Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides

The Western Gneiss Region (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 Scandian (late Palaeozoic) metamorphic and structural imprint. Scandian-aged eclogites have been widely (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 coesite (high pressure 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 eclogites (ultrahigh pressure 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 HP/UHP 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 (P) and temperature (T) across the WGR. Overall, a scenario emerges where P and T progressively increase from 500°C and 16 kbar in Sunnfjord to >800°C 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 5°C/km, 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 HP/UHP 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 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 (Proterozoic lithospheric mantle?) porphyroclast assemblages and Scandian (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.