Intraplate magmatism at a convergent plate boundary: The case of the Cenozoic northern Adria magmatism

The complex European–Adria geodynamic framework, which led to the formation of the Alpine belt, is considered responsible for the orogenic magmatism that occurred in the Central Alps along the Periadriatic/Insubric Line (late Eocene–early Oligocene) and the anorogenic magmatism that occurred in the Southeastern Alps (late Paleocene–early Miocene). While the subduction-related magmatic activities are, as expected, near convergent margins, the occurrence of the intraplate-related magmatism is still puzzling. Therefore, in this work new geochemical and geochronological data of magmatic products from the Veneto Volcanic Province (VVP, north–east Italy) are provided to constrain the Cenozoic intraplate magmatism of the Southeastern Alps. The VVP is formed by dominant basic–ultrabasic (from nephelinites to tholeiites) magmatic products and by localized acid (latitic, trachytic, and rhyolitic) volcanic and subvolcanic bodies. Trace element patterns and ratios suggest that the mantle source of the alkaline magma types was a garnet lherzolite possibly metasomatised by carbonatitic melts and with residual phlogopite. According to the biostratigraphic records and our new 40 Ar/ 39 Ar ages, VVP eruptions occurred in several pulses, reflecting the extensional phases experienced by the Eastern Alpine domain. The volcanism started in the late Paleocene in the western sector of the VVP where activity was widespread also during the Eocene (45.21 ± 0.11 Ma – 38.73 ± 0.44 Ma). In the eastern sector eruptions took place in the early Oligocene (32.35 ± 0.09 Ma – 32.09 ± 0.29 Ma) and in the early Miocene (~23–22 Ma). From the studies so far undertaken, the anorogenic magmatic activity of the VVP was interpreted as resulting from mantle upwellings through slab window(s) following the European slab break-off, which occurred at ~ 35 Ma. However, considering (i) new tomographic images evidencing a continuous subvertical (~ 500 km in depth) slab beneath the Central Alps, and (ii) the onset of magmatic activity in the VVP in the late Paleocene (i.e., before the slab break-off) and its continuation until the Miocene, a better suited geodynamic scenario is required to explain the anorogenic magmatism. The westward rollback of the European slab caused the retreat and steepening of the subducting plate. As a consequence, sub-slab mantle material escaped and upwelled from the front of the slab and created a poloidal mantle flow. The latter induced the breakdown of carbonates in calcareous metasediments and carbonated metabasics within the subducting oceanic slab, providing carbonatitic melts, which could be responsible for the metasomatism of the VVP mantle sources. After that, the poloidal mantle flow also induced (i) the extensional deformation in the overriding Adria microplate, (ii) the decompressional melting of VVP mantle sources, and (iii) the intraplate affinity of the VVP magmatism. During these processes, the Adria microplate also rotated counterclockwise, forming sedimentary basins, and allowing the poloidal mantle flow to affect different portions of the overlying lithosphere, generating syn-estensional magmatism within the VVP.