Magma evolution in a complex geodynamic setting, South Harghita volcanic area, East-Central Europe: Constraints from magma compositions and zircon petrochronology

The southernmost segment of the Călimani-Gurghiu-Harghita (CGH) volcanic chain (East Carpathians, Romania) consists of four aligned volcanoes (Luci-Lazu, Cucu, Pilişca and Ciomadul), which represent the youngest manifestation (from 5.3 to 3.7 Ma and from 3 Ma to 0.03 Ma) of the Late Neogene to Quaternary post-collisional volcanism in the Carpathian-Pannonian Region. Within this restricted, 35 km long segment, two marked changes occurred in the erupted magma composition, with an increase in bulk rock Sr and Ba contents and a decrease of heavy rare earth element values from north to south and from the older to the younger volcanism. Furthermore, a notable increase of Eu/Eu* and Ce/Ce* can be observed in the younger zircon population. New zircon U-Pb ages indicate that these compositional changes took place at 2 Ma and 1 Ma, the latter one following a prolonged eruption gap between ∼1.4 and 1 Ma. The two major geochemical changes correspond to a modification of the geodynamic environment. The 2–2.7 Ma Cucu volcanic activity could have occurred during a catastrophic slab loss period at the Trotuş segment, when asthenospheric mantle ascended into the shallow detachment area and caused melting of the lithospheric mantle metasomatized by aqueous fluids during the former subduction. In contrast, the