Eclogite Formation Research Articles

Eclogites and their geodynamic interpretation: a history

Haüy coined the term eclogite, meaning “chosen rock”, in 1822, but de Saussure had already observed rocks of this type in the Alps four decades earlier. Throughout the 19th century, the origin of eclogite remained an enigma, in spite of great progress in our knowledge of this rock. The first chemical analyses, carried out around 1870, showed that its bulk composition was the same as gabbro. Therefore, eclogite was thought to be either an igneous rock of gabbroic composition or a metamorphosed gabbro. This second hypothesis became preferred when progressive transitions were observed between gabbros and eclogites. In 1903, simply by comparing the molar volumes of gabbroic and eclogite parageneses, Becke inferred that eclogite was the high-pressure equivalent of gabbro. In 1920, eclogite was involved in the conception of the metamorphic facies by Eskola. However, a few researchers denied the existence of an eclogite facies, and claimed that high stress instead of high lithostatic pressure could generate eclogites. In the 1960s, consideration of the water pressure parameter also favoured the belief that eclogite was simply the anhydrous equivalent of amphibolite. Finally, eclogite was definitely considered as a high-pressure metamorphic rock following the development of experimental petrology and the application of thermodynamics. In recent years, the discovery of ultrahigh-pressure coesite-bearing rocks in the crust has drastically changed geologists' ideas concerning the limits of eclogite-facies crustal metamorphism. Eclogites have been involved in several geodynamic theories. Around 1900, kimberlite studies favoured the idea that eclogite might be abundant in the interior of the Earth. In 1912, Fermor predicted the existence of a dense eclogite-bearing zone in the mantle. This “eclogite layer” hypothesis was still envisaged as late as 1970. The alternative “peridotite” hypothesis became preferred when experimental investigations demonstrated that the gabbro-to-eclogite transition could not coincide with a sharp Mohorovičić discontinuity. Before plate tectonics, high-pressure belts were interpreted as remnants of ophiolite-bearing “geosynclines”, metamorphosed by loading during thrust faulting. After the acceptance of plate tectonics, around 1970, the same high-pressure Alpine-type belts came to be considered as former oceanic crust, transformed into eclogite within subduction zones, and subsequently incorporated into mountain belts. Surprisingly, formation of eclogite in “subsidence” zones (i.e. subduction zones) had already been envisaged as early as 1931 by Holmes, the inventor of a convection-current theory. In the 1980s, many authors tried to apply the model of Alpine-type high-pressure belts to eclogites enclosed within the gneisses of ancient orogens, but the question remains obscure nowadays. These eclogites have been involved in the “in situ versus foreign” controversy and in the unresolved enigma of ultrahigh-pressure metamorphism. The latter came under scrutiny in 1984 after the discovery of coesite and diamond in some eclogite-facies rocks. It has been a matter of considerable interest during the last two decades. Currently, the debate is focused on the geodynamic mechanisms responsible for the exhumation of these rocks, a question that will probably remain unresolved for part of the coming century.

A three-dimensional model of the Pyrenean deep structure based on gravity modelling, seismic images and petrological constraints

Summary The present study aims at building a 3-D density model of a whole collisional mountain belt at the lithospheric scale. The geometry of internal structures is constrained by seismic studies, while density values are imposed by petrological considerations. The model must fit both the Bouguer and geoid anomalies. This approach is applied to the Pyrenees, for which numerous geophysical data are available. The crustal structure is constrained from the results of seismic profiles. The most important crustal feature is the large Moho jump which coincides with the surface expression of the North Pyrenean Fault, considered as the suture between the Iberian and Eurasian plates. The Eurasian crust is about 30 km thick, whereas the Iberian crust is 50 km thick in the central part of the range. Thick sedimentary basins at the Piedmont, and two large heavy intracrustal bodies located in the western and central parts of the range, are also important features of the crustal model. At the lithospheric scale, a low-velocity strip under the North Pyrenean Fault down to 80–100 km depth, interpreted as the subduction of the Iberian lower crust, has been revealed by tomographic models. The density of this structure has been constrained using petrological arguments, which assume that the crustal material is transformed into eclogite at about 12–15 kbar (40–50 km depth). An eclogite-type density value, much higher than the mantle density, is thus imposed from 50 to 100 km depth. Theoretical Bouguer and geoid anomalies are computed for this 3-D density model using a summation of density block anomalies. A good agreement is found with observations, giving strong support to the proposed model. The crustal structure, as seen by seismic profiles, explains most of the observed long-wavelength negative Bouguer anomaly, as well the local anomalies related to the intracrustal bodies. A possible mantle origin of these bodies is ruled out. On the other hand, the transformation of the subducted lower crust into dense eclogite provides a successful fit to the long-wavelength positive geoid anomalies. In addition, the water released during eclogite formation may help to explain the high conductivity zone detected down to 80 km south of the North Pyrenean Fault. The 3-D model proposed here thus reconciles petrological considerations and geophysical data.

Formation and exhumation of blueschists and eclogites from NE Oman: new perspectives from Rb–Sr and 40Ar/39Ar dating

AbstractSeven eclogite facies samples from lithologically different units which structurally underlie the Semail ophiolite were dated by the 40Ar/39Ar and Rb–Sr methods. Despite extensive efforts, phengite dated by the 40Ar/39Ar method yielded saddle, hump or irregularly shaped spectra with uninterpretable isochrons. The total gas ages for the phengite ranged from 136 to 85 Ma. Clinopyroxene–phengite, epidote–phengite and whole‐rock–phengite Rb–Sr isochrons for the same samples yielded ages of 78 ± 2 Ma. We therefore conclude that the eclogite facies rocks cooled through 500 °C at c. 78 ± 2 Ma, and that the 40Ar/39Ar dates can only constrain maximum ages due to the occurrence of excess Ar inhomogeneously distributed in different sites.Our new results lead us to conclude that high‐pressure metamorphism of the Oman margin took place in the Late Cretaceous, contemporaneous with ophiolite emplacement. Previously published structural and petrological data lead us to suggest that this metamorphism resulted from intracontinental subduction and crustal thickening along a NE‐dipping zone. Choking of this subduction zone followed by ductile thinning of a crustal mass wedged between deeply subducted continental material and overthrust shelf and slope units facilitated the exhumation of the eclogite facies rocks from depths of c. 50 km to 10–15 km within c. 10 Ma, and led to their juxtaposition against overlying lower grade rocks. Final exhumation of all high‐pressure rocks was driven primarily by erosion and assisted by normal faulting in the upper plate.

Dating the Indian continental subduction and collisional thickening in the northwest Himalaya: Multichronology of the Tso Morari eclogites: Comment and Reply

[De Sigoyer et al. (2000)][1], studying Tso Morari eclogites in northern Ladakh (northwest Himalaya), claim to have “dated eclogite formation and therefore the timing of the onset of India—Asian contact in the northwest Himalaya at 55 ± 6 Ma.” Their ages (Lu-Hf 55 ± 12 Ma; Sm-Nd 55 ± 7 Ma)

Pressure–temperature evolution of retrogressed kyanite eclogites, Weinschenk Island, North–East Greenland Caledonides

Caledonian eclogites and associated high-pressure rocks are present in a 400×100-km long belt along the coast of North–East Greenland. Kyanite and clinozoisite eclogites on Weinschenk Island (77°54′N, 21°17′W) occur as lenses within amphibolite facies, quartzofeldspathic gneisses. The eclogites have been extensively overprinted by lower pressure assemblages and exhibit a variety of coronitic and symplectitic reaction textures that record the exhumation path of the rocks. Textural relationships and mineral composition data indicate that the initial high-pressure, plagioclase-free assemblage was garnet+omphacite±kyanite±clinozoisite+rutile. Compositional growth zoning in garnet, together with distribution of mineral inclusions within it, record a prograde evolution from epidote–amphibolite facies progressing into the eclogite facies. Post-peak pressure reaction textures include clinopyroxene–plagioclase intergrowths after omphacite; the replacement of kyanite by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites; and resorption of garnet by amphibole–plagioclase kelyphites. The replacement textures indicate that post-eclogite facies decompression took place while the rocks remained at elevated temperatures (upper amphibolite/granulite facies conditions). The metamorphic evolution of the eclogites is in agreement with eclogite formation by thick-skinned crustal imbrication and subhorizontal shortening of the Laurentian margin during Caledonian collision, exhumation to amphibolite facies by emplacement of the eclogite-bearing thrust sheet on the Laurentian margin and accompanying erosion, and final exhumation to the surface by post-Caledonian crustal thinning.

TEM and HRTEM Study of Microstructures in Omphacite from UHP Eclogites at Shima, Dabie Mountains, China

AbstractTransmission electron microscope (TEM) and high resolution transmission electron microscope (HRTEM) analyses have been performed on omphacite from ultra‐high pressure (UHP) eclogites at the locality of Shima, Dabie Mountains, China. TEM reveals that the microstructures consist dominantly of dislocation substructures, including free dislocations, loops, tiltwalls, dislocation tangles and subboundaries. They were produced by high‐temperature ductile deformation, of which the main mechanism was dislocation creep. Antiphase domain (APD) boundaries are common planar defects; an age of 470 ± 6 Ma for UHP eclogite formation has been obtained from the equiaxial size of APDs in ordered omphacites from Shima, coincident with ages given by single‐zircon U‐Pb dating (471 ± 2 Ma). HRTEM reveals C2/c and P2/n space groups in different parts of one single omphacite crystal, and no exsolution is observed in the studied samples, which is attributed to rapid cooling. It is suggested that the UHP eclogites underwent a long period of annealing at high temperatures, followed by relatively rapid cooling. These data provide valuable information for the formation and exhumation mechanism of UHP eclogites in the Dabie high‐pressure (HP) and UHP metamorphic belt.

Structural and microstructural constraints on the mechanism of eclogite formation in the Sambagawa belt, SW Japan

Eclogites in the Seba basic schist of the Sambagawa belt show contrasting textures. In one type of eclogite, coarse-grained omphacite is randomly oriented and cross-cuts the schistosity suggesting post-tectonic growth. In contrast, in the second type, fine-grained omphacite is preferentially aligned parallel to the schistosity suggesting pre- to syn-tectonic growth. The former type of eclogite has been regarded as a product of solid-state contact metamorphism after the formation of the schistosity while the latter type has been related to the Sambagawa regional metamorphism. The idea that eclogite can form by contact metamorphism is unconventional and, if true, could have major implications for the interpretation of eclogitic rocks in general. In the Seba basic schist, it has generally been assumed that the dominant schistosity formed during a single phase of deformation. However, field and microstructural studies show: (i) there are two different schistosities, one formed before and the other during eclogite formation; and (ii) the differences in eclogite texture reflect the strength of deformation during the second stage. Therefore, the various textural types of eclogite in the Seba basic schist all formed during a single phase of regional metamorphism and there is no need to invoke contact metamorphism.

Evolution of Alpine eclogites in the Eastern Alps: Implications for Alpine geodynamics

In the Eastern Alps Alpine eclogites are generally associated with rocks of continental lithosphere, while eclogites that are associated with oceanic assemblages are restricted to minor exposures. Such eclogites are exposed both in the Penninic unit of the Tauern Window and in the Austroalpine nappe complex. (1) In the central southern part of the Tauern Window (Eclogite Zone) eclogites and associated high pressure metasediments of a distal continental margin are intercalated between Penninic basement units. A mylonitic eclogitic foliation and stretching lineation are contemporaneous to the high pressure metamorphism and are related to the subduction of distal Penninic continental margin sequences. Continuous subduction of cool lithosphere resulted in blueschist facies overprint of the whole Penninic nappe pile. (2) Within the Middle-AustroAlpine Koralm/Saualm region most eclogites are eclogitic mylonites documenting plastic deformation of omphacite and garnet. The meso- and macroscale structures indicate an overall extensional regime possibly related to a large-scale SE-directed ductile low-angle normal shear zone. The eclogites are associated with migmatite-like structures and are intruded by pegmatites. This indicates decreasing pressure, but isothermal or even increasing temperature conditions during exhumation.These relationships argue for the subduction of Penninic continental lithosphere in the foot-wall of the Austroalpine unit at the time of exhumation of the Koralm/Saualm eclogites. Formation of the Austroalpine eclogites is explained by subduction of continental lithosphere, and subsequent, rapid exhumation in an upper plate tectonic position within an extensional regime.

Constrains on formation of eclogites in the dabie mountains from helium and argon isotopic composition

Constrains on formation of eclogites in the dabie mountains from helium and argon isotopic composition

Thermal calculation for high-pressure contact metamorphism: application to eclogite formation in the Sebadani area, the Sambagawa belt, SW Japan

Eclogite, a high-pressure–temperature metamorphic rock characterized by garnet + omphacite, is usually considered to be a product of regional metamorphism under a low geothermal gradient. However, in the Sebadani area of the Sambagawa metamorphic belt most petrologists agree that the eclogite formed by localized contact metamorphism due to intrusion of a body in the solid-state (the Sebadani mass). This process is termed `high-pressure contact metamorphism'. However, geological considerations suggest that the effect of such a process would be limited, firstly because the speed of emplacement for solid-state material will generally be much lower than that for magma and secondly because in the solid-state there is no heat of fusion in the body available for thermal effects. Thermal modelling of a solid-state intrusion, based on the heat conduction equation, allows the relationship between size of intrusion, velocity of emplacement and thermal effects to be calculated. Two cases have been considered: (1) a hot model, where none of the heat conducted into the surroundings is lost during the rise of the body; and (2) a cold model where all the heat conducted into the surroundings is lost. These models bracket possible thermal histories of the body. Calculations suggest that in the Sebadani region, production of the observed metamorphic features requires unrealistically high velocity and a much larger intruded body than is observed. These conclusions suggest that it is unlikely that eclogite in the Sebadani area was formed by high-pressure contact metamorphism, but rather that it represents the highest-grade part of the regional Sambagawa metamorphism.

Discussion on continental eclogites and the Wilson Cycle

H. Austrheim & A. K. Engrik writes: Recent attempts to model continent collision zones have focused attention on processes in, and the petrophysical properties of, crustal root zones. In particular the density changes during the formation of eclogites and their back reaction to amphibolites and granulites have been used to explain various aspects of collision orogens such as seismic velocity, crustal thickness, orogenic uplift and exposure of high P-rocks ( Dewey et al. 1993 ). We are happy to see that Ryan & Dewey (1997) now also introduce the rheological properties of an eclogitized crust into their models and point to the importance this may have for the opening and closing of oceans (the Wilson cycle). Ryan & Dewey (1997) use observations from the Bergen Arcs of Western Norway as evidence of low shear-strength in an eclogitized crust. We agree that the Bergen example shows that eclogites, at some stage in their evolution, had much lower strength than their granulite facies protolith (see discussions by Austrheim 1991 , 1994 ). A similar situation can also be observed in the Sunnfjord area, Western Gneiss Region (Norway), where granulites of mafic and leucocratic composition form lenses in an eclogite-facies melange-like rock ( Austrheim et al. 1997 ). The geodynamic significance of the weakening must, however, depend on its cause and the duration of the weakening. The possible reasons for this weakening were discussed by Klaper (1990) and Austrheim (1991) .While Ryan & Dewey (1997) model weakening of orogenic lithosphere as a function of heat production

Proterozoic eclogites from the Lofoten islands, northern Norway

Massif lenses and bands of variably retrogressed eclogites occur within the granulite facies Proterozoic basement of the Lofoten islands in northern Norway. Eclogites are often related to shear and mylonite zones suggesting that deformation and fluid access was important for their formation. The eclogitic rocks are restricted to gabbroic or troctolitic rock compositions and show, in the least retrogressed samples, the typical assemblage garnet + omphacite + rutile. The widespread occurrence of eclogite shows that the whole group of the Lofoten islands was affected by high pressure metamorphism. Eclogites were found on three of the five major islands so far (Flakstadøy, Vestvågøy, Austvågøy). Thermobarometry reveals minimum conditions during eclogite formation of 680 °C/15 kbar on Flakstadøy and 540 °C/14 kbar on Austvågøy. Geological, petrological and geochemical data suggest the following succession of events: (a) Intrusion of mafic to mangeritic magmas (at about 1.8 Ga; Griffin et al., 1978). (b) Incomplete recrystallization under high-pressure granulite facies conditions. Associated with granulitization was the formation of Cl-rich hydrosilicates. Granulite formation was incomplete except for zones of intensive deformation. (c) Eclogites formed in response to a significant pressure increase in the same high strain zones. However, scattered eclogite lenses and boudins also occur in gneisses. (d) Post-eclogite retrogression occurred under middle to high pressure amphibolite facies conditions (probably the Leknes event at 1.1 Ga of Griffin et al., 1978). Eclogite formation was related to crustal thickening during the final stages of early Proterozoic magmatism and granulite facies metamorphism.

Petrology and Geochronology of Eclogites from the Lanterman Range, Antarctica

The mafic eclogites of the Lanterman Range are the first record of mineral ages. The inferred P–T path, the microtextural features a well-preserved high-pressure assemblage from the Pacific end of and the overlap of the Sm–Nd garnet ages with the range of the the Transantarctic Mountains. They occur among pods and lenses U–Pb rutile–whole-rock ages indicate fast cooling and suggest (from <1 to ~30 m in size) of mafic and ultramafic metamorphic that the time of the high-pressure event was ~500 Ma. These data rocks that constitute a narrow zone intercalated with amphiboliteplace both the formation and exhumation of eclogite within a facies metasediments. This zone extends along the faulted contact convergent plate margin setting, thus documenting the subduction– between the Wilson Terrane and the Bowers Terrane, in northern accretional nature of the early Palaeozoic Ross Orogen in northern Victoria Land. Most of the amphibolites and retrogressed eclogites Victoria Land. analysed have geochemical compositions that resemble transitional to E-type mid-ocean ridge basalt (MORB) and Nd isotope data indicative of differentiation from a depleted mantle source. The age of the igneous precursors is not well determined, but Sm–Nd whole

Petrology and Geochronology of Eclogites from the Lanterman Range, Antarctica

The mafic eclogites of the Lanterman Range are the first record of a well-preserved high-pressure assemblage from the Pacific end of the Transantarctic Mountains. They occur among pods and lenses (from <1 to 30 m in size) of mafic and ultramafic metamorphic rocks that constitute a narrow zone intercalated with amphibolite-facies metasediments. This zone extends along the faulted contact between the Wilson Terrane and the Bowers Terrane, in northern Victoria Land. Most of the amphibolites and retrogressed eclogites analysed have geochemical compositions that resemble transitional to E-type mid-ocean ridge basalt (MORB) and Nd isotope data indicative of differentiation from a depleted mantle source. The age of the igneous precursors is not well determined, but Sm–Nd whole-rock data indicate a Neoproterozoic age, most probably around 700–750 Ma. For this group of metabasites a tectonic setting of an incipient ocean basin is proposed on geological and geochemical grounds. In contrast, the well-preserved eclogites are characterized by strong enrichment in more incompatible elements and pronounced negative Ta and Nb anomalies in MORB-normalized element patterns. Geochemical and Nd isotope data suggest that they are derived from a different mantle source with an enriched signature. The age of the protolith of the well-preserved eclogites, however, remains unconstrained. In the well-preserved eclogite samples the reaction textures testify to three main metamorphic states: (1) an eclogite facies stage, (2) a medium-pressure amphibolite facies stage, and (3) a low-pressure amphibolite facies stage. The high-pressure event occurred at temperatures of up to 850°C based on garnet and omphacite thermometry and at a minimum pressure of 15 kbar based on the jadeitic content of omphacites. Internal Sm–Nd isochrons from two well-preserved eclogites are 500±5 Ma (rutile, clinopyroxene, amphibole, whole rock and garnet) and 492±3 Ma (rutile, clinopyroxene, whole rock and garnet). Rutile–whole-rock 238U-206Pb ages ( 500 Ma) overlap the range of the Sm–Nd mineral ages. The inferred P–T path, the microtextural features and the overlap of the Sm–Nd garnet ages with the range of the 238U-206Pb rutile–whole-rock ages indicate fast cooling and suggest that the time of the high-pressure event was 500 Ma. These data place both the formation and exhumation of eclogite within a convergent plate margin setting, thus documenting the subduction–accretional nature of the early Palaeozoic Ross Orogen in northern Victoria Land.

U-Pb dating of marble associated with eclogite from the Dabie Mountains, East China

Marble is associated with ultrahigh pressure eclogite in the Dabie Mountains, East China. U-Pb isotope data for the marble define a238U-206Pb isochron age of 435 ± 45 Ma with an initial206Pb/204Pb value of 18.075 ± 0.006. The age of ∼435 Ma is interpreted to represent the time of limestone diagenesis rather than the time of metamorphic resetting accompanied by the formation of the ultrahigh pressure eclogites at Triassic. The paleontologic and stratigraphic studies also favor a local excursion in the carbon isotopic composition of the latest Ordovician water within a sedimentary basin between the Sino-Korean and Yangtze cratons. Because the latest Ordovician limestone was involved in the eclogite formation, the continental collision between the Yangtze and Sino-Korean plates would postdate Ordovician.

Igneous and metamorphic petrology of the eclogitic Seljeneset Meta-anorthosite and related jotunites, Western Gneiss Region, Norway

The Seljeneset Meta-anorthosite consists of deformed and metamorphosed anorthosite and mafic rocks, is situated in the high-grade Western Gneiss Region (WGR) of Norway, and is adjacent to the 1.52 Ga Flatraket quartz monzonite (mangerite). The WGR was metamorphosed to eclogite fades during Caledonian tectonism 414 ± 16 Ma ago. The Meta-anorthosite contains 46–57 wt.% SiO 2 and displays chondrite-normalized REE patterns typical of anorthosite, indicating that metamorphism did not significantly affect REE abundances. Norms indicate that igneous anorthosite consisted of ≥ 90% modal plagioclase, minor mafic silicates, and ~ 1% Fe-Ti oxide. Mafic rocks (43–48 wt.% SiO 2) are rich in total Fe 2O 3 (13–20%), TiO 2 (3–4%), and P 2O 5 (1–3%), poor in Cr (< 4 ppm) and Ni (~ 30 ppm), and are thus classified as jotunites, which are characteristic of Proterozoic anorthosite-mangerite complexes. Jotunite originally consisted of > 50% modal andesine, ∼ 25% mafic silicates, ∼ 10% Fe-Ti oxide, and up to 7% apatite. REE patterns of jotunite are highly variable, which reflects variable igneous abundances and modification by metasomatism. Filter pressing of residual liquid trapped between cumulus plagioclase may have been important in the evolution of anorthosite and jotunite. Anorthosite contains ultramafic xenoliths (high MgO, Cr and Ni and depleted in LREE), which suggests that some mantle-derived peridotite was present in the Baltic crust by mid-Proterozoic time; this may extend the crustal history of Norwegian garnet peridotites by over one billion years. Seljeneset rocks display evidence of retrogression from eclogite through amphibolite facies. Meta-anorthosite consists of ovoidal clots of margarite, quartz, plagioclase, and relict kyanite set in a matrix of zoisite, clinozoisite, plagioclase, quartz, hornblende, and biotite. Phase equilibria analysis of sequential assemblages shows that meta-anorthosite records a retrograde P-T path from ≥ 15–17 kbar to 4.3–8.2 kbar at ~ 450–600 °C. Mafic rocks vary from moderately well-preserved eclogite, through varieties containing symplectite, to dynamically recrystallized amphibolite. In one sample, the rare Na-Ca amphibole magnesio-alumino-taramite appears to be part of the eclogite assemblage. Calculations indicate eclogite formation at P ≥ 16–18 kbar at ~ 750 °C. Some mafic rocks contain veins and segregations of biotite, muscovite, pyrite, allanite, and zircon, that, considered together with geochemical data, indicate metasomatic introduction of K, Rb, Ba, Ga, S and rare-earth and other high-field-strength elements by channelized fluids during retrogression. The Seljeneset Meta-anorthosite, associated jotunites, and adjacent Flatraket quartz monzonite likely constitute a single complex, here named the Nordpollen Plutonic Complex. This complex was emplaced 1.52 Ga ago, metamorphosed to eclogite facies during Silurian continent-continent collision, largely retrograded to amphibolite facies during uplift of the orogen, and locally metasomatised during retrogression.

Temporal and tectonic evolution of the granulite-eclogite association from the Bergen Arcs, western Norway

The U-Pb and Sm-Nd dating of deep crustal rocks from the Bergen Arcs system helps resolve enigmatic aspects of the tectonic evolution of the Caledonian Orogen in western Norway and yields insights into the arrested stages of eclogite development within the granulites of the area. The U-Pb dating of zircon from one of the eclogite facies shear zones yields an upper intercept age of 945 ± 5 Ma [all errors two standard deviations (2σ)], which is similar to other zircon ages from the granulite facies protolith. The age is interpreted to represent the time of late Proterozoic (Sveconorwegian) granulite metamorphism. The U-Pb ages of sphene and epidote show that the eclogites formed early in the evolution of the Caledonian Orogen (pre-Scandian phase) at about 460 Ma. An eclogite facies quartz vein yields a Sm-Nd whole rock-garnet isochron of 440 ± 12 Ma that may reflect the onset of cooling immediately after peak eclogite facies conditions, although the Sm-Nd systematics reveal some isotopic disequilibrium within the sample. In tandem with previous 40Ar 39Ar age determinations from, an adjacent eclogite of 450 Ma for hornblende and 430 Ma for muscovite, these data indicate that < 30 Ma elapsed between formation of the eclogites and the initial stages of cooling and exhumation to at least mid-crustal levels. This corresponds to minimum cooling rates of 14 °C/m.y. The timing relations suggest that the formation and exhumation of these eclogites from the overlying Caledonian Nappe wedge in western Norway are related to an early phase of crustal subduction during or somewhat before the major phase of continent-continent collision. The short period of time between the formation of the eclogites and the initial stages of exhumation and rapid cooling is consistent with the only partial and localized transformation of the granulite to eclogite. Isolated occurrences of eclogite within the granulite, the formation of eclogite along metasomatic fronts and the formation of hydrous eclogite facies minerals within the “dry” granulite all point to the importance of fluids in the transformation and re-equilibration of the granulite to eclogite. Together, field and isotopic data demonstrate that both the localized and limited access of fluids and the rapid cycling of continental crust through the deepest portions of the orogen to upper crustal levels resulted in the preservation of the arrested stages of eclogite formation and survival of the granulites metastably through eclogite facies conditions.

Contraction, extension and timing in the South Norwegian Caledonides: the Sognefjord transect

Abstract The Sognefjord transect through the lower to middle Palaeozoic Caledonian mountain belt in southern Norway provides one of the best and most completely documented examples of late collisional tectonics in an Alpine-type orogen. It exposes a 250 km long cross-section from the cratonic foreland, in the east, through the heavily deformed continental margin (Baltica), to the remains of the Caledonian ocean (Iapetus) at the Norwegian west coast. Exceptionally detailed and complete structural data are available along the whole transect, together with good stratigraphic, radiometric, petrological, and geophysical control. In this synthesis, the structural data are analysed, in terms of the kinematics and relative age of the different deformation phases, and correlated along the whole transect. The analysis is then used, in conjunction with the other data, to carry out a retrodeformation, reconstructing the crustal geometry at different stages backward in time. The earliest of the present reconstructions ( c. 410 Ma) marks the time of formation of the well-known West Norwegian eclogites, in an over-deepened root of Baltica which had developed in the ductile lower crust as a response to extreme crustal shortening. The brittle upper crust took up the shortening by the SE movement of a rigid sheet of Precambrian basement (Jotun complex) above the low-angle Jotunheimen contractional detachment, across a rigid wedge of the Baltic Shield. During the final stages of contraction ( c. 410–395 Ma), the upper crust acted as an orogenic lid, against which the root ‘collapsed’ upwards by sub-vertical shortening and lateral E-W extension. During this process of inverted gravity spreading, the eclogites were carried upwards from 60–70 km to 40 km (exhumation phase 1, rate 2–3 mm a −1 ) and retrograded within their deforming gneissic matrix. At the end of this phase, the strain field in the upper crust changed from contraction to extension, concomitant with a broad up-doming (base Devonian unconformity) which caused a further 10 km exhumation by 385 Ma (exhumation phase 2, 1 mm a −1 ). This was followed by the main phase of crustal extension with the development of low-angle normal top-to-W or NW fault and shear zones, of which the Nordfjord-Sogn detachment was the most important (50 km of normal displacement). Exhumation in this phase took place by rapid uplift and erosion of the footwall of the detachment, causing the currently exposed eclogites in outer Sognefjord to rise the remaining 30 km (exhumation phase 3, 1.5 mm a −1 ), to become juxtaposed against Devonian conglomerates on the hanging wall. The reconstructions confirm the general picture of eclogite exhumation in western Norway, and fill out some of the details. However, they do not support the idea that the process was due to extensional orogenic collapse caused by advective or convective lithospheric thinning. Although gravity played a significant role at various stages in the process, the main phase of crustal extension seems to have been mainly related to changes in Devonian plate motion.

Oxygen and carbon isotopic constraints on the development of eclogites, Holsnøy, Norway

Eclogite formation on the island of Holsnøy required the addition of water to anhydrous granulite-facies protoliths. In order to assess this process, oxygen and carbon isotope ratios of whole rock powders and mineral separates from eclogites and granulites have been measured. Whole rock oxygen isotope ratios range from 7.3 to 6.0%. SMOW in granulites (average = 6.38%.) and 7.2 to 6.1%. in eclogites (average = 6.55%.). Field relations permit identification of the granulite protolith of eclogites. Oxygen isotope measurements show shifts of up to 0.5%. between some eclogites compared to their corresponding granulite protoliths, indicating open system and locally heterogeneous fluid behavior. Mineral pair fractionations in the eclogites show disequilibrium, are incompatible with slow cooling and diffusive exchange between phases, and suggest that open system fluid movement continued after eclogite-facies metamorphism. Carbonate is also present in some of the eclogites as a primary mineral (dolomite) and as part of a retrograde assemblage (calcite). Textural evidence suggests that carbonate formation occurred during and after eclogite formation, however all measured carbonate is out of isotopic equilibrium with eclogite facies minerals, due to the influx of retrograde fluids. Massive calcite marble pods, containing amphibolite facies cale-silicate minerals, have average δ 18O of 9.5 ± 0.6%., while calcite in retrograded eclogites has δ 18O 17.7 ± 2.7%., The δ 13C (≈ −4 ± 0.8%.) is indistinguishable between these two groups. Both whole rock and carbonate stable isotope data are interpreted as indicating a continued history of fluid infiltration during and after peak eclogite facies metamorphism. The most probable source of fluids are from dewatered sediments tectonically juxtaposed during the Caledonian orogeny.

Eclogite at the Antarctic palaeo-Pacific active margin of Gondwana (Lanterman Range, northern Victoria Land, Antarctica)

Well-preserved eclogites were found for the first time in Antarctica, at the Lanterman Range, northern Victoria Land. They are part of a mafic–ultramafic belt that lies between the Wilson Terrane, representing part of the palaeo-Pacific margin of Gondwana, and the Bowers Terrane, a Cambro-Ordovician volcanic are and related sediments, accreted to the margin during the Ross Orogeny. The eclogites formed at temperatures in the range 750–850°C and pressures above 15 kbar and subsequently experienced a decompressional path to low pressure amphibolite facies conditions. The formation and exhumation of eclogites and the attainment of the metamorphic peak in adjacent rock units is consistent with a plate convergent setting model at the palaeo-Pacific margin of Gondwana.