Middle Miocene volcanism in the vicinity of the Middle Hungarian zone: Evidence for an inherited enriched mantle source

Middle Miocene igneous rocks in the vicinity of the Middle Hungarian zone (MHZ) show a number of subduction-related geochemical characteristics. Many of these characteristics appear to be time-integrated, showing a decreasing subduction signature with time. In contrast to previous models, which suggest southward-dipping subduction of European lithosphere beneath the Alcapa microplate (along the Western Carpathians) is responsible for the chemical characteristics seen in middle Miocene volcanics, we propose that source enrichment occurred via the subduction of either the Budva-Pindos or Vardar Oceans. Recent seismic studies have revealed that the proposed southward-dipping subduction was not developed beneath the entire Western Carpathians or, even if it had, was overprinted by the collision of the European plate and the Alcapa unit at 16 Ma. This subduction is thought to have started 30 Ma ago, therefore the time between the onset of subduction and collision cannot account for extensive source enrichment in the overlying mantle wedge. It is also pertinent to note that the middle Miocene igneous rocks of the MHZ in their reconstructed positions are not parallel to the supposed suture expected for subduction-related arc volcanoes. Our review suggests an alternative hypothesis, whereby source enrichment is related to the subduction of either the Budva-Pindos or Vardar Ocean during the Mesozoic-Paleogene. In this model the Alcapa microplate was transferred to its present tectonic position via extrusion and rotations. Geophysical modeling and mantle xenoliths provide evidence that this process occurred at the scale of the lithospheric mantle, indicating that the subduction-modified lithospheric mantle was coupled to the crust. Melting in the lithospheric mantle of the Alcapa unit was triggered by the extension during the formation of the Pannonian Basin. The preserved subduction-related geochemical character of volcanics in intra-plate settings that are otherwise directly unaffected by subduction, can be attributed to tectonic transport of metasomatised mantle from a previous subduction-affected setting. This model provides an alternative approach to understanding the geochemical complexity seen among intra-plate calc-alkaline volcanics, where chemical characteristics can be explained without the involvement of plumes.