Plume-related magmatism in collision zones: examples from the Tertiary-Quaternary magmatism in Italy.

From late Cretaceous to present time, an extensive magmatic activity developed both in Europe and northern Africa, showing a progressive transition with time from calc-alkaline to Na-rich alkaline features in areas tightly connected with subduction systems (Morocco, Algeria, Tunisia, Spain, Italy), while Na-rich basalts and basanites, with minor tholeiitic volcanics, occur at extensional tectonic settings, both associated or not to orogenic dynamics (Valencia trough, Pannonian, Alboran, Tyrrhenian, and Aegean basins, Pantelleria-Etna-Iblean area, Veneto Province, Cenozoic Rift System). The widespread alkaline magmatism in the European and circum Mediterranean area shows a uniform HIMU-DMM type signature which has been recently ascribed to a mantle contamination episode triggered by the rise of the Central Atlantic Plume (CAP) head since Cretaceous time (Piromallo et al., 2008). The occurrence of a HIMU-DMM component within lavas originated in collision environments such as the Veneto Volcanic Province in NE Alps (related to the Tertiary convergence of Europe and Africa plates) or the Roman Magmatic Province (related to the Cenozoic Adria subduction) might be explained in terms of slab breakoff (Macera et al., 2008; Gasperini et al., 2002). Evidence for slab breakoff in these areas comes from seismic tomography (Piromallo and Faccenna, 2004) and geophysical modeling consisting in evaluating the time evolution of buoyancy of oceanic and continental lithosphere during subduction with both constant and time-varying convergence rates (Macera et al., 2008). If the subducted slab intercepts a hot mantle diapir rising from the frayed plume head, the corresponding part of the slab is heated and therefore softened. The softening effect is enhanced if the slab includes continental material. The combination of changes in negative buoyancy caused by continental subduction, and softening of a part of the slab caused by slab-plume interaction, may act as a regulator for the time of slab breakoff and consequently for the time and type variations of magmatism in the overriding lithosphere above a subduction zone. In the Alpine region, we assume that the plume material interacted with the subducting slab causing its heating, softening, and finally its detachment. Ensuing upwelling of plume material through the resulting plate window is supposed to be the responsible for partial melting in the lithospheric mantle wedge and/or decompression melting of the ascending plume material. Plume-related volcanism in subduction zones is possible either before the subducted slab intercepts plume head material , or after slab breakoff. The above considerations provide a framework for the occurrence of plume-related and calc-alkaline magmatism in the southeastern Alps and probably in other similar tectonic settings such as the Central Europe Magmatic Province. The heat released by the hot mantle component to the overriding mantle wedge contributed to its partial melting and production of calc-alkaline mafic magmas. The latter additionally favored the genesis of calc-alkaline felsic magmas through partial melting of the lower crust by underplating. A quantitative approach based on mixing calculations on Sr-Nd and Pb isotopes support these hypotheses.