Importance de la nature et du rôle des discontinuités au sein des ophiolites lors du développement d'un orogène

Le caractère unitaire des ophiolites résulte de processus inéluctables à tout magmatisme qui engendre résidu, cumulat et liquide. Une comparaison des ceintures ophiolitiques entre elles a permis de souligner d'importantes discontinuités pétrologiques, structurales et stratigraphiques liées à des systèmes géotectoniques différents mais aussi à des variations longitudinales le long de la ceinture considérée. La classification pétrologique des ophiolites en deux types proposée précédemment a été affinée tandis que la quantification des fractionnements (majeurs et traces) à partir des exemples Corse et Vourinos a permis de juger de sa validité. En particulier dans les ophiolites de type Il (Vourinos) on retiendra que les liquides observés ont pu subir 40 % d'extraction réalisée principalement dans des chambres magmatiques sous-crustales. La composition des magmas qui évolue avec le temps -diminution des incompatibles et augmentation de la sursaturation en silice- est en accord avec un appauvrissement progressif du manteau. Dans les ophiolites à affinité de ride médioocéanique, (type 1), le manteau est renouvelé lors de l'accrétion à partir des hétérogénéités pétrologiques et structurales observées dans un massif de péridotites de Corse, l'importance des processus de fusion, fractionnement et métasomatisme dans un diapir mantellaire mis en place dans une zone d' accrétion a pu être appréciée et la chronologie des mécanismes précisée. Les ophiolites de type Il ont servi de base à une classification structurale des ophiolites en deux groupes dont la structure interne pourrait être liée aux conditions d'ouverture. En particulier, dans les ophiolites à structures verticalisées qui sont souvent plus pauvres en éléments incompatibles que celles à variations lithologiques et structurales sub-horizontales, la morphologie des chambres a été estimée à partir de l'étude d'un massif en Iran. Dans toutes les ophiolites examinées, la durée de l'accrétion est brève dépassant rarement 40 Ma même pour celles qui montrent le plus d' affinité géochimique avec les échantillons océaniques. Ces différents aspects, de même que la nature des formations géologiques associées, ont conduit à substituer au jeu distension-collision frontale, des coulissements induits par la dérive des plaques majeures. Ces derniers sont responsables de la succession d'ouverture et fermeture plus ou moins obliques par rapport aux ceintures bordant les continents. Si certaines ophiolites correspondent à des tentatives d'océanisation, d'autres sont intégrées dans le développement d' un orogène. Dans ce cas, les ophiolites apparaissent après la formation de bassins à marges passives devenant quelquefois actives avant d'être ensemble rapldement tectonisées.pas de résum

The role of possible transverse faults in the development of the Sabzevar ophiolites, North-East Iran, with special reference to magma chamber tectonics

In the Sabzevar ophiolites, North-East Iran, structural studies have shown that accretion have rather occurred along an oblique spreading axis. A discontinuous ridge may have developed either at a ridge-transform intersection with a small offset or in a pull-apart basin linked to a wrench fault subparallel to the mountain belt. Special attention was paid to the origin of the setting of the layered cumulates which probably were formed in several disrupted magma chambers. The layering, of variable dip ahd strike, results from crystallization taking place either along inclined planes linked to fault activities or near temporary subhorizontal floor regulated by gravitational forces. Tectonics in accretion zones also control the shape and size of the chamber, the evolution of the more or less asymmetric channel, the setting of gabbroic dykes and flow-layered secant cumulates.Dans les ophiolites de Sabzevar, au NE de l'Iran, des études structurales ont montré que l'accrétion avait eu lieu plutôt le long d'un axe d'extension oblique. Celui-ci a pu se développer soit à l'intersection d'une ride avec une zone transformante qui la décale de peu, soit dans des bassins de distension losangiques alignés le long d'un décrochement subparallèle à la chaîne de montagne. L'attention a été particulièrement portée sur l'origine du gisement des cumulats issus probablement de plusieurs chambres magmatiques disloquées. Le rubanement, d'inclinaison et d'orientation variables, résulte de cristallisations effectuées soit le long de plans inclinés liés à l'activité de failles, soit à proximité de planchers temporaires subhorizontaux contrôlés par les forces gravitaires. La tectonique dans les zones d'accrétion contrôle aussi, en partie, la forme et la taille des chambres magmatiques, l'évolution des chenaux plus ou moins dissymétriques et le gisement des dykes de gabbros et des cumulats sécants au laminage magmatique.Ohnenstetter Maryse. The role of possible transverse faults in the development of the Sabzevar ophiolites, North-East Iran, with special reference to magma chamber tectonics. In: Sciences Géologiques. Bulletin, tome 36, n°1, 1983. Ophiolites téthysiennes. pp. 73-90

Trace elements in the Merensky Reef and adjacent norites Bushveld Complex South Africa

International audienceTrace elements were analysed in rocks and minerals from three sections across the Merensky Reef in the Rustenburg Platinum Mine in the Bushveld Complex of South Africa. Whole rocks and separated minerals were analysed by inductively coupled plasmamass-spectrometer (ICP-MS) and in situ analyses were carried out by ion microprobe and by laser-source ICPMS. Merensky Reef pyroxenites contain extremely high concentrations of a wide range of trace elements. These include elements incompatible with normal silicate minerals as well as siderophile and chalcophile elements. For major elements and compatible trace elements, the measured concentrations in cumulus phases and the bulk rock compositions are similar. For highly incompatible elements, however, concentrations in bulk rocks are far higher than those measured in the cumulus phases. In situ analyses of plagioclase have far lower concentrations of Th, Zr and rare earth elements than ICP-MS analyses of bulk separates of plagioclase, a difference that is attributed to the presence of traceelement-rich accessory phases in the bulk mineral separates. We used these data to calculate the trace-element composition of the magmas parental to the Merensky Unit and adjacent norites. We argue that there is no reason to assume that the amount of trapped liquid in the Merensky orthopyroxenite was far greater than in the norites and we found that the pyroxenite formed from a liquid with higher concentrations of incompatible trace elements than the liquid that formed the norites. We propose that the Bushveld Complex was fed by magma from a deeper magma chamber that had been progressively assimilating its crustal wall rocks. The magma that gave rise to the Merensky Unit was the more contaminated and unusually rich in incompatible trace elements, and when it entered the main Bushveld chamber it precipitated the unusual phases that characterize the Merensky Reef. The hybrid magma segregated sulphides or platinum-group-element-rich phases during the course of the contamination in the lower chamber. These phases accumulated following irruption into the main Bushveld chamber to form the Merensky ore deposits

Flow of the partiallymolten Variscan orogenic crust fromsyn-orogenic exhumation of subductedcontinental crust to gravitational collapsealong a convergent plate boundary markedby slab retreat

International audienceThe Variscan belt ofWestern Europe exposed in the French Massif Centralis a perfect example of a collision zone characterized by protractedsyntectonic magmatism and partial melting (from 380 to 280 Ma) witha wide range of petrologic and geochemical signatures (calc-alkaline,high-K, Mg-K, peraluminous) that have been inferred to fingerprint lithosphericsubduction, mantle upwelling and/or partial melting of theorogenic wedge.The nappe pile encompasses an upper gneiss unit (UGU) and a lowergneiss unit (LGU) that are separated by an association of maficultramaficrocks designated as the Leptynite-Amphibolite Group (LAG)and has been interpreted as representing remnants of former small immatureoceanic basins. Both the UGU and the LGU are made of migmatitesbut are distinguished on the basis of their structural position withrespect to the LAG and of their metamorphic record. The UGU has preservedrelics of high-pressure metamorphism whereas the LGU has onlyrecorded a high-temperature metamorphism.We present a synthesis of structural, petrologic, geochemical and geochronologicaldata from the various lithologic-tectonic units exposedalong a transect across the Variscan belt of Western Europe from theFrench Massif Central to the Pyrenees. In particular, the new geochronologicaland geochemical dataset presented by Couzinié et al. (this conference)suggests the contribution of mantle and crustal derived magmaswith a southward younging of syntectonic emplacement. These dataprovide a basis to elaborate a model for the structure of the Laurussia-Gondwana plate boundary at the onset of convergence and for the generationand flow of migmatites during orogenic evolution from the earlystage of subduction of the continental crust to gravitational collapse ofthe orogenic belt in a context of a convergent plate boundary marked bysouthward slab retreat

Uranium mineralization associated with late magmatic ductile to brittle deformation and Na–Ca metasomatism of the Pan-African A-type Zabili syntectonic pluton (Mayo-Kebbi massif, SW Chad)

International audienceThe Mayo-Kebbi massif (Chad) exposes a Neoproterozoic juvenile crustal segment that has been tectonically accreted in the Central African Orogenic Belt and reworked during the Pan-African orogeny. It comprises a syntectonic high-K magmatic suite including the Zabili A-type granitic pluton. The Zabili pluton is made of a highly differentiated granite generated by fractional crystallization of a magma formed by partial melting of a Neoproterozoic juvenile protolith. Syn- to post-magmatic ductile to brittle deformation of the Zabili pluton is associated with metasomatism and deposition of uranium. Primary magmatic U-bearing minerals are zircon, monazite, and uranothorite. Late-magmatic deformation and Na-metasomatism are marked by the development of R’ antithetic shear zones with high-temperature dynamic recrystallization of K-feldspar and Ca-plagioclase phenocrysts coeval with crystallization of albite along deformation bands and grain boundaries, and crystallization of interstitial amphibole, calcite epidote and albite also affected by intracrystalline deformation. At this stage, U-bearing minerals are monazite, uraninite, brannerite (pseudomorphosed in ekanite). This late-magmatic event is dated by U–Th–Pb on monazite at 599 ± 4 Ma. Brittle deformation and Ca-metasomatism are marked by cataclastic zones and veins containing albite, epidote, calcite, chlorite, apatite, metamict zircon, pitchblende, U-silicates, and iron oxides. U-bearing minerals are altered and/or remobilized in ekanite, kasolite, and uranophane. These data suggest that the uranium mineralization hosted by the Zabili pluton records a superposition of processes and traces extreme crustal differentiation of a Neoproterozoic juvenile crustal segment reworked during the Pan-African orogeny