Philippe Rossi

Situation structurale et nature ophiolitique de roches basiques jurassiques associées aux flyschs maghrébins du Rif (Maroc) et de Sicile (Italie)

At both ends of the Maghrebides belt, in the Rif Mountains and Sicily, Middle to Upper Jurassic slices of basic rocks with an E-MORB character are associated with various tectonic units of the Maghrebian flysch zone. This zone, which was located between the internal zones, originally linked to the European plate and the African external zones of this Alpine belt, was therefore oceanized, at least partially, and acted as a transform fault between the mid Atlantic and the Ligurian ocean, both of them being open at that time.

Electron-microprobe dating as a tool for determining the closure of Th-U-Pb systems in migmatitic monazites

Abstract High spatial resolution dating of monazite by the electron-probe microanalyzer (EPMA) enables systematic and detailed studies of small minerals. Like zircon, monazite records the complex history undergone by the host rocks. Recent improvements in the statistical treatment of many in situ data now make it possible to decipher the related thermal events and so obtain reliable and precise ages. Our work shows that a significant number of individual spot analyses is required to reach such precise information (i.e., more than 30.40 data). Using the examples of monazites from three migmatites and one granite, we show how to select the most efficient method of age calculation according to the U and Th geochemistry of the grains, or grain domains, that we are trying to date. Three situations may be met: (1) monazites exhibiting significant Th/U ratio variation, (2) monazites exhibiting a fairly constant Th/U ratio, but significant U + Th heterogeneity, and (3) monazites of constant U and Th concentrations. For the first case, a precise mean age can be calculated using a method of data reduction in the Th/Pb = f(U/Pb) diagram, whereby a precision of ±5−10 Ma (2σ) is commonly achieved. For the second case, an isochron age can be calculated according to the Pb = f(Th*) method, with a common precision of around 20 Ma (2σ), whereas for the third case, a simple weighted average age can be calculated. Using these approaches, coupled with a back-scattered electron image study, we demonstrate that inheritance is probably as common for monazite as for zircon. In addition, the combination of high spatial resolution and precise age determination show the limited extent of Pb diffusion in monazite. Finally, an example from a migmatite from southern French Guiana demonstrates the especially robust behavior of the Th-U-Pb system in monazite. This system remains closed during late migmatization and during the subsequent zircon crystallization and zircon overgrowth of protolith zircons. The monazite yielded exactly the same age as the protolith zircons.

Radiometric dating of granitic rocks from the Central Bohemian Plutonic Complex (Czech Republic): constraints on the chronology of thermal and tectonic events along the Moldanubian-Barrandian boundary

abstract Single-zircon dating by step-wise evaporation has established that successive granitic intrusions were emplaced in the Central Bohemian Plutonic Complex (CBPC) during a short time span of about 10 Ma. In agreement with field data, the Požary trondhjemite, emplaced early at 351 ±11 Ma and subcontemporaneously with the Sazava granodiorite dated at 349 ±12 Ma, was followed by the Blatna granodiorite at 346 ±10 Ma. The magnesium-potassium-rich units (durbachites) indicate younger ages both for the Certovo Břemeno melagranite at 343 ±6 Ma (within the CPBC) and for durbachite from the Třebic Massif (south-east of the CPBC) at 340 ±8 Ma. These data provide evidence that the sequence of intrusion and the age of the emplacement of the CBPC are comparable with those of other western Variscan batholiths (i.e. the Vosges or the French Massif Central) in similar structural environment.

Genesis of a Variscan batholith: Field, petrological and mineralogical evidence from the Corsica-Sardinia batholith

Abstract The genesis of the Corsica-Sardinia batholith (CSB) can be explained in a continental collision setting. During the time of construction of the batholith, which is dated at about 70 Ma, three magmatic associations were emplaced: Mg-K calc-alkaline (U1), composite (mafic-silicic) calc-alkaline (U2) and post-collisional alkaline (U3). In addition, the level of intrusion became progressively shallower (from 5–6 kbar for U1 to less than Ikbar for U3) in a large-scale uplift. Associations U1 and U2 exhibit non-cogenetic mafic and felsic components. The processes and products of interaction between mantle- and crustal-derived melts are however different, and may be related to the thickness of the crust which controls both the parameters T, pCO2, pH2O andPtot, and the time of mingling of the products of anatexis and basaltic melts. Both U1 and U2 granitoids were formed by crystallization of a magma derived by fusion of a source of greywacke composition. The melting occurred successively under granulitic (U1) and amphibolite facies conditions (U2). In association U1, basaltic melts displaying potassic lamprophyre-like composition (up to 8% K2O) are of uncertain affinity. Furthermore, crustal interaction with the basic magma precludes precise definition of the source. In the U2 composite association, the primary tholeiitic character of the mantle-derived basic rocks is constrained by both mineralogical and geochemical data. “Dry” and “hydrated” subunits are distinguished among the basic rocks associated with the granitoids. No hybridization processes have been identified in the core of the mafic complexes of the dry subunits, but they are recognized at the level of emplacement at the margins at the contact with the granitoids. Contamination in the mafic hydrated subunit is due to the increase in water content of crustal origin in the magma during the ascent and emplacement of the basic complexes, leading to the development of abundant green amphibole. Extensive hybridization is restricted to the base of the crust where it led to incomplete mixing, yielding granodiorites and enclaves. The genesis and emplacement of the composite calc-alkaline U2 association took place in an extensional setting in an uplifting basement postdating a collisional event. During the Devonian, an eastern Austro-Alpine block was thrust onto the western “Ebro-Balearic” continent. The U1 Mg-K granitoids resulted from anatexis under conditions of pCO2 >pH2O; this took place at the base of the Austro-Alpine crust which was undergoing fusion due to its adiabatic ascent. The U2 calc-alkaline granitoid rocks were derived, in a hydrated environment, by fusion of the same Austro-Alpine crust in Corsica and of the Ebro-Balearic crust on Sardinia and in the Pyrenees. The conditions of interaction between mafic and felsic magmas may serve as a geodynamic marker providing information on the thickness of the crust in time and space and on the genesis of continental crust. The persistence during the Palaeozoic of dominantly tholeiitic mafic magmatism suggests that hot spot activity occurred in this area of southwestern Europe during pre-Variscan and Variscan times up to the time of opening of the Liguro-Piedmontese ocean, birthplace of the Alpine orogen.

Devonian geodynamic evolution of the Variscan Belt, insights from the French Massif Central and Massif Armoricain

The Paleozoic French Variscan Belt in Massif Central and Massif Armoricain is a collision belt that provides a good example of a suture zone where ophiolites are rare, and the frontal (i.e., the magmatic arc) part of the upper plate is not present. In the lower plate (or Gondwana), the continental rocks are subdivided into an Upper Gneiss Unit (UGU) and a Lower Gneiss Unit (LGU). The UGU experienced a high-pressure (and likely ultra-high-pressure) metamorphism followed by crustal melting during their exhumation. New chemical U-Th-Pb monazite ages and ion-probe U-Pb zircon ages on migmatites allow us to constrain the P-T-t paths followed by the UGU and LGU. By comparison with thermomechanical experiments, a possible geodynamic evolution scenario can be proposed for the Variscan convergence. The high-compression regime of continental subduction developed during the initial subduction of the northern margin of Gondwana under Armorica in Silurian times. This induced the formation of a new subduction zone in the back-arc basin, which is the youngest, hottest, and thus mechanically the weakest part of the overriding plate. As a result, the arc-back-arc basin domain has been almost totally subducted below Armorica. Only a limited part of the back-arc basin rocks remains exposed in the Devonian St-Georges-sur-Loire Unit. Subsequently, the continental subduction of Gondwana resumed with a steeper dip associated with low-compression regime that in turn allowed the high-pressure rocks to be exhumed and partly melted in Late Devonian times. Such a scheme depicts quite well the complexity of the Variscan Belt.

Neurophysiological tests and neuroimaging procedures in non‐acute headache (2nd edition)

Background and purpose:  A large number of instrumental investigations are used in patients with non‐acute headache in both research and clinical fields. Although the literature has shown that most of these tools contributed greatly to increasing understanding of the pathogenesis of primary headache, they are of little or no value in the clinical setting.

Importance du volcanisme calco-alcalin miocène sur la marge sud-ouest de la Corse (campagne MARCO)

Abstract During the MARCO cruise, systematic exploration along the western and northern Corsican margins was carried out by dredging. The compositions of the dredged rocks range from basalt to amphibole-biotite bearing andesite with a broad calc-alkaline character. A Miocene age has been obtained for the amphibole-biotite andesite sample DR02 both by 40Ar-39Ar dating on hornblende (16.0 ± 0.4 My) and using fission-track method on apatite (17.2 ± 0.8 My). The south-western Corsican volcanic zone represents the direct extension of the Miocene Sardinian graben volcanism north of 42 °00 N. It could either be synchronous with or post-date the oceanic opening event. Such an arc volcanism probably results from the subduction to the north of the Liguro-Piemont ocean beneath Europe, the Provencal-Ligurian basin being in back-arc position with respect to the studied volcanic centres.

Middle Carboniferous intracontinental subduction in the Outer Zone of the Variscan Belt (Montagne Noire Axial Zone, French Massif Central): multimethod geochronological approach of polyphase metamorphism

Abstract In the migmatitic dome of the Montagne Noire Axial Zone (Variscan French Massif Central), mafic eclogites yield zircon and rutile U–Pb SHRIMP and secondary-ion mass spectrometry (SIMS) ages of c. 315–308 Ma. These ‘young’ dates, obtained in two different laboratories, do not comply with the geological constraints available for the study area that suggest an older age of the high-pressure–low-temperature (M1) metamorphism. Nevertheless, the Sm–Nd age of the same rock at c. 358 Ma appears in better agreement with the geological constraints, and therefore might reflect the age of the high-pressure (HP) event. Similar 357–352 Ma monazite U–Th–Pbtot ages are obtained from relict grains in the Axial Zone kinzigites that represent restites enclosed in migmatites. Furthermore, monazite grains from biotite–garnet–staurolite micaschists from the dome envelope and Axial Zone kinzigites yield U–Th–Pbtot ages in the range 340–320 Ma. These dates are in good agreement with previously documented zircon and monazite ages from the migmatite and anatectic granites that represent a high-temperature–low-pressure (M2) event. The significance of the zircon and rutile ages in the eclogites is discussed in terms of hydrothermal circulations. A crustal-scale model considers: (1) a north-directed intracontinental subduction, responsible for the high-pressure–low-temperature (M1) metamorphism, coeval with kilometre-scale south-vergent recumbent folds in the Palaeozoic non-metamorphic cover, followed by (2) a high-temperature–low-pressure (M2) event, coeval with the development of the Axial Zone migmatitic dome, and upright folding in the Palaeozoic non-metamorphic series.

La formation conglomératique du Vazzio près d'Ajaccio (Corse-du-Sud), un témoin des dépôts du Chattien supérieur continental synrift en Méditerranée occidentale

Abstract Evidence of Oligocene deposits was for the first time described in Corsica (near Ajaccio) within the Vazzio Formation. The succession is composed of a continental sedimentary sequence, mainly conglomerate, with a thickness of 250 to 300 m. Such conglomerates were deposited in torrential and lacustrine environments. Pollinic association and mammal data date the formation from Late Oligocene age (MP 29). The Vazzio Oligocene formation was deposited in a half-graben upon Hercynian granites. Within the Corsica-Sardinia block, the Vazzio Formation can be considered as an equivalent of the Ussana Formation of Sardinia. A Late-Oligocene palaeogeographic reconstruction is attempted, prior to the rotation of the Corsica-Sardinia block.

Early Permian 90° clockwise rotation of the Maures–Estérel–Corsica–Sardinia block confirmed by new palaeomagnetic data and followed by a Triassic 60° clockwise rotation

Abstract Palaeomagnetic investigations of the Corso-Sardinian block and Maures–Estérel show that there has been a change in their magnetic orientation during the Late Carboniferous–Early Permian period (305–280 Ma). This trend is interpreted in terms of a large-scale 90° clockwise rotation of the southern branch of the Variscan belt that matches the successive change in shortening directions revealed by structural geology. The evidence is based on existing structural studies of the fabrics of syntectonically emplaced granitoids partly based on the anisotropy of magnetic susceptibility, combined with a large database of isotopic ages. The chronological match between the palaeomagnetic and tectonic datasets is interpreted here as a result of large-scale dextral wrench movements in the lithosphere between the Gondwana and Laurussia supercontinents. This wrench deformation is regarded as a sequel to the dextral rotation of the northern branch of the Variscan belt during 330–315 Ma which terminated in frontal collision with Avalonia. The continuation of movement in the southern Variscan realm was due to shearing along the southern margin of the Avalonian block. An additional clockwise rotation is inferred to have taken place during the Triassic period. The age of this motion remains to be determined. Supplementary material: Palaeomagnetic and geochronological data from the Maures–Estérel, Corsica–Sardinia block presented in Figure 7 and discussed in the text are available at http://www.geolsoc.org.uk/SUP18742.