Reconstructing the nature of the metamorphic basement in the Cyclades, Greece, by the isotopic signature of hydrothermal minerals

In complex continental back-arc settings, such as the Cycladic realm of southeast Greece, polymetallic mineralization can provide information about past tectonic events and help to reconstruct the architecture of the metamorphic basement. In the Cyclades archipelago, slab rollback generated Miocene crustal extension and the formation of metamorphic core complexes, where hydrothermal systems occurring in the upper crust track one of the latest events in the geodynamic evolution. Lead isotope ratios of galena (PbS) and Sr isotope ratios of barite (BaSO₄) of the polymetallic ore deposits, which are exposed on most of the islands, reveal two distinct sources of lead and strontium in the underlying basement of the core complexes. A clear division exists between ²⁰⁶Pb/²⁰⁴Pb≤18.84 and ⁸⁷Sr/⁸⁶Sr≥0.711 in the north-central Cyclades and ²⁰⁶Pb/²⁰⁴Pb≥18.84 and ⁸⁷Sr/⁸⁶Sr≤0.711 in the west Cyclades. This regional pattern corresponds with the exposures of the high-pressure/low-temperature Upper and Lower Cycladic Blueschist nappes on the islands, respectively. Moreover, the pattern follows the surface trace of the proposed synorogenic Trans-Cycladic Thrust, a subduction-related structure that imbricated the high-pressure units. In addition, the isotopic composition of the hydrothermal minerals indicate that the structurally lower Cycladic Basement has a stronger influence on the metal sources of deposits occurring in the hanging wall of the Trans-Cycladic Thrust than in the footwall. Observations featuring the isotopic signature of hydrothermal minerals are complemented by a compilation of new and published isotopic and geochemical whole-rock data of the basement rock types. This allows us to further interpret the geodynamic significance of the apparent heterogeneities in the exposed rocks of the metamorphic core complexes and correlate the scattered exposures of the metamorphic basement. Furthermore, regional patterns in the isotopic and geochemical signature of the basement lithologies help to reconstruct the nature of the continental margin prior to Cretaceous-Eocene subduction.