Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Metasedimentary rocks in the transition zone between the Pernambuco-Alagoas Domain and the Sergipano Belt (southern Borborema Province, NE Brazil) were studied in order to place constraints on the geological evolution of this portion of West Gondwana. High-grade metamorphic conditions are recorded by the garnet-sillimanite assemblage and common anatexis in pelitic paragneisses. Two samples dated by LA-ICP-MS in the north yielded a predominance of early Neoproterozoic detrital zircons, with age peaks between 990 Ma and 827 Ma, with the youngest grain suggesting deposition after 670 Ma. Low Th/U zircons (Th/U < 0.1) in these samples display a significant spread in ages, from 691 to 568 Ma. One sample in the south is dominated by a homogenous population of Paleoproterozoic grains, with a 2200 Ma peak. The southernmost sample also contains Paleoproterozoic zircons but the most abundant population is Neoproterozoic, and characterized by age peaks at c. 670, 647 and 623 Ma. These results show that deposition of sediments in the southern PEAL Domain and in the northern Sergipano Belt occurred in the Late Neoproterozoic. The differing age spectra between samples are correlated with potential source rocks in the study area or in its proximity, reflecting variable input from local sources. The data are interpreted to indicate that during the course of an extensional event at c. 673-647 Ma, Tonian granitic intrusions and synextensional metamorphic rocks were unroofed and eroded to provide zircons for sediments deposited in the north, whereas Paleoproterozoic and synextensional magmatic rocks were the main sources for sediments in the south. Peak metamorphic conditions and contractional deformation are constrained to c. 630-600 Ma.
ISSN: 0895-9811

hal-01468011
https://hal.archives-ouvertes.fr/hal-01468011
DOI : 10.1016/j.jsames.2016.09.010

Éditeur(s) : HAL CCSD Elsevier
Résumé : LA-ICP-MS and SIMS U–Pb analyses have been performed on detrital zircon grains from four heavy mineral rich metasediments collected at different levels of the Nathorst Land Group (Eleonore Bay Supergroup, Greenland). Zircons from high-grade samples collected in the sillimanite and migmatite zones exhibit modifications to their structure, which are lacking in grains from a low-grade sample (0.3 GPa and 350–400 °C). In the sillimanite zone (0.5 GPa, 650 °C), thin discontinuous rims (<20 μm) plating detrital zircon grains document a metamorphic overgrowth during the Caledonian event dated at 428 ± 25 Ma (2σ). In the migmatite zone (0.4 GPa, 700 °C), zircons underwent severe recrystallisation processes but no new zircon growth. Ilmenite, which constitutes over 50% of the heavy mineral layers, underwent recrystallisation in both high-grade samples and is likely to represent the main source of Zr available for growth of zircon rims in the sillimanite zone. However, in the migmatite zone sample, the metamorphic conditions allowed titanite overgrowth around ilmenite, which acted as a sink for Zr and inhibited new zircon growth. 207Pb/206Pb ages for 152 detrital zircons broadly range between 2800 and 990 Ma. The detrital zircon age signature is characterized by the large predominance of late Paleoproterozoic (1.85–1.60 Ga) grains in all analysed samples and by lesser amounts of Archean (2.7–2.8 Ga) and Mesoproterozoic (1.2–1.0 Ga) zircons. The overall age range indicates that detritus can be sourced from the Labradorian and Makkovikian provinces of northeastern Laurentia. The youngest grain analysed (987 ± 18 Ma) indicates that deposition took place during the Neoproterozoic, in a post-Grenvillian sedimentary environment and was likely coeval with the Mid-Neoproterozoic episode of aborted rifting that affected the eastern margin of Laurentia. The lack of detritus originating from Amazonia or Baltica, placed adjacent to Laurentia during the early stages of Rodinia fragmentation, suggests that these two continental landmasses did not constitute a topographic high during the Neoproterozoic or that the Mid-Neoproterozoic rifting opened toward an open ocean to the north of a combined Laurentia/Baltica, thus resulting in a general south to north direction of transport of the sediments.
ISSN: 0301-9268

hal-00407204
https://hal.archives-ouvertes.fr/hal-00407204
DOI : 10.1016/j.precamres.2007.01.002

Éditeur(s) : HAL CCSD Société géologique de France
Résumé : International audience
Age constraints on the protoliths, deformation, metamorphism and melting events are key parameters when correlating different continental lithospheric remnants among each other and disentangling their evolution within large-scale orogens. In situ U-Th-Pb chemical dating on monazites using Electron Probe Micro-Analyser (EPMA) has been performed on eight samples throughout the Variscan Maures-Tanneron massif (SE France) in order to date the medium to high-tectonothermal events related to the Variscan orogeny.Results indicate a polyphased crustal evolution : (i) U-Th-Pb ages obtained in polygenetic monazite grain cores gave inherited Upper Ordovician (456 ± 11 Ma) age, highlighting the large scale occurrence of the Ordovician magmatic activity in the North Gondwanian margin. An Early Devonian (404 ± 10 Ma) age may date a protolith emplacement related to calc-alkaline supra-subduction magmatism or could be associated to an early medium-grade metamorphism, prior to collisional stage. (ii) The crustal thickening stage has been further recorded in prograde metamorphic monazites formed during the underthrusting and subsequent nappe stacking events, under amphibolite facies conditions. This stage is dated between 382 ± 11 (Middle Devonian) and 331 ± 5 Ma (Late Visean). (iii) An orogenic partial melting event took place during Middle Carboniferous and is accompanied by the crystallization of crustal peraluminous magmas (Plan-de-la-Tour granite, 329 ± 3 Ma).This contribution demonstrates the capacity of monazite to record the prograde path of rocks during increasing metamorphic conditions related to stages of crustal thickening, and the robustness of the U-Th-Pb chronometer in monazite despite the overprinting of high-grade thermal events, including partial melting. The age ranges of the different orogenic stages reported in this study are in good agreement with those reported in adjacent Variscan Corsica and Sardinia; while correlations with other nearest Variscan massifs like the Argentera massif in the southwestern Alps or the French Massif Central remain more hypothetic. The Internal Zone of the Maures-Tanneron massif, and more widely the Internal Zone of the Maures-Tanneron-Corsica-Sardinia segment, is part of the southern orogenic root system of the Variscan belt.
EISSN: 1777-5817

insu-01140973
https://hal-insu.archives-ouvertes.fr/insu-01140973
DOI : 10.2113/gssgfbull.186.2-3.145

Éditeur(s) : HAL CCSD
Résumé : International audience
The geology of the North Pyrenean Zone in the central Pyrenees allows for the observation in the field of theentire section of the Pyrenean rift, from the mantle to the crust and the Mesozoic cover (pre, syn and post rift).The good knowledge we have of the pre-Alpine history of the Pyrenees allows us to properly constrain the Alpinegeological evolution of the pre-Triassic rocks which record both Variscan and Alpine orogenic cycles.The mantle outcrop as kilometric to centimetric fragments of peridotite dispersed within a carbonate metamorphicbreccia. The study of peridotite serpentinisation shows several events of low-temperature serpentinisation, incontact with seawater. In some locallities, we can observe a mixture of fragments of variously serpentinizedperidotites. This suggests a tectonic context where fragments of peridotites from different structural levels weresampled more or less synchronously.The granulitic basement is characterized by a Variscan syndeformational HT event (300-280 Ma). So farwe have not found any trace of a Cretaceous HT event (> 500C). On the other hand, the basement is affectedby a regional metasomatism that began during the Jurassic and became more spatially focused with time until itwas restricted to the Pyrenean rift during the Aptien, Albian and Cenomanian. The talc-chlorite metasomatism(120-95 Ma) shows an evolution from a static toward a syn-deformation hydrothermal event, under a more or lessnormal geothermal gradient. Extensional deformation is recorded by the reworking of several inherited low-angleVariscan tectonic contacts, but also by dispersed high-angle extensional shear zones formed under greenshistconditions.The metamorphic Mesozoic cover of the basement massifs, which constitute the so-called Internal MetamorphicZone, is an allochtonous unit made of lenses of Mesozoic rocks enclosed into the breccia, whichlocally contains peridotite and basement clasts. The Mesozoic metamorphic carbonates show a first phase ofsyn-metamorphic (450-600C, P < 2 kb) ductile deformation, and subsequent phases of folding and fracturing.The datation of neoformed minerals give a 108-85 Ma time span for the metamorphism. We interpret this brecciaas an abandonment breccia which marks the emergence of the main detachment. The basal contact of the Mesozoiccover has a complex 3D geometry traced by Triassic evaporites. It corresponds to a major pre- and synorogenicpolyphased tectonic contact.All these data show a geometrically complex hyper-extended rift where the crust was not stretched under ahigh geothermal gradient but thinned by the tectonic extraction of relatively thin lenses and perhaps cut by highangle low-grade shear zones. The 3D geometry, as well as the strain records and the breccia lithologies stronglysuggest a non-cylindricity for the exhumation process, probably within a transtentional system
Geophysical Research Abstracts
Vienne, Austria

insu-01510371
https://hal-insu.archives-ouvertes.fr/insu-01510371

Éditeur(s) : HAL CCSD Elsevier
Résumé : Hot collisional orogens are characterized by abundant syn-kinematic granitic magmatism that profoundly affects their tectono-thermal evolutions. Voluminous granitic magmas, emplaced between 360 and 270 Ma, played a visibly important role in the evolution of the Variscan Orogen. In the Limousin region (western Massif Central, France), syntectonic granite plutons are spatially associated with major strike-slip shear zones that merge to the northwest with the South Armorican Shear Zone. This region allowed us to assess the role of magmatism in a hot transpressional orogen. Microstructural data and U/Pb zircon and monazite ages from a mylonitic leucogranite indicate synkinematic emplacement in a dextral transpressional shear zone at 313 +/- 4 Ma. Leucogranites are coeval with cordierite-bearing migmatitic gneisses and vertical lenses of leucosome in strike-slip shear zones. We interpret U/Pb monazite ages of 315 +/- 4 Ma for the gneisses and 316 +/- 2 Ma for the leucosomes as the minimum age of high-grade metamorphism and migmatization respectively. These data suggest a spatial and temporal relationship between transpression, crustal melting, rapid exhumation and magma ascent, and cooling of high-grade metamorphic rocks.;Some granites emplaced in the strike-slip shear zone are bounded at their roof by low dip normal faults that strike N-S, perpendicular to the E-W trend of the belt. The abundant crustal magmatism provided a low-viscosity zone that enhanced Variscan orogenic collapse during continued transpression, inducing the development of normal faults in the transpression zone and thrust faults at the front of the collapsed orogen.
ISSN: 0040-1951

hal-00445260
https://hal.archives-ouvertes.fr/hal-00445260
DOI : 10.1016/j.tecto.2009.03.022

Éditeur(s) : HAL CCSD Springer Verlag (Germany) Springer Verlag
Résumé : International audience
Replenished axial melt lenses at fast-spreading mid-oceanic ridges may move upward and intrude into the overlying hydrothermally altered sheeted dikes, resulting in high-grade contact metamorphism with the potential to trigger anatexis in the roof rocks. Assumed products of this process are anatectic melts of felsic composition and granoblastic, two-pyroxene hornfels, representing the residue after partial melting. Integrated Ocean Drilling Program Expeditions 309, 312, and 335 at Site 1256 (eastern equatorial Pacific) sampled such a fossilized oceanic magma chamber. In this study, we simulated magma chamber roof rock anatectic processes by performing partial melting experiments using six different protoliths from the Site 1256 sheeted dike complex, spanning a lithological range from poorly to strongly altered basalts to partially or fully recrystallized granoblastic hornfels. Results show that extensively altered starting material lacking primary magmatic minerals cannot reproduce the chemistry of natural felsic rocks recovered in ridge environments, especially elements sensitive to hydrothermal alteration (e.g., K, Cl). Natural geochemical trends are reproduced through partial melting of moderately altered basalts from the lower sheeted dikes. Two-pyroxene hornfels, the assumed residue, were reproduced only at low melting degrees (<20 vol%). The overall amphibole absence in the experiments confirms the natural observation that amphibole is not produced during peak metamorphism. Comparing experimental products with the natural equivalents reveals that water activity (aH2O) was significantly reduced during anatectic processes, mainly based on lower melt aluminum oxide and lower plagioclase anorthite content at lower aH2O. High silica melt at the expected temperature (1000–1050 °C; peak thermal overprint of two-pyroxene hornfels) could only be reproduced in the experimental series performed at aH2O = 0.1.
ISSN: 0010-7999

hal-01174139
https://hal.archives-ouvertes.fr/hal-01174139
DOI : 10.1007/s00410-015-1136-5

Éditeur(s) : HAL CCSD
Résumé : International audience
The Kazdağ Massif exposes a metamorphic dome in the Biga Peninsula of northwest Turkey. An extensional origin has been proposed for the dome, limited on both flanks by detachments and/or shear zones. The northern flank is bounded by the extensional Alakeçi Shear Zone (ASZ), whose P-T-t path is still poorly known. We therefore focus on its metamorphic conditions and related temporal history to precise its tectono-metamorphic evolution. The local tectonostratigraphy in structurally ascending order comprises: (i) the high-grade metamorphic core rocks of the Kazdaǧ Massif (gneisses and micaschists intercalated with amphibolites and marbles); (ii) the two kilometer-thick ASZ; (iii) the overlaying unmetamorphosed pre-Cenomanian accretionary Çetmi mélange; and (iv) Neogene sedimentary and volcanic cover rocks. ASZ mylonites were derived from both the core rocks and the mélange lithologies. From the north to the south the mylonitic fabrics in the ASZ depict a top-to-the N-NNE shearing, parallel to the NNE-plunging stretching lineation and NNW-dipping mylonitic foliation. This geometry implies normal sense movement i.e. north-side down-dip extensional displacement along this flank of the Kazdaǧ Massif. The northward transition from ductile to brittle-ductile regime through the ASZ shows that the deformation occurred at decreasing temperatures and degree of metamorphism. The paragenesis in equilibrium within the mylonitic gneisses and schists contains Qtz + Fs + Ms + Bt + Grt ± St ± Sill, with late retrogressive chlorite after biotite and garnet. Four samples of ASZ rocks yielded pressures between 6.9-5.7 kbar and temperatures between 706-587°C. Three samples from the mylonitic rocks supplied in situ isochron 36Ar/40Ar mica ages between 31.2-24.2 Ma, which we interpret to date the cooling of the mylonites following the P-T decrease across the ASZ. The metamorphic and structural results support the extensional character of the ASZ, and sketch transition from sillimanite core gneisses in the deeper structural levels to chlorite schists towards the top of the shear zone. These new data allow to precise the peak P-T conditions and the temporal evolution in the northern flank of the Kazdaǧ Massif, where Late Oligocene extensional exhumation was assisted by NNE-directed ductile-brittle ASZ, which had operated from amphibolite to greenschist facies. At the regional scale, this tectono-metamorphic pattern is similar to those observed on other places of the north Aegean domain.
IOP Conference Series
Austin, TE, United States

hal-00642725
https://hal-brgm.archives-ouvertes.fr/hal-00642725
DOI : 10.1088/1755-1307/2/1/012012

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
In this study, we report new U-Pb zircon ages and amphibole and biotite 40Ar-39Ar cooling ages for igneous and metaigneous rocks from the eastern portion of the Borborema Province (NE Brazil). We combine these results with preliminary thermobarometric estimates and existing data to place constraints on the timing and duration of the Brasiliano-Pan-African Orogeny in northeastern Brazil and on the subsequent late- to post-orogenic history. The Alcantil orthogneiss contains a complex zircon population, but the youngest concordant analyses yielded an age of 632 ± 17 Ma (2σ). These grains have low Th/U ratios (<0.1), and the age of 632 Ma is therefore interpreted as dating the metamorphic event responsible for the acquisition of the gneissic fabric. In the Caruaru biotite amphibole orthogneiss, concordant analyses provided an age of 618 ± 4 Ma (2σ), which we relate to emplacement of the protolith of the Caruaru orthogneiss. Together with available U-Pb data from the literature, the 632-600 Ma age range is regarded as dating development of a regional flat-lying foliation coeval with high-grade metamorphism (upper amphibolite facies conditions). The 586 ± 3 Ma age (2σ) of the Panelas pluton is similar to those of other granitic plutons associated with a voluminous plutonism of high-K calc-alkaline to shoshonitic nature that occurred at 590-580 Ma. This plutonism slightly predated or was synchronous with development of large strike-slip shear zones. Amphibole grains yielded 40Ar-39Ar ages ranging from 584 ± 4 Ma to about 530 Ma, suggesting that regional temperatures remained near the closing temperature of this mineral (≈500 °C) for a long time. Most 40Ar-39Ar biotite ages range from 552 to 533 Ma, but two samples yielded plateau ages of 519 ± 5 Ma and 509 ± 5 Ma. The inferred rather slow cooling of the studied region possibly reflects low rates of unroofing, allowing persistence of a protracted thermal anomaly caused by the massive injection of magmas at the end of the Neoproterozoic.
ISSN: 0895-9811

hal-00745698
https://hal.archives-ouvertes.fr/hal-00745698
DOI : 10.1016/j.jsames.2012.06.003

Éditeur(s) : HAL CCSD Taylor & Francis
Résumé : New structural data and P-T estimates of syn-deformational assemblages within the Beni Bousera peridotites and their crustal envelope are used to explain their Alpine exhumation. The Beni Bousera peridotites occur as thin sheets within high grade crustal units of the lower Sebtides (inner Rif, Morocco) and are composed of weakly deformed spinel lherzolite in the core of the massif and garnet-spinel mylonite at the rim. The main foliation trajectories in both the peridotites and overlying crustal units show systematic rotation towards their mutual contact, indicating a kilometer-scale top to the NW shearing with a dextral component along this crust/mantle contact. Widespread top to the NW shear criteria within the crustal units overlying the peridotite support this feature. Available ages constrain the development of the main foliation in both the peridotites and crustal rocks between 25 and 20 Ma. New P-T data from the peridotites show that deformation occurs during decompression from approximate to 22 kbar, 1050 degrees C to approximate to 9-15 kbar, 800 degrees C. As a consequence, exhumation of the Beni Bousera peridotites takes place during the Oligo-Miocene lithosphere thinning in the footwall of a lithospheric extensional shear zone. The exceptional preservation of garnet within the mylonitic peridotites results from rapid cooling of the border of the massif due to the juxtaposition with colder crustal rocks along this shear zone. Uplifting of the hot mantle rocks simultaneously induces high temperature metamorphism in the overlying crustal units. These new findings allow us to reconstruct the deformation history of the Beni Bousera region and the Alboran domain in the framework of the western Mediterranean geodynamics during the last 40 Myrs.. (C) 2011 Lavoisier SAS. All rights reserved
ISSN: 0985-3111

hal-00767654
https://hal.archives-ouvertes.fr/hal-00767654
DOI : 10.3166/ga.24.49-60

Éditeur(s) : HAL CCSD
Résumé : International audience
Metamorphic soles correspond to m to 500m thick tectonic slices welded beneath most of the large-scale ophiolites.They typically show a steep inverted metamorphic structure where the pressure and temperature conditions ofcrystallization increase upward (from 500100ºC at 0.50.2 GPa to 800100ºC at 1.00.2 GPa), with isogradssubparallel to the contact with the overlying ophiolitic peridotite. The proportion of mafic rocks in metamorphicsoles also increases from the bottom (meta-sediments rich) to the top (approaching the ophiolite peridotites). Thesesoles are interpreted as the result of heat transfer from the incipient mantle wedge toward the nascent slab (associatedwith large-scale fluid transfer and possible shear heating) during the first My of intra-oceanic subduction(as indicated by radiometric ages). Metamorphic soles provide therefore major constraints on early subductiondynamics (i.e. thermal structure, fluid migration and rheology along the nascent slab interface).We present a detailed structural and petrological study of the metamorphic sole from 4 major cross-sections alongthe Oman ophiolite. We show precise pressure–temperature estimates obtained by pseudosection modelling andEBSD measurements performed on both the garnet-bearing and garnet-free high-grade sole. Results allow quantificationof the micro-scale deformation and highlight differences in pressure–temperature–deformation conditionsbetween the 4 different locations, showing that the inverted metamorphic gradient through the sole is not continuousin all locations.Based on these new constraints, we suggest a new tectonic–petrological model for the formation of metamorphicsoles below ophiolites. This model involves the stacking of several homogeneous slivers of oceanic crust leadingto the present-day structure of the sole. In this view, these thrusts are the result of rheological contrasts between thesole and the peridotite as the plate interface progressively cools down. These slivers later underwent several stagesof retrogression (partly mediated by ascending fluids from the slab) from amphibolite- to prehnite/pumpellite-faciesconditions.
Geophysical Research Abstracts
Vienne, Austria

insu-01309369
https://hal-insu.archives-ouvertes.fr/insu-01309369

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : The role of the Karakorum fault zone (KFZ) is debated. South of 33°N, ongoing dextral-oblique slip along the SW edge of the Gar basin exhumes metamorphic and magmatic rocks of the Ayilari range. Minerals have recorded a continuum of deformation from temperatures >600–400°C down to <250_C. The 40Ar/39Ar ages, the oldest being 21.2 ± 1.0 Ma, yield minimum estimates for the initiation of the KFZ. These are in agreement with the U-Th/Pb ages constraining the onset of deformation at ca 25–22 Ma. Thermochronologic results show slow cooling for the period ca 21– 14 Ma, followed by rapid cooling between 14 and 4 Ma. These data demonstrate that right-lateral motion was in progress in the early Miocene and that shear continued at least until 4 Ma, pointing to >20 Ma of deformation along the fault. Greenschist facies deformation superimposed upon the medium- to high-grade deformation marks a kinematic change from pure dextral to dextral-normal motion associated with the onset of rapid cooling. At the regional scale, the coexistence of transtension in the Gar basin with transpression documented along the Pangong range farther north suggests another example of the ‘‘zipper tectonics'' model developed along the Red River fault. The kinematic shift induced the rise of the Ayilari range starting at 16–12 Ma and the incision of major river courses. The Indus River might have become captive of the relief at this time. The river's 120 km of apparent offset implies dextral motion at a long-term rate of ca 8.5 ± 1.5 mm/yr.
ISSN: 0278-7407

hal-00197561
https://hal.archives-ouvertes.fr/hal-00197561
https://hal.archives-ouvertes.fr/hal-00197561/document
https://hal.archives-ouvertes.fr/hal-00197561/file/Valli_Karakorum_thermochron_Tectonics_07_pre_print.pdf
DOI : 10.1029/2005TC001913

Éditeur(s) : HAL CCSD
Résumé : Thrusting fault zone in foreland basins are characterized by highly foliated zones generally enriched in phyllosilicates which can play a major role on the mechanical behaviour of the fault. In this context, investigations of synkinematic clay minerals permit to determine the origin of the fluid from which they precipitated as well as the mechanisms of deformation. Our study is focused on clay mineral assemblages (illite and chlorite) in a major thrust fault located in the Monte Perdido massif (southern Pyrenees), a shallow thrust that affects upper cretaceous-paleocene platform carbonates and lower Eocene marls and turbidites. It implied 3 km of displacement of the Monte Perdido thrust unit with respect to the underlying Gavarnie unit. In this area the cleavage development by pressure-solution is linked to the Monte Perdido and Gavarnie thrust activity. The core zone of the fault, about 6 m thick, consists of an interval of intensely deformed clay-bearing rocks bounded by major shear surfaces. The deformed sediment is markedly darker than the protolith. Calcite-quartz shear veins along the shear planes are abundant. Detailed SEM and TEM observations of highly deformed fault zone samples indicate that clay mineral enrichment in the core zone of the fault is not only related to passive increase by pressure-solution mechanism but that dissolution-recrystallization of phyllosilicates occurs during deformation. A mineral segregation is observed in the highly deformed zone. Newly formed 2M 1 muscovite is present along the cleavage whereas IIb chlorite crystals fill SV2 shear veins suggesting syntectonic growth of phyllosilicates in the presence of fluids in low-grade metamorphic conditions. These mineralogical reactions act as weakening processes and would favour Monte Perdido fault creeping.
Swiss Journal of Geosciences

hal-00795553
https://hal.archives-ouvertes.fr/hal-00795553
DOI : 10.1007/s00015-012-0098-0

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
High-grade migmatitic and mylonitic gneisses from the Day Nui Con Voi massif in northern Vietnam record temperatures of 760-810 °C at pressures of 6-10 kbars. High temperature conditions have resulted in the development of major element diffusional profiles in garnet. Laser ablation ICP-MS analyses of trace elements indicate that REE and Hf closely followed Rayleigh-like fractionation trends but underwent significant post-crystallization modification. Light REE and to a lesser extent, HREE rim-to-rim zonation profiles show progressive flattening with the decreasing garnet size. Nd and Sm are completely homogenized in the crystals smaller than 1.5 mm, while Lu always preserves variable degrees of core-to-rim concentration gradient. The observed REE patterns are interpreted as resulting from the combination of protracted garnet growth of progressively smaller crystals and intracrystalline diffusion. This had profound influence on Sm-Nd geochronology and resulted in isochron ages ranging from 50 to 32 Ma. The youngest age was obtained for a sample, where all garnet crystals are smaller than 2 mm in which light REE profiles are completely or nearly completely homogenized. Thus, only the youngest age represents geologically meaningful event, and 31.7 ± 0.9 Ma age is interpreted as the best estimate of the resetting episode due to high temperature diffusional homogenization of light REE during early Oligocene metamorphism. Older Sm-Nd ages reflect mixed analyses of variably reset individual garnet crystals. Lu-Hf isotopic analyses of bulk garnet fractions, despite yielding high parent/daughter isotopic ratios appeared very scattered and did not allow defining isochron ages. Instead, apparent ages defined by whole rock and individual garnet fractions range from c. 80 to 160 Ma. Very old apparent ages are interpreted as being the consequence of intracrystalline Lu diffusion and preservation of the original Hf distribution, which leads to lower 176Lu/177Hf ratios, and thus steeper (older) isochrons. Back diffusion of Lu during commonly observed resorption played a subordinate role in modifying isotope systematics.
ISSN: 0009-2541

hal-00813949
https://hal.archives-ouvertes.fr/hal-00813949
DOI : 10.1016/j.chemgeo.2012.04.024

Éditeur(s) : HAL CCSD European Geosciences Union
Résumé : International audience
An increasing number of field examples in mountain belts show that the formation of passive margins during extreme continent thinning may occur under conditions of high to very high thermal gradient beneath a thin cover of syn-rift sediments. Orogenic belts resulting from the tectonic inversion of distal margins and regions of exhumed continental mantle may exhibit high-temperature, low-pressure (HT-LP) metamorphism and coeval syn-extensional, ductile deformation. Recent studies have shown that the northern flank of the Pyrenean belt, especially the North Pyrenean Zone, is one of the best examples of such inverted hot, passive margin. In this study, we provide a map of HT-LP metamorphism based on a data set of more than 100 peak-temperature estimates obtained using Raman spectroscopy of the carbona-ceous material (RSCM). This data set is completed by previous PT (pressure and temperature) estimates based on mineral assemblages, and new 40 Ar– 39 Ar (amphibole, micas) and U–Pb (titanite) ages from metamorphic and magmatic rocks of the North Pyrenean Zone. The implications on the geological evolution of the Cretaceous Pyrenean paleomar-gins are discussed. Ages range mainly from 110 to 90 Ma, and no westward or eastward propagation of the metamor-phism and magmatism can be clearly identified. In contrast, the new data reveal a progressive propagation of the thermal anomaly from the base to the surface of the continental crust. Focusing on the key localities of the Mauléon basin, Arguenos–Moncaup, Lherz, Boucheville and the Bas-Agly, we analyze the thermal conditions prevailing during the Cre-taceous crustal thinning. The results are synthetized into a series of three regional thematic maps and into two detailed maps of the Arguenos–Moncaup and Lherz areas. The results indicate a first-order control of the thermal gradient by the intensity of crustal thinning. The highest grades of metamor-phism are intimately associated with the areas where subcon-tinental mantle rocks have been unroofed or exhumed.
ISSN: 1869-9510

insu-01161826
https://hal-insu.archives-ouvertes.fr/insu-01161826
https://hal-insu.archives-ouvertes.fr/insu-01161826/document
https://hal-insu.archives-ouvertes.fr/insu-01161826/file/PL03457.pdf
DOI : 10.5194/se-6-643-2015

Éditeur(s) : HAL CCSD
Résumé : This thesis is devoted to the study of the geodynamic evolution of the panafrican belt in NW Angola, lying on the western margin of the Congo craton. This orogenic belt, resulting from the late neoproterozoic - early paleozoic convergence of the São Francisco and Congo cratons, can be subdivided into two main domains: 1) an eastern foreland made of folded neoproterozoic sediments and 2) a western hinterland made of magmatic and metamorphic rocks. Our structural, petrological, geochemical and geochronological studies lead to the recognition of two main units within this hinterland : 1) an upper eastern unit made of gneiss and micaschists (PTmax : 8-12 kbar, 650-680°C) that preserves D1 deformation structures related to the ENE transport of nappes and 2) a western lower unit made of gneiss and migmatites (P-Tmax : 7-9 kbar, 700-750°C) that were affected by a polyphased deformation. Our geochronological study shows that this belt contains ~2Ga old granites that were involved in a polyphased panafrican evolution between 540 Ma for syn-D1 peak metamorphic conditions and 480 Ma for the last stages of recrystallization and exhumation of high grade rocks. The synthesis of all data leads to a precise reconstruction of P-T-D-t paths in both tectonometamorphic units and to comparison of this panafrican belt with similar belts distributed on both sides of the Atlantic ocean. This work attests of the complexity of the geodynamic processes resulting in the Gondwana assembly and implying both large cratonic landmasses and intercalated microplates.
Cette thèse est consacrée à l'étude de l'évolution géodynamique de la chaîne panafricaine au NW de l'Angola, sur la marge occidentale du craton du Congo. Ce segment orogénique, conséquence de la convergence des cratons du São Francisco et du Congo à la fin du Néoprotérozoïque et début du Paléozoïque, peut être subdivisé en un domaine externe plissé et peu métamorphique et un domaine interne formé de roches magmatiques et métamorphiques. Les études structurales, pétrologiques, géochimiques et géochronologiques conduisent à distinguer deux unités principales dans le domaine interne : - l'unité supérieure de gneiss et micaschistes (P-Tmax : 8-12 kbar, 650-680°C) dont la déformation D1 est synchrone de la mise en place des nappes vers l'ENE; - l'unité inférieure dans la partie occidentale constituée de gneiss et migmatites (P-Tmax : 7-9 kbar, 700-750°C) affectés par une déformation polyphasée. L'étude géochronologique démontre l'existence de protolithes granitiques datés à ~2 Ga impliqués dans l'orogénèse pan africaine caractérisée par une histoire polyphasée entre 540 Ma pour le pic du métamorphisme syn-D1 et 480 Ma pour les dernières phases de recristallisation et d'exhumation des roches métamorphiques de haut grade. La synthèse des données permet ainsi de reconstruire précisément les deux chemins P-T-D-t de ces deux unités tectono-métamorphiques et de comparer ce segment panafricain à ceux présents de part et d'autre de l'Atlantique Sud. Ce travail témoigne de la complexité des processus géodynamiques de convergence amenant à l'assemblage du Gondwana impliquant à la fois de larges masses cratoniques et des microplaques intercalées entre ces masses.
https://tel.archives-ouvertes.fr/tel-00770757

Droits : info:eu-repo/semantics/OpenAccess
tel-00770757
https://tel.archives-ouvertes.fr/tel-00770757
https://tel.archives-ouvertes.fr/tel-00770757/document
https://tel.archives-ouvertes.fr/tel-00770757/file/these2012-Nsungani.pdf

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
Serpentinites from subduction environments represent an important sink for fluid-mobile elements. In order to constrain geochemical behavior of fluid-mobile elements hosted by serpentine phases during subduction processes, we carried out a geochemical study (trace elements and Pb isotopes) of a series of serpentinites and cumulates from the accretionary wedge of Greater Caribbean (Cuba and Dominican Republic). The trace element compositions of the primary and alteration-related phases were analyzed in situ using LA-HR-ICP-MS techniques. The studied samples represent parts of the subducted proto-Atlantic oceanic lithosphere, which has experienced low to high grade metamorphism (greenschist to eclogite facies), before being exhumed; a subset of these samples were derived from the mantle wedge. This sampling provides the opportunity to trace the chemical mobility of fluid-mobile elements during prograde metamorphism along a cold geotherm in an oceanic subduction setting.
ISSN: 0009-2541

hal-00720895
https://hal.archives-ouvertes.fr/hal-00720895
DOI : 10.1016/j.chemgeo.2012.04.009

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) : Elsevier Science Bv
Résumé : In the South Gabon Basin, deep multi-channel seismic reflection and gravity modeling analysis have shed light on key features of the structure of the margin. The thinned continental crust beneath the Gabon Margin appears to be composed of two distinct layers, separated by a clear, strong and more or less continuous reflector running in the 7–10 s TWT window. The lower crust is characterized by a higher density, intermediate between the lower values of the upper crust and the denser values of the mantle. The lower crust is irregularly shaped and presents lateral thickness variations along the direction of thinning and along the coast. In the offshore thinned continental domain, the lower and upper crust form a 20–25 km thick body. Crustal thicknesses point to a relatively sharp and narrow transition, along a few tens of kilometers, between the unthinned and the thinned continental crust. The high density layer identified offshore Gabon presents similar characteristics in density, geometry and spatial distribution, as the underplated magmatic bodies observed along volcanic margins, e.g. along the South Atlantic Namibia Margin or the North Atlantic Vøring Margin. Although this lower crustal body could possibly represent ultra mafic serpentinized rocks or high grade metamorphic crustal rocks, we suggest that it could be composed of mafic rocks. Magmas resulting from partial melting during rifting may underplate the crust and/or be intruded in the lower crust through a system of dykes and sills. In this view, the present-day crustal thicknesses along rifted margins, characterized by magmatic underplating and/or intrusion, are not representative of the thinning that the crust experienced during rifting. Results of this study point to relatively shallow sedimentary basins along the South Gabon Margin. The deepest offshore depocenters located under the westernmost side of the continental platform appear to be associated with the deepest syn-rift basins These basins seem to extend along 20 to 40 km in the ~ NE–SW direction with a present-day average thickness of 7.3 km. Offshore Gabon, whereas the crustal thinning appears significant, the syn-rift deposit are not thick. We suggest that the area was anomalously uplifted during the rifting phase, due to an elevated thermal lithospheric gradient. The conclusions derived from our seismic and gravity analysis are consistent with the implications such a thermal anomaly would have on the tectonic evolution of a rifted margin with 1) an underplated high density lower crustal layer, 2) shallow depth syn-rift basins associated with a relatively thin crust and subsequently 3) elevated recorded subsidence rates in the initial post-rift stages. Research Highlights ► The Gabon Margin appears characterized by an underplated high density lower crustal layer. ► Shallow depth syn-rift basins are associated with a relatively thin crust. ► Seismic and gravity analysis points to an anomalous uplift of the Gabon Margin during the rifting phase. ► Crustal structure, syn-rift infill and post-rift subsidence are consistent an uplift of the Gabon Margin during the rifting phase. ► The uplift of the Gabon Margin is possibly due to an elevated thermal lithospheric gradient.
Tectonophysics (0040-1951) (Elsevier Science Bv), 2011-06 , Vol. 506 , N. 1-4 , P. 31-45

Droits : 2011 Elsevier B.V. All rights reserved.
http://archimer.ifremer.fr/doc/00039/15020/12347.pdf
DOI:10.1016/j.tecto.2011.04.009
http://archimer.ifremer.fr/doc/00039/15020/

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The Borborema Province of northeastern Brazil is divided into three main domains: northern, central, and southern. Several U-Pb zircon ages of plutons and orthogneisses became available in the recent years in the central and northern domains, but similar results are scarce in the southern domain. This study reports U-Pb dates for single zircon grains from one orthogneiss (Jupi orthogneiss) and two plutons (Cachoeirinha syenitic pluton and Cabanas granite) south of the East Pernambuco shear zone system (EPSZ). The results provide geochronological constraints on the timing of deformation and magmatism in this part of the southern domain and allow correlations with the central domain. The Jupi orthogneiss was emplaced and deformed during development of the regional flat-lying foliation. A Pb-206/U-238 weighted apparent mean age of 606 +/- 8 Ma is interpreted as the crystallization age of the protolith of the orthogneiss and consequently the age of high-grade Brasiliano metamorphism. The NNE-trending Cachoeirinha pluton is only locally affected by strike-slip deformation, whereas the ENE-trending Cabanas granite is intensely affected by deformation related to the EPSZ. The 587 8 Ma and 573 4 Ma ages of the Cachoeirinha pluton and Cabanas granite, respectively, bracket the main period of activity of the EPSZ. Tectonomagmatic activity in the study area is similar to the age of Brasiliano events in the central domain, north of the EPSZ. In addition, xenocrystic zircons in the Jupi orthogneiss and Cabanas granite are interpreted as inherited from Paleoproterozoic source rocks, suggesting the presence of widespread reworked old crust in the southern domain, similar to the central domain. These results support the idea that the central and southern domains belonged to the same crustal block before the onset of the Brasiliano orogeny.
ISSN: 0895-9811

hal-00411394
https://hal.archives-ouvertes.fr/hal-00411394
DOI : 10.1016/j.jsames.2007.06.003

Éditeur(s) : HAL CCSD American Geophysical Union (AGU)
Résumé : International audience
The current structure of the South Armorican Domain in the French Variscan Belt, provides an excellent record of the late orogenic evolution, as documented by the complex interactions between extensional structures and partial melting. In the Golfe du Morbihan, the South Brittany Migmatite Belt (SBMB), tectonically overlain by non-melted metamorphic units, was studied by combined structural and 40Ar/39Ar methods. The recognition of successive deformation stages points to a strain continuum initiated at depth at the end of a partial melting event, and continued to near-surface brittle faulting, encompassing ductile strain localization over a major top-to-the-ESE extensional shear zone (SSZ) consistent with an overall WNW-ESE orogen-parallel stretching. This multidisciplinary study making the link between structure, texture, microchemistry and 40Ar/39Ar geochronology allow distinguishing between cooling, recrystallization and syn-tectonic crystallization ages. The first extensional insights, witnessed by dyke intrusion were dated at ~319 ± 6 Ma immediately after the SBMB partial melting. Strain localization along the SSZ, occurred until ~302-298 Ma when SBMB crossed the ductile-brittle transition. Moreover, in situ 40Ar/39Ar analysis, of syn-tectonic white micas sampled in the ductile and brittle structures yield similar ~300 Ma. Formation or reactivation of the SSZ may have occurred during the rapid cooling of the SBMB settled at ~305 Ma by conventional 40Ar/39Ar analysis. Large-scale late orogenic extensional event of the previously overthickened Variscan crust thus lasted over ~20 ± 3 Myr in the same extensional tectonic regime guided by the South Armorican Shear Zone.
ISSN: 0278-7407

insu-00628699
https://hal-insu.archives-ouvertes.fr/insu-00628699
https://hal-insu.archives-ouvertes.fr/insu-00628699/document
https://hal-insu.archives-ouvertes.fr/insu-00628699/file/2010TC002788.pdf
DOI : 10.1029/2010TC002788