Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Maghrebides, Betics and some parts of the Calabrian, NE Sicilian and Tuscan allochtons constitute dismembered fragments of the Alpine belt that resulted from the Cenozoic collision between Africa and Eurasia and the opening of the Western Mediterranean basin. Mineral and whole-rock geochemical analyses have been performed on three distinct outcrops of mantle material from the Edough Massif of NE Algeria, namely the Bou Maiza (BM) gabbros, the La Voile Noire (LVN) amphibolites and the Sidi Mohamed (SM) peridotites. In all samples, Sr isotopes are largely affected by seawater alteration (87Sr/86Sract. > 0.70384 and up to 0.70888) and cannot be used to evaluate the nature of the source reservoirs. SM peridotites display variable depleted mantle Nd isotopic signatures (εNdact. from + 7.0 to + 12.2) and geochemical features suggesting no significant chemical depletion as a result of partial melting and melt extraction (Mg# < 90; slightly LREE-depleted patterns with La/YbN = 0.33-0.39). These rocks are interpreted as parts of the subcontinental lithospheric mantle incorporated into the crustal units of the Edough Massif during the early stages of opening of the Algerian basin. BM gabbros and LVN amphibolites show geochemical signatures indicating derivation from a common Depleted Mantle reservoir (εNd > + 7.9) and are likely cogenetic, but without filiation with the SM peridotites. Pb isotopes indicate a contribution of sediments in the source reservoir, which is attributed to contamination solely by hydrous fluids released from a sedimentary component. This observation, together with a LILE-enrichment, suggests a back-arc basin environment. These results indicate that BM and LVN units were obducted onto the North African margin and subsequently fragmented, probably during doming and exhumation of the lower continental crust of the Edough massif. Doming resulted in opposite movements of the overlying oceanic units, southward for the BM units and northward for LVN amphibolites.
ISSN: 0040-1951

hal-01122777
https://hal.archives-ouvertes.fr/hal-01122777
DOI : 10.1016/j.tecto.2014.03.031

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : National audience
Late Triassic submarine alkali basalts and hawaiites were collected from two superimposed tectonic slices belonging to the Kara Dere – Sayrun unit of the Middle Antalya nappes, southwestern Turkey. New determinations on conodont faunas allow to date this sequence to the Lower Carnian (Julian). The volcanic rocks show rather homogeneous compositions, with high TiO2 and relatively low MgO and Ni contents which suggest olivine fractionation. Their primitive mantle-normalised multi-elements plots show Nb and Ta enrichments relative to La, Pb negative anomalies and heavy rare earth element and Y depletions typical of intraplate ocean island basalts. These characteristics are consistent with the major and trace element compositions of their primary clinopyroxene phenocrysts, which do not show any feature ascribable to crustal contamination. The studied lavas display a restricted range of {varepsilon}Nd (+4.6 to +5.2) which falls within the range of ocean island basalts. Their initial (143Nd/144Nd)i ratios are too low to be explained by a simple mixing line between depleted MORB mantle (DMM) and HIMU components. Their Pb and Nd isotopic compositions plot along a mixing line between HIMU component and an enriched mantle, the composition of which could be the result of the addition of about 5 to 8% of an EM2 component (recycled marine sediments) to DMM. The lack of evidence for any continental crustal component in their genesis could be consistent with their emplacement in an intra-oceanic setting.
EISSN: 1777-5817

insu-00338829
https://hal-insu.archives-ouvertes.fr/insu-00338829
DOI : 10.2113/gssgfbull.179.4.397

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : We present whole-rock major- and trace-element and Nd–Sr–Pb radiogenic isotope data for Cretaceous igneous suites from eastern Cuba. These rocks are related to the Greater Antilles paleo-island arc magmatism and have three different igneous styles. Group 1 consists of tholeiitic basalts and rare basaltic andesites that have normal mid-ocean ridge basalt (N-MORB)-like compositions similar to those found in back-arc basin basalts (TiO2 = 1·2–2·9 wt%; La/Yb(N) = 0·7–0·9, Th/Nb = 0·06–0·08, and initial 208Pb/ 204Pb = 37·65–37·74). Group 2 comprises basaltic and rare basaltic andesitic subvolcanic dykes with major- and trace-element and isotopic compositions similar to those of island arc tholeiites (TiO2 = 0·7–1·4 wt%; La/Yb(N) = 0·6–0·9, Th/Nb = 0·06–0·68, and initial 208Pb/ 204Pb = 37·74–38·25). Group 3 is composed of low-Ti (TiO2 = 0·3–0·9 wt%) calcalkaline igneous rocks that have an unambiguous subduction-related character (La/Yb(N) = 1·1–5·0, Th/Nb = 0·35–1·55, and initial 208Pb/ 204Pb = 37·94–38·39). The parental magmas of the three groups formed by variable melting degrees (< 5–25%) of spinel lherzolite, with more depleted mantle sources for Groups 2 and 3 than Group 1. The trace-element and radiogenic isotope compositions of primitive Group 3 samples are strongly bimodal. One subgroup of samples is characterized by low Ta/Yb (0·02–0·03) and Th/La (0·10–0·13), slightly subchondritic Nb/Ta (13·3–17·3), and relatively high initial 206Pb/204Pb (18·57–18·62) and {varepsilon}Nd (7·6–9·4). The remaining primitive Group 3 samples have higher Ta/Yb (0·06–0·11) and Th/La (0·24–0·32), and highly subchondritic Nb/Ta (7·6–9·1), coupled with lower initial 206Pb/204Pb (18·24–18·29) and {varepsilon}Nd (3·4–5·5). These signatures were induced by two distinct slab components that mainly reflect the contributions of Cretaceous Atlantic marine and North American continental sediments, respectively. Nb/Ta in the first subgroup was influenced by melting of rutile-bearing subducted crust, whereas in the second it was inherited from recycled continental sediments. The involvement of Atlantic and North American sediments in Cuban Cretaceous magmatism indicates that the Proto-Caribbean (North American-Proto Atlantic) lithosphere subducted beneath the Greater Antilles arc during the Late Cretaceous (pre-Campanian), consistent with geotectonic models involving onset of SW-dipping subduction beneath the Greater Antilles paleo-arc during the Aptian. The variable mantle source depletion and magnitude of the subduction component probably reflect different settings across the arc, from the arc front to a back-arc spreading ridge.
ISSN: 0022-3530

hal-00406373
https://hal.archives-ouvertes.fr/hal-00406373
DOI : 10.1093/petrology/egm040

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Parentis and Arzacq-Mauleon basins located in front of the V-shaped oceanic propagator in the Bay of Biscay present evidence for extreme crustal thinning. In this paper we investigate the 3D structure of these rift basins, based on field observations and the interpretation of seismic data. We compare these results with those obtained from two different and independent inversion methods: first a 3D gravity inversion and second the standard Euler deconvolution.;For the Mauleon Basin our results show that the positive gravimetric anomaly identified above its southern margin is the consequence of two shallower high density bodies that are separated by the Pamplona fault and a deeper high density body. The high density bodies can be explained by the presence of mid-crustal and mantle rocks that were exhumed or uplifted at shallower depth during Early Cretaceous rifting before they were reworked and integrated to the Pyrenean chain during compression phase. Also, during this reactivation phase, some slices of the exhumed mid-crustal and mantle rocks were sheared off and were integrated in the present-day thrust belt in the Mauleon basin. For the Parentis Basin we can demonstrate, based on seismic data and gravimetric inversion methods, a decrease in extension from west to east, which is compatible with the V-shape geometry of the overall basin. Along strike, a change in the fault geometry from downward concave top-basement detachment faults to upward concave high-angle faults can be observed eastwards, i.e. towards the termination of the basin. A key structure, controlling the evolution of the Parentis Basin, is the east-west trending Ibis fault. We interpret this fault to have initially formed as a strike slip fault before it was reactivated during later crustal thinning. At present, it forms the limit between an upper plate sag basin to the north and a lower plate sag basin, floored at least locally by a top-basement detachment faults to the south. The strong asymmetry of the basin is supported by the shape of the basin and the results of standard Euler deconvolution.
ISSN: 0040-1951

hal-00523444
https://hal.archives-ouvertes.fr/hal-00523444
DOI : 10.1016/j.tecto.2010.04.023

Éditeur(s) : The Geological Society of London
Résumé : Conservational models, like simple shear, pure shear or polyphase models that exclude exchanges between the lower continental crust and upper mantle, are usually proposed to explain the lithospheric stretching and consequent crustal thinning of passive continental margins. These models need large amounts of horizontal movement, and have, therefore, important implications for plate kinematic reconstructions and intraplate deformation. In this paper we propose to show these implications in the Central Segment of the South Atlantic Ocean. In the Angola–Brazilian system, these models imply about 240 km of horizontal movement. This movement can be compensated by two end-member mechanisms: (1) an intraplate deformation located in Africa; and (2) an intraplate deformation located in South America. We detail for each solution the strong geological and geodynamical implications, and discuss the consequences for the genesis of passive continental margins.
Geological Society, special publication (0375-6440) (The Geological Society of London), 2012-02 , Vol. 369 , P. 75-90

Droits : The Geological Society of London 2012
http://archimer.ifremer.fr/doc/00087/19839/17757.pdf
DOI:10.1144/SP369.5
http://archimer.ifremer.fr/doc/00087/19839/

Éditeur(s) : HAL CCSD Elsevier Masson
Résumé : In Vietnam, the Triassic Indosinian collision affected coevally the Truong Son belt and the Kontum Massif,which were not independent tectonic units, but parts of the same Gondwana-derived Indochina continental block. This thermotectonic event took place synchronously throughout Vietnam, during the Lower Triassic 250-240-Ma time interval, as demonstrated by numerous geochronological data, combining Ar-Ar and U-Pb dating methods. Structural and kinematic investigations, in the Indosinian metamorphic rocks, reveal that the collisional process resulted from a consistent northwest-striking convergence of Indochina with respect to the adjacent blocks. It is suggested that this motion was taken up by a pair of opposite subduction zones: to the north, beneath South China, and to the west, beneath western Indochina, along the Song Ma and Po Ko sutures, respectively. Tectonic markers, calc-alkaline subduction-related volcanism and granitic intrusions and the generation of high-pressure rocks that have been recently discovered support this geodynamic setting, at least along Po Ko. Along the northwest-trending Song Ma zone, the obliquity of the convergence during subduction and subsequent collision resulted in the development, within the Truong Son Belt, of a set of subparallel dextral mylonitic shear zones, under amphibolite-facies metamorphism. The intermediate segments remained weakly metamorphic or even almost devoid of metamorphism. Along Po Ko, the convergence was near-orthogonal, with a left-lateral strike-slip component; the ongoing continental subduction resulted in the reworking of the Kontum granulitic basement and the development of Indosinian HP granulitic conditions; the subsequent extension-related exhumation operated approximately in the same northwestwards direction. This Indosinian evolution, applied on a continental crust that had been probably affected, as in South China, by a Caledonian-related event, as judged by the general unconformity of the Lower Devonian sediments, the widespread occurrence of magmatic crystallisation ages of ca 450 Ma (Ordovician-Silurian), and by the approximately similar age of the primary granulitic episode in the Kontum Massif. The similarities of the Devonian facies over central, northern Vietnam and South China imply a land connection, possibly as a consequence of a Caledonian collision along Song Ma, but this does not preclude a further oceanic opening and a closure during the Indosinian.
ISSN: 1631-0713

hal-00411591
https://hal.archives-ouvertes.fr/hal-00411591
DOI : 10.1016/j.crte.2007.10.005

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Dahomey belt in Togo and Benin is an important segment of the larger West Gondwana Orogen. Here, we review the geodynamic evolution of the Dahomey belt and discuss new U–Pb and Lu–Hf zircon data in light of similar data previously acquired on the geologically related Northern Borborema Province, in NE Brazil. Eighteen samples from different tectonic settings and regions within the belt were collected for zircon isotopic investigation. Passive margin deposits of the Atacora Structural Unit and lower units of the Volta Basin have detrital zircon signatures compatible with the flanking West Africa Craton. The arc-related magmatism resulted from the east-dipping subduction of the Goiás-Pharusian oceanic lithosphere and is represented by a variety of granitoids emplaced in the Benino-Nigerian Shield between 670 and 610 Ma. These granitoids were mainly sourced from crustal reservoirs with subordinate juvenile input. Detrital zircon ages from syn-orogenic deposits in Benino-Nigerian Shield suggest that arc development could have started as early as 780 Ma. The main period of melting in the internal part of the belt, the Benino-Nigerian Shield, is related to crustal thickening and occurs only ca. 30 m.y. after initiation of the continental collision, marked by the ca. 610 Ma ultra-high pressure (UHP) metamorphism recorded at Lato Hills. Foreland development represented by the upper units of the Volta basin developed soon after continental collision and persisted with the development of the west-verging thrust front synchronously with the main period of crustal melting due to collision at ca. 580 Ma. The subvertical Transbrasiliano Lineament in South America, that corresponds to the Kandi Lineament in Africa, provides a present-day fit between NW Africa and NE Brazil. Restoration of the movement of the Transbrasiliano-Kandi Lineament (strike-slip plate boundary) places the Dahomey belt and Borborema Province (NE Brazil) along the same section of the West Gondwana Orogen. This configuration would explain some of the misfits previously discussed in the literature and aligns the UHP eclogites in Togo and NE Brazil.
ISSN: 0301-9268

hal-01358584
https://hal.archives-ouvertes.fr/hal-01358584
DOI : 10.1016/j.precamres.2016.01.032

Éditeur(s) : HAL CCSD
Résumé : The northern Mirdita ophiolite massifs in Albanian Dinarides formed at a slow-spreading ridge, active during the Jurassic (160–165 Ma). They share a common horizontal Jurassic–Lower Cretaceous sedimentary cover showing that they were not deeply and intrinsically affected by later Alpine thrusting. The western massifs of Mirdita, first oceanic core complex (OCC) and detachment shear zone described in ophiolites, compare with OCCs in slow-spreading ridges and provide continuous exposure of the deep internal structure of this system, revealing its kinematics, thanks to detailed structural mapping in peridotites and gabbros. The Mirdita detachments root in the Moho transition zone (MTZ), a weak zone at the top of asthenospheric mantle, where basaltic melts impregnate dunites. The OCC domes are plagioclase-amphibole–bearing mylonitic peridotites, ∼400 m thick, grading downward within 200 m to harzburgitic mantle. The mylonitic detachments crossed Moho beneath a NNE-SSW–trending ridge. On the western side of OCC domes, the hanging wall of the ridge, crustal gabbros, and basalts are still preserved, despite being deeply affected by hydrothermal alteration. From there, the partially molten MTZ was detached as a shear zone, mixing with lower gabbros. The OCC emerged, migrating upsection and eastward over 5 km. Finally, the OCC front is observed in hornblende-rich syntectonic mylonites derived from upper gabbros and from the overlying former lid. Serpentinization is static within these mylonites. A low-temperature detachment fault is expressed as a sheared antigoritic mélange at the margin of the mylonitic shear zone. Asthenospheric flow in the harzburgitic mantle beneath the ridge of origin has been preserved below the OCC rooting. The dominant asthenospheric flow direction trends parallel to the ridge axis. This mantle flow rotates over 200 m into the low-temperature mylonitic detachments, where OCC motion turns transversal to the ridge. Crystal preferred orientation measurements on six samples point to brown hornblende crystal growth during mylonitic flow and illustrate the change of olivine intra-crystalline slip system in mylonites compared to porphyroclastic harzburgite.
ISSN: 1553-040X

hal-01504440
https://hal.archives-ouvertes.fr/hal-01504440
DOI : 10.1130/GES01383.1

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The 1415 m deep IODP Hole U1309D (Mid-Atlantic Ridge, 30 degrees N) is the second deepest hole drilled into slow spreading oceanic lithosphere. The recovered section comprises essentially gabbroic rocks, with a large range of compositions. The most primitive end-members of the gabbroic sequence, herein referred to as olivine-rich troctolites (ol > similar to 70%), have textures and geochemical compositions intermediate between that of mantle peridotites and primitive cumulates, indicative of melt impregnation processes. We carried out a detailed microstructural study to further characterize the petrogenetic processes leading to their formation, as well as discuss their mode of emplacement and relationship with neighboring mantle lithosphere. In olivine-rich troctolites, olivines range from coarse-grained subhedral crystals, commonly containing well-developed subgrains, to medium-grained rounded crystals with fewer or no substructures. They are embedded in large, undeformed pyroxene and plagioclase poikiloblasts. Olivine substructures reveal dislocation creep that is consistent with activation of the main high-temperature slip systems, dominantly (010)[100]. Olivine crystallographic preferred orientation is very weak but generally shows a relatively stronger, uncommon [001] concentration. These unusual olivine fabrics are interpreted as resulting from melt impregnation of a previously deformed olivine matrix: the solid olivine framework is disrupted by olivine corrosion along grain and subgrain boundaries, and the high-temperature plastic fabric is modified in a liquid-dominated regime. Based on mineral composition and fabrics and in comparison with what is observed in impregnated mantle rocks elsewhere, we posit that olivine represents relicts of mantle peridotites disaggregated by large melt influx, although the mantle origin of olivine is not unequivocally demonstrated yet. Whatever the initial lithology, impregnation by large volumes of melt has strongly modified the original composition and microstructure. If the mantle origin hypothesis is correct, the original olivine fabric could have been efficiently weakened by dunitization prior to disruption of the olivine framework by melt impregnation. Incorporation, at the base of the lithosphere, of small slivers of impregnated dunite into gabbroic sections, trapped between successive igneous units, may be a common mechanism of lower crustal accretion at slow spreading ridges. Extensive melt-rock interaction processes are expected to contribute significantly to the final chemical composition of erupted lavas.
ISSN: 1525-2027

hal-00496391
https://hal.archives-ouvertes.fr/hal-00496391
DOI : 10.1029/2009GC002995

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Recent geochemical work shows that subduction-zone serpentinites are repositories for fluid-mobile elements absorbed during interaction with sediment-derived fluids. Unraveling the geochemical fingerprint of these rocks helps to define timing of tectonic accretion of sediments along the subduction interface and the role of serpentinite in element recycling to volcanic arcs. Here we present the trace element and isotopic composition (B–O–H, Sr, Pb) of high-pressure serpentinites from the Voltri Massif (Ligurian Western Alps, Italy), to discuss their role as incompatible element carriers and their contribution to recycling of sediment-derived components in subduction zones.The serpentinites presented here record metamorphic olivine growth during eclogite-facies metamorphism and show undeformed and mylonitic textures. Field relations show that undeformed rocks are enclosed in deformed ones and that no metasedimentary rocks are present nearby. Undeformed serpentinite has very high δ11BSRM951 (from +26‰ to +30‰), low Sr and Pb isotope ratios (87Sr/86Sr = 0.7053–0.7069; 206Pb/204Pb = 18.131–18.205) and low As and Sb contents (0.1 and 0.01 μg/g, respectively). Oxygen and hydrogen isotope compositions are +4.5‰ and −67‰, respectively. In contrast, mylonitic serpentinite shows lower δ11B (from +22‰ to +17‰), significant enrichment in radiogenic Sr and Pb isotopes (87Sr/86Sr up to 0.7105; 206Pb/204Pb up to 18.725), and enrichment in As and Sb (1.3 and 0.39 μg/g, respectively). δ18O of the mylonitic serpentinites reaches values of +5.9‰, whereas δD is comparable with that of undeformed rocks (approximately −70‰). In mylonitic serpentinites, the B and Sr isotopic values and the fluid-mobile element (FME) concentrations are near those for the Voltri metasedimentary rocks (calc- and mica-schists). Pb systematics also reveal influx of a crust-derived component.Our dataset shows that undeformed serpentinite still preserves an oceanic geochemical fingerprint, whereas mylonitic serpentinite is reset in its concentrations of FME and its B, Sr and Pb isotope compositions, due to interaction with sediment- and crust-derived fluids. The environment of this interaction is either compatible with (i) an outer-rise zone setting, with percolation of seawater-derived fluids enriched in sedimentary components into bending-related fault structures, or with (ii) subduction channel domains, where ascending sediment-derived slab fluids infiltrate slices of former oceanic serpentinite accreted to the plate interface domain. Influx of sediment-derived subduction fluids along major deformation zones in serpentinite modifies the element budget of the rocks, with important implications for element recycling and the tectonic history of serpentinite. The B, Sr and Pb isotopic systematics, coupled with FME concentration in serpentinites are particularly helpful geochemical tracers of interaction between different reservoirs in subduction-interface environments, and are more sensitive than the traditionally applied stable oxygen and hydrogen isotope compositions.
ISSN: 0016-7037

hal-01419940
https://hal.archives-ouvertes.fr/hal-01419940
DOI : 10.1016/j.gca.2016.06.034

Éditeur(s) : HAL CCSD Elsevier Masson
Résumé : International audience
Late Jurassic coral reefs are present in the Western High Atlas of Morocco along the eastern paleomargin of the young Atlantic Ocean. The Izwarn section shows two reef units made of scleractinian corals and microbialites, included in a progradational shallowing-upward succession. Changes in reefal composition and structure observed in a ca. 50 cm-thick interval at the base of the reef unit 1 are interpreted to represent the first well-documented ecological succession in Late Jurassic reefs. Bioclastic deposits, sparse and poorly diversified corals (five genera) preserved in life position, and finally a more diversified coral assemblage (11 genera) building a reef framework with stromatolitic crusts reflect pioneer, colonization and diversification stages, respectively. A specific depth window, where light was reduced but sufficient for a catch-up growth mode and below the erosive action of waves, probably at the lower slope of a fore-reef area, has made possible the development and the record of this ecological succession. With a more diversified coral assemblage (16 genera), the reef unit 2 reflects shallower and well-agitated waters of a fore-reef area. Overlaying this second reef unit, the upper part of the section reveals back-reef deposits made of reworked reef elements, whereas a putative reef crest area is not recorded.
ISSN: 0016-6995

hal-00795549
https://hal.archives-ouvertes.fr/hal-00795549
DOI : 10.1016/j.geobios.2012.05.002

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Several recent studies have documented that reactions between melt and crystal mush in primitive gabbroic rocks (via reactive porous flow) have an important control in the formation of the lower oceanic crust and the evolution of MORBs. In this context, olivine-rich rocks can form either by fractional crystallization of primitive melts or by open system reactive percolation of pre-existing (possibly mantle-derived) olivine matrix. To address this question, we performed in-situ trace element analyses (by LA-ICP-MS) of olivine from the Erro-Tobbio ophiolite Unit (Ligurian Alps), where mantle peridotites show gradational contacts with an hectometer-scale body of troctolites and plagioclase wehrlites, and both are cut by later decameter-wide lenses and dykes of olivine gabbros. Previous studies inferred that troctolites and olivine gabbros represent variably differentiated crystallization products from primitive MORB-type melts. Olivines in the three rock types (mantle peridotites, troctolites, olivine gabbros) exhibit distinct geochemical signature and well-defined elemental correlations. As expected, compatible elements (e.g. Ni) show the highest concentrations in peridotites (2580–2730 ppm), intermediate in troctolites (2050–2230 ppm) and lowest in gabbros (1355–1420 ppm), whereas moderate incompatible elements (e.g. Mn, Zn) show the opposite behaviour. By contrast, highly incompatible elements like Zr, Hf, Ti, HREE are variably enriched in olivines of troctolites, and the enrichment in absolute concentrations is coupled to development of significant HFSE/REE fractionation (ZrN/NdN up to 80). AFC modelling shows that such large ZrN/NdN ratios in olivines are consistent with a process of olivine assimilation and plagioclase crystallization at decreasing melt mass, in agreement with textural observations. In-situ trace element geochemistry of olivine, combined with microstructural investigations, thus appears a powerful tool to investigate reactive percolation and the origin of olivine-rich rocks in the lower oceanic crust.
ISSN: 0016-7037

hal-01419938
https://hal.archives-ouvertes.fr/hal-01419938
DOI : 10.1016/j.gca.2016.06.029

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The Calabro-Peloritan belt constitutes the eastward termination of the southern segment of the Alpine Mediterranean belt. This orogenic system was built up during the convergence between the Eurasian and the African plates, roughly directed North-South since the Upper Cretaceous. It was subsequently fragmented during the opening of the Western Mediterranean basins since Oligocene times. The curved shape of the Calabro-Peloritan belt was acquired during the opening of the Tyrrhenian basin since Tortonian. The origin, kinematics and significance of the Calabro-Peloritan tectonic pile are still debated. Our data in the Aspromonte Massif of Southern Calabria reveal an Alpine history marked by two main superimposed kinematic regimes. (i) A first phase corresponds to the piling up of basement nappes with a top-to-the-SE vergence, i.e in a direction orthogonal to the belt trend and towards the Adriatic foreland. This external vergence is similar to what is observed in both Northeastern Sicily and Northern Calabria. In Sicily, the age of nappe piling is Alpine, as evidenced by pinched slices of Mesozoïc sediments. In the Aspromonte Massif, thrusting age is less constrained. Our data suggest remnants of late Hercynian structuration before the Alpine stacking. (ii) A second phase corresponds to the thinning of the continental crust, dated at around 30 Ma by both geochronological and stratigraphical data. This extension is mainly localized on two low-angle detachment contacts, with top-to-the-NE displacement. The lower one corresponds to the reworking of the former main nappe contact. The upper one is a large detachment fault cutting across the pile from upper sedimentary levels down to metamorphic basement. Extension of similar Alpine age and similar internal vergence has been already recognized in other parts of the Calabro-Peloritan Arc: in the basement nappes of Northeastern Sicily and in the ophiolitic units of Northern Calabria. Coming back to the original geometry and position of the Calabro-Peloritan belt, before its bending and the opening of the Liguro-Provençal and Tyrrhenian basins, we evidence a homogeneous Oligocene NE-SW extension all along the Calabro-Peloritan segment of the Alpine Mediterranean belt. This tectonometamorphic history is best explained within the framework of the continuous Tertiary westward dipping subduction of the Tethyan oceanic domain below the European active margin and the progressive eastward retreat of the Apennine trench since Oligocene times.
ISSN: 0278-7407

hal-00283182
https://hal.archives-ouvertes.fr/hal-00283182
https://hal.archives-ouvertes.fr/hal-00283182/document
https://hal.archives-ouvertes.fr/hal-00283182/file/Heymes_et_al_2008.pdf
DOI : 10.1029/2007TC002157

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Tectonic mixing of slab- and mantle-derived materials at the interface between converging plates highly enhances fluid-mediated mass transfer from the slab to the overlying mantle. Subduction mélanges can provide information about the interaction among different slices accreted at plate interface domains, with implications on the tectonic and geochemical evolution of the plate-interface itself. At Cima di Gagnone, pelitic schists and gneiss enclose chlorite harzburgite and garnet peridotite lenses, like in subduction mélanges located in-between downgoing slabs and overlying mantle. These peridotites host MORB-type eclogite and metarodingite, and derive from dehydration of serpentinized mantle protoliths. Their enrichment in fluid-mobile B, As, Sb, U, Th is the result of an early-stage oceanic serpentinization, followed by interaction with host metasediments during subduction burial. Here we define the element exchange process in the Gagnone mélange by means of the B, Sr and Pb isotope analysis of its main lithologies (ultramafic, mafic rocks and paragneiss). The 87Sr/86Sr and 206Pb/204Pb ratios of ultramafic rocks (0.7090–0.7124 and 18.292–18.837, respectively) show enrichments in radiogenic Sr and Pb after exchange with the host paraschist (up to 0.7287 87Sr/86Sr; 18.751 206Pb/204Pb). The δ11B values of peridotites (down to −10‰) point to a combined effect of (1) 11B release to deserpentinization fluids (serpentinized protoliths likely had positive δ11B and lower radiogenic Sr, Pb), and of (2) exchange with fluids from the surrounding metasediments. The whole Gagnone rock-suite is finally overprinted by retrograde fluids that essentially bring to an increase in radiogenic Pb (about 19.0 206Pb/204Pb) and to values of 0.710 87Sr/86Sr and of −10‰ δ11B. The recognition of different stages of interaction between mantle rocks and sedimentary/crustal reservoirs allows us to define the geochemical effects related to the early coupling of such rocks along the plate-interface. Our study shows that ultramafic rocks involved in subduction-zone metamorphism and serpentinization uptake radiogenic Pb and Sr released by associated sedimentary reservoirs. The exchange process envisioned here is not only representative of subduction mélanges: it can also be a proxy of mass transfer between slab and serpentinized supra-subduction mantle, as occurs in forearcs. Dehydration of the Gagnone-type serpentinized mantle releases crust-derived components to arcs, without direct involvement of metasediment dehydration and/or melting in subarc environments. The retention of appreciable amounts of fluid-mobile elements, radiogenic Pb and Sr in dehydrated Gagnone peridotites has implications on element recycling in the deep Earth’s mantle.
ISSN: 0016-7037

hal-01174137
https://hal.archives-ouvertes.fr/hal-01174137
DOI : 10.1016/j.gca.2015.04.024

Éditeur(s) : HAL CCSD
Résumé : International audience
Understanding subduction rheology in both space and time has been a challenge since the advent of plate tectonics. We herein focus on "subduction infancy", that is the first ~1-5 My immediately following subduction nucleation, when a newly born slab penetrates into the upper plate mantle and heats up. The only remnants of this critical yet elusive geodynamic step are thin metamorphic soles, commonly found beneath pristine, 100-1000 km long portions of oceanic lithosphere emplaced on continents (i.e., ophiolites).Through the (i) worldwide compilation of pressure-temperature conditions of metamorphic sole formation augmented by pseudosection thermodynamic modeling, (ii) calculations of the viscosity of materials along the plate interface and (iii) generic numerical thermal models, we provide a conceptual model of dynamic plate interface processes during subduction infancy (and initiation s.l.).We show in particular how major rheological switches across the subduction interface control slab penetration, and the formation of metamorphic soles. Due to the downward progression of hydration and weakening of the mantle wedge with cooling, the lower plate (basalt, sediment) and the upper plate (mantle wedge) rheologies equalize and switch over a restricted temperature-time-depth interval (e.g., at ~800°C and ~1 GPa, during 0.1-2 My, for high-temperature metamorphic sole formation). These switches result in episodes of maximum interplate mechanical coupling, thereby slicing the top of the slab and welding pieces (basalt, sediment) to the base of the mantle wedge. Similar mechanical processes likely apply for the later, deeper accretion and exhumation of high-temperature oceanic eclogites in serpentinite mélanges, or for the accretion of larger tectonic slices.This model provides constraints on the effective rheologies of the crust and mantle and general understanding, at both rock and plate scale, for accretion processes and early slab dynamics.
AGU Fall Meeting 2015
San Francisco, United States

insu-01242103
https://hal-insu.archives-ouvertes.fr/insu-01242103

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The ODP/IODP multileg campaign at ODP Site 1256 (Cocos plate, eastern equatorial Pacific) provides the first continuous in situ sampling of fast spreading ocean crust from the extrusive lavas, through the sheeted dikes and down into the uppermost gabbros. This paper focuses on a detailed petrographic and microanalytical investigation of the gabbro section drilled during IODP Expedition 312. The marked patchy and spotty features that can be observed in many Hole 1256D gabbros is mostly due to a close association of two different lithological domains in variable amounts: (1) subophitic domains and (2) a granular matrix. Major and trace element mineral compositions, geothermometry, and petrological modeling suggest that subophitic and granular domains follow one single magma evolution trend formed by in situ fractionation. The subophitic domains correspond to the relative primitive, high-temperature end-member, compositionally similar to the basalts and dikes from the extrusive unit upsection, while the granular domains fit with a magma evolution by crystal fractionation to lower temperatures, up to a degree of crystallization of similar to 80%. Our results support the following scenario for the fossilization of the axial melt lens at ODP Site 1256: relatively primitive MORB melts under near-liquidus conditions fill the melt lens and feed the upper, extrusive crust. Near the melt lens-sheeted dike boundary at lower temperatures, crystallization starts with first plagioclase before clinopyroxene in a mushy zone forming the subophitic domains. At decreasing temperatures, the subophitic domains continue to crystallize, finally forming a well-connected framework. Evolved, residual melt is finally trapped within the subophitic network, crystallizing at near-solidus conditions to the granular matrix. Another important textural feature in Hole 1256D gabbros is the presence of microgranular domains which are interpreted as relics of stoped/assimilated sheeted dikes (transformed to "granoblastic dikes" by contact metamorphism). All these different domains can be observed in close association, often at the thin section scale, demonstrating the extremely complex petrological record of combined crystallization/assimilation processes ongoing in the axial melt lens. Very similar gabbros with a marked spotty/patchy appearance, and bearing the same close association of lithological domains as observed at Site 1256, are known in the so-called "varitextured gabbro" unit from the Oman Ophiolite located at the same structural level, between cumulate gabbros and granoblastic dikes. The close petrological similarity of the gabbro/dike transition between both IODP Hole 1256D and the Oman ophiolite suggests that in situ fractionation and dike assimilation/contamination are major magmatic processes controlling the dynamics and fossilization of the axial melt lens at fast spreading oceanic ridges.
ISSN: 1525-2027

hal-00644798
https://hal.archives-ouvertes.fr/hal-00644798
DOI : 10.1029/2011GC003655

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 Société géologique de France
Résumé : International audience
The present study proposes to estimate the influence of climate, eustatism and local tectonics on the sedimentation of a basin margin at the syn-rift to post-rift transition. For that, paleomagnetic measurements were performed on a marine marly-calcareous sedimentary succession ranging from Upper Oligocene to Lower Miocene and located on the northern margin of the Liguro-Provençal basin. The magnetostratigraphic record is correlated to the reference geomagnetic polarity scale [ATNTS04, Lourens et al. 2004] with the help of biostratigraphy based on calcareous nannofossils and planctonic foraminifers [Oudet et al., 2010]. The resulting age model shows that the 100 m-thick sedimentary succession covers a time span of 5 m.y. from the Late Chattian to the Early Burdigalian. Despite several exposure surfaces and a change in the sedimentation rate, no significant hiatus of sedimentation is documented. In addition, we also estimate the paleoenvironmental evolution through the sedimentary succession. Comparing the dated paleoenvironmental reconstruction with global {delta}18O and sea level curves [Miller et al., 2005], we show that the Carry-le-Rouet succession is an excellent paleoclimatic archive. Indeed, coral reefs developed at the glacial-interglacial stage transition marking the end of the Oligocene. In addition, the most diversified coral reefs occurred during the warmest period of the Aquitanian. During rifting, bathymetric variations recorded in the studied succession are related to local synsedimentary tectonics whereas, during oceanic crust accretion, global sea level changes influence the sedimentation. This result allows to characterise and to accurately date the break-up unconformity at 20.35 Ma.
EISSN: 1777-5817

hal-00619040
https://hal.archives-ouvertes.fr/hal-00619040
DOI : 10.2113/gssgfbull.182.1.37

Éditeur(s) : HAL CCSD Nature Publishing Group
Résumé : International audience
High 3He/4He ratios in some basalts have generally been interpreted as originating in an incompletely degassed lower-mantle source1, 2, 3, 4, 5, 6, 7, 8, 9. This helium source may have been isolated at the core-mantle boundary region since Earth's accretion4, 5, 6. Alternatively, it may have taken part in whole-mantle convection and crust production over the age of the Earth7, 8, 9; if so, it is now either a primitive refugium at the core-mantle boundary8 or is distributed throughout the lower mantle7, 9. Here we constrain the problem using lavas from Baffin Island, West Greenland, the Ontong Java Plateau, Isla Gorgona and Fernandina (Galapagos). Olivine phenocryst compositions show that these lavas originated from a peridotite source that was about 20 per cent higher in nickel content than in the modern mid-ocean-ridge basalt source. Where data are available, these lavas also have high 3He/4He. We propose that a less-degassed nickel-rich source formed by core-mantle interaction during the crystallization of a melt-rich layer or basal magma ocean5, 6, and that this source continues to be sampled by mantle plumes. The spatial distribution of this source may be constrained by nickel partitioning experiments at the pressures of the core-mantle boundary.
ISSN: 0028-0836

hal-00941025
https://hal.archives-ouvertes.fr/hal-00941025
DOI : 10.1038/nature11771