Éditeur(s) : HAL CCSD University of Chicago Press
Résumé : International audience
In the North China block, the south Liaodong Peninsula massif is an elliptical metamorphic core complex (MCC) with a long axis trending NE‐SW. In cross‐section view, it is asymmetric, with a steeply dipping northwestern flank and a gently dipping southeastern flank. It consists of three lithotectonic units: a gneissic migmatite unit, a Paleo‐ to Meso‐Proterozoic micaschist‐slate unit, and a Neoproterozoic to Mesozoic sedimentary cover. Three deformation events related to extensional tectonics are distinguished in the study area: D1 is a ductile deformation related to the exhumation of the MCC; the following event, D2, corresponds to the development of recumbent folds formed during the early exhumation of the MCC; and the youngest event, D3, corresponds to brittle normal faulting that controlled the opening of a Cretaceous continental half‐graben basin. A pre‐D1 event characterized as northward verging is interpreted as the result of N‐S shortening that occurred in the Late Triassic during the final stages of the collision between the North and South China blocks. The ductile and brittle structures were developed coevally, with synkinematic plutonism and formation of half‐grabens. New 40Ar/39Ar and U/Pb Cretaceous ages obtained from the mylonitic granodiorite, gneissic migmatite, orthogneiss, and granite indicate that the south Liaodong Peninsula MCC is contemporaneous with other Cretaceous extensional structures, such as numerous syntectonic plutons bounded by ductile normal faults, MCC, and half‐graben basins, described in eastern China. Among the several hypotheses proposed to account for the Mesozoic extension along the eastern margin of Eurasia, lithosphere convective removal appears to be the most likely.
ISSN: 0022-1376

insu-00362879
https://hal-insu.archives-ouvertes.fr/insu-00362879
https://hal-insu.archives-ouvertes.fr/insu-00362879/document
https://hal-insu.archives-ouvertes.fr/insu-00362879/file/WeiLin-TheJournalofGeology-2008.pdf
DOI : 10.1086/527456

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Ibituruna quartz-syenite was emplaced as a sill in the Ribeira-Aracuai Neoproterozoic belt (Southeastern Brazil) during the last stages of the Gondwana supercontinent amalgamation. We have measured the Anisotropy of Magnetic Susceptibility (AMS) in samples from the Ibituruna sill to unravel its magnetic fabric that is regarded as a proxy for its magmatic fabric. A large magnetic anisotropy, dominantly due to magnetite, and a consistent magnetic fabric have been determined over the entire Ibituruna massif. The magmatic foliation and lineation are strikingly parallel to the solid-state mylonitic foliation and lineation measured in the country-rock. Altogether, these observations suggest that the Ibituruna sill was emplaced during the high temperature (similar to 750 degrees C) regional deformation and was deformed before full solidification coherently with its country-rock. Unexpectedly, geochronological data suggest a rather different conclusion. LA-ICP-MS and SHRIMP ages of zircons from the Ibituruna quartz-syenite are in the range 530-535 Ma and LA-ICP-MS ages of zircons and monazites from synkinematic leucocratic veins in the country-rocks suggest a crystallization at similar to 570-580 Ma, i.e., an HT deformation >35My older than the emplacement of the Ibituruna quartz-syenite. Conclusions from the structural and the geochronological studies are therefore conflicting. A possible explanation arises from Ar-40-Ar-39 thermochronology. We have dated amphiboles from the quartz-syenite, and amphiboles and biotites from the country-rock. Together with the ages of monazites and zircons in the country-rock, Ar-40-Ar-39 mineral ages suggest a very low cooling rate: <3 degrees C/My between 570 and similar to 500 Ma and similar to 5 degrees C/My between 500 and 460 Ma. Assuming a protracted regional deformation consistent over tens of My, under such stable thermal conditions the fabric and microstructure of deformed rocks may remain almost unchanged even if they underwent and recorded strain pulses separated by long periods of time. This may be a characteristic of slow cooling "hot orogens" that rocks deformed at significantly different periods during the orogeny, but under roughly unchanged temperature conditions, may display almost indiscernible microstructure and fabric.
ISSN: 0040-1951

hal-00445262
https://hal.archives-ouvertes.fr/hal-00445262
DOI : 10.1016/j.tecto.2009.02.039

Éditeur(s) : HAL CCSD Wiley-Blackwell
Résumé : International audience
Serpentinization of the oceanic lithosphere contributes significantly to the geochemical cycle from spreading ridges to subduction zones. In situ trace element analysis of oceanic serpentinites from the Mid-Atlantic Ridge and the Greater Antilles (Cuba, Dominican Republic) shows that all serpentine minerals are enriched in fluid-mobile elements (FME: As, Sb, B, Li, Cs, Pb, U, Ba, Sr). We observe no loss of these elements from abyssal to subduction environments during prograde metamorphism. Moreover, the transition from lizardite/chrysotile to antigorite during subduction is marked by a strong over-enrichment in As and Sb in antigorite, indicating late contamination by a sedimentary source. This suggests that a second stage of serpentinization occurs in the earlier stages of subduction, when newly formed or reactivated normal faults ease fluid penetration, and/or in the subduction channel. Our study shows that, from spreading ridges to forearc environments, serpentines act as sponges for FME. We posit that, until ultimate antigorite breakdown, serpentinites efficiently transport significant amounts of FME down to great depths in the mantle.
ISSN: 0954-4879

insu-00603792
https://hal-insu.archives-ouvertes.fr/insu-00603792
DOI : 10.1111/j.1365-3121.2011.00995.x

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 : International audience
The age of Ultra-high pressure metamorphism and early cooling remains controversial in the Alps. In a new approach to the problem, samples from the undeformed Hercynian metagranite, Brossasco, were studied using an ultra-violet laser ablation microprobe technique for 40Ar-39Ar dating. The results demonstrate the frequent occurence of excess argon with high 40Ar/36Ar ratios (1000-10000) and a strong relationship between apparent ages and metamorphic textures. The highest excess argon ratios are always associated with high closure temperature minerals or large diffusion domains within single mineral phases. The best interpretation of this relationship seems to be that excess argon was incorporated in all phases during the high pressure event, then mixed with an atmospheric component during rapid cooling and retrogression, producing a wide range of argon concentrations and 40Ar/36Ar ratios. In the present case, some ages in the range 60-110 Ma could be explained by the presence of excess argon incorporated around 40-50 Ma ago. Similar results found in other high-pressure terrains in the Alps may reconcile the argon geochronometer with other systems such as Rb/Sr, U/Pb or Sm/Nd. This study therefore calls for an increasing use of high resolution in-situ sampling techniques to clarify the meaning of 40Ar/39Ar ages in many high pressure terrains.
ISSN: 0010-7999

hal-00197563
https://hal.archives-ouvertes.fr/hal-00197563
https://hal.archives-ouvertes.fr/hal-00197563/document
https://hal.archives-ouvertes.fr/hal-00197563/file/Excess_UV_Arnaud_and_Kelley_GCA_Pub_preprint.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Microstructures, U-Pb SHRIMP zircon ages and temperature estimates were used to constrain the fabric evolution and timing of mylonitisation in the Patos shear zone (NE Brazil). Melt-bearing HT-mylonitic orthogneiss displays solid-state fabrics with coarse quartz ribbons and sutured grain boundaries. K-feldspar is often fractured and shows peripheral myrmekite. The textures typically become magmatic at the contacts with the Espinho Branco anatexite. Zircon crystals from two samples of the leucosome show Paleoproterozoic (ca. 2.2 Ga) inherited cores that are enveloped by recrystallised Neoproterozoic rims. These zircon grains define a discordia with lower intercept ages of 558 and 562 Ma but with large analytical errors. A leucogranite with transitional contacts with host diatexites provides a well-constrained zircon mean age of 566 ± 6 Ma (n = 15, 95% conf.), which is considered the best estimate for the peak metamorphic conditions. The transition from HT-mylonites to fine-grained mylonites and ultramylonites is marked by a progressive grain size reduction assisted by recrystallisation. Quartz crystallographic fabrics display [0 0 0 1] concentrations between Z and Y. K-feldspar and plagioclase fabrics record the activity of the (0 1 0)[0 0 1] and (0 1 0)[1 0 0] slip systems and, in the lower-temperature tectonites, the (1 0 0)[0 1 0] slip system. These results suggest a diachronic evolution in which an initial HT northern domain was deformed by intracrystalline slip and melt-assisted granular flow, followed by medium- to low-temperature solid-state creep in the southern shear zone domain. The mylonites therefore record the rheological heterogeneities responsible for the formation of a late continental shear zone that nucleated under partially molten conditions and was later reactivated through melt-absent strain localization following exhumation.
ISSN: 0301-9268

hal-01053497
https://hal.archives-ouvertes.fr/hal-01053497
DOI : 10.1016/j.precamres.2013.12.020

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 Elsevier
Résumé : International audience
Serpentinites are one of the major components of the oceanic lithosphere and are stable in the slab and the mantle wedge up to 100-150 km depth in subduction zones. During oceanic mantle hydration and alteration, they trap trace and fluid mobile (FME: B, Li, As, Sb, Rb, Ba, Cs, Sr, U and Pb) elements that participate to elemental transfer occurring between the dehydrating slab and the mantle wedge in subduction context. The Lanzo massif is an eclogitized oceanic lithosphere that preserved its oceanic structure and recorded different steps of serpentinization/de-serpentinization from oceanic lizardite to prograde antigorite in subduction context, up to its dehydration and secondary olivine crystallization, and finally retrograde antigorite during massif exhumation. It constitutes a suitable place to study trace element behavior during serpentinization/de-serpentinization processes and associated chemical transfers between the different envelopes of the oceanic lithosphere and the mantle wedge. Geochemical analyses of serpentine and associated minerals show that the serpentinization/de-serpentinization of the Lanzo massif took place in a relatively closed system without significant trace element transfer between the different parts of the oceanic lithosphere. In the deeper part of the lithosphere, from the slightly serpentinized mantle peridotites (SSP, < 20% serpentinization) to the paleo-Moho, composed of massive serpentinites (MS, 80% serpentinization), the trace elements mobility is reduced. The chemical composition of lizardite and antigorite is homogenized with the local degree of serpentinization: in SSP, serpentine veins composition is inherited from the host mineral while, in MS, their composition is homogenous between destabilized phases at the scale of the outcrop (~ 5 m). In the shallowest part of the oceanic lithosphere, from the paleo-Moho to the oceanic paleo-seafloor, the serpentinites are foliated (FS, > 90% serpentinization). In that zone, the alpine deformation enhances the mobility of trace elements and permits their redistribution and the homogenization of antigorite composition at massif scale. Locally, in the SSP and MS, the crystallization of metamorphic veins of ~ 1-2 m corresponds to channelized fluid flows that allowed fluid transfers - and thereby trace elements - to longer distance. The successive crystallizations of antigorite and then olivine are accompanied by a diminution of some FME (B, Li, As, Sb, Ba, Rb) and Eu contents attesting that these elements are removed from slab to mantle wedge during subduction.
ISSN: 0009-2541

hal-00937661
https://hal.archives-ouvertes.fr/hal-00937661
DOI : 10.1016/j.chemgeo.2013.08.025

Éditeur(s) : HAL CCSD
Résumé : International audience
Sedimentary basins, through the sedimentary successions and the nature of the deposits, reflect the geology of thearea from which the sediments were derived and thus provide valuable record of hinterland tectonism. As the collisionbetween the South China and the Indochina blocks (i.e. the Indosinian orogeny) is still the object of a numberof controversies regarding, for instance, its timing and the polarity of the subduction, the sedimentary basins associatedwith this mountain belt are likely to provide clues to reconstruct its geodynamic evolution. However, boththe Sam Nua Basin (located to the south of the inner zones of the Indosinian orogeny and the Song Ma ophiolites)and the Song Da Basin (located to the north of the inner zones), northern Vietnam, are still lacking importantinformation regarding the depositional environments and the ages of the main formations that they contain.Using sedimentological and dating analyses (foraminifers biostratigraphy and U-Pb dating on detrital zircon), weprovide a new stratigraphic framework for these basins and propose a geodynamic evolution of the present-daynorthern Vietnam.During the Early Triassic, the Sam Nua Basin was mainly supplied by volcaniclastic sediments originating froman active volcanic activity. Geochemical investigations, combined with sedimentological and structural analyses,support an arc-related setting for this magmatism. This magmatic arc resulted from the subduction of a south dippingoceanic slab that once separated the South China from the Indochina blocks. During the Middle to the LateTriassic, the Sam Nua Basin underwent erosion that lead to the formation of a major unconformity, termed theIndosinian unconformity. This unconformity is interpreted to result from the erosion of the Indosinian mountainbelt, built after the continental collision between the South China and the Indochina blocks. Later, during the LateTriassic, the Sam Nua Basin experienced the deposition of very coarse material, emplaced under continental settingand representing the product of the erosion of the Indosinian mountain belt.To the North, the Song Da Basin is characterized by strongly diachronous deposits over a basal unconformitydeveloped at the expense of volcanic and volcaniclastic deposits related to the Emeishan Large Igneous Province.The sedimentary succession indicates a foreland setting during the Early to the Middle Triassic, which contrastswith the commonly assumed rift setting for these sediments. Thus, the Song Da Basin documents the formation ofthe Indosinian thrust belt, located immediately to the South of the basin.
European Geosciences Union General Assembly 2017
Vienne, Austria

insu-01510331
https://hal-insu.archives-ouvertes.fr/insu-01510331

Éditeur(s) : HAL CCSD
Résumé : International audience
The Alboran Domain, situated at the western end of the Mediterranean subduction system, is characterized by theRonda Peridotites, one of the world largest exposures of sub-continental mantle. Using U-Pb (LA-ICP-MS) and Ar-Ar dating, we precisely dated two tectonic events associated with the Tertiary exhumation of the Ronda Peridotites.First, shearing along the Crust-Mantle Extensional Shear Zone caused, at ca. 22.5 Ma, mantle exhumation, localpartial melting in the deep crust and coeval cooling in the upper crust. Second, the Ronda Peridotites Thrusttriggered the final crustal emplacement of the peridotites onto the continental crust at ca. 21 Ma, as testified bygranitic intrusions in the thrust hanging-wall. The tectonic evolution of the western Alboran Domain is thereforecharacterized by a fast switch from a continental lithosphere extension in a backarc setting, with sub-continentalmantle exhumation, to a rift inversion by thrusting driven by shortening of the subduction upper plate
European Geosciences Union General Assembly 2017
Vienne, Austria

insu-01510393
https://hal-insu.archives-ouvertes.fr/insu-01510393