Eclogite-facies Mineral Research Articles

Eclogite-Facies Mineral Inclusions in Clinozoisite from Paleozoic Blueschist, Central Chugoku Mountains, Southwest Japan: Evidence of Regional Eclogite-Facies Metamorphism

An eclogite-facies assemblage garnet + omphacite + rutile + glaucophane + quartz occurs as inclusions in clinozoisite porphyroblasts in a high-grade blueschist block in the Paleozoic Osayama serpentinite mélange of the Chugoku Mountains, southwest Japan. Textual relations, and parageneses and compositions of minerals, indicate that the high-grade block experienced peak eclogite-facies metamorphism (M 1) and subsequent blueschist-facies overprinting (M 2). Moreover, low-P amphibolite-facies minerals (M 0) that may represent remnants of pre-subduction conditions are locally preserved in the protolith. Omphacite + adjacent garnet inclusion pairs in clinozoisite give a minimum P = 1.1-1.3 GPa at T = ~480-550°C. Many petrologic similarities between the Osayama eclogite and the Hida Mountains eclogite provide evidence for a regional eclogite-facies metamorphism in Paleozoic terranes of southwest Japan. Considering available petrotectonic information regarding eastern Asia, we suggest two alternative models: (1) Japanese Paleozoic eclogites may represent "Pacific-type" subduction prior to collision between the Sino-Korean and Yangtze blocks, which occurred on both western and eastern sides of the Qinling-Dabie-Sulu belt; or (2) Japanese Paleozoic eclogites may represent part of a Paleozoic "Pacific-type" subduction chain, which is continuous from the China-Russia border through Japan southward to North Korea.

Trace element fractionation during fluid-induced eclogitization in a subducting slab: trace element and Lu–Hf–Sm–Nd isotope systematics

Spatially closely associated gabbros and eclogites of central Zambia represent relics of subducted oceanic crust in a suture zone. The eclogites, which formed at 630–690 °C and 2.6–2.8 GPa, yield Lu–Hf ages between 607±14 and 659±14 Ma, suggesting that subduction was active for at least 24 Myr. The trace element- and isotope compositions of the gabbros and eclogites range from those of incompatible-element depleted gabbros from the lower oceanic crust to those of enriched ocean–island basalts. Several eclogites display a large fractionation of the light rare earth elements from heavy rare earth- and high field strength elements, an effect that cannot be of magmatic origin but must have resulted from the passage of fluids through the rocks during metamorphism. In some samples, fluid pathways are marked by veins of eclogite facies minerals. Garnet-whole rock ages based on the Sm–Nd (relatively mobile) and Lu–Hf (relatively immobile) systems are identical, consistent with light rare earth elements being fractionated during eclogitization. Modeling using fluid–mineral partition coefficients suggests that the fractionated rocks have reacted with an amount of fluid equal up to 80% of their mass. The most likely source for such a large volume of fluid is the serpentinized lithospheric mantle of the subducting slab. The Zambian eclogites and their veins represent relict fluid pathways through subducted oceanic crust and provide direct evidence for channelized fluid flow and element transport within a slab. The transformation of dry, metastable slab gabbros to eclogites upon fluid-infiltration, accompanied by the transport of fluid-mobile elements, could be responsible for generating the slab component in arc magmas.

Mass balance during retrogression of eclogite-facies minerals in the Rongcheng eclogite, eastern Sulu ultrahigh-pressure terrane, China

Eclogite from Rongcheng in the easternmost part of the Sulu ultrahigh-pressure metamorphic terrane has broken down (at least partially) with the formation of coronae of spinel + anorthite around kyanite, symplectites of pargasite + plagioclase around garnet, and symplectites of diopside + plagioclase after omphacite. Textural evidence and microprobe analysis indicate that this breakdown was generated by discontinuous reactions. Quantification of material transfer and textural information suggest that the reaction mechanism involved local exchange of components among the three major minerals (omphacite, garnet, and kyanite) during dissolution and discontinuous precipitation, coupled with exchange between eclogites and their host rocks. The symplectite formed after omphacite gained Al and minor Si and Na from its surroundings, and lost Mg, Ca, and minor Fe to its surroundings. Formation of the symplectite around garnet was accompanied by gain of Ca, Al, Si, Na, and H 2 O, and by loss of Mg and Fe 2 + to its surroundings. Coronae around kyanite gained Ca, Fe, Mg, and minor Si, and provided Al to their surroundings. Material exchange between eclogite and host rocks occurred as well: eclogite gained minor Al, Na, and H 2 O from the host rocks, and provided Mg, Fe, and Ca to the host rocks. Pressure and temperature estimation using THERMOCALC yielded a lower temperature (655 ′ 70 °C) and higher pressure (1.3 ′ 0.2 GPa), which values contrast with estimates made by other authors. Strong deformation may have created a path for ingress of hydrous fluids, then resulted in a localized lower T and/or higher P.

The origin and P–T evolution of peridotites and serpentinites of NE New Caledonia: prograde interaction between continental margin and the mantle wedge

AbstractThe Pouébo and Diahot terranes of NE New Caledonia mostly comprise eclogite to blueschist facies metabasite and metasedimentary rocks that experienced c. 40 Ma metamorphism. This Eocene high‐P event has been linked with the SW‐directed obduction of the New Caledonian Ophiolite, an extensive ultramafic nappe that dominates outcrop in the south of the island. In the north, ultramafic lithologies are found only as sheets or discrete lenticular masses interleaved with, but separated from, the eclogites and blueschists by foliated talc–chlorite–serpentine–carbonate‐bearing rocks. The base of the largest and best‐preserved ultramafic body at Yambé is marked by a distinctive (2 m thick) layer of high‐P mylonite that preserves evidence for early blueschist facies conditions (S1) as inclusions in eclogite facies minerals. Textural evidence preserved in olivine‐bearing serpentinites and their bounding mafic mylonites suggest that the ultramafic bodies were emplaced within the structurally highest levels of the high‐P terrane as serpentinite tectonites sourced from hydrated mantle, formerly in the hangingwall of the Eocene subduction zone. Serpentinite emplacement accompanied burial of the NE New Caledonian margin at T<500 °C and P<16 kbar. The ultramafic fragments were buried to depths of 50–60 km in the subduction zone, where olivine was stable and coarse‐grained garnet–omphacite‐rich assemblages developed in low strain domains within enclosing mylonites. Host metabasic and metasedimentary rocks from the structurally highest portions of the high‐P belt have a prograde record identical to that of the ultramafic tectonites. The early emplacement and similar P–T history of host rocks and ultramafic masses suggest that NE New Caledonia preserves a fossil slab/mantle–wedge boundary reactivated during exhumation.

Discovery of Late Paleozoic retrograded eclogites from the middle part of the northern margin of North China Craton

The retrograded eclogites have been discovered in the middle part of the northern margin of the North China Craton, which occur as lens or boudin within biotite-plagioclase gneisses in Paleoproterozoic Hongqiyingzi Group. The peak eclogite facies (P > 1.40–1.50 GPa, T= 680 –730 °C) mineral assemblage is composed of garnet, omphacite and rutile (± quartz), which was overprinted by the granulite facies mineral assemblage of vermicular symplectite of sodic clinopyroxene and plagioclase which replaced the precursory omphacite, and then amphibolite facies retrograded minerals with characterization of Amp+Pl kelyphitic rim and symplectite, and amphibole replaced clinopyroxene. The protolith of retrograded eclogites is oceanic basalt formed at 438 ± 11 Ma.The peak eclogite facies metamorphic age of the retrograded eclogite is 325 ± 4 Ma. These relict eclogites may be formed by the subduction of Paleo-Asian oceanic crust beneath the North China Craton during Late Paleozoic. The discovery of relict eclogite in this paper provides a new insight into farther understanding of tectonic evolution of the northern margin of the North China Craton, and the relationship between the Paleo-Asian Ocean and the North China Craton.

Inhibited eclogite formation: The key to the rapid growth of strong and buoyant Archean continental crust

Subduction is the principal mechanism by which the hydrosphere and interior of Earth interact. Today, subduction involves the dehydration of ocean crust at depths of 60–120 km depending on the age of the slab. Release of the water leads to generation of arc magmas (future continental crust), and the slab is then transformed into denser eclogite that helps to pull more of the slab into the trench. However, it is unlikely that the first continental crust formed this way. Growing geochemical evidence indicates that large volumes of continental crust were produced over a short period of time in the Archean, when the planet was probably too hot for modern plate tectonics to operate. A significant increase in the kinetics of eclogite-forming reactions may have been the key to the transition from Archean to modern tectonics. Under the higher geothermal gradients of the Archean, tectonically buried ocean crust would have been severely dehydrated before reaching eclogite facies pressures. Because rapid eclogitization is dependent on water as a medium for advective ion transport, the very shallow dehydration in the Archean may have inhibited the formation of eclogite facies minerals. The importance of water in eclogite metamorphism is illustrated by a complex of partly eclogitized mafic granulites in Holsnoy, western Norway, in which reaction progress was limited by the availability of water. When water is scarce or absent, metastable granulite facies mineral assemblages can persist at eclogite facies depths owing to the extremely slow reaction kinetics when diffusion is the only chemical transport mechanism. Such dehydrated but uneclogitized mafic crust would have been very strong and too buoyant to sink into the mantle, and it may have formed the substrate for the first continental lithosphere.

Partial eclogitisation of gabbroic rocks in a late Precambrian subduction zone (Zambia): prograde metamorphism triggered by fluid infiltration

Gabbros and eclogites occur closely associated in a 200-km-long and up to 40-km-wide area of the Zambezi Belt in central Zambia. This area is interpreted to represent part of a late Precambrian suture zone, with the mafic rocks being relics of subducted oceanic crust. Gradual stages of prograde transformation from gabbro to eclogite are preserved by disequilibrium textures of incomplete reactions. This resulted in kyanite-omphacite-bearing assemblages for eclogites that have Al-poor bulk compositions. Undeformed eclogites typically preserve features of a former gabbroic texture, reflected by replacements of plagioclase and magmatic pyroxene by eclogite facies minerals. Textures of deformed eclogites range from sheared porphyroclastic to porphyroblastic. Relics of magmatic pyroxene are common and complete eclogitisation occurred only in millimetre to centimetre-scale domains in most of the rocks. No evidence for prograde blueschist or amphibolite facies mineral assemblages was found in eclogites. In contrast, the fine grained intergrowth of omphacite, garnet, kyanite and quartz, which replace former plagioclase or was formed in the pressure shadow of magmatic pyroxene relics, indicates that eclogitisation might have affected the gabbroic protoliths directly without any significant intervening metamorphic reactions. Eclogitisation took place under P-T conditions of 630-690 degreesC and 26-28 kbar, suggesting a large overstepping (>10 kbar) of reaction boundaries. Eclogitisation was initialised and accompanied by a channelised fluid flow resulting in veins with large, subhedral grains of omphacite, kyanite and garnet. The gabbro-to-eclogite transformation was enhanced by a fluid which allowed the necessary material transport for the dissolution-precipitation mechanism that characterises the metamorphic mineral replacements. The process of eclogitisation was limited by reaction kinetics and dissolution-precipitation rates rather than by the metamorphic P-T conditions. Even though ductile deformation occurred and equilibrium phase boundaries were overstepped, the infiltration of fluids was necessary for triggering the gabbro-to-eclogite transformation.

Stability of phengite and biotite in eclogites and characteristics of biotite- or orthopyroxene-bearing eclogites

Stability of phengite and biotite in eclogite is discussed using petrological data of natural eclogites, and the observational data are examined by thermodynamic calculations. Generally, phengite is a major K phase in natural eclogite and is stable in wide range of bulk composition. However, in eclogites from several localities of the Caledonides, biotite occurs as a stable eclogite-facies mineral, and is often associated with orthopyroxene. Bulk compositions of such biotite- or orthopyroxene-bearing eclogites are compared with those of eclogites from the Dabie–Sulu region, China, where phengite is a major K phase in eclogite. The biotite- or orthopyroxene-bearing eclogites from the Western Gneiss Region of the Caledonides are rich in MgO (10–15 wt%) and relatively poor in CaO (7–8 wt%) and Al2O3 (12–16 wt%). The CaO/MgO ratios of the biotite- or orthopyroxene-bearing eclogites are clearly lower than those of eclogites from the Dabie–Sulu region, indicating that MgO-rich and CaO-poor environments should be important for stabilizing of biotite and orthopyroxene in eclogite. Biotite-bearing eclogite from the North-East Greenland Eclogite Province is rich in MgO (≈16 wt%) and CaO (≈15.5 wt%) and extremely poor in Al2O3 (≈8 wt%). To stabilize biotite in eclogite, Al2O3-poor environments are also important. Bulk compositions of these biotite- or orthopyroxene-bearing eclogites are similar to picrite basaltic compositions. To examine these observational data, thermodynamic calculations were carried out in a seven-component system KH2O1.5–Na2O–CaO–FeO–MgO–Al2O3–SiO2, which includes garnet, kyanite, phengite, biotite, quartz, omphacite, orthopyroxene and olivine in conjunction with mass-balance calculations. Firstly, calculations were performed on the average bulk composition of eclogites from the Dabie–Sulu region to lherzolite (KLB-1). The calculation results confirmed that phengite should be stable in eclogite with 'ordinary' basaltic composition, whereas biotite and orthopyroxene should be stable in picrite basaltic compositions (e.g. MgO >11.0 wt%, CaO <9.8 wt%, Al2O3 <15.2 wt% at 700 °C, 2.5 GPa). Further calculations in basaltic system confirmed that increase of MgO content and decrease of CaO and Al2O3 contents were important to stabilize biotite and orthopyroxene in eclogite. Thus, mineral assemblage in picrite basalt system should be completely different from that in normal basaltic system.

Coesite micro-inclusions and the U/Pb age of zircons from the Hareidland Eclogite in the Western Gneiss Region of Norway

We have identified by laser micro-Raman spectroscopy that inclusions of coesite occur together with other eclogite-facies mineral phases within metamorphic zircons separated from the large eclogite body at Ulsteinvik–Dimnøy on Hareidland. This is the first identification of coesite from this portion of the northwestern Western Gneiss Region (WGR) and supports continuity of ultrahigh-pressure (UHP) metamorphism between the documented coesite occurrences on Stadlandet to the south and the microdiamond and coesite pseudomorph localities on Fjørtoft in the Nordøyene to the north. The zircons, first analysed by U–Pb TIMS in 1973, have been re-analysed and have yielded a much more precise age of 401.6±1.6 Ma, that overlaps with the previously determined age. Our discovery of coesite and the indication of a close to 402 Ma formation age add to a growing number of mid–late Early Devonian ages that signal that the UHP metamorphism in this part of west Norway occurred relatively late in the Caledonian orogenic cycle. These observations should be incorporated in geodynamic models for the exhumation of these rocks and for the metastable preservation of eclogite-facies mineralogies.

Prograde and Retrograde P-T Paths of the Late Paleozoic Glaucophane Eclogite from the Renge Metamorphic Belt, Hida Mountains, Southwestern Japan

Late Paleozoic glaucophane eclogite and garnet glaucophane schist are intercalated with pelitic schist from the Omi eclogitic unit in the Renge metamorphic belt, Hida Mountains, southwestern Japan. Eclogite and garnet glaucophane schist have MORB-like bulk compositions (major and trace), but garnet glaucophane schist is free from omphacite, reflecting a higher Mg/(Mg+Fe) ratio bulk-rock composition. In the eclogite, three different metamorphic stages (stages I to III) are defined on the basis of microstructures and mineral zoning. In particular, prograde-zoned porphyroblastic garnet preserves the transition from blueschist to eclogite facies. The inclusion trails in the rims of garnet are parallel to a penetrative foliation S1 in the matrix, and consist of eclogite-facies minerals (omphacite, glaucophane, epidote, rutile, and quartz). Its core contains the preceding epidote blueschist-facies minerals (glaucophane, epidote, titanite, albite, and quartz) instead of eclogite-facies minerals, and has an internal fabric S0 at a high angle to the surrounding S1 foliation. In the foliated matrix, the eclogite-facies mineral assemblage garnet + omphacite + glaucophane + epidote + rutile + quartz + phengite is partly replaced by secondary minerals such as chlorite, titanite, albite, and calcite, which coexist with recrystallized glaucophane. These petrographic features show a "hair-pin"-like P-T path that passes from the epidote blueschist facies (stage I) through the eclogite facies (stage II), and then is retraced on a nearly coincident path to epidote blueschist facies (stage III). The systematic decrease from core to rim in K D value between garnet and omphacite, and a significant increase in jadeite component of omphacite suggest a rise in both temperature and pressure during prograde eclogitization. The host pelitic schist contains the primary mineral assemblage of quartz + paragonite + phengite + garnet + ferroglaucophane + clinozoisite + rutile, and is strongly replaced by secondary albite, chlorite, and titanite. Pseudomorphs after omphacite and possible kyanite are also found in host pelitic schist. Using geothermobarometry for eclogite and a petrogenetic grid for the host pelitic schist, conditions of peak eclogite-facies metamorphism are estimated to be around 550-600°C and at least 1.8 GPa, indicating an apparent paleo-geothermal gradient of ~10°C/km. The eclogite-bearing Renge metamorphic belt in southwestern Japan may be a new candidate for the eastern extension of a suture zone in east-central China. Late Paleozoic blueschist and eclogite metamorphism may be related to subduction of oceanic crust between the Sino-Korean and Yangtze blocks, prior to their collision.

Fractionation of bulk rock composition due to porphyroblast growth: effects on eclogite facies mineral equilibria, Pam Peninsula, New Caledonia

AbstractChemically zoned porphyroblasts in metamorphic rocks indicate that diffusional processes could not maintain equilibrium conditions on a grain scale during porphyroblast growth or establish it afterwards. An effect of this inability to maintain equilibrium is the progressive removal of elements forming garnet cores from any metamorphic reaction that occurs at the porphyroblast boundaries or in the matrix of the rock. To examine this effect on mineral assemblages, the Bence–Albee matrix correction was applied to X‐ray intensity maps collected using eclogite samples from northern New Caledonia in order to determine the chemical composition of all parts of the sample. The manipulation of these element maps allows a quantitative analysis of the fractionation of the bulk rock composition between garnet cores and the matrix. A series of calculated equilibrium‐volume compositions represents the change in matrix chemistry with progressive elemental fractionation as a consequence of prograde garnet growth under high‐P conditions. Pressure–temperature pseudosections are calculated for these compositions, in the CaO–Na2O–FeO–MgO–Al2O3–SiO2–H2O system. Assemblages, modal proportions and mineral textures observed in the New Caledonian eclogites can be closely modelled by progressively ‘removing’ elements forming garnet cores from the bulk rock composition. The pseudosections demonstrate how chemical fractionation effects the peak metamorphic assemblage, prograde textures and the development of retrograde assemblages.

A method for applying matrix corrections to X‐ray intensity maps using the Bence–Albee algorithm and Matlab

AbstractMatlab scripts that apply the Bence &amp; Albee (1968) matrix correction algorithm to X‐ray intensity data collected as element maps on a Cameca SX‐50 microprobe are used to produce two‐dimensional maps of oxide weight percent and cation proportions for SiO2, Al2O3, FeO, MnO, MgO, CaO, Na2O and K2O. Once generated, large data sets of mapped oxide weight percent values or cation numbers that retain spatial information can be used petrologically. The technique is used to evaluate the compositional range of barroisitic amphibole in an eclogite from New Caledonia, to examine aspects of equilibration during the partial hydration of the eclogite facies mineral assemblage.

Crystallographic fabrics of omphacite, rutile and quartz in Vendée eclogites (Armorican Massif, France). Consequences for deformation mechanisms and regimes

This study aims at further understanding of the mechanisms how lattice-preferred orientations (LPO) develop during deformation in the main eclogite minerals. Microstructures and textures of deformed eclogites from the Les Essarts complex (Western France) were investigated using optical microscopy and electron backscatter diffraction (EBSD) in the scanning electron microscope. Microfabric analyses of eclogite-facies minerals are used to identify their deformation mechanisms, which define the rheology at high-pressure metamorphic conditions. Mechanisms of intracrystalline deformation by dislocation movement (dislocation creep) result usually in a non-linear flow law (typically power law), while diffusive processes (diffusion creep) correspond to linear flow laws. General microstructural observations may suggest intracrystalline deformation (dislocation creep) of omphacite. The omphacite LPO vary between S- and L-type and correlate with oblate or prolate grain shape fabrics, respectively. Until now, these LPO types have not been understood by plasticity models based on dislocation glide on the known slip systems in clinopyroxene. An alternative interpretation is given in terms of anisotropic growth and dissolution, with grain boundary diffusion as the rate controlling process. There are further indications suggesting diffusion creep with concomitant anisotropic growth and dissolution as a main deformation mechanism in omphacite. In omphacite around a hollow garnet, crystallographic and shape fabrics align with the c[001] axes parallel to the grain elongations defining the mineral lineation, which rotates locally with the inferred flow direction. In this part, the grain sizes of omphacite and rutile are larger than in the surrounding matrix. The geometry of both the shape and crystallographic fabrics is interpreted to represent the local stress regime (directions and ratios of the principal stresses). The LPO of rutile duplicate the LPO of omphacite and a similar distinction between S- and L-type was used. Rutile deformation mechanisms probably involve dislocation creep as well as diffusion creep. Quartz mainly occurs as an interstitial phase with weak LPO patterns interpreted as random. No representative obliquity of the LPO in omphacite nor rutile with respect to foliation and lineation was observed to be used as potential shear sense criteria. However, the rutile LPO was slightly rotated relative to the omphacite LPO consistently in most samples. The results suggest that diffusion processes are strongly involved in the deformation of eclogites. A linear flow law should be taken into account in tectonic models where eclogites are incorporated.

Interaction between fluid flow, fracturing and mineral growth during eclogitization, an example from the Sunnfjord area, Western Gneiss Region, Norway

In Sunnfjord, Western Gneiss Region of south Norway, a Proterozoic layered gabbro complex displays various degrees of transformation to eclogite. In the unreacted parts, layering in the gabbro is defined by modal variations of plagioclase, olivine, pyroxenes and minor Fe–Ti oxide and spinel. Coronitic and foliated eclogite formed from the gabbro by hydration reactions at T=510–620°C, causing a volume decrease of c. 13%. In the coronitic eclogite, the mafic magmatic phases are replaced by aggregates of omphacite, barroisite, tremolite, talc and rutile, whereas the plagioclase domains are pseudomorphed by omphacite, barroisite, clinozoisite, kyanite, paragonite and garnet. The felsic and mafic domains are separated by a garnet rim up to 5-mm thick. Garnet was also formed along dilational veins connecting and radiating out of coronas, including the same eclogite facies minerals. In addition, microfractures filled by amphibole and omphacite cut through the corona and vein garnet, oriented perpendicular to the garnet layering. The transformation of dry gabbro to eclogite with hydrous minerals requires supply of water. The timing of metamorphic reactions is therefore dependent on the timing of fluid introduction. The inclusion pattern of garnet in the Holt–Tyssedalsvatnet metagabbro complex indicates that transformation started under eclogite facies conditions. Brittle deformation, in form of fractures allowing infiltration of fluids and mobilisation of elements, is shown to be the most important process initiating transformation. Brittle deformation is thereby active in deep crustal levels corresponding to eclogite facies conditions. Fracturing is interpreted as having been caused by a combination of high fluid pressure, volume changes during mineral transformations and external stresses. Ductile deformation started after the initial metamorphic transformation. Garnet chemistry and zoning pattern are controlled by the chemistry of the growth place, the fluid influx and element supply. Abrupt chemical variations across corona garnet may have been formed by an interaction between rapid growth and transport along microfractures between plagioclase and mafic domain. The metamorphic transition proceeded rapidly. The transformation of the deep crust to eclogite can be viewed as a dynamic process where the volume changes result in fracturing, and these fractures funnelling fluid and element enhancing mineral reaction. The densification causes further fracturing, indicating that the process is self-driven provided that fluid is available.

Trace element distribution in Central Dabie eclogites

Coesite-bearing eclogites from Dabieshan (central China) have been studied by ion microprobe to provide information on trace element distributions in meta-basaltic mineral assemblages during high-pressure metamorphism. The primary mineralogy (eclogite facies) appears to have been garnet and omphacite, usually with coesite, phengite and dolomite, together with high-alumina titanite or rutile, or both titanite and rutile; kyanite also occurs occasionally as an apparently primary phase. It is probable that there was some development of quartz, epidote and apatite whilst the rock remained in the eclogite facies. A later amphibolite facies overprint led to partial replacement of some minerals and particularly symplectitic development after omphacite. They vary from very fine-grained dusty-looking to coarser grained Am + Di + Pl symplectites. The eclogite facies minerals show consistent trace element compositions and partition coefficients indicative of mutual equilibrium. Titanite, epidote and apatite all show high concentrations of REE relative to clinopyroxene. The compositions of secondary (amphibolite facies) minerals are clearly controlled by local rather than whole-rock equilibrium, with the composition of amphibole in particular depending on whether it is replacing clinopyroxene or garnet. REE partition coefficients for Cpx/Grt show a dependence on the Ca content of the host phases, with DREECpx/Grt decreasing with decreasing DCa. This behaviour is very similar to that seen in mantle eclogites, despite differences in estimated temperatures of formation of 650–850 °C (Dabieshan) and 1000–1200 °C (mantle eclogites). With the exception of HREE in garnet, trace elements in the eclogites are strongly distributed in favour of minor or accessory phases. In particular, titanite and rutile strongly concentrate Nb and Zr, whilst LREE–MREE go largely into epidote, titanite and apatite. If these minor/accessory minerals behave in a refractory manner during melting or fluid mobilisation events and do not contribute to the melt/fluid, then the resultant melts and fluids will be strongly depleted in LREE–MREE.

Early Tertiary eclogite facies metamorphism in the Monviso Ophiolite

Although crucial for the construction of tectonic models of the Alps the timing of high pressure metamorphism is still poorly determined and controversial. It is likely to vary from one tectonic unit to another depending on when each became involved in subduction. This in turn relates to palaeogeographic position with respect to the active ocean basin. Well defined, reliable geochronological data are too few to test this hypothesis. This paper extends the database by determining Sm–Nd mineral isochrons on two samples from the Monviso Ophiolite in the Piemonte Zone, carefully selected to minimize the problems of Sm–Nd dating of eclogites encountered elsewhere in the Alps. The dated samples have eclogite facies mineral assemblages typical of the Lago Superiore unit of the ophiolite; mineral compositions are similar to previously reported samples and indicate pressures of around 2 GPa and temperatures of 400–500 °C. Sm–Nd isochron ages of 60±12 and 62±9 Ma are defined by garnet and clinopyroxene, while the Rb–Sr age on phengite which is part of the high‐P assemblage is 40±1 Ma. The new data fit an emerging pattern of ages in which high‐P metamorphism in the oceanic realm is Early Tertiary, with slightly older ages in the overlying Sesia Zone and younger, Oligocene ages in the underlying internal basement massifs which only became involved in subduction when closure of the Piemont ocean was complete.

High‐Pressure Granulite Facies Metamorphism in the Pan‐African Dahomeyide Orogen, West Africa

High‐pressure (HP) granulite facies rocks are associated with eclogites in the Pan‐African Dahomeyide orogen, West Africa. The HP‐granulite and eclogite facies mineral assemblages occur in mafic gneisses within the suture zone exposed in southeastern Ghana and the adjoining part of Togo. Thermobarometric calculations and phase equilibria considerations indicate that the HP metamorphic rocks preserve a record of: (i) prograde garnet crystallization during eclogite facies metamorphism as temperature increased to a maximum of ∼600°C and pressure reached >18 kbar, followed by (ii) HP‐granulite facies metamorphism when temperatures reached 800–900°C at P ∼14 kbar, resulting in partial melting to produce leucocratic veins. Decompression along a retrograde path is inferred from granulites that yield temperatures down to ∼700°C at ∼10 kbar. The inferred P‐T path of the metamorphic record defines a clockwise loop in the kyanite stability field, compatible with orogeny involving subduction during the Pan‐African as...

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.

Garnets recording deep crustal earthquakes

Syn-eclogite facies pseudotachylytes of the Bergen Arcs, western Norway, were formed during co-seismic faulting at a depth of 60 km during Caledonian continent collision, concurrent with fluid-induced partial eclogitization of Precambrian anorthositic granulites. The pseudotachylytes contain the high-pressure assemblage omphacite, garnet and kyanite and formed in the metastable granulites at the border of eclogitized areas. Rock volumes corresponding to outcrops of 100 m2 in area were shattered by pseudotachylytes associated with ultramylonites, cataclasites and shear zones. The granulite facies garnets, unzoned and inclusion-free away from the pseudotachylyte veins, display planar and curved fractures, brecciation and melting with decreasing distance from the veins. Sieve-textured garnet developed by the disintegration of granulite facies garnet, crystallization of eclogite facies mineral inclusions and welding together of garnet fragments. Fractured granulite facies garnets trend to more almandine-rich compositions, with variable grossular contents. In the pseudotachylyte veins, a second generation of minute (10–200 μm) euhedral or dendritic garnets has a higher FeMg ratio than the granulite facies garnet and a wide range in grossular content (Grs11Grs89), suggesting rapid growth and cooling. Fracturing of garnet and its preferential disappearance in the pseudotachylyte show that garnet has a relatively low fracture toughness. Seismic faulting caused enhanced reaction rates during ongoing fluid-induced eclogitization of the deep crust by increasing the surface to volume ratio of garnet and its dislocation density. The sudden volume reduction associated with eclogitization of metastable crust may have nucleated further faulting and pseudotachylyte formation.

Pre-collisional ductile extension in the internal western Alps (Sesia zone, Italy)

Early Alpine eclogites in the western Alps were formed in the Cretaceous prior to the main stage of continental collision in the Eocene. These early Alpine eclogites were exhumed to shallow crustal levels by the end of the Cretaceous, but Mesozoic (pre-collisional) extensional structures, required to unroof these rocks, are not common. Most of the extensional structures described in the western Alps are younger, they postdate Eocene continental collision or are coeval with it and may have helped the unroofing of the usually lower grade, collision-related, blueschist facies rocks. The Sesia zone, at the top of the western Alps structural pile, is a slice of a polymetamorphosed continental basement partly dominated by Eoalpine (130-110 Ma) eclogite facies mineral assemblages. The radiometric data show that these eclogites cooled below ca. 350°C at 60 Ma, so that the major part of the orogenic cycle, including burial, high-pressure metamorphism, cooling and decompression of this unit, was completed prior to continental collision. The present study analyses a NW-dipping, 2 km thick zone of medium- P ductile deformation that divides the Sesia zone into a lower, eclogite-bearing (EMS) unit and an upper blueschist facies unit (II DK). Ductile deformation was mainly confined to the footwall, EMS, whereas the hangingwall, II DK, preserves pre-Alpine structures and mineralogy and remained rigid. Rare eclogite facies nodules occur in this zone of ductile deformation, but medium- P dynamic recrystallization in the stability field of albite is predominant. Mineral chemistry and geothermobarometry of a metaquartz diorite, that occupies a significant part of this deformation zone, show that the ductile fabric in this rock formed at estimated P-T conditions of 8–10 kbar and 500°C. ‘S C’ fabric in the vicinity of the EMS-II DK contact indicates a top-to-the-WNW sense of motion, which is down-dip on the NW-dipping foliation in the region, and extensional with respect to the orientation of the NW-dipping tectonic contact. The interpretation of the medium- P fabric in the EMS as extensional is supported also by the fact that the hangingwall II DK has lower (Alpine) P-T. A WNW-directed Cretaceous extension of the type described in the present work may have dispersed the overthickened Adriatic margin towards the still open Piemont ocean. The later was then sufficiently large to provide the space which is required if the exhumation of deeply buried rocks is a result of horizontal spreading of orogenic wedges. However, the scale of the extensional deformation deduced from the metamorphic discontinuity across the II DK-EMS contact (ca. 2 kbar,) is certainly insufficent to account for the entire vertical motion implied by the emergence of the Sesia eclogites.