- dotite bodiesŽ. ``internal'' eclogites are found with prograde-zoned garnets more typical of HP ``exter- nal'' eclogites, possibly indicative of a prograde Caledonian evolutionŽ Jamtveit, 1984; Griffin and Qvale, 1985.Ž. . Carswell et al. 1983 defined several stages of development for the Mg±Cr group, with megacrystsrporphyroclasts having a mantle origin during the mid-ProterozoicŽ. Jamtveit et al., 1991 and grain rims and neoblasts having equilibrated at lower pressures and temperatures, considered to be compatible with external eclogites. The remarkable recent discovery from garnetiferous peridotites on Otrùy and Nordùyane of evidence for majoritic com- positions in some garnet megaclasts is consistent Fig. 7. Garnet±orthopyroxene±clinopyroxene thermobarometry Ž. with a Proterozoic deep mantle plume evolution for calculated from published data references in Table 2 using the average of the Carswell and HarleyŽ. 1990 garnet±orthopyroxene some Mg±Cr peridotites prior to emplacement into thermometer and Krogh-RavnaŽ. 1999 garnet±clinopyroxene ther- the crustŽ van Roermund, 1998; van Roermund and mometers, combined with the Carswell and HarleyŽ. 1990 Drury, 1999; Terry et al., 1999. . garnet±orthopyroxene Al-barometer. Cs±Qz and Dm±Gph equi- We have made a preliminary re-evaluation of libria, and 58C reference geotherm as in Fig. 6.Ž. A External published data for some Opx external eclogites and orthopyroxene eclogites and Fe±Ti garnet peridotites. Squares Ð external Opx eclogites; circles Ð FeTi garnet peridotites, open garnetiferous peridotitesŽ. Table 2 and Fig. 7 . The symbols Ð grain rims; filled symbols Ð grain cores.Ž. B Mg±Cr average of results from the Grt±Cpx and Grt±Opx Ž.mantle-derived garnet peridotites. Filled symbols are for cores of Fe±Mg exchange barometers of Carswell and Harley zoned grains and porphyroclasts, or unzoned grains, and open Ž.1990 and Krogh-Ravna Ž. 1999 was combined with symbols are for rims of zoned grains and porphyroclasts, and the Carswell and HarleyŽ. 1990 Grt±Opx Al barome- neoblasts. ter. For the external Opx eclogites and Fe±Ti peri- dotitesŽ. Fig. 7A , rim compositions give generally mixed HPrUHP zoneŽ. Fig. 3 and gives extreme lower P and T than grain cores, probably due to pressures well within the diamond field, consistent retrogressive diffusive re-adjustment or growth of with those previously determined by Lappin and amphibole as discussed above. Grain cores from SmithŽ. 1978 and Smith Ž. 1988 . Unfortunately, the external Opx eclogites and Fe±Ti peridotites have a Nybù eclogite lacks free silica, phengite or kyanite, broad P±T scatter which is steeper than the trend for making independent thermobarometric corroboration phengite and kyanite eclogites. There is no clear of these results difficult. The most southerly Opx geographic pattern. The most northerly localities from eclogites, at KvalnesetŽ. near Totland, Nordfjord and MoldefjordŽ Solholmen and Skarsholmen on Otrùy, HornindalŽ. Fig. 2 , give the lowest P and T as might and Kolmannskog northwest of Molde. give P±T be expected although T for the Kvalneset eclogite is conditions within the quartz field, in contrast to the much lower than calculated for nearby phengite coesite eclogites along strike on Fjùrtoft. In contrast, eclogites. Overall, the difficulties in identifying grain cores from the Eiksundal Fe±Ti peridotite, well-equilibrated phases in the Opx eclogites con- HareidŽ. Fig. 2 , give a very high pressure, just into tinue to make interpretation of thermobarometric es- the diamond field. Grain cores from the Grytting timates from these rocks problematic and controver- orthopyroxene eclogite on StatlandetŽ. Figs. 2 and 3 sial. give a broad group of values whose lower range is The Mg±Cr peridotites appear to give a more similar to local phengite eclogitesŽ. e.g., Drage but rational patternŽ. Fig. 7B , and probably have the calculated pressures vary by )5 kbar for this single better textural and chronological control for their locality. The Nybù eclogite Sùrpollen lies within the analysesŽ. see Carswell, 1986; Jamtveit et al., 1991 . S.J. Cuthbert et al.rLithos 52() 2000 165±195 185

Nevertheless, interpretation of geothermobarometric variably effective recrystallisation and re-equilibra- results remains difficult. Cores of porphyroclasts or tion to the conditions along the channel. It is notable unzoned large grains give a range of T and P from that the P±T results for grain rims and neoblasts 6398C to 8288C and 21.1 to 43.8 kbar whichŽ espe- Ž.Fig. 7B overlap both the lower end of the porphy- cially at higher pressures. scatters along a linear roclast array and the field of phengite UHP eclogites array lying close to the ;58Crkm field gradient Ž.Fig. 6D , consistent with the hypothesis that they identified for phengite- and kyanite-bearing eclogites crystallised or recrystallised during or after emplace- Ž.Fig. 6D . However, once again, there is no clear ment in the subducted WGC crustŽ Carswell, 1986; geographic pattern; indeed, samples from the same Jamtveit et al., 1991. . Hence, the overall apparent body or area may be widely separated in P and T,as P±T array in Fig. 7B may well correspond to a shown by the two samples from Sandvika, Guskùy composite record of Proterozoic and Scandian P±T and the nearby Gurskebotn sampleŽ. Table 2 . In conditions. Even so, the spread of data for individual contrast, samples from two other areasŽ Ugelvik and bodies is large and obscures any possible relation- Raudhaugene, Otrùy and Kalskaret, Tafjord. form ship between P±T conditions for individual peri- distinct clusters at significantly different positions dotite bodies and their adjacent external eclogites. along the array. Porphyroclast rims and neoblasts lie Consideration of possible disequilibrium dictates in a steep elongate cluster in the P±T planeŽ Fig. that caution is needed in the interpretation of the 7B. , departing from the trend for phengite external peridotite array. The dTrd P gradient of isopleths eclogites and overlapping the low P end of the for the garnet±Al in Opx equilibrium used for pres- porphyroclastrgrain core array. Pressures are usu- sure evaluationŽ. see Carswell and Harley, 1990 ally, but not always, lower than for grain cores from happens to equate with a geothermal gradient of the same body. Only the upper part of the array for around 58Crkm, so it would appear that the linear rims and neoblasts is compatible with conditions for P±T array for garnet peridotite samples may be nearby external eclogites. largely an artifact of the P±T evaluation methodol- While the P±T array for Mg±Cr peridotite grain ogy, under circumstances where the net transfer reac- cores and porphyroclasts might feasibly relate to the tion equilibrium for Al between garnet and orthopy- thermal regime associated with Scandian continental roxeneŽ. monitoring P fails to keep pace with the lithosphere subduction, such an interpretation is in- Fe±Mg cation exchange equilibriaŽ. monitoring T . consistent with the Proterozoic age for the garnet Hence, at this stage, we conclude that there is some peridotite porphyroclast assemblages indicated by limited evidence for individual Mg±Cr peridotites the Sm±Nd data of Jamtveit et al.Ž. 1991 . On the having been sourced from distinct positions in the other hand, with such a low dTrd P gradient, the lithospheric mantle and subsequently partially equili- P±T array is thought unlikely to correspond to an brating under similar conditions to the external ambient mid-Proterozoic mantle geotherm and is even eclogites and their enclosing crustal rocks. However, less likely to represent the thermal regime associated uncertainties in the thermobarometry mask any clear with emplacement into the sub-continental litho- pattern and create artifacts which require that any spheric mantle of a sub-lithospheric mantle plume as apparent ``geothermal gradient'' is viewed with cau- deduced from the relict association of early majoritic tion. garnets and high-Al orthopyroxenes in garnet peri- dotite bodies on OtrùyŽ van Roermund, 1998; van Roermund and Drury, 1999; Terry et al., 1999. . To account for the P±T variations observed for 6. Discussion garnet peridotite samples from within local areas, two factors need to be considered Ð re-equilibration Three key issues in the study of UHP rocks are of older parageneses and disequilibrium. It might be the scale of the UHP unitsŽ indicative of the volume suggested that, as the bodies were entrained by the of buoyant continental crust capable of being sub- crust and moved within the subduction channel dur- ducted. , the extent to which they equilibrated to ing Scandian continental collision, they suffered ambient P±T±fluid conditionsŽ controlling the petro- 186 S.J. Cuthbert et al.rLithos 52() 2000 165±195 physical properties of the subducted crust such as fied during exhumation. , and that the UHP part of density and strength. , and the motion history of UHP the WGC was simply a higher-grade metamorphic units relative to their surrounding crust and mantle zone which was more deeply subducted than con- Ž.reflecting their mechanical response to subduction . tiguous rocks to the south. In this view, the first Our observations of evidence for the distribution of appearance of coesite is an isograd. coesite indicate that the UHP terrane in the WGR is Other geodynamic models for the WGC have significantly more extensive than was previously in- taken a more mobilistic view; for instance, Smith dicated by SmithŽ. 1988; 1995 , although our more Ž.1980 proposed a model of a tectonic mega-melange, conservative estimates are smaller than the excessive and AustrheimŽ. 1991 proposed a melange-like, estimate by Coleman and WangŽ. 1995 of 350=150 gravity-differentiated model based on observations in km2 and their rather optimistic estimates for the the Bergen Arcs Anorthosite Nappe. The large extent extent of the UHP terranes in DabieshanŽ. China and of mappable lithological units across the WGR dic- KokchetavŽ. Russia of 450=100 and 300=150 tates against such extreme, melange-like models ex- km2, respectively. The tectonic boundaries to most cept perhaps on a local scale, but nevertheless, the UHP terranes are poorly defined, due to overprinting WGC is clearly a composite terrane assembled from by late orogenic processes. While some UHP ter- a number of lithotectonic units. The Mg±Cr peri- ranes appear to be well-defined, allochthonous litho- dotites are generally agreed to have been introduced tectonic units of limited thickness are bounded by into the WGC from the mantle, and they are UHP shear zones against low pressure rocksŽ e.g., rocks. Allochthonous or parautochthonous cover units Kokchetav Massif; Kaneko et al., 1998. ; others such are an important component of the WGCŽ Muret, as the Dora Maira Massif have tectonic breaks de- 1960; Brueckner, 1977; Krill, 1985; Bryhni, 1989; fined principally on petrographic criteria, with no Robinson, 1995, 1997; Tveten, 1995. . In the Molde- conclusive evidence relating these to present-day fjord±Nordùyane area, cover and basement appear to structuresŽ. e.g., Michard et al., 1995 . The situation have distinctive assemblages of eclogites. UHP in the WGR is also not well-defined but bears some eclogites are present in the basementŽ. Ulla Gneiss . similarities to the large South-Central Dabie Terrane In the Sñtra and Risberget cover nappes, eclogites in Dabieshan where the eclogites show a general have prograde-zoned garnets typical of HP eclogites northward increase in P and T, but are divided by a Ž.Robinson, 1995 . The BlahùÊ cover nappe has spo- possible tectonic break into an HP unit with zoned radic occurrences of eclogite, but importantly con- garnets and an UHP unit with coesite and microdia- tains the UHP eclogite and diamond-gneiss on mondŽ. Wang et al., 1995; Carswell et al., 1997 . Fjùrtoft, separated from basement by eclogitic my- However, unlike the structurally low WGC, the lonites and pods of garnet peridotite. Evidence for South-Central Dabie Terrane appears to occupy a deep-level emplacement of the BlahùÊ nappe against high structural level in the orogen, at least at its basement is well-preserved hereŽ Terry and Robin- southern margin where it overthrusts other lower P son, 1998; Robinson and Terry, 1999; Terry et al., units near the foreland. 1999. . Some previous geodynamic models for the south- In the Norfjord±Stadlandet area, where evidence ern Scandinavian Caledonides have tended to treat for UHP metamorphism has been most commonly the WGC as a coherent body during subduction or found, UHP eclogites appear to be found in both have indicated only in a very general way that it is basement and cover, and late amphibolite facies likely to have been tectonically imbricatedŽ e.g., re-working has largely obscured any earlier HP fab- Carswell and Cuthbert, 1986; Cuthbert and Carswell, rics which might have recorded the kinematics of 1990; Cuthbert et al., 1983; Andersen et al., 1991; UHP nappe emplacement. Assessment of the coher- Dewey et al., 1993. . A simplistic view of the general ence of the WGC in this area during collisionrsub- trends in eclogites in the WGR described in the duction depends critically on interpretation of the foregoing sections might be that the WGC was, mixed UHP±HP zone. We shall consider below two indeed, a continuous, unbroken structural unit during possible end-member scenarios for the origins of the eclogite facies metamorphismŽ although later modi- mixed zone Ð tectonic juxtaposition of originally S.J. Cuthbert et al.rLithos 52() 2000 165±195 187 separate HP and UHP terranes, and a single terrane the estimate of Krabbendam and WainŽ. 1997 of up exhibiting a metamorphic transitionŽ. isograd . to 4±6 km present-day structural thickness, a bulk The spatial distribution of P across the WGR, vertical shortening from 13 to 26 km across the while admittedly based upon unevenly distributed entire mixed HPrUHP zone can be calculated. How- data, does not appear to be smooth and regular in ever, this bulk figure may be meaningless, owing to spite of the linear array in P±T space. Based upon the close proximity of HP and UHP eclogites within the dataset used in this study, augmented with that of this zone. Krabbendam and WainŽ. 1997 and Wain Ž 1997a; b; While the distribution of eclogite types and calcu- 1998. , pressures vary from about 14±17 kbar in lated pressures does seem to support some shorten- inner Sunnfjord to 20±23 kbar immediately to the ing of crustal section, and hence, tectonic transport south of the mixed HPrUHP zone at Nordfjord, between the HP and UHP zones and possible tec- giving a P change of up to 6"3 kbar over a map tonic mixing within the boundary zone between them, distance of 55 km across the strike of the gneissic the lithological assemblages in the country-rocks in foliation. The southernmost eclogites in the UHP the HP and UHP zones are substantially similar and zone proper give pressures of about 28 kbarŽ Wain, neither zone can be attributed wholly to either base- 1998. , indicating an apparent pressure change of ment or cover units, as both are present in each zone 6.5"1.5 kbar over a map distance of about 10 km and in the mixed zone. Furthermore, the mixed zone across the HPrUHP mixed zone. The highest is not marked by any increase in strain intensity, nor recorded pressures are about 33 kbar, which are are the intervening country-rocks between adjacent found about 40 km across strike from the north side HP and UHP eclogites within the mixed zone. Hence, of the mixed zone, indicating a pressure change of 5 it can be concluded that the HP and UHP zones on kbar over this distance. It therefore appears that the either side of Nordfjord have similar crustal prove- mixed zoned represents a strong telescoping of nances. The fabrics in the country-rock gneisses are eclogite-facies isobars. It is difficult to relate these dominated by amphibolite-facies parageneses defin- apparent pressure gradients to true structural thick- ing L-tectonite fabrics indicating strong vertical flat- ness due to the effects of late folds, but in a more tening and E±W extensionŽ. Andersen et al., 1994 , local study in the Nordfjord±Stad area where struc- sometimes constrictionalŽ Krabbendam and Dewey, tures are reasonably well-constrained, Krabbendam 1998. . These strains are likely to have made a con- and WainŽ. 1997 demonstrated a strong telescoping siderable contribution to the juxtaposition of the HP of isobars across the mixed zone related either to and UHP zones, but as these strains are not unique to differential attenuation of a continuous crustal sec- the mixed zone, some extra transport, specifically tion or a distinct tectonic break. Within the mixed within the mixed zone, and pre-dating the present zone, pressures derived from adjacent UHP and HP fabrics in the country rock gneisses seem likely. The eclogites indicate strong, non-lithostatic apparent similarity of lithologies in the HP and UHP zones pressure gradientsŽ Fig. 5; see also Krabbendam and would suggest that the UHP terrane is not ``exotic'' Wain, 1997; Wain, 1997b. . The maximum estimates to the rest of the WGC, but simply forms a more of D P between adjacent HP and UHP eclogites are deeply subducted part of the same crustal assem- about 8 kbar over 500 m. Foliation dips are steep blage which has been tectonically dismembered. The along this part of Nordfjord, so assuming that this presence of numerous peridotite bodies within the distance approximates to maximum structural thick- mixed zoneŽ Krabbendam and Wain, 1997; Wain, ness and that pressures were purely lithostatic, a 1998. is consistent with such a scenario if they were vertical shortening of about 26 km can be estimated incorporated from the mantle bounding the subduc- between these eclogites. However, given the cautious tion zone; this important observation needs further view of uncertainties in pressure estimates adopted investigation. This may not be the only tectonic here, D P between the two groups of HP and UHP break in the central-southern WGR. In the Dalsford± eclogites could be as low as 4 kbar, equivalent to Fùrde area, inner SunnfjordŽ. Fig. 2 glaucophane about 13 km vertical shortening over a minimum of eclogites in the Jostedal Complex basement give ;500 m. Following the same reasoning and using temperatures up to 1008C lower than adjacent bar- 188 S.J. Cuthbert et al.rLithos 52() 2000 165±195 roisite eclogites in the Fjordane complex cover found, and eclogite bodies appear to be uniform in Ž.Cuthbert, 1985; Krogh-Ravna, unpublished data . petrographic typeŽ. Wain, 1997a, 1998 . Also, within Inference of tectonic juxtaposition and loss of the mixed zone, eclogite bodies of the same type crustal section from P±T conditions recorded in the follow distinct horizons in the gneissŽ. Wain, 1998 , HP and UHP eclogites assume that these eclogites suggesting tectonic interleaving of HP and UHP rock equilibrated at approximately the same time, and that units. The survival of a large volume of plagioclase- equilibration was maintained as P, T and fluid bearing rock in the Flatraket mangerite bodyŽ. Fig. 3 composition changed. There are currently no isotopic would have been more likely if it had only experi- dates available for UHP eclogites, and isotopic dise- enced HP conditions, as the overstep on the plagio- quilibrium makes dating of HP eclogites difficult clase breakdown reaction would have been smaller Ž.Ž.Griffin and Brueckner, 1985 . Wain 1997c; 1998 , than at UHP. The Flatraket body contains HP eclog- AustrheimŽ. 1998 , Austrheim and Engvik Ž. 1997 and ites but is sandwiched between UHP eclogites, so Austrheim et al.Ž. 1997 have shown that disequilib- this line of reasoning requires tectonic juxtaposition rium, as signified by survival of pre-eclogitic plagio- of HP and UHP rock massesŽ. Wain, 1997a,c, 1998 . clase-bearing parageneses, is common in dry igneous However, the occurrences of prograde-zoned garnet or granulite protoliths in eclogite-facies terranes, and in UHP eclogites and their surrounding gneisses at the catalytic effect of fluids and deformation are of Asheimneset,Ê Stadlandet and Vetrhuset, Nordpollen great importance in transforming crust to eclogite indicates that prograde features can, rarely, survive facies parageneses. The efficiency of transformations UHP conditions. In the light of these tantalising within the eclogite facies, such as that between HP exceptions, the influence of kinetic factors such as and UHP rocks, is less well-known, but it seems fluids, temperature and deformation in controlling possible that HP eclogites could survive metastably the transformation from HP to UHP eclogite de- under UHP conditions. If a HP terrane underwent serves closer scrutiny. continued subduction and the overstep in P andror Preservation of zoning and inclusion suites in HP T required for the transformation was large, transi- eclogites precludes their formation by retrograde tions between reacted and unreacted rock masses metamorphism of UHP eclogites at eclogite facies, would record a significant jump in pressure, and but retrograde effects are seen in the form of changes possibly temperature. UHP eclogites could also in phengite chemistry and thin retrograde-zoned rims record younger isotopic ages than HP eclogites, given on garnets. These have the effect of smearing out the a precise-enough dating technique. distribution of calculated P and T and creating large Disequilibrium within a crustal slab that remained uncertainties in determination of any pressure gap coherent during eclogite facies metamorphism could between HP and UHP eclogites. provide an alternative explanation for the nature of Thus, at this stage, the relative roles of tectonic the HP±UHP transition in the WGC. Pressures and kinetic factors in controlling the distribution of recorded in the HP±UHP transitionŽ. mixed zone are HP and UHP eclogites are uncertain, but both have quite close to the coesite±quartz inversion and tem- undoubtedly contributed to the present distribution of peratures are close to those which might be expected eclogite types in the WGC. The existence of UHP to homogenise garnet compositional zoning for rea- eclogites in both basement and cover argues for UHP sonable timescales and grain sizes. Unequivocal evi- metamorphism prior to or during tectonic assembly dence for transformation of HP to UHP eclogite is of lithotectonic units. This assemblage may then currently lacking; WainŽ. 1998 found no evidence of have suffered some dismembering, certainly during fluid or deformation controlling the development of the later stages of exhumation at amphibolite facies, UHP assemblages in eclogites from the Nordford± but probably also at deeper levels. The present distri- Stadlandet area, except in local retrograde effects. bution of HP and UHP eclogites and their recorded Features such as occurrence of UHP eclogite in apparent P±T conditions was controlled by this higher strain zones or fluid infiltration channels deformation, but may also have been affected by within HP eclogites, or volumes of HP eclogite in kinetically controlled survival of HP eclogite and low strain zones in UHP bodies have not yet been later retrogressive adjustment of phase compositions S.J. Cuthbert et al.rLithos 52() 2000 165±195 189

Ž.especially phengite . Better understanding is likely We tentatively suggest that a conservative estimate to come from a more extensive database of thermo- for the size of the UHP terrain is ;5000 km2. barometric data, identification of critical areas where Ž.2 Polycrystalline quartz has been found in the early structures and parageneses are better preserved, BlahùÊ cover nappe and in the adjacent basement and detailed fabric studies to fully evaluate the im- gneiss, close to the locality where microdiamond has portance of fluids, strain and recrystallisation in the been separated by thermochemical methods from a transformation from HP to UHP eclogite. pelitic gneiss, confirming the UHP nature of this Given the broad pattern of systematic P±T change unit. across the WGR, we speculate that the Baltica crust Ž.3 A review of petrographic features in eclogites Ž.possibly with a nappe pile already in place above it such as silica polymorph type, garnet zoning and was subducted and eclogites formed, equilibrating to inclusion patterns, matrix parageneses and retrogres- theŽ. transient? geothermal gradient in the subduc- sive evolution shows a spatial variation indicating an tion zone. The subducted crust became more or less increase of T and P from southeast to northwest imbricated in the subduction zone, and was vertically across the WGR, although with some local varia- flattened during the later stages of exhumation, but tions. The presence of a mixed UHPrHP zone as not disrupted to the extent that the pattern of P and described by WainŽ. 1997b; 1998 and Krabbendam T recorded along the subducted slab was severely and WainŽ. 1997 is confirmed and extended to the modified. ImbricationŽ prior to, during or after UHP east. metamorphism. was apparently effective in entrain- Ž.4 Geothermobarometry using garnet±ompha- ing masses of peridotite from the mantle bounding cite±phengite assemblagesŽ Waters and Martin, 1983; the subduction channel. The peridotites may have Krogh-Ravna, 1999. gives P±T conditions generally been derived from a range of depths in the subduc- consistent with the distribution of silica polymorphs. tion channel as suggested by their distinct character- Given a cautious evaluation of uncertainties Ž P up to istics and clustering of P±T conditions in some "2 kbar, T"758C. , the variation in T across the casesŽ. Fig. 7A and B . They probably had diverse WGR suggested by eclogite petrography and early origins and subduction histories with possibilities for geothermometry studiesŽ. Griffin et al., 1985 is con- entrainment under deep-level UHP conditions and firmed and shown to correlate with an increase in P recrystallisation during transport up the subduction from 16 kbar in the south to )32 kbar in the channel, or transport down the subduction channel northwest. and subjection to prograde HPrUHP metamorphism, Ž.5 P±T conditions for phengite eclogites lie in as seems likely for the Almklovdalen body with its geographical order from south to north along a linear enclosed ferroan eclogites bearing prograde-zoned array in the P±T plane, corresponding to a fairly garnetsŽ. Griffin and Qvale, 1985 . The common low ``geothermal gradient'' of ca. 58Crkm. This location of peridotites along basement-cover contacts gradient could, of course, be diachronous, but exist- indicates that such surfaces were important detach- ing geochronological data are not sufficient to re- ment horizons during subduction or return and, along solve this. with the mixed HP±UHP transition zone, emphasise Ž.6 The mixed HPrUHP zone marks the transi- the importance of taking a, more or less, mobilistic tion between an HP terrane to the south and an UHP view of the development of the WGC. terrane to the north. The mixed zone appears to represent an area of telescoped isobars within the regional trend of increasing P across the WGR. Within this zone, apparent pressure gradients be- 7. Conclusions tween HP and UHP eclogites exceed lithostatic gra- dients, even assuming rather cautious uncertainties in Ž.1 Coesite, or polycrystalline quartz after coesite, P Ž."2 kbar and in estimating true structural thick- has now been found from 32 localities in the WGR ness. While telescoping of isobars probably results, and new discoveries significantly extend the at least in part, from tectonic shortening, the litholog- coesite-bearingŽ. UHP terrane to the north and east. ical similarities of the HP and UHP terranes suggest 190 S.J. Cuthbert et al.rLithos 52() 2000 165±195 that the UHP terrane is not ``exotic'' with respect to tent with previous viewsŽ e.g., Cuthbert et al., 1983; the HP zone and both have a similar crustal prove- Griffin et al., 1985. that the WGC has been sub- nance, their juxtaposition possibly resulting from a ducted towards the northwest during the Scandian -26 km vertical loss of crustal section, over a zone collision episode, and the general order of P and T ;4 km in present-day thickness. Vertical flattening along the subducted slab has been preserved in spite at amphibolite facies may have contributed to this of severe Scandian reworking during exhumation. shortening, with possible earlier crustal imbrication However, evidence from the northern WGRŽ Terry at greater depths to explain the very local juxtaposi- and Robinson, 1998. suggests that significant move- tion of HP and UHP eclogites in this zone. Consider- ment between lithotectonic basement and cover units ation of kinetic factors in the HP±UHP transition has occurred along with entrainment of mantle peri- suggests that survival of HP eclogites under UHP dotite. The extent of tectonic modification and differ- conditions could contribute to the formation of the ent metamorphic regimes in basement and cover in mixed zone, but direct evidence for this is currently the Nordfjord±Stadlandet area is less clear, but the tenuous. mixed HPrUHP zone may represent a significant Ž.7 A review of existing data for orthopyroxene disjunction and is also marked by a concentration of eclogites, all within the UHP terrane, gives results peridotite bodies. Unfortunately, as in all UHP ter- broadly consistent with an UHP origin, but no sys- ranes, regional scale eclogite facies structures, which tematic geographical pattern emerges and the scatter can give direct evidence for deep-level tectonic evo- of data from individual localities is large, re-empha- lution, have been largely obliterated by late, lower sising the problems of recognising equilibrium com- pressure deformation and metamorphism and it is binations of phase compositions in these petrographi- necessary to infer tectonic breaks from discordances cally complex rocks. in recorded P and T. Such discordances could also Ž.8 Mantle-derived, Mg±Cr-enriched garnetifer- result from kinetic factors operating in a coherent ous peridotites are possibly restricted to the UHP and crustal unit. Although WainŽ. 1998 found no evi- mixed zones of the WGR. Peridotites often decorate dence for kinetic controls on the distribution of HP basement-cover contacts and may be useful indica- and UHP mineralogies within eclogites from the tors of imbrication, tectonic transport and crust± mixed HPrUHP transition zone in the WGR, this mantle interaction in the subduction zone. Garnet± cannot be positively ruled out and some tentative opx±cpx geothermobarometry on early Proterozoic evidence supports survival of prograde textures un- porphyroclast assemblages from Mg±Cr garnet peri- der UHP conditions. Hence, the interpretation of dotites gives P±T conditions lying along a linear P±T jumps in tectonic breaks must be treated with array unlikely to correspond to ambient conditions caution. along a Proterozoic subcontinental geotherm. Ž.10 A complete understanding of the dynamics of Neoblast assemblages and rim compositions of prob- continental collision and subduction requires a able Scandian age show a trend of P and T depart- knowledge of the interactions and feedback between ing from the trend for Scandian external phengite external tectonic controls, thermal evolution, meta- eclogites but overlapping with the UHP eclogites. morphic reactions and reaction rates, fluid flux and The overall P±T array for porphyroclastic and changes in petrophysical properties of the crust and neoblastic assemblages in the garnet peridotites mantle. The WGR provides a well-exposed and ac- probably corresponds to a composite record of Pro- cessible example of a UHP terrane with the potential terozoic and Scandian P±T conditions. The similar- to provide information on all these aspects. Further ity of the HPŽ. Proterozoic segment of the array to work should concentrate on more completely defin- that defined by the externalŽ. Caledonian phengite ing the full extent of the UHP terrane, establishing a eclogites is considered to be fortuitous and may be better geochronologic framework for the regional an artifact of the P±T evaluation methodology. field gradient, detailed structural examination of the Ž.9 The overall pattern of petrographic character- nature of basement-cover contacts, examination of istics, silica polymorphs, mantle peridotite distribu- peridotites to determine the nature of crust±mantle tion and P±T conditions across the WGR is consis- interactions, and detailed textural studies to ascertain S.J. Cuthbert et al.rLithos 52() 2000 165±195 191 the influence of kinetic factors in the creation and tween Crust and Mantle. Tectonophysics, Vol. 273, pp. 129± preservation of HP and UHP rocks and related 156. changes in their petrophysical properties. Bailey, D.E., 1989. Metamorphic evolution of the crust of south- ern Norway: an example from Sognefjord. PhD thesis, Univer- sity of Oxford. Bence, A.E., Albee, A.L., 1969. Empirical correction factors for Acknowledgements electron probe microanalyses of silicates and oxides. Journal of Geology 76, 382±403. We acknowledge support for recent studies in Berman, R.G., 1990. Mixing properties of Ca±Mg±Fe±Mn gar- Norway from the Norwegian Research Council, the nets. American Mineralogist 75, 328±344. Bohlen, S.R., Boettcher, A.L., 1982. The quartz±coesite transfor- Norwegian Geological SurveyŽ. NGU , the British mation: a pressure determination and the effects of other Council, the Carnegie Trust for the Scottish Univer- components. Journal of Geophysical Research 87, 7073±7078. sities, the Natural Environment Research Council Boundy, T.M., Essene, E.J., Hall, C.M., Austrheim, H., Halliday, and the Universities of Paisley, Sheffield, Tromsù A.N., 1996. Rapid exhumation of lower crust during conti- and Oxford. 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