From initiation to termination: The evolution of the Ediacaran Volyn large igneous province (SW East European Craton) constrained by comparative geochemistry of proximal tuffs versus lavas and zircon geochronology

Abstract

The Volyn Series, representing a large intra-plate igneous province, is the result of an Ediacaran lithospheric-scale extension related to the breakup of the Rodinia/Pannotia supercontinent. Due to its subsequent burial and thus limited subsurface access, the precise determination of the igneous activity’s initiation and termination, as well as the reconstruction of its geochemical evolution throughout time still remains a challenge. In its western part (the study area) abundantly cored deep boreholes provide an advantage in accessing rock samples. Among them Kaplonosy IG 1, representing the most complete section with the greatest thickness, preserves a record of initiation and termination of volcanic activity. Tuff samples, collected from the top the Volyn Series, were used for U-Pb zircon dating focussed on autocrysts but from the bottom also on antecrysts. The initiation of effusive activity was dated at 580 ± 10 Ma, and its termination at 547 ± 6 Ma. These volcanics have been affected by later alteration processes (hydrothermal and K- metasomatism), although trace element geochemical data hold information on the primary magmas. The data plot along a common geochemical trend, defined by the trace element contents of the lava samples. Detailed geochronological zonation of the vertical section (in this and previous studies) and its good geochemical coverage, allow for the reconstruction of the igneous system’s geochemical evolution through time. Three main phases were recognized. The initial phase, composed of picrite lavas and tuffs (c. 580–567 Ma), is characterised by extremely low concentrations of Zr and TiO2, but elevated Cr and Ni contents. The main phase (c. 567–551 Ma) is characterised by a large number and volume of subalkaline basalt to andesite flows and tuff beds. The eruptions in the terminal phase (c. 551–547 Ma) indicate a significant decrease in volume, longer hiatuses between eruptive episodes and an increase in the silicic components as well as in the most evolved compositions. The ages measured indicate that, during the break-up of Rodinia/Pannotia, the rifting and separation of Baltica from Amazonia occurred later than the separation of Laurentia from Baltica and from Amazonia.