The Sr, Nd, Hf, and Pb isotope geochemistry of Pleistocene Italian mafic volcanics (Etna, Iblean Basin, Vesuvius, Roman and Tuscan magmatic provinces) can be accounted for by an essentially binary mixture of two well-identified components. A first component with a distinctive mantle flavor shows a strong affinity with HIMU basalts but contains a well-mixed contribution from the depleted mantle. It dominates the southern volcanoes (Etna and Iblean Basin). The second component reveals the presence of both terrigenous and pelagic sediments. These sediments are either present in the mantle source of the lavas (EMII component) or were introduced into the lavas upon contamination with the continental crust. This crustal component dominates the northern provinces. The nature of the process that allows a HIMU component, which normally traces mantle plumes of deep mantle origin, to be present in magmas associated with subduction zones is unclear. There is no indication of a hot spot being present under Southern Italy. Outpouring mantle would be expected to be deflected in the south by the plates subducting under Sicily and in the north by the remnants of the Adria plate associated with the Apennine collision, which culminated with the termination of the Oligocene orogenic volcanism in Sardinia. Seismological evidence suggests that the plate currently subducting northward under Sicily is dangling at a steep angle. At the latitude of Vesuvius and, presumably to the north of it, the Adria plate moved in an approximately E-W direction against the European margin until the Appennine collision. E-W subduction is presently blocked by the contact of the two continental fragments and there are strong seismological indications that either under gravity pull or upon entrapment of the plate subducting under Sicily, the deeper part of the Adria plate is being torn off and foundering into the deep mantle. We suggest that, after break-off, a wake of lower mantle material replaces the foundering plate and the upwelling of this mantle material from below the 660 km discontinuity is the main cause of recent Italian volcanic activity. In most places where continental lithosphere is absent, the HIMU component traces the recycling of ancient crust in the lower mantle while the EMII component traces the sedimentary blanket of oceanic crust fragments. Upwelling of lower mantle beneath Italy and mixing of this material with the asthenosphere provides an alternative to plumes for explaining the presence of deep mantle material in subduction zones.

OIB-Type magmas in subduction zones: mantle counterflow above detaching plates

MACERA, PATRIZIA
2000-01-01

Abstract

The Sr, Nd, Hf, and Pb isotope geochemistry of Pleistocene Italian mafic volcanics (Etna, Iblean Basin, Vesuvius, Roman and Tuscan magmatic provinces) can be accounted for by an essentially binary mixture of two well-identified components. A first component with a distinctive mantle flavor shows a strong affinity with HIMU basalts but contains a well-mixed contribution from the depleted mantle. It dominates the southern volcanoes (Etna and Iblean Basin). The second component reveals the presence of both terrigenous and pelagic sediments. These sediments are either present in the mantle source of the lavas (EMII component) or were introduced into the lavas upon contamination with the continental crust. This crustal component dominates the northern provinces. The nature of the process that allows a HIMU component, which normally traces mantle plumes of deep mantle origin, to be present in magmas associated with subduction zones is unclear. There is no indication of a hot spot being present under Southern Italy. Outpouring mantle would be expected to be deflected in the south by the plates subducting under Sicily and in the north by the remnants of the Adria plate associated with the Apennine collision, which culminated with the termination of the Oligocene orogenic volcanism in Sardinia. Seismological evidence suggests that the plate currently subducting northward under Sicily is dangling at a steep angle. At the latitude of Vesuvius and, presumably to the north of it, the Adria plate moved in an approximately E-W direction against the European margin until the Appennine collision. E-W subduction is presently blocked by the contact of the two continental fragments and there are strong seismological indications that either under gravity pull or upon entrapment of the plate subducting under Sicily, the deeper part of the Adria plate is being torn off and foundering into the deep mantle. We suggest that, after break-off, a wake of lower mantle material replaces the foundering plate and the upwelling of this mantle material from below the 660 km discontinuity is the main cause of recent Italian volcanic activity. In most places where continental lithosphere is absent, the HIMU component traces the recycling of ancient crust in the lower mantle while the EMII component traces the sedimentary blanket of oceanic crust fragments. Upwelling of lower mantle beneath Italy and mixing of this material with the asthenosphere provides an alternative to plumes for explaining the presence of deep mantle material in subduction zones.
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/11568/169044
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