Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Tethyan ophiolites show an apparent poorly organized association of ultramafic andmafic rocks. By contrast tothe complete mantle-crustal sections of Semail-type ophiolite sheets, Tethyan ophiolites are characterized by asmaller amount of mafic rocks (gabbros and basalts), by the absence of any sheeted dyke complex and by thefrequent occurrence of oceanic sediments stratigraphically overlying mantle-derived peridotites and associatedgabbroic intrusions. Therefore, they are considered as typical remnants of oceanic lithosphere formed in slowspreadingenvironment or in ocean–continent transition at distal passive margins. In the very first models offormation of the Tethyan ophiolites, in the years 1980, the geodynamical processes leading to mantle unroofingwere poorly understood due to the paucity of data and concepts available at that time from the present-dayoceans. In particular, at that time, little work had focused on the distribution, origin and significance ofmafic rocks with respect to the dominant surrounding ultramafics. Here, we reconsider the geology of sometypical metaophiolites from the Western Alps and Corsica, and we show how results from the past decadeobtained in the current oceans ask for reassessing the significance of the Tethyan ophiolites in general. Revisitedexamples include a set of representative metaophiolites from the blueschists units of the Western Alps (Queyrasregion) and from Alpine Corsica (Golo Valley). Field relationships between the ophiolitic basement andthe metasedimentary/metavolcanic oceanic cover are described, outlining a typical character of the Tethyanophiolite lithological associations. Jurassic marbles and polymictic ophiolite metabreccias are unconformablyoverlying the mantle-gabbo basement, in a way strictly similar to what is observed in the non-metamorphicAppennine ophiolites or Chenaillet massif. This confirms that very early tectonic juxtaposition of ultramafic andmafic rocks occurred in the oceanic domain before subduction. This juxtaposition resulted from tectonic activitythat is now assigned to the development of detachment faults and to the formation of Oceanic Core Complexes(OCCs) at the axis of slow spreading ridges. This fundamental Plate Tectonics process is responsible for the exhumationand for the axial denudation of mantle rocks and gabbros at diverging plate boundaries. In addition, fieldrelationships between the discontinuous basaltic formations and the ultramafic–mafic basement indicate that thistectonic stage is followed or not by a volcanic stage.We discuss this issue in the light of available field constraints.
ISSN: 0012-8252

insu-01096467
https://hal-insu.archives-ouvertes.fr/insu-01096467
https://hal-insu.archives-ouvertes.fr/insu-01096467/document
https://hal-insu.archives-ouvertes.fr/insu-01096467/file/Lagabrielle-EarthSciRev-2014.pdf
DOI : 10.1016/j.earscirev.2014.11.004

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
Rainbow is a dome-shaped massif at the 36°14′N nontransform offset along the Mid-Atlantic Ridge. It hosts three ultramafic-hosted hydrothermal sites: Rainbow is active and high temperature; Clamstone and Ghost City are fossil and low temperature. The MoMARDREAM cruises (2007, 2008) presented here provided extensive rock sampling throughout the massif that constrains the geological setting of hydrothermal activity. The lithology is heterogeneous with abundant serpentinites surrounding gabbros, troctolites, chromitites, plagiogranites, and basalts. We propose that a W dipping detachment fault, now inactive, uplifted the massif and exhumed these deep-seated rocks. Present-day deformation is accommodated by SSW-NNE faults and fissures, consistent with oblique teleseismic focal mechanisms and stress rotation across the discontinuity. Faults localize fluid flow and control the location of fossil and active hydrothermal fields that appear to be ephemeral and lacking in spatiotemporal progression. Markers of high-temperature hydrothermal activity (∼350°C) are restricted to some samples from the active field while a more diffuse, lower temperature hydrothermal activity (<220°C) is inferred at various locations through anomalously high As, Sb, and Pb contents, attributed to element incorporation in serpentines or microscale-sulfide precipitation. Petrographic and geochemical analyses show that the dominant basement alteration is pervasive peridotite serpentinization at ∼160–260°C, attributed to fluids chemically similar to those venting at Rainbow, and controlled by concomitant alteration of mafic-ultramafic units at depth. Rainbow provides a model for fluid circulation, possibly applicable to hydrothermalism at oceanic detachments elsewhere, where both low-temperature serpentinization and magmatic-driven high-temperature outflow develop contemporaneously, channeled by faults in the footwall and not along the detachment fault.
ISSN: 1525-2027

hal-01115281
https://hal.archives-ouvertes.fr/hal-01115281
https://hal.archives-ouvertes.fr/hal-01115281/document
https://hal.archives-ouvertes.fr/hal-01115281/file/Andreani_et_al-2014-Geochemistry%2C_Geophysics%2C_Geosystems.pdf
DOI : 10.1002/2014GC005269

Éditeur(s) : HAL CCSD AGU and the Geochemical Society
Résumé : International audience
The transition between the small melt lens observed on top of fast spreading ridge magma chambers and the overlying sheeted dike complex marks the interface between magma and the hydrothermal convective system. It is therefore critical to our understanding of fast spreading ridge accretion processes. We present maps of two areas of the Oman ophiolite where this transition zone is observed as continuous outcrops. Our observations, which include the base of the sheeted dike being crosscut by gabbros, are consistent with episodic dike injections in a steady state model but also suggest that the root of these dikes is commonly erased by vertical movements of the top of the melt lens. Dike assimilation is a possible mechanism for incorporating hydrated phases, which result from hydrothermal alteration, to the melt lens during upward migrations of its upper boundary. Upward migrations are also responsible for a granoblastic overprint of the root of the dikes that is also observed in the stoped diabase xenoliths. This granoblastic overprint attests to reheating of previously hydrothermally altered lithologies which can even trigger hydrous partial melting due to the lowering of the solidus of mafic lithologies by the presence of a water activity. Clinopyroxenes present in these granoblastic lithologies are typically low in Ti and Al content, thus strongly contrasting with corresponding magmatic clinopyroxene. This may attest to the recrystallization of clinopyroxenes after amphiboles under the peculiar conditions present at the root zone of the sheeted dike complex. Downward migrations of the top of the melt lens result in the crystallization of the isotropic gabbros at its roof, which represent the partly fossilized melt lens. Melt lens fossilization eventually occurs when magma supply is stopped or at the melt lens margins where the thermal conditions become cooler. Melt lens migration, recrystallization of hydrothermally altered sheeted dikes during reheating stages, and assimilation processes observed in the Oman ophiolite are consistent with the observations made in IODP Hole 1256D. We propose a general dynamic model in which the melt lens at fast spreading ridges undergoes upward and downward movements as a result of either eruption/replenishment stages or variations in the hydrothermal/magmatic fluxes.
ISSN: 1525-2027

hal-00445249
https://hal.archives-ouvertes.fr/hal-00445249
DOI : 10.1029/2009GC002652

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
To quantify the seismic properties of lower crustal rocks and to better constrain the origin of the lower crustal seismic reflectivity, we determined the complete 3-D seismic properties of a lower crustal section. Eight representative samples of the main lithologic and structural units outcropping in the Val Sesia (Ivrea zone) were studied in detail. The seismic velocities were calculated using the single crystal stiffness coefficients and the lattice preferred orientation (LPO) of each mineral in all samples. The 21 stiffness coefficients characterizing the elastic behaviour of each rock are determined. Mafic and ultramafic rocks such as pyroxenite and pyroxene-bearing gabbros display complex shear wave properties. These rocks are weakly birefringent (maximum 0.1 kms-1) and it is difficult to find consistent relationships between the seismic properties and the rock structure. On the other hand, seismic properties of deformed felsic rocks are essentially controlled by mica. They display strong S-wave birefringence (0.3 km s-1) and relatively high Vp anisotropy (7.6 per cent). Amphibole also strongly influences the rock birefringence patterns. For both kind of rocks, the foliation is highly birefringent and the fast polarized shear wave is systematically oriented parallel to the foliation. We show that the number of mineral phases in the rock strongly controls the anisotropy. The seismic anisotropy has a complex role in the P-wave reflectivity. Compared to the isotropic case, anisotropy enhances the reflection coefficient for about 60 per cent of the possible lithological interfaces. For 40 per cent of the interfaces, the reflection coefficient is much lower when one considers the medium as anisotropic.
ISSN: 0956-540X

hal-01389122
http://hal.univ-reunion.fr/hal-01389122
http://hal.univ-reunion.fr/hal-01389122/document
http://hal.univ-reunion.fr/hal-01389122/file/Barruol_3D_velocities_GJI1993.pdf
DOI : 10.1111/j.1365-246X.1993.tb01519.x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
CMAS 3D, developed in MATLAB (R), is a program to support visualization of major element chemical data in three dimensions. Such projections are used to discuss correlations, metamorphic reactions and the chemical evolution of rocks, melts or minerals. It can also project data into 2D plots. The CMAS 3D interface makes it easy to use, and does not require any knowledge of Matlab (R) programming. CMAS 3D uses data compiled in a Microsoft Excel (TM) spreadsheet. Although useful for scientific research, the program is also a powerful tool for teaching.
ISSN: 0098-3004

hal-00413570
https://hal.archives-ouvertes.fr/hal-00413570
DOI : 10.1016/j.cageo.2008.07.002

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Hengshan–Wutaishan area forms part of the Trans-North-China Belt developed in response to the final amalgamation of the North China Craton. The Hengshan–Wutaishan domain and the adjacent Fuping massif constitute the largest and the most representative rock exposure of the belt. A structural study allows us to redefine the lithological and tectonic units that compose the architecture of the Hengshan–Wutaishan domain and to propose that the Trans-North-China Belt was built up through a polyphase tectonic evolution within the period 1900–1800 Ma. The first event (D1) corresponds to the emplacement of lower and upper nappes herein called the Orthogneiss and Volcanites Unit (OVU) and the Low Grade Mafic Unit (LGMU), respectively. The OVU mainly consists of rocks formed in an arc system and metamorphosed under amphibolite facies conditions whereas the LGMU represents oceanic crust rocks with an arc component that underwent greenschist facies metamorphism. The syn-metamorphic D1 deformation is characterized by a NW–SE stretching and mineral lineation with a top-to-the SE sense of shear that corresponds to the sense of nappe motion. U–Th–Pb EPM chemical dating on monazite from three metapelites of the OVU gives isochron ages of 1884 ± 11 Ma, 1886 ± 5 Ma and 1887 ± 4 Ma interpreted as the age of the prograde amphibolite facies metamorphism coeval with nappe stacking during the D1 event. The D2 event corresponds to a widespread crustal melting of the OVU and underlying basement rocks in Hengshan related to the exhumation of deep structures about 20–30 Ma after nappe emplacement. The migmatite contains blocks of retrograded eclogite or HP-granulitic restites, and is characterized by the development of a pervasive foliation and lineation. A late D2 event represents the folding stage of the migmatitic foliation during the completion of a dome. The Orthogneiss and Volcanics Unit and the Low Grade Mafic Unit are unconformably covered by weakly metamorphosed to unmetamorphosed sediments belonging to the Hutuo Supergroup. These rocks were deformed by a ductile D3 event, characterized by a NW–SE trending stretching lineation, south verging folds associated with an axial planar slaty cleavage and an E–W crenulation lineation. Lastly, a left-lateral wrenching along the Zhujiafang Shear Zone separates the northern and southern parts of Hengshan. Our structural study shows that there is no significant difference between the southern part of Hengshan and the lower part of Wutaishan, both areas belong to OVU and show a structural continuity. We suggest that this polyphase deformation developed in response to a north-westward subduction of an old ocean, named the Lüliang Ocean, beneath the Western Block, followed by collision with a micro-continent named the Fuping Block at not, vert, similar1890–1880 Ma.
ISSN: 0301-9268

insu-00151594
https://hal-insu.archives-ouvertes.fr/insu-00151594
https://hal-insu.archives-ouvertes.fr/insu-00151594/document
https://hal-insu.archives-ouvertes.fr/insu-00151594/file/Trap-PrecambrainResearch-2007.pdf
DOI : 10.1016/j.precamres.2007.03.001

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 Wiley-Blackwell
Résumé : International audience
Although lawsonite-bearing rocks are rare in exhumed high-pressure (HP) terranes, they are considered to exert a primary role in subduction dynamics. Recent observations in natural settings have shown that fluid–rock interaction at HP conditions, including metasomatism, may lead to unusually high lawsonite amounts even in rocks that originally contained little or no lawsonite. This process may therefore bear important implications for element recycling in subduction zones. A detailed characterization of the geochemical fingerprints associated with lawsonite metasomatism is presented in this contribution. The studied rocks belong to the HP terranes of Alpine Corsica (France), which is the largest documented exposure for lawsonite metasomatism. Metasomatic lawsonite displays complex compositional zoning, including high trace element, Cr and Ti content. The trace element content is much higher compared with the average of non-metasomatic lawsonite, and is in line with the re-incorporation of large amounts of trace elements (e.g. REE, Sr, Pb, Th) in the rock during metasomatism, as shown by mass transfer calculations. Our data suggest that serpentinites represented the main fluid source for the metasomatism, with concurrent contribution of other, possibly Ca-rich lithologies, such as mafic or meta-sedimentary rocks. We propose that the breakdown of metasomatic lawsonite may contribute to the genesis of magmas and their characteristic geochemical signatures.
ISSN: 0263-4929

hal-01107395
https://hal.archives-ouvertes.fr/hal-01107395
DOI : 10.1111/jmg.12074

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Timétrine massif exposed west of the Pan-African suture zone in northeastern Mali belongs to the passive margin of the West African craton facing to the east intra-oceanic arc assemblages and 730 Ma old pre-collisional calc-alkaline plutons. The Timétrine lithologic succession includes from the base to the top Mesoproterozoic cratonic to passive margin formations overlain by deep-sea Fe-Mg schists. Submarine metabasalts and two ultramafic massifs of serpentinized mantle peridotites are inserted as olistoliths towards the top whereas turbidites of continental origin represent the younger unit. Field and petrological data have revealed a distinct metasedimentary sequence attached to the serpentinized peridotites. It essentially consists of impure carbonates, Fe jaspers and polymictic breccias containing altered blocks of mantle peridotites, most rocks being enriched in detrital chromite. This association is interpreted as reworked chemical and detrital sediments derived from the alteration of mafic-ultramafic rocks. It is argued that mantle exhumation above sea floor took place during the Neoproterozoic rifting and crustal thinning period under possible tropical conditions, as suggested by the large volume of silicified serpentinites. In spite of greenschist facies metamorphic overprint characterized by widespread Fe-rich blue amphiboles that are not diagnostic of high-pressure conditions, it is possible to reconstruct a former ocean-continent transition similar to that evidenced for the Mesozoic period, followed by the deposition of syn-to post rift terrigeneous turbidites roughly coeval with ocean spreading some time before 800 Ma. It is concluded that the serpentinite massifs were tectonically emplaced first in an extensional setting, then incorporated within deep-sea sediments as olistoliths and finally transported westward during late Neoproterozoic collisional tectonics onto the West African craton.
ISSN: 0899-5362

hal-00985586
https://hal.archives-ouvertes.fr/hal-00985586
DOI : 10.1016/j.jafrearsci.2013.11.004

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
This paper presents a reappraisal of the tectonic evolution of the Zanhuang Massif that lies at the eastern margin of the Trans-North China Orogen, a continent–continent collision belt that marked the amalgamation of the North China Craton in Late Paleoproterozoic. Detailed field work with focus on geometries of structures and kinematics was performed. This was completed with LA-ICP-MS U–Pb analyses on zircon, EPMA U–Th/Pb dating on monazite and 40Ar/39Ar dating on amphibole. These studies led us to propose a new three-fold litho-tectonic subdivision of the massif: The Western Zanhuang Domain (WZD) made of TTG, migmatite and pink anatectic granite is correlated to the Fuping Massif that crops out to the north-west. Both areas represent a continental block, called the Fuping Block, which acquired most of its architecture around 2100 Ma ago. The Eastern Zanhuang Domain (EZD) made of TTG and migmatite represents the western edge of an Eastern Neoarchean Block. In between, the Central Zanhuang Domain (CZD) is a NE–SW trending stack of supracrustal, gneiss and mafic magmatic rocks thrust sheets displaced toward the ESE upon the Eastern Block. The lithological features suggest that the CZD represents the remnant of an oceanic basin, called the Taihang Ocean that closed during the amalgamation of the Eastern Block and the Fuping Block around 1880–1850 Ma. In agreement with recent work done along the western margin of the belt, in the Lüliang Massif, this study documents the amalgamation of the North China Craton in response to the closure of two oceanic basins, namely the Lüliang Ocean and the Taihang Ocean. West-dipping subductions and collisions involving three distinct continental blocks, called the Western, the Fuping and the Eastern Blocks, took place around 1880–1850 Ma.
ISSN: 0301-9268

insu-00403871
https://hal-insu.archives-ouvertes.fr/insu-00403871
https://hal-insu.archives-ouvertes.fr/insu-00403871/document
https://hal-insu.archives-ouvertes.fr/insu-00403871/file/Trap-PrecambrianResearch-2009.pdf
DOI : 10.1016/j.precamres.2009.03.011

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We carried out detailed petrographic, major and trace element geochemical, microstructural and FTIR analyses on eight characteristic ultramafic xenoliths from Nyos and Barombi Mbo Lakes in the continental sector of the Cameroon Volcanic Line (CVL). The studied xenoliths are spinel lherzolites showing lithologies similar to the other xenoliths reported previously along the CVL. They have protogranular and porphyroclastic textures. One of the Barombi xenolith contains amphibole, which had not been previously reported in this locality. Amphibole is common in the Nyos xenoliths suite. Peridotite xenoliths from both localities show some chemical heterogeneity, but Barombi xenoliths generally are less depleted in basaltic elements with respect to Nyos xenoliths. Trace element compositions of Nyos spinel lherzolites show a moderately depleted initial (premetasomatic) composition and variable enrichment in REE. Evidence for both modal and cryptic metasomatism is present in Nyos xenoliths. Rare earth element patterns of clinopyroxene suggest that interaction between mafic melts and the upper mantle occurred beneath the Nyos locality. Barombi Mbo xenoliths, on the other hand, record a small degree of partial melting. The Barombi Mbo xenoliths have weak, dominantly orthorhombic olivine crystal preferred orientations, whereas Nyos ones have strong axial-[010] patterns, which may have formed in response to transpression. Nominally anhydrous mantle minerals (NAMs) of the Barombi Mbo xenoliths show generally higher bulk concentrations of ‘water’ (70–127 ppm) than Nyos xenoliths (32–81 ppm). The Barombi Mbo xenoliths could originate from a juvenile segment of the lithospheric mantle, which had been originally part of the asthenosphere. It became a part of the lithosphere in response to thermal relaxation following the extension, forming a weakly deformed lower lithospheric mantle region along the CVL. The Nyos xenoliths, however, represent a shallow lithospheric mantle bearing imprints of several depletion and enrichment events probably prior or following the extension (at ∼30 Ma).
ISSN: 0899-5362

hal-01236039
https://hal.archives-ouvertes.fr/hal-01236039
DOI : 10.1016/j.jafrearsci.2015.07.006

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
In the Tifnoute Valley, three plutonic units have been defined: the Askaoun intrusion, the Imourkhssen intrusion and the Ougougane group of small intrusions. They are made of quartz diorite, granodiorite and granite and all contain abundant mafic microgranular enclaves (MME). The Askaoun granodiorite and the Imourkhssen granite have been dated by LA-ICP-MS on zircon at 558 ± 2 Ma and 561 ± 3 Ma, respectively. These granitic intrusions are subcontemporaneous to the widespread volcanic and volcano-detrital rocks from the Ouarzazate Group (580-545 Ma), marking the post-collisional transtensional period in the Anti-Atlas and which evolved towards alkaline and tholeiitic lavas in minor volume at the beginning of the Cambrian anorogenic intraplate extensional period. Geochemically, the Tifnoute Valley granitoids belong to an alkali-calcic series (high-K calc-alkaline) with typical Nb-Ta negative anomalies and no alkaline affinities. Granitoids and enclaves display positive εNd-560Ma (+0.8 to +3.5) with young Nd-TDM between 800 and 1200 Ma and relatively low 87Sr/86Sr initial ratios (Sri: 0.7034 and 0.7065). These values indicate a mainly juvenile source corresponding to a Pan-African metasomatized lithospheric mantle partly mixed with an old crustal component from the underlying West African Craton (WAC). Preservation in the Anti-Atlas of pre-Pan-African lithologies (c. 2.03 Ga basement, c. 800 Ma passive margin greenschist-facies sediments, allochthonous 750-700 Ma ophiolitic sequences) indicates that the Anti-Atlas lithosphere has not been thickened and was never an active margin during the Neoproterozoic. After a transpressive period, the late Ediacaran period (580-545 Ma) is marked by movement on near vertical transtensional faults, synchronous with the emplacement of the huge Ouarzazate Group and the Tifnoute Valley granitoids. We propose here a geodynamical model where the Tifnoute Valley granitoids as well as the Ouarzazate Group were generated during the post-collisional metacratonic evolution of the northern boundary of the West African craton. The convergence with the peri-Gondwanan active margin produced brittle fracturing of the cratonic boundary without thickening, allowing rising of magmas. The Tifnoute Valley granitoids display a metasomatized lithospheric mantle source mixed with a minor ancient (2 Ga) continental crust component from the underlying WAC.
ISSN: 0930-0708

halsde-00782152
https://hal.archives-ouvertes.fr/halsde-00782152
DOI : 10.1007/s00710-012-0245-3