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Genesis and evolution of the upper oceanic crust (ODP-IODP site 1256, East Pacific Rise): inferences from structure and composition of late magmatic veins in a lava pond
No full textEvolution of rodingitic dykes : metasomatism and metamorphism in the Mount Avic serpentinites (Alpine Ophiolites, Southern Aosta Valley)
No full textThe Mount Avic ophiolite mainly consists of a large mass of serpentinites, which constitutes the base of a subunit of the Piemonte ophiolitic nappe. Serpentinites represent the mantle portion of the oceanic lithosphere of the Mesozoic Tethys and consist of antigorite-titanian clinohumite-diopside schists as products of oceanic metasomatism and tectono-metamorphic evolution of the Alpine orogeny, at the expense of abyssal peridotite mineral assemblage. The serpentinite mass includes metagabbro pods (without metasomatic alteration) and associated rodingitic dykes. Various rodingitic dykes can be distinguished on the basis of their mineralogical assemblages. The main assemblage consists of ugranditic garnet, chlorite ± diopside ± epidote ± vesuvianite. We observed also some other peculiar rodingites such as vesuvianite-chlorite-, diopside-epidote-chlorite-, and diopside-chlorite-bearing rodingites, as distinctive of the
Mount Avic massif, as well as rodingitic reaction zones and foliated rodingites with chlorite, diopside, or epidote. These mineral assemblages are strictly related to the chemistry of the protolith (probably mafic dykes within serpentinite), as well as to the oceanic rodingitization during the serpentinization event and to the Alpine evolution that affected the Mount Avic massif
Structural and magmatic evolution of the upper oceanic crust (ODP-IODP Site 1256, East Pacific Rise) : inferences from the texture of late magmatic veins in a lava pond.
No full textOceanic relict textures in the Mount Avic serpentinites, Western Alps
No full textThe Mount Avic serpentinites derive from mantle peridotites that have been involved in several geodynamic episodes, from their formation in the Jurassic Tethys, to the subduction event and suture in the Alpine chain. The main steps of their tectono-metamorphic evolution are inferred by investigating the texture and mineralogy of selected samples. Some serpentinite samples show relict textures and mineralogy related to the emplacement in the oceanic realm and to the early Alpine evolution. Olivine and clinopyroxene porphyroblasts preserve relict mantle textures. Magnetite crystals include relict chromite at their core.
Abundant pseudomorphs of antigorite on former porphyroblasts are present. Mineral composition of the pseudomorphic serpentine partly inherits the composition of former minerals (e.g., relatively Ti-rich antigorite replacing pyroxene). These observations support the hypothesis that the Mount Avic serpentinites derive from original mantle porphyroclastic peridotites. An early formation of Ti-clinohumite during oceanic serpentinization of mantle peridotites at relatively high temperature conditions, followed by HP subduction-related Alpine recrystallization, is proposed. The late orogenic history is characterised by transposition accompanied by greenschist facies minerals crystallization. In these samples, antigorite represents the main mineral phase, marking a pervasive, often mylonitic, foliation. The last generation of serpentine, that occurs as yellow fibres filling late extension veins, was formed during a Neo-Alpine brittle-ductile deformation stage
Oceanic and Alpine evolution of metagabbros from the Mount Avic Ophiolite, Southern Aosta Valley (Western Alps)
No full textGeological map of the Mount Avic massif (Western Alps ophiolites) : is it a fossil Oceanic core complex?
No full textON-AXIS VS OFF-AXIS ACCRETION OF SUPERFAST SPREAD CRUST IN THE PACIFIC OCEAN (EAST PACIFIC RISE, 6°44.2’ N)
No full textGeological map of the Mount Avic massif (Western Alps Ophiolites)
No full textThe Mount Avic massif consists of serpentinized peridotite exposed in the southern Aosta valley (Northwestern Alps), covering an area of ca. 180 km2. The 1:10,000 scale geological map is located in the southern portion of the massif, where serpentinite is in contact with ophiolitic rocks pertaining to the Piemonte Zone, which represents the fossil Mesozoic Tethyan ocean. Southwards, ophiolites are overthrusted by the continental-derived Austroalpine Mont Glacier unit. Serpentinite consists of antigorite, magnetite, and coarse grained Ti-clinohumite, olivine, and diopside, which are reminiscent of the original mantle texture. Rodingitic mafic dykes are intruded within serpentinite; other mafic rocks, consisting of (not rodingitized) metagabbro and metabasalt with relict eclogitic minerals, occur as tectonic slices associated with serpentinite, calcschist and sulphide-rich epidosite. The map gives detailed and updated information on the structure and lithostratigraphy of the Mount Avic ophiolites, providing an insight to the mantle-crust transition of the Tethyan oceanic lithosphere
Felsic segregation during crystallization of a subaqueous lava field (ODP-IODP Site 1256, East Pacific Rise) : Inferences from structure and petrography
No full textMassive basalt flows have been mainly observed and studied in subaerial environments, but data collected during sonar surveys and drilling cruises in ocean highlighted that thick lava fields (up to 70 m in thickness) are widespread in the oceanic crust produced along fast spreading ridges. Most of the thickest lava flows include felsic differentiates consisting of Na-plagioclase + quartz. Drilling at ODP-IODP Site 1256 encountered a >30 m and >70 m-tick massive lava flow near the top of the basement in Holes 1256C and D respectively. Rare (1 to 3%) late magmatic veins (LMVs) and late magmatic domains (LMDs) of felsic material occur within this very thick lava flow. LMVs and LMDs occur in a range of different orientations, attitudes and textures and cut the basalt magmatic assemblage inducing a deformation
ranging from brittle to ductile. Fine-scale structural, microstructural, and petrographic analyses from the giant lava flow suggest that segregation and migration of felsic melt through host basalt were strictly related to the cooling and crystallization of the lava flow, which represents a single stage magmatic event
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