Lower mantle upwelling in subduction zones: an example from the italian south-eastern Alps

The geochemical properties of the SE Alps volcanics (SEAV, Eocene age) call for a plume-related origin of the most primitive basalts in contrast to the widespread calc-alkaline magmatism which developed some million years later northwestwards along the Periadriatic Lineament. The two contrasting magmatic suites define binary mixing relationships in the Sr-Nd and Pb-Pb isotopic space, the two end members being represented by a lower crustal component, and a HIMU-DM component (the SEAV). The latter shows geochemical features comparable to the numerous magmatic provinces of Tertiary-Quaternary age from the western Mediterranean area for which a plume-related origin has been assessed. The occurrence of a HIMU component, which is the hallmark of hotspot basalts, in a tectonic regime dominated by collision tectonics (the Tertiary convergence of Europe and Africa plates) is an intriguing feature. Here, we combine tectonic reconstruction, plate motion and tomographic images to unravel the evolution of the Alps-Apenninic slabs and their interaction with the plume, at the scale of the upper mantle. Our result shows that the plume could have been captured and detached from its deeper root by the Alpine-Apennine subducting slab during the Paleogene. This model can explain the lack of evident track of the plume on the surface during the northward drift of Europe (hot spot reference frame). The latest evolutionary stage of the system is signed by the break of the Alpine slab that can definitively have trapped the plume material. Evidence for the European slab breakoff comes from seismic tomography which shows that the present-day fast velocity feature, interpreted as the European slab subducted below the Alpine chain, is shorter by about 300 km than the total length of the subducted slab estimated by paleotectonic reconstructions. Slab detachment and ensuing upwelling of deep mantle material through the lithospheric gap is proposed as a viable mechanism for HIMU-type basalt extrusions in SE Alps as well as in other subduction zones of the western Mediterranean area. Interactions between the HIMU component and the shallower depleted mantle may account for the geochemical characteristics of the SEAV. Counterflows of plume material towards the top of the subducting slab may also induce heating and partial melting in the overriding mantle wedge, giving rise to the tholeiitic basalts often bordering the Periadriatic intrusions. Upwelling of these basalts and their underplating at the mantle-lower crust interface could cause partial melting in the latter and generate the widespread tonalitic-trondhjemitic-granodioritic associations observed along the Periadriatic Lineament.