Metamorphic evolution of the Assuéros Layered Complex, Argentera Massif (Western Alps).
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<p>Ion probe <sup>208</sup>Pb/<sup>232</sup>Th fissure monazite ages from high pressure regions of the Western Alps and from the Argentera Massif provide new insights on the tectonic evolution of the Western Alps during Cenozoic times. Fissure monazite is a hydrothermal mineral crystallizing during cooling/exhumation in Alpine fissures, an environment where monazite is highly susceptible to fluid-mediated dissolution-(re)crystallization. Fissure monazite ages directly record chemical disequilibrium occurring in a fissure environment, but growing evidences indicate that fissure monazite commonly register tectonic activity. Fissure monazite age domains from this study show that monazite crystallization occurred between ~32-30.5 Ma and ~31.5-30 Ma in the Pi&#233;montais and Brian&#231;onnais Zone of the High Pressure regions, and between ~17-15 Ma and in the north-eastern border of the Argentera Massif. So far, monazite ages were recorded between ~32-23 Ma and at ~20.5 Ma in the Brian&#231;onnais Zone and in the south-western border of the Argentera Massif respectively. Thus the presented dataset corroborate and complement already reported fissure monazite <sup>208</sup>Pb/<sup>232</sup>Th ages from the Western Alps. This new fissure monazite ages compilation supports that Late Oligocene thrusting affected the High Pressure regions of the Western Alps, and that Early and Middle Miocene dextral strike-slips movements respectively affected the south-western and north-eastern margins of the Argentera Massif. Chemical observations provide new hints on fissure monazite growth conditions (e.g. leached host-rock minerals, oxidation conditions) encouraging to pursue chemical studies with a larger dataset on natural fissure monazite to better understand growth conditions under cleft environment.</p>
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The Ivrea zone consists of a metapelitic series and of a main basic body, with related ultramafites (peridotites and pyroxenites). The peridotites are currently considered as mantle material. On the basis of their relationships with the country rocks, bulk and phase composition and factorial analysis, it is demonstrated in this paper that the peridotites are in formed by fractionation and cumulus processes. The main basic body is generally considered as a metamorphic suite of basic granulites, that have suffered the same metamorphic event as the metapelitic series. In this paper the hypothesis that it represents a deep-seated stratiform complex is proposed. This possibility is supported by the igneous contacts between the main basic body and the metapelitic series, by the fractionation pattern of the main basic body and by its stratigraphy. Its geochemical features and dynamic characteristics suggest an emplacement into an orogenic environment, possibly related to a continental margin. The 'granulitic' mineral assemblages of the main basic body are attributed to re-equilibration by slow cooling in deep crust and not to a metamorphic event. This point is supported by the relationships between the temperature and pressure paths from the estimated original conditions to those of re-equilibration, by the preservation of evidence of a thermal increase in the country rock, and by the preservation of primary parageneses. The relationships between the main basic body and the metapelitic series indicate that the emplacement of the first postdates the earlier (Caledonian) metamorphism of the metapelites.
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