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 Calimani-Gurghiu-Harghita (CGH) volcanic chain (East Carpathians, Romania) consists of four aligned volcanoes (Luci-Lazu, Cucu, Pilis,ca 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 similar to 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 Trotus, 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 <2 Ma Pilis,ca and Ciomadul volcanoes developed above a remnant oceanic slab, south of the Trotus, zone. The dense oceanic slab initiated the unzipping of the lower continental lithosphere of the Moesian plate and this caused perturbation in the asthenospheric mantle. The upward mantle flow generated by the slab roll-back resulted in alkali basalt and shoshonitic magma formation in the upwelling asthenosphere and the meta-somatized lithospheric mantle, respectively. The sharp compositional change at the onset of the Ciomadul volcanism can be due to slab melting at the edge of the still sinking oceanic slab beneath Vrancea initiated by toroidal asthenospheric mantle flow and dehydration reactions within the slab. Ultrahydrous slab melts accu-mulated beneath the crust and mixed with shoshonitic magma. High pressure amphibole crystallized in these hybrid magmas at strongly oxidizing conditions, followed by further crystallization and magma mixing in the shallow crustal magma reservoir. The South Harghita -Vrancea area is geodynamically still active and serves as an excellent example of complex interaction between the final stage of subducted slab sinking, the ongoing slab detachment, the asthenospheric mantle flows and the magma generation in a post-collisional setting.