Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
A laser ablation ICP-MS U-Pb age of 17.84 +/- 0.12 Ma (late Burdigalian) was obtained from monazites separated from a leucocratic diatexite collected in close proximity to a small peridotite massif incorporated into the lower crustal sequence of the Edough Massif (north-eastern Algeria), a southern segment of the peri-Mediterranean Alpine Belt. Monazites extracted from a neighbouring deformed leucogranite intruding early Paleozoic phyllites yield a consistent age of 17.4 +/- 1.3 Ma. Zircons occurring in the same leucogranite, with magmatic characteristics, have an age of 308 +/- 7 Ma interpreted as dating magmatic crystallisation of the leucogranite and reflecting partial melting during the Hercynian orogeny. Low Th/U domains (Th/U<0.10) from the same grains substantiate recrystallisation during a younger metamorphic event whose upper age limit is 286 +/- 11 Ma. These results emphasize the polycyclic evolution of basement rocks preserved in the crystalline units of the western Mediterranean and indicate that part of their metamorphic features were inherited from older, Hercynian, events. Taken together with published Ar-Ar dates, the late Burdigalian age of monazites indicates a rapid cooling rate of c. 370 degrees C/Ma and is regarded as closely approximating the emplacement of the peridotites into the Hercynian basement. The monazite ages are significantly younger than those recorded for orogenic peridotites from the Betic-Rif oroctine and for the timing of lithospheric extension forming the Alboran sea. It is also younger than rifting and back-arc extension opening the Liguro-Provencal basin. The late Burdigalian age is interpreted as dating the incipient rifting event that opened the Algerian basin, which is consequently not a continuation of the Liguro-Provencal basin. At the scale of the western Mediterranean, these observations concur with current models supporting slab roll-back and an eastwards migration of extension in the western Mediterranean, but suggest that the Algerian basin opened as a result of torsion and stretching of the Thethyan slab due to its steepening under the Alboran microplate.
ISSN: 0009-2541

hal-00424511
https://hal.archives-ouvertes.fr/hal-00424511
DOI : 10.1016/j.chemgeo.2008.11.016

Éditeur(s) : HAL CCSD American Geophysical Union
Résumé : International audience
We investigate upper mantle anisotropy beneath the Pyrenean range along three N-S profiles across the mountain belt. The results of a first profile that operated in 1993 in the central part of the belt have been presented elsewhere. We present the results of two other profiles that ran in 1995-1996 and 1996-1997 in the eastern and western part of the belt, respectively and propose an interpretation of the whole results. Teleseismic shear waves (SKS, SKKS, and PKS) are used to determine splitting parameters: the fast polarization direction φ and the delay time δt. Teleseismic shear wave splitting in the eastern Pyrenees displays homogeneous φ values trending N100°E and δt values in the range 1.1 to 1.5 s. A station located in the southern Massif Central, 100 km north of the range, is characterized by different splitting parameters (φ = N70°E, δt = 0.7 s). In the western part of the belt, anisotropy parameters are similar across the whole belt (φ = N110°E and δt = 1.3 to 1.5 s). Most of the measured delay times, including those obtained in the central part of the range, are above the global average of the SKS splitting (around 1 s). At the belt scale, φ is generally poorly correlated with recent estimations of the absolute plate motion, which predicts a fast direction ranging between N50°E and N80°E. Instead, the orientation of φ (N100°E) is parallel to the trend of the Pyrenean belt but also to Hercynian preexisting structures. This parallelism supports an anisotropy primarily related to frozen or active lithospheric structures. We show that a signature related to the Pyrenean orogeny is likely for the stations located in the internal domains of the belt. By contrast, the anisotropy measured at the stations located on the external parts of the belt could reflect a pre-Pyrenean (Hercynian) deformation. We suggest that a late Hercynian strike-slip deformation is responsible for this frozen upper mantle anisotropy and that the Pyrenean tectonic fabric developped parallel to this preexisting fabric. Finally, no particularly strong splitting is related to the North Pyrenean Fault, commonly believed to represent the plate boundary between Iberia and Eurasia.
ISSN: 2169-9313

hal-01388841
http://hal.univ-reunion.fr/hal-01388841
http://hal.univ-reunion.fr/hal-01388841/document
http://hal.univ-reunion.fr/hal-01388841/file/Barruol_Aniso_Pyr_JGR1998.pdf
DOI : 10.1029/98JB02790

Éditeur(s) : HAL CCSD Elsevier
Résumé : Upper mantle flow beneath the French Massif Central is investigated using teleseismic shear wave splitting induced by seismic anisotropy. About 25 three component stations (short period, intermediate and broad-band) were installed during the period 1998-1999 in the southern Massif Central, from the Clermont Ferrand volcanic area to the Mediterranean sea. Teleseismic shear waves (SKS, SKKS and PKS) were used to determine the splitting parameters: the fast polarization direction and the delay time. Delay times ranging between 0.7 and 1.5 s have been observed at most of the sites. The azimuths of the fast split shear waves trend homogeneously NW-SE in the southern Massif Central suggesting a homogeneous mantle flow beneath this area. The observed NW-SE direction differs from the N100°E Pyrenean anisotropy further South. It does not appear to be correlated to Hercynian structures nor to the present-day motion of the plate but is well correlated to the Tertiary extension direction. We propose that the opening of the western Mediterranean induced by the rotation of the Corsica-Sardinia lithospheric bloc and the roll-back to the SE of the Tethys slab may have generated a large asthenospheric mantle flow beneath the southern Massif Central and a deflection of the up going plume centered beneath the northern Massif Central toward the SE.
ISSN: 0012-821X

insu-00150214
https://hal-insu.archives-ouvertes.fr/insu-00150214
https://hal-insu.archives-ouvertes.fr/insu-00150214/document
https://hal-insu.archives-ouvertes.fr/insu-00150214/file/Barruol_Granet_EPSL2002_hal.pdf
DOI : 10.1016/S0012-821X(02)00752-5

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The brittle structures of a Hercynian granite (La Borne, French Massif Central) observed at several scales, from regional to microscopic, are presented and interpreted on mechanical terms. Emphasis is placed on strike slip faults, joints, and cracks related to incipient shear fracturing during horizontal compression. Three compressive tectonic phases have been identified according to their brittle structures and characterised by the burial depth at the time of their generation.The two first phases (H1, H2) are Hercynian and occurred while the granite was deeply buried (∼5 to 3 km depth) whereas the last phase (P) is Pyrenean and occurred at very low depth. The geometric organizations of cracks, joints and faults are clearly similar at various scales from about 10 μm to several m.These field observations are strong arguments for the occurrence of shear structures at microscopic to macroscopic scales during tectonic events. Following this inference, it is proposed that, at least in the case of Hercynian phases, cracks, joints and macroscopic strike slip faults have been generated by the same mechanical process under high overburden weight. Therefore, incipient fractures seem to be generated as shearing structures and to evolve by coalescence with neighbouring ones with the same direction.
ISSN: 0191-8141

hal-01115718
https://hal.archives-ouvertes.fr/hal-01115718
DOI : 10.1016/j.jsg.2014.05.002

Éditeur(s) : HAL CCSD
Résumé : International audience
The Achmmach tin mineralization (NE of theMoroccan Central Massif) is associated with tourmaline-richalteration halos, veins, and faults hosted in sandstones andmetapelites of the Upper Visean-Namurian. These depositsare reported to be late Hercynian in age and related to theemplacement of late-orogenic granite not outcropping in thestudied area. Structural and paragenetic studies of theAchmmach tin deposit were conducted in order to establisha general model of the mineralization. From field constraints,the late Hercynian phase is marked by a transition fromtranspression to extension with deformation conditions evolvingfrom ductile to brittle environments. The transpression(horizontal shortening direction roughly trending E-W) is coevalwith the emplacement of the first tourmaline halos alongseveral conjugated trends (N070, N020, and N120).Thereafter, a tourmaline-rich breccia formed in response tothe fracturing of early tourmaline-altered rocks.Subsequently, during the extensional phase, these structureswere reactivated as normal faults and breccias, allowing theformation of the main tin mineralization (cassiterite) associatedwith a wide variety of sulfides (arsenopyrite, chalcopyrite,sphalerite, galena, pyrrhotite, bismuthinite, pyrite, and stannite). This evolution ends with fluorite and carbonate deposition.The hydrothermal fluid flow velocity, calculated byapplying statistical measures on the tourmaline growth bands,varies with the lithology. Values are lower in metapelites andhigher in breccia. In the general evolution model proposedhere, tourmaline alteration makes the rock more competent,allowing for brittle fracturing and generation of open spacewhere the main Sn mineralization was precipitated.
Mineralium Deposita

insu-01240481
https://hal-insu.archives-ouvertes.fr/insu-01240481
DOI : 10.1007/s00126-015-0613-0

Éditeur(s) : HAL CCSD Springer Verlag
Résumé : International audience
During the development of the Variscan orogeny, large amounts of granitic melt were produced, giving rise to the intrusion of granitoids at different structural levels. Despite numerous studies, ages available from previous work on the Cevennes granites remain largely imprecise. In order to better constrain the age and emplacement mode of these granites, we have combined U-Pb dating on monazites and zircons and Ar-40/Ar-39 dating on biotites with petrological observations, major element chemical analysis and SEM zircon imaging on five samples from the Aigoual-St Guiral-Liron and Mont Lozere granitic massifs. The results revealed that granitic intrusions and cooling in Southern Cevennes occurred in a short time span at similar to 306 Ma after the main episode of regional metamorphism. Petrological and chemical data suggest that they result from a mixing between mantle-derived basic magmas (lamprophyres) and lower crust acid magmas. At a regional scale the production of these melts occurred at the end of crustal thickening induced by nappe stacking, at the same time as the late anatectic events recorded further north in the Velay dome and the granulite facies metamorphism recorded in metasedimentary granulite enclaves brought up by Tertiary volcanoes of the Velay area (Bournac).
ISSN: 1437-3254

hal-00412183
https://hal.archives-ouvertes.fr/hal-00412183
DOI : 10.1007/s00531-007-0187-x

Éditeur(s) : HAL CCSD Elsevier
Résumé : International audience
The N-S Languedoc volcanic province between the French Massif Central and the Mediterranean Sea is characterized by sporadic scattered low volume (similar to 2 km(3)) and geochemically homogeneous alkali basaltic activity spanning from 161 to 05 Ma The existence of magmatic activity of such a long duration within such a small area (similar to 140 km long and similar to 60 km wide) in spite of the extensive shift to the East of the European plate (about 2500 km during the last 160 Ma) is problematic Trace-element abundances in lavas suggest low degrees of melting (1-5%) in the spinet-garnet transition zone of an enriched Iherzolitic source The lavas display rather large ranges in Sr isotopic ratios (0 70307-0 70436) The Nd-143/Nd-144 ratio variations are smaller (0 51268-0 51300) and these of Pb-206/Pb-204 Pb-208/Pb-204 and Pb-207/Pb-204 are 18 745-19 515 38 53239 228 and 15 567-15 680 respectively The Languedoc lithospheric mantle as sampled by xenoliths is globally similar to the Pyrenees lithosphere The xenoliths show also rather large Sr Nd and Pb isotopic variations (Sr-87/Sr-86 0 70287-0 70578 Nd-143/Nd-144 0 51256-0 51414 Pb-208/Pb-204 37772-39 041 Pb-206/Pb-204 17 901-19 353) except for Pb-207/Pb-204 (15 570-15 620) The Pb-206/Pb-204 and La/Sm ratios are positively correlated both in xenoliths and lavas The increase of the Pb-206/Pb-204 (which could be interpreted as participation of the European Asthenospheric Reservoir EAR) is probably related to volatile-rich (carbonated?) fluid percolation This is corroborated by LILE and HFSE patterns observed in several xenoliths Therefore our data on lavas and xenoliths suggest a lithospheric origin for this long lived magmatism We propose (1) that the role of the asthenosphere in the Languedoc volcanism was restricted to volatile-rich fluid supplying and (2) that the fluid injection within the lithosphere may be related to the arrival of the Central Atlantic Plume head beneath Western Europe about 70 Ma ago In this model the isotopic signature of the oldest lavas (>70 Ma) would be that of the mantle lithosphere inherited from Hercynian processes The signatures of the subsequent lavas would be driven by the metasomatic component stored within the lithosphere and preferentially mobilized during incipient melting This metasomatised lower lithosphere was close to its solidus and small changes in P (or T) triggered incipient melting leading to basaltic volcanism Successive local re adjustments of the lithospheric blocks which accompanied the Meso-Cenozoic evolution of the Thetys Ligurian margin towards the present Mediterranean margin are the probable cause of these changes and so the sporadic volcanic activity in Languedoc is unrelated to deep asthenospheric processes
ISSN: 0024-4937

hal-00558884
https://hal.archives-ouvertes.fr/hal-00558884
DOI : 10.1016/j.lithos.2010.04.009