gballou clovis blanchard

Both geochemical and experimental petrological research indicate that Archaean continental crust was generated by partial melting of an Archaean tholeiite transformed into a garnet-bearing amphibolite or eclogite. The geodynamic context Ž. Ž. of tholeiite melting is the subject of controversy. It is assumed to be either 1 subduction melting of a hot subducting slab , Ž. Ž. or 2 hot spot melting of underplated basalts. These hypotheses are considered in the light of modern adakite genesis. Ž. Adakites are intermediate to felsic volcanic rocks, andesitic to rhyolitic in composition basaltic members are lacking. They Ž. Ž .Ž. have trondhjemitic affinities high-Na O contents and K OrNa O ; 0.5 and their Mg no. 0.5 , Ni 20–40 ppm and Cr 2 2 2 Ž. Ž. 30–50 ppm contents are higher than in typical calc-alkaline magmas. Sr contents are high) 300 ppm, until 2000 ppm Ž. Ž. and REE show strongly fractionated patterns with very low heavy REE HREE contents Yb F 1.8 ppm, Y F 18 ppm. Consequently, high SrrY and LarYb ratios are typical and discriminating features of adakitic magmas, indicative of melting of a mafic source where garnet andror hornblende are residual phases. Adakitic magmas are only found in Ž. subduction zone environments, exclusively where the subduction andror the subducted slab are young-20 Ma. This situation is well-exemplified in Southern Chile where the Chile ridge is subducted and where the adakitic character of the lavas correlates well with the young age of the subducting oceanic lithosphere. In typical subduction zones, the subducted lithosphere is older than 20 Ma, it is cool and the geothermal gradient along the Benioff plane is low such that the oceanic crust dehydrates before it reaches the solidus temperature of hydrated tholeiite. Consequently, the basaltic slab cannot melt. Ž. The released large ion lithophile element LILE-rich fluids rise up into the mantle wedge, inducing both its metasomatism and partial melting. Afterwards, the residue is made up of olivineq clinopyroxeneq orthopyroxene, such that the partial Ž. Ž. melts are HREE-rich low LarYb and SrrY. Contrarily, when a young-20 Ma and hot oceanic lithosphere is subducted, the geothermal gradient along the Benioff plane is high, so the temperature of hydrated tholeiite solidus is reached before dehydration occurs. Under these conditions, garnet andror hornblende are the main residual phases giving Ž. Ž. rise to HREE-depleted magmas high LarYb. The lack of residual plagioclase accounts for the Sr enrichment high SrrY of the magma. Experimental petrologic data show that the liquids produced by melting of tholeiite in subduction-like P–T conditions are adakitic in composition. However, natural adakites systematically have higher Mg no., Ni and Cr contents, which are interpreted as reflecting interactions between the ascending adakitic magma generated in the subducted slab and the overlying mantle wedge. This interpretation has been recently corroborated by studies on ultramafic enclaves in Batan w lavas where olivine crystals contain glass inclusions with adakitic compositions Schiano,

Structure et mode de mise en place du pluton granitique de Tiébélé (Burkina Faso) et son implication dans la concentration des anomalies en métaux de base (Zn-Pb-Cu) et en or (Au) Résumé. Le pluton granitique de Tiébélé (PGT) se trouve en position centrale dans les formations paléoprotérozoïques, dans l'environnement du gîte d'amas sulfuré polymétallique (Zn-Pb-Cu) et or (Au). Ce granite, visiblement sans structure apparente, présente une microstructure magmatique à sub-magmatique prédominante. L'orthogneissification est localisée. La fabrique magnétique par la méthode de l'anisotropie de la susceptibilité (ASM) montre dans l'ensemble des linéations sub-horizontales avec toutefois une zone de fort plongement (≥ 56°) qui constituerait le conduit d'alimentation en magma. Celui-ci serait mis en place sous forme diapirique à un niveau supracrustal à la faveur d'un cisaillement régional accommodé par un aplatissement in situ lié à un effet de toit. La source d'alimentation en magma coïncide avec la zone de forte concentration en or et métaux de base (particulièrement Zn). La disposition spatiale des corps minéralisés en métaux (Zn-Pb-Cu), à l'origine très rapprochés et orientés NE-SW, aurait été perturbée par les mises en place successives des granitoïdes à amphibole ± biotite du groupe Tonalite-Trondhjémite-Granodiorite (TTG), puis tardivement par le PGT. L'effet thermique induit par le PGT au cours de sa mise en place, aurait: i/ joué un rôle majeur dans la remobilisation et la reconcentration des phases minérales sulfurées notamment la sphalérite, et ii/ occasionné des microbrèchifications sous l'effet de la pression fluide. Les minéralisations aurifères situées au sud du PGT, sont dans des veines de quartz ± tourmaline ouvertes à la faveur de cisaillements NE-SW sous pression fluide générée vraisemblablement par l'anomalie thermique du PGT. Mots-clés : Burkina Faso, Paléoprotérozoïque, Pluton granitique de Tiébélé (PGT), Anisotropie de la Susceptibilité Magnétique (ASM), pétrologie structurale, minéralisations. Abstract. The Tiébélé granite pluton (TGP) is located in a central position of the Palaeproterozoic terrains in an environment made of polymetallic massive sulphide (Zn-Pb-Cu) and gold deposit. The granite, likely undeformed, shows therefore a predominant magmatic and submagmatic structure under microscope. Orthogneissification is localised. The study of magnetic fabric using the magnetic susceptibility anisotropy (MSA) method shows in overall a subhorizontal lineation with however an area of high plunging (≥ 56°), which constitutes the conduct of the magma feeding. The magma should have emplaced by diapirism in a supracrustal level due to a regional shearing accommodated by an in situ flattening imputable to roof effect. The source of the magma coincides with the most prospective area for gold and base metals (especially Zn). The spatial organisation of the base metals (Zn-Pb-Cu), originally very close along a NE-SW trend, has been largely disturbed by the consecutive emplacement of the amphibole ± biotite bearing granitoid of Tonalite-Trondhjemite-Granodiorite (TTG) group, and later by the TGP. The thermal effect induced by TGP during its emplacement should have: i/ played a major role in remobilizing and concentrating sulphide mineral phases notably sphalerite, and ii/causing microbrecciation under fluid pressure effect. Mineralization in the south of the TGP, are hosted by quartz ± tourmaline veins in the NE-SW shearing, on fluid pressure probably generated by the thermal anomaly of the TGP.