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Abstract

Eskola (1921) drew attention to some of the aesthetically impressive eclogites and garnet peridotites that outcrop in the coastal region of west Norway between Bergen and Trondheim. These occurrences lie within the so-called Western Gneiss Region (WGR), the lowest exposed structural level in the southern Scandinavian Caledonides. The WGR is now recognised as a composite tectono-metamorphic terrane that mostly comprises Proterozoic autochthonous to para-autochthonous basement rocks with minor late Proterozoic cover belonging to the leading edge of the Baltic Plate, along with infolds of the main, outboard-derived Caledonian allochthon. Much of this composite edifice experienced short-lived deep level subduction beneath the Laurentian Plate during the Scandian phase of the Caledonian orogeny. Several more recent papers, including those by Andersen et al, (1991); Carswell et al. (2003a); Cuthbert et al. (1983, 2000); Cuthbert & Carswell (1990); Dewey et al. (1993); Griffin et al. (1985); Krogh & Carswell (1995); Smith (1995), have considered the stabilisation and exhumation of eclogites and other cofacial high pressure (HP) and ultrahigh pressure (UHP) rocks in this region, within the context of the tectono-metamorphic development of this segment of the Scandinavian Caledonides.

Historical background to UHPM in western Norway

Eskola (1921) drew attention to some of the aesthetically impressive eclogites and garnet peridotites that outcrop in the coastal region of west Norway between Bergen and Trondheim. These occurrences lie within the so-called Western Gneiss Region (WGR), the lowest exposed structural level in the southern Scandinavian Caledonides. The WGR is now recognised as a composite tectono-metamorphic terrane that mostly comprises Proterozoic autochthonous to para-autochthonous basement rocks with minor late Proterozoic cover belonging to the leading edge of the Baltic Plate, along with infolds of the main, outboard-derived Caledonian allochthon. Much of this composite edifice experienced short-lived deep level subduction beneath the Laurentian Plate during the Scandian phase of the Caledonian orogeny. Several more recent papers, including those by Andersen et al, (1991); Carswell et al. (2003a); Cuthbert et al. (1983, 2000); Cuthbert & Carswell (1990); Dewey et al. (1993); Griffin et al. (1985); Krogh & Carswell (1995); Smith (1995), have considered the stabilisation and exhumation of eclogites and other cofacial high pressure (HP) and ultrahigh pressure (UHP) rocks in this region, within the context of the tectono-metamorphic development of this segment of the Scandinavian Caledonides.

Smith (1984) provided the first description, as well as confirmation by Raman spectroscopy, of an occurrence of coesite within an eclogite in the WGR. This coesite (CD Images 1 & 2) is preserved (armoured within omphacite in turn enclosed in garnet) within a small, partly retrograded, eclogite pod at Grytting, near Selje, in the SW part of the Stadlandet peninsula (Fig. 1). interestingly, but perhaps just coincidentally, this coesite-bearing eclogite outcrops closely adjacent to the more spectacular-looking coarse orthopyroxene eclogite (CD Image 3) described by Eskola (1921). Importantly, thermobarometric evaluation by Lappin & Smith (1978) and Cuthbert et al. (2000) of samples of this orthopyroxene eclogite indicates formation under UHP conditions consistent with the stability of coesite in the nearby pod.

Fig. 1.

A generalised geological map for the Western Gneiss Region between the Sognefjord and Moldefjord areas showing the respective distributions of documented occurrences of UHP (coesite-bearing) eclogites, UHP diamond gneiss and peridotite bodies.

Fig. 1.

A generalised geological map for the Western Gneiss Region between the Sognefjord and Moldefjord areas showing the respective distributions of documented occurrences of UHP (coesite-bearing) eclogites, UHP diamond gneiss and peridotite bodies.

Smith's (1988) expansive review article on WGR eclogites, documented confirmed coesite at only one other eclogite locality named Straumen, some 14 km SW of Grytting.

In addition, he deduced the likely previous presence of coesite from observations of polycrystalline or at least multi-crystalline quartz inclusions within garnet or omphacite in samples from five additional eclogite localities, including Årsheimneset, Drage and Liset on Stadlandet. Such polycrystalline quartz (PCQ) inclusions, sometimes with a distinctive palisade microstructure (CD Image 4), are now widely accepted to be pseudomorphs after earlier coesite crystals, some of which have a distinctive tabular form (CD Image 5).

On this rather limited evidence, Smith (1988) proposed the existence within this coastal region of the WGR of a specific coesite eclogite province containing rocks that had experienced UHPM conditions. However, Smith (1988) did not establish the boundaries for this UHPM province and moreover emphasised that most, if not all, of the intervening “country-rock” gneisses enclosing the various documented eclogite occurrences in this part of the WGR lacked mineralogical evidence that they had witnessed HP (quartz eclogite stable), let alone UHP (coesite eclogite stable), conditions. Accordingly, Smith (1988) in fact concluded that, rather than the WGR incorporating a regionally extensive, structurally coherent, UHPM province or terrane, it in fact comprised a highly imbricated tectonic melange of HP and UHP eclogites enclosed within dominant lower pressure metamorphic rocks. Smith (1995) further expounded his view that the geographically scattered coesite eclogite occurrences represent tectonically dismembered fragments of an early Caledonian (ca. 440 Ma) UHPM nappe within the WGR, and in his elaborate Foreign/In situ/Foreign (FIF) geodynamic model ne introduced the notion that the quartz-stable eclogites may in fact have formed during a later Caledonian (ca. 410 Ma) HPM (Pmax ca. 2.0 GPa) event that affected a substantial segment of mixed acid + basic lithology crust.

A further significant step in the recognition of a possible, regionally extensive, UHPM terrane within the WGR of Norway, was the reported recovery by Dobrzhinetskaya et al. (1993, 1995) of micro-diamonds from dissolution of samples of garnet-kyanite-biotite-rutile-quartz gneiss (CD Image 6) and of garnet-pyroxene-amphibole-biotite gneiss from the north coast of the island of Fjørtoft in Nordøyene, about 80 km to the north of the first coesite eclogite occurrences documented by Smith (1984, 1988). Consequently, in their global overview of the then recognised UHPM terranes, Coleman & Wang (1995) speculated that the UHPM terrane in western Norway might cover an area of roughly 350 × 150 km. Even given more conservative estimates of the size of the UHPM terrane, it is clear from the lithostatic pressures required for UHPM that a substantial mass of initially buoyant continental crust has been inserted (subducted) into the sub-lithospheric mantle, and subsequently exhumed, during the late Silurian to Middle Devonian Scandian plate collision episode.

Most recent discoveries of UHPM rocks in western Norway

Meticulous field and petrographic studies in the outer Nordfjord and Stadlandet areas reported by Wain (1997) have greatly increased the number of eclogite localities recognised to have experienced UHPM. Actual relict coesites were identified as microinclusions in garnet, omphacite or kyanite in eclogite from five new localities and petrographically distinctive PCQ pseudomorphs after coesite were recognised in eclogite minerals at a further twelve localities.

Cuthbert et al. (2000) reported discoveries in the outer Nordfjord and Stadlandet areas of a further relict coesite-bearing eclogite locality at Flister and of PCQ pseudomorphs after coesite in eclogite pods at Maurstad and Sandvikneset. In addition, convincing PCQ inclusions after coesite within garnet were reported from a large eclogite body capping the peaks of Hornet and Bautene to the south of the extensive peridotite outcrops in Almklovdalen and in a smaller eclogite pod in Stigedalen, south of the peridotite outcrops in Bjørkedalen, that together extend the occurrences of UHP eclogites some 40 km east of previously recognised localities (Fig. 1).

The present authors have also recently discovered convincing PCQ inclusions after coesite in garnets within a small flaser-textured eclogite body (CD Image 7) that outcrops virtually adjacent to the classic HP kyanite eclogite body (CD Image 8) at Verpeneset on the north shore of Nordfjord. This then is the most southerly UHP eclogite occurrence so far identified within the WGR.

Cuthbert et al. (2000) also reported the discovery, confirmed by Terry et al. (2000b), of PCQ inclusions after coesite (CD Image 5) in a pod of kyanite-phengite eclogite (CD Image 9) at Fjørtoftvika on the island of Fjørtoft, some 85 km northeast of the UHP eclogites localities recognised in the Nordfjord and Stadlandet areas. Importantly this UHP eclogite locality is only about 2 km along strike from the outcrops of the graphite-bearing garnet-kyanite-biotite gneiss from which scarce micro-diamonds were recovered by Dobrzhinetskaya et al. (1993, 1995).

Compelling new evidence that the rocks exposed along the north coast of Fjørtoft experienced UHPM conditions that extended into the diamond stability field is provided by the recent startling discovery of micro-diamonds in a garnet websterite lens within a small outcrop of peridotite at Bardane. These micro-diamonds are interpreted to have formed in response to infiltration by crustal-derived fluids during Caledonian deformation-induced recrystallisation (van Roermund et al., 2002; Brueckner et al., 2002).

Smith (1988, 1995) reported finding poly- or multi-crystalline quartz thought to have replaced earlier coesite in the large Ulsteinvik–Dimnøy eclogite body on Hareidlandet and in the Hessdalen eclogite body on the opposite side of Vartdalsfjorden, these localities being roughly midway between the occurrences of UHPM rocks on Stadlandet and Fjørtoft (Fig. 1). The UHP status of the Ulsteinvik–Dimnøy eclogite has been confirmed by Carswell et al. (2003b) through the discovery of preserved inclusions of coesite within a zircon separate from this eclogite body. Further evidence that rocks in this part of the WGR also experienced UHPM conditions is provided by the reported discovery by Hacker et al. (2001) of numerous eclogite localities with relict coesite or PCQ pseudomorphs after coesite on the Sørøyane islands to the west of Hareidlandet.

This steady increase over recent years in the number of recognised occurrences of UHPM rocks in the coastal region of the WGR between Nordfjord and the Nordøyene suggests that the coesite eclogite–bearing UHPM terrane in the WGR covers an area of at least 5000 km2 in a coastal strip up to 40 km wide (Fig. 1).

The “foreign” versus “in situ” eclogite controversy: The influences of differential retrogression and metastability

The HP and UHP eclogites in the WGR occur as highly variable sized lenticular pods or tabular layers within highly deformed, dominantly quartzo-feldspathic, gneisses (e.g. CD Image 10). Typically the margins to the eclogite bodies show retrogression to amphibolite facies mineralogies (CD Image 11), co-facial with the mineralogy observed in the surrounding gneisses. Sometimes the amphibolitisation at eclogite margins is a static growth feature (e.g. CD Image 12) apparently triggered by an influx of hydrous fluid but in other instances the strong shear deformation fabric seen in the surrounding gneisses may be seen to have penetrated the margins of the eclogite bodies resulting in the development of a strongly foliated amphibolite (CD Image 13). Not uncommonly, especially towards the margins of the eclogite bodies, omphacites show at least partial replacement by a granulite facies symplectite of secondary jadeite-depleted clinopyroxene plus sodic plagioclase (e.g. CD Images 1 & 2), as a result of a retrogressive, granulite facies, decompression stage that preceded the amphibolitisation.

Given the obvious major contrasts in metamorphic grade between the unretrograded eclogites and the encompassing amphibolite facies gneisses, it has remained uncertain and controversial as to whether or not the “country rock” gneisses enclosing the eclogites, garnet peridotites and other recognised HP and UHP rocks in the WGR experienced comparable HPM or UHPM conditions. Consequently, there has been a prolonged debate over whether the observed HP and UHP rocks were stabilised “in situ” within the gneisses in an essentially structurally coherent metamorphic terrane or alternatively represent “exotic” blocks or lenses of HP and UHP rocks within some sort of highly disrupted tectonic melange, as envisaged for example by Smith (1980).

Krogh (1977) and Griffin et al. (1985) established the existence of a thermal gradient from around 550 °C in the SE to > 800 °C in the coastal areas to the NW (Fig. 2) for the HPM/UHPM across the WGR from consideration of Fe2+/Mg2+ partitioning between garnet and omphacite in eclogite samples. From an updated thermobarometric evaluation of phengite-bearing and orthopyroxene-bearing eclogites, Cuthbert et al. (2000) established that the regional temperature gradient is matched by a gradient of increasing lithostatic pressures, consistent with the stability of coesite to the north of Nordfjord and of diamond only in the most northwesterly exposed part of the WGR in the Nordøyene. This P–T analysis supports two important conclusions. Firstly, that the rocks in the most northwesterly part of the WGR were subducted to deepest levels during the Caledonian plate collision. Secondly, that the subduction-related P–T gradient across the WGR (Fig. 2) has not been greatly disrupted by the widely displayed late orogenic, exhumation related, amphibolite facies, proto-mylonitic fabrics that developed in response to extensional, top-to-west, shear deformation (Andersen et al., 1991).

Fig. 2.

Regional temperature gradient across the Western Gneiss Region of Norway based on Fe2+/Mg2+ partitioning between garnet and omphacite in eclogites (after Griffin et al., 1985) plus indications of the concomitant pressure gradient based on P–T estimates for eclogite samples from various localities indicated by Cuthbert et al. (2000), Carswell et al. (2003a) and Terry et al. (2000b).

Fig. 2.

Regional temperature gradient across the Western Gneiss Region of Norway based on Fe2+/Mg2+ partitioning between garnet and omphacite in eclogites (after Griffin et al., 1985) plus indications of the concomitant pressure gradient based on P–T estimates for eclogite samples from various localities indicated by Cuthbert et al. (2000), Carswell et al. (2003a) and Terry et al. (2000b).

Terry et al. (2000b) have proposed the existence of a major metamorphic discontinuity on the island of Fjørtoft between a higher structural unit/plate that records UHPM conditions of ca. 820 °C and 3.4–3.9 GPa and a lower unit/plate that only records HPM conditions of ca. 780 °C and 1.8 GPa. In contrast to the melange model of Smith (1988), Terry et al. (2000b) placed the HP/UHP junction at the lower contact of a regionally extensive and coherent sequence of thrust nappes (Blåho and Saetra nappes) with para-autochthonous Baltica basement (Ulla gneiss and other migmatitic or augen gneisses). No evidence for UHPM was reported within the lower plate gneisses or their enclosed eclogites. However, pods and layers of garnet peridotite are found within these Baltica basement gneisses, including the important Bardane UHP microdiamond-bearing peridotite on Fjørtoft (see below). Also, the documented UHP kyanite eclogite lies in close proximity to the contact of the upper and lower plates as defined by Terry et al. (2000b), and the high strain state of the gneisses makes exact definition of the boundary problematic, so the assignation of this eclogite to one plate or the other is problematic. When these uncertainties are considered along with the overall rarity of evidence for coesite in the WGR (see next section) and the general lack of useful parageneses for geobarometry in the Baltica basement in the northwestern WGR, the HP vs. UHP status of the lower plate remains, in our view, a somewhat open question, and it is possible that both units were juxtaposed before, or during, UHPM. If the lower plate is eventually proven to have recorded UHPM, then the survival of primary, low P, igneous phases in large masses of partially eclogitised metagabbro (Mørk, 1985a) raises the possibility of extreme metastability under UHP conditions here, as it does further south in the WGR (Wain et al., 2001). Indeed, a key issue in understanding the distribution of HP, UHP and lower P facies is the operation of factors controlling the efficiency of metamorphic transformations. As we argue further below, such factors need to be considered carefully before appealing to the tectonic juxtaposition of non-cofacial rock masses, especially when direct evidence for a tectonic break is lacking.

Cuthbert & Carswell (1990) and Griffin et al. (1985) have previously summarised various lines of evidence and arguments in favour of an essentially “in situ” formation of most crustal protolith eclogites within the WGR during short-lived deep subduction of a slab of continental crust. The two most compelling lines of evidence supporting this interpretation are:

  • 1)

    Widespread occurrences of corona-textured metadolerites (e.g. Gjelsvik, 1952) and metagabbros (e.g. Griffin & Råheim, 1973; Mørk, 1985a, b; Krabbendam et al., 2000) that display incomplete transformation to eclogite (CD Images 14&15). Crucially, some of these preserve primary igneous contacts with granitoid gneisses (Cuthbert, 1985). It is apparent that the extent of eclogitisation is controlled by the extent of influx of fluids and/or concomitant deformation and hence is crucially dependent on reaction kinetics (Austrheim, 1998). Not only do such rocks demonstrate eclogite formation under conditions of increased P and T from low-pressure (high crustal level) protoliths but they also provide clear evidence of plagioclase metastability under eclogite facies conditions.

  • 2)

    Many eclogites, especially the quartz-stable HP types in the vicinity of Nordfjord, contain large garnets that display evidence of a prograde growth history under conditions of increasing P and T (CD Image 16). Such garnets show a compositional zonation with a marked increase in Mg/Fe ratio from core to rim. Not infrequently they also show a zonation in the entrapped mineral inclusions with the blue-green amphiboles in garnet cores and omphacite inclusions within later growth garnet (CD Image 17). The growth of such garnet thus apparently commenced under amphibolite facies conditions and continued under subsequent eclogite facies conditions.

As indicated earlier, the margins of eclogite and garnet peridotite bodies mostly show retrogression to amphibolite, frequently linked to the development of a deformation fabric (e.g. CD Image 13). It might be tempting to link this retrogression with tectonic emplacement of the HPM/UHPM rocks into higher crustal level, lower pressure, gneisses, but then not all eclogite body margins are the focus of such shear deformation and invariably the scale of any observed deformation is inconsistent with the notion that the HPM or UHPM rocks have been thrust upwards by some 30–90 km, to account for the confining pressure contrasts with the enclosing amphibolite facies gneisses.

Hence, in the almost ubiquitous absence of obvious petrographic evidence that the “country rocks” enclosing the HPM and UHPM rocks also witnessed comparable P–T conditions, the marked contrasts in observed metamorphic grade seem best explained by arguing that the late orogenic deformation associated with the exhumation of this HP/UHP terrane has been partitioned largely into the more ductile quartzo-feldspathic gneisses. This resulted in an essentially pervasive metamorphic reworking of the gneisses and the replacement of any earlier HP/UHP mineralogies by retrograde amphibolite facies assemblages. By contrast, the HP/UHP mineralogies have had much better survival rates in the more structurally competent mafic and ultramafic rocks. Hence although pervasive tectonism has undoubtedly been responsible for small-scale relative movements between eclogites and country rocks, it is unlikely to provide an adequate explanation for the large-scale relative motions required to produce the striking contrasts in metamorphic grade between the eclogite pods and the encompassing gneisses observed throughout the WGR. Rather, these contrasts result from the variable efficiency with which the HP or UHP parageneses were overprinted.

Wain (1997), Krabbendam & Wain (1997) and Wain et al. (2000) have provided crucial evidence in support for this differential retrogression interpretation, through their demonstration that small volumes of schists and gneisses in low strain zones immediately adjacent to certain UHP eclogite occurrences on Stadlandet have partly preserved UHP mineralogies, including petrographic evidence of previous coesite stability.

Within the WGR, pelitic paragneisses are relatively scarce but where they do occur, as in outer Nordfjord and on Fjørtoft, they do provide some good petrographic evidence that they experienced HPM or UHPM conditions, even despite their propensity and susceptibility to retrogression. Indications are that they contained Pmax assemblages of garnet + phengite ± kyanite ± zoisite ± omphacite + rutile + quartz/coesite. Only in rare instances, such as in the UHPM schists along the shore outcrops at Vetrhuset on Nordpollen (Fig. 3), has petrographic evidence been preserved for the previous stability of coesite.

Fig. 3.

Map for the Outer Nordfjord and Stadlandet area of the Western Gneiss Region showing the relative distributions of documented occurrences of UHP (coesite-stable) and HP (quartz-stable) eclogites relative to the HP-UHP Transition Zone boundaries (solid lines) indicated by Wain et al. (2000) and the modified boundaries (broken lines) proposed in this paper.

Fig. 3.

Map for the Outer Nordfjord and Stadlandet area of the Western Gneiss Region showing the relative distributions of documented occurrences of UHP (coesite-stable) and HP (quartz-stable) eclogites relative to the HP-UHP Transition Zone boundaries (solid lines) indicated by Wain et al. (2000) and the modified boundaries (broken lines) proposed in this paper.

By comparison, finding evidence that the voluminous Proterozoic acid–intermediate orthogneisses (Lappin et al., 1979; Harvey, 1983) that outcrop extensively within the WGR also experienced HPM or UHPM conditions is a much greater challenge. Unequivocal proof that this was the case may, as with comparable rocks the Dabieshan–Sulu UHPM terrane in central China (Ye et al., 2000; Liu et al., 2002), ultimately depend on searching for preserved micro-inclusions of coesite or other HP/UHP mineral phases, within zircon mineral separates.

Given the previously emphasised indications of extreme mineral metastability in metadolerite and metagabbro bodies within the WGR, metastability should also be seriously considered as an additional or alternative explanation for the apparent lack of HP or UHP mineralogies in the orthogneisses. It is feasible that such rocks may have witnessed the HPM or UHPM conditions but responded in only a limited and incomplete manner.

Some little deformed granitic rocks both on Vagsøy in outer Nordfjord and further north in the Moldefjord region (Carswell & Harvey, 1985) do show limited coronitic development of garnet. Intrusive acid igneous rocks have low inherent H2o contents bound into small amounts of micas and/or hornblende. During prograde, subduction-related HPM or UHPM this limited H2O content is likely to become locked into newly formed phengite and zoisite, minerals that can demonstrably remain stable to UHPM conditions. Thus further reactivity may be inhibited in such rocks unless deformation provides channels for fluid ingress. The fact that many of the orthogneisses are observed to retain porphyroclasts of unmixed high-temperature feldspars is a strong pointer to frequent metastability in these rocks under HPM or UHPM conditions. More intermediate composition meta-igneous rocks, such as the mangerite at Flatraket (Krabbendam et al., 2000; Wain et al., 2001), show more extensive but still incomplete reactivity, again with residual plagioclase metastability.

The HPM to UHPM transition

Wain (1997) and Wain et al. (2000) observed that coesite or PCQ-bearing UHP eclogites apparently had a set of petrographic characteristics that were distinct from eclogites lacking any evidence for coesite (interpreted as quartz-stable HP eclogites). UHP eclogites were reported to generally contain xenoblastic garnets that lack clear prograde compositional zoning and contain only eclogite facies solid inclusion suites, including coesite or PCQ. HP eclogites commonly contain idioblastic garnet, the larger grains of which enclose monocrystalline quartz inclusions throughout, exhibit strong prograde compositional zoning and have amphibolite facies inclusion suites in their cores. This latter type is exemplified by the eclogites at Verpeneset, Almenningen and Kroken along the northern side of Nordfjord (Fig. 3), so we name this petrographic group the “VAK-type” eclogites.

A significant number of eclogites have ambiguous petrographic characteristics, such as those lacking prograde zoned garnet with inclusions of early amphiboles in cores but also lacking evidence of coesite stability. UHP eclogites first appear along the north shore of Nordfjord (Fig. 3) and persist northwards, being particularly in evidence around the southern part of the Stadlandet peninsula. The region stretching south from Nordfjord to Sognfjord exposes only VAK-type, HP eclogites (Krogh & Carswell, 1995; Cuthbert et al., 2000).

An important observation, arising from the work of Wain (1997) and Wain et al. (2000), was that HP-type eclogites persist for a distance of about 10 km north of the coesite-in line (southern solid line in Fig. 3). Accordingly, they defined a mixed, or transitional, HP/UHP zone whose northern boundary stretched from across the southern end of the Stadlandet peninsula westwards towards Nordpollen (northern solid line on Fig. 3). Within this transition zone, Wain et al. (2000) described the discretely separate HP and UHP eclogite bodies as occurring up to a minimum of 100 metres from each other.

Thermobarometric evaluation of mineral reaction equilibria, in particular for phengite-bearing eclogite samples (Wain, 1997; Cuthbert et al., 2000), has for the most part corroborated the barometric distinctions between HP and UHP eclogite samples deduced from petrographic criteria. Even allowing for generous error brackets, significant non-lithostatic apparent pressure gradients have been recorded between adjacent HP and UHP eclogites (Wain et al., 2000; Cuthbert et al., 2000). Wain (1997), Krabbendam & Wain (1997) and Wain et al. (2000) thus attributed the close juxtapositioning of HP and UHP eclogites within the transition zone to the tectonic interleaving of different structural units. Accordingly it was assumed that the lithotectonic unit containing the UHP eclogites was carried to a higher lithospheric level and emplaced against a different lithotectonic unit containing only HP eclogites.

In our opinion, a number of difficulties arise with this tectonic mixing interpretation of the HP-UHP transition zone in the Outer Nordfjord area. Firstly, no obvious zones of higher strain are observed between outcrops of HP and UHP eclogites within the transition zone, nor is this zone as a whole characterised by higher strains than in the rocks on either side. It is, of course, possible that the pervasive, late, exhumation-related amphibolite facies deformation and recrystallisation in the WGR (Andersen et al., 1991; Krabbendam & Wain, 1997) has obliterated fabrics associated with such shear zones. Indeed some relative movement between adjacent eclogite bodies must have happened during development of these late fabrics, but such fabrics are not confined to the transition zone, so it cannot be regarded as a distinct displacement horizon during the later stages of exhumation.

Belts of metasediments associated with meta-anorthosite, at least some of which are likely to be allochthonous, lie across the area north of Nordfjord within the predominant orthogneissic basement, (Bryhni, 1989; see Fig. 3). UHP eclogites are found in both paragneiss and orthogneiss units, and both types of eclogite can be found in the same lithological unit (Cuthbert et al., 2000). Hence we have found that HP or UHP eclogites cannot be exclusively assigned to particular lithotectonic units within the transition zone.

Our second reservation concerning the tectonic mixing interpretation for the transition zone arises from the difficulty in unambiguously identifying the HP and UHP rocks. Identification of some HP eclogites effectively by default, based upon a lack evidence of coesite or PCQ, is unreliable due to the poor preservation potential of both coesite and its delicate PCQ replacement textures. Coesite is, in fact, frustratingly rarely preserved in the WGR compared to other UHP terrains such as the Dabie Shan of central China (Carswell & Zhang, 1999) but it is always possible that a single observation of coesite or PCQ in a prograde-zoned “VAK-type” (apparently HP) garnet will render the other petrographic criteria for identification of a HP eclogite invalid. Examples of prograde-zoned garnets with inclusions of coesite are certainly known from other UHP belts, such as in the Kokchetav Massif of Kazakhstan (Parkinson, 2000).

The Årsheimneset UHP eclogite (Fig. 3) is known to exhibit prograde-zoned garnets with amphibole-rich inclusion suites (Carswell et al., 1985), but the same eclogite body also contains good palisade-textured PCQ (Smith, 1988; Cuthbert et al., 2000). Hence here a single body of eclogite clearly shows petrographic characteristics of both of Wain's (1997) UHP and HP types, and potentially records development from amphibolite facies, through HP eclogite facies to UHPM. Clearly, then, a further weakness of the discriminatory HP and UHP eclogite classification of Wain (1997) is the assertion that eclogites in the transition zone are each exclusively HP or UHP in character. Clearly if both types are found together in the same body, then it is difficult to envisage how they could have been brought together tectonically.

In the light of these difficulties with the tectonic juxtapositioning interpretation, we have undertaken a detailed examination of certain eclogite bodies within the transitional HP/UHP zone and in the adjacent HP and UHP zones of the Nordfjord–Stadlandet region.

A key locality is at Vetrhuset on the eastern shore of Nordpollen (Fig. 3), close to the northern margin of the transition zone and recognised as an UHP, coesite-bearing eclogite by Wain et al. (2000). Here, a swarm of eclogite pods lies within a belt of semi-pelitic schists. PCQ inclusions in both garnet and omphacite are quite common in these pods, in addition to much rarer, actual preserved, coesite. However, large, subidioblastic garnets are frequently, clearly, compositionally zoned with Fe, Mn and Ca-enriched cores and Mg-enriched rims (CD Image 18). The largest of these prograde-zoned grains contain concentrations of hornblende inclusions in the cores. Palisade-textured PCQ inclusions tend to be found in more xenoblastic garnets that may lie only a few millimetres from the zoned garnet grains. The enclosing semi-pelitic, phengite-kyanite-quartz schists contain distinctive, purplish red garnets up to 5 cm in diameter displaying conspicuous prograde compositional zoning. Rarely, PCQ inclusions are found in the narrow, Ca and Mg-enriched garnet rims.

Hence it is now apparent that intimately associated individual eclogite bodies and their host semi-pelitic schists may display the characteristics of both HP “VAK-type” and UHP garnets. The Vetrhuset eclogites display a range of deformation fabrics: coarser-grained eclogites with a weak linear omphacite-shape fabric tend to contain the prograde zoned “VAK-type” garnets, while more strongly lineated eclogites show PCQ inclusions in omphacite and in later grown or dynamically recrystallised garnet, especially in garnet-quartz streaks. Hence our observations at this location within the HP/UHP transition zone have revealed evidence for incomplete transformation from lower P metabasic and semi-pelitic rocks to UHP parageneses, leading to partial preservation of certain petrographic characteristics that are typical of HP eclogites. Coesite crystallisation appears to have been associated with a distinct, later stage of garnet growth that either mantles the earlier amphibolite facies and HP eclogite facies garnet, or with the recrystallisation of earlier garnet and omphacite during deformation.

The coarse-grained, UHP, coesite-kyanite eclogite at Flatraket harbour (Wain et al., 2000) shows similar characteristics to the coarser-grained eclogites at Vetrhuset, with subidioblastic garnets having darker red cores that contain abundant hornblende inclusions (CD Image 19). Rare PCQ inclusions are found within the paler coloured rims of these garnets, so this eclogite clearly demonstrates evolution from amphibolite facies through HP to UHP eclogite facies. Parts of this eclogite body have spongy-textured, highly poikiloblastic garnets with only omphacite and kyanite inclusions, showing more complete recrystallisation to the UHPM paragenesis, perhaps because the original amphibole inclusions were less well-armoured within the garnets and hence more prone to decomposition. Hence the prior textural development of this eclogite seems to have had an important control on the efficiency of UHP recrystallisation in this case. Zircons from this body give TIMS U–Pb age spectra with peaks at ca. 405 Ma and ca. 400 Ma (Hacker et al., 2001). These ages may correspond to the HPM then UHPM growth episodes of garnet (Carswell et al., 2003a).

As discussed above, the Årsheimneset eclogite (Fig. 3) also contains characteristics typical of the HP “VAK-type” eclogites, yet this body outcrops about 5 km into the UHP zone/province as recognised by Wain et al. (2000). An important observation here (CD Image 20) is that the central part of this body comprises mainly bimineralic (orthopyroxene-free) eclogite whereas adjacent to the top and bottom contacts with the country rock gneiss, the main eclogite facies rock type is an orthopyroxene-, phlogopite-and magnesite-bearing eclogite that becomes distinctly pegmatitic towards the lower margin, and is spatially associated with veins and lenses of phlogopite and/or phengite-bearing glimmerites. The central, bimineralic eclogite frequently contains conspicuous, up to cm-sized, prograde zoned garnets with cores containing abundant inclusions of dark blue-green amphibole (CD Image 17). The coarser-grained orthopyroxene-bearing eclogite (CD Image 21) exhibits more irregularly-shaped, large garnets comprising early darker cores with blue-green amphibole inclusions, extensively overgrown by later UHP garnet containing PCQ inclusions after coesite, as well as frequent inclusions of clinopyroxene and phlogopite, indicating that the latter was a stable UHPM phase. Hence the Årsheimneset eclogite is another example that shows evolution of a single eclogite body from an early HP (quartz-stable), essentially bimineralic, eclogite, to a more siliceous, orthopyroxene- and phlogopite-bearing, eclogite in which substantial new growth of garnet, orthopyroxene and clinopyroxene took place under UHPM conditions and consequently trapped coesite inclusions. Thus, in this case, the UHP mineral growth is thought to have occurred in response to a substantial influx of metasomatising fluid from the enclosing continental crust gneisses. The transformation of the early, essentially bimineralic, HP eclogite into a coarser-grained, even pegmatitic, UHP eclogite can be followed along hydraulic fractures penetrating the former. Once again, field relationships give no support to an interpretation of tectonic mixing of HP and UHP eclogites as proposed by Krabbendam & Wain (1997) and Wain et al. (2000).

U–Pb zircon data of Gebauer et al. (1985) for an orthopyroxene- and phlogopite-bearing eclogite sample from the Årsheimneset body (labelled SAN-1 from east of “Sandviknaes”) indicate metamorphic zircon growth at ca. 400 Ma, supporting the argument for UHPM at approximately that time (Carswell et al., 2003a – see below).

The evidence from the Vetrhuset, Flatraket and Årsheimneset eclogite bodies demonstrates that individual eclogites do not necessarily exhibit uniquely “HP” or “UHP” characteristics. This leads us to question the value of using prograde zoning and an apparent absence of PCQ or coesite inclusions in garnet to indicate that the rock has lacked a UHPM history. These examples demonstrate an evolution from amphibolite facies parageneses through HP to UHP eclogite during which the efficiency of the metamorphic transformation was limited. Furthermore, they indicate that a number of processes drove the transformation. The clearest manifestation of the transition to UHP parageneses in both eclogites and semi-pelitic schists is the development of new garnet, by overgrowth, recrystallisation and/or neoblast formation. Garnet growth appears to have been the result of a discontinuous series of reactions amongst its matrix phases. The temporally distinct stages of garnet growth were prompted by distinct deformation events and/or triggered by influxes of externally derived fluids. In the light of these observations it is instructive to also examine some other eclogite bodies that occur within and outside the HP/UHP transition zone.

The spectacularly layered coesite eclogite (CD Image 22) at Saltaneset (Fig. 3) first recognised by Wain (1997) lies in the UHP zone, some 5 km north of the northern boundary of the HP/UHP transition zone as defined by Wain (1997) and Wain et al. (2000). Carswell et al. (2003a) recognised within this eclogite body two generations of garnet growth — aggregates of deeper red, Ca-rich grains characterised by concentrations of tiny rutile needles in their cores, and overgrowths or discrete neoblasts of Ca-poorer and Mg-richer garnet. In conspicuous quartz-garnet layers, the Mg-rich garnet mantles aggregates of the earlier, Ca rich type, or exists as discrete, compositionally homogeneous grains (CD Image 23) containing remarkably abundant inclusions of PCQ after coesite, indicating that these layers were originally garnet coesitite rock. Such layers, or veins, are common in eclogites in the WGR and appear to be metamorphic segregations associated with fracturing and the infiltration of aqueous fluids. In the case of the Saltaneset eclogite the formation of the garnet-coesite layers is clearly associated with growth of the second generation, UHP garnet, so the HPM–UHPM transformation was aided by ingress of an aqueous fluid. Carefully extracted UHP garnet, omphacite plus whole rock from a sample of a garnet coesitite vein have yielded a Sm–Nd isochron age of 408.3 ± 6.7 Ma (Carswell et al, 2003a).

At Flister, a swarm of eclogite pods within semi-pelitic schists and interlayers of meta-anorthosite lies close to the extreme northern edge of Wain’s (1997) transition zone (Fig. 3). Wain et al. (2000) described a typical HP eclogite at this locality. Cuthbert et al. (2000) subsequently reported the discovery of relict coesite at Flister, but it is not clear if this is the same pod as that discovered by Wain et al. (2000). The coesite-bearing body is a flaser-textured eclogite in which granular streaks of paler garnet are sometimes cored by deeper red porphyroclasts, indicating the break-up, recrystallisation and compositional adjustment of an earlier generation of amphibolite facies to HP eclogite facies garnet. Coesite or PCQ has been found only in the late-formed garnet, again indicating a distinct episode of garnet growth or recrystallisation under UHPM conditions, in this case clearly aided by deformation. Such flaser-textured eclogites form a distinctive textural type in the Nordfjord–Stadlandet area. Intriguingly, we have recently discovered the comparable flaser-textured eclogite at Verpeneset, Nordfjord (Fig. 3), that is separated by only a narrow screen of gneiss from what is one of the archetypal HP “VAK-type” eclogites, to also contain PCQ inclusions in neoblastic garnet. This pushes the southern limit of recognised UHP eclogites further south than that shown in Cuthbert et al. (2000). Wain et al., 2000 did not actually extend their boundary as far west as Verpeneset.

It is clear that the location of both our modified coesite-in line and the northern limit of identified, pre-UHPM, HP eclogite relics (dashed grey lines in Fig. 3) remain only provisional until further detailed petrographic and thermobarometric studies are carried out. Nevertheless, it is evident that tectonic juxtapositioning of HP and UHP eclogites is much too simplistic a view, and we would argue that it played no more than a very minor role during the subduction and early exhumation phases of the Scandian orogenic event. Instead, kinetic factors, dictated to a great extent by deformation and fluid activity, controlled the efficiency of transformation of HP and pre-HP parageneses to UHP eclogite facies parageneses as the descending continental slab passed into the stability conditions of coesite, and beyond the stability of amphibole, leading to apparent intermingling of HP and UHP eclogites (and schists) on all scales from millimetres to kilometres. The increase in modal garnet associated with the HP-UHP transition can be expected to have modified the petrophysical properties of the WGR rocks, such as their density and rheology.

The general east-west trend of the coesite-in line (Figure 3) is apparently discordant to the trend of the eclogite isotherms shown in Figure 2. The significance of this is presently unclear, but is likely to be at least partly an artifact of the rather sparse geothermometry upon which the isotherms are based. A more detailed thermobarometric survey will be required to better resolve the regional variation in P and T.

The age of the UHPM

It was widely assumed that the eclogite facies metamorphism in the WGR was Precambrian in age (e.g. Krogh, 1977) prior to publication by Griffin & Brueckner (1980, 1985) of Caledonian Sm-Nd ages for garnet-omphacite pairs from five eclogite samples. Subsequently the consensus view was that the HPM/UHPM event occurred at ca. 425 Ma, this corresponding to the mean Sm-Nd age obtained for these eclogite samples. For example, this age was assumed in the tectonic models for the stabilisation and exhumation for these HPM/UHPM rocks presented by Andersen et al. (1991), Cuthbert & Carswell (1990) and Dewey et al. (1993).

The Sm-Nd mineral ages obtained by Griffin & Brueckner (1980, 1985) did not specifically discriminate between the timing of HPM and UHPM conditions in different parts of the WGR. Based mostly on this dataset, Smith (1988) subsequently speculated that the UHPM rocks formed at ca. 440 Ma and the HPM rocks at ca. 410 Ma, but the limited sample set and large uncertainties in the ages make this difficult to substantiate. Moreover, as documented in the preceding section, our recent petrographic observations on the relative timing of the HPM and UHPM mineralogies strongly point to the latter having been stabilised later rather than earlier than the HPM assemblages. Furthermore, as emphasised in the UHPM timing review paper by Carswell et al. (2003a), recently published U-Pb ages for zircons (Hacker et al., 2001; Carswell et al., 2003b) and for monazites (Terry et al., 2000a) in specifically identified UHPM rocks mostly fall within the rather later 400–410 Ma timeframe.

Compelling evidence that the UHP mineralogies may indeed have formed at close to 400 Ma is provided by the 402 ± 2 Ma U–Pb age obtained for metamorphic zircons from the Hareidland eclogite shown to contain micro-inclusions of UHP minerals, including coesite (Carswell et al., 2003b). In addition, Carswell et al. (2003a) have reported a statistically indistinguishable Sm-Nd garnet-omphacite-whole rock isochron age of 408.3 ± 6.7 Ma for an eclogite sample from Salta (see Fig. 1) that contains abundant petrographic evidence of previous coesite stability (e.g. CD Image 4).

The conclusion that UHPM rocks in the WGR formed at close to 400 Ma, much later in the Scandian phase of the Caledonian orogenic cycle than was previously taken to be the case, has profound implications for the dynamic modelling of this continental plate collision belt and certainly signals extremely rapid exhumation of these UHPM rocks. Hence, Carswell et al. (2003a) have concluded that they were probably exhumed to ca. 35 km depth at a mean rate of ca. 1 cm per year. They suggested that this rapid initial exhumation could well have been driven by residual bouyancy of the deeply subducted crustal slab that resulted from incomplete eclogitisation. They particularly emphasised the survival of metastable, plagioclase-bearing assemblages in the dominant orthogneisses due to limited reactivity and the probable short duration of the UHPM event. Further exhumation to about 8–10 km apparently occurred at a much slower mean rate of ca. 1.3 mm per year with the final unroofing of the UHPM rocks attributable to the late-stage extension collapse of the Caledonian orogen (e.g. Andersen et al. 1991).

The occurrence and interpretation of garnet peridotites

Occurrences of peridotite bodies of metre to kilometre-scale dimensions are a conspicuous feature of the WGR (Fig. 4). Chlorite-amphibole peridotites are most common (CD Image 24) and only a few bodies retain the pyropic garnet-olivine assemblage that is diagnostic of the stabilisation of HP/UHP eclogite facies assemblages (CD Image 25). Nonetheless, petrographic features, including the observed retrograde transitions from granoblastic garnet-bearing peridotite into foliated chlorite-amphibole peridotite within certain bodies, suggest that at least some of the designated chlorite peridotites in Figure 4 may have originally contained pyropic garnets.

Fig. 4.

Map of the principal occurrences of garnet-bearing and chlorite-bearing (garnet-absent) peridotite bodies within the Western Gneiss Region of Norway.

Fig. 4.

Map of the principal occurrences of garnet-bearing and chlorite-bearing (garnet-absent) peridotite bodies within the Western Gneiss Region of Norway.

Carswell et al. (1983) drew attention to the existence of two fundamentally different chemical types of garnet-bearing peridotite bodies in the WGR. The Fe-Ti type garnet peridotites were interpreted to have had a prograde origin from the ultramafic portions of layered low pressure crustal intrusive bodies during Caledonian subduction-related HPM/UHPM. The Eiksunddal eclogite complex on Hareidlandet with its subordinate garnet peridotite layers, well documented by Jamtveit (1987), is an excellent example of this garnet peridotite type. Jamtveit et al. (1991) were unable to obtain a meaningful Sm–Nd age for a garnet peridotite sample from the Eiksunddal complex, due to isotopic disequilibrium between the garnet, clinopyroxene and whole rock. However, they did obtain a Scandian age of 412 ± 12 Ma for coexisting garnet and clinopyroxene from a well-foliated eclogite sample from the same complex.

By contrast, the Mg-Cr type garnet peridotites display characteristic upper mantle mineral and whole rock chemistries (Carswell, 1968) and isotopic ratios (Brueckner, 1977), consistent with derivation from mostly highly depleted sub-continental lithospheric mantle. Medaris (1984), Carswell (1986) and Medaris & Carswell (1990) have documented the complex, multistage metamorphic evolution experienced by these particular peridotite bodies. Carswell (1986) and Medaris & Carswell (1990) concluded that the earliest assemblage contained high temperature pyroxenes that, on the basis of scarce petrographic evidence, probably coexisted with spinel rather than with the oldest generation garnet.

Sm–Nd dating of garnet + clinopyroxene + whole rock in samples of Mg-Cr type garnet peridotite and associated olivine-free garnet pyroxenites by Mearns (1986) and Jamtveit et al. (1991) has importantly shown the earliest garnet-bearing assemblages in these mantle-derived peridotite bodies to be of mid-Proterozoic age and consequently to have been stabilised in the sub-continental mantle long before the Scandian, subduction-related, HPM/UHPM event.

An even more surprising discovery has been the recognition by van Roermund & Drury (1998) and van Roermund et al. (2000, 2001) of pyroxene exsolution lamellae (CD Image 26) in megacrysts of the earliest garnet generation (CD Image 27) in samples from the Mg-Cr type peridotite bodies outcropping on Otrøy. This feature has been taken to indicate that these garnets originally contained a significant majorite component. Van Roermund et al. (2000) used their deduced initial composition of this super-silicic majoritic garnet to estimate minimum pressures, at high temperature, of 6–6.5 GPa – interpreted (see also Drury et al., 2001) to indicate derivation within a rising mantle diapir originating from depths of at least 185 km. Subsequently, similar exsolution microstructures in garnets were discovered by Terry et al. (1999) in garnet peridotite/websterite bodies on Fjørtoft and Flemsøy (Fig. 4).

Further petrographic observations and Sm–Nd isotopic data (Brueckner et al., 2002; van Roermund et al., 2002) on an occurrence of megacrystic garnets, with possible majoritic garnet microstructures, (CD Image 28) within a small peridotite body at Bardane (CD Image 29) on the island of Fjørtoft (Fig. 4), have confirmed that these early garnets formed during the Gothian orogeny at around 1600–1700 Ma and moreover coexisted with high-temperature megacrystic aluminous pyroxenes (CD Image 30). Brueckner et al. (2002) thus considered that the earliest recognisable assemblage in the Mg-Cr type peridotite bodies at Bardane on Fjørtoft and at Ugelvik on otrøy formed at conditions of ca. 3.0–4.5 GPa and 1300–1500 °C in the mid-Proterozoic. Accordingly, the metamorphic evolution of these peridotites as outlined by Carswell (1986) and Medaris & Carswell (1990) must be modified to eliminate the previous indication that the oldest assemblage formed at high temperature but relatively low pressure, with aluminous spinel rather than garnet coexisting with the aluminous pyroxenes.

A revised, multistage metamorphic evolution for these Mg-Cr type peridotite bodies, that indicates instead the early stability of a majoritic garnet formed at UHP conditions deep in the mantle, is presented in Figure 5. Full details of the Proterozoic history and location of these peridotite bodies remain somewhat conjectural given the scarcity of preservation of stage 1 assemblages due to widespread overprinting by much later Palaeozoic/Caledonian assemblages. It seems likely that the stage 1 high P/T majoritic garnet-bearing assemblages were mostly recrystallised and re-equilibrated to stage 2 assemblages, with more normal Cr-pyrope garnets, as these rocks were uplifted (perhaps within a rising mantle diapir) and cooled to the ambient Proterozoic geotherm.

Fig. 5.

Revised outline for the deduced multi-stage metamorphic evolution of the mantle-derived, Mg-Cr type, garnet-bearing peridotite bodies in the Western Gneiss Region of Norway.

Fig. 5.

Revised outline for the deduced multi-stage metamorphic evolution of the mantle-derived, Mg-Cr type, garnet-bearing peridotite bodies in the Western Gneiss Region of Norway.

These mantle-derived, Mg-Cr type, garnet-bearing peridotite bodies in the WGR have been widely interpreted (e.g.. Medaris & Carswell, 1990; Krogh & Carswell, 1995) to have been tectonically emplaced into the subducted slab of continental crust during the Scandian plate collision. Brueckner (1998) and Brueckner & Medaris (2000) have presented new dynamic models for crust–mantle interaction in major continental plate collision zones and emphasised that different locations and timings are possible for the tectonic emplacement of the mantle peridotites into the crustal slab either during subduction or subsequent exhumation.

A further dramatic new discovery with an important bearing on this issue and on the P–T–t evolutionary history experienced by these peridotites has been the discovery of microdiamonds within the Bardane peridotite occurrence on Fjørtoft (van Roermund et al., 2002). Rather surprisingly these microdiamonds apparently did not form in association with the UHP majoritic garnet during the mid-Proterozoic. Instead, combined petrographic, geochemical and isotopic data (Brueckner et al., 2002; van Roermund et al., 2002) provide strong pointers to the microdiamonds having formed in response to an influx of crustal-derived fluids attendant on Scandian-aged deformation and the recrystallisation of the oldest generation Proterozoic assemblage with its megacrystic majoritic garnets and aluminous pyroxenes.

To date, preserved microdiamonds have only been discovered armoured within Cr-spinel grains that are in turn enclosed within later generation garnet that forms a corona network (CD Image 31) around deformed sub-grains of the original orthopyroxene megacrysts (van Roermund et al., 2002). Further ‘secondary’ generation garnet occurring as rational exsolution lamellae within the deformed orthopyroxene megacrysts is seen both at Bardane and in the comparable deformed and partly recrystallised megacrystal orthopyroxenite lens at Ugelvik on Otrøy (Carswell, 1973). It is difficult to extract an unquestionably pure separate of this later generation garnet. Accordingly, the rather equivocal Sm-Nd ages of 518 ± 78 Ma (Brueckner et al., 2002) and of ca. 561 Ma (Jamtveit et al., 1991) obtained for the exsolved garnet + clinopyroxene assemblages at Bardane and Ugelvik, respectively, are probably “mixed” ages but nonetheless signal the strong influence of Caledonian recrystallisation and isotopic re-equilibration. This latter interpetation is further supported by the Sm-Nd age of 437 ± 58 Ma obtained for garnet + clinopyroxene + whole rock by Jamtveit et al. (1991) from a thoroughly recrystallised, granoblastic textured, garnet pyroxenite sample from the Raudhaugene peridotite body on Otrøy.

The deduction that the microdiamond-bearing assemblage within the Bardane peridotite body was probably stabilised at P-T conditions of around 3.4-4.1 GPa and 840-900 °C (Brueckner et al., 2002; van Roermund et al., 2002) during the subduction-related Scandian-aged UHPM requires that this peridotite body must have been emplaced into the enclosing slab of continental crust either before or during its transient deep subduction. This conclusion is consistent with the nearby occurrence of a coesite-bearing kyanite-phengite eclogite within the host quartzo-feldspathic gneisses (Terry et al., 2000b) and also of microdiamond-bearing garnet-kyanite-biotite-rutile-quartz gneisses that outcrop essentially along strike on the north coast of Fjørtoft (Dobrzhinetskaya et al., 1993, 1995).

This surprising new interpretation that mantle-derived garnet-bearing peridotite bodies of the Mg-Cr type, similarly to the contrasting Fe-Ti type, also experienced Scandian, subduction-related, HPM/UHPM is further supported by certain previously published but largely overlooked observations in other garnet-bearing peridotite bodies of the Mg-Cr type within the WGR.

Griffin & Qvale (1985) documented the fact that Fe-rich eclogite lenses within the Mg-Cr type peridotite body at Raudkleivane in Almklovdalen contain garnets that show strong prograde compositional zoning and also contain ferropargasite inclusions in grain cores. This signals that this peridotite body must have been entrained into the subducting crustal slab and hence experienced a Scandian prograde metamorphic evolution from amphibolite to eclogite facies. Secondly, petrographic evidence has been recorded both in the peridotite body at Lien in Almklovdalen (Griffin & Heier, 1973) and in the Sandvika body (CD Images 32&33) on Gurskøy (Carswell et al., 1983) for the growth of new garnet in the outer parts of kelyphites composed of secondary aluminous pyroxenes and spinel around older garnet porphyroclasts. Griffin & Heier (1973) concluded that the apparent reversal of the kelyphite-forming reaction garnet + olivine → orthopyroxene + clinopyroxene + spinel was probably attributable to post-decompressional cooling but, in view of other observations, it is perhaps more appropriate to now attribute it to a later, post-kelyphite, prograde subduction-related HPM/UHPM event.

Accordingly, the P–T evolution of the WGR garnet peridotites shown in Figure 5 incorporates the possibility of the existence of two separate granulite facies kelyphite-forming events, with the older one placed between the two pyropic garnet-bearing stages 2 and 4. It is conceivable that the older kelyphite-forming event occurred during the Sveconorwegian (ca. 1000–1100 Ma) orogeny, based on Sm–Nd systematics of zoned garnets from the Almklovdalen and Sandvika peridotite bodies (Brueckner et al., 1996).

The long time interval between stages 2 and 4 creates uncertainly over exactly where these mantle-derived garnet-bearing peridotites resided. They conceivably may have already by this stage been emplaced in the Proterozoic continental crust. However, it is considered more likely that they resided in the uppermost sub-continental lithospheric mantle and were not incorporated into the crust until the deep subduction of the WGR during the Caledonian plate collision.

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Acknowledgements

The authors wish to acknowledge the support for recent studies of HP and UHP rocks in the WGR of Norway provided by the Norwegian Research Council, the Norwegian Geological Survey, the British Council, the EU “Access to Research Infrastructures” Programme, the Carnegie Trust for Scottish Universities and the Universities of Sheffield and Paisley. The section on the WGR garnet peridotites has benefited from helpful reviews by Herman van Roermund, Hannes Brueckner and Gordon Medaris.

Figures & Tables

Fig. 1.

A generalised geological map for the Western Gneiss Region between the Sognefjord and Moldefjord areas showing the respective distributions of documented occurrences of UHP (coesite-bearing) eclogites, UHP diamond gneiss and peridotite bodies.

Fig. 1.

A generalised geological map for the Western Gneiss Region between the Sognefjord and Moldefjord areas showing the respective distributions of documented occurrences of UHP (coesite-bearing) eclogites, UHP diamond gneiss and peridotite bodies.

Fig. 2.

Regional temperature gradient across the Western Gneiss Region of Norway based on Fe2+/Mg2+ partitioning between garnet and omphacite in eclogites (after Griffin et al., 1985) plus indications of the concomitant pressure gradient based on P–T estimates for eclogite samples from various localities indicated by Cuthbert et al. (2000), Carswell et al. (2003a) and Terry et al. (2000b).

Fig. 2.

Regional temperature gradient across the Western Gneiss Region of Norway based on Fe2+/Mg2+ partitioning between garnet and omphacite in eclogites (after Griffin et al., 1985) plus indications of the concomitant pressure gradient based on P–T estimates for eclogite samples from various localities indicated by Cuthbert et al. (2000), Carswell et al. (2003a) and Terry et al. (2000b).

Fig. 3.

Map for the Outer Nordfjord and Stadlandet area of the Western Gneiss Region showing the relative distributions of documented occurrences of UHP (coesite-stable) and HP (quartz-stable) eclogites relative to the HP-UHP Transition Zone boundaries (solid lines) indicated by Wain et al. (2000) and the modified boundaries (broken lines) proposed in this paper.

Fig. 3.

Map for the Outer Nordfjord and Stadlandet area of the Western Gneiss Region showing the relative distributions of documented occurrences of UHP (coesite-stable) and HP (quartz-stable) eclogites relative to the HP-UHP Transition Zone boundaries (solid lines) indicated by Wain et al. (2000) and the modified boundaries (broken lines) proposed in this paper.

Fig. 4.

Map of the principal occurrences of garnet-bearing and chlorite-bearing (garnet-absent) peridotite bodies within the Western Gneiss Region of Norway.

Fig. 4.

Map of the principal occurrences of garnet-bearing and chlorite-bearing (garnet-absent) peridotite bodies within the Western Gneiss Region of Norway.

Fig. 5.

Revised outline for the deduced multi-stage metamorphic evolution of the mantle-derived, Mg-Cr type, garnet-bearing peridotite bodies in the Western Gneiss Region of Norway.

Fig. 5.

Revised outline for the deduced multi-stage metamorphic evolution of the mantle-derived, Mg-Cr type, garnet-bearing peridotite bodies in the Western Gneiss Region of Norway.

Contents

GeoRef

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