Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A suite of eclogite xenoliths from the Roberts Victor kimberlite (South Africa) is remarkable for its high abundances of base metal sulfides (BMS; up to 2 modal %). However, while sulfides are nearly ubiquitous in Type I eclogites (garnet > 0.07% Na2O), Type II eclogites (garnet < 0.07% Na2O) systematically lack sulfides. Two different sulfide assemblages are recognised within the Type I xenolith suite. Both populations are polyphase Cu-Ni-Fe sulfides, one characterised by the pyrrhotite (Po) + pentlandite (Pn) + chalcopyrite (Cp) assemblage and the other by the smythite/violarite (Smy/Vi) + (Ni)pyrite (Py) + Cp assemblage. The latter is the most abundant assemblage and reflects the supergene alteration of the "primary" Po + Pn + Cp assemblage. This process overprints the original composition of the sulfides by remobilising elements such as S, Fe, Ni, Se and Te. In the Type I eclogites, BMS occur as inclusions within silicates (garnet and clinopyroxene) and as interstitial grains. No chemical differences were observed between enclosed and interstitial sulfides. However, their relative abundances are correlated, indicating a similar origin. The Po + Pn + Cp assemblage is identical to eclogitic sulfides previously described in some Roberts Victor diamonds. Silicate-enclosed sulfides could appear to be early phases, but they are restricted to the outer parts of the silicate grains, suggesting a late incorporation during partial recrystallisation of silicate phases, induced by fluid-rock percolation-reaction during a metasomatic event. Positive whole-rock correlations between Se and (La/Sm)N⁎ and between Cu and ΣLREEN⁎ indicate a direct link between sulfide content and the enrichment of Mg + incompatible elements in the Type I eclogites, further supporting a metasomatic origin of the sulfide component. Similarly, a negative correlation between ΣLREEN⁎ and Cucpx implies that the metasomatic agent was at least partially composed of S-rich fluid (e.g. H2S, SO2) that reacted with the rock and leached Cu out of the silicates. The coupling between sulfides and LREE in the Type I eclogites, as well as the absence of sulfides and metasomatic features (e.g. unequilibrated grain boundaries, melt pockets, fluid inclusions, phlogopite) in the Type II eclogites, demonstrate that Type II eclogites did not undergo such a metasomatic event and therefore may represent the less-modified protoliths of Type I eclogites.
ISSN: 0009-2541

hal-00903856
https://hal.archives-ouvertes.fr/hal-00903856
DOI : 10.1016/j.chemgeo.2013.06.015

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Upon subduction, the oceanic crust transforms to blueschists and eclogites, with seismic properties that gradually become similar to those of the surrounding mantle. In order to evaluate the anisotropy of blueschists and glaucophane-bearing eclogites, the elastic constants of glaucophane single-crystal plates from the Sesia-Lanzo Zone (Aosta Valley, Western Alps) were measured using Brillouin spectroscopy at ambient conditions. The mean P- and S-wave velocities are 7.8 and 4.6 km s−1 respectively, and the anisotropy is high (38.1% (AVP) and 27.3% (AVS)). Glaucophane develops strong LPO, characterized by the [001]-axes concentrated sub-parallel to the lineation, and the {110} poles concentrated sub-perpendicular to the foliation in both blueschist and eclogite rocks. The measured LPO is in good agreement with viscoplastic selfconsistent numerical models. Seismic properties of glaucophane-bearing blueschist and eclogite are calculated by combining measured LPO and the single-crystal elastic moduli of glaucophane with the other main mineral constituents of the rock: mostly epidote for blueschist, and omphacite and garnet for eclogite. Blueschists present stronger anisotropies (AVP=16.1% and AVS=10.3%) than eclogites (AVP=2.9% and AVS=1.7%). The shear-wave splitting and resulting delay times for a 7-km thick layer of eclogite or blueschist are low for the eclogite (b0.03 s), but signi!cant for blueschist (0.16 s). Application to the subducted oceanic crust yields a decrease of velocity contrast with the surrounding mantle and of anisotropy at depth depending on the temperature of the slab. The details of the velocity variations in subducted oceanic crust are dif!cult to reconcile with the blueschist-eclogite transition as probed by exhumed metamorphic rocks and may require additional phases such as lawsonite or chemical variations such as oxidation state.
ISSN: 0040-1951

hal-00564518
https://hal.archives-ouvertes.fr/hal-00564518
DOI : 10.1016/j.tecto.2010.09.011

Éditeur(s) : HAL CCSD Elsevier
Résumé : Understanding chemical element mobility during high-pressure metamorphism is paramount to the knowledge of the transformations occurring during the course of eclogite-facies metamorphism. In particular, the role of deformation and fluid circulation appear essential. In order to better decipher this role, we investigated two eclogite-facies samples from the Lindas Nappe in the Bergen arcs (Norwegian Caledonides), which are related to a fluid/deformation metamorphic event. This study, based on detailed microtextural investigations, coupled with major and trace element analyses, either in situ EMPA and LA-ICP-MS or by conventional ICP-MS nebulisation on mineral fractions, illustrates the complexity of chemical reactions accompanying the eclogite- and amphibolite-facies metamorphism. A single eclogite-facies mineral species two eclogite-facies (i.e. phengite and epidote) depicts various chemical features depending on its textural location at thin-section scale. The correlation with the mineralogical reaction occurring in each textural site demonstrates that the compositional variations are inherited from the precursor minerals. Thus, the composition of eclogite-facies minerals is locally controlled by the composition of few hundred-micron domains. On the contrary, alteration phases of the amphibolite facies (symplectite and calcic amphibole) display clear enrichment in LILE, Pb, Sr and LREE compared to their precursor minerals (omphacite and garnet). This supports that the transport of elements at hand-sample scale was enhanced during the retrogression and elements were efficiently delocalised from one textural site to another. The contrasting behaviour of element mobility between peak eclogite-facies metamorphism and retrogression can be explained in terms of rate processes and fluid/deformation activity. During eclogitisation,deformation allows the formation of textural and mineralogical microdomains. Contemporaneously the fluid circulation favours the development of heterogeneities by transport of elements in excess towards areas protected from deformation where phengite-bearing quartz lenses crystallise. This, together with the very fast recrystallisation of eclogite-facies minerals, results in a heterogeneous redistribution of elements at sample scale and in a chemical disequilibrium between different microdomains of hundred-micron size. During retrogression in the amphibolite facies, coeval with long-term ductile deformation, the fluids act as an efficient vector to redistribute the elements from a given textural site to another and, thus, allow to reach, at hand-sample scale, a new chemical equilibrium between the different crystallising phases.
ISSN: 0024-4937

hal-00404602
https://hal.archives-ouvertes.fr/hal-00404602
DOI : 10.1016/j.lithos.2006.10.001

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We investigated the deformation mechanisms and the seismic properties of 10 eclogite samples from different localities (Alps, Norway, Mali and eastern China) through the analysis of their microstructures and lattice-preferred orientations (LPO). These samples are representative of various types and intensity of deformation under eclogitic metamorphic conditions. Omphacite and garnet LPO were determined from electron backscatter diffraction (EBSD) technique. Garnet appears to be almost randomly oriented whereas omphacite develops strong LPO, characterized by the [001]-axes concentrated sub-parallel to the lineation, and the (010)-poles concentrated sub-perpendicular to the foliation. In order to analyze the deformation mechanisms that produced such omphacite LPO, we compare our observations to LPO simulated by viscoplastic self-consistent numerical models. A good fit to the measured LPO is obtained for models in which the dominant slip systems are 1/2h110i{11 ¯ 0}, [001] {110} and [001] (100). Dominant activation of these slip systems is in agreement with TEM studies of naturally deformed omphacite. Seismic properties of eclogite are calculated by combining the measured LPO and the single crystal elastic constants of omphacite and garnet. Although eclogite seismic anisotropies are very weak (less than 3% for both P-and S-wave), they are generally characterized by a maximum P-wave velocity sub-parallel to the lineation and by a minimum velocity approximately normal to foliation. The mean P-and S-wave velocities are high (respectively, 8.6 and 4.9 km/s). The S-wave anisotropy pattern displays complex relationships with the structural frame but the fast polarization plane generally tends to be parallel to the foliation. Calculated reflection coefficients show that an eclogite/crust interface is generally a good reflector (Rc > 0.1), whereas an eclogite body embedded in the upper mantle would be hardly detectable.
ISSN: 0040-1951

hal-01389715
http://hal.univ-reunion.fr/hal-01389715
http://hal.univ-reunion.fr/hal-01389715/document
http://hal.univ-reunion.fr/hal-01389715/file/Bascou_eclo-sismo_tphy2001_hal.pdf
DOI : 10.1016/S0040-1951(01)00156-1

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
eoproterozoic eclogite and garnet amphibolites, representing retrogressed eclogites, from the Tassendjanet-Tidéridjaouine terrane in the Pan-African Western Hoggar (Tuareg shield, Southern Algeria) form lenses enclosed in 1.8 Ga subalkaline Paleoproterozoic orthogneisses. They lie along a major sinistral shear zone marking the boundary with the Archean/Paleoproterozoic granulitic In Ouzzal terrane and are associated with high-pressure metasediments. According to thermodynamic calculations, peak condition of the eclogitic stage is 650 °C, 20-22 kbar followed by post-peak heating and exhumation at 730 °C, 10-14 kbar and cooling to 610 °C, 7-10 kbar. Their major, trace-element and isotopic compositions are consistent with emplacement of former basalts in the shoulder of a 700-800 Ma continental rift that evolved to an oceanic basin. The eclogite and garnet amphibolites were thus part of the continental portion of a subducting slab that was pulled down by a denser oceanic lithosphere. Sheath and recumbent folds associated with the exhumation of HP units are verging toward the west, meaning that the former slab was dipping to the east. U-Pb zircon dating on metamorphic zircons bordering ilmenite and rutile in the eclogite gave an age of 623 ± 2 Ma, interpreted as the syn-collisional exhumation stage of the high-pressure unit. This event was rapidly followed by transpressional tectonics and abundant magmatism linked to the northward motion of the adjacent In Ouzzal terrane.
ISSN: 0301-9268

hal-01054338
https://hal.archives-ouvertes.fr/hal-01054338
DOI : 10.1016/j.precamres.2014.04.002

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present an integrated study of geochemistry, petrofabrics and seismic properties of strongly sheared eclogites from the Chinese Continental Scientific Drilling (CCSD) project in the Sulu ultrahigh-pressure (UHP) metamorphic terrane, eastern China. First, geochemical data characterize diverse protoliths of the studied eclogites. The positive Eu- and Sr-anomalies, negative Nb anomaly and flat portion of heavy rare earth elements in coarse-grained rutile eclogites (samples B270 and B295) suggest a cumulate origin in the continental crust, whereas the negative Nb anomaly and enrichment of light rare earth elements in retrograde eclogites (samples B504, B15 and B19) imply an origin of continental basalts or island arc basalts. Second, P-wave velocities (V-p) of three typical eclogite samples were measured under confining pressures up to 500 MPa and temperatures to 700 degrees C. At 500 MPa and room temperature, the mean V-p reaches 8.50-8.53 km/s in samples B270 and B295 but drops to 7.86 km/s in sample B504, and the P-wave anisotropy changes from 1.7-2.7% to 5.5%, respectively. The pressure and temperature derivatives of V-p are larger in the retrograde eclogite than in fresh ones. Third, the electron backscatter diffraction (EBSD) measurements of the eclogites reveal random crystal preferred orientation (CPO) of garnet and pronounced CPO of omphacite, which is characterized by a strong concentration of [001]-axes sub-parallel to the lineation and of (010)-poles perpendicular to the foliation. The asymmetric CPO of omphacite in sample B270 recorded a top-to-the-south shear event during subduction of the Yangtze plate. The calculated fastest V-p is generally sub-parallel to the lineation, but a different deformation environment during exhumation could form second-order variations in omphacite CPO and affect the V-p distribution in eclogites (e.g., the fastest V-p is at similar to 35 degrees from the foliation in sample 13295). Comparison between measured and calculated seismic properties indicates that the CPO of omphacite controls the seismic anisotropy of eclogites at high pressure, and compositional layering and retrograde minerals will increase the anisotropy. Calculated P-wave velocities agree well with velocities measured at 500 MPa and room temperature for fresh eclogites, but much higher than those of retrograde eclogite. As a case study, the laboratory-derived V-p-P and V-p-T relationships were used to estimate P-wave velocities of eclogites and peridotites beneath the Western Superior Province, Canada. The results indicate that besides the fabric-induced anisotropy, the direction dependence of pressure and temperature derivatives of V-p can significantly increase seismic anisotropy of eclogites with depth, which results in eclogites being an important candidate for the seismic anisotropy in the upper mantle. Due to their very high density and velocity, garnet-rich eclogites within peridotite could be detected in seismic reflections in subduction zones.
ISSN: 0040-1951

hal-00463512
https://hal.archives-ouvertes.fr/hal-00463512
DOI : 10.1016/j.tecto.2008.09.027

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In many orogens, high-pressure (HP) metamorphic rocks such as eclogite occur as lenses in quartzofeldspathic gneiss that equilibrated at much lower pressures. The pressure–temperature–time (P–T–t) history of eclogite relative to host gneiss provides information about mechanisms and timescales of exhumation of orogenic crust. The Montagne Noire of the southern Massif Central, France, is an eclogite–bearing gneiss (migmatite) dome located at the orogen-foreland transition of the Variscan belt. Results of our study show that it contains the youngest eclogite in the orogen, similar in age to migmatite and granite that crystallized under low-pressure conditions. P–T conditions for an exceptionally unaltered eclogite from the central Montagne Noire were estimated using a pseudosection supplemented by garnet–clinopyroxene and Zr-in-rutile thermometry. Results indicate peak P∼1.4 GPaP∼1.4 GPa and View the MathML sourceT∼725°C for Mg-rich garnet rim (50 mol% pyrope) + omphacite (36 mol% jadeite) + rutile + quartz. U–Pb geochronology (LA-ICP-MS) of 16 zoned zircon grains yielded ∼360 Ma (4 cores) and ∼315 Ma (12 rims and cores). Rare earth element abundances determined by LA-ICP-MS for dated zircon are consistent with crystallization of ∼315 Ma zircon under garnet-stable, plagioclase-unstable conditions that we interpret to indicate high pressure; in contrast, the ∼360 Ma zircon core corresponds to crystallization under lower pressure plagioclase-stable conditions. Based on garnet zoning and inclusion suites, rutile textures and Zr zoning, P–T results, and zircon petrochronology, we interpret the ∼315 Ma date as the age of eclogite-facies metamorphism that only slightly preceded dome formation and crystallization at 315–300 Ma. This age relation indicates that eclogite formation at high pressure and migmatite dome emplacement at low pressure were closely spaced in time. We propose that collapse-driven material transfer from the hot orogen to the cool foreland resulted in thickening of the orogen edge, leading to eclogite facies metamorphism of the deep crust. Soon after, the low-viscosity partially molten crust flowed from the plateau toward the foreland, incorporating and exhuming eclogite. The P–T–t history of the Montagne Noire eclogite shows that some dome material ascended from >40 km depth to shallow crustal levels, likely in a single decompression event, and that migmatite domes are therefore very efficient at exhuming the deep crust.
ISSN: 0012-821X

hal-01243499
https://hal.archives-ouvertes.fr/hal-01243499
DOI : 10.1016/j.epsl.2015.08.026

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : Geochemistry, Geophysics, Geosystems, v. 9, p. Q12001, 2008. http://dx.doi.org/10.1029/2008GC002098
International audience
Rb/Sr, 40Ar/39Ar, and Sm/Nd isotopic data are reported in Caledonian eclogites from the Lindås Nappe, Bergen Arcs, Norway, in order to investigate processes controlling isotopic equilibrium at mineral scale in polymetamorphic rocks. The Bergen Arcs exposes Sveconorwegian ∼950–930 Ma granulites, partially overprinted by Caledonian eclogite-facies metamorphism at ∼425 Ma and amphibolite-facies metamorphism at ∼410 Ma. Geochemical and Rb-Sr data from more than 10 phengite fractions separated from one sample reflect the composition of the microdomain (a few hundred microns in size) in which phengite crystallized. Phengite crystallized after garnet or plagioclase by dissolution-precipitation processes yield apparent age between 700 and 600 Ma. At the time of their crystallization, these phengites inherited the isotopic composition of their precursor minerals, at a microdomain scale. Phengite from quartz veins, which crystallized from elements mobilized by the circulating fluid, yield an age closer to the eclogite-facies metamorphic age. The closed system evolution of the eclogitizing fluid, the segregation of textural and reactional microdomains, the high Sr content of the studied phengite, and the short duration of the recrystallization processes (<1Ma) are interpreted as the main factors responsible for the lack of a Rb/Sr isotopic equilibrium, at the scale of hand samples. Such equilibrium is nevertheless reached in quartz veins where the crystallization of minerals implies that the fluid circulation acted as a factor of isotopic homogenization. The in situ single mineral Ar/Ar datings revealed that both eclogite- and amphibolite-facies minerals are characterized by an excess of radiogenic argon (ages between 425 and 520 Ma). The excess of argon has been inherited from the previous granulite and has been only partially evacuated from the system by the circulating fluid. The apparent Sm/Nd ages from garnets inherited from the granulite-facies metamorphism (c. 930 Ma) are in agreement with previous estimates. This result confirms that the diffusion of Nd in garnet does not occur at temperatures lower than 700°C. This study highlights the complexity of radiochronometer behavior during HP metamorphism and demonstrates that coupling different radiochronometers, such as Ar/Ar and Rb/Sr, does not always guarantee the validity of the geochronological results.
ISSN: 1525-2027

hal-00407885
https://hal.archives-ouvertes.fr/hal-00407885
DOI : 10.1029/2008GC002098

Éditeur(s) : Elsevier
Résumé : High P-wave velocities (7.1-7.8 km/s) lower crustal bodies (LCBs) imaged along volcanic margins are commonly interpreted as plume and break-up-related thick mafic underplating. This interpretation is partly challenged in this paper based on new seismic observations and modelling of the outer Voring Basin (Norway). An exceptional strong amplitude reflection, the T Reflection, is particularly well defined below the North Gjallar Ridge (NGR) between 7 and 8 s TWT. The T Reflection is located near the volcanic lava flows emplaced during the NE Atlantic breakup (similar to 55-54 Ma ago) and coincides with the top of the LCB, forming a mid-crustal dome. Based on structural and temporal relationships, we show that the dome clearly influences the structural development of the NGR and predates the continental breakup at least by 10-15 Ma. Using a thermo-kinematical model, we tried also to investigate and quantify the relationships between the extension, LCB and the magmatic production. Modelling suggests that significant Paleocene-Early Eocene magmatism can be produced without any temperature anomaly in the mantle if differential stretching occurs during the break-up initiation. The conclusion of 2D thermo-kinematical parametric analysis is that the magmatic model predicts, either little extension (beta < 2) with no melting or high extension (beta > 5) with significant melting along the outer Voring Basin. We suggest that the continental part of the LCB could not necessarily be breakup-related and so magmatic, as has often been stated previously. It is concluded here that the continental part of the LCB observed beneath the outer Voring Basin may be partly (or fully) attributed to inherited, high-pressure granulite/eclogite lower crustal rocks. The real amount of mafic material emplaced along the outer Voring Basin could be 20-40% less than thought.
Tectonophysics (0040-1951) (Elsevier), 2006 , Vol. 412 , N. 3-4 , P. 255-278

Droits : 2005 Elsevier B.V. All rights reserved
http://archimer.ifremer.fr/doc/2006/publication-1042.pdf
DOI:10.1016/j.tecto.2005.10.038
http://archimer.ifremer.fr/doc/00000/1042/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Type I and Type II eclogite xenoliths from the Roberts Victor kimberlite (South Africa) show marked differences in terms of microstructures, mineralogy, major-and trace-element compositions and oxygen-isotope compositions. The unequilibrated microstructures of Type I eclogites, their typical accessory assemblages (phologopite, diamond, sulphides, fluid inclusions) and the ubiquitous presence of "melt pockets" in garnets provide strong evidence of metasomatism. Type II eclogites systematically lack such features and are microstructurally more equilibrated. Type I eclogites are more magnesium-rich than most Type II (mean Mg# = 0.56 vs. 0.46), while Type II eclogites are generally more Ca-rich (mean CaO = 9 vs. 12 wt%) and Fe-rich (mean FeO = 10 vs. 12 wt%). Type I eclogites are systematically enriched in LREE, Sr, Ba, alkali elements, HFSE, Th and U compared to the more depleted Type II eclogites. Calculated trace-element patterns of fluids in equilibrium with Type I eclogites are closely similar to those of volatile-rich small-volume mantle melts in the carbonatite-kimberlite spectrum commonly inferred to be responsible for mantle metasomatism. Although oxygen isotopes are often used to argue for a subduction origin of mantle eclogites, correlations between delta O-18 of garnet and typical metasomatic tracers suggest that the metasomatic process also has shifted the oxygen-isotope compositions of the Type I eclogites toward heavier values. Roberts Victor Type I eclogites thus carry the imprint of a metasomatic process that strongly modified their major-element, trace-element and isotopic compositions, while the more pristine Type II eclogites escaped this modification. Therefore, attempts to constrain the origin of Roberts Victor eclogites should not be based on the much more abundant Type I eclogites, which retain little geochemical memory of their protoliths. The most suitable materials for such investigations may be the less metasomatised, but more rare, Type II eclogites.
ISSN: 0016-7037

hal-00669867
https://hal.archives-ouvertes.fr/hal-00669867
DOI : 10.1016/j.gca.2011.08.035

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
At Cima di Gagnone, garnet peridotite and chlorite harzburgite lenses within pelitic schists and gneisses correspond to eclogite-facies breakdown products of hydrated peridotites and are suitable for studying dehydration of serpentinized mantle. Thermobarometry and pseudosection modelling yield peak temperatures of 750-850°C and pressures <3 GPa. The minimum temperature recorded by the garnet peridotite corresponds to the maximum conditions experienced by the chlorite harzburgite, suggesting that these rocks recrystallized cofacially at ∼800°C. Alternatively, they might have decoupled during subduction, as achieved in tectonically active plate interface boundaries. The major and rare earth element (REE) variability of the peridotites was mostly acquired during pre-subduction mantle evolution as a result of partial melting and reactive melt flow. The ultramafic suite is also characterized by fluid-mobile element enrichments (B, Pb, As, Sb, Cs, Li, U, Be), which confirm derivation from variably serpentinized protoliths. Similarity in the U, Pb, B, Li and Sr contents of the Gagnone peridotites to present-day oceanic serpentinites suggests that these elements were partly taken up during initial serpentinization by seawater-derived fluids. Positive Be, As and Sb anomalies suggest involvement of fluids equilibrated with crustal (metasedimentary) reservoirs during subsequent subduction metamorphism and peridotite entrainment in (meta)sediments. Fluid-mobile element enrichment characterizes all peak eclogitic minerals, implying that multiple hydration events and element influx pre-dated the eclogite-facies dehydration. Peak anhydrous minerals retain B, Li, As and Sb concentrations exceeding primitive mantle values and may introduce geochemical anomalies into the Earth's mantle. The relatively low contents of large ion lithophile elements and light REE in the Gagnone peridotites with respect to much higher enrichments shown by metasomatized garnet peridotite pods hosted in migmatites (Ulten Zone, Eastern Alps) suggest that the crustal rocks at Gagnone did not experience partial melting. The Gagnone garnet peridotite, despite showing evidence for chlorite dehydration, retains significant amounts of fluid-mobile elements documenting that no partial melting occurred upon chlorite breakdown. We propose that the Gagnone ultramafic rocks represent a prime example of multi-stage peridotite hydration and subsequent dehydration in a plate interface setting.
ISSN: 0022-3530

hal-01054319
https://hal.archives-ouvertes.fr/hal-01054319
DOI : 10.1093/petrology/egt068

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The late Carboniferous accretionary system of the South Tianshan orogen (North-Western China) underwent complex structural and polymetamorphic evolution. Combined petrological, geochronological and microstructural analysis of (ultra)high-pressure (UHP) metabasites (eclogites and blueschists) enclosed in metapelites show a relict coarse-grained eclogitic fabric S2 surrounded by a dominant fine-grained eclogite and blueschist facies retrograde fabric S2. The S2 fabric is reworked by upright folds F3 that are responsible for a major shortening of the whole accretionary system. For both the eclogite and blueschist, peak and retrograde P–T conditions have been thermodynamically constrained at 25–26 kbar and 425–500 °C and 10–13 kbar and 500−550 °C respectively, suggesting a shared exhumation history. The garnet-whole rock-amphibole isochron in the blueschist yielded Lu–Hf and Sm–Nd ages of 326.0 ± 2.9 Ma and 318.4 ± 3.9 Ma respectively, interpreted to date the prograde to peak metamorphic assemblage. The retrograde path of the eclogite is characterized by heterogeneous omphacite recrystallization into a mylonitic fine-grained matrix and crystallization of blue amphibole. Microstructures in both pristine porphyroclastic and recrystallized fine-grained domains in the eclogite indicate a gradual evolution from constriction-dominated (L>S-type) to flattening-dominated (S>L-type) type of deformation, increase of fabric intensity reflected by gradually growing M-indexes and the development of lattice preferred orientation (LPO) typical for dislocation creep under slightly hydrated conditions. Recrystallization of the matrix in the blueschist is homogeneous, which indicates a matrix dominated channel flow during exhumation. These LPOs evolutions suggest a significant mechanical coupling with the upper plate concomitant with oroclinal bending of the Kazakh orocline. Lock up of Kazakh orocline is responsible for further stress increase resulting in horizontal shortening of South Tianshan accretionary wedge and development of D3 upright folding and steepening of the whole sequence.
ISSN: 1342-937X

hal-01481263
https://hal.archives-ouvertes.fr/hal-01481263
DOI : 10.1016/j.gr.2016.07.007

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
Convergent plate margins typically experience a transition from subduction to collision dynamics as massive continental blocks enter the subduction channel. Studies of high-pressure rocks indicate that tectonic fragments are rapidly exhumed from eclogite facies to mid-crustal levels, but the details of such dynamics are controversial. To understand the dynamics of a subduction channel we report the results of a petrochronological study from the central Sesia Zone, a key element of the internal Western Alps. This comprises two polymetamorphic basement complexes (Eclogitic Micaschist Complex and Gneiss Minuti Complex) and a thin, dismembered cover sequence (Scalaro Unit) associated with pre-Alpine metagabbros and metasediments (Bonze Unit). Structurally controlled samples from three of these units (Eclogitic Micaschist Complex and Scalaro-Bonze Units) yield unequivocal petrological and geochronological evidence of two distinct high-pressure stages. Ages (U-Th-Pb) of growth zones in accessory allanite and zircon, combined with inclusion and textural relationships, can be tied to the multi-stage evolution of single samples. Two independent tectono-metamorphic 'slices' showing a coherent metamorphic evolution during a given time interval have been recognized: the Fondo slice (which includes Scalaro and Bonze rocks) and the Druer slice (belonging to the Eclogitic Micaschist Complex). The new data indicate separate stages of deformation at eclogite-facies conditions for each recognized independent kilometer-sized tectono-metamorphic slice, between ∼85 and 60 Ma, with evidence of intermittent decompression (ΔP ∼ 0*5 GPa) within only the Fondo slice. The evolution path of the Druer slice indicates a different P-T-time evolution with prolonged eclogite-facies metamorphism between ∼85 and 75 Ma. Our approach, combining structural, petrological and geochronological techniques, yields field-based constraints on the duration and rates of dynamics within a subduction channel.
ISSN: 0022-3530

hal-01054335
https://hal.archives-ouvertes.fr/hal-01054335
DOI : 10.1093/petrology/egu029

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
The Lanzo peridotite massif is a fragment of oceanic lithosphere generated in an ocean-continent transition context and eclogitized during alpine collision. Despite the subduction history, the massif has preserved its sedimentary oceanic cover, suggesting that it may have preserved its oceanic structure. It is an exceptional case for studying the evolution of a fragment of the lithosphere from its oceanization to its subduction and then exhumation. We present a field and petrological study retracing the different serpentinization episodes and their impact on the massif structure. The Lanzo massif is composed of slightly serpentinized peridotites (<20% serpentinization) surrounded by an envelope of foliated serpentinites (100% serpentinization) bordered by oceanic metabasalts and metasedimentary rocks. The limit between peridotites and serpentinites defines the front of serpentinization. This limit is sharp: it is marked by the presence of massive serpentinites (80% serpentinization) and, locally, by dykes of metagabbros and mylonitic gabbros. The deformation of these gabbros is contemporaneous with the emplacement of the magma. The presence of early lizardite in the peridotites testifies that serpentinization began during the oceanization, which is confirmed by the presence of meta-ophicarbonates bordering the foliated serpentinite envelope. Two additional generations of serpentine occur in the ultramafic rocks. The first is a prograde antigorite that partially replaced the lizardite and the relict primary minerals of the peridotite during subduction, indicating that serpentinization is an active process at the ridge and in the subduction zone. Locally, this episode is followed by the deserpentinization of antigorite at peak P-T (estimated in eclogitized metagabbros at 2-2.5 GPa and 550-620 °C): it is marked by the crystallization of secondary olivine associated with chlorite and/or antigorite and of clinopyroxene, amphibole and chlorite assemblages. A second antigorite formed during exhumation partially to completely obliterating previous textures in the massive and foliated serpentinites. Serpentinites are an important component of the oceanic lithosphere generated in slow to ultraslow spreading settings, and in these settings, there is a serpentinization gradient with depth in the upper mantle. The seismic Moho limit could correspond to a serpentinization front affecting the mantle. This partially serpentinized zone constitutes a less competent level where, during subduction and exhumation, deformation and fluid circulation are localized. In this zone, the reaction kinetics are increased and the later steps of serpentinization obliterate the evidence of this progressive zone of serpentinization. In the Lanzo massif, this zone fully recrystallized into serpentinite during alpine subduction and collision. Thus, the serpentinite envelope represents the oceanic crust as defined by geophysicists, and the sharp front of serpentinization corresponds to an eclogitized seismic palaeo-Moho.
ISSN: 0263-4929

hal-00823051
https://hal.archives-ouvertes.fr/hal-00823051
DOI : 10.1111/jmg.12008

Éditeur(s) : HAL CCSD Société Géologique de France
Résumé : Xenoliths brought up by kimberlite magmas are rare samples of otherwise inaccessible lithospheric mantle. Eclogite xenoliths are found in most cratons and commonly show a range of mineral and chemical compositions that can be used to better understand craton formation. This study focuses on five new kyanite-bearing eclogites from the Udachnaya kimberlite pipe (367±5 Ma). They are fine-to coarse-grained and consist mainly of “cloudy” clinopyroxene (cpx) and garnet (grt). The clinopyroxene is Al,Na-rich omphacite while the garnet is Ca-rich, by contrast to typical bi-mineral (cpx+grt) eclogites that contain Fe- and Mg-rich garnets. The Udachnaya kyanite eclogites are similar in modal and major element composition to those from other cratons (Dharwar, Kaapvaal, Slave, West African). The kyanite eclogites have lower REE concentrations than bi-mineral eclogites and typically contain omphacites with positive Eu and Sr anomalies, i.e. a “ghost plagioclase signature”. Because such a signature can only be preserved in nonmetasomatised samples, we infer that they were present in the protoliths of the eclogites. It follows that subducted oceanic crust is present at the base of the Siberian craton. Similar compositions and textures are also seen in kyanite eclogites from other cratons, which we view as evidence for an Archean, subduction-like formation mechanism related to craton accretion. Thus, contrary to previous work that classifies all kyanite eclogites as type I (IK), metasomatized by carbonatite/kimberlitic fluids, we argue that some of them, both from this work and those from other cratons, belong to the non-metasomatized type II (IIB). The pristine type IIB is the nearest in composition to protoliths of mantle eclogites because it contains no metasomatic enrichments.
ISSN: 0037-9409

hal-01622617
https://hal.archives-ouvertes.fr/hal-01622617
DOI : 10.1051/bsgf/2017008

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Highly siderophile element concentrations (HSE: Re and platinum-group elements (PGE)) are presented for gabbros, gabbroic eclogites and basaltic eclogites from the high-pressure Zermatt-Saas ophiolite terrain, Switzerland. Rhenium and PGE (Os, Ir, Ru, Rh, Pt, Pd) abundances in gabbro- and eclogite-hosted sulphides, and Re-Os isotopes and elemental concentrations in silicate phases are also reported. This work, therefore, provides whole rock and mineral-scale insights into the PGE budget of gabbroic oceanic crust and the effects of subduction metamorphism on gabbroic and basaltic crust. Chondrite-normalised PGE patterns for the gabbros are similar to published mid-ocean ridge basalts (MORB), but show less inter-element fractionation. Mean Pt and Pd contents of 360 and 530 pg/g, respectively, are broadly comparable to MORB, but gabbros have somewhat higher abundances of Os, Ir and Ru (mean: 64, 57 and 108 pg/g). Transformation to eclogite has not significantly changed the concentrations of the PGE, except Pd which is severely depleted in gabbroic eclogites relative to gabbros (similar to 75% loss). In contrast, basaltic eclogites display significant depletion of Pt (>= 60%), Pd (>85%) and Re (50-60%) compared with published MORB, while Os, Ir and Ru abundances are broadly comparable. Thus, these data suggest that only Pt, Pd and Re, and not Os, Ir and Ru, may be significantly fluxed into the mantle wedge from mafic oceanic crust. Re-Os model ages for gabbroic and gabbroic eclogite minerals are close to age estimates for igneous crystallisation and high-pressure metamorphism, respectively, hence the HSE budgets can be related to both igneous and metamorphic behaviour. The gabbroic budget of Os, Ir, Ru and Pd (but not Pt) is dominated by sulphide, which typically hosts >90% of the Os, whereas silicates account for most of the Re (with up to 75% in plagioclase alone). Sulphides in gabbroic eclogites tend to host a smaller proportion of the total Os (10-90%) while silicates are important hosts, probably reflecting Os inheritance from precursor phases. Garnet contains very high Re concentrations and may account for >50% of Re in some samples. The depletion of Pd in gabbroic eclogites appears linked, at least in part, to the loss of Ni-rich sulphide. Both basaltic and gabbroic oceanic crust have elevated Pt/Os ratios, but Pt/Re ratios are not sufficiently high to generate the coupled Os-186-Os-187 enrichments observed in some mantle melts, even without Pt loss from basaltic crust. However, the apparent mobility of Pt and Re in slab fluids provides an alternative mechanism for the generation of Pt- and Re-rich mantle material, recently proposed as a potential source of Os-187-Os-186 enrichment.
ISSN: 0016-7037

hal-00420072
https://hal.archives-ouvertes.fr/hal-00420072
DOI : 10.1016/j.gca.2008.11.036

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This study addresses the mechanical and chemical behavior of allanite during shear zone formation under high-pressure metamorphism. Understanding physico-chemical processes related to the retention or resetting of Pb isotopes in allanite during geological processes is essential for robust petrochronology. Dating of allanite in meta-granodiorite showing variable amounts of strain (from an undeformed protolith to mylonite) at Monte Mucrone (Sesia Zone, NW Italy) gave surprising results. Based on structural and petrographic observations the shear zones at Mt Mucrone are Alpine, yet allanite located within an eclogite facies mylonite yielded Permian ages (208Pb/232Th average age: 287 ± 7 Ma). These mm-sized allanite grains are rimmed by an aggregate of coarse-grained garnet + phengite, thought to derive from former epidote. These aggregates were immersed in a weak matrix that experienced granular flow, and they were thus chemically and mechanically shielded during Alpine mylonitization. In undeformed samples (8a and 8b), two populations of epidote group minerals were found. Allanite forms either coronas around Permian monazite or individual grains with patchy zoning. Both types yield Permian ages (208Pb/232Th age: 291 ± 5 Ma). On the other hand, grains of REE-rich clinozoisite of Cretaceous age are found in undeformed rocks. These grains appear as small fragments with embayed surface outlines and minute satellites or rims around Permian allanite. These (re)crystallized grains are Sr-rich and show mosaic zoning. These results indicate that allanite crystals retained their chemical and isotopic characteristics, and thus their Permian age, as a result of strong strain partitioning between the epidote group porphyroclasts and the eclogite facies matrix in HP-mylonites. The observed partial mobilization of Pb isotopes, which lead to the Cretaceous-aged rims or grains in undeformed samples was facilitated by (re)crystallization of allanite and not by mere Pb diffusion alone under the HP conditions.
ISSN: 0024-4937

insu-00679730
https://hal-insu.archives-ouvertes.fr/insu-00679730
DOI : 10.1016/j.lithos.2011.01.005

Éditeur(s) : HAL CCSD Elsevier
Résumé : The Borborema Province, in the NE of Brazil, is a rather complex piece in the Brazil-Africa puzzle as it represents the junction of the Dahomeyide/Pharusian, Central African, Aracuai and Brasilia fold belts located between the West-African/Sao Luis, Congo/Sao Francisco and Amazonas craton. The correlation between the Dahomeyides from W-Africa (Ghana, Benin, Togo, and Mali) and the Borborema Province involves the Medio Coreau and Central Ceara domains. The inferred continuation of the main oceanic suture zone exposed in the Dahomeyides of W Africa is buried beneath the Phanerozoic Parnaiba Basin in Brazil (northwest of the Medio Coreau domain) where some high density gravity anomalies may represent hidden remnants of an oceanic suture. In addition to this major suture a narrow, nearly continuous strip composed of mainly mafic pods containing relics of eclogite-facies assemblages associated with partially migmatized granulite-facies metapelitic gneisses has been found further east in the NW Borborema Province. These high pressure mafic rocks, interpreted as retrograded eclogites, are located between the Transbrasiliano Lineament and the Santa Quiteria continental arc and comprise primitive to evolved arc-related rocks with either arc- or MORB-type imprints that can indicate either deep subduction of oceanic lithosphere or roots of continental and oceanic magmatic arcs. Average peak P-T conditions under eclogite-facies metamorphism (T=770 degrees C and P = 17.3 kbar) were estimated using garnet-clinopyroxene thermometry and Jd content in clinopyroxene. Transition to granulite-facies conditions, as well as later widespread re-equilibration under amphibolite facies, were registered both in the basic and the metapelitic rocks and suggest a clockwise P-T path characterized by an increase in temperature followed by strong decompression. A phenomenon possibly related to the exhumation of a highly thickened crust associated with the suturing of the Medio Coreau and Central Ceara domains, two distinct crustal blocks separated by the Transbrasiliano Lineament.
ISSN: 1342-937X

hal-00413584
https://hal.archives-ouvertes.fr/hal-00413584
DOI : 10.1016/j.gr.2009.01.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : Kyanite and zoisite eclogites from the Limousin area in the Variscan French Massif Central (FMC) have been formed by deep subduction to 100 km depth at peak pressure-temperature conditions around 660 degrees C and 2.9 GPa. They belong to a thin tectonic unit comprising ophiolitic bodies devoid of HP-UHP markers and forming a transported suture zone now separating the two main lithotectonic units of the FMC with contrasted tectono-metamorphic features (namely the lower and upper allochthons). Composition of kyanite-eclogites is similar to supra-subduction-zone basalts found in back-arc basins or island arcs (LREE depleted, negative Nb-Ta anomalies, epsilon Nd: +6 to + 9). Zoisite eclogites have the major-element fingerprint of ultrabasic iron-rich plagioclase cumulates common in continental intrusions or forming Fe-Ti oceanic gabbros. The distribution of immobile trace-elements is however more comparable to alkaline Si-undersaturated rocks, but the Sr-Nd isotopic composition (Sr-87/Sr-86: 0.706; epsilon Nd: -3 to -5) suggests the intervention of an old radiogenic crustal component during their petrogenesis. The chemical composition of hydrous zoisite eclogites does not correspond closely to a common magmatic precursor. It is thus suggested that, by analogy with well exposed worldwide HP-UHP terranes, at least a part of the trace-element signature and the isotopic compositions are controlled by channelled fluid-rock interaction at UHP conditions, leading to localised segregation of zoisite that hosts nearly all Sr and LREE budgets of the bulk eclogite. Sr-Nd isotopic composition of these samples is thus controlled by zoisite and probably represents the isotopic composition of the fluid phase present during UHP recrystallisation. Zircons extracted from a zoisite eclogite were dated in-situ by LA-ICP-MS. Crystallisation of the magmatic precursor is bracketed between 489 and 475 Ma and the UHP event is dated at 412 +/- 10 Ma, in agreement with published ages for the Eo-Variscan HP stage in the FMC. A late resetting of the U-Pb isotopic system at 382 +/- 7 Ma is related to an anatectic high pressure-medium pressure event, well known in the Limousin area. The oceanic eclogites dated in this study are tentatively linked to the Galicia-South Brittany Lower Paleozoic Ocean; they show evidence of the existence of an intra-oceanic subduction zone at the Lower Ordovician and of partial closure of this oceanic domain by ocean-continent subduction during Early Devonian.
ISSN: 0024-4937

hal-00512835
https://hal.archives-ouvertes.fr/hal-00512835
DOI : 10.1016/j.lithos.2010.05.013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In the South Tien Shan range (Kyrgyzstan), the Late Paleozoic geodynamic evolution remains debated especially to the west of the Talas-Fergana fault (TFF) fault where suture-related high-pressure (HP) rocks are scarce. We provide new petrological and geochronological data on garnet amphibolites from the Chatkal range, to the west of the TFF, northwest of the South Tien Shan suture. These rocks are retrogressed eclogites. We used a micro-mapping approach combined with forward modeling and empirical thermobarometry to decipher the P–T path of these amphibolitized eclogites. The metamorphic peak conditions culminated at 490 ± 50 °C and 18.5 ± 2 kbar and were followed by higher temperature retrogression (∼560 °C at 11–7 kbar). In order to constrain the age of the HP stage, we dated allanite crystals texturally coeval to the HP mineral assemblage. Allanite grains dated in situ with a U–Pb LA-ICPMS methodology yield an age of 301 ± 15 Ma. Compared with previously published data for the east of the TFF, these P–T constraints allow improving the understanding of the Late Paleozoic geodynamic evolution of the South Tien Shan. To the east of TFF, the Turkestan Ocean closed around 320 Ma with the collision of the Tarim Craton with the Kazakh microcontinent. To the west of TFF, the Turkestan Ocean closed around 300 Ma, when the Alai block collided with the Kazakh microcontinent. This later collision involved nappe-stacking and intense subvertical folding in the western South Tien Shan. This complex folding explains the S-shape of the suture to the west of the TFF that cannot be observed in the eastern part. These new data allow us to propose a distinct tectonic evolution of the two sides of the TFF, which suggests that this fault was a major transform fault before being a strike-slip intra-continental fault.
ISSN: 0264-3707

hal-01347171
https://hal.archives-ouvertes.fr/hal-01347171
DOI : 10.1016/j.jog.2015.06.005