Combined petrological, geochemical, and statistical analysis of Eocene-Oligocene sandstones of the Thrace Basin, Greece and Bulgaria

The Rhodopeian Orogen developed since Late Cretaceous-Lower Eocene during accretionary processes following the closure of the Vardar ocean basin. We concentrate on Paleogene clastic sediments of the Rhodope area, developed synchronous to the post – Cretaceous collisional collapse and the subsequent Tertiary extensional phase. Throughout a multidisciplinary approach, including sedimentary petrology, sandstone geochemistry and compositional data analysis, we argue to reconstruct the unroofing history of the Rhodopian orogen and the abrupt onset of the volcanic activity between Late Eocene and Oligocene across the eastern and southern Rhodopian region. A total of 127 sandstone samples have been analysed. Sandstone detrital modes include three distinctive petrofacies, a quartzolithic, quartzofeldspathic and volcaniclastic. The major contributions are from the metamorphic basement units, represented mostly by low-medium grade lithic fragments for the quartzolithic petrofacies and high grade metamorphic rock fragments for the quartzofeldspathic petrofacies. Volcaniclastic sandstones recorded different composition betweeen eastern Rhodopes and southern Rhodopes samples. Detrital mode evolution testifies contributions from three key source areas corresponding with the two main crystalline tectonic units of the Rhodope Massif, Variegated Complex and Gneiss-Migmatite Complex and from the Circum-Rhodope Belt. The volcaniclastic petrofacies is interbedded with quartzofeldspathic petrofacies reflecting superposition of active volcanic activity. Geochemical analyses for major and trace elements provide useful provenance informations. The Zr/Sc vs. Th/Sc and Cr vs. Cr/Ni plots suggest that sediment recycling is negligible. The use of geochemical diagrams for tectonic setting discrimination confirmed, in most of the cases, the inferred tectonic setting, corresponding mainly to an active continental margin and subordinately to a continental island arc. The idea of a multidisciplinary approach has been represented by the successful attempt to use together the information provided by petrographic and geochemical analyses. Compositional data from Eocene-to-Oligocene sandstones of the Thrace Basin were used to unravel the interplay between tectonics and sedimentation by means of multivariate statistical methods adapted to the particular nature of the available data (concentrations and percentages). The biplot was particularly useful in order to extract details in terms of source area evolution. The indications obtained call for a progressive enrichment from mafic to felsic elements, corresponding to increasing rates of supply from deeper levels of the crust. In conclusion, petrostratigraphic (detrital modes) evolution and geochemical signatures of the Eocene-to-Oligocene sandstone suites of the western portions of the Thrace basin in Greece and Bulgaria is closely related to various geodynamic stages of the Rhodopian region, from collisional to post-collisional orogenic collapse and the superimposed volcanism related to extensional collapse. The type of sedimentary provenance of these Rhodopian Paleogene sandstones, provide an example of the changing nature of orogenic belts through time, and may contribute to the general understanding of similar geodynamic settings.