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Theye, Thomas
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- PublicationRestrictedAlpine subduction zone metamorphism in the Palaeozoic successions of the Monti Romani (Northern Apennines, Italy)(2022)
; ; ; ; ; ; ; ; ; The hinterland of the Cenozoic Northern Apennines fold-and-thrust belt exposes the metamorphic roots of the chain, vestiges of the subduction-related tectono-metamorphic evolution that led to the buildup of the Alpine orogeny in the Mediterranean region. Like in other peri-Mediterranean belts, the tectono-metamorphic evolution of the Palaeozoic continental basement in the Apennines is still poorly constrained, hampering the full understanding of their Alpine orogenic evolution. We report the first comprehensive tectono-metamorphic study of the low-grade metasedimentary (metapsammite/metapelite) succession of the Monti Romani Complex (MRC) that formed after Palaeozoic protoliths and constitutes the southernmost exposure of the metamorphic domain of the Northern Apennines. By integrating fieldwork with microstructural studies, Raman spectroscopy on carbonaceous material and thermodynamic modelling, we show that the MRC preserves a D1/M1 Alpine tectono-metamorphic evolution developed under HP–LT conditions (~1.0–1.1 GPa at T ~ 400°C) during a non-coaxial, top-to-the-NE, crustal shortening regime. Evidence for HP–LT metamorphism is generally cryptic within the MRC, dominated by graphite-bearing assemblages with the infrequent blastesis of muscovite ± chlorite ± chloritoid ± paragonite parageneses, equilibrated under cold palaeo-geothermal conditions (~10°C/km). Results of this study allow extending to the MRC the signature of subduction zone metamorphism already documented in the hinterland of the Apennine orogen, providing further evidence of the syn-orogenic ductile exhumation of the HP units in the Apennine belt. Finally, we discuss the possible role of fluid-mediated changes in the reactive bulk rock composition on mineral blastesis during progress of regional deformation and metamorphism at low-grade conditions.33 34 - PublicationRestrictedMagmatic Mn-rich garnets in volcanic settings: Age and longevity of the magmatic plumbing system of the Miocene Ramadas volcanism (NW Argentina)(2018)
; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ; ;; ;The Miocene “Corte Blanco Tuff” rhyolite deposit is the product of a large volume and high intensity Plinian erup-tion from the solitary and monogenetic Ramadas Volcanic Centre (Central Andes, Province of Salta, NWArgentina). The“Corte Blanco Tuff”consists of vitreous tube pumices with rare euhedral sub-millimetricMn-garnet phenocrysts, typically hosting inclusions of U-phases as zircon and monazite. Here, we present newtextural, major and trace elemental analyses of garnet, zircon and glass that, combined with in situ U-(Th)-Pbzircon and monazitedating, are used to reconstruct thethermobaricenvironment offormation,age andlongevityof the magmatic plumbing system of the Ramadas magma. The results indicate to a crystallization path of aperaluminous rhyolitic melt at shallow crustal levels (≤6 km), as sequentially tracked by the initial nucleationof zircon (780 °C at 9.16 Ma) and garnet (above or at ca. 700 °C), to thefinal monazite growth (660–670 °C, at8.70 Ma) in a water-saturated (H2O=3–5 wt%) environment, shortly before the eruption started. These data(1) define for thefirst time the primary magmatic origin of Mn-garnet in a rhyolitic volcanic setting; (2) providenew partition coefficients of rare earth elements (REE) between natural garnet, zircon and rhyolitic melts; and(3) permit reconstruction of the magmatic processes that resulted in the Ramadas eruption. On a wider scale,our results document the spatio-temporal (P-Tconditions, timing and longevity) time scales involved in the pet-rogenesis of a shallow peraluminous water-saturated rhyolitic magmatic plumbing system that is able to gener-ate the conditions for extremely explosive Plinian eruptions.94 1