Éditeur(s) : HAL CCSD The Geochemical Society of Japan
Résumé : International audience
The Western Cordillera of Ecuador consists of Cretaceous crustal fragments of oceanic plateaux and superimposed insular arcs, which were accreted to the northwestern South American margin during the Late Cretaceous and Paleocene. Slices of high-grade metabasites, ultramafic rocks, gabbros and basalts, unmetamorphosed radiolarian cherts and scarce garnet-bearing metasediments were randomly exhumed along Miocene to Recent transcurrent faults crosscutting the Western Cordillera. The basalts show geochemical characteristics of oceanic plateau basalts (flat REE patterns, La/Nb = 0.85). The gabbros differ from the basalts in having lower REE levels, positive Eu anomalies, and negative Nb and Ta anomalies; they are interpreted as resulting from arc magmatism. The amphibolites and banded amphibolites have major and trace element chemistry similar to that of oceanic plateau basalts (flat REE patterns, La/Nb = 0.86) or to cumulate gabbros. The granulite shares with oceanic plateaus similar trace element chemistry (flat REE patterns, La/Nb < 1) and εNdi values (+7.6). Continent-derived metasediments are depleted in heavy REE (La/Y = 4.8) and have a negative Eu anomaly. Foliated lherzolites, melagabbronorites and pyroxenites consist of serpentinized olivine + cpx + opx ± Ca-plagioclase. Lherzolites, melagabbronorites and pyroxenites are LREE depleted with positive Eu anomalies, while the harzburgite displays a U-shaped REE pattern. The trace element abundances of the ultramafic rocks are very low (0.1 to 1 times the chondritic and primitive mantle values). The ultramafic rocks represent fragments of depleted mantle, deformed cpx-rich cumulate, and continental lithospheric mantle or mantle contaminated by subduction-fluid. Except the scarce quartz-rich metasediments, all these rocks likely represent remnants of accreted oceanic crustal fragments and associated depleted mantle. Since these samples were randomly sampled at depth by the fault, we propose that the Western Cordillera and its crustal root are mainly of oceanic nature.
ISSN: 0016-7002

insu-00679228
https://hal-insu.archives-ouvertes.fr/insu-00679228

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
The Iberian Massif is the best exposed segment of the European Variscan Belt. It includes relatively well preserved terranes that were accreted by transpression along time and resulted in a number of geotectonic units that formed part of the Late Paleozoic assembly of the Pangaea Supercontinent. In SW Iberia, these units are the Central Iberian Zone (CIZ), Ossa Morena Zone (OMZ), and the South Portuguese Zone (SPZ). A 210 km long NE-SW magnetotelluric profile was carried out through the CIZ, from the OMZ-CIZ boundary toward the north, reaching the Tagus (Cenozoic) basin. Data dimensionality analysis resulted in a suitable 2-D electrical resistivity structure, allowing a 2-D inversion of the data set. Complementary available geophysical data (deep seismic, gravity and aeromagnetic) and a comparison with a detailed geological cross section led us to constrain the interpretation of the 2-D electrical resistivity structure of the CIZ crust. The results show, for the upper crust, the existence of diverse conductive/resistive bodies that correlate well with known geological features (sedimentary basins, faults, granitic plutons, mineralized systems). A mild but steady conductive band is located along the middle and lower crust that is interpreted as a mafic granulite basement. The upper section of this band connects with several elongated shallow conductors, providing further evidence for the existence, in the Central Iberian Zone, of a complex decollement system where the major faults are rooted. Such a crustal architecture is viewed as the northward continuation of the Variscan large-scale structures previously recognized in the southern sectors (OMZ and SPZ).
ISSN: 0148-0227

hal-00671036
https://hal.archives-ouvertes.fr/hal-00671036
DOI : 10.1029/2010JB007538

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Field and petrostructural investigations in the Central Mauritanides provide new precisions on the polyorogenic character of the nappe edifice of this belt. The upper structural unit exposed in the Gaoua area includes anatectic gneisses and high-grade metasediments affected by medium temperature, high-pressure metamorphism and cut by granitoids. They represent a basement unconformably overlain by a monometamorphic cover, the Gaoua Group, of assumed lower Paleozoic age. Late Paleozoic regional metamorphism of metapelites from this cover is characterized by pyrophyllite-kyanite-chloritoid assemblages equilibrated at T around 420 degrees C and P = 1 GPa. In the lower structural unit exposed in the south around Boufkerine-Farkaka, metapelites of the Gadel Group display slightly retrogressed garnet-kyanite-staurolite-rutile assemblages that equilibrated at T around 600 degrees C and P >= 1.2 GPa before the intrusion of 639 Ma old plutons. The Gadel Group locally overlies in unconformity polycyclic gneisses. This continental assemblage represents an outboard terrane inserted between two monocyclic greenschist facies terranes: remnants of oceanic lithosphere in the east and arc-derived metasediments and metavolcanics cut by 670 Ma old calc-alkaline plutons in the west. All units have been involved in late Paleozoic nappes emplaced to the east above the West African craton and its late Neoproterozoic to Cambro-ordovician cover. Though an east-directed vergence is locally recorded in the Neoproterozoic units, most early E-W trending stretching and mineral lineations are synchronous with greenschist facies metamorphism developed during the Late Paleozoic and roughly coeval with the Applachian nappe system.
ISSN: 0899-5362

hal-00424501
https://hal.archives-ouvertes.fr/hal-00424501
DOI : 10.1016/j.jafrearsci.2008.09.004

Éditeur(s) : HAL CCSD Taylor & Francis
Résumé : Modern geochronology has moved beyond the acquisition of dates: the goal is to understand the significance of these numbers for the geodynamic evolution of Earth at all scales. The coupling of the laser-ablation microprobe (LAM) to inductively coupled plasma mass spectrometers (ICPMS, multicollector (MC)-ICPMS) has revolutionised geochronology and geochemistry over the last 10 years. These systems enable the rapid and precise in situ analysis of trace-element patterns and isotopic systems, while adding information related to microstructural context and major-element composition. The integration of these multiple sources of data is crucial in constraining the origin of the sample and the processes leading to its formation, so that we can understand the meaning of a date in terms of geological events. LAM-ICPMS measurement of U-Pb ages and trace-element patterns in zircon, coupled with LAM-MC-ICPMS analysis of Hf isotopes in the same grains, gives new insights into the processes of magma genesis. Applied to detrital zircons from modern drainages or sedimentary rocks (the TerraneChron approach), it becomes a powerful tool to investigate problems of crustal evolution on scales ranging from single terranes to continents. The in situ analysis (LAM-MC-ICPMS) of Re-Os systematics in single grains of sulfides in mantle-derived peridotites has demonstrated that most mantle rocks contain several generations of Os-bearing sulfides; whole-rock analyses are mixtures reflecting multiple melting and metasomatic events in the lithospheric mantle. These deep-seated events are commonly mirrored in the crust; Os model-age spectra from xenolith suites show age 'peaks' that correspond to the ages of thermal/tectonic events in the overlying crust, suggesting strong linkages between crust and mantle. Integrated studies of the timing and nature of crustal and mantle events, using these techniques, will be important for understanding the large-scale dynamics of the Earth.
ISSN: 0812-0099

hal-00411328
https://hal.archives-ouvertes.fr/hal-00411328
DOI : 10.1080/08120090802097450

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
We present new Sr-Nd-Pb isotope data of the western Alpujárride metamorphic basement and the pre-Miocene Flysch sediments of the Betic Cordillera (southern Spain). Nd model ages are consistent with an increasing detrital input from the Alborán domain to the Flysch Trough in the western Mediterranean during the late Oligocene. The Alpujárride metamorphic crustal rocks derived from Archean-Paleoproterozoic terranes located along the northern margin of Gondwana in the Neoproterozoic. The heterogeneous isotopic signatures of the Alpujárride units indicate that they have different sedimentary protoliths and underwent contrasted Variscan and pre-Variscan tectono-magmatic evolutions. Melts/fluids derived from the western Alpujárride gneisses contaminated the mantle source of the Ronda high-Mg pyroxenite dykes, implying that the Alpujárride lower crust underthrusted the subcontinental lithospheric mantle of the Alborán domain generating subduction-like magmatism in the late Oligocene. The western Alpujárride upper crust is involved in the Neogene volcanism of the Alborán Sea basin, but only contaminated some LREE-enriched calc-alkaline lavas erupted along the continental margins. On the other hand, tholeiitic lavas in the center of the basin show no isotopic evidence of crustal assimilation. This indicates that most of the crust in the central Alborán Sea accreted by Miocene tholeiitic magmatism and that Alpujárride lower crust is absent and likely foundered close to the continental margins of the basin.
ISSN: 0024-4937

hal-01526912
https://hal.archives-ouvertes.fr/hal-01526912
DOI : 10.1016/j.lithos.2016.10.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Field study, thermochronology and geochemistry of the east Lungmu Co (LMC) range highlight some of the geological events that shaped western Tibet. The LMC fault zone has long been interpreted as the boundary between the Tianshuihai terrane of Laurasian affinity and the Qiangtang block of Gondwanian affinity. In the LMC range, the Paleozoic series is intruded by the Mangtsa leucogranite whose zircon have a U/Pb age of 116.9 ± 1 Ma and by mafic rocks with U/Pb zircon ages ranging from 116.9 ± 1 to 95.1 ± 1.7 Ma. Geochemistry of the mafic rocks indicates that they have been emplaced in a supra-subduction zone setting, probably the north dipping Nujiang suture zone. 40Ar/39Ar micas ages of the granite indicate that cooling below ~ 350 °C occurred between 105 and 85 Ma. 40Ar/39Ar K-feldspar data suggest a fast cooling event at 60-55 Ma, which we relate to the reactivation of the LMC suture zone as a thrust at the onset of the India-Eurasia collision. The last, and still active, deformation event corresponds to left-lateral strike-slip faulting along the ENE-WSW LMC fault.
ISSN: 1342-937X

hal-00660491
https://hal.archives-ouvertes.fr/hal-00660491
DOI : 10.1016/j.gr.2011.07.026

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Alpine Corsica consists of a stack of variably metamorphosed units of continental and Tethys-derived rocks. It represents an excellent example of high-pressure (HP) orogenic belt, such as the Western Alps, exposed over a small and accessible area. Compared to the Western Alps, the geology of Alpine Corsica is poorly unraveled. During the 1970s-80s, based on either lithostratigraphic or metamorphic field observations, various classifications of the belt have been proposed, but these classifications have been rarely matched together. Furthermore, through time, the internal complexity of large domains has been progressively left aside in the frame of large-scale geodynamic reconstructions. As a consequence, major open questions on the internal structure of the belt have remained unsolved. Apart from a few local studies, Alpine Corsica has not benefited of modern developments in petrology and basin research. This feature results in several uncertainties when combining lithostratigraphic and metamorphic patterns and, consequently, in the definition of an exhaustive architecture of the belt. In this paper we provide a review on the geology of Alpine Corsica, paying particular attention to the available lithostratigraphic and metamorphic classifications of the metamorphic terranes. These data are completed by a new and exhaustive metamorphic dataset obtained by means of thermometry based on Raman Spectroscopy of Carbonaceous Material (RSCM). This technique provides reliable insights on the peak temperature of the metamorphic history for CM-bearing metasediments. A detailed metamorphic characterization of metasediments, which have been previously largely ignored due to retrogression or to the lack of diagnostic mineralogy, is thus obtained and fruitfully coupled with the available lithostratigraphic data. Nine main tectono-metamorphic units are defined, from subgreenschist (ca. 280-300 °C) to the lawsonite-eclogite-facies (ca. 500-550 °C) condition. These units are homogeneous in metamorphism, laterally continuous and have characteristic lithostratigraphic features. This study also suggests a direct link between the pre-orogenic extensional setting and the present-day compressional structure of Alpine Corsica, indicating that large sections of subducted lithosphere were subducted and exhumed as coherent domains. These features provide important insight on the mechanism of stacking and exhumation of HP rocks, and make Alpine Corsica a unique reference for mountain-building processes in Tethyan-type orogens.
ISSN: 0012-8252

hal-00814433
https://hal.archives-ouvertes.fr/hal-00814433
DOI : 10.1016/j.earscirev.2012.10.003

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The eastern margin of the Tibetan Plateau is a key region to understand the formation and tectonic evolution of the plateau. Most studies concentrated on the Longmen Shan range that corresponds to the tectonically very active boundary between the Tibetan Plateau and the Sichuan Basin. This work addresses the deformation and exhumation pattern in the poorly understood metamorphic Danba dome situated at the conjunction between the southern Longmen Shan range and the major strike-slip Xian Shui He fault. The Early Mesozoic and Cenozoic evolution of the dome are then compared to the regional evolution of eastern Tibet. The about 830 Ma zircon U–Pb ages obtained on the Gezong and Gongcai gneiss as well as U–Pb ages on inherited zircons in the Early Mesozoic granites confirm the occurrence of Neo-Proterozoic magmatic episodes in the Danba basement at 800–900 Ma and 650–750 Ma. Following the closure of the Palaeotethys Ocean, granitoids were emplaced in the Danba metamorphic basement and Triassic cover from about 230 Ma to about 170 Ma. The western part of the Lian He Kau granitic massif, emplaced (between 210 and 200 Ma) within the Triassic cover sequence of the dome was only slightly affected by the Early Mesozoic deformation. However, in the eastern, deeper level compartment of the massif, ductile deformation occurred between 200 and 190 Ma, contemporaneous with the onset of the Barrovian-type metamorphism dated at 200–180 Ma. That deformation is compatible with the Early Mesozoic deformation observed in the Longmen Shan range and with the development of the large-scale décollement level in the Songpan Garzê area. Apatite fission track data then indicate that, following the Oligocene–Middle Eocene onset of eastward motion of the Songpan Garzê terrane, the Danba dome developed as a crustal-scale antiform during the Late Miocene–Early Pliocene (10–4 Ma). The exhumation was controlled by the Lian He Kau fault (a major transpressive structure parallel to the Xian Shui He fault) and the associated Manai thrust faults. Regionally, while the Oligocene–Middle Miocene exhumation phase seems to have been of regional extent, the Late Miocene to Present day phase is localised along the Longmen Shan range thrusts as well as along the back-thrusts formed by the Lian He Kau, Manai and associated faults.
ISSN: 0743-9547

insu-01117657
https://hal-insu.archives-ouvertes.fr/insu-01117657
DOI : 10.1016/j.jseaes.2015.02.009

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In the Schistes Lustres of Alpine Corsica (France) serpentinized mantle rocks are associated with continental basement and meta-volcanic/-seclimentary cover rocks. The relationships among these different lithologies are especially well exposed in the Monte San Petrone unit, where Alpine metamorphism reached lawsonite-eclogite conditions. The contact between serpentinites and slivers of continental basement, relatively flat-lying over several kilometers, is characterized by evidence of cataclastic deformation pre-dating Alpine High-Pressure ductile fabrics. The serpentinite/continental basement pair is stratigraphically overlain by metasediments with a typical Jurassic-Cretaceous supra-ophiolitic lithostratigraphy, with metaradiolarites passing upward to marbles and cal cschists. Noticeably, no evidence of cataclastic deformation is found in metasediments. These observations indicate that the lithostratigraphy of the Monte San Petrone unit was established during a pre-Alpine polyphase evolution, which culminated in extensive brittle deformation along a flat-lying detachment fault prior to the deposition of Jurassic sediments. We suggest that the inferred Mesozoic extensional tectonics were related to the opening of the Westem Tethys. The Mesozoic architecture of the Monte San Petrone area, which is typical of an Ocean-Continent Transition (OCT) zone, was preserved despite Alpine deformation and metamorphism, when the different lithologies (i.e. meta-ophiolites, continent-derived rocks and metasediments) underwent a common metamorphic evolution, culminating at T=490-550 degrees C and P= 2.2-2.6 GPa. Similar tectonostratigraphic associations are observed in other high-pressure terranes of Alpine Corsica, suggesting that inherited OCT-type domains may be common in Alpine-type orogens.
ISSN: 0024-4937

hal-00617744
https://hal.archives-ouvertes.fr/hal-00617744
DOI : 10.1016/j.lithos.2011.02.013

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Intraplate deformation is most often linked to major stress applied on plate margins. When such intraplate events are accompanied by magmatism, the use of several dating methods integrated within a multidisciplinary approach can bring constraints on the age, nature and source mobilized for generating the magma and in turn on the nature of the intraplate deformation. This study focuses on the large gabbro Arrikine sill (35 km in extension) emplaced within the Silurian sediments of the western margin of the Murzuq cratonic basin in southeastern Algeria. Its emplacement is dated during the early Devonian (415-400 Ma) through the determination of a reliable paleomagnetic pole by comparison with the Gondwana Apparent Polar Wander Path (APWP). This age can be correlated with deep phreatic eruptions before Pragian time thought to be at the origin of sand injections and associated circular structures in Algeria and Libya. For the sill, the K–Ar age of 325.6 ± 7.7 Ma is related to a K-rich aplitic phase that has K-enriched by more than 20% the Devonian gabbro. Laser-ICP-MS U–Pb method dates only inherited zircons mostly at c. 2030 Ma with additional ages at c. 2700 Ma and younger ones in the 766-598 Ma age range. The Arrikine sill is a high-Ti alkaline gabbro having the geochemical composition of a hawaiite akin to several intraplate continental and oceanic provinces, including the contemporaneous Aïr ring complexes province in Niger, but also to the Mauna Loa volcano in Hawaii. This peculiar composition akin to that of the contemporaneous Aïr province is in agreement with a lower Devonian age for the Arrikine sill.The lower Devonian Arrikine sill emplacement is related to a “Caledonian” transtensive reactivation of the western metacratonic boundary of the Murzuq craton. This event also generated in the Saharan platform the so-called “Caledonian unconformity” of regional extension, the Aïr ring complexes and magmatic rocks that produced sand injections. It could be related to rifting of the Hun terranes that occurred at the plate margin to the north (Stampfli and Borel, 2002, Blackey, 2008 and references therein). The mid-Carboniferous (c. 326 Ma) reactivation corresponds to Variscan compression on NW Africa generating aplitic fluids, but also to the major “Hercynian unconformity” of regional extension. The generation of the Arrikine magma is attributed to partial melting through adiabatic pressure release of uprising asthenosphere along tectonically reactivated mega-shear zones, here bordering the relictual Murzuq craton enclosed in the Saharan metacraton.
ISSN: 0899-5362

hal-01306637
https://hal.archives-ouvertes.fr/hal-01306637
DOI : 10.1016/j.jafrearsci.2015.11.019

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
U-Pb zircon and Ar-Ar amphibole and biotite ages from rocks of the West Congo Belt of Angola indicate that this area underwent two main deformation events of amphibolite facies grade at c. 540 and 490 Ma, which were followed by tectonically assisted exhumation during eastward thrusting of the hinterland domain onto the foreland domain. High-grade conditions in the West Congo Belt are 20-40 Ma younger than in the Ribeira-Araçuai Belt of Brazil, its South American counterpart, or than in the westernmost Kaoko Belt of Namibia, its African correlative. In the present state of knowledge, a more appropriate counterpart to the southern part of the West Congo Belt may be restricted to the Cabo Frio Terrane in the eastern Ribeira Belt, which yields a broadly similar evolution.
ISSN: 0954-4879

hal-00716463
https://hal.archives-ouvertes.fr/hal-00716463
DOI : 10.1111/j.1365-3121.2012.01060.x

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : The Triassic fold belt of North Tibet is mainly composed, from west to east, of the Bayan Har, Songpan-Garze, and Yidun (or Litang-Batang) terranes. The Indosinian orogeny results from interactions between the South China, North China and Qiangtang (North Tibet) blocks during the closure of the Palaeotethys. A synthesis of the tectonic and geochronological data available on this belt is presented and a new geodynamic model of its formation is proposed. At the end of the Permian, a synchronous activity along three subduction zones, Kunlun-Anyemaqen to the north, Jinsha to the south and Yushu-Batang to the east, induced the growth of wide accretionary orogens until the end of the Triassic period. The onset of subduction in Tibet is contemporaneous with Indosinian tectonism in Indochina (pre-Norian). However, the main tectonic events that lead to the closure of the Tethysian basin and the subsequent building of the Triassic belts are younger (220-200 Ma).
ISSN: 0010-7999

hal-00411202
https://hal.archives-ouvertes.fr/hal-00411202
DOI : 10.1016/j.crte.2007.10.014

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Dating the timing of motion on crustal shear zones is of tremendous importance for understanding the assembly of orogenic terranes. This objective is achieved in this paper by combining petrological and structural observations with novel developments in in situ U-Th-Pb geochronology of allanite. A greenschist facies shear zone within the Mont Blanc Massif is documented. Allanite is synkinematic and belongs to the mylonitic assemblage. LA-ICP-MS U-Th-Pb isotope analyses of allanite reveal high contents and highly radiogenic isotopic compositions of the common-Pb component. The use of measured Pb-isotope compositions of associated minerals (feldspars and chlorite) is critical for accurate common-Pb correction, and provides a powerful mechanism for linking allanite growth to the metamorphic assemblage. A mean 208Pb/232Th age of 29.44 ± 0.95 Ma is accordingly taken for synkinematic allanite crystallisation under greenschist facies conditions. This age reflects the timing of the Mont Blanc underthrusting below the Penninic Front and highlights the potential of directly dating deformation with allanite.
ISSN: 0954-4879

hal-00963545
https://hal.archives-ouvertes.fr/hal-00963545
DOI : 10.1111/ter.12066

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The crust of the Paleoproterozoic West African Craton in northern Ghana is dominantly made of Tonalite-Trondhjemite-Granodiorite (TTG) suites, low-K to high-K calc-alkaline granites, with minor LILE-enriched diorites, and chemically related lavas. Results of zircon U-Pb dating indicate that all these different rock types were formed together over a prolonged period of ca. 100 Ma, between 2.21 and 2.11 Ga. Zircon Hf isotope compositions of five samples are supra-chondritic (εHft from +1 to +6) indicating that the granitoids derive from the reworking of juvenile crustal components as old as 2.60 Ga. Geochemical data show that the TTGs were derived from the reworking of a low-K mafic crust, while biotite granites formed by the reworking of older felsic crustal rocks, and diorites derived from the melting of LILE-enriched mantle sources.The combined datasets support a three-step model for the formation of the crust of the West African craton in northern Ghana: (1) a juvenile, mafic proto-crust was extracted from the depleted mantle between 2.60 and 2.30 Ga, in an oceanic environment remote from any pre-existing continental nucleus; (2) the mafic proto-crust was reworked in several primitive magmatic arcs or in accreted oceanic plateaux. Crustal reworking initially produced minor felsic magmatism between 2.35 and 2.21 Ga, followed by the 2.21–2.15 Ga emplacement of low- to medium-P TTG and K-rich calc-alkaline granitoid suites, and was coeval with the formation of an enriched mantle source. (3) Various proto-continental terranes collided during the Eburnean orogeny between 2.14 and 2.11 Ga, triggering intense reworking of the composite crust and mantle reservoirs formed in the previous two steps, i.e. remnants of the juvenile mafic crust, older TTGs and biotite granites, and previously enriched mantle sources. The emplacement of high-P TTGs, granites and LILE-enriched diorites resulted from lower crustal heating in response to the delamination of the lower crust and lithospheric mantle. The Eburnean collision therefore marks the final stages of a 200–500 Ma long period of juvenile continental crust formation and stabilisation. The datasets further reveal that the Paleoproterozoic West African Craton comprises a minimum of two distinct crustal blocks, with north-western Ghana lying at their interface.
ISSN: 0301-9268

hal-01467999
https://hal.archives-ouvertes.fr/hal-01467999
DOI : 10.1016/j.precamres.2016.10.011

Éditeur(s) : HAL CCSD Oxford University Press (OUP)
Résumé : International audience
The nature and extent of the regional lithosphere-asthenosphere interaction beneath Ireland and Britain remains unclear. Although it has been established that ancient Caledonian signatures pervade the lithosphere, tertiary structure related to the Iceland plume has been inferred to dominate the asthenosphere. To address this apparent contradiction in the literature, we image the 3-D lithospheric and deeper upper-mantle structure beneath Ireland via non-linear, iterative joint teleseismic-gravity inversion using data from the ISLE (Irish Seismic Lithospheric Experiment), ISUME (Irish Seismic Upper Mantle Experiment) and GRACE (Gravity Recovery and Climate Experiment) experiments. The inversion combines teleseismic relative arrival time residuals with the GRACE long wavelength satellite derived gravity anomaly by assuming a depth-dependent quasilinear velocity-density relationship. We argue that anomalies imaged at lithospheric depths probably reflect compositional contrasts, either due to terrane accretion associated with Iapetus Ocean closure, frozen decompressional melt that was generated by plate stretching during the opening of the north Atlantic Ocean, frozen Iceland plume related magmatic intrusions, or a combination thereof. The continuation of the anomalous structure across the lithosphere-asthenosphere boundary is interpreted as possibly reflecting sub-lithospheric small-scale convection initiated by the lithospheric compositional contrasts. Our hypothesis thus reconciles the disparity which exists between lithospheric and asthenospheric structure beneath this region of the north Atlantic rifted margin.
ISSN: 0956-540X

hal-00617815
https://hal.archives-ouvertes.fr/hal-00617815
DOI : 10.1111/j.1365-246X.2011.04921.x

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