During the Early Oligocene, the ~800 m thick Aveto-Petrignacola volcaniclastic Formation (APF) was deposited in a relatively short time (~32-29 Ma) in the Northern Apennines. The APF volcanic rocks are mostly basaltic andesites, andesites and dacites, with minor rhyolites and gabbroic compositions found as pebbles ~0.001-0.5 m3 in size (with average diameter ranging from ~5 to 30 cm). This volcaniclastic succession is interpreted as the product of subaerial effusive to explosive arc magmatism and closely associated turbidite sedimentation. Petrographic, mineral chemical, major and trace element content, as well as Sr-Nd-Pb isotopic ratios are all compatible with a calc-alkaline magmatism generated as consequence of the metasomatic modifications related to the subduction of oceanic lithosphere. The APF volcanic rocks are a puzzle in the geodynamic evolution of the Italian area. This Formation is now part of the Apennine thrust system, developed as consequence of the West-directed subduction system that involved recycling of the oceanic lithosphere of the Mesogean/Liguride ocean(s) beneath the southern paleo-continental margin of Europe. Such a subduction system was associated with production of abundant calc-alkaline/arc tholeiitic magmatism starting from Late Eocene/Early Oligocene in SE Spain (Malaga arc tholeiitic dykes), Sardinia (e.g., Calabona microdiorite) and SE France (Esterel microdiorite). During the Middle-Late Eocene, the Alpine Tethys subduction system had already ceased, followed by Adria-Europe continental collision. In other words, during the emplacement of the APF subductionrelated volcanic rocks, the Alpine subduction system was already shut down, whereas the Apennines subduction system was fully developing. Notwithstanding this, we propose that the APF volcanic rocks are genetically connected to the Alpine subduction system rather than the Apennines. We propose that the APF volcanic rocks are generated by a mantle source modified by the Alpine slab detached during Middle-Late Eocene, wandering beneath the Adriatic lithosphere after the closure of the Piedmontese (Liguride) Ocean. The effects of the “zombie” slab would be a metasomatic modification of the original peridotitic matrix, allowing hydratation with the consequent formation of amphibole. Local effects of stress release in the Apennines foreland and in the Alps hinterland would have played an important role to trigger magmatism. Paradoxically, the subduction-related igneous activity in the APF area developed several Myrs after the cessation of oceanic lithosphere subduction in an area that, few Myr later, would have been involved in a completely different orogenesis (Apennines tectonics). In summary, the APF volcano (now completely eroded) would have been generated as consequence of Alpine tectonics, but would have been involved in the Apennines thrusts soon after its formation. According to this view, the APF and the Mortara volcanoes, the latter now buried beneath the Po Plain, would have the same origin. The formation of the Po Plain (representing the foreland of the Apennines and the retro-foreland of the Alps back-thrusts would have prevented the Mortara volcano from being eroded, leaving it in its original position on the north-western margin of Adria.

Subduction-unrelated subduction-related magmatism in Northern Apennines. The Early Oligocene Aveto-Petrignacola volcaniclastic succession

BIANCHINI, Gianluca
2011

Abstract

During the Early Oligocene, the ~800 m thick Aveto-Petrignacola volcaniclastic Formation (APF) was deposited in a relatively short time (~32-29 Ma) in the Northern Apennines. The APF volcanic rocks are mostly basaltic andesites, andesites and dacites, with minor rhyolites and gabbroic compositions found as pebbles ~0.001-0.5 m3 in size (with average diameter ranging from ~5 to 30 cm). This volcaniclastic succession is interpreted as the product of subaerial effusive to explosive arc magmatism and closely associated turbidite sedimentation. Petrographic, mineral chemical, major and trace element content, as well as Sr-Nd-Pb isotopic ratios are all compatible with a calc-alkaline magmatism generated as consequence of the metasomatic modifications related to the subduction of oceanic lithosphere. The APF volcanic rocks are a puzzle in the geodynamic evolution of the Italian area. This Formation is now part of the Apennine thrust system, developed as consequence of the West-directed subduction system that involved recycling of the oceanic lithosphere of the Mesogean/Liguride ocean(s) beneath the southern paleo-continental margin of Europe. Such a subduction system was associated with production of abundant calc-alkaline/arc tholeiitic magmatism starting from Late Eocene/Early Oligocene in SE Spain (Malaga arc tholeiitic dykes), Sardinia (e.g., Calabona microdiorite) and SE France (Esterel microdiorite). During the Middle-Late Eocene, the Alpine Tethys subduction system had already ceased, followed by Adria-Europe continental collision. In other words, during the emplacement of the APF subductionrelated volcanic rocks, the Alpine subduction system was already shut down, whereas the Apennines subduction system was fully developing. Notwithstanding this, we propose that the APF volcanic rocks are genetically connected to the Alpine subduction system rather than the Apennines. We propose that the APF volcanic rocks are generated by a mantle source modified by the Alpine slab detached during Middle-Late Eocene, wandering beneath the Adriatic lithosphere after the closure of the Piedmontese (Liguride) Ocean. The effects of the “zombie” slab would be a metasomatic modification of the original peridotitic matrix, allowing hydratation with the consequent formation of amphibole. Local effects of stress release in the Apennines foreland and in the Alps hinterland would have played an important role to trigger magmatism. Paradoxically, the subduction-related igneous activity in the APF area developed several Myrs after the cessation of oceanic lithosphere subduction in an area that, few Myr later, would have been involved in a completely different orogenesis (Apennines tectonics). In summary, the APF volcano (now completely eroded) would have been generated as consequence of Alpine tectonics, but would have been involved in the Apennines thrusts soon after its formation. According to this view, the APF and the Mortara volcanoes, the latter now buried beneath the Po Plain, would have the same origin. The formation of the Po Plain (representing the foreland of the Apennines and the retro-foreland of the Alps back-thrusts would have prevented the Mortara volcano from being eroded, leaving it in its original position on the north-western margin of Adria.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11392/1507313
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